Measurement of high-Q2 charged current cross sections in e−p deep inelastic scattering at HERA

doi:10.1016/S0370-2693(02)02093-2

Physics Letters B

Volume 539, Issues 3–4, 18 July 2002, Pages 197-217

https://doi.org/10.1016/S0370-2693(02)02093-2 Get rights and content

Abstract

Cross sections for e⁻p charged current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV with an integrated luminosity of 16.4 pb⁻¹ using the ZEUS detector at HERA. Differential cross sections dσ/dQ², dσ/dx and dσ/dy are presented for Q²>200 GeV². In addition, $d^{2} σ/(dx dQ^{2})$ was measured in the kinematic range $280 GeV^{2} <Q^{2} <30 000 GeV^{2}$ and 0.015<x<0.42. The predictions of the Standard Model agree well with the measured cross sections. The mass of the W boson, determined from a fit to dσ/dQ², is M_W=80.3±2.1(stat.)±1.2(syst.)±1.0(PDF) GeV.

Introduction

Deep inelastic scattering (DIS) of leptons on nucleons has been vital in the development of our understanding of the structure of the nucleon. In the Standard Model (SM), charged current (CC) DIS is mediated by the exchange of the W boson. In contrast to neutral current (NC) interactions, where all quark and antiquark flavours contribute, only up-type quarks and down-type antiquarks participate at leading order in e⁻p CC DIS reactions. This makes such interactions a powerful tool for flavour-specific investigation of the parton distribution functions (PDFs). Since only left-handed quarks and right-handed antiquarks contribute to CC DIS at HERA, the distribution of the electron–quark centre-of-mass scattering angle, $θ^{∗}$ , is a sensitive probe of the chiral structure of the weak interaction.

Measurements of the CC DIS cross sections at HERA have been reported previously by the H1 [1], [2] and ZEUS [3], [4] collaborations. These data extended the kinematic region covered by fixed-target neutrino–nucleus scattering experiments [5] by about two orders of magnitude in the negative square of the four-momentum transfer, Q². In addition, the double-differential e⁺p CC DIS cross section, $d^{2} σ/(dx dQ^{2})$ , where x is the Bjorken scaling variable, was measured for the first time at high Q² by the HERA collider experiments [6], [7]. The mass of the exchanged boson in the space-like domain, extracted from a fit to the differential cross section dσ/dQ², was consistent with the mass of the W boson measured in time-like processes at LEP and at the Tevatron [8].

This Letter presents measurements of the e⁻p CC DIS single-differential cross-sections dσ/dQ², dσ/dx and dσ/dy, as well as $d^{2} σ/(dx dQ^{2})$ . The results are compared to the expectations of the SM. The measurements are based on 16.4 pb⁻¹ of data collected during the running periods in 1998 and 1999 when HERA collided electrons of energy 27.5 GeV with protons of energy 920 GeV, yielding a centre-of-mass energy of 318 GeV. The data represent an increase of a factor of 20 in integrated luminosity over the previous ZEUS e⁻p measurement [4].

Section snippets

Standard model prediction

The electroweak Born-level CC DIS differential cross section, $d^{2} σ^{CC}_{Born} /(dx dQ^{2})$ , for the reaction e⁻p→ν_eX, with longitudinally unpolarised beams, can be expressed as [9] $d^{2} σ^{CC}_{Born} (e^{−} p) dx dQ^{2} = G^{2}_{F} 4πx M_{W}^{4} (Q^{2} +M_{W}^{2})^{2} Y_{+} F_{2}^{CC} x,Q^{2} −y^{2} F_{L}^{CC} x,Q^{2} +Y_{−} xF_{3}^{CC} x,Q^{2},$ where G_F is the Fermi constant, M_W is the mass of the W boson, x is the Bjorken scaling variable, y=Q²/xs and Y_±=1±(1−y)². The centre-of-mass energy in the electron–proton collision is given by $s =2 E_{e} E_{p}$ , where E_e and E_p are the electron and proton beam

The ZEUS experiment

A detailed description of the ZEUS detector can be found elsewhere [12]. A brief outline of the components most relevant for this analysis is given below.

Charged particles are tracked in the central tracking detector (CTD) [13], which operates in a magnetic field of 1.43 T provided by a thin superconducting coil. The CTD consists of 72 cylindrical drift chamber layers, organised in nine superlayers covering the polar-angle⁵⁴

Monte Carlo simulation

Monte Carlo (MC) simulation was used to determine the efficiency for selecting events and the accuracy of kinematic reconstruction, to estimate the ep background rates and to extract cross sections for the full kinematic region. A sufficient number of events was generated to ensure that the statistical uncertainties arising from the MC simulation were negligible compared to those of the data. The MC samples were normalised to the total integrated luminosity of the data.

The ZEUS detector

Reconstruction of kinematic variables

The principal signature of CC DIS at HERA is the presence of a large missing transverse momentum, P_T,miss, arising from the energetic final-state neutrino that escapes detection. The quantity P_T,miss was calculated from $P_{T,miss}^{2} =P_{X}^{2} +P_{Y}^{2} = ∑ i E_{i} sin θ_{i} cos φ_{i}^{2} + ∑ i E_{i} sin θ_{i} sin φ_{i}^{2},$ where the sums run over all calorimeter energy deposits, E_i (uncorrected in the trigger, but corrected in the offline analysis for energy loss in inactive material etc. [27]) and θ_i and φ_i are the polar and azimuthal angles of

Event selection

Charged current DIS candidates were selected by requiring a large P_T,miss and a reconstructed event vertex consistent with an ep interaction. The main sources of background come from NC scattering and high-E_T photoproduction. The energy resolution of the CAL or the energy that escapes detection can lead to significant missing transverse momentum. Events not from ep collisions, such as beam-gas interactions, beam-halo muons or cosmic rays can also cause substantial apparent imbalance in the

Cross section determination

Monte Carlo events were generated according to Eq. (1), including electroweak radiative effects. The value of the cross section, at a fixed point within a bin, was obtained from the ratio of the number of observed events, from which the estimated background had been subtracted, to the number of events predicted by the MC simulation, multiplied by the cross section obtained using Eq. (1). Consequently, the acceptance, bin-centering and radiative corrections were all taken from the MC simulation.

Systematic uncertainties

Results

The single-differential cross-sections dσ/dQ², dσ/dx and dσ/dy for Q²>200 GeV² are shown in Fig. 2 and compiled⁵⁵ in Table 1 . The cross sections dσ/dQ² and dσ/dx were extrapolated to the full y range using the SM predictions with CTEQ5D PDFs. The SM cross sections derived from Eq. (1)

Electroweak analysis

Eq. (1) shows that the magnitude of the CC DIS cross section is determined by G_F and the PDFs. The fall in the cross section with increasing Q² is dominated by the propagator term, M_W⁴/(Q²+M_W²)². Fig. 3 shows that the Q² dependence of the PDFs is small by comparison. An electroweak analysis, performed by fitting dσ/dQ² with G_F fixed at the PDG [8] value of 1.16639×10⁻⁵ GeV⁻² and M_W treated as a free parameter gives $M_{W} =80.3±2.1(stat.)±1.2(syst.)±1.0(PDF) GeV,$ where the third uncertainty was

Summary

Differential cross sections for charged current deep inelastic scattering, e⁻p→ν_eX, have been measured for Q²>200 GeV² using 16.4 pb⁻¹ of data collected with the ZEUS detector during the period 1998 to 1999. The double-differential cross section $d^{2} σ/(dx dQ^{2})$ has been measured in the kinematic range $280 GeV^{2} <Q^{2} <30 000 GeV^{2}$ and 0.015<x<0.42. The chiral structure of the Standard Model was investigated by plotting the double-differential cross section as a function of (1−y)². The Standard Model gives

Acknowledgements

We appreciate the contributions to the construction and maintenance of the ZEUS detector of the many people who are not listed as authors. The HERA machine group and the DESY computing staff are especially acknowledged for their success in providing excellent operation of the collider and the data-analysis environment. We thank the DESY directorate for their strong support and encouragement.

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⁵⁰: Supported by the US Department of Energy.

⁴¹: Supported by the Italian National Institute for Nuclear Physics (INFN).

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Measurement of high-Q2 charged current cross sections in e−p deep inelastic scattering at HERA

Abstract

Introduction

Section snippets

Standard model prediction

The ZEUS experiment

Monte Carlo simulation

Reconstruction of kinematic variables

Event selection

Cross section determination

Systematic uncertainties

Results

Electroweak analysis

Summary

Acknowledgements

Phys. Lett. B

Z. Phys. C

Phys. Rev. Lett.

Z. Phys. C

Acta Phys. Pol. B

Comput. Phys. Commun.

Comput. Phys. Commun.

Comput. Phys. Commun.

Comput. Phys. Commun.

Comput. Phys. Commun.

Comput. Phys. Commun.

Comput. Phys. Commun.

Nucl. Phys. B

Nucl. Instrum. Methods A

Eur. Phys. J. C

J. Phys. G

Nucl. Phys. (Proc. Suppl.) B

Phys. Lett. B

Z. Phys. C

Measurement of high-Q² charged current cross sections in e⁻p deep inelastic scattering at HERA