arXiv subjects

Ameen Ismail

Publications and source records attributed to Ameen Ismail.

At least 19 recordsLinked to original sources

arXiv2026

Probing Confining Dark Sectors with Cosmological Perturbations

Dark matter may emerge as a composite state of a dark sector which confines in a strongly first-order phase transition (PT). To avoid structure formation constraints on warm dark matter, the dark PT must occur above the keV scale. We investigate the cosmological signatures of this scenario, focusing on a keV- to MeV-scale PT. The stochastic nature of bubble nucleation sources curvature perturbations that can be constrained by various cosmological observations. Composite dark matter inherits the isocurvature perturbations generated during the PT and sources large-scale curvature perturbations. In contrast to a PT that reheats into dark radiation, the slower redshifting of dark matter enhances the infrared tail of the curvature perturbation upon horizon entry. The PT-induced perturbations compete with the suppression of the matter power spectrum due to the free-streaming of composite dark matter. We place limits on the PT strength and temperature from cosmic microwave background anisotropies, the Lyman-$\alpha$ forest, and other probes of the small-scale matter power spectrum. In a minimal scenario where the relic density is determined by the PT parameters, this provides a concrete example of a dark matter model that is testable via measurements of cosmological perturbations -- even in the absence of a sizable nongravitational coupling to the visible sector.

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arXiv2026

Uncool soft-wall transitions and gravitational waves

Theories with warped extra dimensions, like the Randall-Sundrum (RS) model, exhibit a holographic phase transition from a hot, deconfined black brane phase to a cool, confined phase. The standard picture of a first-order, strongly supercooled phase transition is expected to change in variations where the extra dimension is smoothly cut off by a soft-wall curvature singularity, as opposed to a hard brane. To understand this situation, we consider a simple ansatz for the warped geometry which allows us to obtain analytical results while maintaining the essential behavior of a soft wall. Unlike RS with the usual Goldberger-Wise stabilization, the hot, black brane phase only exists above a minimum temperature, which is not much smaller than the critical temperature. We explore the dynamics of the phase transition across the range of possibilities for the asymptotic geometry of a soft wall. This involves calculating an effective 4D action for the location of the black brane horizon. Using the effective action, we show that the phase transition completes rapidly ($\beta/H$ of $10^{3\text{-}4}$ is typical) and with only slight supercooling. We compute the resulting gravitational wave signal for a TeV-scale transition, finding that it is accessible to future space-based interferometers.

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arXiv2025

Asymptotic Freedom for Holographic Energy Correlators

We calculate energy correlators in a holographic model incorporating elements of asymptotic freedom and confinement. We model a running coupling by considering a geometry with a warp factor that deviates logarithmically from anti-de Sitter (AdS). A novel aspect of our bulk metric is that it smoothly interpolates between a Randall-Sundrum solution with a hard wall and a geometry corresponding to a logarithmic running typical of gauge theories. By studying shockwave deformations of this metric, we compute a two-point energy correlator assuming a high-energy scalar source. This extends techniques recently developed for correlators in asymptotically AdS geometries. We use numerical methods to find the profile of shockwaves along the extra dimension, as it does not admit an analytical form. The running coupling leads to a decay of the two-point correlator at small angular separation, unlike the flat correlator one finds in AdS. In the back-to-back limit we observe an exponential falloff similar to other hard-wall models.

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arXiv2025

Ultralight Dilatonic Dark Matter

The dilaton, a pseudo-Nambu-Goldstone boson (pNGB) of broken scale invariance, is an appealing ultralight dark matter (DM) candidate. Its mass is protected by conformal invariance and it can be searched for in tabletop experiments. However, contrary to standard pNGBs of internal symmetries, the dilaton generically has a large non-derivative self-coupling, leading to radiative contributions to its mass of the order of its decay constant. Hence typical ultralight dilatons should also have sub-eV decay constants, which would incur significant deviations from standard DM behavior at structure formation times, in severe tension with observations. Therefore, a fine-tuning is required to generate a hierarchy between the mass and the decay constant. In this work, we consider whether supersymmetry (SUSY) can be used to protect this hierarchy from quantum corrections. To ensure an ultralight dilaton mass robust against realistic SUSY-breaking contributions, we must consider a novel dilaton stabilization mechanism. The observed DM abundance can be produced by the misalignment mechanism for dilaton masses ranging from $10^{-11}$ to $1$ eV. Unfortunately, irreducible SUSY-breaking corrections due to gravity restrict the couplings between the dilaton and the Standard Model to be extremely small, beyond the reach of any current or proposed experiments. Our work demonstrates that constructing a consistent model of ultralight dilaton DM is quite involved.

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arXiv2025

Gauge hierarchy and metastability from Higgs-driven crunching

We present a new solution to the Higgs hierarchy problem based on dynamical vacuum selection in a landscape scanning the Higgs mass. In patches where the Higgs mass parameter takes a natural value, the Higgs potential only admits a minimum with a large and negative energy density. This causes a cosmological crunch, removing such patches from the landscape. Conversely, in patches where the Higgs mass parameter is smaller than a critical value, the Higgs potential admits a metastable minimum with the standard cosmological history. This critical value is determined by the instability scale, where the quartic coupling turns negative due to its running. The ability of this mechanism to explain the observed Higgs mass hinges on new physics at the TeV scale, such as vector-like fermions. We study two simple realizations of this scenario in a heavy neutral lepton model and in the singlet-doublet model, the latter mimicking a Higgsino-bino system. We show that the relevant parts of their parameter spaces can be probed by proposed future colliders, such as the FCC-ee or a muon collider.

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arXiv2024

Holographic Energy Correlators for Soft Walls

We calculate energy correlators in a general holographic model of confinement, involving an asymptotically anti-de Sitter (AdS) warped extra dimension. Building on a recent computation in a minimal hard-wall model of confinement, we show that the shockwave method for efficiently computing energy correlators in AdS generalizes to an arbitrary warped geometry. This is possible because exact, linear shockwave solutions to the 5D field equations exist in any warped background. We apply our formalism to compute the two-point energy correlator for two simple models of confinement with interesting infrared spectra -- one with a gapped continuum spectrum and one with linear Regge trajectories. The results differ from the simple hard-wall model and from each other, demonstrating that the details of the confining dynamics affect the shape of the energy correlator observables.

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arXiv2024

Holographic Energy Correlators for Confining Theories

We present a holographic calculation of energy correlators in a simple model of confinement based on a warped extra dimension with an IR brane. For small distances we reproduce the constant correlators of a strongly-coupled conformal field theory, while for large distances the effects of confinement dominate and the correlators decay exponentially. We find exact shockwave solutions to the Einstein equations in the presence of the IR brane, hence avoiding the need for a perturbative expansion in terms of Witten diagrams. While some of the expected qualitative features of energy correlators in quantum chromodynamics (QCD) are reproduced, our crude model of confinement does not capture the effects of asymptotic freedom nor exhibit jetty behavior. We expect that our method can also be applied to more realistic models of confinement incorporating asymptotic freedom, which should fix some of the deviations from QCD.

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arXiv2024

Axion dark matter from inflation-driven quantum phase transition

We propose a new mechanism to produce axion dark matter from inflationary fluctuations. Quantum fluctuations during inflation are strengthened by a coupling of the axion kinetic term to the inflaton, which we parametrize as an effective curvature $κ$ in the axion equation of motion. A nonvanishing curvature breaks the scale invariance of the axion power spectrum, driving a quantum phase transition with $κ$ as the order parameter. The axion power spectrum is proportional to the inverse comoving horizon to the power of $κ$. For positive $κ$ the spectrum gets a red tilt, leading to an exponential enhancement of the axion abundance as the comoving horizon shrinks during inflation. This enhancement allows sufficient axion production to comprise the entire dark matter relic abundance despite the ultralight mass. Our mechanism predicts a significantly different parameter space from the usual misalignment mechanism. It allows for axion-like particle dark matter with a much lower decay constant and thus a larger coupling to Standard Model particles. Much of the parameter space can be probed by future experiments including haloscopes, nuclear clocks, CASPEr, and CMB-S4. We can also generate heavier QCD axion dark matter than the misalignment mechanism.

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arXiv2023

FIMPzilla dark matter and conformal sectors

We point out a dark matter candidate which arises in a minimal extension of solutions to the hierarchy problem based on compositeness. In such models, some or all of the Standard Model fields are composites of a conformal field theory (CFT) which confines near the electroweak scale. We posit an elementary scalar field, whose mass is expected to lie near the cutoff of the CFT, and whose couplings to the Standard Model are suppressed by the cutoff. Hence it can naturally be ultraheavy and feebly coupled. This scalar can constitute all of the dark matter for masses between $10^{10}$ GeV and $10^{18}$ GeV, with the relic abundance produced by the freeze-in mechanism via a coupling to the CFT. The principal experimental constraints come from bounds on the tensor-to-scalar ratio. We speculate about future detection prospects.

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arXiv2023

Forbidden conformal dark matter at a GeV

We introduce a model of dark matter (DM) where the DM is a composite of a spontaneously broken conformal field theory. The DM is a thermal relic with its abundance determined by the freeze-out of annihilations to dilatons, the Goldstone boson of broken conformal symmetry. If the dilaton is heavier than the DM this is an example of forbidden DM. We explore the phenomenology of this model in its 5D dual description, corresponding to a warped extra dimension with the Standard Model on the ultraviolet brane and the DM on the infrared brane. We find the model is compatible with theoretical and experimental constraints for DM masses in the $0.1$-$10$ GeV range. The conformal phase transition is supercooled and strongly first-order. It can source large stochastic gravitational wave signals consistent with those recently observed at pulsar timing arrays like NANOGrav. The majority of the viable parameter space will be probed by future detectors designed to search for long-lived particles, including most of the region favored by the NANOGrav signal. The rest of the parameter space can be probed at future direct detection experiments.

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arXiv2023

Relevant Dilaton Stabilization

We propose a simple modification of the Goldberger-Wise mechanism for stabilizing the scale of spontaneously broken conformal theories. The source of explicit conformal symmetry breaking is a relevant operator with a small coefficient, as opposed to the usual mechanism of an almost marginal operator with an order-one coefficient. In the warped 5D picture this relevant stabilization corresponds to a small tadpole for the bulk scalar on the UV brane, which can be technically natural if it is the only source for the breaking of a symmetry (for example, a discrete $Z_2$). This modification of the stabilization mechanism has significant consequences for the nature of the conformal phase transition, since the radion/dilaton potential is no longer shallow. The bounce action is significantly reduced, leading to a weaker first-order phase transition instead of the supercooled and strongly first-order transition seen in Goldberger-Wise stabilization. This also leads to reduction of gravitational wave signals which, however, may still be observable at future detectors. We present numerical and analytical studies of the phase transition and the resulting gravitational wave signal strength, assuming that the effective dilaton potential provides a good leading approximation. While the dilaton is not expected to be generically light in this setup, in order to keep perturbative control over the effective theory one needs to mildly tune the dilaton quartic to be somewhat small.

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arXiv2023

The Continuum Dark Matter Zoo

We generalize the recently proposed continuum dark matter model to the case where the dark matter consists of a spin-$1/2$ or spin-$1$ gapped continuum. We construct simple continuum analogs of weakly interacting massive particles annihilating through the $Z$ portal. We discuss all existing experimental constraints, with the strongest bounds arising from indirect detection and limits on continuum decays from the cosmic microwave background. Our models are phenomenologically viable for gap scales of $60$-$200$ GeV (spin-$1/2$) and $35$-$90$ GeV (spin-$1$), owing to the strong kinematic suppression of direct detection bounds which is unique to continuum states. We comment on future prospects for detection and suggest directions for further continuum model building.

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arXiv2022

Higgs Squared

We present a novel construction for a Higgs-VEV sensitive operator, which can be used as a trigger operator in cosmic selection models for the electroweak hierarchy problem. Our operator does not contain any degrees of freedom charged under the SM gauge symmetries, leading to reduced tuning in the resulting models. Our construction is based on the extension of a two Higgs doublet model (2HDM) with a softly broken approximate global $D_8$ symmetry (the symmetry group of a square). A cosmic crunching model based on our extended Higgs sector has only a percent level tuning corresponding to the usual little hierarchy problem. In large regions of parameter space the 2HDM is naturally pushed towards the alignment limit. A complete model requires the introduction of fermionic top partners to ensure the approximate $D_8$ symmetry in the fermion sector. We also show that the same extended Higgs sector can be used for a novel implementation of the seesaw mechanism of neutrino masses.

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arXiv2022

$a$-Anomalous Interactions of the Holographic Dilaton

We explore higher-derivative terms in the low-energy effective action for the dilaton, the Goldstone boson of spontaneously broken scale invariance. Focusing on the simplest holographic realization of spontaneously broken scale invariance, the Randall-Sundrum (RS) scenario, we identify the nonlinear action for the RS dilaton by integrating out Kaluza-Klein graviton modes. The coefficient of a particular four-derivative dilaton self-interaction can be identified with the Weyl $a$-anomaly of the dual conformal field theory, which we use to verify anomaly matching arguments. We also find novel, $a$-dependent couplings of the dilaton to light matter fields. These anomalous interactions can have a significant effect on the collider phenomenology and the cosmology, potentially allowing us to probe the structure of the underlying conformal sector via low-energy physics. The dilaton effective theory also serves as an interesting scalar analog of gravity, and we study solutions to the equation of motion that parallel black holes and cosmologies.

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arXiv2022

The Forward Physics Facility: Sites, Experiments, and Physics Potential

The Forward Physics Facility (FPF) is a proposal to create a cavern with the space and infrastructure to support a suite of far-forward experiments at the Large Hadron Collider during the High Luminosity era. Located along the beam collision axis and shielded from the interaction point by at least 100 m of concrete and rock, the FPF will house experiments that will detect particles outside the acceptance of the existing large LHC experiments and will observe rare and exotic processes in an extremely low-background environment. In this work, we summarize the current status of plans for the FPF, including recent progress in civil engineering in identifying promising sites for the FPF and the experiments currently envisioned to realize the FPF's physics potential. We then review the many Standard Model and new physics topics that will be advanced by the FPF, including searches for long-lived particles, probes of dark matter and dark sectors, high-statistics studies of TeV neutrinos of all three flavors, aspects of perturbative and non-perturbative QCD, and high-energy astroparticle physics.

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arXiv2022

CKM substructure from the weak to the Planck scale

We look for relations among CKM matrix elements that are not consequences of the Wolfenstein parametrization. In particular, we search for products of CKM elements raised to integer powers that approximately equal $1$. We study the running of the CKM matrix elements and resolve an apparent discrepancy in the literature. To a good approximation only $A$ runs, among the Wolfenstein parameters. Using the Standard Model renormalization group we look for CKM relations at energy scales ranging from the electroweak scale to the Planck scale, and we find 19 such relations. These relations could point to structure in the UV, or be numerical accidents. For example, we find that $| V_{td} V_{us} | = | V_{cb}^2 |$, within $2\%$ accuracy, in the $10^9$-$10^{15}$ GeV range. We discuss the implications of this CKM relation for a Yukawa texture in the UV.

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arXiv2021

A benchmark for LHC searches for low-mass custodial fiveplet scalars in the Georgi-Machacek model

The Georgi-Machacek (GM) model is used to motivate and interpret LHC searches for doubly charged scalars decaying to vector bosons pairs. The doubly charged scalars are part of a degenerate fermiophobic custodial fiveplet with states $H_5^{\pm\pm}$, $H_5^\pm$, and $H_5^0$ and common mass $m_5$. The GM model has been extensively studied at the LHC for $m_5 > 200$ GeV but there is a largely unprobed region of parameter space from $120$ GeV $< m_5 < 200$ GeV where light doubly-charged scalars could exist. This region has been neglected by experimental searches due in part to the lack of a benchmark for $m_5 < 200$ GeV. In this paper we propose a new "low-$m_5$" benchmark for the GM model, defined for $m_5 \in (50, 550)$ GeV, and characterize its properties. We apply all existing experimental constraints and summarize the phenomenology of the surviving parameter space. We show that the benchmark populates almost the entirety of the relevant allowed parameter plane for $m_5$ below 200 GeV and satisfies the constraints from the 125 GeV Higgs boson signal strengths. We compute the 125 GeV Higgs boson's couplings to fermion and vector boson pairs and show that they are always enhanced in the benchmark relative to those in the Standard Model. We also compute the most relevant production cross sections for $H_5$ at the LHC, including Drell-Yan production of $H_5$ pairs. The process $pp \to H \to H_5 H_5$ contributes in a small region of parameter space, but is small compared to the Drell-Yan production cross section. Finally we compute the branching ratios of $H_5^0 \to γγ$ and $H_5^{\pm} \to W^{\pm} γ$. The width-to-mass ratio of each of the $H_5$ states is below 1\% over the entire benchmark, so that the narrow-width approximation is well justified.

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arXiv2020

Crunching Dilaton, Hidden Naturalness

We introduce a new approach to the Higgs naturalness problem, where the value of the Higgs mass is tied to cosmic stability and the possibility of a large observable Universe. The Higgs mixes with the dilaton of a CFT sector whose true ground state has a large negative vacuum energy. If the Higgs VEV is non-zero and below $\mathcal{O}({\rm TeV})$, the CFT also admits a second metastable vacuum, where the expansion history of the Universe is conventional. As a result, only Hubble patches with unnaturally small values of the Higgs mass support inflation and post-inflationary expansion, while all other patches rapidly crunch. The elementary Higgs VEV driving the dilaton potential is the essence of our new solution to the hierarchy problem. The main experimental prediction is a light dilaton field in the 0.1-10 GeV range that mixes with the Higgs. Part of the viable parameter space has already been probed by measurements of rare B-meson decays, and the rest will be fully explored by future colliders and experiments searching for light, weakly-coupled particles.

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