Conseil Scientifique et Technique du SPhN STATUS OF EXPERIMENT

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1 Conseil Scientifique et Technique du SPhN STATUS OF EXPERIMENT Title: ALICE Date of the first CSTS presentation: November 18 th Last status report in November 2008 Experiment carried out at: CERN Spokes person(s): Jurgen Schukraft Contact person at SPhN: A. Baldisseri Experimental team at SPhN: A. Baldisseri, H. Borel, J. Castillo, J-L. Charvet, H. Pereira Da Costa, A. Rakotozafindrabe, Sanjoy Pal (post-doctorate), Hongyan Yang (post-doctorate), Claudio Geuna (PhD student) List of IRFU divisions and number of people involved: SEDI+SIS: 5 FTE (till ~ 2009) List of the laboratories and/or universities in the collaboration and number of people involved: In France: IPN Lyon, IPN Orsay, LPC Clermont-Ferrand, SUBATECH Nantes, IPHC Strasbourg, LPSC Grenoble for a total of ~ 109 institutes over ~ 31 countries and more than 1000 people involved. SCHEDULE Starting date of the experiment [including preparation]: Spring 2009 Total beam time allocated: 1 month of heavy ions and 7 months of protons per year Total beam time used: several years Data analysis duration: One year per running period Final results foreseen for: 2015 (?) BUDGET Total Already Used Total investment costs for the collaboration: 915 keuro (IRFU) 915 keuro Share of the total investment costs for SPhN: Total travel budget for SPhN: ~1%(~7% of muon arm ~70 keuro / year Please include in the report references to any published document on the present experiment.

2 ALICE status A. Baldisseri, H. Borel, J. Castillo, J-L. Charvet, H. Pereira Da Costa, A. Rakotozafindrabe Claudio Geuna 1,SanjoyPal 2,HongyanYang 2 CEA, Centre de Saclay, IRFU/SPhN, F Gif-sur-Yvette, France November 15, Introduction ALICE is one of the four experiments at the LHC and is the only one that is specifically designed for the high multiplicity environment of heavy ion collisions. Is is composed of various detection systems among which is the forward MUON spectrometer. ALICE aims to investigate in ultra-relativistic heavy ion collisions the properties of nuclear matter under extreme conditions of temperature and pressure which is expected to lead to the creation of deconfined partonic matter, the Quark Gluon Plasma (QGP). The proton-proton collisions are used as reference and also to constrain particle production models. The IRFU contribution to the ALICE experiment is focused on the largest tracking chambers of the muon spectrometer. On the hardware side, our group has been present and has played a major role through all the phases of the project: the early conception, design, prototyping, construction, installation and commissioning. On the software side, we have contributed to the development of the ALICE computing framework AliRoot which is the simulation, reconstruction and analysis program of ALICE. Our contribution includes the improvement of the simulation of the detector response, of the reconstruction algorithm, the development of the gain calibration and detector alignment algorithms as well as the preparation of the software for the data analysis. The group is now present on the data taking and their analysis. 2 The ALICE detector The ALICE detector is designed to cope with the highest particle multiplicities anticipated for Pb-Pb collisions (dn ch /dy up to 8000) and was operational for the first circulated beams of the LHC in september In addition to heavy systems, the ALICE Collaboration will study collisions of lower-mass ions, which are a means of varying the energy density, and protons (both pp and pa), which primarily provide reference data for the nucleusnucleus collisions. In addition, the pp data will allow for a number of genuine pp physics studies. The detector consists of a central part, which measures event-by-event hadrons, electrons and photons, and of a forward spectrometer to measure muons. The central part, which covers polar angles from 45 to 135 over the full azimuth, is embedded in the large L3 solenoidal magnet. It consists of various tracking detectors and particle identification 1 PhD student 2 Post-doctorate 1

3 arrays among which are the Inner Tracking System (ITS) of high resolution silicon detectors and a cylindrical Time Projection Chamber (TPC). The forward muon arm (covering polar angles )consistsofacomplexarrangementofabsorbers,alargedipolemagnet, and fourteen planes of tracking and triggering chambers and will be further described in section 3. Several smaller detectors (ZDC, FMD, V0...) for global event characterization and triggering are located at forward angles. An array of scintillators (ACORDE) on top of the L3 magnet is used to trigger on cosmic rays. The general layout of the ALICE detector is shown in figure 1. Figure 1: General layout of the ALICE detector. 3 The Muon spectrometer The forward MUON spectrometer is one of the main detectors of the ALICE experiment and was designed to measure and identify muons at large rapidities. A schematic view of the spectrometer is shown in figure 2. It consists of afrontabsorbertostopmosthadrons,electronsandphotonscomingfromtheinteraction point, an inner beam shield to stop re-scattered particles from the beam pipe, 10 tracking planes to allow particle trajectory reconstruction, alargeareawarm0.7tdipolemagnetformomentumdeterminationfromthetrack bending, a passive muon filter wall followed by 4 trigger planes that will provide single muon and muon pair triggers. 2

4 Figure 2: View of the ALICE forward MUON spectrometer. The ALICE forward MUON spectrometer covers an acceptance region from -2.5 to -4.0 in pseudo-rapidity ( ) and has full azimuthal coverage. A minimum cut on the transverse momentum (p T )ofsinglemuonslargerthan1.0gev/cisappliedforbackgroundrejection mainly. Our group was coordinating the whole forward muon spectrometer project from 2001 to 2007 and the muon tracking system since ALICE status The ALICE experiment along with more details concerning the muon spectrometer had been described in the previous status report to the CSTS in November The ALICE 2010 detector configuration is in agreement with the planning. All the expected detectors for the first data taking are complete: ITS, TPC, TOF, HMPID, FMD, T0, V0, ZDC, MUON arm, Acorde, PMD, PHOS(3/5). Concerning the detectors more recently approved, 7 modules of TRD out of 18 and 4 EMCAL modules out of 10 are installed (Fig. 3 left). EMCAL should be completed during the winter shutdown along with 3 more TRD modules (Fig. 3 right). The acquisition is also completed. All the systems are running and taking data. ALICE and among it the MUON arm, participated to all the proton-proton collisions data taking, from the very beginning short S NN =900GeVand S NN =2.36TeVrunsto the main S NN =7TeVrun. The proton data are used for comparison with ion ones: many signals will be relative to p-p collisions. This requires around 10 9 minimum bias events, at somewhat reduced luminosity to cope with the detector capabilities and avoid too large pile-up and dead time. 3

5 Figure 3: Detector configuration in 2010 (left) and expected after the winter shutdown (right). The luminosity is reduced by displacing the colliding beams (Fig. 4). Figure 4: Delivered integrated luminosity with time for the four LHC experiments. Effect of displaced beams for reducing the luminosity is clearly visible for ALICE. The proton data have been used also for comprehensive studies of minimum bias data at LHC and for tuning Monte Carlo. They address mainly soft and semi-hard processes, which are important topics investigated in the central part of ALICE detector. The number of recorded trigger with time is shown in figure 5 (left) and the integrated recorded luminosity in figure 5 (right). More than 750 millions events have been recorded in minimum bias, 50 millions by muon trigger and 20 millions of high multiplicity events. 4

6 Figure 5: Number of recorded trigger with time (left): INT1 B trigger corresponds to minimum bias and MUS1 B to muon trigger. Integrated recorded luminosity with time (right). SH1 B corresponds to high multiplicity events. The luminosity has been increased for a 2 weeks run of higher luminosity in October 2010 for rare processes studies, as for example heavy resonances in the muon spectrometer. The analysis is in progress. The first heavy ion run with lead-lead collisions has started in November 2010, with a low luminosity (1/20 of the nominal luminosity is expected) for all the LHC experiments. The event rate should range between 10 and 100 Hz with an expected number of minimum bias events around 10 7 ( 1-3 b 1 ). Several papers have been already published, concerning global event properties like the multiplicity of charged particles and their distributions for the three sets of available energies SNN =900GeV (1), S NN =900GeVand2.36TeV (2)and S NN =7TeV (3),concerning p/p ratio at S NN =900GeVand7TeV (4),momentumdistributions( S NN =900GeV) (5) and Bose-Einstein correlations ( S NN =900GeV) (6). Other papers are reviewed by the collaboration on identified particles (,,p)and strangeness ( 0,,,, ). Analysis are ongoing on event properties at 7 TeV (spectra, HBT, identified particles, strangeness..), on heavy flavours like charm (D 0,D +,D )andsemi-leptonicdecaysofheavy quarks (c, b) in electrons or muons, on production of J/ decaying in + and e + e... 5 Muon spectrometer status The Muon Trigger is operated successfully as a readout and trigger detector since Dec It consists in 72 resistive plate chambers (RPC) assembled in 4 half-planes (2 chambers). The RPCs, operated in avalanche mode, have shown a stable behavior, with low currents and counting rates, and high efficiency. The RPCs efficiencies are all above 90% with a mean value of 95% (Fig. 6). The current trigger efficiency is above 98% per track. Some minor maintenance operations have been carried out whenever necessary during the LHC technical stops. 5

7 Figure 6: The mean RPC efficiencies are shown by half-plane for the bending plane (Left) and non-bending one (Right) for the three data taking periods. The muon tracking system consists in 10 planes of cathode pad chambers, grouped in 5 stations. Each chamber has two cathode planes in order to provide a two-dimensional hit information in the plane transverse to the beam direction, along the track bending direction (vertical axis y) and along the non-bending direction (horizontal axis x). The third dimension is provided by the longitudinal (z) position of the chamber. The whole system is operated successfully since Dec. 2009, with presently 95% of readout pads fully working with a good noise on the bending planes where the best resolution is required. The intrinsic noise of the FEE has a constant component of about 600 electrons and a dependence on the detector capacitor with a 11 e /pf slope. A noise between 1000 and 1800 electrons had been recorded during test beams, depending on the pads sizes and the connecting lines length. This corresponded to the requirements, leading to good spatial resolutions. On the contrary, a very large noise was encountered in the cavern, coming from two components: a 300 KHz common noise from the LV power supplies, which had to be modified by adding ferrite in the power supplies and a 1 MHz pick-up noise coming from cooling plants which had also to be modified; this latter component was captured by the GMS (Global Monitoring System) cables, then transmitted to the large panel frames which radiate to the slat. The slat side facing the frame was indeed noisier. After modifications, the noise ranges between 1000 and 2500 electrons on the bending side where the resolution has to be around 100 microns (see figure 7). A residual pick-up noise still exists on the non-bending side (facing the frame) but without real consequence on the resolution needed, around 1 mm. The excluded electronic channels are mainly due to pedestal shifts on some readout lines giving a large occupancy rate. Small continuous pedestal shifts are handled by taking regularly pedestal runs ( every 4-5 h) while large sudden shifts on few lines seem coming from connections problems in the conception of the LV bus bars and of the FEE of the slats. Possible solutions will be investigated during the winter shutdown. The values of the average efficiency per chamber are shown in figure 8. This efficiency includes the intrinsic efficiency of each detection element, close to 100%, and the mapping of 6

8 Figure 7: The noise on bending planes is mostly below 2 ADC channels i.e 2000 electrons. the excluded pads. Considering the requirements for reconstructing a track of one hit among chambers 1 and 2, one hit among chambers 3 and 4, one hit among chambers 5 and 6, and three hits among chambers 7, 8, 9 and 10, the total tracking efficiency can be evaluated to 94 ± 0.9 %. The real tracking efficiency is computed from simulations. Figure 8: The average efficiencies (including acceptance) for the 10 tracking chambers. The smaller efficiencies of chambers 6-10 are due to excluded channels giving too high occupancies. 7

9 A first alignment with particles has been performed without magnetic field. The GMS system is operational and will be used first to make the link between magnetic field OFF and ON and also to survey continuously the displacement of the chambers. The work for including the GMS information in the track reconstruction is in progress. 6 Muon analyses The physics program of the muon spectrometer had been described in the status report of the CSTS of November While our main physics interest lies on the Pb-Pb collisions we were eagerly awaiting for the p-p data. Our interest in pp data is three fold. First, it provides the necessary data with a cleaner environment to understand, calibrate and align the detector. Second, it should allow to measure the cc (and bb) which is currently the source of large theoretical uncertainties for the J/ production models. Finally it will provide the necessary reference data for the Pb-Pb measurements. Analyses are ongoing on three main topics: single muon measurements, J/ and low mass vector mesons in p-p collisions. A critical point that made all those analysis possible was the improvement of the geometrical alignment of the spectrometer. For this, we developed an offline program based on the simultaneous minimization of the chi-square of many tracks both as a function of the track parameters and the track-independent alignment parameters. This method has the advantage, over the typical mean-residual shifting iterative method, to be non-iterative and non-biased by the ignorance of the alignment parameters during the first track reconstruction. The method was successfully tested with cosmic muons during the two cosmic-data taking periods in the spring and autum The alignment was then performed using the data acquired during a special run with both L3 and Dipole magnets switched off in May At that time we improved the position resolution in the bending direction from several millimiters to 700 µm, which translated into an improvement of the J/ invariant mass resolution from 230 MeV to 90 MeV, as can be seen in figure 9. AfurtherimprovementofthealignmentwasperformedinOctober2010byre-applying the same method on the same data but reconstructed with the first alignment. With the second alignment the mass resolution of the J/ goes down to 75 MeV, which is close to the expected nominal resolution of 70 MeV. Figure 10 shows the invariant mass peak of the J/ with the first and second alignment. 8

10 Figure 9: Unlike sign dimuon distribution in the J/ and after (right panel) the first alignment. invariant mass region before (left panel) Figure 10: Unlike sign dimuon distribution in the J/ invariant mass region with the first alignment (left panel) and the second alignment (right panel). Low mass vector mesons (,, )domain is related to the question of restoration of chiral symmetry. Lyon and Cagliari groups are particularly interested in this analysis. Figure 11 shows a performance plot of the unlike sign dimuon spectrum in the low mass region. 9

11 Figure 11: First unlike sign dimuon spectrum in the low mass region. Open charm (D mesons) and open beauty (B mesons) production studies are, on one hand, interesting on their own as they will allow to extract the charm and beauty crosssections and thus apply further constraints on perturbative QCD calculations. On the other hand, their measurements could be used as a reference for the quarkonia studies in ion-ion collisions. Their production will be measured in the ALICE forward MUON spectrometer via their muonic decay channels. The first measurement of open charm and open beauty will be extracted from the single muon p T distributions, as shown in Figure 12. Clermont-Ferrand group is looking at the single muon measurements. Figure 12: Single muon p T distribution from charm and beauty. 10

12 Figure 13: Acceptance and efficiency corrected J/ transverse momentum distribution. The study of the production of heavy quark and anti-quark bound states (quarkonia), J/, (cc), (1S), (2S) and (3S)(bb) remains among the priorities for the ALICE forward MUON spectrometer and will be done by the analysis of the invariant mass distribution of opposite sign muon pairs. The 2010 proton data allow to explore the cc resonances with the aim of computing the J/ cross section. The first analysis of the J/ cross-section is based on a small subset of data (about one tenth of the total) which corresponds to an integrated luminosity of 8.5 nb 1. Its aim is to measure the integrated J/ cross-section in p-p collisions at s =7TeV as well as its p T and rapidity dependence. Preliminary results on J/ transverse momentum and rapidity distributions were first presented at the ICHEP 2010 conference in Paris (7). Figure 13 shows the J/ transverse momentum distribution corrected for detector acceptance and trigger and tracking efficiency in arbitrary units. The remaining work for the proper normalization is progressing. A more detailed study of the J/ differential cross-section needs the full data sample accumulated in Claudio Geuna is currently working on this as part of his PhD thesis project, this work is expected to yield a second and more complete publication on J/ production in p-p collisions at s =7TeV and will also be used as reference for the Pb-Pb studies. The firsts Pb-Pb collisions at s NN =2.76 TeV have just been delivered by the LHC 11

13 on November 8th The 2010 Pb-Pb data taking period will continue until December 6th We have estimated the number of quarkonium resonances to be reconstructed in this period to be on the order of 5000 J/ and 50 Υ. Thus, a first measurement of the centrality dependence of the J/ production in Pb-Pb collisions at s NN =2.76 TeV seems feasible and is the objective of our group. A second Pb-Pb data taking period with higher luminosity is foreseen for the autumn With this later data set more detailed studies should be feasible. In particular our group is interested in the following topics: First, a more precise measurement of the centrality dependence of the J/ production in Pb-Pb collisions at s NN =2.76 TeV. Second, the measurement of the J/ elliptic flow. And third, the measurement of the Υ production in Pb-Pb collisions at s NN =2.76 TeV. As explained on our CSTS status report of November 2008, the first two measurements are expected to give a clear view of the dominant J/ production mechanism, i.e. whether quark recombination dominates over sequential screening. To carry out this physics program we submitted in January 2010 a grant request to the ANR s Programme Jeunes Chercheuses et Jeunes Chercheurs Edition This request was not granted, however the external referee encouraged us to resubmit the request after the inclusion of some feasibility studies. Thus, we plan to resubmit the modified project to the ANR s Programme Jeunes Chercheuses et Jeunes Chercheurs Edition It is worth noting that to reach the necessary resolution to separate the three Υ states, in addition to a good alignment we may need to improve the electronics gain calibration. Our group is actively involved in this work, both online and offline. Concerning the Υ measurement, we have submitted a grant request for a post-doc to the GIS Physique des 2 Infinis. References [1] K. Aamodt et al, ALICE collaboration, Eur. Phys. J. C 65, (2010) 111. [2] K. Aamodt et al, ALICE collaboration, Eur. Phys. J. C 68, (2010) 89. [3] K. Aamodt et al, ALICE collaboration, Eur. Phys. J. C 68, (2010) 345. [4] K. Aamodt et al, ALICE collaboration, Phys. Rev. Lett. 105, (2010) [5] K. Aamodt et al, ALICE collaboration, Phys. Lett. B 693, (2010) 53. [6] K. Aamodt et al, ALICE collaboration, arxiv: [7] J. Castillo for the ALICE collaboration, Heavy quark and quarkonium measurements with ALICE at the LHC, ICHEP 2010, Paris, France. 12