Many of the XY Z mesons discovered in the last decade can be identified as bound states in BornOppenheimer (B-O) potentials for a heavy quark and antiquark. They include quarkonium hybrids, which are bound states in excited flavor-singlet B-O potentials, and quarkonium tetraquarks, which are bound states in flavor-nonsinglet B-O potentials. We present simple parameterizations of the deepest flavor-singlet B-O potentials. We infer the deepest flavor-nonsinglet B-O potentials from lattice QCD calculations of static adjoint mesons. Selection rules for hadronic transitions are used to identify XY Z mesons that are candidates for ground-state energy levels in the B-O potentials for charmonium hybrids and tetraquarks. The energies of the lowest-energy charmonium hybrids are predicted by using the results of lattice QCD calculations to calculate the energy splittings between the ground states of different B-O potentials and using the Schroedinger equation to determine the splittings between energy levels within a B-O potential.
In a recent experiment with ultracold trapped 85 Rb atoms, Makotyn et al. studied a quantum-degenerate Bose gas in the unitary limit where its scattering length is infinitely large. We show that the observed momentum distributions are compatible with a universal relation that expresses the high-momentum tail in terms of the two-body contact C 2 and the three-body contact C 3 . We determine the contact densities for the unitary Bose gas with number density n to be C 2 ≈ 20n 4=3 and C 3 ≈ 2n 5=3 . We also show that the observed atom loss rate is compatible with that from 3-atom inelastic collisions, which gives a contribution proportional to C 3 , but the loss rate is not compatible with that from 2-atom inelastic collisions, which gives a contribution proportional to C 2 . We point out that the contacts C 2 and C 3 could be measured independently by using the virial theorem near and at unitarity, respectively.
Many of the XY Z mesons discovered in the last decade can be identified as bound states of a heavy quark and antiquark in Born-Oppenheimer (B-O) potentials defined by the energy of gluon and light-quark fields in the presence of static color sources. The mesons include quarkonium hybrids, which are bound states in excited flavor-singlet B-O potentials, and quarkonium tetraquarks, which are bound states in flavor-nonsinglet B-O potentials. The deepest hybrid potentials are known from lattice QCD calculations. The deepest tetraquark potentials can be inferred from lattice QCD calculations of static adjoint mesons. Selection rules for hadronic transitions are derived and used to identify XY Z mesons that are candidates for ground-state energy levels in the B-O potentials for charmonium hybrids and tetraquarks. A full decade has elapsed since the discovery of the first XY Z meson, the X(3872) [7], but no compelling explanation for the pattern of XY Z mesons has emerged. In simple constituent models, an XY Z meson consists of a heavy quark (Q) and antiquark (Q) and possibly additional constituents that could be gluons (g) or light quarks (q) and light antiquarks (q). The models that have been proposed can be classified according to how the constituents are clustered within the me- The B-O approximation is used in atomic and molecular physics to understand the binding of atoms into molecules. It exploits the large ratio of the time scale for the motion of the atomic nuclei to that for the electrons, which is a consequence of the large ratio of the nuclear and electron masses. The electrons respond almost instantaneously to the motion of the nuclei, which can be described by the Schroedinger equation in a B-O potential defined by the energy of the electrons in the presence of static electric charges. The B-O approximation for QQ mesons in QCD was developed by Juge, Kuti, and Morningstar [13]. It exploits the large ratio of the time scale for the motion of the Q andQ to that for the evolution of gluon fields, which is a consequence of the large ratio of the heavy-quark mass to the nonperturbative momentum scale Λ QCD . The gluon field responds almost instantaneously to the motion of the QQ pair, which can be described by the Schroedinger equation in a B-O potential defined by the energy of the gluon field in the presence of static color sources. Conventional quarkonia are energy levels of a QQ pair in the ground-state B-O potential. The energy levels in the excited-state B-O potentials are quarkonium hybrids. Juge, Kuti, and Morningstar calculated many of the B-O potentials using quenched lattice QCD [13]. They calculated the spectra of charmonium hybrids and bottomonium hybrids by
In a system of ultracold atoms near a Feshbach resonance, pairs of atoms can be associated into universal dimers by an oscillating magnetic field with frequency near that determined by the dimer binding energy. We present a simple expression for the transition rate that takes into account many-body effects through a transition matrix element of the contact. In a thermal gas, the width of the peak in the transition rate as a function of the frequency is determined by the temperature. In a dilute Bose-Einstein condensate of atoms, the width is determined by the inelastic scattering rates of a dimer with zero-energy atoms. Near an atom-dimer resonance, there is a dramatic increase in the width from inelastic atom-dimer scattering and from atom-atom-dimer recombination. The recombination contribution provides a signature for universal tetramers that are Efimov states consisting of two atoms and a dimer.
In systems of ultracold atoms, pairwise interactions can be resonantly enhanced by a new mechanism which does not rely upon a magnetic Feshbach resonance. In this mechanism, interactions are controlled by tuning the frequency of an oscillating parallel component of the magnetic field close to the transition frequency between the scattering atoms and a two-atom bound state. The real part of the resulting s-wave scattering length a is resonantly enhanced when the oscillation frequency is close to the transition frequency. The resonance parameters can be controlled by varying the amplitude of the oscillating field. The amplitude also controls the imaginary part of a which arises because the oscillating field converts atom pairs into molecules. The real part of a can be made much larger than the background scattering length without introducing catastrophic atom losses from the imaginary part. For the case of a shallow bound state in the scattering channel, the dimensionless resonance parameters are universal functions of the dimensionless oscillation amplitude.
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