Elsevier

Nuclear Physics B

Volume 864, Issue 1, 1 November 2012, Pages 1-37
Nuclear Physics B

Inclusive-jet photoproduction at HERA and determination of αs

https://doi.org/10.1016/j.nuclphysb.2012.06.006Get rights and content

Abstract

Inclusive-jet cross sections have been measured in the reaction epe+jet+X for photon virtuality Q2<1 GeV2 and γp centre-of-mass energies in the region 142<Wγp<293 GeV with the ZEUS detector at HERA using an integrated luminosity of 300 pb1. Jets were identified using the kT, anti-kT or SIScone jet algorithms in the laboratory frame. Single-differential cross sections are presented as functions of the jet transverse energy, ETjet, and pseudorapidity, ηjet, for jets with ETjet>17 GeV and 1<ηjet<2.5. In addition, measurements of double-differential inclusive-jet cross sections are presented as functions of ETjet in different regions of ηjet. Next-to-leading-order QCD calculations give a good description of the measurements, except for jets with low ETjet and high ηjet. The influence of non-perturbative effects not related to hadronisation was studied. Measurements of the ratios of cross sections using different jet algorithms are also presented; the measured ratios are well described by calculations including up to O(αs2) terms. Values of αs(MZ) were extracted from the measurements and the energy-scale dependence of the coupling was determined. The value of αs(MZ) extracted from the measurements based on the kT jet algorithm is αs(MZ)=0.12060.0022+0.0023(exp.)0.0035+0.0042(th.); the results from the anti-kT and SIScone algorithms are compatible with this value and have a similar precision.

Introduction

The study of jet production in ep collisions at HERA has been well established as a testing ground of perturbative QCD (pQCD). Jet cross sections provided precise determinations of the strong coupling constant, αs, and its scale dependence. The jet observables used to test pQCD included inclusive-jet [1], [2], [3], [4], [5], [6], [7], dijet [1], [4], [6], [7], [8], [9] and multijet [6], [7], [10], [11], [12] cross sections in neutral current (NC) deep inelastic ep scattering (DIS), inclusive-jet [13], [14], dijet [15], [16], [17], [18], [19], [20] and multijet [21], [22] cross sections in photoproduction and the internal structure of jets in NC [23], [24], [25] and charged current [26], [27] DIS. These studies also demonstrated that the kT cluster algorithm [28] in the longitudinally invariant inclusive mode [29] results in the smallest uncertainties in the reconstruction of jets in ep collisions. Jet cross sections in NC DIS [2] and photoproduction [16] were used by ZEUS [30] as input in a QCD analysis to extract the parton distribution functions (PDFs) of the proton; these data helped to constrain the gluon density at medium- to high-x values, where x is the fraction of the proton momentum carried by the gluon.

The kT algorithm is well suited for ep collisions and yields infrared- and collinear-safe cross sections at any order of pQCD. However, it might not be best suited to reconstruct jets in hadron–hadron collisions, such as those at the LHC. In order to optimise the reconstruction of jet observables in such environments, new infrared- and collinear-safe jet algorithms were recently developed, namely the anti-kT [31], a recombination-type jet algorithm, and the “Seedless Infrared-Safe” cone (SIScone) [32] algorithms. Measurements of jet cross sections in NC DIS using these algorithms were recently published [33] and constituted the first measurements with these new jet algorithms. The results tested the performance of these jet algorithms with data in a well understood hadron-induced reaction and it was shown that pQCD calculations with up to four partons in the final state provide a good description of the differences between jet algorithms.

Measurements of inclusive-jet cross sections in photoproduction are presented in this paper. Two types of QCD processes contribute to jet production in photoproduction; at leading order they can be separated into [34], [35] the direct process, in which the photon interacts directly with a parton in the proton, and the resolved process, in which the photon acts as a source of partons, one of which interacts with a parton in the proton. Due to the presence of the resolved processes, the analysis of jet cross sections in photoproduction with different jet algorithms provides a test of their performance in a reaction closer to hadron–hadron interactions than NC DIS.

In this paper, single-differential inclusive-jet cross sections as functions of the jet transverse energy, ETjet, and pseudorapidity, ηjet, are presented based on the kT, anti-kT and SIScone jet algorithms. The results based on the anti-kT and SIScone jet algorithms are compared to the measurements based on the kT via the ratios of cross sections. In addition, measurements of cross sections are also presented as functions of ETjet in different regions of ηjet, which have the potential to constrain further the gluon density at high x. Next-to-leading-order (NLO) QCD calculations using recent parameterisations of the proton and photon PDFs are compared to the measurements. A determination of αs(MZ) as well as of its energy-scale dependence are also presented. The analyses presented here are based on a data sample with a more than three-fold increase in statistics with respect to the previous study [13].

Section snippets

Experimental set-up

A detailed description of the ZEUS detector can be found elsewhere [36], [37]. A brief outline of the components most relevant for this analysis is given below.

Charged particles were tracked in the central tracking detector (CTD) [38], which operated in a magnetic field of 1.43 T provided by a thin superconducting solenoid. The CTD consisted of 72 cylindrical drift-chamber layers, organised in nine superlayers covering the polar-angle62

Data selection

The data were collected during the running period 2005–2007, when HERA operated with protons of energy Ep=920 GeV and electrons or positrons63 of energy Ee=27.5 GeV, at an ep centre-of-mass energy of s=318 GeV, and correspond to an integrated luminosity of 299.9±5.4 pb1.

A three-level trigger system was used to select events online [37], [43]. At the first level, events were triggered by a

Jet search

In photoproduction, jets are usually defined using the transverse-energy flow in the pseudorapidity–azimuth (ηϕ) plane of the laboratory frame [28], [29], [44]. The procedure to reconstruct jets with the kT algorithm from an initial list of objects (e.g. final-state partons, final-state hadrons or energy deposits in the calorimeter) is described below in some detail. In the following discussion, ETi denotes the transverse energy, ηi the pseudorapidity and ϕi the azimuthal angle of object i.

Monte Carlo simulations

Samples of events were generated to determine the response of the detector to jets of hadrons and the correction factors necessary to obtain the hadron-level jet cross sections. In addition, these samples were used to estimate hadronisation corrections to the NLO calculations (see Section 8).

The MC programs Pythia 6.146 [47] and Herwig 6.504 [48] were used to generate resolved and direct photoproduction events. In both generators, the partonic processes are simulated using leading-order matrix

Transverse-energy and acceptance corrections

The comparison of the reconstructed jet variables for the hadronic and the calorimetric jets in MC-simulated events showed that no correction was needed for the jet pseudorapidity and azimuth. However, ET,caljet underestimates the corresponding hadronic-jet transverse energy by 14% with an r.m.s. of 10%. This underestimation is mainly due to the energy lost by the particles in the inactive material in front of the CAL. The transverse-energy corrections to calorimetric jets, as functions of η

Experimental uncertainties

The following sources of systematic uncertainty were considered for the measured cross sections:

  • the differences in the results obtained by using either Pythia or Herwig to correct the data for detector effects. The resulting uncertainty was typically below ±4%;

  • the effect of the CAL energy-scale uncertainty on Wγp was estimated by varying yJB by ±1% in simulated events. The uncertainty in the cross sections was below ±1% at low ETjet, increasing to ±3% at high ETjet;

  • the effect of the

Next-to-leading-order QCD calculations

The NLO QCD (O(αs2)) calculations used in the analysis presented here were computed using the program by Klasen, Kleinwort and Kramer [59]. The calculations use the phase-space-slicing method [60] with an invariant-mass cut to isolate the singular regions of the phase space. The number of flavours was set to five and the renormalisation (μR) and factorisation (μF) scales were chosen to be μR=μF=μ=ETjet. The strong coupling constant was calculated at two loops with ΛMS¯=226 MeV, corresponding to

Results

Single- and double-differential inclusive-jet cross sections were measured in the kinematic region given by Q2<1 GeV2 and 142<Wγp<293 GeV. These cross sections include every jet of hadrons with ETjet>17 GeV and 1<ηjet<2.5 in each event. The jets were reconstructed using either the kT, the anti-kT or the SIScone jet algorithms. The x region covered by the measurements was determined to be 3103<x<0.95.

Summary and conclusions

Measurements of differential cross sections for inclusive-jet photoproduction at a centre-of-mass energy of 318 GeV using an integrated luminosity of 300 pb1 collected by the ZEUS detector have been presented. The cross sections refer to jets of hadrons of ETjet>17 GeV and 1<ηjet<2.5 identified in the laboratory frame with the kT, anti-kT or SIScone jet algorithms with jet radius R=1. The cross sections are given in the kinematic region of Q2<1 GeV2 and 142<Wγp<293 GeV.

Measurements of

Acknowledgements

We thank the DESY Directorate for their strong support and encouragement. The remarkable achievements of the HERA machine group were essential for the successful completion of this work and are greatly appreciated. We are grateful for the support of the DESY computing and network services. The design, construction, installation and running of the ZEUS detector were made possible owing to the ingenuity and effort of many people who are not listed as authors.

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  • Cited by (0)

    1

    Supported by the US Department of Energy.

    2

    Supported by the Italian National Institute for Nuclear Physics (INFN).

    3

    Supported by the German Federal Ministry for Education and Research (BMBF), under contract No. 05 H09PDF.

    4

    Supported by the Science and Technology Facilities Council, UK.

    5

    Supported by an FRGS grant from the Malaysian government.

    6

    Supported by the US National Science Foundation. Any opinion, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

    7

    Supported by the Polish Ministry of Science and Higher Education as a scientific project No. DPN/N188/DESY/2009.

    8

    Supported by the Polish Ministry of Science and Higher Education and its grants for Scientific Research.

    9

    Supported by the German Federal Ministry for Education and Research (BMBF), under contract No. 05h09GUF, and the SFB 676 of the Deutsche Forschungsgemeinschaft (DFG).

    10

    Supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and its grants for Scientific Research.

    11

    Supported by the Korean Ministry of Education and Korea Science and Engineering Foundation.

    12

    Supported by FNRS and its associated funds (IISN and FRIA) and by an Inter-University Attraction Poles Programme subsidised by the Belgian Federal Science Policy Office.

    13

    Supported by the Spanish Ministry of Education and Science through funds provided by CICYT.

    14

    Supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).

    15

    Partially supported by the German Federal Ministry for Education and Research (BMBF).

    16

    Supported by RF Presidential grant No. 4142.2010.2 for Leading Scientific Schools, by the Russian Ministry of Education and Science through its grant for Scientific Research on High Energy Physics and under contract No. 02.740.11.0244.

    17

    Supported by the Netherlands Foundation for Research on Matter (FOM).

    18

    Supported by the Israel Science Foundation.

    19

    Now at University of Salerno, Italy.

    20

    Now at Queen Mary University of London, United Kingdom.

    21

    Also funded by Max Planck Institute for Physics, Munich, Germany.

    22

    Also Senior Alexander von Humboldt Research Fellow at Hamburg University, Institute of Experimental Physics, Hamburg, Germany.

    23

    Also at Cracow University of Technology, Faculty of Physics, Mathematics and Applied Computer Science, Poland.

    24

    Supported by the research grant No. 1 P03B 04529 (2005–2008).

    25

    Supported by the Polish National Science Centre, project No. DEC-2011/01/BST2/03643.

    26

    Now at Rockefeller University, New York, NY 10065, USA.

    27

    Now at DESY group FS-CFEL-1.

    28

    Now at Institute of High Energy Physics, Beijing, China.

    29

    Now at DESY group FEB, Hamburg, Germany.

    30

    Also at Moscow State University, Russia.

    31

    Now at University of Liverpool, United Kingdom.

    32

    Now at CERN, Geneva, Switzerland.

    33

    Also affiliated with University College London, UK.

    34

    Now at Goldman Sachs, London, UK.

    35

    Also at Institute of Theoretical and Experimental Physics, Moscow, Russia.

    36

    Also at FPACS, AGH-UST, Cracow, Poland.

    37

    Partially supported by Warsaw University, Poland.

    38

    Now at Istituto Nucleare di Fisica Nazionale (INFN), Pisa, Italy.

    39

    Now at Haase Energie Technik AG, Neumünster, Germany.

    40

    Now at Department of Physics, University of Bonn, Germany.

    41

    Now at Biodiversität und Klimaforschungszentrum (BiK-F), Frankfurt, Germany.

    42

    Also affiliated with DESY, Germany.

    43

    Also at University of Tokyo, Japan.

    44

    Now at Kobe University, Japan.

    45

    Supported by DESY, Germany.

    46

    Member of National Technical University of Ukraine, Kyiv Polytechnic Institute, Kyiv, Ukraine.

    47

    Member of National University of Kyiv, Mohyla Academy, Kyiv, Ukraine.

    48

    Partly supported by the Russian Foundation for Basic Research, grant 11-02-91345-DFG_a.

    49

    Alexander von Humboldt Professor; also at DESY and University of Oxford.

    50

    STFC Advanced Fellow.

    51

    Nee Korcsak-Gorzo.

    52

    This material was based on work supported by the National Science Foundation, while working at the Foundation.

    53

    Also at Max Planck Institute for Physics, Munich, Germany, External Scientific Member.

    54

    Now at Tokyo Metropolitan University, Japan.

    55

    Now at Nihon Institute of Medical Science, Japan.

    56

    Now at Osaka University, Osaka, Japan.

    57

    Also at Łódź University, Poland.

    58

    Member of Łódź University, Poland.

    59

    Now at Department of Physics, Stockholm University, Stockholm, Sweden.

    60

    Also at Cardinal Stefan Wyszyński University, Warsaw, Poland.

    61

    Deceased.

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