211 research outputs found
Two-Loop Corrections to the Neutral Higgs Boson Masses in the CP-Violating NMSSM
We present our calculation of the two-loop corrections of to the neutral Higgs boson masses of the CP-violating
Next-to-Minimal Supersymmetric extension of the Standard Model (NMSSM). The
calculation is performed in the Feynman diagrammatic approach in the gaugeless
limit at vanishing external momentum. We apply a mixed
-on-shell (OS) renormalization scheme for the NMSSM
input parameters. Furthermore, we exploit a as well as
an OS renormalization in the top/stop sector. The corrections are implemented
in the Fortran code NMSSMCALC for the calculation of the Higgs spectrum both in
the CP-conserving and CP-violating NMSSM. The code also provides the Higgs
boson decays including the state-of-the-art higher-order corrections. The
corrections computed in this work improve the already available corrections in
NMSSMCALC which are the full one-loop corrections without any approximation and
the two-loop corrections in the gaugeless limit
and at vanishing external momentum. Depending on the chosen parameter point, we
find that the corrections add about
4-7% to the one-loop mass of the SM-like Higgs boson for
renormalization in the top/stop sector and they reduce
the mass by about 6-9% if OS renormalization is applied. For an estimate of the
theoretical uncertainty we vary the renormalization scale and change the
renormalization scheme and show that care has to be taken in the corresponding
interpretation
Temperature dependent band structure of the Kondo insulator
We present a Qantum Monte Carlo (QMC) study of the temperature dependent
dynamics of the Kondo insulator. Working at the so-called symmetrical point
allows to perform minus-sign free QMC simulations and thus reach temperatures
of less than 1% of the conduction electron bandwidth. Study of the temperature
dependence of the single particle Green's function and dynamical spin
correlation function shows a surprisingly intricate low temperature band
structure and gives evidence for two characteristic temperatures, which we
identify with the Kondo and coherence temperature, respectively. In particular,
the data show a temperature induced metal-insulator transition at the coherence
temperature.Comment: RevTex-file, 4 PRB pages with 4 eps figures. Hardcopies of figures
(or the entire manuscript) can be obtained by e-mail request to:
[email protected]
Higgs-boson masses and mixing matrices in the NMSSM: analysis of on-shell calculations
We analyze the Higgs-boson masses and mixing matrices in the NMSSM based on an on-shell (OS) renormalization of the gauge-boson and Higgs-boson masses and the parameters of the top/scalar top sector. We compare the implementation of the OS calculations in the codes NMSSMCALC and NMSSM-FeynHiggs up to O(αtαs). We identify the sources of discrepancies at the one- and at the twoloop level. Finally we compare the OS and DR evaluation as implemented in NMSSMCALC. The results are important ingredients for an estimate of the theoretical precision of Higgs-boson mass calculations in the NMSSM
Signs of composite Higgs pair production at next-to-leading order
In composite Higgs models the Higgs boson arises as a pseudo-Goldstone boson from a strongly-interacting sector. Fermion mass generation is possible through partial compositeness accompanied by the appearance of new heavy fermionic resonances. The Higgs couplings to the Standard Model (SM) particles and between the Higgs bosons themselves are modified with respect to the SM. Higgs pair production is sensitive to the trilinear Higgs self-coupling but also to anomalous couplings like the novel 2-Higgs-2-fermion coupling emerging in composite Higgs models. The QCD corrections to SM Higgs boson pair production are known to be large. In this paper we compute, in the limit of heavy loop particle masses, the next-to-leading order (NLO) QCD corrections to Higgs pair production in composite Higgs models without and with new heavy fermions. The relative QCD corrections are found to be almost insensitive both to the compositeness of the Higgs boson and to the details of the heavy fermion spectrum, since the leading order cross section dominantly factorizes. With the obtained results we investigate the question if, taking into account Higgs coupling constraints, new physics could first be seen in Higgs pair production. We find this to be the case in the high-luminosity option of the LHC for composite Higgs models with heavy fermions. We also investigate the invariant mass distributions at NLO QCD. While they are sensitive to the Higgs non-linearities and hence anomalous couplings, the influence of the heavy fermions is much less pronounced
Screened-interaction expansion for the Hubbard model and determination of the quantum Monte Carlo Fermi surface
We develop a systematic self-consistent perturbative expansion for the self
energy of Hubbard-like models. The interaction lines in the Feynman diagrams
are dynamically screened by the charge fluctuations in the system. Although the
formal expansion is exact-assuming that the model under the study is
perturbative-only if diagrams to all orders are included, it is shown that for
large-on-site-Coulomb-repulsion-U systems weak-coupling expansions to a few
orders may already converge. We show that the screened interaction for the
large-U system can be vanishingly small at a certain intermediate electron
filling; and it is found that our approximation for the imaginary part of the
one-particle self energy agrees well with the QMC results in the low energy
scales at this particular filling. But, the usefulness of the approximation is
hindered by the fact that it has the incorrect filling dependence when the
filling deviates from this value. We also calculate the exact QMC Fermi
surfaces for the two-dimensional (2-D) Hubbard model for several fillings. Our
results near half filling show extreme violation of the concepts of the band
theory; in fact, instead of growing, Fermi surface vanishes when doped toward
the half-filled Mott-Hubbard insulator. Sufficiently away from half filling,
noninteracting-like Fermi surfaces are recovered. These results combined with
the Luttinger theorem might show that diagrammatic expansions for the
nearly-half-filled Hubbard model are unlikely to be possible; however, the
nonperturbative part of the solution seems to be less important as the filling
gradually moves away from one half. Results for the 2-D one-band Hubbard model
for several hole dopings are presented. Implications of this study for the
high-temperature superconductors are also discussed.Comment: 11 pages, 12 eps figures embedded, REVTeX, submitted to Phys. Rev. B;
(v2) minor revisions, scheduled for publication on November 1
Light stop decays into Wb x ~ 10 near the kinematic threshold
We investigate the decays of the light stop in scenarios with the lightest neutralino x~10 being the lightest supersymmetric particle, including flavour-violating (FV) effects. We analyse the region where the three-body decay t~1->;Wb x ~10 is kinematically allowed and provide a proper description of the transition region between the three-body decay and the four-body decay t~1->;Wb x ~10bff\u27. The improved treatment has been implemented in the Fortran package SUSY-HIT and is used for the analysis of this region. A scan over the parameter range including all relevant experimental constraints reveals that the FV two-body decay into charm and x~10 can be as important as the three-, respectively, four-body decays if not dominant and therefore should be taken into account in order to complete the experimental searches for the light stop
Electron-phonon vertex in the two-dimensional one-band Hubbard model
Using quantum Monte Carlo techniques, we study the effects of electronic
correlations on the effective electron-phonon (el-ph) coupling in a
two-dimensional one-band Hubbard model. We consider a momentum-independent bare
ionic el-ph coupling. In the weak- and intermediate-correlation regimes, we
find that the on-site Coulomb interaction acts to effectively suppress the
ionic el-ph coupling at all electron- and phonon- momenta. In this regime, our
numerical simulations are in good agreement with the results of perturbation
theory to order . However, entering the strong-correlation regime, we find
that the forward scattering process stops decreasing and begins to
substantially increase as a function of , leading to an effective el-ph
coupling which is peaked in the forward direction. Whereas at weak and
intermediate Coulomb interactions, screening is the dominant correlation effect
suppressing the el-ph coupling, at larger values irreducible vertex
corrections become more important and give rise to this increase. These vertex
corrections depend crucially on the renormalized electronic structure of the
strongly correlated system.Comment: 5 pages, 4 eps-figures, minor change
Nonlocal Excitations and 1/8 Singularity in Cuprates
Momentum-dependent excitation spectra of the two-dimensional Hubbard model on
the square lattice have been investigated at zero temperature on the basis of
the full self-consistent projection operator method in order to clarify
nonlocal effects of electron correlations on the spectra. It is found that
intersite antiferromagnetic correlations cause shadow bands and enhance the
Mott-Hubbard splittings near the half-filling. Furthermore nonlocal excitations
are shown to move the critical doping concentration , at
which the singular quasiparticle peak is located just on the Fermi level, from
(the single-site value) to .
The latter suggests the occurance of an instability such as the stripe at
.Comment: 4 pages, 5 figures; to be published in the Journal of Korean Physical
Society (ICM12
Kink Structure in the Quasiparticle Band of Doped Hubbard Systems
By making use of the self-consistent projection operator method with
high-momentum and high-energy resolutions, we find a kink structure in the
quasiparticle excitation spectrum of the two-dimensional Hubbard model in the
underdoped regime. The kink is caused by a mixing between the quasiparticle
state and excitations with short-range antiferromagnetic order. We suggest that
this might be the origin of the strong concentration dependence of the 'kink'
found in La_{2-x}Sr_{x}CuO_{4} (x=0.03-0.07).Comment: 3 pages, 4 figures. to be published in J. Phys. Soc. Jpn., Vol. 74,
No. 9, September 15, 200
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