Quarkonia measurements in heavy-ion collisions in CMS
Journal of Physics G: Nuclear and Particle Physics
The production of quarkonia is one of the most promising signals at the LHC for the study of the production properties of Quark Gluon Plasma. In addition to the J/ψ the extent to which Υ is suppressed should give much insight into the new state of matter. The large muon acceptance and the high precision tracker make the CMS detector ideal for studies of this physics. In this note, the performance of the CMS detector for quarkonia measurements in heavy-ion collisions in the dimuon channel is
... ented. Dimuon reconstruction efficiencies and mass resolution are calculated using detailed detector simulation. Mass spectra and signal to background ratios are estimated with a fast Monte Carlo program. Results obtained with the fast Monte Carlo are compared with more detailed simulations. [1, 2] which showed a strong anomalous suppression of J/ψ production in Pb+Pb collisions at √ s N N = 17.3 GeV. RHIC is studying the J/ψ production in detail at √ s N N = 200 GeV in Au+Au and Cu+Cu collisions. The global J/ψ suppression ammounts in factor three between most central and peripheral collisions  . Only part of the suppression can be explained by calculation including shadowing and absorption effects  . However recent theoretical analysis  suggests that the direct J/ψ could survive for temperature as high as 1.5 Tc (The critical temperature for the phase transition is about 200 MeV) which could be out of the range of the RHIC. Although the Υ production cross section is large enough to be observed at RHIC, albeit with limited statistics, its suppression is not expected until the high initial temperatures foreseen at LHC are reached. Thanks to its large muon detector and to the high precision tracker, the CMS detector is particularly well suited to study the quarkonia state production in the dimuon channel. Over than 4 orders of magnitude separate the Pb+Pb inelastic cross section from the production cross section of the Upsilon in the dimuon decay channel. Therefore, a complete and statistically significant simulation of the quarkonia detection using an event generator like HIJING  to simulate a Pb+Pb collision and the official tools of the CMS Collaboration for the tracking of the secondaries, OSCAR , together with the reconstruction of the event, ORCA , would need an unavailable calculation power. A fast simulation method is therefore unavoidable to study the Υ and J/ψ production in the heavy ion collisions. The fast Monte Carlo method is described and compared with the result of the detailed simulation. A full detector and trigger simulation simulation plus reconstruction are carried out for a few 10 7 particles of different types relevant to the muon signal and background (muons, pions, kaons, b−, c− hadrons. The corresponding response functions (trigger acceptances, mass resolutions, reconstruction efficiencies, etc) are parametrized with detailed simulations for the each type of particle. The obtained parametrizations are used in a fast MC to produce the finally corrected yields. The performances of CMS for Υ and J/ψ observation are discussed. CMS detector A detailed description of the detector elements can be found in the corresponding Technical Design Reports [9, 10, 11, 12]. The CMS detector is designed to identify and measure muons, electrons, photons and jets over a large energy and rapidity range. It offers the widest muon acceptance centered at midrapidity. CMS is particularly well suited to study the Υ and J/ψ families, the continuum up to the Z 0 mass. The CMS dilepton capability allows systematic studies of heavy flavour physics. The impact parameter (centrality) of the collision can be determined from measurements of transverse energy production over the range |η| < 5. The central element of CMS is the magnet, a 13 m long, 6 m diameter, high-field solenoid (a uniform 4 T field) with an internal radius of ≈3 m. The hadronic (HCAL) and electromagnetic (ECAL) calorimeters are located inside the coil (except the forward calorimeter) and cover (including the forward calorimeter) from -5 to 5 pseudorapidity units. The HF calorimeter covers the region 3 < |η| < 5. The ECAL calorimeter consists from the 20 cm length PbWO4 crystalls and corresponds to 1.1 interaction length. The central hadron calorimeter (|η| < 3) is a sampling calorimeter: it consists of active material inserted between brass alloy absorber plates. Innemost and outermost absorbers are produced from stainless steel. The overall number of the nuclear interaction length before penetrating muon stations (λ) is 11-16 over |η| < 3 . Muons loose 3 GeV in the calorimeter due-to ionization losses. The probablity for hadron to not interact in calorimeter at al is 0.005%. The first absorber, the electromagnetic calorimeter, is 1.3 m from the interaction point, eliminating a large fraction of the hadronic background. The tracker and muon chambers cover the pseudorapidity region |η| < 2.4, Starting from the beam axis the tracker is composed of two different types of detectors: pixels and silicon strips. The pixel detector consists of 3 barrel layers located at 4,7,11 cm from the beam axis with granularity 150×150µm 2 and 2 forward layers with granularity 150 × 300µm 2 located at the distances of 34 and 43 cm in z from the center of detector. Silicon strip detectors are divided into inner and outer sections and fill the tracker area from 20 cm to 110 cm (10 layers) in the transverse direction and up to 260 cm (12 layers) in longitudinal direction. The strip length for silicon 1 strip counters varies up to 21 cm for outermost layers and the pitch varies from 61 to 205 µm depending on the radius. The η-coverage of tracker detector is from -2.4 to 2.4. The CMS muon stations consist of drift tube chambers (DT) in the barrel region (MB), |η| < 1.2, cathode strip chambers (CSCs) in the endcap regions (ME), 0.9 < |η| < 2.4, and resistive plate chambers (RPCs) in both barrel and endcaps, for |eta| < 1.6. The chambers are mounted within iron wheels in barrel and iron disks in endcap. The RPC detector is dedicated to triggering, while the DT and CSC detectors, used for precise momentum measurements, also have the capability to self-trigger up to |eta| < 2.1. The muon system can thus reconstruct muons in the range |eta| < 2.4. Input to Monte Carlo studies At the LHC energies a possible regeneration of the J/Psi, due to the high production of c-bar c pairs, is expected and may compensate an anomalous suppression  . Therefore, in the work presented here, the shadowing is the only effect taken into account. Dense matter effects like suppression or modification in the high Pt hadron spectra  are not considered.