On the phase transition in the scalar theory

The basic tool for the study of the electroweak phase transition is $V_{eff} (\phi,T)$, the one-loop finite-temperature effective potential, improved by all-loop resummations of the most important infrared contributions. In this paper we perform, as a first step towards a full analysis of the Standard Model case, a detailed study of the effective potential of the scalar theory. We show that subleading corrections to the self-energies lead to spurious terms, linear in the field-dependent mass $m(\phi)$, in the daisy-improved effective potential. Consistency at subleading order requires the introduction of superdaisy diagrams, which prevent the appearance of linear terms. The resulting $V_{eff}(\phi,T)$ for the scalar theory hints at a phase transition which is either second-order or very weakly first-order.Comment: 10 A4 pages, table and figures not included and available (by ordinary mail) upon request, plain LATEX, CERN-TH.6451/92, IEM-FT-56/9

Quantum Gravity Effects in Black Holes at the LHC

We study possible back-reaction and quantum gravity effects in the evaporation of black holes which could be produced at the LHC through a modification of the Hawking emission. The corrections are phenomenologically taken into account by employing a modified relation between the black hole mass and temperature. The usual assumption that black holes explode around $1$TeV is also released, and the evaporation process is extended to (possibly much) smaller final masses. We show that these effects could be observable for black holes produced with a relatively large mass and should therefore be taken into account when simulating micro-black hole events for the experiments planned at the LHC.Comment: 14 pages, 8 figures, extended version of hep-ph/0601243 with new analysis of final products, final version accepted for publication in J. Phys.

Tau Signals of R-parity breaking at LEP 200

The detectability at LEP 200 of explicit $R$-parity breaking by tau-number $(L_\tau)$ violating operators is considered. The assumption of $L_\tau$-violation is motivated by the relative lack of constraints on such couplings but similar considerations apply to explicit $L_e$- or $L_\mu$-violation. The $LSP$, now unstable, and not necessarily neutral, decays via $L_\tau$-violating modes. Only signals from the production and decays of $LSP$ pairs are considered, thereby avoiding any dependence on the sparticle mass spectrum. Rather spectacular signals are predicted: spherical events with $m$ leptons (usually containing at least one $\tau$) and $n$ jets $(m,n \leq 4)$, the most characteristic of which are like-sign $\tau\tau$ events. These signals are enumerated for each $LSP$ candidate and quantitative estimates are provided for the favoured case when the $LSP$ is a neutralino. Other new physics signals, which can mimic these signatures, are also briefly discussed.Comment: 27 pages in PHYZZX, six figures (not included, available on request). TIFR/TH/92-29,UH-511-751-92,CERN-TH.6613/9

Leptonic CP Violation in Supersymmetric Standard Model

We point out the possibility of spontaneous and hard CP-violation in the scalar potential of R-parity broken supersymmetric Standard Model. The existence of spontaneous CP-violation depends crucially on the R-parity breaking terms in the superpotential and, in addition, on the choice of the soft supersymmetry breaking terms. Unlike in theories with R-parity conservation, it is natural, in the context of the present model, for the sneutrinos to acquire (complex) vacuum expectation values. In the context of this model we examine here the global implications, like the strength of the CP-violating interactions and the neutrino masses.Comment: REVTEX, 15 page

Cosmological constraints on R-parity violation from neutrino decay

If the neutrino mass is non-zero, as hinted by several experiments, then R-parity-violating supersymmetric Yukawa couplings can drive a heavy neutrino decay into lighter states. The heavy neutrino may either decay radiatively into a lighter neutrino, or it may decay into three light neutrinos through a Z-mediated penguin. For a given mass of the decaying neutrino, we calculate its lifetime for the various modes, each mode requiring certain pairs of R-parity-violating couplings be non-zero. We then check whether the calculated lifetimes fall in zones allowed or excluded by cosmological requirements. For the latter case, we derive stringent new constraints on the corresponding products of R-parity-violating couplings for given values of the decaying neutrino mass.Comment: 13 pages, Latex, uses axodraw.sty; version to appear in Physical Review

How generic is cosmic string formation in SUSY GUTs

We study cosmic string formation within supersymmetric grand unified theories. We consider gauge groups having a rank between 4 and 8. We examine all possible spontaneous symmetry breaking patterns from the GUT down to the standard model gauge group. Assuming standard hybrid inflation, we select all the models which can solve the GUT monopole problem, lead to baryogenesis after inflation and are consistent with proton lifetime measurements. We conclude that in all acceptable spontaneous symmetry breaking schemes, cosmic string formation is unavoidable. The strings which form at the end of inflation have a mass which is proportional to the inflationary scale. Sometimes, a second network of strings form at a lower scale. Models based on gauge groups which have rank greater than 6 can lead to more than one inflationary era; they all end by cosmic string formation.Comment: 31 pages, Latex, submitted to PR

On the Spontaneous CP Breaking at Finite Temperature in a Nonminimal Supersymmetric Standard Model

We study the spontaneous CP breaking at finite temperature in the Higgs sector in the Minimal Supersymmetric Standard Model with a gauge singlet. We consider the contribution of the standard model particles and that of stops, charginos, neutralinos, charged and neutral Higgs boson to the one-loop effective potential. Plasma effects for all bosons are also included. Assuming CP conservation at zero temperature, so that experimental constraints coming from, {\it e.g.}, the electric dipole moment of the neutron are avoided, and the electroweak phase transition to be of the first order and proceeding via bubble nucleation, we show that spontaneous CP breaking cannot occur inside the bubble mainly due to large effects coming from the Higgs sector. However, spontaneous CP breaking can be present in the region of interest for the generation of the baryon asymmetry, namely inside the bubble wall. The important presence of very tiny explicit CP violating phases is also commented.Comment: 28 pages, 4 figures available upon request, DFPD 94/TH/38 and SISSA 94/81-A preprint

Recycling universe

If the effective cosmological constant is non-zero, our observable universe may enter a stage of exponential expansion. In such case, regions of it may tunnel back to the false vacuum of an inflaton scalar field, and inflation with a high expansion rate may resume in those regions. An ``ideal'' eternal observer would then witness an infinite succession of cycles from false vacuum to true, and back. Within each cycle, the entire history of a hot universe would be replayed. If there were several minima of the inflaton potential, our ideal observer would visit each one of these minima with a frequency which depends on the shape of the potential. We generalize the formalism of stochastic inflation to analyze the global structure of the universe when this `recycling' process is taken into account.Comment: 43 pages, 10 figure