Elsevier

Physics Letters B

Volume 487, Issues 3–4, 17 August 2000, Pages 273-288
Physics Letters B

Measurement of exclusive ω electroproduction at HERA

https://doi.org/10.1016/S0370-2693(00)00794-2Get rights and content

Abstract

The exclusive electroproduction of ω mesons, epeωp, has been studied in the kinematic range 3<Q2<20 GeV2, 40<W<120 GeV and |t|<0.6 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 37.7 pb−1. The ω mesons were identified via the decay ωπ+ππ0. The exclusive cross section in the above kinematic region is σepeωp=0.108±0.014(stat.)±0.026(syst.) nb. The reaction epeφp, φπ+ππ0, has also been measured. The cross sections, as well as the ratios σγp→ωpγp→ρ0p and σγp→ωpγp→φp, are presented as a function of W and Q2. Thus, for the first time, the properties of ω electroproduction can be compared to those of ρ0,φ and J/ψ electroproduction at high W.

Introduction

The exclusive photoproduction of vector mesons, ρ0, ω, φ and J/ψ, has been studied over a wide range of the photon-proton centre-of-mass energy W [1], [2], [3], [4], [5], [6], [7], [8]. For high W and for light vector mesons, these reactions display features characteristic of soft diffractive processes, and are well described within the framework of the vector dominance model (VDM) [9] and Regge phenomenology [10]. In the VDM, the photon is assumed to fluctuate into a vector meson (VM), which subsequently interacts with the target proton. The expectation of the VDM and Regge phenomenology is that the cross sections for exclusive VM production will be in proportions determined by the couplings of the photon to the vector mesons and by the elastic VM-proton cross sections. The couplings, in particular, are determined by the quark current decomposition of the photon and by the quark wave function of the VM. The SU(4) prediction, which ignores the VM mass differences, is that the coupling strengths of the photon to the ρ0,ω,φ and J/ψ mesons are in the ratio48 9:1:2:8. Both at fixed target experiments at W≳8 GeV [1] and at HERA [5], [6], [7], the ratio ω/ρ0 of the exclusive photoproduction cross sections is found to be approximately 1:9, while the φ/ρ0 and (J/ψ)/ρ0 ratios are found to be smaller than the SU(4) predicted ratios of photon-VM couplings.

For high-energy exclusive photoproduction of heavy VMs and for electroproduction of all VMs at large virtualities, Q2, of the exchanged virtual photon, γ, an alternative production mechanism has been proposed [11], [12], [13]: the virtual photon fluctuates into a qq̄ pair before arriving at the target, and it is this qq̄ state that scatters elastically off the proton. The VM is formed well after the interaction. If the transverse size of the qq̄ fluctuation is small enough, the interaction is expected to become flavour independent and the gluons of the proton are resolved [14]. Such a configuration occurs when the mass of the quarks is large or if the photon with large virtuality is longitudinally polarized. In these cases, perturbative QCD can be applied [11], [12], [15], [16], [17] and the expectation for the ratios of VM production cross sections is given by the ratios of the couplings.

The HERA measurements of exclusive electroproduction of ρ0 [18], [19], φ [20], [21] and J/ψ [18], [22] mesons, as well as the exclusive photoproduction of J/ψ [7], [8] and ϒ [8], [23] mesons, are in broad agreement with the expectations from QCD. In particular, the sharp decrease of the cross sections with Q2, the strong rise of the J/ψ cross section with W, the change in the W dependence and the broadening of the t distribution of the ρ0 cross section with increasing Q2 show the expected behaviour. With increasing Q2, the φ/ρ0 and (J/ψ)/ρ0 cross-section ratios rise towards the expected values.

This letter reports the first measurement of the exclusive ω electroproduction cross section for 40<W<120 GeV and 3<Q2<20 GeV2. The ω mesons were identified via the decay ωπ+ππ0. Measurements of the exclusive φ electroproduction cross section using the π+ππ0 final state are also reported. The compatibility of the ω/ρ0 and ω/φ cross-section ratios with the SU(4) hypothesis is investigated. Thus, the data presented here permits, for the first time, a discussion of the ratios of the production cross sections of all VMs related by SU(4).

Section snippets

Experimental conditions

The measurements were performed at the ep collider HERA with the ZEUS detector using an integrated luminosity of 37.7pb−1. During 1996 and 1997 HERA operated with a proton energy of 820 GeV and a positron energy of 27.5 GeV. A detailed description of the ZEUS detector can be found elsewhere [24]. The main components used in this analysis are described below.

The high-resolution uranium-scintillator calorimeter CAL [25] consists of three parts: forward

Kinematics and cross sections

Fig. 1 shows a schematic diagram for the reaction:e(k)p(P)→e(k′)V(v)p(P′)where V is an ω or φ meson and k, k′, P, P′, and v are the four-momenta of the incident positron, scattered positron, incident proton, scattered proton and vector meson, respectively. The kinematic variables used to describe exclusive VM production are:

  • Q2=−q2=−(kk′)2, the negative squared four-momentum of the virtual photon;

  • W2=(q+P)2, the squared centre-of-mass energy of the photon-proton system;

  • y=(P·q)/(P·k), the

Event selection

Events were selected online with a three-level trigger system. Offline, the following requirements were imposed to select candidates for the reaction e+pe+π+ππ0p:

  • the energy of the scattered positron, measured in the CAL, was required to be greater than 10 GeV;

  • the interaction vertex was required to have its Z coordinate within ±50 cm of the nominal interaction point and to lie within a transverse distance of 0.6 cm of the nominal beam position;

  • in addition to a scattered positron, two

Reconstruction of the π0

The two-photon decay of the π0 was reconstructed using signals in the calorimeter combined into condensates, which are objects consisting of adjacent calorimeter cells. To reject background from uranium radioactivity in the calorimeter cells, a minimum individual cell energy of 100 MeV was required. Only condensates consisting solely of cells in the EMC section of the CAL were used. The position and energy, Eγ, of these condensates were then used to determine the invariant mass, Mγγ, of the

Monte Carlo simulation and acceptance corrections

A MC generator for exclusive electroproduction of light VMs [28], interfaced to HERACLES [29] to simulate radiative effects, was used to evaluate the acceptance. In this generator the cross sections were parameterised over the entire W and Q2 range using the ZEUS data on ρ0 electroproduction [18] in terms of σγp→VpL and σγp→VpT, where V=ρ0,ω or φ; s-channel helicity conservation was assumed. The MC samples generated in this way display good agreement with the present data, for both ω and φ

Background

The main source of resonant background to the exclusive reaction epeVp is the proton-dissociative reaction epeVN, where N is a hadronic system produced by the dissociation of the proton. The proton-dissociative events in which the hadronic system N deposits energy around the beam pipe in the FCAL are removed by the selection criteria; the rest are misidentified as epeVp reactions and therefore have to be subtracted. The proton-dissociative fraction of events was estimated in the ZEUS ρ0

Analysis of the mass spectrum

The invariant-mass (M3π) spectrum for the π+ππ0 system after all offline cuts is shown in Fig. 3a. In addition to the ω signal, a second peak is visible from the process epeφp (φπ+ππ0). The spectrum was fitted with the function:f(M)=g1(M)+g2(M)+ζ(M)where g1 and g2 are Gaussian functions to describe the ω and φ mass peaks, and ζ is a second-order polynomial representing the background. The latter is mainly due to the background under the π0 peak, as well as to other processes in

Conclusions

The reaction epeωp has been studied in the +ππ0p final state for 3<Q2<20 GeV2, 40<W<120 GeV and |t|<0.6 GeV2. The cross-sections σepeωp and σγp→ωp have been measured for the first time at high Q2. In addition, the cross-sections σepeφp and σγp→φp have been measured using the same final state.

The cross-section ratio ω/φ=0.49±0.15(stat.)±0.12(syst.), obtained with the present data, is consistent with the SU(4) expectation.

For the light vector mesons, ρ0,ω and φ, the cross-section ratios

Acknowledgements

We thank the DESY Directorate for their strong support and encouragement, and the HERA machine group for their diligent efforts. We are grateful for the support of the DESY computing and network services. The design, construction and installation of the ZEUS detector have been made possible owing to the ingenuity and effort of many people from DESY and home institutes who are not listed as authors. It is also a pleasure to thank M. Strikman for many useful discussions.

References (31)

  • M. Derrick

    Phys. Lett. B

    (1996)
  • M. Derrick

    Phys. Lett. B

    (1996)
  • T.H. Bauer et al., Rev. Mod. Phys. 50 (1978) 261; Rev. Mod. Phys. 51 (1979) 407...
  • R.M. Egloff et al., Phys. Rev. Lett. 43 (1979) 657; R.M. Egloff et al., Phys. Rev. Lett. 43 (1979) 1545; D. Aston et...
  • ZEUS Collaboration, M. Derrick et al., Z. Phys. C 69 (1995) 39; H1 Collaboration, S. Aid et al., Nucl. Phys. B 463...
  • J. Breitweg

    Eur. Phys. J. C

    (1998)
  • M. Derrick

    Z. Phys. C

    (1996)
  • J. Breitweg

    Z. Phys. C

    (1997)
  • H1 Collaboration, C. Adloff et al., DESY Report 00–037 (2000), to be published in Phys. Lett....
  • J.J. Sakurai

    Phys. Rev. Lett.

    (1969)
  • P.D.B. Collins, An Introduction to Regge Theory and High Energy Physics, Cambridge University Press,...
  • S.J. Brodsky

    Phys. Rev. D

    (1994)
  • L. Frankfurt

    Phys. Rev. D

    (1996)
  • W. Köpf et al., in: G. Ingelman et al. (Eds.), Proc. Workshop on Future Physics at HERA, DESY, Hamburg, Germany, 1996,...
  • J.C. Collins et al.

    Phys. Rev. D

    (1997)
  • Cited by (63)

    View all citing articles on Scopus
    46

    Supported by the US Department of Energy.

    36

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

    1

    Now visiting scientist at DESY.

    2

    Also at IROE Florence, Italy.

    3

    Now at Univ. of Salerno and INFN Napoli, Italy.

    4

    Supported by Worldlab, Lausanne, Switzerland.

    View full text