One-loop Decay Research Articles

Phenomenological implications of sterile neutrinos in the [formula omitted]SSM and dark matter

We analyze the role of sterile neutrinos in the framework of the μνSSM, where the presence of right-handed neutrinos provides a simultaneous solution to μ- and ν-problems in supersymmetry. We adopt a minimalistic approach, reproducing light neutrino masses and mixing angles at tree-level using just two right-handed neutrinos as part of the seesaw mechanism. A third right-handed neutrino does not contribute significantly to the mass of the three active ones, behaving as a sterile neutrino with a mass in the range keV-MeV. Furthermore, a sterile neutrino of about 10 keV can be a good candidate for dark matter with a lifetime larger than the age of the Universe. In particular, the three-body decay to active neutrinos gives the dominant contribution to its lifetime. The one-loop decay to gamma and active neutrino is subdominant, but relevant for observations such as astrophysical X-rays. We find regions of the parameter space of the μνSSM, with different values of the sterile neutrino mass, fulfilling not only these constraints but also collider constraints from the Higgs sector.

One-loop formulas for H → Zν l ν l for l = e, µ, τ in 't Hooft-Veltman gauge* *This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under the grant number (103.01-2019.346)

In this paper, we present analytical results for one-loop contributions to the decay processes (for ). The calculations are performed within the Standard Model framework in the 't Hooft-Veltman gauge. One-loop form factors are then written in terms of scalar one-loop functions in the standard notations of . As a result, one-loop decay rates for the decay channels can be evaluated numerically by using the package. Furthermore, we analyze the signals of via the production processes , including the initial beam polarizations at future lepton colliders. The Standard Model backgrounds, such as the processes , are also examined in this study. Numerical results indicate that one-loop corrections make contributions of approximately 10% to the decay rates. These are sizeable contributions and should be taken into account at future colliders. We show that the signals are clearly visible at the center-of-mass energy GeV and are difficult to probe in higher-energy regions owing to the dominant backgrounds.

Unified origin of dark matter self interactions and low scale leptogenesis

We propose a novel and minimal framework where a light scalar field can give rise to dark matter (DM) self-interactions, while enhancing the CP symmetry required for successful baryon asymmetry of the Universe via leptogenesis route. For demonstration purpose we choose to work in a scotogenic seesaw scenario where the lightest among the right handed neutrinos (RHN), introduced for generating light neutrino masses radiatively, play the role of DM while the heavier two can play non-trivial roles in generating DM relic as well as lepton asymmetry. While dark matter self-interactions mediated by an additional singlet scalar can alleviate the small scale issues of cold dark matter paradigm, the same scalar can give rise to new one-loop decay processes of heavy RHN into standard model leptons providing an enhanced contribution to CP asymmetry, even with sub-TeV scale RHN mass. The thermally under-abundant relic of DM due to large annihilation rates into its light mediator receives a late non-thermal contribution from a heavier RHN. With only five new particles involved in the scotogenic seesaw, each having non-trivial roles in generating DM relic and baryon asymmetry, the model can explain non-zero neutrino mass while being verifiable at different experiments related to DM direct detection, flavour physics and colliders. The mechanism we demonstrated here by using a scotogenic seesaw scenario is also applicable to other models.

Loop-corrected Higgs masses in the NMSSM with inverse seesaw mechanism

In this study, we work in the framework of the Next-to-Minimal extension of the Standard Model (NMSSM) extended by six singlet leptonic superfields. Through the mixing with the three doublet leptonic superfields, the non-zero tiny neutrino masses can be generated through the inverse seesaw mechanism. While R-parity is conserved in this model lepton number is explicitly violated. We quantify the impact of the extended neutrino sector on the NMSSM Higgs sector by computing the complete one-loop corrections with full momentum dependence to the Higgs boson masses in a mixed on-shell-overline{text{ DR }} renormalization scheme, with and without the inclusion of CP violation. The results are consistently combined with the dominant two-loop corrections at mathcal{O}(alpha _t(alpha _s+alpha _t)) to improve the predictions for the Higgs mixing and the loop-corrected masses. In our numerical study we include the constraints from the Higgs data, the neutrino oscillation data, the charged lepton flavor-violating decays l_i rightarrow l_j + gamma , and the new physics constraints from the oblique parameters S, T, U. We present in this context the one-loop decay width for l_i rightarrow l_j + gamma . The loop-corrected Higgs boson masses are included in the Fortran code NMSSMCALC-nuSS.

Radiative seesaw dark matter

The singlet majoron model of seesaw neutrino mass is appended by one dark Majorana fermion singlet $\chi$ with $L=2$ and one dark complex scalar singlet $\zeta$ with $L=1$. This simple setup allows $\chi$ to obtain a small radiative mass anchored by the same heavy right-handed neutrinos, whereas the one-loop decay of the standard-model Higgs boson to $\chi \chi + \bar{\chi} \bar{\chi}$ provides the freeze-in mechanism for $\chi$ to be the light dark matter of the Universe.

Higgs\u2013photon resonances

We study models that produce a Higgs boson plus photon (h^0gamma ) resonance at the LHC. When the resonance is a Z' boson, decays to h^0gamma occur at one loop. If the Z' boson couples at tree level to quarks, then the h^0gamma branching fraction is typically of order 10^{-5} or smaller. Nevertheless, there are models that would allow the observation of Z'rightarrow ,h^0gamma at sqrt{s}=13 TeV with a cross section times branching fraction larger than 1 fb for a Z' mass in the 200–450 GeV range, and larger than 0.1 fb for a mass up to 800 GeV. The one-loop decay of the Z' into lepton pairs competes with h^0gamma , even if the Z' couplings to leptons vanish at tree level. We also present a model in which a Z' boson decays into a Higgs boson and a pair of collimated photons, mimicking an h^0gamma resonance. In this model, the h^0gamma resonance search would be the discovery mode for a Z' as heavy as 2 TeV. When the resonance is a scalar, although decay to h^0gamma is forbidden by angular momentum conservation, the h^0 plus collimated photons channel is allowed. We comment on prospects of observing an h^0gamma resonance through different Higgs decays, on constraints from related searches, and on models where h^0 is replaced by a nonstandard Higgs boson.

Schwinger pair production at finite temperature

Thermal corrections to Schwinger pair production are potentially important in particle physics, nuclear physics and cosmology. However, the lowest-order contribution, arising at one loop, has proved difficult to calculate unambiguously. We show that this thermal correction may be calculated for charged scalars using the worldline formalism, where each term in the decay rate is associated with a worldline instanton. We calculate all finite-temperature worldline instantons, their actions and fluctuations prefactors, thus determining the complete one-loop decay rate at finite temperature. The thermal contribution to the decay rate becomes nonzero at a threshold temperature $T=eE/2m$, above which it dominates the zero temperature result. This is the lowest of an infinite set of thresholds at $T=neE/2m$. The decay rate is singular at each threshold as a consequence of the failure of the quadratic approximation to the worldline path integral. We argue that that higher-order effects will make the decay rates finite everywhere, and model those effects by the inclusion of hard thermal loop damping rates. We also demonstrate that the formalism developed here generalizes to the case of finite-temperature pair production in inhomogeneous fields.

Diphoton decay for a 750 GeV scalar boson in a U(1)X model

In the context of a nonuniversal and anomaly free U(1)X extension of the standard model, we examine the decay of a 750 GeV scalar singlet state, ξχ, as a possible explanation of the observed diphoton excess announced by the ATLAS and CMS collaborations at CERN-LHC collider. The one-loop decay to photons is allowed through three heavy singlet quarks and one charged Higgs boson into the loop. We obtain, for different width approximations and for masses of the exotic singlet quarks in the region [900,3000] GeV, a production cross section σ(pp→ξχ→γγ) compatible with ATLAS and CMS collaborations data. We also include another scalar singlet, σ, as a dark matter candidate that may couple with the 750 GeV scalar at tree level with production cross sections in agreement with ATLAS and CMS.

SFOLD — A program package for calculating two-body sfermion decays at full one-loop level in the MSSM

SFOLD (Sfermion Full One-Loop Decays) is a Fortran program package for calculating all sfermion two-body decay widths and the corresponding branching ratios at full one-loop level within the MSSM. The package adopts the SUSY Parameter Analysis convention and supports the SUSY Les Houches Accord input and output format. With the SFOLD package we found non-negligible electroweak corrections in bosonic decays of b̃,t̃ and τ̃. Program summaryProgram title: SFOLDCatalogue identifier: AEMZ_v1_0Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEMZ_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 263346No. of bytes in distributed program, including test data, etc.: 1481697Distribution format: tar.gzProgramming language: Fortran 77.Computer: Workstation, PC.Operating system: Linux.RAM: approx. 500 MbytesClassification: 11.1.External routines: LoopTools 2.6 (http://www.feynarts.de/looptools/), SLHALib 2.2 (http://www.feynarts.de/slha/)Nature of problem:If the MSSM is realized in nature, LHC will produce supersymmetric particles copiously. The best environment for a precise determination of the model parameters would be a high energy e+e− linear collider. Experimental accuracies are expected at the per-cent down to the per-mill level. These must be matched from the theoretical side. Therefore loop calculations are mandatory.Solution method:This program package calculates all sfermion two-body decay widths and the corresponding branching ratios at full one-loop level within the MSSM. The renormalization is done in the DR¯ scheme following the SUSY Parameter Analysis convention. The program supports the SUSY Les Houches Accord input and output format.Running time:The test provided just takes a few seconds to run.

Light stop decay in the MSSM with minimal flavour violation

In supersymmetric scenarios with a light stop particle $\tilde{t}_1$ and a small mass difference to the lightest supersymmetric particle (LSP) assumed to be the lightest neutralino, the flavour changing neutral current decay $\tilde{t}_1 \to c \tilde{\chi}_1^0$ can be the dominant decay channel and can exceed the four-body stop decay for certain parameter values. In the framework of Minimal Flavour Violation (MFV) this decay is CKM-suppressed, thus inducing long stop lifetimes. Stop decay length measurements at the LHC can then be exploited to test models with minimal flavour breaking through Standard Model Yukawa couplings. The decay width has been given some time ago by an approximate formula, which takes into account the leading logarithms of the MFV scale. In this paper we calculate the exact one-loop decay width in the framework of MFV. The comparison with the approximate result exhibits deviations of the order of 10% for large MFV scales due to the neglected non-logarithmic terms in the approximate decay formula. The difference in the branching ratios is negligible. The large logarithms have to be resummed. The resummation is performed by the solution of the renormalization group equations. The comparison of the exact one-loop result and the tree level flavour changing neutral current decay, which incorporates the resummed logarithms, demonstrates that the resummation effects are important and should be taken into account.

Decay widths of all MSSM scalar particles at full one-loop level

We discuss two packages in preparation, which calculate all possible two-body decays at full one-loop level of all scalars particles of the Minimal Supersymmetric extension of the Standard Model. The package HFOLD (Higgs Full One-Loop Decays) calculates the decay widths and branching ratios of the three neutral and charged Higgs bosons. The package SFOLD (Sfermion Full One-Loop Decays) computes the decay widths and branching ratios of squarks and sleptons. The packages comply with the SUSY Parameter Analysis convention and the SUSY Les Houches Accord input and output format.

Electromagnetic leptogenesis

We present a new leptogenesis scenario, where the lepton asymmetry is generated by $CP$-violating decays of heavy electroweak singlet neutrinos via electromagnetic dipole moment couplings to the ordinary light neutrinos. Akin to the usual scenario where the decays are mediated through Yukawa interactions, we have shown, by explicit calculations, that the desired asymmetry can be produced through the interference of the corresponding tree-level and one-loop decay amplitudes involving the effective dipole moment operators. We also find that the relationship of the leptogenesis scale to the light neutrino masses is similar to that for the standard Yukawa-mediated mechanism.

Z boson decay to photon plus Kaluza-Klein graviton in large extra dimensions

In the large extra dimensional ADD scenario, Z bosons undergo a one-loop decay into a photon and Kaluza-Klein towers of gravitons/gravi-scalars. We calculate such a decay width, extending previous arguments about the general form of the four-dimensional on-shell amplitude. The amplitudes calculated are relevant to processes in other extra dimensional models where the Standard Model fields are confined to a 4-brane.

ρ meson broadening and dilepton production in heavy ion collisions

The modification of the width of the rho meson due to in-medium decays and collisions is evaluated. In high temperature and/or high density hadronic matter, the collision width is much larger than the one-loop decay width. The large width of the ρ meson in matter seems to be consistent with some current interpretations of the e + e − mass spectra measured at the CERN/SPS.

$rho; meson broadening and dilepton production in heavy ion collisions*1

The modification of the width of the rho meson due to in-medium decays and collisions is evaluated. In high temperature and/or high density hadronic matter, the collision width is much larger than the one-loop decay width. The large width of the ρ meson in matter seems to be consistent with some current interpretations of the e + e − mass spectra measured at the CERN/SPS.

Radiative decay as the signal of a heavy neutrino at the SSC.

The one-loop decay $N\ensuremath{\rightarrow}\ensuremath{\nu}\ensuremath{\gamma}$ is studied for a heavy neutrino $N$ whose mass cannot be neglected during the loop calculation. The event rates for the resulting signal at the SSC are found to be experimentally accessible if ${m}_{N}<{M}_{W}$, so that $N$ has only three-body decays at the tree level. The usefulness of such decays in identifying a heavy neutrino is also discussed.

Searching for the CP-odd Higgs boson of the minimal supersymmetric model at hadron supercolliders.

The {ital CP}-odd Higgs boson {ital A}{sup 0} of a nonminimal Higgs model possesses no tree-level couplings to vector-boson pairs. As a result, many of the techniques employed to search for the standard model Higgs boson at the Superconducting Super Collider (SSC) and CERN Large Hadron Collider (LHC) are not likely to be useful in searches for the {ital A}{sup 0}. This paper focuses on the phenomenology of {ital A}{sup 0} production and decay in the minimal supersymmetric model. We evaluate a comprehensive set of branching ratios for {ital A}{sup 0} (which includes all two-body tree-level and one-loop decays into nonsupersymmetric final states), including the effects of leading-log radiative corrections. The one-loop decay {ital A}{sup 0}{r arrow}{ital ZZ} leads to the gold-plated'' signature {ital ZZ}{r arrow}{ital l}{sup +}{ital l{minus}}{ital l+}{ital l{minus}} and provides an observable signature for {ital A}{sup 0} production at the SSC and LHC only over a very narrow region of parameter space. More promising signatures for the {ital A}{sup 0} are also discussed.

Searching for CP-even Higgs bosons of the minimal supersymmetric model at hadron supercolliders.

The {ital CP}-odd Higgs boson {ital A}{sup 0} of a nonminimal Higgs model possesses no tree-level couplings to vector-boson pairs. As a result, many of the techniques employed to search for the standard model Higgs boson at the Superconducting Super Collider (SSC) and CERN Large Hadron Collider (LHC) are not likely to be useful in searches for the {ital A}{sup 0}. This paper focuses on the phenomenology of {ital A}{sup 0} production and decay in the minimal supersymmetric model. We evaluate a comprehensive set of branching ratios for {ital A}{sup 0} (which includes all two-body tree-level and one-loop decays into nonsupersymmetric final states), including the effects of leading-log radiative corrections. The one-loop decay {ital A}{sup 0}{r arrow}{ital ZZ} leads to the gold-plated'' signature {ital ZZ}{r arrow}{ital l}{sup +}{ital l{minus}}{ital l+}{ital l{minus}} and provides an observable signature for {ital A}{sup 0} production at the SSC and LHC only over a very narrow region of parameter space. More promising signatures for the {ital A}{sup 0} are also discussed.

Higgs-boson decay into two gluinos.

We present the one-loop calculations of Higgs bosons decaying into gluino pairs in minimal {ital N}=1 supergravity models and compare these rates to the one-loop decay modes with two gluons in the final state. Depending on the unknown low-energy supersymmetry parameters and the mass spectrum, rates for Higgs-boson decay into gluino pairs normalized to the {ital b{bar b}} mode typically vary from 10{sup {minus}5} to 10{sup {minus}3}.

Z 0 decay to gluinos

The induced one-loop decay of physical Z 0s into gluino pairs is calculated,and the relevant couplings and symmetry constraints discussed. For the case of chirally degenerate heavy scalars, as suggested by supergravity, the Z 0 branching ratio to gluino pairs is calculated, and compared to that for gluino pair production in association with quark-antiquark pairs.