Scientific Series

Cosmic-ray-driven instabilities play a crucial role in particle acceleration at shocks and during the propagation of GeV cosmic rays in galaxies and galaxy clusters within the self-confinement picture of CR transport. These instabilities amplify magnetic fields, which, in turn, scatter cosmic rays and thus self-regulate their transport. This leads to a strong coupling between the collisionless cosmic ray population and the thermal background plasma, implying potentially significant dynamic feedback. In this presentation, I discuss a recent discovery of a new cosmic ray-driven instability, referred to as the intermediate-scale instability, which triggers comoving ion-cyclotron electromagnetic waves at sub-ion skin-depth scales. Its growth rate is notably faster compared to the ion gyro scale (streaming) instability, which is commonly assumed to be the dominant instability in the self-confinement picture. Therefore, this new instability could play a vital role in the transport of cosmic rays in galactic and stellar environments. I then explore the implications of this instability for electron acceleration at non-relativistic shocks. Through Particle-in-cell (PIC) simulations, it is demonstrated that the new instability triggers the dominant mechanism for efficient electron acceleration at parallel electron-ion shocks, addressing a persistent issue with electron injection at these shocks. The PIC simulations also reveal that the common practice of using reduced ion-to-electron mass ratios in shock simulations, which artificially suppresses the intermediate instability, not only hinders electron acceleration but also leads to incorrect electron and ion heating in downstream and shock transition areas.

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Zoom link https://pitp.zoom.us/j/91367222746?pwd=REpEdXE3ZGdLeVQ1bnh0NldIQktWQT09

Out of the many lessons quantum mechanics seems to teach us, one is that it seems there are things we cannot experimentally have access to. There is, indeed, a fundamental limit to our ability to experimentally explore the world. In this work we accept this lesson as a fact and we build a general theory based on this principle. We start by assuming the existence of statements whose truth value is not experimentally accessible. That is, there is no way, not even in theory, to directly test if these statements are true or false. We further develop a theory in which experimentally accessible statements are a union of a fixed minimum number of inaccessible statements. For example, the value of truth of the statements a and b is not accessible, but the value of truth of the statement “a or b" is accessible. We do not directly assume probability theory, we exclusively define experimentally accessible and inaccessible statements and build on these notions using the rules of classical logic. We find that an interesting structure emerges. Developing this theory, we relax the logical structure, naturally obtaining a derivation of a constrained quasi-probabilistic theory rich in structure that we name theory of inaccessible information. Surprisingly, the simplest model of theory of inaccessible information is the qubit in quantum mechanics. Along the path for the construction of this theory, we characterise and study a family of multiplicative information measures that we call inaccessibility measures. arXiv:https://arxiv.org/abs/2305.05734

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Zoom link https://pitp.zoom.us/j/91350754706?pwd=V1dVdGM3Zk9MNkp4VlpCYUoxbXg3UT09

For a simple Lie algebra \mathfrak{g}, Kazhdan-Lusztig correspondence states that for certain values of the level k, there is an equivalence between two braided tensor categories: the category of modules of the affine Lie algebra of \mathfrak{g} at level k and the category of modules of the quantum group of \mathfrak{g} at q=e^{\pi i/k}. I will report on recent work to appear with T. Creutzig and T. Dimofte proving such a statement for a class of Lie superalgebras. These Lie superalgebras and their affine VOAs arise from the study of boundary conditions in 3d \mathcal{N}=4 abelian gauge theories. I will also explain how the corresponding supergroups act on the category of matrix factorizations.

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Zoom link https://pitp.zoom.us/j/92338197336?pwd=OG40V2p6V0FmalNCZnV2ZFF1NHAzZz09

Classical simulation techniques are widely used in quantum computation and condensed matter physics. In this talk I will describe algorithms for classically simulating measurement of an n-qubit quantum state in the standard basis, that is, sampling a bit string from the probability distribution determined by the Born rule. Our algorithms reduce the sampling task (known as weak simulation) to computing poly(n) amplitudes of n-qubit states (strong simulation). Two classes of quantum states are considered: output states of polynomial-size quantum circuits and ground states of local Hamiltonians with an inverse polynomial energy gap. We show that our algorithm can significantly accelerate quantum circuit simulations based on tensor network contraction and low-rank stabilizer decompositions. To sample ground state probability distributions we employ the fixed-node Hamiltonian construction, previously used in Quantum Monte Carlo simulations to address the fermionic sign problem. We implement the proposed sampling algorithm numerically and use it to sample from the ground state of Haldane-Shastry Hamiltonian with up to 56 qubits. 

Joint work with Giuseppe Carleo, David Gosset, and Yinchen Liu

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Zoom link https://pitp.zoom.us/j/93297869296?pwd=TVpRdVJmU3lWZjVQM3NNKzBucVVRUT09

In the last few years, a remarkable link has been established between the soft theorems and asymptotic symmetries of quantum field theories: soft theorems are Ward identities of the asymptotic symmetry generators. In quantum electrodynamics, Weinberg's soft photon theorem is nothing but the Ward identity of a gauge transformation whose parameter is non-trivial at infinity. Likewise, Low's tree-level subleading soft photon theorem is the Ward identity of a gauge transformation whose parameter diverges linearly at infinity. More recently, it has been shown that Low's theorem receives loop corrections that are logarithmic in soft photon energy. Then, it is natural to ask whether such corrections are associated with some asymptotic symmetry of the S-matrix. There have been proposals for conserved charges whose Ward identities yield the loop-corrected soft theorems, but a clear symmetry interpretation remains elusive. We explore this question in the context of scalar QED, in hopes of shedding light on the connection between asymptotic symmetries and loop-corrected soft theorems.

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Zoom link https://pitp.zoom.us/j/94420835190?pwd=dEpOSHluRzFpVTg3Qm10OS9PTTU3dz09

The QUEST-DMC experiment, aimed at detecting sub-GeV dark matter, utilizes a unique approach by employing superfluid Helium-3 (He-3) in conjunction with quantum sensors. Superfluid He-3 stands out as an ideal target medium for sub-GeV dark matter searches, especially in the context of spin-dependent interactions. This choice aligns seamlessly with a wide array of theoretically motivated dark matter models. The experiment's projected sensitivity to various dark matter models, as well as its potential to set upper limits on dark matter interactions, will be presented.

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Zoom link https://pitp.zoom.us/j/99386484623?pwd=dGVQdFJKbEpPMUcrRUNLbHdIR2p3Zz09

Anomaly of global symmetry is an important tool to study dynamics of quantum systems. In recent years, new non-invertible global symmetries are discovered in many quantum systems such as the 2d Ising model, Standard Model like theories, and lattice models. I will discuss constraints on the dynamics in 3+1d systems using anomalies of non-invertible symmetries from the perspective of bulk-boundary correspondence. The discussion is based on the work https://arxiv.org/abs/2308.11706 with Clay Cordova and Carolyn Zhang.

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Zoom link https://pitp.zoom.us/j/99162815973?pwd=M01nZXJIN2tCRjhuZlljNU1id01XQT09

Low mass galaxies challenge our picture of galaxy formation and are an intriguing laboratory for the study of star formation, feedback and dark matter physics. I will present results from high resolution, cosmological simulations that contain many (isolated) dwarf galaxies [the MARVEL dwarfs] as well as satellite dwarf galaxies [the DC Justice League]. Together, they create the largest collection of high-resolution simulated dwarf galaxies to date and the first flagship suite to resolve ultra-faint dwarf galaxies in multiple environments. This sample spans a wide range of physical (stellar and halo mass), and evolutionary properties (merger history). I will present results and predictions constraining star formation, feedback and dark matter physics soon testable by telescopes like JWST, Rubin's LSST and the Roman Space Telescope. Finally, I will present new work on measuring galaxy shapes and the diversity of rotation curves in the dwarf galaxy mass regime which may be used to distinguish dark matter model.

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Zoom link: https://pitp.zoom.us/j/99038411436?pwd=OFd1SEdUUXJkd0NLeWtrTUxGR0FCUT09

The performance of neural networks like large language models (LLMs) is governed by "scaling laws": the error of the network, averaged across the whole dataset, drops as a power law in the number of network parameters and the amount of data the network was trained on. While the mean error drops smoothly and predictably, scaled up LLMs seem to have qualitatively different (emergent) capabilities than smaller versions when one evaluates them at specific tasks. So how does scaling change what neural networks learn? We propose the "quantization model" of neural scaling, where smooth power laws in mean loss are understood as averaging over many small discrete jumps in network performance. Inspired by Max Planck's assumption in 1900 that energy is quantized, we make the assumption that the knowledge or skills that networks must learn are quantized, coming in discrete chunks which we call "quanta". In our model, neural networks can be understand as being implicitly a large number of modules, and scaling simply adds modules to the network. In this talk, I will discuss evidence for and against this hypothesis, its implications for interpretability and for further scaling, and how it fits in with a broader vision for a "science of deep learning".

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Zoom link https://pitp.zoom.us/j/93886741739?pwd=NzJrcTBNS2xEUUhXajgyak94LzVvdz09

The rich EM phenomenology in the seconds, minutes, and hours just before, during, and after a compact object merger encodes the magnetization of the binary components, the nature of the post-merger remnant, the neutron star equation of state, the free neutron abundance, and a wide array of other compelling physics. Unfortunately, the requirement to search, find, and classify an electromagnetic counterpart within the large GW localization regions before targeted follow-up with sensitive instruments can begin, excludes access to these earliest times, even for the most well localized GW sources. The ability to promptly localize a GW source to within the field-of-view of a narrow-field sensitive facility, would enable extraordinary science. I will discuss the science cases that require extremely early time observations, and the coordination, instruments, and analyses necessary to achieve it. These include gamma-ray imaging, novel data analysis techniques, pre-merger GW detection, faster space telescopes, and new experiments.

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Zoom link https://pitp.zoom.us/j/96186614241?pwd=R0xpT0dDZVZzek5RT0x4Q1c3Z1RuUT09

Motivated by the static patch of de Sitter space, we discuss timelike surfaces in general relativity and the initial boundary value problem. We consider the Dirichlet problem and a conformal version thereof. Time permitting we discuss some ideas in lower dimensions.

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Zoom link https://pitp.zoom.us/j/98357788662?pwd=b3lHS3Q3T2xkNm53SkR3QVVvTGJMdz09

In general relativity, the remnant black hole of a binary black hole merger emits “ringdown” radiation: a superposition of quasinormal modes with characteristic complex frequencies. The detection of more than one of these frequencies would serve as smoking-gun evidence of a black hole and would enable a test of the no-hair theorem. In this talk, I will discuss strategies for identifying quasinormal modes within simulated ringdown waveforms both in linear perturbation theory and full general relativity. I will show that a rich spectrum of modes could exist in the ringdown, including overtones, retrograde modes and nonlinear modes, and that interesting quasi-universal relationships exist between some of their amplitudes. I will also discuss the spectral instability of quasinormal modes and its potential imprints on the ringdown waveform. These results could guide the analysis of real ringdown signals detected by gravitational-wave detectors.

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Zoom link https://pitp.zoom.us/j/92002625163?pwd=bHNDVVZKYnVNTENLNXJBLzVNKzVSQT09