Seminars WS 18/19

Tue 23.10.2018, 16.30 h (28B110)

Laura Lopez Honorez (Vrije Universiteit Brussel)

EDGES observations of the 21cm line and possible implications for astroparticle physics

The recent measurement of the 21 cm background signal from the dark ages by the EDGES (Experiment to Detect the Global Epoch of reionization Signature) experiment has come with a surge of interest for 21cm cosmology from both particle physics and astrophysics community.
The main reason for this is the unexpected deepness of the absorption trough around
After a brief introduction to 21cm cosmology, I will explain why such feature is difficult to account for and why it represents a call for new physics. I will go through different new physics cases: annihilating dark matter, primordial black holes and non Cold dark matter and discuss why they can not help to reproduce the observed absorption trough. As a result interesting constraints can be derived in some of these scenarios.


Tue 30.10.2018, 16.30 h (28B110)

Thomas Kuhr (LMU Munich)

Flavour physics at Belle 2

The standard model of particle physics is a very successful theory, but can only be an approximation of a more general theory. A complementary approach to the direct search for physics beyond the standard model via the production of new particles in high-energy collisions is the precise measurement of processes and the comparison to theoretical predictions. This is the approach of the Belle II experiment at the SuperKEKB e+e- collider in Tsukuba, Japan. It will collect a sample of events with entangled B meson pairs, charm hadrons, and tau leptons 50 times larger than its predecessor Belle and offer unique opportunities for flavor physics measurements. After the successful commissioning run this year, Belle II will start its physics run early 2019. The status of the experiment and the physics program will be presented.


Tue 13.11.2018, 16.30 h (28B110)

Andrea Pocar (UMass Amherst)

Searching for neutrinoless double beta decay with EXO-200 and nEXO

Neutrinoless double beta (0νββ) decay is a process in which a nucleus (A,Z) decays to (A,Z+2) with the emission of two electrons (but no neutrinos).  Experimental searches for such a decay are the most sensitive test of lepton-number conservation and its discovery would unambiguously prove the Majorana nature of neutrinos, with profound implications for cosmology in addition to particle and nuclear physics. This process is also a sensitive probe of the absolute neutrino mass scale. EXO (Enriched Xenon Observatory) is an experimental program searching for 0νββ decay of 136Xe. The first phase of the program, EXO-200, uses 200 kg of Xenon enriched to 80% in 136Xe, liquefied in a Time Projection Chamber (TPC) with scintillation readout (110 kg active mass), allowing for event calorimetry and 3D localization of ionizing events. EXO-200 has discovered the standard two-neutrino decay mode (2νββ) of 136Xe and holds one of the leading sensitivities for 0νββ decay, at 3.4x10^25 years. A next generation experiment, nEXO, is proposed and in advanced design phase. nEXO is a 5-tonne liquid xenon TPC with a sensitivity to the 0νββ decay half-life of 136Xe of ~10^28 years. It builds on the EXO-200 experience while introducing novel technical solutions. This talk will highlight theresults from EXO-200, and illustrate the recent progress in the R&D program for nEXO.


Tue 20.11.2018, 16.30 h (28B110)

Luca Mastrolorenzo (RWTH Aachen/CERN)

Observation of the Higgs boson decay to bottom quarks with the CMS experiment

The first observation of the Higgs boson decay into a bottom and anti bottom quarks by the ATLAS and CMS Collaboration in summer 2018 represents a discovery of major importance towards the characterization of the Yukawa couplings. Even though this decay channel is the one with the highest predicted branching fraction, the detection of such events is extremely challenging at a hadron collider environment because the overwhelming production of hadronic jets from QCD events. In order to maximise the sensitivity to the H->bb signal sophisticated analysis techniques have been deployed, relying on the exploitation of Deep Neural Networks along the whole analysis chain. In this seminar, the CMS observation of the H->bb decay will be presented focussing on the analysis of the data collected by the experiment during the 2017 Run of the LHC and targeting the identification of events where a Higgs boson is produced in association with a vector boson.


Tue 04.12.2018, 16.30 h (28B110)

Andrei Puiu (Milano Bicocca)

Holmes, the challenge of measuring the neutrino mass

Measuring the neutrino mass is one of the most compelling challenges of modern physics. Neutrinos are the second most abundant particle in the universe, yet one of the most mysterious. Since their existance was postulated in 1930 by Pauli, several experimental efforts have been made to detect them at first and subsequently, to a better understanding of the neutrino properties and nature. Today, thanks to even more precise oscillation measurements we have established that neutrinos have a very tiny, yet larger than zero mass.  Several experiments have been performed and are being set-up in order to assess this important issue, which not only could shed light on some limits of the Standard Model of Elementary Particles, but on the history
and the structure of the universe as well. The direct, or kinematic, measurement of the neutrino mass is a unique tool, given that no complex theoretical model is necessary to probe the mass of the neutrino. Many experimental challenges are to be faced though, so experimentalists and theoretcians across the world are working on solving new issues in order to push the limit on the neutrino mass lower and lower, and possibly measuring this important parameter soon. I will try to give an overview of the current status of the direct measurements of the neutrino mass, with special care devoted to the spectrometric and calorimetric approach, where KATRIN, ECHO and HOLMES are operating.


Tue 11.12.2018, 16.30 h (28B110)

Christian Hoelbling (Wuppertal)

Anatomy of the proton mass

The origin of the mass of nucleons is a prime example where quantitative and qualitative insight into genuinely non-perturbative QCD phenomena is obtained through lattice calculations. I will present the lattice calculation of the nucleon and other baryon masses and the decomposition of its origin into the contributions from three sectors of the standard model: QCD, electromagnetic and Higgs. I will use these results to show what lattice calculations can achieve and where their limitations lie.


Tue 22.01.2019, 16.30 h (28B110)

Babette Döbrich (CERN)

ALP and Axion searches with NA62 and RADES

Pseudoscalar QCD axions and axion-like Particles (ALPs) are an excellent candidate for Dark Matter or can act as a mediator particle for Dark Matter. A look at the allowed axion/ALP parameter space makes it clear that these might exist at low mass (below few eV), as (part of) Dark Matter. Alternatively they might exist at higher mass, above roughly the MeV scale, potentially as a Dark Matter mediator particle. In this talk I will present two very different CERN experiments that have an ongoing search program for low-and high-mass axions and ALPs, describe their reach and put them into context of global efforts for searches for new pseudoscalars.


Tue 29.01.2019, 16.30 h (28B110)

José Espinosa (Barcelona, IFAE)

Implications and Signatures of a Metastable Electroweak Vacuum

The Standard Model electroweak vacuum lies very close to the boundary between stability and metastability, with the last option being the most likely. This can have important cosmological implications that I will discuss but relies on physics at scales beyond 1011 GeV, which makes the scenario difficult to test. Nevertheless, two tantalizing cosmological signatures of such instability could arise if Higgs fluctuations during inflation probe the instability region: 1) Such fluctuations can serve as seeds for dark matter in the form of primordial black holes and 2) they can source gravitational waves observable at future facilities.