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1. March 2016 Tereza Kroupová

The things flying around in particle physics

Second term of this year was admittedly quite though for me. I took most of my modules in the first term and hence I only got one three-hour lecture from quantum field theory a week on Thursdays. Nevertheless the rest of my time was taken by finishing my master's project, writing the final report and largely also making sure that I will have a PhD place after I finish my studies in June.

During the process of applying for doctorates I visited a number of universities and had a chance to speak with many physicists about their research. This experience was extremely exciting and beneficial for me. I applied for PhD programs in experimental particle physics, which includes whole range of topics and experiments. As a result, the work can be very different from one PhD student to another. In the previous post I wrote about neutrinos and one specific experiment, which I did my project on. This time I would like to give an overview of the different branches of experimental particle physics. It surely will be an incomplete list, yet I hope it will be interesting.

Research of tiny particles needs (slightly unintuitively) enormously large and expensive experiments. The most famous are currently on the Larhe Hadron Collider (LHC) at CERN, which also was the topic of one of my posts. LHC is very versatile and focuses on many divisions of particle physics, such as the strong force, which is the force holding together the constituents of protons and neutrons. Another example of LHC abilities is the study of so called CP violation, which could explain the ratio of matter and antimatter in the Universe. In the future, the data from LHC can also provide evidence for new particles or phenomena, and hence prove or refute some of the existing theories (such as supersymmetry or string theory) and shed light on the nature of gravitation and dark matter.

The large size of LHC and the huge number of research carried out there causes that each experiment on LHC employs many people. These people are needed for service, maintenance, development, upgrades and data analysis. The largest groups are around CMS and ATLAS experiments, which contain over 3000 scientists, engineers and students. The whole of LHC produces circa 30 petabytes of data a year (equivalent to more than 6 million DVDs). During its existence, LHC gave rise to more than 60 000 scientific publications.

Another currently growing field of particle physics is neutrino research. The collaborations around the experiments are considerably smaller (usually of the order of tens or hundreds of members). On the other hand, there are multiple such experiments, which differ from one to the other by both design and scientific aim. It could be a giant water tank like in SuperKamiokande in Japan or it can be a collection of small germanium detectors in liquid argon like in the GERDA experiment in Gran Sasso in Italy. The Sun, a nuclear power station in the vicinity of the experiment, artificially prepared beam of neutrinos, decays of various isotopes in the Earth's crust or supernova explosion can serve as the source of neutrinos for these experiments. Common feature of these experiments is that they take place underground. This is due to the fact that on the surface everything is constantly bombarded with various particles from outer space (protons, alpha particles, electrons etc.). It is impossible to observe the very rare interactions of neutrinos in the presence of high rate of interaction of these cosmic ray particles. On the other hand, cosmic rays cannot penetrate deep underneath the surface and hence neutrino interactions can be observed.

Number of experiments is focused on neutrino oscillations. That is the process, in which one type of neutrino can after some time spontaneously transform itself into another neutrino type (altogether we know 3 types/flavours: electron, muon and tau neutrino). Another group of experiments aim to observe a process called neutrinoless double beta decay. An example is the SuperNEMO experiment, which was the topic of my master's project.

Last but not least there are dark matter experiments. Dark matter is believed to form about 25% of all the matter and energy in the Universe. The remaining is formed mostly by dark energy, which we know almost nothing about.  Only around 5% of the Universe is formed by matter which we know and see around us (matter made out of proton, neutrons, electrons etc.) Various candidates for dark matter particles are theoretically proposed, but have never been observed or proved experimentally. Three types of experiments are eager to make such a discovery. As mentioned above, one way to observe dark matter could be in the LHC. The high energy collisions taking place there can in theory result in result in production of dark matter particle. Second way is to study dark matter in space using astronomical telescopes. Finally,  there are direct matter searches. These look for signatures of dark matter passing through Earth interaction directly with a detector. Similar to neutrino research, these particle experiments have to take place deep underground to eliminate the cosmic ray background. The leading experiment of this type is LZ in South Dakota, which contains 7 tons of liquid xenon.

Regarding my PhD, I decided to continue to focus on neutrinos. If nothing goes wrong last minute, I will start my postgraduate study in September in Oxford. I will be working on the SNO+ experiment, which I will write about another time.


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