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Matthew McCullough

Publications and source records attributed to Matthew McCullough.

At least 37 records · Page 2Linked to original sources
arXiv2022

Gegenbauer's Twin

In Twin Higgs models the dominant source of fine-tuning is the cancellation of order $v^2/f^2$ required to obtain a Standard Model-like Higgs, where $v$ and $f$ are the electroweak and new physics scales, respectively. Recently proposed Gegenbauer Goldstone models naturally realise $v^2/f^2 \ll 1$ and hence remove this source of fine-tuning. By combining the two into `Gegenbauer's Twin', we obtain a symmetry-based model for Higgs sector naturalness consistent with current collider measurements without fine-tuning of parameters. Single-Higgs coupling deviations of a few percent and trilinear self-coupling deviations of order one are irreducible in the natural parameter space. Thus, notably, the fingerprints of Gegenbauer's Twin could emerge first through di-Higgs measurements at the High-Luminosity LHC.

hep-ph↗
arXiv2022

The dark side of the proton

We study the sensitivity of the High-Luminosity LHC to a light baryonic dark photon B, primarily coupled to quarks, as a constituent of the proton. This is achieved by allowing for a dark photon parton distribution function (PDF) in the PDF evolution equations. Depending on the mass and coupling of the dark photon, the evolution of standard quark and gluon PDFs is distorted to varying degrees. By analysing the effect of the dark photon on the tails of Drell-Yan invariant mass distributions, we demonstrate the potential of the LHC in determining competitive bounds on dark photon parameter space.

hep-ph↗
arXiv2022

Gegenbauer Goldstones

We investigate radiatively stable classes of pseudo-Nambu-Goldstone boson (pNGB) potentials for approximate spontaneously broken $\mathrm{SO}(N+1)\to\mathrm{SO}(N)$. Using both the one-loop effective action and symmetry, it is shown that a Gegenbauer polynomial potential is radiatively stable, being effectively an `eigenfunction' from a radiative perspective. In Gegenbauer pNGB models, one naturally and automatically obtains $v \propto f/n$, where $n\in 2\mathbb{Z}$ is the order of the Gegenbauer polynomial. For a Gegenbauer Higgs boson, this breaks the usual correlation between Higgs coupling corrections and `$v/f$' tuning. Based on this, we argue that to conclusively determine whether or not the Higgs is a composite pNGB in scenarios with up to $\mathcal{O}(10\%)$ fine-tuning will require going beyond both the Higgs coupling precision and heavy resonance mass reach of the High-Luminosity LHC.

hep-ph↗
arXiv2021

Self-Organised Localisation

We describe a new phenomenon in quantum cosmology: self-organised localisation. When the fundamental parameters of a theory are functions of a scalar field subject to large fluctuations during inflation, quantum phase transitions can act as dynamical attractors. As a result, the theory parameters are probabilistically localised around the critical value and the Universe finds itself at the edge of a phase transition. We illustrate how self-organised localisation could account for the observed near-criticality of the Higgs self-coupling, the naturalness of the Higgs mass, or the smallness of the cosmological constant.

hep-ph↗
arXiv2020

Charting the Fifth Force Landscape

Fifth forces arising from hidden sector scalar operator exchange, as probed in a variety of present and planned experiments, can be captured by a general dispersion relation involving one real, positive, spectral density function. Previously considered scalar fifth forces, from tree-level to more exotic loop-level possibilities, are derived in this formalism, without explicitly performing conventional loop calculations. A variety of experimental observables commonly used in fifth force searches are also presented in the same formalism, allowing the straightforward extraction of limits on any specific model. The speculative possibility of probing hidden sector violations of unitarity, causality, or locality, manifested as a breakdown of the positivity of the spectral density, is also discussed.

hep-ph↗
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

Undulating Dark Matter

We suggest that an interplay between microscopic and macroscopic physics can give rise to dark matter (DM) whose interactions with the visible sector fundamentally undulate in time, independent of celestial dynamics. A concrete example is provided by fermionic DM with an electric dipole moment (EDM) sourced by an oscillating axion-like field, resulting in undulations in the scattering rate. The discovery potential of light DM searches can be enhanced by additionally searching for undulating scattering rates, especially in detection regions where background rates are large and difficult to estimate, such as for DM masses in the vicinity of 1 MeV where DM-electron scattering dominantly populates the single electron bin. An undulating signal could also reveal precious dark sector information after discovery. In this regard we emphasise that, if the recent XENON1T excess of events is due to light DM scattering exothermically off electrons, future analyses of the time-dependence of events could offer clues as to the microscopic origins of the putative signal.

hep-ph↗
arXiv2020

Supersoft Stops

In a supersymmetric (SUSY) theory, the IR-contributions to the Higgs mass are calculable below the mediation scale $Λ_{\text{UV}}$ in terms of the IR field content and parameters. However, logarithmic sensitivity to physics at $Λ_{\text{UV}}$ remains. In this work we present a first example of a framework, dictated by symmetries, to supersoften these logarithms from the matter sector. The result is a model with finite, IR-calculable corrections to the Higgs mass. This requires the introduction of new fields -- the `lumberjacks' -- whose role is to screen the UV-sensitive logs. These models have considerably reduced fine-tuning, by more than an order of magnitude for high scale supersymmetry. This impacts interpretations of the natural parameter space, suggesting it may be premature to declare a naturalness crisis for high-scale SUSY.

hep-ph↗
arXiv2019

The Hyperbolic Higgs

We introduce the Hyperbolic Higgs, a novel solution to the little hierarchy problem that features Standard Model neutral scalar top partners. At one-loop order, the protection from ultraviolet sensitivity is due to an accidental non-compact symmetry of the Higgs potential that emerges in the infrared. Once the general features of the effective description are detailed, a completion that relies on a five dimensional supersymmetric framework is provided. Novel phenomenology is compared and contrasted with the Twin Higgs scenario.

hep-ph↗
arXiv2019

The $\hat{H}$-Parameter: An Oblique Higgs View

We study, from theoretical and phenomenological angles, the Higgs boson oblique parameter $\hat{H}$, as the hallmark of off-shell Higgs physics. $\hat{H}$ is defined as the Wilson coefficient of the sole dimension-6 operator that modifies the Higgs boson propagator, within a Universal EFT. Theoretically, we describe self-consistency conditions on Wilson coefficients, derived from the Källén-Lehmann representation. Phenomenologically, we demonstrate that the process $gg\to h^\ast \to VV$ is insensitive to propagator corrections from $\hat{H}$, and instead advertise four-top production as an effective high-energy probe of off-shell Higgs behaviour, crucial to break flat directions in the EFT.

hep-ph↗
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

MATHUSLA: A Detector Proposal to Explore the Lifetime Frontier at the HL-LHC

The observation of long-lived particles at the LHC would reveal physics beyond the Standard Model, could account for the many open issues in our understanding of our universe, and conceivably point to a more complete theory of the fundamental interactions. Such long-lived particle signatures are fundamentally motivated and can appear in virtually every theoretical construct that address the Hierarchy Problem, Dark Matter, Neutrino Masses and the Baryon Asymmetry of the Universe. We describe in this document a large detector, MATHUSLA, located on the surface above an HL-LHC $pp$ interaction point, that could observe long-lived particles with lifetimes up to the Big Bang Nucleosynthesis limit of 0.1 s. We also note that its large detector area allows MATHUSLA to make important contributions to cosmic ray physics. Because of the potential for making a major breakthrough in our conceptual understanding of the universe, long-lived particle searches should have the highest level of priority.

hep-ex↗
arXiv2018

A Letter of Intent for MATHUSLA: a dedicated displaced vertex detector above ATLAS or CMS

In this Letter of Intent (LOI) we propose the construction of MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles), a dedicated large-volume displaced vertex detector for the HL-LHC on the surface above ATLAS or CMS. Such a detector, which can be built using existing technologies with a reasonable budget in time for the HL-LHC upgrade, could search for neutral long-lived particles (LLPs) with up to several orders of magnitude better sensitivity than ATLAS or CMS, while also acting as a cutting-edge cosmic ray telescope at CERN to explore many open questions in cosmic ray and astro-particle physics. We review the physics motivations for MATHUSLA and summarize its LLP reach for several different possible detector geometries, as well as outline the cosmic ray physics program. We present several updated background studies for MATHUSLA, which help inform a first detector-design concept utilizing modular construction with Resistive Plate Chambers (RPCs) as the primary tracking technology. We present first efficiency and reconstruction studies to verify the viability of this design concept, and we explore some aspects of its total cost. We end with a summary of recent progress made on the MATHUSLA test stand, a small-scale demonstrator experiment currently taking data at CERN Point 1, and finish with a short comment on future work.

physics.ins-det↗
arXiv2018

Clockwork / Linear Dilaton: Structure and Phenomenology

The linear dilaton geometry in five dimensions, rediscovered recently in the continuum limit of the clockwork model, may offer a solution to the hierarchy problem which is qualitatively different from other extra-dimensional scenarios and leads to distinctive signatures at the LHC. We discuss the structure of the theory, in particular aspects of naturalness and UV completion, and then explore its phenomenology, suggesting novel strategies for experimental searches. In particular, we propose to analyze the diphoton and dilepton invariant mass spectra in Fourier space in order to identify an approximately periodic structure of resonant peaks. Among other signals, we highlight displaced decays from resonantly-produced long-lived states and high-multiplicity final states from cascade decays of excited gravitons.

hep-ph↗
arXiv2017

Colorful Twisted Top Partners and Partnerium at the LHC

In scenarios that stabilize the electroweak scale, the top quark is typically accompanied by partner particles. In this work, we demonstrate how extended stabilizing symmetries can yield scalar or fermionic top partners that transform as ordinary color triplets but carry exotic electric charges. We refer to these scenarios as "hypertwisted" since they involve modifications to hypercharge in the top sector. As proofs of principle, we construct two hypertwisted scenarios: a supersymmetric construction with spin-0 top partners, and a composite Higgs construction with spin-1/2 top partners. In both cases, the top partners are still phenomenologically compatible with the mass range motivated by weak-scale naturalness. The phenomenology of hypertwisted scenarios is diverse, since the lifetimes and decay modes of the top partners are model dependent. The novel coupling structure opens up search channels that do not typically arise in top-partner scenarios, such as pair production of top-plus-jet resonances. Furthermore, hypertwisted top partners are typically sufficiently long lived to form "top-partnerium" bound states that decay predominantly via annihilation, motivating searches for rare narrow resonances with diboson decay modes.

hep-ph↗
arXiv2017

Simplified Models for Displaced Dark Matter Signatures

We propose a systematic programme to search for long-lived neutral particle signatures through a minimal set of displaced searches requiring significant missing transverse energy (dMETs). Our approach is to extend the well-established dark matter simplified models to include displaced vertices. The dark matter simplified models are used to describe the primary production vertex. A displaced secondary vertex, characterised by the mass of the long-lived particle and its lifetime, is added for the displaced signature. We show how these models can be motivated by, and mapped onto, complete models such as gauge-mediated SUSY breaking and models of neutral naturalness. We also outline how this approach may be used to extend other simplified models to incorporate displaced signatures and to characterise searches for long-lived charged particles. Displaced vertices are a striking signature with virtually no backgrounds from SM processes, and thus provide an excellent target for the high-luminosity run of the Large Hadron Collider. The proposed models and searches provide a first step towards a systematic broadening of the displaced dark matter search programme.

hep-ph↗
arXiv2016

A Clockwork Theory

The clockwork is a mechanism for generating light particles with exponentially suppressed interactions in theories which contain no small parameters at the fundamental level. We develop a general description of the clockwork mechanism valid for scalars, fermions, gauge bosons, and gravitons. This mechanism can be implemented with a discrete set of new fields or, in its continuum version, through an extra spatial dimension. In both cases the clockwork emerges as a useful tool for model-building applications. Notably, the continuum clockwork offers a solution to the Higgs naturalness problem, which turns out to be the same as in linear dilaton duals of Little String Theory. We also elucidate the similarities and differences of the continuum clockwork with large extra dimensions and warped spaces. All clockwork models, in the discrete and continuum, exhibit novel phenomenology with a distinctive spectrum of closely spaced resonances.

hep-ph↗