Beauty photoproduction measured using decays into muons in dijet events in ep collisions at $\sqrt{s}$=318 GeV

TL;DR

This study measures beauty photoproduction in ep collisions at HERA via events with two jets and a muon, testing perturbative QCD with a substantial differential analysis. It combines NLO QCD predictions (FMNR) with fragmentation and B-hadron decay models, and contrasts them with MC generators (Pythia, Cascade) to interpret direct and resolved photon contributions. The results show good agreement with NLO QCD across multiple differential distributions, and with MCs that include flavour-excitation processes, while not confirming the H1 excess. The findings constrain photon-parton dynamics and heavy-quark production, and align with prior ZEUS electron-channel measurements, strengthening confidence in the theoretical framework for heavy-flavor photoproduction.

Abstract

The photoproduction of beauty quarks in events with two jets and a muon has been measured with the ZEUS detector at HERA using an integrated luminosity of 110 pb$^{- 1}$. The fraction of jets containing b quarks was extracted from the transverse momentum distribution of the muon relative to the closest jet. Differential cross sections for beauty production as a function of the transverse momentum and pseudorapidity of the muon, of the associated jet and of $x_γ^{jets}$, the fraction of the photon's momentum participating in the hard process, are compared with MC models and QCD predictions made at next-to-leading order. The latter give a good description of the data.

Figures (7)

• Figure 1: Distributions for the dijet-plus-muon sample (points) compared to the predictions of the Pythia Monte Carlo (full line) normalized to the data. The shaded histogram shows the beauty component and the dotted line is the sum of charm and beauty. The plots show (a) the transverse momentum of the jet associated with the muon; (b) its pseudorapidity; (c) the transverse momentum of the highest-$p_T$ non-$\mu$-associated jet; (d) its pseudorapidity; (e) the transverse momentum of the muon; (f) its pseudorapidity; (g) the distribution of $x_\gamma^{\rm jets}$.
• Figure 2: (a) The $\hbox{$p_{\rm T}^{\rm rel}$}$ distribution as predicted by the Pythia Monte Carlo for reconstructed muons from beauty ($f_{\mu}^{b \mkern1.5mu\overline{\mkern-1.5mub\mkern-1.mu}\mkern1.mu,{\rm MC}}$, dashed histogram), charm ($f_{\mu}^{c \mkern1.5mu\overline{\mkern-1.5muc\mkern-1.mu}\mkern1.mu,{\rm MC}}$, dotted histogram) and light-flavours ($f_{\mu}^{\rm LF,MC}$, dash-dotted histogram), and for unidentified tracks ($f_{x}^{\rm MC}$, full-line histogram). The distributions are normalized to unity. (b) The $\hbox{$p_{\rm T}^{\rm rel}$}$ distribution of unidentified data tracks (points), compared to the prediction from Pythia (full line). The charm and beauty components are also shown as the dotted- and dashed-line histograms, respectively.
• Figure 3: The $\hbox{$p_{\rm T}^{\rm rel}$}$ distribution for events with muons in the rear, barrel and forward regions defined in Eq. (\ref{['e:mucuts']}). The data (points) are compared to the mixture of beauty and charm+LF background obtained from the $\hbox{$p_{\rm T}^{\rm rel}$}$ fit (full line). The shaded histogram represents the beauty component while the dashed-line histogram is the background.
• Figure 4: Cross sections for muons coming from $b$ decays in dijet events with $p_{T}^{\rm jet_1}>7,p_{T}^{\rm jet_2}>6 {\,\text{Ge}\text{V}}$, $\eta^{\rm jet_1},\eta^{\rm jet_2} <2.5$, $0.2<y<0.8$, $Q^2<1{\,\text{Ge}\text{V}}^2$ passing the selection of Eq. (\ref{['e:mucuts']}). (a) The cross section for the forward, barrel and rear regions (defined in Eq. (\ref{['e:mucuts']})). (b) The differential cross section as a function of $x_\gamma^{\rm jets}$. The data (points) are compared to the predictions of NLO QCD (dotted line: parton-level jets; dashed line: jets corrected to the hadron level). The full error bars are the quadratic sum of the statistical (inner part) and systematic uncertainties. The band around the NLO prediction represents the variation on the theoretical predictions obtained by varying the $b$-quark mass, $\mu_r$ and $\mu_f$, as explained in the text. The data are also compared to the predictions of the Pythia (solid line histogram) and Cascade (dot-dashed line histogram) Monte Carlo models.
• Figure 5: Differential cross sections as a function of (a) the muon pseudorapidity $\eta^\mu$ and (b) transverse momentum $p_{T}^\mu$, for $p_{T}^\mu>2.5{\,\text{Ge}\text{V}}$ and $-1.6<\eta^{\mu}<2.3$, for muons coming from $b$ decays in dijet events with $p_{T}^{\rm jet_1}>7, p_{T}^{\rm jet_2}>6 {\,\text{Ge}\text{V}}$, $\eta^{\rm jet_1},\eta^{\rm jet_2} <2.5$, $0.2<y<0.8$, $Q^2<1{\,\text{Ge}\text{V}}^2$. The data (points) are compared to the predictions of NLO QCD (dotted line: parton-level jets; dashed line: corrected to hadron-level jets). The full error bars are the quadratic sum of the statistical (inner part) and systematic uncertainties. The band around the NLO prediction represents the variation on the theoretical predictions obtained by varying the $b$-quark mass and $\mu_r$ and $\mu_f$ as explained in the text.The data are also compared to the predictions of the Pythia (solid line histogram) and Cascade (dot-dashed line histogram) Monte Carlo models.