In-Medium Modifications of theππInteraction in Photon-Induced Reactions

Abstract

Differential cross sections of the reactions (γ, π • π • ) and (γ, π • π +/− ) have been measured for several nuclei ( 1 H, 12 C, and nat Pb) at an incident-photon energy of Eγ =400-460 MeV at the taggedphoton facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π • π • channel. This dependence is not observed in the π • π +/− channel and is consistent with an in-medium modification of the ππ interaction in the I=J=0 channel. The data are compared to π-induced measurements and to calculations within a chiral-unitary approach.One of the challenges in nuclear physics is to study the properties of hadrons and the modification of these properties when the hadron is embedded in a nuclear manybody system. Although much has been learned about the properties of hadrons in free space, there is a lack of information for particles in a dense environment. In this Letter, an experiment is described which has measured correlated pion pairs photoproduced on nuclei in the scalar-isoscalar J=I=0 channel, also known as the σ mode. In Ref.[1] the σ meson is identified as the f 0 (400-1200). The large natural width in free space of Γ=400-500 MeV [2] makes it doubtful that this particle is a mesonic state, and has initiated many discussions on its nature. An in-medium study of the I=J=0 channel could provide a better insight into the nature of the σ meson.Within some theoretical approaches of quantum chromodynamics (QCD) [3,4,5], the σ is treated as a pure qq state (J P =0 + ) and regarded as the chiral partner of the pion (J P =0 − ). Chiral symmetry is spontaneously broken in the QCD vacuum, resulting in a mass difference between the pion and the σ. For large baryon densities, it is predicted that chiral symmetry is partially restored, leading to a degeneracy in mass of the pion and the σ. Since the pion approximates a Goldstone boson, the pion mass is not expected to change dramatically with increasing nuclear density ρ. Hence, these models predict a significant drop in the mass of the σ. A measurement of the inmedium σ→ ππ mass distribution might be essential for the understanding of the mechanism of chiral-symmetry breaking.Alternatively, the in-medium σ mode can be considered to be a resonant state of two pions [6,7,8,9]. In vacuum, the ππ system is mildly attractive. However, in the nuclear medium the ππ interaction strength could increase, thereby changing width and pole position of the resonant state. Experimental data on the density dependence of pion-pair interactions in the nuclear medium can provide evidence for this phenomenon.The first measurement of the in-medium ππ mass was obtained by a pion-induced experiment by the CHAOS collaboration [10,11,12]. A rising accumulation of strength at low π + π − mass was observed with increasing nuclear mass whereas such an enhancement was not seen in the π + π + -mass distributions. This effect was interpreted as a signature for an in-medium modification of the ππ interaction in the I=J=0 chann...

Double pion photoproduction off 40Ca

“…The measured distributions for two ranges of incident photon energies (400 MeV-500 MeV and 500 MeV -550 MeV) are summarized in fig. 11 and compared to previous results for carbon [41]. The distributions for the two different charge channels and also for the two different nuclei are very similar.…”

“…The distributions for the two different charge channels and also for the two different nuclei are very similar. For 40 Ca we do not observe such an important shift of the π 0 π 0 invariant mass distributions towards small invariant masses as has been found for lead [41], neither relative to the π 0 π ± channel for Ca nor in the comparison to π 0 π 0 off carbon nuclei. For a more detailed comparison the cross section ratios C ππ (Ca/C) and C ππ (π 0 π 0 /π 0 π ± ) defined by:…”

“…First results from a measurement of double π 0 and π 0 π ± photoproduction off carbon and lead have been reported in [41]. A shift of the strength to lower invariant masses was found for the heavier nucleus for the π 0 π 0 channel but not for the mixed charge channel.…”

“…For the simulations the pion pairs have been generated according to phase space distributions in quasifree kinematics taking into account the Fermi motion of the nucleons. FSI was taken into account as in [41] in a simple approximation, where pions scatter randomly off nucleons according to their mean-free path. It was then checked, that missing mass-, kinetic energy-, and angular distributions were in agreement with the simulations.…”

UA(1) Breaking effects and η' at finite temperature

“…The restoration of chiral symmetry has been linked to the vector meson spectral density and has been the subject of great theoretical and experimental interest up to this day (Hayano and Hatsuda, 2010; Leupold et al, 2010). It has also been linked to quenching of the pion decay constant (Jido et al, 2008) and possible observation of the sigma meson in nuclear matter through the π − π correlation (Hatsuda and Kunihiro, 1985;Messchendorp et al, 2002). As for the U A (1) symmetry, its breaking is due to the anomaly, which induces an operator relation that remains broken above the QCD phase transition.…”