arXiv subjects

Maxim Perelstein

Publications and source records attributed to Maxim Perelstein.

At least 19 recordsLinked to original sources

arXiv2026

Probing CP-violating top-quark dipole moments with tomographic observables

Quantum tomography program reconstructs the full spin density matrix of top-quark pairs from dilepton angular correlations at both electron-positron and hadron colliders. We use symmetry arguments to identify tomographic observables sensitive to CP violation in top quark couplings. As a concrete example, we study the sensitivity of these observables to (chromo-)electric dipole moment operators at the LHC and the FCC-ee. We also construct the optimal observable, which combines tomographic and kinematic information to achieve statistically optimal sensitivity for each operator and production process. The analysis is based on fast detector simulation of top pair production and decay in the dilepton channel. We project a $95\%$ CL sensitivity to new-physics scales (the inverse dipole couplings) of $10-15$ TeV for the electroweak electric dipoles at the FCC-ee ($365$ GeV, $3~{\rm ab}^{-1}$) and of 40 (200) TeV for the top chromo-electric dipole moment at the LHC Run 2 (HL-LHC). This sensitivities exceed those of the traditional observables currently used by experiments to use for these operators, indicating the power of the tomographic approach to search for CP violation.

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arXiv2025

WIMP Meets ALP: Coherent Freeze-Out of Dark Matter

We consider the cosmological history of a weakly interacting massive particle (WIMP) coupled to a light axion-like particle (ALP) via a quadratic coupling. Although the coupling is too feeble to thermalize the ALP, coherent forward scattering between the two sectors induces temperature-dependent mass shifts that substantially modify both WIMP freeze-out and ALP misalignment dynamics, giving rise to a novel coherent freeze-out mechanism. At high temperatures, the WIMP thermal bath spontaneously breaks the symmetry of the ALP potential, displacing the field to a new vacuum. The resulting back-reaction reduces the WIMP effective mass and significantly delays its freeze-out. Depending on the strength of the coupling, symmetry restoration occurs via either a first-order phase transition (FOPT) or a crossover. In the FOPT regime, dark matter consists solely of WIMPs, whose delayed freeze-out permits annihilation cross sections up to three orders of magnitude above the standard value, while still yielding the correct relic density. In the crossover regime, both WIMP and ALP can contribute to dark matter. Remarkably, we find an "ALP miracle": a Planck-suppressed quadratic coupling yields an ALP abundance comparable to the observed dark matter density, largely independent of its initial displacement and mass.

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arXiv2025

Collider Searches for Near-Continuum Dark Matter

We study collider constraints on the near-continuum dark matter model, in which the dark sector consists of a tower of closely spaced states with weak-scale masses coupled to the Standard Model through a $Z$-portal. To capture this structure in a model-agnostic way, we introduce a minimal parameterization that encodes the dominant geometric information with three parameters. Using a custom-built Monte-Carlo tool for near-continuous spectra, we simulate DM-pair production at $\sqrt{s}=13$ TeV and subsequent cascade decays via on/off-shell $Z$ bosons, which yield events with large missing transverse momentum and high jet multiplicity. Recasting the CMS multijet$+H_T^{\rm miss}$ analysis of Run-2 data (35.9 fb$^{-1}$), we derive bounds on the model parameter space. Extrapolating these bounds, we provide High-Luminosity LHC projections (3 ab$^{-1}$). We also project sensitivities for future electron-positron colliders at $\sqrt{s}=365$ GeV and $\sqrt{s}=500$ GeV, showing substantial improvements over the HL-LHC. The current LHC sensitivity is beginning to approach the theoretically motivated region of the parameter space, while future colliders will be able to comprehensively test this model.

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arXiv2025

Sommerfeld Enhancement from Quantum Forces for Dark Matter

Quantum forces are long-range interactions that arise only at the loop level. In this work, we study the Sommerfeld enhancement of dark matter (DM) annihilation cross sections caused by quantum forces. One notable feature of quantum forces is that they are subject to coherent enhancement in the presence of a background of mediator particles, which occurs in many situations in cosmology. We show that this effect has important implications for the Sommerfeld enhancement and DM physics. For the first time, we calculate the Sommerfeld factor induced by quantum forces for both bosonic and fermionic mediators, including the background corrections. We observe several novel features of the Sommerfeld factor that do not exist in the case of the Yukawa potential, such as temperature-induced resonance peaks for massless mediators, and having both enhancement and suppression effects in the same model with different DM masses. As direct applications, we discuss the DM phenomenology affected by the Sommerfeld enhancement from quantum forces, including thermal freeze-out, CMB spectral distortion from DM annihilation, and DM indirect detection. We highlight one particularly interesting effect relevant to indirect detection caused by the Sommerfeld enhancement in a non-thermal background of bosonic mediators in the galaxy, in which case the DM mass is shifted due to the background correction and the effective cross section for DM annihilation can be either enhanced or suppressed. This may be important for DM searches in the Milky Way or its satellite galaxies.

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arXiv2025

Conformal Freeze-in Dark Matter: 5D Dual and Phase Transition

Conformal Freeze-in (COFI) scenario postulates a dark sector described by a conformal field theory (CFT) at energies above the ``gap scale" in the keV$-$MeV range. At the gap scale, the dark CFT undergoes confinement, and one of the resulting bound states is identified as the dark matter candidate. In this paper, we study this model in the context of the AdS/CFT correspondence with a focus on the mechanism of the infrared (IR) breaking of conformal invariance in the dark sector. We construct the holographic dual to the conformal dark sector, given by a Randall-Sundrum-like model in 5D, where the Standard Model (SM) fields and the dark matter candidate are placed on the ultraviolet (UV) and IR branes respectively. The separation between the UV and IR branes is stabilized by a bulk scalar field, naturally generating a hierarchy between the electroweak scale and the gap scale. We find that the parameter space of COFI comprises two distinct branches of CFT's living on the Anti-de-Sitter (AdS) boundary, each corresponding to a different UV boundary condition. The two branches of CFT's result in different radion potentials. The confinement of the CFT is dual to the spontaneous symmetry breaking by the 5D radion potential. We then use this dual 5D setup to study the cosmological confining phase transition in the dark sector. We find the viable parameter space of the theory which allows the phase transition to complete promptly without significant supercooling.

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arXiv2024

Conformal Freeze-In from Neutrino Portal

We study a scenario where a dark sector, described by a Conformal Field Theory (CFT), interacts with the Standard Model through the neutrino portal. In this setup, conformal invariance breaks below the electroweak scale, causing the theory to transition into a confined (hadronic) phase. One of the hadronic excitations in this phase can act as dark matter. In the "Conformal Freeze-In" cosmological framework, the dark sector is populated through interactions with the Standard Model at temperatures where it retains approximate conformal symmetry. The dark matter relic density depends on the CFT parameters, such as the dimension of the operator coupled to the Standard Model. We demonstrate that this model can reproduce the DM relic density and meet all observational constraints. The same neutrino portal interaction may also generate masses for the active neutrinos. The dark matter candidate could either be a pseudo-Goldstone boson (PGB) or a composite fermion with the quantum numbers of a sterile neutrino. In the latter case, the model is consistent with the current X-ray constraints, and may be detectable with future X-ray observations.

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arXiv2023

Anomaly Detection in Presence of Irrelevant Features

Experiments at particle colliders are the primary source of insight into physics at microscopic scales. Searches at these facilities often rely on optimization of analyses targeting specific models of new physics. Increasingly, however, data-driven model-agnostic approaches based on machine learning are also being explored. A major challenge is that such methods can be highly sensitive to the presence of many irrelevant features in the data. This paper presents Boosted Decision Tree (BDT)-based techniques to improve anomaly detection in the presence of many irrelevant features. First, a BDT classifier is shown to be more robust than neural networks for the Classification Without Labels approach to finding resonant excesses assuming independence of resonant and non-resonant observables. Next, a tree-based probability density estimator using copula transformations demonstrates significant stability and improved performance over normalizing flows as irrelevant features are added. The results make a compelling case for further development of tree-based algorithms for more robust resonant anomaly detection in high energy physics.

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arXiv2023

Collider Signatures of Near-Continuum Dark Matter

In this paper we study a near-continuum dark matter model, in which dark sector consists of a tower of closely spaced states with weak-scale masses. We construct a five-dimensional model which naturally realizes this spectrum. The dark matter is described by a bulk field, which interacts with the brane-localized Standard Model sector via a Z portal. We then study collider signatures of this model. Near-continuum dark matter states produced in a collider undergo cascade decays, resulting in events with high multiplicity of jets and leptons, large missing energy, and displaced vertices. A custom-built Monte Carlo tool described in this paper allows for detailed simulation of the signal events. We present results of such simulations for the case of electron-positron collisions.

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arXiv2023

Sub-GeV dark matter search at ILC beam dumps

Light dark matter particles may be produced in electron and positron beam dumps of the International Linear Collider (ILC). We propose an experimental setup to search for such events, the Beam-Dump eXperiment at the ILC (ILC-BDX). The setup consists of a muon shield placed behind the beam dump, followed by a multi-layer tracker and an electromagnetic calorimeter. The calorimeter can detect electron recoils due to elastic scattering of dark matter particles produced in the dump, while the tracker is sensitive to decays of excited dark-sector states into the dark matter particle. We study the production, decay and scattering of sub-GeV dark matter particles in this setup in several models with a dark photon mediator. Taking into account beam-related backgrounds due to neutrinos produced in the beam dump as well as the cosmic-ray background, we evaluate the sensitivity reach of the ILC-BDX experiment. We find that the ILC-BDX will be able to probe interesting regions of the model parameter space and, in many cases, reach well below the relic target.

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arXiv2022

Dark Matter from a Conformal Dark Sector

We consider theories in which a dark sector is described by a Conformal Field Theory (CFT) over a broad range of energy scales. A coupling of the dark sector to the Standard Model breaks conformal invariance. While weak at high energies, the breaking grows in the infrared, and at a certain energy scale the theory enters a confined (hadronic) phase. One of the hadronic excitations can play the role of dark matter. We study a "Conformal Freeze-In" cosmological scenario, in which the dark sector is populated through its interactions with the SM at temperatures when it is conformal. In this scenario, the dark matter relic density is determined by the CFT data, such as the dimension of the CFT operator coupled to the Standard Model. We show that this simple and highly predictive model of dark matter is phenomenologically viable. The observed relic density is reproduced for a variety of SM operators ("portals") coupled to the CFT, and the resulting models are consistent with observational constraints. The mass of the COFI dark matter candidate is predicted to be in the keV-MeV range.

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arXiv2022

Dark Z at the International Linear Collider

A dark Z is a massive Abelian gauge boson which is coupled to the Standard Model through both kinetic and mass mixing with the electroweak sector. We study the phenomenology of the dark Z at an energy-frontier electron-positron collider, such as the proposed International Linear Collider (ILC). We show that precision electroweak constraints and the current bounds from the hadron colliders allow a dark Z that is kinematically accessible at the ILC with 250 GeV or 500 GeV center-of-mass energy. Further, the reach of the ILC searches for a dark Z significantly exceeds the expected reach of the high luminosity LHC. If a signal consistent with a dark Z is discovered, it would motivate a dedicated run of the ILC at the center-of-mass energy matching the dark Z mass. We demonstrate that a short one month run at design luminosity could measure the dark Z chiral couplings to fermions with percent precision. This measurement can be used to discriminate between competing theoretical models of the resonance: for example, the dark Z can be distinguished from a dark photon with purely kinetic mixing.

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arXiv2022

The International Linear Collider: Report to Snowmass 2021

The International Linear Collider (ILC) is on the table now as a new global energy-frontier accelerator laboratory taking data in the 2030s. The ILC addresses key questions for our current understanding of particle physics. It is based on a proven accelerator technology. Its experiments will challenge the Standard Model of particle physics and will provide a new window to look beyond it. This document brings the story of the ILC up to date, emphasizing its strong physics motivation, its readiness for construction, and the opportunity it presents to the US and the global particle physics community.

physics.acc-ph↗

arXiv2021

Z-portal Continuum Dark Matter

We examine the possibility that dark matter (DM) consists of a gapped continuum, rather than ordinary particles. A Weakly-Interacting Continuum (WIC) model, coupled to the Standard Model via a Z-portal, provides an explicit realization of this idea. The thermal DM relic density in this model is naturally consistent with observations, providing a continuum counterpart of the "WIMP miracle". Direct detection cross sections are strongly suppressed compared to ordinary Z-portal WIMP, thanks to a unique effect of the continuum kinematics. Continuum DM states decay throughout the history of the universe, and observations of cosmic microwave background place constraints on potential late decays. Production of WICs at colliders can provide a striking cascade-decay signature. We show that a simple Z-portal WIC model provides a fully viable DM candidate consistent with all current experimental constraints.

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arXiv2021

Continuum Dark Matter

We initiate the study of dark matter models based on a gapped continuum. Dark matter consists of a mixture of states with a continuous mass distribution, which evolves as the universe expands. We present an effective field theory describing the gapped continuum, outline the structure of the Hilbert space and show how to deal with the thermodynamics of such a system. This formalism enables us to study the cosmological evolution and phenomenology of gapped continuum DM in detail. As a concrete example, we consider a weakly-interacting continuum (WIC) model, a gapped continuum counterpart of the familiar WIMP. The DM interacts with the SM via a Z-portal. The model successfully reproduces the observed relic density, while direct detection constraints are avoided due to the effect of continuum kinematics. The model has striking observational consequences, including continuous decays of DM states throughout cosmological history, as well as cascade decays of DM states produced at colliders. We also describe how the WIC theory can arise from a local, unitary scalar QFT propagating on a five-dimensional warped background with a soft wall.

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arXiv2021

ILC Study Questions for Snowmass 2021

To aid contributions to the Snowmass 2021 US Community Study on physics at the International Linear Collider and other proposed $e^+e^-$ colliders, we present a list of study questions that could be the basis of useful Snowmass projects. We accompany this with links to references and resources on $e^+e^-$ physics, and a description of a new software framework that we are preparing for $e^+e^-$ studies at Snowmass.

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arXiv2021

Improved Neural Network Monte Carlo Simulation

The algorithm for Monte Carlo simulation of parton-level events based on an Artificial Neural Network (ANN) proposed in arXiv:1810.11509 is used to perform a simulation of $H\to 4\ell$ decay. Improvements in the training algorithm have been implemented to avoid numerical instabilities. The integrated decay width evaluated by the ANN is within 0.7% of the true value and unweighting efficiency of 26% is reached. While the ANN is not automatically bijective between input and output spaces, which can lead to issues with simulation quality, we argue that the training procedure naturally prefers bijective maps, and demonstrate that the trained ANN is bijective to a very good approximation.

hep-ph↗

arXiv2020

Dark Matter as a Solution to Muonic Puzzles

We propose a simple model in which dark matter particle exchanges mediate a new quantum force between muons and nucleons, resolving the proton charge radius puzzle. At the same time, the discrepancy between the measured anomalous magnetic moment of the muon and the Standard Model prediction can be accommodated, and thermal relic abundance of the dark matter candidate is consistent with observations. The dark matter particle mass is in the MeV range. We show that the model is consistent with a variety of experimental and observational constraints.

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arXiv2020

Neural Network-Based Approach to Phase Space Integration

Monte Carlo methods are widely used in particle physics to integrate and sample probability distributions (differential cross sections or decay rates) on multi-dimensional phase spaces. We present a Neural Network (NN) algorithm optimized to perform this task. The algorithm has been applied to several examples of direct relevance for particle physics, including situations with non-trivial features such as sharp resonances and soft/collinear enhancements. Excellent performance has been demonstrated in all examples, with the properly trained NN achieving unweighting efficiencies of between 30% and 75%. In contrast to traditional Monte Carlo algorithms such as VEGAS, the NN-based approach does not require that the phase space coordinates be aligned with resonant or other features in the cross section.

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