Momentum-dependent Corrections Research Articles

Thermoelectric transport properties of Floquet multi-Weyl semimetals

We discuss the circularly polarized light (of amplitude $A_0$ and frequency $\omega$) driven thermo-electric transport properties of type-I and type-II multi-Weyl semimetals (mWSMs) in the high frequency limit. Considering the low energy model, we employ the Floquet-Kubo formalism to compute the thermal Hall and Nernst conductivities for both types of mWSMs. We show that the anisotropic nature of the dispersion for arbitrary integer monopole charge $n>1$ plays an important role in determining the effective Fermi surface behavior; interestingly, one can observe momentum dependent corrections in Floquet mWSMs in addition to momentum independent contribution as observed for Floquet single WSMs. Apart from the non-trivial tuning of the Weyl node position $\pm Q \to \pm Q- A_0^{2n}/\omega$, our study reveals that the momentum independent terms result in leading order contribution in the conductivity tensor. This has the form of $n$ times the single WSMs results with effective chemical potential $\mu \to \mu -A_0^{2n}/\omega$. On the other hand, momentum dependent corrections lead to sub-leading order terms which are algebraic function of $\mu$ and are present for $n>1$. Remarkably, this analysis further allows us to distinguish type-I mWSMs from their type-II counterparts. For type-II mWSMs, we find that the transport coefficients for $n\geq 2$ exhibit algebraic dependence on the momentum cutoff in addition to the weak logarithmic dependence as noticed for $n=1$ WSMs. We demonstrate the variation and qualitative differences of transport coefficients between type-I and type-II mWSM as a function of external driving parameter $\omega$.

Noncommutative spacetime geometry and one-loop effects in primordial cosmology

We study the effect of noncommutative spacetime geometry on one-loop corrections to the primordial curvature two-point function, arising from various forms of massless spectator matter fields interacting gravitationally with the inflaton. After deforming the algebra of functions on the inflationary background to a spatially noncommutative one, we find that this induces momentum-dependent corrections to one-loop terms which imply that the vacuum fluctuation of the energy-momentum tensor sources that of the curvature fluctuation even for distances beyond horizon scales. The one-loop corrections break spatial isotropy by being functions of the noncommutative parameters lying in the tranverse plane while reducing smoothly to the commutative limit. This furnishes an example of how UV/IR mixing manifests itself in the context of noncommutative field theories defined on inflationary backgrounds, and demonstrates how in principle, the primordial spectrum could carry a signature of nonlocality and anisotropy in the setting of noncommutative spacetime geometry.

Tetragonal and collapsed-tetragonal phases ofCaFe2As2: A view from angle-resolved photoemission and dynamical mean-field theory

We present a study of the tetragonal to collapsed-tetragonal transition of CaFe2As2 using angle-resolved photoemission experiments and dynamical mean field theory-based electronic structure calculations. We observe that the collapsed-tetragonal phase exhibits reduced correlations and a higher coherence temperature due to the stronger Fe-As hybridization. Furthermore, a comparison of measured photoemission spectra and theoretical spectral functions shows that momentum-dependent corrections to the density functional band structure are essential for the description of low-energy quasiparticle dispersions. We introduce those using the recently proposed combined "Screened Exchange + Dynamical Mean Field Theory" scheme.

Renormalization scheme dependence of the two-loop QCD corrections to the neutral Higgs-boson masses in the MSSM.

Reaching a theoretical accuracy in the prediction of the lightest MSSM Higgs-boson mass, M_h, at the level of the current experimental precision requires the inclusion of momentum-dependent contributions at the two-loop level. Recently two groups presented the two-loop QCD momentum-dependent corrections to M_h (Borowka et al., Eur Phys J C 74(8):2994, 2014; Degrassi et al., Eur Phys J C 75(2):61, 2015), using a hybrid on-shell-overline{mathrm {DR}} scheme, with apparently different results. We show that the differences can be traced back to a different renormalization of the top-quark mass, and that the claim in Ref. Degrassi et al. (Eur Phys J C 75(2):61, 2015) of an inconsistency in Ref. Borowka et al. (Eur Phys J C 74(8):2994, 2014) is incorrect. We furthermore compare consistently the results for M_h obtained with the top-quark mass renormalized on-shell and overline{mathrm {DR}}. The latter calculation has been added to the FeynHiggs package and can be used to estimate missing higher-order corrections beyond the two-loop level.

Numerical multi-loop calculations with the program SecDec

SecDec is a program which can be used for the evaluation of parametric integrals, in particular multi-loop integrals. For a given set of propagators defining the graph, the program constructs the graph polynomials, factorizes the endpoint singularities, and finally produces a Laurent series in the dimensional regularization parameter, whose coefficients are evaluated numerically. In this talk we discuss various features of the program, which extend the range of applicability. We also present a recent phenomenological example of an application entering the momentum dependent two-loop corrections to neutral Higgs boson masses in the MSSM.

Limits on Lorentz violation from the highest energy cosmic rays

We place several new limits on Lorentz violating effects, which can modify particles' dispersion relations, by considering the highest energy cosmic rays observed. Since these are hadrons, this involves considering the partonic content of such cosmic rays. We get a number of bounds on differences in maximum propagation speeds, which are typically bounded at the 10^{-21} level, and on momentum dependent dispersion corrections of the form v = 1 +- p^2/Lambda^2, which typically bound Lambda > 10^{21} GeV, well above the Planck scale. For (CPT violating) dispersion correction of the form v = 1 + p/Lambda, the bounds are up to 15 orders of magnitude beyond the Planck scale.

On the [formula omitted] two-loop corrections to the neutral Higgs boson masses in the MSSM

We compute the O(α t 2) two-loop corrections to the neutral CP-even Higgs boson mass matrix in the Minimal Supersymmetric Standard Model, for arbitrary values of m A and of the parameters in the stop sector, in the effective potential approach. In a large region of parameter space these corrections are sizeable, increasing the prediction for m h by several GeV. We present explicit analytical formulae for a simplified case. We discuss the inclusion of momentum-dependent corrections and some possible ways of assigning the input parameters.

Non equilibrium spectra of degenerate relic neutrinos

We calculate the exact kinetic evolution of cosmic neutrinos until complete decoupling, in the case when a large neutrino asymmetry exists. While not excluded by present observations, this large asymmetry can have relevant cosmological consequences and in particular may be helpful in reconciling Primordial Nucleosynthesis with a high baryon density as suggested by the most recent observations of the Cosmic Microwave Background Radiation. By solving numerically the Boltzmann kinetic equations for the neutrino distribution functions, we find the momentum-dependent corrections to the equilibrium spectra and briefly discuss their phenomenological implications.

The dilepton-production cross section in principal value resummation

Using a recent calculation of the perturbative hard part for dilepton production that sums large threshold corrections to all orders in perturbative QCD, we compute the corresponding cross sections. The hard part has been evaluated using principal value resummation and contains all singular momentum-dependent corrections. We also include a resummation of large Sudakov terms, which are independent of parton momenta. We give predictions for the dilepton-mass distribution, the rapidity distribution and the rapidity-integrated K-factor at fixed-target energies and compare with various experimental results in several kinematic regimes. We find that principal value resummation produces cross sections that are finite and well-behaved. For both protons and anti-protons on fixed targets, the resummed cross sections are, in general, in excellent agreement with the data.

Comment on "Recoil saturation of the self-energy in atomic systems"

The effective potential in low-energy two-body scattering obtained by the ``self-energy method'' is identical to the effective potential commonly defined as the Fourier transform of the scattering transition amplitude (the T matrix) with respect to the momentum transfer. Because of the convenience that may occur in the exchange of the order of integration, nonadiabatic recoil corrections may be disguised as momentum-dependent corrections. These momentum-dependent corrections are distinct from energy-dependent corrections. In certain forms, they may violate either T invariance or Hermiticity. Their origin is attributable to a mathematical artifact and there exists a systematic method of converting the momentum-dependent correction terms into truly local form. When recoil effect is important, these nonadiabatic corrections cannot be ignored even at threshold energy when all energy-dependent corrections vanish. In certain cases, such as in the extrapolation of the effective potential to short-distance behavior, each of the nonadiabatic corrections may be comparable to the leading term. On resummation, these may greatly alter the behavior of the effective potential indicated by the leading term alone. The previous analysis of J. R. Manson and R. H. Ritchie [Phys. Rev. A 35, 5249 (1987)] on the saturation effect due to recoil in the scattering of Coulombic systems is based on neglect of these nonadiabatic corrections disguised as momentum-dependent corrections while extrapolating the effective potential to short-distance behavior. The present Comment points out the flaw in such a procedure.

Nonadiabatic, momentum-dependent, and energy-dependent corrections in the effective-potential description for low-energy scattering of spinless systems: Their relations and validity.

In the effective-potential description for low-energy scattering involving a spinless complex (a body with internal structure), the nonadiabatic corrections are sometimes disguised in momentum-dependent terms. These are distinct from energy-dependent corrections. A general procedure is given here by which all the momentum-dependent corrections can be converted into nonadiabatic corrections in truly local form. Circumstances under which an expansion of the effective potential, in terms of the adiabatic term plus nonadiabatic and energy-dependent corrections is allowed and forbidden, are discussed. An example for the latter is in the case of near degeneracy in the spectrum of the complex or in the extrapolation of the effective potential to short-distance behavior. This indicates that certain claims of 'saturation effect' at short distances in low-energy electron-atom scattering are invalid.

Momentum-dependent effects in the decays of charmonium and upsilon

Abstract Decay widths of quarkonia are determined in a calculation in which the momentum distribution resulting from the localization of the quarks is taken into account. The e + e − decay modes of charmonium and upsilon are considered with the momentum-dependent corrections to the well-known van Royen-Weisskopf formula for leptonic widths. The momentum-dependent effects are found to be significant, and amount to approximately a 20% correction. The γη(1440) decay mode is also examined to investigate a decay involving a propagating quark in the bound state. Here the momentum-dependent corrections to the static approximation are very large and suppress the decay widths by well over a factor of 3. Therefore one must exercise caution in using a static approximation.

A Bethe-Salpeter basis for meson and baryon spectra under harmonic confinement

The Bethe-Salpeter equation forq−q andqqq systems derived in the preceding paper [8] in the instantaneous approximation are solved algebraically for harmonic confinement. The approximateq−q spectrum for all flavour is expressible asF(M)=N+3/2, where $$\begin{gathered} F\left( M \right) = \left( {M^2 - 4m_q^2 } \right)\Omega _{_{\rm M} }^{ - 1} - \Omega _{\rm M} M^{ - 2} \gamma ^{ - 2} \hfill \\ \cdot \left( {2J \cdot S - 3 - Q_N } \right) + F_{QCD} \hfill \\ \end{gathered} $$ \(\Omega _{\rm M} = 8(Mm)_q )^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \tilde \omega \gamma \) is a mass-dependent FKR-like spring constant\(\tilde \omega \)(=0.15 GeV) a universal flavourindependent parameter, and γ(≈1) a slowly varying quantity. J·S represents the spin-dependent effects andQN, a quadratic function ofN, comprises some significant momentum dependent corrections, whileFQCD is a small additional correction due to shortrange gluon exchange effects. An identical equation\(\bar F\)(M)=N+3 holds for non-strangeqqq excitations, with a very similar definition of\(\bar F\)(M) in terms of the same parameters\(\tilde \omega \) andmq. The calculated values ofF(M) and\(\bar F\)(M) in terms of theobserved masses,M, andmud=0.28,ms=0.35,mc=1.40 (all in GeV), conform rather well to the principal features of the predictions, viz. (i) spin and flavour degeneracy ofq\(\bar q\) supermultiplet members at theF(M) level, despite huge variations in their actual masses (e. g. P vs V); and likewise forqqq members (e.g.,NL,ΔL) at the\(\bar F\)(M) level, and (ii) fulfilment of the unit spacing rule ΔF=1, Δ\(\bar F\)=1 for successive h.o. supermultiplets. TheP—V degeneracy at theF(M) level leads to the prediction ψ−η c ≈100±20 MeV. Finally, theP→l\(\bar l\) amplitudesfπ,k, as well as the principalV→e+ e− widths are fairly well reproduced without extra parameters.

One-boson-exchange potential model for the ΛN interaction

A one-boson-exchange potential model for the ΛN interaction is described. The effect of the ΣN channel is included explicitly. The potential includes contributions from the following bosons: pseudoscalar and vector octets, and scalar and vector unitary singlets. The approximations needed to obtain a potential for a general baryon-baryon interaction are discussed, and certain ambiguities are pointed out. In particular, we consider to what extent the addition of momentum-dependent corrections would be an improvement over the extreme nonrelativistic limit. By numerical calculations, we then adjust the parameters of our model to reproduce experimental low-energy Λp data. We use SU(3) to relate the meson-baryon couplings. The principal search parameters are the mass and coupling of the scalar unitary singlet boson, the vector coupling of the vector unitary singlet boson, and the radius of a phenomenological spin-independent hard core. Λp cross sections are then calculated from zero energy up through the Σ-production threshold. Some of the ambiguities present in a local potential are investigated numerically. For our model we find that the values of the search parameters are not especially sensitive to these ambiguities.