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西 隆博*; 橋本 直; 他46名*
Nature Physics, 19(6), p.788 - 793, 2023/06
被引用回数：2 パーセンタイル：89.39(Physics, Multidisciplinary)According to quantum chromodynamics, vacuum is not an empty space, because it is filled with quark-antiquark pairs. The pair has the same quantum numbers as the vacuum and forms a condensate because the strong interaction of the quantum chromodynamics is too strong to leave the vacuum empty. This quark-antiquark condensation, the chiral condensate, breaks the chiral symmetry of the vacuum. The expectation value of the chiral condensate is an order parameter of the chiral symmetry, which is expected to decrease at high temperatures or high matter densities where the chiral symmetry is partially restored. Head-on collisions of nuclei at ultra-relativistic energies have explored the high-temperature regime, but experiments at high densities are rare. Here we measure the spectrum of pionic 
Sn atoms and study the interaction between the pion and the nucleus. We find that the expectation value of the chiral condensate is reduced at finite density compared to the value in vacuum. The reduction is linearly extrapolated to the nuclear saturation density and indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus.
hadrons in 
Au and 
collisions at 
GeVAdare, A.*; 長谷川 勝一; 今井 憲一; 永宮 正治; 佐甲 博之; 佐藤 進; 谷田 聖; PHENIX Collaboration*; 他468名*
Physical Review C, 98(1), p.014912_1 - 014912_16, 2018/07
被引用回数：7 パーセンタイル：55.14(Physics, Nuclear)We present measurements of two-particle angular correlations between high-transverse-momentum (
GeV/
) 
observed at midrapidity (
) and particles produced either at forward (
) or backward (
) rapidity in Au and collisions at GeV. The azimuthal angle correlations for particle pairs with this large rapidity gap in the Au-going direction exhibit a ridge-like structure that persists up to 
GeV/ and which strongly depends on collision centrality, which is a similar characteristic to the hydrodynamical particle flow in A+A collisions. The ridge-like structure is absent in the -going direction as well as in collisions, in the transverse-momentum range studied. The results indicate that the ridge-like structure is shifted in the Au-going direction toward more central collisions, similar to the charged-particle pseudorapidity distributions.
+ Au collisions at 
= 200, 62.4, 39, and 19.6 GeVAidala, C.*; 長谷川 勝一; 佐甲 博之; 佐藤 進; 谷田 聖; PHENIX Collaboration*; 他246名*
Physical Review C, 96(6), p.064905_1 - 064905_21, 2017/12
被引用回数：32 パーセンタイル：92.97(Physics, Nuclear)We present measurements of the elliptic flow, 
, as a function of transverse momentum (), pseudorapidity (
), and centrality in Au collisions at 
200, 62.4, 39, and 19.6 GeV. The beam-energy scan of Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero signal at all four collision energies, which, at midrapidity and low , is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the 
Monte Carlo generator) show good agreement with the data at midrapidity to forward (-going) rapidities and low . At backward (Au-going) rapidities and 
GeV/, the data diverges from calculations of relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity (
) as a function of in central Au collisions at the same energies. We find that in Au collisions at 200 GeV the scales with over all in the PHENIX acceptance. At 62.4, and 39 GeV, scales with at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the scaling may be a further indication of nonflow effects dominating at backward rapidity.
collisions at 
= 510 GeVAdare, A.*; 長谷川 勝一; 今井 憲一; 永宮 正治; 佐甲 博之; 佐藤 進; 谷田 聖; PHENIX Collaboration*; 他462名*
Physical Review D, 95(7), p.072002_1 - 072002_19, 2017/04
被引用回数：11 パーセンタイル：51.15(Astronomy & Astrophysics)Dihadron and isolated direct photon-hadron angular correlations are measured in collisions at = 510 GeV. Correlations of charged hadrons of 
GeV/ with mesons of 
GeV/ or isolated direct photons of 
GeV/ are used to study nonperturbative effects generated by initial-state partonic transverse momentum and final-state transverse momentum from fragmentation. The nonperturbative behavior is characterized by measuring the out-of-plane transverse momentum component 
perpendicular to the axis of the trigger particle, which is the high- direct photon or . Nonperturbative evolution effects are extracted from Gaussian fits to the away-side inclusive-charged-hadron yields for different trigger-particle transverse momenta (
). The Gaussian widths and root mean square of are reported as a function of the interaction hard scale to investigate possible transverse-momentum-dependent evolution differences between the -
and direct photon-
correlations and factorization breaking effects. The widths are found to decrease with , which indicates that the Collins-Soper-Sterman soft factor is not driving the evolution with the hard scale in nearly back-to-back dihadron and direct photon-hadron production in collisions. This behavior is in contrast to Drell-Yan and semi-inclusive deep-inelastic scattering measurements.
and 
mesons from protons in the photon energy range from 1.5 to 3.0 GeV森野 雄平*; Hwang, S.; 今井 憲一; 谷田 聖; LEPS Collaboration*; 他46名*
Progress of Theoretical and Experimental Physics (Internet), 2015(1), p.013D01_1 - 013D01_11, 2015/01
被引用回数：9 パーセンタイル：56.56(Physics, Multidisciplinary)We report the measurement of differential cross sections for and 
photoproduction from protons at backward angles (
) using linearly polarized photons at 
-
GeV. The differential cross sections for mesons are larger than the predicted 
-channel contribution based on amplitude analysis in the energy range 
GeV. The differential cross sections for and mesons become closer to the predicted -channel contribution at 
GeV. A bump structure in the 
dependence of the differential cross sections for mesons was observed at 
GeV.
