Masahiro Ibe

Constraints on Dark Matter from Colliders

We show that colliders can impose strong constraints on models of dark matter, in particular, when the dark matter is light. We analyze models where the dark matter is a fermion or scalar interacting with quarks and/or gluons through an effective theory containing higher dimensional operators which represent heavier states that have been integrated out of the effective field theory. We determine bounds from existing Tevatron searches for monojets as well as expected LHC reaches for a discovery. We find that colliders can provide information which is complementary or in some cases even superior to experiments searching for direct detection of dark matter through its scattering with nuclei. In particular, both the Tevatron and the LHC can outperform spin-dependent searches by an order of magnitude or better over much of the parameter space, and if the dark matter couples mainly to gluons, the LHC can place bounds superior to any spin-independent search.

Constraints on Light Majorana dark Matter from Colliders

Abstract We explore model-independent collider constraints on light Majorana dark matter particles. We find that colliders provide a complementary probe of WIMPs to direct detection, and give the strongest current constraints on light DM particles. Collider experiments can access interactions not probed by direct detection searches, and outperform direct detection experiments by about an order of magnitude for certain operators in a large part of parameter space. For operators which are suppressed at low momentum transfer, collider searches have already placed constraints on such operators limiting their use as an explanation for DAMA.

Breit-Wigner Enhancement of Dark Matter Annihilation

Dark matter of the universe has been discussed since 1933, yet its nature still remains elusive[1]. Seventy-five years later we only managed to restrict its mass between 10 31 and 10 50 GeV, demonstrating our lack of understanding. However, the thermal relic of a Weakly Interacting Massive Particle (WIMP) is arguably best theoretically motivated because it has the same mass scale as the anticipated new physics that would explain why our universe is a superconductor (electroweak symmetry breaking). Hopes are high to discover WIMPs at the forthcoming LHC experiments, to detect them directly in sensitive underground experiments, as well as to observe signals of WIMP annihilations from the galactic center or the halo in high-energy cosmic rays. Recent observations of the PAMELA[2], ATIC[3], and PPB-BETS[4] experiments strongly suggest the existence of a new source of positrons (and electrons) in cosmic rays. The most interesting interpretation of these results is the annihilation of the dark matter with a mass at the TeV scale. However, such interpretation requires that the annihilation cross section of the dark matter in the galactic halo is much larger (by a factor of O(100)) than the one appropriate to explain the dark matter relic density precisely measured by the WMAP experiment[5]. The enhancement of the dark matter annihilation in the galactic halo is called a “boost factor.” So far, there have been several proposals to explain the origin of the boost factor both from astrophysics such as the enhanced local dark matter density, and from particle physics such as the Sommerfeld enhancement due to an attractive force among the dark matter particles[6]. In this letter, we propose a new explanation of the boost factor. We consider the dark matter which annihilates via a narrow Breit–Wigner resonance just below the threshold. When the resonance mass is just below twice the dark matter mass, the annihilation cross section becomes sensitive to the velocity of the dark matter. In such a case, the time evolution of the dark matter abundance is quite different from the one in the usual non-resonant case of annihilation, and we find that the annihilation cross section in the halo is enhanced compared to what is inferred from the relic abundance. As we will show, the cross section required from the dark matter density can be large enough to explain the PAMELA, ATIC, and PPB-BETS results, and hence, we do not need in our proposal any additional boost factor due to an overdense region in the halo or the Sommerfeld enhancement. Cross Section Just Above a Pole. In this study, we assume that the dark matter with mass m annihilates via a narrow resonance R. For a simplicity, we consider a scalar resonance, although generalization to arbitrary spins is straight forward. The general formula for the scattering cross section via a resonance R is given by σ = 16π E 2 cm ¯ βiβi M 2 2

Gamma ray line constraints on effective theories of dark matter

Abstract A monochromatic gamma ray line results when dark matter particles in the galactic halo annihilate to produce a two body final state which includes a photon. Such a signal is very distinctive from astrophysical backgrounds, and thus represents an incisive probe of theories of dark matter. We compare the recent null results of searches for gamma ray lines in the galactic center and other regions of the sky with the predictions of effective theories describing the interactions of dark matter particles with the Standard Model. We find that the null results of these searches provide constraints on the nature of dark matter interactions with ordinary matter which are complementary to constraints from other observables, and stronger than collider constraints in some cases.

Thermal leptogenesis and gauge mediation

We show that a mini-thermal inflation occurs naturally in a class of gauge mediation models of supersymmetry (SUSY) breaking, provided that the reheating temperature T_R of the primary inflation is much higher than the SUSY-breaking scale, say T_R>10^{10} GeV. The reheating process of the thermal inflation produces an amount of entropy, which dilutes the number density of relic gravitinos. This dilution renders the gravitino to be the dark matter in the present universe. The abundance of the gravitinos is independent of the reheating temperature T_R, once the gravtinos are thermally produced after the reheating of the primary inflation. We find that the thermal leptogenesis takes place at T_L\simeq 10^{12-14} GeV for m_{3/2}\simeq 100 keV - 10 MeV without any gravitino problem.

Nambu-Goldstone Dark Matter and Cosmic Ray Electron and Positron Excess

We propose a model of dark matter identified with a pseudo-Nambu-Goldstone boson in the dynamical supersymmetry breaking sector in a gauge mediation scenario. The dark matter particles annihilate via a below-threshold narrow resonance into a pair of R-axions each of which subsequently decays into a pair of light leptons. The Breit-Wigner enhancement explains the excess electron and positron fluxes reported in the recent cosmic ray experiments PAMELA, ATIC and PPB-BETS without postulating an overdensity in halo, and the limit on anti-proton flux from PAMELA is naturally evaded.

Studying Gaugino Mass in Semi-Direct Gauge Mediation

We study gaugino mass generation in the context of semi-direct gauge mediation models, where the messengers are charged under both the hidden sector and the standard model gauge groups while they do not play important roles in dynamical supersymmetry breaking. We clarify the cancellation of the leading contributions of the supersymmetry breaking effects to the gaugino mass in this class of models in terms of the macroscopic effective theory of the hidden sector dynamics. We also consider how to retrofit the model so that we obtain the non-vanishing leading contribution to the gaugino mass.

R-axion detection at LHC

Supersymmetric models with spontaneously broken approximate R-symmetry contain a light spin 0 particle, the R-axion. The properties of the particle can be a powerful probe of the structure of the new physics. In this paper, we discuss the possibilities of the R-axion detection at the LHC experiments. It is challenge to observe this light particle in the LHC environment. However, for typical values in which the mass of the R-axion is a few hundred MeV, we show that those particles can be detected by searching for displaced vertices from R-axion decay.

Strongly coupled semidirect mediation of supersymmetry breaking

Strongly coupled semidirect gauge mediation models of supersymmetry breaking through massive mediators with standard-model charges are investigated by means of composite degrees of freedom. Sizable mediation is realized to generate the standard-model gaugino masses for a small mediator mass without breaking the standard-model symmetries.

Cosmic Ray Spectra in Nambu-Goldstone Dark Matter Models

We discuss the cosmic ray spectra in annihilating/decaying Nambu-Goldstone dark matter models. The recent observed positron/electron excesses at PAMELA and Fermi experiments are well fitted by the dark matter with a mass of 3 TeV for the annihilating model, while with a mass of 6 TeV for the decaying model. We also show that the Nambu-Goldstone dark matter models predict a distinctive gamma-ray spectrum in a certain parameter space.