MIT Center for Theoretical Physics — a Leinweber Institute Publications

Despite its successes, the large-$N$ holographic dictionary remains incomplete. Key features of gravitational path integrals–most notably Euclidean wormholes and the associated failure of factorization–lack a clear interpretation in the standard large-$N$ framework. A related challenge is the possibility of erratic $N$-dependence in CFT observables, behavior with no evident semiclassical gravitational counterpart. We argue that these puzzles point to a missing ingredient in the dictionary: a large-$N$ filter. This filter projects out the erratic $N$-dependence of CFT quantities when mapping them to semiclassical bulk physics, providing an intrinsic boundary definition of gravitational “averages.” It also offers a boundary explanation of wormhole contributions and a boundary prediction of their amplitudes, thereby giving a natural resolution of the factorization puzzle. In addition, we derive an infinite tower of inequalities constraining wormhole amplitudes and argue that internal wormholes do not induce random couplings in the low-energy effective theory. Beyond resolving factorization, the large-$N$ filter supplies a generalized framework from which richer Lorentzian spacetime structures can emerge, including closed universes and black hole interiors. We argue that, as a consequence of erratic large-$N$ behavior, both closed universes and black hole interiors are quantum volatile, and that an AdS spacetime entangled with a baby universe is likewise quantum volatile. This volatility may allow an observer in AdS to infer the existence of the baby universe, whereas for an infalling observer, the ability to make measurements near a black hole horizon may become fundamentally limited–even if they may not live long enough to notice.

The Lyman-alpha (Lya) forest is a unique probe of cosmology and the intergalactic medium at high redshift and small scales. The statistical power of the ongoing Dark Energy Spectroscopic Instrument (DESI) demands precise theoretical tools to model the Lya forest. We present a hybrid effective field theory (HEFT) forward model in redshift space that leverages the accuracy of non-linear particle displacements computed using the N-body simulation suite AbacusSummit with the predictive power of an analytical, perturbative bias forward model in the framework of the effective field theory (EFT). The residual noise between the model and the simulated Lya field has a nearly white (scale-and orientation-independent) power spectrum on quasi-linear scales, substantially simplifying its modeling compared to a purely perturbative description. As a consequence of the improved control over the 3D Lya forest stochasticity, we find agreement between the modeled and the true power spectra at the 5 per cent level down to scales of k <= 1 h/Mpc. This procedure offers a promising path toward constructing efficient and accurate emulators to predict large-scale clustering summary statistics for full-shape cosmological analyses of Lya forest data from both DESI and its successor, DESI-II.

The optical dump at the LUXE experiment has the potential to create a large flux of $\mathcal{O}({\rm GeV})$ photons that can be used to look for new physics when directed at a solid material dump. The LUXE-NPOD extension of LUXE, which focuses the hard photons onto a slab of tungsten, offers two interaction points (laser-electron and photon-tungsten), making it well-suited to test theories containing two or more new particles. We examine the dark axion portal, a scenario involving both dark photons (DPs) and axion-like particles (ALPs) and their mutual interactions, and its implications on the phenomenology at LUXE-NPOD. To simulate the spectra of particle populations generated at the electron-laser interaction point, we solve a set of extended 1D cascade equations. We recover a photon spectrum consistent with previous analyses and present previously unconsidered DP and ALP spectra. We derive the overall sensitivity of LUXE-NPOD to various parameters of the new particles, and show that it is capable of probing previously uncharted regions in the dark axion parameter space. For ALPs in the $10$-$500$ MeV mass range and DPs either heavier ($\sim{\rm GeV}$) or significantly lighter $\mathcal{O}(10^{-18}$-$10^{-13})$ eV, we obtain novel constraints on DP kinetic mixing parameters smaller than $10^{-2}$ and on $\mathcal{O}(10^{-4}$-$10^{-3})$ GeV$^{-1}$ ALP-photon couplings. We find that restrictions on $\mathcal{O}\left(10^{-3}\right)$ kinetic mixing can be extracted for arbitrarily small DP masses. Our discussion aims to be systematic and demonstrates a practical method of analyzing constraints on multi-dimensional parameter spaces.

We consider the propagation of heavy quarks through a dense, hydrodynamically flowing QCD medium, representative of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions. Working in the high-energy limit, we identify two novel mass-dependent effects arising from the heavy quark coupling to the local medium flow. The first is the emergence of a tensorial jet transport coefficient, $\hat{q}_{ij}$, which encodes the directional structure of transverse-momentum broadening. The second, named the directional dead-cone effect, corresponds to an anisotropic suppression of medium-induced radiation aligned with the hydrodynamic flow. We discuss how these effects manifest in jet observables and identify distinctive signature of heavy quark dynamics in an evolving medium.

Motivated by the requirement of the LSZ reduction formula to remove self-energy contributions on external legs, we examine quark self-energy contributions in soft-collinear effective (SCET) theory. We examine an operator basis that follows directly from full quantum chromodynamics (QCD) (upon application of the SCET equations of motion to express small Dirac components in terms of large Dirac components). We find that, for this basis, the self-energy contributions can be identified from their diagrammatic topologies, as in full QCD. However, for an alternative operator basis that is obtained from the direct-QCD basis by an application of Wilson-line identities, interactions are shifted from a covariant derivative to a Wilson line. Consequently, some self-energy contributions are hidden in diagrams involving Wilson lines, making their identification subtle.

We use a combination of perturbation theory, holography, supersymmetric localization, integrability, and numerical conformal bootstrap methods to constrain the energy-energy correlator in $\text{SU}(N_c)$${\mathcal N}=4$ SYM at finite coupling. For finite $N_c$, we derive lower bounds on the second and fourth multipoles of the energy-energy correlator at different couplings, along with a smeared energy-energy correlator as a function of the angle between the two detectors. We present evidence that our lower bounds on the multipoles are nearly saturated by the ${\cal N} = 4$ SYM theory. In the planar limit, we further use dispersive functionals to obtain tight two-sided bounds on both the first three non-trivial multipoles and on the angular dependence of the energy-energy correlator. As the coupling is varied from weak to strong, the energy-energy correlator exhibits a transition from single-trace to double-trace operator dominance in the collinear limit, which we characterize quantitatively. A similar phenomenon occurs in QCD, where a parton-hadron transition is observed as detectors are brought closer together.

Jet substructure observables serve as essential tools for probing the quark-gluon plasma produced in relativistic heavy-ion collisions. Their interpretation, however, is often complicated by edge effects, which arise when correlated particles fall outside the reconstructed jet radius, introducing distortions that obscure the underlying QCD dynamics. In this work, we present a comprehensive phenomenological study of edge effects in soft-insensitive angular observables, taking the two-point energy correlator (EEC) as a representative example. We argue that these distortions scale linearly with the average angular separation between the winner-take-all and $E$-scheme axes $\langle ϕ\rangle$, and validate this behavior across proton-proton (p-p) simulations with Pythia8 and Herwig7, as well as lead-lead (Pb-Pb) simulations using JEWEL and CoLBT. In p-p collisions, edge effects are strongly suppressed, scaling as $(R_L/R)^4$, whereas medium-modified jets can exhibit larger distortions, with contributions scaling as $(R_L/R)^2$ and $(R_L/R)^4$. Taking Pb-Pb/p-p ratios of the EEC substantially reduces, but does not completely eliminate, these distortions, highlighting the need of accounting for edge effects in the interpretation of heavy-ion jet substructure measurements. Since edge effects are largely governed by the $\langle ϕ\rangle$ distribution, studying this distribution provides a new handle for benchmarking and constraining the modeling of edge effects in heavy-ion event generators.

We consider planar $\mathcal{N}=4$ super Yang-Mills at finite temperature with chemical potentials that couple either to the $R$-charges or the spins of the operators. We find expressions for the Hagedorn temperatures at both zero coupling by explicitly counting states, and at strong coupling using the string theory dual. We then apply the quantum spectral curve (QSC) to this problem, which adds additional twists to the $Q$-functions. For a single chemical potential $μ$ coupled to one of the $R$-charges, we find the analytic weak-coupling Hagedorn temperature to one-loop order for any value of $μ$, and to two-loop order for $μ=1/2$. We then solve the QSC numerically, showing that at strong coupling there is good agreement with the string theory prediction to order $1/λ^{1/4}$. This provides further evidence for a recent conjecture of Harmark for the form of the world-sheet zero-point shift. We also use the QSC to find the analytic one-loop correction to the Hagedorn temperature with non-zero chemical potentials coupled to the spins.

Large superconducting quantum circuits have a number of important applications in quantum computing. Accurately predicting the performance of these devices from first principles is challenging, as it requires solving the many-body Schrödinger equation. This work introduces a new, general ab-initio method for analyzing large quantum circuits based on lattice field theory, a tool commonly applied in nuclear and particle physics. This method is competitive with state-of-the-art techniques such as tensor networks, but avoids introducing systematic errors due to truncation of the infinite-dimensional Hilbert space associated with superconducting phases. The approach is applied to fluxonium, a specific many-component superconducting qubit with favorable qualities for quantum computation. A systematic study of the influence of impedance on fluxonium is conducted that parallels previous experimental studies, and ground capacitance effects are explored. The qubit frequency and charge noise dephasing rate are extracted from statistical analyses of charge noise, where thousands of instantiations of charge disorder in the Josephson junction array of a fixed fluxonium qubit are explicitly averaged over at the microscopic level. This is difficult to achieve with any other existing method.

Supersymmetric localization and Ward identities have been used in the past several years to derive two integral constraints on the four-point function of the stress-tensor multiplet in $\mathcal{N} = 4$ super-Yang-Mills theory. These constraints are powerful tools for studying the theory, especially when used in tandem with analytic and/or numerical bootstrap techniques. In this paper, we consider three additional integral constraints that can be derived starting from the $\mathcal{N} = 4$ super-Yang-Mills theory placed on a squashed four-sphere. These constraints are technically much more challenging to derive than the ones in the literature, and much of the paper is devoted to developing techniques to make this computation tractable. Our end result is that these three constraints are implied by the two constraints already appearing in the literature.

Models of cosmic inflation generically predict a weak but potentially detectable amount of primordial non-Gaussianity (PNG), which can be used to obtain insights into the degrees of freedom during inflation and their interactions. The simplest types of PNG are the local and non-local (equilateral and orthogonal) shapes of the primordial three-point correlators, which are predicted by models with multiple light fields and derivative interactions in single-field inflation, respectively. In this paper we place constraints on local, equilateral, and orthogonal non-Gaussianities using the power spectrum and bispectrum extracted from first public release of the Dark Energy Spectroscopic Instrument (DESI). Our analysis makes use of higher-order clustering information through a consistent effective field theory (EFT) model for both the power spectrum and bispectrum at one-loop order. Using robust scale cuts where the EFT description is valid, we find the following constraints on PNG amplitudes: $f^{\rm loc}_{\rm NL}=-0.1\pm 7.4$, $f^{\rm equil}_{\rm NL}=719\pm 390$, $f^{\rm orth}_{\rm NL}=-200\pm 100$ (at $68\%$ CL). Non-local PNG constraints can be further improved by combining high-redshift DESI with legacy BOSS data and using simulation-based priors on bias parameters, yielding the strongest large-scale structure constraints to date $f^{\rm equil}_{\rm NL}=200\pm 230$, $f^{\rm orth}_{\rm NL}=-24\pm 86$. Our constraint on $f^{\rm loc}_{\rm NL}$ is competitive with the cosmic microwave background (CMB) limit; the combination gives $f^{\rm loc}_{\rm NL}=-0.0\pm 4.1$, $18\%$ stronger than the CMB only result, which represents the strongest bound on multi-field inflation yet obtained.

How is the production of primordial black holes (PBHs) in single-field models of inflation impacted by the presence of additional scalar fields? We consider the effect of a spectator field – a free scalar field with sub-Hubble mass, no direct coupling to the inflaton, and which makes a subdominant contribution to the total energy density – in the context of single-field models of inflation featuring a transient phase of ultra-slow roll (USR) evolution. Despite the modest title, a spectator field can have a dramatic impact: the slow-roll evolution of the spectator prevents the combined inflaton-and-spectator system from entering into USR, which naively might be expected to preclude the production of PBHs. However, we demonstrate that the growth of perturbations is maintained or enhanced by the spectator, through the rich interplay of curvature and isocurvature perturbations. We show in a model-independent way that the single-field phase of ultra-slow-roll is replaced by two turns in field space encompassing a phase of tachyonic instability for the isocurvature perturbations and a transfer of power from isocurvature to curvature modes. Furthermore, we highlight a degeneracy between the fine-tuning of the feature in the inflaton potential and the parameters of the spectator, leading to an overall resilience of model predictions to parameter variations. This makes it easier for the underlying PBH model to accommodate both high-precision CMB constraints and production of PBHs in the asteroid-mass range.

This work proposes a direct link between the hierarchy problem and Weakly Interacting Massive Particles (WIMPs): we suggest that the small mass of the Higgs boson arises from being dynamically driven to the scale of the WIMP. Such a special electroweak vacuum is singled out by lying close to the critical boundary of a phase transition, as recently explored in a new class of cosmological solutions to the hierarchy problem. They generically predict the Higgs potential to be destabilised just above the weak scale. Intriguingly, the requirement for new physics to achieve this coincides with two independently well-motivated expectations: a split spectrum of light fermions and heavy bosons, as anticipated from naturalness, and the so-called “WIMP miracle”. A WIMP with mass around the weak scale not only happens to have the correct thermal relic abundance to be the dark matter (DM), it can also give rise to the necessary critical boundary at the TeV scale through its Yukawa couplings to the Higgs. We use a higgsino-like singlet-doublet model to illustrate our Higgs-DM criticality scenario and show that if this WIMP DM mass is observed to be greater than ~1.2 TeV then it necessarily implies a strong bound on the Higgs mass and an upper bound on the scale of heavy new physics that restores vacuum stability. It can be thoroughly probed in direct detection experiments, astrophysical signals and future collider searches, further motivating a comprehensive exploration of the remaining heavy WIMP parameter space.

We identify a symmetry that enforces every symmetric model to have a Fermi surface. These symmetry-enforced Fermi surfaces are realizations of a powerful form of symmetry-enforced gaplessness. The symmetry we construct exists in quantum lattice fermion models on a $d$-dimensional Bravais lattice, and is generated by the onsite U(1) fermion number symmetry and non-onsite Majorana translation symmetry. The resulting symmetry group is a non-compact Lie group closely related to the Onsager algebra. For a symmetry-enforced Fermi surface $\cal{F}$, we show that this UV symmetry group always includes the subgroup of the ersatz Fermi liquid L$_{\cal{F}}$U(1) symmetry group formed by even functions ${f(\mathbf{k})\in\mathrm{U}(1)}$ with ${\mathbf{k}\in \cal{F}}$. Furthermore, we comment on the topology of these symmetry-enforced Fermi surfaces, proving they generically exhibit at least two non-contractible components (i.e., open orbits).

When the gauge group of a theory has infinite volume, defining the inner product on physical states becomes subtle. This is the case for gravity, even in exactly solvable models such as minisuperspace or low-dimensional theories: the physical states do not inherit an inner product in a straightforward manner, and different quantization procedures yield a priori inequivalent prescriptions. This is one of the main challenges when constructing gravitational Hilbert spaces. In this paper we study a quantization procedure known as group averaging, which is a special case of the BRST/BV formalism and has gained popularity as a promising connection between Dirac quantization and gravitational path integrals. We identify a large class of theories for which group averaging is ill-defined due to isometry groups with infinite volume, which includes Jackiw-Teitelboim gravity. We propose a modification of group averaging to renormalize these infinite volumes and use it to quantize Jackiw-Teitelboim gravity with a positive cosmological constant in closed universes. The resulting Hilbert space naturally splits into infinite-dimensional superselection sectors and has a positive-definite inner product. This is the first complete Dirac quantization of this theory, as we are able to capture all the physical states for the first time.

Recent results from DESI BAO analyses suggest that dark energy may not be a cosmological constant and is in fact dynamical. Furthermore, the data suggest that the equation of state may have been in the phantom regime in the distant past, recently undergoing a phantom crossing. In this work, we investigate whether this preference can be realized within a kinetically mixed axion-dilaton (KMIX) quintessence model, a string-motivated system in which an axion-like field couples exponentially to a dilaton-like (moduli) field. Crucially, KMIX can appear phantom in a standard Chevallier-Polarski-Linder (CPL) based analysis. To confront the model with data, we develop a fast pipeline based on normalizing flows that (i) learns a theory-informed prior on $(w_0,w_a)$ from KMIX realizations and (ii) provides an inverse mapping from CPL parameters back to the physical KMIX parameters. By importance-sampling pre-computed CPL chains using this framework, we effectively transform generic phenomenological constraints into direct, computationally efficient constraints on the underlying KMIX theory, avoiding the prohibitive cost of full parameter space exploration. Applied to Planck+DESI DR2 BAO measurements, our framework finds support for KMIX at $2.5σ$ compared to the base CPL fit at $3.1σ$, demonstrating that KMIX may account for the DESI preference without invoking true phantom behavior. When additionally including Type Ia supernovae data, we find that the preference remains above $3σ$ for Union3 and DES Y5, but drops to $2.1σ$ with Pantheon+. The latter, combined with the DESI full-shape power spectrum and bispectrum data, further reduces the preference to $1.7σ$. Ultimately, should the DESI deviation persist with future data, KMIX may offer a theoretically well-motivated explanation for the phantom-like signatures inferred from phenomenological fits.

We carry out an independent re-analysis of the Dark Energy Spectroscopic Instrument (DESI) public dataset, focusing on extensions to the standard cosmological model, $Λ$CDM. Utilizing the dataset and Effective Field Theory (EFT)-based pipeline described in Paper 1, we constrain cosmological models with massive neutrinos ($Λ$CDM+$M_ν$), spatial curvature ($oΛ$CDM), dynamical dark energy ($w_0w_a$CDM), and their combinations using the power spectrum and bispectrum of DESI galaxies and quasars. Our work also presents the first measurements of relevant non-minimal cosmological parameters from the combination of cosmic microwave background (CMB) and DESI full-shape (FS) data, which are made possible thanks to carefully chosen priors on EFT parameters. We find that the addition the FS likelihood to DESI’s baryon acoustic oscillation (BAO) data improves the limits on the spatial curvature by a factor of two over the BAO only results, though the improvements are less significant with the CMB data. The dark energy equation of state figure-of-merit increases both with and without the supernovae data (SNe), by $\approx30\%$ and $\approx20\%$ relative to the CMB+BAO and CMB+BAO+SNe results, respectively. Our FS likelihood also yields the strongest CMB-independent constraint on the total neutrino mass $M_ν<0.32\,{\rm eV}$, with the $30\%$ improvement due to the bispectrum. In combination with the CMB, we find a $14\%$ improvement assuming the $Λ$CDM+$M_ν$ model (yielding $M_ν<0.059\,{\rm eV}$), but this increases to $22\%$ when using non-minimal backgrounds: $M_ν<0.097\,{\rm eV}$ in $oΛ$CDM+$M_ν$ and $M_ν<0.13\,{\rm eV}$ in $w_0w_a$CDM+$M_ν$. Overall, our work illustrates that robust and substantial gains in constraining power can be obtained by incorporating the FS power spectrum and bispectrum measurements in analyses of non-minimal cosmological models.

We introduce a holographic framework for analyzing the steady states of repeated quantum channels with strong symmetries. Using channel-state duality, we show that the steady state of a $d$-dimensional quantum channel is holographically mapped to the boundary reduced density matrix of a $(d+1)$-dimensional wavefunction generated by a sequential unitary circuit. From this perspective, strong-to-weak spontaneous symmetry breaking (SWSSB) in the steady state arises from the anyon condensation on the boundary of a topological order in one higher dimension. The conditional mutual information (CMI) associated with SWSSB is then inherited from the bulk topological entanglement entropy. We make this duality explicit using isometric tensor network states (isoTNS) by identifying the channel’s time evolution with the transfer matrix of a higher-dimensional isoTNS. Built on isoTNS, we further construct continuously tunable quantum channels that exhibit distinct mixed-state phases and transitions in the steady states.

The excellent sensitivities of quantum sensors are a double-edged sword: minuscule quantities can be observed, but any undesired signal acts as noise. This is challenging when detecting quantities that are obscured by such noise. Decoupling sequences improve coherence times and hence sensitivities, though only AC signals in narrow frequency bands are distinguishable. Alternatively, comagnetometers operate gaseous spin mixtures at high temperatures in the self-compensating regime to counteract slowly varying noise. These are applied with great success in various exotic spin-interaction searches. Here, we propose a method that decouples specific DC fields from DC and AC magnetic noise. It requires any spin cluster where the effect on each individual spin is different for the target field and local magnetic fields, which allows for a different approach compared to comagnetometers. The presented method has several key advantages, including an orders-of-magnitude increase in noise frequencies to which we are resistant. We explore electron-spin nuclear-spin pairs in nitrogen-vacancy centres in diamond, with a focus on their merit for light dark-matter searches. Other applications include gradient sensing, quantum memory, and gyroscopes.

Primordial non-Gaussianity (PNG) is a common prediction of a wide class of inflationary models. Equilateral-type PNG, generically predicted by single-field inflationary models with higher-derivative interactions, imprints subtle but measurable signatures on the large-scale distribution of matter. An important parameter of these imprints is the PNG-induced bias coefficient $b_ψ$, which quantifies how the abundance and clustering of dark matter halos and galaxies respond to mode coupling in the initial conditions. Measuring $b_ψ$ is important for constraining equilateral PNG, yet it is notoriously challenging due to its degeneracy with Gaussian scale-dependent bias contributions. In this work, we present the first precision measurements of equilateral $b_ψ$ for dark matter halos using effective field theory at the field level. We show that this approach disentangles PNG effects from those of the Gaussian bias by virtue of noise variance cancellation. We compare our results with the phenomenological predictions based on the Peak-Background Split model, finding some agreement at the qualitative level on the redshift and mass dependence, but poor agreement at the quantitative level. We present a fitting formula for $b_ψ$ as a function of the linear bias, which can be used to set priors in PNG searches with ongoing and future galaxy surveys.