Physics Department
Quarkonia have long been recognized as unique probes to study the properties of the quark-gluon plasma (QGP) created in high-energy heavy-ion collisions. Specifically, their yields in heavy-ion collisions are expected to be suppressed compared to that in vacuum due to medium-induced dissociation, which is closely related to QGP deconfinement, temperature and dynamics. Furthermore, different quarkonia of different binding energies are predicted to suffer from different levels of suppression, providing an additional handle in studying the medium's properties. In this talk, I will present recent progress in measurements of quarkonium modification in heavy-ion collisions at RHIC and discuss their physics implications.
Dark matter makes up 85% of the matter in our Universe, but we have yet to learn its identity. While most experimental searches focus on Weakly Interacting Massive Particles (WIMPs) with masses above the proton (about 1 GeV/c2), many natural dark-matter candidates have masses below the proton and are invisible in traditional WIMP searches. In this talk, I will discuss the search for dark matter with masses between about 500 keV/c2 to 1 GeV/c2 ("sub-GeV dark matter"), which has seen tremendous progress in the last few years. I will describe several direct-detection strategies, and discuss how to search for dark matter interactions with electrons and nuclei in various target materials, such as noble liquids and semiconductors. I will in particular highlight SENSEI, a funded experiment that will use ultra-low-threshold silicon CCD detectors ("Skipper CCDs") capable of detecting even single electrons. I will describe the latest results from SENSEI, and how SENSEI and other experiments expect to probe orders of magnitude of novel dark matter parameter space in the next few years.
There exists enormous, but rather hidden, potential of the existing CERN accelerator infrastructure to conduct new research programs in a very broad domain of science with novel, unprecedented-quality tools. This potential needs to be explored to assure the prominent place of CERN - as the accelerator-based research leader - even in the case in which its planned, large-cost, high-energy-frontier machines (such as FCC, CLIC or Muon Collider) are nor constructed.
In this talk, I shall discuss an initiative which may lead to significant broadening the present CERN research programme by including a new component - the novel-type light source. The proposed, partially-stripped-ion-beam-driven light source is the backbone of the Gamma Factory project. It could be realized at CERN by re-using the infrastructure of the already existing accelerators and by profiting from the recent progress in the laser technology. It could extend the scientific life of the LHC storage rings beyond its HL-LHC phase. Gamma Factory could push the intensity limits of the presently operating light-sources by at least 7 orders of magnitude, reaching the flux of up to 1018 photons/s, in the particularly interesting gamma-ray energy domain of 0.1 - 400 MeV, which is presently nor accessible to the FEL photon sources. The partially stripped ion beams, the unprecedented-intensity energy-tuned gamma beams, together with the gamma-beam-driven secondary beams of polarized positrons, polarized muons, neutrinos, neutrons, and radioactive ions constitute the basic research tools of the Gamma Factory. A broad spectrum of new research opportunities, in a vast domain of uncharted fundamental and applied physics territories, could be opened by the Gamma Factory. Examples of new research opportunities and the status of the project development will be presented in this talk.
The most powerful means of exploring nature at small length scales is through the use of particle colliders. Colliders smash particles together at high energies, briefly producing new particles through quantum fluctuations, which then decay into complicated sprays of energy in surrounding detectors. Much in analogy with how the details of our cosmic history are imprinted in the cosmic microwave background, the detailed features of the interactions of elementary particles are imprinted into macroscopic correlations in the energy flow of the collision products. Understanding the underlying microscopic physics in collider experiments therefore relies on our ability to decode these complicated correlations in energy flow. In turn, the desire to understand how to compute collider observables from an underlying quantum field theory (QFT) description has been a driver of theoretical developments and insights into the structure of QFT itself.
In this talk I will present some recent highlights in the quest to better understand the strong nuclear force at collider experiments, driven by recent theoretical developments in the understanding of a class of observables called ``Energy Correlators". I will then apply these developments to explore a variety of interesting phenomena in Quantum Chromodynamics, ranging from from weighing the heaviest quark, to imaging the most perfect fluid.
The turn of the 21st century witnessed a sudden shift in our fundamental understanding of particle physics. While the minimal Standard Model predicts that neutrino masses are exactly zero, the discovery of neutrino oscillations proved the Standard Model wrong. Neutrino oscillation measurements, however, shed light neither on the scale of neutrino masses, nor on the mechanism by which those are generated. The neutrino mass scale is most directly accessed by studying the energy spectrum generated by beta decay or electron capture - a technique dating back to Enrico Fermi's formulation of radioactive decay. I will review the methods and techniques aimed at measuring neutrino masses kinematically, with a focus on recent experimental developments that have emerged in the past decade. Finally, we provide an outlook of what future experiments might be able to achieve.
Neutrinos are some of the most fascinating particles that occur in nature. Over one billion times lighter than the proton, the neutrino was once thought to be massless and to travel at the speed of light. The Nobel-Prize winning discovery of neutrino oscillations demonstrated that neutrinos have non-zero mass, which opens up the unique possibility of the neutrino being its own antiparticle, known as a Majorana fermion. This property, combined with observations of CP violation, could help to explain the dominance of matter in our Universe. This talk will discuss the physics landscape, and present recent technological advances that enable a new kind of "hybrid" neutrino experiment, which would combine two highly successful detection techniques: the topological information of Cherenkov detectors, with the high light yield of scintillators. The Theia detector would be capable of combining both signals to achieve unprecedented levels of particle and event identification, offering a rich program of science across high-energy particle, nuclear and astrophysics.
This talk will report on the substantial progress made over the last ~2 decades on both experimental and theoretical fronts in the global enterprise to search for neutrinoless double-beta decay, a hypothetical ultra-rare nuclear process in which matter (two new electrons) is created without any antimatter, as is predicted generically by many Standard Model extensions. This progress has culminated in the mounting of experiments with the capability of identifying a single decaying nucleus in the midst of tens to hundreds of kg of detector material or more, reaching half-life sensitivities on the order of 1018 times the age of the universe. Such sensitivity gives very exciting prospects for discovery in the coming round of experimental searches.
Neutrons are stable inside atomic nuclei. Outside the confines of the nucleus, they decay into a proton, electron, and antineutrino, with a lifetime of approximately 880 s. The rate of decay can be precisely calculated, using the theory of electroweak interactions, with an uncertainty on the order of 1e-4. Recent measurements using bottled neutrons have achieved uncertainties below 1 s (0.1%), but other measurements observing neutron decay in flight disagree by 10 s. Attempts to resolve this discrepancy have spawned much experimental effort as well as exotic theoretical conjectures, thus far without a clear conclusion. In this talk, I will discuss the challenges of precision measurement of the neutron lifetime, illustrating the UCNtau experiment. It eliminates the dominant loss mechanisms present in previous bottle experiments by levitating polarized ultracold neutrons above the surface of a large Halbach magnetic trap. Using this approach, a new result, 877.75 ± 0.28 (stat) +0.22/-0.16 (sys) s [PRL 127, 162501 (2021)], is the most precise measurement of the lifetime. This result, together with improved measurements of the axial coupling constant, will provide a determination of the CKM matrix element Vud, independent of nuclear decays, and address the recent tension in the test of CKM unitarity.
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