Masaaki Kitaguchi

Experimental signature of medium modifications for rho and omega mesons in the 12 GeV p+A reactions.

The invariant mass spectra of e+e- pairs produced in 12 GeV proton-induced nuclear reactions are measured at the KEK Proton Synchrotron. On the low-mass side of the meson peak, a significant enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.

Observation of rho/omega meson modification in nuclear matter.

We measured the invariant mass spectra of electron-positron pairs in the target rapidity region of 12-GeV p+A reactions. We have observed a significant difference in the mass spectra below the omega meson between p+C and p+Cu interactions. This difference indicates that the spectral shape of mesons is modified at normal nuclear-matter density.

Observation of the Spatial Distribution of Gravitationally Bound Quantum States of Ultracold Neutrons and its Derivation Using the Wigner Function

Ultracold neutrons (UCNs) can be bound by the potential of terrestrial gravity and a reflecting mirror. The wave function of the bound state has characteristic modulations. We carried out an experiment to observe the vertical distribution of the UCNs above such a mirror at the Institut Laue-Langevin in 2011. The observed modulation is in good agreement with that prediction by quantum mechanics using the Wigner function. The spatial resolution of the detector system is estimated to be 0.7  μm. This is the first observation of gravitationally bound states of UCNs with submicron spatial resolution.

Nuclear-matter modification of decay widths in the phi-->e+e- and phi-->K+K- channels.

The invariant mass spectra of phi-->K+K- are measured in 12 GeV p+A reactions in order to search for the in-medium modification of phi mesons. The observed K+K- spectra are well reproduced by the relativistic Breit-Wigner function with a combinatorial background shape in three betagamma regions between 1.0 and 3.5. The nuclear mass number dependence of the yields of the K+K- decay channel is compared to the simultaneously measured e+e- decay channel for carbon and copper targets. We parameterize the production yields as sigma(A)=sigma0Aalpha and obtain alphaphi-->K+K- -alphaphi-->e+e- to be 0.14+/-0.12. Limits are obtained for the partial decay widths of the phi mesons in nuclear matter.

Experimental signature of in-medium mass modification of vector mesons at normal nuclear density

We measured invariant mass spectra of e+e− pairs produced in 12 GeV p + A interaction to investigate in-medium mass modification of vector mesons. We observed an enhancement over the known hadronic sources on the low-mass side of the ω meson mass peak. The obtained ρ/ω ratio indicates that this enhancement mainly results from ρ mesons whose mass was modified in nuclear matter. We performed a toy model calculation based on the prediction using the QCD sum rule. It reproduced well the tendency of our data.

Development of a Pixel Detector for Ultra-Cold Neutrons

Abstract A pixel detector with high spatial resolution and temporal information for ultra-cold neutrons is developed based on a commercial CCD on which a neutron converter is attached. 10 B and 6 Li are tested for the neutron converter and 10 B is found to be more suitable based on efficiency and spatial resolution. The pixel detector has an efficiency of 44.1 ± 1.1 % and a spatial resolution of 2.9 ± 0.1 μ m (1 sigma).

Nuclear mass number dependence of inclusive production of {omega} and {phi} mesons in 12 GeV p+A collisions

The inclusive production of {omega} and {phi} mesons is studied in the backward region of the interaction of 12 GeV protons with polyethylene, carbon, and copper targets. The mesons are measured in e{sup +}e{sup -} decay channels. The production cross sections of the mesons are presented as functions of rapidity y and transverse momentum p{sub T}. The nuclear mass number dependences (A dependences) are found to be A{sup 0.710{+-}}{sup 0.021(stat){+-}}{sup 0.037(syst)} for {omega} mesons and A{sup 0.937{+-}}{sup 0.049(stat){+-}}{sup 0.018(syst)} for {phi} mesons in the region of 0.9

Development of time projection chamber for precise neutron lifetime measurement using pulsed cold neutron beams

Abstract A new time projection chamber (TPC) was developed for neutron lifetime measurement using a pulsed cold neutron spallation source at the Japan Proton Accelerator Research Complex (J-PARC). Managing considerable background events from natural sources and the beam radioactivity is a challenging aspect of this measurement. To overcome this problem, the developed TPC has unprecedented features such as the use of polyether-ether-ketone plates in the support structure and internal surfaces covered with 6 Li-enriched tiles to absorb outlier neutrons. In this paper, the design and performance of the new TPC are reported in detail.

A study of resolution function on a MIEZE spectrometer

We have installed a MIEZE (modulated intensity by zero effort) spectrometer in the C3-1-2-2(MINE1) port at JRR-3M. In the MIEZE spectrometer, a sample is placed after the analyzer. Thus the contrast of the MIEZE signal can be observed without reduction even in the case of magnetic scattering where the signal for NSE (neutron spin echo) and NRSE (neutron resonance spin echo) instruments is less than half. The MIEZE signal is sensitive to the dispersion of neutron trajectories from the sample to the detector. In other words, the MIEZE signal is reduced only due to the dispersion even without in- and quasi-elastic scattering. We studied the correlation between experimental configuration and its resolution function using the Monte Carlo simulation. We prepared a magnetite ferrofluid sample with a typical particle diameter of 84 A, and examined the simulation model by experiments.

Pulsed neutron time-dependent intensity modulation for quasi-elastic neutron scattering spectroscopy

We propose a basic formula and demonstration for a high-resolution quasi-elastic neutron scattering (QENS) by combining the time-of-flight (TOF) method with Modulation of Intensity by Zero Effort (MIEZE) type neutron spin echo spectroscopy. The MIEZE technique has the potential to develop a unique approach to study on slow dynamics of condensed matter; however, the energy resolution is limited owing to the hypersensitivity of the MIEZE signal contrast to the echo condition, which is strongly affected by the alignment of the instruments and the sample. The narrow allowance of the optimal alignment is a major obstacle to the wide use of this technique. Combining the TOF method with MIEZE (TOF-MIEZE), the hypersensitivity of MIEZE signals is significantly alleviated with a short pulsed beam. This robustness is very useful to optimize experimental alignments and enables accurate measurements of QENS. The experimental results demonstrate the characteristic of the TOF-MIEZE technique and are well described by the formula presented in this study.