arXiv subjects

Kazuki Sakurai

Publications and source records attributed to Kazuki Sakurai.

67 records · Page 4Linked to original sources

arXiv2013

Reconstruction of Higgs bosons in the di-tau channel via 3-prong decay

We propose a method for reconstructing the mass of a particle, such as the Higgs boson, decaying into a pair of tau leptons, of which one subsequently undergoes a 3-prong decay. The kinematics is solved using information from the visible decay products, the missing transverse momentum, and the 3-prong tau decay vertex, with the detector resolution taken into account using a likelihood method. The method is shown to give good discrimination between a 125 GeV Higgs boson signal and the dominant backgrounds, such as Z decays to tau tau and W plus jets production. As a result, we find an improvement, compared to existing methods for this channel, in the discovery potential, as well as in measurements of the Higgs boson mass and production cross section times branching ratio.

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arXiv2013

Can R-parity violation hide vanilla supersymmetry at the LHC?

Current experimental constraints on a large parameter space in supersymmetric models rely on the large missing energy signature. This is usually provided by the lightest neutralino which stability is ensured by the R-parity. However, if the R-parity is violated, the lightest neutralino decays into the standard model particles and the missing energy cut is not efficient anymore. In particular, the UDD type R-parity violation induces the neutralino decay to three quarks which potentially leads to the most difficult signal to be searched at hadron colliders. In this paper, we study the constraints on the R-parity violating supersymmetric model using a same-sign dilepton and a multijet signatures. We show that the gluino and squarks lighter than a TeV are already excluded in constrained minimal supersymmetric standard model with R-parity violation if their masses are approximately equal. We also analyze constraints in a simplified model with R-parity violation. We compare how R-parity violation changes some of the observables typically used to distinguish a supersymmetric signal from standard model backgrounds.

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arXiv2012

Determining the helicity structure of third generation resonances

We examine methods that have been proposed for determining the helicity structure of decays of new resonances to third generation quarks and/or leptons. We present analytical and semi-analytical predictions and assess the applicability of the relevant variables in realistic reconstruction scenarios using Monte Carlo-generated events, including the effects of QCD radiation and multiple parton interactions, combinatoric ambiguities and fast detector simulation.

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arXiv2012

Constraining compressed supersymmetry using leptonic signatures

We study the impact of the multi-lepton searches at the LHC on supersymmetric models with compressed mass spectra. For such models the acceptances of the usual search strategies are significantly reduced due to requirement of large effective mass and missing E_T. On the other hand, lepton searches do have much lower thresholds for missing E_T and p_T of the final state objects. Therefore, if a model with a compressed mass spectrum allows for multi-lepton final states, one could derive constraints using multi-lepton searches. For a class of simplified models we study the exclusion limits using ATLAS multi-lepton search analyses for the final states containing 2-4 electrons or muons with a total integrated luminosity of 1-2/fb at \sqrt{s}=7 TeV. We also modify those analyses by imposing additional cuts, so that their sensitivity to compressed supersymmetric models increase. Using the original and modified analyses, we show that the exclusion limits can be competitive with jet plus missing E_T searches, providing exclusion limits up to gluino masses of 1 TeV. We also analyse the efficiencies for several classes of events coming from different intermediate state particles. This allows us to assess exclusion limits in similar class of models with different cross sections and branching ratios without requiring a Monte Carlo simulation.

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arXiv2012

LHC constraints on Yukawa unification in SO(10)

LHC constraints on the recently proposed SUSY SO(10) GUT model with top-bottom-tau Yukawa unification are investigated. In this model, various phenomenological constraints are in concord with the Yukawa unification thanks to the negative sign of μ, D-term splitting in the soft scalar masses and non-universal gaugino masses generated by a non-zero F-term in a 24-dimensional representation of SU(5) \subset SO(10). After discussing the impact of the CP-odd Higgs boson mass bound on this model, we provide a detailed analysis of the recent direct SUSY searches performed by ATLAS and investigate the constraints on this SO(10) model. At 95% confidence level, the lower limit on the gluino mass is found to be 675 GeV. Assuming an integrated luminosity of 10 fb^{-1}, this bound may be extended to 1.1 TeV if the right-handed down squark is lighter than about 1 TeV.

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arXiv2011

Constraint from recent ATLAS results on non-universal sfermion mass models and naturalness

We investigate the impact of recent direct supersymmetry (SUSY) searches on a non-universal sfermion mass scenario focusing on naturalness. One of the advantages of this scenario is that the non-universality between third generation and first two generation sfermion masses can relax the tension between naturalness and constraints from flavour and CP violating observables. In the parameter region, where various phenomenological constraints are satisfied, the constraints to this scenario from ATLAS 165pb-1 "0-lepton" search and 35pb-1 "b-jet" search are much weaker than those to the constrained minimal SUSY standard model, because of differences in the main SUSY production processes and the main decay chains. Naturalness can be easily achieved in this scenario in accord with the current direct SUSY searches. An additional dedicated analysis may be needed to discover/exclude this scenario.

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arXiv2011

Polynomials, Riemann surfaces, and reconstructing missing-energy events

We consider the problem of reconstructing energies, momenta, and masses in collider events with missing energy, along with the complications introduced by combinatorial ambiguities and measurement errors. Typically, one reconstructs more than one value and we show how the wrong values may be correlated with the right ones. The problem has a natural formulation in terms of the theory of Riemann surfaces. We discuss examples including top quark decays in the Standard Model (relevant for top quark mass measurements and tests of spin correlation), cascade decays in models of new physics containing dark matter candidates, decays of third-generation leptoquarks in composite models of electroweak symmetry breaking, and Higgs boson decay into two tau leptons.

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arXiv2011

Searching for third-generation composite leptoquarks at the LHC

Fermion masses may arise via mixing of elementary fermions with composite fermions of a strong sector in scenarios of strongly-coupled electroweak symmetry breaking. The strong sector may contain leptoquark states with masses as light as several hundred GeV. In the present study we focus on the scalar modes of such leptoquarks since their bosonic couplings are determined completely and hence their production cross sections only depend on their masses. We study all the possible gauge-invariant non-derivative and single-derivative couplings of the scalar leptoquarks to the quarks and leptons, which turn out to be, predominantly, of the third generation. We examine their phenomenology and outline search strategies for their dominant decay modes at the LHC.

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arXiv2010

Controlling ISR in sparticle mass reconstruction

Use of inclusive MT2 distribution for sparticle mass determination is discussed. We define new parameters MT2(min) and MT2mod(min), which are a kind of minimum of sub-systerm MT2 values. Their endpoints are less affected by initial state radiations. We demonstrate that both masses of squark and gluino can be extracted from the endpoints of the distributions in the wide region of parameter space expected in CMSSM. We also present a comparison with MTGen distributions.

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arXiv2010

Reconstructing particle masses from pairs of decay chains

A method is proposed for determining the masses of the new particles N,X,Y,Z in collider events containing a pair of effectively identical decay chains Z to Y+jet, Y to X+l_1, X to N+l_2, where l_1, l_2 are opposite-sign same-flavour charged leptons and N is invisible. By first determining the upper edge of the dilepton invariant mass spectrum, we reduce the problem to a curve for each event in the 3-dimensional space of mass-squared differences. The region through which most curves pass then determines the unknown masses. A statistical approach is applied to take account of mismeasurement of jet and missing momenta. The method is easily visualized and rather robust against combinatorial ambiguities and finite detector resolution. It can be successful even for small event samples, since it makes full use of the kinematical information from every event.

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arXiv2010

Gauge-mediated supersymmetry breaking with generalized messenger sector at LHC

We consider the generalized gauge mediated supersymmetry breaking (GMSB) models with the messenger fields which do not form the complete multiplets of SU(5) GUT symmetry. Such a situation may happen in the anomalous U(1) GUT scenario because the mass spectrum of the superheavy particle does not respect SU(5) GUT symmetry, although the success of the gauge coupling unification can be explained. In this paper, we assume that one pair of the messenger fields gives the dominant contribution, and the LHC signature for the two possible messengers, X + \bar{X} and Q + \bar{Q}, are examined. We investigate the possibility to measure the deviation from the usual GUT relation of the gaugino masses which is one of the most important features of these scenarios.

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arXiv2009

LHC signature of supersymmetric models with non-universal sfermion masses

We study the LHC signature of the minimal supersymmetric standard model with non-universal sfermion masses. In the model, soft masses of gauginos and the 3rd generation of 10 of SU(5) are around the weak scale, while other sfermion soft mass is universal and around a few TeV. Such sfermion mass spectrum is motivated not only from flavor, CP and naturalness constraints but also from E_6 grand unified model with non-Abelian horizontal (flavor) symmetry. The characteristic signature of the model at the LHC is the dominance of the events with 4 b partons in the final state together with high rate of mildly boosted top quark arising from gluino decay. The prominent high p_T jet also arises from squark decay. We show it is possible to find the characteristic signature in the early stage of the LHC. The discrimination of our scenario from some CMSSM model points with similar signature may be possible with large integrated luminosity. The result of sparticle mass measurement using exclusive channel with the help of hemisphere analysis, and inclusive measurement of gluino and squark masses using M_{T2} and M_{T2}^{min} in some representative model points are presented.

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arXiv2008

Handling jets + missing E_T channel using inclusive mT2

The ATLAS and CMS experiments at the Large Hadron Collider (LHC) may discover the squarks (\tilde{q}) and gluino (\tilde{g}) of the minimal supersymmetric standard model (MSSM) in the early stage of the experiments if their masses are lighter than 1.5 TeV. In this paper we propose the sub-system m_{T2} variable, which is sensitive to the gluino mass when m_{\tilde{q}}>m_{\tilde{g}}. Using it with the inclusive m_{T2} distribution proposed earlier, \tilde{q} and \tilde{g} masses can be determined simultaneously in the early stage of the experiments. Results of Monte Carlo simulations at sample MSSM model points are presented both for signal and background.

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