Search for the Θ+\Theta^{+} pentaquark via the π−p→K−X\pi^-p\to K^-X reaction at 1.92 GeV/cc

The $\Theta^+$ pentaquark baryon was searched for via the $\pi^-p\to K^-X$ reaction in a missing-mass resolution of 1.4 MeV/$c^2$(FWHM) at J-PARC. $\pi^-$ meson beams were incident on the liquid hydrogen target with the beam momentum of 1.92 GeV/$c$. No peak structure corresponding to the $\Theta^+$ mass was observed. The upper limit of the production cross section averaged over the scattering angle of 2$^{\circ}$ to 15$^{\circ}$ in the laboratory frame was obtained to be 0.26 $\mu$b/sr in the mass region of 1.51$-$1.55 GeV/$c^2$.The upper limit of the $\Theta^+$ decay width using the effective Lagrangian approach was obtained to be 0.72 MeV/$c^2$ and 3.1 MeV/$c^2$ for $J^P_{\Theta}=1/2^+$ and $J^P_{\Theta}=1/2^-$, respectively.Comment: 5 pages, 3 figures, 1 tabl

Design and construction of the cylindrical drift chamber for the COMET Phase-I experiment

Sato A., Yoshida H., Moritsu M., et al. Design and construction of the cylindrical drift chamber for the COMET Phase-I experiment. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 1069, 169926 (2024); https://doi.org/10.1016/j.nima.2024.169926.COMET is an experiment at J-PARC that searches for the charged lepton flavor violating process of neutrino-less muon-to-electron conversion (μ→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 μ→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 μ→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

High-resolution search for the Θ+\Theta^{+} pentaquark via a pion-induced reaction at J-PARC

The pentaquark $\Theta^+$ has been searched for via the $\pi^-p \to K^-X$ 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$^{\circ}$ to 15$^{\circ}$ 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 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.Comment: 12 pages, 9 figures; published versio

Search for the pentaquark at J-PARC

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 1.92 GeV/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 0.26 μb/sr. In order to make a more stringent constraint we have also performed a measurement at 2.0 GeV/c. Present analysis status of this new data is reported

Beam and SKS spectrometers at the K1.8 beam line

High-resolution spectrometers for both incident beams and scattered particles have been constructed at the K1.8 beam line of the Hadron Experimental Facility at J-PARC. A point-to-point optics is realized between the entrance and exit of QQDQQ magnets for the beam spectrometer. Fine-pitch wire chamber trackers and hodoscope counters are installed in the beam spectrometer to accept a high rate beam up to 107 Hz. The superconducting kaon spectrometer for scattered particles was transferred from KEK with modifications to the cryogenic system and detectors. A missing-mass resolution of 1.9 ± 0.1 MeV/c2 (FWHM) was achieved for the ∑ peaks of (π±, K+) reactions on a proton target in the first physics run of E19 in 2010

Search for the jet-induced diffusion wake in the quark-gluon plasma via measurements of jet-track correlations in photon-jet events in Pb+Pb collisions at sNN=5.02TeV with the ATLAS detector

This paper presents a measurement of jet-track correlations in photon-jet events, using 1.72 nb^(-1) of Pb+Pb data at root(sNN) = 5.02 TeV recorded with the ATLAS detector at the LHC. Events with energetic photon-jet pairs are selected, where the photon and jet are approximately back-to-back in azimuth. The angular correlation between jets and charged-particle tracks with transverse momentum (pT) in the range 0.5-2.0 GeV in the hemisphere opposite to the jet, | phi(jet,track)| > pi/2, is measured as a function of their relative pseudorapidity difference, | eta(jet,track)|. In central Pb+Pb collisions, these correlations are predicted to be sensitive to the diffusion wake in the quark-gluon plasma resulting from the lost energy of high-pT partons traversing the plasma, with a characteristic modification as a function of | eta(jet,track)|. The correlations are examined with different selections on the jet-to-photon pT ratio to select events with different degrees of energy loss. No diffusion wake signal is observed within the current sensitivity and upper limits at 95% confidence level on the diffusion wake amplitude are reported

Test of lepton flavour universality in W-boson decays into electrons and τ-leptons using pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

A measurement of the ratio of the branching fractions, Rτ/e = B(W → τν)/B(W → eν), is performed using a sample of W bosons originating from top-quark decays to final states containing τ-leptons or electrons. This measurement uses pp collisions at s =13TeV, collected by the ATLAS experiment at the Large Hadron Collider during Run 2, corresponding to an integrated luminosity of 140fb−1. The W → τντ (with τ → eνeντ) and W →eνe decays are distinguished using the differences in the impact parameter distributions and transverse momentum spectra of the electrons. The measured ratio of branching fractions Rτ/e = 0.975±0.012(stat.)±0.020(syst.), is consistent with the Standard Model assumption of lepton flavour universality in W-boson decays

Search for vector-like leptons coupling to first- and second-generation Standard Model leptons in pp collisions at s \sqrt{s} = 13 TeV with the ATLAS detector

A search for pair production of vector-like leptons coupling to first- and secondgeneration Standard Model leptons is presented. The search is based on a dataset of proton-proton collisions at root s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140fb-1. Events are categorised depending on the flavour and multiplicity of leptons (electrons or muons), as well as on the scores of a deep neural network targeting particular signal topologies according to the decay modes of the vector-like leptons. In each of the signal regions, the scalar sum of the transverse momentum of the leptons and the missing transverse momentum is analysed. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No significant excess above the Standard Model background expectation is observed and limits are set at 95% confidence level on the production cross-sections of vector-like electrons and muons as a function of the vector-like lepton mass, separately for SU(2) doublet and singlet scenarios. The resulting mass lower limits are 1220GeV (1270GeV) and 320GeV (400GeV) for vector-like electrons (muons) in the doublet and singlet scenarios, respectively

Configuration, Performance, and Commissioning of the ATLAS b-jet Triggers for the 2022 and 2023 LHC data-taking periods

In 2022 and 2023, the Large Hadron Collider produced approximately two billion hadronic interactions each second from bunches of protons that collide at a rate of 40 MHz. The ATLAS trigger system is used to reduce this rate to a few kHz for recording. Selections based on hadronic jets, their energy, and event topology reduce the rate to O(10) kHz while maintaining high efficiencies for important signatures resulting in b-quarks, but to reach the desired recording rate of hundreds of Hz, additional real-time selections based on the identification of jets containing b-hadrons (b-jets) are employed to achieve low thresholds on the jet transverse momentum at the High-Level Trigger. The configuration, commissioning, and performance of the real-time ATLAS b-jet identification algorithms for the early LHC Run 3 collision data are presented. These recent developments provide substantial gains in signal efficiency for critical signatures; for the Standard Model production of Higgs boson pairs, a 50% improvement in selection efficiency is observed in final states with four b-quarks or two b-quarks and two hadronically decaying tau-leptons