arXiv subjects

Toby Falk

Publications and source records attributed to Toby Falk.

At least 19 recordsLinked to original sources

arXiv2000

Supersymmetric Dark Matter in the Light of LEP and the Tevatron Collider

We analyze the accelerator constraints on the parameter space of the Minimal Supersymmetric extension of the Standard Model, comparing those now available from LEP II and anticipating the likely sensitivity of Tevatron Run II. The most important limits are those from searches for charginos, neutralinos and Higgs bosons at LEP, and searches for stop squarks, charginos and neutralinos at the Tevatron Collider. We also incorporate the constraints derived from b --> s + gamma decay, and discuss the relevance of charge- and colour-breaking minima in the effective potential. We combine and compare the different constraints on the Higgs-mixing parameter mu, the gaugino-mass parameter m_{1/2} and the scalar-mass parameter m0, incorporating radiative corrections to the physical particle masses. We focus on the resulting limitations on supersymmetric dark matter, assumed to be the lightest neutralino, incorporating coannihilation effects in the calculation of the relic abundance. We find that m_chi > 51 GeV and tan(beta) > 2.2 if all soft supersymmetry-breaking scalar masses are universal, including those of the Higgs bosons, and that these limits weaken to m_chi > 46 GeV and tan(beta) > 1.9 if non-universal scalar masses are allowed. Light neutralino dark matter cannot be primarily Higgsino in composition.

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arXiv2000

CP Violating Phases and the Dark Matter Problem

New CP violating phases in the MSSM can affect both the abundance and detection of neutralino dark matter. We discuss the effect of including cosmological constraints in the limits on new sources of CP violation in the MSSM and the effects of new CP violating parameters on dark matter densities and detection.

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arXiv2000

Cosmological Constraints on the MSSM

I discuss recent developments in the study of cosmological limits on the Minimal Supersymmetric Standard Model (MSSM). In particular, I focus on the effect of neutralino-stau coannihilation on the relic abundance of neutralinos, and I give examples where the cosmologically derived limits on the supersymmetric parameters are relaxed, and one example (CP violating phases) where they are not.

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arXiv1999

New Contributions to Neutralino Elastic Cross-sections from CP Violating Phases in the MSSM

We compute the four-Fermi neutralino-quark interaction Lagrangian including contributions from the CP violating phases in the MSSM. We find that neutralino-nucleus scattering cross-sections relevant for direct detection experiments show a strong dependence on the value of the CP-violating phase associated with the $μ$ parameter, $θ_μ$. In some cases, for a broad range of non-zero $θ_μ$, there are cancellations in the cross-sections which reduce the cross-section by more than an order of magnitude. In other cases, there may be enhancements as one varies $θ_μ$.

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arXiv1999

Variations of the Neutralino Elastic Cross-Section with CP Violating Phases

We analyze the neutralino-nucleus elastic cross-section in the MSSM, including contributions from CP-violating phases, using the four-fermi neutralino-quark interaction. Over a wide range of the MSSM parameter space we show the variations in the cross-sections due to these phases. We further concentrate on the regions which are consistent with constraints from the electric dipole moment of the electron, neutron and mercury atom. In the regions we examine in detail, we find suppressions by up to a factor of two, while enhancements in the cross-sections are no greater than $\sim$ 10%.

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arXiv1999

Dark Matter Abundance and Electroweak Baryogenesis in the CMSSM

The Minimal Supersymmetric Standard Model has a candidate dark matter particle in its spectrum and may be able to generate the baryon asymmetry of the Universe (BAU) at the electroweak phase transition. In the Constrained MSSM, we find the area of parameter space which is allowed by accelerator and precision tests, which produces a relic dark matter abundance in the observationally favored window 0.1<Omega h^2<0.3, and where baryon plus lepton number violating processes are out of equilibrium after the electroweak phase transition.

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arXiv1999

Higgsinos in SUSY Models with Gaugino Mass Unification

In the MSSM, the assumptions of a common sfermion mass parameter m_0 and a common gaugino mass parameter m_1/2, along with the requirements from radiative electroweak symmetry breaking, lead to relatively large values of the Higgs mixing parameter mu, and consequently to a gaugino-like lightest neutralino. Lifting the requirement that the Higgs mass parameters unify with the sfermion masses is known to allow for smaller mu. We show that a mu parameter sufficiently small to yield a Higgsino-like neutralino requires a precise adjustment of the Higgs mass parameter m_Hu. Consequently a gaugino-type neutralino is still preferred in SUSY models with gaugino mass unification.

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arXiv1999

MSSM Predictions for the Electric Dipole Moment of the $^{199}$Hg Atom

The Minimal Supersymmetric Standard Model can possess several CP-violating phases beyond the conventional Cabibo-Kobayashi-Maskawa phase. We calculate the contribution of these phases to T-violating nuclear forces. These forces induce a Schiff moment in the $^{199}$Hg nucleus, which is strongly limited by experiments aimed at the detection of the electric dipole moment of the mercury atom. The result for $d_{Hg}$ is found to be very sensitive to the CP-violating phases of the MSSM and the calculation carries far fewer QCD uncertainties than the corresponding calculation of the neutron EDM. In certain regions of the MSSM parameter space, the limit from the mercury EDM is stronger than previous constraints based on either the neutron or electron EDMs. We present combined constraints from the mercury and electron EDMs to limit both CP-violating phases of the MSSM. We also present limits in mSUGRA models with unified gaugino and scalar masses at the GUT scale.

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arXiv1998

Charge and Colour Breaking in the Constrained MSSM

We show that the physical minimum of the Constrained MSSM is only free from dangerous charge and colour breaking minima in the region of parameter space bounded by 110< m_1/2< 400 GeV and 80< m_0< 170 GeV. In the remaining regions, the cosmology is severely constrained.

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arXiv1998

Neutralino-Stau Coannihilation and the Cosmological Upper Limit on the Mass of the Lightest Supersymmetric Particle

We consider the effects of neutralino-stau coannihilations on the cosmological relic density of the lightest supersymmetric particle (LSP) $χ$ in the minimal supersymmetric extension of the Standard Model (MSSM), particularly in the constrained MSSM in which universal supergravity inputs at the GUT scale are assumed. For much of the parameter space in these models, $χ$ is approximately a U(1) gaugino ${\tilde B}$, and constraints on the cosmological relic density $Ω_{\tilde B} h^2$ yield an upper bound on the bino mass. We show that in regions of parameter space for which the cosmological bound is nearly saturated, coannihilations of the $\tilde B$ with the stau, the next lightest sparticle, are important and may reduce significantly the bino relic density. Including also bino coannihilations with the selectrons and smuons, we find that the upper limit on $m_χ$ is increased from about 200 GeV to about 600 GeV in the constrained MSSM, with a similar new upper limit expected in the MSSM.

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arXiv1998

Charginos and Neutralinos in the Light of Radiative Corrections: Sealing the Fate of Higgsino Dark Matter

We analyze the LEP constraints from searches for charginos $χ^{\pm}$ and neutralinos $χ_i$, taking into account radiative corrections to the relations between their masses and the underlying Higgs-mixing and gaugino-mass parameters $μ, m_{1/2}$ and the trilinear mass parameter $A_t$. Whilst radiative corrections do not alter the excluded domain in $m_{χ^{\pm}}$ as a function of $m_{χ^{\pm}} - m_χ$, its mapping into the $μ, m_{1/2}$ plane is altered. We update our previous lower limits on the mass of gaugino dark matter and on tan$β$, the ratio of Higgs vacuum expectation values, in the light of the latest LEP data and these radiative corrections. We also discuss the viability of Higgsino dark matter, incorporating co-annihilation effects into the calculation of the Higgsino relic abundance. We find that Higgsino dark matter is viable for only a very limited range of $μ$ and $m_{1/2}$, which will be explored completely by upcoming LEP runs.

hep-ph↗

arXiv1998

More on Electric Dipole Moment Constraints on Phases in the Constrained MSSM

We reconsider constraints on $\cp$-violating phases in the Constrained Minimal Supersymmetric Standard Model. We include the recent calculations of Ibrahim and Nath on the chromoelectric and purely gluonic contributions to the quark electric dipole moment and combine cosmological limits on gaugino masses with experimental bounds on the neutron (and electron) electric dipole moments. The constraint on the phase of the Higgs mixing mass $μ$, $|\thm|$, is dependent on the value of the trilinear mass parameter, $A$, in the model and on $\tan β$. For values of $|A| < 300 \gev$ at the GUT scale, we find $|\thm|/π\la 0.05$, while for $|A| < 1500 \gev$, $|\thm|/π\la 0.3$. Thus, we find that in principle, large CP violating phases are compatible with the bounds on the electric dipole moments of the neutron and electron, as well as remaining compatible with the cosmological upper bound on the relic density of neutralinos. The other $\cp$-violating phase $\tha$ is essentially unconstrained.

hep-ph↗

arXiv1998

LEP Constraints on Neutralino Relic Densities, and the Fate of Higgsino Dark Matter

We examine the current state of neutralino dark matter and consider how the LEP constraints on the Minimal Supersymmetric Standard Model parameters are squeezing the available dark matter regions. We also show how cosmological constraints augment bounds coming from collider searches to further constrain the MSSM parameter space.

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arXiv1997

Constraints on Neutralino Dark Matter from LEP 2 and Cosmology

A significant lower limit on the mass of the lightest neutralino χcan be obtained by combining the results from sparticle searches at LEP at centre-of-mass energies up to 172 GeV with cosmological considerations, if it is assumed that the χis stable. Exclusion domains from slepton searches close m_χ\sim 0 loopholes that were left open by previous lower-energy LEP searches for charginos and neutralinos, leading to the lower limit $m_χ\ga 17$ GeV. The constraints on supersymmetric parameter space are strengthened significantly if LEP constraints on supersymmetric Higgs bosons are taken into account, and further if the relic neutralino density is required to fall within the range favoured by astrophysics and cosmology. These bounds are considerably strengthened if universality at the GUT scale is assumed for soft supersymmetry-breaking scalar masses, including those of the Higgs bosons. In this case, the Higgs searches play a dramatic role, and we find that m_χ\ga 40 GeV. Furthermore, we find that if tanβ\la 1.7 for μ<0, or tanβ\la 1.4 for μ>0, the cosmological relic density is too large for all values of m_χ.

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arXiv1996

Constraints from Inflation and Reheating on Superpartner Masses

Flat directions in the Minimal Supersymmetric Standard Model (MSSM) can become unstable due to radiative corrections, and the global minimum of the (zero temperature) potential can lie at large values of the squark and/or slepton fields. Here we show that, in inflationary models of early cosmology, the universe is very likely to be in the domain of attraction of this global minimum at the end of inflation. While the minimum at the origin of field space is global at sufficiently high temperatures, depending on details of the model, the universe may be trapped in the non-zero minimum until it becomes the global minimum at low temperatures. Parameter values leading to this scenario are therefore ruled out.

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arXiv1996

Supersymmetric Dark Matter in the Light of LEP 1.5

We discuss the lower limit on the mass of the neutralino $χ$ that can be obtained by combining data from $e^+e^-$ annihilation at LEP and elsewhere with astrophysical and theoretical considerations. Loopholes in the purely experimental analysis of ALEPH data from the Z peak and LEP 1.5, which appear when $μ<0$ for certain values of the sneutrino mass $m_{\tildeν}$ and the ratio $\tanβ$ of supersymmetric Higgs vacuum expectation values, may be largely or totally excluded by data from lower-energy $e^+e^-$ data, the hypothesis that most of the cosmological dark matter consists of $χ$ particles, and the assumption that electroweak symmetry breaking is triggered by radiative corrections due to a heavy top quark. The combination of these inputs imposes $m_χ \ge 21.4~\gev$, if soft supersymmetry-breaking masses are assumed to be universal at the grand-unification scale.

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arXiv1996

CP-Violating Phases in the MSSM

We combine experimental bounds on the electric dipole moments of the neutron and electron with cosmological limits on the relic density of a gaugino-type LSP neutralino to constrain certain CP-violating phases appearing in the MSSM. We find that in the Constrained MSSM, the phase |θ_μ| < π/10, while the phase θ_A remains essentially unconstrained.

hep-ph↗

arXiv1996

Electric Dipole Moment Constraints on Phases in the Constrained MSSM

We consider constraints on $\cp$-violating phases in the Constrained Minimal Supersymmetric Standard Model. We find that by combining cosmological limits on gaugino masses with experimental bounds on the neutron and electron electric dipole moments, we can constrain the phase of the Higgs mixing mass $μ$ to be $|θ_μ| < π/10$, independent of choices of the other mass parameters in the model. The other $\cp$-violating phase $θ_A$ is essentially unconstrained.

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