Gaugino Sector Research Articles

Improving sensitivity of trilinear R-parity violating SUSY searches using machine learning at the LHC

In this work, we have explored the sensitivity of multilepton final states in probing the gaugino sector of a R-parity violating supersymmetric scenario with specific lepton number violating trilinear couplings (λijk) being nonzero. The gaugino spectrum is such that the charged leptons in the final state can arise from the R-parity violating decays of the lightest supersymmetric particle (LSP) as well as R-parity conserving decays of the next-to-LSP (NLSP). Apart from a detailed cut-based analysis, we have also performed a machine learning-based analysis using a boosted decision tree algorithm, which provides much better sensitivity. In the scenarios with nonzero λ121 and/or λ122 couplings, the LSP pair in the final states decays to 4l (l=e,μ)+ET final states with a 100% branching ratio. We have shown that under this circumstance, a final state with ≥4l has the highest sensitivity while probing for gaugino masses. We also discuss how the sensitivity can change in the presence of τ lepton(s) in the final state due to other choices of trilinear couplings. We present our results through the estimation of the discovery and exclusion contours in the gaugino mass plane for both the high luminosity LHC (HL-LHC with s=14 TeV and L=3000 fb−1) and high energy LHC (HE-LHC with s=27 TeV and L=3000 fb−1). For the λ121 and/or λ122 nonzero scenario, the projected 2σ exclusion limit on NLSP masses reaches upto 2.37 TeV and 4 TeV for the HL-LHC and the HE-LHC, respectively, by using a machine learning based algorithm. We observe an enhancement of ∼380 (190) GeV in the projected 2σ exclusion limit on the NLSP masses at the 27 (14) TeV LHC. Considering the same final state (Nl≥4) for the λ133 and/or λ233 nonzero scenario, we find that the corresponding 2σ projected limits are ∼1.97 TeV and ∼3.25 TeV for the HL-LHC and HE-LHC, respectively. Published by the American Physical Society 2024

First steps towards the reconstruction of the squark flavour structure

Assuming the observation of a squark at the Large Hadron Collider, we investigate methods to access its flavour content and thus gain information on the underlying flavour structure of the theory. Based on simple observables, we apply a likelihood inference method to determine the top-flavour content of the observed particle. In addition, we employ a multivariate analysis in order to classify different flavour hypotheses. Both methods are discussed within a simplified model and the more general Minimal Supersymmetric Standard Model including most general squark mixing. We conclude that the likelihood inference may provide an estimation of the top-flavour content if additional knowledge, especially on the gaugino sector is available, while the multivariate analysis identifies different flavour patterns and can accommodate a more minimalistic set of observables.

Long-lived bino and wino in supersymmetry with heavy scalars and higgsinos

We point out that there is a parameter region in supersymmetry with heavy scalars and higgsinos, in which the heavier of bino and wino becomes long-lived as a consequence of the heavy higgsinos. In this region these electroweak gaugino sectors are secluded from each other with very small mixings that are inversely proportional to the higgsino mass. We revisit the bino and bino decays and provide simple formulae for the partial decay rates and the lifetimes in the limit of heavy higgsinos. We discuss the collider signatures of the long-lived binos and winos in this scenario.

Stop on top: SUSY parameter regions and fine-tuning constraints

We analyze minimal supersymmetric models in order to determine in what parameter regions with what amount of fine-tuning they are capable of accomodating the LHC-allowed top-stop degeneracy window. The stops must be light enough to enable Higgs naturalness yet heavy enough to induce a 125 GeV Higgs boson mass. These two constraints imply a large mass splitting. By an elaborate scan of the parameter space, we show that stop-on-top scenario requires at least Delta_CMSSM ~ O(10^4) fine-tuning in the CMSSM. By relaxing the CMSSM parameter space with nonuniversal Higgs masses, we find that Delta_NUHM1 ~ O(10^4). The CMSSM with gravitino LSP works slightly better than the NUHM1 model. Compared to all these, the CMSSM with mu<0 and nonuniversal gauginos yield a much smaller fine-tuning Delta_{mu,g} ~ O(100). Our results show that gaugino sector can pave the road towards a more natural stop-on-top scenario.

Naturalness of the non-universal MSSM in the light of the recent Higgs results

We analyse the naturalness of the Minimal Supersymmetric Standard Model (MSSM) in the light of recent LHC results from the ATLAS and CMS experiments. We study non-universal boundary conditions for the scalar and the gaugino sector, with fixed relations between some of the soft breaking parameters, and find a significant reduction of fine-tuning for non-universal gaugino masses. For a Higgs mass of about 125 GeV, as observed recently, we find parameter regions with a fine-tuning of O(10), taking into account experimental and theoretical uncertainties. These regions also survive after comparison with simplified model searches in ATLAS and CMS. For a fine-tuning less than 20 the lightest neutralino is expected to be lighter than about 400 GeV and the lighter stop can be as heavy as 3.5 TeV. On the other hand, the gluino mass is required to be above 1.5 TeV. For non-universal gaugino masses, we discuss which fixed GUT scale ratios can lead to a reduced fine-tuning and find that the recent Higgs results have a strong impact on which ratio is favoured. We also discuss the naturalness of GUT scale Yukawa relations, comparing the non-universal MSSM with the CMSSM.

Magic messengers in gauge mediation and signal for 125 GeV boosted Higgs boson

We consider a renormalizable messenger sector with magic messenger fields instead of usual SU(5) complete multiplets. We derive the soft supersymmetry breaking terms and show that the gaugino sector can be parameterized by only two parameters. These parameters can be chosen appropriately to obtain various patterns of gaugino masses and different ratios among them. The sfermion sector can also be characterized by two independent parameters which can be adjusted to change the relative masses of squarks and sleptons. A judicious choice of parameters also allows us to achieve the lightest Higgs boson mass about 125 GeV. In this paper we focus on a scenario where a comparatively large hierarchy exists between the U(1) and SU(2) gaugino mass parameters. In such a case, the lightest Higgs boson originating from the decay of the next-to-lightest neutralino, following the direct production of chargino neutralino pair, can be considerably boosted. We show that a boosted Supersymmetric Higgs signal with a decent signal to background ratio can be obtained using the jet substructure technique at LHC with 8 TeV (14 TeV) center of mass energy and an integrated luminosity of about 30 fb$^{-1}$ (15 fb$^{-1}$).

Diagnosis of supersymmetry breaking mediation schemes by mass reconstruction at the LHC

If supersymmetry is discovered at the LHC, the next question will be the determination of the underlying model. While this may be challenging or even intractable, a more optimistic question is whether we can understand the main contours of any particular paradigm of the mediation of supersymmetry breaking. The determination of superpartner masses through endpoint measurements of kinematic observables arising from cascade decays is a powerful diagnostic tool. In particular, the determination of the gaugino sector has the potential to discriminate between certain mediation schemes (not all schemes, and not between different UV realizations of a given scheme). We reconstruct gaugino masses, choosing a model where anomaly contributions to supersymmetry breaking are important (KKLT compactification), and find the gaugino unification scale. Moreover, reconstruction of other superpartner masses allows us to solve for the parameters defining the UV model. The analysis is performed in the stop and stau coannihilation regions where the lightest neutralinos are mainly gauginos, to additionally satisfy dark matter constraints. We thus develop observables to determine stau and stop masses to verify that the coannihilation mechanism is indeed operational, and solve for the relic density.

Implications of different supersymmetry breaking patterns for the spectrum and decay of neutralinos and charginos

We consider different patterns of supersymmetry breaking gaugino masses, and implications of these patterns for the phenomenology of neutralinos and charginos in models of low energy supersymmetry. We outline a general procedure for obtaining approximate values for the neutralino masses relevant for our analysis, and describe the constraints on the gaugino mass parameters which follow from the present experimental limits on the mass of the lightest chargino. We evaluate an upper bound on the mass of the lightest neutralino that follows from the structure of the mass matrix in different models for the gaugino mass parameters. Using the experimental lower limit for the chargino mass, we examine the lower bound for the neutralino masses. Using a sum rule for the squared masses of charginos and neutralinos, we discuss how to distinguish between different models for the supersymmetry breaking scenarios. We discuss in detail the decay modes of neutralinos and charginos in different models of supersymmetry breaking. Our analysis shows that by measuring the masses and decay properties of the neutralinos, one can distinguish between different patterns of supersymmetry breaking in the gaugino sector. We then compare the dark matter characteristics that arise in different models of supersymmetry breaking.

Dark matter as a guide toward a light gluino at the LHC

Motivated by specific connections to dark matter signatures, we study the prospects of observing the presence of a relatively light gluino whose mass is in the range ∼(500–900) GeV with a wino-like lightest supersymmetric particle with mass in the range of ∼(170–210) GeV. The light gaugino spectra studied here is generally different from other models, and in particular those with a wino dominated LSP, in that here the gluinos can be significantly lighter. The positron excess reported by the PAMELA satellite data is accounted for by annihilations of the wino LSP and their relic abundance can generally be brought near the WMAP constraints due to the late decay of a modulus field re-populating the density of relic dark matter. We also mention the recent FERMI photon constraints on annihilating dark matter in this class of models and implications for direct detection experiments including CDMS and XENON. We study these signatures in models of supersymmetry with non-minimal soft breaking terms derived from both string compactifications and related supergravity models which generally lead to non-universal gaugino masses. At the LHC, large event rates from the three-body decays of the gluino in certain parts of the parameter space are found to give rise to early discovery prospects for the gaugino sector. Excess events at the 5 sigma level can arise with luminosity as low as O(100) pb−1 at a center of mass energy of 10 TeV and ≲O(1) fb−1 at s=7 TeV.

Testing Gaugino Mass Unification Directly at the LHC

We report on the first step of a systematic study of how gaugino mass unification can be probed at the LHC in a quasi-model independent manner. Here we focus our attention on the theoretically well-motivated mirage pattern of gaugino masses, a one-parameter family of models of which universal (high scale) gaugino masses are a limiting case. Using a statistical method to optimize our signature selection we arrive at three ensembles of observables targeted at the physics of the gaugino sector, allowing for a determination of this non-universality parameter without reconstructing individual mass eigenvalues or the soft supersymmetry-breaking gaugino masses themselves. In this controlled environment we find that approximately 80% of the supersymmetric parameter space would give rise to a model for which our method will detect non-universality in the gaugino mass sector at the 10% level with approximately 10 inverse femptobarns of integrated luminosity.

Constraining the MSSM with universal gaugino masses and implication for searches at the LHC

Using a Markov chain Monte Carlo approach, we find the allowed parameter space of a MSSM model with seven free parameters. In this model universality conditions at the GUT scale are imposed on the gaugino sector. We require in particular that the relic density of dark matter saturates the value extracted from cosmological measurements assuming a standard cosmological scenario. We characterize the parameter space of the model that satisfies experimental constraints and illustrate the complementarity of the LHC searches, B-physics observables and direct dark matter searches for further probing the parameter space of the model. We also explore the different decay chains expected for the coloured particles that would be produced at LHC.

Studying gaugino mass unification at the LHC

We begin a systematic study of how gaugino mass unification can be probed at the CERN Large Hadron Collider (LHC) in a quasi-model independent manner. As a first step in that direction we focus our attention on the theoretically well-motivated mirage pattern of gaugino masses, a one-parameter family of models of which universal (high scale) gaugino masses are a limiting case. We improve on previous methods to define an analytic expression for the metric on signature space and use it to study one-parameter deviations from universality in the gaugino sector, randomizing over other soft supersymmetry-breaking parameters. We put forward three ensembles of observables targeted at the physics of the gaugino sector, allowing for a determination of this non-universality parameter without reconstructing individual mass eigenvalues or the soft supersymmetry-breaking gaugino masses themselves. In this controlled environment we find that approximately 80% of the supersymmetric parameter space would give rise to a model for which our method will detect non-universality in the gaugino mass sector at the 10% level with an integrated luminosity of order 10 inverse femptobarns. We discuss strategies for improving the method and for adding more realism in dealing with the actual experimental circumstances of the LHC.

Radiative [formula omitted] symmetry breaking and the [formula omitted] mediated SUSY breaking

We explore a mechanism of radiative B–L symmetry breaking in analogous to the radiative electroweak symmetry breaking. The breaking scale of B–L symmetry is related to the neutrino masses through the see-saw mechanism. Once we incorporate the U(1)B–L gauge symmetry in SUSY models, the U(1)B–L gaugino, Z˜B–L appears, and it can mediate the SUSY breaking (Z-prime mediated SUSY breaking) at around the scale of 106 GeV. Then we find a links between the neutrino mass (more precisely the see-saw or B–L scale of order 106 GeV) and the Z-prime mediated SUSY breaking scale. It is also very interesting that the gluino at the weak scale becomes relatively light, and almost compressed mass spectra for the gaugino sector can be realized in this scenario, which is very interesting in scope of the LHC.

Squark pair production in the MSSM with explicitCPviolation

We analyze effects of the $CP$-odd soft phases in the MSSM on the pair productions of colored superpartners in $pp$ collisions at the LHC energies. We find that, among all pair-production processes, those of the scalar quarks in the first and second generations are particularly sensitive to the $CP$-odd phases, more precisely, to the phases of the gluinos and neutralinos. We compute pair-production cross sections, classify various production modes according to their dependencies on the gluino and neutralino phases, perform a detailed numerical analysis to determine individual as well as total cross sections, and give a detailed discussion of 2. electric dipole moment (EDM) bounds. We find that pair productions of first and second generation squarks serve as a viable probe of the $CP$ violation sources in the gaugino sector of the theory even if experiments cannot determine chirality, flavor and electric charge of the squarks produced.

Dark matter and collider searches in the MSSM

We study the complementarity between dark matter experiments (direct detection and indirect detection) and accelerator experiments (the CERN LHC and a $\sqrt{s}$ =1 TeV e+e- linear collider) within the framework of the constrained minimal supersymmetric standard model (MSSM). We show how non-universality in the scalar and gaugino sectors can affect the experimental prospects to discover the supersymmetric particles. The future experiments will explore a large part of the parameter space of the MSSM favored by the Wilkinson Microwave Anisotropy Probe (WMAP) constraint on the relic density, but there still exist some regions beyond reach for certain extreme (fine tuned) values of the supersymmetric parameters. Whereas the focus point region characterized by heavy scalars will be easily probed by experiments searching for dark matter, the regions with heavy gauginos and light sfermions will be accessible more easily by collider experiments. More information on both supersymmetry and astrophysics parameters can thus be obtained by combining the different signals.

Naturalness priors and fits to the constrained minimal supersymmetric Standard Model

We examine the effect of a prior that favours low values of fine-tuning on Bayesian multi-dimensional fits of the constrained minimal supersymmetric Standard Model (CMSSM or mSUGRA) to current data. The dark matter relic density, the anomalous magnetic moment of the muon and the branching ratio of b→sγ are all used to constrain the model via a Markov chain Monte Carlo sampler. As a result of the naturalness prior, posterior probability distributions skew towards lighter Higgs and sparticle masses, the effect being most pronounced in the gaugino sector. Interestingly, slepton masses are an exception and skew towards heavier masses. The lightest CP-even Higgs h0-pole annihilation mechanism becomes allowed at the 2σ level for the latest combination of measurements of mt=172.7±2.9 GeV, provided we allow for a theoretical error in the prediction of its mass mh0. mh0 is constrained to be less than 120 GeV at the 95% C.L. Probing the branching ratio of Bs→μ+μ− to the level of 2×10−8, as might be achieved by the Tevatron experiments, would cover 32% of the probability density, irrespective of which of the two priors is used.

Adiabatic compression and indirect detection of supersymmetric dark matter

Recent developments in the modelling of the dark matter distribution in our Galaxy pointout the necessity to consider some physical processes to satisfy observational data. Inparticular, models with adiabatic compression, which include the effect of the baryonic gasin the halo, increase significantly the dark matter density in the central region of the MilkyWay. On the other hand, the non-universality in the scalar and gaugino sectors ofsupergravity models can also increase significantly the neutralino annihilation cross section.We show that the combination of both effects gives rise to a gamma-ray flux arising fromthe galactic centre largely reachable by future experiments like GLAST. We also analyse inthis framework the EGRET excess data above 1 GeV, as well as the recent data fromCANGAROO and HESS. The analysis has been carried out imposing the most recentexperimental constraints, such as the lower bound on the Higgs mass, the b → sγ branching ratio, and the muong − 2. In addition, the recently improved upper bound on B(Bs → μ+μ-) has also been taken into account. The astrophysical (WMAP) bounds on the dark matterdensity have also been imposed on the theoretical computation of the relic neutralinodensity through thermal production.

Gaugino CP phases and EDMs in the extended gauge mediation SUSY breaking

We study phenomenological aspects of the soft supersymmetry breaking parameters in a model with the extended gauge mediation supersymmetry breaking. In this model gaugino masses can be non-universal and as its result physical CP-phases remain in the gaugino sector even after the R-transformation. These phases contribute to the electric dipole moment (EDM) of an electron and a neutron. We show that their experimental bounds can be satisfied even for the situation such that there exist the order one CP-phases and the masses of superpartners are of the order of 100 GeV.

Gaugino-induced quark and lepton masses in a truly minimal left-right model

It has recently been proposed that all fundamental fermion masses (whether Dirac or Majorana) come from effective dimension-five operators in the context of a truly minimal left-right model. We show how a particularly economical scheme emerges in a supersymmetric framework, where chiral symmetry breaking originates in the gaugino sector.

Soft supersymmetry breaking masses in a unified model with doublet-triplet splitting

We study soft supersymmetry breaking parameters in a supersymmetric unified model which potentially solves the doublet-triplet splitting problem. In the model the doublet-triplet splitting is solved by the discrete symmetry which is allowed to be introduced due to the direct product structure of the gauge group. The messenger fields for the gauge mediated supersymmetry breaking are naturally embedded in the model. The discrete symmetry required by the doublet-triplet splitting makes the gaugino masses non-universal and also induces a different mass spectrum for the scalar masses from the ordinary minimal gauge mediation model. Independent physical CP phases can remain in the gaugino sector even after the R-transformation.