Parton Distribution Functions for Discovery at Next Generation Colliders

By 2030 the High-Luminosity Large Hadron Collider and the Electron Ion Collider may record their first events. These are only two of several facilities around the world that will allow physicists to investigate the structure of the proton with unprecedented precision and unveil various, yet unexplored, aspects of the underlying laws of Nature. I will discuss the role played by Parton Distribution Functions and their determination in this endeavour. I will focus on the theoretical and methodological challenges that ought to be addressed to meet the needs of accuracy and precision for discovery at future colliders.

Wednesday, 17th December 2021, 12:00 — Sala Wataghin

Flavor Phenomenology of the QCD Axion

I will discuss the possibility to search for the QCD axion with high-precision flavor facilities, complementary to the usual axion searches with helio- and haloscopes. After a short introduction to the strong CP problem and the QCD axion, I will summarize the bounds on the most general flavor-violating axion couplings. These constraints arise mainly from 2-body meson and lepton decays with missing energy, and could be substantially improved by dedicated searches at e.g. Belle-II. I will also discuss novel constraints on Hyperon decays with missing energy from SN1987A, which can provide the strongest limits on flavored dark sectors.

Wednesday, 19th May 2021, 14:30 — Zoom seminar

Leading singularities in Baikov representation and Feynman integrals with uniform transcendental weight

Differential equations are a powerful tool for computing Feynman integrals. Their solution is straightforward if one can find a transformation to a certain ‘canonical’ form. An algorithmic way to construct the necessary transformation is therefore highly desired, and recent years have seen numerous works in this direction. In this talk, I will first introduce the relevant set of Feynman integrals and their differential equations for a given scattering problem and then explain how a canonical basis of integrals can be found through analysing their leading singularities. In addition, I will introduce a method for finding more general (hypergeometric) canonical integrals and show how these can be related to the desired family of Feynman integrals. A two-loop family with three external masses is used to provide examples, as well as show the applicability of the method to state-of-the-art problems.

Wednesday, 26th May 2021, 14:30 — Zoom seminar

Charting the structure of hadrons with Transverse Momentum Distributions

In this seminar I will introduce some key aspects of hadron femtography, the investigation of the structure of hadrons by means of Quantum Chromodynamics (QCD). In particular, I will focus on the imaging of hadron structure and of hadron formation in momentum space, based on the Transverse Momentum Distributions (TMDs). After discussing the potential of current and future experimental facilities to provide crucial information about hadron structure and formation, I will present some of the most recent studies of TMDs and discuss potential implications for high-energy physics.

*Tuesday, 19th October 2021, 14:30 — Sala Wataghin *

Duals of Feynman Integrals: Differential Equations

We elucidate the vector space (twisted relative cohomology) that is Poincar\'e dual to the vector space of Feynman integrals (twisted cohomology) in general spacetime dimension. The pairing between these spaces - an algebraic invariant called the intersection number - extracts integral coefficients for a minimal basis, bypassing the generation of integration-by-parts identities. Dual forms turn out to be much simpler than their Feynman counterparts: they are supported on maximal cuts of various sub-topologies (boundaries). Thus, they provide a systematic approach to generalized unitarity: the reconstruction of amplitudes from on-shell data. In this talk, we will introduce the idea of dual forms and study their mathematical structures. As an application, we derive compact differential equations satisfied by arbitrary one-loop integrals in non-integer spacetime dimension. Our methods are expected to generalize to higher loop order and we provide a simple 2-loop example as evidence.

Friday, 4th June 2021, 14:30 — Zoom seminar

Non-abelian infrared divergences on the celestial sphere

We consider the infrared factorisation of non-abelian multi-particle scattering amplitudes, and we study the form of the universal colour operator responsible for infrared divergences, when expressed in terms of coordinates on the ‘celestial sphere’ intersecting the future light-cone at asymptotic distances. We find that colour-dipole contributions to the infrared operator, to all orders in perturbation theory, have a remarkably simple expression in these coordinates, with scale and coupling dependence factorised from kinematics and colour. Generalising earlier suggestions in the abelian theory, we then show that the infrared operator can be computed as a correlator of vertex operators in a conformal field theory of Lie-algebra-valued free bosons on the celestial sphere. We verify by means of the OPE that the theory correctly predicts the all-order structure of collinear limits, and the tree-level factorisation of soft real radiation.

Wednesday, 9th June 2021, 14:00 — Zoom seminar

Dissecting the collinear structure of quark splitting at NNLL

Higher-order splitting kernels are a subject of continued interest as they comprise an essential ingredient for enhancing the logarithmic accuracy of parton showers. We use the groomed jet mass spectrum, in e+e- collisions, as an anchor to dissect the collinear dynamics of quark splitting at NLO. In the first part of the talk, I will show how to obtain the NNLL structure of the groomed jet mass directly from triple-collinear splitting functions. I will then move to discussing how one may extract B_2^q(z), an object that defines a differential version of the coefficient B_2^q which enters the quark form factor at NLO. This object should offer a starting point towards designing higher-order parton showers. As a bonus, I will use the results to shed light on the resummation structure of generic groomed observables in a QCD-based approach.

*Tuesday, 2nd November 2021, 14:30 — Zoom seminar *

Standard Model prediction of the Bc lifetime

Applying an operator product expansion approach an updated Standard Model prediction of the Bc lifetime is presented. The non-perturbative velocity expansion is carried out up to third order in the relative velocity of the heavy quarks. Scheme dependence is studied using three different mass schemes for the b and c quarks, resulting in three different values consistent with each other and with experiment. Uncertainties resulting from scale dependence, neglecting the strange quark mass, non-perturbative matrix elements and parametric uncertainties are discussed in detail. The resulting uncertainties are still rather large compared to the experimental ones, and therefore do not allow for clear-cut conclusions concerning New Physics effects in the Bc decay.

Tuesday, 5th October 2021, 14:30 — Zoom seminar

Understanding Event-Generation Networks via Uncertainties

Following the growing success of generative neural networks in LHC simulations, the crucial question is how to control the networks and assign uncertainties to their event output. We show how Bayesian normalizing flow capture uncertainties from the training. Fundamentally, the interplay between density and uncertainty estimates indicates that these networks learn functions in analogy to parameter fits rather than binned event counts

Wednesday, 16th March 2021, 14:30 — Zoom seminar

Leading hadronic contribution to the muon magnetic moment from lattice quantum chromodynamics

Twenty years ago in an experiment at Brookhaven National Laboratory, physicists measured the muon's anomalous magnetic moment, $a_\mu=(g_\mu-2)/2$, with a remarkable precision of 0.54 parts per million. Since then, the standard model prediction for $a_\mu$ has exhibited a discrepancy with experiment of over 3 standard deviations, raising the tantalizing possibility of physical particles or forces as yet undiscovered. On April 7 a new experiment at Fermilab presented its first results, brilliantly confirming Brookhaven's measurement and bringing the discrepancy with the standard model to a near discovery level of 4.2 sigma. To fully leverage this and future measurements, and possibly claim the presence of new fundamental physics, it is imperative to check the standard model prediction with independent methods, and to reduce its uncertainties. After an introduction and a discussion of the current experimental and theoretical status of $a_\mu$, I will present a precise lattice QCD calculation, by the BMW collaboration, of the contribution to this quantity that most limits the precision of the standard model prediction. The result of this calculation significantly reduces the gap between the standard model and experiment, and suggests that new physics may not be needed to explain the current, experimental, world-average value of $a_\mu$.

Wednesday, 23rd June 2021, 14:30 — Zoom seminar