# 750 GeV diphoton excess

Composition Possible production and decay mechanism of the digamma resonance at LHC. Elementary particle suspected bosonic Refuted; absent in August 2016 data[1][2] Ϝ,[3] Ϝ(750),[4] ϕ,[5] X,[6] ηzy[7] Resonance of mass ≈750 GeV decaying into two photons could have been seen by CERN in 2015[8][9] (though sufficient statistical significance never reached) ≈ 750 GeV/c2 (CMS + ATLAS)[8][9] < 50 GeV/c2[8][9] two photons (hinted in 2015 data;[8][9] absent in 2016 data[1][2]) two Z-bosons (predicted) one photon + one Z-boson (predicted) two W bosons (predicted) two gluons (predicted)

The 750 GeV diphoton excess in particle physics was an anomaly in data collected at the Large Hadron Collider (LHC) in 2015, which could have been an indication of a new particle or resonance.[8][9] The anomaly was absent in data collected in 2016, suggesting that the diphoton excess was a statistical fluctuation.[1][2] In the interval between the December 2015 and August 2016 results, the anomaly generated considerable interest in the scientific community, including about 500 theoretical studies.[10] The hypothetical particle was denoted by the Greek letter Ϝ (pronounced digamma) in the scientific literature, owing to the decay channel in which the anomaly occurred.[3] The data, however, were always less than five standard deviations (sigma) different from that expected if there was no new particle, and, as such, the anomaly never reached the accepted level of statistical significance required to announce a discovery in particle physics.[11] After the August 2016 results, interest in the anomaly sank as it was considered a statistical fluctuation.[12] Indeed, a Bayesian analysis of the anomaly found that whilst data collected in 2015 constituted “substantial” evidence for the digamma on the Jeffreys’ scale, data collected in 2016 combined with that collected in 2015 was evidence against the digamma.[13]

## December 2015 data

On December 15, 2015, the ATLAS and CMS collaborations at CERN presented results from the second operational run of the Large Hadron Collider (LHC) at the center of mass energy of 13 TeV, the highest ever achieved in proton-proton collisions. Among the results, the invariant mass distribution of pairs of high-energy photons produced in the collisions showed an excess of events compared to the Standard Model prediction at around 750 GeV/c2. The statistical significance of the deviation was reported to be 3.9 and 3.4 standard deviations (locally) respectively for each experiment.[8][9]

The excess could have been explained by the production of a new particle (the digamma) with a mass of about 750 GeV/c2 that decayed into two photons. The cross-section at 13 TeV centre of mass energy required to explain the excess, multiplied by the branching fraction into two photons, was estimated to be

${\displaystyle \sigma (pp\to \digamma )\times {\rm {Br}}(\digamma \to \gamma \gamma )\approx 5\,{\rm {fb}}}$

(fb = femtobarns)

This result, while unexpected, was compatible with previous experiments, and in particular with the LHC measurements at a lower centre of mass energy of 8 TeV.

## August 2016 data

Analysis of a larger sample of data, collected by ATLAS and CMS in the first half 2016, did not confirm the existence of the Ϝ particle, which indicates that the excess seen in 2015 was a statistical fluctuation.[1][2]

## Implications for particle physics research

The non-observation of the 750 GeV bump in follow-up searches by the ATLAS and CMS experiments had a significant impact on the particle physics community.[14] Despite the initial significance being lower than the discovery threshold of five sigma, many physicists treated the initial excess as tantamount to a discovery, as evidenced by the extreme interest particularly by the theory community, leading to the authorship of over 500 articles. The event highlighted the desire in the community for the LHC to discover a fundamentally new particle, and the difficulties in searching for a signal which is unknown a priori.[15]

## References

1. ^ a b c d "Search for resonant production of high mass photon pairs using 12.9 fb−1 of proton-proton collisions at √s = 13 TeV and combined interpretation of searches at 8 and 13 TeV" – via CERN Document Server.
2. ^ a b c d Search for scalar diphoton resonances with 15.4 fb−1 of data collected at √s = 13 TeV in 2015 and 2016 with the ATLAS detector. 2016 – via CERN Document Server.
3. ^ a b Strumia, Alessandro (5 Aug 2016). "Interpreting the 750 GeV digamma excess: A review". arXiv:1605.09401 [hep-ph].
4. ^ Franceschini, Roberto; Giudice, Gian F.; Kamenik, Jernej F.; McCullough, Matthew; Riva, Francesco; Strumia, Alessandro; Torre, Riccardo (July 2016). "Digamma, what next?". Journal of High Energy Physics. 2016 (7): 150. arXiv:1604.06446. Bibcode:2016JHEP...07..150F. doi:10.1007/JHEP07(2016)150.
5. ^ Nakai, Yuichiro; Sato, Ryosuke; Tobioka, Kohsaku (12 April 2016). "Footprints of New Strong Dynamics via Anomaly and the 750 GeV Diphoton". Physical Review Letters. 116 (15): 151802. arXiv:1512.04924. Bibcode:2016PhRvL.116o1802N. doi:10.1103/PhysRevLett.116.151802. PMID 27127957.
6. ^ Dutta, Bhaskar; Gao, Yu; Ghosh, Tathagata; Gogoladze, Ilia; Li, Tianjun (22 March 2016). "Interpretation of the diphoton excess at CMS and ATLAS". Physical Review D. 93 (5): 055032. arXiv:1512.05439. Bibcode:2016PhRvD..93e5032D. doi:10.1103/PhysRevD.93.055032.
7. ^ Zhang, Yu-Jie; Zhou, Bin-Bin; Sun, Jia-Jia (5 Jan 2016). "The Fourth Generation Quark and the 750 GeV Diphoton Excess". arXiv:1602.05539 [hep-ph].
8. Aaboud, M.; et al. (ATLAS Collaboration) (September 2016). "Search for resonances in diphoton events at √s = 13 TeV with the ATLAS detector". Journal of High Energy Physics. 2016 (9): 001. arXiv:1606.03833. Bibcode:2016JHEP...09..001A. doi:10.1007/JHEP09(2016)001.
9. Khachatryan, V.; Sirunyan, A. M.; Tumasyan, A.; Adam, W.; Asilar, E.; Bergauer, T.; Brandstetter, J.; Brondolin, E.; Dragicevic, M.; et al. (CMS Collaboration) (28 July 2016). "Search for resonant production of high-mass photon pairs in proton-proton collisions at √s = 8 and 13 TeV". Physical Review Letters. 117 (5): 051802. arXiv:1606.04093. Bibcode:2016PhRvL.117e1802K. doi:10.1103/PhysRevLett.117.051802. PMID 27517765.
10. ^ "#Run2Seminar and subsequent γγ-related arXiv submissions". jsfiddle.net. Retrieved 2016-08-11.
11. ^ Lyons, Louis (4 Oct 2013). "Discovering the Significance of 5 sigma". arXiv:1310.1284 [physics.data-an].
12. ^ Coldham, K. (2016-08-05). "Chicago sees floods of LHC data and new results at ICHEP". CERN Document Server. Retrieved 26 January 2017.
13. ^ Fowlie, Andrew (2016). "Bayes-factor of the ATLAS diphoton excess". The European Physical Journal Plus. 132 (1): 46. arXiv:1607.06608. Bibcode:2017EPJP..132...46F. doi:10.1140/epjp/i2017-11340-1. ISSN 2190-5444.
14. ^ "Horizon: Inside CERN". BBC. Retrieved 29 October 2018.
15. ^ "And so to bed for the 750 GeV bump". PhysicsWorld. 2016-08-05.