arXiv subjects

W. Porod

Publications and source records attributed to W. Porod.

At least 55 records · Page 3Linked to original sources

arXiv2010

Impact of squark generation mixing on the search for squarks and gluinos at LHC

We study the effect of squark-generation mixing on production and decays of squarks and gluinos at LHC in the Minimal Supersymmetric Standard Model (MSSM). We show that the mixing effects can be very large in a significant range of the squark-generation mixing parameters despite the very strong constraints on quark-flavour violation (QFV) from experimental data on B mesons. We find that under favourable conditions the QFV decay branching ratio B(gluino -> c bar{t} (t bar{c}) neutralino_1) can be as large as about 50%, which may lead to significant QFV signals at LHC. We also find that the squark generation mixing can result in a novel multiple-edge (3- or 4-edge) structure in the charm-top quark invariant mass distribution. Further we show that the two lightest up-type squarks ~u_{1,2} can have very large branching ratios for the decays ~u_i -> c neutralino_1 and ~u_i -> t neutralino_1 simultaneously due to the mixing effect, resulting in QFV signals 'pp -> c bar{t} (t bar{c}) + missing-E_T + X' at a significant rate at LHC. These remarkable signatures could provide a powerful test of supersymmetric QFV at LHC and could have an important impact on the search for squarks and gluinos and the determination of the MSSM parameters at LHC.

hep-ph↗

arXiv2010

Supersymmetric type-III seesaw: lepton flavour violating decays and dark matter

We study a supersymmetric version of the seesaw mechanism type-III. The model consists of the MSSM particle content plus three copies of 24 superfields. The fermionic part of the SU(2) triplet contained in the 24 is responsible for the type-III seesaw, which is used to explain the observed neutrino masses and mixings. Complete copies of 24 are introduced to maintain gauge coupling unification. These additional states change the beta functions of the gauge couplings above the seesaw scale. Using mSUGRA boundary conditions we calculate the resulting supersymmetric mass spectra at the electro-weak scale using full 2-loop renormalization group equations. We show that the resulting spectrum can be quite different compared to the usual mSUGRA spectrum. We discuss how this might be used to obtain information on the seesaw scale from mass measurements. Constraints on the model space due to limits on lepton flavour violating decays are discussed. The main constraints come from the bounds on the decay mu to e and gamma but there are also regions where the decay tau to mu and gamma gives stronger constraints. We also calculate the regions allowed by the dark matter constraint. For the sake of completeness, we compare our results with those for the supersymmetric seesaw type-II and, to some extent, with type-I.

hep-ph↗

arXiv2010

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(${\tildeτ}_R \to μχ^0_1$)/Br(${\tildeτ}_L \to μχ^0_1$) 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 $μ^+ \to e^+ γ$, 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.

hep-ph↗

arXiv2010

Probing Neutrino Oscillations in Supersymmetric Models at the Large Hadron Collider

The lightest supersymmetric particle may decay with branching ratios that correlate with neutrino oscillation parameters. In this case the CERN Large Hadron Collider (LHC) has the potential to probe the atmospheric neutrino mixing angle with sensitivity competitive to its low-energy determination by underground experiments. Under realistic detection assumptions, we identify the necessary conditions for the experiments at CERN's LHC to probe the simplest scenario for neutrino masses induced by minimal supergravity with bilinear R parity violation.

hep-ph↗

arXiv2010

Impact of squark generation mixing on the search for gluinos at LHC

We study gluino decays in the Minimal Supersymmetric Standard Model (MSSM) with squark generation mixing. We show that the effect of this mixing on the gluino decay branching ratios can be very large in a significant part of the MSSM parameter space despite the very strong experimental constraints on quark flavour violation (QFV) from B meson observables. Especially we find that under favourable conditions the branching ratio of the the QFV gluino decay gluino -> c bar{t} (bar{c} t) + neutralino_1 can be as large as about 50%. We also find that the squark generation mixing can result in a multiple-edge (3- or 4-edge) structure in the charm-top quark invariant mass distribution. The appearance of this remarkable structure provides an additional powerful test of supersymmetric QFV at LHC. These could have an important impact on the search for gluinos and the determination of the MSSM parameters at LHC.

hep-ph↗

arXiv2010

Strong dark matter constraints on GMSB models

We reconsider the dark matter problem in supersymmetric models with gauge mediated supersymmetry breaking, with and without R-parity breaking. In these classes of models, a light gravitino forms the dark matter.Consistency with the experimental data, in particular the dark matter abundance and the small-scale power spectrum, requires additional entropy production after the decoupling of the gravitino from the thermal bath. We demonstrate that the usual mechanism via messenger number violating interactions does not work in models where the messenger belongs to SU (5) representations. This is mainly a consequence of two facts: (i) there are at least two different types of lightest messenger particles and (ii) the lightest messenger particle with SU (2) quantum numbers decays dominantly into vector bosons once messenger number is broken, a feature which has been overlooked so far. In case of SO(10) messenger multiplets we find scenarios which work if the SM gauge singlet component is rather light.

hep-ph↗

arXiv2010

The Hunt for New Physics at the Large Hadron Collider

The Large Hadron Collider presents an unprecedented opportunity to probe the realm of new physics in the TeV region and shed light on some of the core unresolved issues of particle physics. These include the nature of electroweak symmetry breaking, the origin of mass, the possible constituent of cold dark matter, new sources of CP violation needed to explain the baryon excess in the universe, the possible existence of extra gauge groups and extra matter, and importantly the path Nature chooses to resolve the hierarchy problem - is it supersymmetry or extra dimensions. Many models of new physics beyond the standard model contain a hidden sector which can be probed at the LHC. Additionally, the LHC will be a top factory and accurate measurements of the properties of the top and its rare decays will provide a window to new physics. Further, the LHC could shed light on the origin of neutralino masses if the new physics associated with their generation lies in the TeV region. Finally, the LHC is also a laboratory to test the hypothesis of TeV scale strings and D-brane models. An overview of these possibilities is presented in the spirit that it will serve as a companion to the Technical Design Reports (TDRs) by the particle detector groups ATLAS and CMS to facilitate the test of the new theoretical ideas at the LHC. Which of these ideas stands the test of the LHC data will govern the course of particle physics in the subsequent decades.

hep-ph↗

arXiv2009

FCNC and CP Violation Observables in a SU(3)-flavoured MSSM

A non-Abelian flavour symmetry in a minimal supersymmetric standard model can explain the flavour structures in the Yukawa couplings and simultaneously solve the SUSY flavour problem. Similarly the SUSY CP problem can be solved if CP is spontaneously broken in the flavour sector. In this work, we present an explicit example of these statements with a SU(3) flavour symmetry and spontaneous CP violation. In addition, we show that it is still possible to find some significant deviation from the SM expectations as far as FCNC and CP violation are concerned. We find that large contributions can be expected in lepton flavour violating decays, as $μ\to e γ$ and $τ\to μγ$, electric dipole moments, $d_e$ and $d_n$ and kaon CP violating processes as $ε_K$. Thus, these flavoured MSSM realizations are phenomenologically sensitive to the experimental searches in the realm of flavor and CP vioation physics.

hep-ph↗

arXiv2009

SUSY Les Houches Accord 2

The Supersymmetry Les Houches Accord (SLHA) provides a universal set of conventions for conveying spectral and decay information for supersymmetry analysis problems in high energy physics. Here, we propose extensions of the conventions of the first SLHA to include various generalisations: the minimal supersymmetric standard model with violation of CP, R-parity, and flavour, as well as the simplest next-to-minimal model.

hep-ph↗

arXiv2009

SUSY Les Houches Accord: Interfacing SUSY Spectrum Calculators, Decay Packages, and Event Generators

An accord specifying a unique set of conventions for supersymmetric extensions of the Standard Model together with generic file structures for 1) supersymmetric model specifications and input parameters, 2) electroweak scale supersymmetric mass and coupling spectra, and 3) decay tables is defined, to provide a universal interface between spectrum calculation programs, decay packages, and high energy physics event generators.

hep-ph↗

arXiv2009

SU(3) Flavour Symmetries and CP Violation

In order to satisfy current FCNC and CP violation bounds, SUSY flavour structures cannot be generic. An interesting solution to these SUSY Flavour and CP Problems lies on the use of an SU(3) family symmetry which spontaneously breaks CP symmetry. Typical observables of such a model are electric dipole moments and LFV processes. In addition, these models can give contributions to CP violation in neutral meson mixing. We show how the latter can be used to restrict the allowed SUSY parameter space, and induce correlations between LFV and EDM predictions.

hep-ph↗

arXiv2009

Dark matter in minimal supergravity with type-II seesaw

We calculate the relic density of the lightest neutralino in a supersymmetric seesaw type-II (``triplet seesaw'') model with minimal supergravity boundary conditions at the GUT scale. The presence of a triplet below the GUT scale, required to explain measured neutrino data in this setup, leads to a characteristic deformation of the sparticle spectrum with respect to the pure mSugra expectations, affecting the calculated relic dark matter (DM) density. We discuss how the DM allowed regions in the (m_0,M_{1/2}) plane change as a function of the (type-II) seesaw scale. We also compare the constraints imposed on the models parameter space form upper limits on lepton flavour violating (LFV) decays to those imposed by DM. Finally, we briefly comment on uncertainties in the calculation of the relic neutralino density due to uncertainties in the measured top and bottom masses.

hep-ph↗

arXiv2009

LHC phenomenology of the $μν$SSM

The $μν$SSM has been proposed to solve simultaneously the $μ$-problem of the MSSM and explain current neutrino data. The model breaks lepton number as well as R-parity. In this paper we study the phenomenology of this proposal concentrating on neutrino masses and the decay of the lightest supersymmetric particle (LSP). At first we investigate in detail the $μν$SSM with one generation of singlets, which can explain all neutrino data, once 1-loop corrections are taken into account. Then we study variations of the model with more singlets, which can generate all neutrino masses and mixings at tree-level. We calculate the decay properties of the lightest supersymmetric particle, assumed to be the lightest neutralino, taking into account all possible final states. The parameter regions where the LSP decays within the LHC detectors but with a length large enough to show a displaced vertex are identified. Decay branching ratios of certain final states show characteristic correlations with the measured neutrino angles, allowing to test the model at the LHC. Finally we briefly discuss possible signatures, which allow to distinguish between different R-parity breaking models.

hep-ph↗

arXiv2009

Flavour violation at the LHC: type-I versus type-II seesaw in minimal supergravity

We reconsider the role that the possible detection of lepton flavour violating (LFV) decays of supersymmetric particles at the Large Hadron Collider (LHC) can play in helping reconstruct the underlying neutrino mass generation mechanism within the simplest high-scale minimal supergravity (mSUGRA) seesaw schemes. We study in detail the LFV scalar tau decays at the LHC, assuming that the observed neutrino masses arise either through the pure type-I or the simpler type-II seesaw mechanism. We perform a scan over the mSUGRA parameter space in order to identify regions where lepton flavour violating decays of $χ^0_2$ can be maximized, while respecting current low-energy constraints, such as those coming from the bounds on Br($μ\to e γ$). We estimate the cross section for $χ^0_2 \to χ^0_1 + τ+ μ$. Though insufficient for a full reconstruction of the seesaw, the search for LFV decays of supersymmetric states at the LHC brings complementary information to that coming from low energy neutrino oscillation experiments and LFV searches.

hep-ph↗

arXiv2009

Majoron emission in muon and tau decays revisited

In models where the breaking of lepton number is spontaneous a massless Goldstone boson, the Majoron ($J$), appears. We calculate the theoretically allowed range for the branching ratios of Majoron emitting charged lepton decays, such as Br($μ\to e J$) and Br($μ\to e J γ$), in a supersymmetric model with spontaneous breaking of R-parity. Br($μ\to eJ$) is maximal in the same region of parameter space for which the lightest neutralino decays mainly invisibly. A measurement of Br($μ\to eJ$) thus potentially provides information on R-parity violation complementary to accelerator searches. We also briefly discuss existing bounds and prospects for future improvements on the Majoron coupling to charged leptons.

hep-ph↗

arXiv2008

Supersymmetric seesaw type II: CERN LHC and lepton flavour violating phenomenology

We study the supersymmetric version of the type-II seesaw mechanism assuming minimal supergravity boundary conditions. We calculate branching ratios for lepton flavour violating (LFV) scalar tau decays, potentially observable at the LHC, as well as LFV decays at low energy, such as $l_i \to l_j + γ$ and compare their sensitivity to the unknown seesaw parameters. In the minimal case of only one triplet coupling to the standard model lepton doublets, ratios of LFV branching ratios can be related unambigously to neutrino oscillation parameters. We also discuss how measurements of soft SUSY breaking parameters at the LHC can be used to indirectly extract information of the seesaw scale.

hep-ph↗

arXiv2008

Probing minimal supergravity in the type-I seesaw mechanism with lepton flavour violation at the CERN LHC

The most general supersymmetric seesaw mechanism has too many parameters to be predictive and thus can not be excluded by any measurements of lepton flavour violating (LFV) processes. We focus on the simplest version of the type-I seesaw mechanism assuming minimal supergravity boundary conditions. We compute branching ratios for the LFV scalar tau decays, ${\tilde τ}_2 \to (e,μ) + χ^0_1$, as well as loop-induced LFV decays at low energy, such as $l_i \to l_j + γ$ and $l_i \to 3 l_j$, exploring their sensitivity to the unknown seesaw parameters. We find some simple, extreme scenarios for the unknown right-handed parameters, where ratios of LFV branching ratios correlate with neutrino oscillation parameters. If the overall mass scale of the left neutrinos and the value of the reactor angle were known, the study of LFV allows, in principle, to extract information about the so far unknown right-handed neutrino parameters.

hep-ph↗

arXiv2008

Reconstructing SUSY and R-Neutrino Masses in SO(10)

We report on the extrapolation of scalar mass parameters in the lepton sector to reconstruct SO(10) scenarios close to the unification scale. The method is demonstrated for an example in which SO(10) is broken directly to the Standard Model, based on the expected precision from coherent LHC and ILC collider analyses. In addition to the fundamental scalar mass parameters at the unification scale, the mass of the heaviest right-handed neutrino can be estimated in the seesaw scenario.

hep-ph↗