Measurement of the fraction of \Y1S originating from \chib1P decays in $pp$ collisions at $\sqrt{s} = 7\tev$

TL;DR

This paper measures the fraction of Υ(1S) mesons produced in pp collisions at √s = 7 TeV that originate from χb1P decays, via the radiative chain χb1P → Υ(1S) γ with Υ(1S) → μ+ μ−, using 32 pb−1 of LHCb data. The Υ(1S) candidates are combined with photons to form χb1P, and the signal is extracted from a mass-difference fit, yielding εcond(χb) ≈ 9.4% and a χb1P signal of 201 ± 55 events. The fraction of Υ(1S) from χb1P decays is measured to be 20.7% with statistical, systematic, and polarization-related uncertainties, and no strong pT dependence is observed across four pT bins. This feed-down contribution is non-negligible and must be accounted for in Υ(1S) production cross-section and polarization analyses at hadron colliders.

Abstract

The production of \chib1P mesons in $pp$ collisions at a centre-of-mass energy of $7\tev$ is studied using $32\invpb$ of data collected with the \lhcb detector. The $\chib1P$ mesons are reconstructed in the decay mode $\chib1P \to \Y1S\g \to \mumu\g$. The fraction of \Y1S originating from \chib1P decays in the \Y1S transverse momentum range $6 < \pt^{\Y1S} < 15\gevc$ and rapidity range $2.0 < y^{\Y1S} < 4.5$ is measured to be $(20.7\pm 5.7\pm 2.1^{+2.7}_{-5.4})%$, where the first uncertainty is statistical, the second is systematic and the last gives the range of the result due to the unknown \Y1S and \chib1P polarizations.

Figures (3)

• Figure 1: Distribution of the $\upmu\xspace^+\upmu\xspace^-\xspace$ mass for selected $\Upsilon\xspace(nS)$ candidates (black points), together with the result of the fit (solid blue curve), including the background (dotted blue curve) and the signal (dashed magenta curve) contributions.
• Figure 2: Distribution of the mass difference $m(\upmu\xspace^+\upmu\xspace^-\xspace\upgamma\xspace\xspace) - m(\upmu\xspace^+\upmu\xspace^-\xspace)$ for selected $\upchi\xspace_{\mathrm{b}\xspace\xspace}{(1P)}$ candidates (black points), together with the result of the fit (solid blue curve), including background (dotted blue curve) and signal (dashed magenta curve) contributions. The solid (red) histogram is an alternative background estimation using simulated events containing a $\Upsilon\xspace{(1S)}$ that does not originate from a $\upchi\xspace_{\mathrm{b}\xspace\xspace}{(1P)}$ decay, normalized to the data. It is used for evaluation of the systematic uncertainty due to the choice of fitting model. The bottom insert shows the pull distribution of the fit. The pull is defined as the difference between the data and fit value divided by the data error.
• Figure 3: Fraction of $\Upsilon\xspace{(1S)}\xspace$ originating from $\upchi\xspace_{\mathrm{b}\xspace\xspace}{(1P)}\xspace$ decays for different $\hbox{$p_{\rm T}$}\xspace^{\Upsilon\xspace{(1S)}\xspace}$ bins, assuming production of unpolarized $\Upsilon\xspace{(1S)}\xspace$ and $\upchi\xspace_{\mathrm{b}\xspace\xspace}{(1P)}\xspace$ mesons, shown with solid circles. The vertical error bars are statistical only. The result determined for the range $6 < \hbox{$p_{\rm T}$}\xspace < 15{\mathrm{\,Ge V\!/}c}\xspace$ is shown with the horizontal solid line, its statistical uncertainty with the dash-dotted lines, and its total uncertainty (statistical and systematic, including that due to the unknown polarization) with the shaded (light blue) band.