Measurement of <em>σ_χc2B(χ_c2 -&gt; J/ψγ)/σχ_c1B(χc1 -&gt; J/ψγ)</em> in <em>pp</em> Collisions at s√=1.96  TeV

Article

Reference

Measurement of σ

B(χ

-> J/ψγ)/σχ

B(χc1 -> J/ψγ) in pp Collisions at s√=1.96  TeV

CDF Collaboration

CAMPANELLI, Mario (Collab.), et al.

Abstract

We measure the ratio of cross section times branching fraction, Rp=σχc2B(χc2→J/ψγ)/σχc1B(χc1→J/ψγ), in 1.1  fb−1 of pp collisions at s√=1.96  TeV. This measurement covers the kinematic range pT(J/ψ)>4.0  GeV/c, |η(J/ψ)1.0  GeV/c. For events due to prompt processes, we find Rp=0.395±0.016(stat)±0.015(syst). This result represents a significant improvement in precision over previous measurements of prompt χc1,2 hadro production.

CDF Collaboration, CAMPANELLI, Mario (Collab.), et al . Measurement of σ

B(χ

->

J/ψγ)/σχ

B(χc1 -> J/ψγ) in pp Collisions at s√=1.96  TeV. Physical Review Letters , 2007, vol. 98, no. 23, p. 232001

DOI : 10.1103/PhysRevLett.98.232001

Available at:

http://archive-ouverte.unige.ch/unige:38405

Disclaimer: layout of this document may differ from the published version.

1 / 1

Measurement of

B

! J= =

B

! J= in p p Collisions at p s

1:96 TeV

A. Abulencia,²⁴J. Adelman,¹³T. Affolder,¹⁰T. Akimoto,⁵⁶M. G. Albrow,¹⁷D. Ambrose,¹⁷S. Amerio,⁴⁴D. Amidei,³⁵ A. Anastassov,⁵³K. Anikeev,¹⁷A. Annovi,¹⁹J. Antos,¹⁴M. Aoki,⁵⁶G. Apollinari,¹⁷J.-F. Arguin,³⁴T. Arisawa,⁵⁸ A. Artikov,¹⁵W. Ashmanskas,¹⁷A. Attal,⁸F. Azfar,⁴³P. Azzi-Bacchetta,⁴⁴P. Azzurri,⁴⁷N. Bacchetta,⁴⁴W. Badgett,¹⁷ A. Barbaro-Galtieri,²⁹V. E. Barnes,⁴⁹B. A. Barnett,²⁵S. Baroiant,⁷V. Bartsch,³¹G. Bauer,³³F. Bedeschi,⁴⁷S. Behari,²⁵

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J. C. Yun,¹⁷L. Zanello,⁵²A. Zanetti,⁵⁵I. Zaw,²²X. Zhang,²⁴J. Zhou,⁵³and S. Zucchelli⁵ (CDF Collaboration)

1Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, Republic of China

2Argonne National Laboratory, Argonne, Illinois 60439, USA

3Institut de Fisica d‘Altes Energies, Universitat Autonoma de Barcelona, E-08193, Bellaterra (Barcelona), Spain

4Baylor University, Waco, Texas 76798, USA

5Istituto Nazionale di Fisica Nucleare, University of Bologna, 1-4-0127 Bologna, Italy

6Brandeis University, Waltham, Massachusetts 02254, USA

7University of California, Davis, Davis, California 95616, USA

8University of California, Los Angeles, Los Angeles, California 90024, USA

9University of California, San Diego, La Jolla, California 92093, USA

10University of California, Santa Barbara, Santa Barbara, California 93106, USA

11Instituto de Fisica de Cantabria, CSIC-University of Cantabria, 39005 Santander, Spain

12Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA

13Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

14Comenius University, 842 48 Bratislava, Slovakia; Institute of Experimental Physics, 040 01 Kosice, Slovakia

15Joint Institute for Nuclear Research, RU-141980 Dubna, Russia

16Duke University, Durham, North Carolina 27708

17Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

18University of Florida, Gainesville, Florida 32611, USA

19Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, I-OOO44 Frascati, Italy

20University of Geneva, CH-1211 Geneva 4, Switzerland

21Glasgow University, Glasgow G12 8QQ, United Kingdom

22Harvard University, Cambridge, Massachusetts 02138, USA

23Division of High Energy Physics, Department of Physics, University of Helsinki and Helsinki Institute of Physics, FIN-00014, Helsinki, Finland

232001-2

24University of Illinois, Urbana, Illinois 61801, USA

25The Johns Hopkins University, Baltimore, Maryland 21218, USA

26Institut fu¨r Experimentelle Kernphysik, Universita¨t Karlsruhe, 76128 Karlsruhe, Germany

27High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305, Japan

28Center for High Energy Physics: Kyungpook National University, Taegu 702-701, Korea;

Seoul National University, Seoul 151-742, Korea;

and SungKyunKwan University, Suwon 440-746, Korea

29Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

30University of Liverpool, Liverpool L69 7ZE, United Kingdom

31University College London, London WC1E 6BT, United Kingdom

32Centra de Investigaciones Energeticas Medioambientales y Tecnologicas, E-28040 Madrid, Spain

33Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

34Institute of Particle Physics: McGill University, Montreal, Canada H3A 2T8;

and University of Toronto, Toronto, Canada M5S 1A7

35University of Michigan, Ann Arbor, Michigan 48109, USA

36Michigan State University, East Lansing, Michigan 48824, USA

37Institution for Theoretical and Experimental Physics, ITEP, Moscow 117259, Russia

38University of New Mexico, Albuquerque, New Mexico 87131, USA

39Northwestern University, Evanston, Illinois 60208, USA

40The Ohio State University, Columbus, Ohio 43210, USA

41Okayama University, Okayama 700-8530, Japan

42Osaka City University, Osaka 588, Japan

43University of Oxford, Oxford 0X1 3RH, United Kingdom

44University of Padova, Istituto Nazionale di Fisica Nucleare, Sezione di Padova-Trento, 1-35131 Padova, Italy

45LPNHE, Universite Pierre et Marie Curie/IN2P3-CNRS, UMR7585, Paris, F-75252 France

46University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

47Istituto Nazionale di Fisica Nucleare Pisa, Universities of Pisa, Siena and Scuola Normale Superiore, 1-56127 Pisa, Italy

48University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

49Purdue University, West Lafayette, Indiana 47907, USA

50University of Rochester, Rochester, New York 14627, USA

51The Rockefeller University, New York, New York 10021, USA

52Istituto Nazionale di Fisica Nucleare, Sezione di Roma 1, University of Rome ‘‘La Sapienza’’, 1-00185 Roma, Italy

53Rutgers University, Piscataway, New Jersey 08855, USA

54Texas A&M University, College Station, Texas 77843, USA

55Istituto Nazionale di Fisica Nucleare, University of Trieste/Udine, Italy

56University of Tsukuba, Tsukuba, Ibaraki 305, Japan

57Tufts University, Medford, Massachusetts 02155, USA

58Waseda University, Tokyo 169, Japan

59Wayne State University, Detroit, Michigan 48201, USA

60University of Wisconsin, Madison, Wisconsin 53706, USA

61Yale University, New Haven, Connecticut 06520, USA (Received 15 March 2007; published 6 June 2007)

We measure the ratio of cross section times branching fraction, R_pc2B_c2! J= =_c1B_c1!J= , in1:1 fb¹ ofppcollisions at

ps

1:96 TeV. This measurement covers the kinematic rangep_TJ= >4:0 GeV=c,jJ= <1:0, andp_T>1:0 GeV=c. For events due to prompt processes, we findRp0:3950:016stat 0:015syst. This result represents a significant improvement in precision over previous measurements of prompt_c1;2hadro production.

DOI:10.1103/PhysRevLett.98.232001 PACS numbers: 13.60.Le, 13.85.Qk

Since it was first observed, charmonium (cc) production in hadronic collisions has been a subject of considerable theoretical interest. Recent approaches to understanding charmonium production make use of nonrelativistic QCD [1,2] to calculate hadro-production rates at the Tevatron and elsewhere. While most experimental observations of charmonium production consist of J= measurements, a significant contribution ofJ= production is indirect, re- sulting from the decay of higher mass states [3]. In par-

ticular, the radiative decay of the_cJstates [4] accounts for a significant fraction (30%–40%) of theJ= production seen in hadronic collisions, and any calculation of J=

production must include_cJ production as well.

Measurements of hadronic _cJ production have been made in a variety of beam types and energies [5] by observing the decay process _cJ!J= . Experimental results available until now have suffered from large statis- tical uncertainties, and no measurement has had the preci-

sion to test the consistency of the cross-section ratio _c2=_c1 with the simple spin-state counting expectation of ⁵₃ for _cJ mesons that are directly produced in the interaction [6]. Knowledge of this ratio is needed in cal- culations of J= production through radiative_cJ decay, and can be an important standard for comparing production models.

In this Letter, we report a measurement of the relative cross section times branching fractions of the_c1 and_c2 mesons produced in pp collisions at a center of mass energy of 1.96 TeV using the CDF II detector at the Fermilab Tevatron. We study the inclusive processpp ! _cJX, where _cJ!J= , and J= !, in a data sample with a time-integrated luminosity of1:1 fb¹. The final state photon is reconstructed through its conversion into ee, which provides the mass resolution needed to distinguish the_c1and_c2states. The spatial resolution of the vertex allows separation of prompt _cJ pro- duction from events where the_cJ meson is a B-hadron decay product. We measure the ratio of the cross section times branching fraction R_pc2B_c2 !J= = _c1B_c1 !J= for promptly produced _cJ mesons.

In addition, we obtain a result for the analogous quan- tity in B decay events, R_BBBB!_c2XBc2 ! J= =_BBB!_c1XBc1 !J= , which provides a measurement ofBB!_c2X=BB!_c1Xfor theB hadrons produced in the Tevatron environment.

This analysis makes use of the tracking, muon identi- fication, and trigger systems. The CDF II detector has been described in detail elsewhere [7,8]. The tracking system consists of a seven-layer silicon microstrip detector and an open-cell drift chamber (COT) that operate inside a sole- noid with a 1.4 T magnetic field. Muon candidates from the decay J= ! are identified by two sets of drift chambers located outside the electromagnetic and hadronic calorimeters. The central muon chambers cover the pseu- dorapidity region jj<0:6, and are sensitive to muons having transverse momentump_T >1:4 GeV=c[9]. A sec- ond muon system covers the region0:6<jj<1:0and is sensitive to muons havingp_T>2:0 GeV=c. Muon trigger- ing and identification are based on matching tracks mea- sured in the muon system to COT tracks.

The analysis of the data begins with a selection of well- measuredJ= !candidates. These are selected by requiring events that contain two oppositely charged muon candidates, each with a match between the COT and muon chamber tracks. We also require that both muon tracks have measurements in at least three layers of the silicon- detector and a two-track invariant mass within 80 MeV=c² of the world-average J= mass [10]. The J= candidates are required to fall within kinematic bounds of p_TJ= >4:0 GeV=c and jJ= j<1:0 which correspond to the approximate limits of our accep- tance. A simultaneous mass and vertex constrained fit is performed on the muon tracks, where the dimuon mass is constrained to the world-averageJ= mass.

The search for photon conversion candidates begins with a scan of all additional tracks withp_T>400 MeV=cfound in eachJ= event. Two oppositely charged tracks are each assigned the electron mass, and have their track parameters recalculated by subjecting them and their uncertainties to a fit that has constraints consistent with the photon conver- sion hypothesis. Specifically, the two tracks are con- strained to be parallel at their point of intersection, and the momentum vector of the pair is constrained to originate from the dimuon vertex. A displacement of 12.0 cm or more from the beam line in the direction of the track pair‘s transverse momentum is required to omit conversions whose momentum is poorly measured due to bremsstrah- lung in the inner detector material. We also require p_T>1:0 GeV=c. Finally, a constrained fit is performed on the four tracks that combines the J= mass constraint with the photon conversion hypothesis. The invariant mass distribution of all J= combinations is shown in Fig.1, which clearly demonstrates that theJ= mass resolution achieved by this technique is sufficient to resolve the _cJ states.

The lifetime ofBhadrons allows the transverse displace- ment of the dimuon vertex from the beam line to be used as a tool for their identification. Since anyJ= combination that originates from B decay represents only a partial reconstruction of the Bhadron, the proper lifetime is not directly measurable. We therefore use the quantity ct LMJ= F p_TJ= =p_TJ= , where MJ= and p_TJ= are the mass and transverse momentum, respec-

FIG. 1 (color online). The J= mass distribution (points) with the projection of the likelihood fit overlaid on the data.

The masses of the_cJmesons and the contributions of the signal and background components are indicated.

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tively, of the J= candidate, and L is the measured dis- placement of the dimuon vertex in the direction of p_TJ= . The correction factor,F p_TJ= , was obtained by a Monte Carlo simulation ofB hadron decay [3], and provides an average correction between the measured dis- placement and the lifetime of the decayingBhadron.

Mass resolution, acceptance, and reconstruction effi- ciencies forJ= final states of different invariant mass have been studied with a Monte Carlo simulation that gen- erates events uniformly in rapidity and with a transverse momentum distribution that matches the measured distri- bution forJ= events [7]. The simulated events were pro- cessed through our reconstruction and analysis algorithms, and provided templates for the expected signal shape of the final J= invariant mass distribution as a function of p_TJ= . In particular, the simulated events enabled an estimate of the mass resolution and radiative tail due to scattering and radiation by the conversion electrons as they passed through the material in the detector.

We used an unbinned likelihood fit to calculate the yield of _cJ events for both prompt and B-decay production processes. The probability density function used for the fit is a function of both J= invariant mass and ct.

Independent signal and background distributions are cal- culated for both processes. The mass distributions of the signals are constrained to the templates obtained through simulation. The mass distributions of the backgrounds are modeled by polynomials, and the probability density func- tion for each event uses the calculated uncertainty on the invariant mass andct. Thectdistribution is used to sepa- rate the production processes, and is modeled as a sum of prompt (Gaussian resolution) and B-decay (exponential, convoluted with resolution) contributions. Our fit to the data gives an event yield (N_cJ) ofN_c0 4120,N_c1 214360, andN_x2 103540for promptly produced events. ForBdecay events, the yields areN_c0 2916, N_c1 38435, andN_c26616. Projections of the un- binned likelihood fit are overlaid onto the mass and ct distributions shown in Figs.1and2. The relatively small yield of _c0 candidates is due to the small branching fraction into the J= final state [10], and is the reason they were not used in the subsequent analysis.

For our acceptance calculation, we have analyzed our simulated events assuming every photon converted, and created electron-positron pairs according to the Bethe- Heitler distribution [11]. We then counted the number of events that would have been accepted if all final state products were to pass our kinematic requirements. Our low transverse momentum limit of 400 MeV=c for the electrons results in a dependence of our acceptance on the invariant mass of the parent. The overall ratio of the _c1 and _c2 meson acceptances (_c2=_c1) is listed in TableIfor several ranges ofp_TJ= . The acceptance ratio is then combined with our yield ratios to provide measure- ments ofR_pandR_Bfor several ranges ofp_TJ= .

Several systematic effects that might change the recon- struction efficiency ratio _c2=_c1 were studied. First, the simulated event sample size used for the acceptance cal- culation provides an overall relative uncertainty of0:005 on the ratio. A comparison between the full event simula- tion and reconstruction and the simpler simulation based on the electron energy distribution yields a relative system- atic uncertainty of 0:020. Another effect considered is that polarization of one of the two _cJ states would also introduce a systematic shift. We have evaluated the effect of having one state decay with a distribution given by I /1cos², where is the polar angle of thein theJ= rest frame, and we take a0:13 0:15 as was done for a separate J= cross-section mea- surement [7]. A variation ofby this uncertainty for one of FIG. 2 (color online). Thectdistribution (points) for events in the_c1(a) and_c2(b) mass ranges. The projection of the fit is overlaid on the data, with the contribution of each signal and background component indicated.

TABLE I. The acceptance ratio and ratios of cross section times branching fractions of the_cJstates for the prompt events andBdecay events. Uncertainties listed are statistical only.

p_TJ=

(GeV=c)

_c2=_c1 R_p R_B

4 – 6 1:270:01 0:4570:039 0:1500:087 6 –8 1:170:01 0:3840:034 0:0800:094 8–10 1:140:01 0:4550:053 0:1160:070

>10 1:100:01 0:3090:045 0:1970:082

>4 1:230:01 0:3950:016 0:1430:042

the two_cJstates implies a relative shift of0:030on our reconstruction efficiency ratio.

We have also considered the sources of systematic un- certainty in the yield ratio calculation. We varied the invariant mass signal shape obtained from the simulation within the uncertainty of its parametrization, and found that the relative shift in the yield ratio is 0:005. The uncertainty on parameters used in thectdefinition corre- sponds to a variation of theBfraction of theJ= events of0:007of its value, giving a systematic uncertainty of 0:0020:010in the yield ratio for the prompt (Bdecay) sample. Finally, we explored the possibility that our data contain partially reconstructed h_c!J= ⁰, ⁰! events. This has been studied by simulating this process, parametrizing the resultingJ= invariant mass distribu- tion, and including this possible background in our like- lihood fits for the signal yield. The possibleh_cbackground contribution was found to be negligible in this data sample, so no systematic uncertainty was assigned for this process.

Differences in the two states due to production angular or p_T distributions would require different production mechanisms for the_c1and_c2mesons, and are, therefore, considered to be unlikely. Consequently, we did not assign a systematic uncertainty on the _cJ acceptance due to production dynamics. A summary of the systematic un- certainties on the cross section times branching-fraction ratio is listed in TableII. The individual uncertainties are combined in quadrature to give the total systematic uncertainty.

Our final result on the relative rate of production for promptly produced_cJstates isR_p0:3950:016stat 0:015systfor_cJwithp_TJ= >4 GeV=candp_T>

1 GeV=c. For _cJ resulting from B decay over the same kinematic range we find R_B 0:1430:042stat 0:005syst. These results provide the most precise mea- surement of the _cJ production ratio obtained in any hadronic interactions. Conversion of this measurement into the direct cross-section ratio _c2=_c1 requires a knowledge of the branching fractions, which are not mea- sured in this experiment, and a small correction due to 2S !_cJdecays. Based on the existing measurement of the prompt 2S cross section [12], and the_cJ con- tribution to the promptJ= production cross section [3], we estimate that 4:01:05:01:0% of our prompt

_c1c2 sample is due to decay of promptly produced 2Smesons.

Prior measurements of the prompt cross-section ratio have been severely limited in their precision due to the statistical uncertainties inherent to small data samples [5].

The relative precision of previous measurements has typi- cally been approximately 30% on the cross-section ratio, and provides weak guidance for production models. This work, combined with the best branching-fraction ratio measurementR_J= B_c1!J= =B_c2!J=

1:910:10 available [13], gives R_pR_J= 0:75 0:03stat 0:03syst 0:04BF, where the last term in the uncertainty is due to the branching-fraction (BF) ratio uncertainty. This level of precision should serve to inform any future developments in the calculation of hadronic charmonium production.

We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions.

This work was supported by the U. S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A.P. Sloan Foundation; the Bundes- ministerium fur Bildung und Forschung, Germany; the Korean Science and Engineering Foundation and the Korean Research Foundation; the Particle Physics and Astronomy Research Council and the Royal Society, UK;

the Institut National de Physique Nucleaire et Physique des Particules/CNRS; the Russian Foundation for Basic Research; the Comision Interministerial de Ciencia y Tecnologia, Spain; in part by the European Community‘s Human Potential Programme under contract No. HPRN- CT-2002-00292; and the Academy of Finland.

aVisiting scientist from University of Athens.

bVisiting scientist from University of Bristol.

cVisiting scientist from University Libre de Bruxelles.

dVisiting scientist from Cornell University, Ithaca, New York, USA.

eVisiting scientist from University of Cyprus.

fVisiting scientist from University of Dublin.

gVisiting scientist from University of Edinburgh.

hVisiting scientist from University of Heidelberg.

iVisiting scientist from Universidad Iberoamericana.

jVisiting scientist from University of Manchester.

kVisiting scientist from Nagasaki Institute of Applied Science.

lVisiting scientist from University de Oviedo.

mVisiting scientist from University of London, Queen Mary and Westfield College.

TABLE II. Relative systematic uncertainties onRpandRB.

Effect Uncertainty

Simulation Sample Size 0:005

Photon Conversion Simulation 0:020

Polarization Effects 0:030

Invariant Mass Resolution 0:005

Prompt=BSeparation 0:002(0:010forB)

Total 0:037(0:038forB)

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nVisiting scientist from Texas Tech University, Lubbock, Texas 79409, USA.

oVisiting scientist from IFIC(CSIC-Universitat de Valencia).

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