Seminare WS 21/22

Di. 26.10.2021, 16:30 (Online)

Tristan du Pree (Nikhef)

Search for the Higgs boson decay to charm quarks at the ATLAS experiment



Since the discovery of the Higgs boson by the ATLAS and CMS experiments, almost a decade ago, its couplings to massive vector bosons and third-generation fermions have been firmly established. Also, the first evidence of the Higgs boson coupling to muons now starts to appear at the LHC. However, the couplings to lighter quarks are still very much unconstrained.
In this seminar, I will present the most recent search for the Higgs boson decay to charm quarks by the ATLAS collaboration, using the full LHC Run-2 dataset. To study the coupling to quarks of the second generation, we use dedicated flavour tagging algorithms to identify jets originating from beauty, charm, and lighter quarks. We categorise the events according to the number of jets, charm quarks, and leptons, and perform a combined fit to all data in 44 signal and control regions. As a validation of our analysis method, we measure diboson production with heavy vector boson decays to one or two charm quarks and we observe significances for these processes of 3.8 and 2.6 standard deviations. We don't observe Higgs boson decays to charm quarks and exclude enhanced signal strengths greater than 26 times the predicted value. For the first time, we place a direct constraint on the Higgs-charm coupling, at most 8.5 times the interaction strength in the Standard Model.
I'll end the seminar with an overview of complementary methods to probe the Higgs-charm coupling, a comparison with other experiments, as well as the future prospects for the LHC and beyond.


Host: Alexander Schmidt



Di. 23.11.2021, 16:30 (Online)

Juan Rojo (Vrije Universiteit Amsterdam & Nikhef)

The Path to Proton Structure at One-Percent Accuracy



In this talk I will review recent progress in our understanding of proton structure, as quantified by the parton distribution functions (PDFs). This progress not only enables the precision phenomenology program of the (HL)-LHC, but also makes possible addressing key open questions in QCD from the origin of nucleon mass and spin to the antimatter and heavy quark content of the nucleon. In particular I will present the recent NNPDF4.0 determination, a state-of-the-art global QCD analysis based on machine learning techniques and combining the information provided by almost 100 independent datasets. I will also discuss how the software framework used to produce NNPDF4.0 is made available as an open-source package together with documentation and examples. I will review the implications of NNPDF4.0 for important LHC processes, as well as for key topics in nucleon structure from strangeness and the large-x gluon to intrinsic charm.


Host: Malgorzata Worek



Di. 07.12.2021 16:30

Gordon Watts (University of Washington)

New Computing Paradigms for HEP Analysis



The physics program for the High Luminosity run of the Large Hadron Collider is ambitious. A combination of x10 more luminosity, x10 more data, and changes in the shape of modern day computing are forcing a revolution through out the field.  In this talk I'll discuss some of the forces that are prompting this rethinking and some of the projects that are leading the change. I'll concentrate on the ROOT and python ecosystems, the two most prevalent in particle physics, talking a bit about  hardware advances, Machine Learning, Analysis, and a bit about preservation and open data, time permitting.


Host: Dennis Noll



Di. 14.12.2021 16:30 (Online)

Michele Lupattelli (RWTH Aachen University)

ttbb at the LHC: on the size of corrections and b-jet definitions



Since the discovery of the Higgs boson in 2012, its properties have been widely tested. The coupling of the Higgs boson to the heaviest of the quarks, the top-quark, has been probed first indirectly and then directly, in the ttH process, first observed in 2018. The main decay channel of the Higgs boson is the H->bb, but ttH(bb) suffers from a significant background because of the large numbers of b-jets in the final state. In this Colloquium I will talk about one of the main backgrounds, ttbb, presenting the latest experimental results and discussing the theoretical modeling, with a focus on the latest theoretical predictions.


Host: Malgorzata Worek



Dienstag, 21.12.2021, 16.30 (Online)

Sebastian Wuchterl (DESY)

Experimental aspects of probing the top quark mass



The top quark is the most massive elementary particle known. Its mass is a fundamental parameter of the Standard Model (SM), and its value needs to be determined experimentally. From the theoretical point of view, the top quark mass cannot be defined uniquely, while experimentally, the usage of multi-purpose event generators complicates the interpretation of the results of direct top mass measurements. The attempt of shedding light on this issue led to the development of complementary theoretical and experimental techniques to measure this parameter. The value and precision of the top quark mass are of particular importance since in the SM the masses of the top quark, the W boson, and the Higgs boson are closely related. Further, the value and the uncertainty of the top quark mass are driving predictions for the energy dependence of the Higgs quartic coupling, which determines the stability of the electroweak vacuum. Currently, the most precise measurements of the top quark mass are achieved at the Large Hadron Collider (LHC) in proton-proton collisions by the ATLAS and CMS collaborations.Processes like top quark-antiquark pair or single top quark production can be used, for instance, to extract its value in different renormalization schemes or directly measure it by reconstruction of the final state particles. In this seminar, I will show different ways to measure the top quark mass and present recent experimental results. Their benefits and drawbacks will be reviewed, and comparisons to theoretical predictions will be shown. Finally, I will discuss prospects for future measurements at the LHC and lepton colliders.



Host: Michal Czakon



Di. 18.01.2022 16:30 (Online)

Xavier Coubez (Brown University and RWTH Aachen University)

Jet tagging techniques


Abstract: tbd

Host: Alexander Schmidt




Di. 25.01.2022 16:30 (Online)

Dennis Noll

Status & Prospects of Higgs Pair Production Searches at the LHC



This year marks the 10th anniversary of the Higgs boson discovery. In
the recent years, many properties of the Higgs boson have been
successfully measured. Yet some central building blocks are still
hidden: How large is the Higgs-self coupling? What is the shape of the
Higgs potential? Is it connected to electroweak baryogenesis or even
cosmic inflation? These and other open questions can be studied in LHC
collisions with two Higgs bosons in the final state. I will give an
overview of the ongoing searches and present their latest results.
Finally, we will take a look into the future and see which strategy to
take along this exciting journey.


Host: Martin Erdmann



Postponed to SS2022 - 16:30 (Online)

Urs Wiedemann (CERN)

Heavy Ion Collisions at the LHC


Abstract: tbd


Host: Robert Harlander