119188244
LHC phenomenology of lowest massive Regge recurrences in the Randall-Sundrum orbifold
We consider string realizations of the Randall-Sundrum effective theory for electroweak symmetry breaking and explore the search for the lowest massive Regge excitation of the gluon and of the extra (color singlet) gauge boson inherent in D-brane constructions. In these curved backgrounds, the higher-spin Regge recurrences of standard model fields localized near the IR brane are warped down to close to the TeV range and hence can be produced at collider experiments. Assuming that the theory is weakly coupled, we make use of four gauge boson amplitudes evaluated near the first Regge pole to determine the discovery potential of LHC. We study the inclusive dijet mass spectrum in the central rapidity region |y{sub jet}| =}2.5 TeV. We find that with an integrated luminosity of 100 fb{sup -1}, the 5{sigma} discovery reach can be as high as 4.7 TeV. Observations of resonant structures in pp{yields}direct{gamma}+jet can provide interesting corroboration for string physics up to 3.0 TeV. We also study the ratio of dijet mass spectra at small and large scattering angles. We show that with the first fb{sup -1} such a ratio can probe lowest-lying Regge states for masses {approx}2.5 TeV.
119271491
Searching for string resonances in $e^+e^-$ and $\gamma \gamma$ collisions
If the fundamental mass scale of superstring theory is as low as few TeVs, the massive modes of vibrating strings, Regge excitations, will be copiously produced at the Large Hadron Collider (LHC). We discuss the complementary signals of low mass superstrings at the proposed electron-positron facility (CLIC), in e^+e^- and \gamma \gamma collisions. We examine all relevant four-particle amplitudes evaluated at the center of mass energies near the mass of lightest Regge excitations and extract the corresponding pole terms. The Regge poles of all four-point amplitudes, in particular the spin content of the resonances, are completely model independent, universal properties of the entire landscape of string compactifications. We show that \gamma \gamma \to e^+ e^- scattering proceeds only through a spin-2 Regge state. We estimate that for this particular channel, string scales as high as 4 TeV can be discovered at the 11\sigma level with the first fb^{-1} of data collected at a center-of-mass energy \approx 5 TeV. We also show that for e^+e^- annihilation into fermion-antifermion pairs, string theory predicts the precise value, equal 1/3, of the relative weight of spin 2 and spin 1 contributions. This yields a dimuon angular distribution with a pronounced forward-backward asymmetry, which will help distinguishing between low mass strings and other beyond the standard model scenarios.
119185690
Hermiticity of the Dirac Hamiltonian in curved spacetime
In previous work on the quantum mechanics of an atom freely falling in a general curved background spacetime, the metric was taken to be sufficiently slowly varying on time scales relevant to atomic transitions that time derivatives of the metric in the vicinity of the atom could be neglected. However, when the time dependence of the metric cannot be neglected, it was shown that the Hamiltonian used there was not Hermitian with respect to the conserved scalar product. This Hamiltonian was obtained directly from the Dirac equation in curved spacetime. This raises the paradox of how it is possible for this Hamiltonian to be non-Hermitian. Here, we show that this non-Hermiticity results from a time dependence of the position eigenstates that enter into the Schroedinger wave function, and we write the expression for the Hamiltonian that is Hermitian for a general metric when the time dependence of the metric is not neglected.
119182228
S-dual inflation: BICEP2 data without unlikeliness
Abstract We show that S -dual inflationary potentials solve the unlikeliness problem manifested in Planck data and explain the excess B -mode power observed by the BICEP2 experiment as arising from primordial tensor fluctuations.
118565086
Stringy origin of Tevatron Wjj anomaly
Abstract The invariant mass distribution of dijets produced in association with W bosons, recently observed by the CDF Collaboration at Tevatron, reveals an excess in the dijet mass range 120 – 160 GeV / c 2 , 3σ beyond Standard Model expectations. We show that such an excess is a generic feature of low mass string theory, due to the production and decay of a leptophobic Z ′ , a singlet partner of SU ( 3 ) gluons coupled primarily to the U ( 1 ) baryon number. In this framework, U ( 1 ) and SU ( 3 ) appear as subgroups of U ( 3 ) associated with open strings ending on a stack of 3 D-branes. In addition, a minimal model contains two other stacks to accommodate the electro-weak SU ( 2 ) ⊂ U ( 2 ) and the hypercharge U ( 1 ) . Of the three U ( 1 ) gauge bosons, the two heavy Z ′ and Z ″ receive masses through the Green–Schwarz mechanism. For a given Z ′ mass, the model is quite constrained. Fine tuning three of its free parameters is just sufficient to simultaneously ensure: a small Z – Z ′ mixing in accord with the stringent LEP data on the Z mass; very small (less than 1%) branching ratio into leptons; and a large hierarchy between Z ″ and Z ′ masses. The heavier neutral gauge boson Z ″ is within the reach of LHC.
119576534
Graviton Propagator in a Covariant Massive Gravity Theory
We study the massive gravity theory proposed by Arkani-Hamed, Georgi and Schwartz. In this theory, the graviton becomes massive when general covariance is spontaneously broken through the introduction of a field that links two metrics, one of the which will eventually decouple. The excitation of this "link" field acts like a Goldstone boson in giving mass to the graviton. We work out the graviton and Goldstone boson propagators explicitly by means of gauge fixing terms similar to the renormalizability gauges used in gauge theories. With these propagators, we calculate the lowest order tree-level interaction between two external energy momentum tensors. The result is independent of the gauge parameter, but different from the prediction of massless gravity theory, i.e., general relativity. This difference remains even if the mass of the graviton goes to zero, in which case it gives the van Dam-Veltman-Zakharov (vDVZ) discontinuity between the propagators of a massive and massless linearized graviton. Moreover, we show that the Fierz-Pauli graviton mass term can be considered as the ``unitary gauge'' of a more general theory with an extra scalar field. We explicitly construct such a theory in which the vDVZ discontinuity arises with a graviton mass term that is different from the Fierz-Pauli mass term. This theory has a local Weyl symmetry under conformal transformations of the metric. In the case when the mass goes to zero, the Weyl summetry becomes a global symmetry. It is possible that the local Weyl symmetry will give a hint as to the form of the corresponding fully nonlinear theory having a nonzero graviton mass.
119239509
BCS-BEC crossover at finite temperature in spin-orbit coupled Fermi gases
By adopting a $T$-matrix-based method within the $G_0G$ approximation for the pair susceptibility, we study the effects of the pairing fluctuation on the three-dimensional spin-orbit coupled Fermi gases at finite temperature. The critical temperatures of the superfluid/normal phase transition are determined for three different types of spin-orbit coupling (SOC): (1) the extreme oblate (EO) or Rashba SOC, (2) the extreme prolate (EP) or equal Rashba-Dresselhaus SOC, and (3) the spherical (S) SOC. For EO- and S-type SOC, the SOC dependence of the critical temperature signals a crossover from BCS to BEC state; at strong SOC limit, the critical temperature recover those of ideal BEC of rashbons. The pairing fluctuation induces a pseudogap in the fermionic excitation spectrum in both superfluid and normal phases. We find that, for EO- and S-type SOC, even at weak coupling, sufficiently strong SOC can induce sizable pseudogap. Our research suggests that the spin-orbit coupled Fermi gases may open new means to the study of the pseudogap formation in fermionic systems.
16264551
Neutrino Emission From Inhomogeneous Pion Condensed Quark Matter
It is believed that quark matter can exist in neutron star interior if the baryon density is high enough. When there is a large isospin density, quark matter could be in a pion condensed phase. We compute neutrino emission from direct Urca processes in such a phase, particularly in the inhomogeneous Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states. The neutrino emissivity and specific heat are obtained, from which the cooling rate is estimated.
118657682
New constraints on primordial minihalo abundance using cosmic microwave background observations
It was proposed that the massive compact halo objects (MCHOs) would be produced during an earlier epoch of cosmology if the density perturbations are between $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and 0.3. Then these objects can accrete dark matter particles onto them due to their high density. If the dark matter is in the form of the weakly interacting massive particles, the MCHOs can have a significant effect on the evolution of cosmology due to the dark matter annihilation within them. Using the WMAP-7 years data, we investigated the constraints on the current abundance of MCHOs (${f}_{\mathrm{MCHOs}}$) formed during the ${e}^{+}{e}^{\ensuremath{-}}$ annihilation phase transition. We have found that the $2\ensuremath{\sigma}$ constraint is ${f}_{\mathrm{MCHOs}}\ensuremath{\lesssim}{10}^{\ensuremath{-}4}$ for dark matter masses in the range between 1 GeV and 1 TeV.
197499640
Unusual Zeeman Field Effects in 2D Spin-Orbit Coupled Fermi Superfluids
We investigate the Zeeman field effects on the bulk superfluid properties and the collective modes in 2D attractive atomic Fermi gases with Rashba-type spin-orbit coupling. In the presence of a large spin-orbit coupling, the system undergoes a quantum phase transition to a topological superfluid state at a critical Zeeman field. We show that the non-analyticities of the thermodynamic functions as well as other physical quantities at the quantum phase transition originate from the infrared singularities caused by the gapless fermionic spectrum. The same argument applies also to the BCS-BEC evolution in 2D fermionic superfluids with $p$- or d-wave pairing. The superfluid density $n_s$ and the velocity of the Goldstone sound mode $c_s$ behave oppositely in the normal and the topological superfluid phases: they are suppressed by the Zeeman field in the normal superfluid phase, but get enhanced in the topological superfluid phase. The velocity of the Goldstone sound mode also shows non-analyticity at the quantum phase transition. For large Zeeman field, we find $n_s\rightarrow n$ and $c_s\rightarrow \upsilon_{\rm F}$, where $n$ is the total fermion density and $\upsilon_{\rm F}$ is the Fermi velocity of non-interacting system. The unusual behaviors of the superfluid density and the collective modes can be understood by the fact that the spin-orbit coupled superfluid state at large Zeeman field can be mapped to the $p_x+ip_y$ superfluid state of spinless fermions.
118532372
Constraints on ultracompact minihalos from extragalactic {\gamma}-ray background
It has been proposed that ultracompact minihalos (UCMHs) might be formed in earlier epoch. If dark matter consists of Weakly Interacting Massive Particles (WIMPs), UCMHs can be treated as the γ-ray sources due to dark matter annihilation within them. In this paper, we investigate the contributions of UCMHs formed during three phase transitions (i.e., electroweak symmetry breaking, QCD confinement and e+e− annihilation) to the extragalactic γ-ray background. Moreover, we use the Fermi-LAT observation data of the extragalactic γ-ray background to get the constraints on the current abundance of UCMHs produced during these phase transitions. We also compare these results with those obtained from Cosmic Microwave Background (CMB) observations and find that the constraints from the Fermi-LAT are more stringent than those from CMB.