MIT Center for Theoretical Physics – a Leinweber Institute

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.