Of Particular Significance

The Two-Photon Excess at LHC Brightens Slightly

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 03/18/2016

Back in December 2015, there was some excitement when the experiments ATLAS and CMS at the Large Hadron Collider [LHC] — especially ATLAS — reported signs of an unexpectedly large number of proton-proton collisions in which

  • two highly energetic photons [particles of light] were produced, and
  • the two photons could possibly have been produced in a decay of an unknown particle, whose mass would be about six times the mass of the Higgs particle (which ATLAS and CMS discovered in 2012.)

This suggested the possibility of an unknown particle of some type with rest mass of 750 GeV/c².  However, the excess could just be a statistical fluke, of no scientific importance and destined to vanish with more data.

The outlook for that bump on a plot at 750 GeV has gotten a tad brighter… because not only do we have ATLAS’s plot, we now have increasing evidence for a similar bump on CMS’s plot. This is thanks largely to some hard work on the part of the CMS experimenters.  Some significant improvements at CMS,

  1. improved understanding of their photon energy measurements in their 2015 data,
  2. ability to use 2015 collisions taken when their giant magnet wasn’t working — fortunately, the one type of particle whose identity and energy can be measured without a magnet is… a photon!
  3. combination of the 2015 data with their 2012 data,

have increased the significance of their observed excess by a moderate amount. Here’s the scorecard.*

  • CMS 2015 data (Dec.): excess is 2.6σ local, < 1σ global
  • CMS 2015 data (improved, Mar.) 2.9σ local, < 1σ global
  • CMS 2015+2012 data: 3.4σ local, 1.6σ global
  • ATLAS 2015 data (Dec. and Mar.): 3.6σ local, 2.0σ global to get a narrow bump [and 3.9σ local , 2.3σ global to get a somewhat wider bump, but notice this difference is quite insignificant, so narrow and wider are pretty much equally ok.]
  • ATLAS 2015+2012 data: not reported, but clearly goes up a bit more, by perhaps half a sigma?

You can read a few more details at Resonaances.

*Significance is measured in σ (“standard deviations”) and for confidence in potentially revolutionary results we typically want to see local significance approaching 5σ and global approaching 3σ in both experiments. (The “local” significance tells you how unlikely it is to see a random bump of a certain size at a particular location in the plot, while the “global” significance tells you how unlikely it is to see such a bump anywhere in the plot … obviously smaller because of the look-elsewhere effect.)

This is good news, but it doesn’t really reflect a qualitative change in the situation. It leaves us slightly more optimistic (which is much better than the alternative!) but, as noted in December, we still won’t actually know anything until we have either (a) more data to firm up the evidence for these bumps, or (b) a discovery of a completely independent clue, perhaps in existing data. Efforts for (b) are underway, and of course (a) will get going when the LHC starts again… soon!  Next news on this probably not til June at the earliest… unless we’re very lucky!

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24 Responses

  1. G’day Professor Strassler , what is a dimension of space ? How do the three dimensions of space come into being? From whence do they originate? How does one dimension of space become two dimensions working in unison? How does that then become three dimensions of space all working together in complete unison even when travelling at light speed? For light to travel even at the far reaches of the universe it has to have the three dimensions of space to travel into, from whence and via what mechanism does this three dimensional space come into being. Surely for three dimensions of space to come into being, energy is required, so what is this energy ,what are its origins and how does it work? Never mind the possible 10 or 11 dimensions,start with something simple like how do you get 2 or 3 dimensional plains working and moving together. Are the dimensions of space ( length/ energy ) continuous or discrete and what is the working relationship with light speed and relative motion? How does this effect entropic balance throughout the universe? Now for some light comedy- am I a mug for asking such questions? Kindly respond at your leisure and pleasure. Cheers. Pete. Stiphout .

    Sent from my iPad


  2. G’day Professor Strassler , the dark energy distribution is always smooth and always remains constant over time and does not dilute as the universe expands. The dark energy density- energy not carried by particles or matter – remains the same. For this reason,physicists refer to this type of energy as a cosmological constant. To quote Professor Susan Randall! This being so I ask ,where does this energy come from? For the dark energy density to remain constant over time and universal expansion can only mean that energy is coming into the universe. That does rather seem to contradict the idea that the amount of energy in the universe is constant ,unchanged since the Big Bang. A universe and even a multi-verse is much more easily explained mathematically via the use of a singular dimension infinite length/energy string . Then constructing a three dimensional universe then add relative motion and expansion . All quite easily done using dual inputs. What I call Duality systems may be similar or same to Transcendence maths systems. On the shape of the universe , the complete lack of observable curvature is highly intriguing and I wonder could that shape be related to the structure of the Higgs Field and might that structure be determined by observation of three different types of Higgs Boson. Thanks to you and Professor Randall just for having beautiful minds and sharing them. Cheers. Pete. Stiphout . Sent from my iPad


  3. G’day Professor, may I have the new mass/energy numbers for the up and down quark, that you mentioned. As I have developed a system of patterns to describe particles but have no accurate figures for quark m/e. Thanks for your well chosen words on physics, cheers. Pete. S.

    Sent from my iPad


  4. Also the micro black holes generate the gravitational waves distorting the spacetime the quantum vacuum, for the discrete spacetime, that break , the continuity and the smoothness in the 4dimensional worlds, deforming the fábrics of curved spacetime given by a complex geometry as the of riemann.the hawking radiation could not evaporate the micro, because there are integral path generated by múltiples violations of PT that measure the spacetime curves with differents metrics

  5. Forgive me for bringing up another topic, nuclear fusion, but these experiments are making me nervous. The sun is a nuclear fusion reactor and stars of a certain mass can create a black hole if and when the collapse.

    So, is it possible under the right setup to create a micro black hole in a nuclear fusion experiment?

    1. I believe that if it did, it would almost immediately evaporate due to Hawkings Radiation.

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  7. “A second Higgs (spin-0) of some type is possible, but only if you add still more new particles that are not in the minimal version of supersymmetry.” What would be the minimum number of such new particles that would have to be added? Why are there 3 generations of fermions? Why are there not 3 generations of bosons?

    1. The number of gauge bosons is determined by the interactions-so the fact that there are eight gluons is a consequence of the properties of the charges of the strong force, similarly that there are four gauge bosons for the electroweak interactions. Why there are three generations of fermions isn’t known-but it is known that there must be as many generations of leptons-particles that have only electroweak interactions-as quarks, particles that, also, have strong interactions. The number of scalars is considerably more complicated to determine; in the Standard Model one needs at least four such fields, three become parts of the massive gauge bosons and one is the Higgs, for example.

  8. Based on the configurations of the Atlas and CMS, wouldn’t it be harder to detect the higher Higgs at 750 GeV than the one at 125 GeV? To this point could a relationship be derived to calculation the ratio of detection of one over the other, based the known measured parameters?

    Layman’s speculation, 750/125 = 6 … a nice round number!

  9. G’day Matt, from my own work I would suggest that the energy of the graviton is exactly equal to that of a gluon and that gluons are the exact equal and opposite of each other, furthermore the energy of 2 photons is exactly equal to the graviton or gluon. Also the universe is constructed solely of a singular dimensional string of infinite length/energy which is composed entirely of gluons and graviton. I have sent the maths of my work to Professor M.T.Lough,formerly of Murray State University as I know of no other Professors as I am myself a self taught person and work mostly as a Bushman in Australia. Cheers. Pete. S.

  10. Those two photon excess are due without doubts for the supersymmetry or extradimensions,could believe that the photons have differents energias and frequencies and can appear as anihillations of positrons and electrons that have several energy level.some once occur the somatorium of two or mores quantic states.with thendiscovered of anomalies in the spectrum of energy for antineutrinos,is possible that such neutrinos as antineutrinos appear as energy,momentum and carrying spins,more complex,or the speed of light is not constant,maybe limit and these diferences of energy and mass appear in these experiments

  11. Thanks Matt. It’s great to experience how our understanding of the universe unfolds slowly but surely.

  12. Matt, I feel this is a stretch but I’ll ask anyway. Is there any proposed SUSY theory that would allow for the speculated particle to possibly be a Higgsino?

    1. A Higgsino is spin-1/2 and cannot decay to two photons. So there is *no* possibility at all, I am afraid.

      A second Higgs (spin-0) of some type is possible, but only if you add still more new particles that are not in the minimal version of supersymmetry. It’s best to say that this particle is not predicted by supersymmetric models but isn’t obviously incompatible with them either.

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