arXiv subjects

Ethan Carragher

Publications and source records attributed to Ethan Carragher.

5 recordsLinked to original sources

arXiv2026

A more effective QCD string at colliders: Decay of excited strings and the worldsheet axion

The confining flux tube of $(3+1)$d QCD is described by an effective string theory with $(1+1)$d worldsheet action that extends the Nambu-Goto form by the addition of a massive pseudoscalar worldsheet ``axion". As argued in companion papers concerning the modified phenomenology of the Lund string model at colliders, QCD flux tubes produced by high-energy collisions are likely to involve excitation of both worldsheet Nambu-Goldstone and axion modes, although the standard Lund model assumes a constant tension ground-state string. Here we detail the path-integral computation of the modified Schwinger-like process of string breaking via nucleation of quark-antiquark pairs in the presence of excitations above the string ground state. We find that the worldsheet axion leads to the dominant change in the string breaking process, the axion excitations producing, among other effects, a varying effective tension of the string, which can exponentially enhance or suppress the string breaking rate depending on the local phase of the excitation. Our computation employs a version of the Schwinger-Keldysh complex time contour method with initial state data specified by a density matrix. In an excited background the Euclidean saddle point is generically complex, but its continuation gives real initial data for post-decay evolution. Our results are of relevance for hadronisation models with excited QCD strings.

hep-ph↗

arXiv2026

Metastable cosmic strings are broken at the start

We show that metastable cosmic strings break at early times, either via finite-temperature effects or by attaching to pre-existing monopoles during network percolation. The resulting segments can be initially super-horizon in size and thus persist for a significant amount of time. If the strings do not re-percolate, the network's eventual destruction is typically due to this early-time breaking rather than late-time quantum tunnelling. Survival of strings to epochs probed by NANOGrav requires $m_M^2/\mu\gtrsim 10^3$, where $m_M$ and $\mu$ are the monopole mass and the string tension respectively, over an order of magnitude larger than previous estimates. We also revisit quantum-tunnelling induced breaking. Results from numerical simulations suggest that this occurs mainly at rare high-tension points on the strings, yielding a rate much larger than is usually assumed. We briefly discuss the related scenario of flux tubes in a dark QCD-like hidden sector with dark-quark masses above the confinement scale.

hep-ph↗

arXiv2024

Extending Global Fits of 4D Composite Higgs Models with Partially Composite Leptons

We perform the first convergent Bayesian global fits of 4D Composite Higgs Models with partially-composite third generation quarks and leptons based on the minimal $SO(5) \rightarrow SO(4)$ symmetry breaking pattern. We consider two models with the $τ$ lepton and its associated neutrino in different representations of $SO(5)$. Fitting each model with a wide array of experimental constraints allows us to analyse the Bayesian evidence and currently-observed fine-tuning of each model by calculating the Kullback-Leibler divergence between their respective priors and posteriors. Notably both models are found to be capable of satisfying all constraints simultaneously at the $3σ$ level at scales of $< 5$ TeV. From a Bayesian viewpoint of naturalness the model with leptons in the $\mathbf{14}$ and $\mathbf{10}$ representations is preferred over those in the $\mathbf{5}$ representation due to its lower fine-tuning. Finally, we consider the experimental signatures for the preferred parameters in these models, including lepton partner decay signatures and gluon-fusion produced Higgs signal strengths, and discuss their potential phenomenology at future high-luminosity LHC runs.

hep-ph↗

arXiv2021

Convergent Bayesian Global Fits of 4D Composite Higgs Models

Models in which the Higgs boson is a composite pseudo-Nambu-Goldstone boson offer attractive solutions to the Higgs mass naturalness problem. We consider three such models based on the minimal $SO(5) \rightarrow SO(4)$ symmetry breaking pattern, and perform convergent global fits on the models under a Bayesian framework in order to find the regions of their parameter spaces that best fit a wide range of constraints, including recent Higgs measurements. We use a novel technique to analyse the fine-tuning of the models, quantifying the tuning as the Kullback-Leibler divergence from the prior to the posterior probability on the parameter space. Each model is found to be able to satisfy all constraints at the $3σ$ level simultaneously. As a by-product of the fits, we analyse the collider phenomenology of our models in these viable regions. In two of the three models, we find that the $g g \rightarrow H \rightarrow γγ$ cross section is less than ${\sim}90$% that predicted by the SM, which is already in slight tension with experiment and could potentially be ruled out in the future high-luminosity run of the LHC. In addition, the lightest fermions $F$ arising from the new strong dynamics in these models are seen in general to lie above ${\sim}1.1$ TeV, with the $F \rightarrow tW^{+}$ and $F \rightarrow \bar{b}W^{+}$ decays offering particularly promising channels for probing these models in future collider searches.

hep-ph↗

arXiv2020

Minimal 4D Composite Higgs Models Under Current LHC Constraints

We present preliminary results of the first convergent global fits of several minimal composite Higgs models. Our fits are performed using the differential evolution optimisation package $\texttt{Diver}$. A variety of physical constraints are taken into account, including a wide range of exclusion bounds on heavy resonance production from Run 2 of the LHC. As a by-product of the fits, we analyse the collider phenomenology of the lightest new up-type and down-type resonances in the viable regions of our models, finding some low-mass resonances that can be probed in future collider searches.

hep-ph↗