750 GeV diphoton excess: Difference between revisions

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| composition = [[Elementary particle]]
| composition = [[Elementary particle]]
| statistics = [[Bosonic]]
| statistics = [[Bosonic]]
| status = Dead. Absent in August 2016 data<ref name='CMS-PAS-EXO-16-027'/><ref name='ATLAS-CONF-2016-059'/>
| interaction = unknown
| 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)
| particle =
| symbol = Ϝ<ref name='1605.09401'/>,
| antiparticle =
Ϝ(750)<ref>{{cite arXiv|eprint=1604.06446|title=Digamma, what next?|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|class=hep-ph|year=2016}}</ref>,
| status = A resonance decaying to photons of mass ≈750 GeV could have been seen by CERN in 2015
ϕ<ref>{{cite journal|arxiv=1512.04924|title=Footprints of New Strong Dynamics via Anomaly and the 750 GeV Diphoton |journal=Physical Review Letters |volume=116 |issue=15 |pages=151802 |doi=10.1103/PhysRevLett.116.151802 |pmid=27127957 |date= |year=2016 |last1=Nakai |first1=Yuichiro |last2=Sato |first2=Ryosuke |last3=Tobioka |first3=Kohsaku |bibcode=2016PhRvL.116o1802N }}</ref>,
| discovered = disproved 2016
| symbol = Ϝ, Ϝ(750),<ref>{{cite arXiv|eprint=1604.06446|title=Digamma, what next?|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|class=hep-ph|year=2016}}</ref> ϕ,<ref name="Nakai_etal">{{cite journal|arxiv=1512.04924|title=Footprints of New Strong Dynamics via Anomaly and the 750 GeV Diphoton |journal=Physical Review Letters |volume=116 |issue=15 |pages=151802 |doi=10.1103/PhysRevLett.116.151802 |pmid=27127957 |date= |year=2016 |last1=Nakai |first1=Yuichiro |last2=Sato |first2=Ryosuke |last3=Tobioka |first3=Kohsaku |bibcode=2016PhRvL.116o1802N }}</ref> X,<ref name="Dutta_etal">{{cite journal|arxiv=1512.05439|title=Interpretation of the diphoton excess at CMS and ATLAS |journal=Physical Review D |volume=93 |issue=5 |pages=055032 |doi=10.1103/PhysRevD.93.055032 |date= |year=2016 |last1=Dutta |first1=Bhaskar |last2=Gao |first2=Yu |last3=Ghosh |first3=Tathagata |last4=Gogoladze |first4=Ilia |last5=Li |first5=Tianjun |bibcode=2016PhRvD..93e5032D }}</ref> η<sub>zy</sub>,<ref>{{cite arXiv |eprint=1602.05539|last1=Zhang|first1=Yu-Jie|title=The Fourth Generation Quark and the 750 GeV Diphoton Excess|last2=Zhou|first2=Bin-Bin|last3=Sun|first3=Jia-Jia|class=hep-ph|year=2016}}</ref>
X<ref>{{cite journal|arxiv=1512.05439|title=Interpretation of the diphoton excess at CMS and ATLAS |journal=Physical Review D |volume=93 |issue=5 |pages=055032 |doi=10.1103/PhysRevD.93.055032 |date= |year=2016 |last1=Dutta |first1=Bhaskar |last2=Gao |first2=Yu |last3=Ghosh |first3=Tathagata |last4=Gogoladze |first4=Ilia |last5=Li |first5=Tianjun |bibcode=2016PhRvD..93e5032D }}</ref>,
η<sub>zy</sub><ref>{{cite arXiv |eprint=1602.05539|last1=Zhang|first1=Yu-Jie|title=The Fourth Generation Quark and the 750 GeV Diphoton Excess|last2=Zhou|first2=Bin-Bin|last3=Sun|first3=Jia-Jia|class=hep-ph|year=2016}}</ref>
| mass =
| mass =
{{nowrap|≈ 750 [[Electronvolt (mass)|GeV/''c''<sup>2</sup>]]}} (CMS + ATLAS)<ref name='1606.03833'/><ref name='1606.04093'/>
{{nowrap|≈ 750 [[Electronvolt (mass)|GeV/''c''<sup>2</sup>]]}} (CMS+ATLAS)<ref>{{cite web | publisher = CERN | department = ATLAS note | id = ATLAS-CONF-2015-081 | url = https://cdsweb.cern.ch/record/2114853 | title = Search for resonances decaying to photon pairs in 3.2 fb<sup>−1</sup> of ''pp'' collisions at √''s'' = 13 TeV with the ATLAS detector | date = 15 Dec 2015 | author = The ATLAS collaboration }}</ref><ref>{{cite web | publisher = CERN | department = CMS note | id = CMS-PAS-EXO-15-004 | url = https://cds.cern.ch/record/2114808 | title = Search for new physics in high mass diphoton events in proton-proton collisions at 13 TeV | date = December 15, 2015 | author = CMS Collaboration }}</ref>
|width = < {{val|50|ul=GeV/c2}}
|width = < {{val|50|ul=GeV/c2}}<ref name='1606.03833'/><ref name='1606.04093'/>
| decay_particle = {{plainlist|
| decay_particle = {{plainlist|
* two [[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 [[photon]]s (potentially observed)
* two [[Z-boson]]s (predicted)
* two [[Z-boson]]s (predicted)
* one [[photon]] + one [[Z-boson]] (predicted)
* one [[photon]] + one [[Z-boson]] (predicted)
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| electric_charge = 0 ''e''
| electric_charge = 0 ''e''
| colour_charge = 0
| colour_charge = 0
| spin =
| spin = not known
| parity = not known
| num_spin_states =
}}
}}


The '''750 GeV diphoton excess''' in [[particle physics]] is an anomaly in data from the [[Large Hadron Collider]] (LHC) in 2015, which could have been an indication of a new particle or [[Resonance (particle physics)|resonance]]. The name for the hypothetical particle used in the scientific discourse is the [[Greek alphabet|Greek]] letter [[Digamma|Ϝ]] (pronounced digamma), owing to the decay channel in which the anomaly occurred,<ref name="arxiv1">{{cite arXiv |eprint=1605.09401|last1=Strumia|first1=Alessandro|title=Interpreting the 750 GeV digamma excess: A review|class=hep-ph|year=2016}}</ref> though other symbols such as Ϝ(750), ϕ and X are also used. The digamma would have been the first elementary particle beyond the [[Standard Model]] (SM) to be discovered. The diphoton excess was not expected to be caused by an experimental or theoretical [[systematic error]].<ref name="arxiv1"/> The data were always less than five [[standard deviation]]s (sigma) different from that expected if there was no new particle, and, as such, the anomaly did not reach the accepted level of [[statistical significance]] applied in the field.
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 (particle physics)|resonance]].<ref name='1606.03833'>
{{cite arXiv |eprint=1606.03833}}</ref>
<ref name='1606.04093'>
{{cite arXiv |eprint=1606.04093}}</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|url=https://cds.cern.ch/record/2205245|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}}
</ref>
<ref name='ATLAS-CONF-2016-059'>
{{cite web|url=https://inspirehep.net/record/1480039|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}}
</ref>
In the interval between the 2015 and 2016 data, the anomaly generated considerable interest in the scientific community, including about 500 theoretical studies.<ref>
⚫
{{Cite web|url=http://jsfiddle.net/adavid/bk2tmc2m/show/|title=#Run2Seminar and subsequent γγ-related arXiv submissions|last=|first=|date=|website=jsfiddle.net|publisher=|access-date=2016-08-11}}
</ref>
The hypothetical particle was denoted by the [[Greek alphabet|Greek]] letter [[Digamma|Ϝ]] (pronounced digamma) in the scientific literature, owing to the decay channel in which the anomaly occurred.<ref name='1605.09401'>
{{cite arXiv |eprint=1605.09401|last1=Strumia|first1=Alessandro|title=Interpreting the 750 GeV digamma excess: A review|class=hep-ph|year=2016}}
</ref>
The data, however, were always less than five [[standard deviation|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.<ref>
{{cite arXiv |eprint=1310.1284}}
</ref>
The digamma is now an [[Superseded scientific theories|obsolete scientific theory]].


== Diphoton excess (2015) ==
== December 2015 data ==


On December 15, 2015, the [[ATLAS]] and [[Compact Muon Solenoid|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 distribution of pairs of high-energy photons produced in the proton collisions showed an excess of events compared to the [[Standard Model]] prediction at an [[invariant mass]] of the two photons of around 750&nbsp;GeV/''c''<sup>2</sup>. The [[statistical significance]] of the deviation was reported to be 3.9 and 3.4 [[standard deviation]]s (locally) respectively for each experiment.
On December 15, 2015, the [[ATLAS]] and [[Compact Muon Solenoid|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 distribution of pairs of high-energy photons produced in the proton collisions showed an excess of events compared to the [[Standard Model]] prediction at an [[invariant mass]] of the two photons of around 750&nbsp;GeV/''c''<sup>2</sup>. The [[statistical significance]] of the deviation was reported to be 3.9 and 3.4 [[standard deviation]]s (locally) respectively for each experiment.


If it is not a statistical fluctuation, the excess can be explained by the production of a proposed new particle Ϝ(750) with a mass around 750&nbsp;GeV/''c''<sup>2</sup> that decays into two photons. The [[Cross section (physics)|cross-section]] at 13&nbsp;TeV centre of mass energy required to explain the excess, multiplied by the [[branching fraction]] into two photons, is estimated to be
The excess could have been explained by the production of a new particle (the digamma) with a mass of about 750&nbsp;GeV/''c''<sup>2</sup> that decayed into two photons. The [[Cross section (physics)|cross-section]] at 13&nbsp;TeV centre of mass energy required to explain the excess, multiplied by the [[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>
:<math>\sigma(pp\to \digamma) \times {\rm Br}(\digamma\to \gamma\gamma)\approx 5\,{\rm fb}</math>


(fb=[[Barn (unit)|femtobarns]])
(fb=[[Barn (unit)|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&nbsp;TeV.
This result, while unexpected, is compatible with previous experiments, and in particular with the LHC measurements at a lower centre of mass energy of 8&nbsp;TeV. Further investigations are under way in early 2016 that may confirm or disprove the existence of Ϝ during 2016.<ref>{{Cite journal |title=750 GeV Diphoton Excess May Not Imply a 750 GeV Resonance|journal=Physical Review Letters |volume=116 |issue=15 |pages=151805 |date= |doi=10.1103/PhysRevLett.116.151805 |pmid=27127960 |year=2016 |last1=Cho |first1=Won Sang |last2=Kim |first2=Doojin |last3=Kong |first3=Kyoungchul |last4=Lim |first4=Sung Hak |last5=Matchev |first5=Konstantin T. |last6=Park |first6=Jong-Chul |last7=Park |first7=Myeonghun |bibcode=2016PhRvL.116o1805C |lay-url=http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.116.151805 |lay-source=Physics |lay-date=April 12, 2016}}</ref>


⚫
== August 2016 data ==
The width of the potential resonance is unknown. ATLAS results initially hinted at a width of around 50&nbsp;GeV/''c''<sup>2</sup>, while data from CMS fits better to a narrower width. Most theoretical explanations prefer a narrow width {{Citation needed|date=July 2016}}.


A larger sample of data collected in 2016 could not confirm the existence of the [[Digamma|Ϝ]] particle, suggesting that the excess seen in 2015 was a statistical fluctuation.<ref name='CMS-PAS-EXO-16-027'/><ref name='ATLAS-CONF-2016-059'/>
== Interpretation ==
If confirmed, the diphoton excess would imply that the Standard Model of particle physics must be extended with new particles and possibly new interactions.

The excess observed by ATLAS and CMS can be explained through the existence of a new particle with zero electric charge and a mass of about 750&nbsp;GeV/''c''<sup>2</sup>, called Ϝ or digamma, decaying into two photons. The spin of the resonance could either be 0 or 2 (spin-1 is excluded by the [[Landau-Yang theorem]]) to allow for a decay to two photons, but theoretical arguments strongly favour the spin-0 hypothesis. To be produced at LHC with significant cross-section, Ϝ(750) should couple to some of the constituents of the proton, mostly [[quarks]] and [[gluon]]s.

Several mechanisms of production and decay have been proposed. Compatibility with LHC measurements at 8&nbsp;TeV and the size of the cross-section favour production through gluon fusion as illustrated by the [[Feynman diagram]] in the figure. This was the dominant production mechanism for the [[Higgs boson]] discovered in 2012 at the LHC. Given that the particle decays into photons, it should also decay into [[W and Z bosons]], although the relative probability of these decays is not uniquely predicted.

⚫
After the announcement, over 500 theoretical studies appeared investigating possible explanations and implications of the proposed new particle.<ref>{{Cite web|url=http://jsfiddle.net/adavid/bk2tmc2m/show/|title=#Run2Seminar and subsequent γγ-related arXiv submissions|last=|first=|date=|website=jsfiddle.net|publisher=|access-date=2016-08-10}}</ref> The models can be roughly divided into two categories, depending on the nature of the new particle:
* Elementary Ϝ(750): the new resonance is an elementary particle similar to the [[Higgs boson]] in the SM.
* Composite Ϝ(750): the new resonance is a bound state of new interactions, similar for example to [[pions]] in [[quantum chromodynamics]].

In all models, the coupling of a neutral particle to photons would require the existence of other particles beyond the SM, carrying electric charge.
If Ϝ(750) couples to gluons, new particles with [[color charge|color]] are also required. As a consequence, in all scenarios considered a whole new sector of particles must be added to the SM, some of which are likely to be within the energy reach of the LHC. For example, many scenarios predict the existence of new [[fermion]]s with SM charges that, unlike the known fermions, do not acquire mass through the Higgs mechanism. If F(750) is elementary, the new fermions could be produced, while in the composite scenarios bound states are more likely to be formed.

It is also possible that Ϝ(750) has invisible decays into particles without SM charges.
These decays could be connected to the existence of [[dark matter]] in the Universe.<ref>{{Cite journal|arxiv=1512.06562|title= Scalar Dark Matter Explanation of Diphoton Excess at LHC |journal= Nuclear Physics B |volume= 907 |issue= 180 |pages= 180 |date= |last1= Han |first1= Huayong |last2= Wang |first2= Shaoming |last3= Zheng |first3= Sibo |year= 2015 |doi= 10.1016/j.nuclphysb.2016.04.002 }}</ref>

⚫
== August 2016 data results ==

A first analysis of 2016 data by the experiments could not confirm the existence of the [[Digamma|Ϝ]] particle. The excess seen in 2015 seems to be a statistical fluctuation.<ref>{{cite web|url=https://press.cern/press-releases/2016/08/chicago-sees-floods-lhc-data-and-new-results-ichep-2016-conference|title=Chicago sees floods of LHC data and new results at the ICHEP 2016 conference|date=5 August 2015|accessdate=5 August 2015}}</ref><ref>{{cite web | url = https://cds.cern.ch/record/2205245/files/EXO-16-027-pas.pdf | title = CMS Physics Analysis Summary | author = The CMS Collaboration | publisher = CERN | accessdate = August 4, 2016 }}</ref><ref>{{cite news | url = http://www.bbc.com/news/science-environment-36976777 | title = New particle hopes fade as LHC data 'bump' disappears | publisher = BBC News | first = Pallab | last = Ghosh | date = 5 August 2016 }}</ref>


==References==
==References==
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[[Category:2016 in science]]
[[Category:2016 in science]]
[[Category:Large Hadron Collider]]
[[Category:Large Hadron Collider]]
[[Category:Obsolete scientific theories]]

Revision as of 13:10, 11 August 2016

Digamma
Possible production and decay mechanism of the digamma resonance at LHC.
CompositionElementary particle
StatisticsBosonic
StatusDead. Absent in August 2016 data[1][2]
SymbolϜ[3],

Ϝ(750)[4], ϕ[5], X[6],

ηzy[7]
DiscoveredResonance of mass ≈750 GeV decaying into two photons could have been seen by CERN in 2015[8][9] (though sufficient statistical significance never reached)
Mass≈ 750 GeV/c2 (CMS + ATLAS)[8][9]
Decay width< 50 GeV/c2[8][9]
Decays into
Electric charge0 e
Colour charge0
Spinnot known

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 2015 and 2016 data, 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] The digamma is now an obsolete scientific theory.

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 distribution of pairs of high-energy photons produced in the proton collisions showed an excess of events compared to the Standard Model prediction at an invariant mass of the two photons of 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.

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

(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

A larger sample of data collected in 2016 could not confirm the existence of the Ϝ particle, suggesting that the excess seen in 2015 was a statistical fluctuation.[1][2]

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".
  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".
  3. ^ a b Strumia, Alessandro (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 (2016). "Digamma, what next?". arXiv:1604.06446 [hep-ph].
  5. ^ Nakai, Yuichiro; Sato, Ryosuke; Tobioka, Kohsaku (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 (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 (2016). "The Fourth Generation Quark and the 750 GeV Diphoton Excess". arXiv:1602.05539 [hep-ph].
  8. ^ a b c d e A bot will complete this citation soon. Click here to jump the queue arXiv:1606.03833.
  9. ^ a b c d e A bot will complete this citation soon. Click here to jump the queue arXiv:1606.04093.
  10. ^ "#Run2Seminar and subsequent γγ-related arXiv submissions". jsfiddle.net. Retrieved 2016-08-11.
  11. ^ A bot will complete this citation soon. Click here to jump the queue arXiv:1310.1284.
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