Seminare SS 20

Di. 14.04.2020 16:30 (Zoom)

Juliette Alimena (Ohio State University)

Searches for Long-Lived Particles at the LHC

Particles with large lifetimes are an important possibility in the search for new phenomena, especially in the search for the particle nature of dark matter. Conventional searches at the CERN LHC target promptly decaying particles, but standard model (SM) particle lifetimes span a very wide range, and long lifetimes can generically appear in scenarios beyond the standard model (BSM). In fact, long-lived particles (LLPs) are predicted by many not yet excluded BSM scenarios such as gauge-mediated supersymmetry breaking models, heavy neutral leptons, and dark photons, to name a few. Thus, LLPs have a great potential for discovery. When produced at the LHC, these LLPs have a distinct experimental signature: they can decay far from the primary proton-proton interaction, or even completely pass through a detector, such as ATLAS or CMS, before decaying. After introducing LLP searches at the LHC, I will present a search for the decays of heavy LLPs that come to rest in the CMS detector. Their decays would be visible during periods of time well separated from proton-proton collisions. Two decay scenarios of stopped LLPs will be shown: a hadronic decay detected in the calorimeter and a decay into muons detected in the muon system. I will also present a proposal for a Helmholtz Young Investigator Group, to be based at DESY and in collaboration with RWTH Aachen University.

Host: Lutz Feld


Di. 12.05.2020 16:30 (Zoom)

Alexander Grohsjean (DESY)

Top quark spin: A window to new physics?


High precision measurements of the properties of elementary particles are essential
to understand the Universe at smallest scales. However, they also might give us exciting insights
to the largest scales of the Universe, such as the understanding of Dark Matter, and may provide
an avenue to physics beyond the Standard Model (BSM). The analysis of observables with small
systematic uncertainties in the phase space region of new physics are of particular interest.

In this talk, the first measurement of the full ttbar spin density matrix at the CMS experiment in
proton-proton scattering at the Large Hadron Collider is presented. The general analysis concept and
its application in the quest for BSM physics are highlighted. The measurement provides an important
answer to an anomaly observed at ATLAS in 2018 and is used to constrain anomalous couplings in
an Effective Field Theory framework with greatest sensitivity. It will also be shown how the tt spin
density matrix can be utilized to enhance the sensitivity in searches for new scalar particles and how this
could shed light on the nature of Dark Matter.


Host:Danilo Meuser


Di. 26.05.2020 16:30 (Zoom)

Andreas Hinzmann (Hamburg University)

From raw data to published experimental results at the LHC


The collider experiments at the LHC are among the most complex particle detectors ever built and operated. Likewise, the process of analysing the raw data collected by these experiments to publish results on fundamental interactions of elementary particles is as complex as the detectors themselves. It required algorithm development for more than a decade. This talk gives a basic overview of the most important aspects of data reconstruction, calibration and analysis necessary for the study of fundamental interactions, taking as example the process of analysing raw data collected by the CMS experiment and covering state of the art techniques.

Host: Alexander Schmidt


Di. 09.06.2020 16:30 (Zoom)

Melissa Beekveld (Nikhef / Radboud University Nijmegen)

Power up your logarithm: the large impact of 'subleading’ terms

Abstract: Cross-sections in perturbative QCD are plagued by large corrections from soft and collinear radiation. The most singular terms are known to be universal, which allows their resummation to all orders in the coupling, hereby curing the cross-section of the large uncertainties. In this talk, I will give a broad introduction as to how these large corrections arise, how resummation solves the issue, and whether we can expect the same universal structure for a class of next-to-singular contributions. We also discuss their numerical impact.

Host: Michael Krämer


Di. 23.06.2020 16:30 (Zoom)

Steffen Schumann (Universität Göttingen)


I will discuss concepts and techniques used for the matching and merging of next-to-leading order hard-scattering matrix elements with QCD parton showers in modern Monte Carlo event generators. Furthermore, recent developments to also include next-to-leading order electroweak corrections will be highlighted.

Host: Robert Harlander