arXiv subjects

St. Kolb

Publications and source records attributed to St. Kolb.

9 recordsLinked to original sources

arXiv2004

The $\bm{e^- e^-}$ mode of LC: opportunities to discover loop-level lepton flavor (number) violation

The $e^- e^-$ mode with lepton number $L=+2$ of the initial state is particularly suitable for studying lepton and flavor number violating reactions (LFV). In this brief report we give a summary of the study of two loop-level reactions which violate lepton and flavor numbers: we first consider seesaw type models with heavy Majorana neutrinos at the TeV scale and study the reactions $e^{-}e^{-} \to \ell^{-}\ell^{-}$, and then similar reactions $e^{-}e^{-} \to \ell^{-}e^{-}$ ($\ell=μ,τ$) in supersymmetric models where LFV is due to slepton mixing.

hep-ph↗

arXiv2003

Lepton flavour violation in e^{\pm}e^{-}\to \ell^{\pm}e^{-} (\ell = μ,τ) induced by R-conserving supersymmetry

The lepton flavour violating signals $e^{+}e^{-}\to \ell^{+}e^{-}$ and $e^{-}e^{-}\to \ell^{-}e^- (\ell=μ,τ)$ are studied in the context of low energy R-parity conserving supersymmetry at center of mass energies of interest for the next generation of linear colliders. Loop level amplitudes receive contributions from electroweak penguin and box diagrams involving sleptons and gauginos. Lepton flavour violation is due to off diagonal elements in $SU(2)_L$ doublet slepton mass matrix. These masses are treated as model independent free phenomenological parameters in order to discover regions in parameter space where the signal cross section may be observable. The results are compared with (a) the experimental bounds from the non-observation of rare radiative lepton decays $μ, τ\to eγ$ and (b) the general mSUGRA theoretical scenario with seesaw mechanism where off diagonal slepton matrix entries are generated by renormalization group evolution of neutrino Yukawa couplings induced by the presence of new energy scales set by the heavy $SU(2)_L$ singlet neutrino masses. It is found that in $e^- e^-$ collisions the ($eτ$) signal can be observable with a total integrated luminosity of 100 fb$^{-1}$ and the that the background can be easily suppressed. In $e^+ e^-$ collisions the cross section is smaller and higher luminosities are needed. The experimental bound on the decay $μ\to e γ$ prevents the ($eμ$) signal from being observable.

hep-ph↗

arXiv2003

On the heavy Majorana neutrino and light sneutrino contribution to \bm{$e^{-}e^{-} \to \ell^{-} \ell^{-}$}, (\bm{$\ell=μ,τ)$}

The cross section for the reaction $e^{-}e^{-} \to \ell^-\ell^-$ ($\ell=μ,τ$) is calculated in models with heavy Majorana neutrinos mediating lepton number violating amplitudes at the loop level. The contributing four-point functions are evaluated exactly (numerically) taking into account the full propagator dependence on external momenta, thereby extending %an earlier approximate low energy calculation to the energy range of interest for the next linear colliders an earlier approximate low energy calculation. The amplitude shows a non-decoupling behaviour relative to the heavy Majorana neutrino masses, but due to the stringent bounds on heavy-light mixing the signal cross section attains observable values only for the less constrained $τ$ signal. The cross section induced by lepton number violation in the $SU(2)_L$ doublet sneutrino sector of supersymmetric extensions of the standard model is constrained by the upper limits on neutrino masses and probably too tiny to be observable.

hep-ph↗

arXiv2001

Sneutrino-induced like sign dilepton signal with conserved R-parity

Lepton number violation could be manifest in the sneutrino sector of supersymmetric extensions of the standard model with conserved R-parity. Then sneutrinos decay partly into the ``wrong sign charged lepton'' final state, if kinematically accessible. In sneutrino pair production or associated single sneutrino production, the signal then is a like sign dilepton final state. Under favourable circumstances, such a signal could be visible at the LHC or a next generation linear collider for a relative sneutrino mass-splitting of order ${\cal O}(0.001)$ and sneutrino width of order ${\cal O}$(1 GeV). On the other hand, the like sign dilepton event rate at the TEVATRON is probably too small to be observable.

hep-ph↗

arXiv2001

Recent Results from the HDMS Experiment

The status of dark matter search with the HDMS experiment is reviewed. After one year of running the HDMS prototype detector in the Gran Sasso Underground Laboratory, the inner crystal of the detector has been replaced with a HPGe crystal of enriched 73Ge. The results of the operation of the HDMS prototype detector are discussed.

hep-ph↗

arXiv2000

Neutrino Majorana Mass and Baryon Number of the Universe below the Electroweak Symmetry breaking Scale

If the neutrino is Majorana type and the electroweak phase transition is second or weak first order, neutrino-induced interactions together with sphaleron transitions have the potential to erase a previously generated baryon asymmetry of the universe. Taking correctly into account the evolution of the vacuum expectation of the Higgs field the effective light neutrino masses are constrained to be lighter than order 10 MeV, while the effective heavy masses are constrained to be heavier than order 10^7 GeV.

hep-ph↗

arXiv2000

Collider Signatures of Sneutrino Cold Dark Matter

Decays of sneutrinos are considered in the case that in the presence of lepton-number violation in the sneutrino sector the lighter tau-sneutrino is the Lightest Supersymmetric Particle and the Cold Dark Matter in the Universe. In such circumstances the signals from sparticle decays differ considerably from the ``standard'' case where the lightest neutralino is the Lightest Supersymmetric Particle and it is found that in a wide range of parameters compatible with the sneutrino Cold Dark Matter hypothesis signatures characteristic for such a scenario should be easily observable at for example a Next Linear Collider.

hep-ph↗

arXiv2000

Light Lepton Number Violating Sneutrinos and the Baryon Number of the Universe

Recent results of neutrino oscillation experiments point to a nonvanishing neutrino mass. Neutrino mass models favour Majorana-type neutrinos. In such circumstances it is natural that the supersymmetric counterpart of the neutrino, the sneutrino, bears also lepton number violating properties. On the other hand, the fact that the universe exhibits an asymmetry in the baryon and antibaryon numbers poses constraints on the extent of lepton number violation in the light sneutrino sector if the electroweak phase transition is second or weak first order. From the requirement that the Baryon Asymmetry of the Universe should not be washed out by sneutrino induced lepton number violating interactions and sphalerons below the critical temperature of the electroweak phase transition we find that the mass splitting of the light sneutrino mass states is compatible with the sneutrino Cold Dark Matter hypothesis only for heavy gauginos and opposite sign gaugino mass parameters.

hep-ph↗

arXiv1999

Limits on the Majorana neutrino mass in the 0.1 eV range

The Heidelberg-Moscow experiment gives the most stringent limit on the Majorana neutrino mass. After 24 kg yr of data with pulse shape measurements, we set a lower limit on the half-life of the neutrinoless double beta decay in 76Ge of T_1/2 > 5.7 * 10^{25} yr at 90% C.L., thus excluding an effective Majorana neutrino mass greater than 0.2 eV. This allows to set strong constraints on degenerate neutrino mass models.

hep-ex↗