Conseil Scientifique et Technique du SPhN

STATUS OF EXPERIMENT

Title: ALICE Date of the first CSTS presentation: November 18th 1997

Experiment carried out at: CERN

Spokes person(s): J. Schukraft

Contact person at SPhN: H. Borel

Experimental team at SPhN: A. Baldisseri, H. Borel, J. Castillo, J-L. Charvet, H. Pereira Da Costa, A.

Rakotozafindrabe, C. Silvestre-Tello (PhD student)

List of IRFU divisions and number of people involved: SEDI+SIS: 5 FTE.

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:

(core cost)

915 kEuro (IRFU) 915 kEuro

Share of the total investment costs for SPhN: ~1%(~7% of muon arm) Total travel budget for SPhN: ~70 kEuro / year Please include in the report references to any published document on the present experiment.

ALICE: Status and plans for data analysis

A. Baldisseri, H. Borel, J. Castillo, J-L. Charvet,H. Pereira Da Costa, A. Rakotozafindrabe, C. Silvestre-Tello

CEA, Centre de Saclay, IRFU/SPhN, F-91191 Gif-sur-Yvette, France

October 22, 2008

1 Introduction

In ultra-relativistic heavy ion collisions, we aim to investigate the properties of nuclear matterunder extreme conditions of temperature and pressure which is expected to lead to the creationof deconfined partonic matter, the Quark Gluon Plasma (QGP). With the objective of studyingthe QGP, heavy ion collisions were studied at the CERN SPS up to a center of mass energy√

sNN

=17.2 GeV and are currently under investigation at RHIC at BNL with an energy upto√

sNN

=200 GeV. The Large Hadron Collider (LHC) at CERN which circulated its firstproton beams in September 2008 will collide Pb to Pb ions at

√sNN

=5.5 TeV providing so farunprecedented conditions to study the QGP.

ALICE is one of the four experiments at the LHC and is the only one that is specificallydesigned for the high multiplicity environment of heavy ion collisions. It is composed of variousdetection systems among which is the forward MUON spectrometer.

The IRFU contribution to the ALICE experiment is focused on the largest tracking cham-bers of the dimuon spectrometer. On the hardware side, our group has been present andhas played a major role through all the phases of the project: from the early conception, de-sign, prototyping, construction, installation up to the ongoing commissioning phase. On thesoftware side, we have contributed to the development of the ALICE computing frameworkAliRoot which is the simulation, reconstruction and analysis program of ALICE. Our contribu-tion includes the improvement of the simulation of the detector response, of the reconstructionalgorithm, the development of the gain calibration and detector alignment algorithms as wellas the preparation of the software for the data analysis.

In the following, we describe the ALICE detector and its forward muon spectrometer andwe present the current status of the detector. Then we give a short overview of the physicsprogram of the forward muon spectrometer and we present our plans for data analysis.

2 The ALICE experiment at LHC

ALICE is a general-purpose heavy-ion experiment designed to study the physics of stronglyinteracting matter and the quark-gluon plasma in nucleus–nucleus collisions at the LHC. Itcurrently involves more than 900 physicists and senior engineers, from both the nuclear andhigh-energy physics sectors, from over 90 institutions in about 30 countries.

1

Figure 1: General layout of the ALICE detector.

The ALICE detector is designed to cope with the highest particle multiplicities anticipatedfor Pb–Pb collisions (dNch/dy up to 8000) and was operational for the first circulated beamsof the LHC in september 2008. In addition to heavy systems, the ALICE Collaboration willstudy collisions of lower-mass ions, which are a means of varying the energy density, andprotons (both pp and pA), which primarily provide reference data for the nucleus–nucleuscollisions. 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, electronsand photons, and of a forward spectrometer to measure muons. The central part, which coverspolar angles from 45◦ to 135◦ over the full azimuth, is embedded in the large L3 solenoidalmagnet. It consists of various tracking detectors and particle identification arrays among whichare the Inner Tracking System (ITS) of high resolution silicon detectors and a cylindrical Time-Projection Chamber (TPC). The forward muon arm (covering polar angles 171◦ − −178◦ )consists of a complex arrangement of absorbers, a large dipole magnet, and fourteen planes oftracking and triggering chambers and will be further described in section 3. Several smallerdetectors (ZDC, FMD, V0...) for global event characterization and triggering are located atforward angles. An array of scintillators (ACORDE) on top of the L3 magnet will be used totrigger on cosmic rays. The general layout of the ALICE detector is shown in figure 1.

3 The ALICE forward MUON spectrometer

The forward MUON spectrometer is one of the main detectors of the ALICE experiment andwas designed to measure and identify muons at large rapidities. A schematic view of thespectrometer is shown in figure 2. It consists of

• a front absorber to stop most hadrons, electrons and photons coming from the interactionpoint,

2

Figure 2: View of the ALICE forward MUON spectrometer.

• an inner beam shield to stop re-scattered particles from the beam pipe,

• 10 tracking planes to allow particle trajectory reconstruction,

• a large area warm 0.7 T dipole magnet for momentum determination from the trackbending,

• a passive muon filter wall followed by 4 trigger planes that will provide single muon andmuon pair triggers.

The ALICE forward MUON spectrometer covers an acceptance region from -2.5 to -4.0in pseudo-rapidity (η) and has full azimuthal coverage. A minimum cut on the transversemomentum (p⊥) of single muons larger than 1.0 GeV/c is applied for background rejectionmainly.

Our group was coordinating the whole forward muon spectrometer project from 2001 to2007.

3.1 Tracking system

The tracking system of the ALICE forward MUON spectrometer consist of 10 cathode padchambers grouped in 5 stations. Each chamber has two cathode planes in order to providea two-dimensional hit information in the plane transverse to the beam direction, along thetrack bending direction (vertical axis y) and along the non-bending direction (horizontal axisx). The third dimension is provided by the longitudinal (z) position of the chamber.

Stations 1 and 2 are located between the front absorber and the dipole magnet. Eachchamber of stations 1 and 2 consists of 4 independents quadrants (see figure 3). The Orsay-IN2P3 group is in charge of the 8 quadrants of station 1 while the Indian groups of Kolkataand Aligarh are in charge of the 8 quadrants of station 2

3

Figure 3: Station 1 viewed from inside the dipole magnet.

Station 3 is located inside the dipole magnet and plays a crucial role in determining thetrack bending. In view of the large surface covered by the station 3 a modular design basedon a slat geometry is therefore used (see figure 4). Each chamber consists of 18 independentslats. The 18 slats of each chamber are also mounted into 2 independent support structures(carbon/epoxy fiber panels).

Finally, stations 4 and 5 are located between the dipole magnet and the muon filter wall.A slat geometry is also used for stations 4 and 5 and each chamber consists of 26 independentslats mounted into 2 half chambers as for station 3. Four laboratories, INFN-Cagliari, PNPIGatchina, SUBATECH-IN2P3 Nantes and CEA-IRFU Saclay are in charge of the 140 slats ofstations 3, 4 and 5.

Therefore, the tracking system of the ALICE forward MUON spectrometer consists of 156independent detection elements, each of them providing two-dimensional hit position. Alldetection elements were designed to provide a combined spatial resolution of 100 µm for thebending coordinate and 1 mm for the non-bending coordinate.

The initial position of the half-chambers will be measured by the CERN survey groupwith 1 mm resolution in the 3 directions. The displacements of the half-chambers relativeto a reference chamber will be periodically monitored by the Geometrical Monitoring System(GMS), developed by the Lyon-IN2P3 group, with ∼ 50 µm resolution in the 3 directions [3, 4].

The mechanical precision on the attach points of the detection elements into their respectivesupport structures is ∼ 500 µm in all cases. A survey by a photogrammetry method willprovide the initial position of the detection elements with respect to their support structurewith an expected resolution of 100 µm along the beam axis and 50 µm in the 2 directions

4

Figure 4: Station 4 and 5 viewed from inside the dipole magnet.

perpendicular to the beam axis. However, the photogrammetry method will only be performedonce before the start-up of the LHC. Alignment with tracks will be needed to find the finalposition of each detection element.

The Front End Electronics (FEE) cards, called MANU, are plugged on the detectors. Four16 channels MANAS chips collect, pre-amplify and shape the analogical charge signal fromeach pad which is then converted into a digital signal by two 12 bits ADCs. A MARC chipcontrols the MANU card and makes the zero-suppression. The signal from the MANU issent through a data bus on the printed circuit board to a translator card for adaptation ofthe signal. From the translator card on the detector, the digital signal is sent through a flatcable (bus patch) to a signal concentrator crate (CROCUS) located close to the chambers. Itreceives the signal in a front board (FRT), sends it to a concentrator board (CRT) which thensends it to the acquisition (DAQ) through an optical fiber. About 1.1 million channels areread form the eighteen thousands MANU cards. Figure 5 schematically describes the aboveread-out chain of the spectrometer. The Orsay-IN2P3 group is responsible for the electronicsof all the tracking stations.

More detailed information on the ALICE forward MUON spectrometer design can be foundin its technical design report [1, 2].

3.2 Construction and Installation

The quadrants were built in the laboratories of Orsay for station 1 and of Kolkata for station2. The construction of the 160 slats (including spares) was shared by the four laboratoriesmentioned above. The construction of the slats started in 2004 an was successfully completedby 2006. The assembly phase took also 2 years, from 2005 to 2006 in a Hall at CERN. Theslats coming from the four laboratories were tested with gas and High Voltage. The finalelectronics was then plugged and read-out. Finally, the slats were mounted on the support

5

Figure 5: Schematic description of the read-out chain of the spectrometer. See text for details.

structures to form a half-chamber. The half-chambers are then transported and installed inthe ALICE cavern. The first half-chamber was installed in the ALICE cavern in July 2006while the last one was installed in March 2008. In addition to construction of 40 slats, ourgroup, in close collaboration with other IRFU’s services (SEDI and SIS), also took in chargethe construction of the 12 support panels and was responsible as well of the integration of thechambers, the services (Low Voltage (LV), HV, read-out cables, gas pipes, . . . ) and of thecooling studies. Our group has also been coordinating the muon tracking project since 2004.

3.3 Commissioning

The commissioning of the detectors in the cavern lasted for more than one year. The startingphase shifted, waiting for the installation of the cables in the cavern and of the Low VoltagePower Supplies (LVPS) and for the final read-out system. We encountered two major problemsthat slow down the commissioning. The first one is a lack of read-out stability due mainly tothe FEE and the low voltage bus bar connections, while the second one is the noise on stations3, 4 and 5. Reaching a stable read-out takes time and careful handedness. Slats and quadrantsare not grounded in the same way since slats are fully in the acceptance of the spectrometer.It appears that slats are more sensitivity to noise than quadrants. A first positive step onthis noise issue was taken, after several long tests could be carried out and finally pointedout a common mode noise, with the modification of the LVPS by the supplier. This reducedthe noise on stations 4 and 5 to an acceptable level. However, a very important noise is stillpresent on station 3, which is inside the dipole magnet, and suggests other components likepick-up noise. Tests are in progress to understand and solve this problem.

Three cosmic runs (December 2007 and March and June 2008) were taken with enlarging

6

but still reduced configuration of the read-out chambers. Nevertheless, they allowed to improvethe tracking DAQ in order to be compliant with the global ALICE DAQ and Trigger, to test thewhole DAQ chain (from pad read-out to mass-storage) and the track reconstruction software.The final version of the slow control system (DCS for Detector Control System) was intensivelyused for the commissioning work as well as during cosmic runs. The same can be said for themonitoring tool for experts (MOOD).

At the beginning of September, the ALICE cavern was closed waiting for the first beams.We had therefore selected a stable, but reduced, detector configuration: the 2 chambers ofstation 1 (8 quadrants), the 2 chambers of station 2 (except one quadrant with HV problem),one half-chamber (the other half-chamber being very close to be ready)of chamber 5 (station3), one half-chamber of station 4 and 3 out of 4 half-chambers of station 5 (the last one alsobeing close to be ready).

The LHC incident on September 19 2008 changed completely the planning of running andset a long shutdown period, starting beginning of October for at least 6 months. The muonTracking project has then decided to resume the commissioning. The tasks and planning havebeen fixed as follow: complete the commissioning of the missing chambers; perform tests tounderstand the noise on station 3 and implement a solution; improve noise on stations 4 and5; and finish the installation of GMS. Concerning GMS, the transverse part measuring thepossible deformation of the detection planes is installed, along with the external part, linkingchamber 9 to the wall of the cavern. The longitudinal system (LMS) links the chambers insidea station and also the stations between them and has still to be installed for stations 3, 4 and5. The teams have already restarted the commissioning work at the beginning of October.

4 Physics program of the ALICE forward MUON spec-

trometer

One of the mains objectives of the ALICE experiment is to study the properties of nuclearmatter under extreme conditions of temperature and pressure produced in ultra relativisticheavy ion collisions at the LHC. It is expected that deconfined partonic matter, the QGP willbe created.

While several observables have been proposed to characterize the QGP, the study of heavyquarks c and b production is thought to be one of the most powerful probes. The forwardMUON spectrometer is specially designed to study heavy quarks production in heavy ioncollisions at the LHC.

A detailed description of the physics program of the ALICE forward MUON spectrometercan be found in chapters 6.6 and 6.7 of the ALICE Physics Performance Report - volumeII [5]. We will give a brief review of this program below.

4.1 Quarkonia production

The study of the production of heavy quark and anti-quark bound states (quarkonia), J/ψ ,Ψ′ (cc̄), Υ (1S), Υ (2S) and Υ (3S) (bb̄) is among the priorities for the ALICE forward MUONspectrometer and would be done by the analysis of the invariant mass distribution of oppositesign muon pairs. It was first proposed that quarkonia resonances will sequentially dissociate bycolor screening in the presence of a QGP [6], thus a suppression of quarkonia production in Ion-

7

Figure 6: J/ψ RAA vs. Npart at SPS compared to RHIC [10].

Ion collisions compared with proton-Ion collisions was predicted as a signature for the QGPformation. Later, it was also proposed that additional quarkonia production mechanisms,such as quark recombination in the QGP [7], could add-up to the prompt production byinitial hard scattering. In this case an enhanced production of quarkonia resonances will beobserved in Ion-Ion collisions. The recombination scenarios are expected to be important forthe charmonium states at the RHIC energies and even more at the LHC energies.

The current results at lower energies, at SPS and RHIC, are so far inconclusive and evenpuzzling. Figure 6 shows the J/ψ nuclear modification factor

RAA =dN

J/ψAA /dp⊥dy

NcolldNJ/ψpp /dp⊥dy

, (1)

from PHENIX compared to earlier results at SPS energies. We observe a similar J/ψ sup-pression at RHIC than at SPS energies as well as a larger suppression at forward than atmid rapidity. Both observations are surprising and cannot be explained by any suppressionmodel based on the medium energy density. Also, both results suggest the competing effect ofprimary J/ψ suppression and J/ψ regeneration at RHIC energies. However, recent theoreticaldevelopments seem to go in the same direction [8].

The results from the LHC should allow us to differentiate between the different quarkoniaproduction scenarios. The ALICE forward MUON spectrometer must then be able to clearlymeasure the above mentioned quarkonia resonances and in particular it should be able toseparate the 3 bb̄ states. This requires a maximum invariant mass resolution of 100 MeV/c2

at the mass of the Υ .

8

4.2 Open charm and beauty production

Open charm (D mesons) and open beauty (B mesons) production studies are also among thepriorities for the ALICE forward MUON spectrometer. On one hand, they are interesting ontheir own as they will allow to extract the charm and beauty cross-sections and thus applyfurther constraints on perturbative QCD calculations. On the other hand, open charm andopen beauty measurements could be used as a reference for the quarkonia studies in Ion-Ioncollisions.

Open charm and open beauty production will be measured in the ALICE forward MUONspectrometer via their muonic decay channels. The first measurement of open charm andopen beauty will probably be extracted from the single muon p⊥ distributions. Decay muonsfrom π and K will dominate at low transverse momentum while D and B contributionsshould dominate at p⊥ larger than 5 − 8 GeV/c. Open charm and open beauty can alsobe measured with muon pairs. Indeed, like and unlike sign muon pairs originating from thesame hard scattering or same heavy quark, will present a residual correlation in the low(1− 3 GeV/c2) and high (4− 8 GeV/c2) invariant mass regions. Correlated unlike-sign muonpairs in the higher invariant mass region will be mainly produced by semi-muonic decays ofD − D̄ and B − B̄ mesons from the same hard scattering. The lower invariant mass regionwill be primary populated by the B −D (B̄ − D̄) semi-muonic decay chain from the same b(b̄) quark fragmentation.

5 Plans for data analysis

The startup plan for the LHC run in 2009 is not yet finalized due to the LHC incident onseptember 19 2008. It’s however expected that the LHC will startup during the spring 2009followed by a long proton–proton run at

√sNN

=10 TeV. It is not yet clear whether a Lead–Lead run will take place in 2009. While our main physics interest lies on the Pb–Pb collisionswe will eagerly process the p–p data. Our interest in pp data is three fold. First, it willprovide the necessary data with a cleaner environment to understand, calibrate and align thedetector. Second, it should allow to measure the cc̄ (and bb̄) which is currently the sourceof large theoretical uncertainties for the J/ψ production models. Finally it will provide thenecessary reference data for the Pb–Pb measurements.

5.1 Alignment and calibration

Two key issues to reach the momentum and mass resolution needed for achieving the forwardmuon spectrometer physics program are the pads gain calibration and the detector alignment.As an example, figure 7 shows the expected Mass (top) and Width (bottom) of the Υ peakas a function of the detection elements alignment resolution in the non-bending(bending)planes, σx(y). We observe a rapid increase in the invariant mass resolution for the Υ as afunction of the alignment resolution. We conclude that we should restrain the misalignmentto σx(y) ≤ 50 < µm to keep the increase of the invariant mass resolution smaller than ∼ 10%.

Our group has taken the responsibilities for the pad gain calibration and for the alignmentof the forward muon spectrometer. Concerning the gain calibration, we have developed anddeployed the algorithm for the online gain calibration using special calibration runs and anoffline program for the gain calibration evaluation, validation and/or improvement is being

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Figure 7: Mass (top) and Width (bottom) of the Υ as a function of the alignment resolutionσx(y).

developed. Concerning the alignment, we developed an offline program based on the simulta-neous minimization of the chi-square of many tracks both as a function of the track parametersand the track-independent alignment parameters. This method has the advantage, over thetypical mean-residual shifting iterative method, to be non-iterative and non-biased by theignorance of the alignment parameters during the first track reconstruction.

Figure 8 shows the performance of the alignment algorithm in terms of the obtained Mean(left panels) and RMS (right panels) of the δmx −δrx (top), δmy −δry (center) and δmφ −δφx (bottom)distributions as a function of the number of tracks (Ntracks) used by the alignment program.δmx , δ

my and δ

mφ are the simulated misalignments for the x and y translations and for the

azimuthal rotation. The superscript r is used for the misalignment found by our alignmentalgorithm. We observe that about Ntracks ∼ 150000 are needed for the alignment, whichshould represent a few hours of data taking.

Therefore as the first p–p data are taken our group will be devoted to an intensive alignmentand calibration phase. The foreseen plan is to have an early run of zero field data and performfirst the detector alignment over this data. Once a good alignment of the detector has beenachieved, the online calibration will be evaluated and validated or improved with the offlinegain calibration program.

It is worth noting that the final word on the validity of the alignment and gain calibrationwill be given by the obtained J/ψ (and later Υ ) mass resolution. The first analysis of theJ/ψ production in p–p collisions at

√sNN

=10 TeV (or 14 TeV) will be the natural extensionof our initial work on detector alignment and calibration. As mentioned earlier, this resultswill be of extreme importance as it will provide the needed reference data for the RAA typeof J/ψ suppression (or enhancement) analysis in Pb–Pb.

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Figure 8: Mean (top panels) and RMS (bottom panels) of the δmx −δrx (left), δmy −δry (middle)and δmφ − δφx (right) distributions as a function of the number of tracks (Ntracks) used by thealignment program.

5.2 Secondary production of J/ψ

One of the lessons from the puzzling results at RHIC is that a good knowledge of the sourcesof secondary J/ψ is crucial for the interpretation of the suppression results. This knowledge ispoor, the commonly accepted understanding is that about ∼ 40% of the produced J/ψ comefrom the decay of the excited states ψ′ (∼ 10%)and χc (∼ 30%), both at SPS and RHIC. AtLHC the situation will be even worst. Indeed, in addition to those J/ψ originating from theexcited charmonia states, the contribution from B hadrons decays will also be important. Itis expected that at LHC energies about ∼ 20% of the produced J/ψ originate from B hadronsdecays.

Our group has been interested on this issue. First, ψ′ contribution will be estimated fromthe direct ψ′ measured via their dimuon decay channel in the forward muon spectrometer.However, simulations have shown that the significance of this measurement will be marginal [5].Second, the contribution from the χc could be measured via the χc → J/ψ+γ. Unfortunatelywe have shown that ALICE does not have the acceptance to measure this contribution whenmeasuring the decayed J/ψ in the forward muon spectrometer.Indeed, our simulations showedthat when the J/ψ from the χc decay is in the forward muon spectrometer acceptance theγ, which by kinematics follows the heavier J/ψ , cannot be detected by any of the ALICEphoton detectors (PHOS or EMCAL) which are in the central barrel. This result triggered astudy of the capability to do this measurement in the central arm by measuring the J/ψ in itsdielectron channel and the γ by conversion to e+e−. Third, the contribution from B hadronsdecays can be measured directly in the central arm with a displaced vertex method using thegood secondary vertex resolution of the ITS. Currently a study is being carried to explore

11

the possibility to measure this contribution using the forward muon spectrometer by studying3 muon correlations. Although the person doing those studies is no longer in our group, weclosely follow those developments.

5.3 J/ψ elliptic flow

At LHC the J/ψ RAA measurement could allow us to distinguish between a QGP suppressionscenario and a regeneration scenario if either, a smaller RAA than at SPS and RHIC is observedor a RAA above one is seen. Any other intermediate result will be inconclusive. Recently, ithas been proposed that the measurement of the azimuthal anisotropy of the J/ψ will give aclearer answer. The azimuthal anisotropy can be characterized by the v2 parameter whichis the second order coefficient of the Fourier expansion of the particle azimuthal distributionwith respect to the reaction plane. The reaction plane is defined by the plane containingthe beams axis and the impact parameter vector. In non central heavy ion collisions theformed medium will exhibit an initial spatial anisotropy or ”almond” shape, longer in theout-of-plane than in the in-plane direction. On one hand, for non central collisions, primariesJ/ψ will show an effective v2 which will originate from the different path length of the mediumtraversed, causing more in-medium suppression in the out-of-plane direction. On the otherhand, J/ψ originating from regeneration will inherit the v2 from the constituent c and c̄ quarks.In this last case, the elliptic flow is generated from the initial spatial anisotropy of the systemcreated in non-central collisions by rescatterings among the constituents of the system, whichproduces stronger pressure gradients in-plane than out-of-plane.

Several theoretical calculations indicate that the observed v2 for regenerated J/ψ shouldbe larger than that for the primaries ones. As an example, the right panel of figure 9 showthe v2 as a function of the transverse momentum both for the primaries and the regeneratedJ/ψ in a QGP model including both production mechanisms [11]. Those calculations alsoindicate that at LHC energies the regenerated J/ψ should dominate the observed v2.

A pioneering measurement of the J/ψ v2 using the PHENIX detector at RHIC was per-formed by our group [12], the results are shown in figure 10. However, the statistical uncer-tainties are too large for a conclusive interpretation. Benefiting from this experience and theexpertise in our group on identified particles elliptic flow measurements in heavy ion collisions,we plan to carry out the J/ψ v2 analysis in Pb–Pb collisions with ALICE at LHC.

The simulation work to study the sensibility of the forward muon spectrometer to suchmeasurement has just started and will continue in parallel to other previously discussed topicsuntil the first Pb–Pb run which will provide the required statistics. The forward muon spec-trometer is well suited for the J/ψ reconstruction for the v2 measurement thanks to its highreconstruction efficiency and full azimuthal coverage.

The v2 measurement also requires the determination of the reaction plane. Typically,the reaction plane can be estimated by the so called event plane, which can be determinedfrom the azimuthal distribution of the reconstructed particles [13]. Among the detectors thatwill be read out along with the forward muon spectrometer three could potentially be usedfor such determination. First, the Silicon Pixel Detector which constitute the two inner-most layers of the ITS could be used. The SPD should allow the determination of the eventplane using the azimuthal distribution of the tracklets, defined by one hit in each layer ofthe SPD and the primary vertex. Second, the Forward Multiplicity Detectors will providethe azimuthal distribution of the event multiplicity in two regions, forward and backward, of

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Figure 9: LeftPanel: Number of regenerated over number of primary produced J/ψ at RHICand LHC energies as a function of the number of binary collisions. Right panel: v2 parameterfor initial, regenerated and total J/ψ in non-central collisions at RHIC. Within this modeland given the large N reg/N ini ratio at LHC, the J/ψ v2 is therefore predicted to follow theregeneration calculations [11]

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at freeze-out (PLB595,202)in transport model (PLB655,126)in fireball (PRL97:232301)+ initial mix (Zhao, Rapp priv. comm.)

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rapidity. Third, the Zero Degree Calorimeters will measure the azimuthal distribution of thespectators neutrons and protons, this will provide the first order reaction plane (useful fordirect flow studies). In all cases the figure of merit will be given by the resolution of the eventplane determination [13]. The event plane resolution will determine the sensitivity for theJ/ψ v2 measurement. Therefore, the first steps of our simulation studies will be to evaluatethe event plane resolution using the above mentioned detectors.

6 Summary

The commissioning work has restarted and would continue throughout the LHC shutdownperiod until the spring 2009. As the first p–p collisions are recorded the alignment of thedetector and the calibration of the pad gains will be performed. This work will naturallyallow us to participate in the first measurement of the J/ψ production in p–p collisions. Inparallel, simulation will be carried out to prepare the analysis of the J/ψ elliptic flow in Pb–Pbcollisions.

References

[1] ALICE collaboration, Technical Design Report of the Dimuon Forward Spectrometer,CERN/LHCC 99-22 (1999).

[2] ALICE collaboration, Addendum to the Technical Design Report of the Dimuon ForwardSpectrometer, CERN/LHCC 2000-046 (2000).

[3] R. Tieulent et al., Internal note, ALICE-INT-2005-009 (2005).

[4] P. Pillot, J.-Y. Grossiord, V. Kakoyan and R. Tieulent, Internal Note, ALICE-INT-2005-020 (2005).

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