Dr Kristin Lohwasser

Photo of Kristin Lohwasser

Research Fellow

Contact details

Group web pages

Particle Physics and Particle Astrophysics Group (PPPA)

Particle Physics and Particle Astrophysics Research Group - ATLAS

General

As a researcher at the University of Sheffield I am investigating fundamental particles and their interactions as a member of the ATLAS collaboration. With an ERC starting grant, I am currently building up a small research group.

I completed my diploma studies successfully at the University of Dortmund with a thesis on the efficiency of a fast track trigger at the H1 experiment to select D-star bosons for the measurement of quark-gluon fusion, sensitive to the charm content of the proton. During my studies, I spent half a year in Prague to study Astrophysics at the Charles University. In addititon to my diploma in physics, I completed a degree in journalism, including a one-year internship at a newspaper, with a thesis on a science popularisation in museum for mathematics.

After my undergraduate studies, I read Particle Physics in Oxford and completed a DPhil as a member of the ATLAS Collaboration. The topic of my thesis were studies on directly reconstructing the W asummetry at the LHC and using this measurement to constrain the parton distribution functions. I further went to work as post-doctoral researcher at Freiburg University in Germany and at the DESY laboratory in Zeuthen, near Berlin.

Research Summary

Particle physics attempts to describe the building blocks of matter and the fundamental forces between them. Our current understanding is encoded in the Standard Model (SM) theory of particle physics which combines our description of quantum chromodynamics (QCD) and electroweak (EWK) theory. The SM allows us to make very precise predictions that have been confronted with experimental measurements at high-energy colliders over the past century – with incredible success. The last missing piece of the SM was the experimental proof of the actual existence of the Higgs boson that had been hypothesised for more than 50 years. Despite this obvious success of the SM, there are however flaws in the theory: There are observations in astrophysics and cosmology that the SM does not account for. Thus the pressing goal of particle physics today is to go beyond the SM so as to explain these contradictory observations. In order to do so, particle physicists strive to discover new physics phenomena.

A technique exploited by a majority of physicists at the current LHC experiments to search for new physics phenomena are direct searches for new physics, testing very specific theories which only cover parts of possible new physics models. An alternative powerful strategy to discover new physics is precision measurements of well-known processes. This requires a profound knowledge of the underlying theory and also a very good understanding of the detector performance – which is both extremely challenging but at the same time extremely rewarding in terms of possible advancement of knowledge of particle physics. Since so far no other new phenomena have been directly discovered at the LHC, it is paramount to pursue precision measurements with sensitivity to new phenomena.

One class of processes, suited to do so, is the production of pairs of bosons at the LHC, in particular of W and Z bosons. These fundamental particles are the “force carriers” of the SM, mediating how the other particles of the SM interact with one another. The interactions between multiple W and Z bosons are particularly delicate towards the influences of new physics phenomena at higher scales. These can be introduced based on effective field theory (EFT). New physics will manifest itself in these additional degrees of freedom, so called higher-dimensional operators which will modify how pairs of W and Z bosons are produced in high-energy collisions at the LHC. Measuring the pair production of W bosons, also called diboson production processes, is therefore a window towards new phenomena at higher energy scales.

Research Topics

1. Measurements of the production of pairs of W bosons and ATGC studies and extractions

Pairs of W bosons can be produced at the LHC either via proton-proton collisions or through the interaction of two photons or two W bosons that were emitted by protons. Measurements of these processes and a combined interpretation can achieve the best possible sensitivity to new physics models at the highest energy scales.

2. Improvements of Electron reconstruction and identification

A number of precision measurements and as well as searches for new physics rely crucially on the capability of the ATLAS detector to detect and accurately measure electrons. I have been involved in the efforts to develop reliable measurements of the electron selections efficiencies and to prepare the Run-2 data taking by optimizing the identification criteria for electrons.

3. Measurement of single boson production and Parton distribution functions (PDFs)

Reliable and accurate SM cross section predictions are needed to confront the data to extract hints for new physics scenarios at proton colliders. These are a convolution of the partonic cross sections and the PDFs. I have been worked on a number of precision measurements and used them to constrain the parton distributions from these measurements.

4. Upgrade of the ATLAS detector

The Upgrade of the ATLAS detector will allow to gather the data necessary to measure processes with a low cross-section. The ITk projects targets the upgrade of the current ATLAS inner tracking detector. My main focus was the testing of alternative sensor designs, which could also benefit further future experiments.

Outreach

High energy physics (HEP) is concerned with some of the most fundamental questions imaginable. It has become a popular topic with movies and TV series. Evoking general interest in science is a good thing, but we have much more responsibility as scientists. Studies show, that performance depends crucial on the locus of control - the expectancy of how able one is to master a specific task though dedicated effort (as opposed to elusive “talent”). Scientific thinking and logical reasoning rank high amongst those tasks, that people often do not believe they possess. One of our goals should be to foster the idea, that people can understand the basic ideas and principles of (particle) physics. Making people believe they are capable of logical thinking is indeed in the truest sense of the word enlightenment.

Sharing the fascination for physics through teaching or outreach activities is a great pleasure to me. In Oxford, I organised a cinema screening followed by science discussion. I was the local outreach coordinator in Freiburg and coordinated events in schools, more long-term research projects for students and further training for teachers. At DESY, I continued to participate in the school events and other outreach events such as the “Open days” and “Science fairs''.

I was involved in a project for the further educating secondary school teachers in Malawi that used to be part of the Institute of Physics “IOP for Africa'' programme, where I was one of the international coordinators. We have built up a laboratory at a school in the rural Nkhata Bay district, which serves as resource centre for schools in the region by loaning out equipment and providing training courses.

Achievements and Publications
  • 2016 -- ERC Starting Grant
  • 2015 -- USA Travel Grant -- Programme des Projektbezogenen Personenaustauschs (PPP) with University of Pennsylvania
  • 2009 -- DAAD Scholarship
  • 2006-2008 -- Michael Foster Memorial Scholarship, Oxford University
  • 2016-2017 -- ATLAS-Convenor of the Electroweak Working group
  • 2015-2016 -- ATLAS-Convenor of the Parton Distribution Functions Forum
  • 2012-2015 -- ATLAS-Convenor for electron efficiency measurements

Major Publication

  • K. Lohwasser, J. Ferrando, and C. Issever. -- On direct measurement of the W production charge asymmetry at the LHC -- JHEP, 09:079, 2010.
  • ATLAS Collaboration -- Measurement of the W -> l nu and Z/gamma -> ll production cross sections in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector -- JHEP, 12:001, 2010.
  • ATLAS Collaboration -- Measurement of the production of a W boson in association with a charm quark in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector -- JHEP, 1405:068, 2014.
  • ATLAS Collaboration -- Electron reconstruction and identification efficiency measurements with the ATLAS detector using the 2011 LHC proton-proton collision data -- Eur. Phys. J., C74:2941, 2014.
  • S. Alekhin et al. -- HERAFitter, Open Source QCD Fit Project -- Eur.Phys.J. C75 (2015) no.7, 304.
  • ATLAS Collaboration -- Measurement of the WW production cross section in proton-proton collisions at sqrt(s) = 8 TeV with the ATLAS detector -- JHEP 09 (2016) 029.
  • ATLAS Collaboration -- Measurement of W+W- production in association with one jet in proton-proton collisions at √s=8 TeV with the ATLAS detector -- Phys. Lett. B 763 (2016) 114.
  • L. Poley, A. Blue et. al. -- Characterisation of strip silicon detectors for the ATLAS Phase-II Upgrade with a micro-focused X-ray beam, JINST 11 (2016) no.07, P07023.
  • R. Mori et al. -- Long term performance stability of silicon sensors -- Nucl.Instrum.Meth. A796 (2015) 131-135.