MIT Center for Theoretical Physics — a Leinweber Institute Publications

Classifying hadronic jets using their constituents’ kinematic information is a critical task in modern high-energy collider physics. Often, classifiers are designed by targeting the best performance using metrics such as accuracy, AUC, or rejection rates. However, the use of a single metric can lead to the use of architectures that are more model-dependent than competitive alternatives, leading to potential uncertainty and bias in analysis. We explore such trade-offs and demonstrate the consequences of using networks with high performance metrics but low resilience.

In this paper we propose a two-field model of warm inflation motivated from a heterotic string construction. The model contains an axion and a dilaton-like field. We show that while warm inflation can take place in the axion-field direction, thermal corrections coming from the radiation gauge fields, which couples to both the axion and the dilaton, prevent warm inflation to happen in the dilaton-field direction. We explore the background dynamics for different parameters, and identify a diversity of dynamical behaviors allowed in this model, denoting different regimes of warm inflation.

Public outreach in quantum science and technologies has many goals, ranging from generating interest and dampening hype to making the fascinating and complex topic more accessible. In this work, we present a play on quantum science and technologies aimed at a science-curious audience, which aims to lift the curtains on different researchers’ perspectives on this rapidly evolving field. These notes expand on the ideas presented in the play and provide additional sources that may be useful for educators and directors in future productions. In “A Quantum of Hope”, the end of the world has never looked so bureaucratic. When an alien civilization announces that Earth will be demolished to make way for a research station, humanity is given five years to prove its value. The evaluation criteria? Randomly chosen to be quantum science and technologies. Five experts, sampled from quantum academia and industry, are locked in a room to select a single project proposal to unite global efforts. Douglas Adams meets 12 Angry Men in a galactic-grade grant review committee, where scientific ideas, moral convictions, and unruly egos clash.

We study baby closed universes in AdS, focusing on the Antonini-Sasieta-Swingle (AS$^2$) cosmology, which arises in the gravitational description of partially entangled thermal states (PETS), as well as the classical example of Maldacena-Maoz (MM). We show that the algebraic formulation of AdS/CFT–and in particular the subregion-subalgebra duality–provides a natural framework for describing such universes within the standard AdS/CFT setting, phrased in terms of an operator algebra ${\cal M}$ and a state $ω$ on that algebra, with no need to introduce observers. The algebra encodes all physical operations in the closed universe, and, in principle, all physical observables are obtainable from the pair $({\cal M},ω)$. Along the way, we propose a resolution to a puzzle raised by Antonini and Rath (AR) and reinforced by recent arguments of Engelhardt and Gesteau and a no-go theorem of Gesteau: that a semiclassical baby universe in the AS$^2$ cosmology cannot be understood from the boundary in the usual manner. Our analysis motivates an averaged large-$N$ limit as part of the AdS/CFT dictionary and points toward a unified treatment of spacetimes of all asymptotics in terms of operator algebras. Finally, our boundary descriptions of closed universes indicate that for small but finite $G_N$ there should exist a semiclassical description of a macroscopic closed universe, rather than a one-dimensional Hilbert space, and we discuss weaknesses in arguments favoring the latter.

Under conservative assumptions, it is established at a mathematical level of rigor that if correlation functions of single trace operators in a sequence of states of $O(N^0)$ energy of a two-sided holographic conformal field theory admit a large $N$ limit, then they must be described a pure state in the large $N$ Hilbert space of free field theory on two copies of vacuum AdS. This result clarifies recent discussions concerning the possible emergence of a semiclassical baby universe in the large $N$ limit of a low-energy partially entangled thermal state. The proof heavily relies on dominated convergence arguments from mathematical analysis. Some comments are also offered on the relationship between various recently proposed ways of restoring the emergence of the baby universe and the relaxation of different assumptions of the theorem.

We propose a new statistical ensemble of toric bases for elliptic Calabi-Yaus used in F-theory models, by focusing on only the convex hull of the base, i.e., the base polytope. This physically motivated coarse-graining greatly simplifies the combinatorial complexity of the part of the 4D F-theory landscape with toric bases. We develop a Monte Carlo approach that randomly samples the base polytopes within fixed boxes, with proper statistical weights. We first apply the algorithm to the set of 2d base polytopes, generating an enlarged set of toric 2d bases that include certain types of codimension-two (4,6) points, and we validate our approach against exact numbers. We then explore the set of 3d base polytopes which fit in a set of “maximal” 3d boxes, and estimate the total number of inequivalent 3d base polytopes to be $\sim 10^{85}$–$10^{90}$. We provide statistical data such as the distribution of non-Higgsable gauge groups on these bases. Amusingly, a similar method can also be applied to generate reflexive polytopes in various dimensions. In both the reflexive and base polytope cases, the number of relevant polytopes obeys a Gaussian distribution as a function of the number of vertices, which can be understood in terms of other results on random polytopes in the math literature.

Recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) have been interpreted to suggest that dark energy may be evolving. In this work, we examine how prior choices affect such conclusions. Specifically, we study the biases introduced by the customary use of uniform priors on the Chevallier-Polarski-Linder (CPL) parameters, $w_0$ and $w_a$, when assessing evidence for evolving dark energy. To do so, we construct theory-informed priors on $(w_0, w_a)$ using a normalizing flow (NF), trained on two representative quintessence models, which learns the distribution of these parameters conditional on the underlying $Λ$CDM parameters. In the combined $\textit{Planck}$ CMB + DESI BAO analysis we find that the apparent tension with a cosmological constant in the CPL framework can be reduced from $\sim 3.1σ$ to $\sim 1.3σ$ once theory-informed priors are applied, rendering the result effectively consistent with $Λ$CDM. For completeness, we also analyze combinations that include Type Ia supernova data, showing similar shifts toward the $Λ$CDM limit. Taken together, the observed sensitivity to prior choices in these analyses arises because uniform priors – often mischaracterized as “uninformative” – can actually bias inferences toward unphysical parameter regions. Consequently, our results underscore the importance of adopting physically motivated priors to ensure robust cosmological inferences, especially when evaluating new hypotheses with only marginal statistical support. Lastly, our NF-based framework achieves these results by post-processing existing MCMC chains, requiring $\approx 1$ hour of additional CPU compute time on top of the base analysis – a dramatic speedup over direct model sampling that highlights the scalability of this approach for testing diverse theoretical models.

We introduce a novel numerical method to obtain the gluon splitting rates in an anisotropic QCD plasma in the AMY formalism, suitable for an anisotropic collision kernel. The method extends previous works by decomposing the additional angular information into Fourier modes, resulting in a significantly larger system of differential equations to solve numerically. It is then tested by calculating the rates for a simple anisotropic model for the collision kernel, which is compared to a thermal system. Remarkably, the obtained rates can be well-approximated by the rates calculated from an angular-averaged collision kernel, while still deviating significantly from equilibrium. An isotropic model for the collision kernel that is commonly used in QCD kinetic theory simulations, and which relies on the infrared temperature $T_*$ and an effective Debye mass, leads to rates that significantly deviate from the nonequilibrium rate, particularly at smaller parton energies.

We show that recent measurements of substructure-dependent jet suppression constrain the value of the resolution length of the droplets of quark-gluon plasma (QGP) formed in heavy ion collisions. This resolution length, $L_{\rm res}$, is defined such that the medium can only resolve partons within a jet shower that are separated by more than $L_{\rm res}$. We first use Hybrid Model calculations to reproduce ALICE measurements of the scaled Soft Drop angle $θ_g$ for anti-$k_t$$R = 0.2$ jets reconstructed from charged-particle tracks. We find that the narrowing of the $θ_g$-distribution in PbPb collisions compared to pp collisions that is seen in the ALICE data rules out a picture of fully coherent energy loss ($L_{\rm res} = \infty$) where each entire parton shower loses energy to the plasma as if it were a single unresolved colored object. We then use Hybrid Model calculations to reproduce ATLAS measurements of $dR_{12}$, the Soft Drop angle obtained by applying the Soft Drop grooming procedure to all charged-particle tracks in $R=1$ jets reconstructed from $R = 0.2$ skinny subjets. Our analysis demonstrates that the ATLAS measurements of $R_{\rm AA}$ for such $R = 1$ jets as a function of $dR_{12}$ are inconsistent with a picture of fully incoherent energy loss ($L_{\rm res} = 0$) in which every splitting in a parton shower is immediately resolved by the plasma. We find that our Hybrid Model calculations agree best with the ATLAS measurements if QGP has a finite, nonzero, resolution length $L_{\rm res}\sim (1-2)/(πT)$. For the first time, jet substructure measurements are constraining the resolution length of QGP from below, as well as from above.

We examine the status of benchmark simplified dark matter models that have been proposed to explain the GeV gamma-ray Galactic Center excess. We constrain the available parameter space using updated observations from indirect detection, cosmology, direct detection, and accelerators. We show that there is still unconstrained parameter space in both classes of models we consider (a secluded dark sector with a vector portal coupling, and a two-Higgs doublet with a pseudoscalar mediator), and discuss the prospects for potential improvement of these constraints in future experiments.

Quark-gluon plasma (QGP), when viewed at length scales of order the inverse of its temperature, behaves as a strongly-coupled liquid. However, when it is probed with sufficiently high momentum transfer, asymptotic freedom mandates the presence of quark- and gluon-like quasi-particles. High energy partons within jets can trigger these high-momentum exchanges, making jets valuable probes for revealing the presence of such quasi-particles. Such elastic scatterings are implemented in the Hybrid Model, where a jet parton that scatters is deflected, kicking a medium parton, which recoils. Before and after a scattering, as one and then both partons propagate through the medium, they lose energy and momentum, exciting wakes in the QGP droplet. We use two-point and three-point energy-energy correlators (EECs) to reveal the relevant angular regions at which (modified) parton showers and wakes in the QGP each dominate, offering a new way with which to visualize and constrain the corresponding dynamics. We compare our calculations to recent CMS and ALICE measurements of two-point EECs of charged-particle tracks in jets produced in PbPb collisions. We show that our calculations are closest to the experimental measurements when elastic scattering is included and when the elastically scattered recoil-partons produce their own wakes. We also propose a new variant of the measurement that is especially sensitive to jet wakes.

We propose a Symmetry Topological Field Theory (SymTFT) for continuous spacetime symmetries. For a $d$-dimensional theory, it is given by a $(d+1)$-dimensional BF-theory for the spacetime symmetry group, and whenever $d$ is even, it can also include Chern-Simons couplings that encode conformal and gravitational anomalies. We study the boundary conditions for this SymTFT and describe the general setup to study symmetry breaking of spacetime symmetries. We then specialize to the conformal symmetry case and derive the dilaton action for conformal symmetry breaking. To further substantiate that our setup captures spacetime symmetries, we demonstrate that the topological defects of the SymTFT realize the associated spacetime symmetry transformations. Finally, we study the relation to gravity and holography. The proposal classically coincides with two-dimensional Jackiw-Teitelboim gravity for $d=1$ as well as the topological limit of four-dimensional gravity in the $d=3$ case.

The pion light-cone distribution amplitude (LCDA) is an essential non-perturbative input for a range of high-energy exclusive processes in quantum chromodynamics. Building on our previous work, the continuum limit of the fourth Mellin moment of the pion LCDA is determined in quenched QCD using quark masses which correspond to a pion mass of $m_π= 550$ MeV. This calculation finds $\langleξ^2\rangle = 0.202(8)(9)$ and $\langle ξ^4 \rangle = 0.039(28)(11)$ where the first error indicates the combined statistical and systematic uncertainty from the analysis and the second indicates the uncertainty from working with Wilson coefficients computed to next-to-leading order. These results are presented in the $\overline{\text{MS}}$ scheme at a renormalization scale of $μ= 2$ GeV.

Measuring the energy loss of energetic jet partons may provide experimental opportunities to constrain the initial nonequilibrium stages in heavy-ion collisions, which requires theoretical predictions for the jet-medium potential. In this letter, we go beyond the usual harmonic approximation, extract this potential for the first time using QCD kinetic theory simulations, and provide a simple cutoff-independent small-distance form. We find significant differences to thermal media at early times in momentum and position space, as well as substantial angular dependence. Applying our results to the gluon splitting rates using a novel computational method reveals tensions in the current kinetic theory implementations for early times.

Quantum systems of physical interest are often local, but there are at least three competing perspectives on how “locality” should be formalized: an algebraic framework, a path-integral framework, and a lattice framework. One puzzle in this competition is that systems with higher-form symmetries, which are perfectly local from the path-integral and lattice perspectives, can violate the algebraic principle of “additivity”. In this paper, we propose a resolution to this puzzle by introducing a weaker locality principle, “disjoint additivity”, which together with Haag duality should always be satisfied in local quantum systems. As evidence, we give examples in which disjoint additivity is preserved when ordinary additivity is violated; we show that Haag duality and disjoint additivity are satisfied in rather general lattice systems with local symmetry constraints; we give examples of nonlocal theories in which either disjoint additivity or Haag duality is violated; and finally we give examples of systems with nonlocal symmetry constraints in which disjoint additivity is violated, but can be restored by passing to a local “SymTFT” system in one higher dimension.

We present simulations of the real time dynamics of a fluid in the vicinity of a critical endpoint in the phase diagram. The relevant hydrodynamic theory is known as model H, and it is expected to describe the long-distance dynamics of QCD matter in the vicinity of a possible critical endpoint in the QCD phase diagram.

The spectrum of coherent gluon radiation from a quark-anti-quark pair experiencing multiple scatterings within a coloured medium is central for understanding in-medium parton cascades. Despite its foundational importance, current results are limited by reliance on simplified scattering rates, such as the harmonic oscillator approximation, valid only in restricted phase-space regions. Using the formalism introduced in a previous article, we express the gluon emission spectrum as a set of differential equations that can be solved numerically, circumventing conventional approximations. We present the transverse momentum and energy distributions of emitted gluons for realistic interaction models, illustrating the breakdown of colour coherence across the entire accessible phase-space, and consequently enabling a higher-precision description of jet observables.

This work reports the first calculation of the gravitational form factors (GFFs) of the scalar glueball, performed via lattice field theory in Yang-Mills theory at a single lattice spacing. The glueball GFFs are compared with those of other hadrons as determined in previous lattice calculations, providing strong indications that glueballs have a different gluonic structure than typical hadronic states. A mass radius of 0.263(31) fm is predicted, supporting previous suggestions that the scalar glueball is significantly smaller than other hadrons. These results point towards a potential smoking-gun characteristic to target by experimental glueball searches.

We summarize and update using new NICER measurements the results of arXiv:2401.16253, in which we used various astrophysical neutron-star observations to set an upper bound on the CFL color-superconducting gap in a range of baryon chemical potentials $μ_B \in [2.1,3.2]$, above those reached within neutron stars. We also corroborate the “reasonable” constraint from arXiv:2401.16253 on the maximum value of the color-superconducting gap by performing a new Bayesian analysis using a prior that extends a two-segment Gaussian process connecting the whole density range between CEFT and pQCD.

Two-point correlation functions of systems with baryon number $B \in \{1,2,3,4\}$ are investigated using lattice Quantum Chromodynamics (QCD). In particular, the empirical distributions of importance-sampling Monte-Carlo samples of these correlation functions are examined as a function of the spacetime separation between the two points and the baryon number. While the exact forms of these distributions are not known for QCD, recent work has determined asymptotic expressions for analogous correlation function distributions in simpler theories such as scalar field theory and the disordered phase of the $O(N)$ model. The theoretical O(N) model distributions are found to provide an accurate description of the empirical QCD distributions at zero momentum over a wide range of temporal separations for each baryon number when assessed with a range of different statistical tests. In particular, the behaviour of the baryon number $B$ QCD correlation function at large temporal separation is well-reproduced by the $O(N \sim 2/B)$ model distribution.