121477317
Neutrinos in a Robertson-Walker universe
A solution to the Einstein-Dirac system of equations in a Robertson-Walker spacetime is presented using the separation of variables method. With an axially symmetric and a non-axially symmetric boundary conditions, we find that there are no solutions for the massless fermions and only null or `ghost' solutions for the massive fermions. The `ghost' solutions carry a current but have a vanishing energy-momentum tensor and correspond to the special case when the Robertson-Walker spacetime reduces to flat spacetime.
121494708
Requirements for scattering solutions and the causal Green's function of the Bethe--Salpeter equation
We investigate the conditions necessary for the existence of scattering solutions to the Bethe-Salpeter equation. We work with the causal Green's function in both timelike and spacelike regions of $x\ensuremath{-}{x}^{\ensuremath{'}}$. Our results indicate that high-momentum or small-relative-time limits must be taken. In the small-relative-time limit, suitable restrictions on the interaction $I(x)$ must also be considered.
122431079
Massive photons in General Relativity
In order to avoid a speed-of-light catastrophe in General Relativity with an electromagnetic source, gauge invariance with respect to the electric charge is broken with the photon acquiring mass. The general equations for the Einstein-Maxwell system are derived for the case with massive photons. Nonminimal couplings which might compete with the small minimal photon mass term are included.
121710663
SPIN-2 GLUON - GRAVITON MODEL WITH BARYON NUMBER NONCONSERVATION
We have shown how a weak-strong gravity model with Higgs fields can contain a baryon-number-violating mechanism. If more than one set of Higgs particles are allowed, the Higgs-fermion coupling will generate the violation of baryon-number symmetry spontaneously. To illustrate our point, we present some numerical examples on quark and nucleon decay. We find that some of the quarks can decay into leptons, antileptons, and pions with very short lifetimes. The proton is found to be much more stable than the neutron and both have possible baryon-number-violating modes of decay with very long lifetimes.
122132298
On the Green's Functions for the Bethe‐Salpeter Equation
Considering the Bethe‐Salpeter equation as a relativistic equation, we have studied the free particle Green's functions for unequal mass scalar bosons, interacting via a translationally invariant potential. The scattering Green's function found by Huang and DeFacio (HD) is presented for all possible Ω‐plane contours for the unequal masses case. Only two k‐plane contours are found to be physically interesting. The causal, advanced, and retarded Green's functions are listed in an appendix for unequal masses for the physical contour. The timelike Green's functions again require severe restrictions on the interaction potential as found by HD, and possible Bethe‐Salpeter bound states are briefly discussed.
122685473
Ladder approximation, scalar-exchange mesons, and Bethe--Salpeter scattering states
121270760
The complex EAS hybrid arrays in Tibet
The Tibet hybrid experiment is an extensive air shower experiment that operated at Yangbajing (E90°31', N30°06'; 4300 m above sea level) in Tibet, China, since 1996. This multi-detector system is used to observe air showers of high energy celestial gamma-ray origin and that of nuclear-component origin. Both observations aim to investigate the origin of cosmic rays using different methods. In this paper, based on the energy spectrum of the nuclear component and its chemical composition around the knee, we would like to discuss whether they could be described by the theoretical picture of the diffusive shock acceleration at SNRs. The consistency of the experimental data with rigidity dependent cutoff spectrum of CRs is examined using Tibet data and KASCADE data. It is found that both experimental data cannot be fully interpreted by such a simple approach. The sharp knee observed by Tibet experiment suggests the contribution of nearby source(s).
1384985
HerMES: SPECTRAL ENERGY DISTRIBUTIONS OF SUBMILLIMETER GALAXIES AT z > 4*
We present a study of the infrared properties for a sample of seven spectroscopically confirmed submillimeter galaxies (SMGs) at z > 4.0. By combining ground-based near-infrared, Spitzer IRAC and MIPS, Herschel SPIRE, and ground-based submillimeter/millimeter photometry, we construct their spectral energy distributions (SEDs) and a composite model to fit the SEDs. The model includes a stellar emission component at λ_(rest) 50 μm. Six objects in the sample are detected at 250 and 350 μm. The dust temperatures for the sources in this sample are in the range of 40-80 K, and their L_(FIR) ~ 10^(13) L_☉ qualifies them as hyper-luminous infrared galaxies. The mean FIR-radio index for this sample is around 〈q〉 = 2.2 indicating no radio excess in their radio emission. Most sources in the sample have 24 μm detections corresponding to a rest-frame 4.5 μm luminosity of Log_(10)(L_(4.5)/L_☉) = 11 ~ 11.5. Their L_(4.5)/L_(FIR) ratios are very similar to those of starburst-dominated SMGs at z ~ 2. The L_(CO) – L_(FIR) relation for this sample is consistent with that determined for local ULIRGs and SMGs at z ~ 2. We conclude that SMGs at z > 4 are hotter and more luminous in the FIR but otherwise very similar to those at z ~ 2. None of these sources show any sign of the strong QSO phase being triggered.
16233799
Bulge formation in very late-type galaxies
The dynamical evolution of super star clusters (SSCs) moving in the background of a dark matter halo has been investigated as a possible event responsible for the formation of bulges in late-type spirals. The underlying physical processes include sinking of SSCs due to the dynamical friction and stripping of SSCs on their way to the center. Based on the assumption of a universal density profile for the dark matter halo, and an isothermal model for the SSCs, our simulations have yielded bulges that are similar in many aspects to the observational ones. In particular, the derived surface density profiles can be well fitted by an exponential structure with nuclear cusps, which is consistent with HST observations.The preliminary simulations with the Burkert density profile yield very instructive predictions on the bulge formation, which is surely worth further investigating.
117991499
Zero and one jet combined next-to-leading order analysis of the top quark forward-backward asymmetry
We present an analysis of the forward{backward asymmetry in the production of top quark pairs at the Tevatron collider. We use novel Monte Carlo methods for merging matrix elements and parton showers to combine NLO QCD predictions fort t andt t+jet production. Theoretical uncertainties are quantied in detail. We nd agreement with experimental data on the transverse momentum dependence of the asymmetry. t t events with large invariant mass, mt t 450 GeV. The inclusive parton-level asymmetry was measured as 0:164 0:047 considering all pair masses, and 0:295 0:067 for mt t 450 GeV. This needs to be compared to theoretical predictions from various event generators in the unrestricted and high t t mass region: 0:067 0:020 and 0:089 0:027 from MC@NLO (6, 7), 0:066 0:020 and 0:100 0:030 from POWHEG (8, 9), and 0:073 0:022 and 0:110 0:033 from MCFM (4, 10). In all cases, electroweak corrections had been applied. The linear behavior of AFB with increasing y and mt t persists,
119331816
An Equilibrium Dark Matter Halo with the Burkert Profile
In this paper a prescription for generating an equilibrium spherical system depicted with cuspy density profiles is extended to a core density profile, the Burkert profile, which is observationally more suitable to dwarfs. By using a time-saving Monte Carlo method instead of N-body simulations, we show that the Burkert halo, which is initialized with a distribution function that depends only on energy, is in a practically stable equilibrium. The one generated with the local Maxwellian approximation is unstable, where the flat core density structure tends to steepen. This is fundamentally different from a previous study on a halo with a cuspy density profile. The deviation of the unstable “Burkert” from the initial Burkert profile is found to be closely related to taking a Gaussian as the velocity distribution at any given point. The significance of not using the local Maxwellian approximation is further demonstrated by exploring the dynamical evolution of compact super star clusters (SSCs) in the Burkert halo. In particular, the local Maxwellian approximation results in underestimating the dynamical friction and overestimating sinking time scale. Accordingly, this leads to a lower probability of forming massive bulges and young nuclear star clusters in bulgeless galaxies.