Measurements of the dependence of the J/Psi production cross section on its
kinematic variables as well as on the target atomic numbers for 920 GeV/c
protons incident on different targets have been made with the Hera-B detector.
The large collected di-lepton sample allows to study the production ratio of
Psi(2S) to J/Psi and of Chic to J/Psi . We also report on measurements of the
b-bbar and Upsilon production cross section.Comment: 9 pages, 11 figure

(for the HERA-B Collaboration)

Alexopoulos T

Baier R

Beneke M

Bodwin G T

Cho P

Granier de Cassagnac R

Joachim Spengler

Moreno G

Schub M H

English

arXiv

Measurements of the dependence of the J/ψ production cross section on its kinematic variables as well as on the target atomic numbers for 920 GeV/c protons incident on different targets have been made with the HERA-B detector. The large collected di-lepton sample allows us to study the production ratio of ψ(2S) to J/ψ and of χc to J/ψ. We also report on measurements of the and Υ production cross section

Spengler, J.

MPG.PuRe

Quarkonia Production with the HERA-B Experiment

Crossref

Quarkonia production with the HERA-B experiment

Measurements of the dependence of the J/ψ production cross section on its kinematic variables as well as on the target atomic numbers for 920 GeV/c protons incident on different targets have been made with the HERA-B detector. The large collected di-lepton sample allows us to study
the production ratio of ψ(2S) to J/ψ and of χc to J/ψ. We also report on measurements of the  and Υ production cross section

Spengler, Joachim

Max Planck Society eDoc Server

arXiv:hep-ex/0403043 v1   29 Mar 2004Quarkonia production with the HERA BexperimentJoachim Spengler for the HERA B collaboration‡Max-Planck-Institut fu¨r Kernphysik, Postfach 103989, 69029 Heidelberg, GermanyE-mail: joachim.spengler@desy.deAbstract. Measurements of the dependence of the J/ψ production cross section onits kinematic variables as well as on the target atomic numbers for 920 GeV/c protonsincident on different targets have been made with the HERA B detector. The largecollected di-lepton sample allows to study the production ratio of ψ(2S) to J/ψ and ofχc to J/ψ. We also report on measurements of the bb and Υ production cross section.PACS numbers: 25.40.Ep, 13.20.Gd, 13.85.Qk, 12.39.-xSubmitted to: J. Phys. G: Nucl. Phys.1. IntroductionHERA B is a fixed-target experiment which studies collisions of protons with the nucleiof atoms in target wires positioned in the halo of HERA’s 920 GeV proton beam. Thelarge acceptance of the HERA B spectrometer coupled with high-granularity particle-identification devices and a precision vertex detector allow for detailed studies of complexmulti-particle final states. HERA B’s sophisticated di-lepton trigger is capable offinding rare interactions containing two leptons coming, for example, from the decay of aJ/ψ amidst an overwhelming background of more ordinary proton-nucleus interactions.By using target wires made from a variety of materials (carbon, aluminum, titaniumand tungsten) HERA B is able to study the dependence of various properties of proton-nucleus interactions as a function of atomic number.Analysis of modest data samples which had been taken in the commissioning run ofyear 2000 was completed by early 2003 and led to publications on the bb cross section [1],the ratio of χc to J/ψ production cross sections [2] and on the production cross sectionsof Λ0 and K0s [3].During the HERA run which began in Summer 2002 and ended in March 2003,HERA B accumulated 150 million events with its di-lepton trigger. The di-leptonsample includes 300,000 events with a J/ψ in the final state, roughly equally divided‡ Talk given at the Quark Matter 2004Quarkonia production with the HERA B experiment 20100002000030000400005000060000700000 1 2 3 4 5e+e- Invariant mass GeV/c2Entries/(0.033 GeV/c2)J/ y05000100001500020000250003000035000400000 1 2 3 4 5Figure 1. The µ+µ− (left) and e+e− (right) invariant mass spectrum.between di-muon and di-electron decays. This sample is more than 50 times larger thanthe sample accumulated in the 2000 run, but thanks to the large number of improvementsin detector and trigger systems could be recorded in a similar amount of beam time.Thanks to improvements in the data acquisition system, recording rates of a 1000 eventsper second (more than 1 Terabyte per day) for minimum bias data were achieved whichallowed collection of a 200 million events minimum bias sample in just one week ofcalendar time. This represents an increase in sample size by more than a factor of100 with respect to the 2000 run. Having successfully accumulated these interestingdata samples, the HERA B Collaboration decided in early 2003 to definitively finishdata-taking activities at the end of running period and to concentrate on analyzingthe accumulated data. Several studies are in progress on both the di-lepton and theminimum bias samples.2. Preliminary results2.1. The Di-lepton sampleThe µ+µ− invariant mass plot from 80% of the full sample is shown in Fig. 1. Aprominent peak containing 150,000 di-muon pairs at the mass of the J/ψ is seen. Otherfeatures of the plot include peaks at the masses of the ψ(2S) , the φ(1020), the ω(782) andthe ρ(770). The same features, although at lower resolution due to the bremsstrahlungemitted by the electrons, can be seen in the e+e− mass plot shown in Fig. 1. The J/ψ→ e+e− peak contains 100,000 events.2.2. Differential distributions of J/ψThe large data sample in connection with the large acceptance of the HERA Bspectrometer allows for a precise determination of the kinematical distributions of J/ψ.The pT-distribution is plotted Fig. 2 for 80% of the e+e−sample. Note the wide rangeof up to 4.8 GeV/c in transverse momentum.Quarkonia production with the HERA B experiment 3pT         GeV/cdN/dp T210 210 310 410 50 1 2 3 4 5Figure 2. The pT distribu-tion of J/ψ → e+e− .xFdN/dx F10 310 4-0.4 -0.3 -0.2 -0.1 0 0.1 0.2Figure 3. The xF distribu-tion of J/ψ → e+e− .The average pT is determined by fitting the following expression to the datadσdpT2= β ·1 +(35 · π · pT256 · < pT >)2−6.The outcome of this analysis can be found in Table 1. The HERA B data confirm thewell known trend of pT-broadening with increasing atomic number as it was reportedduring this conference for example by Phenix [4].Table 1. Average transverse momentum of J/ψ. For the HERA B results onlystatistical errors are given.Target Exp. Range in pT < pT >(e±) < pT >(µ±) errorC 920 Hera-B ≤ 4.8 1.22 1.22 0.01W 920 Hera-B ≤ 4.8 1.29 1.29 0.01Si 800 E771 [5] ≤ 3.5 1.20 0.01Au 800 E789 [6] ≤ 2.6 1.29 0.009The range in xF of J/ψ which is accessible with the HERA B detector can beseen from Fig. 3. It is for the first time that J/ψ-production can be measured in thebackward region. However, due to still ongoing acceptance calculations, only a smallfraction of about 10% of the e+e− sample is shown in this plot. In order to comparethis distribution with other results, the expressiondσdxF= β · (1− |xF|)cwas fitted to our data. For the time being, only a range for the slope c can be quoted(see Table 2).Quarkonia production with the HERA B experiment 4Table 2. Exponent c of xF distribution of J/ψ. For the HERA B results onlystatistical errors are given.Target Exp. Range in xF c(e±) c(µ±) errorC, W 920 Hera-B -0.35 ≤ xF≤ 0.15 5 - 6.5 0.3Si 800 E771 [5] -0.05 ≤ xF≤ 0.25 6.54 0.23Au 800 E789 [6] -0.03 ≤ xF≤ 0.13 4.91 0.18Cu 800 E789 [7] 0.30 ≤ xF≤ 0.95 5.21 0.04Due to the simultaneously recorded data sets with carbon and tungsten target,HERA B is well suited to study the A-dependence of J/ψ-production, which is usuallyparametrized asσpA = σpN · Aα; α = 1log(AW/AC)· log(NWNCLCLWǫCǫW).Thus the measurement of α requires the ratio of the number of observed J/ψ ’s, theratio of luminosities (LC,W ) and the ratio of the reconstruction efficiencies (ǫC,W ) forthe two different target materials. Up to now, the absolute ratio of luminosities is stillunder evaluation. Therefore our data points, determined from 25% of the e+e− sample(see Fig. 4), were normalized to the E866 result [8] in the overlap region of the xFdistribution.xFaHERA-B preliminaryE8660.60.70.80.911.11.21.31.4-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4Figure 4. α versus xF for J/ψ. The HERA B points were normalized to the E866data in the region of overlap.2.3. Production ratio of ψ(2S) to J/ψA precise measurement of the ψ(2S) cross section allows to test the importance of coloroctet production channels [9]. Analyzing 30% of the e+e− data sample (Carbon target),a clear ψ(2S) peak can be seen in Fig. 5. In order to minimize systematic uncertainties,the following ratioRψ(2S) =σ(ψ(2S) )σ(J/ψ )=n(ψ(2S) )n(J/ψ )· Br(J/ψ → e+e− )Br(ψ(2S) → e+e− ) ·ǫ(J/ψ )ǫ(ψ(2S) )Quarkonia production with the HERA B experiment 5e+e- Invariant mass GeV/c2                   .Entries/(0.033 GeV/c2)J/y10 310 42.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2Figure 5. The e+e− invari-ant mass spectrum.Ebeam (GeV)Figure 6. The ψ(2S) crosssection as function of theproton beam energy.is determined. We obtain a result of Rψ(2S) = 0.13± 0.02 which is in good agreementwith the result of the Muon channel. For the time being, no systematic error can bequoted because studies are ongoing. Based on results of E771 [10] and E789 [11], weuse Rψ(2S) = (357± 8 ± 27) nb/nucleon [1] to determine the ψ(2S) production crosssection σ(ψ(2S)) = (46 ± 12) nb/nucleon. This preliminary value is shown in Fig. 6together with results of other experiments.2.4. Production ratio of χc to J/ψA measurement of the production ratio of χc to J/ψ will help to choose betweenmodels [12, 13, 14, 15] attempting to describe the production of quarkonium states.Using the year-2000 data set, HERA B measured the fraction of J/ψ coming from χcdecaysRχc =∑2i=1 σ(χci ) · Br(χci → J/ψ γ)σ(J/ψ )=n(χc )n(J/ψ )· ǫ(J/ψ )ǫ(χc) · ǫ(γ)based on 380±74 reconstructed χc mesons to be Rχc = 0.32±0.06±0.04 [2] which is ingood agreement with the NRQCD calculations [14]. The state χc0 was neglected due toits small branching ratio. Fig. 7 shows the mass difference ∆m = m(l+l−γ)−m(l+l−)for 15% of the µ+µ− data sample recorded during the 2002/03 period. For the fullsample over 10,000 reconstructed χc mesons are expected in both final states which willlead to a much more precise measurement.2.5. bb Cross SectionA precise measurement of the b cross section in hadronic interactions at fixed targetenergies would be welcome in part as a test of calculations, in part to give guidance forsetting the parameters of the theory [16, 17].Quarkonia production with the HERA B experiment 6  55.15    /    45P1   1281.   190.3P2  0.4190  0.5706E-02P3  0.3592E-01  0.5224E-02P4  0.2001E-01  0.6846E-040100020003000400050000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1  55.15    /    45P1   1281.   190.3P2  0.4190  0.5706E-02P3  0.3592E-01  0.5224E-02P4  0.2001E-01  0.6846E-04-200-10001002003004005006000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Figure 7. Spectrum of the mass difference ∆m = m(l+l−γ)−m(l+l−) before (left)and after subtraction (right) of the combinatorial background.The principle of the HERA B B cross section determination is to measure the rateof J/ψ’s which originate typically 1 cm downstream of the target and compare it to therate of directly produced J/ψ’s. Using this technique and the data collected during theyear-2000 running period, a total bb production cross section of σ(bb) = (32± 13± 6)nb/nucleon was derived. The much larger new sample will allow a considerably moreprecise measurement. Fig. 8 shows the e+e− (left) and µ+µ− (right) invariant massspectra from 35% of the data sample after applying vertex cuts to suppress the directly-produced J/ψ’s. The downstream samples clearly show the J/ψ but not the upstreamsamples. For the full sample we expect approximately 100 J/ψ’s from b decay. Cutoptimization and systematic studies are underway. A preliminary estimate of the crosssection/nucleon, which is 1.5σ below the 2000 result, is shown in Fig. 9 together with theresults of E771 [10] and E789 [11]. The latest QCD calculations [16, 17] are indicatedby their central values as well as upper and lower bounds.2.6. Production cross section of ΥIn order to search for Υ-mesons, both C and W data samples were combined assumingan A-dependence of α = 1. As can be seen in Fig. 11, an Υ-signal is visible in bothµ+µ− and e+e− final states. The shape of the signals was derived from a Monte Carlosimulation with the relative production rates of the three Υ-states fixed on E605 [18]data. The background is described by a term for the combinatorial background and aDrell-Yan term dominating at high invariant masses.As for the previous analyses, the cross section is determined relative to the J/ψcross section:σΥ = σ(J/ψ ) ·n(Υ )n(J/ψ )· Br(J/ψ → ℓ+ℓ− )Br(Υ → ℓ+ℓ− ) ·ǫ(J/ψ )ǫ(Υ)The results of both subsamples are in good agreement. We obtain a combined productionQuarkonia production with the HERA B experiment 70246810121416182 2.25 2.5 2.75 3 3.25 3.5 3.75 4 4.25 e+e- invariant mass (GeV) entries11 ± 5  J/y  → e+e-Downstream (signal region)Upstream (background)0510152025302.5 3 3.5 4 4.5 5 m+m- invariant mass (GeV) entries 19 ± 5  J/y  → m +m -Downstream (signal region)Upstream (background)Figure 8. Invariant mass distribution of e+e−(left) and µ+µ−-pairs (right) afterapplication of vertex cuts. Points with error bars: lepton pairs downstream of thevertex. Shaded histogram: lepton pairs which, due to finite measurement resolution,appear to originate upstream of the main interaction point.11010 2400 500 600 700 800 900R. BoncianiN. Kidonakiset al.et al.E789E771HERA-B 2000Preliminary 2004 Proton energy (GeV)s(    ) (nb/nucleon)bbFigure 9. Comparison of theHERA B results of 2000 and2004 with other experimentsand theoretical predictions.10-210-111020 25 30 35 40 45 50 55 60 65B ds/dy, pb/nucleonHera-B, preliminarynot usedin fitFigure 10. Br · dσdy|y=0 for Υproduction as function of thecenter-of-mass energy. The fitto the data is described in thetext.cross section times branching ratio at mid-rapidity of Br · dσdy|y=0 = (3.4 ± 0.8) pb/N.In Fig. 10 the HERA B measurement is compared to the results of other experiments.The new point from HERA B will help to clarify the experimental situation which iscontroversial in the energy regime of√s ∼ 40 GeV. The energy dependence of the crosssection can be described with the parameterization [19] Br · dσdy|y=0 = σ0 · exp(−m0√s ).Quarkonia production with the HERA B experiment 810-111010 210 35 6 7 8 9 10 11 12 m+m- invariant mass (GeV) entries 33 –  7  U  fi  m + m -comb. BG  Drell-Yan10-111010 210 36 8 10 12 14 e+e- invariant mass (GeV) entries 31 –  10  U  fi  e+e -comb. BG Drell-YanFigure 11. The µ+µ− (left) and e+e− (right) invariant mass spectrum.3. SummaryThe final data taking period of the HERA B experiment was used to accumulate 150million events with its di-lepton trigger. These events were produced in pC and pWinteractions of the 920 GeV proton beam of Hera. The di-lepton sample includes 300,000events with a J/ψ decaying into electon- or muon-pairs. Studies of the dependence ofthe J/ψ production cross section on its kinematic variables as well as on the targetatomic number were shown which turn to advantage the large acceptance in pT andthe backward hemisphere. Preliminary results of the ψ(2S) to J/ψ as well as χc to J/ψproduction cross section ratios were shown. New results on measurements of the bb andΥ production cross section were also discussed.References[1] HERA B Collaboration 2003 Eur.Phys.J. C 26 345[2] HERA B Collaboration 2003 Phys.Lett. B 561 61[3] HERA B Collaboration 2003 Eur.Phys.J. C 29 181[4] de Cassagnac R G 2004 to be published in J. Phys. G Proceedings of Quark Matter 2004[5] Alexopoulos T et al 1997 Phys.Rev. D 55 3927[6] Schub M H et al 1995 Phys.Rev. D 52 1307[7] Kowitt M S et al 1994 Phys.Rev.Lett. 72 1318[8] Leich M J et al 2000 Phys.Rev.Lett. 84 3256[9] Beneke M and Rothstein I Z 1996 Phys.Rev. D 54 2005[10] Alexopoulos T et al 1999 Phys.Rev.Lett. 82 41[11] Jansen D M et al 1995 Phys.Rev.Lett. 74 3118[12] Baier R and Ru¨ckl R 1981 Phys.Lett. B 102 364; 1983 Z. Phys. C 19 251[13] Bodwin G T, Braaten E and Lepage G P 1995 Phys.Rev. D 51 1125[14] Cho P and Leibovich A 1996 Phys.Rev. D 53 6203[15] Vogt R 2002 Nucl.Phys. A 700 539[16] Bonciani R et al 1998 Nucl.Phys. B 529 424[17] Kidonakis N et al 2001 Phys.Rev. D 64 114001Quarkonia production with the HERA B experiment 9[18] Moreno G et al 1991 Phys.Rev. D 43 2815[19] Craigie N S 1978 Phys.Rep. 47 1

Quarkonia production with the Hera-B experiment

Quarkonia production with the Hera-B experiment

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