Measurement of Open Beauty Production at HERA

Abstract

The first observation of open b production in ep collisions is reported. An event sample containing muons and jets has been selected which is enriched in semileptonic b quark decays. The visible cross section σ(ep -> b \bar{b}X -> μX') for Q^2 < 1 GeV^2, 0.1 < y < 0.8 is measured to be 0.176+-0.016(stat.)+0.026-0.017(syst.) nb for the muons to be detected in the range 35 deg < θ^μ< 130 deg and \pt^μ> 2.0 GeV in the laboratory frame. The expected visible cross section based on a NLO QCD calculation is 0.104+-0.017 nb. The cross sections for electroproduction with Q^2<1 GeV^2 and photoproduction are derived from the data and found to be σ(ep-> e b\bar{b}X) = 7.1+-0.6(stat.)+1.5-1.3(syst.) nb and σ(γp-> b\bar{b} X) = 111+-10(stat.)+23-20(syst.) at an average <W_{γp}> \sim 180 GeV, respectively.

Figures (4)

• Figure 1: Fractional energy flow within jets as a function of the distance to the jet axis. This fraction is the ratio of the energy in an annulus at radius $r$ to the total energy in the jet. The data points (filled dots) correspond to a data sample of mainly charm decays and the full histogram to a Monte Carlo simulation where only charm quarks are generated. Both the data and Monte Carlo are normalized so that the integral of the distributions equals one.
• Figure 2: Momentum (a) and polar angle (b) distributions for fake muons in a pion sample from the decay $K^0_S\rightarrow\pi^+\pi^-$. The filled points show the fake muon yield as measured in the data. The solid histogram gives the estimate of the muon yield as obtained by assigning ${\cal P}^{\mu}_h(p,\theta)$ as a weight to every pion and summing the weights over the entire pion sample. The measured yield is 64 fake muons; the estimate amounts to 69.8.
• Figure 3: The measured $p_{T, rel}^\mu$ distribution in the data and the fitted sum (solid line) of the contributions of beauty (dashed line), charm (dotted line) and the fixed fake muon background (dashed-dotted line).
• Figure 4: The total photoproduction cross section, $\sigma(\gamma p\rightarrow b\bar{b} X)$. The horizontal error bar represents the range of the measurement. The solid curve shows the expectation of the FMNR NLO QCD calculation (full line) with $m_b=4.75 \,\hbox{GeV}$ and the MRSG and GRV-HO structure functions for the proton and the photon, respectively. The shaded area corresponds to the uncertainty if the factorisation scale changes by a factor of 2. A change of the renormalisation scale by a factor of 2 leads to a similar result. The dashed line represents the prediction of the FMNR NLO QCD calculation if the MRST mrst structure function for the proton is used.