Long-lived neutral-kaon flux measurement for the KOTO experiment

Abstract

The KOTO (K 0 at Tokai) experiment aims to observe the CP-violating rare decay K L → π 0 ν ν by using a long-lived neutral-kaon beam produced by the 30 GeV proton beam at the Japan Proton Accelerator Research Complex. The K L flux is an essential parameter for the measurement of the branching fraction. Three K L neutral decay modes, K L → 3π 0 , K L → 2π 0 , and K L → 2γ were used to measure the K L flux in the beam line in the 2013 KOTO engineering run. A Monte Carlo simulation was used to estimate the detector acceptance for these decays. Agreement was found between the simulation model and the experimental data, and the remaining systematic uncertainty was estimated at the 1.4% level. The K L flux was measured as (4.183 ± 0.017 stat. ± 0.059 sys. ) × 10 7 K L per 2 × 10 14 protons on a 66-mm-long Au target.

Search for KL→π0νν¯ and Ahn1, Beckford2, Beechert3 et al. 2019Phys. Rev. Lett.Self Cite168111581View full textAdd to dashboardCite

“…The neutral beam, composed of neutrons, photons, and K L 's, was collimated by two collimators made of iron and tungsten to a size of 8 × 8 cm 2 by the end of the 20-m-long beam line. The peak K L momentum was 1.4 GeV=c, and the K L flux was measured [24,25] as 4.2 × 10 7 K L 's per 2 × 10 14 protons on the target at the exit of the beam line. The neutron (kinetic energy > 100 MeV) and photon (energy > 10 MeV) fluxes were estimated to be 6 and 7 times larger than the kaon, respectively.…”

Imprint of a new light particle at KOTO?

“…In the following, we will explain our procedure and compare our result on K L → π 0 X with KOTO's to verify our simulation. We first generate initial K L particles according to the momentum distribution of K L measured experimentally [43], which will have a certain probability of decay in the detector. All of the K L decay modes in this paper produce two photons, and the distributions of the energies and positions of the photons are largely dependent on the probability distribution functions.…”

Cosmological implications of the KOTO excess

“…The layout of the KOTO beam line and the KOTO detector is described, e.g., in [53]. We start by generating K L momenta p K L , and K L decay vertex locations z K L based on the distribution…”

“…where z exit ¼ 20 m is the distance of the beam exit from the target, and gðp K L Þ is the measured K L momentum distribution at the beam exit from [53]. We include a small transverse component of the K L momentum such that the beam profile at the beam exit is constant within an 8.5 cm × 8.5 cm square and zero outside [53].…”