Towards New Frontiers with B→πKB\to\pi K Decays

Exploring correlations between the CP asymmetries of $B^0_d\to\pi^0K_{\rm S}$ following from an isospin relation, we uncover new tensions with the Standard Model in data for neutral $B\to\pi K$ decays. Should this intriguing picture originate from New Physics, a modified electroweak penguin sector provides a key scenario. It includes models with extra $Z'$ bosons, which offer attractive ways to resolve anomalies in $B\to K^{(*)}\ell^+\ell^-$ measurements. We present a new strategy to reveal the underlying physics, apply it to current $B\to\pi K$ data, and discuss the excellent prospects for Belle II.Comment: 7 pages, 3 figure

Decoding (Pseudo)-Scalar Operators in Leptonic and Semileptonic BB Decays

We consider leptonic $B^-\to \ell^- \bar\nu_\ell$ and semileptonic $\bar B \to \pi \ell^- \bar\nu_\ell$, $\bar B \to \rho \ell^- \bar\nu_\ell$ decays and present a strategy to determine short-distance coefficients of New-Physics operators and the CKM element $|V_{ub}|$. As the leptonic channels play a central role, we illustrate this method for (pseudo)-scalar operators which may lift the helicity suppression of the corresponding transition amplitudes arising in the Standard Model. Utilising a new result by the Belle collaboration for the branching ratio of $B^-\to \mu^- \bar\nu_\mu$, we explore theoretically clean constraints and correlations between New Physics coefficients for leptonic final states with $\mu$ and $\tau$ leptons. In order to obtain stronger bounds and to extract $|V_{ub}|$, we employ semileptonic $\bar B \to \pi \ell^- \bar\nu_\ell$ and $\bar B \to \rho \ell^- \bar\nu_\ell$ decays as an additional ingredient, involving hadronic form factors which are determined through QCD sum rule and lattice calculations. In addition to a detailed analysis of the constraints on the New Physics contributions following from current data, we make predictions for yet unmeasured decay observables, compare them with experimental constraints and discuss the impact of CP-violating phases of the New-Physics coefficients.Comment: 35 pages, 19 figures, matches published versio

CP Violation in Leptonic Rare Bs0B^0_s Decays as a Probe of New Physics

The decay $B^0_s\to\mu^+\mu^-$ is a key probe for the search of physics beyond the Standard Model. While the current measurements of the corresponding branching ratio agree with the Standard Model within the uncertainties, significant New-Physics effects may still be hiding in $B^0_s\to\mu^+\mu^-$. In order to reveal them, the observable $\mathcal{A}^{\mu\mu}_{\Delta \Gamma_s}$, which is provided by the decay width difference $\Delta\Gamma_s$ of the $B^0_s$-meson system, plays a central role. We point out that a measurement of a CP-violating observable ${\cal S}_{\rm \mu\mu}$, which is induced through interference between $B^0_s$-$\bar B^0_s$ mixing and $B_s\to\mu^+\mu^-$ decay processes, is essential to obtain the full picture, in particular to establish new scalar contributions and CP-violating phases. We illustrate these findings with future scenarios for the upgrade(s) of the LHC, exploiting also relations which emerge within an effective field theory description of the Standard Model, complemented with New Physics entering significantly beyond the electroweak scale.Comment: 36 pages, 15 figures, improved presentation, to appear in Eur. Phys. J.

Towards new frontiers in the exploration of charmless non-leptonic B decays

Non-leptonic $B$ decays into charmless final states offer an important laboratory to study CP violation and the dynamics of strong interactions. Particularly interesting are $B^0_s\to K^-K^+$ and $B^0_d\to\pi^-\pi^+$ decays, which are related by the $U$-spin symmetry of strong interactions, and allow for the extraction of CP-violating phases and tests of the Standard Model. The theoretical precision is limited by $U$-spin-breaking corrections and innovative methods are needed in view of the impressive future experimental precision expected in the era of Belle II and the LHCb upgrade. We have recently proposed a novel method to determine the $B_s^0$-$\bar{B}_s^0$ mixing phase $\phi_s$ from the $B_s^0\to K^-K^+$, $B_d^0\to \pi^-\pi^+$ system, where semileptonic $B^0_s\to K^-\ell^+\nu_\ell$, $B^0_d\to \pi^-\ell^+\nu_\ell$ decays are a new ingredient and the theoretical situation is very favourable. We discuss this strategy in detail, with a focus on penguin contributions as well as exchange and penguin-annihilation topologies which can be probed by a variety of non-leptonic $B$ decays into charmless final states. We show that a theoretical precision as high as ${\cal O}(0.5^\circ)$ for $\phi_s$ can be attained in the future, thereby offering unprecedented prospects for the search for new sources of CP violation.Comment: 50 pages, 25 figure

Testing lepton flavour universality with (semi)-leptonic

Data in B-meson decays indicate violations of lepton flavour universality, thereby raising the question about such phenomena in the charm sector. We perform a model-independent analysis of NP contributions in (semi)-leptonic decays of $$D_{(s)}$$ mesons which originate from $$c \rightarrow d \bar{{\ell }} \nu _l$$ and $$c \rightarrow s \bar{{\ell }} \nu _{\ell }$$ charged-current interactions. Starting from the most general low-energy effective Hamiltonian containing four-fermion operators and the corresponding short-distance coefficients, we explore the impact of new (pseudo)-scalar, vector and tensor operators and constrain their effects through the interplay with current data. We pay special attention to the elements $$|V_{cd}|$$ and $$|V_{cs}|$$ of the Cabibbo–Kobayashi–Maskawa matrix and extract them from the $$D_{(s)}$$ decays in the presence of possible NP decay contributions, comparing them with determinations utilizing unitarity. We find a picture in agreement with the Standard Model within the current uncertainties. Using the results from our analysis, we make also predictions for leptonic $$D_{(s)}^+ \rightarrow e^+ \nu _e$$ modes which could be hugely enhanced with respect to their tiny Standard Model branching ratios. It will be interesting to apply our strategy at the future high-precision frontier

In pursuit of new physics with B s,d 0 → ℓ + ℓ −

Abstract Leptonic rare decays of B s,d 0 mesons offer a powerful tool to search for physics beyond the Standard Model. The B s 0  → μ + μ − decay has been observed at the Large Hadron Collider and the first measurement of the effective lifetime of this channel was presented, in accordance with the Standard Model. On the other hand, B s 0  → τ + τ − and B s 0  → e + e − have received considerably less attention: while LHCb has recently reported a first upper limit of 6.8 × 10 −3 (95% C.L.) for the B s 0  → τ + τ − branching ratio, the upper bound 2.8 × 10 −7 (90% C.L.) for the branching ratio of B s 0  → e + e − was reported by CDF back in 2009. We discuss the current status of the interpretation of the measurement of B s 0  → μ + μ −, and explore the space for New-Physics effects in the other B s,d 0  → ℓ + ℓ − decays in a scenario assuming flavour-universal Wilson coefficients of the relevant four-fermion operators. While the New-Physics effects are then strongly suppressed by the ratio m μ /m τ of the lepton masses in B s 0  → τ + τ −, they are hugely enhanced by m μ /m e in B s 0  → e + e − and may result in a B s 0  → e + e − branching ratio as large as about 5 times the one of B s 0  → μ + μ −, which is about a factor of 20 below the CDF bound; a similar feature arises in B d 0  → e + e −. Consequently, it would be most interesting to search for the B s,d 0  → e + e − channels at the LHC and Belle II, which may result in an unambiguous signal for physics beyond the Standard Model