Elsevier

Physics Letters B

Volume 487, Issues 1–2, 10 August 2000, Pages 53-73
Physics Letters B

Measurement of the proton structure function F2 at very low Q2 at HERA

https://doi.org/10.1016/S0370-2693(00)00793-0Get rights and content

Abstract

A measurement of the proton structure function F2(x,Q2) is presented in the kinematic range 0.045GeV2<Q2<0.65GeV2 and 6·10−7<x<1·10−3. The results were obtained using a data sample corresponding to an integrated luminosity of 3.9pb−1 in e+p reactions recorded with the ZEUS detector at HERA. Information from a silicon-strip tracking detector, installed in front of the small electromagnetic calorimeter used to measure the energy of the final-state positron at small scattering angles, together with an enhanced simulation of the hadronic final state, has permitted the extension of the kinematic range beyond that of previous measurements. The uncertainties in F2 are typically less than 4%. At the low Q2 values of the present measurement, the rise of F2 at low x is slower than observed in HERA data at higher Q2 and can be described by Regge theory with a constant logarithmic slope lnF2/ln(1/x). The dependence of F2 on Q2 is stronger than at higher Q2 values, approaching, at the lowest Q2 values of this measurement, a region where F2 becomes nearly proportional to Q2.

Introduction

A remarkable feature of the proton structure function F2(x,Q2) is its rapid rise at low x, observed by the H1 and ZEUS collaborations [1] at HERA. First observed for Q2 values above 10GeV2, the persistence of this rise down to small Q2 [2], [3], [4] challenges our understanding of QCD. In a recent publication [4], the ZEUS collaboration has discussed the transition from deep inelastic scattering to photoproduction. It was found that standard non-perturbative approaches which apply in photoproduction fail to describe the data in the region above Q2=0.9GeV2. Next-to-leading-order QCD fits are successful when taken down to these low Q2 values. As Q2 approaches 1GeV2, however, these fits yield vanishing gluon densities at low x, while the sea-quark density remains finite. Such a “valence-like” gluon distribution, vanishing as x→0, seems unnatural even at low Q2 and has led to much discussion [5]. Precise measurements at low Q2 are important in elucidating this subject.

This letter presents a measurement of F2 at low Q2 (0.045GeV2<Q2<0.65GeV2) and low x (6·10−7<x<1·10−3). The data used correspond to an integrated luminosity of 3.9pb−1 and were taken with dedicated triggers during six weeks of e+p running in 1997. Compared to a previous result51 [2], the new measurement covers a larger kinematic region with an improved statistical precision and systematic accuracy. This was made possible by the addition of a Beam Pipe Tracker in front of the Beam Pipe Calorimeter used for measuring the energy of the final-state positron at small scattering angles, and by an enhanced simulation of the hadronic final state.

Section snippets

Kinematic variables and cross sections

Inclusive deep inelastic positron-proton scattering, e+pe+X, can be described in terms of two kinematic variables, x and Q2, where x is the Bjorken scaling variable and Q2 the negative of the square of the four-momentum transfer. They are defined as Q2=−q2=−(kk′)2 and x=Q2/(2P·q), where k and P are the four-momenta of the incoming positron and proton, respectively, and k′ is the four-momentum of the scattered positron. The fractional energy transferred to the proton in its rest frame, y, is

Experimental setup and kinematic reconstruction

The ZEUS detector has been described in detail previously [10]. In the present analysis, the scattered positron was detected in the Beam Pipe Calorimeter (BPC) and Beam Pipe Tracker (BPT) [11], [12]. The BPC was installed in 1995 to enhance the acceptance of the ZEUS detector for low-Q2 events, where the positron is scattered through a small angle, and was used for a previous measurement of F2 [2]. In 1997, the BPT was installed in front of the BPC to complement the calorimetric energy

Trigger, event selection and background

The event selection was based mainly on the requirement of a well-reconstructed positron in the BPC and in the BPT, while additional cuts on the hadronic final state suppressed background and limited effects of resolution smearing (event migrations) and radiative corrections.

Events were selected online by the ZEUS three-level trigger system. The trigger required a minimum energy deposit in the BPC, a timing compatible with an ep interaction, and imposed requirements on energy deposits from the

Monte Carlo simulation

Monte Carlo (MC) simulations were used to characterize the accuracy of the kinematic reconstruction, to determine the efficiency of selecting events, to estimate the background rate, and to extract F2. Non-diffractive processes including first-order QED radiative corrections were simulated using the HERACLES 4.6.1 program with the DJANGOH 1.1 interface [19] to the QCD programs. The program RAPGAP 2.06 [20] was used to simulate diffractive processes in which the incoming proton emits a Pomeron

Results

The measurement of F2 presented here uses data in the kinematic region 0.04GeV2<Q2<0.74GeV2 and 5.3·10−7<x<1.6·10−3, corresponding to 0.005<y<0.84. The values of F2 extracted in 70 bins are listed in Table 1. Fig. 3 shows these F2 values as a function of x for different bins of Q2, together with previous ZEUS and H1 data at low Q2 [2], [4], [3] and with data at higher x from the fixed-target experiment E665 [30]. The curve denoted as “ZEUS Regge fit” represents the parameterization of Eq. (5),

Summary

The proton structure function F2(x,Q2) has been measured in the kinematic range 0.045GeV2<Q2<0.65GeV2 and 6·10−7<x<1·10−3, using an e+p data sample corresponding to an integrated luminosity of 3.9pb−1. The addition of a Beam Pipe Tracker in front of the Beam Pipe Calorimeter and an enhanced simulation of the hadronic final state have resulted in coverage of a larger kinematic region and in improved statistical precision and systematic accuracy compared to previous results obtained with the Beam

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 by the ingenuity and effort of many people from DESY and home institutes who are not listed as authors. It is a pleasure to thank H. Spiesberger and H. Jung for useful discussions.

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    49

    Supported by the US Department of Energy.

    39

    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.

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