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 121Sn 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. In quantum chromodynamics, the condensation of quark–antiquark pairs breaks the chiral symmetry of vacuum. Experiments with pionic tin atoms demonstrate that the symmetry is partially restored at high densities.

Modern theories of physics tell that the vacuum is not an empty space [1, 2]. Hidden in the vacuum is a structure of anti-quarks ¯ q and quarks q . The ¯ q and q pair has the same quantum number as the vacuum and condensates in it since the strong interaction of the quantum chromo-dynamics (QCD) is too strong to leave it empty. The ¯ qq condensation breaks the chiral symmetry of the vacuum. The expectation value (cid:104) ¯ qq (cid:105) is an order parameter [3]. For higher temperature or higher matter-density, | (cid:104) ¯ qq (cid:105) | decreases reflecting the restoration of the symmetry [4]. In contrast to these clear-cut arguments, experimental evidence is so far limited. First of all, the ¯ qq is noth-ing but the vacuum itself. It is neither visible nor perceptible. In this article, we unravel this invis-ible existence by high precision measurement of pionic atoms, π − -meson–nucleus bound systems. Using the π − as a probe, we demonstrate that | (cid:104) ¯ qq (cid:105) | is reduced in the nucleus by a factor of 58 ± 4% compared with that in the vacuum. This reduction indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus. The present experimental result clearly exhibits the existence of the hidden structure, the chiral condensate, in the vacuum.

The magnetic moment of the isomeric state of the neutron-rich ^75Cu nucleus was measured using a highly spin-aligned beam produced via a two-step reaction scheme. In the experiment carried out at the BigRIPS at RIBF, we achieved to produce spin alignment reaching 30% by employing the one-proton removal from ^76Zn to produce ^75Cu. In the magnetic moment measurement, a method of time-differential perturbed angular distribution (TDPAD) was employed. Precession of the isomeric state with spin parity of 3/2^− was clearly observed with significance larger than 5σ in the TDPAD spectrum. The magnetic moment of the isomeric state of ^75Cu was determined to be μ = 1.40(6)μ_N.

The dependence of the single-particle strength on the difference between proton and neutron separation energies is studied for oxygen isotopes in a wide range of isospins. The cross sections of the quasi-free (p, 2p) reaction on O-14,O-16,O-18,O-22,O-24 were measured at intermediate energies. The measured cross sections are compared to predictions based on the distorted wave impulse approximation and shell-model psd valence-space spectroscopic factors. The reduction factors, which are the ratio of the experimental cross sections to the theoretical predictions, show no apparent dependence on the proton-neutron separation energy difference. The result is compatible with the result of the (e, e'p) reaction on stable targets and with the predictions of recent ab initio calculations.

We conduct an experimental project to make spectroscopy of deeply bound pionic atoms systematically over wide range of nuclei. We aim at studying the strong interaction in the low energy region, which has close connection to spontaneous chiral symmetry breaking and its partial restoration in nuclear matter. First experimental results show improved spectral resolution and much better statistical sensitivity than previous experiments. Present status of the experiment is reported.

We performed a precision missing-mass spectroscopy experiment of the deeply bound pionic states in a 121Sn atom using the (d,3He) reaction near the π− emission threshold. The experiment serves as a pilot experiment for our new ‘pionic atom factory project’ at RIBF, which aims at precision spectroscopy of the energy spectrum of the pionic atom of isotopes and isotones. The result of the pilot experiment demonstrated the potentiality of BigRIPS and of the RIBF facilities for the project. The current status of the analysis is reported.

The degree of freedom of spin in quantum systems serves as an unparalleled laboratory where intriguing quantum physical properties can be observed, and the ability to control spin is a powerful tool in physics research. We propose a method for controlling spin in a system of rare isotopes which takes advantage of the mechanism of the projectile fragmentation reaction combined with the momentum-dispersion matching technique. The present method was verified in an experiment at the RIKEN RI Beam Factory, in which a degree of alignment of 8% was achieved for the spin of a rare isotope 32Al. The figure of merit for the present method was found to be greater than that of the conventional method by a factor of more than 50. Enhanced control of the nuclear spin orientation of rare isotopes has now been demonstrated. This technique is considerably more efficient than traditional methods and significantly broadens the domain of accessible nuclei, promising insights in nuclear physics and applications in material science.