Charged-particle nuclear modification factors in XeXe collisions at $$ \sqrt{s_{\mathrm{NN}}} = 5.44 $$ TeV

Abstract

The differential yields of charged particles having pseudorapidity within |η| < 1 are measured using xenon-xenon (XeXe) collisions at √ s NN = 5.44 TeV. The data, corresponding to an integrated luminosity of 3.42 µb −1 , were collected in 2017 by the CMS experiment at the LHC. The yields are reported as functions of collision centrality and transverse momentum, p T , from 0.5 to 100 GeV. A previously reported p T spectrum from proton-proton collisions at √ s = 5.02 TeV is used for comparison after correcting for the difference in center-of-mass energy. The nuclear modification factors using this reference, R * AA , are constructed and compared to previous measurements and theoretical predictions. In head-on collisions, the R * AA has a value of 0.17 in the p T range of 6-8 GeV, but increases to approximately 0.7 at 100 GeV. Above ≈6 GeV, the XeXe data show a notably smaller suppression than previous results for lead-lead (PbPb) collisions at √ s NN = 5.02 TeV when compared at the same centrality (i.e., the same fraction of total cross section). However, the XeXe suppression is slightly greater than that for PbPb in events having a similar number of participating nucleons.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 solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, and a brass and scintillator hadron calorimeter, each composed of a barrel and two endcap sections. Forward calorimeters extend the η coverage provided by the barrel and endcap detectors. Muons are detected in gas-ionization chambers embedded in the steel flux-return yoke outside the solenoid.The silicon tracker measures charged particles within the range |η| < 2.5. It consists of 1856 silicon pixel and 15 148 silicon strip detector modules. For nonisolated particles of 1 < p T < 10 GeV and |η| < 1.4, the track resolutions are typically 25-90 (45-150) µm in the transverse (longitudinal) impact parameter [25].The hadron forward (HF) calorimeter uses steel as an absorber and quartz fibers as the sensitive material. The two halves of the HF are located 11.2 m from the interaction region, one on each end, and together they provide coverage in the range 3.0 < |η| < 5.2.Events of interest are selected using a two-tiered trigger system [26]. During XeXe operation the first level trigger (L1), composed of custom hardware processors, uses information from the calorimeters to select events at a rate of around 4 kHz within a time interval of less than 4 µs. The second level, known as the high-level trigger (HLT), consists of a farm of processors running a version of the full event reconstruction software optimized for fast processing, and reduces the event rate to around 2 kHz before data storage.A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in Ref. [27].

Latest results on jet suppression and jet substructure in heavy-ion collisions with CMS

“…Recently the CMS collaboration has measured nuclear modification factors from XeXe collisions at √ s NN = 5.44 TeV [6]. The results for the 0-5 % most central collisions compared to previous results for PbPb collisions at √ s NN = 5.02 TeV are shown of the left plot of Fig.…”

“…We present recent results from the CMS collaboration studying the nuclear modification factors [6], jet shapes [7,8,9], shared momentum fractions [10] and groomed jet mass [11]. All the presented pp and PbPb results use datasets collected by the CMS experiment [12] in 2015 at collision energy √ s NN = 5.02 TeV corresponding to integrated luminosities of 404 µb −1 for PbPb and 27.4 pb −1 for pp.…”

Jet quenching from heavy to light ion collisions

“…We have adjusted the free parameter κ by χ 2 fitting of the LHC data on R AA for heavy ions collisions for centrality 30% and the transverse momentum 10 < p T < 120 GeV. We have used data from ALICE [75], ATLAS [76], and CMS [77] for 2.76 TeV Pb+Pb collisions; from ALICE [78], ATLAS [79], and CMS [80] for 5.02 TeV Pb+Pb collisions; and for 5.44 TeV Xe+Xe collisions from ALICE [81], ATLAS [82], and CMS [83]. As usual, we define χ 2 as…”

Quenching of Hadron Spectra in Heavy Ion Collisions at the LHC

“…Data include charged hadron production measured by CMS in PbPb collisions in five centrality classes at both colliding energies [8,9]. In this update, we have also analyzed the CMS measurements of J/ψ production in PbPb collisions at √ s = 5.02 TeV [4] and charged hadron production in XeXe collisions at √ s = 5.44 TeV [5].…”

“…In that spirit, we analyze in these proceedings the quenching of J/ψ production in PbPb collisions at √ s = 5.02 TeV [4] and that of charged hadrons in XeXe collisions at √ s = 5.44 TeV [5], recently made available by the CMS collaboration and shown at this conference.…”