Heavy Ions in CMS

EPJ Web of Conferences 60, 13010 (2013) DOI: 10.1051/epjconf/201360 13010 © Owned by the authors, published by EDP Sciences, 2013 Heavy Ions in CMS Thiago Rafael Fernandez Perez Tomei1,2 , a on behalf of the CMS Collaboration 1 Institute for Theoretical Physics - Univ. Estadual Paulista, R. Dr. Bento Teobaldo Ferraz, 271, Barra Funda, CEP 01140-070 São Paulo, SP, Brazil 2 CERN – European Organization for Nuclear Research, CH-1211 Geneva 23, Switzerland Abstract. The capabilities of the CMS experiment allow to investigate various hard probes, as well as bulk particle production and collective phenomena, using the calorimetry, muon and tracking systems covering a large range in pseudorapidity. In this paper selected results of the CMS experiment from p-p and Pb-Pb collisions p p at sNN = 2.76 TeV are discussed. First results from the recent p-Pb Run at sNN = 5.02 TeV are also be presented. 1 Introduction This paper reports on the latest results on heavy ions physics obtained by the CMS Collaboration. The Compact Muon Solenoid (CMS) is a high-energy physics experiment located at the Large Hadron Collider (LHC), CERN. The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the superconducting solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass/scintillator hadron calorimeter (HCAL). Muons are measured in gas-ionization detectors embedded in the steel return yoke outside the solenoid. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors; iron forward calorimeters (HF) with quartz fibers, read out by photomultipliers, extend the calorimeter coverage up to |⌘| = 5.0 and are used to classify the p-Pb and Pb-Pb collisions. A more detailed description can be found in Ref. [1]. All the detailed simulations of the interaction of the collision products with the CMS detector shown in these studies are made with geant4 [2]. 2 Results of p-Pb Run in 2013 In the beginning of 2013, the LHC collider provided 1 µb-1 p of proton-lead collisions at sNN = 5.02 TeV. Those data are used as an alternate environment, independent of PbPb collisions, for the study of hot nuclear matter. We detail in this section the results of three measurements: two-particle correlations, dijet balance and charged hadron spectra. a e-mail: 2.1 Two-particle correlations in p-Pb collisions This analysis [3] focuses on long-range, near-side twoparticle correlations. Events are collected online through usage of a track-based minimum bias trigger, where events are accepted if there is at least one track with pT > 400 MeV in the pixel tracker. In the o✏ine analysis, a coincidence of at least one HF calorimetric tower with E > 3 GeV on both the positive and negative sides of HF is required, in order to select hadronic collisions. Additionally, the presence of at least one reconstructed primary vertex with two tracks and a minimum fraction of good quality tracks is required in order to minimise beam-induced background. Those o✏ine requirements are collectively called the "basic hadronic selection". Tracks are considered for analysis only if they are "high purity" tracks, with pT > 100 MeV and |⌘| < 2.4. The events are divided in four track multiplicity categories: Ntrk < 35, 35–89, 90–109, > 110 tracks. The two-particle correlation function is given by: 1 d2 N pair S ( ⌘, = B(0, 0) ⇥ Ntrig d ⌘d B( ⌘, ) ) (1) where ⌘, are the di↵erences in ⌘ and of the two particles. The signal distribution S is the per-trigger-particle yield for pairs from the same event, while B is the analogous distribution for pairs coming from di↵erent events, which are not correlated. In Fig. 1, we can see the presence of a long-range structure in the azimuthal correlations for 2 < | ⌘| < 4, in the near-side region ( ⇠ 0). This result is qualitatively p similar for that observed previously both in s = 7 TeV p-p collisions and in nucleus-nucleus collisions. Figure 2 shows the associated yield for both 7 TeV p-p data and 5.02 TeV p-Pb data. The left plot shows the yield as a function of pT for Ntrk > 110, while the right plot shows the yield as function of multiplicity for 1 < pT < 2 GeV. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20136013010 EPJ Web of Conferences offline sNN = 5.02 TeV, Ntrk ≥ 110 verse energy measured by the HF detector. Figure 3 shows the HF transverse energy (ETHF ) distribution for the selected dijet events and minimum bias events. It can be seen that the selection of high-pT dijet event leads to a bias in the ETHF distribution towards higher values. The data are categorised in five ETHF bins: 0–20, 20–25, 25–30, 30–40 and 40–100 GeV, and compared both to pythia [7] (pp collisions) and pythia+hijing [8] (p-Pb) simulations. (b) pair 2 1 dN Ntrig dΔη dΔφ 1 < pT < 3 GeV/c 1.8 1.7 1.6 4 2 Δφ 0 -4 0 -2 Δη 2 4 10 Fraction of minimum bias events CMS pPb (a) CMS offline Ntrk 0.04 ≥ 110 Associated Yield / (GeV/c) Associated Yield / (GeV/c) Figure 1. 2D two-particle correlation function for 5.02 TeV p-Pb collisions for pairs of charged particles with 1 < pT < 3 GeV for Ntrk < 35. 0.04 (b) 1 < p < 2 GeV/c T pPb sNN = 5.02 TeV pp s = 7 TeV 0.02 0.00 0 2 4 pT(GeV/c) 6 0.02 1 CMS Preliminary Minimum bias events Dijet events -3 10 10-4 10-5 10-6 10-7 0 10 20 30 40 50 60 70 80 90 100 HF[|η|>4] ET 100 Noffline trk 150 Figure 2. Associated yield for the near-side of the correlation function integrated over the region 2 < | ⌘| < 4 and | ⌘| < 1.2 in 7 TeV pp collisions (open circles) and 5.02 TeV p-Pb collisions. -1 10-2 10-8 50 ∫ L dt=18.48 nb 10-1 0.00 0 pPb (GeV) Figure 3. Probability distribution of the raw ET measured by the HF detector in the pseudorapidity interval |⌘| > 4 for minimum bias collisions (black open histogram) and dijet events (red hatched histogram). 0.28 It can be seen that not only the absolute associated yield is larger in p-Pb collisions, but also that the correlations start to increase for Ntrk ' 40. 0.26 0.78 CMS Preliminary pPb anti-kT(PFlow) R=0.3 0.74 Δφ1,2 > 2π/3, |η|<3 0.72 0.7 T,2 0.22 0.2 0.68 0.66 0.64 0.18 2.2 Dijet balance and pseudorapidity in p-Pb collisions 0.76 T,1 <p /p > -1 1,2 σ(Δ φ ) 0.24 ∫ L dt=18.48 nb 0.62 0.16 0 10 20 30 40 50 60 0 10 HF[|η|>4] ET (GeV) -0.2 1.1 pT,2 > 30 GeV/c 1.05 50 60 pPb sNN=5.02 TeV PYTHIA + HIJING PYTHIA 1 dijet > -0.4 σ(η dijet <η 40 (GeV) 1.15 pT,1 > 120 GeV/c ) -0.3 30 HF[|η|>4] ET -0.1 This analysis [4] studies dijet events in p-Pb collisions as a function of the forward ca (...truncated)