Measurement of High-Q^2 Charged-Current e^+p Deep Inelastic Scattering Cross Sections at HERA

TL;DR

The paper reports a precision measurement of high-$Q^2$ CC deep inelastic scattering in $e^+p$ collisions at HERA with the ZEUS detector, using 47.7 pb$^{-1}$ to map $d\sigma/dQ^2$, $d\sigma/dx$, $d\sigma/dy$, and $d^2\sigma/dxdQ^2$ across $Q^2=200$–$6\times10^4$ GeV$^2$. It employs a detailed MC-driven unfolding approach, robust background rejection, and careful radiative corrections, enabling a direct test of SM predictions with modern PDFs (CTEQ4D and NLO fits). The results show good agreement with SM expectations, demonstrate the necessity of both quark and antiquark PDFs in CC DIS, and reveal the expected transition where weak and electromagnetic forces become comparable at high $Q^2$ due to the $W$ propagator. An electroweak analysis of the $d\sigma/dQ^2$ distribution yields values for $G_F$ and $M_W$ consistent with other determinations, thereby validating the SM in the space-like regime and across a wide range of momentum transfer.

Abstract

The e^+p charged-current deep inelastic scattering cross sections, $dσ/dQ^2$ for Q^2 between 200 and 60000 GeV^2, and $dσ/dx$ and $dσ/dy$ for Q^2 > 200 GeV^2, have been measured with the ZEUS detector at HERA. A data sample of 47.7 pb^-1, collected at a center-of-mass energy of 300 GeV, has been used. The cross section $dσ/dQ^2$ falls by a factor of about 50000 as Q^2 increases from 280 to 30000 GeV^2. The double differential cross section $d^2σ/dxdQ^2$ has also been measured. A comparison between the data and Standard Model (SM) predictions shows that contributions from antiquarks ($\bar{u}$ and $\bar{c}$) and quarks (d and s) are both required by the data. The predictions of the SM give a good description of the full body of the data presented here. A comparison of the charged-current cross section $dσ/dQ^2$ with the recent ZEUS results for neutral-current scattering shows that the weak and electromagnetic forces have similar strengths for Q^2 above $M^2_W, M^2_Z$. A fit to the data for $dσ/dQ^2$ with the Fermi constant $G_F$ and $M_W$ as free parameters yields $G_F = (1.171 \pm 0.034 (stat.) ^{+0.026}_{-0.032} (syst.) ^{+0.016}_{-0.015} (PDF)) \times 10^{-5} GeV^{-2}$ and $M_W = 80.8 ^{+4.9}_{-4.5} (stat.) ^{+5.0}_{-4.3} (syst.) ^{+1.4}_{-1.3} (PDF) GeV$. Results for $M_W$, where the propagator effect alone or the SM constraint between $G_F$ and $M_W$ have been considered, are also presented.

Figures (10)

• Figure 1: (a) A schematic diagram of charged-current positron-proton scattering. (b) A view of a charged-current candidate event in the ZEUS detector, projected in the plane parallel to the beam. The filled boxes indicate energy deposits in the calorimeter. The transverse momentum imbalance can be clearly seen in the calorimeter and also from the tracks of charged particles measured in the central tracking detector.
• Figure 2: Distribution of the selected CC candidates in the $(x,Q^2)$ plane. Open (full) circles show the events selected with (without) tracking vertex. The curve shows the $P_T\space/\space$ cut of 12 GeV. The bin boundaries are shown by the dotted lines, delimited by the diagonal dotted line of the kinematic limit, $y=1$. The bins used in the double differential cross section measurement are marked with solid lines.
• Figure 3: Comparison of the final CC data sample (solid points) with the expectations of the MC (histograms), normalized to the luminosity of the data. The distributions of (a) the missing transverse momentum, $P_T\space/\space$, (b) the variable $\delta$, (c) $P_T\space/\space/E_T$, the ratio of missing transverse momentum to total transverse energy and (d) the variable $\gamma_h$, are shown. In (e) and (f), the distributions of the $Z$ position of the event vertex and the number of tracks assigned to the primary vertex, N$_{\rm VT}$, are shown for selected events with CTD vertex (see Sect. \ref{['ss:StanEvSel']}). In (g) and (h), the distribution of $Z_{\rm VTX}$ and $E_{\rm FCAL}$ are shown for events passing the selection with the timing vertex (see Sect. \ref{['ss:LowGEvSel']}).
• Figure 4: (a) The $e^+p$ CC DIS Born cross section $d\sigma/dQ^2$ for data (solid points) and the Standard Model (SM) expectation evaluated using the CTEQ4D PDFs. (b) The ratio of the measured cross section $d\sigma/dQ^2$ to the SM expectation evaluated using the CTEQ4D PDFs. The statistical errors are indicated by the inner error bars (delimited by horizontal lines), while the full error bars show the total error obtained by adding the statistical and systematic contributions in quadrature. Also shown by a dot-dashed line is the result of the NLO QCD fit together with the associated PDF uncertainties (shaded band).
• Figure 5: (a) The $e^+p$ CC DIS Born cross section $d\sigma/dx$ for data (solid points) and the Standard Model (SM) expectation evaluated using the CTEQ4D PDFs. (b) The ratio of the measured cross section $d\sigma/dx$ to the SM expectation evaluated using the CTEQ4D PDFs. The statistical errors are indicated by the inner error bars (delimited by horizontal lines), while the full error bars show the total error obtained by adding the statistical and systematic contributions in quadrature. Also shown by a dot-dashed line is the result of the NLO QCD fit together with the associated PDF uncertainties (shaded band). The dashed line represents the result of modifying the $d/u$ ratio with $\delta(d/u)=0.1x(x+1)$. The dotted line shows the prediction from MRST PDFs.