arXiv subjects

H. K. Dreiner

Publications and source records attributed to H. K. Dreiner.

At least 19 recordsLinked to original sources

arXiv2014

Resurrecting light stops after the 125 GeV Higgs in the baryon number violating CMSSM

In order to accommodate the observed Higgs boson mass in the CMSSM, the stops must either be very heavy or the mixing in the stop sector must be very large. Lower stop masses, possibly more accessible at the LHC, still give the correct Higgs mass only if the trilinear stop mixing parameter $|A_t|$ is in the multi-TeV range. Recently it has been shown that such large stop mixing leads to an unstable electroweak vacuum which spontaneously breaks charge or colour. In this work we therefore go beyond the CMSSM and investigate the effects of including baryon number violating operators $λ'' \bar{\bf U} \bar{\bf D}\bar{\bf D}$ on the stop and Higgs sectors. We find that for $λ'' \simeq {\mathcal{O}}(0.3)$ light stop masses as low as 220 GeV are consistent with the observed Higgs mass as well as flavour constraints while allowing for a stable vacuum. The light stop in this scenario is often the lightest supersymmetric particle. We furthermore discuss the importance of the one-loop corrections involving R-parity violating couplings for a valid prediction of the light stop masses.

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arXiv2013

$B_{s,d}^0 \to μ\barμ$ and $B\to X_sγ$ in the R-parity violating MSSM

The recent measurements of $B_s^0 \to μ\barμ$ decay candidates at the LHC consistent with the standard model rate, and the improving upper limits for $B_d^0 \to μ\barμ$ can strongly constrain beyond the standard model physics. For example, in supersymmetric models with broken R-parity (RpV), they restrict the size of the new couplings. We use the combination of the public software packages SARAH and SPheno to derive new bounds on several combinations of R-parity violating couplings. We improve existing limits for the couplings which open tree-level decay channels and state new limits for combinations which induce loop contributions. This is the first study which performs a full one-loop analysis of these observables in the context of R-parity violation. It turns out that at one-loop despite the strong experimental limits only combinations of R-parity violating couplings are constrained which include third generation fermions. We compare our limits with those obtained via $B\to X_sγ$ and discuss the differences.

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arXiv2012

Improving the Hadron Physics of Non-Standard-Model Decays: Example Bounds on R-parity Violation

Using the example of selected decays driven by R-parity-violating supersymmetric operators, we demonstrate how strong final-state interactions can be controlled quantitatively with high precision, thus allowing for a more accurate extraction of effective parameters from data. In our examples we focus on the lepton-flavor-violating decays tau --> mu pi+ pi-. In R-parity violation these can arise due to the product of two couplings. We find bounds that are an order of magnitude stronger than previous ones.

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arXiv2012

General NMSSM signatures at the LHC

We study the possible LHC collider signatures in the next-to-minimal supersymmetric Standard Model (NMSSM). The general NMSSM consists of 29 supersymmetric (SUSY) particles which can be mass ordered in $29! \simeq 9 \cdot 10^{30}$ ways. To reduce the number of hierarchies to a more manageable amount we assume a degeneracy of the sfermions of the first two generations with the same quantum numbers. Further assumptions about the neutralino and chargino masses leave 15 unrelated parameters. We check all $15! \approx 10^{12}$ relevant mass orderings for the dominant decay chains and the corresponding collider signatures at the LHC. As preferred signatures, we consider charged leptons, missing transverse momentum, jets, and W, Z or Higgs bosons. We present the results for three different choices of the singlet to Higgs coupling $λ$: (a) small: $O(λ)< O(Y_τ)$, (b) large: $O(λ)\simeq O(Y_{top},Y_b, Y_τ)$ and (c) dominant: $O(λ)>O(Y_{top})$. We compare these three scenarios with the MSSM expectations as well as among each other. We also mention a possible mass hierarchy leading to 7 jets plus 1 lepton signatures at the LHC and comment briefly on the consequence of possible R-parity violation.

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arXiv2012

New bounds on trilinear R-parity violation from lepton flavor violating observables

Many extensions of the leptonic sector of the Minimal Supersymmetric Standard Model (MSSM) are known, most of them leading to observable flavor violating effects. It has been recently shown that the 1-loop contributions to lepton flavor violating three-body decays $l_i \to 3 l_j$ involving the $Z^0$ boson may be dominant, that is, much more important than the usual photonic penguins. Other processes like $μ$-$e$ conversion in nuclei and flavor violating $τ$ decays into mesons are also enhanced by the same effect. This is for instance also the case in the MSSM with trilinear R-parity violation. The aim of this work is to derive new bounds on the relevant combinations of R-parity violating couplings and to compare them with previous results in the literature. For heavy supersymmetric spectra the limits are improved by several orders of magnitude. For completeness, also constraints coming from flavor violating $Z^0$-decays and tree-level decay channels $l \to l_i l_j l_k$ are presented for a set of benchmark points.

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arXiv2012

Benchmark Models, Planes, Lines and Points for Future SUSY Searches at the LHC

We define benchmark models for SUSY searches at the LHC, including the CMSSM, NUHM, mGMSB, mAMSB, MM-AMSB and p19MSSM, as well as models with R-parity violation and the NMSSM. Within the parameter spaces of these models, we propose benchmark subspaces, including planes, lines and points along them. The planes may be useful for presenting results of the experimental searches in different SUSY scenarios, while the specific benchmark points may serve for more detailed detector performance tests and comparisons. We also describe algorithms for defining suitable benchmark points along the proposed lines in the parameter spaces, and we define a few benchmark points motivated by recent fits to existing experimental data.

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arXiv2012

Bounds on R-parity Violation from Resonant Slepton Production at the LHC

We consider the ATLAS and CMS searches for dijet resonances, as well as the ATLAS search for like-sign dimuon pairs at the LHC with 7 TeV center of mass energy. We interpret their exclusions in terms of bounds on the supersymmetric R-parity violating parameter space. For this we focus on resonant slepton production followed by the corresponding decay.

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arXiv2011

Using rates to measure mixed modulus-anomaly mediated supersymmetry breaking at the LHC

If SUSY is discovered at the LHC, the task will immediately turn to determining the model of SUSY breaking. Here, we employ a Mixed Modulus-Anomaly Mediated SUSY Breaking (MMAMSB) model with very similar LHC phenomenology to the more conventionally studied Constrained Minimal SUSY Model (CMSSM) and minimal Anomaly Mediated SUSY Breaking (mAMSB) models. We then study whether the models can be distinguished and measured. If we only fit to the various mass edges and mass end-points from cascade decay chains that are normally studied, a unique determination and measurement of the model is problematic without substantial amounts of LHC data. However, if event rate information is included, we can quickly distinguish and measure the correct SUSY model and exclude alternatives.

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arXiv2011

Constraining Selectron LSP Scenarios with Tevatron Trilepton Searches

The Tevatron collaborations have searched for associated production of charginos and neutralinos via trilepton final states. No events above the Standard Model prediction were observed. We employ these results to put stringent bounds on R-parity violating models with a right-handed scalar electron as the lightest supersymmetric particle. We work in the framework of lepton number violating minimal supergravity. We find that within these models the complete parameter space consistent with the anomalous magnetic moment of the muon can be excluded at 90% confidence level. We also give prospects for Tevatron trilepton searches assuming an integrated luminosity of 10 fb^{-1}. We find that Tevatron will be able to test selectron LSP masses up to 170 GeV.

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arXiv2011

Discovery Potential of Selectron or Smuon as the Lightest Supersymmetric Particle at the LHC

We investigate the LHC discovery potential of R-parity violating supersymmetric models with a right-handed selectron or smuon as the lightest supersymmetric particle (LSP). These LSPs arise naturally in R-parity violating minimal supergravity models. We classify the hadron collider signatures and perform for the first time within these models a detailed signal over background analysis. We develop an inclusive three-lepton search and give prospects for a discovery at a center-of-mass energy of 7 TeV as well as 14 TeV. There are extensive parameter regions which the LHC can already test with 7 TeV and an integrated luminosity of 1 inverse femtobarn. We also propose a method for the mass reconstruction of the supersymmetric particles within our models at 14 TeV.

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arXiv2011

Stau as the Lightest Supersymmetric Particle in R-Parity Violating SUSY Models: Discovery Potential with Early LHC Data

We investigate the discovery potential of the LHC experiments for R-parity violating supersymmetric models with a stau as the lightest supersymmetric particle (LSP) in the framework of minimal supergravity. We classify the final states according to their phenomenology for different R-parity violating decays of the LSP. We then develop event selection cuts for a specific benchmark scenario with promising signatures for the first beyond the Standard Model discoveries at the LHC. For the first time in this model, we perform a detailed signal over background analysis. We use fast detector simulations to estimate the discovery significance taking the most important Standard Model backgrounds into account. Assuming an integrated luminosity of 1 inverse femtobarn at a center-of-mass energy of 7 TeV, we perform scans in the parameter space around the benchmark scenario we consider. We then study the feasibility to estimate the mass of the stau-LSP. We briefly discuss difficulties, which arise in the identification of hadronic tau decays due to small tau momenta and large particle multiplicities in our scenarios.

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arXiv2010

Bounds on R-parity Violating Couplings at the Grand Unification Scale from Neutrino Masses

We consider the embedding of the supersymmetric Standard Model with broken R-parity in the minimal supergravity (mSUGRA) model. We restrict ourselves to the case of broken lepton number, the B3 mSUGRA model. We first study in detail how the tree-level neutrino mass depends on the mSUGRA parameters. We find in particular a strong dependence on the trilinear supersymmetry breaking A-parameter, even in the vicinity of the mSUGRA SPS1a point. We then reinvestigate the bounds on the trilinear R-parity violating couplings at the unification scale from the low-energy neutrino masses including dominant one-loop contributions. These bounds were previously shown to be very strict, as low as O(10^{-6}) for SPS1a. We show that these bounds are significantly weakened when considering the full mSUGRA parameter space. In particular the ratio between the tree-level and 1-loop neutrino masses is reduced such that it may agree with the observed neutrino mass hierarchy. We discuss in detail how and in which parameter regions this effect arises.

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arXiv2009

Comments on a Massless Neutralino

We consider first an interesting connection between the development of physics and the Boston Red Sox. We then discuss in detail the collider phenomenology, as well as precision electroweak observables of a very light neutralino. We conclude by considering also the astrophysics and cosmology of a very light neutralino. We find that a massless neutralino is consistent with all present data.

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arXiv2009

All Possible Lightest Supersymmetric Particles in R-Parity Violating mSUGRA Models and their Signals at the LHC

We consider minimal supergravity (mSUGRA) models with an additional R-parity violating operator at the grand unification scale. This can change the supersymmetric spectrum leading on the one hand to a sneutrino, smuon or squark as the lightest supersymmetric particle (LSP). On the other hand, a wide parameter region is reopened, where the scalar tau is the LSP. It is vital to know the nature of the LSP, because supersymmetric particles normally cascade decay down to the LSP at collider experiments. We investigate in detail the conditions leading to non-neutralino LSP scenarios. We also present some typical LHC signatures.

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arXiv2009

Mass Bounds on a Very Light Neutralino

Within the Minimal Supersymmetric Standard Model (MSSM) we systematically investigate the bounds on the mass of the lightest neutralino. We allow for non-universal gaugino masses and thus even consider massless neutralinos, while assuming in general that R-parity is conserved. Our main focus are laboratory constraints. We consider collider data, precision observables, and also rare meson decays to very light neutralinos. We then discuss the astrophysical and cosmological implications. We find that a massless neutralino is allowed by all existing experimental data and astrophysical and cosmological observations.

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arXiv2009

Rare meson decays into very light neutralinos

We investigate the bounds on the mass of the lightest neutralino from rare meson decays within the MSSM with and without minimal flavor violation. We present explicit formulae for the two-body decays of mesons into light neutralinos and perform the first complete calculation of the loop-induced decays of kaons to pions and light neutralinos and B mesons to kaons and light neutralinos. We find that the supersymmetric branching ratios are strongly suppressed within the MSSM with minimal flavor violation, and that no bounds on the neutralino mass can be inferred from experimental data, i.e. a massless neutralino is allowed. The branching ratios for kaon and B meson decays into light neutralinos may, however, be enhanced when one allows for non-minimal flavor violation. We find new constraints on the MSSM parameter space for such scenarios and discuss prospects for future kaon and B meson experiments. Finally, we comment on the search for light neutralinos in monojet signatures at the Tevatron and at the LHC.

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arXiv2009

All Possible Lightest Supersymmetric Particles in R-Parity Violating Minimal Supergravity Models

We investigate, which lightest supersymmetric particles can be obtained via a non-vanishing lepton- or baryon-number violating operator at the grand unification scale within the R-parity violating minimal supergravity model. We employ the full one-loop renormalization group equations. We take into account restrictions from the anomalous magnetic moment of the muon and b -> s gamma, as well as collider constraints from LEP and the Tevatron. We also consider simple deformations of minimal supergravity models.

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arXiv2009

Stau LSP Phenomenology: Two- versus Four-Body Decay Modes and Resonant Single Slepton Production at the LHC as an Example

We investigate B3 mSUGRA models, where the lightest scalar tau, stau_1, is the LSP. B3 models allow for lepton number and R-parity violation; the LSP can thus decay. We assume one non-zero B3 coupling lambda'_ijk at M_GUT, which generates further B3 couplings at M_Z. We study the RGEs and give numerical examples. The new couplings lead to additional stau_1 decays, providing distinct collider signatures. We classify the stau_1 decays and describe their dependence on the mSUGRA parameters. We exploit our results for single slepton production at the LHC. As an explicit numerical example, we investigate single smuon production, focussing on like-sign dimuons in the final state. Also considered are final states with three or four muons.

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