Current Seminars

The stress-energy tensor, also known as the energy-momentum tensor, encodes the internal energy, spin, and stress distributions within hadrons, which sheds new light on hadron structures and fundamental QCD problems such as confinement and the origin of the hadron mass. On the other hand, this observable poses a particular challenge for strongly coupled systems due to its dynamical nature. In this talk, I will discuss our recent progress in investigating this quantity and the associated gravitational form factors based on a nonperturbative light-front Hamiltonian approach. The main result is a nonperturbative light-front wave function representation of the hadronic stress-energy tensor, which provides an adequate microscopic description. We started with a strongly coupled (3+1)-dimensional scalar field theory and renormalized the stress-energy tensor with a Fock sector-dependent scheme. We further analyze the hadron matrix elements of the stress-energy tensor and identify three “good currents” to extract the physical gravitational form factors. These currents are free from spurious contributions and are consistent with the covariant perturbation theory in the perturbative limit. Finally, we apply this light-front wave function representation to two systems: a strongly coupled scalar field theory with three-body Fock sector truncation and a phenomenological model of charmonium in basis light-front quantization. We will also discuss our results in combination with recent progress in physical interpolations of the stress-energy tensor.

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I will present my work on assessing the uncertainty of nuclear theory inputs required for Beyond the Standard Model (BSM) physics searches. In particular, I will discuss the work I have done on the uncertainty quantification of the nuclear matrix elements (NMEs) of neutrinoless double beta decay, where I developed a novel machine learning algorithm to quickly emulate results of many-body methods. With this emulator, I obtained the sensitivity of the NMEs to each low energy constant (LEC) of the underlying nuclear force, something that requires millions of samples and would take an unfeasible amount of time without emulation. I found that the NMEs are very dependent on one LEC in particular, which fixes the 1S0 partial wave phase-shift, and consequently that the NMEs highly correlate with said phase-shift. I will also discuss how the other uncertainties of the calculations have been assessed and combined to obtain the first complete uncertainty on this quantity. Furthermore, I will present advances done to improve the possible uses of emulator. In particular, I will present a new algorithm based on Bayesian neural networks which allows to emulate across multiple nuclei simultaneously. In particular, I will show how we can study the dependency to the underlying LECs of the ground state energies and the nuclear radii in multiple isotopic chains.

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The QCD axion is one of the most elegant solutions to the strong CP problem and for some masses could also be a constituent of dark matter. Although axion couplings to photons are well-studied in the laboratory, the defining coupling of the QCD axion to gluons is less constrained. In the dense matter of a neutron star, the effective mass of the axion may become negative due to its coupling to gluons, leading to the axion field condensing with the QCD-theta angle taking on a value of pi in the core of the neutron star. This can have striking effects on neutron star structure, leading to neutron stars with different phases than ordinarily expected. I will review neutron star structure, the effects that axion condensation has on nuclear physics and the properties of nucleons and hadrons, and what properties we expect an axion condensed neutron star to have, ultimately excluding portions of axion phase space based on observations of the thermal relaxation of neutron stars in x-ray binaries, isolated neutron star cooling, and the glitches of the Vela pulsar.

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Using first-principles quantum-field theoretical methods, we investigate electrical charge transport in hot magnetized plasma. The common technique for calculating electrical conductivity is kinetic theory. Generally, however, it is not suitable for plasmas in quantizing magnetic fields when quantum states of charged particles are labelled by discrete Landau levels, n = 0, 1, 2, . . ., rather than continuous transverse momenta. In this case, one must use first-principles methods of quantum field theory to calculate transport properties. By employing Kubo's linear response theory, we express the electrical conductivity tensor in terms of the fermion damping rate in the Landau-level representation. We derive the transverse and longitudinal conductivities by utilizing leading-order results for the damping rates from a recent study. The analytical expressions reveal drastically different mechanisms that explain the high anisotropy of charge transport in a magnetized plasma. Specifically, the transverse conductivity is suppressed while the longitudinal conductivity is enhanced by a strong magnetic field. This is generally expected, as the motion of charge carriers is restricted perpendicular to the background magnetic field but not along it. As usual, at the zero magnetic field, longitudinal conduction is determined by the probability of charge carriers remaining in their quantum states without damping. In contrast, transverse conduction critically relies on quantum transitions between Landau levels, effectively lifting charge trapping in localized Landau orbits. We examine the temperature and magnetic field dependence of the transverse and longitudinal electrical conductivities over a wide range of parameters and briefly address the effects of a nonzero chemical potential. Additionally, we extend our analysis to strongly coupled quark-gluon plasma and study the impact of the coupling constant on the anisotropy of electrical charge transport.

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Just before a nucleus undergoes fission, a neck is formed between the emerging fission fragments. It is widely accepted that this neck undergoes a rather violent rupture, despite the absence of unambiguous experimental evidence. The main difficulty in addressing the neck rupture and saddle-to-scission stages of fission is that both are highly non-equilibrium processes. Here, I present the first fully microscopic characterization of the scission mechanism, along with the spectrum and the spatial distribution of scission neutrons (SNs), and some upper limit estimates for the emission of charged particles. The spectrum of SNs has a distinct angular distribution, with neutrons emitted in roughly equal numbers in the equatorial plane and along the fission axis. They carry an average energy around 3 +/- 0.5 MeV for the fission of 236-U, 240-Pu and 252-Cf, and a maximum of 16 – 18 MeV. We estimate a conservative lower bound of 9−14 % of the total emitted neutrons are produced at scission.

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We identify and describe a new class of three-nucleon forces (3NF) in the heavy baryon Chiral Perturbation Theory (ChiPT). ChiPT is an effective field theory that systematically describes the interactions of pions and nucleons, allowing the construction of nuclear forces. Although the largest contributions come from two-body potentials, three-nucleon (3N) interactions can play an important role in dense systems like nuclei or neutron stars. The leading Three-Nucleon Force (3NF) emerges at the next-to-next-to-leading order (N2LO) in the Effective Field Theory (EFT) expansion. At this order, the 3NF contains a long-range contribution from the exchange of two pions, an intermediate-range interaction from one-pion exchange, and a short-range contribution. However, the current 3NF derivation does not account for the effects of contact operators that involve four nucleon and two pion fields. One of these operators is related to the quark mass-dependent four-nucleon contact operator, while two others arise from terms that depend on the pion momenta. Although these interactions are suppressed in conventional power-counting estimates, Kaplan, Savage, and Wise showed that renormalization requires these terms already at the leading order. In our work, we investigate the consequences of this new class of operators, which induce new 3NFs through loop diagrams. We estimate the resulting contributions to the energy of neutrons and nuclear matter. We find that it leads to a significant contribution that has not been accounted for so far and is comparable to that of the leading-order 3N force. This effect is larger than the uncertainties currently quoted, implying that the new 3NF will have a significant impact on the equations of state of neutron matter and symmetric nuclear matter.

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