MIT Center for Theoretical Physics — a Leinweber Institute Publications

The ATLAS Collaboration has developed a method to analyze large-radius jets composed of skinny $R=0.2$ subjets in heavy-ion collisions. We first demonstrate that the measurements pioneered by ATLAS constrain the value of $L_{res}$, the resolution length of QGP — and rule out any picture in which an entire parton shower loses energy coherently as a single entity. We then analyze the response of the medium to the passage of large-radius $R=2$ jets containing two skinny subjets in gamma-jet events. We introduce novel jet-shape observables that allow us to visualize how the internal structure of large-radius jets shapes the wakes they excite in the QGP. We find that even when the subjets are $\sim 0.8$ radians apart, the angular shape of the soft hadrons originating from their wake forms a single broad structure. Only when the two subjets are even farther apart are two sub-wakes revealed. We show that the way in which jet structure shapes the structure of jet-induced wakes can be visualized with similar clarity in experiments by using only low-$p_T$ hadrons. The observables we introduce offer a new and distinctive way of seeing jet-induced wakes — and wake substructure — in heavy-ion collision data.

The study of hadronic jets and their substructure at hadronic colliders is crucial for improving our understanding of QCD, and searching for new physics. As such, there has been a significant effort to improve their theoretical description. In the small radius limit, inclusive jet production exhibits a universal factorization, enabling the resummation of logarithms which greatly stabilizes theoretical predictions. In this paper, we show how to combine a recently introduced framework for small-$R$ resummation with the STRIPPER subtraction formalism for fragmentation, enabling next-to-next-to-leading order calculations of small-$R$ inclusive jet production for a wide variety of processes at the LHC. We extract the two-loop constants for the jet functions, enabling for the first time next-to-next-to-leading logarithmic resummation matched to next-to-next-to-leading order perturbative calculation. We compare with CMS data for small-$R$ jet production, and find that our results greatly improve the accuracy of the predictions at small-$R$, and stabilize the perturbative convergence and error estimates at larger $R$. Our approach is applicable to a wide class of jet substructure observables exhibiting similar factorization theorems, opening the door to an NNLO jet substructure program at the LHC.

The algebraic approach to quantum field theory focuses on the properties of local algebras, whereas the study of (possibly non-invertible) global symmetries emphasizes global aspects of the theory and spacetime. We study connections between these two perspectives by examining how either of two core algebraic properties — “additivity” or “Haag duality” — is violated in a 1+1D CFT or lattice model restricted to the symmetric sector of a general global symmetry. For the Verlinde symmetry of a bosonic diagonal RCFT, we find that additivity is violated whenever the symmetry algebra contains an invertible element, while Haag duality is violated whenever it contains a non-invertible element. We find similar phenomena for the Kramers-Wannier and Rep(D${}_8$) non-invertible symmetries on spin chains.

Recent work has shown that the (block) Lanczos algorithm can be used to extract approximate energy spectra and matrix elements from (matrices of) correlation functions in quantum field theory, and identified exact coincidences between Lanczos analysis methods and others. In this work, we note another coincidence: the Lanczos algorithm is equivalent to the well-known Rayleigh-Ritz method applied to Krylov subspaces. Rayleigh-Ritz provides optimal eigenvalue approximations within subspaces; we find that spurious-state filtering allows these optimality guarantees to be retained in the presence of statistical noise. We explore the relation between Lanczos and Prony’s method, their block generalizations, generalized pencil of functions (GPOF), and methods based on the generalized eigenvalue problem (GEVP), and find they all fall into a larger “Prony-Ritz equivalence class”, identified as all methods which solve a finite-dimensional spectrum exactly given sufficient correlation function (matrix) data. This equivalence allows simpler and more numerically stable implementations of (block) Lanczos analyses.

We present the first search for evidence of neutrino self-interaction with two new, state-of-the-art likelihoods for eBOSS Lyman-$\alpha$ data. These are an effective field theory (EFT) likelihood with priors from the Sherwood simulation suite, and a compressed likelihood derived from an emulator built using the PRIYA simulation suite. Previous analyses that combined Planck measurements with eBOSS Lyman-$\alpha$ likelihoods based on earlier simulations found a preference for neutrino self-interactions. In contrast, using either of the new eBOSS Lyman-$\alpha$ likelihoods, we find that a joint analysis with the cosmic microwave background (CMB) data from Planck prefers a negligible level of neutrino self-interaction, and derive new constraints on the neutrino self-coupling: ${log}_{10}\left(G_{eff}{MeV}^2\right)=-{5.57}_{-0.58}^{+0.98}$ for Planck + EFT Lyman-$\alpha$, and ${log}_{10}\left(G_{eff}{MeV}^2\right)=-{5.26}_{-1.49}^{+0.87}$ for Planck + PRIYA Lyman-$\alpha$, at 68% confidence. We also consider Planck in combination with DESI BAO data, and find that the latter does not provide significant constraining power for neutrino self-interactions.

We study the scaling of meson-meson scattering amplitudes with the number of colors, $N_{\text{c}}$. We use lattice calculations in a theory with $N_{\text{f}}=4$ degenerate flavors, with $N_{\text{c}}=3-6$ and pion mass $M_{\pi }\approx 560$ MeV. We focus on three different scattering channels, two of which have the same quantum numbers as some tetraquark candidates recently found at LHCb: the $T_{cs0}^0\left(2900\right)$, $T_{c\overline{s}0}^{++}\left(2900\right)$, $T_{c\overline{s}0}^0\left(2900\right)$ and $T_{c\overline{s}1}^0\left(2900\right)$ states. Finite-volume energies are extracted using a large set of operators, containing two-particle operators with the form of two pions or two vector mesons, and local tetraquark operators. The resulting energy spectra is used to constrain the infinite-volume scattering amplitude by means of Lüscher’s quantization condition. We consider polynomial parametrizations of the phase shift, as well as one-loop chiral perturbation theory (ChPT) predictions. We find that our lattice results follow the expected $N_{\text{c}}$ scaling and are sensitive to subleading $N_{\text{c}}$ corrections. In addition, we constrain the scaling of different combinations of low-energy constants from matching to large $N_{\text{c}}$ ChPT. The results for the channel corresponding to a $({\pi }^{+}{D}_s^{+}-K^{+}{D}^{+})$ state show evidence of a virtual bound state with energy $E_{\text{virtual}}=1.63\left(10\right)M_{\pi }$ for $N_{\text{c}}=3$, while this pole disappears at $N_{\text{c}}>3$. This may be connected to the exotic states found in experiment.

The currently observing Dark Energy Spectroscopic Instrument (DESI) places sub-percent constraints on the Baryon Acoustic Oscillations (BAO) scaling parameters from the Lyman-$\alpha$ (Ly-$\alpha$) forest. However, no systematic error budget stemming from non-linearities in the 3D clustering of the Ly-$\alpha$ forest is included in the DESI-Ly-$\alpha$ analysis. In this work, we measure the size of the shift of the BAO peak using large Ly-$\alpha$ forest mocks produced on the $N$-body simulation suite \textsc{AbacusSummit}. Specifically, we measure the Ly-$\alpha$ auto-correlation and the Ly-$\alpha$-quasar cross-correlation functions. We use the DESI Ly-$\alpha$ forest fitting pipeline, \textsc{Vega}, with the publicly available covariance matrix from eBOSS DR16. To mitigate the noise, we adopt a linear control variates (LCV) technique, reducing the error bars by a factor of up to $\sim \sqrt{50}$ on large scales. From the auto-correlation, we detect a small positive shift in radial direction of $\Delta {\alpha }_{\parallel }=0.35\%$ at the 3$\sigma$ level and virtually no shift in the transverse direction, ${\alpha }_{\perp }$. From the cross-correlation, we see a similar shift to $\Delta {\alpha }_{\parallel }$, albeit with larger error bars, and a small negative shift, $\Delta {\alpha }_{\perp }=\sim$0.25%, at the 2$\sigma$ level. We also make a connection with the Ly-$\alpha$ forest effective field theory (EFT) framework and find that the one-loop EFT power spectrum yields unbiased measurements of the BAO shift parameters in radial and transverse direction for Ly-$\alpha$ auto and the Ly-$\alpha$-quasar cross-correlation measurements. When using the one-loop EFT framework, we find that we can recover the BAO parameters without a shift, which has important implications for future Ly-$\alpha$ forest analyses based on EFT. This work paves the way for the full-shape analysis of DESI and future surveys.

The pseudoscalar meson light-cone distribution amplitudes (LCDAs) are essential non-perturbative inputs for a range of high-energy exclusive processes in quantum chromodynamics. In this proceedings, progress towards a determination of the low Mellin moments of the pion and kaon LCDAs by the HOPE Collaboration is reported.

Effective field theory (EFT)-based full-shape analysis with simulation-based priors (SBPs) is a novel approach to galaxy clustering data analysis, which significantly boosts the constraining power by efficiently incorporating field-level simulation information from small scales. So far, SBPs have been mostly extracted from a large set of mock catalogs generated with the Halo Occupation Distribution (HOD) approach. We show that given a halo mass function model and assuming that the EFT parameters of halos depend only on the peak height, HOD-based priors can be computed analytically for the standard 7-parameter HOD model. We derive the relevant expressions for deterministic EFT parameters from the halo model. The halo model, however, fails to accurately describe stochastic EFT parameters, for which we use a physically motivated phenomenological prescription. We compare our analytic priors with simulations and find an excellent agreement. Our approach provides analytic insights into the physics behind the EFT parameters of galaxies, and allows one to reduce the computation time of SBPs to virtually nothing. As an application, we produce a set of 100,000 EFT parameters by sampling both HOD and cosmological models, and explicitly demonstrate that any cosmology dependence can be completely absorbed by relatively minor shifts of the HOD parameters. Our approach can be generalized to other variants of the HOD and cosmological models beyond $\Lambda$CDM.

The so-called metastability bound on the Higgs mass suggests that the smallness of the Higgs mass may be a byproduct of the metastability of the electroweak vacuum. A significantly strong bound requires new physics capable of lowering the scale where the Higgs quartic coupling turns negative through renormalization group effects, without destabilizing the electroweak vacuum entirely. We analyze in this context the low-scale Majoron model of neutrino masses, which automatically contains two key elements for a viable scenario: heavy fermions to lower the instability scale and a extended scalar sector to stabilize the potential and achieve realistic lifetimes for the electroweak vacuum. We show how the metastability bound can be generalized to theories with multiple scalars and present an efficient way of calculating the tunneling rate in such potentials. We also demonstrate that FCC will probe regions of the parameter space relevant for metastability: large regions of the fermionic sector at FCC-ee and some reach to the scalar sector at FCC-hh.

We provide a general prescription for gauging finite non-invertible symmetries in 1+1d lattice Hamiltonian systems. Our primary example is the Rep(D${}_8$) fusion category generated by the Kennedy-Tasaki transformation, which is the simplest anomaly-free non-invertible symmetry on a spin chain of qubits. We explicitly compute its lattice F-symbols and illustrate our prescription for a particular (non-maximal) gauging of this symmetry. In our gauging procedure, we introduce two qubits around each link, playing the role of “gauge fields” for the non-invertible symmetry, and impose novel Gauss’s laws. Similar to the Kramers-Wannier transformation for gauging an ordinary $Z_2$, our gauging can be summarized by a gauging map, which is part of a larger, continuous non-invertible cosine symmetry.

Axion-like particles can couple to Standard Model gluons, electroweak gauge bosons, and massive fermions. A future multi-TeV muon collider provides a favorable environment to probe axion-like particles through multiple production channels, including vector boson fusion via electroweak gauge boson couplings and the top-associated production mediated by direct fermionic couplings. Motivated by the quality issue of the QCD axion, we focus on axion-like particles with masses and decay constants around the TeV scale. We explore how different axion-like particle couplings shape its production and decay modes, revealing a rich and intricate phenomenological landscape.

We study systems of two and three mesons composed of pions and kaons at maximal isospin using four CLS ensembles with $a\approx 0.063$fm, including one with approximately physical quark masses. Using the stochastic Laplacian-Heaviside method, we determine the energy spectrum of these systems including many levels in different momentum frames and irreducible representations. Using the relativistic two- and three-body finite-volume formalism, we constrain the two and three-meson K matrices, including not only the leading $s$ wave, but also $p$ and $d$ waves. By solving the three-body integral equations, we determine, for the first time, the physical-point scattering amplitudes for $3{\pi }^{+}$, $3K^{+}$, ${\pi }^{+}{\pi }^{+}{K}^{+}$ and $K^{+}{K}^{+}{\pi }^{+}$ systems. These are determined for total angular momentum $J^P=0^{-}$, $1^{+}$, and $2^{-}$. We also obtain accurate results for $2{\pi }^{+}$, ${\pi }^{+}{K}^{+}$, and $2K^{+}$ phase shifts. We compare our results to Chiral Perturbation Theory, and to phenomenological fits.

We use lattice QCD calculations of the finite-volume spectra of systems of two and three mesons to determine, for the first time, three-particle scattering amplitudes with physical quark masses. Our results are for combinations of ${\pi }^{+}$ and $K^{+}$, at a lattice spacing $a=0.063$fm, and in the isospin-symmetric limit. We also obtain accurate results for maximal-isospin two-meson amplitudes, with those for ${\pi }^{+}{K}^{+}$ and $2K^{+}$ being the first determinations at the physical point. Dense lattice spectra are obtained using the stochastic Laplacian-Heaviside method, and the analysis leading to scattering amplitudes is done using the relativistic finite-volume formalism. Results are compared to chiral perturbation theory and to phenomenological fits to experimental data, finding good agreement.

We present the first study of anti-isolated Upsilon decays to two muons (${\rm Y}\to {\mu }^{+}{\mu }^{-}$) in proton-proton collisions at the Large Hadron Collider. Using a machine learning (ML)-based anomaly detection strategy, we “rediscover” the ${\rm Y}$ in 13 TeV CMS Open Data from 2016, despite overwhelming anti-isolated backgrounds. We elevate the signal significance to $6.4\sigma$ using these methods, starting from $1.6\sigma$ using the dimuon mass spectrum alone. Moreover, we demonstrate improved sensitivity from using an ML-based estimate of the multi-feature likelihood compared to traditional “cut-and-count” methods. Our work demonstrates that it is possible and practical to find real signals in experimental collider data using ML-based anomaly detection, and we distill a readily-accessible benchmark dataset from the CMS Open Data to facilitate future anomaly detection developments.

The energy created in deuterium-tritium fusion reactors originates from a high-intensity neutron flux interacting with the reactor’s inner walls. The neutron flux can also be used to produce a self-sustaining reaction by lining the walls with lithium-rich `breeding blankets’, in which a fraction of neutrons interacts with lithium, creating the tritium fuel. The high-intensity neutron flux can also result in the production of dark sector particles, feebly interacting light scalars or pseudoscalars, via nuclear transitions within the breeding blanket. We estimate the potential size of such dark sector flux outside the reactor, taking into account all current constraints, and consider possible detection methods at current and future thermonuclear fusion reactors. As a by-product, we also recast the SNO axion bound for a CP even scalar. We find that year-long searches at current and future reactors can set leading constraints on dark scalar — and dark pseudoscalar — nucleon couplings.

A global fit for ${\alpha }_s\left(m_Z\right)$ is performed on available $e^{+}{e}^{-}$ data for the heavy jet mass distribution. The state-of-the-art theory prediction includes $O\left({\alpha }_s^3\right)$ fixed-order results, N${}^{}$LL${}^{}$ dijet resummation, N${}^{}$LL Sudakov shoulder resummation, and a first-principles treatment of power corrections in the dijet region. Theoretical correlations are incorporated through a flat random-scan covariance matrix. The global fit results in ${0.1145}_{-0.0019}^{+0.0021}$, compatible with similar determinations from thrust and $C$-parameter. Dijet resummation is essential for a robust fit, as it engenders insensitivity to the fit-range lower cutoff; without resummation the fit-range sensitivity is overwhelming. In addition, we find evidence for a negative power correction in the trijet region if and only if Sudakov shoulder resummation is included.

We present a new solution to the Higgs hierarchy problem based on a dynamical vacuum selection mechanism in a landscape which scans the Higgs mass. In patches where the Higgs mass parameter takes a natural value, the Higgs potential only admits a minimum with a large and negative energy density. This causes a cosmological crunch, removing such patches from the landscape. Conversely, in patches where the Higgs mass parameter is smaller than a critical value, the Higgs potential admits a metastable minimum with the standard cosmological history. This critical value is determined by the instability scale, where the quartic coupling turns negative due to its running. The ability of this mechanism to explain the observed Higgs mass hinges on new physics at the TeV scale, such as vector-like fermions. We study two simple realizations of this scenario in a heavy neutral lepton model and in the singlet-doublet model, the latter mimicking a Higgsino-bino system. We show that the relevant parts of their parameter spaces can be probed by proposed future colliders, such as the FCC-ee or a muon collider.

Composite Higgs models are a class of Beyond the Standard Model (BSM) models proposed to address the hierarchy and naturalness problems associated with the Standard Model (SM) Higgs. A new QCD-like strongly interacting sector based on $SU\left(2\right)$ with two fundamental flavours can be used to build a composite Higgs model which is not yet ruled out by experiment. The role of the singlet scalar resonance will affect Higgs phenomenology at the LHC. In this project our goal is to understand the properties of the singlet scalar state in the new strongly interacting sector in isolation as a first step to understanding the role of this state in composite Higgs models. We present here the first lattice results for the mass of the $\sigma$ in $SU\left(2\right)$ with two fundamental flavours using exponential clover Wilson fermions.

The maximum entropy principle is foundational for statistical analyses of complex dynamics. This principle has been challenged by the findings of a previous work [Phys. Rev. X 7, 031034 (2017)], where it was argued that a quantum system driven in time by a certain aperiodic sequence without any explicit symmetries, dubbed the Thue-Morse drive, gives rise to emergent nonergodic steady states which are underpinned by effective conserved quantities. Here, we resolve this apparent tension. We rigorously prove that the Thue-Morse drive achieves a very strong notion of quantum ergodicity in the long-time limit: the time evolution of any initial state uniformly visits every corner of its Hilbert space. On the other hand, we find the dynamics also approximates a Floquet drive for arbitrarily long albeit finite periods of time with no characteristic timescale, resulting in a scale-free ergodic dynamics we call critically slow complete Hilbert-space ergodicity (CS-CHSE). Furthermore, numerical studies reveal that CS-CHSE is not specific to the Thue-Morse drive and is in fact exhibited by many other aperiodic drives derived from morphic sequences, i.e., words derived from repeatedly applying substitution rules on basic characters. Our work presents a new class of dynamics in time-dependent quantum systems where full ergodicity is eventually attained, but only after astronomically long times.