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Yukinari Sumino

Publications and source records attributed to Yukinari Sumino.

33 records · Page 2Linked to original sources
arXiv2017

$WW$ scattering in a radiative electroweak symmetry breaking scenario

A classically scale invariant (CSI) extension of the standard model (SM) induces radiative electroweak symmetry breaking and predicts anomalously large Higgs self-interactions. Hence, $W_LW_L$ scattering processes can be a good probe of the symmetry breaking mechanism. We develop a theoretical framework for perturbative computation and calculate $WW$ scattering amplitudes in a CSI model. It is shown that $W_LW_L$ scattering amplitudes satisfy the equivalence theorem, and that a large deviation of $W_LW_L$ differential cross sections from the SM predictions is predicted depending on the c.m. energy and scattering angle. The results are more accurate than those based on the effective-potential approach. A prescription to implement predictions of the CSI model to Monte Carlo event generators is also presented.

hep-ph↗
arXiv2016

On enhanced corrections from quasi-degenerate states to heavy quarkonium observables

It is well known that in perturbation theory existence of quasi-degenerate states can rearrange the order counting. For a heavy quarkonium system, naively, enhanced effects ($l$-changing mixing effects) could contribute already to the first-order and third-order corrections to the wave function and the energy level, respectively, in expansion in $α_s$. We present a formulation and note that the corresponding (lowest-order) corrections vanish due to absence of the relevant off-diagonal matrix elements. As a result, in the quarkonium energy level and leptonic decay width, the enhanced effects are expected to appear, respectively, in the fifth- and fourth-order corrections and beyond.

hep-ph↗
arXiv2016

Computation of Heavy Quarkonium Spectrum in Perturbative QCD

Non-relativistic bound state theories for QED and QCD have become fairly mature and amenable to a textbook-level understanding and computation. In this talk we give an introductory review of the following subjects related to the recent computation of the heavy quarkonium spectrum using perturbative QCD: (1) Technological developments in higher-order computation, (2) Physics predictions, (3) Challenge towards analytic evaluation of the 3-loop static QCD potential.

hep-ph↗
arXiv2016

Bound-state effects on kinematical distributions of top quarks at hadron colliders

First we present a theoretical framework to compute the fully differential cross sections for the top-quark productions and their subsequent decays at hadron colliders, incorporating the bound-state effects which are important in the t\bar{t} threshold region. We include the bound-state effects such that the cross sections are correct in the LO approximation both in the threshold and high-energy regions. Then, based on this framework we compute various kinematical distributions of top quarks as well as of their decay products at the LHC, by means of Monte-Carlo event-generation. These are compared with the corresponding predictions based on conventional perturbative QCD. In particular, we find a characteristic bound-state effect on the (bW^+)-(\bar{b}W^-) double-invariant-mass distribution, which is deformed to the lower invariant-mass side in a correlated manner.

hep-ph↗
arXiv2015

A Scale-invariant Higgs Sector and Structure of the Vacuum

In view of the current status of measured Higgs boson properties, we consider a question whether only the Higgs self-interactions can deviate significantly from the Standard-Model (SM) predictions. This may be possible if the Higgs effective potential is irregular at the origin. As an example we investigate an extended Higgs sector with singlet scalar(s) and classical scale invariance. We develop a perturbative formulation necessary to analyze this model in detail. The behavior of a phenomenologically valid potential in the perturbative regime is studied around the electroweak scale. We reproduce known results: The Higgs self-interactions are substantially stronger than the SM predictions, while the Higgs interactions with other SM particles are barely changed. We further predict that the interactions of singlet scalar(s), which is a few to several times heavier than the Higgs boson, tend to be fairly strong. If probed, these features will provide vivid clues to the structure of the vacuum. We also examine Veltman's condition for the Higgs boson mass.

hep-ph↗
arXiv2015

Weight function method for precise determination of top quark mass at Large Hadron Collider

We propose a new method to measure a theoretically well-defined top quark mass at the LHC. This method is based on the "weight function method," which we proposed in our preceding paper. It requires only lepton energy distribution and is basically independent of the production process of the top quark. We perform a simulation analysis of the top quark mass reconstruction with $t\bar{t}$ pair production and lepton+jets decay channel at the leading order. The estimated statistical error of the top quark mass is about $0.4$ GeV with an integrated luminosity of $100$ fb$^{-1}$ at $\sqrt{s}=14$ TeV. We also estimate some of the major systematic uncertainties and find that they are under good control.

hep-ph↗
arXiv2014

Study of top quark pair production near threshold at the ILC

We report on a study of top pair production at the International Linear Collider (ILC) around center of mass energy (E$_{\rm CM}$) = 350 GeV using an ILD detector simulator based on the Detailed Baseline Design (DBD) configuration. Here we will report on a result of 6-Jet final state, $t\ bar{t} \rightarrow bWbW \rightarrow bqqbqq$. A result for the 4-Jet final state, $t\ bar{t} \rightarrow bWbW \rightarrow bqqblν$, which has almost the same statics as that of the 6-Jet final state will be included in the future. For an energy scan of 11 center of mass energy points (340 - 350GeV) and two beam polarization combinations (P($e^+,\ e^-$) = ($\pm$0.3, $\mp$0.8)) with 10 fb$^{-1}$ each, the statistical errors on the top quark Yukawa coupling, its mass and width are estimated. The results are $δy_t$ = 4.2%, $δm_t$ = 16 MeV in potential subtracted scheme (PS), and $δΓ_t$ = 21 MeV.

hep-ex↗
arXiv2012

Boost-invariant Leptonic Observables and Reconstruction of Parent Particle Mass

We propose a class of observables constructed from lepton energy distribution, which are independent of the velocity of the parent particle if it is scalar or unpolarized. These observables may be used to measure properties of various particles in the LHC experiments. We demonstrate their usage in a determination of the Higgs boson mass.

hep-ph↗
arXiv2011

Measuring the top Yukawa coupling at the ILC at sqrt(s) = 500 GeV

We report on the feasibility of the direct measurement of the top Yukawa coupling g_t at the International Linear Collider (ILC) during its first phase of operation with a center-of-mass energy of 500 GeV. The signal and background models incorporate the non-relativistic QCD corrections which enhance the production cross section near the t-tbar threshold. The e+e- -> t tbar H signal is reconstructed in the 6-jet + lepton and the 8-jet modes. The results from the two channels are combined. The background processes considered are e+e- -> t bbar W- / tbar b W+ (which includes e+e- -> t tbar), e+e- -> t tbar Z, and e+e- -> t tbar g* -> t tbar b bar. We use a realistic fast Monte-Carlo detector simulation. Signal events are selected using event shape variables, through jet clustering, and by identifying heavy flavor jets. Assuming a Higgs mass of 120 GeV, polarized electron and positron beams with (Pe-,Pe+) = (-0.8,+0.3), and an integrated luminosity of 1 ab-1, we estimate that the e+e- -> t tbar H events can be seen with a statistical significance of 5.2 sigma, corresponding to the relative top Yukawa coupling measurement accuracy of |Delta g_t / g_t| = 10%.

hep-ph↗
arXiv2010

Tests of a Family Gauge Symmetry Model at 10^3 TeV Scale

Based on a specific model with U(3) family gauge symmetry at $10^3$ TeV scale, we show its experimental signatures to search for. Since the gauge symmetry is introduced with a special purpose, its gauge coupling constant and gauge boson mass spectrum are not free. The current structure in this model leads to family number violations via exchange of extra gauge bosons. We investigate present constraints from flavor changing processes and discuss visible signatures at LHC and lepton colliders.

hep-ph↗
arXiv2008

Bound-state Effects on Top Quark Production at Hadron Colliders

We study bound-state effects on the t\bar{t} production cross section in the threshold region at hadron colliders. The bound-state effects are important particularly at the LHC where the gluon fusion is the dominant subprocess. Due to the formation of t\bar{t} resonances in the J=0 color-singlet channel of gg \to t\bar{t} and the large width of the top quark, the t\bar{t} invariant-mass distribution peaks at a few GeV below the t\bar{t} threshold, and it is significantly enhanced over the naive NLO prediction until several GeV above the threshold. We present predictions of the t\bar{t} invariant-mass distribution which incorporate both the bound-state effects and initial-state radiations up to NLO. The bound-state effects would lead to a substantial deformation of top-quark kinematical distributions in the threshold region.

hep-ph↗
arXiv2006

QCD Correction to Neutralino Annihilation Process and Dark Matter Density in Supersymmetric Models

We calculate QCD correction to the neutralino annihilation cross section into quark anti-quark final state and discuss its implications to the calculation of neutralino relic density. We see that the QCD correction enhances the pair-annihilation cross section by O(10 %) when final-state quarks are non-relativistic. Consequently, when the lightest neutralinos dominantly annihilate into a t\bar{t} pair, the relic density of the lightest neutralino is significantly affected by the QCD correction, in particular when the lightest-neutralino mass is close to the top-quark mass.

hep-ph↗
arXiv2003

How Well Can We Reconstruct the ttbar System Near its Threshold at Future e+e- Linear Colliders?

We developed a new method for full kinematical reconstruction of the ttbar system near its threshold at future linear e+e- colliders. In the core of the method lies likelihood fitting which is designed to improve measurement accuracies of the kinematical variables that specify the final states resulting from ttbar decays. The improvement is demonstrated by applying this method to a Monte-Carlo ttbar sample generated with various experimental effects including beamstrahlung, finite acceptance and resolution of the detector system, etc. In most cases the fit brings a broad non-Gaussian distribution of a given kinematical variable to a nearly Gaussian shape, thereby justifying phenomenological analyses based on simple Gaussian smearing of parton-level momenta. The standard deviations of the resultant distributions of various kinematical variables are given in order to facilitate such phenomenological analyses. A possible application of the kinematical fitting method and its expected impact are also discussed.

hep-ph↗
arXiv2002

Kinematical Reconstruction of the ttbar System Near its Threshold at Future e+e- Linear Colliders

We developed a new method for full kinematical reconstruction of the ttbar system near its threshold at future linear e+e- colliders. In the core of the method lies likelihood fitting which is designed to improve measurement accuracies of the kinematical variables that specify the final states resulting from ttbar decays. The improvement is demonstrated by applying this method to a Monte-Carlo ttbar sample generated with various experimental effects including beamstrahlung, finite acceptance and resolution of the detector system, etc. A possible application of this method and its expected impact are also discussed.

hep-ex↗