Diphantom

Diphantom, a spectral knight made of two crossing beams of light, carries a small flickering bump at his heart while a skyline of histogram bars falls away behind him.

The Physics

On 15 December 2015 the ATLAS and CMS collaborations each showed a small excess of photon pairs with invariant mass near 750 GeV in the first 13 TeV LHC data. A new heavy boson decaying to two photons would show up as exactly such a bump above the smoothly falling background. Hundreds of theory papers followed. With roughly four to five times more data (ATLAS 15.4 fb⁻¹ in 2016 against 3.2 fb⁻¹ in 2015; CMS 12.9 fb⁻¹ against 3.3 fb⁻¹), both experiments reported at ICHEP on 5 August 2016 that the excess had gone. It was a statistical fluctuation.

m_γγ² c⁴ = 2 E₁ E₂ (1 − cos θ)

• m_γγ: invariant mass of the photon pair (GeV/c²)
• E₁, E₂: measured photon energies (GeV)
• θ: opening angle between the two photons in the lab frame
• c: speed of light

When many mass bins are searched, an excess somewhere is likely by chance (the look-elsewhere effect), so its local significance overstates the global one. Sources: ATLAS briefing, “High-mass di-photon resonances: the first 2016 ATLAS results” (Aug 2016); CMS ICHEP 2016 public results; Physics World, Aug 2016.

Honest caveat: Nothing was wrong with the detectors or analyses; small samples fluctuate. The data rule out this particular bump, not every heavy diphoton resonance.

In character

For eight months I wore a crown of a thousand papers. Then the data grew, and I was what I always was: noise, standing a little too tall.

On the hoodie

Front: two crossing photon lines with a hollow dot at the crossing. Front label: “750 GeV DIPHOTON · FADED 2016”. Back: Diphantom in full pixel art with “BORN 15 DEC 2015 - DIED 5 AUG 2016” and “A FLUCTUATION WEARING A CROWN.” Gildan 18500 Heavy Blend hoodie, Black, S to 5XL.

Part of the Ghost Particles That Were Wrong set with Sir Aetheron, Enrayne, Calorus, Prematura, Thetarch. Browse all physics hoodies, more particle & nuclear physics designs, or the Deep Cuts.