Measurement of D*+- production in deep inelastic ep scattering at HERA

TL;DR

This work measures inclusive $D^{*}(2010)$ production in deep inelastic $ep$ scattering with the ZEUS detector, covering $1.5<Q^2<1000$ GeV$^2$ and $0.02<y<0.7$, and $1.5<p_T(D^*)<15$ GeV with $|(D^*)|<1.5$. The results are compared to NLO QCD predictions using the ZEUS NLO PDF fit and to MC models (Aroma, Cascade), revealing sensitivity to the gluon density and good overall agreement with NLO QCD; a component from beauty is accounted for in the cross sections. The differential cross sections are used to extract the open-charm contribution $F_2^{c\bar{c}}(x,Q^2)$ via extrapolation to full phase space, highlighting that low-$Q^2$ uncertainties are competitive with PDF uncertainties and that the data can constrain the gluon density in future PDF fits. Together, the measurements extend previous D$^{*}$ studies to higher $Q^2$ with increased precision and demonstrate the potential of DIS charm production to probe the proton's gluon structure.

Abstract

Inclusive production of $D^*(2010)$ mesons in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.9 pb$^{-1}$. The decay channel $D^{* +}\to D^0 π^+ $ with $D^0\to K^-π^+$ and corresponding antiparticle decay were used to identify $D^*$ mesons. Differential $D^*$ cross sections with $1.5<Q^2<1000$ GeV$^2$ and $0.02<y<0.7$ in the kinematic region $1.5<p_T(D^*)<15$ GeV and $|η(D^*)|<1.5$ are compared to different QCD calculations incorporating different parameterisations of the parton densities in the proton. The data show sensitivity to the gluon distribution in the proton and are reasonably well described by next-to-leading-order QCD with the ZEUS NLO QCD fit used as the input parton density in the proton. The observed cross section is extrapolated to the full kinematic region in $p_T(D^*)$ and $η(D^*)$ in order to determine the open-charm contribution, $F_2^{\rm charm}(x,Q^2)$, to the proton structure function, $F_2$. Since, at low $Q^2$, the uncertainties of the data are comparable to those from the QCD fit, the measured differential cross sections in $y$ and $Q^2$ should be used in future fits to constrain the gluon density.

Figures (8)

• Figure 1: The distribution of the mass difference, $\Delta M=(M_{K\pi\pi_s} - M_{K\pi})$, for $D^*$ candidates (solid dots). The $\Delta M$ distribution from wrong-charge combinations, normalised in the region $0.15 < \Delta M < 0.165$ GeV, is shown as the histogram. The solid line shows the result of the fit described in the text. The $M_{K\pi}$ distribution for the $D^0$ candidates in the range $0.143 < \Delta M < 0.148$ GeV is shown as an inset. The fit is the sum of a modified Gaussian to describe the signal and a second-order polynomial to describe the background.
• Figure 2: Reconstructed DIS variables for events with $D^*$ candidates (after background subtraction) for data (points) compared to detector-level Rapgap predictions (shaded histograms): (a)-(d) show the distributions for $1.5<Q^2<1000$ GeV$^2$, while (e)-(h) are the same distributions but for $40<Q^2<1000$ GeV$^2$. All histograms are normalised to unit area.
• Figure 3: Differential $D^*$ cross sections, for $e^-p$ and $e^+p$ data combined, as a function of (a) $Q^2$, (b) $x$, (c) $p_T(D^*)$ and (d) $\eta(D^*)$ compared with MC predictions. The inner error bars show the statistical uncertainties and the outer bars show the statistical and systematic uncertainties added in quadrature. Predictions from the Aroma (dashed line) and Cascade (solid line) MC programs are shown. The ratios of the cross sections for $e^-p$ and $e^+p$ data are also shown beneath each plot.
• Figure 4: Differential $D^*$ cross sections, for $e^-p$ and $e^+p$ data combined, as a function of (a) $Q^2$, (b) $x$, (c) $p_T(D^*)$ and (d) $\eta(D^*)$ compared to the NLO QCD calculation of HVQDIS. The inner error bars show the statistical uncertainties and the outer bars show the statistical and systematic uncertainties added in quadrature. Predictions from the ZEUS NLO QCD fit are shown for $m_c = 1.35$ GeV (solid line) with its associated uncertainty (shaded band) as discussed in the text. Predictions using the CTEQ5F3 PDF (dashed-dotted line) and an alternative hadronisation scheme (dotted line) are displayed. The ratios of the cross sections to the central HVQDIS prediction are also shown beneath each plot.
• Figure 5: Differential $D^*$ cross sections, for $e^-p$ and $e^+p$ data combined, as a function of (a) $p_T(D^*)$ and (b) $\eta(D^*)$ for $Q^2 > 40$ GeV$^2$. The inner error bars show the statistical uncertainties and the outer bars show the statistical and systematic uncertainties added in quadrature. Predictions from the ZEUS NLO QCD fit are shown for $m_c = 1.35$ GeV (solid line) with its associated uncertainty (shaded band) as discussed in the text. Predictions using the CTEQ5F3 PDF (dashed-dotted line) and an alternative hadronisation scheme (dotted line) are displayed. The ratios of the cross sections for $e^-p$ and $e^+p$ data and for $e^-p$ and $e^+p$ data combined to the central HVQDIS prediction are also shown beneath each plot.