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.
14725224
On the nature of the electroweak phase transition
We discuss the finite-temperature effective potential of the Standard Model, Veff(�,T), with emphasis on the resummation of the most important infrared contributions. We compute the one-loop scalar and vector boson self-energies in the zero-momentum limit. By solving the corresponding set of gap equations, with the inclusion of subleading contributions, we find a non-vanishing magnetic mass for the SU(2) gauge bosons. We comment on its possible implications for the nature of the electroweak phase transition. We also discuss the range of validity of our approximations and compare this with other approaches.
6717436
Standard model stability bounds for new physics within LHC reach
Abstract We analyse the stability lower bounds on the Standard Model Higgs mass by carefully controlling the scale independence of the effective potential. We include resummed leading and next-to-leading-log corrections, and physical pole masses for the Higgs boson, MH, and the top-quark, Mt. Particular attention is devoted to the cases where the scale of new physics Λ is within LHC reach, i.e. Λ ≤ 10 TeV, which have been the object of recent controversial results. We clarify the origin of discrepancies and confirm our earlier results within the error of our previous estimate. In particular for Λ = 1 TeV we find that M H [GeV]>52+0.64(M 1 [GeV]−175)−0.50 α s (M Z −0.118 0.006 For fixed values of Mt and αs(MZ), the error from higher effects, as the lack of exact scale invariance of the effective potential and higher-order radiative corrections, is conservatively estimated to be ≲ 5 GeV.
117909069
Naturalness of Electroweak Symmetry Breaking while Waiting for the LHC
After revisiting the hierarchy problem of the Standard Model and its implications for the scale of New Physics, I consider the finetuning problem of electroweak symmetry breaking in several scenarios beyond the Standard Model: SUSY, Little Higgs and “improved naturalness” models. The main conclusions are that: New Physics should appear on the reach of the LHC; some SUSY models can solve the hierarchy problem with acceptable residual tuning; Little Higgs models generically suffer from large tunings, many times hidden; and, finally, that “improved naturalness” models do not generically improve the naturalness of the SM.
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.
282605
Novel Opportunities for EW Breaking from Low-Scale SUSY Breaking
In supersymmetric scenarios with a low scale of SUSY breaking [sqrt{F}=O(TeV)] the conventional MSSM Higgs sector can be substantially modified, mainly because the Higgs potential contains additional effective quartic terms. The Higgs spectrum can be dramatically changed, and the lightest state can be much heavier than in usual SUSY scenarios. Novel opportunities for electroweak breaking arise, and the electroweak scale may be obtained in a less fine-tuned way for wide ranges of tan(beta) and the Higgs mass, offering a possible solution to the MSSM fine-tuning problem.
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.