arXiv subjects

Brooks Thomas

Publications and source records attributed to Brooks Thomas.

At least 37 records · Page 2Linked to original sources

arXiv2022

More is Different: Non-Minimal Dark Sectors and their Implications for Particle Physics, Astrophysics, and Cosmology -- 13 Take-Away Lessons for Snowmass 2021

The phrase "more is different" is often used to refer to the new, unexpected collective phenomena that can arise when the number of states in a given system is large. In this contribution to the Snowmass 2021 Study, we describe 13 unexpected collective phenomena that can arise when the dark sector contains a large number of states, contrary to the usual assumptions. These 13 take-away lessons stretch across all of the domains of relevance for dark-matter physics, including collider signatures, direct-detection signatures, indirect-detection signatures, new perspectives on dark-matter complementarity, and even unexpected astrophysical and cosmological phenomena that transcend those normally associated with single-component dark-matter scenarios. These lessons -- and the phenomena on which they are based -- thereby illustrate the need to maintain a broad perspective when contemplating the possible signatures and theoretical possibilities associated with non-minimal dark sectors.

hep-ph↗

arXiv2022

Stasis in an Expanding Universe: A Recipe for Stable Mixed-Component Cosmological Eras

One signature of an expanding universe is the time-variation of the cosmological abundances of its different components. For example, a radiation-dominated universe inevitably gives way to a matter-dominated universe, and critical moments such as matter-radiation equality are fleeting. In this paper, we point out that this lore is not always correct, and that it is possible to obtain a form of "stasis" in which the relative cosmological abundances $Ω_i$ of the different components remain unchanged over extended cosmological epochs, even as the universe expands. Moreover, we demonstrate that such situations are not fine-tuned, but are actually global attractors within certain cosmological frameworks, with the universe naturally evolving towards such long-lasting periods of stasis for a wide variety of initial conditions. The existence of this kind of stasis therefore gives rise to a host of new theoretical possibilities across the entire cosmological timeline, ranging from potential implications for primordial density perturbations, dark-matter production, and structure formation all the way to early reheating, early matter-dominated eras, and even the age of the universe.

astro-ph.CO↗

arXiv2020

An Update to the Letter of Intent for MATHUSLA: Search for Long-Lived Particles at the HL-LHC

We report on recent progress in the design of the proposed MATHUSLA Long Lived Particle (LLP) detector for the HL-LHC, updating the information in the original Letter of Intent (LoI), see CDS:LHCC-I-031, arXiv:1811.00927. A suitable site has been identified at LHC Point 5 that is closer to the CMS Interaction Point (IP) than assumed in the LoI. The decay volume has been increased from 20 m to 25 m in height. Engineering studies have been made in order to locate much of the decay volume below ground, bringing the detector even closer to the IP. With these changes, a 100 m x 100 m detector has the same physics reach for large c$τ$ as the 200 m x 200 m detector described in the LoI and other studies. The performance for small c$τ$ is improved because of the proximity to the IP. Detector technology has also evolved while retaining the strip-like sensor geometry in Resistive Plate Chambers (RPC) described in the LoI. The present design uses extruded scintillator bars read out using wavelength shifting fibers and silicon photomultipliers (SiPM). Operations will be simpler and more robust with much lower operating voltages and without the use of greenhouse gases. Manufacturing is straightforward and should result in cost savings. Understanding of backgrounds has also significantly advanced, thanks to new simulation studies and measurements taken at the MATHUSLA test stand operating above ATLAS in 2018. We discuss next steps for the MATHUSLA collaboration, and identify areas where new members can make particularly important contributions.

physics.ins-det↗

arXiv2020

Deciphering the Archaeological Record: Cosmological Imprints of Non-Minimal Dark Sectors

Many proposals for physics beyond the Standard Model give rise to a dark sector containing many degrees of freedom. In this work, we explore the cosmological implications of the non-trivial dynamics which may arise within such dark sectors, focusing on decay processes which take place entirely among the dark constituents. First, we demonstrate that such decays can leave dramatic imprints on the resulting dark-matter phase-space distribution. In particular, this distribution need not be thermal -- it can even be multi-modal, exhibiting a non-trivial pattern of peaks and troughs as a function of momentum. We then proceed to show how these features can induce modifications to the matter power spectrum. Finally, we assess the extent to which one can approach the archaeological "inverse" problem of deciphering the properties of an underlying dark sector from the matter power spectrum. Indeed, one of the main results of this paper is a remarkably simple conjectured analytic expression which permits the reconstruction of many of the important features of the dark-matter phase-space distribution directly from the matter power spectrum. Our results therefore provide an interesting toolbox of methods for learning about, and potentially constraining, the features of non-minimal dark sectors and their dynamics in the early universe.

astro-ph.CO↗

arXiv2020

Non-Minimal Dark Sectors: Mediator-Induced Decay Chains and Multi-Jet Collider Signatures

A preponderance of astrophysical and cosmological evidence indicates that the universe contains not only visible matter but also dark matter. In order to suppress the couplings between the dark and visible sectors, a standard assumption is that these two sectors communicate only through a mediator. In this paper we make a simple but important observation: if the dark sector contains multiple components with similar quantum numbers, then this mediator also generically gives rise to dark-sector decays, with heavier dark components decaying to lighter components. This in turn can even give rise to relatively long dark decay chains, with each step of the decay chain also producing visible matter. The visible byproducts of such mediator-induced decay chains can therefore serve as a unique signature of such scenarios. In order to examine this possibility more concretely, we examine a scenario in which a multi-component dark sector is connected through a mediator to Standard-Model quarks. We then demonstrate that such a scenario gives rise to multi-jet collider signatures, and we examine the properties of such jets at both the parton and detector levels. Within relatively large regions of parameter space, we find that such multi-jet signatures are not excluded by existing monojet and multi-jet searches. Such decay cascades therefore represent a potential discovery route for multi-component dark sectors at current and future colliders.

hep-ph↗

arXiv2020

Partially Composite Dynamical Dark Matter

In this paper, we consider a novel realization of the Dynamical Dark Matter (DDM) framework in which the ensemble of particles which collectively constitute the dark matter are the composite states of a strongly-coupled conformal field theory. Cosmological abundances for these states are then generated through mixing with an additional, elementary state. As a result, the physical fields of the DDM dark sector at low energies are partially composite -- i.e., admixtures of elementary and composite states. Interestingly, we find that the degree of compositeness exhibited by these states varies across the DDM ensemble. We calculate the masses, lifetimes, and abundances of these states -- along with the effective equation of state of the entire ensemble -- by considering the gravity dual of this scenario in which the ensemble constituents are realized as the Kaluza-Klein states associated with a scalar propagating within a slice of five-dimensional anti-de Sitter (AdS) space. Surprisingly, we find that the warping of the AdS space gives rise to parameter-space regions in which the decay widths of the dark-sector constituents vary non-monotonically with their masses. We also find that there exists a maximum degree of AdS warping for which a phenomenologically consistent dark-sector ensemble can emerge. Our results therefore suggest the existence of a potentially rich cosmology associated with partially composite DDM.

hep-ph↗

arXiv2020

Constraining Dark-Matter Ensembles with Supernova Data

The constraints on non-minimal dark sectors involving ensembles of unstable dark-matter species are well established and quite stringent in cases in which these species decay to visible-sector particles. However, in cases in which these ensembles decay exclusively to other, lighter dark-sector states, the corresponding constraints are less well established. In this paper, we investigate how information about the expansion rate of the universe at low redshifts gleaned from observations of Type Ia supernovae can be used to constrain ensembles of unstable particles which decay primarily into dark radiation.

astro-ph.CO↗

arXiv2019

Enlarging the Space of Viable Inflation Models: A Slingshot Mechanism

The viability of a given model for inflation is determined not only by the form of the inflaton potential, but also by the initial inflaton field configuration. In many models, field configurations which are otherwise well-motivated nevertheless fail to induce inflation, or fail to produce an inflationary epoch of duration sufficient to solve the horizon and flatness problems. In this paper, we propose a mechanism which enables inflation to occur even with such initial conditions. Our mechanism involves multiple scalar fields which experience a time-dependent mixing. This in turn leads to a "re-overdamping" phase as well as a parametric resonance which together "slingshot" the inflaton field from regions of parameter space that do not induce inflation to regions that do. Our mechanism is flexible, dynamical, and capable of yielding an inflationary epoch of sufficiently long duration. This slingshot mechanism can therefore be utilized in a variety of settings and thereby enlarge the space of potentially viable inflation models.

hep-th↗

arXiv2019

Long-Lived Particles at the Energy Frontier: The MATHUSLA Physics Case

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of Standard Model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the $μ$m scale up to the Big Bang Nucleosynthesis limit of $\sim 10^7$m. Neutral LLPs with lifetimes above $\sim$ 100m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. In this white paper we study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

hep-ph↗

arXiv2019

Cosmological Constraints on Unstable Particles: Numerical Bounds and Analytic Approximations

Many extensions of the Standard Model predict large numbers of additional unstable particles whose decays in the early universe are tightly constrained by observational data. For example, the decays of such particles can alter the ratios of light-element abundances, give rise to distortions in the cosmic microwave background, alter the ionization history of the universe, and contribute to the diffuse photon flux. Constraints on new physics from such considerations are typically derived for a single unstable particle species with a single well-defined mass and characteristic lifetime. In this paper, by contrast, we investigate the cosmological constraints on theories involving entire ensembles of decaying particles --- ensembles which span potentially broad ranges of masses and lifetimes. In addition to providing a detailed numerical analysis of these constraints, we also formulate a set of simple analytic approximations for these constraints which may be applied to generic ensembles of unstable particles which decay into electromagnetically-interacting final states. We then illustrate how these analytic approximations can be used to constrain a variety of toy scenarios for physics beyond the Standard Model. For ease of reference, we also compile our results in the form of a table which can be consulted independently of the rest of the paper. It is thus our hope that this work might serve as a useful reference for future model-builders concerned with cosmological constraints on decaying particles, regardless of the particular model under study.

hep-ph↗

arXiv2019

Dynamical Dark Matter, MATHUSLA, and the Lifetime Frontier

MATHUSLA is a proposed surface detector at CERN that would be able to observe the decays of non-hadronic electrically neutral long-lived particles (LLPs) with almost no background or trigger limitations. This would allow MATHUSLA to probe sub-GeV to TeV masses and lifetimes up to $cτ\sim 10^7~{\rm m}$. MATHUSLA can play an important role in probing dark-matter scenarios involving extended hidden sectors, where additional dark states often manifest as LLPs. A prime example of such a scenario is furnished by the Dynamical Dark Matter (DDM) framework, which intrinsically gives rise to large ensembles of dark states exhibiting a broad range of masses and lifetimes. In this paper, we examine the extent to which MATHUSLA can probe the DDM parameter space, and we demonstrate that MATHUSLA may be capable of providing direct confirmation of certain unique aspects of the DDM framework which might be difficult to probe in other ways.

hep-ph↗

arXiv2018

Dynamical Dark Matter from Thermal Freeze-Out

In the Dynamical Dark Matter (DDM) framework, the dark sector comprises a large number of constituent dark particles whose individual masses, lifetimes, and cosmological abundances obey specific scaling relations with respect to each other. In particular, the most natural versions of this framework tend to require a spectrum of cosmological abundances which scale inversely with mass, so that dark-sector states with larger masses have smaller abundances. Thus far, DDM model-building has primarily relied on non-thermal mechanisms for abundance generation such as misalignment production, since these mechanisms give rise to abundances that have this property. By contrast, the simplest versions of thermal freeze-out tend to produce abundances that increase, rather than decrease, with the mass of the dark-matter component. In this paper, we demonstrate that there exist relatively simple modifications of the traditional thermal freeze-out mechanism which "flip" the resulting abundance spectrum, producing abundances that scale inversely with mass. Moreover, we demonstrate that a far broader variety of scaling relations between lifetimes, abundances, and masses can emerge through thermal freeze-out than through the non-thermal mechanisms previously considered for DDM ensembles. The results of this paper thus extend the DDM framework into the thermal domain and essentially allow us to "design" our resulting DDM ensembles at will in order to realize a rich array of resulting dark-matter phenomenologies.

hep-ph↗

arXiv2017

Off-Diagonal Dark-Matter Phenomenology: Exploring Enhanced Complementarity Relations in Non-Minimal Dark Sectors

In most multi-component dark-matter scenarios, two classes of processes generically contribute to event rates at experiments capable of probing the nature of the dark sector. The first class consists of "diagonal" processes involving only a single species of dark-matter particle -- processes analogous to those which arise in single-component dark-matter scenarios. By contrast, the second class consists of "off-diagonal" processes involving dark-matter particles of different species. Such processes include inelastic scattering at direct-detection experiments, asymmetric production at colliders, dark-matter co-annihilation, and certain kinds of dark-matter decay. In typical multi-component scenarios, the contributions from diagonal processes dominate over those from off-diagonal processes. Unfortunately, this tends to mask those features which are most sensitive to the multi-component nature of the dark sector. In this paper, by contrast, we point out that there exist natural, multi-component dark-sector scenarios in which the off-diagonal contributions actually dominate over the diagonal. This then gives rise to a new, enhanced picture of dark-matter complementarity. In this paper, we introduce a scenario in which this situation arises and examine the enhanced picture of dark-matter complementarity which emerges.

hep-ph↗

arXiv2017

Kaluza-Klein Towers in the Early Universe: Phase Transitions, Relic Abundances, and Applications to Axion Cosmology

We study the early-universe cosmology of a Kaluza-Klein (KK) tower of scalar fields in the presence of a mass-generating phase transition, focusing on the time-development of the total tower energy density (or relic abundance) as well as its distribution across the different KK modes. We find that both of these features are extremely sensitive to the details of the phase transition and can behave in a variety of ways significant for late-time cosmology. In particular, we find that the interplay between the temporal properties of the phase transition and the mixing it generates are responsible for both enhancements and suppressions in the late-time abundances, sometimes by many orders of magnitude. We map out the complete model parameter space and determine where traditional analytical approximations are valid and where they fail. In the latter cases we also provide new analytical approximations which successfully model our results. Finally, we apply this machinery to the example of an axion-like field in the bulk, mapping these phenomena over an enlarged axion parameter space that extends beyond those accessible to standard treatments. An important by-product of our analysis is the development of an alternate "UV-based" effective truncation of KK theories which has a number of interesting theoretical properties that distinguish it from the more traditional "IR-based" truncation typically used in the extra-dimension literature.

hep-ph↗

arXiv2017

Lines and Boxes: Unmasking Dynamical Dark Matter through Correlations in the MeV Gamma-Ray Spectrum

Identifying signatures of dark matter at indirect-detection experiments is generally more challenging for scenarios involving non-minimal dark sectors such as Dynamical Dark Matter (DDM) than for scenarios involving a single dark particle. This additional difficulty arises because the partitioning of the total dark-matter abundance across an ensemble of different constituent particles with different masses tends to "smear" the injection spectra of photons and other cosmic-ray particles that are produced via dark-matter annihilation or decay. As a result, the imprints of the dark sector on these cosmic-ray flux spectra typically take the form of continuum features rather than sharp peaks or lines. In this paper, however, we identify an unambiguous signature of non-minimal dark sectors such as DDM which can overcome these issues and potentially be observed at gamma-ray telescopes operating in the MeV range. We discuss the specific situations in which this signature can arise, and demonstrate that this signature can be exploited in order to significantly enhance our ability to resolve the unique spectral features of DDM and other non-minimal dark sectors at future gamma-ray facilities.

hep-ph↗

arXiv2017

Boxes, Boosts, and Energy Duality: Understanding the Galactic-Center Gamma-Ray Excess through Dynamical Dark Matter

Many models currently exist which attempt to interpret the excess of gamma rays emanating from the Galactic Center in terms of annihilating or decaying dark matter. These models typically exhibit a variety of complicated cascade mechanisms for photon production, leading to a non-trivial kinematics which obscures the physics of the underlying dark sector. In this paper, by contrast, we observe that the spectrum of the gamma-ray excess may actually exhibit an intriguing "energy-duality" invariance under $E_γ\rightarrow E_\ast^2/E_γ$ for some $E_\ast$. As we shall discuss, such an energy duality points back to a remarkably simple alternative kinematics which in turn is realized naturally within the Dynamical Dark Matter framework. Observation of this energy duality could therefore provide considerable information about the properties of the dark sector from which the Galactic-Center gamma-ray excess might arise, and highlights the importance of acquiring more complete data for the Galactic-Center excess in the energy range around 1 GeV.

hep-ph↗

arXiv2016

Dynamical Dark Matter from Strongly-Coupled Dark Sectors

Dynamical Dark Matter (DDM) is an alternative framework for dark-matter physics in which the dark sector comprises a vast ensemble of particle species whose decay widths are balanced against their cosmological abundances. Previous studies of this framework have focused on a particular class of DDM ensembles --- motivated primarily by KK towers in theories with extra dimensions --- in which the density of states scales roughly as a polynomial of mass. In this paper, by contrast, we study the properties of a different class of DDM ensembles in which the density of states grows exponentially with mass. Ensembles with this Hagedorn-like property arise naturally as the "hadrons" associated with the confining phase of a strongly-coupled dark sector; they also arise naturally as the gauge-neutral bulk states of Type I string theories. We study the dynamical properties of such ensembles, and demonstrate that an appropriate DDM-like balancing between decay widths and abundances can emerge naturally --- even with an exponentially rising density of states. We also study the effective equations of state for such ensembles, and investigate some of the model-independent observational constraints on such ensembles that follow directly from these equations of state. In general, we find that such constraints tend to introduce correlations between various properties of these DDM ensembles such as their associated mass scales, lifetimes, and abundance distributions. For example, we find that these constraints allow DDM ensembles with energy scales ranging from the GeV scale all the way to the Planck scale, but the total present-day cosmological abundance of the dark sector must be spread across an increasing number of different states in the ensemble as these energy scales are dialed from the Planck scale down to the GeV scale. Numerous other correlations and constraints are also discussed.

hep-ph↗

arXiv2016

Randomness in the Dark Sector: Emergent Mass Spectra and Dynamical Dark Matter Ensembles

In general, non-minimal models of the dark sector such as Dynamical Dark Matter posit the existence of an ensemble of individual dark components with differing masses, cosmological abundances, and couplings to the Standard Model. Perhaps the most critical among these features is the spectrum of masses, as this goes a long way towards determining the cosmological abundances and lifetimes of the corresponding states. Many different underlying theoretical structures can be imagined for the dark sector, each giving rise to its own mass spectrum and corresponding density of states. In this paper, by contrast, we investigate the spectrum of masses that emerges statistically from underlying processes which are essentially random. We find a density of states $n(m)$ which decreases as a function of mass and actually has an upper limit $m_{\rm max}$ beyond which $n(m)=0$. We also demonstrate that this "emergent" density of states is particularly auspicious from the perspective of the Dynamical Dark Matter framework, leading to cosmological abundances and decay widths that are suitably balanced against each other across the dark-matter ensemble. Thus randomness in the dark sector coexists quite naturally with Dynamical Dark Matter, and we examine the prospects for observing the signals of such scenarios in dark-matter indirect-detection experiments.

hep-ph↗