Diphantom: 750 GeV Diphoton Excess
Nuclear & Particle Physics arc, page 186 of 193. Back to The Pixelated Physics Universe
Who they are
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.
Diphantom is a character of the Nuclear & Particle Physics arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
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.mγγ² c⁴ = 2 E₁ E₂ (1 − cos θ) mγγ: 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.
No single equation is shown for this character; the physics above is stated in words because it is an observation or a piece of history, and we do not invent formulas.
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 characterFor 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.
Crossovers
Diphantom meets Sigmarel (Five Sigma Discovery). Diphantom's 750 GeV bump excited physicists in 2015, but it never reached Sigmarel's 5σ and faded with more data in 2016. The gate held.
Diphantom meets Trialsworn (Look-Elsewhere Effect). Trialsworn always asks Diphantom how many mass bins were searched. The look-elsewhere effect cut the bump's global significance well below its local value.
Diphantom meets Thetarch (Θ⁺(1540) Pentaquark). Diphantom's bump faded with more data. Thetarch's pentaquark peak was reported in 2003, then high-statistics searches did not confirm it. Both are reminders that a peak is not yet a particle.
Wear the lore
Concept keywords: 750 GeV Diphoton Excess, Diphantom, Nuclear & Particle Physics, physics lore, Pixelated Physics