Development of water Čerenkov detector for on-line proton rejection in Ξ− hypernuclear spectroscopy via the (K

Abstract

The missing mass spectroscopy of Ξ − hypernuclei with the (K − ,K + ) reaction is planned to be performed at the J-PARC K1.8 beam line by using a new magnetic spectrometer, Strangeness −2 Spectrometer (S-2S). AČerenkov detector with a radiation medium of pure water (refractive index of 1.33) is designed to be used for on-line proton rejection for a momentum range of 1.2 to 1.6 GeV/c in S-2S. Prototype waterČerenkov detectors were developed and tested with positron beams and cosmic rays to estimate their proton-rejection capability. We achieved an average number of photoelectrons of greater than 200 with the latest prototype for cosmic rays, which was stable during an expected beam time of one month. The performance of the prototype in the cosmic-ray test was well reproduced with a Monte Carlo simulation in which some input parameters were adjusted. Based on the Monte Carlo simulation, we expect to achieve > 90% proton-rejection efficiency while maintaining > 95% K + survival ratio in the whole S-2S acceptance. The performance satisfies the requirements to conduct the spectroscopic study of Ξ − hypernuclei at J-PARC.

Preparation status of missing-mass spectroscopy for 𝚵 hypernuclei with S-2S magnetic spectrometer

“…TOF and WC [10,11] were newly designed. On the other hand, AC was used for the experiment a long time ago.…”

The New Stage Of S = -2 Hypernuclear Study Opened With A New High-resolution Spectrometer

“…The design momentum resolution of the S − 2S is 5 × 10 −4 FW HM . The K + trigger signals are generated with a time-of-flight scintillation counter (TOF), an aerogelČerenkov counter (AC: refractive index n=1.055) for π + veto, and a waterČerenkov counter (WC: n=1.33) for proton veto [10], as T OF ⊗ AC ⊗ WC. The particle identification is carried out with the TOF counter in off-line analyses by correcting the flight path and momentum obtained from the tracking in S − 2S.…”

Spectroscopy of Nuclei with Multi-Strangeness by Using New S-2S Spectrometer at J-PARC

“…Scattered K + s, which are produced in the (K − , K + ) reaction at the forward scattering angle (θ Lab K + ≤ 0.25 rad), have momenta of approximately 1.3 GeV/c, and they will be measured with a new magnetic spectrometer, S-2S [14]. The momentum coverage of S-2S ranges from 1.2 to 1.6 GeV/c with an acceptance-solid angle of 60 msr for 1.3-GeV/c particles [15]. One of the important features of S-2S is a better momentum resolution with respect to that of SKS by more than a factor of five.…”

“…On the other hand, we newly designed a waterČerenkov detector to separate protons from K + s, taking into account the S-2S geometrical and momentum acceptances [15,21]. The performance of the latest prototype coupled to S-2S was evaluated by a Monte Carlo simulation, in which some parameters were adjusted to reproduce results of a cosmic-ray test such as the number of photoelectrons, position dependence and so on.…”

“…The performance of the latest prototype coupled to S-2S was evaluated by a Monte Carlo simulation, in which some parameters were adjusted to reproduce results of a cosmic-ray test such as the number of photoelectrons, position dependence and so on. As a result, the waterČerenkov detector is expected to have a capability to reject > 95% of protons with maintaining a K + survival of > 90% at the trigger level [15]. The full-scale waterČerenkov detector is under construction, and will be completed in 2016.…”