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{{Infobox particle
| name = Digamma
| image = S750loops.svg
| caption = Possible production and decay mechanism of the digamma resonance at LHC.
| composition = [[Physics:Elementary particle|Elementary particle]]
| statistics = suspected [[Physics:Bosonic|bosonic]]
| status = Refuted; absent in August 2016 data<ref name='CMS-PAS-EXO-16-027'/><ref name='ATLAS-CONF-2016-059'/>
| discovered = Resonance of mass ≈750 GeV decaying into two photons could have been seen by CERN in 2015<ref name='1606.03833'/><ref name='1606.04093'/> (though sufficient statistical significance never reached)
| symbol = Ϝ,<ref name='1605.09401'/>
Ϝ(750),<ref>
{{cite journal
|last1=Franceschini |first1=Roberto
|last2=Giudice |first2=Gian F.
|last3=Kamenik |first3=Jernej F.
|last4=McCullough |first4=Matthew
|last5=Riva |first5=Francesco
|last6=Strumia |first6=Alessandro
|last7=Torre |first7=Riccardo
|date=July 2016
|title=Digamma, what next?
|journal=[[Physics:Journal of High Energy Physics|Journal of High Energy Physics]]
|volume=2016 |issue=7 |page=150
|arxiv=1604.06446
|bibcode=2016JHEP...07..150F
|doi=10.1007/JHEP07(2016)150
|doi-access=free
}}</ref> Φ,<ref>
{{cite journal
|last1=Nakai |first1=Yuichiro
|last2=Sato |first2=Ryosuke
|last3=Tobioka |first3=Kohsaku
|date=12 April 2016
|title=Footprints of New Strong Dynamics via Anomaly and the 750 GeV Diphoton
|journal=[[Physics:Physical Review Letters|Physical Review Letters]]
|volume=116 |issue=15 |article-number=151802
|arxiv=1512.04924
|bibcode=2016PhRvL.116o1802N
|doi=10.1103/PhysRevLett.116.151802
|pmid=27127957
|s2cid=10541107
}}</ref> X,<ref>
{{cite journal
|last1=Dutta |first1=Bhaskar
|last2=Gao |first2=Yu
|last3=Ghosh |first3=Tathagata
|last4=Gogoladze |first4=Ilia
|last5=Li |first5=Tianjun
|date=22 March 2016
|title=Interpretation of the diphoton excess at CMS and ATLAS
|journal=[[Physics:Physical Review D|Physical Review D]]
|volume=93 |issue=5 |article-number=055032
|arxiv=1512.05439
|bibcode=2016PhRvD..93e5032D
|doi=10.1103/PhysRevD.93.055032
|s2cid=118557231
}}</ref> ηzy<ref>
{{cite arXiv
|last1=Zhang |first1=Yu-Jie
|last2=Zhou |first2=Bin-Bin
|last3=Sun |first3=Jia-Jia
|date=5 Jan 2016
|title=The Fourth Generation Quark and the 750 GeV Diphoton Excess
|eprint=1602.05539
|class=hep-ph
}}</ref>
| mass =
{{nowrap|≈ 750 GeV/''c''2}} (CMS + ATLAS)<ref name='1606.03833'/><ref name='1606.04093'/>
|width = < {{val|50|ul=GeV/c2}}<ref name='1606.03833'/><ref name='1606.04093'/>
| decay_particle = {{plainlist|
* two [[Physics:Photon|photon]]s (hinted in 2015 data;<ref name='1606.03833'/><ref name='1606.04093'/> absent in 2016 data<ref name='CMS-PAS-EXO-16-027'/><ref name='ATLAS-CONF-2016-059'/>)
* two Z-bosons (predicted)
* one [[Physics:Photon|photon]] + one Z-boson (predicted)
* two [[Physics:W boson|W boson]]s (predicted)
* two [[Physics:Gluon|gluon]]s (predicted)}}
| electric_charge =
| colour_charge =
| spin =
}}

The '''750 GeV diphoton excess''' in [[Physics:Particle physics|particle physics]] was an anomaly in data collected at the [[Physics:Large Hadron Collider|Large Hadron Collider]] (LHC) in 2015, which could have been an indication of a new particle or [[Physics:Resonance (particle physics)|resonance]].<ref name='1606.03833'>
{{cite journal|last1=Aaboud |first1=M.
|collaboration=ATLAS Collaboration
|display-authors=0
|date=September 2016
|title=Search for resonances in diphoton events at √s = 13 TeV with the ATLAS detector
|journal=Journal of High Energy Physics
|volume=2016 |issue=9 |page=001
|arxiv=1606.03833
|bibcode=2016JHEP...09..001A
|doi=10.1007/JHEP09(2016)001
|doi-access=free
}}</ref><ref name='1606.04093'>
{{cite journal
|last1=Khachatryan |first1=V.
|last2=Sirunyan |first2=A. M.
|last3=Tumasyan |first3=A.
|last4=Adam |first4=W.
|last5=Asilar |first5=E.
|last6=Bergauer |first6=T.
|last7=Brandstetter |first7=J.
|last8=Brondolin |first8=E.
|last9=Dragicevic |first9=M.
|collaboration=CMS Collaboration
|date=28 July 2016
|title=Search for resonant production of high-mass photon pairs in proton-proton collisions at √s = 8 and 13 TeV
|journal=[[Physics:Physical Review Letters|Physical Review Letters]]
|volume=117 |issue=5 |article-number=051802
|arxiv=1606.04093
|bibcode=2016PhRvL.117e1802K
|doi=10.1103/PhysRevLett.117.051802
|pmid=27517765
|s2cid=207852166
}}</ref> The anomaly was absent in data collected in 2016, suggesting that the diphoton excess was a statistical fluctuation.<ref name='CMS-PAS-EXO-16-027'>
{{cite web
|title=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
|date=2016
|url=https://cds.cern.ch/record/2205245
|via=CERN Document Server
|collaboration=CMS Collaboration
}}</ref><ref name='ATLAS-CONF-2016-059'>
{{cite book
|title=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
|url=https://cds.cern.ch/record/2206154
|via=CERN Document Server
|collaboration=CMS Collaboration
|year=2016
}}</ref> 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.<ref>{{Cite journal|last=Garisto|first=Robert|date=2016-04-12|title=Editorial: Theorists React to the CERN 750 GeV Diphoton Data|journal=Physical Review Letters|language=en|volume=116|issue=15|article-number=150001|doi=10.1103/PhysRevLett.116.150001|bibcode=2016PhRvL.116o0001G |issn=0031-9007|doi-access=free}}</ref><ref>{{Cite journal|last1=Cao|first1=Junjie|last2=Shang|first2=Liangliang|last3=Su|first3=Wei|last4=Zhang|first4=Yang|last5=Zhu|first5=Jinya|year=2016|title=Interpreting the 750 GeV diphoton excess in the minimal dilaton model|journal=The European Physical Journal C|language=en|volume=76|issue=5|page=239|doi=10.1140/epjc/s10052-016-4098-5|arxiv=1601.02570 |bibcode=2016EPJC...76..239C |issn=1434-6044|doi-access=free}}</ref><ref name=":0">
{{cite web
|title=#Run2Seminar and subsequent γγ-related arXiv submissions
|url=http://jsfiddle.net/adavid/bk2tmc2m/show/
|website=jsfiddle.net
|access-date=2016-08-11
}}</ref><ref>{{Cite web|date=2021-01-14|title=A decade in LHC publications|url=https://cerncourier.com/a/a-decade-in-lhc-publications/|access-date=2021-01-15|website=CERN Courier|language=en-GB}}</ref> 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.<ref name='1605.09401'>
{{cite arXiv
|last1=Strumia |first1=Alessandro
|date=5 Aug 2016
|title=Interpreting the 750 GeV digamma excess: A review
|eprint=1605.09401
|class=hep-ph
}}</ref> The data, however, were always less than five [[Standard deviation|standard deviation]]s (sigma) different from that expected if there was no new particle, and, as such, the anomaly never reached the accepted level of [[Statistical significance|statistical significance]] required to announce a discovery in particle physics.<ref>
{{cite arXiv
|last1=Lyons |first1=Louis
|date=4 Oct 2013
|title=Discovering the Significance of 5 sigma
|eprint=1310.1284
|class=physics.data-an
}}</ref> After the August 2016 results, interest in the anomaly sank as it was considered a statistical fluctuation.<ref>
{{cite journal
|last1=Coldham |first1=K.
|date=2016-08-05
|title=Chicago sees floods of LHC data and new results at ICHEP
|url=https://cds.cern.ch/record/2212324
|website=CERN Document Server
|access-date=26 January 2017
}}</ref>
Indeed, a [[Bayesian statistics|Bayesian analysis]] of the anomaly found that whilst data collected in 2015 constituted "''substantial''" evidence for the digamma on the [[Bayes factor|Jeffreys scale]], data collected in 2016 combined with that collected in 2015 was evidence against the digamma.<ref>{{cite journal|last1=Fowlie|first1=Andrew|title=Bayes-factor of the ATLAS diphoton excess|journal=The European Physical Journal Plus|date=2016|volume=132|issue=1|page=46|doi=10.1140/epjp/i2017-11340-1|arxiv=1607.06608|issn=2190-5444|bibcode = 2017EPJP..132...46F |s2cid=119305800 }}</ref>

== December 2015 data ==

On December 15, 2015, the [[Physics:ATLAS experiment|ATLAS]] and [[Physics:Compact Muon Solenoid|CMS]] collaborations at [[Organization:CERN|CERN]] presented results from the second operational run of the [[Physics:Large Hadron Collider|Large Hadron Collider]] (LHC) at the [[Physics:Center-of-momentum frame|centre-of-momentum]] energy of 13 TeV, the highest ever achieved in proton-proton collisions. Among the results, the [[Physics:Invariant mass|invariant mass]] distribution of pairs of high-energy photons produced in the collisions showed an excess of events compared to the [[Physics:Standard Model|Standard Model]] prediction at around {{val|750|u=GeV/c2}}. The [[Statistical significance|statistical significance]] of the deviation was reported to be 3.9 and 3.4 [[Standard deviation|standard deviation]]s (locally) respectively for each experiment.<ref name='1606.03833' /><ref name='1606.04093' />
[[File:ArXiv submissions citing 750 GeV diphoton CMS and ATLAS data.png|thumb|upright=1.5|Cumulative distribution of arXiv submissions, referencing CMS and ATLAS, that gave rise to the "750 GeV diphoton" episode in the CERN LHC history.<ref name=":0" />]]
The excess could have been explained by the production of a new particle (the digamma) with a mass of about 750 GeV/''c''2 that decayed into two photons. The [[Physics:Cross section|cross-section]] at 13 TeV centre-of-momentum energy required to explain the excess, multiplied by the [[Physics:Branching fraction|branching fraction]] into two photons, was estimated to be
:<math>\sigma(pp\to \digamma) \times {\rm Br}(\digamma\to \gamma\gamma)\approx 5\,{\rm fb}</math>

(fb = [[Physics:Barn (unit)|femtobarn]])

This result, while unexpected, was compatible with previous experiments, and in particular with the LHC measurements at a lower centre-of-momentum 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.<ref name='CMS-PAS-EXO-16-027'/><ref name='ATLAS-CONF-2016-059'/>

== 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.<ref>{{cite web |title=Horizon: Inside CERN |url=https://www.bbc.co.uk/programmes/b07nsxkk |website=BBC |access-date=29 October 2018}}</ref> 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''.<ref>{{cite web |title=And so to bed for the 750 GeV bump |url=https://physicsworld.com/a/and-so-to-bed-for-the-750-gev-bump/ |website=PhysicsWorld|date=2016-08-05 }}</ref>

== See also ==
* [[Physics:Sgoldstino|Sgoldstino]]

== References ==
{{reflist|30em}}

[[Category:Hypothetical elementary particles]]
[[Category:Hypothetical composite particles]]
[[Category:Physics beyond the Standard Model]]
[[Category:Large Hadron Collider]]
[[Category:Obsolete theories in physics]]

{{Sourceattribution|750 GeV diphoton excess}}

Physics:750 GeV diphoton excess - Revision history

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