Scientific Series

A high energy muon collider complex can provide new and complementary discovery potential to the LHC or future hadron colliders. New spin-1 bosons are a motivated class of exotic new physics models. In particular leptoquarks, dark photons, and Lμ — Lτ models have distinct production channels at hadron and lepton machines. We study a vector leptoquark model at a muon collider with √ s = 3, 14 TeV within a set of both UV and phenomenologically motivated flavor scenarios. We compute which production mechanism has the greatest reach for various values of the leptoquark mass and the coupling between leptoquark and Standard Model fermions. We find that we can probe leptoquark masses up to an order of magnitude beyond √ s with perturbative couplings. Additionally, we can also probe regions of parameter space unavailable to flavor experiments. In particular, all of the parameter space of interest to explain recent low-energy anomalies in B meson decays would be covered even by a √ s = 3 TeV collider. We also consider other applications of a muon collider complex in the hunt for new physics.

Zoom link:  https://pitp.zoom.us/j/97081554839?pwd=bFBmdUExV0VSdm4vcm5iMTVtaTFrUT09

Cosmic strings arise as remnants of phase transitions in the early Universe, often related to theories of grand unification (GUTs). If such a phase transitions occurs at high energies, the resulting cosmic string network generates a sizable amount of gravitational waves. Most work so far has focused on the gravitational wave signal from topologically stable cosmic strings. In this talk I will introduce metastable cosmic strings, which are a generic consequence of many GUTs. I will discuss how this idea can be probed in various ongoing and upcoming gravitational wave experiments, from pulsar timing arrays to space- and ground-based interferometers. In the final part of my talk I will discuss a recent proposal on using the radio telescopes to probe this and other sources of ultra high frequency gravitational waves.

Strong gravity tests indicate that general relativity is a very accurate description of the classical dynamics of spacetime even at extreme regimes. Yet, the same dynamics can be described by "alternative" versions of general relativity such as unimodular gravity. In the quest for a quantum theory of the gravitational field, it is unclear if the quantization of such classically equivalent theories leads to the same physical predictions. In this talk, I will report on some recent results regarding this issue in the framework of continuum and perturbative quantum field theory. With a view towards ultraviolet completion, I will discuss some evidence for asymptotic safety in unimodular quantum gravity. Moreover, I will comment on the role of matter fields which couple very differently to gravity in those settings.   

If dark matter is composed at least partially of primordial black holes (PBHs) then structure formation occurs very differently than in standard particle dark matter scenarios.  PBH binaries, halos and other structures can form at very early redshifts and the resulting nonlinear dynamics can change constraints on the abundance of PBHs.  In this talk I will describe this structure formation history using results from cosmological simulations and discuss constraints on solar mass and heavier PBHs from LIGO and the CMB.  Lastly, I will discuss how the baryons may be impacted by such nonlinear structures.

It is known that continuous variable quantum information cannot be protected against naturally occurring noise using Gaussian states and operations only. Noh et al. (PRL 125:080503, 2020) proposed bosonic oscillator-to-oscillator codes relying on non-Gaussian resource states as an alternative, and showed that these encodings can lead to a reduction of the effective error strength at the logical level as measured by the variance of the classical displacement noise channel. An oscillator-to-oscillator code embeds K logical bosonic modes (in an arbitrary state) into N physical modes by means of a Gaussian N-mode unitary and N-K auxiliary one-mode Gottesman-Kitaev-Preskill-states.
Here we ask if - in analogy to qubit error-correcting codes - there are families of oscillator-to-oscillator codes with the following threshold property: They allow to convert physical displacement noise with variance below some threshold value to logical noise with variance upper bounded by any (arbitrary) constant. We find that this is not the case if encoding unitaries involving a constant amount of squeezing and maximum likelihood error decoding are used. We show a general lower bound on the logical error probability which is only a function of the amount of squeezing and independent of the number of modes. As a consequence, any physically implementable family of oscillator-to-oscillator codes combined with maximum likelihood error decoding does not admit a threshold.

Abstract: TBD

Zoom Link: https://pitp.zoom.us/j/96516977019?pwd=WVlpZG5WTTUwbFJVZ2wvcXdNWUR5Zz09

It is commonplace that quantum theory can be viewed as a ``non-classical" probability calculus. This observation has inspired the study of more general non-classical probabilistic theories modeled on QM, the so-called generalized probabilistic theories or GPTs. However, the boundary between these putatively non-classical probabilistic theories and classical probability theory is somewhat blurry, and perhaps even conventional. This is because, as is well known, any probabilistic model can be understood in classical terms if we are willing to embrace some form of contextuality. In this talk, I want to stress that this can often be done functorially: given a category $\Cat$ of probabilistic models, there are functors $F : \Cat \rightarrow \Set_{\Delta}$ where $\Set_{\Delta}$ is the category of sets and stochastic maps. In addition to the familiar Beltrametti-Bugajski representation, I'll exhibit two others that are less well known, one involving the ``semi-classical cover" and another, slightly more special, that allows one to represent a probabilistic model with sufficiently strong symmetry properties by a model having a completely classical probabilistic structure, in which any ``non-classicality" is moved into the dynamics, in roughly the spirit of Bohmian mechanics. 

(Based on http://philsci-archive.pitt.edu/16721/)

Zoom Link: https://pitp.zoom.us/j/91838172434?pwd=SUltOGlURWI5MDN6Qk45dnVRelBOQT09

Asymptotically flat spacetimes are invariant under an infinite-dimensional symmetry group comprised of superrotations and supertranslations. These symmetries are spontaneously broken, leading to an infinite degeneracy of gravitational vacua in asymptotically flat spacetimes. Starting from an analysis of four-dimensional asymptotically flat gravity in first order formulation, I will describe how superrotation parametrization modes labelling distinct superrotation vacua are governed by an Alekseev–Shatashvili action on the celestial sphere. I will also comment on a recent construction of a two-dimensional effective action for the Goldstone modes of broken supertranslation invariance.

We study the problem of learning a Hamiltonian given copies of its Gibbs state at a known inverse temperature. Anshu et al. recently studied the sample complexity (number of copies of the Gibbs state needed) of this problem for geometrically local Hamiltonians. In the high-temperature regime, their algorithm has sample complexity polynomial in the system size, temperature, and accuracy. Their algorithm can also be implemented with polynomial, but suboptimal, time complexity. Here, we study the same question for a more general class of Hamiltonians and present an algorithm that solves this problem with improved sample complexity and time complexity linear in the sample size. Furthermore, we prove a matching lower bound showing that our algorithm's sample complexity is optimal, and hence our time complexity is also optimal. Joint work with Robin Kothari and Ewin Tang.

Academic science reflects the context in which science is conducted.  Thus, the academic scientific enterprise struggles with privilege, racism, sexism, ableism, and homophobia, in Canada, as around the world.  In this talk, I will address the uncomfortable and sometimes difficult conversations we must have in order to get to the real work of identifying and removing barriers that limit access and engagement in the scientific enterprise for the full diversity of humanity.  We live within a myth of meritocracy in academia and are failing to achieve our full potential as a sector.  Moving on from self-awareness as individuals or organizations, we can start to identify the tools and skill sets that individuals and collectives need in order to create cultures of care that achieve inclusive excellence and the very best scientific outputs.  

Zoom Link: https://pitp.zoom.us/j/91715376839?pwd=TnBIM1NNek4zOFNyVWFnMHMySE02Zz09

The standard operational probabilistic framework (within which Quantum Theory can be formulated) is time asymmetric.  This is clear because the conditions on allowed operations include a time asymmetric causality condition.  This causality condition enforces that future choices do not influence past events.  To formulate operational theories in a time symmetric way I modify the basic notion of an operation allowing classical incomes as well as classical outcomes.  I provide a new time symmetric causality condition which I call double causality.  I apply these ideas to Quantum Theory proving, along the way, a time symmetric version of the Stinespring extension theorem using double causality.  I also propose the idea of a conditional frame of reference.  We can transform from the time symmetric frame of reference to a forward or a backward frame of reference.  This talk is based on arXiv:2104.00071.

We introduce the basic elements of tensorial group field theories (TGFTs) for quantum gravity, emphasizing how they encode quantum geometry and their relation with canonical loop quantum gravity and spin foam models. Next, we discuss briefly the issue of continuum limit and how it could be understood in this framework.

In the bulk of the talk, we overview the work on the extraction of an effective cosmological dynamics from TGFTs, inspired by the idea of the universe as a quantum gravity condensate. In this context, we emphasize: the need for appropriately coarse-grained states capturing collective dynamics and the role of relational observables and their construction. We discuss what the theory says (so far) about the fate of the big bang singularity at the beginning of our universe and how it suggests a quantum gravity origin for (phantom) dark energy.