Extracting short-distance physics from KL,S→π0e+e− decays

The kaon sector is characterised by several processes which are under active investigation across different experiments. In this work, we present the global picture that emerges from a study of the different decay modes. We begin by revisiting the theoretical component of these decays and providing up-to-date predictions of the Standard Model as well as the corresponding uncertainties. Several new features emerge, in particular for $K_{S,L}\to \mu\bar\mu$, and are presented in considerable detail. This offers an ideal platform for extracting the parameter space supported by the existing data. Motivated by possible lepton flavour universality violation in $B$ decays, we investigate such Beyond the Standard Model effects also in rare kaon decays. Without loss of generality, our primary analyses correspond to the paradigm where the Wilson coefficients for operators involving tau leptons are chosen to be equal to that involving the muon, i.e. $\delta C^\tau=\delta C^\mu$. We conclude by presenting the possible picture that can be achieved towards the end of the run of data accumulation in the planned experiments. This includes assumptions on possible sensitivity goals that the experiments can aim to achieve, in order to extract the kind of physics highlighted in this paper. 

/pdf/anatomy-of-kaon-decays-and-prospects-for-lepton-flavour-sjpr03wi.pdf

Anatomy of kaon decays and prospects for lepton flavour universality violation

A bstract Rare kaon decays offer a powerful tool for investigating new physics in s → d transitions. Currently, many of the interesting decay modes are either measured with rather large uncertainties compared to their theoretical predictions or have not yet been observed. The future HIKE programme at CERN will provide unprecedented sensitivity to rare kaon decays, allowing for strong constraints on new physics scenarios with lepton flavour universality violation. We present the overall picture that emerges from a study of the different decay modes with a global analysis considering projections based on the HIKE programme, both with and without KOTO-II future measurements. We also highlight the most relevant decays and identify that in addition to the “golden channel”, $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> , the rare $$ {K}_L\to {\pi}^0\ell \overline{\ell} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>K</mml:mi> <mml:mi>L</mml:mi> </mml:msub> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mn>0</mml:mn> </mml:msup> <mml:mi>ℓ</mml:mi> <mml:mover> <mml:mi>ℓ</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> decay, especially in the electron sector offers strong constraints on short-distance physics. 

Beyond the Standard Model prospects for kaon physics at future experiments

We report on a new measurement of the branching ratio B(K{sub L}{yields}{pi}{sup 0}e{sup +}e{sup -}{gamma}) using the KTeV detector. This analysis uses the full KTeV data set collected from 1997 to 2000. We reconstruct 139 events over a background of 14, which results in B(K{sub L}{yields}{pi}{sup 0}e{sup +}e{sup -}{gamma})=(1.62{+-}0.14{sub stat}{+-}0.09{sub syst})x10{sup -8}. This result supersedes the earlier KTeV measurement of this branching ratio.

Measurement of the decay KL→π0e+e-γ

On the electron–positron energy asymmetry in KL→π0e+e−

We report on the first observation of the decay K{sub L}{yields}{pi}{sup 0}e{sup +}e{sup -} {gamma} by the KTeV E799 experiment at Fermilab. Based upon a sample of 48events with an estimated background of 3.6{+-}1.1 events, we measure the K{sub L}{yields}{pi}{sup 0}e{sup +}e{sup -} {gamma} branching ratio to be (2.34{+-}0.35{+-}0.13) x 10{sup -8} . Our data agree with recent O(p{sup 6}) calculations in chiral perturbation theory that include contributions from vector meson exchange through the parameter a{sub V} . A fit was made to the K{sub L}{yields}{pi}{sup 0}e{sup +}e{sup -} {gamma} data for a{sub V} with the result -0.67{+-}0.21{+-}0.12 , which is consistent with previous results from KTeV.

First observation of the decay K(L) --> pi0 e+ e- gamma

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First observation of the decay K(L) --> pi0 e+ e- gamma