Measurement of the J/psi inclusive production cross-section in pp collisions at sqrt(s)=7TeV with ALICE at the LHC
aa r X i v : . [ h e p - e x ] J un MEASUREMENT OF THE J/ ψ INCLUSIVE PRODUCTION CROSS-SECTIONIN PP COLLISIONS AT √ s =7 TeV WITH ALICE AT THE LHC J. WIECHULA for the ALICE COLLABORATION
Physikalisches Institut der Universit¨at T¨ubingen, Auf der Morgenstelle 14,72076 T¨ubingen, Germany
ALICE measures the
J/ψ production at mid-rapidity ( | y | < .
9) in the di-electron decaychannel as well as at forward rapidity (2 . < y < .
0) in the di-muon decay channel. In bothchannels the acceptance goes down to zero transverse momentum. We present the rapiditydependence of the inclusive
J/ψ production cross-section and transverse momentum spectra.
The measurement of the
J/ψ production cross-section in pp collisions is crucial for testingQCD models of quarkonium production in the new energy regime provided by the LHC. Severaltheoretical models 1 , J/ψ production. However none ofthem consistently describes the production cross-section, the transverse momentum and rapiditydependences and the polarisation. Measurements of these variables are therefore essential to helpunderstanding the underlying production mechanisms.The main focus of ALICE 3 is to study the deconfined hot and dense QCD phase which isexpected to be formed in heavy-ion collisions. The
J/ψ is an essential observable for this stateof matter and the cross-section measurement in pp collisions is important as a reference.ALICE measures the
J/ψ production in two rapidity windows: at mid-rapidity ( | y | < . . < y < J/ψ productioncross-section as well as its p T and rapidity dependence in pp collisions at √ s = 7 TeV 4. Thedata were not corrected for feed-down contributions from other charmonium states ( χ c , ψ ′ ) orB-hadron decays (inclusive production). J/ψ measurement with ALICE
ALICE is a general purpose heavy-ion experiment. It consists of a central part with a pseudorapidity coverage of | η | < . φ and a muon spectrometer placed atforward rapidity ( − < η < − . J/ψ production is measured in both rapidity regions, atmid-rapidity in the di-electron and at forward rapidity in the di-muon decay channelThe main detectors used for the analysis at mid-rapidity are the Inner Tracking System 3 , E/ d x ) of particles in the detector gas. Due to the excellent d E/ d x resolution of ∼ . / p / c using TPC information only.The muon spectrometer consists of a 3 T · m dipole magnet, 5 tracking stations made of twoplanes of Cathode Pad Chambers each, a set of muon filters and a trigger system. Particlesemerging from the collision in the forward direction have to traverse a front absorber with 10 λ I ,removing most hadrons and electrons. The remaining particles are reconstructed in the trackingsystem with an intrinsic position resolution of about 70 µ m in the bending direction. An ironwall (7.2 λ I ) is placed between the last tracking station and the muon trigger system. Thefront absorber combined with the muon filter stop muons with momenta less than 4 GeV/ c .The trigger consists of 2 stations with 2 planes of Resistive Plate Chambers achieving a timeresolution of ∼ µ -MB condition).For the analysis in the di-electron channel a total of 2 . · MB events were analysed,corresponding to an integrated luminosity of L i nt = 3 . − . Events are required to have areconstructed vertex with a z position within | z vtx | <
10 cm from the nominal interaction point.Several cuts are applied on the level of the single track. Tracks are required to be in the detectoracceptance ( | η | < .
9) and have a transverse momentum larger than 1 GeV/ c . In addition thetracks have to fulfil certain reconstruction quality criteria. They need to be well defined in theITS and TPC, the χ per cluster needs to be less than 4, at least 70 out of 159 clusters need tobe attached in the TPC and a hit in at least one of the first two layers of the ITS is required.The latter helps to reject electrons from photon conversions. Finally electrons are identified bycutting on the d E/ d x signal of the TPC in terms of number of sigma, where σ is the width of thegaussian response of the TPC (d E/ d x -resolution). The requirements are to be within ± σ fromthe electron expectation and more than 3 σ (3 . σ ) away from the proton (pion) expectation.The obtained invariant mass spectrum of the opposite-sign (OS) di-electron pairs is shown inFig. 1 (left, top panel, red points). The remaining background is described by combining like-sign(LS) di-electron pairs as N ++ + N −− and scaling the LS spectrum to match the integral of theOS spectrum in the invariant mass range of 3 . − c (Fig. 1, left, top panel, blue points).The arithmetic mean was preferred over the geometric mean in order to remove the bias thatwould occur in the scaling due to bins with zero entries in the LS spectrum. After backgroundsubtraction the number of J/ψ candidates is extracted by bin counting in the invariant massrange 2 . − .
16 GeV/ c . This yields N J/ψ = 249 ±
27 (stat.) ±
20 (syst.). The systematicuncertainties are described below.The analysis in the di-muon channel was performed on a data sample of 1 . · µ -MBevents ( L int = 15 . − ). For further analysis only events were selected for which at leastone muon candidate has a match in the muon trigger chambers. This significantly reduces thebackground of hadrons which are produced in the absorber. Further selection criteria are areconstructed vertex in the ITS, the rejection of muons emitted under very small angles, whichcross a significant fraction of the beam pipe and requiring the di-muon pair to be in the detectoracceptance (2 . < y <
4) in order to minimise edge effects.Applying all selection criteria 1 . · OS muon pairs are found. Figure 1 (right) showsthe invariant mass spectrum of OS di-muon pairs of a subset of data. The signal is extracted bysimultaneously fitting a Crystal Ball function to describe the signal shape and two exponentialsfor the background. By integrating the Crystal Ball function over the full mass range, the2 .5 2 2.5 3 3.5 4 4.5 5 C oun t s pe r M e V / c OSLS*1.25 =7 TeVsALICE pp ) (GeV/c ee m -20020406080100 OS-1.25*LS/dof=1.1) χ MC ( ) (GeV/c µµ m C oun t s pe r M e V / c OS Fit =7 TeVsALICE pp
Figure 1: Invariant mass spectra for the di-electron (left) and di-muon (right) decay channel 4. extracted
J/ψ yield of the complete analysed statistics is N
J/ψ = 1924 ± ± p T and rapidity distributions used as input for the MC studies,muon trigger efficiency, reconstruction efficiency, error on the luminosity measurement and theuncertainty of the branching ratio. The total uncertainty is 12.6 % in the di-muon and 16.1 % inthe di-electron channel. However, the largest uncertainty results from the unknown polarisationof the J/ψ due to its influence on the acceptance corrections. To quantify the effect, the impactof fully transverse ( λ = 1) and fully longitudinal ( λ = −
1) polarisation for the case of the helicity(HE) as well as the Collins-Soper (CS) reference frame was investigated. Maximum variationsbetween -15 % and +32 % were estimated and will be quoted separately. For details see 4.
The production cross-section is determined by normalising the efficiency and acceptance cor-rected signal ( N corrJ/ψ = N J/ψ / ( A × ǫ )) to the integrated luminosity or the cross-section of areference process. For this analysis the minimum-bias cross-section itself was chosen as a refer-ence. Thus the J/ψ cross-section is given by σ J/ψ = N corrJ/ψ BR ( J/ψ → l + l − ) × σ MB N MB , where BR ( J/ψ → l + l − ) = (5 . ± . J/ψ to di-leptons, N MB isthe number of analysed minimum bias events and σ MB = 62 . ± . ± . A × ǫ ) is 10.0 % for the di-electron, 32.9 % for the di-muon decay channel.The measured integrated cross-sections for the two rapidity ranges are: σ J/ψ ( | y | < .
9) = 10 . ± . ± . λ HE = 1) - 2.3 ( λ HE = -1) µ b and σ J/ψ (2 . < y <
4) = 6 . ± .
25 (stat.) ± .
80 (syst.) +0 . λ CS = 1) − . λ CS = − µ b.In addition for both rapidity ranges the differential cross-sections d σ J/ψ / d p T d y and d σ J/ψ / d y ( p T >
0) were determined. The spectra are presented in Fig. 2 and compared with results fromATLAS 8 ( p T > c , | y | < . p T > . c , | y | < .
2) and LHCb 10 ( p T > . < y < p T = 0 an thus is complementary to those of ATLAS and CMS.3 (GeV/c) T p b / G e V / c ) µ d y ( T / dp ψ J / σ d -2 -1 , |y|<0.9 - e + ALICE e , 2.5 Figure 2: Differential J/ψ production cross-sections as a function of p T (left) and rapidity (right) 4. We have presented first results on the J/ψ inclusive production cross-section measured with theALICE detector system. In addition the p T -differential cross-section and the rapidity dependencewere shown and compared to the other LHC experiments. ALICE is the only experiment atLHC measuring J/ψ at mid-rapidity down to zero transverse momentum.With increased statistics and a different PID strategy, the large statistical as well as system-atic uncertainty of the measurement at mid-rapidity will be reduced significantly. To providea better electron identification other detectors will be used in addition to the TPC. Under in-vestigation are the Time Of Flight detector, to extend the electron identification towards lowermomenta ( p / c ) as well as the Transition Radiation Detector to allow a better electronto pion separation at higher momenta ( p ' c ).Ongoing analyses in both decay channels are the measurement of the feed-down from B-hadron decays, the analysis of pp data at √ s = 2 . 76 TeV and the multiplicity dependence of the J/ψ production in pp collisions. In addition the measurement of the polarisation will help toput limits on the largest contribution to the systematic uncertainty.In parallel to the analysis of pp collision, the data from the Pb–Pb run at √ s NN = 2 . 76 TeVof November/December 2010 are being analysed. First results were presented at the QuarkMatter 2011 conference 11. References 1. N. Brambilla et al.(Quarkonium Working Group), Eur. Phys. J. C71 , 1534 (2011)2. J.P. Lansberg, Eur. Phys. J. C61 , 693 (2009)3. K. Aamodt et al. (ALICE Collaboration), JINST , S08002 (2008)4. K. Aamodt et al. (ALICE Collaboration), arXiv:1105.0380, submitted to Phys. Lett. B.5. K. Aamodt et al. (ALICE Collaboration), JINST , P03003 (2010).6. J. Alme et al. (ALICE Collaboration), Nucl. Instrum. Methods A622 , 316 (2010)7. K. Oyama et al. (ALICE Collaboration), proc. of the Workshop “LHC Lumi Days”, tobe published.8. G. Aad et al. (ATLAS Collaboration), arXiv:1104.3038, submitted to Nucl. Phys. B.9. V. Khachatryan et al. (CMS Collaboration), Eur. Phys. J. C71 , 1575 (2011)10. R. Aaij et al.(LHCb Collaboration), Eur. Phys. J.