Centrality in nucleus-nucleus collisions A.Kurepin, A.Litvinenko, E.Litvinenko Institute for Nuclear Research RAS, Moscow Joint Institute for Nuclear Research, Dubna Contents: • Introduction • Conception of centrality in collider and fixed target experiments • Methods of centrality determination • Accuracy of the centrality and impact parameter measurements • Simulation of possible multiplicity detector for MPD/NICA • Conclusions EMIN2018, October 8 - 11, 2018 , INR, Moscow, Russia. 1
Phys.Rev. C88 (2013) no.4, 044909 ALICE Collaboration Centrality dependence of d N ch = d h per participant pair as a function of N part, measured in the Pb–Pb data at ps NN = 2 : 76 TeV fitted with various parametrizations of N part and N coll, calculated with the Glauber model. The fit parameters are given in the figure
N ch = d h per centrality bin from each of the three detectors used. The error Phys.Lett. B726 (2013) 610-622 bars correspond to the total ALICE Collaboration statistical and systematic error
d N ch = d h per centrality class compared to model predictions Phys.Lett. B726 (2013) 610-622
Phys.Lett. B734 (2014) 314-327 ALICE Collaboration Centrality dependence of the nuclear modification factor, R AA, of inclusive J =y production in Pb-Pb collisions at ps NN =2 : 76TeV, measured at mid-rapidity and at forward-rapidity
Phys.Rev. C91 (2015) no.6, 064905 ALICE Collaboration Distribution of the neutron energy spectrum measured in the Pb- remnant side ZN calorimeter. The distribution is compared with the corresponding distribution from the SNM-Glauber model shown as a line. Centrality classes are indicated in the figure. The inset shows a zoom-in on the most peripheral events.
Phys.Lett. B772 (2017) 567-577 ALICE Collaboration Comparison of d N ch = d h in the 0– 5% (0–6% for KLN) most central collisions of two versions of HIJING, KLN, and EPOS-LHC model calculations to the measured distribution.
Centrality and pseudorapidity dependence of the charged-particle multiplicity density in Xe-Xe collisions at 5.44 TeV ALICE Collaboration arXiv:1805.04432 Comparison of d N ch = d h as a function of h in the 0–5% central class to model predictions. The bottom panel shows the ratio of the models to the data. Boxes around the points reflect the total uncorrelated systematic uncertainties.
Eur.Phys.J. A54 (2018) no.5, 85 Centrality determination of Au + Au collisions at 1.23A GeV with HADES The cross section as a function of N hits TOF+RPC for the sum of TOF and RPC hits. The panel shows a simulation based on UrQMD eventsnfiltered through the detailed detector simulation (blue symbols) and in addition through an emulator of the PT3 trigger (green symbols) in comparison with the Glauber MC model (red histogram)
Sketches of VZERO-A and VZERO-C arrays showing their segmentation. Scintillator thicknesses are 2.5 and 2 cm respectively. 11
Distribution of the sum of amplitudes in the two VZERO arrays (black histogram) in Pb–Pb collisions at √ s NN = 2.76 TeV. The red line shows the fit with a Glauber model. The shaded areas define the different centrality classes of hadronic collisions. The inset shows the low amplitude part of the distribution. 12
The ALICE Diffractive detector AD is formed by two main stations, each station consists of 2 layers with 4 plastic scintillator pads each (8 pads per side). Each scintillator measures roughly 18cm x 21cm . Each scintillator plastic is coupled to a PMT through a wave length shifting bar and an array of clear optic fibers. Not to scale For trigger generation, a coincidence between adjacent pads is required. ,,
XeXe run Used data: Event selection: – pass1 AOD data – CINT7ZAC (CINT7 + ZDC) – ADC saturation was corrected in CPass1 – AD AND (online+offline BB) – charges were normalized to have the same 90% quantile Glauber ntuples for XeXe (thanks to Alberica Toia) 14
Evo volution o of FHCAL Compact ZDC of 20 modules Extended HCAL of 120 modules with high granularity. with high granularity transverse sizes 10x10 cm 2 Simplicity, cheap. Low acceptance. Poor event plane angular resolution. High acceptance, perfect event plane resolution. Very expensive, complicated . transverse sizes 15x15 cm 2 FHCAL of 45 modules: Moderate segmentation, High acceptance, Nice event plane resolution, Reasonably simple and cheap. 15
Centrality determination from the energy depositions in calorimeter. = S 11 GeV NN M.Golubeva et al. “ Nuclear-Nuclear Collision Centrality Determination by the Spectators Calorimeter for the MPD Setup at the NICA Facility “ Yad.Fiz. 76 ( 2013 ) 2-17 The ambiguity in centrality determination can be resolved by taking track multiplicity in TPC. 16
Место для уравнения. The correlation between energy deposition in FHCal and charged tracks multiplicity in TPC for the Au-Au collisions with energy 5 GeV (left) and 9 GeV (right)
FHCAL alone can measure the centrality (without any other detectors). Together with energy deposition E dep in FHCAL another E out observable “ energy asymmetry” is introduced: E in As=(E in -E ou t )/( E in +E ou t ). By taking the horizontal cut at asymmetry and the vertical bins at energy deposition one can resolve the ambiguity in centrality determination. 11 GeV 5 GeV Energy asymmetry Energy asymmetry 18
The resolution of impact parameter obtained in separate bins of the energy depositions in FHCal for beam energy √ s = 5 GeV (left) and √ s= 11 GeV (right). Blue and red points correspond to the one and two parts of FHCal, respectively. Green points - estimation with allowance for spectator number fluctuations. 19
V. Klochkov, I Seluuzhenkov, CBM centrality determination Acta Physica Polonica 10 ( 2017 procedure with PSD ) 919
Multiplicity distribution for STS at CBM ( left ). Impact parameter resolution with different centrality estimators ( right )
Number of primary particles/event hitting FWALL surface (hole d=10 cm) Including neutrons Au+Au √s = 5 AGeV Au+Au √s = 11 AGeV M.Golubeva, A.Ivashkin, A.Kurepin “Study of nuclear fragmentation at MPD/NICA “ EPJ Web of Conferences 138, 11001 ( 2017 ) Baldin ISHEPP XXIII 22
Au+Au √s = 5 AGeV Number of primary particles/event hitting FWALL surface Without neutrons ( from generator ) Hole d=10 cm Hole d=70 cm Hole d=0.5 cm 23
Au+Au √s = 11 AGeV Number of primary particles/event hitting FWALL surface Without neutrons (Using info from MC tracks category) Hole d=10 cm Hole d=70 cm 24
Sketch of FMD array showing their segmentation. Scintillator thicknesses are 2.5 cm. Radius of the hole is 6 cm. Outer radius is 70 cm. 25
Multiplicity and energy distributions of protons, pions and fragments hitting the FMD detector 26
Multiplicity and energy distributions of protons, pions and fragments with energy less 3.5 GeV hitting the FMD detector 27
Dependence of the total energy in GeV of protons on the impact parameter b in fm 28
Conclusions 1. Centrality in nucleus-nucleus collisions is the important conception in the relativistic nuclear collisions. However due to the multiple interactions by passing the nuclei the impact parameter could not be determined with accuracy better than 10 % 2. Additional constraints arise at energies of the order of 3-10 AGeV because of the contribution of spectators 3. For MPD/NICA by using the Forward Multiplicity Detector the determination of the impact parameter could be obtained with accuracy better than with Forward Hadron Calorimeter of the order of 10 % for the central and semi- central collisions 32
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