Colloquium
Spring 2023 Colloquia will be held in Room 307 of the Science and Engineering Research Facility (unless slated as virtual in the schedule below) on Mondays at 3:30 PM, EST.
The understanding of neutron star properties from fundamental physics is a very active field of theoretical, observational and experimental research, but it is still far from being completed. One of the reasons is that the theory for strong force, the so-called QCD theory, does not apply in a simple way to neutron star matter at a few times the nuclear saturation density. It is only at very high density, never realized in neutron stars, that QCD is perturbative, but some first attempts to connect this limit to the densities in neutron stars are currently investigated with some success. At low density, chiral effective field theory is also fixing a limit which is closely related to QCD and can be incorporated in the description of the crust of neutron stars. Finally, astrophysics observation (gravitational wave, x-rays, radio) and nuclear physics experiment (collective motion, heavy ion collisions) are used to constrain the equation of state for neutron stars and to answer to the question of the properties of the strong interaction in dense matter. But which quantity shall be measured in priority and what is important to observe in the near future?
"Who Ordered That?" --- Muons For New Physics
Presentation Slides
Who Ordered That? I.I. Rabi asked this question when a new particle, the muon, was discovered in 1936. Ever since, this unexpected particle has constantly brought us more surprises, including the pion discovery, parity violation, J/psi discovery, neutrinos and flavor physics etc., opening an avenue in front of us to new physics and new technology. In this talk, I will discuss a new aspect — a high energy muon collider. Due to the recent technological breakthroughs for muon cooling, the muon collider program has regained its momentum. I will present the idea and the current status for a muon collider, and discuss the rich physics potential in exploring the physics beyond the Standard Model, for two representative scenarios: the Higgs factory for the resonant Higgs production and the multi-TeV muon collider at the energy frontier.
Using circularly-polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of spatial chirality and magnetization M. The former is central for asymmetric synthesis in chemistry and homochirality in bio-molecules, while the latter is of great interest for ferromagnetic spintronics. In this paper, the authors report the surprising observation of helicity-dependent optical control of fully-compensated antiferromagnetic (AFM) order in 2D even-layered MnBi2Te4, a topological Axion insulator with neither chirality nor M. They demonstrated helicity-dependent optical creation of AFM domain walls by double induction beams and the direct reversal of AFM domains by ultrafast pulses. The control and reversal of AFM domains and domain walls by light helicity have never been achieved in any fully-compensated AFM. To understand this optical control, the authors studied an AFM circular dichroism (CD) proportional to the AFM order, which only appears in reflection but is absent in transmission. They showed that the optical control and CD both arise from the optical Axion electrodynamics. The Axion induction provides the possibility to optically control a family of PT-symmetric AFMs such as Cr2O3, even-layered CrI3 and possibly pseudo-gap state in cuprates. In MnBi2Te4, this further opens the door for optical writing of dissipationless circuit formed by topological edge states.
Detection of ultrahigh energy (UHE) neutrinos is key to identifying the most energetic objects and processes in the universe. These are the sources of UHE cosmic rays, which have been detected at earth with energies exceeding 1 Joule per nucleon (roughly the kinetic energy of a bird in flight). As UHE cosmic messengers, neutrinos are unparalleled for their ability to travel from source to Earth, interacting only weakly with matter and therefore able to traverse great distances unimpeded. For this same reason, however, they are very difficult to detect (and additionally at high energies, a vanishingly small number arrive at earth).
In this talk, I will discuss the general challenges in detecting UHE neutrinos, and the extensive experimental work that has been done so far to meet these challenges using various detection strategies. I'll focus on a forthcoming experimental effort, the Radar Echo Telescope (RET), which uses well-known radar technology to attempt detection of the cascade produced by these elusive neutrinos as they interact in polar ice. I'll discuss the theory and storied history of astroparticle physics, the radar echo method, recent laboratory work, and our current experimental efforts in service of UHE neutrino detection with RET.
Although Dark Matter (DM) comprises most of the mass of the Universe, the physical nature of DM remains unknown. The hypothesis that DM made of heavy Super Symmetric particles was refuted by LHC. Very light DM candidates axion cannot be found after several decades of searches. Direct searches for DM lack any signal of heavy and light DM particles. We will discuss a model where dark matter particles are a twin-copy of our ordinary particles with the same masses, charges, spins, interactions, thus forming similar atoms, molecules, planets, stars, etc. These DM particles are separated from us by small extra dimension, which makes them invisible for us but does not exclude their gravitational interaction with ordinary matter and between themselves. Such a model of Dark Matter was named a Mirror Matter model. Neutral particles in the ordinary and mirror worlds can be mixed with each other via extra dimensional tunneling and form, in this way, an "interaction portal" between two worlds. Thus, the neutron of our world (n) can be mixed with the mirror neutron (n') leading to quantum-mechanical transformations n -> n' and n' -> n, the process called oscillations. Interaction of neutron components with the environment in both worlds (magnetic field, matter gas) might lead to the observable and reproducible effects that we will addressed in this talk. Some controversial observations pointing to the possible presence of the mirror neutron effects and results of the first searches for "mirror neutrons" will be discussed.
Neutron star mergers are connected to some of the most pressing open questions in physics and astrophysics, ranging from the nature of strong gravity, to the behavior of QCD in the non-perturbative regime, to the origin of the heavy elements. Multimessenger observations of these events hold the key to unlock these mysteries. However, theory is needed to turn data into answers. In this talk, I will discuss our efforts to model binary neutron star mergers in numerical relativity. I will talk about new developments in the simulation technology and I will present some recent results. Finally, I will talk about challenges and prospectives in this field.
Quantum materials provide responses and states of matter with no classical analogues. As such they offer opportunities to create an array of platforms for future devices crucial to human health, energy efficiency, communications and imaging. I will begin by describing the physics challenges and sensing opportunities these materials offer. I will then focus on our use of the relativistic electrons in graphene for biosensing. Specifically we have developed a new platform for multiplexed, rapid, easy to use detectors of biological analytes. I will discuss the unique aspects of graphene involved resulting in our demonstration of the detection of antibiotic resistant bacteria, decease biomarkers in saliva, opioids in waste water and respiratory infection at clinically relevant levels. Time permitting I will explain our efforts to use quantum materials to create new quantum simulators.
In my talk, I review recent and not so recent works aiming to understand whether a nominally repulsive Coulomb interaction can give rise to superconductivity. I discuss a generic scenario of the pairing, put forward by Kohn and Luttinger back in 1965, and briefly review modern studies of the electronic mechanisms of superconductivity in the lattice systems, which model cuprates, Fe-based superconductors, and even doped graphene. I show that the pairing in all three classes of materials can be viewed as a lattice version of Kohn-Luttinger physics, despite that the pairing symmetries are different. I discuss under what condition the pairing occurs and rationalize the need to do renormalization-group studies. I also discuss most recent work on the pairing near a quantum-critical point, particularly the interplay between superconductivity and non-Fermi liquid physics.
This talk discusses two examples of how a combination of analytical and computational methods can serve to connect basic theoretical ideas about correlated states to quantum information quantities and neutron scattering experiments. The ground state of a chain of antiferromagnetically interacting spins (the 1D "Heisenberg model") is one of the solvable hydrogen atoms of many-body physics, but its dynamics remained opaque for eighty years. We introduce the Heisenberg model's novel fluid-like dynamical regime at high temperatures and describe its realization in a variety of recent experiments ranging from neutron scattering on crystals to optical lattice emulation with atoms. It turns out that the dynamics of spins in this canonical model are described by the Kardar-Parisi-Zhang dynamical universality class, which is well known from classical problems such as driven interfaces. For frustrated systems in higher dimensions, controlled comparisons between theory and experiment are more difficult except for a small number of tractable cases. We forge ahead nevertheless and present theoretical arguments that a chiral spin liquid is likely to appear near the Mott transition in some triangular lattice materials, and second, that other kinds of spin liquids and quantum critical points are suggested in recent experiments.
Long-baseline neutrino oscillation experiments present some of the most compelling paths towards beyond-the-standard-model physics through measurement of PMNS matrix elements and observation of the degree of leptonic CP violation. State-of-the-art long-baseline oscillation experiments, like NOvA and T2K, are currently statistically limited, however uncertainty in neutrino-nucleus scattering represents an important source of systematic uncertainty in future experiments like DUNE and Hyper-Kamiokande. Neutrino cross-section uncertainties can be reduced through high-statistics measurement of neutrino interactions on light nuclei, but creating a detector with an appropriate light target has proved elusive since the hydrogen bubble chambers designed in the 70's. Modern bubble chamber-based dark matter detectors like PICO and the Scintillating Bubble Chamber have demonstrated that advances in sensor technology, computing, and automation would allow a modern bubble chamber to fully utilize the megawatt scale intensity LBNF beam. This talk will review the broad physics program and the construction of a hydrogen bubble chamber for use with neutrinos at Fermilab.
Majorana zero modes are fermion-like excitations that were originally proposed in particle physics by Ettore Majorana and are characterized as being their own anti-particle. In condensed matter systems, Majorana zero modes occur as fractionalized excitations with topologically protected degeneracy associated with such excitations. For over a decade, the only candidate systems for observing Majorana zero modes were the non-Abelian fractional quantum Hall state and chiral p-wave superconductors. In this colloquium, I will start by explaining the basic ideas of topological quantum computation using Majorana zero modes. This will be followed by a status update on transport experiments on potential Majorana systems including the recent experiments on Microsoft as well as those in iron superconductors. I will then provide a more detailed explanation of braiding, Majorana operators, and the associated topological degeneracy. I will end with my outlook on the challenges and future directions.
The CERN Large Hadron Collider (LHC) has discovered the Higgs boson and confirmed the predictions for many of its properties given by the "Standard Model" of particle physics. However, this does not mean that particle physics is solved. Mysteries that the Standard Model does not address are still with us and, indeed, stand out more sharply than ever. To understand these mysteries, we need experiments at still higher energies. In this colloquium, I will argue that we should be planning for a particle collider reaching energies of about 10 times those of the LHC in the collisions of elementary particles. Today, there is no technology that can produce such energies robustly and at a reasonable cost. However, many solutions are under study, including colliders for protons, muons, electrons, and photons. I will review the status of these approaches to the design of the next great energy-frontier accelerator.
The merging of two neutron stars is a true multimessenger event that includes gravitational waves, an electromagnetic signal, and the emission of enormous numbers of neutrinos. In order to understand these signals we need a careful accounting of the microphysics that occurs during and after the merger. I will focus on the elements produced in these objects and the effect of two aspects of this microphysics; nuclear models/reactions and neutrino flavor transformation physics. In particular, I will discuss the importance of new developments in these areas to predictions of r-process observables and the astrophysical origin of the r-process.