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Register a new userStudy of in-medium mass modification at J-PARC
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Authors
Kazuya Aoki
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Study of in-medium mass modification has attracted interest in terms of the restoration of the spontaneously broken chiral symmetry, which is responsible for the generation of hadron mass. Many experiments were performed to measure in-medium property of hadrons but there is no consensus yet. J-PARC E16 has been proposed to study in-medium property of vector mesons via dilepton decay channel. The status of spectrometer R & D is explained. Other related experiments planned at J-PARC are also introduced.
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With an appropriate hard scale, exclusive hadronic processes could provide novel information of the internal quark-gluon configurations of hadrons. The availability of 10-20 GeV secondary meson beam in the coming high-momentum beam line of Hadron Hall at J-PARC offers an unique opportunity to carry out the measurements of the exclusive hard processes. We address this interesting approach by two possibilities: (a) $pi^- p to K^0 Lambda(1405)$ for the constituent quark structure of $Lambda(1405)$, and (b) exclusive pion-induced Drell-Yan process $pi^- p to gamma^* n to l^+ l^- n$ for the generalized parton distributions (GPDs) of nucleons. Realization of such measurements at J-PARC will open up a new way of accessing the internal quark structure of exotic hadrons and also the nucleon GPDs based on a solid theoretical foundation of perturbative QCD.
The neutron lifetime is one of the basic parameters in the weak interaction, and is used for predicting the light element abundance in the early universe. Our group developed a new setup to measure the lifetime with the goal precision of 0.1% at the polarized beam branch BL05 of MLF, J-PARC. The commissioning data was acquired in 2014 and 2015, and the first set of data to evaluate the lifetime in 2016, which is expected to yield a statistical uncertainty of O(1)%. This paper presents the current analysis results and the future plans to achieve our goal precision.
Invariant mass spectra of e+e- pairs have been measured in 12 GeV p+A reactions to detect possible in-medium modification of vector mesons. Copper and carbon targets are used to study the nuclear-size dependence of e+e- invariant mass distributions. A significant excess on the low-mass side of the phi meson peak is observed in the low beta gamma (=beta/sqrt(1-beta^2)) region of phi mesons (beta gamma < 1.25) with copper targets. However, in the high beta gamma region (beta gamma > 1.25), spectral shapes of phi mesons are well described by the Breit-Wigner shape when experimental effects are considered. Thus, in addition to our earlier publication on rho / omega modification, this study has experimentally verified vector meson mass modification at normal nuclear density.
We studied the feasibility of an experiment searching for sub-millicharged particles ($chi$s) using 30 GeV proton fixed-target collisions at J-PARC. The detector is composed of two layers of stacked scintillator bars and PMTs and is proposed to be installed 280 m from the target. The main background is a random coincidence between two layers due to dark counts in PMTs, which can be reduced to a negligible level using the timing of the proton beam. With $N_textrm{POT}=10^{22}$ which corresponds to running the experiment for three years, the experiment provides sensitivity to $chi$s with the charge down to $5times10^{-5}$ in $m_chi<0.2$ $textrm{GeV}/textrm{c}^2$ and $8times10^{-4}$ in $m_chi<1.6$ $textrm{GeV}/textrm{c}^2$. This is the regime largely uncovered by the previous experiments. We also explored a few detector designs to achieve an optimal sensitivity to $chi$s. The photoelectron yield is the main driver, but the sensitivity does not have a strong dependence on the detector configuration in the sub-millicharge regime.
The committee for the study of the extension of the Hadron Experimental Facility was formed under the Hadron Hall Users Association in August, 2015. This document is a summary of the discussions among the committee members, and documented by a part of the members listed below.
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