High Energy Physics

In this talk, we explore new techniques to probe the analytic structure of scattering amplitudes in perturbative Quantum Field Theory. The goal of this approach is to leverage symmetries, limits, and analyticity in order to circumvent the explicit evaluation of multi-loop Feynman integrals. First, we generalize the cutting rules by relating sequential discontinuities (discontinuities of discontinuities) to multiple cuts through the corresponding Feynman diagram. Then, we determine the logarithmic branch-cut structure of massive scattering amplitudes by expanding around their branch points. As a corollary, we prove a conjectured bound on the transcendental weight of polylogarithmic Feynman integrals. These results pave a new path towards bootstrapping scattering amplitudes in perturbation theory.

Finding cosmological models that are consistent with a wide range of observations, including those that indicate a high Hubble constant (the current rate of expansion of space), has proved to be very challenging. In this talk I explore why. Our exploration leads us to fundamental sources of length scales in cosmological models: gravitational collapse time scales and photon electron-scattering mean free paths. We find that scaling down these quantities leaves cosmic microwave background (CMB) maps, and many other cosmological observables, nearly invariant, while boosting up the expansion rate. I then introduce a model that takes advantage of this symmetry to boost up model predictions of the Hubble constant given CMB and other cosmological data. Gravitational collapse time scales are scaled down from standard cosmological values by the introduction of a `mirror world' dark sector, and photon mean free paths are scaled down by reducing the amount of helium, thereby freeing up more electrons. I speculate about alternative ways to reduce photon mean free paths, as consistency with the Riess et al. (2021) measurement of the Hubble constant requires there to be less helium in the universe than is observed. 

The universe has been studied using light since the dawn of astronomy.  

But deep down in the dark glacial ice of the South Pole, Antarctica, a very different kind of telescope is getting a new view of the universe. Operated by a team of more than 300 physicists from 12 countries, the IceCube Neutrino Observatory captures the universe in high-energy neutrinos.  

Neutrinos are particles a lot like light (photons), but with one remarkable property that makes them a powerful medium for studying the universe. Physicist Naoko Kurahashi Neilson has travelled to the snow-swept IceCube Neutrino Observatory to study these elusive particles. In her October 6 Perimeter Public Lecture webcast, she will share more about the insights neutrinos can offer and what it’s like conducting research in one of the least habitable places on Earth.

Kurahashi Neilson is an associate professor at Drexel University and the recipient of a CAREER award from the National Science Foundation. Symmetry magazine featured her among 10 early-career experimentalists of note in 2019.

After her undergraduate degree from University of California, Berkeley, Kurahashi Neilson obtained her PhD at Stanford University while “listening” for extremely high-energy neutrinos in the ocean in the Bahamas. She now lives outside Philadelphia with her husband and three young children, and is devoted to STEM outreach, particularly aimed at middle- and high-school girls.

In this talk, I will connect physical properties of scattering amplitudes to the Riemann zeta function. Specifically, I will construct a closed-form amplitude, describing the tree-level exchange of a tower with masses m^2_n = \mu^2_n, where \zeta(\frac{1}{2}\pm i \mu_n) = 0. Requiring real masses corresponds to the Riemann hypothesis, locality of the amplitude to meromorphicity of the zeta function, and universal coupling between massive and massless states to simplicity of the zeros of \zeta. Unitarity bounds from dispersion relations for the forward amplitude translate to positivity of the odd moments of the sequence of 1/\mu^2_n.

Cosmic inflation provides an environment similar to particle colliders that can produce new particles and record the resulting signal. In this talk, I will describe a scenario in which new particles much heavier than the Hubble scale are produced during inflation via couplings to the inflaton. These heavy particles propagate classically and give rise to localized spots on the cosmic microwave background following their production. Momentum conservation during particle production dictates that these localized spots come in pairs. I will discuss the properties of such pairs of CMB spots and the prospect of their detection from the thermal fluctuation background in a position space search.

In this talk we discuss aspects of the non-relativistic two-body problems in AdS spacetime and their CFT duals. Specifically, we focus on understanding the spectrum of double-twist operators in the dual CFT as well as realizing the flat space scattering and bound states from the CFT correlator.

To understand the double-twist operator spectrum we use quantum perturbation theory in the bulk. We then match the result with the inversion formula for consistency. Next, we show how to obtain the flat space scattering from the correlator by using Euclidean time evolution to construct the scattering states; finally we demonstrate how to get the bound states through the WKB approximation. Time permitting, we will also discuss how to obtain physical quantities such as precession of the near-circular orbits in AdS from the data of the double twist operators lying on the first two Regge trajectories.

The COHERENT collaboration made the first measurement of coherent elastic neutrino nucleus scattering(CEvNS) in 2017 using a low-background, 14.6-kg CsI[Na] detector at the SNS. Since initial detection, this detector has opened a new era of precision CEvNS measurements by doubling the detector exposure and improving understanding of the detector response. We these improvements, we now use CsI[Na] data to make competitive constraints of beyond-the-standard-model physics.
We will focus on our recent search for dark matter particles produced at the SNS. With our experience measuring CEvNS, we are sensitive to analogous coherent dark matter induced recoils in our detector. This is a novel approach for accelerator-based dark matter experiments. Searching in this channel is also very powerful, allowing relatively small detectors to explore new parameter space inaccessible to much larger detectors. We will briefly discus other BSM opportunities with COHERENT, showing current results from CsI[Na] along with future sensitivity.

Multi-matrix integrals in  planar, large N limit are genuine functional integrals, in general very difficult to compute.  Apart from some known solvable examples (one-matrix model, two-matrix model with specific interaction, etc) one has to rely on perturbation theory or Monte-Carlo at large enough N.  Matrix bootstrap (MB), initiated by Anderson and Kruczenski, and Lin,  is an interesting alternative to these methods. MB deals with the planar loop equations for loop moments -- the averages of traces of "words" built out of products of matrix variables.   The number of unknowns - loop moments - grows with the length of words quicker then the number of  loop equations. The needed extra conditions come from the positivity of correlation matrix of loop moments. This allows, at a given cut-off on the length of words, to establish the upper and lower limits for particular, lowest loop moments, sometimes with an excellent precision.  The main difficulty of the previous works is the non-linearity of loop equations, leading to a  non-convex  optimization procedure. In our recent paper with Zechuan Zheng, we propose to complete this scheme with the relaxation procedure:   the non-linear loop equations are incorporated into the relaxation matrix  as linear inequalities. The problem becomes the standard SDP, allowing for longer loops and thus a better precision. We demonstrate the relaxed matrix bootstrap (RMB) on the example of an analytically unsolvable 2-matrix model. The RMB for Z_2 symmetric states  gives a very satisfactory precision for generic parameters,  up to 6 digits.  We also managed  to apply   RMB  to  more challenging, Z_2 symmetry breaking solutions, though with  less of precision.  We also prove analytically, using the Hamburger problem, that MB for the 1-matrix model converges to physical solutions, with eigenvalues distributed only on the real axis.

 The existence of right-handed neutrinos may shed light on the origin of neutrino masses. It is also conceivable that if these particles exist, they may have a new set of interactions and symmetries of their own. In this talk, I will discuss "lamppost" models where MeV to GeV heavy neutrinos interact with a dark photon, and discuss some novel experimental signatures at neutrino detectors, e+e− colliders, and kaon decays. I will also comment on some connections to MiniBooNE and the (g-2) of the muon.

I will discuss a set of calculations of dark matter interactions with low threshold semi-conductor detectors such as SENSEI and superCDMS. As a warm-up, I'll show how to rephrase traditional DM-electron scattering calculations in terms of the energy loss function of the material. We will then apply this framework to compute the rate for the Migdal effect in semi-conductor targets. This is needed because the traditional calculations of the Migdal effect do not apply in low threshold semi-conductor detectors, as the delocalized nature of the valence electrons is not taken into account.

We consider quantum field theory on a rigid de Sitter space. We show that the perturbative expansion of late-time correlation functions to all orders can be equivalently generated by a non-unitary Lagrangian on a Euclidean AdS geometry. We use this to infer the analytic structure of the spectral density that captures the conformal partial wave expansion of a late-time four-point function, to derive an OPE expansion, and to constrain the operator spectrum. Generically, dimensions and OPE coefficients do not obey the usual CFT notion of unitarity. Instead, unitarity of the de Sitter theory manifests itself as the positivity of the spectral density. We illustrate and check these properties by explicit calculations in a scalar theory by computing first tree-level, and then full one-loop-resummed exchange diagrams.