[Paper Review] LHCb Results on Semileptonic B/B$_s$/$\Lambda_b$ Decays

C. Bozzi|arXiv (Cornell University)|Mar 18, 2013

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper presents LHCb's measurements of semileptonic decays of B, B$_s$, and Λ$_b$ hadrons using 3 fb$^{-1}$ of pp collision data at √s = 7 and 8 TeV. By reconstructing charm-muon pairs and leveraging vertex separation to determine b-hadron flight directions, LHCb achieves precise determinations of b-hadron production fractions and performs the most accurate measurement of the semileptonic CP asymmetry in the B$_s$ system to date.

ABSTRACT

Studies of semileptonic decays of b hadrons with the LHCb experiment are reported. In particular, measurements of the b hadron production fractions at the LHC and Bs semileptonic decays in P-wave charmed mesons are presented. An outlook of the LHCb potential in measuring the CKM matrix elements Vub and Vcb, as well as decays involving taus, is given.

Motivation & Objective

• To measure the production fractions of B$_s$ and Λ$_b$ hadrons relative to B$^0$ and B$^+$ mesons at the LHC.
• To perform the most precise measurement of the semileptonic CP asymmetry in the B$_s$ system to date.
• To enable future determinations of the CKM matrix elements |V$_{cb}$| and |V$_{ub}$| through exclusive semileptonic decay analyses.
• To investigate the composition of inclusive semileptonic widths by reconstructing P-wave charm mesons and radial excitations.
• To explore the feasibility of measuring B → D(∗)τ⁻ν decays despite challenges in neutrino reconstruction.

Proposed method

• Reconstruct semileptonic decays via final states with D$^0$, D$^+$, D$_s^+$, or Λ$_c^+$ and a muon, using vertex separation to identify b-hadron origins.
• Apply impact parameter requirements to suppress prompt charm background.
• Use two-dimensional fits to q² and hadronic mass (m(D$_s^+$μ) or m(Λ$_c^+$μ)) to separate signal components and estimate backgrounds.
• Employ neutrino momentum reconstruction with two-fold ambiguity using primary and secondary vertex separation in the b-hadron rest frame.
• Measure production fractions in bins of pseudorapidity (η) and transverse momentum (p$_T$) using a 3 fb$^{-1}$ data sample.
• Account for cross-feed contributions via reconstruction of D$^0$K$^\pm$ or D$^0$p final states and use isospin symmetry to estimate feed-down effects.

Experimental results

Research questions

• RQ1 What are the production fractions of B$_s$ and Λ$_b$ hadrons relative to B$^0$ and B$^+$ mesons at the LHC?
• RQ2 What is the most precise measurement of the semileptonic CP asymmetry in the B$_s$ system?
• RQ3 Can exclusive semileptonic decays of B$_s$ and Λ$_b$ be used to determine |V$_{cb}$| and |V$_{ub}$| with high precision?
• RQ4 What is the contribution of P-wave charm mesons and radial excitations to the inclusive semileptonic width?
• RQ5 Is the measurement of B → D(∗)τ⁻ν decays feasible at LHCb despite neutrino reconstruction challenges?

Key findings

• The B$_s$ production fraction is measured as fs/(fu + fd) = 0.134 ± 0.004(stat) ± 0.011(syst) −0.010 (syst) with no significant p$_T$ dependence.
• The Λ$_b$ production fraction is measured as f$_{\Lambda b}$/(fu + fd) = (0.404 ± 0.017(stat) ± 0.027(syst) ± 0.105(syst)) × [1 − (0.031 ± 0.004)p$_T$] in GeV, showing a p$_T$ dependence.
• The first observation of the semileptonic decay B$_s$ → D$_{s2}^{*+}$ Xμν is reported, along with the most precise measurement of B$_s$ → D$_{s1}^{+}$ Xμν.
• The q² resolution achieved in B$_s$ → D$_s^+$ Xμν decays is ~0.4 GeV², comparable to B-factory measurements.
• A two-dimensional fit to q² and m(D$_s^+$μ) successfully separates signal components, including P-wave D mesons and D$^{*+}$.
• The analysis demonstrates feasibility for measuring B → D(∗)τ⁻ν decays using three-prong τ decays and neutrino reconstruction with four-fold ambiguity.