117092560
Response to D.T. Son's comment on ``Is there a `most perfect fluid' consistent with quantum field theory?''
This is a response to the comment, arXiv:0709.4651. It is noted that while the comment raises an extremely interesting and subtle point, the original conclusion that theoretically consistent exceptions exist for the proposed general bound for the ratio of shear viscosity to entropy density, appears to remain unaffected by the issue raised.
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.
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.
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.
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.
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.
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.
17675959
Leptophilic dark matter from the lepton asymmetry.
We present a model of weak scale dark matter (DM) where the thermal DM density is set by the lepton asymmetry due to the presence of higher dimension lepton violating operators. In these models there is generically a separation between the annihilation cross section responsible for the relic abundance (through lepton violating operators) and the annihilation cross section that is relevant for the indirect detection of DM (through lepton preserving operators). This implies a perceived boost in the annihilation cross section in the Galaxy today relative to that derived for canonical thermal freeze-out, giving a natural explanation for the observed cosmic ray electron and positron excesses, without resorting to a Sommerfeld enhancement. These models motivate continued searches for DM with apparently nonthermal annihilation cross sections. The DM may also play a role in radiatively generating Majorana neutrino masses.
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.
119108935
Supersymmetric baryogenesis from exotic quark decays
In a simple extension of the minimal supersymmetric standard model, out-of-equilibrium decays of TeV-scale exotic vectorlike squarks may generate the baryon asymmetry of the universe. Baryon number and CP violation are present in the superpotential, so this mechanism does not rely on CP violation in supersymmetry breaking parameters. We discuss phenomenological constraints on the model as well as potential signals for the Large Hadron Collider and electronic dipole moment experiments. A variation on the TeV-scale model allows the exotic squarks to be the messengers of gauge mediated supersymmetry breaking.
119221065
SUSY simplified models at 14, 33, and 100 TeV proton colliders
A bstractResults are presented for a variety of SUSY Simplified Models at the 14 TeV LHC as well as a 33 and 100 TeV proton collider. Our focus is on models whose signals are driven by colored production. We present projections of the upper limit and discovery reach in the gluino-neutralino (for both light and heavy flavor decays), squark-neutralino, and gluino-squark Simplified Model planes. Depending on the model a jets + $ E_T^{\mathrm{miss}} $, mono-jet, or same-sign di-lepton search is applied. The impact of pileup is explored. This study utilizes the Snowmass backgrounds and combined detector. Assuming 3000 fb−1 of integrated luminosity, a gluino that decays to light flavor quarks can be discovered below 2.3 TeV at the 14 TeV LHC and below 11 TeV at a 100 TeV machine.