EPJ2308.DVI Digital Object Identifier (DOI) 10.1140/epjc/s2005-02308-8 Eur. Phys. J. C (2005) THE EUROPEAN PHYSICAL JOURNAL C Charm, beauty and charmonium production at HERA–B The HERA–B Collaboration A. Zoccoli6,a, I. Abt23, M. Adams10, M. Agari13, H. Albrecht12, A. Aleksandrov29, V. Amaral8, A. Amorim8, S. J. Aplin12, V. Aushev16, Y. Bagaturia12,36, V. Balagura22, M. Bargiotti6, O. Barsukova11, J. Bastos8, J. Batista8, C. Bauer13, Th. S. Bauer1, A. Belkov11,†, Ar. Belkov11, I. Belotelov11, A. Bertin6, B. Bobchenko22, M. Böcker26, A. Bogatyrev22, G. Bohm29, M. Bräuer13, M. Bruinsma28,1, M. Bruschi6, P. Buchholz26, T. Buran24, J. Carvalho8, P. Conde2,12, C. Cruse10, M. Dam9, K. M. Danielsen24, M. Danilov22, S. De Castro6, H. Deppe14, X. Dong3, H. B. Dreis14, V. Egorytchev12, K. Ehret10, F. Eisele14, D. Emeliyanov12, S. Essenov22, L. Fabbri6, P. Faccioli6, M. Feuerstack-Raible14, J. Flammer12, B. Fominykh22, M. Funcke10, Ll. Garrido2, B. Giacobbe6, P. Giovannini6, J. Gläß20, D. Goloubkov12,33, Y. Golubkov12,34, A. Golutvin22, I. Golutvin11, I. Gorbounov12,26, A. Gorǐsek17, O. Gouchtchine22, D. C. Goulart7, S. Gradl14, W. Gradl14, F. Grimaldi6, Yu. Guilitsky22,35, J. D. Hansen9, J. M. Hernández29, W. Hofmann13, T. Hott14, W. Hulsbergen1, U. Husemann26, O. Igonkina22, M. Ispiryan15, T. Jagla13, C. Jiang3, H. Kapitza12, S. Karabekyan25, N. Karpenko11, S. Keller26, J. Kessler14, F. Khasanov22, Yu. Kiryushin11, E. Klinkby9, K. T. Knöpfle13, H. Kolanoski5, S. Korpar21,17, C. Krauss14, P. Kreuzer12,19, P. Križan18,17, D. Krücker5, S. Kupper17, T. Kvaratskheliia22, A. Lanyov11, K. Lau15, B. Lewendel12, T. Lohse5, B. Lomonosov12,32, R. Männer20, S. Masciocchi12, I. Massa6, I. Matchikhilian22, G. Medin5, M. Medinnis12, M. Mevius12, A. Michetti12, Yu. Mikhailov22,35, R. Mizuk22, R. Muresan9, M. zur Nedden5, M. Negodaev12,32, M. Nörenberg12, S. Nowak29, M. T. Núñez Pardo de Vera12, M. Ouchrif28,1, F. Ould-Saada24, D. Peralta2, R. Pernack25, R. Pestotnik17, M. Piccinini6, M. A. Pleier13, M. Poli6,31, V. Popov22, D. Pose11,14, S. Prystupa16, V. Pugatch16, Y. Pylypchenko24, J. Pyrlik15, K. Reeves13, D. Reßing12, H. Rick14, I. Riu12, P. Robmann30, I. Rostovtseva22, V. Rybnikov12, F. Sánchez13, A. Sbrizzi1, M. Schmelling13, B. Schmidt12, A. Schreiner29, H. Schröder25, A. J. Schwartz7, A. S. Schwarz12, B. Schwenninger10, B. Schwingenheuer13, F. Sciacca13, N. Semprini-Cesari6, S. Shuvalov22,5, L. Silva8, L. Sözüer12, S. Solunin11, A. Somov12, S. Somov12,33, J. Spengler13, R. Spighi6, A. Spiridonov29,22, A. Stanovnik18,17, M. Starič17, C. Stegmann5, H. S. Subramania15, M. Symalla12,10, I. Tikhomirov22, M. Titov22, I. Tsakov27, U. Uwer14, C. van Eldik12,10, Yu. Vassiliev16, M. Villa6, A. Vitale6, I. Vukotic5,29, H. Wahlberg28, A. H. Walenta26, M. Walter29, J. J. Wang4, D. Wegener10, U. Werthenbach26, H. Wolters8, R. Wurth12, A. Wurz20, Yu. Zaitsev22, M. Zavertyaev12,13,32, G. Zech26, T. Zeuner12,26, A. Zhelezov22, Z. Zheng3, R. Zimmermann25, T. Živko17 1 NIKHEF, 1009 DB Amsterdam, The Netherlands a 2 Department ECM, Faculty of Physics, University of Barcelona, E-08028 Barcelona, Spain b 3 Institute for High Energy Physics, Beijing 100039, P.R. China Institute of Engineering Physics, Tsinghua University, Beijing 100084, P.R. China 4 Institut für Physik, Humboldt-Universität zu Berlin, D-12489 Berlin, Germany c,d 5 Dipartimento di Fisica dell’ Università di Bologna and INFN Sezione di Bologna, I-40126 Bologna, Italy 6 Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA e 7 LIP Coimbra, P-3004-516 Coimbra, Portugal f 8 Niels Bohr Institutet, DK 2100 Copenhagen, Denmark g 9 Institut für Physik, Universität Dortmund, D-44221 Dortmund, Germany d 10 Joint Institute for Nuclear Research Dubna, 141980 Dubna, Moscow region, Russia 11 DESY, D-22603 Hamburg, Germany 12 Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany d 13 Physikalisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany d 14 Department of Physics, University of Houston, Houston, TX 77204, USA e 15 Institute for Nuclear Research, Ukrainian Academy of Science, 03680 Kiev, Ukraine h 16 J. Stefan Institute, 1001 Ljubljana, Slovenia i 17 University of Ljubljana, 1001 Ljubljana, Slovenia 18 University of California, Los Angeles, CA 90024, USA j 19 Lehrstuhl für Informatik V, Universität Mannheim, D-68131 Mannheim, Germany 20 University of Maribor, 2000 Maribor, Slovenia 21 Institute of Theoretical and Experimental Physics, 117259 Moscow, Russia k a e-mail: zoccoli@bo.infn.it A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B 22 Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, D-80805 München, Germany d 23 Dept. of Physics, University of Oslo, N-0316 Oslo, Norway l 24 Fachbereich Physik, Universität Rostock, D-18051 Rostock, Germany d 25 Fachbereich Physik, Universität Siegen, D-57068 Siegen, Germany d 26 Institute for Nuclear Research, INRNE-BAS, Sofia, Bulgaria 27 Universiteit Utrecht/NIKHEF, 3584 CB Utrecht, The Netherlands a 28 DESY, D-15738 Zeuthen, Germany 29 Physik-Institut, Universität Zürich, CH-8057 Zürich, Switzerland m 30 visitor from Dipartimento di Energetica dell’ Università di Firenze and INFN Sezione di Bologna, Italy 31 visitor from P.N. Lebedev Physical Institute, 117924 Moscow B-333, Russia 32 visitor from Moscow Physical Engineering Institute, 115409 Moscow, Russia 33 visitor from Moscow State University, 119899 Moscow, Russia 34 visitor from Institute for High Energy Physics, Protvino, Russia 35 visitor from High Energy Physics Institute, 380086 Tbilisi, Georgia Received: 14 March 2005 / Revised version: 5 April 2005 / Published online: 27 July 2005 – c© Springer-Verlag / Società Italiana di Fisica 2005 Abstract. HERA B is a fixed target experiment working on the 920 GeV proton beam of the HERA acceler- ator at the DESY laboratory in Hamburg. During the last data taking period (2002–2003), about 150 mil- lion dilepton triggers, 220 million minimum bias events and 35 million hard photon triggers were acquired. These large statistics allow detailed studies on the production of charmonium states in proton-nucleus p-A collisions, which extend for the first time into the negative Feynman-x (xF) region. Measurements of the inclusive bb, Υ and open charm cross sections are also ongoing. After a brief discussion of the detector and of the data samples, we report on preliminary results obtained on these physics topics. PACS. 24.85.p, 13.85.Ni A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B 1 Introduction HERA B is a fixed target experiment [1] working on the 920 GeV proton beam ( √ s = 41.6 GeV) of the HERA ring at DESY in Hamburg. † deceased a supported by the Foundation for Fundamental Research on Matter (FOM), 3502 GA Utrecht, The Netherlands b supported by the CICYT contract AEN99-0483 c supported by the German Research Foundation, Graduate College GRK 271/3 d supported by the Bundesministerium für Bildung und Forschung, FRG, under contract numbers 05-7BU35I, 05-7DO55P, 05-HB1HRA, 05-HB1KHA, 05-HB1PEA, 05- HB1PSA, 05-HB1VHA, 05-HB9HRA, 05-7HD15I, 05-7MP25I, 05-7SI75I e supported by the U.S. Department of Energy (DOE) f supported by the Portuguese Fundação para a Ciência e Tec- nologia under the program POCTI g supported by the Danish Natural Science Research Council h supported by the National Academy of Science and the Min- istry of Education and Science of Ukraine i supported by the Ministry of Education, Science and Sport of the Republic of Slovenia under contracts number P1-135 and J1-6584-0106 j supported by the U.S. National Science Foundation Grant PHY-9986703 k supported by the Russian Ministry of Education and Science, grant SS-1722.2003.2, and the BMBF via the Max Planck Re- search Award l supported by the Norwegian Research Council m supported by the Swiss National Science Foundation The physics program of the experiment covers a wide range of topics and is centered on the heavy flavor produc- tion. Other topics under investigation are strangeness and hyperon production, hard photon production, searches of exotic states (like pentaquarks, glueballs or hybrids), and the search of flavor-changing neutral currents (FCNC). Emphasis is placed on the study of charmonium pro- duction (e.g. J/ψ, ψ(2S) and χc) and its nuclear depen- dence in p-A collisions. For the first time it is possible to perform measurements in the negative xF region. This will provide important tests for many different models at- tempting to describe the inclusive single-particle produc- tion and its modification in the nuclear matter. In this context the basic color singlet and color octet mechanisms [2] of quarkonium production are complemented with the inclusion of processes which try to account for the inter- action with the nuclear matter, like the final state ab- sorption, interactions with comovers, shadowing of parton distributions and parton energy loss [3]. The understand- ing of these particle production mechanisms will provide a solid baseline for the interpretation of the particle sup- pression studies in heavy-ion collisions, performed with the aim to observe quark-gluon-plasma formation. HERA B is also studying the production of b and c quarks in p-A collisions, by providing a new precise mea- surement of the bb cross section and of the Υ production, and by measuring the open charm cross section. This will provide important inputs and put further constraints on NLO QCD predictions. In Sect. 2 we provide a short discussion of the detec- tor and of trigger of the experiment, as well as a short A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B Ring Imaging Cherenkov Counter 250 mrad 220 mrad Magnet Si-Strip Vertex Detector Calorimeter TRD Muon Detector Target Wires 0 m 5 10 15 20 Proton Beam Electron Beam Vertex Vessel Inner / Outer Tracker high-pt Al Beam Pipe x z Fig. 1. Top view of the HERA B detector summary of the main data samples acquired. Section 4 describes the main preliminary results obtained on char- monium production. Sections 5 and 6 discuss the open charm production and the open and hidden beauty pro- duction. Finally the conclusions are drawn in Sect. 7. 2 The HERA B detector The HERA B experiment is a forward magnetic spectrom- eter with an acceptance extending from 10 to 220 mrad horizontally and to 160 mrad vertically [1, 4]. This large angular coverage allows us to study kinematic regions (like the negative xF range for charmonium states) not acces- sible in previous high energy experiments. A top view of the detector is shown in Fig. 1. The first part of the spec- trometer is devoted to tracking and vertex measurements and consists of a target, a silicon vertex detector, a mag- net and a tracking system. The second part is focused on the particle identification and includes a Ring Imaging Cherenkov detector, an electromagnetic calorimeter and a muon detector. The target system consists of two stations of 4 wires each, of different materials (C, Al, Pd, Ti, W), separated by 4 cm along the beam direction. It is placed in the halo of the HERA proton beam. The wire positions can be continuously adjusted in order to keep a constant interac- tion rate, in the range between 1 and 40 MHz. During the physics data taking only single-wire (mainly C or W) and double-wire (C and W) configurations were used, with a typical interaction rate of about 5 MHz. The vertex detector (VDS) is placed between the tar- get and the magnet and consists of 8 stations of double- sided silicon microstrip detectors (50 × 70 mm2, 50 µm pitch). Each station consists of four “quadrants” arranged if four different stereo views. This system provides a pri- mary vertex resolution of σz ∼ 500 µm along the beam direction and σx,y ∼ 50 µm in the transverse plane. A dipole magnet with 2.13 Tm field-integral is po- sitioned before the tracking system. Each tracking sta- tion consists of several planes of MSGC/GEM chambers placed near the beam pipe (Inner Tracker, ITR) and sev- eral planes of Honeycomb Drift chambers which cover the rest of the acceptance (Outer Tracker, OTR). The detec- tor segmentation is set in order to cope with the particle flux variation with the distance from the beam pipe. Typ- ical momentum resolutions of σp/p ∼ 1% are achieved. The particle identification for charged tracks (protons, kaons, etc.) is provided by a Cherenkov detector (RICH) installed downstream of the magnet. The electromagnetic calorimeter (ECAL), which provides the electron pretrig- ger seeds and the e/π separation, is installed after the RICH and the tracking system. The ECAL is a Shashlik sampling calorimeter with Pb or W as absorber and scin- tillator as active material. The Muon detector (MUON) provides the muon pretrigger seeds and the muon identifi- cation and is located in the most downstream part of the detector. It consists of four superlayers embedded in an iron loaded concrete absorber. The sensitive area close to the beam pipe is covered by pixel chambers, while in the rest of the acceptance tube chambers are used. 3 Trigger and data samples The trigger is based on a multi-level scheme and has been designed to select with high efficiency the two leptons from the J/ψ → �+�− decay and to provide a large back- ground suppression, reducing the initial interaction rate (∼5 MHz) to the typical logging rate of ∼ 100 Hz, without dead time. Pretrigger seeds from the ECAL or the MUON detectors are sent to the First Level Trigger (FLT). The FLT is a hardware tracking device based on Kalman fil- tering. It selects the lepton candidates with a maximum latency of 12 µs providing a rate reduction factor of ∼ 250. The events accepted by the FLT are sent to the Second Level Trigger (SLT) which provides a refined track recon- struction including the VDS information. The SLT is im- plemented on a farm of 240 Linux-PCs and has a typical latency of 10 ms with another rate reduction factor of ∼ 200. The full reconstruction of the events passing the trigger selection is performed online on a second farm of A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B 0 5000 10000 15000 20000 25000 2.5 3 3.5 4 0 10000 20000 30000 40000 50000 60000 2.5 3 3.5 4 e + e - Invariant mass (GeV/c 2 ) E n tr ie s/ (3 3 M eV /c 2 ) 110000 J/ψ 2200 ψ(2S) µ+ µ- Invariant mass (GeV/c2) E n tr ie s/ (2 8 .5 M eV /c 2 ) 177000 J/ψ 3000 ψ(2S) Fig. 2. e+e− (left) and µ+µ− (right) invariant mass distributions in the region of the J/ψ and ψ(2S) signals about 200 Linux-PCs, providing a final logging rate on tape of ∼ 100 Hz. The scheme of this system allows to implement differ- ent trigger configurations, besides the main dilepton trig- ger. As a matter of fact, data have been taken requiring at least one energy deposit in ECAL with large transverse momentum (Hard Photon trigger). Another configuration required at least one inelastic interaction in the target (Minimum Bias trigger). The data acquisition was performed in two different periods: a first short period, during the year 2000, mainly devoted to the debugging of the detector and of the trig- ger; a second longer period, from October 2002 to March 2003, for the physics program of the experiment. The physics data taking has been performed by using three main different trigger configurations: – Dilepton trigger. This trigger selects di-lepton events (e.g. e+e− or µ+µ−) with large transverse momentum. The total acquired statistics in this configuration is about 150 million of events, with an average DAQ rate of about 100 Hz. – Minimum Bias (MB) trigger. This trigger requires at least one inelastic interaction in the target, by checking the presence of a minimum energy deposition in the RICH or in the ECAL detectors. The total collected statistics is about 220 million events, with an average DAQ rate larger than 1000 Hz. – Hard photon trigger. It requires the presence in the ECAL of at least one cluster of high transverse energy (typically > 3 GeV). The total collected statistics cor- responds to about 35 million events. The results presented in this paper are derived from the first two trigger samples. The charmonium, the b-hadron and the Υ production studies are performed on the dilep- ton trigger sample, while the open charm cross section is measured in the MB sample. 4 Studies on charmonium production The charmonium states are reconstructed exploiting their dilepton decay modes (e+e−and µ+µ−). The reconstruc- tion requires the presence of two trigger tracks of opposite charge, with a common vertex and a good lepton identifi- cation. In the e+e− channel, where a large hadronic back- ground is present, the electrons are identified by applying an E/p cut (where E is the electron energy measured by ECAL and p is its momentum measured by the tracking system), by performing a cluster shape analysis and by requiring a good match between the cluster in ECAL and the charged track. Moreover, possible energy losses due to the emission of a bremsstrahlung photon in the region before the magnet are taken into account. In the µ+µ− channel, the request of a good muon likelihood is suffi- cient to strongly suppress the background. Following these criteria about 300 000 J/ψ have been reconstructed in both decay channels. The corresponding invariant mass distributions are shown in Fig. 2. In the e+e− distribution a clear peak containing ∼ 110 000 J/ψ events is visible at the correct mass position [5], while in the muon channel the J/ψ peak contains ∼ 177 000 events. Also ψ(2S) events have been reconstructed in the same invariant mass distributions, ∼ 2200 in the e+e− channel and ∼ 3000 in the µ+µ− channel. In both channels the main contribution to the background is coming from pion and kaon decays, while physics processes like charm and beauty production give a small effect. 4.1 J/ψ differential distributions and nuclear dependence These large statistics allowed detailed studies on the J/ψ pT and xF differential distributions and to measure the nuclear dependence of its production cross-section. Figure 3 shows the preliminary results obtained on the electron channel for the pT (a) and the xF (b) differential A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B Table 1. Experimental situation on J/ψ differential distributions. The definition of the fitting parameters (〈pT〉 and c) is given in Eqs. 1 and 2. The kinematic intervals and the fitting parameters obtained from the e+e− channel of the HERA B analysis are compared to fixed-target results at √ s = 38.8 GeV [6] Exp. target (A) pT range 〈pT〉 xF range c (GeV/c) (GeV/c) HERA–B C (12) 0 ÷ 5.0 1.22 ± 0.01 −0.375 ÷ 0.125 5 ÷ 6.5 W (184) 0 ÷ 5.0 1.29 ± 0.01 (±0.2stat) E771 Si (28) 0 ÷ 3.4 1.20 ± 0.01 −0.05 ÷ 0.25 6.54 ± 0.23 E789 Be (9) 0.30 ÷ 0.95 5.32 ± 0.05 Cu (64) 0.30 ÷ 0.95 5.21 ± 0.04 Au (197) 0 ÷ 2.6 1.29 ± 0.01 −0.035 ÷ 0.135 4.91 ± 0.18 E672/E706 Be (9) 0 ÷ 3.0 1.22 ± 0.01 0 ÷ 0.6 6.18 ± 0.16 10 2 10 3 10 4 10 5 0 2 4 6 pT (GeV/c) d N /d p T 2 xF . d N /d x F 10 4 10 5 -0.4 -0.2 0 0.2 Fig. 3. J/ψ differential distributions, obtained in p-C collisions from the e+e− channel, as a function of pT a and xF b distributions, where the lines represent the fit results. The pT and xF distributions are fitted respectively with the functions: dN(J/ψ ) dp2T = A · [ 1 + ( 35 · π · pT 256 · 〈pT 〉 )2]−6 (1) and dN(J/ψ ) dxF = B · (1 − |xF|)c (2) where A and B are arbitrary normalization factors. The preliminary fit results on the average pT and on the c exponent are reported in Tab. 1. As one can see, HERA B extends significantly the pT and xF ranges for J/ψ’s by accessing for the first time the negative xF region. By comparing the results on the average pT obtained from the p-C and the p-W data samples we confirm the tendency of an increase of the 〈pT〉 with the mass number. The preliminary results on the xF slopes are in the range 5–6.5 and the achievable final statistical accuracy is estimated to be ±0.2. The nuclear effects in heavy quark production are com- monly parameterized by using the power law σpA = σpN · Aα(pT,xF), where σpA is the production cross section in p-A collisions and σpN is the elementary proton-nucleon cross section. In order to measure the α exponent for the J/ψ production as a function of pT and xF only data taken Fx -0.4 -0.2 0 0.2 0.4 ) Fx( α 0.85 0.9 0.95 1 1.05 1.1 HERA-B Prel. E866 NA50 ]c [GeV/Tp 0 1 2 3 4 5 ) T p( α 0.8 0.9 1 1.1 1.2 1.3 HERA-B Prel. E866 Fig. 4. Nuclear suppression parameter, α, as a function of xF (left) and pT (right), as obtained from the full muon sample. Results from the E866 [7] (open circles) and NA50 [8] (open squares) experiments are also shown. The errors include sta- tistical and systematic uncertainties by using simultaneously the carbon and the tungsten tar- gets have been used. This allowed to minimize the system- atic effects depending on changes in the trigger and detec- tor performances. Preliminary results obtained on the full muon sample are shown in Fig. 4 as a function of xF and pT. In the xF range covered by HERA B a small flat sup- pression is observed. The obtained average value of the nuclear suppression parameter α is: α = 0.969 ± 0.003(stat) ± 0.021(syst) . (3) In the overlap region (i.e. for xF > −0.1), our results are compatible with the E866 data [7] and are larger than the NA50 values [8]. Moreover, α increases with increasing pT and, for large transverse momenta, an enhancement of J/ψ production (α > 1) is observed. This is a consequence of the pT broadening, in agreement with the observations of the E866 experiment [7]. This study is important for the verification of the different model predictions currently available for the nuclear suppression [9], and will be com- pleted with the addition of the result from the e+e− chan- nel. A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B Table 2. Preliminary results on the ratio R(l+l−) = B(ψ(2S) → l+l−) · σ(ψ(2S)) / B(J/ψ → l+l−) · σ(J/ψ). The errors represent only the statistical uncertainties Target R(e+e−) (%) R(µ+µ−) (%) Carbon 1.60 ± 0.20 1.65 ± 0.10 Tungsten 1.80 ± 0.40 1.55 ± 0.20 4.2 ψ(2S) production The comparison between ψ(2S) and J/ψ production can further contribute to the understanding of nuclear absorp- tion processes: different cross section behaviors of the two charmonium states, as a function of kinematic variables like xF and pT, may be interpreted in terms of the size and of the binding energy of these two charmonium states. Preliminary results on the total yield ratio R(l+l−) = B(ψ(2S) → l+l−) · σ(ψ(2S)) / B(J/ψ → l+l−) · σ(J/ψ) have been obtained both from the e+e− and the µ+µ− samples and are summarized in Table 2. These values con- firm the apparent independence of the ratio R from the target mass number. Moreover, these results are in agree- ment with the world average result and show no depen- dence of R on the center of mass energy. 4.3 χc production An additional check of the hadronic charmonium produc- tion models can be provided by the measurement of the fraction (Rχc ) of J/ψ produced via the χc radiative decay χc → J/ψγ. Out of the three χc states, the χc0 contribu- tion is negligible, due to its small branching ratio [5], while the χc1 and χc2 states, separated by 46 MeV/c2, cannot be resolved due to the insufficient energy resolution of the ECAL. For these reasons the ratio is quoted as: Rχc = Σ2i=1σ(χci)B(χci → J/ψγ) σ(J/ψ) (4) where σ(J/ψ) and σ(χci) are respectively the total pro- duction cross section of the J/ψ and of the χc states. Figure 5 shows the ∆M = M(J/ψ γ) − M(J/ψ) distri- bution, referring to 15% of the total muon data sample, before (left) and after (right) the subtraction of the back- ground. The main contributions to the background are the random combinations of J/ψ and photon candidates, and decays of heavier mesons into J/ψX. After background subtraction, we see a clear peak, corresponding to the two χc states, containing about 1300 events. The preliminary result, from this sub-sample of events, is: Rχc = 0.21 ± 0.05stat , (5) where the error includes only statistical uncertainties. This result is in agreement with the previous HERA B result Rχc = 0.32 ± 0.06(stat) ± 0.04(syst). Moreover, it seems to favor the NRQCD predictions [2], even if a better Table 3. Preliminary results obtained on D0, D+ and D∗+ production cross sections (µb/nucl) and cross section ratios. The results are obtained from the full MB data sample, by summing up the data samples taken with different target mate- rials. The second column gives the cross section in the HERA B kinematic range, the third one gives the cross sections extrap- olated to the full phase space using Pythia. The first error is due to statistics, the second to systematic uncertainties Channel Cross section Cross section −0.1 < xF < 0.05 Full xF range σD0 21.4 ± 3.2 ± 3.6 56.3 ± 8.5 ± 9.5 σD+ 11.5 ± 1.7 ± 2.2 30.2 ± 4.5 ± 5.8 σD∗+ 10.0 ± 1.9 ± 1.4 27.8 ± 5.2 ± 3.9 σD+/σD0 0.54 ± 0.11 ± 0.14 σD∗+/σD0 0.49 ± 0.12 ± 0.10 precision is needed to draw final conclusions. The number of χc expected in the full data sample is ∼ 15 000 which means an increase in statistics of about a factor 20 with respect to this analysis [10]. 5 Open charm production Signals of D0 → K−π+, D+ → K−π+π+ and D�+ → D0π+ (and charge conjugate decays) are obtained from the minimum bias data sample. The HERA B acceptance is limited to the mid-rapidity range, xF ∈ [−0.1, 0.05], while the pT acceptance covers the range pT ∈ [0.0, 2.5] GeV/c. The D mesons are reconstructed requiring a good particle identification for the kaons and pions produced in the decay. Moreover the D-meson vertex is required to be separated from the primary one with high significance and the prolonged D-meson track must pass through the primary vertex. In this way it has been possible to recon- struct 189 ± 20 D0, 198 ± 12 D+ and 43 ± 8 D�+. The corresponding production cross sections have been determined following the expression σD = ND � · B · ΣAiLi , (6) where ND is the number of reconstructed mesons, � is the reconstruction efficiency, B is the corresponding branch- ing ratio and L = ΣAiLi is the sum of the integrated lu- minosities over different target materials. The integrated luminosities have been measured by the ratio of the num- ber of inelastic interactions (Ninel) recorded during the data taking period and the A-dependent inelastic cross section. Ninel has been determined in different ways: by looking at the signals of plastic scintillators placed behind the magnet, by measuring the total energy deposition in the ECAL, or from the probability to observe an empty event in a given subdetector [11]. The obtained prelimi- nary results, summarized in Table 3, can be used to check the predictions of different QCD models [12] and to com- pare with previous experiments. A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B 55.15 / 45 P1 1281. 190.3 P2 0.4190 0.5706E-02 P3 0.3592E-01 0.5224E-02 P4 0.2001E-01 0.6846E-04 0 1000 2000 3000 4000 5000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 55.15 / 45 P1 1281. 190.3 P2 0.4190 0.5706E-02 P3 0.3592E-01 0.5224E-02 P4 0.2001E-01 0.6846E-04 -200 -100 0 100 200 300 400 500 600 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ∆ M (GeV/ c2)∆ M (GeV/ c2) Fig. 5. χc signals in the ∆M = M(J/ψγ) − M(J/ψ) invariant mass distribution, before (left) and after (right) background subtraction 0 5 10 15 20 25 30 35 40 2.25 2.5 2.75 3 3.25 3.5 3.75 4 4.25 4.5 46.2 ± 8.2 J/ψ µ+µ− invariant mass (GeV/c2) E n tr ie s/ 50 M eV /c 2 10 10 2 10 3 0 1 2 3 4 5 6 7 proper time (ps) E ff w ei gh te d e n tr ie s (a .u .) τ = 1.39 ± 0.19 ps Fig. 6. Left: Dilepton mass for detached events from the full muon data sample. A J/ψ signal of about 50 events is clearly visible. Right: proper time distribution for the detached events sitting in the J/ψ mass region. The line represents the result of an unbinned likelihood fit of the events 6 Open and hidden beauty production The open beauty production cross section, σ(bb), is mea- sured in the inclusive decay channel B → J/ψX, by look- ing at the J/ψ → l+l− decay modes. The B → J/ψX events are selected by requiring J/ψ’s detached with re- spect to the primary interaction vertex, exploiting the long lifetime of B-mesons. Moreover, both leptons have to be inconsistent with being produced in the primary interac- tion, by having large impact parameters to the target. In Fig. 6 (left) the mass of the selected dimuon candidates with a vertex downstream of the target, from the full data sample, is shown. A clear J/ψ peak of about 50 events is visible over a smooth background, which is mainly com- posed of π and K decays, and bb̄ double semileptonic de- cay events. The right part of the figure shows the distri- bution of proper time for the detached J/ψ’s, corrected for the selection efficiency. The line represents the result of an unbinned likelihood fit, which gives a lifetime of τ = 1.39 ± 0.19 ps, well in agreement with the expected value for B-meson decays [5]. Combining the results in the e+e− and µ+µ− channels, we obtain the following preliminary value for the cross section ratio in the HERA B acceptance: R∆σ = ∆σ(bb̄) ∆σ(J/ψ) = 0.033±0.005(stat)±0.004(syst) . (7) Here, ∆σ represents the cross section in the HERA B ac- ceptance. In order to compare this result to other measure- ments and to theoretical predictions [13], we extrapolate the R∆σ ratio to the full xF range and then, by using the prompt J/ψ cross section value σ(J/ψ) = 357 ± 2 ± 36 nb/nucl [14], we obtain the preliminary value for the total bb production cross section: σ(bb̄) = (9.9 ± 1.5 ± 1.4) nb/nucl . (8) The statistics of bb events largely superseeds that of all earlier fixed target experiments. The result is within 2 standard deviations compatible with the previous HERA B result [14]. In the high part of the dilepton mass spectra clear signals corresponding to the Υ states are observed. This allowed to perform a preliminary measurement of the Υ cross section, σ(Υ), times the branching ratio Br(Υ → l+l−) at mid-rapidity. The preliminary combined e+e− µ+µ− result yields: dσ(Υ) dy |y=0 · Br(Υ → l+l−) = (3.4 ± 0.8) pb/nucl , (9) with an uncertainty comparable or better than that of earlier experiments. Again, this result can be used to check and constrain the prediction of QCD models. A. Zoccoli et al. (HERA–B Coll.): Charm, beauty and charmonium production at HERA–B 7 Conclusions A brief overview on the studies performed by the HERA B experiment in the field on heavy flavor production in p- A interactions has been given here. 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