Discovery Center Annual Report 2017 TABLE OF CONTENTS Introduction 2 News from the atlas group 4 News from the cmb group 7 News from the theory group 8 News from the alice group 11 News from the icecube group 14 Search for hidden particles 16 Defended msc theses in 2017 17 Defended phd theses in 2017 17 Seminars, workshops and courses 18 Discovery center financing 18 Discovery Center people 19 Scientific staff 19 PhD students 20 MSc students 20 Discovery associates 21 Discovery scientific advisory board 21 Visitors 22 Discovery publications 2017 23 INTRODUCTION in the atmosphere, the search for neutrinos from the galactic center and in the understanding instrumental Starting in the neutrino sector, we had a share in the noise. 2017 “Breakthrough in Science” prize for the first multi-messenger observation of a neutron star merg- The elusive neutrino may indeed hold the answers to er. This consolidated the Nobel prize given to LIGO some of the deepest questions we can ask about the in 2017 for the first observation of gravitational Universe. Thus, the phenomenology group discov- waves. Although IceCube actually saw no neutrinos ered a possible deep connection between the model from this merger event, we did look for them and widely believed to be responsible for neutrino mass significantly improved the strategies for searching for generation and the emergence of the Higgs potential. point sources of astrophysical neutrinos, thus raising Also the possibility of an astrophysical abundance of the chances of yet another discovery around the cor- a sterile right-handed neutrino was investigated in a ner. The Discovery IceCube group also made further series of papers and found to be consistent with pres- progress in the search for the appearance of tau neu- ent observations, such as Lyman-alpha forest mea- trinos that were originally created as muon neutrinos surements. 2 | DISCOVERY CENTER FOR PARTICLE PHYSICS In 2017 the LHC machine again delivered an impres- In the opposite end of the energy spectrum, the CMB sive luminosity at 13 TeV. The ATLAS run II data set group has made a reanalysis of the galactic dust emis- now integrates to 80 fb-1 and the analysis of this data sion, an important foreground to the coveted CMB set is progressing well. Among the milestone papers B-mode polarization signal. New data from the Dis- submitted in 2017 were a new measurement of the covery led, Greenland based, experiment, GreenPol, W mass (the first from the LHC), the top quark mass will shed further light on the unresolved features in and various Higgs production and decay channels. foreground emissions, due to the large number of fre- The observed Higgs production rate divided by the quency bands that will be measured. standard model expectation is now 0.99+-0.15 with constitutes an important test of the model. Also many These achievements have been supported by several searches were published, among those a Discovery led new successful grants in 2017: Villum Young Investi- search for di-boson resonances where one boson de- gator and ERC Starting Grant (Jacob Bourjaily), Vil- cays into leptons and the other one into quarks. This lum Starting Grant (Oleg Ruchayskiy), Marie Curie search sets new limits on the existence of new scalar or Fellowship (von Hippel) and SV fellowship (Chris- vector resonances as well as on a spin-2 Kaluza-Klein tian Bierlich). graviton. In the LHC experiments, three Discovery members In the Discovery ALICE group, colliding “small sys- were elected to central management posts: Kris Guld- tems”, pp and pA collisions, attract a lot of attention brandsen (ALICE Run Coordinator), Stefania Xella since these systems appear to exhibit some of the same (ATLAS Trigger Coordinator), Peter Hansen (ATLAS features which in high-energy head-on nucleus-nucle- TRT Project Leader). In addition Mogens Dam was us collisions are interpreted in terms of a quark-gluon appointed Convenor of the Detector Design Group plasma. This is totally unexpected and possible expla- for the CERN Future Circular Collider Study. Finally nations are currently being pursued in close collabo- Jürgen Schukraft, member of the external advisory ration with theorists from the Lund phenomenology board for the Discovery center, was awarded the group. In 2017 we furthermore published the charged Niels Bohr medal of honor. particle distributions from 13 TeV Pb+Pb collisions and took a first look at Xe+Xe collisions. March 2018 Peter Hansen, Director of the Discovery Center ANNUAL REPORT 2016 | 3 NEWS FROM THE ATLAS GROUP In 2017 the LHC accelerator once again brought more than doubling of the total data volume for the ATLAS experiment. The maturing of the vari- ous data analysis has put the challenge of very high beam intensity more into focus, as this makes the cur- rent reconstruction algorithms less efficient. When the experiment started only one proton collision took place at a time. In 2017 the typical number of simultaneous proton collisions is 40-50 and in the future we plan for this number of increase to 200+. We have therefore worked on ensuring the optimal Figure 1: The plot shows how well one can get rid of background (not electrons) while retaining a certain efficiency for selecting sig- performance of the electron identification in the nal (electrons). The red curve represents the current method used Transition Radiation Tracker (TRT), as electrons play in the ATLAS experiment, which is very good but has a hard time coping with the growing number of simultaneous proton colli- a central role in the ATLAS physics program. This sions. The three other curves show the capability of the developed TRT information is an input to the overall ATLAS ML methods, which can in general suppress the background by electron identification, which we have shown can be a factor two while retaining the same amount of signal. In the future, we expect to further improve this result. further improved using Machine Learning (ML). One of the breakthroughs in this achievement was One of the 2017 highlights from the ATLAS col- the invention of a method to train ML algorithms laboration was the notorious W mass measurement, directly on the data, which we expect more of in the which is possibly the most difficult measurement in future. The improvement is quantified in the figure all of particle physics. We have in the Discovery cen- below and will in the future be able to improve the ter contributed to some of the essential ingredients Higgs measurements. to this measurement (see last years report), which has now yielded results. 4 | DISCOVERY CENTER FOR PARTICLE PHYSICS The crucial precision reached in this first W mass measurement from a LHC experiment is 0.019 GeV compared to the W mass itself of 80.385 GeV (1 part in 20000), which rivals the single best measurement ever made. It will be interesting to see, what further understanding and analysis improvements will bring to this measurement, which indirectly probes for new particles (it was through this measurement that the first indications of the Higgs particle and its mass was obtained). No other LHC experiments have pub- lished a W mass measurement yet. Figure 2: The upper plot shows the distribution of electron trans- verse momenta (pT) from the W decay. The typical pT is half of the W mass, and this distribution is thus sensitive to the W mass. If the W mass had been different, the simulated W mass (histo- gram) would have to have been different in order for it to the match data (black points). At the bottom, the ratio between the data and the simulation is shown, which proves that we understand the data to a precision better than 1%. The meas- urement of the W mass from this specific decay (to electrons) and observable (electron pT) yields a value of 80.347 GeV. The lower plot shows all the W mass measurement when subdi- vided according to decay, observable, and charge. The measure- ment from the upper plot is represented as a blue bar 7th row from the top. ANNUAL REPORT 2016 | 5 We also contribute to the ALFA detector, which measures the total cross section that is the prob- ability for protons to collide (crucial for the AT- LAS experiment), and a parameter called rho, which can be used to determine what cross section to ex- pect at (much) higher collision energies (important for other experiments). The analysis of the special LHC runs for these measurements are challenging and the Discovery center plays a central role in this. Last but not least, we participate strongly in the Phase Figure 3: The plot shows what a future circular e+e- accelerator (FCC-ee in green) will be able to explore in terms of sterile neutri- I and II upgrade of the ATLAS detector, as well as nos, which is one of the possible candidates for dark matter. On in the development of potential future accelerators the x-axes is the mass of such a particle and on the y-axis how much it interacts. In full are shown the limits from previous and and experiments. One of the most promising can- current experiments (top) and theoretical and cosmological con- didates is a circular accelerator with electrons col- siderations (bottom). liding, which will be able to explore very widely, not the least the possible origins of dark matter. As can be seen in the below figure, a Future Circular Collider with e+ e- collision (FCC-ee) will be able to exclude a huge portion of the parameter space for a sterile neutrino, especially in conjunction with the proposed SHIP experiment. The Discovery Center is working on the development of both these potential future experiments.