Measurement of Beauty Photoproduction near Threshold using Di-electron Events with the H1 Detector at HERA

TL;DR

The study measures beauty photoproduction near threshold in ep collisions at HERA using di-electron final states, enabling access to low <P_T(b)> with a dedicated online/offline electron identification strategy. It employs a hemisphere-based reconstruction for b-quarks, a template unfolding to separate uds/charm/J/ψ backgrounds, and an unfolding framework to extract differential and total cross sections. The results, including a total cross section of about 3.79 nb, are consistent with NLO QCD within large uncertainties and extend measurements toward threshold, validating pQCD in a challenging regime and demonstrating the effectiveness of low-momentum electron tagging. Overall, the analysis reinforces previous HERA findings and provides a robust near-threshold test of heavy-flavor photoproduction dynamics at high-energy ep colliders.

Abstract

The cross section for ep -> e b\bar{b} X in photoproduction is measured with the H1 detector at the ep-collider HERA. The decay channel b\bar{b} -> ee X' is selected by identifying the semi-electronic decays of the b-quarks. The total production cross section is measured in the kinematic range given by the photon virtuality Q^2 <= 1 GeV^2, the inelasticity 0.05 <= y <= 0.65 and the pseudorapidity of the b-quarks |eta(b)|,|eta(\bar{b})| <= 2. The differential production cross section is measured as a function of the average transverse momentum of the beauty quarks <P_T(b)> down to the threshold. The results are compared to next-to-leading-order QCD predictions.

Figures (12)

• Figure 1: Generic leading order diagrams for $b\bar{b}$ production in $ep$ collisions. The diagram a) is referred to as direct or pointlike, the diagram b) is referred to as resolved or hadronlike.
• Figure 2: Normalized discriminator distributions for the separation of electrons and pions as obtained from $J/\psi \rightarrow e^+e^-$ and $K_s^0 \rightarrow \pi^+ \pi^-$ decays using the tag and probe method. a) the track seeded, calorimeter based discriminator $D_{\mathrm{calo}}$, b) the discriminator $D_{\mathrm{d}E/\mathrm{d}x}$ based on the measurement of the specific energy loss in the CTD and c) their combination $D_{\mathrm{ele}}$. Data are represented by circles and Monte Carlo simulations by histograms.
• Figure 3: Schematic illustration of the determination of the thrust axis in the plane transverse to the $ep$ beams. The transverse thrust axis, indicated by the dashed arrow, maximizes the sum of momenta projected onto it in this plane. The thrust axis allows the event to be divided into two hemispheres, each containing the decay products of a beauty quark, used to reconstruct the average transverse beauty mass $m_{T, \mathrm{rec}}(b)$ as defined in equation \ref{['eq: M_T_est']}.
• Figure 4: Correlation between the reconstructed transverse beauty mass $m_{T, \mathrm{rec}}(b)$ and the transverse mass $\langle m_T(b) \rangle$ calculated from the quadratically averaged transverse momentum of the generated beauty quarks. The inner line on the diagonal indicates the correlation of $m_{T, \mathrm{rec}}(b)$ and $\langle m_T(b) \rangle$, and the outer two lines show the $1\sigma$ error band. The used binning (dotted grey lines) for the vectors $\mathbf{x}$ and $\mathbf{y}$ entering the unfolding procedure are also shown.
• Figure 5: Templates used to separate the light quarks (uds) from the heavy quark flavours as obtained by the Monte Carlo simulation. For the definition of the background enhanced regions $B1$-$B3$ and the signal enhanced region $S$ see table \ref{['tab:udsTemplatesDef']} and text.