9655408
Phase separation and an upper bound for a generalized superfluid gap for cold fermi fluids in the unitary regime.
An upper bound is derived for Delta for a cold dilute fluid of equal amounts of two species of fermion in the unitary limit k(f)a--> infinity (where k(f) is the Fermi momentum, a is the scattering length, and Delta is a pairing energy: the difference in energy per particle between adding to the system a macroscopic number (but infinitesimal fraction) of particles of one species compared to adding equal numbers of both. The bound is delta < or =5/3 [2(2xi)(2/5)-(2xi)] where xi=epsilon/epsilon(FG), delta=2Delta/epsilon(FG); epsilon is the energy per particle and epsilon(FG) is the energy per particle of a noninteracting Fermi gas. If the bound is saturated, then systems with unequal densities of the two species will separate spatially into a superfluid phase with equal numbers of the two species and a normal phase with the excess. If the bound is not saturated, then Delta is the usual superfluid gap. If the superfluid gap exceeds the maximum allowed by the inequality, phase separation occurs.
9144282
QCD inequalities for the nucleon mass and the free energy of baryonic matter.
The positivity of the integrand of certain Euclidean space functional integrals for two flavor QCD with degenerate quark masses implies that the free energy per unit volume for QCD with a baryon chemical potential mu(B) (and zero isospin chemical potential) is greater than the free energy with an isospin chemical potential mu(I)=(2 mu(B)/N(c)) (and zero baryon chemical potential). The same result applies to QCD with any number of heavy flavors in addition to the two light flavors so long as the chemical potential is understood as applying to the light quark contributions to the baryon number. This relation implies a bound on the nucleon mass: there exists a particle X in QCD (presumably the pion) such that M(N)> or =(N(c) m(X)/2 I(X)) where m(X) is the mass of the particle and I(X) is its isospin.
118729457
A sticky business: the status of the conjectured viscosity/entropy density bound
There have been a number of forms of a conjecture that there is a universal lower bound on the ratio, η/s, of the shear viscosity, η, to entropy density, s, with several different domains of validity. We examine the various forms of the conjecture. We argue that a number of variants of the conjecture are not viable due to the existence of theoretically consistent counterexamples. We also note that much of the evidence in favor of a bound does not apply to the variants which have not yet been ruled out.
36009299
Doubly heavy hadrons and the domain of validity of doubly heavy diquark-antiquark symmetry
In the limit of heavy quark masses going to infinity, a symmetry is known to emerge in QCD relating properties of hadrons with two heavy quarks to analogous states with one heavy antiquark. A key question is whether the charm mass is heavy enough so that this symmetry is manifest in at least an approximate manner. The issue is crucial in attempting to understand the recent reports by the SELEX Collaboration of doubly charmed baryons. We argue on very general grounds that the charm quark mass is substantially too light for the symmetry to emerge automatically via color coulombic interactions. However, the symmetry could emerge approximately depending on the dynamical details of the nonperturbative physics. To treat the problem systematically, a new expansion that simultaneously incorporates NRQCD and HQET is needed.
126291695
Nuclear and Hadronic Physics at Large N c
This talk briefly surveys some of the advances in large Nc QCD in nuclear and hadronic physics during the past decade or so. It highlights recent results including an analytic result for the total nucleon-nucleon cross section in nucleon-nucleon scattering in the extreme large Nc limit (logNc 1)); it is given by s total = 2p log 2 (Nc) m 2 for p s much larger than the QCD scale. The talk also stresses a demonstration that given certain technical assumptions, it can be shown that QCD must have a Hagedorn spectrum (i.e., a spectrum where the number of hadrons grows exponentially with hadron mass) . Another theme in the talk is the phenomenology of a variant of the large Nc limit based on quarks in the two-index antisymmetric representation. This variant of large Nc QCD is qualitatively different from the conventional one in which quarks are in the fundamental representation and corresponds to a distinct large Nc limit with a distinct 1=Nc expansion. The description of nuclear forces and nuclear matter in both variants of the largeNc limit are discussed.
37449512
Is there a "most perfect fluid" consistent with quantum field theory?
It was recently conjectured that the ratio of the shear viscosity to entropy density eta/s for any fluid always exceeds [formula: see text]. A theoretical counterexample to this bound can be constructed from a nonrelativistic gas by increasing the number of species in the fluid while keeping the dynamics essentially independent of the species type. The question of whether the underlying structure of relativistic quantum field theory generically inhibits the realization of such a system and thereby preserves the possibility of a universal bound is considered here. Using rather conservative assumptions, it is shown here that a metastable gas of heavy mesons in a particular controlled regime of QCD provides a realization of the counterexample and is consistent with a well-defined underlying relativistic quantum field theory. Thus, quantum field theory appears to impose no lower bound on eta/s, at least for metastable fluids.
118583650
Comments on Gaugino Screening
Gauge mediated models of supersymmetry breaking often exhibit 'gaugino screening,' where to leading order in F, gaugino masses are unaffected by higher dimensional Kahler potential interactions between the supersymmetry breaking spurion and the messengers. We provide a derivation of this phenomenon which utilizes the gaugino counterterm originally proposed in the context of anomaly mediation by Dine and Seiberg. We argue that this counterterm is present when there are non-zero messenger F-terms, and can cancel the leading order Feynman diagram contribution to the gaugino mass. We provide a nontrivial check of the regulator independence of our results by performing the computation using both dimensional reduction and Pauli-Villars. This analysis reconciles an apparent contradiction between diagrammatics and analytic continuation into superspace.
8964651
Electroweak Baryogenesis and Colored Scalars
We consider the 2-loop finite temperature effective potential for a Standard Model-like Higgs boson, allowing Higgs boson couplings to additional scalars. If the scalars transform under color, they contribute 2-loop diagrams to the effective potential that include gluons. These 2-loop effects are perhaps stronger than previously appreciated. For a Higgs boson mass of 115 GeV, they can increase the strength of the phase transition by as much as a factor of 3.5. It is this effect that is responsible for the survival of the tenuous electroweak baryogenesis window of the Minimal Supersymmetric Standard Model. We further illuminate the importance of these 2-loop diagrams by contrasting models with colored scalars to models with singlet scalars. We conclude that baryogenesis favors models with light colored scalars. This motivates searches for pair-produced di-jet resonances or jet(s) + = E{sub T}.
119227010
Wino dark matter under siege
A fermion triplet of SU(2)L — a wino — is a well-motivated dark matter candidate. This work shows that present-day wino annihilations are constrained by indirect detection experiments, with the strongest limits coming from H.E.S.S. and Fermi. The bounds on wino dark matter are presented as a function of mass for two scenarios: thermal (winos constitute a subdominant component of the dark matter for masses less than 3.1 TeV) and non-thermal (winos comprise all the dark matter). Assuming the NFW halo model, the H.E.S.S. search for gamma-ray lines excludes the 3.1 TeV thermal wino; the combined H.E.S.S. and Fermi results completely exclude the non-thermal scenario. Uncertainties in the exclusions are explored. Indirect detection may provide the only probe for models of anomaly plus gravity mediation where the wino is the lightest superpartner and scalars reside at the 100 TeV scale.
118615137
Correlation between the spin-independent and spin-dependent direct detection of dark matter
We study the correlation between spin-independent and spin-dependent scattering in the context of minimal supersymmetric standard model neutralino dark matter for both thermal and nonthermal histories. We explore the generality of this relationship with reference to other models. We discuss why either fine-tuning or numerical coincidences are necessary for the correlation to break down. We derive upper bounds on spin-dependent scattering mediated by a ${Z}^{0}$ boson.
59031368
Illuminating the 130 GeV gamma line with continuum photons
A bstractThere is evidence for a 130 GeV γ-ray line at the Galactic Center in the Fermi Large Area Telescope data. Dark matter candidates that explain this feature should also annihilate to Standard Model particles, resulting in a continuous spectrum of photons. To study this continuum, we analyze the Fermi data down to 5 GeV, restricted to the inner 3◦ of the Galaxy. We place a strong bound on the ratio of continuum photons to monochromatic line photons that is independent of uncertainties in the dark matter density profile. The derived constraints exclude neutralino dark matter as an explanation for the line.