Last month, Berlin 'hosted' the 37th International Cosmic Ray Conference (ICRC) - the major conference for IceCube physics. It is a chance to meet, present new results and chat informally with colleagues from different experiments around the world - an important opportunity to exchange ideas, plan for future experiments, network, and, for the younger people, formally or informally job hunt.
Unfortunately, Covid forced us into the virtual world. Although the organizers worked very hard and did a good job, it just isn't the same. Virtual meeting rooms may be getting better, but they're nowhere near in-person meetings, and the time differences limited the opportunity to interact.
The ICRC program included 693 talks and 687 posters, with 84 presentations from IceCube. To cope with the time differences, the talks were pre-recorded, viewable at leisure. Posters were also made available, accompanied by short 'flash' talks by the presenters. The organizers scheduled discussion sessions, clustering talks on similar topics. I found these were quite valuable, although there was so much to cover that some presentations did not get the attention than they deserved.
Although no major new results were presented (by IceCube or by other experiments), it was still a good opportunity to assess progress in the field. There was steady progress in most areas. IceCube presented a host of new searches for astrophysical neutrino searches, plus progress reports on a number of new studies of the diffuse (aggregate) neutrino flux, two measurements of the neutrino-nucleon cross-sections, and several contributions on neutrino oscillation studies. IceCube Gen2 also received some attention, with reports on the science case and hardware developments. The IceCube talks are linked to a master arXiv submission available here.
One subfield with some nice developments is high-energy gamma-ray astronomy. The Chinese Large High Altitude Air Shower Observatory (LHAASO) is now operational in Tibet. It features a large surface array consisting of water Cherenkov detectors to detect air shower particles that reach ground level, buried muon detectors to separate gamma-ray and hadronic showers, and Cherenkov telescopes for further gamma/hadron rejection. The large-area surface coverage and high-altitude site give it good acceptance for gamma-rays with energies down to 500 GeV. At TeV energies, it is the most sensitive observatory we have.
Although it is still early days, LHAASO has presented observations of a twelve sources, including seeing photons with energies up to 1.4 PeV. One has to be careful about claims of maximum photon energies, but the events look good, and this is a considerable step up from previous maximum energies. At least most of the sources are likely to be in our galaxy. This is expected since photons with energies above about 50 TeV are attenuated in-flight, through interaction with lower-energy photons from the cosmic microwave background radiation. Even within our galaxy, only about 1/3 of the most energetic photons survive the trip to Earth.
