arXiv subjects

Helmut Eberl

Publications and source records attributed to Helmut Eberl.

15 recordsLinked to original sources

arXiv2025

Underground Production of Electromagnetic Dark States by MeV-scale Electron Beams and Detection with CCDs

In this work we explore the possibility of new light fermionic particles with millicharge or electromagnetic form factor interactions and their underground production via an electron beam in the 100 MeV range and their subsequent detection using a CCD-sensor. We evaluate the S-matrix elements and the phase spaces for production analytically, and then calculate the corresponding cross sections numerically. For millicharged fermions this set-up could be able to probe a window in parameter space, yet unconstrained by direct detection experiments. The electric or magnetic dipole moment of a light fermion could feasibly be probed with enough beam time or an increased beam energy.

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arXiv2024

Gravitino Thermal Production, Dark Matter, and Reheating of the Universe

We present a full one-loop calculation of the gravitino thermal production rate, beyond the so-called hard thermal loop approximation, using the corresponding thermal spectral functions in numerical form on both sides of the light cone. This framework requires a full numerical evaluation. We interpret our results within the framework of a general supergravity-based model, remaining agnostic about the specifics of supersymmetry breaking. In this context, assuming that gravitinos constitute the entirety of the dark matter in the Universe imposes strict constraints on the reheating temperature. For example, with a gluino mass at the current LHC limit, a maximum reheating temperature of $T_\mathrm{reh} \simeq 10^9$ GeV is compatible with a gravitino mass of $m_{3/2} \simeq 1$ TeV. Additionally, with a reheating temperature an order of magnitude lower at $T_\mathrm{reh} \simeq 10^8$ GeV, the common gaugino mass $M_{1/2}$ can range from $2$ to $4 $ TeV within the same gravitino mass range. For much higher values of $M_{1/2}$, which are favored by current accelerator and cosmological data in the context of supersymmetric models, such as $M_{1/2} = 10$ TeV, and for $m_{3/2} \simeq 1$ TeV the reheating temperature compatible with the gravitino dark matter scenario is $ 10^7$ GeV. If other dark matter particles are considered, the reheating temperature could be much lower.

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arXiv2021

The h(125) decays to c cbar, b bbar, b sbar, photon photon and gluon gluon in the light of the MSSM with quark flavor violation

We study the Higgs boson decays $h^0 \to c \bar{c}, b \bar{b}, b \bar{s}, \gamma \gamma, g g$ in the Minimal Supersymmetric Standard Model (MSSM) with general quark flavor violation (QFV), identifying the $h^0$ as the Higgs boson with a mass of 125 GeV. We compute the widths of the $h^0$ decays to $c \bar c, b \bar b, b \bar s (s \bar b)$ at full one-loop level. For the $h^0$ decays to photon photon and gluon gluon we compute the widths at NLO QCD level. We perform a systematic MSSM parameter scan respecting all the relevant constraints, i.e. theoretical constraints from vacuum stability conditions and experimental constraints, such as those from K- and B-meson data and electroweak precision data, as well as recent limits on Supersymmetric (SUSY) particle masses and the 125 GeV Higgs boson data from LHC experiments. From the parameter scan, we find that the deviations of these MSSM widths from the Standard Model (SM) values can be quite sizable. All of these large deviations in the $h^0$ decays are due to large scharm-stop mixing, large scharm/stop involved trilinear couplings $T_{U23}, T_{U32}, T_{U33}$, large sstrange-sbottom mixing, and large sstrange/sbottom involved trilinear couplings $T_{D23}, T_{D32}, T_{D33}$. International Linear Collider (ILC) can observe such large deviations from the SM at high signal significance. In case the deviation pattern shown here is really observed at ILC, then it would strongly suggest the discovery of QFV SUSY (the MSSM with QFV).

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arXiv2021

Gravitino thermal production

In this talk we present a new calculation of the gravitino production rate, using its full one-loop corrected thermal self-energy, beyond the hard thermal loop approximation. Gravitino production $2 \to 2$ processes, that are not related to its self-energy have been taken properly into account. Our result, compared to the latest estimation, differs by almost 10%. In addition, we present a handy parametrization of our finding, that can be used to calculate the gravitino thermal abundance, as a function of the reheating temperature.

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arXiv2020

Gravitino thermal production revisited

We calculate the gravitino production rate, computing its one-loop thermal self-energy. Gravitino production processes that do not result through thermal cuts of its self-energy, have been identified and taken into account. Correcting analytical errors and numerical approximations in the previous calculations, we present our result. This deviates from the latest estimation by almost 10%. More importantly, we provide a convenient formula, for calculating the gravitino production rate and its thermal abundance, as a function of the reheating temperature of the Universe.

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arXiv2017

Gluino two-body decays at full one-loop level in the MSSM with quark-flavour violation

We study the two-body decays of the gluino at full one-loop level in the Minimal Supersymmetric Standard Model with quark-flavour violation (QFV) in the squark sector. The renormalization is done in the DRbar scheme and hard gluon and photon radiations are included by adding the corresponding three-body decay widths. In the numerics the dependence of the gluino decay widths on the QFV parameters is discussed with special emphasis put on the separation of the electroweak and the SUSY QCD corrections. The main dependence stems from the $\tilde c_R - \tilde t_R$ mixing in the decays to up-type squarks because there hold strong constraints from B-physics on the other quark-flavour mixing parameters. Including the full one-loop corrections, the changes of the gluino decay widths are mostly negative and of the order of about -10%. The QFV part stays small in the total width but can vary up to -8% for the decay width into the lightest squark. For the corresponding branching ratio, however, the effect is smaller by at least a factor of two. The electroweak corrections can become 35% of the SUSY QCD corrections.

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arXiv2017

Two-body decays of gluino at full one-loop level in the quark-flavour violating MSSM

We study the two-body decays of the gluino at full one-loop level in the Minimal Supersymmetric Standard Model with quark-flavour violation (QFV) in the squark sector. The renormalization is done in the $\overline{\rm DR}$ scheme. The gluon and photon radiations are included by adding the corresponding three-body decay widths. We discuss the dependence of the gluino decay widths on the QFV parameters. The main dependence stems from the $\tilde c_R - \tilde t_R$ mixing in the decays to up-type squarks, and from the $\tilde s_R - \tilde b_R$ mixing in the decays to down-type squarks due to the strong constraints from B-physics on the other quark-flavour mixing parameters. The full one-loop corrections to the gluino decay widths are mostly negative and of the order of about -10\%. The QFV part stays small in the total width but can vary up to -8\% for the decay width into the lightest squark. For the corresponding branching ratio the effect is somehow washed out by at least a factor of two. The electroweak corrections can be as large as 35\% of the SUSY QCD corrections.

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arXiv2015

GravitinoPack and late decays involving gravitinos

In this talk, we present the package GravitinoPack that calculates decays of unstable supersymmetric particles, involving gravitinos in the final or initial state. If the gravitino is the dark matter particle and therefore stable, the package calculates the decays of the lightest neutralino, and the lighter stau or stop NLSP into the gravitino LSP and one or two Standard Model particles. On the other hand, assuming that the gravitino is unstable, GravitinoPack calculates all its two-body and the three-body decay widths to the neutralino LSP and Standard Model particles. Since all these decays, involving the gravitino, are of gravitational nature, the lifetime of the decaying particle can be of the order of seconds are more, hence called "late decays". The precise knowledge of all these partial decay widths enables the user to apply accurately the relevant cosmological constraints.

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arXiv2015

GravitinoPack and decays of supersymmetric metastable particles

We present the package GravitinoPack that calculates the two- and three-body decays of unstable supersymmetric particles involving the gravitino in the final or initial state. In a previous paper, we already showed results for the gravitino decays into two and three particles. In this paper, we incorporate the processes where an unstable neutralino, stau or stop decays into a gravitino and Standard Model particles. This is the case in gravitino dark matter supersymmetric models, where the gravitino is the lightest SUSY particle. We give instructions for the installation and the use of the package. In the numerical analysis, we discuss various MSSM scenarios. We show that the calculation of all the decay channels and the three-body decay branching ratios is essential for the accurate application of cosmological bounds on these models.

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arXiv2013

Three-body gravitino decays in the MSSM

We present results from a new calculation of two- and three-body decays of the gravitino including all possible Feynman graphs. The work is done in the R-parity conserving Minimal Supersymmetric Standard Model, assuming that the gravitino is unstable. We calculate the full width for the three-body decay process Gravitino -> \chi_1 X Y, where \chi_1 is the lightest neutralino that plays the role of the lightest supersymmetric particle, and X, Y are standard model particles. For calculating the squared amplitudes we use the packages FeynArts and FormCalc, with appropriate extensions to accommodate the gravitino with spin 3/2. Our code treats automatically the intermediate exchanged particles that are on their mass shell, using the narrow width approximation. This method considerably simplifies the complexity of the evaluation of the total gravitino width and stabilizes it numerically. At various points of the supersymmetric parameter space, we compare the full two-body and the three-body results with approximations used in the past. In addition, we discuss the relative sizes of the full two-body and the three-body gravitino decay widths. Our results are obtained using the package GravitinoPack, that will be publicly available soon, including various handy features.

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arXiv2010

CP Violating Asymmetry in Stop Decay into Bottom and Chargino

In the MSSM with complex parameters, loop corrections to the decay of a stop into a bottom quark and a chargino can lead to a CP violating decay rate asymmetry. We calculate this asymmetry at full one-loop level and perform a detailed numerical study, analyzing the dependence on the parameters and complex phases involved. If the stop can decay into a gluino, the self-energy and the vertex correction dominate due to the strong coupling. It is shown that the vertex contribution is always suppressed. We therefore give a simple approximate formula for the asymmetry. We account for the constraints on the parameters coming from several experimental limits. Asymmetries up to 25 percent are obtained. We also comment on the feasibility of measuring this asymmetry at the LHC.

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arXiv2009

CP Violating Asymmetry in Stop Decay into Bottom and Chargino

In the MSSM with complex parameters, loop corrections to the decay of a stop into a bottom quark and a chargino can lead to a CP violating decay rate asymmetry. We calculate this asymmetry at full one-loop level and perform a detailed numerical study, analyzing the dependence on the parameters and complex phases involved. In addition, we take the Yukawa couplings of the top and bottom quark running. We account for the constraints on the parameters coming from several experimental limits. Asymmetries of several percent are obtained. We also comment on the feasibility of measuring this asymmetry at the LHC.

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arXiv2008

CP violation in associated production of a charged Higgs boson and a top quark at the LHC

We study effects of CP violation in an associated production of a charged Higgs boson and a top quark at the LHC: $pp \to tH^\pm +X$. We calculate the CP violating asymmetry between the total cross section for $H^+$ and $H^-$ production at next-to-leading order in the MSSM, and perform a detailed numerical analysis. In the production only the asymmetry is of the order of 20%. The asymmetry in the production and any subsequent decay of an on-shell charged Higgs boson is to a good approximation the sum of the asymmetry in the production and the asymmetry in the decay. We consider subsequent decays of $H^\pm $ to $tb$, $ν_ττ^\pm$ and $Wh^0$. In the case of subsequent $H^\pm \to tb$ decay, the $W^\pm - H^\pm$ self energy contributions from the production and the decay cancel. However, the remaining effect, mainly due to CP violating box graphs with gluino can go up to $\sim 13$%.

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arXiv2008

CP violation in charged Higgs boson decays

CP violation in H^\pm decays into the three possible decay modes into ordinary particles, 1) H^\pm \to tb, 2) H^\pm \to ντand 3) H^\pm \to W^\pm h^0 is considered. Analytic expressions and numerical results for the CP violating decay rate asymmetries in the MSSM are obtained. Increasing \tanβthe asymmetries for the fermionic decays, H^\pm \to tb and H^\pm \to ντ, decrease and it increases for H^\pm \to W^\pm h^0. The asymmetry of H^\pm \to tb is most sensitive to the phase of $A_t and can go up to 20%, the asymmetries of 2) and 3) depend mainly on the phases of A_τand M_1. The asymmetry of 2) is smaller than 0.5% and of 3) can reach up to 2%.

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arXiv1996

Impact of squark pair decay modes on search for supersymmetric neutral Higgs bosons

We present a detailed study of the decays of the neutral Higgs bosons $H^0$ and $A^0$ within the Minimal Supersymmetric Standard Model. We find that the supersymmetric modes ${\tilde{t}}\bar{\tilde{t}}$ and ${\tilde{b}}\bar{\tilde{b}}$ can dominate the $H^0$ and $A^0$ decays in a wide range of the model parameters due to large Yukawa couplings and mixings of ${\tilde{t}}$ and ${\tilde{b}}$. Compared to the conventional modes $t\bar{t}$ and $b\bar{b}$, these modes have very distinctive signatures. This could have a decisive impact on the neutral Higgs boson searches at future colliders.

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