Measurement of F_2^{c\bar{c}} and F_2^{b\bar{b}} at Low Q^2 and x using the H1 Vertex Detector at HERA

TL;DR

This study extends heavy-flavour structure-function measurements to low Q^2 and small x in e+p DIS using the H1 vertex detector. By tagging heavy flavours with track impact parameters and performing a sophisticated flavor separation, it provides the first F2_bbbar measurement in this regime and a precise F2_ccbar determination, both compatible with perturbative QCD. The charm contribution is found to be around 24% of the inclusive cross section, while the beauty contribution grows from ~0.4% to ~1.5% as Q^2 increases, with results broadly agreeing with VFNS and CCFM-based predictions. Overall, the work demonstrates the viability of vertex-based heavy-flavour tagging at low Q^2 and strengthens the validation of pQCD in the heavy-flavour sector of DIS.

Abstract

Measurements are presented of inclusive charm and beauty cross sections in e^+p collisions at HERA for values of photon virtuality 12 \le Q^2 \le 60 GeV^2 and of the Bjorken scaling variable 0.0002 \le x \le 0.005. The fractions of events containing charm and beauty quarks are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 vertex detector. Values for the structure functions F_2^{c\bar{c}} and F_2^{b\bar{b}} are obtained. This is the first measurement of F_2^{b\bar{b}} in this kinematic range. The results are found to be compatible with the predictions of perturbative quantum chromodynamics and withprevious measurements of F_2^{c\bar{c}}.

Figures (9)

• Figure 1: The azimuthal difference between the $D^*$ and the quark axis for those events where the quark axis is defined (a) by a jet and (b) by $180^\circ-\phi_{\rm elec}$. Included in the figure is the expectation from the Monte Carlo simulation normalized to the number of data events.
• Figure 2: The distribution of the signed impact parameter $\delta$ of a track to the primary vertex in the $x$--$y$ plane. Included in the figure is the expectation from the DJANGO Monte Carlo simulation for light quarks and that from the RAPGAP Monte Carlo simulation for $c$ and $b$ quarks. The contributions from the various quark flavours are shown after applying the scale factors obtained from the fit to the subtracted significance distributions of the data (see section \ref{['quarkflavourseparation']}).
• Figure 3: The significance $\delta /\sigma(\delta)$ distribution (a) of the highest absolute significance track ($S_1$), (b) of the track with the second highest absolute significance ($S_2$) and (c) of the track with the third highest absolute significance ($S_3$). Included in the figure is the expectation from the DJANGO Monte Carlo simulation for light quarks and that from the RAPGAP Monte Carlo simulation for $c$ and $b$ quarks. The contributions from the various quark flavours are shown after applying the scale factors obtained from the fit to the subtracted significance distributions of the data.
• Figure 4: The subtracted significance distributions of (a) $S_1$, (b) $S_2$ (c) $S_3$. Included in the figure is the result from the fit to the data of the Monte Carlo distributions of the various quark flavours.
• Figure 5: The measured reduced cross section $\tilde{\sigma}^{c\bar{c}}$ shown as a function of $x$ for 5 different $Q^2$ values. The inner error bars show the statistical error, the outer error bars represent the statistical and systematic errors added in quadrature. The measurements of $\tilde{\sigma}^{c\bar{c}}$ from H1 at high values of $Q^2$Aktas:2004az, the measurements obtained from $D^*$ mesons from H1 and ZEUSH1DstarZEUSDstar and predictions of QCD are also shown.