[Paper Review] B0s Decays and B Hadron Leptonic Decays

R. Van Kooten|ArXiv.org|Jun 2, 2006

Particle physics theoretical and experimental studies15 references3 citations

TL;DR

This paper presents updated measurements of $B^0_s$ decays into CP eigenstates and leptonic decays of $B$ hadrons, combining results from the Tevatron and $B$ factories to determine the world average of the $B^0_s$ width difference $\Delta\Gamma_s$, which is found to be 2.3$\sigma$ from zero and consistent with the Standard Model. It also reports the first observation of $B^+ \to \tau^+ \nu_\tau$ by Belle, providing the first direct measurement of $f_B|V_{ub}|$.

ABSTRACT

Branching fractions of B^0s decays into specific CP eigenstates are presented, and these and other results are combined in world averages to evaluate implications on the width difference between mass or CP eigenstates, ΔΓ_s. New results on purely leptonic decays of B hadrons from the Tevatron and B factories are also presented.

Motivation & Objective

• To determine the world average of the $B^0_s$ width difference $\Delta\Gamma_s$ using branching fractions into CP eigenstates and lifetime measurements.
• To constrain new physics through purely leptonic $B$ decays, particularly $B^0_d \to \tau^+\tau^-$ and $B^+ \to \tau^+\nu_\tau$, which are sensitive to new physics beyond the Standard Model.
• To provide the first direct measurement of the $B^+$ decay parameter $f_B|V_{ub}|$ from the observation of $B^+ \to \tau^+ \nu_\tau$.
• To improve constraints on the CKM matrix element $|V_{ub}|$ and the $B$-meson decay constant $f_B$ using leptonic decays.
• To test the consistency of $\Delta\Gamma_s$ with Standard Model predictions, particularly given its sensitivity to new physics in the $B^0_s$ mixing amplitude.

Proposed method

• Measure the branching fraction of $B^0_s \to D_s^{(\ast)+}D_s^{(\ast)-}$, a CP-even final state, using fully reconstructed decays in CDF, DØ, and Belle data.
• Use the measured branching fraction in the formula $\Delta\Gamma_s/\Gamma_s \approx 2Br / (1 - Br/2)$ to extract $\Delta\Gamma_s$, assuming $\phi = 0$.
• Combine $B^0_s$ lifetime, branching fraction, and $\Delta m_s$ measurements from multiple experiments to compute a world average of $\Delta\Gamma_s$.
• Search for $B^0_d \to \tau^+\tau^-$ using fully reconstructed $B^0_d$ decays and event topology in BaBar data, applying kinematic and topological cuts to isolate signal.
• Use a signal-enhancing variable, excess energy in the electromagnetic calorimeter, to identify $B^+ \to \tau^+ \nu_\tau$ decays in Belle data.
• Fit the excess energy distribution to extract the signal significance and measure the branching fraction $Br(B^+ \to \tau^+ \nu_\tau)$, translating it into $f_B|V_{ub}|$ using the HFAG world average for $|V_{ub}|$.

Experimental results

Research questions

• RQ1 What is the world average value of the $B^0_s$ width difference $\Delta\Gamma_s$, and how does it compare to the Standard Model prediction?
• RQ2 How do the latest measurements of purely leptonic $B$ decays constrain new physics beyond the Standard Model?
• RQ3 What is the first direct measurement of $f_B|V_{ub}|$ from the observation of $B^+ \to \tau^+ \nu_\tau$?
• RQ4 What are the current limits on $Br(B^0_d \to \tau^+\tau^-)$, and how do they compare to the SM prediction?
• RQ5 To what extent do $B^0_s$ decays into CP eigenstates provide a model-independent probe of $\Delta\Gamma_s$?

Key findings

• The world average of $\Delta\Gamma_s$ is found to be 2.3$\sigma$ from zero, consistent with the Standard Model prediction.
• The first observation of $B^+ \to \tau^+ \nu_\tau$ is reported by Belle with a significance of 4.0$\sigma$, yielding $Br(B^+ \to \tau^+ \nu_\tau) = (0.106^{+0.034+0.018}_{-0.028-0.016})\%$.
• The measurement translates into the first direct determination of $f_B|V_{ub}| = (7.73^{+1.24+0.066}_{-1.02-0.058}) \times 10^{-4}$ GeV.
• Using the HFAG world average for $|V_{ub}|$, the paper derives $f_B = 0.176^{+0.028+0.020}_{-0.023-0.018}$ GeV, the first direct measurement of this parameter.
• The limit on $Br(B^0_d \to \tau^+\tau^-)$ from BaBar is $< 3.4 \times 10^{-3}$ at 90% C.L., approaching the SM prediction.
• The $B^0_s$ branching fraction measurement $Br(B^0_s \to D_s^{(\ast)+}D_s^{(\ast)-})$ is used to extract $\Delta\Gamma_s$, with the result consistent with the SM.