Getting ready for first physics in ALICE

Transcription

1 SUPLEMENTO REVISTA MEXICANA DE FÍSICA 55 (2) DICIEMBRE 2009 Getting ready for first physics in ALICE G. Paić Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Apartado Postal , México D.F , México. Recibido el 11 de marzo de 2009; aceptado el 13 de marzo de 2009 The initial proton-proton collisions at the LHC promise to open a completely new and exciting perspective in the soft sector. The first hours of data taking will permit an insight in a new regime dominated above all by hard interactions. In the present paper we are trying to account for the first measurements planned in ALICE and their connection with the existing theoretical predictions. Keywords: LHC; heavy ions; quark gluon plasma. Las colisiones protón-protón iniciales en el LHC prometen de abrir una perspectiva completamente nueva y exitante en el sector suave. Las primeras horas de colectar datos van a permitir un entendimiento en un régimen nuevo, dominado sobre todo de interacciónes duras. En la contribución presente tratamos de tomar en cuenta las primeras medidas planeadas en ALICE y sus conexiones con prediciones teóricas existentes. Descriptores: LHC; ions pesados; plasma de quark y gluons. PACS: h; Mh 1. Introduction 2009 will be the first year of collisions at the Large Hadron Collider in CERN. The huge effort of thousands of scientists and engineers to build the collider and the experiments will finally bring long awaited results. The sintagma first physics denotes the ensemble of physics projects that can be achieved with a few data - typically a few tens to a few hundred thousands events. Since the LHC will start with protonproton collisions the present article limits to them, although a no less interesting program of first physics for heavy ion collisions. The center of mass energies to be considered here is 10 TeV. Other possibilities exist of runs at 900 MeV and other intermediate energies however the basic approach does not change very much. The completely new energy regime opens very interesting possibilities. The main novelty is the absolute dominance of hard processes as shown in Fig. 1 [1]. We will give a short review of the status of the ALICE experiment followed by the expected first physics observables achievable, generally speaking with a small number of events where small is a number between several tens thousands and several hundred thousand. A special part of the ALICE program is the ACORDE detector, which in the preparatory period serves a a trigger for the commissioning of detectors and in its own right can provide interesting physics related to cosmic ray bundles of high multiplicity. momentum range and the large multiplicities anticipated has dictated the detector design resulting a in a 2. The ALICE experiment The ALICE experiment is located in point 2 of the LHC ring. The experiment is designed as a general purpose experiment for heavy-ion physics. The requirement to be able to measure with precision in a wide FIGURE 1. Ratio of the hard processes with respect to the inelastic cross section in function of the cms energy as calculated by the dual parton model and compared to experimental data.

2 104 G. PAIĆ A detailed description of the detector system and its capabilities can be found in Ref. 2. The year 2008 has been the year of commissioning, calibration and alignment of the central detectors TPC and SPD, primarily. The Silicon pixel detector was aligned to a spatial resolution xy of 52 µm compared with simulation result of 43 µm. The TPC has also been characterized in many details. Here we will only mention that the momentum and de/dx resolution are in line with the expectations. In Fig. 2 we show the results of real data measured with cosmic rays for the de/dx. The relativistic rise is clearly visible. FIGURE 2. Measurement of the de/dx in the TPC with the cosmic rays. 3. The multiplicity trigger 3.1. Cosmic rays During 2008 there has been cosmic ray runs using the silicon pixel trigger which is part of the ITS, and the ACORDE array consisting of plastic scintillator modules (60 at present), placed on the top 3 sides of the central ALICE magnet. Beyond providing triggers of cosmic rays necessary for the calibration of the central barrel detectors it has also the task to search in conjunction with the TPC for events of high muon multiplicities ( 100) - the so called muon bundles. The transverse size of muon bundles, their number and their energy distributions are sensitive to the mass of the primary nucleus inducing the Extended Air Shower (EAS) and therefore are of astrophysical interest. In Fig. 3 we show a muon bundle registered in ALICE Triggers for day-one physics FIGURE 3. A muon bundle triggered by ACORDE (the modules in red show the hits in the scintillators) and traced in the TPC. midrapidity region dedicated to the tracking and particle identification. We have in that region the inner tracking system (ITS), the Time projection chamber (TPC) a time of flight array (TOF) and the transition radiator detector (TRD). These are the detectors with a whole azimuthal coverage. On top of those we have smaller detectors in the central region: the high momentum particle identification HMPID, the photon spectrometer PHOS and at a later stage the Electromagnetic Calorimeter EMCAL. The forward regions are covered with detectors for triggering and particle multiplicity measurement (the forward multiplicity detector FMD, the VZERO and T0 trigger detectors, the photon multiplicity detector PMD, and the zero degree calorimeter ZDC). On one side of the interaction region there is a muon spectrometer spectrometer ( 4 < η < 2.4). Finally on top of the magnet there is ACORDE a large area of scintillator modules dedicated to the measurement of cosmic rays. The trigger system of ALICE, which is designed for effectively triggering heavy ion collisions, is capable of triggering most of the inelastic proton- proton collisions at high energy. For the proton-proton runs ALICE will use two of the ALICE sub-detectors (SPD and VZERO) in order to form the most effective trigger with the best possible background rejection. The proposed proton-proton minimum bias triggers are sensitive to interactions corresponding to 90% of the total inelastic cross section (and 98% of the non-diffractive cross section) and still reject the majority of beam gas interactions. The SPD is capable to generate a prompt trigger based on an internal Fast-OR. This feature will be of enormous importance in getting early results on the multiplicity distributions. Each pixel chip provides a Fast-OR digital pulse when one or more of the pixels in the matrix are hit The Fast-OR signals of the 10 chips on each of the 120 half staves are transmitted every 100 ns on the 120 optical links that are also used for the data readout. They are processed in a separate processor unit according to a variety of predefined trigger algorithms to generate a signal that can contribute to the level zero (L0) trigger decision in the ALICE central trigger processor (CTP).

3 GETTING READY FOR FIRST PHYSICS IN ALICE The day-one measurements 4.1. Multiplicity and pseudorapidity distributions We have several generators and models (Pythia, PHOJET and QGSM [3]) that are predicting several basic parameters of pp collisions especially the multiplicity distributions and psedorapidity densities. However the above mentioned generators yield very different results at the level of the multiplicity distributions. The possibility to trigger on different multiplicity bins will be of great importance to determine the knowledge we can gather from the multiplicity distribution. Using three high multiplicity threshold in the SPD electronics we will be able to cover with a reasonable trigger frequency (of 13Hz) high multiplicity events about five order of magnitude less frequent than the mean multiplicity events, reaching events with 20 times the mean multiplicity. With about 20K events one may reach multiplicities 8 times larger than the average one. This is also an important feature of pp collisions, namely the collisions have a large range of multiplicities reaching an order of magnitude. This means that, in the high multiplicity events, taking into account the dimensions of the colliding systems and the achieved multiplicities one may expect final states of densities comparable to the ones achieved in heavy ion collisions. The pseudo rapidity distributions are displayed in Fig. 4 where are the predictions of PHOJET, Pythia and QGSM [4]. The original QGSM plot which predicts the rapidity densities (dotted line in Fig. 4.) was converted to pseudorapidity density using conversion factors extracted from PHOJET [5]. One observes that there exists a large discrepancy between the models something that should be settled in the first pp runs at LHC The baryon number transfer previsions are really poor when compared with experimental results. With 20K events we will be able to reach up to 10 GeV/c transverse momenta. With 100 million events (one month of running) momenta up to 50 GeV/c will be reachable. Measuring the mean transverse momentum in function of multiplicity bins one will be able to study the behavior of the mean p t - a dependence that is not well understood as is illustrated in Fig. 4. The CDF collaboration has reported an interesting fact: the mean of an event does not depend on the center of mass energy but uniquely on the multiplicity of the events. At the same multiplicity the same mean p t is measured! FIGURE 4. Pseudo rapidity distributions predicted by Phojet, Pythia and QGSM. In the string models baryons are considered as configurations consisting of three strings attached to three valence quarks and connected in a point called string junction [6]. Thus the string-junction has a nonperturbative origin in QCD. It is very interesting to understand the role of the string-junction in the dynamics of high-energy hadronic interactions, in particular in the processes of baryon number transfer [7]. Experimentally the effect is measured by the asymmetry between baryons and anti baryons at midrapidity: A = 2 B B B + B 4.3. The mean transverse momentum versus multiplicity (1) The correlation between mean p t and charged multiplicity is known since UA1 [8] and successively investigated at the ISR [9] and at the Tevatron Collider energies [10], but its theoretical explanation is still not complete yet. Among the proposed interpretations is that the increase is due to the contribution of high E t interactions (minijets), but quantitative FIGURE 5. Mean p t vs multiplicity at 1800 GeV compared with theoretical predictions (from Ref. 11).

4 106 G. PAIĆ TABLE I. Suggested cuts in R and 1 T to select different topologies of jets. Region Kind of event Variables A Dijets R 0.35, τ 0.03 B Monojets R 0.9, τ 0.03 C Mercedes R 0.4, τ 0.25 FIGURE 6. Shape variables at hadron colliders are defined over particles within a central region. FIGURE 8. Azimuthal correlation for MB Pythia sample: dijets (red-region A), monojets (blue-region B) and the trijets (greenregion C). In this plot the comparison between the true spectrum (solid line) and the measured spectrum (dotted line) is shown. The leading particle is not shown. To compute the shape variables we used the ptcutoff = 1.5 GeV/c, events were analyzed. 5. Jets using the Event Shape Structure (ESA) The jets in the new energy range opened by the LHC will be of special interest. The study of the jet shapes of the k t and j t distribution represent challenges to the experiments. Namely, due to the high energies plentiful production of jets will occur. It is interesting to study the extent to which one can, in minimum bias events, isolate distinct simple configurations of two and three jets, allowing to study the jet finding algorithms and to start on the study of specific jet parameters. Recently, it was proposed to apply an event shape analysis modified from the e + e experiment to treat only the thrust in the transverse plane [12, 13]. The thrust (T ) is defined as in the e + e case i T max p t,i n t }{{} i (2) p t,i n t FIGURE 7. Distribution of R vs (1 T ) for the MB Pythia sample: proton-proton collisions at 10 TeV in the c. m. The distributions correspond to: a) generator level (upper), in this case we use primary charged MC particles; and b) reconstruction level (bottom), in this case we used TPC tracks from primaries. To compute the shape variables we demanded at least three particles (or tracks) with p t > 1.5 GeV/c and we used the cut: η 1. The sample is composed by events. In the literature it is more common to find the following definition: τ 1 T (3) corresponding to the sphericity of the event. The other parameter is due to the incomplete acceptance The recoil term R is simply the vector sum of the transverse momentum: (Fig. 6). 1 R i p t,i p ti (4) i

5 GETTING READY FOR FIRST PHYSICS IN ALICE 107 This quantity measures the balance of momenta in the event. For example for a di-jet event, with only one jet inside the acceptance of the detector (monojet in the further text): R tends to 1, because there are no vectorial cancellations in the numerator which appear in the definition of R. Otherwise, in the case of the perfect 2 back-to-back jet completely inside the acceptance: R tends to 0. Plotting the sphericity and the recoil values in a twodimensional map we get for events of minimum bias gives the map shown in Fig. 7. Selecting defined parts of the map as defined in Table I, one can among other things, draw the azimuthal correlations of the particles above 1.5 GeV/c with respect to the leading particle in the event. The resulting correlations are shown in Fig. 8. One clearly identifies three cases: the dijets belonging to the region A, the single jets from the region B belonging to high R values (meaning that the back-to-back jet is not in the detector acceptance, and finally a not very numerous category (about 0.2% of all the events)belonging to events with three jets equally spaced in the azimuth. One sees that even with a very limited sample one will be able to study the lower energy jets. 6. Summary The initial operation of ALICE has already begun using cosmics for calibrations. We have demonstrated that even with a few hours runs in pp collision mode one will be in position to tackle very interesting and fundamental problems of the soft pp regime. Acknowledgments I would like to acknowledge A. Ortiz for the development of ESA and his help in preparing the manuscript, and J.F. Grosse-Oetringhaus for useful comments on the multiplicity predictions in various models The work was performed under the projet IN and Conacyt P79764-F. 1. Capella, J. Tran Thanh Van, and J. Kwiecinski, Phys. Rev. Lett. 58 (1987) [ALICE Collaboration], J. Phys. G30 (2004) A. Capella, A. Kaidalov, and J. Tran Thanh Van, Heavy Ion Phys. 9 (1999) J.F. Grosse-Oetringhaus, Measurement of the Charged-Particle Multiplicity in Proton-Proton Collisions with the ALICE Detector, thesis University of Muenster, to be published J.F Grosse-Oetringhaus (private communication). 6. G.C. Rossi and G. Veneziano, Nucl. Phys. B 123 (1977) D. Kharzeev, Phys. Lett. B 378 (1996) G. Arnison et al., Phys.Lett. B118 (1982) A. Breakstone et al., Phys. Lett. B 132 (1983) 463; Phys. Lett. B 183 (1987) T. Alexopoulos et al., Phys. Lett B 336 (1994) D. Acosta et al., Phys. Rev. D 65 (2002) A. Ayala et al., Fine structure in the azimuthal transverse momentum correlations at s NN = 200 GeV using the event shape analysis, (2009) [arxiv:hep-ph / ] 13. A. Ortiz and G. Paić, Event Shape Abalysis in ALICE, ALICE Internal Note to be published.