First results from the HMPID detector in theALICE experiment at LHC

Francesco Barile for the ALICE Collaboration

Dipartimento Interateneo di Fisica “M. Merlin” and INFN Sezione di Bari, via E. Orabona 4,70126 Bari, Italy

Abstract. The ALICE experiment is dedicated to the study of heavy-ion collisions at LHC energies.ALICE has a high-quality hadron identification system in thecentral region e.g. a Inner TrackingSystem, a large Time-Projection-Chamber, a Time-Of-Flight and the High Momentum ParticleIdentification Detector (HMPID). The ALICE HMPID detector has been designed to identifycharged hadrons (π, K, p) in the momentum range 1

FIGURE 1. Event display of a p-p event in ALICE. The ALICE central system includes, from theinteraction vertex to the outside, the Inner Tracking System (ITS), the Time-Projection-Chamber (TPC),the Transition-Radiation-Detector (TRD), and the Time-Of-Flight (TOF). The central system is com-plemented by the High-Momentum-Particle-Identification-Detector (HMPID), the PHOton-Specrometer(PHOS) and the electromagnetic calorimeter (EMCal).

FIGURE 2. Left: view of the seven modules of the HMPID mounted on the cradle; Right: workingprinciple of a RICH detector employing CsI thin films deposited onto the cathode plane of a MWPC. TheCherenkov cone refracts out of the liquid radiator of C6F14 and expands in the proximity volume of CH4before reaching the MWPC photon detector. Electrons released by ionizing particles in the proximity gapare prevented to enter the MWPC volume by a positive polarization of the collection electrode close tothe radiator.

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FIGURE 3. MIP charge distribution for the seven chambers at√

s = 7 TeV and B=0 T.

val attainable through energy loss (in ITS and TPC) and time-of-flight measurements (inTOF). The detector was optimized to extend the useful range for π/K and K/p discrimi-nation, on a track-by-track basis, up to 3 and 5 GeV/c−1 respectively.

The HMPID is based on proximity-focusing Ring Imaging Cherenkov (RICH) coun-ters and consists of seven modules of about 1.5× 1.5 m2 each, mounted in an inde-pendent support cradle (see Fig.2). The cradle [4] is fixed tothe space frame in thetwo o’clock position. The radiator, which defines the momentum range covered by theHMPID, is a 15 mm thick layer of low chromaticity C6F14 (perfluorohexane) liquid withan index of refraction ofn = 1.2989 @ 175 nm corresponding toβmin = 0.77 (i.e. a mo-mentum thresholdpth = 1.21m, wherem is the particle mass). The photons detection isprovided by multiwire chamber coupled with pad-segmented CsI photocatode (CsI Q.E.≈ 25 % @ 175 nm, pad size 0.8x0.84 cm2). The amplification gas is CH4 at atmosphericpressure and the anode-cathode gap is 2 mm. A positive voltage of 2050 V, applied tothe anodes, while cathodes are grounded, provides a total gas gain of almost few 104.The 42 photocatodes are segmented in 3840 pads with individual analog readout. Thenoise level is 1 ADC channel (1000 e−) and dead or noisy pads are less than 300 out of161280.

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FIGURE 4. Number of reconstructed photons at√

s= 7 TeV and B=0 T.

Physics with HMPID

The main tasks of the HMPID are listed below:

• Extension of theπ, K, p spectra to high momentum range;• Particle ratios vspT (p-bar/p, p/π, K/π);• Measurement of resonance production such asΦ (1020)−→ K+ K−;• Jet physics:

– Study of jet fragmentation with the identification of particle in the jet;– Study of the flavor of the leading particle;

• Identification of light nuclei (d, t, 3He,α).

ANALYSIS AND RESULTS

ALICE has recorded data during the first LHC pp runs at√

s = 7 TeV in March 2010(B=0.5 T). The HMPID detector has successfully collected enough statistics to allowthe detector alignment and to evaluate the calibration parameters needed to estimate itsperformance. In Fig.1 the event display of a p-p event in ALICEis shown.

In Fig.3 the distribution of MIP charge for the seven chambers corresponding to 106

p-p events at 7 TeV without magnetic field is shown. The distributions are well fitted bya Landau function with mean value of about 400 ADC. A cut on the distance of the MIP

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FIGURE 5. Cherenkov angle distributions forπ+, K+ at 0.5

to the extrapolated point on the HMPID chambers are applied (d < 0.7 cm). Using thesame set of data the distributions of the number of reconstructed photons as a functionof sin2θCh (blue points) for the seven chambers have been extracted (Fig.4). Red pointsrepresent the same quantity corrected by taking into account detector acceptance (deadzones between CsI photocathodes). For the 7 TeV data with a magnetic field (B = 0.5 T),the analysis has been restricted to the momentum range 0.5