Rare radiative and semileptonic B meson decays

Recent results at the LHCb and B-factory experiments have suggested that rare processes in B →Vγ and B → V ℓ¯ℓ decays, where V is a vector meson, show some deviation from Standard Model predictions. Although these anomalies are not yet at the level to constitute a formal discovery, they are certainly suggestive of potential New Physics effects in flavour-changing neutral currents. However, explanations within the Standard Model cannot yet be ruled out. This thesis contributes to the understanding of such anomalies in two ways. Firstly, the angular distribution of the B → KJ (→ Kπ)ℓ1¯ℓ2 decay is derived, for the full dimension-six effective weak Hamiltonian, using a generalisation of the helicity formalism to effective theories mediating b → sℓ1¯ℓ2 transitions. This approach sheds light on the origin of the underlying structure, and in the process extends the general angular distribution to decays in which the two leptons in the final state, ℓ1¯ℓ2, are not necessarily identical. An additional benefit of the derivation of the angular distribution presented in this manner is that it lends itself to a moments analysis of the decay. It is shown how the angular distribution changes in the presence of new operators, predicted to be vanishingly small in the Standard Model. Such operators could be sizeable in the presence of New Physics, but using a moments analysis enables the contribution of such operators to be assessed. Secondly, an analysis is presented of the three-particle vector and axial meson distribution amplitudes. It is shown that the distribution amplitudes of both particles are, up to QCD corrections, nearly identical. These results are applied to a new calculation of the long-distance charm loop contribution to radiative B → Vγ decays, and it is shown that the approximate symmetry can be exploited to provide an improved theoretical control in the search for New Physics contributions to right-handed currents in radiative decays

Revisiting lifetimes of doubly charmed baryons

We present updated predictions for lifetimes of doubly charmed baryons, within the heavy quark expansion, including available NLO $\alpha_s$ contributions and newly-computed terms in the $1/m_c$ series. Our improved results confirm the expected hierarchy $$\tau(\Xi_{cc}^{+}) < \tau(\Omega_{cc}^{+}) < \tau(\Xi_{cc}^{++}) \,,$$ while the predicted lifetime $\tau(\Xi_{cc}^{++}) = 0.32 \pm 0.5 ^{+0.8}_{-0.7} \,\textrm{ps}$ is consistent with the recent LHCb determination. We provide predictions for the lifetime ratios of the $\Xi_{cc}^{+}$ and $\Omega_{cc}^+$ baryons relative to the $\Xi_{cc}^{++}$ baryon, namely $\tau(\Xi_{cc}^{+})/\tau(\Xi_{cc}^{++})=0.22\pm 0.05\pm 0.04$ and $\tau(\Omega_{cc}^{+})/\tau(\Xi_{cc}^{++})=0.52\pm 0.13^{+0.03}_{-0.02}$.Comment: 25 pages, 2 figures; v2: few clarifications added, one reference added, matches version accepted for publicatio

Right-handed Currents Searches and Parity Doubling

The extraction of right-handed currents, beyond the Standard Model, faces theoretical challenges from long-distance contributions. We show that these effects can be controlled by combining, for example, studies of $B \to V(1^-) \gamma$ and $B \to A(1^+) \gamma$ observables. The sum of the long-distance contributions can be extracted without compromise, and the individual pieces follow from a ratio predicted by theory. This leads to significant reduction in the uncertainty of long-distance contributions. The ideas extend to charm decays and the low $q^2$-region of $B \to V \ell \bar{\ell}$, and open the prospect of checking input affecting the angular $B \to K^* \mu \mu$-anomaly.Comment: QCD-Moriond 2018 proceedings, 6pp + 1fi

Parity Doubling as a Tool for Right-handed Current Searches

Abstract The V-A structure of the weak interactions leads to definite amplitude hierarchies in exclusive heavy-to-light decays mediated by b → (d, s)γ and b→d,sℓℓ¯ $$b\to \left(d,\ s\right)\ell \overline{\ell}$$. However, the extraction of right-handed currents beyond the Standard Model is contaminated by V-A long-distance contributions leaking into right-handed amplitudes. We propose that these quantum-number changing long-distance contributions can be controlled by considering the almost parity-degenerate vector meson final states by exploiting the opposite relative sign of left- versus right-handed amplitudes. For example, measuring the time-dependent rates of a pair of vector V (J P = 1−) and axial A(1+) mesons in B → (V, A)γ, up to an order of magnitude is gained on the theory uncertainty prediction, controlled by long-distance ratios to the right-handed amplitude. This renders these decays clean probes to null tests, from the theory side

Generalized helicity formalism, higher moments, and the B→KJK(→Kπ)¯¯¯ℓ1ℓ2 angular distributions

We generalise the Jacob-Wick helicity formalism, which applies to sequential decays, to effective field theories of rare decays of the type $B \to K_{J_K}(\to K \pi) \bar{\ell}_1 \ell_2$. This is achieved by reinterpreting local interaction vertices $\bar b \Gamma'_{\mu_1 ..\mu_n} s \bar \ell \Gamma^{\mu_1 ..\mu_n} \ell$ as a coherent sum of $1 \to 2$ processes mediated by particles whose spin ranges between zero and $n$. We illustrate the framework by deriving the full angular distributions for $B \to K\bar{\ell}_1 \ell_2$ and $B \to K^*(\to K \pi) \bar{\ell}_1 \ell_2$ for the complete dimension-six effective Hamiltonian for non-equal lepton masses. Amplitudes and decay rates are expressed in terms of Wigner rotation matrices, leading naturally to the method of moments in various forms. We discuss how higher-spin operators and QED corrections alter the standard angular distribution used throughout the literature, potentially leading to differences between the method of moments and the likelihood fits. We propose to diagnose these effects by assessing higher angular moments. These could be relevant in investigating the nature of the current LHCb anomalies in $R_K = {\cal B}( B \to K \mu^+\mu^-) /{\cal B}( B \to K e^+e^-)$ as well as angular observables in $B \to K^* \mu^+\mu^-$.Comment: identical to version appeared in PRD. typo in Eq below C.9 corrected, includes Mathematica notebook with angular distribution same as v

Revisiting lifetimes of doubly charmed baryons

Abstract We present updated predictions for lifetimes of doubly charmed baryons, within the heavy quark expansion, including available NLO α s contributions and newly-computed terms in the 1/m c series. Our improved results confirm the expected hierarchy τ Ξ cc + < τ Ω cc + < τ Ξ cc + + , $$\tau \left({\Xi}_{cc}^{+}\right)<\tau \left({\Omega}_{cc}^{+}\right)<\tau \left({\Xi}_{cc}^{++}\right),$$ while the predicted lifetime τ Ξ cc + + $$\tau \left({\Xi}_{cc}^{++}\right)$$ = 0.32 ± 0.5 − 0.7 + 0.8 $${0.5}_{-0.7}^{+0.8}$$ ps is consistent with the recent LHCb determination. We provide predictions for the lifetime ratios of the Ξ cc + $${\Xi}_{cc}^{+}$$ and Ω cc + $${\Omega}_{cc}^{+}$$ baryons relative to the Ξ cc + + $${\Xi}_{cc}^{++}$$ baryon, namely τ Ξ cc + / τ Ξ cc + + $$\tau \left({\Xi}_{cc}^{+}\right)/\tau \left({\Xi}_{cc}^{++}\right)$$ = 0.22 ± 0.05 ± 0.04 and τ Ω cc + / τ Ξ cc + + $$\tau \left({\Omega}_{cc}^{+}\right)/\tau \left({\Xi}_{cc}^{++}\right)$$ = 0.52 ± 0.13 − 0.02 + 0.03 $${0.13}_{-0.02}^{+0.03}$$