Higgs Mass Bounds, Type II SeeSaw and LHC

In type II seesaw utilized to explain the observed neutrino masses and mixings, one extends the Standard Model (SM) by introducing scalar fields which transform as a triplet under the electroweak gauge symmetry. New scalar couplings involving the Higgs doublet then appear and, as we show, these have important implications for the Higgs boson mass bounds obtained using vacuum stability and perturbativity arguments. We identify, in particular, regions of the parameter space which permit the SM Higgs boson to be as light as 114.4 GeV, the LEP2 bound. The triplet scalars include doubly charged particles whose masses could, in principle, be in the few hundred GeV range, and so they may be accessible at the LHC. 1 E-mail: [email protected] 2 E-mail: [email protected] 3 E-mail: [email protected] The discovery of the Standard Model (SM) Higgs boson is arguably the single most important mission for the LHC. Under the somewhat radical assumption that the next energy frontier lies at Planck scale (MP l), it has been found that the SM Higgs boson mass lies in the range 127 GeV ≤ MH ≤ 170 GeV [1]. Here the lower bound of 127 GeV on MH derives from arguments based on the stability of the SM vacuum (more precisely, that the Higgs quartic coupling does not turn negative at any scale between MZ and MP l). Thus, from the point of view of the SM it is perhaps not too surprising that the Higgs boson has not yet been found. The upper bound of about 170 GeV on MH comes from the requirement that the Higgs quartic coupling remains perturbative and does not exceed √ 4π, say, during its evolution between MZ and MP l. It has become abundantly clear in recent years that an extension of the SM is needed to explain a number of experimental observations. These include solar and atmospheric neutrino oscillations [2], existence of non-baryonic dark matter [3], the observed baryon asymmetry of the universe, etc. Neutrino oscillations, in particular, cannot be understood within the SM, even after including dimension five operators with Planck scale cutoff. These operators yield neutrino masses of order 10 eV or less, which is far below the 0.05-0.01 eV scale needed to explain the observed atmospheric and solar neutrino oscillations, respectively. Two attractive seesaw mechanisms exist for explaining the measured neutrino masses (more accurately, mass differences squared). In the so-called type I seesaw [4], new physics is added to the SM by introducing at least two heavy right-handed neutrinos. The seesaw mechanism then ensures that the observed neutrinos acquire masses which are suppressed by the heavy right-handed neutrino mass scale(s). One expects that the heaviest right-handed neutrino has a mass less than or of order 10 GeV (This, roughly speaking, comes from the seesaw formula mD/MR for the light neutrino mass, where mD and MR denote the Dirac and right-handed neutrino masses, respectively, and assumes that mD is less than or of order the electroweak scale). In type II seesaw [5], the SM is supplemented by a SU(2)L triplet scalar field ∆ which also carries unit hypercharge. There exist renormalizable couplings l∆l which enable the neutrinos to acquire their tiny (observed) masses through the non-zero VEV of ∆. From our point of view one of the most interesting features in type II seesaw derives from the fact that the SU(2)L triplet ∆ interacts with the SM Higgs doublet via both cubic and quartic scalar couplings. These, as we will show in this letter, can have far reachings implications for the SM bounds on MH , which can be studied by employing the coupled renormalization group equations(RGEs) involving the SM Higgs doublet φ and ∆. We find, in particular, that for a plausible choice of parameters, the SM Higgs boson mass MH can be as low as the LEP2 bound