arXiv subjects

Michihisa Takeuchi

Publications and source records attributed to Michihisa Takeuchi.

53 records · Page 3Linked to original sources

arXiv2014

Disentangling Higgs-Top Couplings in Associated Production

In the presence of CP violation, the Higgs-top coupling may have both scalar and pseudoscalar components, $κ_t$ and ${\tilde κ_t}$, which are bounded indirectly but only weakly by the present experimental constraints on the Higgs-gluon-gluon and Higgs-$γ$-$γ$ couplings, whereas upper limits on electric dipole moments provide strong additional indirect constraints on ${\tilde κ_t}$, if the Higgs-electron coupling is similar to that in the Standard Model and there are no cancellations with other contributions. We discuss methods to measure directly the scalar and pseudoscalar Higgs-top couplings by measurements of Higgs production in association with ${\bar t} t$, single $t$ and single ${\bar t}$ at the LHC. Measurements of the total cross sections are very sensitive to variations in the Higgs-top couplings that are consistent with the present indirect constraints, as are invariant mass distributions in ${\bar t} t H$, $tH$ and ${\bar t} H$ final states. We also investigate the additional information on $κ_t$ and ${\tilde κ_t}$ that could be obtained from measurements of the longitudinal and transverse $t$ polarization in the different associated production channels, and the ${\bar t} t$ spin correlation in ${\bar t} t H$ events.

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arXiv2013

Muon g-2 and LHC phenomenology in the $L_μ-L_τ$ gauge symmetric model

In this paper, we consider phenomenology of a model with an $L_μ-L_τ$ gauge symmetry. Since the muon couples to the $L_μ-L_τ$ gauge boson (called $Z''$ boson), its contribution to the muon anomalous magnetic moment (muon g-2) can account for the discrepancy between the standard model prediction and the experimental measurements. On the other hand, the $Z''$ boson does not interact with the electron and quarks, and hence there are no strong constraints from collider experiments even if the $Z''$ boson mass is of the order of the electroweak scale. We show an allowed region of a parameter space in the $L_μ-L_τ$ symmetric model, taking into account consistency with the electroweak precision measurements as well as the muon g-2. We study the Large Hadron Collider (LHC) phenomenology, and show that the current and future data would probe the interesting parameter space for this model.

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arXiv2013

Buckets of Higgs and Tops

We show that associated production of a Higgs with a top pair can be observed in purely hadronic decays. Reconstructing the top quarks in the form of jet buckets allows us to control QCD backgrounds as well as signal combinatorics. The background can be measured from side bands in the reconstructed Higgs mass. We back up our claims with a detailed study of the QCD event simulation, both for the signal and for the backgrounds.

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arXiv2013

Buckets of Tops

Reconstructing hadronically decaying top quarks is a key challenge at the LHC, affecting a long list of Higgs analyses and new physics searches. We propose a new method of collecting jets in buckets, corresponding to top quarks and initial state radiation. This method is particularly well suited for moderate transverse momenta of the top quark, closing the gap between top taggers and traditional top reconstruction. Applying it to searches for supersymmetric top squarks we illustrate the power of buckets.

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arXiv2012

Tagging single Tops

Top taggers which identify and reconstruct boosted top quarks have been established as novel tools for a multitude of LHC analyses. We show how single top production in association with a light-flavor or bottom jet can be observed in the boosted phase space regime. The full top reconstruction as part of the tagging algorithm allows us to define a distinctive kinematic angle which clearly separates different single top production processes.

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arXiv2012

Measuring Higgs CP and couplings with hadronic event shapes

Experimental falsification or validation of the Standard Model of Particle Physics involves the measurement of the CP quantum number and couplings of the Higgs boson. Both ATLAS and CMS have reported an SM Higgs-like excess around m_H=125 GeV. In this mass range the properties of the Higgs boson can be extracted from an analysis of the azimuthal angle distribution of the two jets in pp->Hjj events. This channel is also important to measure the couplings of the Higgs boson to electroweak gauge bosons and fermions, hereby establishing the exceptional role of the Higgs boson in the Standard Model. Instead of exploiting the jet angular correlation, we show that hadronic event shapes exhibit substantial discriminative power to separate a CP even from a CP odd Higgs. Some event shapes even show an increased sensitivity to the Higgs CP compared to the azimuthal angle correlation. Constraining the Higgs couplings via a separation of the weak boson fusion and the gluon fusion Higgs production modes can be achieved applying similar strategies.

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arXiv2012

Stop searches in 2012

For this year's 8TeV run of the LHC we lay out different strategies to search for scalar top pairs. We show results for the hadronic and for the semi-leptonic channels based on hadronic top tagging. For the di-lepton channel we illustrate the impact of transverse mass variables. Each of our signal-to-background ratios ranges around unity for a stop mass around 400GeV. The combined signal significances show that dedicated stop searches are becoming sensitive over a non-negligible part of parameter space.

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arXiv2011

How to Improve Top Tagging

In time for the first tests on LHC data we introduce a set of improvements and tests of purely kinematic top tagging algorithms. First, we show how different jet algorithms can be used for different transverse momentum regimes. Combining pruning and filtering in the reconstruction can enhance the signal over background ratio significantly, while larger jet radii only give minor improvements. Finally, bottom tagging can be added to the top tagger, but at least for the HEPTopTagger does not improve the kinematic selection algorithm.

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arXiv2011

W+Jets at CDF: Evidence for Top Quarks

Recently, an anomaly of W+jets events at large invariant masses has been reported by CDF. Many interpretations as physics beyond the Standard Model are being offered. We show how such an invariant mass peak can arise from a slight shift in the relative normalization of the top and WW backgrounds.

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arXiv2011

$A^t_{FB}$ Meets LHC

The recent Tevatron measurement of the forward-backward asymmetry of the top quark shows an intriguing discrepancy with Standard Model expectations, particularly at large $\ttbar$ invariant masses. Measurements of this quantity are subtle at the LHC, due to its $pp$ initial state, however, one can define a forward-central-charge asymmetry which captures the physics. We study the capability of the LHC to measure this asymmetry and find that within the SM a measurement at the $5σ$ level is possible with roughly 60 fb$^{-1}$ at $\sqrt{s} = 14$ TeV. If nature realizes a model which enhances the asymmetry (as is necessary to explain the Tevatron measurements), a significant difference from zero can be observed much earlier, perhaps even during early LHC running at $\sqrt{s} = 7$ TeV. We further explore the capabilities of the 7 TeV LHC to discover resonances or contact interactions which modify the $\ttbar$ invariant mass distribution using recent boosted top tagging techniques. We find that TeV-scale color octet resonances can be discovered, even with small coupling strengths and that contact interactions can be probed at scales exceeding 6 TeV. Overall, the LHC has good potential to clarify the situation with regards to the Tevatron forward-backward measurement.

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arXiv2011

Boosted Semileptonic Tops in Stop Decays

Top partner searches are one of the key aspects of new physics analyses at the LHC. We correct an earlier statement that supersymmetric top searches based on decays to semileptonic tops are not promising. Reconstructing the direction of the boosted leptonic top quark and correlating it with the measured missing transverse energy vector allows us to reduce the top pair background to an easily manageable level. In addition, reconstructing the full momentum of the leptonic top quark determines the stop mass based on an M_{T2} endpoint.

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arXiv2010

Stop Reconstruction with Tagged Tops

At the LHC combinatorics make it unlikely that we will be able to observe stop pair production with a decay to a semi-leptonic top pair and missing energy for generic supersymmetric mass spectra. Using a Standard-Model top tagger on fully hadronic top decays we can not only extract the stop signal but also measure the top momentum. To illustrate the promise of tagging tops with moderate boost we include a detailed discussion of our HEPTopTagger algorithm.

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arXiv2009

Dark matter and collider phenomenology of split-UED

We explicitly show that split-universal extra dimension (split-UED), a recently suggested extension of universal extra dimension (UED) model, can nicely explain recent anomalies in cosmic-ray positrons and electrons observed by PAMELA and ATIC/PPB-BETS. Kaluza-Klein (KK) dark matters mainly annihilate into leptons because the hadronic branching fraction is highly suppressed by large KK quark masses and the antiproton flux agrees very well with the observation where no excess is found . The flux of cosmic gamma-rays from pion decay is also highly suppressed and hardly detected in low energy region (E<20 GeV). Collider signatures of colored KK particles at the LHC, especially q_1 q_1 production, are studied in detail. Due to the large split in masses of KK quarks and other particles, hard p_T jets and missing E_T are generated, which make it possible to suppress the standard model background and discover the signals.

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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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arXiv2008

Study of the top reconstruction in top-partner events at the LHC

In the Littlest Higgs model with T-parity (LHT), top-partners (T_-) are produced in pairs at the Large Hadron Collider (LHC). Each top-partner decays into a top quark (t) and the lightest T-odd gauge partner A_H. We demonstrate reconstruction of the ttbar system decaying hadronically, and measurement of the top-partner mass from the m_{T2} distribution. A top quark from a T_{-} decay is polarized, and we discuss the effect of this polarization on the decay distributions. Because the events consist of highly collinear jets which occasionally overlap each other, we compare distributions using different jet reconstruction algorithms (Snowmass cone, kt, Cambridge, SISCone). We find clustering algorithms are advantageous for studying top polarization effects.

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arXiv2008

Unparticle physics at the photon collider

Recently, a conceptually new physics beyond the Standard Model (SM), unparticle, has been proposed, where a hidden conformal sector is coupled to the SM sector through higher dimensional operators. In this setup, we investigate unparticle physics at the photon collider, in particular, unparticle effects on the gamma gamma to gamma gamma process. Since this process occurs at loop level in the SM, the unparticle effects can be significant even if the cutoff scale is very high. In fact, we find that the unparticle effects cause sizable deviations from the SM results. The scaling dimension of the unparticle d_U reflects the dependence of the cross section on the final state photon invariant mass, so that precision measurements of this dependence may reveal the scaling dimension of the unparticle.

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arXiv2007

The study of sq_L sq_L production at LHC in the l^\pm l^\pm channel and sensitivity to other models

$\tilde{q}_L \tilde{q}_L$ production is one of the main SUSY production processes in the MSSM, which occurs due to the chirality flip caused by the Majorana gluino mass. This process is one of the sources of same sign two lepton (SS2$l$) events, however, gluino production also contributes to this channel. In this paper, we develop a method to identify gluino and squark production separately in the SS2$l$ channel, based on cuts on the kinematical configuration of the jets. We applied the method to the MSSM, a model with an extended gluino sector, and the Littlest Higgs model with T-parity (LHT), and found a distinctive difference between these models when considering the numbers of SS2$l$ events selected by the cuts.

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