Measurement of F_2^ccbar and F_2^bbbar at High Q^2 using the H1 Vertex Detector at HERA

TL;DR

This work measures inclusive charm and beauty contributions to DIS structure functions at high $Q^2$ using the H1 vertex detector, leveraging track impact parameters to statistically separate $c$ and $b$ hadron decays. By fitting lifetime-significance templates to data and cross-checking with a secondary-vertex method, the authors extract $F_2^{c\bar{c}}$ and $F_2^{b\bar{b}}$ in several kinematic bins, and report integrated cross sections with minimal extrapolation. The results, including the first $F_2^{b\bar{b}}$ measurement, are consistent with NLO QCD predictions across VFNS and FFNS schemes, and are in agreement with ZEUS $D^*$-based charm results. The analysis demonstrates the robustness of lifetime-based heavy-flavour tagging in high-$Q^2$ DIS and provides important constraints on heavy-flavour PDFs at HERA.

Abstract

Measurements are presented of inclusive charm and beauty cross sections in e^+p collisions at HERA for values of photon virtuality Q^2 > 150 GeV^2 and of inelasticity 0.1 < y < 0.7. The charm and beauty fractions 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. The data are divided into four regions in Q^2 and Bjorken x, and values for the structure functions F_2^{c\bar{c}} and F_2^{b\bar{b}} are obtained. The results are found to be compatible with the predictions of perturbative quantum chromodynamics.

Figures (9)

• Figure 1: The polar angle distribution (a) and transverse momentum distribution (b) of all HFS CST tracks. Included in the figure is the expectation from the RAPGAP Monte Carlo simulation, showing the contributions from the various quark flavours after applying the scale factors obtained from the fit to the subtracted significance distributions of the data (see section \ref{['sec:fit']}).
• Figure 2: The polar angle distribution (a) and transverse momentum distribution (b) of the highest $p_T$ jet which contains at least one reconstructed CST track within a cone of radius 1. If there are no reconstructed jets the complete hadronic final state is used to define the jet axis. Included in the figure is the expectation from the RAPGAP Monte Carlo simulation showing the contributions from the various quark flavours after applying the scale factors obtained from the fit to the subtracted significance distributions of the data.
• Figure 3: The number of reconstructed central silicon tracker (CST) tracks per event associated to the jet axis. Each CST track is required to have at least two CST hits and $p_T>0.5~{\rm GeV}$. Included in the figure is the expectation from the RAPGAP Monte Carlo simulation, showing the contributions from the various quark flavours after applying the scale factors obtained from the fit to the subtracted significance distributions of the data.
• Figure 4: The signed impact parameter $\delta$ of a track to the primary vertex in the $x$--$y$ plane (a) and the significance $\delta / \sigma (\delta)$ (b), where $\sigma(\delta)$ is the error on $\delta$, for all CST tracks associated to the jet axis. The cut $|\delta| < 0.1$ cm has been applied in figure (b). Included in the figure is the expectation from the RAPGAP Monte Carlo simulation, showing the contributions from the various quark flavours after applying the scale factors obtained from the fit to the subtracted significance distributions of the data.
• Figure 5: The significance $S_1=$$\delta /\sigma(\delta)$ distribution per event (a) for events that contain one reconstructed CST track associated to the jet axis and the significance $S_2= \delta /\sigma (\delta)$ distribution per event (b) of the track with the second highest absolute significance for events with $\ge 2$ reconstructed CST tracks associated to the jet. Included in the figure is the expectation from the RAPGAP Monte Carlo simulation, showing the contributions from the various quark flavours after applying the scale factors obtained from the fit to the subtracted significance distributions of the data.