Every year the CMS Thesis Award Committee recognizes and rewards excellence in the CMS PhD student research. As highlighted by Andrea Giammanco, the Award Committee Chair,  “The quality of theses submitted for this award is increasing year after year, making harder the decision of the committee that selects the winners.”
Doing a PhD at CERN is a challenging experience requiring, among others, hard motivation, starved curiosity and large open-mindness. For these reasons all CMS PhD students should deserve a special mention. However, despite the difficulties, the committee recognizes one (in special cases more than one) PhD thesis that stands out in terms of the quality of content, originality, clarity of writing, and impact within CMS.

This years’ awardees are telling us about their research, career paths and their life as a PHD student.

Daniel Salerno, Universität Zürich

I was born and raised in Australia where I completed a double bachelor degree at the University of Melbourne. During this time I did a one-semester exchange in St. Louis, USA and a one-semester exchange in Bologna, Italy. After the exchange, I knew I wanted to live, at least for a few years, in Italy. I worked for three years in corporate finance but then I decided to return to science and took the opportunity to move to Rome where I completed my masters in Physics. After this, I wanted to move to Zurich to be with my fiancée and I was lucky enough to get a PhD position at the University of Zurich working on the CMS experiment.

The main task of my PhD research was to find the Higgs boson when it is produced together with a pair of top quarks, a process known as “ttH production”. The three particles decay very rapidly into new particles which live long enough to be detected in the CMS detector. I was looking for a very particular decay mode in which the Higgs boson decays to a pair of bottom quarks and both top quarks decay to a bottom quark and a W boson, which in turn decays to two light-flavour quarks. These eight quarks all hadronise in the detector and can be seen as bunches of several charged and uncharged particles, which are called jets. My task was therefore to look for collisions that produce eight jets that could be consistent with those coming from ttH production. After sorting through billions of collisions (called events) for interesting particles I then had to analyse millions of remaining events in more detail. My research contributed to the overall search for ttH production, which found enough events that can only be explained by the existence of this Higgs production process. Furthermore, we can use these results to measure the rate of ttH production and from this we can calculate the coupling strength between the Higgs boson and the top quark.

I really enjoyed being a PhD student at CMS. There is no doubt that it is a lot of hard work and there are times of extreme pressure. However, it is very special to be able to produce your own work contributing to fundamental research, and all the effort pays off in the end. Additionally, working with CERN gives you access to a great network of physicists from all over the world, some of whom will become good friends.

Joosep Pata, ETH Zürich

My name is Joosep Pata and I’m a particle physicist from Estonia. Currently I work in the US at the California Institute of Technology as a postdoc.
It has been 11 years since I started studying physics and I have been working on the CMS experiment for the past 7 years. I was attracted to fundamental science and particle physics in particular, as there is a possibility to discover and understand things about our world that we might not have known before.

For my PhD,  I worked on understanding how fundamental matter particles acquire mass. This can be done by studying how the Higgs boson and the top quark interact. The top quark is a very massive particle - about 100,000 times heavier than the light quarks that protons and neutrons of ordinary matter are made of. If quarks were grains of sand on a beach, the top quark would be an odd grain that weighs around a kilogram but looks no different than the rest. It behaves differently than other quarks and we believe it might have a special role to play in particle physics. The Higgs boson is the only known fundamental particle with no spin - a scalar. Since it is the first fundamental scalar we have discovered it is very exciting to study it experimentally.
I worked on a search for the interaction of the top quark and the Higgs boson. If we could measure the interaction strength of this process, we could directly determine if the Higgs mechanism is working as predicted for the top quark. The precise measurement of this top-Higgs process will need more data and improvements in the experimental methods, however knowing that it exists is already a very good sign.

As for the life of a PhD student, I think it was quite fun, challenging but sometimes frustrating. I really enjoyed being able to immerse myself in a topic and trying to solve a big problem such as finding the top - Higgs interaction. The challenges came from not always knowing what approach will work, so there were a lot of failures and new problems found along the way. Learning from the experts at CERN was also a great opportunity. It was also common to get stuck and be unsure of the way forward, this could sometimes feel frustrating. Overall, I feel really lucky and thankful that I was able to work on such an interesting project.

Thomas Owen James, Imperial College London

I am Thomas James. I come from London and I have been working on the CMS experiment for about 5 years. I completed my PhD at Imperial College London, and am now a postdoctoral researcher at the same Institute. The topic of my thesis deals with an important aspect of preparing the CMS detector for the High Luminosity upgrade of the Large Hadron Collider at CERN.

Even with the fastest and most sensitive electronics available, it is impossible to capture and store all the data from CMS. For that reason it is essential to select events of potential interest using a “trigger” system. This system relies mainly on signals from the muon detectors and electromagnetic and hadron calorimeters, but in the future, these signals alone are insufficiently selective to reduce the trigger rate to levels with which CMS can cope. The most promising solution to this problem is to exploit information from the tracking detectors in the experiment, which has hitherto not been used in the first level of triggering. This is immensely challenging because for triggering purposes about 13,000 modules comprising about 214 million sensor elements must be used. The entire system must provide essentially all fully reconstructed trajectories for particles and these must be available within a few microseconds. This is unprecedented, and especially difficult in a hadron collider environment which generates multiple events per beam crossing with a very large number of outgoing charged particles from each interaction. My thesis describes an approach to finding and fitting these trajectories within four microseconds, using custom hardware. I investigated the feasibility of Hough Transform and Kalman Filter algorithms, which were shown to meet the required performance metrics. I was able to build a demonstrator system to prove the concept would work by using existing custom hardware, designed originally for the CMS calorimeter trigger.

Studying for a PhD is more like a full-time job than it is like an undergraduate degree. On the other hand, while I did work very hard, I was also afforded a lot of flexibility to choose how I spent my time, and which areas of research I would focus on. I was lucky to have a very strong group of mentors, both at Imperial and at CERN. In my personal life, I listen to heavy metal, play electric guitar, and enjoy making sushi with my fiancée.

A video interview of the winners is also available at the following link