arXiv subjects

Roberto Vega-Morales

Publications and source records attributed to Roberto Vega-Morales.

At least 19 recordsLinked to original sources

arXiv2026

Schwinger dark matter production

Building on recently constructed inflationary vector dark matter production mechanisms as well as studies of magnetogenesis, we show that an inflationary dark Schwinger mechanism can generate the observed dark matter relic abundance for `dark electron' masses as light as $\sim 0.1$ eV and as heavy as $10^{12}$ GeV. The dark matter can interact very weakly via the exchange of light dark photons with a power spectrum which is peaked at very small scales, thus evading isocurvature constraints. This mechanism is viable even when (purely) gravitational particle production is negligible. Thus dark matter can be produced solely via the Schwinger effect during inflation including for light masses.

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arXiv2026

Dark photon dark matter from a rolling inflaton

We study in detail a recently proposed mechanism for producing non-thermal dark photon dark matter at the end of inflation in the mass range $μ\,{\rm eV} \lesssim m \lesssim 10\,{\rm TeV}$. A tachyonic instability induced by a rolling inflaton leads to the coherent production of dark (abelian) gauge bosons with a peak in the power spectrum corresponding to the Hubble scale at the end of inflation. As the Universe expands after inflation the dark photons redshift and, at some point in their cosmic evolution, they obtain a mass. We focus in particular on the case where the dark photons are relativistic at the time their mass is generated and examine the associated cosmic evolution to compute the relic abundance today. We also examine the late time power spectrum demonstrating explicitly that it preserves the peak generated at the end of inflation. We show that the peak corresponds to small physical scales today, $\ell_{\rm today} \sim {\rm cm} - 100\,{\rm km}$, with large density fluctuations at $\ell_{\rm today}$ leading to a clumpy nature for the dark photon dark matter. We also discuss potential phenomenology and future directions, briefly commenting on the non-relativistic case.

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arXiv2026

Schwinger Current in de Sitter Space

We study classical background electric fields and the Schwinger effect in de Sitter space. We show that having a constant electric field in de Sitter requires the photon to have a tachyonic mass proportional to the Hubble scale. This has physical implications for the induced Schwinger current which affect its IR behaviour. To study this we recompute the Schwinger current in de Sitter space for charged fermions and minimally coupled scalars imposing a physically consistent renormalization condition. We find a finite and positive Schwinger current even in the massless limit. This is in contrast to previous calculations in the literature which found a negative IR divergence. We also obtain the first result of the Schwinger current for a non-minimally coupled scalar, including for a conformally coupled scalar which we find has very similar behaviour to the fermion current. Our results may have physical implications for both magnetogenesis and inflationary dark matter production.

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arXiv2026

Classical constant electric fields and the Schwinger effect in de Sitter

We study constant classical electric fields and the Schwinger effect in de Sitter space, with potential implications for magnetogenesis and inflationary dark matter production. Treating the photon as a dynamical field, we show that sustaining a constant electric field in de Sitter requires a tachyonic photon mass of order the Hubble scale. This observation has physical implications, as it alters the infrared behaviour of the induced Schwinger current. Using an on-shell renormalization condition consistent with a tachyonic photon, we recompute the current for charged fermions and scalars, finding it to be finite and positive even in the massless limit of the charge carriers-contrary to earlier results predicting a puzzling negative IR divergence. For scalars, we include a non-minimal coupling to the Ricci curvature, enabling us to analyze the conformal limit, where the current closely matches that of charged fermions.

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arXiv2026

Global fits and the 95 GeV diphoton excesses in the Supersymmetric Georgi-Machacek Model

Recently the ATLAS and CMS experiments have reported modest excesses in the diphoton channel at around 95 GeV.~A number of recent studies have examined whether these could be due to an extended electroweak symmetry breaking (EWSB) sector, including the well known Georgi-Machacek (GM) model.~Here we examine whether the excesses can be explained by a light exotic Higgs boson in the \emph{Supersymmetric} GM (SGM) model which has the same scalar spectrum as the conventional GM model, but with a more constrained Higgs potential and the presence of custodial Higgsino fermions.~We perform a global fit of the SGM model including all relevant production and decay channels, some of which have been neglected in previous studies, which severely constrain the parameter space.~We find that the SGM model can fit the data if the LHC diphoton excesses at 95\,GeV are due to the lightest custodial singlet Higgs boson which contributes $(5-7)\%$ to EWSB, but \emph{cannot} accommodate the LEP $b\bar{b}$ excess, in contrast to other recent studies of the GM model.~Since the SGM model has a highly constrained Higgs potential, the rest of the mass spectrum is sharply predicted, allowing for targeted searches at the LHC or future colliders.~We also compare the SGM model with the non-supersymmetric GM model and identify how they can be distinguished at the LHC or future colliders.

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arXiv2024

Golden Probe of the Top Yukawa

We perform a preliminary study of the ability of the Higgs decay to four leptons to shed light on the top quark Yukawa couplings. In particular we examine whether the $h\to 4\ell$ `golden channel' is sensitive to the $CP$ properties of the top quark couplings to the Higgs boson. We show that kinematic distributions are sensitive to interference of the next-to-leading order electroweak corrections with the tree level $ZZ$ contribution. This translates into a sensitivity to the top quark Yukawa couplings such that meaningful constraints on their $CP$ properties can begin to be obtained once $\sim 300$ fb$^{-1}$ of data has been collected at $\sim 14$ TeV, with significant improvements at higher luminosity or with a higher energy hadron collider. This makes the $h\to4\ell$ channel a useful probe of the top quark Yukawa couplings that is qualitatively different from already established searches in $h\to Vγ$ two body decays, $tth$, and $gg\to h$. We also briefly discuss other potential possibilities for probing the top Yukawa $CP$ properties in $h\to2\ellγ$ and $\ell^+\ell^-\to h Z, hγ$.

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arXiv2024

Super heavy dark matter from inflationary Schwinger production

We consider a simple setup with a dark sector containing dark electrons charged under an abelian $U(1)_D$ gauge symmetry. We show that if the massless dark photon associated to the $U(1)_D$ is produced during inflation in such a way as to form a classical dark electric field, then dark electron-positron pairs are also produced close to the end of inflation via the Schwinger effect even if they are very massive. For large enough dark electric force, dark electrons with masses larger than the Hubble scale can be produced which are non-relativistic at production and throughout their cosmic evolution. They can account for the dark matter abundance today for masses in the range $\sim$ 100 GeV to $10^{17}$ GeV and up to six orders of magnitude larger than the Hubble scale at the end of inflation where purely gravitational production is exponentially suppressed. We examine the regime where the dark electrons do not thermalize with the dark photons throughout their cosmic history and assume negligible kinetic mixing with the visible $U(1)$ so they remain decoupled from the Standard Model thermal bath as well. Thus the final dark matter relic abundance is determined only by the initial inflationary Schwinger production and redshifting after reheating.

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arXiv2021

On New Physics Contributions to the Higgs Decay to $Zγ$

We explore models of new physics that can give rise to large (100% or more) enhancements to the rate of Higgs decay to $Zγ$ while still being consistent with other measurements. We show that this is impossible in simple models with one additional multiplet and also in well motivated models such as the MSSM and folded SUSY. We do find models with several multiplets that carry electroweak charge where such an enhancement is possible, but they require destructive interference effects. We also show that kinematic measurements in Higgs decay to four leptons can be sensitive to such models. Finally we explore the sensitivity of four lepton measurements to supersymmetric models and find that while the measurement is difficult with the high luminosity LHC, it may be possible with a future high energy hadron collider.

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arXiv2019

A Golden Probe of Nonlinear Higgs Dynamics

The most salient generic feature of a composite Higgs boson resides in the nonlinearity of its dynamics, which arises from degenerate vacua associated with the pseudo-Nambu-Goldstone (PNGB) nature of the Higgs boson. It has been shown that the nonlinear Higgs dynamics is universal in the IR and controlled only by a single parameter $f$, the decay constant of the PNGB Higgs. In this work we perform a fit, for the first time, to Wilson coefficients of ${\cal O}(p^4)$ operators in the nonlinear Lagrangian using the golden H $\to$ 4L decay channel. By utilizing both the "rate" information in the signal strength and the "shape" information in the fully differential spectra, we provide limits on the Goldstone decay constant $f$, as well as ${\cal O}(p^4)$ Wilson coefficients, using Run 2 data at the LHC. In rate measurements alone, the golden channel prefers a negative $ξ=v^2/f^2$ corresponding to a non-compact coset structure. Including the shape information, we identify regions of parameter space where current LHC constraint on $f$ is still weak, allowing for $ξ\lesssim 0.5$ or $ξ\gtrsim -0.5$. We also comment on future sensitivity at the high-luminosity upgrade of the LHC which could allow for simultaneous fits to multiple Wilson coefficients.

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arXiv2019

Vector dark matter production at the end of inflation

It has been shown that the longitudinal mode of a massive vector boson can be produced by inflationary fluctuations and account for the dark matter content of the Universe. In this work we examine the possibility of instead producing the transverse mode via the coupling $ϕF \tilde F$ between the inflaton and the vector field strength. Such a coupling leads to a tachyonic instability and exponential production of one transverse polarization of the vector field, reaching its maximum near the end of inflation. At production the mass is negligible and the vectors add up coherently to form a dark electromagnetic field. As the Universe expands, the energy density of the dark electromagnetic field then redshifts like radiation until its wavelength stretches to beyond its Compton wavelength. After this point the vectors become non-relativistic and their energy density redshifts like matter. We show that these polarized transverse vectors can account for the observed dark matter relic density in the mass range $μ$eV to hundreds of GeV. We also find that the tachyonic production mechanism of the transverse mode can accommodate larger vector masses and lower Hubble scales of inflation compared to the production mechanism for the longitudinal mode via inflationary fluctuations.

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arXiv2018

Light (and darkness) from a light hidden Higgs

We examine light diphoton signals from extended Higgs sectors possessing (approximate) fermiophobia with Standard Model (SM) fermions as well as custodial symmetry. This class of Higgs sectors can be realized in various beyond the SM scenarios and is able to evade many experimental limits, even at light masses, which are otherwise strongly constraining. Below the $WW$ threshold, the most robust probes of the neutral component are di and multi-photon searches. Utilizing the dominant Drell-Yan Higgs pair production mechanism and combining it with updated LHC diphoton data, we derive robust upper bounds on the allowed branching ratio for masses between $45 - 160$ GeV. Furthermore, masses $\lesssim 110$ GeV are ruled out if the coupling to photons is dominated by $W$ boson loops. We then examine two simple ways to evade these bounds via cancellations between different loop contributions or by introducing decays into an invisible sector. This also opens up the possibility of future LHC diphoton signals from a light hidden Higgs sector. As explicit realizations, we consider the Georgi-Machacek (GM) and Supersymmetric GM (SGM) models which contain custodial (degenerate) Higgs bosons with suppressed couplings to SM fermions and, in the SGM model, a (neutralino) LSP. We also breifly examine the recent $\sim 3σ$ CMS diphoton excess at $\sim 95$ GeV.

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arXiv2018

Golden Probe of the di$-Υ$ Threshold

Recent studies indicate that the ground state of a QCD four bottom quark system may form a resonance in the range $\sim 18 -19$ GeV, near the di-$Υ$ threshold, which may be observable at the LHC in the four lepton final state. These studies also predict a variety of possible resonances associated with the various excited states and allow for a number of possible spin and $CP$ quantum numbers. Of course, even if a resonance is observed in four leptons in the predicted mass range, it will be prudent to experimentally confirm its four bottom quark QCD nature or whether it is perhaps something more exotic. We initiate an investigation in this direction by exploring the ability of the \emph{normalized} fully differential decay width for decays to four leptons to probe the underlying nature of a putative resonance including $CP$ and tensor properties. We assume the first observed state is a spin-0 boson decaying to at least one on-shell $Υ$, but allow for decays to four leptons through $ΥΥ,\,Υγ$, and $ΥZ$ vector boson pairs. We consider a range of resonance masses around the di-$Υ$ threshold and find excellent prospects at the LHC for establishing its $CP$ and tensor properties, perhaps not long after a discovery depending on (unknown) production cross sections and the exact resonance mass.

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arXiv2018

The Supersymmetric Georgi-Machacek Model

We show that the well known Georgi-Machacek (GM) model can be realized as a limit of the recently constructed Supersymmetric Custodial Higgs Triplet Model (SCTM) which in general contains a significantly more complex scalar spectrum. We dub this limit of the SCTM, which gives a weakly coupled origin for the GM model at the electroweak scale, the Supersymmetric GM (SGM) model. We derive a mapping between the SGM and GM models using it to show how a supersymmetric origin implies constraints on the Higgs potential in conventional GM model constructions which would generically not be present. We then perform a simplified phenomenological study of diphoton and ZZ signals for a pair of benchmark scenarios to illustrate under what circumstances the GM model can mimic the SGM model and when they should be easily distinguishable.

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arXiv2016

Golden Probe of Electroweak Symmetry Breaking

The ratio of the Higgs couplings to $WW$ and $ZZ$ pairs, $λ_{WZ}$, is a fundamental parameter in electroweak symmetry breaking as well as a measure of the (approximate) custodial symmetry possessed by the gauge boson mass matrix. We show that Higgs decays to four leptons are sensitive, via tree level/1-loop interference effects, to both the magnitude and, in particular, overall sign of $λ_{WZ}$. Determining this sign requires interference effects, as it is nearly impossible to measure with rate information. Furthermore, simply determining the sign effectively establishes the custodial representation of the Higgs boson. We find that $h\to4\ell$ ($4\ell \equiv 2e2μ, 4e, 4μ$) decays have excellent prospects of directly establishing the overall sign at a high luminosity 13 TeV LHC. We also examine the ultimate LHC sensitivity in $h\to4\ell$ to the magnitude of $λ_{WZ}$. Our results are independent of other measurements of the Higgs boson couplings and, in particular, largely free of assumptions about the top quark Yukawa couplings which also enter at 1-loop. This makes $h\to4\ell$ a unique and independent probe of the electroweak symmetry breaking mechanism and custodial symmetry.

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arXiv2016

New vector bosons and the diphoton excess

We consider the possibility that the recently observed diphoton excess at $\sim 750$ GeV can be explained by the decay of a scalar particle ($φ$) to photons. If the scalar is the remnant of a symmetry-breaking sector of some new gauge symmetry, its coupling to photons can be generated by loops of the charged massive vectors of the broken symmetry. If these new $W^\prime$ vector bosons carry color, they can also generate an effective coupling to gluons. In this case the diphoton excess could be entirely explained in a simplified model containing just $φ$ and $W^\prime$. On the other hand if $W^{\prime}$ does not carry color, we show that, provided additional colored particles exist to generate the required $φ$ to gluon coupling, the diphoton excess could be explained by the same $W^{\prime}$ commonly invoked to explain the diboson excess at $\sim 2$ TeV. We also explore possible connections between the diphoton and diboson excesses with the anomalous $t\bar{t}$ forward-backward asymmetry.

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arXiv2016

Diphoton and Diboson Probes of Fermiophobic Higgs Bosons at the LHC

Extensions of the Standard Model Higgs sector with electroweak charged scalars can possess exotic `Higgs' bosons with vanishing or suppressed couplings to Standard Model fermions. These `fermiophobic' scalars, which cannot be produced via gluon fusion, are constrained by LHC measurements of the 125 GeV Higgs boson to have a small vacuum expectation value. This implies that vector boson fusion and associated vector boson production are in general suppressed rendering conventional Higgs searches insensitive. However, Drell-Yan Higgs pair production, which is not present in the SM, can be sizeable even in the limit of vanishing exotic Higgs vacuum expectation value. We utilize this to show that diphoton searches at 8 TeV LHC already rule out a large class of neutral fermiophobic Higgs bosons below around 110 GeV. This includes fermiophobic scalars found in two Higgs doublet as well as Higgs triplet and Georgi-Machacek type models. Our results extend the only relevant limit on fermiophobic Higgs bosons obtained by a recent CDF analysis of $4γ+ X$ Tevatron data. Furthermore, diphoton limits are independent of the decay of the second Higgs boson and thus apply even for degenerate masses in contrast to the CDF search. We also find that if the fermiophobic Higgs has very enhanced couplings to photons, masses as large as $\sim 150$ GeV can be ruled out while if these couplings are somehow highly suppressed, masses below $\sim 90$ GeV can still be ruled out. Finally, we show that $WW$ and $ZZ$ diboson searches may serve as complementary probes for masses above the diphoton limit up to $\sim 250$ GeV and discuss prospects at 13 TeV LHC.

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arXiv2016

The Virtual Diphoton Excess

Interpreting the excesses around 750 GeV in the diphoton spectra to be the signal of a new heavy scalar decaying to photons, we point out the possibility of looking for correlated signals with virtual photons. In particular, we emphasize that the effective operator that generates the diphoton decay will also generate decays to two leptons and a photon, as well as to four leptons, independently of the new resonance couplings to $Zγ$ and $ZZ$. Depending on the relative sizes of these effective couplings, we show that the virtual diphoton component can make up a sizable, and sometimes dominant, contribution to the total $2\ell γ$ and $4\ell$ partial widths. We also discuss modifications to current experimental cuts in order to maximize the sensitivity to these virtual photon effects. Finally, we briefly comment on prospects for channels involving other Standard Model fermions as well as more exotic decay possibilities of the putative resonance.

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arXiv2015

Golden Probe of the Top Yukawa

We describe how the Higgs decay to four leptons can be used to probe the nature and $CP$ structure of the top Yukawa coupling.

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