arXiv subjects

Tony Gherghetta

Publications and source records attributed to Tony Gherghetta.

At least 55 records · Page 3Linked to original sources

arXiv2017

Inflation and High-Scale Supersymmetry with an EeV Gravitino

We consider inflation and supersymmetry breaking in the context of a minimal model of supersymmetry in which the only "low" energy remnant of supersymmetry is the gravitino with a mass of order an EeV. In this theory, the supersymmetry breaking scale is above the inflaton mass, m ~ 3 x 10^{13} GeV, as are all sfermion and gaugino masses. In particular, for a no-scale formulation of Starobinsky-like inflation using the volume modulus T, we show that inflation can be accommodated even when the supersymmetry breaking scale is very large. Reheating is driven through a gravitational coupling to the two Higgs doublets and is enhanced by the large mu-parameter. This leads to gravitino cold dark matter where the mass is constrained to be in the range 0.1 EeV $\lesssim m_{3/2} \lesssim 1000$ EeV.

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arXiv2017

Low-Scale D-term Inflation and the Relaxion

We present a dynamical cosmological solution that simultaneously accounts for the early inflationary stage of the Universe and solves the supersymmetric little hierarchy problem via the relaxion mechanism. First, we consider an inflationary potential arising from the $D$-term of a new $U(1)$ gauge symmetry with a Fayet--Iliopolous term, that is independent of the relaxion. A technically natural, small $U(1)$ gauge coupling, $g\lesssim 10^{-8}$, allows for a low Hubble scale of inflation, $H_I\lesssim 10^5$ GeV, which is shown to be consistent with Planck data. This feature is then used to realize a supersymmetric two-field relaxion mechanism, where the second field is identified as the inflaton provided that $H_I\lesssim 10$ GeV. The inflaton controls the relaxion barrier height allowing the relaxion to evolve in the early Universe and scan the supersymmetric soft masses. After electroweak symmetry is broken, the relaxion settles at a local supersymmetry-breaking minimum with a range of $F$-term values that can naturally explain supersymmetric soft mass scales up to $10^6$ GeV.

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arXiv2016

Neutral Naturalness with Bifundamental Gluinos

We study constraints on one-loop neutral naturalness at the LHC by considering gluon partners which are required to ameliorate the tuning in the Higgs mass-squared arising at two loops. This is done with a simple orbifold model of folded supersymmetry which not only contains color-neutral stops but also bifundamental gluinos that are charged under the Standard Model color group $SU(3)_C$ and a separate $SU(3)_C'$ group. The bifundamental gluinos reduce the Higgs mass tuning at two loops and maintain naturalness provided the gluinos are lighter than approximately 1.9 TeV for a 5 TeV cutoff scale. Limits from the LHC already forbid bifundamental gluinos below 1.4 TeV, and other non-colored states such as electroweakinos, $Z'$ bosons and dark sector bound states may be probed at future colliders. The search for bifundamental gluinos therefore provides a direct probe of one-loop neutral naturalness that can be fully explored at the LHC.

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arXiv2016

Naturalizing Supersymmetry with a Two-Field Relaxion Mechanism

We present a supersymmetric version of a two-field relaxion model that naturalizes tuned versions of supersymmetry. This arises from a relaxion mechanism that does not depend on QCD dynamics and where the relaxion potential barrier height is controlled by a second axion-like field. During the cosmological evolution, the relaxion rolls with a nonzero value that breaks supersymmetry and scans the soft supersymmetric mass terms. Electroweak symmetry is broken after the soft masses become of order the supersymmetric Higgs mass term and causes the relaxion to stop rolling for superpartner masses up to $\sim 10^9$ GeV. This can explain the tuning in supersymmetric models, including split-SUSY models, while preserving the QCD axion solution to the strong CP problem. Besides predicting two very weakly-coupled axion-like particles, the supersymmetric spectrum may contain an extra Goldstino, which could be a viable dark matter candidate.

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arXiv2016

A Visible QCD Axion from an Enlarged Color Group

We consider the possibility of an enlarged QCD color group, SU(3+$N^\prime$) spontaneously broken to SU(3)$_c\times$SU($N^\prime$) with extra vector-like quarks transforming in the fundamental representation. When the heavy quarks are integrated out below the PQ-breaking scale, they generate an axion coupling which simultaneously solves the strong CP problem for both gauge groups. However, the axion mass now receives a new nonperturbative contribution from the SU($N^\prime$) confinement scale, which can be substantially larger than the QCD scale. This can increase the axion mass to be at or above the electroweak scale. This visible axion can then decay into gluons and photons giving rise to observable signals at Run-II of the LHC. In particular, if the mass is identified with the 750 GeV diphoton resonance then the new confinement scale is $\sim$ TeV and the PQ-breaking scale is $\sim$ 10 TeV. This predicts vector-like quarks and a PQ scalar resonance in the multi-TeV range, with the possibility that dark matter is an SU($N^\prime$) baryon.

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arXiv2016

Long-Lived, Colour-Triplet Scalars from Unnaturalness

Long-lived, colour-triplet scalars are a generic prediction of unnatural, or split, composite Higgs models where the spontaneous global-symmetry breaking scale $f \gtrsim 10$ TeV and an unbroken $SU(5)$ symmetry is preserved. Since the triplet scalars are pseudo Nambu-Goldstone bosons they are split from the much heavier composite-sector resonances and are the lightest exotic, coloured states. This makes them ideal to search for at colliders. Due to discrete symmetries the triplet scalar decays via a dimension-six term and given the large suppression scale $f$ is often metastable. We show that existing searches for collider-stable R-hadrons from Run-I at the LHC forbid a triplet scalar mass below 845 GeV, whereas with $300\,\mathrm{fb}^{-1}$ at 13 TeV triplet scalar masses up to 1.4 TeV can be discovered. For shorter lifetimes displaced-vertex searches provide a discovery reach of up to 1.8 TeV. In addition we present exclusion and discovery reaches of future hadron colliders as well as indirect limits that arise from modifications of the Higgs couplings.

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arXiv2015

SUSY Implications from WIMP Annihilation into Scalars at the Galactic Centre

An excess in gamma-rays emanating from the galactic centre has recently been observed in the Fermi-LAT data. This signal can be interpreted as resulting from WIMP annihilation, with the spectrum well-fit by dark matter annihilating dominantly into either bottom-quark or Higgs pairs. Supersymmetric models provide a well-motivated framework to study the implications of this signal in these channels. With a neutralino dark matter candidate, the gamma-ray excess cannot be easily accommodated in the minimal supersymmetric model, which in any case requires tuning below the percent level to explain the observed Higgs mass. Instead we are naturally led to consider the next-to-minimal model with a singlet superfield. This not only allows for the annihilation channel into bottom-quark pairs to be implemented, but also provides new possibilities for annihilation into Higgs-pseudoscalar pairs. We show that the fit to the gamma-ray excess for the Higgs-pseudoscalar channel can be just as good as for annihilation into bottom-quark pairs. Moreover, in the parameter range of interest, the next-to-minimal supersymmetric model solves the mu-problem and can explain the 125 GeV Higgs mass with improved naturalness. We also consider an extension by adding a right-handed neutrino superfield with the right-handed sneutrino acting as a dark matter candidate. Interestingly, this allows for the annihilation into pseudoscalar pairs which also provide a good fit to the gamma-ray excess. Furthermore, in the case of a neutralino LSP, the late decay of a sneutrino NLSP can non-thermally produce the observed relic abundance. Finally, the WIMP annihilation into scalar pairs allows for the possibility of detecting the Higgs or pseudoscalar decay into two photons, providing a smoking-gun signal of the model.

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arXiv2015

The Unnatural Composite Higgs

Composite Higgs models can trivially satisfy precision-electroweak and flavour constraints by simply having a large spontaneous symmetry breaking scale, f > 10 TeV. This produces a 'split' spectrum, where the strong sector resonances have masses greater than 10 TeV and are separated from the pseudo Nambu-Goldstone bosons, which remain near the electroweak scale. Even though a tuning of order 10^{-4} is required to obtain the observed Higgs boson mass, the big hierarchy problem remains mostly solved. Intriguingly, models with a fully-composite right-handed top quark also exhibit improved gauge coupling unification. By restricting ourselves to models which preserve these features we find that the symmetry breaking scale cannot be arbitrarily raised, leading to an upper bound f < 100-1000 TeV. This implies that the resonances may be accessible at future colliders, or indirectly via rare-decay experiments. Dark matter is identified with a pseudo Nambu-Goldstone boson, and we show that the smallest coset space containing a stable, scalar singlet and an unbroken SU(5) symmetry is SU(7) / SU(6) x U(1). The colour-triplet pseudo Nambu-Goldstone boson also contained in this coset space is metastable due to a residual symmetry. It can decay via a displaced vertex when produced at colliders, leading to a distinctive signal of unnaturalness.

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arXiv2015

A Soft-Wall Dilaton

We study the properties of the dilaton in a soft-wall background using two solutions of the Einstein equations. These solutions contain an asymptotically AdS metric with a nontrivial scalar profile that causes both the spontaneous breaking of conformal invariance and the generation of a mass gap in the particle spectrum. We first present an analytic solution, using the superpotential method, that describes a CFT spontaneously broken by a finite dimensional operator in which a light dilaton mode appears in the spectrum. This represents a tuning in the vanishing of the quartic coupling in the effective potential that could be naturally realised from an underlying supersymmetry. Instead, by considering a generalised analytic scalar bulk potential that quickly transitions at the condensate scale from a walking coupling in the UV to an order-one $β$-function in the IR, we obtain a naturally light dilaton. This provides a simple example for obtaining a naturally light dilaton from nearly-marginal CFT deformations in the more realistic case of a soft-wall background.

hep-th↗

arXiv2015

Affleck-Dine Sneutrino Inflation

Motivated by the coincidence between the Hubble scale during inflation and the typical see-saw neutrino mass scale, we present a supergravity model where the inflaton is identified with a linear combination of right-handed sneutrino fields. The model accommodates an inflaton potential that is flatter than quadratic chaotic inflation, resulting in a measurable but not yet ruled out tensor-to-scalar ratio. Small CP-violation in the neutrino mass matrix and supersymmetry breaking yield an evolution in the complex plane for the sneutrino fields. This induces a net lepton charge that, via the Affleck-Dine mechanism, can be the origin of the observed baryon asymmetry of the universe.

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arXiv2014

UV descriptions of composite Higgs models without elementary scalars

We consider four-dimensional UV descriptions of composite Higgs models without elementary scalars, in which four-fermion interactions are introduced to an underlying gauge theory like in the gauged NJL model. When the anomalous dimension of the fermion bilinear is large, these interactions drive the spontaneous global symmetry breaking in the model, with the Higgs identified as a Nambu-Goldstone boson. The UV descriptions support composite top partner operators, also with large anomalous dimensions, thereby providing an explicit realisation of the idea of partial compositeness. In particular, the composite SO(6)/SO(5) model can be described by an Sp gauge theory with four flavours of fermion, together with a vector-like pair of fermions transforming in the antisymmetric representation and charged under SU(3) colour. These fermions confine to produce both the Higgs and top partner bound states. Our methods can also be applied to different coset groups, suggesting that four-fermion operators can describe the underlying UV dynamics of other composite Higgs models.

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arXiv2014

The price of being SM-like in SUSY

We compute the tuning in supersymmetric models associated with the constraints from collider measurements of the Higgs couplings to fermions and gauge bosons. In supersymmetric models, a CP-even state with SM Higgs couplings mixes with additional, heavier CP-even states, causing deviations in the Higgs couplings from SM values. These deviations are reduced as the heavy states are decoupled with large soft masses, thereby exacerbating the tuning associated with the electroweak scale. This new source of tuning is different from that derived from collider limits on stops, gluinos and Higgsinos. It can be offset with large tan beta in the MSSM, however this compensating effect is limited in the NMSSM with a large Higgs-singlet coupling due to restrictions on large tan beta from electroweak precision tests. We derive a lower bound on this tuning and show that the level of precision of Higgs coupling measurements at the LHC will probe naturalness in the NMSSM at the few-percent level. This is comparable to the tuning derived from superpartner limits in models with a low messenger scale and split families. Instead the significant improvement in sensitivity of Higgs coupling measurements at the ILC will allow naturalness in these models to be constrained at the per-mille level, beyond any tuning derived from direct superpartner limits.

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arXiv2013

Radiative corrections to the composite Higgs mass from a gluon partner

In composite Higgs models light fermionic top partners often play an important role in obtaining a 126 GeV Higgs mass. The presence of these top partners implies that coloured vector mesons, or massive gluon partners, most likely exist. Since the coupling between the top partners and gluon partners can be large there are then sizeable two-loop contributions to the composite Higgs mass. We compute the radiative correction to the Higgs mass from a gluon partner in the minimal composite Higgs model and show that the Higgs mass is in fact reduced. This allows the top partner masses to be increased, easing the tension between having a light composite Higgs and heavy top partners.

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arXiv2013

The Scale-Invariant NMSSM and the 126 GeV Higgs Boson

The recent LHC discovery of a Higgs-like resonance at 126 GeV suggests that the minimal supersymmetric standard model must be modified in order to preserve naturalness. A simple extension is to include a singlet superfield and consider the scale-invariant NMSSM, whose renormalizable superpotential contains no dimensionful parameters. This extension not only solves the μ-problem, but can easily accommodate a 126 GeV Higgs. We study the naturalness of the scale-invariant NMSSM taking into account the recent constraints from LHC searches, flavor physics and electroweak precision tests. We show that TeV-scale stop masses are still allowed in much of the parameter space with 5% tuning for a low messenger scale of 20 TeV, split families (with third-generation sleptons decoupled) and Higgs-singlet coupling λof order one. For larger values of the Higgs-singlet coupling, which can relieve the tuning in the Higgs VEV, an additional tuning in the Higgs mass limits increasing the (lightest) stop mass beyond 1.2 TeV, the gluino mass above 3 TeV, and electroweak charginos and neutralinos beyond 400 GeV for a combined tuning better than 5%. This implies that the natural region of parameter space for the scale-invariant NMSSM will be fully explored at the 14 TeV LHC.

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arXiv2012

Natural gauge mediation with a bino NLSP at the LHC

Natural models of supersymmetry with a gravitino LSP provide distinctive signatures at the LHC. For a neutralino NLSP, sparticles can decay to two high energy photons plus missing energy. We use the ATLAS diphoton search with 4.8 fb^{-1} of data to place limits in both the stop-gluino and neutralino-chargino mass planes for this scenario. If the neutralino is heavier than 50 GeV, the lightest stop must be heavier than 580 GeV, the gluino heavier than 1100 GeV and charginos must be heavier than approximately 300-470 GeV. This provides the first nontrivial constraints in natural gauge mediation models with a neutralino NLSP decaying to photons, and implies a fine tuning of at least a few percent in such models.

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arXiv2012

Radion Dynamics and Phenomenology in the Linear Dilaton Model

We investigate the properties of the radion in the 5D linear dilaton model arising from Little String Theory. A Goldberger-Wise type mechanism is used to stabilise a large interbrane distance, with the dilaton now playing the role of the stabilising field. We consider the coupled fluctuations of the metric and dilaton fields and identify the physical scalar modes of the system. The wavefunctions and masses of the radion and Kaluza-Klein modes are calculated, giving a radion mass of order the curvature scale. As a result of the direct coupling between the dilaton and Standard Model fields, the radion couples to the SM Lagrangian, in addition to the trace of the energy-momentum tensor. The effect of these additional interaction terms on the radion decay modes is investigated, with a notable increase in the branching fraction to photons. We also consider the effects of a non-minimal Higgs coupling to gravity, which introduces a mixing between the Higgs and radion modes. Finally, we calculate the production cross section of the radion at the LHC and use the current Higgs searches to place constraints on the parameter space.

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arXiv2012

Split families unified

We present a simple supersymmetric model of split families consistent with flavor limits that preserves the successful prediction of gauge coupling unification and naturally accounts for the Higgs mass. The model provides an intricate connection between the Standard Model flavor hierarchy, supersymmetric flavor problem, unification and the Higgs mass. In particular unification favors a naturally large Higgs mass from D-term corrections to the quartic couplings in the Higgs potential. The unification scale is lowered with a stable proton that can account for the success of b-tau Yukawa coupling unification. The sparticle spectrum is similar to that of natural supersymmetry, as motivated by the supersymmetric flavor problem and recent LHC bounds, with a heavy scalar particle spectrum except for a moderately light stop required for viable electroweak symmetry breaking. Finally, Higgs production and decays, NLSP decays, and new states associated with extending the Standard Model gauge group above the TeV scale provide signatures for experimental searches at the LHC.

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arXiv2011

A Distorted MSSM Higgs Sector from Low-Scale Strong Dynamics

We show that when supersymmetry is broken at the TeV scale by strong dynamics, the Higgs sector of the MSSM can be drastically modified. This arises from possible sizeable mixings of the Higgs with the resonances of the strong sector. In particular the mass of the lightest Higgs boson can be significantly above the MSSM bound (~130 GeV). Furthermore only one Higgs doublet is strictly necessary, because the Yukawa couplings can have a very different structure compared to the MSSM. Using the AdS/CFT correspondence electroweak precision observables can be calculated and shown to be below experimental bounds. The most natural way to generate sparticle masses is through mixing with the composite states. This causes the gauginos and Higgsinos to easily obtain Dirac masses around 200 GeV, while scalar masses can be generated either from extra D-terms or also through mixing with the strongly-coupled states. Finally one of the most interesting predictions of these scenarios is the sizeable decay width of the Higgs boson into a very light gravitino (~ 10^{-4} eV) and a Higgsino.

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