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Measurement of open beauty production in photoproduction at HERA

ZEUS Collaboration, J. Breitweg

TL;DR

The paper measures open beauty production in photoproduction at HERA by selecting events with two jets and an electron from semileptonic heavy-quark decays, using $p_T^{\rm rel}$ to separate $b$ from $c$ contributions. Differential cross sections in $p_T^{\rm rel}$ and $x_{\gamma}^{obs}$ are extracted with MC-based acceptance corrections, and the $b$ cross section is extrapolated to the parton level using extrapolation factors derived from MC. The resulting extrapolated cross section, $\sigma^{\rm ext}_{ep\to e^+bX}=1.6^{+0.4}_{-0.5}\,(stat.)^{+0.3}_{-0.5}\,(ext.)$ nb, lies somewhat above the NLO QCD prediction of $0.64$ nb in the same region, consistent with similar discrepancies observed in beauty production. This work confirms the need for higher-order effects or refined modelling in heavy-quark photoproduction and provides a benchmark for future theoretical and experimental studies.

Abstract

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process $e^+p -> e^+ + {dijet} + e^- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 ${\rm pb^{-1}}$. Events with photon-proton centre-of-mass energies, $W_{γp}$, between 134 and 269 GeV and a photon virtuality, Q^2, less than 1 ${\rm GeV^2}$ were selected requiring at least two jets of transverse energy $E_T^{\rm jet1(2)} >7(6)$ GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, $p_T^{\rm rel}$. The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum $p_T^{b} > p_T^{\rm min} =$ 5 GeV and pseudorapidity $|η^{b}| <$ 2. The extrapolated cross section is $1.6 \pm 0.4 (stat.)^{+0.3}_{-0.5} (syst.) ^{+0.2}_{-0.4} (ext.) {nb}$. The result is compared to a perturbative QCD calculation performed to next-to-leading order.

Measurement of open beauty production in photoproduction at HERA

TL;DR

The paper measures open beauty production in photoproduction at HERA by selecting events with two jets and an electron from semileptonic heavy-quark decays, using to separate from contributions. Differential cross sections in and are extracted with MC-based acceptance corrections, and the cross section is extrapolated to the parton level using extrapolation factors derived from MC. The resulting extrapolated cross section, nb, lies somewhat above the NLO QCD prediction of nb in the same region, consistent with similar discrepancies observed in beauty production. This work confirms the need for higher-order effects or refined modelling in heavy-quark photoproduction and provides a benchmark for future theoretical and experimental studies.

Abstract

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process {\rm pb^{-1}}W_{γp}{\rm GeV^2}E_T^{\rm jet1(2)} >7(6)p_T^{\rm rel}p_T^{b} > p_T^{\rm min} =|η^{b}| <1.6 \pm 0.4 (stat.)^{+0.3}_{-0.5} (syst.) ^{+0.2}_{-0.4} (ext.) {nb}$. The result is compared to a perturbative QCD calculation performed to next-to-leading order.

Paper Structure

This paper contains 21 sections, 5 equations, 9 figures.

Table of Contents

  1. Introduction
  2. Experimental conditions
  3. Analysis
  4. Offline cuts and event selection
  5. Electron finding
  6. Electron-enriched and hadronic samples
  7. Measurement of $dE/dx$
  8. Tracking requirements
  9. Background electrons from converting photons
  10. Electrons from Dalitz decays, $\pi^0 \rightarrow e^+e^-\gamma$
  11. Electron signal
  12. Monte Carlo event simulation
  13. Event characteristics
  14. Cross section measurements
  15. Differential cross sections
  16. ...and 6 more sections

Figures (9)

  • Figure 1: (a) The distribution of the quality factor, $D$, (see Section \ref{['sec:dedx']}) for conversion candidates. (b) the invariant mass, $M_{e^+e^-}$, for conversion candidates. In (a) and (b), the conversion candidates resulting in pairs with zero net charge are shown as points; those pairs having non-zero net charge are shown as the crosses.
  • Figure 2: (a) The $dE/dx$ distribution of photon-conversion candidates and (b) the dependence on the polar angle, $\theta^{\rm trk}$, for electrons. In (a), photon-conversion candidates having two tracks of opposite charge are shown as open circles whereas those with tracks of the same sign are shown as the crosses; the solid circles show the difference between these two distributions. A Gaussian fit is shown in (a) for illustration.
  • Figure 3: The measured distribution of $dE/dx$ against momentum, $p^{\rm trk}$, for negative tracks in the range $|\eta^{\rm trk}|~<~1.1$, as for the analysis. The curves show the expected average values for particular types of particles as derived from the Bethe-Bloch formula bb. The events are a sub-sample of those that pass the dijet trigger requirements.
  • Figure 4: Comparison of the number of electrons found in the data (points) with the prediction from pair production (solid line) for conversions in which both tracks have a momentum greater than 0.2 GeV. The prediction for no cut on the momentum of the tracks is also shown as the dashed line.
  • Figure 5: (a) The $dE/dx$ distribution for the hadronic sample (crosses) and electron-enriched (open squares) sample normalised to each other in the hatched region shown. (b) The difference between the electron-enriched and hadronic samples (solid circles), together with the background arising from photon conversions (open circles). The data in the region with $dE/dx>1.4$ mips shown by the dashed line were used to extract the results in this paper.
  • ...and 4 more figures