Measurement of high-Q2 charged current cross sections in e−p deep inelastic scattering at HERA
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, , 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, , 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 . 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, , for the reaction e−p→νeX, with longitudinally unpolarised beams, can be expressed as [9] where GF is the Fermi constant, MW 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 , where Ee and Ep 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, PT,miss, arising from the energetic final-state neutrino that escapes detection. The quantity PT,miss was calculated from where the sums run over all calorimeter energy deposits, Ei (uncorrected in the trigger, but corrected in the offline analysis for energy loss in inactive material etc. [27]) and θi and φi are the polar and azimuthal angles of
Event selection
Charged current DIS candidates were selected by requiring a large PT,miss and a reconstructed event vertex consistent with an ep interaction. The main sources of background come from NC scattering and high-ET 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 GF and the PDFs. The fall in the cross section with increasing Q2 is dominated by the propagator term, MW4/(Q2+MW2)2. Fig. 3 shows that the Q2 dependence of the PDFs is small by comparison. An electroweak analysis, performed by fitting dσ/dQ2 with GF fixed at the PDG [8] value of 1.16639×10−5 GeV−2 and MW treated as a free parameter gives where the third uncertainty was
Summary
Differential cross sections for charged current deep inelastic scattering, e−p→νeX, 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 has been measured in the kinematic range 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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Supported by the US Department of Energy.
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Supported by the Italian National Institute for Nuclear Physics (INFN).