COMET is an experiment at J-PARC that searches for the charged lepton flavor violating process of neutrinoless muon-to-electron conversion (mu -> e conversion) in a muonic atom. The COMET Phase-I experiment, which represents the first stage of its staged approach, utilizes the cylindrical drift chamber (CDC) as the main detector to measure the momentum of electrons emitted from mu -> e conversion in a 1 T magnetic field. The CDC is designed for optimal performance to achieve the best momentum resolution and effectively separate the mu -> e conversion electron signal from background signals. This design entails adapting He:i-C4H10 (90:10) as its chamber gas and employing an all-stereo wire configuration. This document provides a comprehensive overview of the CDC detector, encompassing design considerations, mechanical structure, construction methodologies, and results of performance evaluations conducted using cosmic-rays.

In an emulsion-counter hybrid experiment performed at J-PARC, a Ξ^{-} absorption event was observed which decayed into twin single-Λ hypernuclei. Kinematic calculations enabled a unique identification of the reaction process as Ξ^{-}+^{14}N→_{Λ}^{10}Be+_{Λ}^{5}He. For the binding energy of the Ξ^{-} hyperon in the Ξ^{-}-^{14}N system a value of 1.27±0.21  MeV was deduced. The energy level of Ξ^{-} is likely a nuclear 1p state which indicates a weak ΞN-ΛΛ coupling.

The performance of a small prototype of a cylindrical drift chamber (CDC) used in the COMET Phase-I experiment was studied by using an electron beam. The prototype chamber was constructed with alternating all-stereo wire configuration and operated with the He-iC(4)H(10)(90/10) gas mixture without a magnetic field. The drift space-time relation, drift velocity, dE/dx resolution, hit efficiency, and spatial resolution as a function of distance from the wire were investigated. The average spatial resolution of 150 mu m with the hit efficiency of 99% was obtained at applied voltages higher than 1800 V. We have demonstrated that the design and gas mixture of the prototype match the operation of the COMET CDC.

The Technical Design for the COMET Phase-I experiment is presented in this paper. COMET is an experiment at J-PARC, Japan, which will search for neutrinoless conversion of muons into electrons in the field of an aluminium nucleus ($\mu-e$ conversion, $\mu^- N \to e^- N$); a lepton flavor violating process. The experimental sensitivity goal for this process in the Phase-I experiment is $3.1\times10^{-15}$, or 90 % upper limit of branching ratio of $7\times 10^{-15}$, which is a factor of 100 improvement over the existing limit. The expected number of background events is 0.032. To achieve the target sensitivity and background level, the 3.2 kW 8 GeV proton beam from J-PARC will be used. Two types of detectors, CyDet and StrECAL, will be used for detecting the \mue conversion events, and for measuring the beam-related background events in view of the Phase-II experiment, respectively. Results from simulation on signal and background estimations are also described.

A double-$\Lambda$ hypernucleus, ${}_{\Lambda\Lambda}\mathrm{Be}$, was observed by the J-PARC E07 collaboration in nuclear emulsions tagged by the $(K^{-},K^{+})$ reaction. This event was interpreted as a production and decay of $ {}_{\Lambda\Lambda}^{\;10}\mathrm{Be}$, ${}_{\Lambda\Lambda}^{\;11}\mathrm{Be}$, or ${}_{\Lambda\Lambda}^{\;12}\mathrm{Be}^{*}$ via $\Xi^{-}$ capture in ${}^{16}\mathrm{O}$. By assuming the capture in the atomic 3D state, the binding energy of two $\Lambda$ hyperons$\,$($B_{\Lambda\Lambda}$) of these double-$\Lambda$ hypernuclei are obtained to be $15.05 \pm 0.11\,\mathrm{MeV}$, $19.07 \pm 0.11\,\mathrm{MeV}$, and $13.68 \pm 0.11\,\mathrm{MeV}$, respectively. Based on the kinematic fitting, ${}_{\Lambda\Lambda}^{\;11}\mathrm{Be}$ is the most likely explanation for the observed event.

The first commissioning of a bunched slow extraction of a 8 GeV proton beam was performed at J-PARC for the COMET experiment in January and February, 2018. The beam profile of the extracted beam was measured at the primary proton beamline in Hadron Facility and the resulting 2σ emittance was 0.78 π mm mrad and 4.6 π mm mrad in horizontal and vertical direction, respectively. The beam extinction factor of the secondary beam was also measured for the first time at the K1.8 beamline.

We searched for the bound state of the neutron-rich Lambda-hypernucleus H-6(Lambda), using the Li-6(pi(-), K+)X double charge-exchange reaction at a pi-beam momentum of 1.2 GeV/c at J-PARC. A total of 1.4 x 10(12) pi(-) was driven onto a Li-6 target of 3.5-g/cm(2) thickness. No event was observed below the bound threshold, i.e., the mass of H-4(Lambda) + 2n, in the missing-mass spectrum of the Li-6(pi(-), K+) X reaction in the 2 degrees < theta(pi K) < 20 degrees angular range. Furthermore, no event was found up to 2.8 MeV/c(2) above the bound threshold. We obtained the double-differential cross section spectra of the Li-6(pi(-), K+)X reaction in the angular range of 2 degrees < theta(pi K) < 14 degrees. An upper limit of 0.56 nb/sr (90% C.L.) was obtained for the production cross section of the H-6(Lambda) hypernucleus bound state. In addition, not only the bound state region, but also the Lambda continuum region and part of the Sigma(-) quasifree production region of the Li-6(pi(-), K+) reaction were obtained with high statistics. The present missing-mass spectrum will facilitate the investigation of the Sigma(-) -nucleus optical potential for Sigma(-) -He-5 through spectrum shape analysis.

The pentaquark Theta(+) has been searched for via the pi(-) p --> K- X reaction with beam momenta of 1.92 and 2.01 GeV/c at J-PARC. A missing mass resolution of 2 MeV (FWHM) was achieved but no sharp peak structure was observed. The upper limits on the production cross section averaged over the scattering angle from 2 degrees to 15 degrees in the laboratory frame were found to be less than 0.28 mu b/sr at the 90% confidence level for both the 1.92- and 2.01- GeV/c data. The systematic uncertainty of the upper limits was controlled within 10%. Constraints on the Theta(+) decay width were also evaluated with a theoretical calculation using an effective Lagrangian. The present result implies that the width should be less than 0.36 and 1.9 MeV for the spin-parity of 1/2(+) and 1/2(-), respectively.

We carried out an experiment to produce the neutron-rich hypernucleus Λ6H via the (π−, K+) reaction on 6Li target at the pion beam momentum of 1.2 GeV/c (J-PARC E10). In order to calibrate the scale of the missing-mass or of the Λ binding energy of the hypernucleus, we also measured the 12C(π+, K+) Λ12C, p(π−, K+)Σ− and p(π+, K+)Σ+reactions. The experiment was performed at the J-PARC Hadron Hall K1.8 beam line in December 2012 and January 2013. The overall collected data sample corresponds to an integrated beam intensity of 1.65 × 1012 pions.

We have carried out an experiment to search for a neutron-rich hypernucleus, by the 6(Lambda)H by the 6Li(pi(-), K+) reaction at p(pi)- = 1.2 GeV/c. The obtained missing-mass spectrum with an estimated energy resolution of 3.2 MeV (FWHM) showed no peak structure corresponding to the 6(Lambda)H hypernucleus neither below nor above the 4(Lambda)H+2n particle decay threshold. An upper limit of th production cross section for the bound 6(Lambda)H hypernucleus was estimated to be 1.2 nb/sr at 90% confidence level. (c) 2014 The Authors. Published by Elsevier B.V. All rights reserved.

An antikaon and a nucleus may form a bound state (a kaonic nucleus), due to the strong attraction of K bar -N in I=0. A K − pp bound state, which is the bound state of an antikaon and two nucleons, with the total charge +1 and the isospin 1/2, is expected to be the simplest kaonic nucleus. Various theoretical calculations for a K − pp bound state have been carried out and the existence of the bound state is supported in these calculations. However, the binding energy and the decay width depend on the K bar -N interactions and the calculation methods [1]. Several experiments have been performed to look for a signal of kaonic nuclei [2,3]. However, any firm experimental evidence has not been found. An experimental search for the K − pp bound state is performed by using the d(π + , K + ) reaction at J-PARC K1.8 beam line (J-PARC E27experiment). Fig.1 shows the experimental setup of this experiment. The first data taking as a pilot run was carried out in June, 2012. The missing-mass spectrum of this reaction studied at the beam momentum of 1.7 GeV/c, which allows the production of Λ(1405), was obtained for the first time and a significant peak shift by ~30 MeV was observed in the Y* region. In a preliminary proton-coincidence analysis, a sharp spike due to the ΣN-ΛN coupling and a broad enhancement around 2.3 GeV/c 2 , which might be attributed to the K − pp bound state, was clearly observed. At this symposium, the preliminary results of the inclusive and coincidence analysis will be discussed.

We have been searching for the Θ+ pentaquark via the π−p→K−X reaction at the J-PARC hadron facility. No peak structure was observed in the missing mass spectrum obtained at 1.92GeV/c beam momentum. The upper limit for the production cross section averaged over the scattering angle from 2° to 15° in the laboratory frame was derived to be 0.26μb/sr. In order to make a more stringent constraint we have also performed a measurement at 2.0GeV/c. Present analysis status of this new data is reported.

A novel experiment, aiming at demonstrating the feasibility of hypernuclear spectroscopy with heavy ion beams, was conducted. Using the invariant mass method, the spectroscopy of hypernuclear products of Li-6 projectiles on a carbon target at 2 A GeV was performed. Signals of the Lambda-hyperon and H-3(Lambda) and H-4(Lambda) hypernuclei were observed for final states of p + pi(-), He-3 + pi(-) and He-4 + pi(-), respectively, with significance values of 6.7, 4.7 and 4.9 sigma. By analyzing the proper decay time from secondary vertex distribution with the unbinned maximum likelihood fitting method, their lifetime values were deduced to be 262(-43)(+56) +/- 45 ps for Lambda, 183(-32)(+42) +/- 37 ps for H-3(Lambda), and 140(-33)(+48) +/- 35 ps for H-4(Lambda). (c) 2013 Elsevier B.V. All rights reserved.

The first experiment at the J-PARC hadron facility, the J-PARC E19 experiment, aims at searching for the Θ + pentaquark in the hadronic reaction π − p → K − X using the missing-mass technique. Based on a superconducting magnet excited at 2.5 T, the spectrometer achieved the high mass resolution of 1.4 MeV/c2 for the Θ + production process. The first data taking was performed in the autumn of 2010. No significant structure was observed in the missing-mass spectrum. The upper limit obtained for the differential cross section is 0.26 μb/sr in the laboratory frame at a 90 % CL.