ANNULLATO Hunting the Higgs boson at high energy using the ATLAS detector at the LHC @ Aula 500
Apr 30@2:30 pm–3:30 pm

Seminario di Fenomenologia delle Particelle Elementari
Dr. Francesco di Bello (University of Geneva)
Hunting the Higgs boson at high energy using the ATLAS detector at the LHC

With the discovery of the Higgs boson in 2012, an extensive physics program has been carried out by the ATLAS experiment to characterize its properties. Among them, one of the most important is the coupling between the Higgs boson and the fermion, known as Yukawa coupling. Of particular interest is the coupling with the b-quark, the heaviest lepton accessible in the direct decay of the Higgs boson and therefore the one with highest branching ratio. Due to the overwhelming QCD background, an inclusive search for H->bbar has tought to be impossible. However, with the development of theoretical and experimental techniques for boosted object, it was possible to find a strategy to probe the Higgs boson at high transverse momentum. During the seminar, I will discuss an inclusive search for the standard model Higgs boson produced with high transverse momentum decaying to a bottom-antibottom quark pair using a data-set of pp collisions at a center-of-mass energy of 13 TeV collected with the ATLAS experiment at the LHC. The data sample corresponds to an integrated luminosity of 80.5 fb−1. This is the first time the analysis is ever been performed within the ATLAS experiment. The analysis also provided a measurement of W/Z+jets processes in boosted regime with b-quarks final states observed at a significance of 5 standard deviation. A small excess, compatible at 1.6 sigma with the background-only hypothesis, is observed at 125 GeV. In addition, exclusions limits for dark matter resonant model are placed. The seminar will give an overview of the analysis along with its future

ANNULLATO Impact of small-x resummation on HERA data @ A500
Mag 7@1:45 pm–2:45 pm

Seminario di Fenomenologia delle Particelle Elementari

Dr. Francesco Giuli (Roma Due)
Fits to the final combined HERA deep-inelastic scattering cross-section data within the conventional DGLAP framework of QCD have shown some tension at low-x and low-Q2. A resolution of this tension incorporating ln(1/x)-resummation into the HERAPDF fits is investigated using the xFitter program. Such resummation not only gives a better description of the data, but it also results in a gluon PDF which is steeply rising at low x for low scales, contrary to the fixed-order result. Then, a new parametrization for PDFs designed to be flexible in the small-x region has been studied and new fits to HERA data have been performed. The new parametrization leads to a better fit quality, both at fixed order and at resummed level, and thus removes a signficant bias present in the previously used parametrizations. The benefits of including small-x resummation remains signficant also with the new more flexible parametrization, thus giving more robustness to the previous results.
martedì 7 maggio 2019
Ore 13:45, aula A500

ANNULLATO Hot TOP(phys)ics @ A500
Mag 13@2:30 pm–3:30 pm

Seminario di Fenomenologia delle Particelle Elementari

Dr. Yvonne Peters (University of Manchester)

The heaviest known elementary particle, the top quark, provides an interesting area of research. Its high mass and short lifetime make it to be of special interest in the hunt for physics beyond the standard model. In my talk I will present a potpourri of recent analyses in the top sector, using LHC proton-proton collision data at 13TeV centre-of-mass energy, recorded with the ATLAS detector. The presented analyses are a new result on top quark spin correlations, a study of QCD colour-flow in top events, and the connection of the top to the Higgs boson.

Condensed Krypton Source and inter-spectrometer Penning trap at the KATRIN experiment @ Aula 500
Giu 26@3:00 pm–4:00 pm

M. Fedkevych, University of Münster, the KATRIN collaboration

The KArlsruhe TRItium Neutrino experiment (KATRIN) is a direct low-background measurement of the neutrino
mass from the kinematics of tritium -decay with an intended sensitivity of 0.2 eV=c2 (90 % C.L.). It uses
a tandem of two electrostatic spectrometers of MAC-E filter type, called pre- and main spectrometers, to analyze
energies of -electrons generated in WGTS (windowless gaseous tritium source). To achieve the sensitivity
goal, background minimization, as well as accurate energy calibration, monitoring and precise determination of
transmission function of the main spectrometer are required.
After an introduction of the current status of the KATRIN experiment I will present two topics of my own
work in more details.
In the first part of my talk, I will report about elimination of one of the important background sources, the
inter-spectrometer Penning trap. The trap is created by the negative retarding potentials of the spectrometers
combined with the magnetic field produced by a common superconducting magnet. Even at the ultra-high
vacuum conditions of KATRIN electrons may get trapped in this Penning trap creating additional background.
They could even produce discharges which may interrupt the data-taking process and damage parts of the
spectrometer and detector section of KATRIN. As a countermeasure, electron catchers were implemented in the
beamline part between the two spectrometers to remove trapped electrons. The system was tested at various
pressure conditions and showed its effectiveness for suppression of the Penning trap effects. In this talk I will
explain details of the measurements and experimental results.
In the second part, I will present the Condensed Krypton calibration Source (CKrS) developed in Münster,
one of the several calibration sources used in KATRIN, which utilizes the nearly monoenergetic conversion
electrons from cryo-adsorbed 83mKr. The CKrS can be used for frequent measurements due to its relative
simplicity; moreover, as a point-like source it allows for per-pixel calibration of KATRIN focal plane detector
(FPD), with comparatively high rates. The cleanliness of the substrate together with quality of frozen radioactive
films being crucial for the stability and reproducibility of the conversion electron spectrum are monitored by
means of laser ellipsometry. The CKrS was installed in 2017 at the KATRIN cryogenic pumping section (CPS).
Here, I will present the characterization measurements with the CKrS at different vacuum conditions (before
and after bake-out of the system) and will discuss analysis and interpretation of the stability, spectroscopy and
ellipsometry data with different krypton films.

Search for light dark matter and new physics with NA64: new results and prospects @ Aula 500
Set 3@3:00 pm–4:00 pm

Dr. Sergei Gninenko (CERN)
What makes up most of the Universe mass? – is the central question of modern science. A possible answer to this question is: It is thermal dark matter (DM) with mass below the electroweak scale, m_DM << 100 GeV. The NA64 experiment is a fixed-target experiment designed to search for light dark matter and sub-GeV New Physics in missing energy events from high-energy electron, muon and hadron interactions in an active target at the CERN SPS. In this talk, I will discuss the motivation for light dark matter, describe in details the NA64 concept and performance. I will also present the latest NA64 results and discuss projections of the expected sensitivities
from running after LHC long shutdown 2 and beyond.