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A What are the implications of a new 750GEV particle to GUTs?

  1. May 3, 2016 #1
    What are the implications of a new 750GEV particle to current GUTs?

    in the next couple of months, hopefully the LHC will either confirm or not, a new previously unknown particle at 750GEV based on a diphoton excess.

    can currently researched GUTs like SU(5), SO(10) etc accommodate a new particle, spin 0 or spin 2, of mass 750 GEV?

    and if the LHC does confirm a new 750GEV, how would it effect the Higgs hierarchy problem?

    is there a fine-tuning problem for the Higgs with the scale of new physics occurring at 750GEV?

    how would the 750GEV particle affect Higgs hierarchy proposed solutions like SUSY and Technicolor?
     
  2. jcsd
  3. May 8, 2016 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
     
  4. May 10, 2016 #3

    ohwilleke

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    A new 750 GeV particle would almost certainly imply multiple other particles to go with it in a class of particles that do not currently exist. Most GUTs are formulated with the idea that they should predict few particles other than those of the Standard Model, so many won't have an obvious place to fit this although heroic efforts will be made to do so. Now obviously, you can formulate a GUT with arbitrarily many particles and forces as you can imagine (and this 750 GeV particle would probably imply the need for another Standard Model gauge coupling force as well), so I'm sure someone could come up with some way to fit it into a GUT scheme, but it would probably be one of the less parsimonious models.

    A number of alternative gravity theories feature both a massive and a massless graviton, but generally, the massive graviton in those theories is much lighter and much longer lived.

    Unless it contributes to the Higgs field and acquires mass by the mechanism it wouldn't impact the Higgs hierarchy problem at all, and we certainly don't know if this particle acquires mass by this means as opposed, for example, to being a composite particle of some sort that acquires most of its mass via the binding force that holds it together the way that hadrons do. The fact that the vacuum expectation value of the Higgs field is equal to the sum of the square of all of the Standard Model fundamental particle masses (almost exactly) hints at least that this particle's mass does not derive from the Higgs field or Higgs physics - and its properties have little or nothing in common with a hypothetical heavy Higgs boson from two or more Higgs doublet models. A heavy Higgs ought to decay differently than this apparently particle appears to. And, so it isn't a good fit to the hypothetical A or H particles of SUSY theories with multiple Higgs doublets.

    A neutral electric charge both implies the need for intermediate charged particles at some point in the decay/production chain (since photons aren't produced by particles without electric charge making diphoton decays impossible), but the pool of possible unstable spin-0 SUSY particles with neutral electric charge is not terribly large. If it is produced via gluon fusion as some theorists suggest, it would also need to have strong force color charge, which sleptons and squarks in SUSY theories both lack. SUSY gluinos are fermions while this, if it exists, if a boson. Bottom line - you need a pretty elaborate SUSY model to be a good fit.

    At least two preprints address the possibility that this resonance is a techni-pion. http://arxiv.org/abs/1604.02037 and http://arxiv.org/abs/1512.05564
     
    Last edited: May 10, 2016
  5. May 10, 2016 #4
    thanks for replying.

    i've heard that the 750 while it rules out MSSM does not rule out NMSSM

    if the 750 is a heavier higgs, how would that address the fine tuning problem?

    since they found a higgs at 126 is technicolor still viable? since the higgs is what breaks ew-symmetry is there a well motivated reason to continue to pursue technicolor?

    regards
     
  6. May 11, 2016 #5

    Haelfix

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    It's very much too early to say anything definitive. We don't know enough about the purported particles properties to really get into deep theoretical issues. Certainly *if* this turns out to be a real physical effect (and not some sort of systematic error), the devil will be in the details. The couplings, what channels it participates in, and so forth.

    Clearly every model with the word "minimal" in it, will need to be revised. This isn't the first time new physics has come out of left field and surprised everyone, the discovery of the muon was exactly like that.

    Many of the assumptions and arguments going into words like naturalness, GUTs and so forth will need to be rethought in light of what could be an entirely new landscape. I know this isn't satisfying, but unfortunately model building this beast is anything but simple. It really does suggest complicated new physics if it turns out to be true.

    Regarding technicolor, the original version of technicolor was ruled out long ago, and most of the remaining parameter space of the *simplest* versions have now been ruled out with the recent discovery of the Higgs.
     
  7. May 11, 2016 #6

    mfb

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    If it is a particle, we don't know its lifetime. The best ATLAS fit corresponds to some width but the narrow width fit works as well, and in CMS data a narrow width is favored. That gives some minimal lifetime - it cannot be too wide. A maximal lifetime is much more challenging, you cannot use the decay width and with the photons you don't even have precise tracking. It could fly a few millimeters before the calorimeter photon pointing would notice it. At nonrelativistic speeds (small pT of the photon pairs), a few millimeters are tens of picoseconds.

    Edit:
    A systematic error is the most unlikely result. A statistical fluctuation: possible. But there is no way to get a peak in the diphoton spectrum by doing something wrong.
     
    Last edited: May 11, 2016
  8. May 11, 2016 #7
    what about SUSY ? its my understanding MSSM is ruled out if this particle is real.

    how soon can LHC confirm at current data collection?
     
  9. May 11, 2016 #8

    mfb

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    Optimistic: ~2-3/fb for analyses shown at ICHEP (3rd to 10th of August). That is smaller than the 2015 dataset (~4/fb), but if the excess is real at the 2015 signal strength, both experiments should have something like 2.5-3.5 sigma local significance in 2016 data alone, and people will get really excited. Combinations might even reach 5 sigma local significance per experiment, combining both experiment could give that as global significance. Such a combination is unlikely, however, as the experiments will focus on analyzing larger datasets.
    The full 2016 dataset, currently expected to be 20/fb, will then confirm the existence beyond reasonable doubt and will allow a reasonable cross section measurement.

    Pessimistic for LHC performance but with particle: 1 to 1.5/fb shown at ICHEP, ~2 sigma if the excess is real and the signal strength is as in 2015. Very interesting, but probably not sufficient to celebrate. A few weeks more should be a few 1/fb more, sufficient to get high significances.

    Optimistic for LHC performance but without particle: No excess in 2-3/fb, combination with 2015 still has some significance, but interest will drop rapidly.

    Pessimistic: 1 to 1.5/fb, no excess, but the dataset is too small to draw any conclusion. Interest will still drop a bit.
     
  10. May 11, 2016 #9
    thanks, any specific dates when cern lhc will announce the data results analysis from the new run 2016 starting now, that i can mark on my calender? thanks in advance
     
  11. May 11, 2016 #10

    mfb

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    Probably at ICHEP, 3rd to 10th of August. There is no detailed schedule available yet for the conference, but typically they try to move important things to the front so talks in the parallel sessions can show details.

    I would expect an update on the diphoton mass spectrum later in the year, and an analysis of the full dataset ~1.5-2 months after data taking ends (~1st of November), probably before Christmas like last year.

    All those things are just guesses based on typical timescales of analyses and previous publication schedules.
     
  12. May 11, 2016 #11
    how would this impact SUSY? 750 gev spin 0 is not a SUSy of any known SM partner, and thus far gluinos have not shown up
     
  13. May 11, 2016 #12

    Haelfix

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    Yea I've seen it argued several ways now, and with all due respect i'm skeptical. The diphoton channel is indeed very clean theoretically, so I don't think it is a misunderstood background attribution error, and the systematic checks from the CMS and Atlas groups looks solid, nevertheless large statistical fluctuations of this size (even with the trial factor) in both detectors don't happen every day. There was also some tension with run 1, so yea I'm not entirely sold on the clean systematic hypothesis quite yet.
     
  14. May 11, 2016 #13
    How would this particle imply other particles? If its a new particle that the standard model didn't predict, then how could you possibly know it implies anything at all? Why couldn't it just be one particle on its own?
     
  15. May 11, 2016 #14
    What's the probability that this particle isn't a statistical fluke? Are there good odds that this is the real deal?
     
  16. May 12, 2016 #15

    mfb

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    Systematic effects that large, in such a clean channel, would be something completely new I think. A statistical fluctuation like that can happen. Remember the fluctuation around 140(?) GeV in the Higgs search? It was not as significant, but still interesting and both ATLAS and CMS had it. Went away with more data. LHCb had a 3.5 sigma deviation in delta A_CP, which is a single measurement value so there is no local significance. Went away with more data.
    If it is a new particle, it is uncharged, but there has to be some interaction that leads to photons - it has to be another particle in a loop. If that particle is a standard model particle, then the 750 GeV particle would be heavy enough to directly decay to a pair of that particles - much more frequent than the diphoton decay. We would have seen that already. We also have various other constraints - it cannot influence the anomalous magnetic moment of electrons or muons too much, for example.
    Depends on who you ask, but I think most particle physicists see the probability for an actual particle somewhere between 1% and 20%.
     
  17. May 13, 2016 #16

    ohwilleke

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    There is absolutely uncertainty in the width of a potential 750 GeV mass resonance. But, any proposed massive graviton would have to have a lifetime sufficient to travel at a minimum from galaxies to their satellite galaxies at speeds of a something slightly less than the speed of light. Certainly, mass gravitons would be expected to have mean lifetimes longer than the mean lifetime of a free neutron (i.e. longer than 14 minutes before adjusting for special relativity), which would mean that any of its decays would happen outside the range of the detectors at the LHC. Any particle that decays fast enough for its decays to be detected by a detector at the LHC has a mean lifetime too short to be a massive graviton.

    Also, I disagree with you on the issue of systemic error. There are an infinite number of ways that systemic error can arise, and some of them could absolutely produce a peak in the diphoton spectrum. To cite just one example, there are probably half a dozen ways that some serious but overlooked screwup in the computer code that takes the raw data from the LHC and converts it into output used by scientists at the LHC could create such a result. Some systemic errors are just way out there - consider the loose electrical connection in the Opera superluminal neutrino debacle. You probably don't get a peak in the diphoton spectrum from the usual systemic errors like calibration and imprecision issues, but the fact that you can rule out the "usual suspects" for causes of systemic error doesn't mean that you can rule out the completely unexpected ones. Murphy's Law is a powerful thing and unlike statistical fluctuations, is much less subject to being quantified in an accurate way.
     
  18. May 13, 2016 #17

    ohwilleke

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    The MSSM has been pretty much ruled out already with or without this particle. The MSSM continues to be used for a benchmark for much the same reasons that we still use Newtonian gravity for many purposes. Even though we know that both are not accurate depictions of Nature, they are accurate enough to use as toy models that simplify reality in respects that often aren't important for the purposes we want to use them for.

    Newtonian gravity is adequate for lots of dark matter modeling in many body problems at the galactic level. The MSSM is sufficient to model the properties that the lightest supersymmetric particles ought to have if any mainstream less minimal form of SUSY is real when setting up computer models to distinguish potential SUSY events from clearly non-SUSY events at a collider.
     
    Last edited: May 13, 2016
  19. May 13, 2016 #18

    ohwilleke

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    One of the easier ways to know that this can't be one particle on its own is that it is overall electrically neutral, but it is decaying in a diphoton mode. An electrically neutral fundamental particle, by definition, cannot decay to or couple with photons (e.g. a neutrino and an anti-neutrino cannot directly annihilate each other into photons). Electric charge is defined as propensity to couple to photons. So, the particle would have to decay to a charged particle and a charged antiparticle which in turn could give rise to a diphoton decay path (or it could be a composite particle with charged components). But, there are no Standard Model intermediate particles that are obvious fits because we'd see additional channels of decay from Standard Model charged particles with 375 GeV of mass-energy. This isn't the only reason that there need to be other particles, but it is the most straightforward to explain.
     
  20. May 13, 2016 #19

    ohwilleke

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    Lubos Motl, one of the most "optimistic" physics bloggers out there is on record saying there is a 50% likelihood that it is real. I call that irrational exuberance and tend to concur with mfb that 1% to 20% is closer to the mark.
     
  21. May 13, 2016 #20
    i've not heard MSSM is ruled out, is it bc of not finding gluinos below 1.5 TEV range? so are researchers now into nMSSM?

    just as gluons w z photons are all spin 1 could you have a spin-2 particle but not be a graviton,
     
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