[Paper Review] Review of experimental results on rare radiative, semileptonic and leptonic B decays

Thomas Schietinger|ArXiv.org|May 17, 2006

Particle physics theoretical and experimental studies3 citations

TL;DR

This review synthesizes experimental results on rare radiative, semileptonic, and leptonic B decays from B factories, emphasizing precision measurements of $b\to s\gamma$, $b\to s\ell^+\ell^-$, and purely leptonic decays. It highlights agreement with the Standard Model, constraints on new physics, and the role of these decays as clean probes of flavor physics and CKM parameters.

ABSTRACT

We review recent experimental progress in the domain of rare radiative, semileptonic and leptonic B decays. The statistical precision attained for these decays has reached a level where they start to impose meaningful constraints on the Cabibbo-Kobayashi-Maskawa matrix, which are complementary to those obtained from hadronic decays. While the current data indicate no deviations from Standard Model predictions, there is still some room for new physics in these decays.

Motivation & Objective

• To summarize recent experimental progress in rare radiative, semileptonic, and leptonic B decays using data from B factories.
• To assess the sensitivity of these decays to new physics beyond the Standard Model.
• To provide updated branching fraction measurements and limits, particularly for $B\to\mu^+\mu^-$, $B\to\tau^+\tau^-$, and $B\to\nu\bar{\nu}$.
• To evaluate the role of these decays in constraining CKM matrix elements, especially $|V_{ub}|$ and $|V_{td}/V_{ts}|$.
• To examine the potential of photon energy spectra and CP asymmetries in probing $b$-quark dynamics and new physics.

Proposed method

• Utilizes data from the B A B AR and Belle $B$ factories operating at $\Upsilon(4S)$ resonance, with integrated luminosities up to 120M $B\bar{B}$ pairs.
• Applies fully inclusive and semi-inclusive analysis techniques to extract $b\to s\gamma$ branching fractions, with background suppression via event-shape and energy-flow variables.
• Employs lepton-tagging in inclusive analyses to enhance signal-to-background ratios and reduce continuum backgrounds.
• Uses extrapolation based on isospin symmetry and Monte Carlo simulations to account for unobserved exclusive modes in semi-inclusive $b\to s\gamma$ measurements.
• Applies shape function models to correct for low-energy photon extrapolations, with systematic uncertainties quantified.
• Performs model-independent limits on rare decays such as $B^0\to\mu^+\mu^-$, $B^0\to\nu\bar{\nu}$, and $B^0\to\gamma\gamma$ using event reconstruction and missing energy techniques.

Experimental results

Research questions

• RQ1 What is the current precision of the inclusive $b\to s\gamma$ branching fraction, and how does it constrain new physics via Wilson coefficient $C_7$?
• RQ2 To what extent do measured CP asymmetries in $b\to s\gamma$ deviate from Standard Model predictions?
• RQ3 What are the most stringent experimental limits on purely leptonic $B$ decays such as $B^+\to\tau^+\nu_\tau$, $B^0\to\mu^+\mu^-$, and $B^0\to\nu\bar{\nu}$?
• RQ4 How do rare semileptonic decays like $B\to K^*\ell^+\ell^-$ probe the Wilson coefficients and CKM parameters?
• RQ5 Can the photon energy spectrum in $b\to s\gamma$ provide information on the $b$-quark mass and dynamics inside the $B$ meson?

Key findings

• The inclusive $b\to s\gamma$ branching fraction is measured as $\mathcal{B}(b\to s\gamma) = 355 \pm 24^{+9}_{-10} \pm 3 \times 10^{-6}$, in excellent agreement with the Standard Model prediction of $357 \pm 30 \times 10^{-6}$.
• The CP asymmetry in $b\to s\gamma$ is measured as $A_{CP} = (2.5 \pm 5.0 \pm 1.5)\%$ by B A B AR and $(0.2 \pm 5.0 \pm 3.0)\%$ by Belle, consistent with zero and the Standard Model.
• The limit on $B^0\to\mu^+\mu^-$ is $<0.083 \times 10^{-6}$ from B A B AR, improved to $<0.032 \times 10^{-6}$ in a Tevatron combination, constraining supersymmetric models.
• The limit on $B^0\to\nu\bar{\nu}$ is $<220 \times 10^{-6}$ from B A B AR, based on 126k $B\to D^{(*)}\ell\nu$ events, with a limit on the radiative mode $B^0\to\nu\bar{\nu}\gamma < 47 \times 10^{-6}$.
• The limit on $B^0\to\gamma\gamma$ is $<0.62 \times 10^{-6}$ from Belle, based on 111M $B\bar{B}$ pairs, still about 20 times above the SM prediction of ~1.2.
• The first observation of $b\to d\gamma$ is achieved, and the $B^+\to\tau^+\nu_\tau$ decay is on the verge of observation, with a limit of $<0.083 \times 10^{-6}$ from B A B AR.