MIT Center for Theoretical Physics – a Leinweber Institute

Recent JWST observations of "Little Red Dots" (LRDs) at $z gtrsim 4$ suggest the existence of early supermassive black holes (SMBHs) with masses $gtrsim 10^6,M_odot$. Their rapid emergence challenges conventional seeding and growth scenarios based on accretion and mergers of Population III stellar remnants, motivating renewed interest in primordial black holes (PBHs) formed from primordial perturbations. Explaining such massive seeds requires power spectrum enhancements that are constrained by cosmic microwave background (CMB) spectral distortions. In this work we perform the first comprehensive Bayesian exploration of non-Gaussian primordial perturbations capable of producing SMBH seeds while remaining consistent with CMB spectral distortion constraints, and examine the role of accretion history in constraining these models. We perform a suite of Markov chain Monte Carlo analyses of power spectrum enhancements (lognormal and cut-power-law) with varying local non-Gaussianity and accretion histories, satisfying the COBE/FIRAS $μ$-distortion constraint with and without the requirement that the model produces SMBH seeds. We find that both models can seed LRDs and evade $μ$-distortion bounds for $|f_{rm NL}|= O(1)$ and peak amplitude $A= O(10^{-3})$, when standard Eddington-limited accretion is also incorporated. Interestingly, the results exhibit a preference for both positive and negative values of $f_{rm NL}$. The preference for negative $f_{rm NL}$ is strongly dependent on the accretion history, and is removed if the black holes have a small duty cycle, $f_{rm duty}lesssim 0.1$. These results demonstrate that the JWST high-$z$ LRD population can be seeded by primordial perturbations while remaining compatible with CMB data, and highlight the importance of next-generation CMB spectral distortion missions to test this hypothesis.

We compute the pion quark momentum fraction in the large-$N_c$ limit using the Twisted Eguchi-Kawai (TEK) reduced model. Gauge configurations for different values of the lattice spacing are generated with the pure-gauge TEK action, while the pion correlators are computed for different quark masses with maximally twisted mass fermions. The bare matrix element corresponding to the quark momentum fraction is obtained using the Feynman-Hellmann method, and we use perturbative renormalization factors to connect to the $overline{mathrm{MS}}$ scheme at $2,mathrm{GeV}$. A global analysis at several lattice spacings, pion masses, and values of $N_c$ yields $langle xrangle_q^{overline{mathrm{MS}}}(2,mathrm{GeV}) = 0.267(105)$ at the physical pion mass in the large-$N_c$ limit. This value is significantly smaller than existing $N_c=3$ determinations from phenomenology and lattice QCD, indicating that, in the large-$N_c$ limit, the pion's light-cone momentum is carried predominantly by gluons.

Machine learning (ML) methods have been explored as a means of optimizing lattice field theory calculations, with increasing interest in recent years. This chapter gives an introduction and summary to the use of ML methods to optimize lattice field theory without introducing any uncontrolled approximations. We focus on those directions which are clearly established as of late 2025, marking the 50th anniversary of the beginning of the study of lattice gauge theory. This chapter is a contribution to the online book ``Lattice QCD at 50 years,'' (LQCD@50), edited by Tanmoy Bhattacharya, Maarten Golterman, Rajan Gupta, Laurent Lellouch, and Steve Sharpe.

The geometric organization and statistical properties of center-vortex networks provide a window into the infrared dynamics underlying confinement in Yang-Mills theory. Motivated by this connection, we investigate new consequences of a previously proposed confinement mechanism based on physically equivalent elementary Cartan fluxes. We show that, in the percolating phase, these fluxes dynamically reorganize into center-charge sectors whose dominant vortex tensions satisfy a Casimir scaling law, similar to that observed for confining string tensions. We then confront this result with the first nontrivial Yang-Mills case: the doubly charged vortex chains recently observed in SU(4) lattice simulations. To extract the parameters needed to connect the lattice ensemble with the effective description, we characterize how center-vortex chains grow and bend in SU(3) and SU(4) using polymer-inspired techniques not previously applied to center vortices. We find good quantitative agreement for the SU(4) doubly charged sector, providing further evidence for the proposed mechanism.