Softly fine-tuned Standard Model and the scale of inflation

The direct coupling between the Higgs field and the spacetime curvature, if finely tuned, is known to stabilize the Higgs boson mass. The fine-tuning is soft because the Standard Model (SM) parameters are subject to no fine-tuning thanks to their independence from the Higgs-curvature coupling. This soft fine-tuning leaves behind a large vacuum energy ∝ ΛUV 4 which inflates the Universe with a Hubble rate ∝ ΛUV, UV being the SM ultraviolet (UV) boundary. This means that the tensor-to-scalar ratio inferred from cosmic microwave background polarization measurements by BICEP2, Planck and others lead to the determination of UV. The exit from the inflationary phase, as usual, is accomplished via decays of the vacuum energy. Here, we show that, identification of UV with the inflaton, as a sliding UV scale upon the SM, respects the soft fine-tuning constraint and does not disrupt the stability of the SM Higgs boson.Scientific and Technical Research Council of Turkey (113C002 / 115F212

Effects of a Real Singlet Scalar on Veltman Condition

We revisit the fine-tuning problem in the Standard Model (SM) and show the modification in Veltman condition by virtue of a minimally-extended particle spectrum with one real SM gauge singlet scalar field. We demand the new scalar to interact with the SM fields through Higgs portal only, and that the new singlet to acquire a vacuum expectation value, resulting in a mixing with the CP-even neutral component of the Higgs doublet in the SM. The experimental bounds on the mixing angle are determined by the observed best-fit signal strength $\sigma/\sigma_{\rm SM}$. While, the one-loop radiative corrections to the Higgs mass squared, computed with an ultraviolet cut-off scale $\Lambda$, comes with a negative coefficient, the quantum corrections to the singlet mass squared acquires both positive and negative values depending on the parameter space chosen, which if positive might be eliminated by introducing singlet or doublet vector-like fermions. However, based upon the fact that there is mixing between the scalars, when transformed into the physical states, the tree-level coupling of the Higgs field to the vector-like fermions worsen the Higgs mass hierarchy problem. Therefore, the common attempt to introduce vector-like fermions to cancel the divergences in the new scalar mass, might not be a solution, if there is mixing between the scalars.Comment: 6pp, 3 figs, accepted for publication in Physics Letters

Dark Matter from Conformal Sectors

We show that a conformal-invariant dark sector, interacting conformally with the Standard Model (SM) fields through the Higgs portal, provides a viable framework where cold dark matter (CDM) and invisible Higgs decays can be addressed concurrently. Conformal symmetry naturally subsumes the Z_2 symmetry needed for stability of the CDM. It also guarantees that the weaker the couplings of the dark sector fields to the SM Higgs field, the smaller the masses they acquire through elektroweak breaking. The model comfortably satisfies the bounds from Large Hadron Collider (LHC) and Planck Space Telescope (Planck 2013).Comment: 9 pages, 3 figure

Higgs Bosons in a minimal R-parity conserving left-right supersymmetric model

We revisit the Higgs sector of the left-right supersymmetric model. We study the scalar potential in a version of the model in which the minimum is the charge conserving vacuum state, without R-parity violation or additional non-renormalizable terms in the Lagrangian. We analyze the dependence of the potential and of the Higgs mass spectrum on the various parameters of the model, pinpointing the most sensitive ones. We also show that, contrary to previous expectations, the model can predict light neutral flavor-conserving Higgs bosons, while the flavor-violating ones are heavy, and within the limits from K0-K0bar, D0-D0bar and B0-B0bar mixings. We study variants of the model in which at least one pair of doubly-charged Higgs bosons is light, and show that the parameter space for such Higgs masses and mixings is very restrictive, thus making the model more predictive.Comment: 28 pages, 3 figure

Higgsed Stueckelberg Vector and Higgs Quadratic Divergence

Here we show that, a hidden vector field whose gauge invariance is ensured by a Stueckelberg scalar and whose mass is spontaneously generated by the Standard Model Higgs field contributes to quadratic divergences in the Higgs boson mass squared, and even leads to its cancellation at one-loop when Higgs coupling to gauge field is fine-tuned. In contrast to mechanisms based on hidden scalars where a complete cancellation cannot be achieved, stabilization here is complete in that the hidden vector and the accompanying Stueckelberg scalar are both free from quadratic divergences at one-loop. This stability, deriving from hidden exact gauge invariance, can have important implications for modelling dark phenomena like dark matter, dark energy, dark photon and neutrino masses. The hidden fields can be produced at the LHC.Comment: 5pp, 1 fig. Improved exposition, rectified concurrency to broken and unbroken electroweak vacua, added reference

Higgs and Radion Phenomenology Beyond the Standard Model

In this thesis we study models Beyond the Standard Model including Left-Right Supersymmetric Model and Warped Extra Dimensional Models with a Fourth Generation. First, we revisit the Higgs sector of Left-Right Supersymmetric Model by studying the scalar potential in a version of the model in which the minimum is the charge and R-parity conserving vacuum state, and there are no additional non-renormalizable terms in the Lagrangian. We try to find a parameter space predicting at least one light doubly-charged Higgs boson, light neutral avor-conserving Higgs bosons. The Flavor-violating ones are heavy, and within the limits from Delta F = 1,2 mixings. The parameter space for such Higgs masses and mixings is very restrictive, thus making the model more predictive. Subsequently, we study warped extra-dimensional scenarios in the presence of a fourth family of fermions and with the fermion fields lying in the bulk. We concentrate on the flavor structure of the Higgs couplings with fermions in the flavor anarchy ansatz. The occupancy of the fourth family in the model typically enhances the misalignment effects and we show that one should expect them to be highly nonsymmetrical in the (34) inter-generational mixing. The radiative corrections from the new fermions and their flavor violating couplings to the Higgs affect negligibly known experimental precision measurements such as the oblique parameters and Z -> b \bar{b}or Z -> mu^ +mu^-. On the other hand, Delta F = 1, 2 processes, mediated by tree-level Higgs exchange, as well as radiative corrections to b -> s gamma and mu to e gamma put some pressure on the allowed size of the flavor violating couplings. These couplings produce distinguishable signals in high energy colliders as they alter the Higgs decay patterns as well as those of the new fermions. These signals might become very important indirect signals for these type of models as they would be present even when the Kaluza-Klein mass scale is high and no heavy Kaluza-Klein particle is discovered. Afterwards, we focus on the radion phenomenology in the same scenario with and without an additional fourth family of fermions. The radion couplings with the fermions are also generically misaligned with respect to the Standard Model fermion mass matrices as in the Higgs case, therefore producing some amount of flavor violating couplings and potentially influencing production and decay rates of the radion. We present simple analytic expressions for the radion-fermion couplings with three or four families. We also update and analyze the current experimental limits on radion couplings and on the model parameters. The modified decay branching ratios of the radion with an emphasis on the new channels involving flavor diagonal and flavor violating decays into fourth generation quarks and leptons are provided. Finally, we study the Higgs-radion mixing in a warped extra dimensional model in the same scenario. The fourth generation Higgs is now severely constrained by Large Hadron Collider data due to the large enhancement in the Higgs production cross-section in the absence of Higgs-radion mixing. We analyze the production and decay rates of the two physical states emerging from the mixing and confront them with present Large Hadron Collider data. We show that the current signals observed can be compatible with the presence of one, or both, of these Higgs-radion mixed states, although with a severely restricted parameter space. We also present the modified decay branching ratios of the mixed Higgs-radion states, including flavor violating decays into fourth generation quarks and leptons. The windows of allowed parameter space obtained are very sensitive to the increased precision of upcoming Large Hadron Collider data. During the present year, a clear picture of this scenario will emerge, either confirming or further severely constraining this scenario

Hidden Spin-3/2 Field in the Standard Model

Here we show that a massive spin-3/2 field can hide in the SM spectrum in a way revealing itself only virtually. We study collider signatures and loop effects of this field, and determine its role in Higgs inflation and its potential as Dark Matter. We show that this spin-3/2 field has a rich linear collider phenomenology and motivates consideration of a neutrino-Higgs collider. We also show that study of Higgs inflation, dark matter and dark energy can reveal more about the neutrino and dark sector.Comment: 8 pages, 8 figures, accepted for publication in Eur. Phys. J.

Saving the fourth generation Higgs with radion mixing

We study Higgs-radion mixing in a warped extra dimensional model with Standard Model fields in the bulk, and we include a fourth generation of chiral fermions. The main problem with the fourth generation is that, in the absence of Higgs-radion mixing, it produces a large enhancement in the Higgs production cross-section, now severely constrained by LHC data. We analyze the production and decay rates of the two physical states emerging from the mixing and confront them with present LHC data. We show that the current signals observed can be compatible with the presence of one, or both, of these Higgs-radion mixed states (the $\phi$ and the $h$), although with a severely restricted parameter space. In particular, the radion interaction scale must be quite low, Lambda_\phi ~ 1-1.3 TeV. If m_\phi ~ 125 GeV, the $h$ state must be heavier (m_h>320 GeV). If m_h ~ 125 GeV, the $\phi$ state must be quite light or close in mass (m_\phi ~ 120 GeV). We also present the modified decay branching ratios of the mixed Higgs-radion states, including flavor violating decays into fourth generation quarks and leptons. The windows of allowed parameter space obtained are very sensitive to the increased precision of upcoming LHC data. During the present year, a clear picture of this scenario will emerge, either confirming or further severely constraining this scenario.Comment: 22 pages, 5 figures, 4 table

Higgs Phenomenology in Warped Extra-Dimensions with a 4th Generation

We study a warped extra-dimension scenario where the Standard Model fields lie in the bulk, with the addition of a fourth family of fermions. We concentrate on the flavor structure of the Higgs couplings with fermions in the flavor anarchy ansatz. Even without a fourth family, these couplings will be generically misaligned with respect to the SM fermion mass matrices. The presence of the fourth family typically enhances the misalignment effects and we show that one should expect them to be highly non-symmetrical in the ${(34)}$ inter-generational mixing. The radiative corrections from the new fermions and their flavor violating couplings to the Higgs affect negligibly known experimental precision measurements such as the oblique parameters and $Z\to b {\bar b}$ or $Z \to \mu^+ \mu^-$. On the other hand, $\Delta F=1,2$ processes, mediated by tree-level Higgs exchange, as well as radiative corrections to $b \to s \gamma$ and $\mu \to e\gamma$ put some generic pressure on the allowed size of the flavor violating couplings. But more importantly, these couplings will alter the Higgs decay patterns as well as those of the new fermions, and produce very interesting new signals associated to Higgs phenomenology in high energy colliders. These might become very important indirect signals for these type of models as they would be present even when the KK mass scale is high and no heavy KK particle is discovered.Comment: 39 pages, 6 figure