Towards the identification of new physics through quark flavour violating processes

Standard Model predictions for rare K and B decays without new physics infection

The Standard Model (SM) does not contain by definition any new physics (NP) contributions to any observable but contains four CKM parameters which are not predicted by this model. We point out that if these four parameters are determined in a global fit which includes processes that are infected by NP and therefore by sources outside the SM, the resulting so-called SM contributions to rare decay branching ratios cannot be considered as genuine SM contributions to the latter. On the other hand genuine SM predictions, that are free from the CKM dependence, can be obtained for suitable ratios of the K and B rare decay branching ratios to Δ M s , Δ M d and | ε K | , all calculated within the SM. These three observables contain by now only small hadronic uncertainties and are already well measured so that rather precise SM predictions for the ratios in question can be obtained. In this context the rapid test of NP infection in the Δ F = 2 sector is provided by a | V cb | - γ plot that involves Δ M s , Δ M d , | ε K | , and the mixing induced CP-asymmetry S ψ K S . As with the present hadronic matrix elements this test turns out to be negative, assuming negligible NP infection in the Δ F = 2 sector and setting the values of these four observables to the experimental ones, allows to obtain SM predictions for all K and B rare decay branching ratios that are most accurate to date and as a byproduct to obtain the full CKM matrix on the basis of Δ F = 2 transitions alone. Using this strategy we obtain SM predictions for 26 branching ratios for rare semileptonic and leptonic K and B decays with the μ + μ - pair or the ν ν ¯ pair in the final state. Most interesting turn out to be the anomalies in the low q 2 bin in B + → K + μ + μ - ( 5.1 σ ) and B s → ϕ μ + μ - ( 4.8 σ ).

Exploring B → π π , π K decays at the high-precision frontier

The B → π π , π K system offers a powerful laboratory to probe strong and weak interactions. Using the isospin symmetry, we determine hadronic B → π π parameters from data where new measurements of direct CP violation inB d 0 → π 0 π 0 resolve a discrete ambiguity. With the help of the SU(3) flavour symmetry, the B → π π parameters can be converted into their B → π K counterparts, thereby allowing us to make predictions of observables. A particularly interesting decay isB d 0 → π 0 K S as it exhibits mixing-induced CP violation. Using an isospin relation, complemented with a robust SU(3) input, we calculate correlations between the direct and mixing-induced CP asymmetries ofB d 0 → π 0 K S , which are the theoretically cleanest B → π K probes. Interestingly, they show tensions with respect to the Standard Model. Should this B → π K puzzle originate from New Physics, electroweak penguins offer an attractive scenario for new particles to enter. We present a strategy to determine the parameters characterising these topologies and obtain the state-of-the-art picture from current data. In the future, this method will allow us to reveal the B → π K dynamics and to obtain insights into the electroweak penguin sector with unprecedented precision.

The Dual QCD @ Work: 2018

The Dual QCD (DQCD) framework, based on the ideas of ’t Hooft and Witten, and developed by Bill Bardeen, Jean-Marc Gérard and myself in the 1980s is not QCD, a theory of quarks and gluons, but a successful low energy approximation of it when applied to K → ππ decays and K¯0 - K0 mixing. After years of silence, starting with 2014, this framework has been further developed in order to improve the SM prediction for the ratio ε'/ε, the ΔI = 1/2 rule and B^K. Most importantly, this year it has been used for the calculation of all K → ππ hadronic matrix elements of BSM operators which opened the road for the general study of ε'/ε in the context of the SM effective theory (SMEFT). This talk summarizes briefly the past successes of this framework and discusses recent developments which lead to a master formula for ε'/ε valid in any extension of the SM. This formula should facilitate the search for new physics responsible for the ε'/ε anomaly hinted by 2015 results from lattice QCD and DQCD.

Universal Unitarity Triangle 2016 and the tension between [Formula: see text] and [Formula: see text] in CMFV models

Motivated by the recently improved results from the Fermilab Lattice and MILC Collaborations on the hadronic matrix elements entering [Formula: see text] in [Formula: see text]-[Formula: see text] mixing, we determine the universal unitarity triangle (UUT) in models with constrained minimal flavour violation (CMFV). Of particular importance are the very precise determinations of the ratio [Formula: see text] and of the angle [Formula: see text]. They follow in this framework from the experimental values of [Formula: see text] and of the CP-asymmetry [Formula: see text]. As in CMFV models the new contributions to meson mixings can be described by a single flavour-universal variable S(v), we next determine the CKM matrix elements [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] as functions of S(v) using the experimental value of [Formula: see text] as input. The lower bound on S(v) in these models, derived by us in 2006, implies then upper bounds on these four CKM elements and on the CP-violating parameter [Formula: see text], which turns out to be significantly below its experimental value. This strategy avoids the use of tree-level determinations of [Formula: see text] and [Formula: see text], which are presently subject to considerable uncertainties. On the other hand, if [Formula: see text] is used instead of [Formula: see text] as input, [Formula: see text] are found to be significantly above the data. In this manner we point out that the new lattice data have significantly sharpened the tension between [Formula: see text] and [Formula: see text] within the CMFV framework. This implies the presence of new physics contributions beyond this framework that are responsible for the breakdown of the flavour universality of the function S(v). We also present the implications of these results for [Formula: see text], [Formula: see text] and [Formula: see text] within the Standard Model.

Quark flavour observables in the Littlest Higgs model with T-parity after LHC Run 1

The Littlest Higgs model with T-parity (LHT) belongs to the simplest new physics scenarios with new sources of flavour and CP violation. The latter originate in the interactions of ordinary quarks and leptons with heavy mirror quarks and leptons that are mediated by new heavy gauge bosons. Also a heavy fermionic top partner is present in this model which communicates with the SM fermions by means of standard [Formula: see text] and [Formula: see text] gauge bosons. We present a new analysis of quark flavour observables in the LHT model in view of the oncoming flavour precision era. We use all available information on the CKM parameters, lattice QCD input and experimental data on quark flavour observables and corresponding theoretical calculations, taking into account new lower bounds on the symmetry breaking scale and the mirror quark masses from the LHC. We investigate by how much the branching ratios for a number of rare K and B decays are still allowed to depart from their SM values. This includes [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. Taking into account the constraints from [Formula: see text] processes, significant departures from the SM predictions for [Formula: see text] and [Formula: see text] are possible, while the effects in B decays are much smaller. In particular, the LHT model favours [Formula: see text], which is not supported by the data, and the present anomalies in [Formula: see text] decays cannot be explained in this model. With the recent lattice and large N input the imposition of the [Formula: see text] constraint implies a significant suppression of the branching ratio for [Formula: see text] with respect to its SM value while allowing only for small modifications of [Formula: see text]. Finally, we investigate how the LHT physics could be distinguished from other models by means of indirect measurements and discuss the consequences for quark flavour observables of not finding any LHT state in the coming years.