The Les Houches 2021 Dark Matter Summer School
July 26th - August 20th , 2021
Paul Martens (Kyoto University)
From July the 26th, for four weeks onward, Les Houches School of Physics (France) held its annual Summer School, this year on the mysterious dark matter.
Back in February, I decided to apply, and with the support of my supervisor, Prof. Mukohyama, was happy to be accepted. While I had been hoping the coronavirus pandemic would have slowed when the courses begin, it did not, as is now common knowledge. Therefore, I followed the school as an online participant, shifting my daily routine into nighttime for the school duration, as, in Japan, the courses would begin at 16h and end at 2h in the morning.
The summer school aimed to provide a broad and extensive overview of the current state of research on dark matter. That is why courses tackled a wide variety of potential candidates for dark matter, from the popular WIMPs to modified theories of gravity, but also going through fuzzy dark matter, sterile neutrinos, axions, black holes, dark atoms… These were of course also linked to either their experimental search, using e.g. the Large Hadron Collider or deep underground detectors; as well as their observational signatures, using e.g. numerical simulation at various scale or the Cosmological Microwave Background properties.
To cover this large variety of research avenues, Les Houches School of Physics managed to gather researchers from many different locations. These sometimes gave the courses in person at Les Houches and sometimes gave it remotely, if, like me, they had not been able to come in person at the school. In either cases, it was pleasing to not only benefit from their various quality insight, and their openness to questions, but also to have them share their enthusiasm for their research topics. Furthermore, given the range of topics tackled by the school, it is also a wide range of physical concepts that is exploited, including e.g. fluid equation, chemistry, quantum field theory, numerical stability, electromagnetism… And yet, despite these extensive research efforts, dark matter remains an elusive mystery, which, in spite of stronger and stronger constraints regularly trickling down, retains all its intriguing power.
In the end, to me, the total number of questions I had did not drop; on the opposite, this school was the occasion to spring multiple new interrogations on what could possibly account for what we call dark matter. Personally, I cannot determine which candidate stands particularly out as they all seem equally perfectly reasonable and adequate, as they seem unlikely. On a brighter side, several experiments or observations planned for the coming decade or so are expected to definitively either rule out tracks, or, in case of luck, light up the dawn of new exciting physics.