Antonio Racioppi

Towards Completing the Standard Model: Vacuum Stability, EWSB and Dark Matter

We study the standard model (SM) in its full perturbative validity range between

Physical Naturalness and Dynamical Breaking of Classical Scale Invariance

\Lambda_QCD

Non-minimal CW inflation, electroweak symmetry breaking and the 750 GeV anomaly

and the

Embedding inflation into the Standard Model — More evidence for classical scale invariance

U(1)_Y

Linear inflation from quartic potential

Landau pole, assuming that a yet unknown gravitational theory in the UV does not introduce additional particle thresholds, as suggested by the tiny cosmological constant and the absence of new stabilising physics at the EW scale. We find that, due to dimensional transmutation, the SM Higgs potential has a global minimum at 10^26 GeV, invalidating the SM as a phenomenologically acceptable model in this energy range. We show that extending the classically scale invariant SM with one complex singlet scalar S allows us to: (i) stabilise the SM Higgs potential; (ii) induce a scale in the singlet sector via dimensional transmutation that generates the negative SM Higgs mass term via the Higgs portal; (iii) provide a stable CP-odd singlet as the thermal relic dark matter due to CP-conservation of the scalar potential; (iv) provide a degree of freedom that can act as an inflaton in the form of the CP-even singlet. The logarithmic behaviour of dimensional transmutation allows one to accommodate the large hierarchy between the electroweak scale and the Landau pole, while understanding the latter requires a new non-perturbative view on the SM.

Electroweak symmetry breaking from the soft portal into dark matter and prediction for direct detection.

We propose a model of a confining dark sector, dark technicolor, that communicates with the Standard Model (SM) through the Higgs portal. In this model electroweak (EW) symmetry breaking and dark matter (DM) share a common origin, and the EW scale is generated dynamically. Our motivation to suggest this model is the absence of evidence for new physics from recent Large Hadron Collider (LHC) data. Although the conclusion is far from certain at this point, this lack of evidence may suggest that no mechanism exists at the EW scale to stabilize the Higgs mass against radiative corrections from ultraviolet (UV) physics. The usual reaction to this puzzling situation is to conclude that the stabilizing new physics is either hidden from us by accident, or that it appears at energies that are currently inaccessible, such that nature is indeed fine-tuned. In order to re-examine the arguments that have led to this dichotomy, we review the concept of naturalness in effective field theories, discussing in particular the role of quadratic divergences in relation to different energy scales. This leads us to suggest classical scale invariance as a guideline for model building, implying that explicit mass scales are absent in the underlying theory.

Implications of the effective axial-vector coupling of the gluon on top-quark charge asymmetry at the LHC

A bstractWe study whether the hinted 750 GeV resonance at the LHC can be a Coleman-Weinberg inflaton which is non-minimally coupled to gravity. Since the inflaton must couple to new charged and coloured states to reproduce the LHC diphoton signature, the same interaction can generate its effective potential and trigger the electroweak symmetry breaking via the portal coupling to the Higgs boson. This inflationary scenario predicts a lower bound on the tensor-to-scalar ratio of r ≳ 0.006, where the minimal value corresponds to the measured spectral index ns ≃ 0.97. However, we find that the compatibility with the LHC diphoton signal requires exotic new physics at energy scales accessible at the LHC. We study and quantify the properties of the predicted exotic particles.

Stückelino dark matter in anomalous U(1)′ models

A bstractIf cosmological inflation is due to a slowly rolling single inflation field taking trans-Planckian values as suggested by the BICEP2 measurement of primordial tensor modes in CMB, embedding inflation into the Standard Model challenges standard paradigm of effective field theories. Together with an apparent absence of Planck scale contributions to the Higgs mass and to the cosmological constant, BICEP2 provides further experimental evidence for the absence of large MP induced operators. We show that classical scale invariance — the paradigm that all fundamental scales in Nature are induced by quantum effects — solves the problem and allows for a remarkably simple scale-free Standard Model extension with inflaton without extending the gauge group. Due to trans-Planckian inflaton values and vevs, a dynamically induced Coleman-Weinberg-type inflaton potential of the model can predict tensor-to-scalar ratio r in a large range, converging around the prediction of chaotic m2ϕ2 inflation for a large trans-Planckian value of the inflaton vev. Precise determination of r in future experiments will single out a unique scale-free inflation potential, allowing to test the proposed field-theoretic framework.

Phase transition and gravitational wave phenomenology of scalar conformal extensions of the Standard Model

A bstractWe show that if the inflaton has a non-minimal coupling to gravity and the Planck scale is dynamically generated, the results of Coleman-Weinberg inflation are confined in between two attractor solutions: quadratic inflation, which is ruled out by the recent measurements, and linear inflation which, instead, is in the experimental allowed region. The minimal scenario has only one free parameter — the inflaton’s non-minimal coupling to gravity — that determines all physical parameters such as the tensor-to-scalar ratio and the reheating temperature of the Universe. Should the more precise future measurements of inflationary parameters point towards linear inflation, further interest in scale-invariant scenarios would be motivated.

Minimal but non-minimal inflation and electroweak symmetry breaking

Scalar dark matter (DM) can have dimensionful coupling to the Higgs boson-the soft portal into DM-which is predicted to be unsuppressed by the underlying SO(10) grand unified theory (GUT). The dimensionful coupling can be large, μ/v>>1, without spoiling the perturbativity of low energy theory up to the GUT scale. We show that the soft portal into DM naturally triggers radiative electroweak symmetry breaking (EWSB) via large 1-loop DM corrections to the effective potential. In this scenario, EWSB, the DM thermal freeze-out cross section, and DM scattering on nuclei are all dominated by the same coupling, predicting the DM mass range to be 700  GeV<M(DM)<2  TeV. The spin-independent direct detection cross section is predicted to be just below the present experimental bounds.