arXiv subjects

Chan Beom Park

Publications and source records attributed to Chan Beom Park.

33 records · Page 2Linked to original sources

arXiv2014

Collider signatures of a light NMSSM pseudoscalar in neutralino decays in the light of LHC results

We investigate signatures induced by a very light pseudoscalar Higgs in neutralino decays in the Next-to-Minimal Supersymmetric Standard Model (NMSSM) and determine their observability at the LHC. We concentrate on scenarios which feature two light scalar Higgs bosons (one of them is SM-like with a mass of 125 GeV and a singlet-like lighter one) with a very light (singlet-like) pseudoscalar Higgs in the mass range 2m_tau < m_a1 < 2m_b. We consider neutralino-chargino pair production and the subsequent decay X_{2,3}-> X_1 a1, which leads to topologies involving multi-leptons and missing transverse energy. We determine a set of selection cuts that can effectively isolate the signal from backgrounds of the Standard Model or the Minimal Supersymmetric Standard Model. We also exemplify the procedure with a set of benchmark points, for which we compute the expected number of events and signal strength for LHC with 8 TeV center of mass energy. We show that this signal can already be probed for some points in the NMSSM parameter space.

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arXiv2014

MT2-reconstructed invisible momenta as spin analizers, and an application to top polarization

Full event reconstruction is known to be challenging in cases with more than one undetected final-state particle, such as pair production of two states each decaying semi-invisibly. On the other hand, full event reconstruction would allow to access angular distributions sensitive to the spin fractions of the decaying particles, thereby dissecting their production mechanism. We explore this possibility in the case of Standard-Model t-tbar production followed by a leptonic decay of both W bosons, implying two undetected final-state neutrinos. We estimate the t and tbar momentum vectors event by event using information extracted from the kinematic variable MT2. The faithfulness of the estimated momenta to the true momenta is then tested in observables sensitive to top polarization and t-tbar spin correlations. Our method thereby provides a novel approach towards the evaluation of these observables, and towards testing t-tbar production beyond the level of the total cross section. While our discussion is confined to t-tbar production as a benchmark, the method is applicable to any process whose decay topology allows to construct MT2.

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arXiv2013

Probing the two light Higgs scenario in the NMSSM with a low-mass pseudoscalar

In this article we propose a simultaneous collider search strategy for a pair of scalar bosons in the NMSSM through the decays of a very light pseudoscalar. The massive scalar has a mass around 126 GeV while the lighter one can have a mass in the vicinity of 98 GeV (thus explaining an apparent LEP excess) or be much lighter. The successive decay of this scalar pair into two light pseudoscalars, followed by leptonic pseudoscalar decays, produces clean multi-lepton final states with small or no missing energy. Furthermore, this analysis offers an alternate leptonic probe for the 126 GeV scalar that can be comparable with the ZZ* search channel. We emphasize that a dedicated experimental search for multi-lepton final states can be an useful probe for this scenario and, in general, for the NMSSM Higgs sector. We illustrate our analysis with two representative benchmark points and show how the LHC configuration with 8 TeV center-of-mass energy and 25 inverse femtobarns of integrated luminosity can start testing this scenario, providing a good determination of the light pseudoscalar mass and a relatively good estimation of the lightest scalar mass.

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arXiv2012

Peccei-Quinn NMSSM in the light of 125 GeV Higgs

We study the phenomenology of the Peccei-Quinn invariant extension of the next-to-minimal supersymmetric standard model (NMSSM) in view of the recent discovery of a 125 GeV Higgs boson. The minimal model having no quadratic and cubic terms of the NMSSM singlet field predicts a light singlino-like lightest supersymmetric particle (LSP). The model is strongly constrained by the Higgs invisible decay and the dark matter characteristic of the LSP, while some constraints can be relaxed by assuming that the saxion, the CP-even companion of the axion in the Peccei-Quinn sector, causes a late-time entropy production diluting the thermal LSP density. The collider signal of the model contains multi-jet and $h/W/Z$ plus missing energy, which can be discovered in the early stage of the 14 TeV LHC running.

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arXiv2011

Reducing combinatorial uncertainties: A new technique based on MT2 variables

We propose a new method to resolve combinatorial ambiguities in hadron collider events involving two invisible particles in the final state. This method is based on the kinematic variable MT2 and on the MT2-assisted-on-shell reconstruction of invisible momenta, that are reformulated as `test' variables Ti of the correct combination against the incorrect ones. We show how the efficiency of the single Ti in providing the correct answer can be systematically improved by combining the different Ti and/or by introducing cuts on suitable, combination-insensitive kinematic variables. We illustrate our whole approach in the specific example of top anti-top production, followed by a leptonic decay of the W on both sides. However, by construction, our method is also directly applicable to many topologies of interest for new physics, in particular events producing a pair of undetected particles, that are potential dark-matter candidates. We finally emphasize that our method is apt to several generalizations, that we outline in the last sections of the paper.

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arXiv2011

Reconstructing the heavy resonance at hadron colliders

We investigate the method for constructing the invariant mass using the M_T2-assisted on-shell (MAOS) approximation to the invisible particle momenta in the cascade decays of a new particle resonance produced at hadron colliders. We note that the MAOS reconstruction can be defined in several different ways, while keeping the efficiency of approximation at a similar level, and one of them provides a unique solution for each event. It is shown that the invariant mass distribution constructed with the MAOS momenta exhibits a peak at the heavy resonance mass, regardless of the chosen MAOS scheme and the detailed mass spectrum of the particles in the cascade. We stress that the MAOS invariant mass can be used as a clean signal of new particle resonance produced at the LHC, as well as a model-independent method to measure the masses of new particles involved in the event.

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arXiv2011

Measuring the Higgs boson mass with transverse mass variables

We provide a comparative study of the Higgs boson mass measurements based on two approaches to the dileptonic decay of W bosons produced by the Higgs boson decay, one using the kinematic variable M_T^true and the other using the M_T2-assisted on-shell reconstruction of the invisible neutrino momenta. We find that these two approaches can determine the Higgs boson mass with a similar accuracy for both of the two main production mechanisms of the SM Higgs boson at the LHC, i.e. the gluon-gluon fusion and the weak vector boson fusion. We also notice that the Higgs signal distribution for the gluon-gluon fusion becomes narrower under the M_T2 cut, while the corresponding background distribution becomes flatter, indicating that one might be able to reduce the systematic uncertainties of mass measurement with an appropriate M_T2 cut.

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arXiv2010

Sparticle masses from transverse mass kinks at the LHC: the case of Yukawa-unified SUSY GUTs

We explore, in a concrete example, to which extent new particle mass determinations are practicable with LHC data. Our chosen example is that of Yukawa-unified SUSY GUTs, whose viability has been recently studied for two general patterns of soft SUSY-breaking terms. We note that both patterns of SUSY spectra do not admit long decay chains, which would make it possible to determine the masses of the SUSY particles involved using endpoints or mass relations. We thus take the so-called mT2-kink method as our key strategy, since it does not rely on the presence of long decay chains. We then discuss a procedure allowing to determine the masses of the gluino, of the lightest chargino as well as of the first two neutralinos and, for the scenario where a stop is lighter than the gluino, the mass of the light stop too. Our worked example of Yukawa-unified SUSY GUTs may offer a useful playground for dealing with other theories which predict similar patterns of SUSY spectra.

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arXiv2009

Mass and Spin Measurement with M_T2 and MAOS Momentum

We discuss the M_T2-kink method to determine the masses of both the dark matter WIMP and its mother particle produced at the LHC. We then introduce a new kinematic variable, the M_T2-Assisted-On-Shell (MAOS) momentum, that provides a systematic approximation to the invisible particle momenta in hadron collider events producing a pair of invisible particles, and apply it to certain SUSY processes and their UED equivalents to determine the spin of gluino/KK-gluon and of slepton/KK-lepton. An application of the MAOS momentum to the SM Higgs mass measurement is briefly discussed also.

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arXiv2009

Reconstructing the Higgs boson in dileptonic W decays at hadron collider

We examine the prospect to measure the Higgs boson mass using the recently introduced kinematic variable, the M_T2-Assisted On-Shell (MAOS) momentum, that provides a systematic approximation to the invisible neutrino momenta in dileptonic decays of W-boson pair. For this purpose, we introduce a modified version of the MAOS momentum, that is applicable even when one or both of the W-bosons from the Higgs decay are in off-shell. It is demonstrated that the MAOS Higgs mass distribution, constructed with the MAOS neutrino momenta, shows a clear peak at the true Higgs boson mass when an event cut selecting higher value of M_T2 is employed. We perform the likelihood analysis for this MAOS mass distribution to determine the Higgs boson mass, and find it can improve the accuracy of the Higgs mass measurement. Our results indicate that the MAOS Higgs mass can be useful also for the discovery or exclusion of the Higgs boson in certain mass range.

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arXiv2009

M_T2-assisted on-shell reconstruction of missing momenta and its application to spin measurement at the LHC

We propose a scheme to assign a 4-momentum to each WIMP in new physics event producing a pair of mother particles each of which decays to an invisible weakly interacting massive particle (WIMP) plus some visible particle(s). The transverse components are given by the value that determines the event variable M_T2, while the longitudinal component is determined by the on-shell condition on the mother particle. Although it does not give the true WIMP momentum in general, this M_T2-assisted on-shell reconstruction of missing momenta provides kinematic variables well correlated to the true WIMP momentum, and thus can be useful for an experimental determination of new particle properties. We apply this scheme to some processes to measure the mother particle spin, and find that spin determination is possible even without a good knowledge of the new particle masses.

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arXiv2008

Comment on `A collider signature of the supersymmetric golden region'

We show that $Z$ polarization measurement is useful for distinguishing the signal decay chain of the `golden region' in the minimal supersymmetric standard model(MSSM) parameter space, which involves the stop decay of $\tilde t_2 \to \tilde t_1 + Z$, from other decay chains involving the neutralino decay of $\tildeχ_2^0 \to \tildeχ_1^0 + Z$ in different MSSM scenarios.

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arXiv2008

Measuring the top quark mass with m_T2 at the LHC

We investigate the possibility to measure the top quark mass using the collider variable $m_{T2}$ at the LHC experiment. Monte Carlo studies of $m_{T2}$ are performed with the events corresponding to the dilepton decays of $t\bar{t}$ produced at the LHC with 10 $fb^{-1}$ integrated luminosity. Our analysis suggests that the top quark mass can be determined by the $m_{T2}$ variable alone with a good precision at the level of 1 GeV.

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arXiv2007

Measuring superparticle masses at hadron collider using the transverse mass kink

We present a detailed study of the collider observable $m_{T2}$ applied for pair-produced superparticles decaying to visible particles and a pair of invisible lightest supersymmetric particles (LSPs). Analytic expressions of the maximum of $m_{T2}$ over all events ($m_{T2}^{\rm max}$) are derived. It is noticed that if the decay product of each superparticle involves more than one visible particles, $m_{T2}^{\rm max}$ being a function of the {\it trial} LSP mass ${m}_χ$ has a kink structure at ${m}_χ=$ true LSP mass, which can be used to determine the mother superparticle mass and the LSP mass simultaneously. To see how well $m_{T2}^{\rm max}$ can be constructed from collider data, a Monte-Carlo analysis of the gluino $m_{T2}$ is performed for some superparticle spectra.

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arXiv2007

Gluino Stransverse Mass

We introduce a new observable, 'gluino stransverse mass', which is an application of the Cambridge $m_{T2}$ variable to the process where gluinos are pair produced in proton-proton collision and each gluino subsequently decays into two quarks and one LSP, $i.e. \tilde{g}\tilde{g} \to qq\tildeχ_1^0\ qq\tildeχ_1^0$. We show that the gluino stransverse mass can be utilized to measure the gluino and the lightest neutralino masses separately, and also the (1st and 2nd generation) squark masses if lighter than the gluino mass, thereby providing a good first look at the pattern of sparticle masses experimentally.

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