arXiv subjects

J. R. Espinosa

Publications and source records attributed to J. R. Espinosa.

At least 37 records · Page 2Linked to original sources

arXiv2016

One-loop non-renormalization results in EFTs

In Effective Field Theories (EFTs) with higher-dimensional operators many anomalous dimensions vanish at the one-loop level for no apparent reason. With the use of supersymmetry, and a classification of the operators according to their embedding in super-operators, we are able to show why many of these anomalous dimensions are zero. The key observation is that one-loop contributions from superpartners trivially vanish in many cases under consideration, making supersymmetry a powerful tool even for non-supersymmetric models. We show this in detail in a simple U(1) model with a scalar and fermions, and explain how to extend this to SM EFTs and the QCD Chiral Langrangian. This provides an understanding of why most "current-current" operators do not renormalize "loop" operators at the one-loop level, and allows to find the few exceptions to this ubiquitous rule.

hep-ph↗

arXiv2016

The 750 GeV Diphoton Excess as a First Light on Supersymmetry Breaking

One of the most exciting explanations advanced for the recent diphoton excess found by ATLAS and CMS is in terms of sgoldstino decays: a signal of low-energy supersymmetry-breaking scenarios. The sgoldstino, a scalar, couples directly to gluons and photons, with strength related to gaugino masses, that can be of the right magnitude to explain the excess. However, fitting the suggested resonance width, Gamma ~ 45 GeV, is not so easy. In this paper we explore efficient possibilities to enhance the sgoldstino width, via the decay into two Higgses, two Higgsinos and through mixing between the sgoldstino and the Higgs boson. In addition, we present an alternative and more efficient mechanism to generate a mass splitting between the scalar and pseudoscalar components of the sgoldstino, which has been suggested as an interesting alternative explanation to the apparent width of the resonance.

hep-ph↗

arXiv2015

Cosmological Higgs-Axion Interplay for a Naturally Small Electroweak Scale

Recently, a new mechanism to generate a naturally small electroweak scale has been proposed. It exploits the coupling of the Higgs to an axion-like field and a long era in the early universe where the axion unchains a dynamical screening of the Higgs mass. We present a new realization of this idea with the new feature that it leaves no signs of new physics up to a rather large scale, 10^9 GeV, except for two very light and weakly coupled axion-like states. One of the scalars can be a viable Dark Matter candidate. Such a cosmological Higgs-axion interplay could be tested with a number of experimental strategies.

hep-ph↗

arXiv2015

Higgs Inflation as a Mirage

We discuss a simple unitarization of Higgs inflation that is genuinely weakly coupled up to Planckian energies. A large non-minimal coupling between the Higgs and the Ricci curvature is induced dynamically at intermediate energies, as a simple ratio of mass scales. Despite not being dominated by the Higgs field, inflationary dynamics simulates the `Higgs inflation' one would get by blind extrapolation of the low-energy effective Lagrangian, at least qualitatively. Hence, Higgs inflation arises as an approximate `mirage' picture of the true dynamics. We further speculate on the generality of this phenomenon and show that, if Higgs-inflation arises as an effective description, the details of the UV completion are necessary to extract robust quantitative predictions.

hep-ph↗

arXiv2014

The mold integration method for the calculation of the crystal-fluid interfacial free energy from simulations

The interfacial free energy between a crystal and a fluid, γ cf, is a highly relevant parameter in phenomena such as wetting or crystal nucleation and growth. Due to the difficulty of measuring γ cf experimentally, computer simulations are often used to study the crystal-fluid interface. Here, we present a novel simulation methodology for the calculation of γ cf . The methodology consists in using a mold composed of potential energy wells to induce the formation of a crystal slab in the fluid at coexistence conditions. This induction is done along a reversible pathway along which the free energy difference between the initial and the final states is obtained by means of thermodynamic integration. The structure of the mold is given by that of the crystal lattice planes, which allows to easily obtain the free energy for different crystal orientations. The method is validated by calculating γ cf for previously studied systems, namely, the hard spheres and the Lennard-Jones systems. Our results for the latter show that the method is accurate enough to deal with the anisotropy of γ cf with respect to the crystal orientation. We also calculate γ cf for a recently proposed continuous version of the hard sphere potential and obtain the same γ cf as for the pure hard sphere system. The method can be implemented both in Monte Carlo and Molecular Dynamics. In fact, we show that it can be easily used in combination with the popular Molecular Dynamics package GROMACS.

cond-mat.stat-mech↗

arXiv2014

Homogeneous ice nucleation evaluated for several water models

In this work, we evaluate by means of computer simulations the rate for ice homogeneous nucleation for several water models such as TIP4P, TIP4P/2005,TIP4P/ICE, and mW (following the same procedure as in Sanz et al. [J. Am. Chem. Soc.135, 15008 (2013)]) in a broad temperature range. We estimate the ice-liquid interfacial free-energy, and conclude that for all water models γ decreases as the temperature decreases. Extrapolating our results to the melting temperature, we obtain a value of the interfacial free-energy between 25 and 32 mN/m in reasonable agreement with the reported experimental values. Moreover, we observe that the values of γ depend on the chosen water model and this is a key factor when numerically evaluating nucleation rates, given that the kinetic prefactor is quite similar for all water models with the exception of the mW (due to the absence of hydrogens). Somewhat surprisingly the estimates of the nucleation rates found in this work for TIP4P/2005 are slightly higher than those of the mW model, even though the former has explicit hydrogens. Our results suggest that it may be possible to observe in computer simulations spontaneous crystallization of TIP4P/2005 at about 60 K below the melting point.

cond-mat.stat-mech↗

arXiv2014

Taming Infrared Divergences in the Effective Potential

The Higgs effective potential in the Standard Model (SM), calculated perturbatively, generically suffers from infrared (IR) divergences when the (field-dependent) tree-level mass of the Goldstone bosons goes to zero. Such divergences can affect both the potential and its first derivative and become worse with increasing loop order. In this paper we show that these IR divergences are spurious, we perform a simple resummation of all IR-problematic terms known (up to three loops) and explain how to extend the resummation to cure all such divergences to any order. The method is of general applicability and would work in scenarios other than the SM. Our discussion has some bearing on a scenario recently proposed as a mechanism for gauge mediation of scale breaking in the ultraviolet, in which it is claimed that the low-energy Higgs potential is non-standard. We argue that all non-decoupling effects from the heavy sector can be absorbed in the renormalization of low-energy parameters leading to a SM-like effective theory.

hep-ph↗

arXiv2014

High precision fundamental constants at the TeV scale

This report summarizes the proceedings of the 2014 Mainz Institute for Theoretical Physics (MITP) scientific program on "High precision fundamental constants at the TeV scale". The two outstanding parameters in the Standard Model dealt with during the MITP scientific program are the strong coupling constant $α_s$ and the top-quark mass $m_t$. Lacking knowledge on the value of those fundamental constants is often the limiting factor in the accuracy of theoretical predictions. The current status on $α_s$ and $m_t$ has been reviewed and directions for future research have been identified.

hep-ph↗

arXiv2013

Homogeneous ice nucleation at moderate supercooling from molecular simulation

Among all the freezing transitions, that of water into ice is probably the most relevant to biology, physics, geology or atmospheric science. In this work we investigate homogeneous ice nucleation by means of computer simulations. We evaluate the size of the critical cluster and the nucleation rate for temperatures ranging between 15K and 35K below melting. We use the TIP4P/2005 and the TIP4P/Ice water models. Both give similar results when compared at the same temperature difference with the model's melting temperature. The size of the critical cluster varies from $\sim$8000 molecules (radius$ = 4$nm) at 15K below melting to $\sim$600 molecules (radius$ = 1.7$nm) at 35K below melting. We use Classical Nucleation Theory (CNT) to estimate the ice-water interfacial free energy and the nucleation free energy barrier. We obtain an interfacial free energy of 29(3)mN/m from an extrapolation of our results to the melting temperature. This value is in good agreement both with experimental measurements and with previous estimates from computer simulations of TIP4P-like models. Moreover, we obtain estimates of the nucleation rate from simulations of the critical cluster at the barrier top. The values we get for both models agree within statistical error with experimental measurements. At temperatures higher than 20K below melting we get nucleation rates slower than the appearance of a critical cluster in all the water of the hydrosphere in the age of the universe. Therefore, our simulations predict that water freezing above this temperature must necessarily be heterogeneous.

cond-mat.soft↗

arXiv2013

Higgs windows to new physics through d = 6 operators: Constraints and one-loop anomalous dimensions

The leading contributions from heavy new physics to Higgs processes can be captured in a model-independent way by dimension-six operators in an effective Lagrangian approach. We present a complete analysis of how these contributions affect Higgs couplings. Under certain well-motivated assumptions, we find that 8 CP-even plus 3 CP-odd Wilson coefficients parametrize the main impact in Higgs physics, as all other coefficients are constrained by non-Higgs SM measurements. We calculate the most relevant anomalous dimensions for these Wilson coefficients, which describe operator mixing from the heavy scale down to the electroweak scale. This allows us to find the leading-log corrections to the predictions for the Higgs couplings in specific models, such as the MSSM or composite Higgs, which we find to be significant in certain cases.

hep-ph↗

arXiv2013

Renormalization of dimension-six operators relevant for the Higgs decays $h\rightarrow γγ,γZ$

The discovery of the Higgs boson has opened a new window to test the SM through the measurements of its couplings. Of particular interest is the measured Higgs coupling to photons which arises in the SM at the one-loop level, and can then be significantly affected by new physics. We calculate the one-loop renormalization of the dimension-six operators relevant for $h\rightarrow γγ, γZ$, which can be potentially important since it could, in principle, give log-enhanced contributions from operator mixing. We find however that there is no mixing from any current-current operator that could lead to this log-enhanced effect. We show how the right choice of operator basis can make this calculation simple. We then conclude that $h\rightarrow γγ, γZ$ can only be affected by RG mixing from operators whose Wilson coefficients are expected to be of one-loop size, among them fermion dipole-moment operators which we have also included.

hep-ph↗

arXiv2012

First Glimpses at Higgs' face

The 8 TeV LHC Higgs search data just released indicates the existence of a scalar resonance with mass ~ 125 GeV. We examine the implications of the data reported by ATLAS, CMS and the Tevatron collaborations on understanding the properties of this scalar by performing joint fits on its couplings to other Standard Model particles. We discuss and characterize to what degree this resonance has the properties of the Standard Model (SM) Higgs, and consider what implications can be extracted for New Physics in a (mostly) model-independent fashion. We find that, if the Higgs couplings to fermions and weak vector bosons are allowed to differ from their standard values, the SM is ~ 2 sigma from the best fit point to current data. Fitting to a possible invisible decay branching ratio, we find BR_{inv} = 0.05\pm 0.32\ (95% C.L.) We also discuss and develop some ways of using the data in order to bound or rule out models which modify significantly the properties of this scalar resonance and apply these techniques to the global current data set.

hep-ph↗

arXiv2012

Fingerprinting Higgs Suspects at the LHC

We outline a method for characterizing deviations from the properties of a Standard Model (SM) Higgs boson. We apply it to current data in order to characterize up to which degree the SM Higgs boson interpretation is consistent with experiment. We find that the SM Higgs boson is consistent with the current data set at the 82 % confidence level, based on data of excess events reported by CMS and ATLAS, which are interpreted to be related to the mass scale mh = 124-126 GeV, and on published CL_s exclusion regions. We perform a global fit in terms of two parameters characterizing the deviation from the SM value in the gauge and fermion couplings of a Higgs boson. We find two minima in the global fit and identify observables that can remove this degeneracy. An update for Moriond 2012 data is included in the Appendix, which finds that the SM Higgs boson is now consistent with the current data set at only the 94 % confidence level (which corresponds to ~ 2 sigma tension compared to the best fit point).

hep-ph↗

arXiv2012

Composite Higgs under LHC Experimental Scrutiny

The LHC has been built to understand the dynamics at the origin of the breaking of the electroweak symmetry. Weakly coupled models with a fundamental Higgs boson have focused most of the attention of the experimental searches. We will discuss here how to reinterpret these searches in the context of strongly coupled models where the Higgs boson emerges as a composite particle. In particular, we use LHC data to constrain the compositeness scale. We also briefly review the prospects to observe other bosonic and fermionic resonances of the strong sector.

hep-ph↗

arXiv2011

Ultraviolet Properties of the Higgs Sector in the Lee-Wick Standard Model

The Lee-Wick (LW) Standard Model (SM) offers a new solution to the hierarchy problem. We discuss, using effective potential techniques, its peculiar ultraviolet (UV) behaviour. We show how quadratic divergences in the Higgs mass Mh cancel as a result of the unusual dependence of LW fields on the Higgs background (in a manner reminiscent of Little Higgses). We then extract from the effective potential the renormalization group evolution of the Higgs quartic coupling lambda above the LW scale. After clarifying an apparent discrepancy with previous results for the LW Abelian Higgs model we focus on the LWSM. In contrast with the SM case, for any Mh, lambda grows monotonically and hits a Landau pole at a fixed trans-Planckian scale (never turning negative in the UV). Then, the perturbativity and stability bounds on Mh disappear. We identify a cutoff ~10^{16} GeV for the LWSM due to the hypercharge gauge coupling hitting a Landau pole. Finally, we also discuss briefly the possible impact of the UV properties of the LW models on their behaviour at finite temperature, in particular regarding symmetry nonrestoration.

hep-ph↗

arXiv2010

New Physics at the LHC. A Les Houches Report: Physics at TeV Colliders 2009 - New Physics Working Group

We present a collection of signatures for physics beyond the standard model that need to be explored at the LHC. First, are presented various tools developed to measure new particle masses in scenarios where all decays include an unobservable particle. Second, various aspects of supersymmetric models are discussed. Third, some signatures of models of strong electroweak symmetry are discussed. In the fourth part, a special attention is devoted to high mass resonances, as the ones appearing in models with warped extra dimensions. Finally, prospects for models with a hidden sector/valley are presented. Our report, which includes brief experimental and theoretical reviews as well as original results, summarizes the activities of the "New Physics" working group for the "Physics at TeV Colliders" workshop (Les Houches, France, 8-26 June, 2009).

hep-ph↗

arXiv2010

On the Naturalness of Higgs Inflation

We critically examine the recent claim that the Standard Model Higgs boson ${\cal H}$ could drive inflation in agreement with observations if $|{\cal H}|^2$ has a strong coupling $ξ\sim 10^4$ to the Ricci curvature scalar. We first show that the effective theory approach upon which that claim is based ceases to be valid beyond a cutoff scale $Λ=m_p/ξ$, where $m_p$ is the reduced Planck mass. We then argue that knowing the Higgs potential profile for the field values relevant for inflation ($|{\cal H}|>m_p/\sqrtξ\gg Λ$) requires knowledge of the ultraviolet completion of the SM beyond $Λ$. In absence of such microscopic theory, the extrapolation of the pure SM potential beyond $Λ$ is unwarranted and the scenario is akin to other ad-hoc inflaton potentials afflicted with significant fine-tuning. The appealing naturalness of this minimal proposal is therefore lost.

hep-ph↗

arXiv2009

The Probable Fate of the Standard Model

Extrapolating the Standard Model to high scales using the renormalisation group, three possibilities arise, depending on the mass of the Higgs boson: if the Higgs mass is large enough the Higgs self-coupling may blow up, entailing some new non-perturbative dynamics; if the Higgs mass is small the effective potential of the Standard Model may reveal an instability; or the Standard Model may survive all the way to the Planck scale for an intermediate range of Higgs masses. This latter case does not necessarily require stability at all times, but includes the possibility of a metastable vacuum which has not yet decayed. We evaluate the relative likelihoods of these possibilities, on the basis of a global fit to the Standard Model made using the Gfitter package. This uses the information about the Higgs mass available directly from Higgs searches at LEP and now the Tevatron, and indirectly from precision electroweak data. We find that the `blow-up' scenario is disfavoured at the 99% confidence level (96% without the Tevatron exclusion), whereas the `survival' and possible `metastable' scenarios remain plausible. A future measurement of the mass of the Higgs boson could reveal the fate of the Standard Model.

hep-ph↗