8156739
Exact four-spinon dynamical correlation function of the Heisenberg model
Abstract In this paper we derive the exact expression of the four-spinon contribution to the dynamical correlation function of the spin S = 1 2 anisotropic (XXZ) Heisenberg model in the antiferromagnetic regime. We extensively study its isotropic (XXX) limit and derive perturbatively the Ising one. Our method relies on the quantum affine symmetry of the model, which allows for a systematic diagonalization of the Hamiltonian in the thermodynamic limit and for an exact calculation of matrix elements of local spin operators. In fact, we argue that the familiar criticism of this method related to the complication of these matrix elements is not justified. First, we give, in the form of contour integrals, an exact expression for the n-spinon contribution. After we compile recently found results concerning the two-spinon contribution, we specialize the n-spinon formula to the new case n = 4. Then we give an explicit series representation of this contribution in the isotropic limit. Finally, after we show that this representation is free of divergences, we discuss the Ising limit in which a simple expression is found up to first order in the anisotropy parameter.
16544882
Damping of very soft moving quarks in high-temperature QCD
Abstract. We determine the analytic expression of the damping rates for very soft moving quarks in an expansion to second order in powers of their momentum in the context of QCD at high temperature. The calculation is performed using the hard-thermal-loop-summed perturbation scheme. We describe the range of validity of the expansion and make a comparison with other calculations, particularly those using a magnetic mass as a shield from infrared sensitivity. We discuss the possible occurrence of infrared divergences in our results and argue that they are due to magnetic sensitivity.
54663418
Phenomenology of a light cold dark matter two-singlet model
We study the implications of phenomenological processes on a two-singlet extension of the Standard Model we introduced in a previous work to describe light cold dark matter. We look into
119344257
Infrared behavior of the dispersion relations in high-temperature scalar QED
We investigate the infrared properties of the next-to-leading-order dispersion relations in scalar quantum electrodynamics at high temperature in the context of hard-thermal-loop perturbation theory. Specifically, we determine the damping rate and the energy for scalars with ultrasoft momenta. We show by explicit calculations that an early external-momentum expansion, before the Matsubara sum is performed, gives exactly the same result as a late one. The damping rate is obtained up to fourth order included in the ultrasoft momentum and the energy up to second order. The damping rate is found sensitive in the infrared whereas the energy is not.
14577284
An infrared singularity in the damping rate for longitudinal gluons in hot QCD
Abstract We calculate γ l (0), the damping rate for longitudinal on-shell gluons with zero momentum in hot QCD using the hard-thermal-loop scheme, and find it to be divergent in the infrared. We argue that this result suggests that the scheme may be sensitive in this sector and thus may need to be improved upon. We discuss the validity of our result and emphasize the fact that it depends on the way we perform our calculation. In particular, we do not exclude the possibility that alternative approaches that we briefly comment on may in principle yield infrared-safe results.
26502186
ULTRASOFT QUARK DAMPING IN HOT QCD
We determine the quark damping rates in the context of next-to-leading order hard-thermal-loop summed perturbation of high-temperature QCD where weak coupling is assumed. The quarks are ultrasoft. Three types of divergent behavior are encountered: infrared, light-cone and at specific points determined by the gluon energies. The infrared divergence persists and is logarithmic whereas the two others are circumvented.
6263787
Leptogenesis in SO(10) models with a left-right symmetric seesaw mechanism
We study leptogenesis in supersymmetric SO(10) models with a left-right symmetric seesaw mechanism, including flavour effects and the contribution of the next-to-lightest right-handed neutrino. Assuming M_D = M_u and hierarchical light neutrino masses, we find that successful leptogenesis is possible for 4 out of the 8 right-handed neutrino mass spectra that are compatible with the observed neutrino data. An accurate description of charged fermion masses appears to be an important ingredient in the analysis.
124762744
Heavy flavor meson decay constants with Wilson fermions at β = 6.4
We present an extensive lattice study of the physical properties of mesons, composed of a heavy, H, and a light, q, quark, at β = 6.4 on a 243 × 60 lattice, using the Wilson action in the quenched approximation. We have studied the mass spectrum and the decay constants of vector and pseudoscalar mesons. We find significant violations of the mass scaling law fP√MP = const. ( ∼ 50% for D-mesons and ∼ 20% for B-mesons). The results using quenched but propagating quarks are remarkably consistent with the static results when the scale is taken from the pion decay constant. Combining the results obtained by several calculations of the pseudoscalar decay constants as a function of the meson mass, at different values of the lattice spacing, we obtain by extrapolation fB√BB = (220 ± 40) MeV (fB = (205 ± 40) MeV and BB = 1.16 ± 0.07), where BB is the renormalization group invariant B-parameter relevant for Bd-Bd mixing. We also find fBs2BB,fBd2BBd = 1.19 ± 0.10. This last result is relevant in experimental studies of B3-B3 mixing. The vector-pseudoscalar mass splittings do not follow the predicted behaviour, MV2 - MP2 ∼ const., which is expected (and found experimentally) in the limit of large heavy quark masses (i.e. when mQ ⪢ ΛQCD).
123404433
Meson spectroscopy and decay constants with Wilson fermions at β = 6.4
Abstract We present an extensive lattice study of the physical properties of mesons, composed of a heavy, H, and a light, q, quark, at β = 6.4 on a 24 3 × 60 lattice, using the Wilson action in the quenched approximation. We have studied the mass spectrum and the decay constants of vector and pseudoscalar mesons. We find significant violations of the mass scaling law ƒ P √M P = const. . (∼ 50% for D-mesons and ∼ 20% for B-mesons). The results using quenched but propagating quarks are remarkably consistent with the static results when the scale is taken from the pion decay constant. Combining the results obtained by several calculations of the pseudoscalar decay constants as a function of the meson mass, at different values of the lattice spacing, we obtain by extrapolation ƒ B √B B = (220 ± 40) MeV (ƒ B = (205 ± 40) MeV and B B = 1.16 ± 0.07) , where B B is the renormalization group invariant B -parameter relevant for B d − B d mixing. We also find ƒ B s 2 B B s /ƒ B d 2 B B d = 1.19 ± 0.10 . This last result is relevant in experimental studies of B s − B s mixing. The vector-pseudoscalar mass splittings do not follow the predicted behaviour, M V 2 − M P 2 const., which is expected (and found experimentally) in the limit of large heavy quark masses (i.e. when m H ⪢ Λ QCD ).
73561661
Tree-level lepton universality violation in the presence of sterile neutrinos: impact for RK and Rπ
A bstractWe consider a tree-level enhancement to the violation of lepton flavour universality in light meson decays arising from modified Wℓν couplings in the standard model minimally extended by sterile neutrinos. Due to the presence of additional mixings between the active (left-handed) neutrinos and the new sterile states, the deviation from unitarity of the leptonic mixing matrix intervening in charged currents might lead to a tree-level enhancement of RP = Γ(P → eν)/Γ(P → μν), with P = K, π. We illustrate these enhancements in the case of the inverse seesaw model, showing that one can saturate the current experimental bounds on ΔrK (and Δrπ), while in agreement with the different experimental and observational constraints.
38599601
Finite-size effects on multibody neutrino exchange
The effect of multibody massless neutrino exchanges between neutrons inside a finite-size neutron star is studied. We use an effective Lagrangian, which incorporates the effect of the neutrons on the neutrinos. Following Schwinger, it is shown that the total interaction energy density is computed by comparing the zero point energy of the neutrino sea with and without the star. It has already been shown that in an infinite-size star the total energy due to neutrino exchange vanishes exactly. The opposite claim that massless neutrino exchange would produce a huge energy is due to an improper summation of an infrared-divergent quantity. The same vanishing of the total energy has been proved exactly in the case of a finite star in a one-dimensional toy model. 
Here we study the three-dimensional case. We first consider the effect of a sharp star border, assumed to be a plane. We find that there is a non- vanishing of the zero point energy density difference between the inside and the outside due to the refraction index at the border and the consequent non-penetrating waves. An analytical and numerical calculation for the case of a spherical star with a sharp border confirms that the preceding border effect is the dominant one. The total result is shown to be infrared-safe, thus confirming that there is no need to assume a neutrino mass. The ultraviolet cut-offs, which correspond in some sense to the matching of the effective theory with the exact one, are discussed. Finally the energy due to long distance neutrino exchange is of the order of $10^{-8} -- 10^{-13} GeV per neutron$, i.e. negligible with respect to the neutron mass density.