arXiv subjects

Takeo Moroi

Publications and source records attributed to Takeo Moroi.

180 records · Page 10Linked to original sources

arXiv1999

Multi-TeV Scalars are Natural in Minimal Supergravity

For a top quark mass fixed to its measured value, we find natural regions of minimal supergravity parameter space where all squarks, sleptons, and heavy Higgs scalars have masses far above 1 TeV and are possibly beyond the reach of the Large Hadron Collider at CERN. This result is simply understood in terms of ``focus point'' renormalization group behavior and holds in any supergravity theory with a universal scalar mass that is large relative to other supersymmetry breaking parameters. We highlight the importance of the choice of fundamental parameters for this conclusion and for naturalness discussions in general.

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arXiv1999

Supernatural Supersymmetry: Phenomenological Implications of Anomaly-Mediated Supersymmetry Breaking

We discuss the phenomenology of supersymmetric models in which supersymmetry breaking terms are induced by the super-Weyl anomaly. Such a scenario is envisioned to arise when supersymmetry breaking takes place in another world, i.e., on another brane. We review the anomaly-mediated framework and study in detail the minimal anomaly-mediated model parametrized by only 3+1 parameters: M_aux, m_0, \tanβ, and sign(μ). The renormalization group equations exhibit a novel "focus point" (as opposed to fixed point) behavior, which allows squark and slepton masses to be far above their usual naturalness bounds. We present the superparticle spectrum and highlight several implications for high energy colliders. Three lightest supersymmetric particle (LSP) candidates exist: the Wino, the stau, and the tau sneutrino. For the Wino LSP scenario, light Wino triplets with the smallest possible mass splittings are preferred; such Winos are within reach of Run II Tevatron searches. Finally, we study a variety of sensitive low energy probes, including b -> s gamma, the anomalous magnetic moment of the muon, and the electric dipole moments of the electron and neutron.

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arXiv1999

Discovering Supersymmetry at the Tevatron in Wino LSP Scenarios

In supersymmetric models, Winos, partners of the SU(2) gauge bosons, may be the lightest supersymmetric particles (LSPs). For generic parameters, charged and neutral Winos are highly degenerate. Charged Winos travel macroscopic distances, but can decay to neutral Winos and extremely soft leptons or pions before reaching the muon chambers, thereby circumventing conventional trigger requirements based on energetic decay products or muon chamber hits. However, these charginos are detectable, and can be triggered on when produced in association with jets. In addition, we propose a new trigger for events with a high p_T track and low hadronic activity. For Tevatron Run II with luminosity 2 fb^-1, the proposed searches can discover Winos with masses up to 300 GeV and explore a substantial portion of the parameter space in sequestered sector models.

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arXiv1999

Wino Cold Dark Matter from Anomaly-Mediated SUSY Breaking

The cosmological moduli problem is discussed in the framework of sequestered sector/anomaly-mediated supersymmetry (SUSY) breaking. In this scheme, the gravitino mass (corresponding to the moduli masses) is naturally 10 - 100 TeV, and hence the lifetime of the moduli fields can be shorter than $\sim 1 sec$. As a result, the cosmological moduli fields should decay before big-bang nucleosynthesis starts. Furthermore, in the anomaly-mediated scenario, the lightest superparticle (LSP) is the Wino-like neutralino. Although the large annihilation cross section means the thermal relic density of the Wino LSP is too small to be the dominant component of cold dark matter (CDM), moduli decays can produce Winos in sufficient abundance to constitute CDM. If Winos are indeed the dark matter, it will be highly advantageous from the point of view of detection. If the halo density is dominated by the Wino-like LSP, the detection rate of Wino CDM in Ge detectors can be as large as $0.1 - 0.01$ event/kg/day, which is within the reach of the future CDM detection with Ge detector. Furthermore, there is a significant positron signal from pair annihilation of Winos in our galaxy which should give a spectacular signal at AMS.

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arXiv1998

Electric Dipole Moments in Gauge Mediated Models and a Solution to the SUSY CP Problem

The SUSY CP problem in the framework of gauge mediated SUSY breaking model is considered. We first discuss the electric dipole moments of the electron and neutron, which are likely to be larger than the experimental upper bound if all the phases in the Lagrangian are O(1). We derive a constraint on the phases in the so-called $μ$- and $B_μ$-parameters and gaugino masses. Then, we discuss a model in which the CP violating phase can be adequately suppressed. If the $μ$- and $B_μ$-parameters originate from the same superpotential interaction as the SUSY breaking field, the CP violating phase vanishes. However, in this class of models, the ratio $B_μ/μ$ becomes too large, and we discuss a possible scenario to fix this problem.

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arXiv1998

Cosmological Moduli Problem in a Supersymmetric Model with Direct Gauge Mediation

Recently, an interesting class of the direct gauge mediation supersymmetry (SUSY) breaking models are proposed, in which the minimum of the potential of the SUSY breaking field is determined by the inverted hierarchy mechanism. We consider their cosmological implications. In this class of models, SUSY breaking field has a very flat potential, which may have a cosmological importance. Assuming the initial amplitude of the SUSY breaking field to be of the order of the Planck scale, it can be a source of a large entropy production. A special attention is paid to the cosmological moduli problem, and we will see the cosmological mass density of the moduli field can be significantly reduced.

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arXiv1998

Axionic Hot Dark Matter in the Hadronic Axion Window

Mixed dark matter scenario can reconcile the COBE data and the observed large scale structure. So far the massive neutrino with a mass of a few eV has been the only discussed candidate for the hot dark matter component. We point out that the hadronic axion in the so-called hadronic axion window, f_a \sim 10^6 GeV, is a perfect candidate as hot dark matter within the mixed dark matter scenario. The current limits on the hadronic axion are summarized. The most promising methods to verify the hadronic axion in this window are the resonant absorption of almost-monochromatic solar axions from M1 transition of the thermally excited $^{57}$Fe in the Sun, and the observation of the ``axion burst'' in water Čerenkov detectors from another supernova.

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arXiv1998

Tevatron Signatures of Long-lived Charged Sleptons in Gauge-Mediated Supersymmetry Breaking Models

In supersymmetric models with gauge-mediated supersymmetry breaking, charged sleptons are the next lightest supersymmetric particles and decay outside the detector for large regions of parameter space. In such scenarios, supersymmetry may be discovered by searches for a number of novel signals, including highly ionizing tracks from long-lived slow charged particles and excesses of multi-lepton signals. We consider this scenario in detail and find that the currently available Tevatron data probes regions of parameter space beyond the kinematic reach of LEP II. Future Tevatron runs with integrated luminosities of 2, 10, and 30 fb-1 probe right-handed slepton masses of 110, 180, and 230 GeV and Wino masses of 310, 370, and 420 GeV, respectively, greatly extending current search limits.

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arXiv1998

Third Generation Familons, B Factories, and Neutrino Cosmology

We study the physics of spontaneously broken family symmetries acting on the third generation. Massless familons (or Majorons) $f$ associated with such broken symmetries are motivated especially by cosmological scenarios with decaying tau neutrinos. We first note that, in marked contrast with the case for the first two generations, constraints on third generation familon couplings are poor, and are, in fact, non-existent at present in the hadronic sector. We derive new bounds from $B^0$--$\bar{B}^0$ mixing, $B^0 \to l^+ l'^-$, $b\to sν\barν$, and astrophysics. The resulting constraints on familon decay constants are still much weaker than those for the first and second generation. We then discuss the promising prospects for significant improvements from searches for $τ\to l f$, $B\to (π, K) f$, and $b\to (d,s) f$ with the current CLEO, ARGUS, and LEP data. Finally, we note that future constraints from CLEO III and the $B$ factories will probe decay constants beyond 10^8 GeV, well within regions of parameter space favored by proposed scenarios in neutrino cosmology.

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arXiv1997

Sneutrino Cold Dark Matter With Lepton-Number Violation

The tau sneutrino is proposed as a candidate for galactic halo dark matter, and as the cold dark matter (CDM) component of the universe. A lepton-number-violating sneutrino mass, \tildeν\tildeν, splits the tau sneutrino into two mass eigenstates: \tildeν \rightarrow \tildeν_\pm. The absence of a Z \tildeν_- \tildeν_- coupling implies that the lighter mass eignestate, \tildeν_-, does not annihilate via the s-channel Z-exchange to a low cosmological abundance, and furthermore, halo sneutrinos do not scatter excessively in Ge detectors. For the majority of the relevant parameter space, the event rate in Ge detectors is \geq 10^{-2} events/kg/day. The lepton number violation required for sneutrino CDM implies that the tau neutrino mass is m_{ν_τ} \gtrsim 5 MeV, large enough to be excluded by B factory experiments. Events of the form l^+ l^- \not E or jj \not E , with low m_{ll} or m_{jj}, may be observed at LEP2. A seesaw mechanism is investigated as the origin for the lepton number violation, and several other cosmological and particle physics consequences of sneutrino CDM are discussed.

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arXiv1997

Determining $\tanβ$ at the NLC with SUSY Higgs Bosons

We examine the prospects for determining $\tanβ$ from heavy Higgs scalar production in the minimal supersymmetric standard model at a future $e^+e^-$ collider. Our analysis is independent of assumptions of parameter unification, and we consider general radiative corrections in the Higgs sector. Bounds are presented for $\sqrt{s} = 500$ GeV and 1 TeV, several Higgs masses, and a variety of integrated luminosities. For all cases considered, it is possible to distinguish low, moderate, and high $\tanβ$. In addition, we find stringent constraints for $3\lesssim\tanβ\lesssim 10$, and, for some scenarios, also interesting bounds on high $\tanβ$ through $tbH^{\pm}$ production. Such measurements may provide strong tests of the Yukawa unifications in grand unified theories and make possible highly precise determinations of soft SUSY breaking mass parameters.

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arXiv1997

The Muon Anomalous Magnetic Dipole Moment in the Minimal Supersymmetric Standard Model

The muon anomalous magnetic dipole moment (MDM) is calculated in the framework of the minimal supersymmetric standard model (MSSM). In this paper, we discuss how the muon MDM depends on the parameters in MSSM in detail. We show that the contribution of the superparticle-loop becomes significant especially when $\tanβ$ is large. Numerically, it becomes $O(10^{-8}-10^{-9})$ in a wide parameter space, which is within the reach of the new Brookhaven E821 experiment.

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arXiv1997

New Mechanism of Flavor Symmetry Breaking from Supersymmetric Strong Dynamics

We present a class of supersymmetric models in which flavor symmetries are broken dynamically, by a set of composite flavon fields. The strong dynamics that is responsible for confinement in the flavor sector also drives flavor symmetry breaking vacuum expectation values, as a consequence of a quantum-deformed moduli space. Yukawa couplings result as a power series in the ratio of the confinement to Planck scale, and the fermion mass hierarchy depends on the differing number of preons in different flavor symmetry-breaking operators. We present viable non-Abelian and Abelian flavor models that incorporate this mechanism.

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arXiv1997

Cosmology of Supersymmetric Models with Low-energy Gauge Mediation

We study the cosmology of supersymmetric models in which the supersymmetry breaking effects are mediated by gauge interactions at about the 10^5 GeV scale. We first point out that the gravitino is likely to overclose the Universe in this class of models. This requires an entropy production, which prefers a baryogenesis mechanism at a relatively low temperature. The Affleck-Dine mechanism for baryogenesis is one of the possibilities to generate enough baryon asymmetry, but the analysis is non-trivial since the shape of the potential for the flat direction differs substantially from the conventional hidden sector case. To see this, we first perform a 2-loop calculation to determine the shape of the potential. By combining the potential with the supergravity contribution, we then find that the Affleck-Dine baryogenesis works efficiently to generate sufficient baryon asymmetry. On the other hand, we also point out that string moduli fields, if present, are stable and their coherent oscillations overclose the Universe by more than 15 orders of magnitude. One needs a very late inflationary period with an e-folding of N \gtrsim 5 and an energy density of \lesssim (10^7 GeV)^4. A thermal inflation is enough for this purpose. Fortunately, the Affleck-Dine baryogenesis is so efficient that enough baryon asymmetry can survive the late inflation.

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arXiv1996

Determining $tan β$ from the SUSY Higgs Sector at Future e^+e^- Colliders

We examine the prospects for determining $tan β$ from heavy Higgs scalar production in the minimal supersymmetric standard model at a future $e^+ e^-$ collider. Our analysis is independent of assumptions of parameter unification, and we consider general radiative corrections in the Higgs sector. Bounds are presented for $\sqrt{s} = 500$ GeV and 1 TeV, several Higgs masses, a variety of integrated luminosities and b-tagging efficiencies, and in scenarios with and without supersymmetric decays of the Higgs bosons. We find stringent constraints for $3 \alt tan β\alt 10$, and, for some scenarios, also interesting bounds on high $tan β$ through $tbH^{\pm}$ production. These bounds imply that simple Yukawa unifications may be confirmed or excluded. Implications for soft scalar mass determination and top squark parameters are also discussed.

hep-ph↗

arXiv1996

Solving the Crisis in Big-Bang Nucleosynthesis by the Radiative Decay of an Exotic Particle

We discuss a new mechanism which can solve the crisis in standard big-bang nucleosynthesis. A long-lived particle $X$ ($10^4 sec \lesssim τ_X \lesssim 10^6 sec$) which decays into photon(s) will induce cascade photons, and destroy significant amounts of D and $^3$He without destroying $^4$He or too much $^7$Li. We numerically investigate this process and derive a constraint on the properties of $X$ such that the theoretical values of the primordial light-element abundances agree with observation. We also present some candidates for the unstable particle $X$.

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arXiv1996

Non-unified gaugino masses in supersymmetric missing partner models with hypercolor

The gaugino mass relations $m_3/{g_3^2} = m_2/{g_2^2} = m_1/{g_1^2}$ are considered to be robust signals for supersymmetric grand unification. In this letter, we point out that these relations may be significantly modified in an interesting class of models which solve the doublet-triplet splitting problem using a missing partner mechanism together with a strong hypercolor gauge group. The observation of non-unified gaugino masses, together with unified sfermion masses, provides a distinctive signature for these models.

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arXiv1995

Effects of the Gravitino on the Inflationary Universe

Gravitino problem is discussed in detail. We derive an upperbound on the reheating temperature from the constraints of the big-bang nucleosynthesis and the present mass density of the universe. Compared to previous works, we have improve the following three points; (i) the gravitino production cross sections are calculated by taking all the relevant terms in the supergravity lagrangian into account, (ii) high energy photon spectrum is obtained by solving the Boltzmann equations numerically, and (iii) the evolutions of the light elements (D, T, $^3$He, $^4$He) at the temperature lower than $\sim$1MeV are calculated by using modified Kawano's computer code.

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