arXiv subjects

Vincenzo Branchina

Publications and source records attributed to Vincenzo Branchina.

At least 37 records · Page 2Linked to original sources

arXiv2014

The lifetime of the electroweak vacuum and sensitivity to Planck scale physics

If the Standard Model (SM) is valid up to extremely high energy scales, then the Higgs potential becomes unstable at approximately $10^{11}$ GeV. However, calculations of the lifetime of the SM vacuum have shown that it vastly exceeds the age of the Universe. It was pointed out by two of us (VB,EM) that these calculations are extremely sensitive to effects from Planck scale higher-dimensional operators and, without knowledge of these operators, firm conclusions about the lifetime of the SM vacuum cannot be drawn. The previous paper used analytical approximations to the potential and, except for Higgs contributions, ignored loop corrections to the bounce action. In this work, we do not rely on any analytical approximations and consider all contributions to the bounce action, confirming the earlier result. It is surprising that the Planck scale operators can have such a large effect when the instability is at $10^{11}$ GeV. There are two reasons for the size of this effect. In typical tunneling calculations, the value of the field at the center of the critical bubble is much larger than the point of the instability; in the SM case, this turns out to be numerically within an order of magnitude of the Planck scale. In addition, tunneling is an inherently non-perturbative phenomenon, and may not be as strongly suppressed by inverse powers of the Planck scale. We include effective $Φ^6$ and $Φ^8$ Planck-scale operators and show that they can have an enormous effect on the tunneling rate.

hep-ph↗

arXiv2014

Top mass determination, Higgs inflation, and vacuum stability

The possibility that new physics beyond the Standard Model (SM) appears only at the Planck scale $M_P$ is often considered. However, it is usually argued that new physics interactions at $M_P$ do not affect the SM stability phase diagram, so the latter is obtained neglecting these terms. According to this diagram, for the current experimental values of the top and Higgs masses, our universe lives in a metastable state (with very long lifetime), near the edge of stability. Contrary to these expectations, however, we show that the stability phase diagram strongly depends on new physics and that, despite claims to the contrary, a more precise determination of the top (as well as of the Higgs) mass will not allow to discriminate between stability, metastability or criticality of the electroweak vacuum. At the same time, we show that the conditions needed for the realization of Higgs inflation scenarios (all obtained neglecting new physics) are too sensitive to the presence of new interactions at $M_P$. Therefore, Higgs inflation scenarios require very severe fine tunings that cast serious doubts on these models.

hep-ph↗

arXiv2014

Stability of the EW vacuum, Higgs boson, and new physics

The possibility that the Standard Model (SM) is valid up to the Planck scale $M_P$, i.e. that new physics occurs only around $M_P$, is nowadays largely explored. For a metastable EW vacuum, we show that new physics interactions can have a great impact on its lifetime, and, differently from previous analyses, they cannot be neglected. Therefore, contrary to usual believes, the stability phase diagram of the SM depends on new physics. This has far reaching consequences. Beyond SM theories can be tested against their prediction for the stability of the EW vacuum. Moreover, despite of some recent claims, higher precision measurements of the top and Higgs masses cannot provide any definite answer on the SM stability properties. Finally, doubts on Higgs inflation scenarios, all based on results obtained neglecting new physics interactions, are also cast.

hep-ph↗

arXiv2014

Stability, Higgs Boson Mass and New Physics

When the particle with mass $\sim 126$ GeV discovered at LHC is identified with the Higgs boson of the Standard Model, intriguing and challenging questions arise. Among them, the issue of the EW vacuum stability. We find that, despite claims to the contrary, the latter strongly depends on new physics interactions. In particular, if $τ$ is the lifetime of the EW vacuum, new physics can turn $τ$ from $τ>> T_U$ to $τ<< T_U$, where $T_U$ is the age of the Universe.

hep-ph↗

arXiv2013

A compared analysis of the susceptibility in the O(N) theory

The longitudinal susceptibility $χ_L$ of the O(N) theory in the broken phase is analyzed by means of three different approaches, namely the leading contribution of the 1/N expansion, the Functional Renormalization Group flow in the Local Potential approximation and the improved effective potential via the Callan-Symanzik equations, properly extended to $d=4$ dimensions through the expansion in powers of $ε=4-d$. The findings of the three approaches are compared and their agreement in the large $N$ limit is shown. The numerical analysis of the Functional Renormalization Group flow equations at small $N$ supports the vanishing of $χ_L^{-1}$ in $d=3$ and $d=3.5$ but is not conclusive in $d=4$, where we have to resort to the Callan-Smanzik approach. At finite $N$ as well as in the limit $N\to\infty$, we find that $χ^{-1}_L$ vanishes with $J$ as $J^{ε/2}$ for $ε>0$ and as $(\ln (J))^{-1}$ in $d=4$.

hep-th↗

arXiv2010

Dilution of zero point energies in the cosmological expansion

The vacuum fluctuations of all quantum fields filling the universe are supposed to leave enormous energy and pressure contributions which are incompatible with observations. It has been recently suggested that, when the effective nature of quantum field theories is properly taken into account, vacuum fluctuations behave as a relativistic gas rather than as a cosmological constant. Accordingly, zero-point energies are tremendously diluted by the universe expansion but provide an extra contribution to radiation energy. Ongoing and future cosmological observations could offer the opportunity to scrutinize this scenario. The presence of such additional contribution to radiation energy can be tested by using primordial nucleosynthesis bounds or measured on Cosmic Background Radiation anisotropy.

astro-ph.CO↗

arXiv2010

Mapping Fermion and Boson systems onto the Fock space of harmonic oscillators

The fluctuation-dissipation theorem (FDT) is very general and applies to a broad variety of different physical phenomena in condensed matter physics. With the help of the FDT and following the famous work of Caldeira and Leggett, we show that, whenever linear response theory applies, any generic bosonic or fermionic system at finite temperature $T$ can be mapped onto a fictitious system of free harmonic oscillators. To the best of our knowledge, this is the first time that such a mapping is explicitly worked out. This finding provides further theoretical support to the phenomenological harmonic oscillator models commonly used in condensed matter. Moreover, our result helps in clarifying an interpretation issue related to the presence and physical origin of the Bose-Einstein factor in the FDT.

cond-mat.stat-mech↗

arXiv2009

Dark energy and Josephson junctions

It has been recently claimed that dark energy can be (and has been) observed in laboratory experiments by measuring the power spectrum $S_I(ω)$ of the noise current in a resistively shunted Josephson junction and that in new dedicated experiments, which will soon test a higher frequency range, $S_I(ω)$ should show a deviation from the linear rising observed in the lower frequency region because higher frequencies should not contribute to dark energy. Based on previous work on theoretical aspects of the fluctuation-dissipation theorem, we carefully investigate these issues and show that these claims are based on a misunderstanding of the physical origin of the spectral function $S_I(ω)$. According to our analysis, dark energy has never been (and will never be) observed in Josephson junctions experiments. We also predict that no deviation from the linear rising behavior of $S_I(ω)$ will be observed in forthcoming experiments. Our findings provide new (we believe definite) arguments which strongly support previous criticisms.

gr-qc↗

arXiv2009

Fluctuation-dissipation theorem and harmonic oscillators

The question of the "physical meaning" and "origin" of the Bose-Einstein (BE) factor in the fluctuation-dissipation theorem (FDT) is often raised and this term is sometimes interpreted as originating from a real harmonic oscillator composition of the physical system. Such an interpretation, however, is not really founded. Inspired by the famous work of Caldeira and Leggett, we have been able to show that, whenever linear response theory is applicable, which is the main hypothesis under which the FDT is established, any generic bosonic and/or fermionic system at temperature $T$ can be mapped onto a fictitious system of harmonic oscillators so that the suscettivity and the mean square of the fluctuating observable of the real system coincide with the corresponding quantities of the fictitious one. We claim that it is in this sense, and only in this sense, that the BE factor can be interpreted in terms of harmonic oscillators, no other physical meaning can be superimposed to it. At the best of our knowledge, this is the first time that such a mapping is explicitly worked out.

cond-mat.stat-mech↗

arXiv2008

Effective potential and vacuum stability

By following previous work on this subject, we investigate the issue of the instability of the electroweak vacuum against the top loop corrections by performing an accurate analysis of a Higgs-Yukawa model. We find that, when the physical cutoff is properly implemented in the theory, the potential does not exhibit any instability. Moreover, contrary to recent claims, we show that this instability cannot be understood in terms of the very insightful work of Wu and Weinberg on the non-convexity of the one-loop effective potential of a scalar theory. Some of the theoretical and phenomenological consequences of our results are briefly discussed.

hep-ph↗

arXiv2005

Effective potential (in)stability and lower bounds on the scalar (Higgs) mass

It is widely believed that the top loop corrections to the Higgs effective potential destabilise the electroweak (EW) vacuum and that, imposing stability, lower bounds on the Higgs mass can be derived. With the help of a scalar-Yukawa model, we show that this apparent instability is due to the extrapolation of the potential into a region where it is no longer valid. Stability turns out to be an intrinsic property of the theory (rather than an additional constraint to be imposed on it). However, lower bounds for the Higgs mass can still be derived with the help of a criterium dictated by the properties of the potential itself. If the scale of new physics lies in the Tev region, sizeable differences with the usual bounds are found. Finally, our results exclude the alternative meta-stability scenario, according to which we might be living in a sufficiently long lived meta-stable EW vacuum.

hep-th↗

arXiv2004

Electronic contribution to the oscillations of a gravitational antenna

We carefully analyse the contribution to the oscillations of a metallic gravitational antenna due to the interaction between the electrons of the bar and the incoming gravitational wave. To this end, we first derive the total microscopic Hamiltonian of the wave-antenna system and then compute the contribution to the attenuation factor due to the electron-graviton interaction. As compared to the ordinary damping factor, which is due to the electron viscosity, this term turns out to be totally negligible. This result confirms that the only relevant mechanism for the interaction of a gravitational wave with a metallic antenna is its direct coupling with the bar normal modes.

gr-qc↗

arXiv2001

RG generated fermion mass

Chiral symmetry breaking in a purely fermionic theory is investigated by the help of the renormalization group method. The RG equation for the running mass $m_k$ admits a solution with vanishing bare mass and finite physical mass. The running Fermi coupling constant, $G_k$, converges to a finite (renormalized) physical value. It is also shown that the RG equation for $\tilde G$, the dimensionless Fermi coupling, has an UV fixed point $\tilde G_{UV}$. Contrary to a previous result however, it is proven that the chiral symmetry breaking point $\tilde G_c$ does not coincide with $\tilde G_{UV}$.

hep-ph↗

arXiv2000

Non perturbative renormalization group potentials and quintessence

New solutions to the non perturbative renormalization group equation for the effective action of a scalar field theory in the Local Potential Approximation having the exponential form $e^{\pmϕ}$ are found. This result could be relevant for those quintessence phenomenological models where this kind of potentials are already used, giving them a solid field theoretical derivation. Other non perturbative solutions, that could also be considered for the quintessence scenario, are also found. Apart from this particular cosmological application, these results could be relevant for other models where scalar fields are involved, in particular for the scalar sector of the standard model.

hep-ph↗

arXiv1999

Flow Equations for U_k and Z_k

By considering the gradient expansion for the wilsonian effective action S_k of a single component scalar field theory truncated to the first two terms, the potential U_k and the kinetic term Z_k, I show that the recent claim that different expansion of the fluctuation determinant give rise to different renormalization group equations for Z_k is incorrect. The correct procedure to derive this equation is presented and the set of coupled differential equations for U_k and Z_k is definitely established.

hep-th↗

arXiv1998

Instability Induced Renormalization

It is pointed out that models with condensates have nontrivial renormalization group flow on the tree level. The infinitesimal form of the tree level renormalization group equation is obtained and solved numerically for the phi4 model in the symmetry broken phase. We find an attractive infrared fixed point that eliminates the metastable region and reproduces the Maxwell construction.

cond-mat.stat-mech↗

arXiv1998

Global Renormalization Group

The motivation and the challenge in applying the renormalization group for systems with several scaling regimes is briefly outlined. The four dimensional $ϕ^4$ model serves as an example where a nontrivial low energy scaling regime is identified in the vicinity of the spinodal instability region. It is pointed out that the effective theory defined in the vicinity of the spinodal instability offers an amplification mechanism, a precursor of the condensation that can be used to explore nonuniversal forces at high energies.

hep-th↗

arXiv1997

Renormalization Group with Condensate

The renormalization group is applied to the phi4 model in the symmetry broken phase in order to identify different scaling regimes. The new scaling laws reflect nonuniversal behavior at the phase transition. The extension of the analysis to finite temperature is briefly outlined. It is mentioned that the coupling constants can be found in the mixed phase by taking into account the saddle points of the blocking procedure.

hep-th↗