Seminars WS 20/21

Tue 09.02.2021, 16.30 h (Zoom)

Mikael Kuusela (Department of Statistics and Data Science, Carnegie Mellon University)
Unfolding in High Energy Physics: A Statistician's Perspective



Differential cross section measurements in particle physics experiments are smeared by the finite resolution of the particle detectors. Using the smeared observations to infer the true particle-level spectrum is an ill-posed inverse problem, typically referred to as unfolding or unsmearing. The defining feature of this problem is that it is easy to go from the particle-level spectrum to the smeared observations but the inverse direction of inferring the quantity of interest based on the smeared data tends to produce highly unstable solutions. It is customary to address this using regularization which reduces the variance of the estimates at the expense of increased bias. While this can lead to well-behaved point estimates, it is extremely challenging to provide rigorous frequentist uncertainties for the regularized estimates. In this talk, I will first give an overview of the statistical techniques that are commonly used for regularized unfolding at the LHC. I will then demonstrate that some of these methods may seriously underestimate the statistical uncertainty and will explain why that is the case. I will then describe approaches that can be used to obtain improved frequentist uncertainty quantification in unfolding. I will argue that the key is to avoid explicit regularization and instead infer functionals of the unknown spectrum, such as integrals over wide histogram bins, that implicitly regularize the problem.


Host: Lutz Feld




Tue 02.02.2021, 16.30 h (Zoom)

Thomas Becher (University of Bern)

Effective Theory for Jet Processes


Since its early days Soft-Collinear Effective Theory (SCET) has been advertised as an effective theory for jet processes, but the observables that have been resummed using this framework are so far mostly hadronically inclusive cross sections. As an example, I’ll discuss transverse momentum resummation and present precise results for the spectra of both single and multiple electroweak bosons. I’ll then turn to jet cross sections and explain why their resummation is much more involved. As an example of a resummed jet observable, I’ll present results for top production with a central jet veto.


Host: Michal Czakon




Tue 26.01.2021, 16.30 h (Zoom)

Maria Ubiali (University of Cambridge)

New frontiers in the determination of the proton structure



The parametrization of the proton's structure via the determination of Parton Distribution Functions (PDFs) is a crucial theoretical input at the LHC. The PDF uncertainty is often a limiting factor in the accuracy of theoretical predictions. At the same time the LHC is delivering a number of precise measurements that provide precious information on the proton's structure.  In this seminar, I will give a broad overview on the theory behind and on the state-of-the-art of PDF determinations. I will then mention the new challenges that modern fits of PDFs face, due to the unprecedented precision of the LHC data.


Host: Malgorzata Worek




Tue 19.01.2021, 16.30 h (Zoom)

Merijn van de Klundert (DESY)

Analysis of the CP structure of the Yukawa coupling between the Higgs boson and τ leptons in proton-proton collisions at 13 TeV



The Higgs boson constitutes a pivotal particle in the Standard Model of
particle physics. It was discovered in 2012 by the ATLAS and CMS
experiments, and since then its characteristics have been reviewed by
both experiments.
This presentation focuses on the CP-structure of the couplings of the
Higgs boson to fermions and gauge bosons. After introducing the topic,
an overview is provided on the state-of-the-art results on Higgs-CP
measurements at the LHC from both the ATLAS and CMS experiment. The
recent CMS result on the CP-structure of the Higgs-tau Yukawa coupling.


Host: Werner Bernreuther




Tue 15.12.2020, 16.30 h (Zoom)

Thorben Quast (CERN)

CMS HGCAL Overview



The CMS collaboration is preparing to build replacement endcap
calorimeters for the HL-LHC era. The new calorimeter endcap will be a
highly-granular sampling calorimeter (HGCAL) featuring unprecedented
transverse and longitudinal readout segmentation for both its
electromagnetic and hadronic compartments. The granularity together with
a foreseen timing precision on the order of a few tens of picoseconds
will allow for measuring the fine structure of showers, will enhance
pileup rejection and particle identification, whilst still achieving
good energy resolution. The regions exposed to higher-radiation levels
will use silicon as active detector material. The lower-radiation
environment will be instrumented with scintillator tiles with on-tile
SiPM readout. In addition to the hardware aspects, the reconstruction of
signals, both online for triggering and offline, represents a
challenging task - one where modern machine learning approaches are well
In this talk, the reasoning and ideas behind the HGCAL, the
proof-of-concept of its design in test beam experiments, and the
challenges ahead will be presented.


Host: Martin Erdmann 




Tue 08.12.2020, 16.30 h (Zoom)

Giulio Settanta (Forschungzentrum Jülich)

First detection of solar neutrinos from the CNO cycle with the Borexino detector



Neutrinos are elementary particles which are known since many years as fundamental messengers from the interior of the Sun. The Standard Solar Model, which gives a theoretical description of all nuclear processes which happen in our star, predicts that roughly 99% of the energy produced is coming from a series of processes known as the “pp-chain”. Such processes have been studied in detail over the last years by means of neutrinos, thanks also to the important measurements provided by the Borexino experiment. The remaining 1% is instead predicted to come from a separate loop-process, known as the “CNO cycle”. This sub-dominant process is theoretically well understood, but has so far escaped any direct observation. Another fundamental aspect is that the CNO cycle is indeed the main nuclear engine in stars more massive than the Sun.

In 2020, thanks to the unprecedented radio-purity and temperature control achieved by the Borexino detector over recent years, the first ever detection of neutrinos from the CNO cycle has been finally announced. The milestone result confirms the existence of this nuclear fusion process in our Universe.  Here, the details of the detector stabilization and the analysis techniques adopted are reported. Final results are discussed, together with the implications for solar physics and astrophysics.


Host: Livia Ludhova




Tue 01.12.2020, 16.30 h (Zoom)

Christoph Weniger (University of Amsterdam) 

Turbocharging dark matter research with simulator-based inference and differentiable programming



Dark matter constitutes the majority of mass in the Universe, but still its nature remains unknown. One way to study the mass of dark matter particles is to analyse the distribution and clumping of dark matter at small (sub-galactic) scales. Information about this small scale structure can be obtained from the analyses of gravitationally strongly lensed images of distant galaxies. However, due to the complexity of galaxy images and degeneracies between lens and source variations, such images are notoriously difficult to analyse. I will present a new fast, flexible and powerful analysis pipeline of strong lensing images that brings together a wide range of modern machine learning and differentiable programming methodologies, and that makes strong lensing image analysis fit for upcoming observations.


Host: Felix Kahlhoefer




Tue 10.11.2020, 16.30 h (Zoom)

Joachim Kopp (University of Mainz & CERN)

Neutrino Physics: Status and Prospects 



With the planned DUNE, Hyper-Kamiokande, and JUNO detectors, as well as
numerous smaller experiments, neutrino physics has become one of the
flagship disciplines of particles physics. In this colloquium, we review
some of the most exciting current and future developments in neutrino
physics. We begin by outlining the physics program of current and future
long-baseline neutrino oscillation experiments. We then discuss the
potential implication of various anomalies observed in neutrino
oscillation searches on particle physics and cosmology. Finally, we
highlight some of the interconnections between neutrinos and dark matter.


Host: Dr. Malgorzata Worek