arXiv subjects

Spencer Chang

Publications and source records attributed to Spencer Chang.

At least 19 recordsLinked to original sources

arXiv2026

Next-to-Leading-Order Electroweak Corrections to Quantum Observables in Lepton-Lepton Collisions

We study how virtual next-to-leading-order electroweak corrections affect the quantum observables of entanglement and magic in lepton--lepton collisions. Treating the outgoing particles as multi-qudit systems, $\textit{i.e.}$, qubits for spin-$\tfrac{1}{2}$ fermions and qutrits for massive vector bosons, we compute entanglement entropy and the stabilizer second R\'enyi entropy for the processes $\ell^+\ell^- \to \tau^-\tau^+,\, ZZ$ at one-loop accuracy. We show that radiative corrections coherently modify the quantum structure of the final state, shifting regions of maximal entanglement and altering the generated magic relative to leading order. Our results demonstrate that these quantum observables provide novel, precision-sensitive probes of electroweak dynamics at future lepton colliders and open a path toward their use in searches for new physics.

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arXiv2025

Quantifying EFT Uncertainties in LHC Searches

Effective Field Theory (EFT) is a general framework to parametrize the low-energy approximation to a UV model that is widely used in model-independent searches for new physics. The use of EFTs at the LHC can suffer from a 'validity' issue, since new physics amplitudes often grow with energy and the kinematic regions with the most sensitivity to new physics have the largest theoretical uncertainties. We propose a method to account for these uncertainties with the aim of producing robust model-independent results with a well-defined statistical interpretation. In this approach, one must specify the new operators being studied as well as the new physics cutoff $M$, the energy scale where the EFT approximation breaks down. At energies below $M$, the EFT uncertainties are accounted for by adding additional higher dimensional operators with coefficients that are treated as nuisance parameters. The size of the nuisances are governed by a prior likelihood function that incorporates information about dimensional analysis, naturalness, and the scale $M$. At energies above $M$, our method incorporates the lack of predictivity of the EFT, and we show that this is crucial to obtain consistent results. We perform a number of tests of this method in a simple toy model, illustrating its performance in analyses aimed at new physics exclusion as well as for discovery. The method is conveniently implemented by the technique of event reweighting and is easily ported to realistic LHC analyses. We find that the procedure converges quickly with the number of nuisance parameters and is conservative when compared to UV models. The paper gives a precise meaning and offers a principled and practical solution to the widely debated 'EFT validity issue'.

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arXiv2024

General Signals for Charged Lepton Flavor Violating Decays

We explore the most general phenomenology of charged lepton flavor violating (CLFV) decays of muon and tau leptons to the three body final states $(\bar{e}ee, \bar{\mu}\mu\mu, \bar{e}\mu\mu, \bar{\mu}\mu e,\bar{\mu}ee, \bar{e}e\mu)$. By constructing a complete basis of operators at each dimension, we derive the most general amplitudes for these decay processes. By considering constraints from unitarity and LEP, we show that operators of mass dimension 6 and 7 are the most likely to be observed in next generation experiments. Focusing on these dimensions, we compute the results of unpolarized (spin-averaged) decays parameterized in terms of the invariant masses of the daughter particles. We also compute the differential decay rates for polarized decays, in anticipation of the experimental search Mu3e, which expects to have a muon beam with $\sim 90\%$ polarization, and the Chiral Belle proposal, which aims to have a $70\%$ polarized electron beam. To determine the extent to which the operators may be distinguished experimentally, we plot the differential distributions for each operator, showing that they leave only a few possible degenerate explanations. Through a statistical analysis, we estimate the number of events needed to break the degeneracies using the angular information. These results are adapted to treat $\ell\to \ell' \nu \bar{\nu}$, where the angular distribution of the outgoing charged lepton has enhanced distinguishing power. With many Standard Model extensions predicting these CLFV decays, these results will better enable upcoming searches to identify and/or constrain physics beyond the Standard Model.

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arXiv2024

Consequences of Minimal Entanglement in Bosonic Field Theories

In this paper, we study a recently discovered connection between scattering that minimally entangles and emergent symmetries. In a perturbative expansion, we have generalized the constraints of minimal entanglement scattering, beyond qubits, to general qudits of dimension $d.$ Interestingly, projecting on any qubit subspaces, the constraints factorize, so that it is consistent to analyze minimal entanglement by looking at all such subspaces. We start by looking at toy models with two scalar fields, finding that minimal entanglement only allows quartic couplings which have instabilities at large field values and no symmetries. For the two Higgs doublet model, by considering $H^+ H^-\to H^+ H^-$ scattering, we show that minimal entanglement in this channel does not allow an interacting parameter point with enhanced symmetries. These results show that the connection between minimal entanglement and symmetries depends strongly on the scattering channels analyzed and we speculate on the potential resolutions.

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arXiv2023

Primary Observables for Electroweak Gauge Boson Collider Signals

In this paper, we determine a basis for the on-shell 4-point amplitudes $VVVV$ for Standard Model gauge bosons $V=W^\pm, Z, \gamma, g$. Following previous work, this completes the analysis of 3- and 4-point amplitudes for the Standard Model and could be used for model independent searches for beyond the Standard Model physics at colliders. Our results include a Lagrangian parametrization for the "primary" amplitudes, where including additional derivatives leads to the Mandelstam "descendant" amplitudes, and upper bounds on the coupling constants from imposing unitarity. We also perform an estimate for the sensitivity for new $Z$ decays at the HL-LHC, finding that $Z\to \gamma \bar{\ell}\ell$ could be searched for, but that other decay modes, like $Z\to (\gamma\gamma\gamma, \gamma gg)$, are too small to be discovered after imposing unitarity constraints.

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arXiv2023

Primary Observables for Top Quark Collider Signals

At the HL-LHC and future high energy colliders, a sample of a billion top quarks will be produced, allowing precision searches for new physics in top quark decay and production. To aid in this endeavor, we characterize the independent three and four point on-shell amplitudes involving top quarks, under the assumption of $SU(3)_c\times U(1)_{em}$ invariance. The four point amplitudes factorize into primary and descendent amplitudes, where descendants are primaries multiplied by Mandelstam variables. By enumerating the allowed amplitudes, we can check for amplitude redundancies to find the number of independent terms and convert those into a Lagrangian which parameterizes these amplitudes. These results are then cross checked by utilizing the Hilbert series to count the number of independent Lagrangian operators. Interestingly, we find situations where the Hilbert series has cancellations which, if naïvely interpreted, would lead to the incorrect conclusion that that there are no primary operators at a given mass dimension. We characterize the four fermion ($ffff$) and two fermion, two gauge boson ($ffVV$) operators respectively up to dimension 12 and 13. Finally, by combining unitarity bounds on the coupling strengths and simple estimates of the branching ratio sensitivities, we highlight interesting amplitudes for top quark decay that should be studied more closely at the HL-LHC. Of those highlighted, there are both new charge current and flavor changing neutral current decays that occur at dimension 8 and 10 in SMEFT.

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arXiv2023

Primary Observables for Indirect Searches at Colliders

We consider the complete set of observables for collider searches for indirect effects of new heavy physics. They consist of $SU(3)_{\rm C}\times U(1)_{\rm EM}$ invariant interaction terms/operators that parameterize deviations from the Standard Model. We show that, under very general assumptions, the leading deviations from the Standard Model are given by a finite number of `primary' operators, with the remaining operators given by `Mandelstam descendants' whose effects are suppressed by powers of Mandelstam variables divided by the mass scale $M$ of the heavy physics. We explicitly determine all 3 and 4-point primary operators relevant for Higgs signals at colliders by using the correspondence between on-shell amplitudes and independent operators. We give a detailed discussion of the methods used to obtain this result, including a new analytical method for determining the independent operators. The results are checked using the Hilbert series that counts independent operators. We also give a rough sketch of the phenomenology, including unitarity bounds on the interaction strengths and rough estimates of their importance for Higgs decays at the HL-LHC. These results motivate further exploration of Higgs decays to $Z\bar{f}f$, $W\bar{f}f'$, $γ\bar{f}f$, and $Zγγ$.

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arXiv2023

Labels, Information, and Computation: Efficient Learning Using Sufficient Labels

In supervised learning, obtaining a large set of fully-labeled training data is expensive. We show that we do not always need full label information on every single training example to train a competent classifier. Specifically, inspired by the principle of sufficiency in statistics, we present a statistic (a summary) of the fully-labeled training set that captures almost all the relevant information for classification but at the same time is easier to obtain directly. We call this statistic "sufficiently-labeled data" and prove its sufficiency and efficiency for finding the optimal hidden representations, on which competent classifier heads can be trained using as few as a single randomly-chosen fully-labeled example per class. Sufficiently-labeled data can be obtained from annotators directly without collecting the fully-labeled data first. And we prove that it is easier to directly obtain sufficiently-labeled data than obtaining fully-labeled data. Furthermore, sufficiently-labeled data is naturally more secure since it stores relative, instead of absolute, information. Extensive experimental results are provided to support our theory.

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arXiv2022

Creating Simple, Interpretable Anomaly Detectors for New Physics in Jet Substructure

Anomaly detection with convolutional autoencoders is a popular method to search for new physics in a model-agnostic manner. These techniques are powerful, but they are still a "black box," since we do not know what high-level physical observables determine how anomalous an event is. To address this, we adapt a recently proposed technique by Faucett et al., which maps out the physical observables learned by a neural network classifier, to the case of anomaly detection. We propose two different strategies that use a small number of high-level observables to mimic the decisions made by the autoencoder on background events, one designed to directly learn the output of the autoencoder, and the other designed to learn the difference between the autoencoder's outputs on a pair of events. Despite the underlying differences in their approach, we find that both strategies have similar ordering performance as the autoencoder and independently use the same six high-level observables. From there, we compare the performance of these networks as anomaly detectors. We find that both strategies perform similarly to the autoencoder across a variety of signals, giving a nontrivial demonstration that learning to order background events transfers to ordering a variety of signal events.

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arXiv2022

Snowmass 2021 White Paper: Higgs Coupling Sensitivities and Model-Independent Bounds on the Scale of New Physics

In this Snowmass white paper, we describe how unitarity bounds can convert sensitivities for Higgs couplings at future colliders into sensitivities to the scale of new physics. This gives a model-independent consequence of improving these sensitivities and illustrate the impact they would have on constraining new physics. Drawing upon past successful applications of unitarity as a guide for future colliders (e.g. the Higgs mass bound and discovering it at the LHC), we hope this data will be useful in the planning for next generation colliders.

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arXiv2021

Higgs Coupling Measurements and the Scale of New Physics

A primary goal of present and future colliders is measuring the Higgs couplings to Standard Model (SM) particles. Any observed deviation from the SM predictions for these couplings is a sign of new physics whose energy scale can be bounded from above by requiring tree-level unitarity. In this paper, we extend previous work on unitarity bounds from the Higgs cubic coupling to Higgs couplings to vector bosons and top quarks. We find that HL-LHC measurements of these couplings compatible with current experimental bounds may point to a scale that can be explored at the HL-LHC or a next-generation collider. Our approach is completely model-independent: we assume only that there are no light degrees of freedom below the scale of new physics, and allow arbitrary values for the infinitely many couplings beyond the SM as long as they are in agreement with current measurements. We also extend and clarify the methodology of this analysis, and show that if the scale of new physics is above the TeV scale, then the deviations can be described by the leading higher-dimension gauge invariant operator, as in the SM effective field theory.

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arXiv2021

Distinguishing $W'$ Signals at Hadron Colliders Using Neural Networks

We investigate a neural network-based hypothesis test to distinguish different $W'$ and charged scalar resonances through the $\ell+\require{cancel}\cancel{E}_T$ channel at hadron colliders. This is traditionally challenging due to a four-fold ambiguity at proton-proton colliders, such as the Large Hadron Collider. Of the neural network approaches we studied, we find a multi-class classifier based on a fully-connected neural network trained upon 2D histograms made from kinematic variables of the final state $\ell$ to be the most powerful. Furthermore, by considering the 1-jet processes, we demonstrate that one can generalize to multiple $2D$ histograms to represent different variable pairs. Finally, as a comparison to traditional approaches, we compare our method with Bayesian hypothesis testing and discuss the pros and cons of each approach. The neural network scheme presented in this paper is a powerful tool that can help probe the properties of charged resonances.

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arXiv2020

The Higgs Trilinear Coupling and the Scale of New Physics

We consider modifications of the Higgs potential due to new physics at high energy scales. These upset delicate cancellations predicted by the Standard Model for processes involving Higgs bosons and longitudinal gauge bosons, and lead to a breakdown of the theory at high energies. We focus on modifications of the Higgs trilinear coupling and use the violation of tree-level unitarity as an estimate of the scale where the theory breaks down. We obtain a completely model-independent bound of ~ 13 TeV for an order-1 modification of the trilinear. We argue that this bound can be saturated only in fine-tuned models, and the scale of new physics is likely to be much lower. The most stringent bounds are obtained from amplitudes involving multiparticle states that are not conventional scattering states. Our results show that a future determination of the Higgs cubic coupling can point to a well-defined scale of new physics that can be targeted and explored at future colliders.

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arXiv2018

What is the Machine Learning?

Applications of machine learning tools to problems of physical interest are often criticized for producing sensitivity at the expense of transparency. To address this concern, we explore a data planing procedure for identifying combinations of variables -- aided by physical intuition -- that can discriminate signal from background. Weights are introduced to smooth away the features in a given variable(s). New networks are then trained on this modified data. Observed decreases in sensitivity diagnose the variable's discriminating power. Planing also allows the investigation of the linear versus non-linear nature of the boundaries between signal and background. We demonstrate the efficacy of this approach using a toy example, followed by an application to an idealized heavy resonance scenario at the Large Hadron Collider. By unpacking the information being utilized by these algorithms, this method puts in context what it means for a machine to learn.

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arXiv2017

Don't be left in the dark: Improving LHC searches for dark photons using lepton-jet substructure

Collider signals of dark photons are an exciting probe for new gauge forces and are characterized by events with boosted lepton jets. Existing techniques are efficient in searching for muonic lepton jets but due to substantial backgrounds have difficulty constraining lepton jets containing only electrons. This is unfortunate since upcoming intensity frontier experiments are sensitive to dark photon masses which only allow electron decays. Analyzing a recently proposed model of kinetic mixing, with new scalar particles decaying into dark photons, we find that existing techniques for electron jets can be substantially improved. We show that using lepton-jet-substructure variables, in association with a boosted decision tree, improves background rejection, significantly increasing the LHC's reach for dark photons in this region of parameter space.

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arXiv2016

Correlated Signals at the Energy and Intensity Frontiers from Nonabelian Kinetic Mixing

We show that when a dark abelian gauge sector and SU$(2)_{L}$ kinetically mix it necessarily generates a relation between the kinetic mixing strength and the mass of the mediating particle. Remarkably, this correspondence maps the weak scale directly to the kinetic mixing strengths being probed by modern fixed-target experiments and next generation flavor factories. This illuminates the exciting possibility of correlated discoveries of a new particle at the LHC and a dark photon at intensity frontier experiments. To motivate the scenario, we construct a simple model and explore its phenomenology and constraints.

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arXiv2016

A Simple $U(1)$ Gauge Theory Explanation of the Diphoton Excess

The recent ATLAS and CMS diphoton resonance excesses are explored in a simple $U(1)$ gauge theory extension of the Standard Model where the resonance is the Higgs boson of the $U(1)$ symmetry breaking, $ϕ$. This particle couples to exotic quarks which, through loops, can produce a large enough rate to explain the excess. Due to the choice of $U(1)$ charges, flavor constraints are naturally suppressed, allowing arbitrary flavor violation in the decays of the new quarks to up-type quarks, modifying their signal topologies. An additional heavy quark in the model decays to the lighter exotic quark by emitting either $ϕ$ or the $U(1)$ gauge boson, $A_x$, giving extra signals containing diphoton and digluon resonances. Finally, the new Higgs can decay into $γA_x$ and $Z A_x$, followed by $A_x$ decaying into Standard Model fermions through kinetic mixing. Thus, this model gives interesting modified signals to the general class of exotic quark models explaining the diphoton resonance.

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arXiv2014

Phenomenology of Induced Electroweak Symmetry Breaking

We study the phenomenology of models of electroweak symmetry breaking where the Higgs potential is dominated by a positive quadratic term destabilized by a tadpole arising from the coupling to an "auxiliary" Higgs sector. The auxiliary Higgs sector can be either perturbative or strongly coupled, similar to technicolor models. Since electroweak symmetry breaking is driven by a tadpole, the cubic and quartic Higgs couplings can naturally be significantly smaller than their values in the standard model. The theoretical motivation for these models is that they can explain the 125 GeV Higgs mass in supersymmetry without fine-tuning. The auxiliary Higgs sector contains additional Higgs states that cannot decouple from standard model particles, so these models predict a rich phenomenology of Higgs physics beyond the standard model. In this paper we analyze a large number of direct and indirect constraints on these models. We present the current constraints after the 8 TeV run of the LHC, and give projections for the sensitivity of the upcoming 14 TeV run. We find that the strongest constraints come from the direct searches $A^0 \to Zh$, $A^0 \to t\bar{t}$, with weaker constraints from Higgs coupling fits. For strongly-coupled models, additional constraints come from $ρ^+ \to WZ$ where $ρ^+$ is a vector resonance. Our overall conclusion is that a significant parameter space for such models is currently open, allowing values of the Higgs cubic coupling down to 0.4 times the standard model value for weakly coupled models and vanishing cubic coupling for strongly coupled models. The upcoming 14 TeV run of the LHC will stringently test this scenario and we identify several new searches with discovery potential for this class of models.

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