Particle Physics Seminar Spring 2025

18.02.2025

Christopher Monahan
^{(Colorado College)}

11.15 - HPT C103

Extracting $x$-dependent nucleon structure from lattice quantum chromodynamics

Protons and neutrons comprise 95% of the mass of the visible universe, but many of their properties are still poorly understood. The direct calculation of the internal structure of nucleons is a long-standing goal for nuclear physics, in part because a refined understanding of their structure is important to interpreting data at hadron colliders. First principles' determinations of nucleon structure have recently become possible, using new developments in lattice quantum chromodynamics (QCD), the numerical solution of the strong nuclear force. I will introduce these approaches and review the Hadstruc Collaboration's on-going efforts to extract the quark and gluon structure of the nucleon via the reduced pseudo-distribution framework. As part of this discussion, I will summarise the recent developments in our knowledge of the gluon contribution to the spin of the proton.

25.02.2025

Duarte Fontes
^{(KIT, Karlsruhe)}

11.15 - HPT C103

Precision in bound muon physics with EFT techniques

Muon conversion — the process of a bound muon decaying into an energetic electron — provides one of the most stringent limits on charged lepton flavor violation. The experimental limit is soon expected to improve by four orders of magnitude, thus calling for precise theoretical predictions. The latter are usually described with perturbation theory using an Effective Field Theory (EFT) valid at the nuclear mass. However, muon conversion involves a multiplicity of scales below the nuclear mass, which spoil the convergence of perturbation theory. In this talk, I present a comprehensive framework to overcome this problem, by resorting to a series EFTs. Combining Heavy Quark Effective Theory (HQET), Non-Relativistic QED (NRQED), potential NRQED, Soft-Collinear Effective Theory I and II and boosted HQET, I derive a factorization theorem and present the renormalization group equations.

04.03.2025

Giancarlo Ferrera
^{(University of Milan)}

11.15 - HPT C103

Sudakov resummation of thrust distribution in electron-positron annihilation

We present a resummed QCD calculation of the thrust distribution in electron-positron annihilation in the back-to-back region up to N^4LL accuracy, matched to fixed-order results up to NNLO. We perform the resummation of the large Sudakov logarithms in the Laplace-conjugated space, and we show that the results differ significantly from those obtained through resummation in thrust space. We include non-perturbative corrections using an analytic hadronization model that depends on two free parameters. Finally, we present a comparison of our predictions with experimental data at the Z-boson peak, and we extract a value for the QCD coupling that is fully consistent with the world average.

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11.03.2025

Jeppe Andersen
^{(Durham University & CERN)}

11.15 - HPT C103

Cell Resampling - Reducing the impact of negative events in MC integrations

Perturbative predictions beyond the first order have cancelling contributions stemming from the infra-red behaviour, which often spoil the numerical convergence and dramatically increase the required computational effort. We review the approach of Cell Resampling, which reduces the impact on analyses by organising the numerical cancellation before events are used for analysis. The method requires the introduction of a metric on the set of events. We will discuss this, and how the underlying ideas in Cell Resampling could be used beyond their current application.

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25.03.2025

Florian Herren
^{(University of Zurich)}

11.15 - HPT C103

New methods for four-body semileptonic decays

Charged-current semileptonic B-meson decays play a prominent role for determinations of CKM matrix elements and tests of lepton-flavour universality, but also comprise large backgrounds for rare processes. While most theoretical and experimental studies focus on three-body decays, i.e. decays with one final-state hadron, processes with two or more hadrons received relatively little attention. In this talk, I will present a new model-independent dispersive framework for the study of semileptonic B-meson decays with two final-state hadrons, connecting methods from hadron spectroscopy with recent experimental measurements.

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01.04.2025

Ramon Winterhalder
^{(University of Milan)}

11.15 - HPT C103

ML-driven Event Generation and the Road to MadGraph7

High-precision simulations based on first principles are a cornerstone of any modern physics research. As we approach the HL-LHC era, there is an ever-increasing demand for both accuracy and speed in simulations. Modern Machine Learning (ML) is emerging as a beacon of hope, offering new ways to overcome the limitations of traditional Monte Carlo methods. In this talk, I will first outline the key challenges of current LHC event generation methodologies and introduce the role of ML in addressing them. Afterwards, I will delve into the MadNIS framework, highlighting recent advancements in neural importance sampling and differentiable event generation. Additionally, I will discuss the potential of surrogate amplitudes—fast, ML-driven approximations of high-precision calculations—as a crucial ingredient for accelerating event generation. Finally, I will outline how these approaches pave the way towards MadGraph7.

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08.04.2025

Matthias Neubert
^{(University of Mainz)}

11.15 - HPT C103

Factorization and Resummation for LHC Jet Processes

We present a systematic formalism, based on a factorization theorem in soft-collinear effective theory, to describe non-global observables at hadron colliders, such as gap-between-jets cross sections. By solving the evolution equation for the hard functions for arbitrary $2\to M$ jet processes in the leading logarithmic approximation, we accomplish for the first time the all-order resummation of the so-called "super-leading logarithms". We study the numerical size of the corresponding effects for the gap-between-jets cross section at the LHC, including all partonic channels and interference effects. We find that the asymptotic fall-off of the resummed series is much weaker than for standard Sudakov form factors. Analyzing the low-energy dynamics of gap-between-jets cross sections, we uncover a new mechanism by which the breaking of color coherence and collinear factorization is made compatible with unbroken PDF factorization of the cross section below the jet-veto scale. Glauber-gluon effects, which are unambiguously defined without regulators beyond dimensional regularization, play a crucial role in restoring factorization.

15.04.2025

Kara Farnsworth
^{(University of Geneva)}

11.15 - HPT C103

Hamiltonian Truncation and Effective Field Theory

Hamiltonian truncation is a non-perturbative, numerical method for calculating observables of any quantum field theory by truncating the Hilbert space to states with energy below a maximum energy cutoff. In this talk I will provide an overview of effective field theory techniques and discuss how they can be applied to Hamiltonian truncation calculations. This effective field theory approach provides a systematic way of improving the accuracy of calculations without increasing the computational cost. I will demonstrate this in detail with numerical results in a two dimensional example, and talk about ongoing and future applications of this method to more complicated theories. 

29.04.2025

Andrew McLeod
^{(University of Edinburgh)}

11.15 - HPT C103

From Singularities to Scattering Amplitudes

Scattering amplitudes—which constitute some of the key ingredients for making ​predictions in particle physics—have long been known to be constrained by physical principles such as causality and locality; however, the explicit form of these constraints has remained difficult to work out in practice. In this talk, I will describe recent work that allows us to sidestep a number of these historic difficulties, thereby allowing us to derive strong new "genealogical constraints" on the analytic structure of Feynman integrals, which hold to all orders in dimensional regularization. I will further describe how these constraints can be fed into a bootstrap approach, via which the functional form of Feynman integrals can be determined from just knowledge of their singular behaviour.

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06.05.2025

Arnau Bas i Beneito
^{(University of Valencia)}

11.15 - HPT C103

New methods to probe Standard Model Extensions via Proton Decay and Neutrino Masses

Baryon and lepton number are excellent low-energy symmetries of the Standard Model (SM) that tightly constrain the form of its extensions. In this paper we investigate the possibility that these accidental symmetries are violated in the deep UV, in such a way that one multiplet necessary for their violation lives at an intermediate energy scale M above the electroweak scale. We write down the simplest effective operators containing each multiplet that may couple linearly to the SM at the renormalisable level and estimate the dominant contribution of the underlying UV model to the pertinent operators in the SMEFT: the dimension-5 Weinberg operator and the baryon-number-violating operators up to dimension 7. Our results are upper bounds on the scale M for each multiplet–operator pair, derived from neutrino-oscillation data as well as prospective nucleon-decay searches. We also analyse the possibility that both processes are simultaneously explained within a natural UV model. In addition, we advocate that our framework provides a convenient and digestible way of organising the space of UV models that violate these symmetries.

13.05.2025

Maria Ramos
^(CERN)

11.15 - HPT C103

Multiple QCD axions

The QCD axion is a well-motivated candidate for new physics and a primary focus of an ambitious global experimental program. It offers a compelling solution to both the strong CP and the dark matter problems within a narrow region of its parameter space, known as the QCD axion band. Nevertheless, rich theoretical frameworks lead us to expect that the axion is not the only exotic scalar produced in Nature, as it is often produced alongside several axion-like particles (ALPs). In this talk, I will discuss the impact of a non-minimal scalar sector in the properties of the QCD axion and its role in addressing the strong CP and the dark matter problems. I will show that - due to an enlarged misalignment between the flavor and mass basis - the strong CP problem can be solved in a new displaced band, with the displacement determined by the number of ALPs in the theory. This setup can therefore open radically new regions of signal for QCD axions. Motivated by these results, I will present a UV setup that can generate these signals based on extra spacetime dimensions and finally I will also discuss general implications of multiple axions to astrophysical and cosmological probes.

20.05.2025

Miguel Escudero Abenza
^(CERN)

11.15 - HPT C103

CP violation in B Meson Oscillations and its connection with Baryogenesis

The B-Mesogenesis mechanism directly relates the CP violation in neutral B meson oscillations to the baryon asymmetry of the Universe and provides also an explanation of the dark matter of the Universe. In this talk, I will present the essence and experimental status of B-Mesogenesis. In particular, I will discuss a recent systematic analysis of how large could the CP violation in B meson mixing be and highlight its implications for the B-Mesogenesis mechanism.

27.05.2025

Paolo Parotto
^{(University of Turin)}

11.15 - HPT C103

Finite density QCD equation of state: critical point and a lattice-based expansion

In the phase diagram of QCD, it is expected that a critical point separates the low density crossover from a high density first order transition, belonging to the universality class of the three dimensional Ising model. Huge experimental efforts are in place to search for signatures of criticality, which need assistance from theory in the interpretation of the data. In particular, in order to properly simulate the hydrodynamic evolution of the collision systems, a realistic equation of state is necessary. Lattice QCD at finite density suffers from a complex action problem, yet a number of strategies have been devised to extrapolate to as large a chemical potential as possible. I will describe the current knowledge on the QCD equation of state at finite density, and present a procedure to construct a family of parametric equations of state including critical behavior in the correct universality class, that can be readily employed in realistic simulations of heavy ion collisions.

03.06.2025

Rajan Gupta
^{(Los Alamos National Lab)}

11.15 - HPT C103