Cen entralit ity de determin inatio ion in in MPD at t NICA ICA: App pplic icatio ion of of ha hadron cal alorim imeters Volkov Vadim INR RAS 25/08/2020 Workshop on analysis techniques for centrality determination and flow measurements at FAIR and NICA 1
Ov Overv rview • Can FHCal measure the centrality with spectators? • FHCal detects not only energies but the space distribution of energies! • A few methods for centrality determination are discussed: • a) Correlations of transverse and longitudinal energy components, • b) 2D-fit of FHCal energy distributions, • c) Subtraction of pion e nergy contamination and evaluation of spectator’s energy. Tools: • Simulations in MpdRoot; • Au-Au at ; S 11 GeV NN • Two, LA-QGSM and DCM-SMM fragmentation models are used and compared. 2
FH FHCal@ l@MPD • The main purpose of the FHCal is to detect spectators and to provide an experimental measurement of a heavy-ion collision centrality and orientation of its reaction plane. • There is an ambiguity in FHCal energy deposition for central/peripheral events due to the fragments (bound spectators) leak into beam hole. • FHCal measures not only spectator’s but also pion’s energies. Two upstream/downstream parts S 11 GeV NN 44 individual modules ambiguity Non- spectator’s Beam hole contributions 3
Energy Ene y dep deposit itions in FH FHCal l for or di different t models LA-QGSM DCM-SMM Transverse energy distributions are wider for central events and narrower for the peripheral collisions. Non- spectator’s contributions • Energy depositions are quite different for different fragmentation models. • Results would depend on the fragmentation model. • FHCal detects not only the spectators but also the LA-QGSM produced particles and wounded nucleons from Impact parameter b<= 6 Impact parameter b>6 participant region. This feature can be used for the separation of central/peripheral events. 4
Corr orrelatio ion be between transverse an and long longitudinal en energies in in FHC FHCal • LA-QGSM and DCM-SMM LA-QGSM DCM-SMM models for √ S = 11 AGeV are used. • The E T and E L energies are transverse and longitudinal energies: respectively. Each color bin is 10% fractions • The (E T -E L ) histograms are divided of the total number of events. Each color bin is 10% fractions into ten parts, 10% of events in each of the total number of events. part, 10%-clusters are separated from one another by perpendiculars 0.5 to the envelope. 0.45 DCM-SMM counts 0.4 • b-distributions for each centrality LA-QGSM DCM-SMM 0.35 bin are fitted by Gauss. 0.3 0.25 • The separation of central and σ b /b 0.2 peripheral events with DCM-SMM 0.15 model is clearly worse. 0.1 0.05 0 0 20 40 60 80 100 Centralit ity % New approaches are needed Impact parameters [fm] Dependence of resolution of impact parameter 5 on centrality
2D 2D-li linear fi fit t meth thod (linear approach) Energy distribution in FHCal modules Fitted event Single event E [MeV] E [GeV] • In this method the space energy distribution in FHCal modules is used. • The energy in the histogram is uniformly distributed in FHCal modules according to the polar angle. • The histogram is fitted by a symmetrical cone (linear approximation). • Weight of each bin is proportional of the energy deposited in corresponding FHCal module. • This fit provides the new observables: radius, height of the cone. Volume of cone corresponds to the reconstructed energy (E rec ). 6
Corr Correla lation be between ob obtain ined fi fit t par parameters. s. LA LA-QGSM Initially we have experimental Experimental energy deposition vs Maximum energy in energy deposition E dep in FHCal. reconstructed energy from the fitted event central bin vs radius E [MeV] E_max (height) E rec [GeV] radius Experimental energy deposition vs maximum energy in central bin After linear fit we have: • E rec is reconstructed energy (volume of cone); • E max – maximum energy in central bin (in FHCal hole); • Radius of spectator spot at FHCal is defined by the scattering spot of spectators. This correlation can be used for the centrality determination 7
Centralit Cen lity res esolu lutio tion for or E dep vs E max Edep [a.u] 0.45 LA-QGSM DCM-SMM 0.4 LA-QGSM 0.35 0.3 σ b /b 0.25 0.2 0.15 0.1 0.05 0 E max [a.u] 0 20 40 60 80 100 Edep [a.u] Centrality % DCM-SMM Dependence of resolution of impact parameter on centrality DCM-SMM E max [a.u] 8
2D D line near fi fit t me meth thod od (with subtraction of pion contribution) Fitted and uniformly Single event E [MeV] distributed event E [GeV] Experimental energy deposition vs reconstructed energy from the fitted event Pion contribution E rec [GeV] • Narrow cone radius indicates that the outer FHCal modules detect the pions mainly, while the spectators are detected by inner modules. • Energy in outer modules can be regarded as pure non-spectator (pion) contribution. • Let’s try to evaluate pion contribution in full FHCal. 9
Evaluation on of of pion pion ene energy con ontr tributio tion Pion energy distribution Pion energy distribution Energy [GeV] E [MeV] 1D-case 2D-case y=kx+b y=-kx+b b Distance from center [cm] Pion contribution is subtracted E [MeV] • Linear fit with y=kx+b background, • b is known from outer FHCal modules, • According to simulation k = (2.2 - 2.7) depending on centrality, • In this presentation k = 2.5 is used 10
Centrality reso esolutio ion for or E dep vs s E re rec (after subtraction of pion contribution ) 0.45 E rec is a volum ume of a cone ne LA-QGSM DCM-SMM 0.4 0.35 LA-QGSM 0.3 E [MeV] 0.25 σ b /b 0.2 0.15 0.1 0.05 0 0 20 40 60 80 100 Centrality % E rec [GeV] Dependence of resolution of impact parameter on centrality 16 DCM-SMM 14 12 10 Mean 8 6 4 2 0 0 20 40 60 80 100 Centrality % Dependence of impact parameter on centrality E rec [GeV] 11
Energy deposition can be decomposed in two components: energy of free spectators and non-spectators energy LA-QGSM DCM-SMM By using the subtraction of the non-spectator ’s contribution, the energy deposition can be decomposed into two components. E_dep Free Non- LA-QGSM DCM-SMM spectators spectators energy energy (E_rec) (E_pions) Both energies can be used for centrality determination. 12
Comparis Co ison of of result lts fr from di different methods Dependence of resolution of impact parameter on centrality 0.5 0.5 LA-QGSM DCM-SMM 0.45 0.45 0.4 0.4 Et El Et El 0.35 0.35 Edep Erec Edep Erec 0.3 0.3 σ b /b σ b /b 0.25 0.25 0.2 0.2 0.15 0.15 0.1 0.1 0.05 0.05 0 0 0 20 40 60 80 100 0 20 40 60 80 100 Centrality % Centrality % • Application of linear fit method improves the resolution for the most central events; • DCM-SMM model provides worse results comparing to LA-QGSM one. 13
Conclusion • The ability of FHCal to measure the collision centrality was considered. • Only the spectators for the centrality reconstruction were used. • Three methods for the centrality determination have been demonstrated: • Transverse-longitudinal energies correlation; • 2D-linear fit method; • 2D-linear fit with pion contribution subtraction method. • A few new observables were introduced for the centrality determination. • The usage of the introduced observables allows to determine the centrality more accurately, especially for the DCM-SMM model. • DCM-SMM model provides worse centrality resolution because this model has much more heavy fragments which escape in FHCal beam hole. • The subtraction of the pion contribution makes possible to measure the energy of free (protons/neutrons) spectators. • Number of free spectators can be estimated more accurately. It can be used for the centrality 14 measurements.
Thank you for your attention! This work was supported by the RFBR 18-02-40065 mega grant 15
BACKUPS 16
Comparison LA-QGSM 11 GeV FULL MINUS BACKGROUND 17
LA-QGSM 11 GeV Energy in the central bin vs impact parameter FULL MINUS BACKGROUND radius After subtracting the pion contribution, the energies for the central Spectators scattering angle vs impact parameter events become less 18
Centrality reso solution for E dep vs vs E ma max (after subtraction of pion contribution) ba backup Edep [a.u] 0.45 DCM-SMM 0.4 0.35 LA-QGSM 0.3 σ b /b 0.25 0.2 0.15 0.1 LA-QGSM 0.05 0 0 20 40 60 80 100 E max [a.u] Centrality % Dependence of resolution of impact parameter on centrality Edep [a.u] 16 14 12 10 Mean 8 6 4 2 DCM-SMM 0 0 20 40 60 80 100 Centrality % 19 E max [a.u] Dependence of impact parameter on centrality
Comparison DCM-SMM 11 GeV бэкап FULL MINUS BACKGROUND Energy in the central bin vs impact parameter FULL MINUS BACKGROUND Spectators scattering angle vs impact parameter 20
5 GeV example for LA-QGSM and DCM-SMM models LA-QGSM DCM-SMM Each color bin is 10% fractions of the total number of events. E rec E rec 21
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