Beauty photoproduction using decays into electrons at HERA

Photoproduction of beauty quarks in events with two jets and an electron associated with one of the jets has been studied with the ZEUS detector at HERA using an integrated luminosity of $120{\mathrm{pb}}^{-1}$. The fractions of events containing $b$ quarks, and also of events containing $c$ quarks, were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections for beauty and charm production were measured and compared with next-to-leading-order QCD calculations and Monte Carlo models.

Figure 1

The mean $\mathrm{d}E/\mathrm{d}x$ measured in the CTD, $⟨\mathrm{d}E/\mathrm{d}x⟩$, as a function of $\beta \gamma$ for different samples of identified particles as denoted in the figure. The curve shows a physically motivated parametrization of the $⟨\mathrm{d}E/\mathrm{d}x⟩$ dependence on $\beta \gamma$.

Figure 2

Distribution of the relative difference between the observed ($\mathrm{d}E/\mathrm{d}{x}_{\mathrm{obs}}$) and predicted ($\mathrm{d}E/\mathrm{d}{x}_{\mathrm{pred}}$) specific energy loss for the track sample with $n_{\mathrm{trunc}}=23$. The inset shows the same distribution with a logarithmic ordinate scale.

Figure 3

Normalized distributions of the five input variables used in the likelihood for the electron candidates extracted from the Monte Carlo events, without applying the cuts on $\mathrm{d}E/\mathrm{d}x$ and $E_{\mathrm{EMC}}/E_{\mathrm{CAL}}$. The solid line shows the distribution for electrons from semileptonic $b$-quark decays, the dashed line for $c$-quark decays and the dotted line the background (Bkg).

Figure 4

The distribution of the likelihood ratio for electron candidates, $N_{\mathrm{cand}}$, in data compared to the Monte Carlo expectation after the fit described in the text. The arrow indicates the region included in the fit ($-2\ln T<10$). The shaded areas show the fitted contributions from $b$ quarks, $c$ quarks and background as denoted in the figure.

Figure 5

Distributions of the five input variables of the likelihood for the electron candidates used in the fit ($-2\ln T<10$). All cuts have been applied except $\mathrm{d}E/\mathrm{d}x>1.1$ in (a) and $E_{\mathrm{EMC}}/E_{\mathrm{CAL}}>0.9$ in (b) (the cuts are indicated in the figure). The shaded areas show the contributions from $b$ quarks, $c$ quarks and background as denoted in the figure, after applying the scale factors from the fit.

Figure 6

Distributions of the five input variables of the likelihood for a beauty-enriched sample (candidates with $-2\ln T<1$). In these plots the ordinate is shown on a linear scale. Other details as in the caption of Fig. 5.

Figure 7

Distributions of the five input variables of the likelihood for a charm enriched sample (candidates with $-2\ln T_{e,c}<1.5$). In these plots the ordinate is shown on a linear scale. Other details as in the caption of Fig. 5.

Figure 8

Distributions of $E_T^{e\mathrm{jet}}$ and ${\eta }_T^{e\mathrm{jet}}$ of the jet associated with the electron, and $p_T^e$ of the electron candidate. Figures (a)–(c) contain all electron candidates satisfying $-2\ln T_{e,b}<10$; (d)–(f) and (g)–(i) show the same distributions for the beauty ($-2\ln T_{e,b}<1$) and charm ($-2\ln T_{e,c}<1.5$) enriched samples, respectively. Other details as in the caption of Fig. 5.

Figure 9

Total cross sections for electrons from $b$ and $c$ quarks in photoproduction events, $Q^2<1{\mathrm{GeV}}^2$ and $0.2<y<0.8$, with at least two jets with $E_T>7(6)\mathrm{GeV}$, $|\eta |<2.5$ and the subsequent semileptonic decay to an electron with $p_T>0.9\mathrm{GeV}$ and $|\eta |<1.5$. The measurements are shown as points. The inner error bar shows the statistical uncertainty and the outer error bar shows the statistical and systematic uncertainties added in quadrature. The solid line shows the NLO QCD prediction after hadronization corrections, with the theoretical uncertainties indicated by the band; the dashed line shows the prediction from pythia.

Figure 10

Differential cross sections as a function of (a), (c) the transverse momentum and (b), (d) the pseudorapidity of the electrons. Plots (a) and (b) are for $b$-quark production while (c) and (d) are for $c$-quark production. The measurements are shown as points. The inner error bar shows the statistical uncertainty and the outer error bar shows the statistical and systematic uncertainties added in quadrature. The solid line shows the NLO QCD prediction after hadronization corrections, with the theoretical uncertainties indicated by the band; the dashed line shows the scaled prediction from pythia.

Figure 11

Differential cross sections as a function of $x_{\gamma }^{\mathrm{obs}}$. (a) shows the distribution for electrons from $b$-quark production while (b) shows $c$-quark production. Other details as in the caption of Fig. 10.

Figure 12

Differential cross sections as a function of (a), (c) the transverse energy and (b), (d) the pseudorapidity of the highest-energy jet. Plots (a) and (b) show the distributions for electrons from $b$-quark production while plots (c) and (d) show those for $c$-quark production. Other details as in the caption of Fig. 10.

Figure 13

Differential cross sections for (a) $b$-quark and (b) $c$-quark production as a function of the transverse energy of the jet associated to the electron. Other details as in the caption of Fig. 10.

Figure 14

Differential cross section for $b$-quark production as a function of transverse momentum, $p_T^b$, compared to the results of previous ZEUS measurements as indicated in the figure. The measurements are shown as points. The inner error bar shows the statistical uncertainty and the outer error bar shows the statistical and systematic uncertainties added in quadrature. The solid line shows the NLO QCD prediction from the FMNR program with the theoretical uncertainty shown as the shaded band.