arXiv subjects

James M. Cline

Publications and source records attributed to James M. Cline.

At least 181 records · Page 10Linked to original sources

arXiv2002

Quest for a Self-Tuning Brane-World Solution to the Cosmological Constant Problem

It has been proposed that the geometry of an extra dimension could automatically adjust itself to compensate for an arbitrary energy density on the 3-D brane which we are presumed to inhabit, such that a static solution to Einstein's equation results. This would solve the long-standing cosmological constant problem, of why our universe is not overwhelmed by the enormous energy of the quantum vacuum fluctuations predicted by quantum field theory. I will review some of the attempts along these lines, and present a no-go theorem showing that these attempts are doomed, at least within one of the most promising classes of models.

hep-th↗

arXiv2002

Electroweak Phase Transition and Baryogenesis

I review the status of the strength of the electroweak phase transition, and electroweak baryogenesis in the Minimal Supersymmetric Standard Model (work done with K. Kainulainen and M. Joyce). The emphasis is on new brane-inspired ideas about electroweak baryogenesis, and improvements in the semiclassical treatment of CP violation at a first order electroweak phase transition.

hep-ph↗

arXiv2001

Erratum for "Supersymmetric Electroweak Baryogenesis"

We correct a numerical error which led to an overestimate of the baryon asymmetry from supersymmetric electroweak baryogenesis in our paper JHEP07(2000)018. Updated dependences of the baryon asymmetry on chargino mass parameters and the bubble wall velocity are shown. We also include LEP-II-constraints on the chargino mass parameters. Combined with our corrected results for the baryon asymmetry these constraints imply that the phase in the chargino mass matrix must violate CP nearly maximally in order to generate a large enough baryon asymmetry. A number of other typographical errors are also corrected.

hep-ph↗

arXiv2001

No-Go Theorem for Horizon-Shielded Self-Tuning Singularities

We derive a simple no-go theorem relating to self-tuning solutions to the cosmological constant for observers on a brane, which rely on a singularity in an extra dimension. The theorem shows that it is impossible to shield the singularity from the brane by a horizon, unless the positive energy condition (rho+p >= 0) is violated in the bulk or on the brane. The result holds regardless of the kinds of fields which are introduced in the bulk or on the brane, whether Z_2 symmetry is imposed at the brane, or whether higher derivative terms of the Gauss-Bonnet form are added to the gravitational part of the action. However, the no-go theorem can be evaded if the three-brane has spatial curvature. We discuss explicit realizations of such solutions which have both self-tuning and a horizon shielding the singularity.

hep-th↗

arXiv2001

Quintessence, Cosmological Horizons, and Self-Tuning

We point out that quintessence with an exponential potential V_0 exp(- beta phi / 3^{1/2} M_p) can account for the present observed acceleration of the universe, without necessarily leading to eternal acceleration. This occurs for 2.4 < beta < 2.8. Thus a cosmological horizon, which is supposed to be problematic within the context of string theory, can be avoided. We argue that this class of models is not particularly fine-tuned. We further examine this question in the context of a modified Friedmann equation, H^2 ~ rho + p, which is suggested by higher dimensional self-tuning approaches to the cosmological constant problem. It is shown that the self-tuning case can also be consistent with observations, if 1.8 < beta < 2.4. Future observations of high-z supernovae will be able to test whether beta lies in the desired range.

hep-ph↗

arXiv2000

A Small Cosmological Constant from Warped Compactification with Branes

We present a possible explanation for the smallness of the observed cosmological constant using a variant of the Randall-Sundrum(RS)- Goldberger-Wise paradigm for a warped extra dimension. In contrast to RS, we imagine that we are living on the positive tension Planck brane, or on a zero-tension TeV brane. In our model there are two solutions for the scalar field in the bulk and the corresponding brane separations, one of which is tuned to have zero cosmological constant. We show that in the other solution, which is a false vacuum state, the 4-D cosmological constant can be naturally small, due to exponential suppression by the warp factor. The radion is in the milli-eV mass range, and if we live on a TeV brane its couplings are large enough that it can measurably alter the gravitational force at submillimeter distances. In this case the Kaluza-Klein excitations of the graviton can also contribute to submillimeter deviations from Newtonian gravity, and we have in addition the collider phenomenology of the usual TeV-scale radion.

hep-ph↗

arXiv2000

A New Source for Electroweak Baryogenesis in the MSSM

One of the most experimentally testable explanations for the origin of the baryon asymmetry of the universe is that it was created during the electroweak phase transition, in the minimal supersymmetric standard model. Previous efforts have focused on the current for the difference of the two Higgsino fields, $H_1-H_2$, as the source of biasing sphalerons to create the baryon asymmetry. We point out that the current for the orthogonal linear combination, $H_1+H_2$, is larger by several orders of magnitude. Although this increases the efficiency of electroweak baryogenesis, we nevertheless find that large CP-violating angles $\ge 0.15$ are required to get a large enough baryon asymmetry.

hep-ph↗

arXiv2000

5-Dimensional Warped Cosmological Solutions With Radius Stabilization by a Bulk Scalar

We present the 5-dimensional cosmological solutions in the Randall-Sundrum warped compactification scenario, using the Goldberger-Wise mechanism to stabilize the size of the extra dimension. Matter on the Planck and TeV branes is treated perturbatively, to first order. The back-reaction of the scalar field on the metric is taken into account. We identify the appropriate gauge-invariant degrees of freedom, and show that the perturbations in the bulk scalar can be gauged away. We confirm previous, less exact computations of the shift in the radius of the extra dimension induced by matter. We point out that the physical mass scales on the TeV brane may have changed significantly since the electroweak epoch due to cosmological expansion, independently of the details of radius stabilization.

hep-th↗

arXiv2000

Supersymmetric Electroweak Baryogenesis

We re-examine the generation of the baryon asymmetry in the minimal supersymmetric standard model (MSSM) during the electroweak phase transition. We find that the dominant source for baryogenesis arises from the chargino sector. The CP-violation comes from the complex phase in the mu parameter, which provides CP-odd contributions to the particle dispersion relations. This leads to different accelerations for particles and antiparticles in the wall region which, combined with diffusion, leads to the separation of Higgsinos and their antiparticles in the front of the wall. These asymmetries get transported to produce perturbations in the left-handed chiral quarks, which then drive sphaleron interactions to create the baryon asymmetry. We present a complete derivation of the semiclassical WKB formalism, including the chargino dispersion relations and a self-consistent derivation of the diffusion equations starting from semiclassical Boltzmann equations for WKB-excitations. We stress the advantages of treating the transport equations in terms of the manifestly gauge invariant physical energy and kinetic momentum, rather than in the gauge variant canonical variables used in previous treatments. We show that a large enough baryon asymmetry can be created for the phase of the complex mu parameter as small as ~ 0.001, which is consistent with bounds from the neutron electric dipole moment.

hep-ph↗

arXiv2000

Brane-World Cosmology of Modulus Stabilization with a Bulk Scalar Field

We point out that the potential of Goldberger and Wise for stabilizing the distance between two 3-branes, separated from each other along an extra dimension with a warp factor, has a metastable minimum when the branes are infinitely separated. The classical evolution of the radion (brane separation) will place it in this false minimum for generic initial conditions. In particular, inflation could do this if the expansion rate is sufficiently large. We present a simplified version of the Goldberger-Wise mechanism in which the radion potential can be computed exactly, and we calculate the rate of thermal transitions to the true minimum, showing that model parameters can be chosen to ensure that the universe reaches the desired final state. Finiteness of bulk scalar field brane potentials can have an important impact on the nucleation rate, and it can also significantly increase the predicted mass of the radion.

hep-ph↗

arXiv1999

Cosmological Expansion in the Presence of an Extra Dimension

It has recently been pointed out that global solutions of Einstein's equations for a 3-brane universe embedded in 4 spatial dimensions give rise to a Friedmann equation of the form H ~ rho on the brane, instead of the usual H ~ (rho)^{1/2}, which is inconsistent with cosmological observations. We remedy this problem by adding cosmological constants to the brane and the bulk, as in the recent scenario of Randall and Sundrum. Our observation allows for normal expansion during nucleosynthesis, but faster than normal expansion in the very early universe, which could be helpful for electroweak baryogenesis, for example.

hep-ph↗

arXiv1999

Inflation from Extra Dimensions

The radial mode of n extra compact dimensions (the radion, b) can cause inflation in theories where the fundamental gravity scale, M, is smaller than the Planck scale M_P. For radion potentials V(b) with a simple polynomial form, to get the observed density perturbations, the energy scale of V(b) must greatly exceed M ~ 1 TeV: V(b)^{1/4} = M_v ~ 10^{-4} M_P. This gives a large radion mass and reheat temperature ~ 10^9 GeV, thus avoiding the moduli problem. Such a value of M_v can be consistent with the classical treatment if the new dimensions started sufficiently small. A new possibility is that b approaches its stable value from above during inflation. The same conclusions about M_v may hold even if inflation is driven by matter fields rather than by the radion.

hep-ph↗

arXiv1999

Was the Electroweak Phase Transition Preceded by a Color-Broken Phase?

It has been suggested, in connection with electroweak baryogenesis in the Minimal Supersymmetric Standard Model (MSSM), that the right-handed top squark has a negative mass squared parameter, such that its field could condense prior to the electroweak phase transition (EWPT). Thus color and electric charge could have been broken just before the EWPT. Here we investigate whether the tunneling rate from the color-broken vacuum can ever be large enough for the EWPT to occur in this case. We find that, even when all parameters are adjusted to their most favorable values, the nucleation rate is many orders of magnitude too small. We conclude that, without additional physics beyond the MSSM, the answer to our title question is ``no.'' This gives constraints in the plane of the light stop mass versus parameters related to stop mixing. However it may be possible to get color breaking in extended models, such as those with R-parity violation.

hep-ph↗

arXiv1999

Recent Progress in Electroweak Baryogenesis

The status of electroweak baryogenesis in the minimal supersymmetric standard model is reviewed. I discuss the strength of the phase transition, CP violation and transport at the bubble walls, and the possibility of a two stage transition involving charge and color breaking.

hep-ph↗

arXiv1999

Recent Progress in Baryogenesis

I give a synopsis, specifically aimed at nonexperts, of some of the recent developments in electroweak baryogenesis. The focus of the talk is on the present status of electroweak baryogenesis in supersymmetric models, since this is a plausible and realistic possibility that is currently being probed by experimental searches for the Higgs boson and the top squark. The question of whether it is viable to have a period of color-breaking during the electroweak phase transition is also discussed.

hep-ph↗

arXiv1998

Numerical Study of Hawking Radiation Photosphere Formation around Microscopic Black Holes

Heckler has recently argued that the Hawking radiation emitted from microscopic black holes has sufficiently strong interactions above a certain critical temperature that it forms a photosphere, analogous to that of the sun. In this case, the visible radiation is much cooler than the central temperature at the Schwarzschild radius, in contrast to the naive expectation for the observable spectrum. We investigate these ideas more quantitatively by solving the Boltzmann equation using the test particle method. We confirm that at least two kinds of photospheres may form: a quark-gluon plasma for black holes of mass M_{BH} < 5 times 10^{14} g and an electron-positron-photon plasma for M_{BH} < 2 times 10^{12} g. The QCD photosphere extends from the black hole horizon to a distance of 0.2--4.0 fm for 10^9 g < M_{BH} < 5 10^{14} g, at which point quarks and gluons with average energy of order Λ_{QCD} hadronize. The QED photosphere starts at a distance of approximately 700 black hole radii and dissipates at about 400 fm, where the average energy of the emitted electrons, positrons and photons is inversely proportional to the black hole temperature, and significantly higher than was found by Heckler. The consequences of these photospheres for the cosmic diffuse gamma ray and antiproton backgrounds are discussed: bounds on the black hole contribution to the density of the universe are slightly weakened.

hep-ph↗

arXiv1998

Predictions for SUSY Particle Masses from Electroweak Baryogenesis

In collaboration with G.D. Moore, the electroweak phase transition in the minimal supersymmetric standard model is studied using the two-loop effective potential. We make a comprehensive search of the MSSM parameter space consistent with electroweak baryogenesis, taking into account various factors: the latest experimental constraints on the Higgs boson mass and the rho parameter, the possibility of significant squark and Higgs boson mixing, and the exact rate of bubble nucleation and sphaleron transitions. Most of the baryogenesis-allowed regions of parameter space will be probed by LEP 200, hence the Higgs boson is likely to be discovered soon if the baryon asymmetry was indeed created during the electroweak phase transition.

hep-ph↗

arXiv1998

Dynamics of Two Higgs Doublet CP Violation and Baryogenesis at the Electroweak Phase Transition

We quantitatively study the charge transport mechanism of electroweak baryogenesis in a realistic two-Higgs-doublet model, comparing the contributions from quarks and leptons reflecting from electroweak domain walls, and comparing the exact profile of the CP-violating phase with a commonly used ansatz. We note that the phenomenon of spontaneous CP violation at high temperature can occur in this model, even when there is no CP violation at zero temperature. We include all known effects which are likely to influence the baryon production rate, including strong sphalerons, the nontrivial dispersion relations of the quasiparticles in the plasma, and Debye screening of gauged charges. We confirm the claim of Joyce, Prokopec and Turok that the reflection of tau leptons from the wall gives the dominant effect. We conclude that this mechanism is marginally strong enough to produce the observed baryon asymmetry of the universe.

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