D* Production in Deep Inelastic Scattering at HERA

TL;DR

This work measures D*± production in neutral-current DIS at HERA using the ZEUS detector, testing photon-gluon fusion as the primary charm production mechanism. By reconstructing D* mesons through D0 → Kπ and a soft pion, the authors obtain differential and integrated cross sections in a defined kinematic region and extrapolate to full phase space to extract the charm structure function F2ccbar. The data favor PGF and are reasonably described by NLO QCD with GRV-type gluon densities, including the predicted rise of F2ccbar toward low x. The results provide an independent constraint on the proton gluon density at low x and complement H1 measurements in DIS and earlier photoproduction studies.

Abstract

This paper presents measurements of D^{*\pm} production in deep inelastic scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The data have been taken with the ZEUS detector at HERA. The decay channel $D^{*+}\to (D^0 \to K^- π^+) π^+ $ (+ c.c.) has been used in the study. The $e^+p$ cross section for inclusive D^{*\pm} production with $5<Q^2<100 GeV^2$ and $y<0.7$ is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region {$1.3<p_T(D^{*\pm})<9.0$ GeV and $| η(D^{*\pm}) |<1.5$}. Differential cross sections as functions of p_T(D^{*\pm}), $η(D^{*\pm}), W$ and $Q^2$ are compared with next-to-leading order QCD calculations based on the photon-gluon fusion production mechanism. After an extrapolation of the cross section to the full kinematic region in p_T(D^{*\pm}) and $η$(D^{*\pm}), the charm contribution $F_2^{c\bar{c}}(x,Q^2)$ to the proton structure function is determined for Bjorken $x$ between 2 $\cdot$ 10$^{-4}$ and 5 $\cdot$ 10$^{-3}$.

Figures (4)

• Figure 1: (a) LO diagram for photon-gluon fusion (PGF). (b) The ($x,Q^2$) plane with the $Q^2$, $y$ region and bins chosen for the $F_2^{c\bar{c}}$ analysis. Large dots correspond to the $D^\ast$ candidates; small dots correspond to a subsample of DIS events. The second constant $y$ line corresponds to $y=0.2$. (c) Normalized $e^+p \rightarrow e^+\hbox{$D^{*\pm}$} X$ cross section for $5<Q^2<100$ GeV$^2$, $y<0.7$ in the restricted kinematic region $1.3<\hbox{$p_{\rm T}$}(\hbox{$D^{*}$})<9.0$ GeV and $|\eta(\hbox{$D^{*}$})|<1.5$ as a function of $x_{D^\ast}$. The inner error bars show the statistical errors and the outer ones the statistical and systematic errors added in quadrature. The horizontal bars represent the bin widths. The prediction for PGF as calculated with AROMA (solid histogram) and the charm sea contribution as calculated with LEPTO 6.1 selecting QPM events (dashed histogram) are also shown (see text).
• Figure 2: (a) $\Delta\,M$ mass distribution for $K\pi$ combinations in the $M(K\pi)$ signal region (1.80 $<~M(K\pi)~<$ 1.92 GeV), full dots, and for the wrong charge combinations, solid histogram. (b) $M(K\pi)$ mass distribution for the $K\pi\pi_s$ combinations in the $\Delta\,M\,=\,M(K\pi\pi_s)\,-\,M(K\pi)$ signal region (143 $<~\Delta~M~<$ 148 MeV), full dots, and for the wrong charge combinations, solid histogram. The solid lines in both figures show the result from the fits (see text for details).
• Figure 3: Differential $e^+p \rightarrow e^+\hbox{$D^{*\pm}$} X$ cross sections for $5<Q^2<100$GeV$^2$, $y<0.7$ in the restricted kinematic region $1.3<\hbox{$p_{\rm T}$}(\hbox{$D^{*}$})<9.0$ GeV and $|\eta(\hbox{$D^{*}$})|<1.5$ as a function of $p_{\rm T}$($D^{*}$) (a), $\eta(\hbox{$D^{*}$})$ (b), $W$ (c) and $Q^2$ (d). The inner error bars show the statistical errors and the outer ones correspond to the statistical and systematic errors added in quadrature. The horizontal bars represent the bin widths. The NLO QCD prediction for different charm quark masses is shown by the band (see text). The LO prediction for the GRV(LO) gluon density is shown by the histogram (see text). The predicted cross sections in (a) and (b) are shown without the $p_{\rm T}$($D^{*}$) and $\eta(\hbox{$D^{*}$})$ cuts respectively.
• Figure 4: The charm contribution, $F_2^{c\bar{c}}$, to the proton structure function $F_2$ as derived from the inclusive $D^\ast$(ZEUS and H1) and $D^0$(H1) production compared with the NLO QCD predictions based on the GRV parton distribution using different charm quark masses for $Q^2$=7, 12, 25 and 45 GeV$^2$ The upper (lower) limit of the band corresponds to a charm quark mass of 1.35 (1.7) GeV (see text). The results from the EMC collaboration are shown as crosses. For the ZEUS data, the inner error bars show the statistical errors and the outer ones correspond to the statistical and systematic errors added in quadrature. The error bars from H1 show the statistical and systematic errors added in quadrature. The error bars for EMC are within the symbol.