Evidence for a scalar stateI=0 0++(750)from measurements ofπ

Abstract

Measurements of the reactions a + n 7 -a+a-p at 5.98 and 11.85 GeV/c and a-p +a-n'n at 17.2 GeV/c enable model-independent amplitude analyses which yield two solutions for moduli and cosines of certain relative phases of the two S-wave and six P-wave production amplitudes describing the pion production process below the dipion mass of 1000 MeV. We obtain four solutions for the S-and P-wave intensities in the physical region of the TN-+a+a-N reaction. The results for the S-wave intensity provide solution-independent evidence for a new resonant state I = 0 0++(750). Its a+a-decay width depends on the solution and is estimated to be in the range of 100-250 MeV. The I Z 0 0++(750) meson is best understood as the lowest-mass scalar gluonium O++(gg). Our results emphasize the need for a systematic study of pion production on the level of amplitudes in a new generation of dedicated experiments with spin at the recently proposed high-intensity hadron facilities. PACS number(s1: 14.40.Cs, 13.75.Gx, 13.85.Hd, 13.88.+e The isoscalar meson o with a mass near mass has a long and still controversial history. Following the discovery in 1961 of the p meson in aN +an-N reactions [I], large and energy-independent differences were observed [2-41 in the a-n-O and a-a+ angular distributions over a broad region [5-71 of the dipion mass m. The T-.no forward-backward asymmetry changes sign [7] as it goes through the p-mass while the n--a+ distribution shows a maximum [7] at the mass in its large forwardbackward asymmetry.he forward-backward asymmetry A -Repo, measures S-and P-wave interference.The behavior of A ( n--TO corresponds to a large and resonating P wave interfering with a nonresonant and essentially real S wave. Two possible explanations were proposed [4] to account for the behavior of the asymmetry A (a-a+ ): (1) a strong isoscalar S wave or (2) the existence of an isoscalar resonance 0'' near o0 mass. Early models of pion production invoking both alternatives described well the n--ao and a-a+ unpolarized data [8] and later [9] the data on a polarized target in n--pt +n--a+n. Experiments on unpolarized targets do not measure enough observables to isolate the two contributing S-wave amplitudes, and thus are not selective.An alternative is to search for resonances in an--+an-reactions. In the absence of colliding pion beams, model-dependent extrapolations of the unpolarized data on aN-n-n-N into the unphysical region of -t were used to obtain estimates of the phase shifts in n-a-n-n-scattering. Two solutions [lo-131 similar to the alternatives (1) and (2) were invariably produced for the I = 0 S-wave phase shift 6; in n--n-++n--a+. In principle, a solution can be selected from the study of n--n-++son-o where the S wave can be determined without P-wave interference. Originally, the nonresonating solution was selected [12] by a comparison with voao mass spectrum from n--p +n-Oa0n at 8 GeV/c (Ref. [14]). A more recent study of a-n-+ -n-OaO based on measurement of a+p+noao A + + at 8 GeV/c (Ref.[15]) prefers the resonating solution...

New data on theKK¯threshold region and the nature of thef0(

“…Among reports of I = 0 scalar signals, one should mention a new amplitude analysis of peripheral dipion production by Svec et al [65] which claims to reinstate the possibility of a narrow scalar resonance close to the p mass. Such behavior is normally excluded by examining how the f,(S*) effect manifests itself in TT scattering [7,50,661.…”

Soft-core hyperon-nucleon potentials

“…In a similar analysis of data on K + n → K + π − p, evidence was found for an I = 1 2 , 0 + (887) strange scalar meson under the peak of the K * (892) meson [32]. In the latest issue of the Particle Data Group [33] this analysis is cited with reserve, asserting that the ε parameters of [31] cannot be correct because the f 0 (980) is neglected in the analysis.…”

“…From then on, this broad ε(760) has been used in the Nijmegen OBE models. A recent analysis of π production in πN scattering with polarized nucleons claimed to have found unambiguous evidence for a broad isoscalar J P C = 0 ++ state under the peak of the ρ meson [31]. This was based on an amplitude analysis involving besides π exchange also a 1 exchange in the production mechanism.…”

Excess low energy photon pairs from pion annihilation at the chiral phase transition

“…[7,8,13] for details of the derivation of Eqs. (5), (6). Mass and decay width of the σ meson are given within this model as…”