30403586
Scale-independent mixing angles
A radiatively-corrected mixing angle has to be independent of the choice of renormalization scale to be a physical observable. At one-loop in MS-bar, this only occurs for a particular value, p*, of the external momentum in the two-point functions used to define the mixing angle: p*^2=(M1^2+M2^2)/2, where M1, M2 are the physical masses of the two mixed particles. We examine two important applications of this to the Minimal Supersymmetric Standard Model: the mixing angle for a) neutral Higgs bosons and b) stops. We find that this choice of external momentum improves the scale independence (and therefore provides a more reliable determination) of these mixing angles.
17644926
Complete Two-loop Dominant Corrections to the Mass of the Lightest CP-even Higgs Boson in the Minimal Supersymmetric Standard Model
Abstract Using an effective potential approach, we compute two-loop radiative corrections to the MSSM lightest CP -even Higgs boson mass M h 0 to O (α t 2 ) for arbitrary left-right top-squark mixing and tan β . We find that these corrections can increase M h 0 by as much as 5 GeV; assuming a SUSY scale of 1 TeV, the upper bound on the Higgs boson mass is M h 0 ≈129±5 GeV for the top quark pole mass 175±5 GeV. We also derive an analytical approximation formula for M h 0 which approximates our numerical results to a precision of ≲0.5 GeV for most of the parameter space and suitable to be further improved by including renormalization group resummation of leading and next-to-leading order logarithmic terms. Our final compact formula admits a clear physical interpretation: radiative corrections up to the two-loop level can be well approximated by a one-loop expression with parameters evaluated at the appropriate scales, plus a smaller finite two-loop threshold correction term.
119373296
The baryogenesis window in the MSSM
Thermal two-loop QCD corrections associated with light stops have a dramatic effect on the strength of the MSSM electroweak phase transition, making it more strongly first order as required for the viability of electroweak baryogenesis. We perform a perturbative analysis of the transition strength in this model, including these important contributions, extending previous work to arbitrary values of the pseudoscalar Higgs boson mass, m_A. We find a strong enough transition in a region with 2120 GeV, a light Higgs boson with nearly standard couplings, and mass below 85 GeV within the reach of LEP II, and one stop not much heavier than the top quark. In addition, we give a qualitative discussion of the parameter space dependence of the transition strength and comment on the possibility that the transition turns to a crossover for sufficiently large Higgs masses.
117085675
Finetuning in EW Symmetry Breaking
After revisiting the hierarchy problem of the Standard Model and its implications for the scale of New Physics, I consider the finetuning problem in two main scenarios beyond the Standard Model: Supersymmetry (both in the Minimal Supersymmetric Standard Model and alternative models) and Little Higgs models. The main conclusions are that New Physics should appear on the reach of the LHC; that some supersymmetric models can solve the hierarchy problem with acceptable residual finetuning and, finally, that Little Higgs models generically suffer from large tunings, many times hidden.
15139906
On the electroweak phase transition in the minimal supersymmetric standard model
We study the finite-temperature effective potential of the Minimal Supersymmetric Standard Model, in the limit of only one light Higgs boson. Because of the large top Yukawa coupling, there can be significant differences with respect to the Standard Model case: for given values of the Higgs and top masses, little supersymmetry breaking in the stop sector can make the phase transition more strongly first-order. After including the full structure of the stop mass matrix, the most important experimental constraints and the leading plasma effects, we find that the present limits of Higgs and squark masses are still compatible with the scenario of electroweak baryogenesis, in a small region of parameter space corresponding to mh ⪅ 70 GeV and mb1 ⪅ 105 GeV.
5209981
Improved metastability bounds on the standard model Higgs mass
Abstract Depending on the Higgs-boson and top-quark masses, M H and M t , the effective potential of the Standard Model at finite (and zero) temperature can have a deep and non-standard stable minimum 〈 φ ( T )〉 at values of the field much larger than G F −1 2 . We have computed absolute lower bounds on M H , as a function of M t , imposing the condition of no decay by thermal fluctuations, or quantum tunnelling, to the stable minimum. Our effective potential at zero temperature includes all next-to-leading logarithmic corrections (making it extremely scale-independent), and we have used pole masses for the Higgs-boson and top-quark. Thermal corrections to the effective potential include plasma effects by one-loop ring resummation of Debye masses. All calculations, including the effective potential and the bubble nucleation rate, are performed numerically, and so the results do not rely on any kind of analytical approximation. Easy-to-use fits are provided for the benefit of the reader. Conclusions on the possible Higgs detection at LEP-200 are drawn.
41578027
Nuclear Star Formation Activity and Black Hole Accretion in Nearby Seyfert Galaxies
Recent theoretical and observational works indicate the presence of a correlation between the star-formation rate (SFR) and active galactic nucleus (AGN) luminosity (and, therefore, the black hole accretion rate, Ṁ_(BH) of Seyfert galaxies. This suggests a physical connection between the gas-forming stars on kpc scales and the gas on sub-pc scales that is feeding the black hole. We compiled the largest sample of Seyfert galaxies to date with high angular resolution (~0.''4-0.''8) mid-infrared (8-13 μm) spectroscopy. The sample includes 29 Seyfert galaxies drawn from the AGN Revised Shapley-Ames catalog. At a median distance of 33 Mpc, our data allow us to probe nuclear regions on scales of ~65 pc (median value). We found no general evidence of suppression of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) emission in the vicinity of these AGN, and we used this feature as a proxy for the SFR. We detected the 11.3 μm PAH feature in the nuclear spectra of 45% of our sample. The derived nuclear SFRs are, on average, five times lower than those measured in circumnuclear regions of 600 pc in size (median value). However, the projected nuclear SFR densities (median value of 22 M_☉ yr^(–1) kpc^(–2)) are a factor of 20 higher than those measured on circumnuclear scales. This indicates that the SF activity per unit area in the central ~65 pc region of Seyfert galaxies is much higher than at larger distances from their nuclei. We studied the connection between the nuclear SFR and Ṁ_(BH) and showed that numerical simulations reproduce our observed relation fairly well.