MIT Center for Theoretical Physics — a Leinweber Institute Publications

Charge-dependent directed flow provides a sensitive probe of early electromagnetic fields and baryon stopping in relativistic heavy-ion collisions. Recent STAR measurements show a centrality-dependent sign change in the directed flow splitting (the difference between the directed flow of protons and antiprotons), indicating that electromagnetic effects alone are not sufficient to describe this observable and that the baryon stopping $-$ in particular the component of the stopped proton distribution that is odd in rapidity and odd under reflection in the impact parameter direction $-$ must also be included. We develop a semi-analytic hydrodynamic framework that combines spectator-induced electromagnetic fields with a Glauber-based description of baryon stopping, built upon an analytic solution for the background hydrodynamic flow due to Gubser together with the simplifying assumption of a constant electrical conductivity. For Au+Au collisions at $sqrt{s_{NN}}=200$ GeV, we find that baryon stopping gives a positive contribution to the directed flow splitting that decreases for more peripheral collisions, while electromagnetic fields give a negative contribution that is larger for more peripheral collisions. The competition between these two effects naturally reproduces the observed sign change in Au+Au collisions as a function of centrality, describes the observed rapidity dependence in the $50$--$80%$ centrality interval, and reproduces trends seen in U+U collisions. Although the simplifying assumptions that we have made regarding the analytic background and constant conductivity limit our ability to make quantitative comparisons, our model provides a transparent explanation of how transported baryon number and spectator-induced electromagnetic fields jointly shape charge-dependent directed flow.

Recent progress in collider physics has reformulated phenomenological questions in terms of detector correlation functions and advanced their theoretical understanding. These advances have primarily focused on correlators of the average null energy operator, $mathcal{E}(vec{n})$, known as energy correlators. However, colliders have access to a much broader class of detector operators, $mathcal{E}^n_R(vec{n})$, which measure powers of the energy on a subset of hadrons $R$, such as charged hadrons. These generalized detectors are generically not infrared and collinear safe, and their description requires nonperturbative matching between the hadronic detectors measured in the infrared and the partonic detectors used in ultraviolet calculations. We develop a framework for computing their multi-point correlation functions. We introduce universal nonperturbative matching coefficients, termed "detector functions", that implement this infrared-ultraviolet matching. For the $mathcal{E}_R^n$ operators studied here, these coefficients are represented by energy-weighted moments of single- and multi-hadron fragmentation functions. We present their renormalization group structure, derive QCD factorization theorems for the projected correlators, and compute jet functions through next-to-leading order. In the fixed-coupling pure Yang-Mills limit, we derive the light-ray OPE of hadronic detectors and connect it to the QCD factorization framework. We also identify universal nonperturbative power corrections generated by soft radiation, which are enhanced in the collinear limit and can substantially modify the perturbative angular scaling. A parton shower study finds qualitative agreement with the predicted perturbative and nonperturbative scaling behaviors. Our work significantly broadens the space of detector operators under theoretical control, with potential phenomenological applications.

Motivated by searches for weak coherent drives, we formulate repeated quantum sensing with a fixed shot budget as an asymmetric composite hypothesis test. Taking Rabi sensing as a concrete example, we benchmark detection power, sensitivity, and Type-II error exponents in the resonant case with unknown signal amplitude and phase. We compare non-adaptive population and transverse readouts with a myopic Bayesian policy that selects each projective axis by maximizing the expected information gain in one step. The common decision statistic is a log Bayes factor, with a policy specific threshold calibrated under the null to enforce a common Type-I error. A weak signal expansion shows that population readout is phase independent but quadratic in amplitude, giving $n^{-1/4}$ sensitivity, whereas transverse readout is linear in amplitude and permits $n^{-1/2}$ sensitivity without adaptation, but is phase-dependent. In Monte Carlo pseudoexperiments, the adaptive policy exploits posterior information about the unknown direction to guide subsequent readouts; its sensitivity is consistent with $n^{-1/2}$ over the simulated large $n$ range and, at the largest simulated shot counts, outperforms the fixed transverse schedules. Its phase-averaged effective Type-II exponent also exceeds the non-adaptive references over the simulated range. These results demonstrate the finite budget value of exploiting nuisance parameter information under calibrated false positive control.

The kinetic Sunyaev-Zeldovich (kSZ) effect is a sensitive probe of the free-electron momentum field, tapping into spatial correlations relevant for both large-scale cosmology and smaller-scale astrophysics. Precision kSZ measurements will provide a robust channel for constraining inflationary physics and equivalence principle violation, while simultaneously characterizing the small-scale gas distribution and associated baryonic feedback. However, robust kSZ measurement for cosmological applications is challenging, as the primary source of large-scale information is convolved with unknown small-scale dynamics in a degenerate combination. In this paper we address this challenge using the Effective Field Theory (EFT) framework that provides a principled way of treating the effects of small-scale dynamics on large scale-structure. Extracting the kSZ signal can be reformulated as a measurement of the one-loop galaxy-galaxy-electron momentum bispectrum. We compute here a pure dark matter version of this bispectrum in EFT and compare it to simulations. We derive the EFT counterterms that capture small-scale backreaction and find that two of them acquire a transverse direction. One transverse counterterm gets generated entirely by the vorticity of the velocity field. Our computation of the bispectrum dipole agrees with simulation data to $5%$ up to $k_{rm max}=0.23~iM$, which is significantly larger than the reach of tree level theory, which breaks down beyond $k_{rm max}=0.07~iM$ (at redshift $z=0.5$). For the bispectrum computed with the projected momentum field, relevant to measuring the kSZ field, we find a similar $k_{rm max}$, and make the first detection of the transverse counterterms from simulations. Our results suggest that one-loop bispectrum computations can play an important role in advancing kSZ cosmology.

We carry out a search for the relative baryon-cold dark matter (CDM) density and velocity modes in the DESI full-shape (FS) data, and investigate whether DESI's preference for evolving dark energy can be affected by these relative perturbations, which are a known contaminant of the baryon acoustic oscillations. For the first time, we use a holistic effective field theory (EFT) treatment of the power spectrum and bispectrum both for the standard terms (without relative perturbations), modeled in EFT to one-loop order, and for the relative perturbations. For the latter we implement an efficient computation of the one-loop power spectrum that allows us to scan over different cosmologies. We obtain robust constraints on the relative density and velocity bias parameters $b_{δ_{bc}}$, and $b_{v^2_{bc}}$ for all the DESI DR1 tracers, finding e.g. $b_{δ_{bc}}=5 pm 6$, $b_{v^2_{bc}}=-0.031pm 0.023$ (at 68% CL) for the LRG2 sample in a DESI FS-only analysis within $Λ$CDM, supplemented by CMB priors on the baryon density and spectral tilt. Assuming instead a $w_0w_a$CDM cosmological model, and combining DESI FS with the CMB primary anisotropies, CMB lensing, and Pantheon+ supernovae, our LRG2 constraints read $b_{δ_{bc}}=2 pm 5$, $b_{v^2_{bc}}=-0.032pm 0.022$. Adding the relative velocity divergence biases we find $b_{δ_{bc}}=-17_{-17}^{+19}$, $b_{θ_{bc}}=-760_{-680}^{+700}$, $b_{v^2_{bc}}=-0.032pm 0.024$ for the same sample. The constraints on the dark energy equation of state parameters are virtually unchanged when marginalizing over the baryon-CDM modes. We conclude that DESI's preference for evolving dark energy is robust with respect to the baryon-CDM relative perturbations.

The Sommerfeld enhancement (SE) from long-range interactions, and the related bound state formation rate, can be important non-perturbative inputs to dark matter (DM) annihilation signals. In the presence of singular interaction potentials, the conventional boundary conditions of the Schrödinger equation fail, obfuscating the computation of SE, its physical origin, and its relation to bound states in such potentials. In this work, we clarify the origin of SE in singular potentials in a two-fold manner: using the framework of velocity power counting in non-relativistic effective field theory (NREFT), and via position-space regularization of singular potentials at short distances to compute SE and bound states. We illustrate our findings through the case of pseudoscalar mediators interacting with massive Dirac DM. We find that when such a system arises from a UV-complete theory at weak coupling, no SE is generated. However, in the case of a derivatively coupled pseudoscalar, where the interaction is described by a higher-dimension operator in an effective theory, SE (and bound states) can occur for weak couplings if there is a large hierarchy between the cutoff scale of the effective theory and the dark matter mass. This SE is sensitive to the choice of the UV completion of the potential at short distances, but a non-negligible SE can persist even when the UV physics alone would not generate any SE; we elucidate the interplay of UV and IR physics in this case.

In a broad class of inflationary models, the leading non-Gaussian bispectrum arises from one-loop rather than tree-level processes. However, realistic one-loop contributions remain largely unexplored for phenomenologically relevant masses beyond the simplest bubble diagrams. This is insufficient for observational purposes because, for generic masses and couplings, triangle contributions need not be suppressed relative to their bubble counterparts. We present a numerical method for evaluating scalar one-loop in-in diagrams contributing to the inflationary bispectrum at fully general external momenta and masses. First, the Witten-Feynman parameterization reduces de Sitter loop integrals to generalized Euler-Mellin integrals governed by Symanzik graph polynomials. Unfortunately, the exponents of the polynomials in the integrand become complex for sufficiently heavy masses, leading to sign problems when evaluating the integral using standard numerical techniques. To remedy this problem, we introduce the reduced Schwinger method, which evaluates a highly oscillatory subintegral analytically to obtain a Gauss hypergeometric kernel, leaving the remaining integrals to standard numerical quadrature. We validate the algorithm by reproducing known analytic results for the tree-level bispectrum in terms of ${}_3F_2$ functions, and then apply it to the one-loop bubble and triangle contributions at general kinematics. This yields the first direct numerical evaluation of the complete one-loop bispectrum, valid across the full kinematic range. Using these numerical results, we construct bispectrum templates and compare them with CMB data.

We solve in closed form a one-dimensional relativistic system: two masses interacting only through elastic collisions with a massless mediator bouncing between them. Momenta, times, and positions are hyperbolic functions of the collision index. The mediator energy, interpreted as the pair's effective potential, obeys an exact discrete Coulomb law, $Vpropto 1/r$, with a Lorentz-invariant action as coupling. A massive Newtonian mediator instead transmits a $1/r^{3}$ force; one adiabatic invariant traces both laws to the mediator's dispersion relation. Continued to negative mediator energy, the closed forms turn trigonometric, binding a one-dimensional mechanical analog of the Coulomb atom.

We study the growth of massive $SO(9)$ super-representations in type II and type I string theories. We do this first directly by finding an empirical formula that specifies which representations appear at any level, and then compute the multiplicities from a refined partition function evaluated over finite fields up to level 501 for individual representations, and level 226 for all representations. We then derive asymptotic formulae for the growth of any representation, which are constructed by integrating about the peaks of the refined partition function. Using a Rademacher sum we can find very accurate approximations for the multiplicities of the representations. Unlike the superstring partition function, which only receives contributions from the odd Rademacher terms, the multiplicities for any representation have contributions from both even and odd terms. Finally, we apply the same techniques to the ordinary representations, where a simplified refined partition function allows for better computational speed and simpler expressions for the asymptotic approximations.

Quantum thermalization explains how an isolated many-body system naturally evolves towards a thermal state, rendering information about the initial conditions inaccessible to local measurements. This is precisely the mechanism utilized in quantum error correction, where information is protected by design through a nonlocal encoding. In this work, we leverage this connection to port the rigorous framework of (approximate) quantum error correction to the study of quantum thermalization. Treating typical late-time states as codewords, we characterize the error-correcting properties of generic thermalizing dynamics. We numerically uncover a universal relationship between the encoding rate, distance, and thermal entropy density of the emergent code. At infinite temperature, this universal curve saturates the quantum Singleton bound, achieving the same optimal limit as Haar-random codes. At finite temperature, we introduce a code family based on the Scrooge ensemble, the natural thermal analogue of the Haar ensemble, and prove it saturates the entropic quantum Singleton bound, establishing this family as optimal within entropic constraints. Our extracted universal curve independently saturates this same bound, revealing that finite-temperature thermalization is itself optimal. Finally, we show how conserved quantities limit the error-correcting behavior of thermalization: codewords with differing energies, or other conserved charges, leak only classical information, and correctability persists until the difference reaches the scale of thermal fluctuations. Our results reveal a universal optimal coding structure in thermalizing dynamics, while introducing new optimal codes that achieve fundamental limits of approximate quantum error correction.

We present predictions for the collinear energy-energy correlator (EEC) measured inside $Z$-tagged jets in proton-proton collisions across the confinement transition. Our framework combines transverse-momentum-dependent factorization for back-to-back $Z+$jet production with an exclusive differential EEC jet function incorporating perturbative evolution and flavor-dependent non-perturbative fragmentation. The EEC jet function is evaluated at $mathrm{NLL}^{prime}$ accuracy, with separate quark and gluon non-perturbative scales constrained by existing $e^+e^-$ and inclusive-jet data. We provide predictions for forward $Z+$jet production at $sqrt{s}=8~mathrm{TeV}$ and $13~mathrm{TeV}$ in the jet transverse-momentum intervals $(20, 30)$, $(30, 50)$ and $(50, 100) , mathrm{GeV}$. The EEC distributions as functions of $z_χ$ exhibit finite plateaus at small $z_χ$, while the corresponding angular distributions display the characteristic turnover associated with the confinement transition. A flavor decomposition shows that this transition becomes increasingly dominated by quark-initiated jets as the jet transverse momentum increases. Forward $Z$-tagged jets therefore provide a clean probe of non-perturbative quark fragmentation and offer a direct test of whether the characteristic confinement scale extracted from $e^+e^-$ collisions remains universal in a hadronic environment.

The strong coupling constant $α_s$ is a fundamental parameter of the Standard Model. Its precise determination is essential for accurately predicting, studying, and understanding processes at the Large Hadron Collider and future experiments such as the Future Circular Collider. Event shape and correlator observables measured at electron-positron colliders provide one of the cleanest environments for extracting $α_s$, thanks to their sensitivity to $α_s$ and the availability of high-precision data from the Large Electron-Positron Collider. More broadly, such observables provide an ideal setting to develop and test our understanding of the perturbative and non-perturbative elements of Quantum Chromodynamics, which will underpin the field's precision and discovery frontiers for decades to come. Despite these advances, significant discrepancies persist between different determinations of $α_s$ from event shapes, both in the extracted central values and estimated uncertainties. This document motivates the establishment of a dedicated Task Force to coordinate a community-wide effort addressing these open questions. We report on the first two-day meeting held at CERN in November 2025, summarizing the scientific discussion and documenting the experimental analyses identified as priorities during the meeting, as well as the concrete list of tasks to be carried out by the theory community in preparation for future meetings.

We present a robust protocol to reshape interactions in a spin ensemble based on global control pulse sequences applied to multiple subensembles in parallel. This setting arises naturally from ensembles of solid-state defects or multi-species atomic arrays. We show that it is provably computationally hard to find pulse sequences that simultaneously engineer interactions both within and between subensembles. Despite its formal hardness, we identify a set of necessary or sufficient conditions under which one can synthesize a target Hamiltonian from the native one. Moreover, we introduce efficient numerical strategies for designing pulse sequences that engineer target Hamiltonians robust against common control imperfections. As a specific application, we discuss the robust generation of two-mode spin squeezing in dual-species atomic ensembles, which is a challenging task without subensemble control. Our results provide a practical toolbox to design novel quantum simulation and sensing experiments with minimal control overhead.

We construct a constant-degree and constant-gap quantum expander on $n$ qubits where each unitary can be implemented by a depth-$1$ and 1D circuit of Pauli or CNOT gates. We provide two applications of this expander. First, we use it to construct a family of frustration-free 1D Hamiltonians whose ground states obey the entanglement-gap relation $S = Θ(Δ^{-1/2})$; this is believed to be optimal, but achieving it had been open. Second, we use it to provide a streaming protocol that tests for closeness to a class of 1D volume-law entangled states. Moreover, we extend our quantum expander to a constant-degree and constant-gap expander on the unitary group where each unitary is a single $T$ gate, a single $T^{dagger}$ gate, or a depth-$1$ Clifford circuit. This implies that a random sequence of unitaries from the expander yields a gapped walk on a dense subgroup of the unitary group. This improves upon previous work by Bourgain and Gamburd which did not control the dependence of the gap on the dimension.

Quantum anomalies strongly constrain the possible behavior of many-body systems. A prime example is the Lieb-Schultz-Mattis (LSM) theorem, which relates UV symmetry and filling constraints to IR features of the energy spectrum and ground-state structure. Here, we ask how LSM constraints shape dynamical signatures and temporal correlations in open quantum systems. Using a spacetime duality, we show that the Liouvillian of a $d$-dimensional repeated quantum channel with a mixed anomaly between strong $S$ and weak $G$ symmetry can be mapped to a $(d{+}1)$-dimensional mixed-state symmetry-protected topological (mSPT) phase. Under this correspondence, the initial and steady states of the channel are identified with boundary states of the higher-dimensional mSPT in the presence of bulk projection. We further introduce the twisted Renyi-$N$ correlator (TRNC) as a probe of temporal correlations in the channel and demonstrate that it is dual to the mSPT strange correlator, providing a direct bulk-boundary route to diagnose long-range temporal order implied by the LSM anomaly. Finally, we identify the textit{Liouvillian singular spectrum}, rather than the Liouvillian spectrum itself, as a more fundamental diagnostic of quantum anomalies, and show that it is dual to the operator entanglement spectrum of the mSPT.

The dynamics of heavy quarks in quark-gluon plasma (QGP) formed in heavy ion collisions provide a unique window to characterize its properties. Existing approaches to describe heavy quarks in medium rely either on quasiparticle-based models of QGP, or on assuming that the momentum transfer from the medium follows Gaussian statistics. However, neither of these assumptions can be taken for granted in QCD. In fact, in the prototypical theory that is strongly coupled $mathcal{N}=4$ SYM, it has been long known there are no momentum-carrying quasiparticles, and furthermore, we have recently shown that the momentum transfer from the medium is far from being Gaussian [arXiv:2501.06289]. Since then, we showed that the asymmetry of said momentum transfer between energy loss and energy gain -- which affects all moments of the distribution, not only its Gaussian characteristics -- is, in fact, universal [arXiv:2504.21139]. Therefore, in order to connect the initial heavy quark production cross section with the final hadron spectrum in a way that is consistent with QFT principles, new methods are needed. In this talk, we present a new transport description of heavy quarks that encodes all of the non-Gaussian features of the momentum transfer from the medium, all of which can be defined and in principle calculated in QCD, without relying on any assumptions regarding the strength of the coupling. As a demonstrative example, we discuss heavy quark equilibration in $mathcal{N}=4$ SYM. This paves the way towards extracting novel information about fundamental properties of QGP.

We propose projected energy correlators measured on the recoiling QCD radiation of a $Z/γ$ as a benchmark observable for precision QCD at the LHC. Using the $Z/γ$ as a hard scale prevents the need for a jet algorithm, simplifying both the perturbative and non-perturbative corrections. We develop a framework to combine state-of-the-art fixed-order amplitudes, high order resummation, and universal non-perturbative matrix elements. Our approach is based on numerically computed inclusive hard functions, allowing flexibility in the process and the inclusion of realistic experimental cuts. We perform detailed numerical studies of the projected energy correlators at next-to-leading order + next-to-next-to-leading logarithm (NLO+NNLL) to verify the stability of our setup. We present numerical results at NLO+NNLL, which are the first complete matched predictions for energy correlators at the LHC at this order. We discuss the prospects for extensions to higher orders, outlining a path to NNLO calculations of energy correlators at the LHC.

We explain how the 'bar-cobar' construction for homotopy algebras reformulates the equations of motion of arbitrary gauge theories as gauge-covariant quadratic equations for an extended set of fields. The linear term of the new equations encodes the interactions of the original theory, while the quadratic term is universal. The extended fields include type-I multilocal fields, which depend on a set of coordinates and type-II multilocal fields, which depend on several sets of coordinates. The new equations of motion are the Maurer-Cartan equations of a differential graded associative algebra or Lie algebra. We show that every solution of the new equations is gauge equivalent to a solution with only type-I fields, that represents a solution of the original equations of motion. In string theory type-I fields are entangled states of the associated CFT, to be inserted across multiple punctures of a Riemann surface. General type-II states can also represent disconnected Riemann surfaces.

We propose a framework to determine the vacuum polarization functions in QED in the presence of an electromagnetic background field in the spacelike region on the lattice. This method consists in reweighting lattice Monte Carlo data generated without the background with the fermion determinant ratio evaluated in the continuous spacetime using worldline formalism. This proposal can be further extended to other applications in lattice gauge theory, such as QCD in finite chemical potential.

The Chinese remainder theorem is used in metrology for extending the range of quantum clocks/radar/interferometry, where the phase of a signal is known relative to a set of oscillators with different periods. This paper investigates the performance of a quantum-mechanical Chinese remainder clock, consisting of atoms/oscillators with pairwise coprime periods. We provide the optimal initial state and the optimal Heisenberg-limited quantum measurements for measuring time up to the product of the periods. We introduce a novel fault-tolerant post-processing protocol that allows reconstruction of the correct time even in the presence of errors in the remainders.