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

New physics searches at kaon and hyperon factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

Status of the NA62 Experiment

The decay $ {K^ + } \to {\pi ^ + }v\bar v $, with a very precisely predicted branching ratio of less than 10−10 in the Standard Model framework, is one of the best candidates to reveal indirect effects of new physics at the highest mass scales. The NA62 experiment at CERN SPS is designed to measure the branching ratio of such decay with a decay-in-flight technique, novel for this channel. The main goal of NA62 is to measure such Branching Ratio of $ {K^ + } \to {\pi ^ + }v\bar v $ with an accuracy better than 20%. This will be achieved by collecting up to 100 $ {K^ + } \to {\pi ^ + }v\bar v $ events with a background contamination at the level of 10%. The NA62 detector was commissioned in 2014 and 2015 and the experiment took physics data from 2015 to 2018. NA62 has already published the result of the 2016 data analysis. The 2017 data analysis is in progress, the single event sensitivity reached and the evaluation of the main backgrounds will be shown in this contribution.

Results and prospects for K → πνν̄ at NA62 and KOTO

The K → πνν̄ ultra-rare decays are precisely computed in the Standard Model (SM) and are ideal probes for physics beyond the SM. The NA62 experiment at the CERN SPS is designed to measure the charged channel with a precision of 10%. The statistics collected in 2016 allows to reach the SM sensitivity. The KOTO experiment at J-PARC aims at reaching the SM sensitivity before performing a measurement with ∼ 100 signal events. The NA62 preliminary result for the charged channel is presented, together with the current experimental status of the neutral channel and their prospects for the coming years.

First Results on the Rare Decay K+ → π+νν from the NA62 Experiment at CERN

The NA62 experiment at CERN Super Proton Synchrotron (SPS) is currently taking data to measure the ultra-rare decay K + → π + ν ν ¯ . This decay, whose Branching Ratio (BR) is predicted with high precision within the Standard Model (SM), is one of the best candidates to reveal the indirect effects of New Physics (NP) at the highest mass scales. The NA62 experiment is designed to measure BR ( K + → π + ν ν ¯ ) with a decay-in-flight technique, novel for this channel. NA62 took data in 2016, 2017 and 2018; statistics collected in 2016 allows NA62 to reach the SM sensitivity for this decay, reaching the single event sensitivity (SES) and showing the proof of principle of the experiment. The preliminary result on BR ( K + → π + ν ν ¯ ) from the analysis of the 2016 data set is described.

Search for Exotic Particles at the NA62 Experiment

The NA62 experiment at the CERN Super Proton Synchrotron (SPS) is currently taking data to measure the ultra-rare decay K + → π + ν ν ¯ . The high-intensity setup, trigger flexibility, detector performance and high-efficiency vetoes make NA62 also suitable for direct searches of long-lived, beyond-the-Standard-Model particles, such as Heavy Neutral Leptons (HNLs), Axion-Like Particles (ALPs) and Dark Photons (DPs); moreover, many rare and forbidden decays are studied at NA62. The status of all these searches is reviewed, together with prospects for future data taking at NA62 after the CERN Long Shutdown 2 (LS2).

Direct CP violation in [Formula: see text]: Standard Model Status

In 1988 the NA31 experiment presented the first evidence of direct CP violation in the [Formula: see text] decay amplitudes. A clear signal with a [Formula: see text] statistical significance was later established with the full data samples from the NA31, E731, NA48 and KTeV experiments, confirming that CP violation is associated with a [Formula: see text] quark transition, as predicted by the Standard Model. However, the theoretical prediction for the measured ratio [Formula: see text] has been a subject of strong controversy along the years. Although the underlying physics was already clarified in 2001, the recent release of improved lattice data has revived again the theoretical debate. We review the current status, discussing in detail the different ingredients that enter into the calculation of this observable and the reasons why seemingly contradictory predictions were obtained in the past by several groups. An update of the Standard Model prediction is presented and the prospects for future improvements are analysed. Taking into account all known short-distance and long-distance contributions, one obtains [Formula: see text], in good agreement with the experimental measurement.

Recent results from NA62 and NA48

This paper reviews recent results form the NA62 and NA48 experiments at CERN. NA62 has searched for the K → πνν¯ with an in-flight decay technique. The analysis of 1.2 · 1011 decays has found one event compatible with the Standard Model prediction, giving an upper limit for the branching ratio of 1.4 · 10-9 (95% CL) in the background assumption. Prospects for improvements in the measurement are given. Results on the Kl3 decay form factors from NA48/2 are presented as well as the first observation of the decay K± → π ± π0e+e- and the measurement of its branching ratio from the same experiment.

First results on the K+ → π+vv¯ decay search from NA62

The precise measurement of the branching ratio of an ultrarare decay K+ → π+vv¯ (~10-10 according to the calculation within the Standard Model) allows to probe New Physics via indirect effects at mass scales higher than those accessible at the LHC. The NA62 experiment at the CERN SPS is aimed at measuring this branching ratio with the 10% precision. To achieve such level of precision, a novel decay-in-flight technique is used. The statistics collected during the first NA62 physics run in 2016 allowed to demonstrate the proof of the experimental method and obtain O(10-10) single event sensitivity. The preliminary results based on the 2016 data set are described.

Final state interactions in [Formula: see text] decays: [Formula: see text] rule vs. [Formula: see text]

Dispersive effects from strong [Formula: see text] rescattering in the final state interaction (FSI) of weak [Formula: see text] decays are revisited with the goal to have a global view on their relative importance for the [Formula: see text] rule and the ratio [Formula: see text] in the standard model (SM). We point out that this goal cannot be reached within a pure effective (meson) field approach like chiral perturbation theory in which the dominant current-current operators governing the [Formula: see text] rule and the dominant density-density (four-quark) operators governing [Formula: see text] cannot be disentangled from each other. But in the context of a dual QCD approach, which includes both long-distance dynamics and the UV completion, that is, QCD at short-distance scales, such a distinction is possible. We find then that beyond the strict large N limit, N being the number of colours, FSIs are likely to be important for the [Formula: see text] rule but much less relevant for [Formula: see text]. The latter finding diminishes significantly hopes that improved calculations of [Formula: see text] would bring its SM prediction to agree with the experimental data, opening thereby an arena for important new physics contributions to this ratio.

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.