Feasibility of top quark measurements at LHCb and constraints on the large-xx gluon PDF

The forward LHCb acceptance opens interesting possibilities of studying precision Standard Model hard processes in a kinematical region beyond the reach of ATLAS and CMS. In this paper we perform a feasibility study for cross-section measurements of top quark pairs with the LHCb detector, with an analysis of signal and background rates for selected final states, and determine the potential precision achievable at $\sqrt{s} =$ 7 and 14~TeV. We then study the dependence of theoretical uncertainties on the pseudorapidity distribution of top quarks produced in pair production at NLO, and observe that a cross-section measurement at high pseudorapidity has enhanced sensitivity to probe the high-$x$ gluon PDF as compared to measurements in the central-region. Based on simulated pseudodata, the impact of a 14~TeV cross-section measurement on the gluon PDF and charge asymmetry is quantified.Comment: 25 pages, 14 figures, 8 tables. Extended PDF analysis to include ABM11 5flv nlo. Added summary plots of LHCb cross-sections. Include shift of gluon PDF central value for CT10, NNPDF sets assuming different pseudodata values. Matches version to be published in JHE

Understanding forward BB hadron production

The LHCb collaboration has recently performed a measurement of the production rate of inclusive $B$ hadron production ($pp\to BX$) at both 7 and 13~TeV centre-of-mass (CoM) energies. As part of this measurement, the ratio of these two cross section measurements has been presented differentially in $B$ hadron pseudorapidity within the range of $\eta_B \in [2.0,5.0]$. A large tension ($4\sigma$) is observed for the ratio measurement in the lower pseudorapidity range of $\eta_B \in [2.0,3.0]$, where the data is observed to exceed theoretical predictions, while consistency is found at larger $\eta_B$ values. This behaviour is not expected within perturbative QCD, and can only be achieved by introducing ad-hoc features into the structure of the non-perturbative gluon PDF within the region of $x\in[10^{-3},10^{-4}]$. Specifically, the gluon PDF must grow extremely quickly with decreasing $x$ within this kinematic range, closely followed by a period of decelerated growth. However, such behaviour is highly disfavoured by global fits to proton structure. Further studies of the available LHCb $B$ and $D$ hadron cross section data, available for a range of CoM energies, indicate systematic tension in the (pseudo)rapidity region of $[2.0,2.5]$.Comment: 31 page