Golubeva, M.

Kugler, A.

Mikhaylov, V.

Seddiki, S.

Selyuzhenkov, I.

Tlusty, P.

GSI Repository

Collision centrality determination in the CBM experiment∗S. Seddiki1, M. Golubeva2, A. Kugler3, V. Mikhaylov3,4, I. Selyuzhenkov1, and P. Tlusty31GSI, Darmstadt, Germany; 2INR RAS, Troitsk, Russia; 3NPI of ASCR, ˇRezˇ, Czech Republic; 4CTU, Prague, CzechRepublicThe magnitude of the impact parameter b in a heavy-ioncollision is not known experimentally. Estimating b is im-portant for the study by CBM of many physics observables,e.g. the event-by-event fluctuations of conserved quanti-ties at mid-rapidity or collective flow. The multiplicity ofproduced particles in the overlap zone of the nuclei can beused as an experimental proxy of the b value. All eventsare sorted in centrality classes, with most central one be-ing the collisions with highest multiplicity (b ≈ 0) of theproduced particles and peripheral one with the lowest mul-tiplicity (when b value is about the sum of the radii of thetwo nuclei). Since the b value and particle multiplicity arecorrelated only on average, for a given multiplicity (cen-trality) class of events only an average b value and its spreadσb can be estimated.The projectile Spectator Detector (PSD) of the CBM ex-periment is designed to register forward spectator nucle-ons and fragments emitted in nucleus-nucleus collisions atvery small polar angles. The multiplicity of spectators canalso be used as an independent way to determine central-ity. In the case of spectator measurements, the most centralevents correspond to a low spectator multiplicity (small en-ergy deposition in the PSD), while peripheral events resultin a large amount of spectators (large energy deposition inthe PSD).The performance of the centrality determination wasstudied using the DCM-QGSM heavy-ion collision eventgenerator [1]. The PSD was either used as a standalone de-tector utilizing correlations between the energy depositedin different PSD subevents (segments), or in a combinationwith the CBM Silicon Tracking System (STS) which mea-sures the multiplicity of produced particles at midrapidity.In the case of the PSD standalone analysis, it was requiredto have at least 40 GeV of energy in the PSD1 subeventor a total energy in two PSD2 and PSD3 subevents above15 GeV to exclude very peripheral collisions with only fewheavy fragments.Figure 1 shows the performance of the centrality deter-mination for Au+Au collisions at Ebeam = 10 AGeV. ThePSD is positioned at 8 m from the target. Centrality classesare defined by using various detector subevent combina-tions. The top panel shows the average impact parame-ter value b (central value) and σb (as the error bars) versusthe centrality estimate from different subevent correlations.The bottom panel presents the same information in terms ofimpact parameter resolution σb/b of different centrality es-∗Work supported by the European Community FP7 - Capacities, con-tract HadronPhysics3 n◦283286.Figure 1: Average b and width σb of the impact parameterdistribution (top); impact parameter resolution (σb/b) vs.centrality (bottom).timators. The b and σb were determined from Gaussian fitsof the impact parameter distribution for a given centralityclass.The results in Fig. 1 demonstrate that the PSD can beused standalone for the centrality determination and, de-pending on the collision energy, has a comparable impactparameter resolution σb/b to that of the STS. This pro-vides an independent method in the CBM experiment forthe centrality determination with detected spectator frag-ments. When used together with the STS detector, the PSDhelps to improve the overall centrality determination in thecentrality range of 0-40% and allows for centrality deter-mination in narrow classes with a width of at least 5%.References[1] The SHIELD code, www.inr.ru/shield/index.html.MU-NQM-CBM-24 GSI SCIENTIFIC REPORT 201460 DOI:10.15120/GR-2015-1-MU-NQM-CBM-24

Collision centrality determination in the CBM experiment

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Collision centrality determination in the CBM experiment

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