Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userImplications of the top (and Higgs) mass for vacuum stability
66
0
0.0
(
0
)
Authors
Jose R. Espinosa
Ask ChatGPT about the research
No Arabic abstract
The discovery of the Higgs boson by the LHC and the measurement of its mass at around 125 GeV, taken together with the absence of signals of physics beyond the standard model, make it possible that we might live in a metastable electroweak vacuum. Intriguingly, we seem to be very close to the boundary of stability and this near-criticality makes our vacuum extremely long-lived. In this talk I describe the state-of-the-art calculation leading to these results, explaining what are the ingredients and assumptions that enter in it, with special emphasis on the role of the top mass. I also discuss possible implications of this metastability for physics beyond the standard model and comment on the possible impact of physics at the Planck scale on near-criticality.
rate research
Read More
The ATLAS and CMS experiments observed a particle at the LHC with a mass $approx 126$ GeV, which is compatible with the Higgs boson of the Standard Model. A crucial question is, if for such a Higgs mass value, one could extrapolate the model up to high scales while keeping the minimum of the scalar potential that breaks the electroweak symmetry stable. Vacuum stability requires indeed the Higgs boson mass to be $M_H gsim 129 pm 1$ GeV, but the precise value depends critically on the input top quark pole mass which is usually taken to be the one measured at the Tevatron, $m_t^{rm exp}=173.2 pm 0.9$ GeV. However, for an unambiguous and theoretically well-defined determination of the top quark mass one should rather use the total cross section for top quark pair production at hadron colliders. Confronting the latest predictions of the inclusive $p bar p to tbar t +X$ cross section up to next-to-next-to-leading order in QCD to the experimental measurement at the Tevatron, we determine the running mass in the $bar{rm MS}$-scheme to be $m_t^{bar{rm MS}}(m_t) = 163.3 pm 2.7$ GeV which gives a top quark pole mass of $m_t^{rm pole}= 173.3 pm 2.8$ GeV. This leads to the vacuum stability constraint $M_H geq 129.8 pm 5.6$ GeV to which a $approx 126$ GeV Higgs boson complies as the uncertainty is large. A very precise assessment of the stability of the electroweak vacuum can only be made at a future high-energy electron-positron collider, where the top quark pole mass could be determined with a few hundred MeV accuracy.
A general prediction from asymptotically safe quantum gravity is the approximate vanishing of all quartic scalar couplings at the UV fixed point beyond the Planck scale. A vanishing Higgs doublet quartic coupling near the Planck scale translates into a prediction for the ratio between the mass of the Higgs boson $M_H$ and the top quark $M_t$. If only the standard model particles contribute to the running of couplings below the Planck mass, the observed $M_Hsim125,{rm GeV}$ results in the prediction for the top quark mass $M_tsim 171,{rm GeV}$, in agreement with recent measurements. In this work, we study how the asymptotic safety prediction for the top quark mass is affected by possible physics at an intermediate scale. We investigate the effect of a $SU(2)$ triplet scalar and right-handed neutrinos, needed to explain the tiny mass of left-handed neutrinos. For pure seesaw II, with no or very heavy right handed neutrinos, the top mass can increase to $M_tsim 172.5,{rm GeV}$ for a triplet mass of $M_Deltasim 10^8{rm GeV}$. Right handed neutrino masses at an intermediate scale increase the uncertainty of the predictions of $M_t$ due to unknown Yukawa couplings of the right-handed neutrinos and a cubic interaction in the scalar potential. For an appropriate range of Yukawa couplings there is no longer an issue of vacuum stability.
We consider the introduction of a complex scalar field carrying a global lepton number charge to the Standard Model and the Higgs inflation framework. The conditions are investigated under which this model can simultaneously ensure Higgs vacuum stability up to the Planck scale, successful inflation, non-thermal Leptogenesis via the pendulum mechanism, and light neutrino masses. These can be simultaneously achieved when the scalar lepton is minimally coupled to gravity, that is, when standard Higgs inflation and reheating proceed without the interference of the additional scalar degrees of freedom. If the scalar lepton also has a non-minimal coupling to gravity, a multi-field inflation scenario is induced, with interesting interplay between the successful inflation constraints and those from vacuum stability and Leptogenesis. The parameter region that can simultaneously achieve the above goals is explored.
Current Higgs data at the Large Hadron Collider is compatible with a SM signal at the 2$sigma$ level, but the central value of the signal strength in the diphoton channel is enhanced with respect to the SM expectation. If the enhancement resides in the diphoton partial decay width, the data could be accommodated in the Minimally Supersymmetric Standard Model (MSSM) with highly mixed light staus. We revisit the issue of vacuum instability induced by large mixing in the stau sector, including effects of a radiatively-corrected tau Yukawa coupling. Further, we emphasize the importance of taking into account the $tanbeta$ dependence in the stability bound. While the metastability of the Universe constrains the possible enhancement in the Higgs to diphoton decay width in the light stau scenario, an increase of the order of 50% can be achieved in the region of large $tanbeta$. Larger enhancements may be obtained, but would require values of $tanbeta$ associated with non-perturbative values of the tau Yukawa coupling at scales below the GUT scale, thereby implying the presence of new physics beyond the MSSM.
We discuss the issue of vacuum stability of standard model by embedding it within the TeV scale left-right universal seesaw model (called SLRM in the text). This model has only two coupling parameters $(lambda_1, lambda_2)$ in the Higgs potential and only two physical neutral Higgs bosons $(h, H)$. We explore the range of values for $(lambda_1, lambda_2)$ for which the light Higgs boson mass $M_h=126$ GeV and the vacuum is stable for all values of the Higgs fields. Combining with the further requirement that the scalar self couplings remain perturbative till typical GUT scales of order $10^{16}$ GeV, we find (i) an upper and lower limit on the second Higgs $(H)$ mass to be within the range: $0.4 leq frac{M_H}{v_R}leq 0.7$, where the $v_R$ is the parity breaking scale and (ii) that the heavy vector-like top, bottom and $tau$ partner fermions ($P_3, N_3, E_3$) mass have an upper bound $M_{P_3, N_3, E_3} leq v_R$. We discuss some phenomenological aspects of the model pertaining to LHC.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
