Anielski, J.

Bathen, B.

Dietel, T.

Dyba, S.

Feldkamp, L.

Heide, M.

Klein-Bösing, C.

Kohn, M.

Passfeld, A.

Sicking, E.

Tegeder, M.

Wessels, J. P.

Westerhoff, U.

Wilde, M.

Zimmermann, M.

GSI Repository

HBOM method for Event-Background Fluctuations in Pb-Pb Collisions at theLHC∗M. Zimmermann1, J. Anielski1, B. Bathen1, T. Dietel1, S. Dyba1, L. Feldkamp1, M. Heide1,C. Klein-Bo¨sing1,2, M. Kohn1, A. Passfeld1, E. Sicking1, M. Tegeder1, M. Wilde1, J.P. Wessels1, andU. Westerhoff1 for the ALICE Collaboration11Institut fu¨r Kernphysik, University of Mu¨nster, Germany; 2ExtreMe Matter Institute EMMI, GSI Darmstadt, GermanyWith the measurement of jets and their modification inheavy-ion collisions the quark-gluon plasma can be inves-tigated tomographically. However, the determination ofthe jet momentum in Pb–Pb-collisions is complicated bythe background of the underlying event and its fluctua-tions. The fluctuations of the jet momentum scale can bedescribed by the distribution of residuals (δpT ) after back-ground subtraction of the momentum in a cone placed ran-domly in ϕ and η in real Pb–Pb events. The standard devi-ation σ(δpT) of the resulting distribution is used to classifythese fluctuations [1].The measured fluctuations depend on the correlationswithin the heavy-ion event as well as on the detected par-ticle number. The correlations in the event are a-priori un-known, which is the motivation for a data-driven correctionof detector effects on the width of the fluctuations. Thisis done here with the so called Hit Backspace Once More(HBOM) method [2] for the 10% most central heavy ionevents measured in Pb-Pb collisions at 2.76 TeV with AL-ICE. It is a model independent method to back-extrapolatethe data to no detector effect (truth) [3].The method works as follows. First the σ(δpT) is de-termined for the measured events as usual. Then the ef-ficiency is applied again to all particles in the event by aMonte Carlo method that only accepts a particle, if a ran-dom number is lower than the pT and ϕ dependent detec-tion efficiency. Doing this once for all particles is calledone detector hit. After the detector hit the σ(δpT) is deter-mined again. This is done up to five times. The fit to 0–4detector hits is back-extrapolated to -1 detector hits whichcorresponds to the efficiency-corrected value of the stan-dard deviation [3].The method has been validated in simulations that em-ulate the generic features of heavy-ion collisions suchas the multiplicity, the pT spectrum, and flow (v2, v3).In Fig. 1 the back-extrapolation is shown. Since thetruth value is known in the simulation the number of fitpoints and its functional form can be optimized. Withthe best configuration, a corrected σ(δpT) = (9.43 ±0.02(stat) ± 0.05(syst))GeV/c is achieved and the truthvalue σ(δpT) = (9.458± 0.005)GeV/c is reproduced [3].After the validation, the method has been applied to mea-surements of 2010 and 2011 Pb–Pb collisions as shown inFig. 2 and a value of σ(δpT) = (11.85 ± 0.14)GeV/c is∗Work supported by BMBF and Helmholtz Alliance HA216/EMMIFigure 1: HBOM back-extrapolation for σ(δpT) in simu-lated Pb–Pb events. The point at zero detector hits corre-sponds to the measured spectrum [3].Figure 2: HBOM back-extrapolation for σ(δpT) in Pb–Pbcollisions from 2010. The red point at minus one shows thecorrected value [3].obtained consistently in both Pb–Pb data sets. A compari-son with the simulated value is not meaningful because thesimulation does not consider all correlations [3].References[1] Abelev, Betty and others. JHEP, 1203:053, 2012.[2] Monk, James W. and Oropeza-Barrera, Cristina.Nucl.Instrum.Meth, A701:17-24, 2011.[3] M. Zimmermann. Master Thesis, Westfa¨lische Wilhelms-Universita¨t Mu¨nster, 2012.GSI SCIENTIFIC REPORT 2012 PHN-NQM-EXP-70101

HBOM method for Event-Background Fluctuations in Pb-Pb Collisions at the LHC

http://repository.gsi.de/record/52004/files/PHN-NQM-EXP-70.pdf

HBOM method for Event-Background Fluctuations in Pb-Pb Collisions at the LHC

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