Top polarization in stop production at the LHC

We survey the expected polarization of the top produced in the decay of a scalar top quark, t -> t((chi) over tildei)(0), i = 1 - 2. The phenomenology is quite interesting, since the expected polarization depends both on the mixing in the stop and neutralino sectors and on the mass differences between the stop and the neutralino. We find that a mixed stop behaves almost like a right-handed stop due to the larger hypercharge that enters the stop/top/gaugino coupling and that these polarisation effects disappear, when m((t) over tilde1) approximate to m(t) +m((chi) over tildei)(0). After a discussion on the expected top polarization from the decay of a scalar top quark, we focus on the interplay of polarization and kinematics at the LHC. We discuss different probes of the top polarization in terms of lab-frame observables. We find that these observables faithfully reflect the polarization of the parent top-quark, but also have a non-trivial dependence on the kinematics of the stop production and decay process. In addition, we illustrate the effect of top polarization on the energy and transverse momentum of the decay lepton in the laboratory frame. Our results show that both spectra are softened substantially in case of a negatively polarized top, particularly for a large mass difference between the stop and the neutralino. Thus, the search strategies, and the conclusions that can be drawn from them, depends not just on the mass difference m((t) over tilde) - m((chi) over tildei)(0) due to the usual kinematic effects but also on the effects of top polarization on the decay kinematics the extent of which depends in turn on the said mass difference.

Looking for an invisible Higgs signal at the LHC

While the recent discovery of a Higgs-like boson at the LHC is an extremely important and encouraging step towards the discovery of the complete Standard Model (SM), the current information on this state does not rule out possibility of beyond standard model (BSM) physics. In fact the current data can still accommodate reasonably large values of the branching fractions of the Higgs into a channel with `invisible' decay products, such a channel being also well motivated theoretically. In this study we revisit the possibility of detecting the Higgs in this invisible channel for both choices of the LHC energies, 8 and 14 TeV, for two production modes; vector boson fusion (VBF) and associated production (ZH). We perform a comprehensive collider analysis for all the above channels and project the reach of LHC to constrain the invisible decay branching fraction for both 8 and 14 TeV energies. For the ZH case we consider decays of the Z boson into a pair of leptons as well as a b (b) over bar pair. For the VBF channel the sensitivity is found to be more than 5 sigma for both the energies up to an invisible branching ratio (Br-inv) similar to 0.80, with luminosities similar to 20/30 fb(-1). The sensitivity is further extended to values of Br-inv similar to 0.25 for 300 fb(-1) at 14 TeV. However the reach is found to be more modest for the ZH mode with leptonic final state; with about 3.5 sigma for the planned luminosity at 8 TeV, reaching 8 sigma only for 14 TeV for 50 fb(-1). In spite of the much larger branching ratio (BR) of the Z into a b (b) over bar channel compared to the dilepton case, the former channel, can provide useful reach up to Br-inv greater than or similar to 0.75, only for the higher luminosity (300 fb(-1)) option using both jet-substructure and jet clustering methods. (C) 2013 Elsevier B.V. All rights reserved.

Charge and color breaking constraints in MSSM after the Higgs discovery at LHC

We revisit the constraints on the parameter space of the Minimal Supersymmetric Standard Model (MSSM), from charge and color breaking minima in the light of information on the Higgs from the LHC so far. We study the behavior of the scalar potential keeping two light sfermion fields along with the Higgs in the pMSSM framework and analyze the stability of the vacuum. We find that for lightest stops a parts per thousand(2) 1 TeV and small mu a parts per thousand(2) 500 GeV, the absolute stability of the potential can be attained only for . The bounds become stronger for larger values of the mu parameter. Note that this is approximately the value of Xt which maximizes the Higgs mass. Our bounds on the low scale MSSM parameters are more stringent than those reported earlier in literature. We reanalyze the stau sector as well, keeping both staus. We study the connections between the observed Higgs rates and vacuum (meta)stability. We show how a precision study of the ratio of signal strengths, (mu (gamma gamma) /mu (ZZ) ) can shed further light.

Role of supersymmetric heavy Higgs boson production in the self-coupling measurement of 125 GeV Higgs boson at the LHC

Measurement of the self-coupling of the 125 GeV Higgs boson is one of the most crucial tasks for a high luminosity run of the LHC, and it can only be measured in the di-Higgs final state. In the minimal supersymmetric standard model, heavy CP even Higgs (H) can decay into a lighter 125 GeV Higgs boson (h) and, therefore, can influence the rate of di-Higgs production. We investigate the role of single H production in the context of measuring the self-coupling of h. We have found that the H -> hh decay can change the value of Higgs (h) self-coupling substantially, in a low tan beta regime where the mass of the heavy Higgs boson lies between 250 and 600 GeV and, depending on the parameter space, it may be seen as an enhancement of the self-coupling of the 125 GeV Higgs boson.

Light stop in the MSSM after LHC Run 1

The discovery of a Higgs boson with a mass of 126 GeV at the LHC when combined with the non-observation of new physics both in direct and indirect searches imposes strong constraints on supersymmetric models and in particular on the top squark sector. The experiments for direct detection of dark matter have provided with yet more constraints on the neutralino LSP mass and its interactions. After imposing limits from the Higgs, flavour and dark matter sectors, we examine the feasibility for a light stop in the context of the pMSSM, in light of current results for stop and other SUSY searches at the LHC. We only require that the neutralino dark matter explains a fraction of the cosmologically measured dark matter abundance. We find that a stop with mass below similar to 500 GeV is still allowed. We further study various probes of the light stop scenario that could be performed at the LHC Run-II either through direct searches for the light and heavy stop, or SUSY searches not currently available in simplified model results. Moreover we study the characteristics of heavy Higgs for the points in the parameter space allowed by all the available constraints and illustrate the region with large cross sections to fermionic or electroweakino channels. Finally we show that nearly all scenarios with a small stop-LSP mass difference will be tested by Xenon1T provided the NLSP is a chargino, thus probing a region hard to access at the LHC.

Interplay of vector-like top partner multiplets in a realistic mixing set-up

The ATLAS and CMS collaborations at the LHC have performed analyses on the existing data sets, studying the case of one vector-like fermion or multiplet coupling to the standard model Yukawa sector. In the near future, with more data available, these experimental collaborations will start to investigate more realistic cases. The presence of more than one extra vector-like multiplet is indeed a common situation in many extensions of the standard model. The interplay of these vector-like multiplet between precision electroweak bounds, flavour and collider phenomenology is a important question in view of establishing bounds or for the discovery of physics beyond the standard model. In this work we study the phenomenological consequences of the presence of two vector-like multiplets. We analyse the constraints on such scenarios from tree-level data and oblique corrections for the case of mixing to each of the SM generations. In the present work, we limit to scenarios with two top-like partners and no mixing in the down-sector.

Status of the MSSM Higgs sector using global analysis and direct search bounds, and future prospects at the High Luminosity LHC

In this paper, we search for the regions of the phenomenological minimal supersymmetric standard model (pMSSM) parameter space where one can expect to have moderate Higgs mixing angle (alpha) with relatively light (up to 600 GeV) additional Higgses after satisfying the current LHC data. We perform a global fit analysis using most updated data (till December 2014) from the LHC and Tevatron experiments. The constraints coming from the precision measurements of the rare b-decays B-s -> mu(+)mu(-) and b -> s gamma are also considered. We find that low M-A(less than or similar to 350) and high tan beta(greater than or similar to 25) regions are disfavored by the combined effect of the global analysis and flavor data. However, regions with Higgs mixing angle alpha similar to 0.1-0.8 are still allowed by the current data. We then study the existing direct search bounds on the heavy scalar/pseudoscalar (H/A) and charged Higgs boson (H-+/-) masses and branchings at the LHC. It has been found that regions with low to moderate values of tan beta with light additional Higgses (mass <= 600 GeV) are unconstrained by the data, while the regions with tan beta > 20 are excluded considering the direct search bounds by the LHC-8 data. The possibility to probe the region with tan beta <= 20 at the high luminosity run of LHC are also discussed, giving special attention to the H -> hh, H/A -> t (t) over bar and H/A -> tau(+)tau(-) decay modes.

Status of the 98-125 GeV Higgs bosons scenario with updated LHC-8 data

In the context of the minimal supersymmetric standard model (MSSM), we discuss the possibility of the lightest Higgs boson with mass M-h = 98 GeV to be consistent with the 2.3 sigma excess observed at the LEP in the decay mode e(+)e(-) -> Zh, with h -> b (b) over bar. In the same region of the MSSM parameter space, the heavier Higgs boson (H) with mass M-H similar to 125 GeV is required to be consistent with the latest data on Higgs coupling measurements at the end of the 7 + 8 TeV LHC run with 25 fb(-1) of data. While scanning the MSSM parameter space, we impose constraints coming from flavor physics, relic density of the cold dark matter as well as direct dark matter searches. We study the possibility of observing this light Higgs boson in vector boson fusion process and associated production with W/Z-boson at the high luminosity (3000 fb(-1)) run of the 14 TeV LHC. Our analysis shows that this scenario can hardly be ruled out even at the high luminosity run of the LHC. However, the precise measurement of the Higgs signal strength ratios can play a major role to distinguish this scenario from the canonical MSSM one.

Future collider signatures of the possible 750 GeV state

If the recent indications of a possible state I broken vertical bar with mass similar to 750 GeV decaying into two photons reported by ATLAS and CMS in LHC collisions at 13 TeV were to become confirmed, the prospects for future collider physics at the LHC and beyond would be affected radically, as we explore in this paper. Even minimal scenarios for the I broken vertical bar resonance and its gamma gamma decays require additional particles with masses . We consider here two benchmark scenarios that exemplify the range of possibilities: one in which I broken vertical bar is a singlet scalar or pseudoscalar boson whose production and gamma gamma decays are due to loops of coloured and charged fermions, and another benchmark scenario in which I broken vertical bar is a superposition of (nearly) degenerate CP-even and CP-odd Higgs bosons in a (possibly supersymmetric) two-Higgs doublet model also with additional fermions to account for the gamma gamma decay rate. We explore the implications of these benchmark scenarios for the production of I broken vertical bar and its new partners at colliders in future runs of the LHC and beyond, at higher-energy pp colliders and at e (+) e (-) and gamma gamma colliders, with emphasis on the bosonic partners expected in the doublet scenario and the fermionic partners expected in both scenarios.