K Rare Decays with NA62

TL;DR

This paper discusses the NA62 experiment's efforts to measure ultra-rare kaon decays with high precision, testing the Standard Model and exploring CP violation through various decay channels and asymmetry measurements.

Contribution

It presents the experimental setup, preliminary results on lepton universality, and new measurements of rare charged kaon decays from the NA48/2 data, advancing precision in kaon decay studies.

Findings

Preliminary R_K measurement consistent with Standard Model

Collected large samples of rare decay events for precise analysis

Reported new measurements of branching fractions and asymmetries

Abstract

The long-term goal of NA62 is to measure the ultra rare K+/- -> pi+/- nu anti-nu decay with a sensitivity of 10^-12 per event. This is done by using the decay - in - flight technique which allows a signal acceptance of ~10% . The aim is to collect about 100 signal events in two years of data taking with a background to signal ratio smaller than 10%. The principle of the experimental measurement and the layout of the detector are presented. During 2007/2008 a dedicated run devoted to NA62 prototype tests and study of K_e2 decays was taken. The first phase of the NA62 experiment is aiming at a high precision test of the lepton universality by measuring the helicity suppressed ratio R_K. The preliminary result based on 40% of the 2007 NA62 data sample, R_K = K_e2/K_mu2 = (2.500 +/- 0.016) x 10^-5, which is the first result with a precision better than 1%, is consistent with the Standard Model. Aiming at charge asymmetry measurements, the NA48/2 experiment collected an unprecedented amount of charged K_(3 pi) events. The large samples allowed a precision measurement of rare charged kaon decays. New measurements of the K+/-_(pi l l) decays based on the full NA48/2 data sample collected during 2003/2004 are reported in this paper. Samples of about 7200 reconstructed K+/- -> pi+/- e+ e- events, and more than 3000 K+/- -> pi+/- mu+ mu- events, with a few percent background contamination, have been collected. A precise measurement of the branching fractions and the form factors of the rare decays K+/-_(pi l l)$ were performed. Measurements of the CP-violating and the forward-backward asymmetries are reported.