Celebrating 25 years of Planck

9 November 2023
Planck 2023 participants
Joining forces Planck 2023 focused on the interface between particle physics and cosmology. Credit: M Mikołajczyk/Candela Foundation

On its silver jubilee, the Planck 2023 conference took place at the University of Warsaw from 22 to 26 May, attracting around 180 participants. Initiated by a meeting in a small town near Warsaw, Kazimierz Dolny, in 1998 and hosted each year by theory groups across Europe, the series has become one of the key conferences on beyond-the-Standard Model physics. Plenary talks covering the latest topics in theory and phenomenology as well as many parallel talks given by young researchers are the core of the conference programme, following the evolving trends in particle physics and cosmology from the Planck to the electroweak scales.

This year’s conference focused on “Hot topics in particle physics and cosmology: theory facing experimental prospects”. The first day’s plenaries were mainly devoted to machine-learning techniques and to collider physics. Enthusiastic speakers on the former met with some reservation in parts of the audience, which stressed the need for a good balance between new techniques and new physics ideas, while the collider talks emphasised the importance of precision Higgs physics and its prospects at the LHC and HL-LHC for a full understanding of the Brout–Englert–Higgs mechanism. On the theory side, the approach of effective field theory was strongly advocated. A separate session was devoted to flavour physics, in which new ideas on the origin of flavour were presented. A review of rare decays as precision tests of the Standard Model (SM) and as probes of new physics complemented the experimental summary.

The conference was dominated by topics at the interface between particle physics and cosmology. Covered in the many talks were axion couplings and search strategies, axions in rare decays, models of CP violation with nucleon and atomic electric dipole moments with or without the QCD axion, searches for very light and weakly interacting axion-like particles as a complementarity to heavy new particle searches in colliders, and much more.

The conference was very successful in connecting new theoretical ideas with planned experimental programmes

Another issue vividly under consideration was dark matter. Among important theoretical questions is the role of gravity in the production of dark matter. Avoiding overabundance of gravitationally produced dark matter is an important constraint on effective quantum gravity. Similar logic concerns right-handed neutrinos as a dark-matter candidate in simple extensions of the SM. Both were analysed in a number of presentations. Anticipating the results from the Nanograv experiment (CERN Courier September/October 2023 p7), various concrete sources of such signals were reviewed, such as primordial black-hole production, domain walls, cosmic strings and phase transitions in the early universe. Selected theoretical aspects of dark-matter models (such as accidental dark matter with its several realisations) and the analysis of their experimental signatures through new theoretical developments in computing high-energy photon signals from heavy classic WIMPs were presented.

More exotic problems at the interface between particle physics and cosmology were also touched upon. One example is how annihilating dark matter can affect late stellar evolution and the spectrum of black holes, which can be tested with gravitational-wave observations. Another is how the apparent anomaly in the primordial abundance of 4He can be linked to a neutrino–antineutrino asymmetry in the early universe that impacts Big Bang nucleosynthesis. Gravitational waves as a probe of beyond-the-SM physics were discussed at length, also including possible new-physics signals from pulsar timing arrays.

The conference was very successful in connecting new theoretical ideas with planned experimental programmes. The next Planck meeting will be held in Lisbon.

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