Abstract:The Large Hadron Collider (LHC) will directly probe distance scales as short as 10 −19 m, corresponding to energy scales at the level of a few TeV. In order to reach even higher resolutions before the advent of future high-energy colliders, it is necessary to consider indirect probes of New Physics (NP), a prime example being ∆F = 2 neutral meson mixing processes, which are sensitive to much shorter distance scales. However ∆F = 2 processes alone cannot tell us much about the structure of NP beyond the LHC scales. To identify for instance the presence of new quark flavour-changing dynamics of a left-handed (LH) or right-handed (RH) nature, complementary results from ∆F = 1 rare decay processes are vital. We therefore address the important question of whether NP could be seen up to energy scales as high as 200 TeV, corresponding to distances as small as O(10 −21 ) m -the Zeptouniverse -in rare K and B s,d decays, subject to present ∆F = 2 constraints and perturbativity. We focus in particular on a heavy Z gauge boson. If restricted to purely LH or RH Z couplings to quarks, we find that rare K decays, in particular K + → π + νν and K L → π 0 νν, allow us to probe the Zeptouniverse. On the other hand rare B s and B d decays, which receive stronger ∆F = 2 constraints, allow us to reach about 15 TeV. Allowing for both LH and RH couplings a loosening of the ∆F = 2 constraints is possible, and we find that the maximal values of M Z at which NP effects could be found that are consistent with perturbative couplings are approximately 2000 TeV for K decays and 160 TeV for rare B s,d decays. Because Z exchanges in the B s,d → µ + µ − rare decays are helicity suppressed, we also consider tree-level scalar exchanges for these decays, for which we find that scales close to 1000 TeV can be probed for the analogous pure and combined LH and RH scenarios. We further present a simple idea for an indirect determination of M Z that could be realised at the next linear e + e − or µ + µ − collider and with future precise flavour data.

Abstract:We present an analysis of the rare exclusive B decays B → Kνν and B → K * νν within the Standard Model (SM), in a model-independent manner, and in a number of new physics (NP) models. Combining new form factor determinations from lattice QCD with light-cone sum rule results and including complete two-loop electroweak corrections to the SM Wilson coefficient, we obtain the SM predictions BR(B + → K + νν) = (4.0 ± 0.5) × 10 −6 and BR(B 0 → K * 0 νν) = (9.2 ± 1.0) × 10 −6 , more precise and more robust than previous estimates. Beyond the SM, we make use of an effective theory with dimension-six operators invariant under the SM gauge symmetries to relate NP effects in b → sνν transitions to b → s + − transitions and use the wealth of experimental data on B → K ( * ) + − and related modes to constrain NP effects in B → K ( * ) νν. We then consider several specific NP models, including Z models, the MSSM, models with partial compositeness, and leptoquark models, demonstrating that the correlations between b → sνν observables among themselves and with B s → µ + µ − and b → s + − transitions offer powerful tests of NP with new right-handed couplings and non-MFV interactions.

In view of the recent start of the NA62 experiment at CERN that is expected to measure the K + → π + νν branching ratio with a precision of 10%, we summarise the present status of this promising decay within the Standard Model (SM). We do likewise for the closely related K L → π 0 νν, which will be measured by the KOTO experiment around 2020. As the perturbative QCD and electroweak corrections in both decays are under full control, the dominant uncertainties within the SM presently originate from the CKM parameters |V cb |, |V ub | and γ. We show this dependence with the help of analytic expressions as well as accurate interpolating formulae. Unfortunately a clarification of the discrepancies between inclusive and exclusive determinations of |V cb | and |V ub | from tree-level decays will likely require results from the Belle II experiment available at the end of this decade. Thus we investigate whether higher precision on both branching ratios is achievable by determining |V cb |, |V ub | and γ by means of other observables that are already precisely measured. In this context ε K and ∆M s,d , together with the expected progress in QCD lattice calculations will play a prominent role. We find B(K + → π + νν) = (9.11 ± 0.72) × 10 −11 and B(K L → π 0 νν) = (3.00 ± 0.30) × 10 −11 , which is more precise than using averages of the present tree-level values of |V cb |, |V ub | and γ. Furthermore, we point out the correlation between B(K + → π + νν), B(B s → µ + µ − ) and γ within the SM, that is only very weakly dependent on other CKM parameters. Finally, we update the correlation of K L → π 0 νν with the ratio ε /ε in the SM taking the recent progress on ε /ε from lattice QCD and the large N approach into account.