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Neutrinos from Nowhere

Thanks to the University of Geneva Swiss astroparticle physics participates in IceCube - a huge experiment in neutrino research at the South Pole. The scientists aim to probe with IceCube the exact origin of the neutrinos coming from the universe. For over two years the young scientist Tessa Carver (24) has been part of the experiment.

The astro particle physicist Tessa Carver (24) writes her PhD theses in the field of neutrino research at the University of Geneva.
Image: privat / privata / private

The inhabitants of the British Isle prefer to stay among themselves - that's what the political discussion about the 'Brexit' makes us believe. It is actually true that a majority of the British voted in favor of Britain's leave of the European Union in June 2016. But for researchers, confining to the own homeland usually makes no sense. Science needs international exchange; in physics, whose laws are universally valid, that is even more true. So Tessa Carver did not want to limit her life to the British Isles, even though she grew up in Oxford, the city that houses one of the world's top universities.

After Tessa Carver had graduated from highschool in Oxford, the daughter of a scientist and an economist went first to the capital. There she studied physics at Imperial College London. "I have chosen this field of study because physics deals with the most fundamental questions," says Tessa Carver. At the countries leading technical university the British-American double citizen obtained her master's degree in 2015. The third year of her studies she had spent in Paris, at the 'Université de Paris Sud'. She worked in paris as part of the Virgo collaboration looking for Gravitational wave signals from exotic sources.

Part of the IceCube consortium

"After my master's, I wanted to see something new," says Tessa Carver. She had a friend who did her master's degree at the Swiss Federal Institute of Technology in Lausanne (EPFL). Since she felt at home in a francophone neighbourhood, she moved to the University of Geneva for her doctoral studies at the chair of astrophysicist Prof. Teresa Montaruli. Tessa Carver is there investigating neutrinos that are coming from the universe to the earth. By probing these particles, astroparticle researchers hope to obtain new information about our universe.

Scientists in Teresa Montaruli's research group usually investigate astrophysical sources of high energy particles. Among them are the neutrinos, which are probed by the IceCube experiment. Montaruli is the Swiss link to the IceCube experiment. IceCube is a single detector composed of 5000 optical modules which are embedded in the ice of the South Pole in a volume of one cubic kilometer. Neutrinos are very transient guys who can hardly be observed. IceCube now enables participating scientists to observe at least a fraction of the neutrinos passing through the south pole. Since 2005 the experiment has been operating and taking data, and it has been running as a complete detector since 2011. Hundreds of thousands of neutrinos have since been recorded. Around 400 scientists from twelve countries are involved in the experiment.

Around 80 high energy neutrinos discovered

"Most neutrinos observed by IceCube are created in the Earth's atmosphere through the interactions of cosmic rays," says Tessa Carver. "However, our Geneva research group is interested in those neutrinos that come to us from the far universe and therefore can tell us a lot about the construction and history of the cosmos. Cosmic neutrinos should have higher rates at higher energy compared to atmoshperic neutrinos. Therefore by selecting high energy events we can try and have an event selection with a higher fraction of cosmic events." Since 2010, IceCube has detected 80 very high energy neutrinos having a high probability of coming from a cosmic source. They have in common a very high energy (30 to 2600 TeV) - that's a multiple of the energy that elementary particles reach in CERN's LHC particle accelerator.

"We want to find out where exactly these neutrinos come from," says Tessa Carver. To determine the origin, the scientists use the information provided by the IceCube experiment. Since every neutrino arriving from the cosmos is detected by several ice cube modules, their direction can be estimated. If you follow the trajectory backwards, you know the direction from which the neutrino comes. "We choose to investigate positions in the sky where other experiments have already observed sources we deem interesting. We then determine whether we have a significant number of neutrino events coming from these directions," explains Carver.

A new gaze at the universe

A definite answer to the question of the origin of cosmic neutrinos has not yet been found by the Geneva astroparticle physicists. Different ideas are discussed from where the neutrinos could come from. One possible source is called Blazars, a type of galaxy that emits a lot of high-energy radiation from its center. Another hypothesis is that the neutrinos come from the remnants of supernovae.

The IceCube experiment is run continuously and always provides new data. The more data will be evaluated, the better the question of the origin of the cosmic neutrinos can be clarified, says Tessa Carver. Scientists today are confident that they will be able to use neutrinos as an instrument to unravel the secrets of the cosmos. "For a long time, we could only use visible light, other electromagnetic waves or charged particles to observe the universe. Today we are at the beginning of a new period of looking at the universe. That's possible also thanks to gravitational waves since 2015. Neutrinos could open up completely new possibilities in just a few years. "

Excite girls for physics

The personal scientific goal of Tessa Carver is now the completion of the doctoral thesis. In a year, it should be done. In addition to her own scientific career, the researcher of the University of Geneva also cares for the promotion of young female scientists. Through various activities in the framework of the EU project GENERA, she tries to get girls interested in science and physics in particular. "In the IceCube experiment there are around 25% of women. That's a lot compared to other physics experiments. This shows how much more needs to be done to promote female scientists, "says Tessa Carver.

Author: Benedikt Vogel

The astro particle physicist Tessa Carver (24) writes her PhD theses in the field of neutrino research at the University of Geneva.
The astro particle physicist Tessa Carver (24) writes her PhD theses in the field of neutrino research at the University of Geneva.Image: privat / privata / private


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