• 01.07.2020
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Claudia Merlassino wins the CHIPP Prize 2020

The unknown partner of the top quark

Claudia Merlassino, CHIPP Prize 2020
Claudia Merlassino, CHIPP Prize 2020
Claudia Merlassino, CHIPP Prize 2020

Claudia Merlassino was born in Genoa, studied physics in Milan and completed her doctorate at the University of Bern in October 2019, since then she has been doing research at Oxford University in the UK. At the age of 28, the Italian experimental physicist has already made a remarkable journey as a researcher. She is now receiving the PhD prize in Swiss particle physics - among other things for her findings in the context of the most massive of all elementary particles.

Particle physics is present in teaching and research at practically all Swiss universities as well as at the Swiss Federal Institutes of Technology in Zurich and Lausanne. All these institutions cooperate in the Swiss Institute of Particle Physics (CHIPP). Every year, CHIPP awards the CHIPP Prize to a young researcher for outstanding scientific achievements.

In the current year, this honour is being bestowed on Dr. Claudia Merlassino. The prize will be awarded to her on 1st July 2020 at the annual meeting of the Swiss Physical Society. As this year's conference is being held as an online conference due to the corona crisis, Merlassino will not give the traditional award speech until autumn, on the occasion of the CHIPP Plenary Meeting on 15 October 2020. Merlassino will be awarded - as the CHIPP jury put it in its laudation - "for her outstanding contribution in the development of new analysis strategies in the search for physics beyond the Standard Model at the LHC experiments, and for having conceived and conducted an innovative study about the radiation damage of the ATLAS detector in view of the high-luminosity phase of the LHC".

From Bern to Oxford

The jury's appreciation is difficult to understand at first glance for people who are not particle physicists. If you want to know more, it is best to ask Claudia Merlassino directly. The journalist reaches the physicist via Skype in the south of the city of Oxford. She currently spends most of her time in a rented house because the premises of Oxford University are still locked for health reasons. From here, she teaches with video tutorials and performs complex data analysis in a computer network. You can sense from her words that she lacks the personal exchange with colleagues.

"After several years of working on my doctoral thesis, I am very proud of this award," says Claudia Merlassino, "when I can accept the prize in person in October, it will give me the opportunity to return to Switzerland, where my doctoral thesis was written". That was from 2016 to 2019 at the Physics Institute of the University of Bern. There, Merlassino had discovered the advantages of a relatively small research group. Now, at Oxford, she's part of a much larger, but also less manageable research institute.

Does LHC data reveal a new particle?

But what do we mean by the "new analysis strategies" mentioned in the CHIPP jury's laudation, which could open the door to a new, previously unknown world of physics? In her doctoral thesis, Claudia Merlassino analysed data from proton-proton collisions recorded at the Large Hadron Collider (LHC) between 2015 and 2018. In the huge data pool of the CERN particle accelerator, she searched for a new, previously hypothetical particle that would be similar to the top quark discovered in 1995 - the most massive of all elementary particles - but which would have a different spin.

The particle is called a 'top squark' (or briefly 'stop'). The top squark is predicted by physicists within the framework of the theory of supersymmetry, which proclaims partner particles to all known Standard Model particles. The top squark would be the 'superpartner' of the top quark. Like the Higgs boson, it would not possess any spin. The mass of the top squark, if it exists, is not defined by the theoretical framework of supersymmetry and necessarily would be much higher than the mass of the ordinary top quark. "In my doctoral thesis I was able to show that the particle does not exist in a light version; if the top squark really exists, it must have a relatively large mass of around four to five times the mass of the ordinary top, depending of the model considered" says Claudia Merlassino, summing up the quintessence of her analysis. In her current research work in Oxford, the scientist continues to work with the LHC data from 2015/18, exploiting her search for the unknown partner of the top quark, to provide a more precise characterisation of the Higgs boson discovered in 2012. In particular she is investigating the decay mode of the Higgs in so-called invisible particles, that can’t be directly detected by the ATLAS experiment.

Protecting the detector from radiation damage

Those who receive the CHIPP prize have not only demonstrated their analytical skills in their dissertation, but also their experimental skills. So Merlassino set a second important accent in her dissertation. This refers to that part of the doctoral thesis which the CHIPP jury described in its laudation as an "innovative study about the radiation damage of the ATLAS detector". This study aimed to ensure that CERN's large ATLAS detector is going to continue to operate reliably even if the LHC is going to operate at a higher power ('luminosity') from 2025 onwards.

For this purpose, Claudia Merlassino examined four types of silicon sensors for their robustness against radiation in her dissertation (more information can be found here. Although Claudia Merlassino has successfully completed her project, the research community of the ATLAS experiment has meanwhile decided to install another sensor in the ATLAS detector. "Which component is best suited for a detector always depends on many criteria. Even though I was able to show that the sensors I was investigating are very robust against radiation, the ATLAS collaboration decided to use a different sensor for other reasons," says Claudia Merlassino.

A life as a researcher

But also in this way Merlassino has contributed to the continuation of CERN research. And that was probably not her last contribution. In October 2020 - as is currently planned - the European Laboratory for Particle Physics is to resume its work after the corona-related lockdown. Merlassino hopes that she will then be able to move her job to CERN in Geneva on behalf of the University of Oxford. For her it is clear: "I see my professional future as a researcher in particle physics."

Author: Benedikt Vogel



c/o Prof. Dr. Rainer Wallny
ETH Zürich
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