Measurement of beauty production in deep inelastic scattering at HERA

TL;DR

This work presents the first measurement of bottom-quark production in deep inelastic scattering at HERA, using ZEUS data to select events with a muon and a hard jet in the Breit frame. The b-decay contribution is extracted from the muon’s transverse momentum relative to the associated jet, yielding a visible cross section of 40.9 pb, about 2.5 standard deviations above the NLO QCD prediction of 20.6 pb (hadronisation included). Differential spectra largely agree with NLO QCD, though tensions appear in low Q^2, low x, high muon η, and high jet E_T^Breit regions; Cascade (CCFM) describes the data well in many bins, while RAPGAP tends to underestimate. The results provide a crucial test of heavy-flavor production in DIS and the applicability of perturbative QCD in a multi-scale regime, highlighting areas where hadronisation and evolution scheme effects are important.

Abstract

The beauty production cross section for deep inelastic scattering events with at least one hard jet in the Breit frame together with a muon has been measured, for photon virtualities Q^2 > 2 GeV^2, with the ZEUS detector at HERA using integrated luminosity of 72 pb^-1. The total visible cross section is sigma_b-bbar (ep -> e jet mu X) = 40.9 +- 5.7 (stat.) +6.0 -4.4 (syst.) pb. The next-to-leading order QCD prediction lies about 2.5 standard deviations below the data. The differential cross sections are in general consistent with the NLO QCD predictions; however at low values of Q^2, Bjorken x, and muon transverse momentum, and high values of jet transverse energy and muon pseudorapidity, the prediction is about two standard deviations below the data.

Figures (4)

• Figure 1: (a) $p_{T}^{\rm{rel}}$ distribution measured for unidentified tracks in an inclusive DIS sample compared with the Rapgap MC simulation ( see text). Data (dots) and the Rapgap MC (solid line) distributions after the final event selection for: (b) the measured $p_{T}^{\rm{rel}}$ distribution; (c) muon momentum; (d) muon pseudorapidity; (e) transverse energy in the Breit frame; and (f) pseudorapidity in the laboratory frame of the associated jet. The solid line represents all MC contributions while the hatched histograms show the contribution from $b$ quarks according to the percentage given by the fit (see Section \ref{['sec-res']}). The error bars are statistical only.
• Figure 2: Differential $b$-quark cross section as a function of (a) $Q^2$ and (b) Bjorken $x$ for events with at least one jet reconstructed in the Breit frame and a muon, compared to the NLO QCD calculations. The error bars on the data points correspond to the statistical uncertainty (inner error bars) and to the statistical and systematic uncertainties added in quadrature (outer error bars). The solid line shows the NLO QCD calculations with the hadronisation corrections and the dashed line the same calculation without the hadronisation corrections. The shaded bands show the uncertainty of the NLO QCD prediction due to the variation of the renormalisation and factorisation scale, $\mu$, and the $b$-quark mass, $m_b$. Differential $b$-quark cross sections as a function of (c) $Q^2$ and (d) Bjorken $x$, compared with the LO QCD MC programs Cascade (solid line) and Rapgap (dashed line).
• Figure 3: Differential $b$-quark cross section as a function of (a) the muon transverse momentum $p_T^{\mu}$ and (b) muon pseudorapidity $\eta^{\mu}$ in the laboratory frame, compared to the NLO QCD calculations. Other details are as described in the caption to Fig. \ref{['fig-q2x']}. Differential $b$-quark cross section as a function of (c) $p_T^{\mu}$ and (d) $\eta^{\mu}$, compared with LO QCD MC programs Cascade (solid line) and Rapgap (dashed line).
• Figure 4: (a) differential $b$-quark cross section as a function of the transverse energy of the jet in the Breit frame $E_{T,\rm{jet}}^{\rm{Breit}}$. The data (dots) are compared to the NLO QCD calculations (a). Other details are as described in the caption to Fig. \ref{['fig-q2x']}. (b) differential $b$-quark cross sections as a function of $E_{T,\rm{jet}}^{\rm{Breit}}$ compared with LO QCD MC programs Cascade (solid line) and Rapgap (dashed line).