Experimental limits on the width of the reported Theta(1540)

LBNL-54065 Ep erimental Limits on the Width of the Reported ©(ISO) Robert N. Cahn and George H. Trilling Lawrence Berkeley National Laboratory 1 Cyclotron Rd., Berkeley, GA 94720) (Dated: November 19, 2003) Using data on K collisions on xenon and deuterium we derive values and limits on the width of the reported 0(154O) exotic baryon resonance. The xenon experiment gives a width of 0.9 ± 0.3 MeV. The other experiments give upper limits in the range 1-4 MeV. PACS numbers: 14.20.-c, 13.75.-n I. INTRODUCTION where we have taken the values for K n collisions at a resonant mass mo of 1540 MeV and assumed that the res- onance has J = 1/2. As we shall see, the mass resolution of the experiments is always broader than the natural width of the resonance so that the observable quantity is the integral of the resonant cross section: The general features of the spectroscopy of mesons and baryons can be understood from the simple rules that a meson is made of a quark and an antiquark, while a baryon is made from three quarks. These rules are consistent with the principles of quantum chromodynam- ics, which show that physical particles are neutral under color-SI/(3). However, QCD does not preclude the ex- istence of other colorless configurations, including glu- ons or additional quarks and antiquarks. Recent results from a diverse collection of experiments [1-6] show evi- dence for a state @(1540) , whose quantum numbers are those of the combination uudds and which thus cannot be composed simply of three quarks. Such states have been predicted [7] in a Skyrmion model. t _(m^m ) Bi Bf II. AT + T /4 TTT —^-(To (107 mb) x The @(1540) has not been detected in data from a number of early experiments in which one might expect it to appear. However, definitive negative conclusions cannot be drawn in the absence of reliable predictions for production cross sections. r /4 dm B Bf ao BiBfT K°p IN XENON In situations where the @(1540) is or ought to be formed resonantly, 3S 3X1 intermediate state in a scatter- ing experiment, it is possible to draw conclusions about its width from existing data. Such results can provide guidance to the structure of the resonance, or more im- portant, to the likelihood that there truly is a such reso- nance. The resonant cross section is determined entirely by F, the width of the resonance and its branching ratios B and Bf into the initial and final channels according to the Breit-Wigner form: t a(m) r /4 = Bi Bf CT _(m^m ) +r /4. With k as the CM momentum, si and s the incident spins and J the spin of the resonance, we have 2J+ 1 4TT (2si + l)(2s + l) k 68 mb In the DIANA experiment [2], in which a K beam with momentum 750 MeV entered a xenon bubble cham- ber, the signal for the @(1540) was observed by measur- ing the pKs invariant mass spectrum in the final state. If we treat the scattering as simply a two-body process, K n —¥ K°p, resonance occurs when the combination of the incident momentum of the K and the Fermi mo- mentum of the neutron give the invariant mass of the @(1540) . Without the Fermi momentum, this would occur for a K momentum of 440 MeV, to which the incident beam is reduced by ionization losses after pen- etrating a sufficient distance through the xenon. By ob- serving the final-state invariant mass, reconstructed from the pKs, the effects of Fermi motion and incident beam degradation are removed, provided that we can ignore rescattering within the nucleus. The signal in this experiment emerges only after mak- ing cuts that are believed to reduce the effect of rescat- tering. We make the assumption that, in the mass region near the resonance, it is the charge-exchange process on a single nucleon that is observed and that the cuts reduce the resonant and non-resonant charge-exchange processes by the same factor. The apparent resonant signal is con- tained within two 5-MeV bins. The background varies smoothly in this region at a value near 22 events per