Determination of theη′-Proton Scattering Length in Free Space

Abstract

Taking advantage of both the high mass resolution of the COSY-11 detector and the high energy resolution of the low-emittance proton-beam of the Cooler Synchrotron COSY we determine the excitation function for the pp → ppη ′ reaction close-to-threshold. Combining these data with previous results we extract the scattering length for the η ′ -proton potential in free space to be Re(a pη ′ ) = 0 ± 0.43 fm and Im(a pη ′ ) = 0.37 In this letter we report the determination of the scattering length for the interaction of the η ′ meson with the proton based on the shape of the excitation function for the pp → ppη ′ reaction measured close to the kinematic threshold. Using the high mass resolution of the updated COSY-11 detector [1,2] and the low-emittance proton-beam of the Cooler Synchrotron COSY [3] the excitation function was determined down to excess energy Q = 0.76 MeV above threshold, with the precision ∆Q = 0.1 MeV improved by more than a factor of five with respect to previous measurements. The improved resolution enabled quantitative extraction of the η ′ proton scattering length in free space.The scattering lengths describing interaction potentials between mesons and nucleons are of fundamental importance in hadron physics. However, they are not well established especially for those flavor neutral mesons that are characterized by very short life times making investigations of the meson-nucleon potential in the standard way via scattering experiments impossible. So far, based on the shift and width of the ground state of pionic hydrogen atoms [4], only the scattering length of the π 0 -nucleon potential is accurately determined with a precision of about 0.001 fm. The scattering length for the η-nucleon potential is determined more than two orders of magnitude less precisely, with phenomenological values quoted for the real part between ∼0.2 fm and ∼1 fm depending on the analysis method [5]. Until now the η ′ -nucleon scattering length had been estimated only qualitatively [6].Measurements of the η-and η ′ -nucleon and nucleus systems are sensitive to dynamical chiral and axial U(1) symmetry breaking in low energy QCD. While pions and kaons are would-be Goldstone bosons associated with chiral symmetry, the isosinglet η and η ′ mesons are too massive by about 300-400 MeV for them to be pure Goldstone states. They receive extra mass from non-perturbative gluon dynamics associated with the QCD axial anomaly. This OZI violation is also expected to influence the η ′ -nucleon interaction [7]. Without the gluonic mass contribution the η ′ would be a strange quark state after η-η ′ mixing (and the η would be a light-quark state degenerate with the pion), mirroring the situation with isoscalar φ and ω vector mesons. To the extent that coupling to nucleons and nuclear matter is induced by light-quark components in the meson, any observed scattering length and mass shift in medium is induced by the QCD axial anomaly that generates part of the η ′ mass [8].In COSY-11 the η ′ meson was produced in p-p collisions of the COSY ...

Theoretical study of photoproduction of anη′Nbound state on a deuteron target with forward proton emission

“…[30], where they measured the π − p → η n cross section just above the η n threshold. Production experiments of the η meson in other reactions, such as γp → η p [31][32][33][34] and pp → η pp [35][36][37], also give us a good ground to study the η N interaction.…”

Measurement of Excitation Spectra in the C12(p,d) Reaction near the Tanaka1, Itahashi2, Fujioka3 et al. 2016Phys. Rev. Lett.

“…Thus, a strongly attractive potential of V 0 ∼ −150 MeV predicted by the NJL model [6] is rejected for a relatively shallow imaginary potential of jW 0 j ∼ 10 MeV [14]. The present experiment has only limited sensitivity for relatively weak attraction implied by the η 0 N scattering length [15] and suggested by the η 0 photoproduction experiments [13]. In the near future, we will extend the experimental sensitivity by constructing a detector system to efficiently select events originating from the formation of η 0 -mesic nuclei by tagging the decay particles in an experiment at GSI/FAIR.…”

“…Furthermore, the excitation function and the momentum distribution of η 0 mesons, produced and emitted from 12 C, were compared with a model calculation with the real part of the potential as a free parameter leading to a potential depth of −37 AE 10ðstatÞ AE 10ðsystÞ MeV [13]. In contrast, a small η 0 N scattering length with a real part consistent with zero was obtained from the excitation function of the pp → ppη 0 reaction very close to the threshold [15].…”

Mass Modifications of Light Mesons in Nuclear Matter with the Three Flavor Extended Linear Sigma Model

“…Then, another experiment called E16 experiment is planned at J-PARC. Besides, the η mesic nuclei experiments at GSI [18][19][20] and at the University of Bonn [21][22][23][24], and pp → ppη reaction experiment at the COSY accelerator complex [25] have been operated, regarding the investigation of the change of U (1) A axial anomaly in nuclear matter. Another η mesic nuclei experiments is ongoing at SPring-8 [26] (see Ref.…”