Measurement of B anti-B Angular Correlations based on Secondary Vertex Reconstruction at sqrt(s)=7 TeV

TL;DR

This study reports the first measurement of angular correlations between B and anti-B hadrons in proton-proton collisions at 7 TeV, using an inclusive secondary-vertex approach to access small opening angles. It presents differential cross sections in DeltaR and DeltaPhi within the visible B-hadron phase space across three leading-jet pT scales, based on 3.1 pb^-1 of CMS data. The results show a substantial collinear bbbar component that increases with event scale, and comparisons with LO/NLO QCD predictions reveal that no model fully captures the DeltaR distribution, highlighting gaps in current gluon-splitting and higher-order treatment. These measurements provide stringent tests of perturbative QCD and refine understanding of bbbar production dynamics in the LHC era.

Abstract

A measurement of the angular correlations between beauty and anti-beauty hadrons (B B-bar) produced in pp collisions at a centre-of-mass energy of 7 TeV at the CERN LHC is presented, probing for the first time the region of small angular separation. The B hadrons are identified by the presence of displaced secondary vertices from their decays. The B hadron angular separation is reconstructed from the decay vertices and the primary-interaction vertex. The differential B B-bar production cross section, measured from a data sample collected by CMS and corresponding to an integrated luminosity of 3.1 inverse picobarns, shows that a sizable fraction of the B B-bar pairs are produced with small opening angles. These studies provide a test of QCD and further insight into the dynamics of b b-bar production.

Figures (6)

• Figure 1: The measured transverse momentum distributions of the leading jet in the event (left) and measured efficiency to trigger an event on the high-level trigger as a function of jet $p_{\mathrm{T}}$ (right), for three different trigger thresholds.
• Figure 2: Properties of the reconstructed ${\mathrm{B}}$ candidates: vertex mass distribution (left) and flight distance significance distribution (right). The inset in the right plot shows a zoom of the flight distance significance distribution with narrower bins and linear scale. The data are shown by the solid points. The decomposition into the different sources, beauty, charm and light quarks, is shown for the pythia Monte Carlo simulation. The simulated distributions are normalised to the total number of data events. All selection cuts apart from those on the shown quantities are applied.
• Figure 3: Resolution of the $\Delta R$ reconstruction, obtained using simulation for the leading jet $p_{\mathrm{T}}\xspace > 84$ GeV sample. Left: $\Delta R$ values reconstructed between the two secondary vertices $\Delta R_{VV}$ versus the values between the original ${\mathrm{B}}$ hadrons $\Delta R_{BB}$, in the visible ${\mathrm{B}}$ hadron phase space (see text). Right: projection onto the diagonal ($\Delta R_{VV} - \Delta R_{BB}$). The numbers in the boxes represent the number of events reconstructed in that particular bin.
• Figure 6: Differential ${\mathrm{B}}$$\mathrm{\overline{B}}$ production cross sections as a function of $\Delta R$ (left) and $\Delta \phi$ (right) for the three leading jet $p_{\mathrm{T}}$ regions. For clarity, the $p_{\mathrm{T}}\xspace > 56$ and 84 GeV bins are offset by a factor 4 and 2, respectively. For the data points, the error bars show the statistical (inner bars) and the total (outer bars) uncertainties. A common uncertainty of 47% due to the absolute normalisation on the data points is not included. The symbols denote the values averaged over the bins and are plotted at the bin centres. The pythia simulation (shaded bars) is normalised to the region $\Delta R > 2.4$ or $\Delta \phi > 2.4$, as indicated by the shaded normalisation regions. The widths of the shaded bands indicate the statistical uncertainties of the predictions.
• Figure 7: Left: ratio between the ${\mathrm{B}}$$\mathrm{\overline{B}}$ production cross sections in $\Delta R < 0.8$ and $\Delta R>2.4$, $\rho_{\Delta R}=\sigma_{\Delta R < 0.8}$ / $\sigma_{\Delta R >2.4}$, as a function of the leading jet $p_{\mathrm{T}}$. Right: asymmetry between the two regions, $(\sigma_{\Delta R < 0.8} - \sigma_{\Delta R >2.4})$ / $(\sigma_{\Delta R < 0.8} + \sigma_{\Delta R >2.4})$. The symbols denote the data averaged over the bins and are plotted at the mean leading jet $p_{\mathrm{T}}$ of the bins. For the data points, the error bars show the statistical (inner bars) and the total (outer bars) errors. Also shown are the predictions from the pythia and MadGraph simulations, where the widths of the bands indicate the uncertainties arising from the limited number of simulated events.