Decay Modes of the 960-MeVηππResonance

Abstract

The discovery of a new meson of mass about 960 MeV has been recently announced by two independent groups. 1 ' 2 The method of production (K" +p-~A+X°) shows that it must have isospin T = 0 or 1. The only well-established decay mode is^° -7r + +7r" + r7, which includes about half of the identified X° events, almost all of the remaining events consisting of decays of the type X°~all neutrals. Within the available statistics, the number of neutral decays is consistent with X° -7T° + 7T° + 77 being the companion mode to H^H'T). The absence of measurable decay into two, three, or four pions, together with the observation that five-pion decays almost always include three pions having total charge zero and effective mass corresponding to the 7? meson, leads to the conjecture that spin-par ity-G-parity assignment is either 0" + or 1 ++ , with 0" + being favored by the low rate of Assuming that X° has TJ PG = 00" + and that it decays mainly via the strong interactions, we show that the dominance of the TTTTT] mode implies that the principal mechanism iswhere a is an isosinglet neutral scalar meson. 5 We estimate the decay width for this process using an extension of the Coleman-Glashow notion of a unitary-symmetric scalar-meson coupling of universal strength. 6 We also obtain the branching ratios into subsidiary decay modes, including the electromagnetic modes. An interesting point is implied by this work: If the "A invariance" of Bronzan and Low 7 is relevant to decay processes, then X° must have A= -1.Disregarding for the moment the a meson, the allowed strong decays are into the modes mrr], 47r, and 6?r; we shall assume the last to be negligible. A conservative estimate of the four-pion partial decay width is r^o(47r)< 1 keV. This is based on the lowest possible angular-momentum configuration, that corresponding to two neutral di-pions having the quantum numbers of the p° meson. This rate is negligible compared with other partial rates discussed below. A straightforward calculation of X° -ir + + TT~ + 77, assuming a structureless vertex with effective coupling G'X°TT + iT~ri, gives the result 1^(7^-7?) = 2G' 2 keV,where G' is expected to be of order unity.The calculation of the rate for process (1) is exactly analogous to the calculation of 7j-~CT+7r° given in reference 5, except for omission of the factor a = 1/137 in the present case. The results areo o X 0 ' cp(w) = (w 2 -m r f) 1/2 (m x0 2 +m 2 -4^2-2ra__ 0 a>) : 7] ^ % X* 1/2 x (m^2 +rw^2-2w^)-1/2 , ^m^{ w^^m v'^2 )/2m^ (3) (4) (5) (6) (7) (8)and \x is the mass of the charged pion. The quantity g is the a7r + 7r~ coupling and is related to the mass m 0 and the width T 0 of the a meson by £-2 = r (327rm 2 )/3(m 2 -4/i 2 ) 1/2 .With m a = 400 MeV and T a = 100 MeV, one obtains (^2/4rrm a 2 ) = 0.93. The rate for X° -ir° + ir° + 77 is given by one half of the expression (3) with JLL 0 , the neutral pion mass, substituted everywhere for /i. As a first approach, we may consider equating the coupling constant at the X°TJ(J vertex (which we have denoted by gG) with ...

Possible Anisotropies in the Production and Decay Correlations of theη′(958)

“…If a width r ~1 MeV were found, then J = 0 would be fa-vored. 12 However, if r < 0.1 MeV, then either J = 0 or 2 would be possible. A width T ~ 50 keV for a J = 0 r\' is expected using a Duffin-Kemmer-Petiau description, 13 as opposed to the ~l-MeV value from a Klein-Gordon description, whereas for J = 2, r ~ 5 keV is expected.…”

“…This value is about a factor of 30 low according to Fischbach et al 13 This ratio was expected to be about 10 according to Brown and Faier. 12 and m w m^9 T v are the 77 and pion masses and the 77 kinetic energy, and Q is the energy release in the decay. 14 For 77(549) and K° decay to three pions, the corresponding parameter is a'** -0 o 5, 15 Charged decay modes of the kaon have different large values 11 for a.…”

Vector-Vector—Pseudoscalar Interactions

“…14 ' 15 Since the p° and co would be identical in these decay processes except for A value, it seems clear that A x =A y Ap= + l as opposed to AyA^ -1. The estimates of Brown and Faier 16 for the X -> 2wr] mode must be reduced by ~l/36 because of A violation; but with extreme allowance for the a resonance, this could still leave T(2<irri)~0.2G 2 MeV. Using the formula from Eq.…”