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Spectroscopy of deeply bound pionic states via 124Sn(p, 2He) reaction

渡辺, 珠以 東京大学 DOI:10.15083/0002001839

2021.10.04

概要

In this thesis, an experimental study of deeply bound pionic states via 124Sn(p,2He) reaction is described. Spontaneous breaking of chiral symmetry and its restoration in dense circumstance have been an important topic in the hadron physics. Spectroscopy of deeply bound pionic states is one of the promising methods to experimentally investigate the symmetry restoration in a nuclear medium. A potential parameter of the strong interaction between pion and nucleus (b1 parameter) is known to be related to chiral condensate ⟨qq⟩, which is an order parameter of the symmetry breaking. Since the pion is partly embedded in the core nucleus in the deeply bound pionic states, the strong interaction is modified because of the partial restoration of chiral symmetry in the nucleus and the binding energy is shifted accordingly. Thus, by measuring the binding energy of the pionic states, the change of chiral condensate in the nuclear medium can be quantitatively evaluated.

　Experimental studies of the deeply bound pionic states have been performed by the missing- mass spectroscopy using (d,3He) reactions. The spectroscopic experiments were performed in GSI by employing Sn and Pb isotopes as targets. Form the measured binding energy and width of the pionic states, the potential parameter in the nuclear medium was determined. It was concluded that the in-medium b1 parameter is modified from the in-vacuum value and the evidence of the symme-try restoration was obtained. Recently, a new experimental project using the (d,3He) reaction have been carried out at RI beam factory in RIKEN (RIBF). In the project, they plan to study the pionic atoms in Sn isotopes in order to determine the in-medium b1 parameter more precisely, by taking advantage of a large intensity beam and a large acceptance spectrometer. A pilot experiment was carried out in 2010 by employing the 122Sn(d,3He) reaction. Despite of a short measurement time of 18 hours, a clear peak structure of pionic states was observed. However, the experiment posed a new puzzule about the formation cross section of the pionic states.

　It was reported that there is a large discrepancy of the formation cross section between the theoretical calculation and the experiment; the measured cross section of the pionic 1s state is five times smaller than the theoretically calculated one, but the cross section of the pionic 2p state is almost consistent. Although a good theoretical understanding of the formation process is important in the study of deeply bound pionic states, there is no satisfactory explanation of the discrepancy.

　In order to understand the discrepancy, we newly planned to perform a spectroscopic exper- iment employing the 124Sn(p,2He) reaction. The (p,2He) reaction is different from the (d,3He) reaction especially in the distortion effect in the nucleus, which is a possible source of the dis- crepancy. Thus, we may be able to obtain a clue to understand the discrepancy, by measuring the formation cross sections of the 1s and 2p states and comparing them with the theoretically calculated ones in the same way as the previous study.

　The experiment was performed in Research Center for Nuclear Physics at Osaka University (RCNP). Figure 1 schematically depicts the experimental setup. A 350 MeV proton beam was transported to the target position and the reaction took place. The ejected two protons were an- alyzed by the high-resolution spectrometer Grand Raiden at 4.5◦. A typical beam intensity was about 30 nA. The position and angle of the ejectile particles were measured by a set of drift chambers at the focal plane. The incident beam was guided to a beam dump at the wall of the experimental hall through the newly constructed GRAF beamline.

　In the experiment, several calibration measurements were adopted in order to precisely eval- uate the excitation energy and formation cross section of the pionic states. Especially, the measure- ment of the 12C(p,2He)11B reaction played an important role. Since the Q-value of the 12C(p,2He)11B reaction is known, the accuracy of the excitation energy could be investigated by measuring the reaction. In addition, the excitation energy dependence of the detection efficiency of the (p,2He) reaction was studied by measuring the 12C(p,2He)11B reaction with several spectrometer settings.

　After the detailed data analysis and simulations, we successfully obtained the excitaiton energy spectrum of the pionic states. We fitted the obtained spectrum with the theoretically calculated one by assuming three kinds of background. As a result, the significance of pionic 2p state was more than 2σ in all background models, and there seems to be a non-negligible 1s state contribution to the spectrum. It was indicated that the deeply bound pionic states are observed. The ratio of the scaling factors I1s/I2p was also evaluated, where I1s(2p) is a scaling factor of theoretically calculated cross section of pionic 1s (2p) states in the fitting. The estimated ratio tends to be larger than that of the past experiment (I1s/I2p≈0.2), but this small value was not excluded because of the poor statistical sensitivity.

　Although the statistical sensitivity was not good, the method of the spectroscopy of deeply bound pionic states in the (p,2He) reaction was established by the present study. The excitation energy of the pionic states are determined with an accuracy of 70 keV by performing a dedicated measurement of the beam energy. A good spectral resolution of 250 keV was achieved by correct- ing the long-term variation of the beam energy. The accidental coincidence background and the detection efficiency of the (p,2He) reaction were well understood by the simulations.

　Based on the established method by the present study, it is expected that the statistical sensi- tivity is improved and the formation cross section of the pionic states is precisely evaluated in the future experiments. Especially, an experiment with a 136Xe gas target is advantageous in respect of the signal-to-noise ratio; a part of the background events can be rejected by a dedicated analysis and the formation cross section is expected to be larger than the reaction with the 124Sn target. We have proposed a new experiment with the 136Xe gas target and the preparation is ongoing.

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