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We propose a model for the recently discovered Θ + exotic KN resonance as a novel kind of a pentaquark with an unusual color structure: a 3 c ud diquark, coupled to 3 c uds triquark in a relative P -wave. The state has J P = 1/2 + , I = 0 and is an antidecuplet of SU (3) f . A rough mass estimate of this pentaquark is close to experiment.

We discuss the implications of a possible quasinuclear DK bound state at 2.32 GeV. Evidence for such a state was recently reported in D s ϩ 0 by the BaBar Collaboration. We first note that a conventional quark model cs assignment is implausible, and then consider other options involving multiquark systems. An Iϭ0 csnn baryonium assignment is one possibility. We instead favor a DK meson molecule assignment, which can account for the mass and quantum numbers of this state. The higher-mass scalar cs state expected at 2.48 GeV is predicted to have a very large DK coupling, which would encourage formation of an Iϭ0 DK molecule. Isospin mixing is expected in hadron molecules, and a dominantly Iϭ0 DK state with some Iϭ1 admixture could explain both the narrow total width of the 2.32 GeV state as well as the observed decay to D s ϩ 0 .Additional measurements that can be used to test this and related scenarios are discussed.

Lipkin Replies: Two coherence effects must be considered in superradiance 1 : (1) single-photon transitions involving a single excitation from different single atoms, and (2) multiphoton coherence from simultaneous excitation of many atoms. My paper 2 treated only single excitations, specifically excluded multiple excitations, and does not contain the conclusion regarding observability of nuclear superradiance quoted above by Hartmann, Rotman, and Garcia.My main purpose was to show how the trick of integrating the intensity of an outgoing wave over all angles before summing over the atoms in the crystal confirms in a simple way the refutations of a suggested TV 2 dependence in the total neutrino-scattering cross section. 3 For wavelengths much shorter than the interatomic spacings in a crystal, the enhanced intensities proportional to TV 2 in certain directions are mainly at the expense of radiation otherwise emitted in other directions.The same trick was applied to superradiance but only for single excitations. 2 It can also provide physical insight for the multiexcitation case, as a complement to the SU(rt) Lie-algebra approach to collective excitations in fl-level systems first developed for the nuclear shell model 4 and used in both Comments above. The Lie algebra gives general results for many different models with completely different physics by emphasizing common algebraic properties at the price of obscuring the basic physical differences. Let us use the method 2 of integrating over angles to calculate the norm of a state of TV bosons emitted isotropically from each of TV atoms at different points in space,where r^ denotes the position of an atom, al is a creation operator for a boson with momentum k, we neglect spin, and TV n is a normalization factor for the integral over the direction of k denoted by n^. We have WN I V/v> = Vyv-i I yyv-i)+ 2 P-1 V sin 2 a|r^-r p |) k 2 \r N -^ I 2 where TV-1 ^-2>-nWnJ\lk /k ' r "dn k .M = i flr*P The degree of coherence and enhancement of stimulated emission is given by the norm (2a) which has the value TV! if the boson states are all identical and unity if they are all different. For small interatomic distances the factors in Eq. (2a) having the form (sinx)/x all become unity, and the value TV! is obtained by induction: WN I y/v> = Vyv-i I yfN-\) + (N -1) 2 = 1. (3b)A more detailed investigation is needed to determine when the summations are negligible. All terms decrease with distance like k 2 \ r M -r v | 2 . The double sum in Eq. (2a) consists of terms each depending upon at least three interatomic distances with phases that are oscillating functions of these distances. The single sum in Eq. (2a) consists of positive definite terms, each depending only upon the distance between one pair of atoms. In a large ^-dimensional system containing TV atoms, the sum of these terms is of order N^n~1 )ln if the positive definite terms are added c...

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