Lepton Photon shines down under

9 November 2023
Melbourne’s skyline and the Yarra river
Bright lights Melbourne’s skyline and the Yarra river. Credit: rudi1976 –

The cold was biting the morning of 17 July, when Wurundjeri Elder Uncle Tony Garvey welcomed 219 particle physicists to the unceded lands of the Wurundjeri, Bunurong and Wadawurrung peoples for the 31st International Conference on Lepton Photon Interactions, hosted in Melbourne, Australia. Although the distance to Melbourne is considerable, a broad range of nationalities were represented, and about a third of participants were students.

Over five days of pronouncements, presentations and posters, topics included current and future prospects in detector technologies, advances in theoretical calculations (with a particular focus on effective field theories), and improving diversity and outreach in physics. Results from a large number of experiments were presented, many of which are building excitement for the next generation of measurements that seek to provide even more rigorous tests of the Standard Model (SM) and improved searches for physics beyond it (BSM).

The results presented were too numerous to review comprehensively. However, they tended to skew towards flavour physics, with a particular emphasis on searches for CP- and lepton flavour-violation and tests of lepton-flavour universality (LFU). Overall, tensions between the SM and experimental measurements of LFU remain. In particular, Kazuki Kojima (Nagoya University) presented a measurement of R(D*), which is a test of LFU performed with B-meson decays, finding the ratio R(D*) = 0.267+0.041 –0.039 (stat.)+0.028 –0.033 (syst.). While compatible with the SM, it increases the tension with theory from 3.2σ to 3.3σ when all measurements of R(D) and R(D*) are combined.

Not to be outdone, the LHC experiments presented a range of precision measurements of SM parameters, further reducing the available parameter space for BSM physics. In particular, Linda Finco (INFN Torino) from ATLAS presented the most precise measurement of the Higgs-boson mass: 125.11 ± 0.09 (stat.) ± 0.06 (syst.) GeV, using the full Run 1 and Run 2 datasets for both the H  ZZ  4ℓ and H γγ channels. This is one of the most precisely determined masses of any SM particle, a real achievement of precision physics.

Now that the available parameter space for BSM models is shrinking, more innovative approaches to particle physics are needed. One such approach, presented by Ling Sun (Australian National University), is to use the phenomenon of superradiance to search for ultralight bosons around rapidly rotating black holes. The boson clouds extract angular momentum from the black hole when the superradiance condition is met, producing gravitational radiation that could be measured by current and future gravitational-wave detectors. Such a method provides an avenue to measure particles that interact only through gravity, opening a novel avenue for exploring particles beyond the SM.

On the penultimate evening, Alan Duffy (Swinburne University) and Suzie Sheehy (University of Oxford and University of Melbourne) delivered a public lecture “How to discover a universe” to a mix of conference participants, high-school students and the interested public, stressing that science is cultural as well as technological. The best poster was awarded to Emily Filmer (University of Adelaide) for “Searches for BSM physics using challenging long-lived signatures with the ATLAS detector”, while the “people’s choice” was awarded to Eliot Walton (Monash University) for her poster “The Queer History of Physics”. Australia’s small but growing particle-physics community was extremely well represented, and the exposure of the global community to us made Lepton Photon 2023 a resounding success.

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