arXiv subjects

Kaoru Hagiwara

Publications and source records attributed to Kaoru Hagiwara.

At least 55 records · Page 3Linked to original sources

arXiv2011

The MSSM confronts the precision electroweak data and the muon g-2

We update the electroweak study of the predictions of the Minimal Supersymmetric Standard Model (MSSM) including the recent results on the muon anomalous magnetic moment, the weak boson masses, and the final precision data on the Z boson parameters from LEP and SLC. We find that the region of the parameter space where the slepton masses are a few hundred GeV is favored from the muon g-2 for \tanβ\ltsim 10, whereas for \tanβ\simeq 50 heavier slepton mass up to \sim 1000 GeV can account for the reported 3.2 σdifference between its experimental value and the Standard Model (SM) prediction. As for the electroweak measurements, the SM gives a good description, and the sfermions lighter than 200 GeV tend to make the fit worse. We find, however, that sleptons as light as 100 to 200 GeV are favored also from the electroweak data, if we leave out the jet asymmetry data that do not agree with the leptonic asymmetry data. We extend the survey of the preferred MSSM parameters by including the constraints from the b \to s γtransition, and find favorable scenarios in the minimal supergravity, gauge-, and mirage-mediation models of supersymmetry breaking.

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arXiv2011

The earth matter effects in neutrino oscillation experiments from Tokai to Kamioka and Korea

We study the earth matter effects in the Tokai-to-Kamioka-and-Korea experiment (T2KK), which is a proposed extension of the T2K (Tokai-to-Kamioka) neutrino oscillation experiment between J-PARC at Tokai and Super-Kamiokande (SK) in Kamioka, where an additional detector is placed in Korea along the same neutrino beam line.By using recent geophysical measurements, we examine the earth matter effects on the oscillation probabilities at Kamioka and Korea. The average matter density along the Tokai-to-Kamioka baseline is found to be 2.6 g/cm^3, and that for the Tokai-to-Korea baseline is 2.85, 2.98, and 3.05 g/cm^3 for the baseline length of L = 1000, 1100, and 1200 km, respectively. The uncertainty of the average density is about 6%, which is determined by the uncertainty in the correlation between the accurately measured sound velocity and the matter density. The effect of the matter density distribution along the baseline is studied by using the step function approximation and the Fourier analysis. We find that the nu_mu -> nu_e oscillation probability is dictated mainly by the average matter density, with small but non-negligible contribution from the real part of the first Fourier mode. We also find that the sensitivity of the T2KK experiment on the neutrino mass hierarchy does not improve significantly by reducing the matter density error from 6% to 3%, since the measurement is limited by statistics for the minimum scenario of T2KK with SK at Kamioka anda 100 kt detector in Korea considered in this report. The sensitivity of the T2KK experiment on the neutrino mass hierarchy improves significantly by splitting the total beam time into neutrino and anti-neutrino runs, because the matter effect term contributes to the oscillation amplitudes with the opposite sign.

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arXiv2011

Simulating graviton production at hadron colliders

Spin-2 particles and in particular gravitons are predicted in many new physics scenarios at the TeV scale. Depending on the details of models such new states might show up as a continuum, massless particles, or TeV scale resonances. Correspondingly, very different discovery signatures should be exploited, from the search of excesses in events with multi jets and large missing transverse energy, to resonances in weak boson or jet pair productions. We present a very general and flexible implementation in MadGraph/MadEvent of spin-2 particles interacting with the standard model particles via the energy momentum tensor, which encompasses all of the most popular TeV scale models featuring gravitons. By merging matrix elements with parton shower, we can generate inclusive samples of graviton+jets at the hadron colliders in several scenarios (ADD, zero-mass graviton and RS). We compare and validate our results against the corresponding next-to-leading order QCD calculations.

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arXiv2011

Generating QCD amplitudes in the color-flow basis with MadGraph

We propose to make use of the off-shell recursive relations with the color-flow decomposition in the calculation of QCD amplitudes on MadGraph. We introduce colored quarks and their interactions with nine gluons in the color-flow basis plus an Abelian gluon on MadGraph, such that it generates helicity amplitudes in the color-flow basis with off-shell recursive formulae for multi-gluon sub-amplitudes. We demonstrate calculations of up to 5-jet processes such as $gg\rightarrow 5g$, $u\bar{u}\rightarrow 5g$ and $uu\rightarrow uuggg$. Although our demonstration is limited, it paves the way to evaluate amplitudes with more quark lines and gluons with Madgraph.

hep-ph↗

arXiv2010

Antiproton Flux in Cosmic Ray Propagation Models with Anisotropic Diffusion

Recently a cosmic ray propagation model has been introduced, where anisotropic diffusion is used as a mechanism to allow for $\mathcal{O}(100)$ km/s galactic winds. This model predicts a reduced antiproton background flux, suggesting an excess is being observed. We implement this model in GALPROP v50.1 and perform a $χ^2$ analysis for B/C, $^{10}$Be/$^{9}$Be, and the recent PAMELA $\bar{p}/p$ datasets. By introducing a power-index parameter $α$ that dictates the dependence of the diffusion coefficient $D_{xx}$ on height $|z|$ away from the galactic plane, we confirm that isotropic diffusion models with $α=0$ cannot accommodate high velocity convective winds suggested by ROSAT, while models with $α=1$ ($D_{xx}\propto |z|$) can give a very good fit. A fit to B/C and $^{10}$Be/$^{9}$Be data predicts a lower $\bar{p}/p$ flux ratio than the PAMELA measurement at energies between approximately 2 GeV to 20 GeV. A combined fit including in addition the $\bar{p}/p$ data is marginal, suggesting only a partial contribution to the measured antiproton flux.

astro-ph.HE↗

arXiv2010

Angular distribution of thrust axis with power-suppressed contribution in e+e- annihilation

Structure function of e+e- to hadrons cross section proportional to the longitudinal part of the hadron tensor is power suppressed with respect to an event shape variable in the two-jet region. In the SCET framework, we study the event shape distribution for this structure function to NLL level of accuracy. As, a result we obtain the angular distribution of hadron jets as a function of the thrust, in the two jet region. We further examine effects of non-perturbative hadronization corrections by adopting a shape function that reproduce the observed event shape distributions. Impacts of our findings on the electroweak measurements via the jet angular forward-backward asymmetry are discussed.

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arXiv2009

Testing supersymmetry at the LHC through gluon-fusion production of a slepton pair

Renormalizable quartic couplings among new particles are typical of supersymmetric models. Their detection could provide a test for supersymmetry, discriminating it from other extensions of the Standard Model. Quartic couplings among squarks and sleptons, together with the SU(3) gauge couplings for squarks, allow a new realization of the gluon-fusion mechanism for pair-production of sleptons at the one-loop level. The corresponding production cross section, however, is at most of ${\cal O}(1) $fb for slepton and squark masses of ${\cal O}(100) $GeV. We then extend our investigation to the gluon-fusion production of sleptons through the exchange of Higgs bosons. The cross section is even smaller, of ${\cal O}(0.1) $fb, if the exchanged Higgs boson is considerably below the slepton-pair threshold, but it is enhanced when it is resonant. It can reach ${\cal O}(10) $fb for the production of sleptons of same-chirality, exceeding these values for $\widetildeτ$'s of opposite-chirality, even when chirality-mixing terms in the squark sector are vanishing. The cross section can be further enhanced if these mixing terms are nonnegligible, providing a potentially interesting probe of the Higgs sector, in particular of parameters such as $A$, $μ$, and $\tanβ$.

hep-ph↗

arXiv2009

Bound-state effects on gluino-pair production at hadron colliders

We study bound-state effects on the pair production of gluinos at hadron colliders, in a context of the minimal supersymmetric extension of the standard model. Due to the expected large mass and the octet color-charge of gluinos, the bound-state effects can be substantial at the LHC. We find significant deformation of the invariant-mass distributions of a gluino-pair near the mass threshold, as well as an additional correction to the total cross-section. Both the invariant-mass distribution and the correction to the total cross section depend crucially on the decay width of the gluino.

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arXiv2009

Jet angular correlation in vector-boson fusion processes at hadron colliders

Higgs boson and massive-graviton productions in association with two jets via vector-boson fusion (VBF) processes and their decays into a vector-boson pair at hadron colliders are studied. They include scalar and tensor boson production processes via weak-boson fusion in quark-quark collisions, gluon fusion in quark-quark, quark-gluon and gluon-gluon collisions, as well as their decays into a pair of weak bosons or virtual gluons which subsequently decay into $\ell\bar\ell$, $q\bar q$ or $gg$. We give the helicity amplitudes explicitly for all the VBF subprocesses, and show that the VBF amplitudes dominate the exact matrix elements not only for the weak-boson fusion processes but also for all the gluon fusion processes when appropriate selection cuts are applied, such as a large rapidity separation between two jets and a slicing cut for the transverse momenta of the jets. We also show that our off-shell vector-boson current amplitudes reduce to the standard quark and gluon splitting amplitudes with appropriate gluon-polarization phases in the collinear limit. Nontrivial azimuthal angle correlations of the jets in the production and in the decay of massive spin-0 and -2 bosons are manifestly expressed as the quantum interference among different helicity states of the intermediate vector-bosons. Those correlations reflect the spin and the CP nature of the Higgs bosons and the massive gravitons.

hep-ph↗

arXiv2009

Re-evaluation of the T2KK physics potential with simulations including backgrounds

The Tokai-to-Kamioka-and-Korea (T2KK) neutrino oscillation experiment under examination can have a high sensitivity to determine the neutrino mass hierarchy for a combination of relatively large (\sim 3.0^circ) off-axis angle beam at Super-Kamiokande (SK) and small (\sim 0.5^circ) off-axis angle at L \sim 1,000km in Korea. We elaborate previous studies by taking into account smearing of reconstructed neutrino energy due to finite resolution of electron or muon energies, nuclear Fermi motion and resonance production, as well as the neutral current π^0 production background to the nu_mu to nu_e oscillation signal. It is found that the mass hierarchy pattern can still be determined at 3sigma level if sin^22thata_rct \quiv 4|U_{e3}|^2(1-|U_{e3}|^2) \gsim 0.08(0.09) when the hierarchy is normal (inverted) with 5 \times10^{21} POT exposure, or 5 years of the T2K experiment, if a 100kton water cherenkov detector is placed in Korea. The π^0 backgrounds deteriorate the capability of the mass hierarchy determination, whereas the events from nuclear resonance productions contribute positively to the hierarchy discrimination power. We also find that the π^0 backgrounds seriously affect the CP phase measurement. Although δ_mns can still be constrained with an accuracy of \sim \pm 45^circ (\pm 60^circ) at 1sigma level for the normal (inverted) hierarchy with the above exposure if sin^22theta_rct \gsim 0.04, CP violation can no longer be established at 3sigma level even for delta_mns=\pm 90^circ and sin^22theta_rct=0.1. About four times higher exposure will be needed to measure delta_mns with \pm 30^circ accuracy.

hep-ph↗

arXiv2008

Measuring the Higgs-Vector boson Couplings at Linear $e^{+} e^{-}$ Collider

We estimate the accuracy with which the coefficient of the CP even dimension six operators involving Higgs and two vector bosons (HVV) can be measured at linear $e^+ e^-$ colliders. Using the optimal observables method for the kinematic distributions, our analysis is based on the five different processes. First is the WW fusion process in the t-channel ($e^+e^- \to \barν_e ν_e H$), where we use the rapidity y and the transverse momentum $\pT$ of the Higgs boson as observables. Second is the ZH pair production process in the s-channel, where we use the scattering angle of the Z and the Z decay angular distributions, reproducing the results of the previous studies. Third is the t-channel ZZ, fusion processes ($e^+e^- \to e^+e^ -H$), where we use the energy and angular distributions of the tagged $e^+$ and $e^-$. In the fourth, we consider the rapidity distribution of the untagged $e^+e^-H$ events, which can be approximated well as the $γγ$ fusion of the bremsstrahlung photons from $e^+$ and $e^-$ beams. As the last process,we consider the single tagged $e^+e^- H$ events, which probe the $γe^{\pm} \to H e^{\pm}$ process. All the results are presented in such a way that statistical errors of the constraints on the effective couplings and their correlations are read off when all of them are allowed to vary simultaneously, for each of the above processes, for $m_H=120 $ GeV, at $\sqrt{s}=250\GEV$, $350\GEV$ $500\GEV$ and $1\TEV$, with and without $e^-$ beam polarization of 80%.

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↗

arXiv2008

HELAS and MadGraph/MadEvent with spin-2 particles

Fortran subroutines to calculate helicity amplitudes with massive spin-2 particles (massive gravitons), which couple to the standard model particles via the energy momentum tensor, are added to the {\tt HELAS} ({\tt HEL}icity {\tt A}mplitude {\tt S}ubroutines) library. They are coded in such a way that arbitrary scattering amplitudes with one graviton production and its decays can be generated automatically by {\tt MadGraph} and {\tt MadEvent}, after slight modifications. All the codes have been tested carefully by making use of the invariance of the helicity amplitudes under the gauge and general coordinate transformations.

hep-ph↗

arXiv2008

Graviton production with 2 jets at the LHC in large extra dimensions

We study Kaluza-Klein (KK) graviton production in the large extra dimensions model via 2 jets plus missing transverse momentum signatures at the LHC. We make predictions for both the signal and the dominant Zjj and Wjj backgrounds, where we introduce missing P_T-dependent jet selection cuts that ensure the smallness of the 2-jet rate over the 1-jet rate. With the same jet selection cuts, the distributions of the two jets and their correlation with the missing transverse momentum provide additional evidence for the production of an invisible massive object.

hep-ph↗

arXiv2007

T-odd asymmetries in radiative top-quark decays

We study the angular distribution of the charged lepton in the top-quark decay into a bottom quark and a W boson which subsequently decays into \ellν_{\ell}, when a hard gluon is radiated off. The absorptive part of the t \to bWg decay amplitudes, which gives rise to T-odd asymmetries in the distribution, is calculated at the one-loop level in perturbative QCD. The asymmetries at a few percent level are predicted, which may be observable at future colliders.

hep-ph↗

arXiv2007

Constraints on the electroweak chiral Lagrangian from the precision data

In the framework of the effective field theory method, we use the experimental data and the perturbative unitarity bounds to determine the values and uncertainty of all the 11 chiral coefficients ($\al_i, i=0, ..., 10$) of the standard electroweak chiral Lagrangian. Up to linear terms in $\al_i$, we provide the one-loop renormalization group equations of all the chiral coefficients, which are calculated in the Feynman-'t Hooft gauge using the modified minimal subtraction scheme. With the improved renormalization group equations to sum over the logarithmic corrections, we analyze the current experimental uncertainty of oblique correction parameters, $S(Λ)$ and $T(Λ)$. We find that, due to the large uncertainty in the triple gauge-boson coupling measurements, the parameter space of positive $S(Λ)$ for $Λ> 1$ TeV is still allowed by the current experimental data. $T(Λ)$ tends to increase with $Λ$ even in the presence of the operators that contribute to the triple and quartic gauge-boson couplings.

hep-ph↗

arXiv2007

Physics potential of T2KK: An extension of the T2K neutrino oscillation experiment with a far detector in Korea

We study physics potential of placing a far detector in the east coast of Korea, where the off-axis beam from J-PARC at Tokai village for the T2K project has significant intensity at a few GeV range. In particular we examine the capability of determining the mass hierarchy pattern and the CP phase of the lepton-flavor-mixing matrix when a 100 kt water Cerenkov detector is placed at various locations in Korea for the off-axis beam (OAB) of 2.5 deg. and 3 deg. at the Super-Kamiokande site. The best results are found for a combination of 3 deg. OAB at SK (L = 295 km) and 0.5 deg. OAB at L = 1000km, where the mass hierarchy pattern can be determined at 3-σlevel for sin^2 2θ_{RCT} \simgt 0.05 (0.06) when the hierarchy is normal (inverted), after 5 years of running (5 \times 10^{21}POT). We also find that the leptonic CP phase, δ_{MNS}, can be constrained uniquely, without invoking anti-neutrino beams, as long as the mass hierarchy pattern is determined. Those results are obtained by assuming that the charged current quasi-elastic events can be separated from the other backgrounds with high efficiency, the neutrino energy can be reconstructed with a hundred MeV uncertainty, and the earth matter density along the baseline can be determined with 3% accuracy.

hep-ph↗

arXiv2006

Parity-Odd Asymmetries in W-Jet Events at the Tevatron

Parity-odd asymmetries in the decay angular distribution of a W boson produced with a hard jet in p\bar{p} collisions arise only from QCD rescattering effects. If observed, these asymmetries will provide a first demonstration that perturbative QCD calculation is valid for the absorptive part of scattering amplitudes. We propose a simple observable to measure these asymmetries and perform realistic Monte Carlo simulations at Tevatron energies. It is shown that the Tevatron Run-II should provide sufficient statistics to test the prediction.

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