LHCf. Study of forward physics in p snn =8.1 TeV proton-lead ion. collisions with the LHCf detector at. the LHC

Transcription

1 LHCf Letter of Intent for a p-pb run in 2016 CERN-LHCC / LHCC-I /03/2016 Study of forward physics in p snn =8.1 TeV proton-lead ion collisions with the LHCf detector at the LHC

2 The LHCf collaboration O. Adriani 1,2,E.Berti 1,2,L.Bonechi 1,M.Bongi 1,2,G.Castellini 3, R. D Alessandro 1,2, M. Haguenauer 4, Y. Itow 5,6,T.Iwata 7, K. Kasahara 7, Y. Makino 5,K.Masuda 5,E.Matsubayashi 5, Y. Matsubara 5, H. Menjo 8, Y. Muraki 5, Y. Okuno 5,P.Papini 1, S. Ricciarini 3,T.Sako 5,6, N. Sakurai 9,T.Suzuki 7, Y. Shimizu 10, T. Tamura 10, A. Tiberio 1,2,S.Torii 7, A. Tricomi 11,12, W. C. Turner 13, M. Ueno 5,K.Yoshida 14,andQ.D.Zhou 5 1 INFN Florence, Italy 2 University of Florence, Italy 3 IFAC-CNR, Florence, Italy 4 École-Polytechnique, Paris, France 5 Institute for Space-Earth Environmental Research, Nagoya University, Japan 6 Kobayashi Maskawa Institute for the Origin of Particles and the Universe, Nagoya, Japan 7 Waseda University, Tokyo, Japan 8 Graduate School of Science, Nagoya University, Japan 9 Tokushima University, Japan 10 Kanagawa University, Yokohama, Japan 11 INFN Catania, Italy 12 University of Catania, Italy 13 LBNL, Berkeley, California, USA 14 Shibaura Institute of Technology, Japan February 28, 2016

3 Abstract The LHCf detectors were installed for the first time in the TAN regions on both sides of IP1 at the beginning of the LHC run in The goal of the experiment is the measurement of neutral particle production at very high pseudo-rapidity values ( >8.4) in proton-proton (p+p) and proton-ion (p+a) collisions. Until now the experiment has achieved successful measurements for p+p collisions at p s =900 GeV, 2.78 TeV, 7 TeV and 13 TeV and for p+pb collision at p s NN =5.0TeV. These data will be extremely useful in the near future for the calibration of hadronic interaction models that are commonly used for the study of the development of Extensive Air Showers (EAS) produced by extremely energetic cosmic-rays (CR) interacting with the atmospheric gas. Even though the most frequent collisions of cosmic ray protons in atmosphere involve mainly nitrogen and oxygen nuclei, the study of both the p+p and p+pb systems at the LHC allows providing important information for the calibration of hadronic interactions models. A marked reduction of cross section values in p+a interactions with respect to p+p collisions, due to nuclear screening e ects, has been observed in previous measurements performed at the RHIC accelerator at lower values of pseudo-rapidity and energy with respect to LHCf, and confirmed by LHCf at the LHC energy in the first p+pb run at the beginning of The last upgrade of the LHC, followed by the successful p+p run at 13 TeV, could allow now producing p+pb collisions at p s NN =8.1TeV. In this case the equivalent energy of the colliding protons in the laboratory frame (i.e. the frame at rest with respect to the target Lead nuclei) would be approximately 3 times greater than the highest ever reached before, thus representing a significant opportunity for our collaboration and hadronic model developers. For this reason we propose to install one of the LHCf detectors for the p+pb run that is under discussion for 2016, with the goal of reaching a better understanding of the nuclear e ects in a pseudo-rapidity and energy configuration that is very significant for CR physics and never investigated before.

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5 i Contents 1 Introduction 1 2 Physics motivation First results of the 2013 p+pb run at p s NN =5TeV Combined LHCf/ATLAS data taking Detector selection and installation Hardware setup Installation requirements Estimation of activation at TAN after the p+p 2016 run at high luminosity 8 4 Simulation of p+pb events at p s NN =8.1TeV Proton remnant side Hit multiplicities Photon and neutron spectra Neutral pions Selection of the LHCf events by using information at low pseudorapidity Lead remnant side The LHCf ideal run Minimum physics program Pile-up e ect and signal overlap Radiation damage Summary (ideal LHCf run) Non-ideal run: study of the realistic case Luminosity Radiation damage Pile-up and signal overlap Beam Crossing Angle Beam optics Data acquisition time Summary (realistic non-ideal run) Acknowledgments 26

6 ii List of Figures 1 Invariant mass distribution of pairs of photons data Transverse momentum spectra data Nuclear Modification Factor data Neutron spectra data for p+p at 7 TeV Number of secondary particles produced in p+pb collisions Multiplicities of photons and neutron hits on the proton-remnant side Energy spectra of single photons hitting the Arm2 detector on the proton-remnant side Energy spectra of neutrons hitting the Arm2 detector on the protonremnant side Invariant mass of gamma-ray pairs Energy spectrum of fully reconstructed pion events Selection of LHCf events with info from ATLAS Two dimensional projection of the LHCf Arm2 detector for zero beam crossing angle Two dimensional projection of the LHCf Arm2 detector 340 µrad beam crossing angle and upward going beams Invariant mass distribution in case of upward going beams and 370µrad beam crossing angle Neutral pion measurement with upward going beams: standard detector position Neutral pion measurement with upward going beams: detector 44 mm lower than the standard position Study of single ray p t spectra with =0.4m List of Tables 1 Statistics of relevant classes of events in 10 7 p+pb collisions at p s NN = 8.1TeV

7 1 1 Introduction The LHCf experiment at LHC is carrying out an extensive study of neutral particles emitted in high energy p+p or p+a collisions at very small angles with respect to the interaction line, by accessing the rapidity range from y = 8.4 toinfinity. Resultsfrom the data collected so far represents a reference for the calibration of hadronic interaction models that are commonly used for the simulation of the development of cosmic-ray showers in the Earth atmosphere. Cosmic-ray showers, or extensive air showers (EAS), are huge cascades of secondary particles produced by the interaction of primary cosmic rays (elementary particles coming from outer space with energies up to ev) with the atmosphere. The energy flow in the showers is dominated by particles and nuclear fragments that are emitted at very small angles with respect to the arrival direction of the incoming projectile. For this reason LHCf is designed in such a way to cover the rapidity region from 8.4 up to infinity (very forward region). In the last few years the LHCf collaboration has published several results concerning the measurement, at extreme values of the pseudo-rapidity [1, 2, 3, 4, 5], of single gamma ray, neutral pion and neutron spectra in p+p collisions at p s =900GeV, 2.76 TeV, 7TeV, 13 TeV and for p+pb collisions at p s NN =5.02TeV. The investigated configurations correspond to an incoming proton energy ranging from approximately ev up to slightly more than ev in the Earth laboratory frame (LAB). These results have shown a disagreement between all the main hadronic interaction models and the experimental data. In fact none of the models is able to reproduce forward data over the whole energy range within the experimental errors. This is especially true for the hadronic component [5], which is extremely important to understand the anomalous muon multiplicity observed in EAS by di erent experiments. This means that it is possible to use the LHCf results to further improve models, even if it is not a trivial work to integrate the LHCf results in the models themselves. One of the main points recently investigated with model developers is the possibility to identify the contributions by di ractive and non-di ractive events in the LHCf data. According to model developers these two classes of events are treated in completely independent ways in the software implementations of the models and the separation of their contributions in the LHCf data could facilitate the inclusion of the LHCf data. For this purpose, since the recent 13 TeV p+p run the LHCf and ATLAS collaborations have implemented a trigger sharing system that allows collecting common data sets. Exploiting the information by the ATLAS detector on the activity at low pseudo-rapidity it is possible to make this separation. Due to the previous positive experience, both the collaborations are available to confirm this trigger configuration during an eventual LHCf run. 2 Physics motivation The interaction of CRs, mainly protons, in the Earth s atmosphere necessarily involves light nuclei, like nitrogen and oxygen nuclei. Actually the incoming projectile particles

8 2 Figure 1: Invariant mass distribution of pairs of photons detected by the LHCf detector in the 2013 p+pb run at p s NN =5TeV[4]. Thepeakcorrespondingtothe 0 semitted in the range of rapidity 9.4 <Y < 9.2 isclearlyvisible,centeredatthevalueof the 0 mass. No other peaks can be found. Selection of 0 candidates and evaluation of the background are also shown in figure. can be simply protons, but they can also be light or heavy nuclei. Therefore, the study of forward physics in p+p and p+a collisions at high energy at LHC is of extreme importance for the understanding of the properties of hadronic interactions at CR energies. Before the LHC era, this kind of physics has been addressed only by the UA7 collaboration[6], that measured neutral particle production in p+ p collisions at lower energy and rapidity values than those accessible with the LHCf detector. The study of forward physics in p+a collisions provides complementary information to the p+p case and is necessary to further improve the understanding of how the physics parameters of the interaction change because of nuclear e ects. In principle the ideal and most direct way to provide these important results to the cosmic ray physicists, is the measurement of forward emitted particles in proton interactions with nitrogen or oxygen nuclei. In case this opportunity is granted by the LHC in the future, the LHCf collaboration is motivated to propose a new measurement. Anyway this seems not to be possible at the moment either at the LHC or at other facilities, at least at TeV energies. The LHC though, has already provided LHCf in 2013 the possibility to collect data for the study of nuclear e ects in the forward region of the collisions between protons and Lead nuclei at the TeV scale. The corresponding proton energy in the LAB frame was of approximately ev. In a certain way the measurement with very heavy nuclei is an advantage this sense with respect to the light ion case, because stronger nuclear e ects are expected as the mass number of the nucleus increases. Previous measurements with Gold nuclei at other facilities [7] reported a marked reduction of particle production cross section for low transverse momentum events in the forward

9 3 Figure 2: Transverse momentum spectra for di erent bins of the rapidity in the 2013 p+pb run at p s NN =5TeV[4]. region of heavy ion interactions, with respect to what could be expected considering the events as simple binary collisions between nucleons. These results, reported as the ratio between cross sections measured in p+a (or A+A) and in p+p collisions (Nuclear Modification Factor, NMF), are currently explained with nucleon screening e ects and saturation of partonic densities, but so far a poor knowledge of the mechanisms involved has been achieved, especially at extreme rapidity, where at present only the LHCf measurements can provide new significant experimental data. 2.1 First results of the 2013 p+pb run at p s NN =5TeV At the end of 2012 the LHCf collaboration installed one of the two LHCf detectors (called Arm2) in the same location that we are proposing for the 2016 run. The purpose was to make a measurement during p+pb collisions on the p-remnant side and for a short time also on the Pb-remnant side. The run was successful and we achieved the required statistics in a few days of running. A first test of common data taking with the ATLAS collaboration was done in view of the 2015 p+p run at 13 TeV. This test was successful as well, but it didn t allow to accumulate enough statistics for a full combined data analysis. Data analysis of the 2013 run, even if not completed yet, has already given interesting results. Until now only neutral pions have been investigated. In figure 1 the invariant mass distribution of pairs of photons detected separately by the two calorimeter towers of the Arm2 detector is shown. A peak centered at the 0 mass is evident in this plot. Neutral pion events are identified as the events under this peak. The small background is evaluated and subtracted in the final analysis. In figure

10 4 Figure 3: Dependence of the Nuclear Modification Factor for 0 production on p t, measured in di erent rapidity intervals with data from the 2013 p+pb run at p s NN = 5TeV [4]. 2thetransversemomentum(p t )spectrafordi erentbinsoftherapidityareshown. Results show that 0 data are described quite well by the three hadronic models that have been simulated. Data points reported by LHCf are always delimited between the expectations of the di erent models and almost compatible with all three within the errors. The error bars associated to data are anyway too high to make an accurate comparison and this is due in part to the limited statistics of the collected neutral pions. A larger statistics could be collected in the next p+pb run at higher energy. The NMF has also been measured in the same bins of particle rapidity. Results are reported in figure 3. At very low p t the LHCf data confirm the production suppression previously observed experimentally at RHIC. The error bars are anyway too large to allow selecting among the di erent hadronic models. The study of single photons and of hadrons, the latter extremely interesting to understand the anomalous muon multiplicity observed at ground level in cosmic ray showers, for the for the 2013 run have still to be finalized Combined LHCf/ATLAS data taking As anticipated in the introduction (section 1) the LHCf and ATLAS collaboration have tested for the first time the procedures for a common data taking during the p+pb run in 2013, in view of the p+p run at 13 TeV of The test was successful. The LHCf trigger was included in the ATLAS minimum bias trigger to trigger the

11 5 Figure 4: Left: di erence between the event time stamps recorded by ATLAS and LHCf detectors. Right: comparison of BCID values assigned by the two detectors, based on the event time stamps. ATLAS detector, prescaled down in such a way to have a common trigger with a rate of 10 Hz approximately. The identification of common events worked very well. A first combined study has been recently performed jointly by the LHCf and ATLAS groups. Preliminary results are summarized in an o cial note signed by the two collaborations [8]. In figure 4, taken from this note, the identification of common events is checked comparing the event time stamps and the colliding bunch ID (BCID) attached by the two independent DAQ systems to data. Time stamps are identical at the level of a few nanoseconds, while the BCID, assigned on the basis of the time stamp, is in agreement in the 99.7% of all triggered events. For further details and results refer to [8]. After the successful test in 2013 the same setup for common data taking between ATLAS and LHCf has been implemented for the 2015 p+p run. An agreement was signed by both the collaboration for the inclusion of the LHCf trigger as part of the ATLAS minimum bias trigger, prescaled down to 100 Hz. The new common run was successful. Also the ALFA data were included in the final stream, to allow both LHCf- ATLAS and LHCf-ALFA combined analysis, which are on-going. On the basis of the previous discussions, the 2016 run at LHC would allow LHCf to provide an important data set for many reasons: the corresponding energy of the projectile proton in the LAB frame, approximately ev, is almost three times greater that in the 2013 run ahighertotalenergyallowsaccessingaregionofphasespaceathigherp t,where discrepancies from models are more accentuated the common data taking with ATLAS, performed only partially in 2013, would allow removing UPC events and selecting non-di ractive from di ractive collisions

12 6 the measurement at a higher energy allows a study of the Feynman scaling, which is a powerful tool to extrapolate model predictions at high energy ahigherstatisticscouldbecollectedtoimprovetheanalysisofneutralpion events and 4) more rare particles, like the meson, could be detected (as shown in section Detector selection and installation The LHCf experiment is based on two sampling electromagnetic calorimeters called Arm1 and Arm2, made of GSO scintillator and tungsten layers. Each detector is composed of two independent calorimeter towers, complemented by a tracking system, assembled in a cm 3 box which contains also some part of the front-end electronics. Detailed information on the hardware are published in [9, 10, 11]. The LHCf detectors are located in the same locations as the ATLAS ZDC calorimeters. The installation of the LHCf detectors requires replacing some modules of the ZDC (a discussion between the LHCf and ATLAS collaborations is on-going and the ATLAS spokesperson kindly supports the LHCf installation in case the LHCf run is approved). During data taking the LHCf detectors are positioned in such a way that one of the calorimeter towers, the smallest one, lies directly on the beam line, at zero degrees. The other calorimeter tower, slightly larger, is located upward the former. The LHCf standard run configuration, used during all the previous runs except for the p+pb run, requires the installation of both detectors inside the reserved slots of the two TAN absorbers located 140 m on opposite sides of the Interaction Point (IP1). The Arm1 detector is located in between IP1 and IP8 (LSS1L sector), while the Arm2 detector is located in between IP1 and IP2 (LSS1R sector). This allows a comparison of the results between the two detectors and also the study of double di ractive events. Although it is important to consider these aspects, in order to minimize the impact on the ATLAS ZDC physics program during the 2013 p+pb run, the LHCf collaboration, in agreement with the ATLAS management, proposed the installation of only one detector in the TAN absorber located between IP1 and IP2, that is the LSS1R sector, thus defining a di erent configuration with respect to the nominal one. In this configuration LHCf measured mainly in the p-remnant side. Thanks to the beam swap we had the possibility to take data for a short time also in the Pb-remnant side, but due to the high hit multiplicity the Arm2 detector was operated in a non-standard position, few centimeters far from the beam line. Based on this premise, in case the p+pb run at p s NN =8.1TeV under consideration for 2016 is approved, we propose to install the Arm2 detector in the usual location between IP1 and IP2 (LSS1R) and to collect data for approximately one day only in the p-remnant side and at a luminosity of cm 2 s 1. With a very low priority and only in case it does not represent a problem to the other experiments, LHCf would be also interested in some hours data taking

13 7 at higher luminosity. The purpose of this additional activity, for which no requests would be done, is to enhance the statistics for the most rare type of events by using special trigger configurations. In case the time slot reserved to p+pb collision is shared between p+pb at p s NN =8.1TeV and p s NN =5.0TeV and only in the same previous hypothesis (no interference with the programs of the other experiments), the LHCf collaboration would be also interested in a short run at lower energy. This would help increasing the limited statistics of common LHCf-ATLAS triggers collected in Hardware setup The detector that we propose to install for the p+pb run is referred to as Arm2 in all LHCf documents and publications [1, 9, 10]. With respect to Arm1, it exploits the excellent spatial resolution of eight micro-strip silicon sensors arranged, as X/Y layers, placed at di erent depths inside the calorimeter. These silicon detectors have been mainly used in the previous data analysis for the measurement of particle impact points and for the identification of multiple hits, thanks to their fine segmentation. They are suitable also to be exploited for particle identification, allowing the reconstruction of the lateral shape of showers developing inside the calorimeter. Moreover they can also independently measure the energy released in the calorimeter with a good enough resolution, as demonstrated in [11], and can thus therefore be used, thanks to their radiation hardness, to monitor the stability of the energy measurements performed with the GSO scintillator system. For the 2015 p+p run at 13 TeV, the Arm2 silicon system has been upgraded with new silicon sensors, front-end electronics and a di erent readout scheme, in such a way to reduce the e ect of saturation of the front-end chips for high energy showers developing in the calorimeter. Finally all the original parts made with plastic scintillator were replaced by GSO for an improvement in radiation hardness. The upgraded detector has been tested at the SPS accelerator and successfully used for the LHCf p+p special run in June For a more detailed explanation of the Arm2 detector and DAQ refer to [10, 11, 12]. 3.2 Installation requirements The installation procedure of both LHCf detectors was discussed in details before the first installation of the detectors in the LHC tunnel. A detailed report can be found on the CERN EDMS database [13]. To minimize the time required to perform all the operation needed to install and remove the detectors to and from the TAN slot, additional work was done for the 2013 p+pb run. According to the original scheme the Arm2 detector was kept in the run position by a motorized system, called manipulator, that was fixed on the top of the TAN massive absorber. Two boxes, containing the readout electronics for the micro-strip silicon sensors and the preamplifiers for the PMTs used for the scintillator tiles of the calorimeter, were also independently fixed on top of the

14 8 TAN. Within this scheme, the most practical way to install or remove the detector was to have it disconnected from the electronic boxes and to position it by using a dedicated crane moving along rails fixed to the tunnel ceiling. This procedure was initially approved by the Radiation Protection Group (DGS-RP Group) because it satisfied all the requirements for safe operations. The time needed for cable handling was of the order of one hour and the total time for installation or removal of the Arm2 detector was approximately of two hours. Because the installation operation for the 2013 p+pb run was foreseen during the technical shutdown after the high luminosity p+p run, due to the activation of the TAN region we were required to rethink the installation procedure in such a way to reduce the necessary time and avoid health risks for the personnel involved. Following this request we completely reviewed the assembling of the LHCf detectors, producing a strong mounting plate to house all the originally separated blocks of our detectors. In this new scheme, the internal detector s cabling is completed and tested in our labs before the installation at the TAN, which is finally done by means of a mini-crane provided by the LHC and remotely controlled by specialized technicians. A few cables and optical fibers need to by connected at the end of the installation and the risks for involved personnel are substantially reduced. 3.3 Estimation of activation at TAN after the p+p 2016 run at high luminosity A discussion with dr. Cristina Adorisio, person in charge of the studies of material activation at the LHC, has already started for evaluating the expected activation in the TAN region after the main high luminosity p+p run at 13 TeV of 2016, before switching to p+pb. The LHCf detector would in fact be installed during the technical stop at the end of this run. A preliminary evaluation has been performed by taking into account the machine parameters that have been discussed at the recent workshop hold in Chamonix, where the possible scenario for the 2016 run has been outlined. As a result, from the point of view of the radioprotection, there are not impediments for an installation of the LHCf Arm2 detector in the TAN slot in the LSS1R region, by exploiting the remote handling system that was installed in the past years and already used for the 2013 and 2015 runs. On the basis of the detailed installation procedure, presented by Adorisio at the 13 th LTEX meeting in September 2012, the estimation of the individual dose for the most a ected worker is 300 µsv, considering that the installation is done during the 2016 TS3 and including 1 week cooling due to the foreseen MD week. The maximum residual dose rate in proximity of TAN is 140 µsv/h, while 30 µsv/h is expected in the corridor near the TAN. These estimates have been done assuming the LHC operational values announced by OP: peak luminosity of Hz cm 2 at 13 TeV. The measurements done around TAN during the 2015 TS3 have been also used. The installation is thus classified as ALARA level II, for which the submission of a Work and Dose Planning (WDP) and the usage of both personal and operational

15 9 Figure 5: Single neutron spectra measured during the 2010 p+p run at 7 TeV. Comparison of the LHCf results with model predictions at small tower ( >10.76) and large towers (8.99 < <9.22 and 8.81 < <8.99). The black markers and gray hatched areas show the combined results of the LHCf Arm1 and Arm2 detectors and the systematic errors, respectively [5]. dosimeters (DIS and DMC respectively) are mandatory. 4 Simulation of p+pb events at p s NN =8.1 TeV Looking at the physics issue, the asymmetry of the proton-lead ion collision leads to critical di erences in the events observed by the LHCf detector, depending on whether it is installed on the proton-remnant or Lead-remnant side. The proton-remnant side is evidently the most interesting from the point of view of CR physics, because on this side we can measure in principle all the particles that are emitted in the very forward region, once we translate the event back to the LAB rest frame. Due to its location along the LHC tunnel, LHCf cannot detect all of these particles, because the charged components are swept away by the D1 magnetic system located upstream. The neutral components, mainly photon and neutral pions, are studied instead, achieving very precise measurements of their energy and transverse momentum. Neutrons can also be measured, although with lower precision, due to the limited depth of the calorimeter (44 X 0,1.5 I ). Results on forward neutrons are really important for understanding the inelasticity of the collisions, defined as the fraction of the available energy released for multiple particle production in inelastic hadronic interactions. This is an important point, as claimed by theorists, for the interpretation of CR data and it is directly related to the anomalous muon abundance observed in cosmic ray showers. Moreover a study of the neutral-particle cross sections on this side can give information about the saturation of partonic densities and nuclear modification factors (NMF). In figure 5 the neutron spectra measured in p+p collisions during the LHCf special run at 7 TeV of 2010 are reported. Models gives incompatible predictions

16 10 Event type DPMJET EPOSLHC QGSJET II-04 HIJING (small) (large) (small+large) n(small) n(large) Table 1: summary of relevant events expected in 10 7 p+pb collisions at p s NN = 8.1 TeV. The first three lines show the number of events with a single detected - ray hit in the small and large tower of Arm2 and the number of events with single -ray hits on both the towers (i.e. candidate 0 events with background). The last two lines refer to single neutron events. and does not describe data accurately, especially at extreme pseudo-rapidity. Data seems to indicate a higher elasticity in this region with respect to models except for QGSJET II-03, which in turn does not describe well data at higher pseudo-rapidities. Concerning the nucleus-remnant side, it turns also to be significant because on this side we find all the debris produced in the breakup of the nucleus and therefore the nuclear fragmentation region can be studied in some details. A simulation of the LHCf apparatus in these two di erent configurations has been implemented, considering only the Arm2 detector geometry, to verify the feasibility of the measurement and the quality of the expected results. The Cosmic Ray Monte Carlo (CRMC) simulator has been used to generate collision products using four di erent hadronic models: DPMJET [14], EPOSLHC [15], QGSJET II-04 [16] and HIJING [17]. A total of 10 7 events have been generated for each model, considering a 6.5 TeV projectile proton colliding with a 2.56 TeV/nucleon Lead ion with a null crossing angle and random impact parameter. The total energy in the nucleon-nucleon center of mass frame (NNCM 1 )is approximately 8.1 TeV. As a results of the simulations, similarly to the 2013 run, on the Pb-remnant side the hit multiplicity expected in LHCf is too high and the detector is not suitable for any significant study. Thus, in the following part of this document we present only results of the simulation for the p-remnant side. In table 1 the expected number of interesting events on the p-remnant side are shown for all the considered hadronic interaction models. It is easy to identify some remarkable di erence in these values. 4.1 Proton remnant side The distributions of the number of secondary particles generated by the four hadronic interaction models are compared in figure 6. These distributions include all particles 1 The NNCM frame is defined as the frame at rest with respect to the center of mass of a pair of nucleons, one from the projectile and one from the target.

17 11 Figure 6: Distribution of the number of secondary particles expected in p+pb collisions with di erent hadronic interaction models. without any energy or angular limitations and result quite di erent. Some problem is found with the DPMJET 3.0-6, for which, di erently from the other models, the distributions begin around 200. This is probably due to the inclusion of the surviving nucleons of projectile and target nuclei in the set of secondary particles. The HIJING and QGSJET II-04 models have a long tail towards very populated events. The distribution simulated with DPMJET for p+pb collisions at a lower energy is also compared and looks like almost identical to that obtained with EPOSLHC at higher energy, apart from a shift. All these aspects have not been investigated yet and will be studied in the next months together with model developers, with whom we are constantly in touch. In the next sections the results of this preliminary simulation are shown with some details: hit multiplicity (section 4.1.1), photon and neutron spectra (section 4.1.2), neutral pions (section 4.1.3) and the potential of the combined LHCf-ATLAS run (section 4.1.4). All the results include only the QCD events and do not consider the unavoidable e.m. Ultra Peripheral Collisions (UPC), whose contribution must be carefully estimated and finally subtracted from real data. The reported error bars include only statistical uncertainties Hit multiplicities Multiplicities of neutral particles hitting the two calorimeter towers of the LHCf Arm2 detector, located on the proton-remnant side, are shown in figure 7. The four distributions in this figure refer to photons (top) and neutrons (bottom) both on the small tower (left), which is located on the beam line in this simulation, and the large tower

18 12 Figure 7: Multiplicities of photons and neutron hits on the proton-remnant side. Top left: on small tower; top right: on big tower; bottom left: n on small tower; bottom right: n on big tower. (right). From these pictures we see that the HIJING model shows a poor agreement with the other models, which are more commonly used for cosmic ray physics. The number of expected events can di er for a factor 10 among the di erent models. No cut on particle energy is applied for this comparison and no UPC events are included. Overall we find low values of the multiplicities for the majority of the events. Using 10 7 collisions generated with the DPMJET model, we find 1.63 million events with at least one hit in the Arm2 detector, of which 1.16 million events with only one hit. The total acceptance (i.e. the fraction of collisions in which we have hits in the Arm2 detector) is then 16.3% (these numbers must be anyway multiplied by a factor 2 approximately to take into account the contribution due to UPC events). The number of events with more than one hit in a single tower (multi-hit) is approximately Therefore, we can quantify such an e ect at the level of 7%. Multi-hit events can be recognized by exploiting the fine spatial resolution of the microstrip silicon detectors, which allow identifying events with multiple peaks visible in their transverse profile Photon and neutron spectra The energy spectra of photons entering the two calorimeter towers are shown in the top part of figure 8. Left and right plots refers to the small and large calorimeter towers respectively. Only events with a single e.m. hit in the fiducial region of the detector have been selected for this study. The measured energy resolution for the two calorimeter towers has been included in the simulation. On the bottom part of the same figure the ratios between the QGSJET II-04 and DPMJET spectra are shown for

19 13 Figure 8: Top graphs: energy spectra of single photons, normalized to the number of inelastic collisions. Bottom graphs: QGSJET II-04 to DPMJET ratio at p snn =8.1 TeV. both towers. The generated statistics is enough to bring out a clear disagreement between the models, especially for photon energies beyond 1 TeV. This discrepancy is more important for the small tower, which detect particles at extreme values of the pseudo-rapidity (greater than 9.9), but it is also evident for the large tower. In particular, the DPMJET model seems to be the hardest among the considered models, while the HIJING and QGSJET models the softest ones. Data could be further divided in di erent bins of rapidity, in order to present the results in a more suitable form for the model developers. The neutron energy spectra for the two calorimeter towers are shown in top part of figure 9. Only one model, QGSJET, predict a peak of the very forward neutron spectrum at very high energy. In the middle plots the previous graphs are reproduced including an energy spread due to a detector s energy resolution of 35% over the whole energy range (it explains the tails exceeding 6.5 TeV in the energy distribution). Even if it is not a realistic assumption, it represents a quite pessimistic case. From simulations based on the DPMJET model we find that this value of the resolution is reasonable for neutrons with an energy of a few TeV, while it is approximately 20 % at low energy (few hundred GeV) and reaches 39 % for 6 TeV neutrons. The worsening at high energy is mainly due to the longitudinal energy leakage due to the limited depth of the LHCf calorimeters. The ratio between the spectra produced by the QGSJET and DPMJET models is shown in the plots at the bottom of the same figure. At extreme pseudorapidity predictions by di erent models are completely in disagreement and a factor up to 10 for very high energy neutrons is found. As demonstrated recently for p+p collisions at 7 TeV, even though the LHCf detectors are not optimized for neutron

20 14 Figure 9: Top graphs: energy spectra of neutrons, normalized to the number of inelastic collisions. Middle graphs: same including the neutron energy resolution. Bottom graphs: QGSJET II-04 to DPMJET ratio at p s NN =8.1 TeV. energy measurements due to their limited depth (44X 0, 1.5 i ), the achieved detector s performance allow us to exploit the neutron analysis as a powerful tool, complementary to photons and neutral pions, for the verification of model predictions Neutral pions Apreliminarystudyofneutralpionshasbeenperformed,reconstructingeventswith two gamma rays entering the acceptance of the Arm2 detector. The expected distribution of the invariant mass of gamma-ray pairs is shown in figure 10 for the DPMJET model. For comparison also the distribution for p+pb collisions at lower energy obtained with the DPMJET has been reported. Only events with two gamma rays separately hitting the two calorimeter towers have been included in this analysis. A huge clear peak corresponding to the gamma-ray pairs produced by neutral pion decays can be easily identified and can be used for a consistency check of the energy scale calibration for the calorimeters. The fraction of collisions that produce a neutral pion signature in the Arm2 calorimeter is much higher at the energy considered in this proposal with respect to the 2013 run (a factor 5 greater). The selection of neutral

21 15 Figure 10: distribution of invariant mass of gamma-ray pairs for events with two photons hitting separately each calorimeter tower. Figure 11: left side: energy spectrum of pions for events with two photons hitting separately each calorimeter tower. Right side graph shows the ratio between DPMJET- III and EPOS expected 0 energy spectra. pion events can be performed with a background of the order of 5-7 % depending on the selection cut. This background is basically due to gamma-ray pairs originated in di erent processes and entering the detector at the same time. The 0 energy spectra predicted by the di erent models are shown on the left side of figure 11. Even with relatively low statistics, a disagreement between the models is evident particularly for pion energy beyond 2 TeV. This inconsistency is more accentuated for the HIJING model with respect to the other models and can be better quantified looking the graph at the right side of the figure, where the ratio between the HIJING and QGSJET spectra is shown. Between the spectra produced by these two models there is a factor which runs from 0.4 at low energy down to 0.2 at high energy. The DPMJET model predict the hardest spectrum, while HIJING the softest one. Around 9000 neutral pion events have been selected, using the DPMJET distribution, with a cut on the invariant

22 16 mass around the peak. This statistics does not allow anyway to perform a much more sophisticated analysis, by selecting for example events falling into di erent intervals of the pseudo-rapidity. A larger statistics of collisions is required for an improved analysis of the neutral pion component (see section 5). Afurtherpositiveresultexpectedonthebasisofthesimulationsistheevidence for a new peak in the two photons invariant mass distribution (figure 10), which was not statistically significant in the 2013 p+pb run at lower energy. The new peak is centered approximately at w =0.55GeV, which correspond to the mass of the eta meson. Therefore, a limited but significant analysis can be performed also for this particle, which is not expected to su er from the background due to UPC collisions. Approximately 230 events are found in the identified peak predicted by the DPMJET model. A statistic 10 time greater than that used in this simulation could thus allow a study of the energy spectrum of forward emitted eta mesons Selection of the LHCf events by using information at low pseudorapidity As previously anticipated, in case the LHCf request is approved there is already an agreement with the ATLAS collaboration for a common data taking, as implemented for the previous p+pb 2013 and p+p 2015 runs. The information on the activity registered in the ATLAS central detector can be exploited to classify the LHCf events. For example the di ractive contribution can be selected requiring no activity in ATLAS and non di ractive events requiring at least N tracks reconstructed in the ATLAS tracker. An example of the results achievable with this method, proposed by T. Pierog, is shown in figure 12. Note: UPC events, characterized by the interaction of a very high energy proton with a low energy virtual photon produced by the target nucleus, do not give activity in the central detector and can be removed when applying the selection used in this example. 4.2 Lead remnant side Due to the high multiplicity expected in the LHCf calorimeter within a distance of several centimeters from the beam center, we don t plan to take data in the Lead remnant side. In case both the configuration p+pb and Pb+p are activated during the proton-lead run, LHCf will be kept in a safe position (the so called garage position) to avoid radiation damage. 5 The LHCf ideal run In this section we summarize the requirements of the LHCf collaboration for a fruitful run with p-pb collisions at p s NN =8.1 TeV.

23 17 Figure 12: Selection of the LHCf events with a cut on the number of tracks of charged particles registered by the ATLAS central detector at small values of the pseudorapidity. First of all we want to recall again that, with the goal of minimizing the interference between the LHCf and the ATLAS ZDC experiments, we propose to install only one of the LHCf detectors. According to the previous experience achieved in the p+pb run of 2013, this solution is appropriate to ensure a safe data collection and a good data quality. In particular we would like to install the Arm2 detector in the TAN located between IP1 and IP2 (LSS1R). This location should be the proton-remnant side as determined by the usual beam configuration defined by the LHC (BEAM1 = protons, BEAM2 = ions). The choice of the Arm2 detector is due essentially to the better spatial resolution of microstrip silicon detectors, that allows a better multi-hit rejection with respect to the scintillating fibers installed in the Arm1 detector, and the faster front-end electronics. Atotalbeamcrossingangleintherangebetween290µrad and 370 µrad, with downward going beams, is useful to allow extending the accessible range of particle transverse momentum. This is obtained by moving the detector along the vertical direction, thus making a position scan. In paragraph 5.1 and 5.2 we report on the ideal plan to complete the minimum physics program for the p-pb run and we make an estimate of the pile-up e ect and signal overlap in LHCf electronics. We assume an interaction cross section int = 2.2 barn, a nominal luminosity L nom =10 28 cm 2 s 1 and a nominal number of bunches n nom =400,withauniformbunchspacingofapproximately220ns. Currentlythe LHCf trigger setup requires a minimum bunch spacing of around 150 ns, that could be probably reduced to 100 ns with some e ort, if required.

24 Minimum physics program The LHCf minimum physics program is defined as the production of single photon and neutron spectra separately in the two calorimeter towers, up to X F ' 1, and the reconstruction of N > neutral pion events, to be exploited both for physics studies and for the detector s energy calibration. The main limitation for the collection of enough data to complete the minimum physics program is related to the common LHCf-ATLAS operations, for which the LHCf final trigger will be prescaled down and used to trigger the ATLAS detector. In the previous weeks the two collaborations confirmed the scheme used for the 2015 run, when the the LHCf trigger was prescaled down to 100 Hz. According to the previous experience and the new simulations, the minimum LHCf-ATLAS physics program can be achieved collecting at least common triggers. More than 50% of trigger are associated with electromagnetic UPC events, whose contribution has to be subtracted to allow a QCD-driven study. About 30% of triggers are due to single photon hits. To collect this statistics with a common trigger rate of 100 Hz takes approximately 17 hours, assuming a 100% LHC e ciency. With this assumption the corresponding integrated luminosity provided by LHC for this run, assuming a luminosity L =10 28 cm 2 s 1,is L int =0.6nb 1. A discussion between the LHCf and ATLAS collaboration is on-going to verify the possibility to increase the rate allowed by ATLAS for the common operations, from 100 Hz to approximately 400 Hz. This would allow reducing the necessary time required for the LHCf data taking. Half a day with this common rate would allow for example to collect a larger statistics (with respect to that requested for the minimum physics program) and to have some margin for eventual ine ciencies. 5.2 Pile-up e ect and signal overlap The event pile-up e ect is a real problem for the LHCf measurement. It is in fact impossible for LHCf to distinguish secondary particles produced in di erent collisions in the same bunch crossing. Therefore it is important to run with a low value of the µ parameter, defined as the mean number of interactions per single bunch crossing. Considering a nominal luminosity L nom =10 28 cm 2 s 1 we find: µ = N t = Lnom int n nom f rev = 1028 cm 2 s cm s 400 ' At this value of µ and considering also UPC events and the geometrical acceptance of the Arm2 detector, the fraction of triggers which are a ected by the pile-up e ect is approximately 0.25% and does not represent a real problem for the measurement. An additional and potentially critical limit for the LHCf Arm2 detector is represented by the time gate that is used to integrate the signals produced by the scintillator system of the calorimeter. The fast anode signals of the Arm2 PMTs go from the TAN region to the counting room in USA15, 200 meters far, and are then integrated in a

25 19 QDC module. The time gate for the integration is 500 ns because the signal widths increase while traveling along the BNC cables from the detector to USA15. If after a particle hit, a new particle hits the Arm2 calorimeter within this time interval, a superposition of signals occurs and multiple signals are integrated at once. Considering that in 500 ns we have additional bunch crossings, we can estimate the probability of such an e ect. Clearly the eventual collision in these additional bunch crossings determine a signal overlap only when they produce secondaries within the Arm2 acceptance. If we consider also UPC collisions, the probability to have such an e ect, based on the Poisson statistics, is of the order of 0.4%. In conclusion, in the considered configuration the sum of the two e ects remains below the acceptable level of 1%. 5.3 Radiation damage Studies of the degradation of the GSO signal due to radiation have been performed in Japan by irradiating GSO plates with di erent ions. Results show that the light signal begins to change at a dose of the order of few hundreds Gy. At 400 Gy we have a variation of a few percent. Calculations of dose rate at the Arm2 detector have been performed taking into account both the contributions by QCD and UPC to the expected photon and neutron spectra at the entrance of the detector. For a value of luminosity of cm 2 s 1 the resulting maximum dose rate inside the detector is of the order of 40 Gy/d. In this condition we don t expect significant e ects after one day of data taking. 5.4 Summary (ideal LHCf run) In this short section we summarize the ideal configuration for the LHCf run and the necessary time to collect enough statistics for the minimum physics program. The ideal LHCf run is summarized in the following points: p+pb run at p s NN =8.1TeV Maximum luminosity apple cm 1 s 1 (constraint due to radiation damage, pile-up and signal overlap) & 10 m (constraint due to beam angular aperture) Number if bunches apple 600 (constraint due to trigger logic) Installation of Arm2 detector on LSS1R side Beam1 = protons; Beam2 = Lead nuclei (constraint due to high multiplicity in Pb-remnant side)

26 20 Beam Crossing Angle: µrad with downward going beams (constraint due to the size and shape of the geometrical region accessible for physics measurements inside the TAN) 6 Non-ideal run: study of the realistic case Due to the several constraints to which the LHC could be subject to implement both the 5 TeV and 8.1 TeV p+pb runs, we have investigated the possible problems that LHCf might su er in case of a not-ideal run. The considered aspects concern the luminosity, the beam crossing angle scheme, the value of and the allowed data acquisition time. 6.1 Luminosity We evaluate here the impact of a luminosity of the order of cm 2 s 1 on the eventual LHCf run in terms of radiation damage, event pile-up and signal overlap in LHCf. Di erently from other experiments, which are requesting to reach quickly a luminosity of the order of cm 2 s 1, the LHCf ideal value is around a factor 10 lower. This is in part necessary to protect the detector from radiation damage, but the other two factor are surely more relevant Radiation damage From the point of view of radiation damage, after one day exposure at a luminosity of cm 2 s 1, some GSO plate of the Arm2 calorimeter integrate a dose of approximately 400 Gy. At this value we have already a variation of a few percent of the GSO light output. Even if the e ect is at the level of a few percent, we prefer to run at lower luminosity, at least until a set of data su cient to achieve the minimum physics program has been collected Pile-up and signal overlap The probability of these e ects, shortly discussed in section 5.2, increase with increasing luminosity. With respect to the case of cm 2 s 1,thefractionofeventssubject to these two e ect increases of a factor 10, reaching 3% and 4% respectively. In this situation the LHCf measurements would be unreliable. 6.2 Beam Crossing Angle According to the strategy previously defined by the LHC management and experts, each year the beam crossing angles should be flipped, in such a way to avoid directing the beams always towards the same direction and irradiating the downstream LHC instrumentation always on the same place. Flipping the BCA allows to preserve as much as possible the LHC magnetic systems and to extend their life, quantified in

27 21 Figure 13: Cross section of TAN and Arm2 detector in the default LHCf running configuration for zero beam crossing angle. 300 fb 1. For the 2016 p+p run the BCA at IP1 should be set, according to this choice, still in the vertical plane, but with upward going beams instead of downward going. The extrapolation of the beam line from IP1 to the TAN region would hit the LHCf Arm2 detector approximately 24 mm above the beam pipe center. To clarify the geometrical aspects a plot of the run configuration assuming zero beam crossing angle is shown in figure 13. This might represent a critical problem for LHCf. In fact the beam pipe in correspondence of the D1 dipole is very narrow and has an elliptical shape. Its projection at the LHCf Arm2 location is represented by the blue ellipse in figure 13. The inner part of this ellipse defines the geometrical limit of the fiducial region which can be accessed by forward detectors for physics measurements. When the BCA is null, the vertical position of the Arm2 detector is chosen in such a way that the small calorimeter tower stays in the center of the beam pipe projection. The large tower, located above the small one, covers in this case some limited part of the allowed ellipse above the center of the beam pipe projection. In the configuration foreseen for 2016 the projection of the beam center should move up due to a non-zero beam crossing angle with upward going beams. The LHCf detector should be moved upward accordingly, to keep the small tower measuring at infinite pseudo-rapidity and to have the possibility to determine the center of the beam, as shown in figure 14. As a result, in this configuration the large tower of Arm2 goes behind the shadow of the beam pipe and becomes useless. In particular we would not have the possibility to detect events with two photons hitting separately the two calorimeter towers, which are used both to identify the 0 events (type I 0 s), by

28 22 Figure 14: Cross section of TAN and Arm2 detector in the configuration with 340 µrad total beam crossing angle and upward going beams. reconstructing the double photon invariant mass distribution and selecting the peak centered at the 0 mass, and to confirm the detector s energy calibration. Moreover this configuration does not allow detecting the rare events like the! decay (see figure 10). Under this hypothesis only the small calorimeter tower can be used for the measurement. Anyway, events with two photons hitting the small tower at once can be identified and their invariant mass reconstructed to select neutral pions (type II 0 s). The complete set of expected 0 events, including both type I and type II, is shown in the left plot of figure 16, where the events are represented in the p t x F plane. As it is underlined in this plot, events at high transverse momentum would not be detected because of the beam pipe shadow. Only type II events measured with the small tower would be collected. In the right plot of figure 16 the energy distribution for type I 0 s (non measurable) and type II 0 s(measurablewiththesmallcalorimetertoweronly) are compared. All the low energy events, below approximately 1.5 TeV, would be lost. A partial workaround to this problem has been investigated, with the assumption of upward going beams and 340 µrad beam crossing angle. After a first measurement in the previous configuration, required to have a measurement at high rapidity and to determine the beam center position, the detector could be moved down for a position scan. Two di erent positions have been considered: a) 24 mm below the initial position and b) 44 mm below the initial position. Configuration b) is nearly the lowest position to which the detector can be moved. In these two configurations the detector does not cover the beam center, but both the calorimeter towers are free from the beam pipe shadow and we have the possibility to detect also type I 0 s events. In figure 17 the

29 23 Figure 15: Invariant mass distribution of photon pair simulated in the configuration with the LHCf Arm2 small tower centered on the beam line. The hypothetical type I events, which are not detectable in case of upward going beams, have been shown for comparison. Figure 16: Left: distribution of all the selected 0 events in the p t x F plane; only the region within the black closed line would be measured in case of upward going beams. Right: comparison of the 0 energy spectra for type I (not measurable in the considered configuration) and type II events. same plots shown in figure 16 are shown for this configuration. The left plot, including both type I and type II events, shows that, even if the coverage to higher values of p t is possible, the accessible region in the plane p t x F is almost complementary to the region accessible in the nominal configuration (with the small tower centered on the beam center). In conclusion, the choice of upward going beams has a non negligible impact on a possible run of LHCf for p+pb in 2016: Geometry. If the Arm2 detector is set to the nominal position (with the small calorimeter tower intercepting the center of the beam spot), the large tower goes outside of the good region for physics measurements, covered by the shadow of the beam pipe; it cannot be use almost completely.

30 24 Figure 17: Left: distribution of all the selected 0 events in the p t x F plane in case the detector is moved 44 mm lower with respect to the beam spot; both type I and type II events can be measured. Right: 0 energy spectra for type I and type II events. E ciency. The loss of use of half detector introduce a large ine ciency, which would not be a negligible e ect, given the short time eventually reserved for this run. Physics. In the nominal configuration a large part of the acceptance in the p t x F for neutral pion events would be lost (for p t < 0.2 GeV/c and p t > 1 GeV/c). Additionally we would have a higher threshold value for the 0 events. This problem can be partially recovered by moving the detector downward a few centimeters. On the contrary, if the beams are moving downward at IP1, by design the detector can be set to the default running position (small tower centered on the beam spot) with both towers contained in the ellipse of figures 13 amnd 14. This position would be in the lower part of the ellipse, therefore it would be possible also to extend the coverage in transverse momentum by moving up the detector a few centimeters with respect to the beam center. 6.3 Beam optics Another important parameter that have been taken into consideration is,which characterizes the angular aperture of the beam at the Interaction Point. In general the LHCf collaboration would prefer running with high values of this parameter. In fact assuming for arealisticvaluesforthe2016run, =0.4m, and for the normalized emittance the design value, N =3.75 µm, the angular width of the beam at IP1 would be of the order of 37 µrad (because the normalized emittance in the realistic case is expected to be lower that the design value, around 2.0 µm, the angular width is expected to be lower than this limit value). Projecting the beam particles at the TAN position, 140 m far from IP1, it means an uncertainty on the beam center, collision by collision, of approximately 5 mm, which is much worst of the spatial resolution of the Arm2 detector in the measurement of impact point of secondary particles, which is better