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Mihoko M. Nojiri

Publications and source records attributed to Mihoko M. Nojiri.

At least 37 records · Page 2Linked to original sources
arXiv2016

Apparent unitarity violation in top quark's mass off-shell region from a new physics at high energy colliders

Perturbative unitarity conditions have been playing an important role by estimating the energy scale of new physics, including the Higgs mass as one of the most important examples. In this letter, we show that there is a possibility to see the hint of a new physics (top quark partner) indirectly by observing an "apparent" unitarity violation in $M_{bw}$ distribution well above top quark mass in a process of a heavy resonance decaying into a pair of top quarks.

hep-ph↗
arXiv2016

Heavy Fermion Bound States for Diphoton Excess at 750GeV $\sim$ Collider and Cosmological Constraints $\sim$

A colored heavy particle with sufficiently small width may form non-relativistic bound states when they are produced at the large hadron collider\,(LHC), and they can annihilate into a diphoton final state. The invariant mass of the diphoton would be around twice of the colored particle mass. In this paper, we study if such bound state can be responsible for the 750 GeV diphoton excess reported by ATLAS and CMS. We found that the best-fit signal cross section is obtained for the SU(2)$_L$ singlet colored fermion $X$ with $Y_X=4/3$. Having such an exotic hypercharge, the particle is expected to decay through some higher dimensional operators, consistent with the small width assumption. The decay of $X$ may involve a stable particle $χ$, if both $X$ and $χ$ are odd under some conserved $Z_2$ symmetry. In that case, the particle $X$ suffers from the constraints of jets + missing $E_T$ searches by ATLAS and CMS at 8 TeV and 13 TeV. We found that such a scenario still survives if the mass difference between $X$ and $χ$ is above $\sim$ 30 GeV for $m_X \sim 375$ GeV. Even assuming pair annihilation of $χ$ is small, the relic density of $χ$ is small enough if the mass difference between $X$ and $χ$ is smaller than $\sim$ 40 GeV.

hep-ph↗
arXiv2016

Prospects for Spin-1 Resonance Search at 13 TeV LHC and the ATLAS Diboson Excess

Motivated by ATLAS diboson excess around 2 TeV, we investigate a phenomenology of spin-1 resonances in a model where electroweak sector in the SM is weakly coupled to strong dynamics. The spin-1 resonances, W' and Z', are introduced as effective degrees of freedom of the dynamical sector. We explore several theoretical constraints by investigating the scalar potential of the model as well as the current bounds from the LHC and precision measurements. It is found that the main decay modes are V' -> VV and V' -> Vh, and the V' width is narrow enough so that the ATLAS diboson excess can be explained. In order to investigate future prospects, we also perform collider simulations at the 13 TeV LHC, and obtain a model independent expected exclusion limit for the process pp -> W' -> WZ -> JJ. We find a parameter space where the diboson excess can be explained, and are within a reach of the LHC at the integrated luminosity of 10 fb-1 and 13 TeV.

hep-ph↗
arXiv2015

Renormalization-Scale Uncertainty in the Decay Rate of False Vacuum

We study radiative corrections to the decay rate of false vacua, paying particular attention to the renormalization-scale dependence of the decay rate. The decay rate exponentially depends on the bounce action. The bounce action itself is renormalization scale dependent. To make the decay rate scale-independent, radiative corrections, which are due to the field fluctuations around the bounce, have to be included. We show quantitatively that the inclusion of the fluctuations suppresses the scale dependence, and hence is important for the precise calculation of the decay rate. We also apply our analysis to a supersymmetric model and show that the radiative corrections are important for the Higgs-stau system with charge breaking minima.

hep-ph↗
arXiv2015

Footprints of Supersymmetry on Higgs Decay

Motivated by future collider proposals that aim to measure the Higgs properties precisely, we study the partial decay widths of the lightest Higgs boson in the minimal supersymmetric standard model with an emphasis on the parameter region where all superparticles and heavy Higgs bosons are not accessible at the LHC. Taking account of phenomenological constraints such as the Higgs mass, flavor constraints, vacuum stability, and perturbativity of coupling constants up to the grand unification scale, we discuss how large the deviations of the partial decay widths from the standard model predictions can be. These constraints exclude large fraction of the parameter region where the Higgs widths show significant deviation from the standard model predictions. Nevertheless, even if superparticles and the heavy Higgses are out of the reach of 14TeV LHC, the deviation may be large enough to be observed at future $e^+e^-$ collider experiments.

hep-ph↗
arXiv2015

130GeV gamma-ray line through axion conversion

We apply the axion-photon conversion mechanism to the 130 GeV $γ$-ray line observed by the Fermi satellite. Near the Galactic center, some astrophysical sources and/or particle dark matter can produce energetic axions (or axionlike particles), and the axions convert to $γ$ rays in Galactic magnetic fields along their flight to the Earth. Since continuum $γ$-ray and antiproton productions are sufficiently suppressed in axion production, the scenario fits the 130 GeV $γ$-ray line without conflicting with cosmic ray measurements. We derive the axion production cross section and the decay rate of dark matter to fit the $γ$-ray excess as functions of axion parameters. In the scenario, the $γ$-ray spatial distributions depend on both the dark matter profile and the magnetic field configuration, which will be tested by future $γ$-ray observations, e.g., H.E.S.S. II, CTA, and GAMMA-400. As an illustrative example, we study realistic supersymmetric axion models, and show the favored parameters that nicely fit the $γ$-ray excess.

astro-ph.CO↗
arXiv2015

Associated jet and subjet rates in light-quark and gluon jet discrimination

We show that in studies of light quark- and gluon-initiated jet discrimination, it is important to include the information on softer reconstructed jets (associated jets) around a primary hard jet. This is particularly relevant while adopting a small radius parameter for reconstructing hadronic jets. The probability of having an associated jet as a function of the primary jet transverse momentum ($p_T$) and radius, the minimum associated jet $p_T$ and the association radius is computed upto next-to-double logarithmic accuracy (NDLA), and the predictions are compared with results from Herwig++, Pythia6 and Pythia8 Monte Carlos (MC). We demonstrate the improvement in quark-gluon discrimination on using the associated jet rate variable with the help of a multivariate analysis. The associated jet rates are found to be only mildly sensitive to the choice of parton shower and hadronization algorithms, as well as to the effects of initial state radiation and underlying event. In addition, the number of $k_T$ subjets of an anti-$k_T$ jet is found to be an observable that leads to a rather uniform prediction across different MC's, broadly being in agreement with predictions in NDLA, as compared to the often used number of charged tracks observable.

hep-ph↗
arXiv2014

Compressed SUSY search at the 13 TeV LHC using kinematic correlations and structure of ISR jets

The LHC search for nearly degenerate gluinos and neutralinos, which can occur, for example, in SUSY axion models, is limited by the reduced missing transverse momentum and effective mass in the events. We propose the use of kinematic correlations between jets coming from initial state radiation (ISR) in gluino pair production events at the 13 TeV LHC. A significant improvement in the signal to background ratio is obtained for the highly compressed gluino-neutralino search, by using cuts on the rapidity and azimuthal angle separation between the pair of tagged jets with the highest transverse momenta. Furthermore, the distribution of the azimuthal angle difference between the tagged jets in the gluino-pair+jets process is found to be distinctly different from the dominant background process of Z+jets. We also find quark and gluon jet tagging methods to be useful in separating the signal, which contains a higher fraction of gluon initiated jets compared to the dominant backgrounds.

hep-ph↗
arXiv2014

CP violation in $B\to Dτν_τ$ using multi-pion tau decays

Present experimental datas have shown a 3.8$σ$ level discrepancy with the standard model in $\overline{B}\to D^{(*)}τ\barν_τ$. Some new physics models have been considered to explain this discrepancy possibly with new source of the CP violation. In this paper, we construct CP violating observables by using multi-pion decays in $B\to Dτν_τ$, and estimate sensitivity of these observables to generic CP violating operators. We also discuss possibilities of CP violation in leptoquark models and in 2HDM of type-III.

hep-ph↗
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.

hep-ph↗
arXiv2013

Phenomenology of Light Fermionic Asymmetric Dark Matter

Asymmetric dark matter (ADM) has been an attractive possibility attempting to explain the observed ratio of baryon to dark matter abundance in the universe. While a bosonic ADM is constrained by the limits from existence of old neutron stars, a fermionic ADM requires an additional light particle in order to annihilate its symmetric component in the early universe. We revisit the phenomenology of a minimal GeV scale fermionic ADM model including a light scalar state. The current constraints on this scenario from cosmology, dark matter direct detection, flavour physics and collider searches are investigated in detail. We estimate the future reach on the model parameter space from next-generation dark matter direct detection experiments, Higgs boson property measurements and search for light scalars at the LHC, as well as the determination of Higgs invisible branching ratio at the proposed ILC.

hep-ph↗
arXiv2013

Physics Case for the ILC Project: Perspective from Beyond the Standard Model

The International Linear Collider (ILC) has recently proven its technical maturity with the publication of a Technical Design Report, and there is a strong interest in Japan to host such a machine. We summarize key aspects of the Beyond the Standard Model physics case for the ILC in this contribution to the US High Energy Physics strategy process. On top of the strong guaranteed physics case in the detailed exploration of the recently discovered Higgs boson, the top quark and electroweak precision measurements, the ILC will offer unique opportunities which are complementary to the LHC program of the next decade. Many of these opportunities have connections to the Cosmic and Intensity Frontiers, which we comment on in detail. We illustrate the general picture with examples of how our world could turn out to be and what the ILC would contribute in these cases, with an emphasis on value-added beyond the LHC. These comprise examples from Supersymmetry including light Higgsinos, a comprehensive bottom-up coverage of NLSP-LSP combinations for slepton, squark, chargino and neutralino NLSP, a stau-coannihilation dark matter scenario and bilinear R-parity violation as explanation for neutrino masses and mixing, as well as generic WIMP searches and Little Higgs models as non-SUSY examples.

hep-ph↗
arXiv2013

Model Independent Analysis of Interactions between Dark Matter and Various Quarks

Present and future expected limits on interactions between dark matter and various quarks are thoroughly investigated in a model-independent way. In particular, the constraints on the interactions from the Large Hadron Collider (LHC) experiment are carefully considered with focusing on mono jet + missing transverse energy, mono b-jet + missing transverse energy, and top quark(s) + missing transverse energy channels. Model-independent upper limits (expected limits) on the cross section times acceptance for non-standard model events are derived at 7 TeV (8 or 14 TeV) running of the LHC experiment. With assuming that the dark matter is a singlet real scalar or a singlet Majorana fermion, we also put constraints on several operators describing its interactions with up, down, strange, charm, bottom and top quarks. These constraints are compared to those obtained by cosmological and astrophysical observations of the dark matter.

hep-ph↗
arXiv2013

Testing Little Higgs Mechanism at Future Colliders

In the framework of the little higgs scenario, coupling constants of several interactions are related to each other to guarantee the stability of the higgs boson mass at one-loop level. This relation is called the little higgs mechanism. We discuss how accurately the relation can be tested at future e+e- colliders, with especially focusing on the top sector of the scenario using a method of effective lagrangian. In order to test the mechanism at the top sector, it is important to measure the Yukawa coupling of the top partner. We consider higgs associated production and threshold production of the top partner, and find that the mechanism can be tested precisely using the associate production when the center of mass energy is large enough. The threshold production also allows us to test it even if the center mass energy is not so large.

hep-ph↗
arXiv2012

Search for the Top Partner at the LHC using Multi-b-Jet Channels

Vector-like quarks are introduced in various new physics models beyond the standard model (SM) at the TeV scale. We especially consider the case that the quark is singlet (triplet) under the SU(2)$_L$ (SU(3)$_c$) gauge group and couples only to the third generation quarks of the SM. The vector-like quark of this kind is often called a top partner. The top partoner $t_p$ decays into $bW, tZ$ and $th$. In the ATLAS and CMS collaborations, the top partner has been searched in the final states of $bW$ and $tZ$, while the search based on the decay mode $t_p\to th$ has not been started yet. However, the decay into $th$ is important since it is significantly enhanced if some strong dynamics exists in the TeV scale. In the presence of a light higgs boson, the decay mode $t_p\to th$ followed by $h\to b\bar{b}$ produces three bottom quarks. We study the sensitivity for the top partner using multi-b-jet events at the 8 TeV run of the LHC experiment. The multi-b-jet eventss turn out to play a complementary role to the existing $t_p\rightarrow bW$ and $tZ$ searches by the ATLAS and CMS collaborations.

hep-ph↗
arXiv2011

Improved discovery of a nearly degenerate model: MUED using MT2 at the LHC

We study the discovery potential of the Minimal Universal Extra Dimension model (MUED) and improve it utilizing the multijet + lepton mode at the LHC. Since the MUED has a nearly degenerate spectrum, most events only have soft jets and small Emiss. The signature is challenging to search. We apply MT2 for the event selection and set the invisible particle mass of MT2 (test mass) to zero. The test mass is much smaller than the invisible particle mass of MUED. In that case, MT2 of the signal can be large depending on Up-Stream Radiations (USR) which includes initial state radiations (ISR). On the other hand, MT2 of the background is mainly below the top quark mass. Hence, the signal is extracted from the background in the high MT2 region. Since we use the leading jets for MT2, there is a combinatorics effect. We found the effect also enhances the signal to background ratio for high MT2. We perform a detailed simulation with the Matrix Element correction to the QCD radiations. The discovery potential of the MUED is improved by the MT2 cut, and especially, the improvement is significant for the most degenerate parameter we consider, ΛR = 10.

hep-ph↗
arXiv2011

Mass measurement in boosted decay systems at hadron colliders

We report a new possibility of using the $\mct2$ (Constransverse mass) variable for mass measurement in single step decay chains involving missing particles with moderate transverse momentum. We show that its experimental feasibility is enhanced compared to the corresponding $\mt2$-kink method. We apply this method to reconstruct a pair of chargino decay chains.

hep-ph↗