Quo vadis flavour physics? FPCP2017 Theory Summary and Outlook

Abstract

We review the recent highlights of theoretical flavour physics, based on the theory summary talk given at FPCP2017. Over the past years, a number of intriguing anomalies have emerged in flavour violating K and B meson decays, constituting some of the most promising hints for the presence of physics beyond the Standard Model. We discuss the theory status of these anomalies and outline possible future directions to test the underlying New Physics. PoS(FPCP2017)042Quo vadis flavour physics? Monika Blanke New Physics sensitivity of flavour physicsDespite the impressive amount of data collected and analysed during run 1 and early run 2 of the LHC, until now no clear signal of new particles has been found in the direct searches for New Physics (NP). While we might have simply been unlucky in the choice of NP discovery modes, it is also conceivable that the NP scale is larger than the energies directly accessible at the LHC. Together with the continuing efforts at the high-p T frontier, it is therefore of utmost importance to explore alternative routes to access the nature of NP.Precision tests of Standard Model (SM) observables provide complementary access to NP contributions. As in such low energy processes new particles contribute via quantum effects, they are not plagued by the same strict energy cut-off as the LHC. The sensitivity in this case is limited by the cleanliness of the theoretical predictions in the SM, in addition to the obtained experimental precision. Particularly useful in this context are observables whose SM contribution is suppressed, increasing their potential NP sensitivity.Flavour and CP-violating observables hence play a key role in the hunt for NP. In the SM, flavour changing neutral current processes are strongly suppressed not only by a loop factor, but also by the smallness of the off-diagonal CKM elements and the GIM mechanism. NP contributions, even if generated well beyond the TeV scale, can therefore be significant, provided the theoretical and experiment precision is sufficient. The highest NP sensitivity is obtained in rare kaon decays due to their strong CKM suppression by V * ts V td and their theoretical cleanliness, in particular in the K → πνν decays. Indeed the latter have been shown to have the potential to probe NP scales beyond 1000 TeV [1]. B decays on the other hand, while being less sensitive to extremely high scales, have the advantage to offer a multitude of observables suitable to disentangle the underlying NP structure at work.In deciphering the NP flavour structure, the study of correlations between flavour and CPviolating observables will be a crucial task. In this context, we can distinguish between two different types of correlations:• Correlations between observables within a given meson system give information on the underlying NP operator structure. In this way, contributions from vector or scalar mediators can be distinguished, or the chirality of the NP coupling to the SM quarks and leptons can be determined.• Correlations between related observables in dif...

The dawn of the post-naturalness era

“…Of course, the main focus of today's CERN program is the LHC: data taking is in full swing and the operation of the ten-year high-luminosity phase will start in 2026. As of today, the most tantalising LHC news comes from B physics, in the form of combined anomalies in various decay processes [51]. The surprising aspect of these results is that they imply violation of lepton universality, which is not the first place at the high-energy frontier where one would have expected new physics to come up.…”

b → cτ$$ \overline{\nu} $$e,μ contributions to R(D(*))

“…• The contributions from individual scalar or tensor operators are disfavored by their different contributions to R(D) and R(D * ), by the B c lifetime and by their modification of the differential decay rates with respect to the SM [20][21][22][23]. An exception to this statement is provided by a specific combination of scalar and tensor operators.…”

Measuring CP violation in b→cτ−ν¯τ using excited charm

“…Ref. [18] and references therein) that if a single mediator is responsible for the anomaly, then there are four possible candidate mediators, labeled W µ ∼ (1, 3) 0 , U µ ∼ (3, 1) 2/3 , S ∼ (3, 1) −1/3 , V µ ∼ (3, 2) −5/6 . The main role of those mediators is to generate a significant contribution to O In what follows we study the D * πτ −ν τ final state of the D * * decays.…”

“…Under these assumptions, one finds that only four operators can break lepton universality in b → c − ν transitions [18]:…”

“…If the disagreement with the SM becomes significant, it will constitute a clear signal of physics beyond the SM. New physics (NP) explanations for this puzzle have been widely discussed in the literature, where the most popular framework is that of effective field theory (EFT) with new dimension-six operators that enhance the taunic decays by about 30% (for a review, see, e.g., [18] and references therein). In order to explain the enhancement of the central value, all NP solutions introduce hard breaking of lepton flavor symmetry.…”