Detection of heavy quark resonances with heavy ion beams in CMS

Abstract

Performances of CMS in measuring J/ψ and Υ production in heavy ion collisions are investigated. Resonance cross-sections and secondary particle multiplicities are calculated as a function of the collision impact parameter. The most unfavourable assumptions are considered for the hadron multiplicity in central collisions and their PT distributions. Results for lead and calcium beams are presented. The Υ acceptance reaches an acceptable value when the analysis is restricted to the barrel region (|η| < 1.5) and to muons with a transverse momentum PT > 3.5 GeV/c. The dimuon mass spectra obtained for one month of data taking are presented. Statistics and Signal/Background ratios allow to conclude that CMS has good capabilities for thess studies. 1 Introduction The dissociation in a deconfining medium of the heavy quark vector mesons (i.e. J/ψ, Ψ and Υ family resonances) is one of the most promising signatures of QGP formation [1]. Such a suppression has been observed at SPS energies by the NA50 Collaboration [2] detecting the J/ψ produced in Pb-Pb collisions. However this experiment could not conclude firmly about the formation of QGP in the most central collisions; at √ sNN ≈17 GeV, only the J/ψ and less easily the Ψ can be studied. At the future LHC energy, the Υ family resonances are accessible to the experiment. One of the main goal of the CMS heavy ion program is to measure the production of these resonances, via their muonic decay as a function of the atomic number of the beam. In the proton case, although the energy density can locally reach high values, the volume of the hot medium is too small to create a plasma. In Pb-Pb interactions, which involve a much larger number of nucleons, the colour screening effect should lead to the suppression of heavy quark bound state production in the most central collisions, in which all the conditions for QGP formation are achieved. In this note we present the results of dimuon detection simulations in nucleus-nucleus (A-A) collisions. The muon detection in CMS is performed in a wide pseudo-rapidity range, |η| <2.4. However, we will often limit the detector to its barrel part characterized by |η| <1.5. In the 2nd section, we describe how we simulate a A-A collision. The quarkonium production cross-sections are then estimated and their acceptances in the CMS detector are shown. In the 4th section the main sources of dimuon background are studied. The 5th section presents the rates of events with at least two impacts in muon chambers. In the last section we discuss the expected dimuon mass spectra. We present the J/ψ and the Υ mass regions separately and give the corresponding N(resonance)/continuum and the expected statistics for one month running time. 2 General A-A collisions Description of a nucleus-nucleus collision at LHC 2.1 Luminosity Scanning from p-p to Pb-Pb collisions leads to strongly different experimental conditions. Some of them are summarized in table 1 for the Pb and Ca beams. The luminosity values quoted in the table are those adopted in the simulations. For Pb beams, the nominal luminosity ranges between 0.85 and 1.8 × 10 depending on the luminosity lifetime [3]. If two (resp. 3) experiments are running simultaneously the available luminosity has to be divided approximately by 3 (resp. 5). For Ca beams the maximum luminosity is expected to be 2500 times higher than for Pb beams [4]. Whereas the pile-up effect will be almost inexistent with Pb-beam it cannot be neglegted anymore for Ca beams. To limit it at a low level (≈3%) we shall probably have to reduce the Ca beam luminosity by one order of magnitude, and not exceed 2.5× 10 cms. 2.2 NNcol and π/K multiplicity The general parameters for a given A-A collision are: impact parameter b, number of nucleon-nucleon collisions NNcol or hadronic multiplicity dN ± dy . They are determined using HIJING [5]. We did not use the CMS simulation package coupled with HIJING generator for our studies mainly for CPU time reasons. We have therefore developped a separate program using the HIJING results as an input. First, the impact parameter b is randomly choosen according to a fit of the distribution given by HIJING. The choice of b governs several important characteristics of a A-A collison. iFor a given b we can deduce the number of nucleon-nucleon collisions, NNcol. It is given by the following parametrized function: NNcol = N0 × e b b0 ) 2 − a0 × b where N0= 1550 (180), b0= 7 (4.5) fm and a0= 0.5 (0.)for Pb (resp. Ca) collisions (see figure 1) and respectively (180, 4.5, 0.) for the Ca case. The resonance production cross-sections are estimated from NNcol (see next section) as well as the contribution to the dimuon background of the open b and c channels (see farther). iiThe main source of dimuon background comes from the huge number of particles emitted in the ion-ion collision. This number depends obviously on the impact parameter and is proportional to the multiplicity of charged particles reached in one central collision (b=0), ( dN dy b=0 y=0 . It is presently difficult to give a precise value for this variable at the LHC energies. Several event generators like VENUS, HIJING, DPM and FRITIOF predict a multiplicity ranging from 3000 to 8000 in the case of very central Pb-Pb collision. Obviously, smaller the multiplicity 2 Table 1: General parameters for Pb and Ca beams used in the present study. Pb Ca √ SNN (TeV) 5.5 7.0 LA (cms) 10 2.5×1029 σ(A-A) (barn) 7.6 2.1

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