Scalar Leptons Research Articles

Heavy scalar tau decays in the complex MSSM: a full one-loop analysis

We evaluate all two-body decay modes of the heavy scalar tau in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM) and no generation mixing. The evaluation is based on a full one-loop calculation of all decay channels, also including hard and soft QED radiation. The renormalization of the relevant sectors is briefly reviewed. The dependence of the heavy scalar tau decay on the relevant cMSSM parameters is analyzed numerically, including also the decay to Higgs bosons and another scalar lepton or to a tau and the lightest neutralino. We find sizable contributions to many partial decay widths and branching ratios. They are mostly of O(5-10%) of the tree-level results, but can go up to 20%. These contributions are potentially important for the correct interpretation of scalar tau decays at the LHC and, if kinematically allowed, at the ILC or CLIC. The evaluation of the branching ratios of the heavy scalar tau will be implemented into the Fortran code FeynHiggs.

Dark matter and LHC phenomenology in a left-right supersymmetric model

Left-right symmetric extensions of the Minimal Supersymmetric Standard Model can explain neutrino data and have potentially interesting phenomenology beyond that found in minimal SUSY seesaw models. Here we study a SUSY model in which the left-right symmetry is broken by triplets at a high scale, but significantly below the GUT scale. Sparticle spectra in this model differ from the usual constrained MSSM expectations and these changes affect the relic abundance of the lightest neutralino. We discuss changes for the standard stau (and stop) co-annihilation, the Higgs funnel and the focus point regions. The model has potentially large lepton flavour violation in both, left and right, scalar leptons and thus allows, in principle, also for flavoured co-annihilation. We also discuss lepton flavour signals due to violating decays of the second lightest neutralino at the LHC, which can be as large as 20 fb$^{-1}$ at $\sqrt{s}=14$ TeV.

LHC and lepton flavour violation phenomenology of a left-right extension of the MSSM

We study the phenomenology of a supersymmetric left-right model, assuming minimal supergravity boundary conditions. Both left-right and (B-L) symmetries are broken at an energy scale close to, but significantly below the GUT scale. Neutrino data is explained via a seesaw mechanism. We calculate the RGEs for superpotential and soft parameters complete at 2-loop order. At low energies lepton flavour violation (LFV) and small, but potentially measurable mass splittings in the charged scalar lepton sector appear, due to the RGE running. Different from the supersymmetric 'pure seesaw' models, both, LFV and slepton mass splittings, occur not only in the left- but also in the right slepton sector. Especially, ratios of LFV slepton decays, such as Br(${\tilde\tau}_R \to \mu \chi^0_1$)/Br(${\tilde\tau}_L \to \mu \chi^0_1$) are sensitive to the ratio of (B-L) and left-right symmetry breaking scales. Also the model predicts a polarization asymmetry of the outgoing positrons in the decay $\mu^+ \to e^+ \gamma$, A ~ [0,1], which differs from the pure seesaw 'prediction' A=1$. Observation of any of these signals allows to distinguish this model from any of the three standard, pure (mSugra) seesaw setups.

Lepton flavor equilibration and leptogenesis

We study the role played in leptogenesis by the equilibration of lepton flavors, as could be induced in supersymmetric models by off diagonal soft breaking masses for the scalar lepton doublets m̃αβ, or more generically by new sources of lepton flavor violation. We show that if m̃αβ≳1 GeV and leptogenesis occurs below ∼ 100 TeV, dynamical flavor effects are irrelevant and leptogenesis is correctly described by a one-flavor Boltzmann equation. We also discuss spectator effects in low scale leptogenesis by taking into account various chemical equilibrium conditions enforced by the reactions that are in thermal equilibrium. We write down the Boltzmann equation for low scale supersymmetric leptogenesis that includes flavor and spectator effects in the presence of lepton flavor equilibration, and we show how it reduces to a particularly simple form.

Kalman filter based tracker study for lepton flavor violation experiments

A tracking detector is proposed forlepton flavor violation experiments (μ → e conversion, μ → e+γ, μ → 3e)consisting of identical chambers which can be reconfigured tomeet the requirements for all three experiments.A pattern recognition and track reconstructionprocedure based on the Kalman filter techniqueis presented for this detector.The pattern recognition proceeds in two stages.At the first stage only hit straw tube center coordinates, without drift time information, areused to reduce the background to a manageable level. At the second stage the drift time information is incorporated and a deterministic annealing filteris applied to reach the final level of background suppression.The final track momentum reconstruction is provided by a combinatorial drop filterwhich is effective in hit-to-track assignment.The momentum resolution of the tracker in measuring monochromatic leptons is found to be σp = 0.17 and 0.26 MeV for the μ → e conversionand μ+ → e++γ processes, respectively.The tracker reconstruction resolution for the total scalar lepton momentum is σp = 0.33 MeV for the μ → 3e process.The obtained tracker resolutions allow an increase in sensitivity to the branching ratios for these processes bya few orders of magnitude over current experimental limits.

Low-scale gaugino mass unification

We study a new class of scenarios with the gaugino mass unification at the weak scale. The unification conditions are generally classified and then, the mirage gauge mediation is explored where the low-energy mass spectrum is governed by a mirage of unified gauge coupling which is seen by low-energy observers. The gaugino masses have natural and stable low-scale unification. The mass parameters of scalar quarks and leptons are given by gauge couplings but exhibit no large mass hierarchy. They are nonuniversal even when mediated at the gauge coupling unification scale. In addition, the gravitino is rather heavy and not the lightest superparticle. These facts are in contrast to existing gauge and mirage mediation models. We also present several explicit models for dynamically realizing the TeV-scale unification.

A clean slepton mixing signal at the LHC

In supersymmetric scenarios where the scalar tau lepton is stable or long-lived, a search for a decay mode chi0 --> stau + mu at the LHC has a good sensitivity to the flavor mixing in the scalar lepton sector. We demonstrate that the sensitivities to the mixing angle at the level of sin(theta)=0.15 are possible with an integrated luminosity of 100fb^{-1} if the total production cross section of supersymmetric particles is of the order of 1pb. The sensitivity to the mixing parameter can be better than the experimental bound from the tau --> mu + gamma decay depending on model parameters.

Stau-catalyzed 6Li production in big-bang nucleosynthesis

If the gravitino mass is in the region from a few GeV to a few 10's GeV, the scalar lepton X such as stau is most likely the next lightest supersymmetry particle. The negatively charged and long-lived X− may form a Coulomb bound state (AX−) with a nucleus A and may affect the big-bang nucleosynthesis through catalyzed fusion process. We calculate a production cross section of 6Li from the catalyzed fusion (He4X−)+d→Li6+X− by solving the Schrödinger equation exactly for three-body system of 4He, d and X. We utilize the state-of-the-art coupled-channel method, which is known to be very accurate to describe other three-body systems in nuclear and atomic reactions. The importance of the use of appropriate nuclear potential and the exact treatment of the quantum tunneling in the fusion process are emphasized. We find that the astrophysical S-factor at the Gamow peak corresponding to T=10keV is 0.038 MeV barn. This leads to the 6Li abundance from the catalyzed process as Li6|CBBN≃4.3×10−11(D/2.8×10−5)([nX−/s]/10−16) in the limit of long lifetime of X. Particle physics implication of this result is also discussed.

Big-bang nucleosynthesis with long-lived charged slepton

We consider constraints on long-lived charged scalar leptons l˜± in supersymmetric models, where gravitino is the lightest superparticle. We study the decay and hadronization processes of l˜±. We also take into account the significant enhancement of 6L production due to the formation of the bound-state (He4l˜−); for this purpose, we use the reaction rate given by the most recent calculation based on coupled-channel method.

Dark matter from gaugino mediation

We study dark matter (DM) for gaugino-mediated supersymmetry breaking and compactdimensions of order of the grand unification scale. Higgs fields are bulk fields and ingeneral their masses differ from those of squarks and sleptons at the unificationscale. As a consequence, at different points in parameter space, the gravitino, aneutralino or a scalar lepton can be the lightest (LSP) or next-to-lightest (NLSP)superparticle. We investigate the constraints from primordial nucleosynthesis on thedifferent scenarios. While neutralino DM and gravitino DM with a NLSP are consistent for a wide range of parameters, gravitino DM with a NLSP is strongly constrained. Gravitino DM with aχ0 NLSP is excluded.

Sneutrino production at e + e - linear colliders: addendum to slepton production

Complementing the preceding study of charged scalar leptons, the sector of the neutral scalar leptons, sneutrinos, is investigated in a high-precision analysis for future e+e- linear colliders. The theoretical predictions for the cross-sections are calculated at the thresholds for non-zero widths and in the continuum including higher-order corrections at the one-loop level. Methods for measuring the sneutrino masses and the electron-sneutrino-gaugino Yukawa couplings are presented, addressing theoretical problems specific for the sneutrino channels.

Testing the mechanism of R-parity breaking with slepton LSP decays

In supersymmetric models R-parity can be violated through either bilinear or trilinear terms in the superpotential, or both. If charged scalar leptons are the lightest supersymmetric particles, their decay properties can be used to obtain information about the relative importance of these couplings. We show that in some specific scenarios it is even possible to decide whether bilinear or trilinear terms give the dominant contribution to the neutrino mass matrix.

Sleptonarium (Constraints on the CP and flavour pattern of scalar lepton masses)

The constraints on the flavour and CP structure of scalar lepton mass matrices are systematically collected. The display of the resulting upper bounds on the lepton–slepton misalignment parameters is designed for an easy inspection of very large classes of models and the formula are arranged so as to suggest useful approximations. Interferences among the different contributions to lepton flavour violating transitions and lepton electric and magnetic dipole moments of generic character can either tighten or loose the bounds. A combined analysis of all rare leptonic transitions can disentangle the different contributions to yield hints on several phenomenological issues. The possible impact of these results on the study of the slepton misalignment originated in the seesaw mechanism and grand-unified theories is emphasized since the planned experiments are getting close to the precision required in such tests.

Sfermion precision measurements at a linear collider

At prospective e ± e − linear colliders, the large cross-sections and clean signals of scalar fermion production—in particular for the scalar leptons — allow very precise measurements of their masses and couplings and the determination of their quantum numbers. Various methods are proposed for extracting these parameters from the data at the sfermion thresholds and in the continuum. At the same time, NLO radiative corrections and non-zero width effects have been calculated in order to match the experimental accuracy. The substantial mixing expected in the third generation opens up additional opportunities. Techniques are presented for determining potential CP-violating phases and for extracting tan β from the stau sector, in particular at high values. The consequences of possible large mass differences in the stop and sbottom system are explored in dedicated analyses.

Search for long-lived charged massive particles in pp collisions at square root s = 1.8 TeV.

We report a search for the production of long-lived charged massive particles in a data sample of 90 pb(-1) of square root[s]=1.8 TeV pp collisions recorded by the Collider Detector at Fermilab. The search uses the muonlike penetration and anomalously high ionization energy loss signature expected for such a particle to discriminate it from backgrounds. The data are found to agree with background expectations, and cross section limits of O(1) pb are derived using two reference models, a stable quark and a stable scalar lepton.

Probing neutrino properties with charged scalar lepton decays

Supersymmetry with bilinear R-parity violation provides a predictive framework for neutrino masses and mixings in agreement with current neutrino oscillation data. The model leads to striking signals at future colliders through the R-parity violating decays of the lightest supersymmetric particle (LSP). Here we study charged scalar lepton decays and demonstrate that if the scalar tau is the LSP (i) it will decay within the detector, despite the smallness of the neutrino masses, (ii) the relative ratio of branching ratios $\mathrm{Br}({\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{1}\ensuremath{\rightarrow}e\ensuremath{\sum}{\ensuremath{\nu}}_{i})/\mathrm{Br}({\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{1}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\sum}{\ensuremath{\nu}}_{i})$ is predicted from the measured solar neutrino angle, and (iii) scalar muon and scalar electron decays will allow us to test the consistency of the model. Thus, bilinear R-parity breaking SUSY will be testable at future colliders also in the case where the LSP is not the neutralino.

Associated production of sfermions and gauginos at high-energy ee colliders

In the context of the Minimal Supersymmetric extension of the Standard Model, we analyze the production at future high-energy $e^+e^-$ colliders of second and third generation scalar leptons as well as scalar quarks in association with neutralinos and charginos, $e^+e^-\to f\tilde{f}\chi$. In the case of third generation squarks, we also discuss the associated production with gluinos. We show that the cross sections for some of these three-body final state processes could be significant enough to allow for the detection of scalar fermions with masses above the kinematical two--body threshold, $\sqrt{s}= 2m_{\tilde{f}}$. We then discuss, taking as a reference example the case of scalar muons, the production cross sections in various approximations and make a comparison with the full four-body production process, $e^+e^- \to f\bar{f} \chi \chi$, in particular around the two-sfermion threshold.

Associated production of sfermions and gauginos at high-energy ee colliders: The case of selectrons and electronic sneutrinos

We analyze the associated production at future high--energy $e^+e^-$ colliders, of first generation sleptons with neutralinos and charginos in the modes $e^+ e^-$ and $e \gamma$, in the framework of the Minimal Supersymmetric extension of the Standard Model. We show that the production rates, in particular for associated production of right--handed selectrons and the lightest neutralino which in general is the first accessible kinematically, can be much larger than the corresponding ones for second and third generation scalar leptons and for scalar quarks. With the high--luminosities expected at these colliders, the detection of first generation sleptons with masses significantly above the kinematical two--body threshold, $\sqrt{s}= 2m_{\tilde{l}}$, is thus possible in favourable regions of the parameter space.

Search for scalar leptons in e+e− collisions at centre-of-mass energies up to 209 GeV

A search for selectron, smuon and stau pair production is performed with the data collected by the ALEPH detector at LEP at centre-of-mass energies up to 209 GeV. The numbers of candidate events are consistent with the background predicted by the Standard Model. Final mass limits from ALEPH are reported.

Search for R-parity violating decays of supersymmetric particles in e+e− collisions at LEP

A search, in e^+e^- collisions, for chargino, neutralino, scalar lepton and scalar quark pair-production is performed, without assuming R-parity conservation in decays, in the case that only one of the coupling constants lambda_ijk or lambda''_ijk is non-negligible. No signal is found in data up to a centre-of-mass energy of 208GeV. Limits on the production cross sections and on the masses of supersymmetric particles are derived.