Searcharxiv⌕ Search

arXiv subjects

Mihoko M. Nojiri

Publications and source records attributed to Mihoko M. Nojiri.

At least 73 records · Page 4Linked to original sources
arXiv2006

Hunting for the Top Partner in the Littlest Higgs Model with T-parity at the LHC

We study the Littlest Higgs model with T-parity at the LHC through pair productions of the T-odd top quark partner (T_-) which decays into the top quark and the lightest T-odd particle. We identify the region of parameters favored by the electroweak and cosmological considerations. The signal and background events are simulated with fast detector simulation to study the discovery potential at the LHC. We find that the hemisphere analysis recently proposed by the CMS collaboration is very useful to separate the signal from the t\bar{t} background. We discuss the observability of the top tagged signal in the effective mass (M_{eff}) versus the transverse missing energy (E_{Tmiss}) plane. We show that, for all our sample parameter sets with M_{T_-} \leq 900 GeV, the excess of the signal over the background can be visible as a bump structure in the E_{Tmiss} distribution for 50 fb^{-1} at relatively high M_{eff} intervals.

hep-ph↗
arXiv2006

Prospects to study a long-lived charged next lightest supersymmetric particle at the LHC

If the scalar tau $\stau$ is the next lightest supersymmetric particle and decays into a gravitino ($\gravitino$) being the lightest supersymmetric particle, it will have generally a very long lifetime. In this paper, we investigate the possibility to study the decay of such a long lived scalar tau at the LHC. If we can add to the present LHC experiments additional detectors which are able to stop the stau particles and measure the produced decay products, the decay characteristics can be studied precisely at the LHC. We identify a maximum "stopper detector" that could be added in the CMS cavern, and estimate the sensitivity to the lifetime of the stau and to the mass of gravitino with this detector. The decay of the scalar tau may be significantly modified if the decay channel to the axino $\axino$ is open. We study the possibility to distinguish such decays from decays into gravitinos by measuring the process $\stau\to \axino (\gravitino) τγ$ using the stopper detector.

hep-ph↗
arXiv2006

Discovery of supersymmetry with degenerated mass spectrum

Discovery of supersymmetric (SUSY) particles at the Large Hadron Collider(LHC) has been studied for the models where squarks and gluino are much heavier than the lightest supersymetric particle (LSP). In this paper, we investigate the SUSY discovery in the models with degenerated mass spectrum up to m_LSP~0.7 m_sq. Such mass spectrum is predicted in certain parameter region of the mixed modulas anomaly mediation (MMAM) model. We find that the effective transverse mass of the signal for the degenerated parameters shows the distribution similar to that of the background. Experimental sensitivity of the SUSY particles at the LHC therefore depends on the uncertainty of the background in this class of model. We also find that SUSY signal shows an interesting correlation between M_eff and ETmiss which may be used to determine the signal region properly to enhance the S/N ratio even if the sparticle masses are rather degenerated. The structure is universal for the models with new heavy colored particles decaying into visible particles and a stable neutral particle, dark matter.

hep-ph↗
arXiv2006

Investigation of Possible Dark Matter Direct Detection in Electron Accelerators

We investigate a possibility of neutralino dark matter (DM) direct detection in the future electron accelerators. That is counting of high p_T electron recoil events by neutralinos in halo. If selectron and neutralino masses would be precisely measured in future collider experiments, the beam energy could be tuned so that the scatterings are dominated by on-pole selectron exchange. When selectron and neutralino mass difference is smaller than O(10) GeV, the elastic cross section exceeds over micro barn. Discovery of the high p_T electron events would be a firm prove of the neutralino DM component in halo. In the experiment, the electron beam energy must be tuned within O(10) MeV and the electron beam with high currents of O(100)A is required for the detectors of the total length of a few hundred meters so that the sufficient event rate is obtained. The dependence of the event rate on the DM velocity distribution in halo is also discussed. This method might be applicable to other DM candidates.

hep-ph↗
arXiv2005

Non-Perturbative Effect on Dark Matter Annihilation and Gamma Ray Signature from Galactic Center

Detection of gamma rays from dark matter annihilation in the galactic center is one of the feasible techniques to search for dark matter. We evaluate the gamma ray flux in the case that the dark matter has an electroweak SU(2)_L charge. Such dark matter is realized in the minimal supersymmetric standard model (MSSM) when the lightest SUSY particle is the Higgsino- or Wino-like neutralino. When the dark matter is heavy compared to the weak gauge bosons, the leading-order calculation of the annihilation cross sections in perturbation breaks down due to a threshold singularity. We take into account non-perturbative effects by using the non-relativistic effective theory for the two-body states of the dark matter and its SU(2)_L partner(s), and evaluate precise cross sections relevant to the gamma ray fluxes. We find that the annihilation cross sections may be enhanced by several orders of magnitude due to resonances when the dark matter mass is larger than 1 TeV. Furthermore, the annihilation cross sections in the MSSM may be changed by factors even when the mass is about 500 GeV. We also discuss sensitivities to gamma ray signals from the galactic center in the GLAST satellite detector and the large Air Cerenkov Telescope arrays.

hep-ph↗
arXiv2005

Mass and Cross-section Measurements of Chargino at Linear Colliders in Large tan(beta) Case

The lighter chargino pair production ($e^+e^- \to \tildeχ^{+}_{1} \tildeχ^{-}_{1}$) is one of key processes for determination of supersymmetric parameters at a linear collider. If $\tanβ$ is a large value, the lighter stau ($\tildeτ^{\pm}_{1}$) might be lighter than the $\tildeχ^{\pm}_{1}$, while the other sleptons stay heavier. This case leads a cascade decay, $\tildeχ^{\pm}_{1} \to \tildeτ^{\pm}_{1} ν_τ$ followed by $\tildeτ^{\pm}_{1} \to \tildeχ^{0}_{1} τ^{\pm}$, where $\tildeχ^{0}_{1}$ is the lightest supersymmetric particle. This paper addresses to what extent this cascade decay may affect the measurements of the chargino mass and its production cross section.

hep-ph↗
arXiv2005

Direct Detection of the Wino- and Higgsino-like Neutralino Dark Matters at One-Loop Level

The neutralino-nucleon (\tildeχ^0-N) scattering is an important process for direct dark matter searches. In this paper we discuss one-loop contributions to the cross section in the wino-like and Higgsino-like LSP cases. The neutralino-nucleon scattering mediated by the Higgs \tildeχ^0\tildeχ^0 and Z\tildeχ^0\tildeχ^0 couplings at tree level is suppressed by the gaugino-Higgsino mixing at tree level when the neutralino is close to a weak eigenstate. The one-loop contribution to the cross section, generated by the gauge interaction, is not suppressed by any SUSY particle mass or mixing in the wino- and Higgsino-like LSP cases. It may significantly alter the total cross section when σ_{\tildeχ^0 N}\sim 10^{-45} cm^2 or less.

hep-ph↗
arXiv2004

Supersymmetry at the LHC and Beyond --Ultimate Targets--

At the LHC, superpartners with the masses lighter than a few TeV may be found, and the masses of the supersymmetric (SUSY) particles and their interactions will be studied. The information would be the base to consider the SUSY breaking mechanism and cosmology related to the SUSY dark matter. We review the recent studies that aims for the ``precise SUSY study'' at the LHC.

hep-ph↗
arXiv2004

A study of late decaying charged particles at future colliders

In models where the gravitino is the lightest supersymmetric particle (LSP), the next-to-lightest supersymmetric particle (NLSP) is long-lived. We consider an important charged NLSP candidate, the scalar tau $\tildeτ$. Slow charged NLSPs may be produced at future colliders and they may be stopped in a massive stopper which simultaneously serves as a detector for the NLSP and its decay products. We found the number of events at a 1 kton to O(10) kton detector could be significant enough to study the NLSP decays with lifetime shorter than $10^{10}$ sec at the LHC. The performance of existing 1 kton detectors may be good enough to do such studies at the LHC, if they can be placed close to the ATLAS/CMS detectors. At a future $e^- e^-$ collider, scalar electrons $\tilde{e}^-$s are copiously produced. Slow NLSPs may be produced from the $\tilde{e}^-$ decay. The number of stopped NLSPs at a future linear collider could be large enough to study rare decay modes of the NLSP.

hep-ph↗
arXiv2003

Study of the Gauge Mediation Signal with Non-pointing Photons at the CERN LHC

In this paper we study the gauge mediation signal with the ATLAS detector at the CERN LHC. We focus on the case where the NLSP is the long-lived lightest neutralino ($\tildeχ^0_1$) which decays dominantly into a photon ($γ$) and a gravitino ($\tilde{G}$). A non-pointing photon from the neutralino decay can be detected with good position and time resolutions by the electormagnetic calorimeter (ECAL), while the photon momentum would be precisely measured if the photon is converted inside the inner tracking detector before reaching the ECAL. A new technique is developed to determine the masses of the slepton ($\tilde{\ell}$) and the neutralino from events with a lepton and a converted non-pointing photon arising from the cascade decay $\tilde{\ell}\to \ell\tildeχ^0_1\to \ellγ\tilde{G}$. A Monte Carlo simulation at a sample point shows that the masses would be measured with an error of 3% for $\cal{O}$(100) selected $\ellγ$ pairs. Once the sparticle masses are determined by this method, the decay time and momentum of the neutralino are solved using the ECAL data and the lepton momentum only, for all $\ellγ$ pairs without the photon conversion. We estimate the sensitivity to the neutralino lifetime for $cτ=10$ cm to $\cal{O}$(10) m.

hep-ph↗
arXiv2003

Explosive Dark Matter Annihilation

If the Dark Matter (DM) in the Universe has interactions with the standard-model particle, the pair annihilation may give the imprints in the cosmic ray. In this paper we study the pair annihilation processes of the DM, which is neutral, however has the electroweak (EW) gauge non-singlet. In this estimation the non-relativistic (NR) effective theory in the EW sector is a suitable technique. We find that if the DM mass is larger than about 1 TeV, the attractive Yukawa potentials induced by the EW gauge bosons have significant effects on the DM annihilation processes, and the cross sections may be enhanced by several orders of magnitude, due to the zero energy resonance under the potentials. Especially, the annihilation to two $γ$'s might have a comparable cross section to other tree-level processes, while the cross section under the conventional calculation is suppressed by a loop factor. We also discuss future sensitivities to the $γ$ ray from the galactic center by the GLAST satellite detector and the Air Cerenkov Telescope (ACT) arrays.

hep-ph↗
arXiv2003

Study of stop and sbottom at LHC

In supersymmetric models a gluino can decay into $tb\tildeχ^{\pm}_1$ through a stop or a sbottom. The decay chain produces an edge structure in the $m_{tb}$ distribution. Monte Carlo simulation studies show that the end point and the edge height would be measured at the CERN LHC by using a sideband subtraction technique. The stop and sbottom masses as well as their decay branching ratios are constrained by the measurement. We study interpretations of the measurement.

hep-ph↗
arXiv2003

A Detailed Study of the Gluino Decay into the Third Generation Squarks at the CERN LHC

In supersymmetric models a gluino can decay into tb\tildeχ^{\pm}_1 through a stop or a sbottom. The decay chain produces an edge structure in the m_{tb} distribution. Monte Carlo simulation studies show that the end point and the edge height would be measured at the CERN LHC by using a sideband subtraction technique. The stop and sbottom masses as well as their decay branching ratios are constrained by the measurement. We study interpretations of the measurement in the minimal supergravity model. We also study the gluino decay into tb and \tildeχ^{\pm}_2 as well as the influence of the stop left-right mixing on the m_{bb} distribution of the tagged $tb$ events.

hep-ph↗
arXiv2003

Unitarity and Higher-Order Corrections in Neutralino Dark Matter Annihilation into Two Photons

The neutralino pair annihilation into two photons in our galactic halo gives a robust dark matter signal, since it would give a quasi-monotonic gamma ray. This process is radiatively-induced, and the full-one loop calculation was done previously. However, for the heavy wino-like or Higgsino-like neutralino, the one-loop cross section violates unitarity, therefore the higher-order corrections may be important. We construct a non-relativistic theory for chargino and neutralino two-body states, and estimate all-order QED corrections and two-loop corrections by $Z$ and/or $W$ exchange. We find that the critical mass, above that the two-loop contribution is larger than one-loop one, is about 8 TeV (O(10) TeV) in the limit where neutralino is wino (Higgsino)-like, respectively. Around and above the critical mass, the all-order Z and/or W exchange must be included to estimate the cross section. On the other hand, the QED corrections depend on the mass difference between the neutralino and chargino. In the wino-like limit where neutralino is highly degenerate with chargino in mass, we find that QED corrections enhance the pair annihilation cross section by 1.5-2.

hep-ph↗
arXiv2002

Supersymmetry and the Linear Collider

We present a pedagogical introduction to supersymmetry and supersymmetric models and give an overview of the potential of the linear collider for studying them. If supersymmetry is found, its discovery will bring with it many more questions than answers. These include: -Are the newly discovered particles really superpartners? -If not all superpartners are discovered, where are the rest of them? -Do the electromagnetic, weak, and strong forces unify? -Is a supersymmetric particle the dark matter? -How are the supersymmetric flavor and CP problems solved? -What is the scale of supersymmetry breaking? -What are the fundamental interactions at the Planck scale? We review how the linear collider will provide definitive answers to some of these and may shed light on the rest, even if only one or a few superpartners are kinematically accessible.

hep-ph↗
arXiv2002

Scenery from the Top: Study of the Third Generation Squarks at CERN LHC

In the minimal supersymmetric standard model (MSSM) properties of the third generation sfermions are important from the viewpoint of discriminating the SUSY breaking models and in the determination of the Higgs boson mass. If gluinos are copiously produced at CERN LHC, gluino decays into tb through stop and sbottom can be studied using hadronic decays of the top quark. The kinematical endpoint of the gluino decays can be evaluated using a W sideband method to estimate combinatorial backgrounds. This implies that fundamental parameters related to the third generation squarks can be reliably measured. The top-quark polarization dependence in the decay process may also be extracted by looking at the b jet distribution near the kinematical endpoint.

hep-ph↗
arXiv2002

Slepton Oscillation at Large Hadron Collider

Measurement of Lepton-Flavor Violation (LFV) in the minimal SUSY Standard Model (MSSM) at Large Hadron Collider (LHC) is studied based on a realistic simulation. We consider the LFV decay of the second-lightest neutralino, $\tildeχ^0_2 \to \tilde{l} l' \to l l' \tildeχ^0_1$, in the case where the flavor mixing exists in the right-handed sleptons. We scan the parameter space of the minimal supergravity model (MSUGRA) and a more generic model in which we take the Higgsino mass $μ$ as a free parameter. We find that the possibility of observing LFV at LHC is higher if $μ$ is smaller than the MSUGRA prediction; the LFV search at LHC can cover the parameter range where the $μ\to e γ$ decay can be suppressed by the cancellation among the diagrams for this case.

hep-ph↗