What happened at 750GeV? (Amelia Brennan)

The particle physics community was left shuffling its feet in mild embarrassment recently as the new 750 GeV di-photon resonance, which had inspired upwards of 500 submitted papers, turned out to be just another statistical fluctuation in the data.

The apparent resonance was first publicly announced by CERN in December last year, when both the ATLAS and CMS Collaborations, using new 13 TeV data produced by the Large Hadron Collider and collected by the two multi-purpose detectors, reported seeing more events than predicted by the Standard Model in data containing two high-energy photons. Significantly, both experiments saw this excess occurring when the two photons had an invariant mass close to 750 GeV (indicating a single new heavy particle at the same mass), with local significance measures of 3.9 sigmas (ATLAS) and 3.4 sigmas (CMS). Typically, a significance of 5 sigmas is required before a discovery can be claimed.

During the first half of 2016, the experimentalists worked hard on collecting new data and checking their analyses, while the theorists – including many from around Australia – wrote paper after paper explaining the potential new particle. A resonance decaying into two photons would most likely be a spin-0 or spin-2 boson, so might be compatible with the popular models of an expanded Higgs sector (which predicts heavier versions of the spin-0 Higgs boson) or of extra dimensions (which predicts a heavy spin-2 graviton). Indeed, many new theories of physics beyond the Standard Model can easily incorporate a photon-decaying, 750 GeV mass boson without too much manipulation, which was another point in favour of the resonance maybe, possibly, hopefully being real.

So many di-photon papers were written that it became a source of amusement in the community, and inspired work that modelled the production of papers following hints of new physics (by M. Backovic, cheekily titled ‘A Theory of Ambulance Chasing’), though ultimately even this work underestimated the proliferating papers.

Inevitably, connections were drawn to the Higgs boson, discovered at CERN in 2012, the experimental evidence for which also began as a similar-looking bump in the di-photon data. However, while the Higgs is a component of the Standard Model and was therefore expected to be found sooner rather than later, the new resonance would have been a signal of completely new physics, and potentially an even greater discovery.

By August, the ATLAS and CMS experiments had collected new data, and the 750 GeV bump was disappointingly diminished. This was confirmed by a CERN announcement at the recent ICHEP conference in Chicago, and indicates that the apparent new resonance was just a statistical fluctuation after all. The level-headed among us are quick to point out that the global significance, which takes into account the so-called Look Elsewhere effect, was never that large to begin with, but it is still disappointing to see the most exciting hint of new physics yet turn into just a flash in the pan.

So what next for the particle physics community? It’s back to business as usual; there are still plenty of open questions in particle physics, observed phenomena (like the presence of dark matter) not yet explained by theory, and big theories (like supersymmetry) not yet confirmed by observation. And when the next tantalising hint comes along, the inevitable excitement will perhaps be tempered by our experience with the 750 GeV bump that is no more.

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Amelia Brennan
University of Melbourne