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Featured I LHCb observes 5 new particles (excited hadrons)

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  1. Mar 18, 2017 #1

    mfb

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    Plain old standard model baryons, but 5 at the same time, and with crystal clear peaks in the decay to ##\Xi_c^+ K^-##. Each peak in the figure is a particle never seen before, and the significances of those peaks are about 20 sigma (10 sigma for the 5th one). A broad 6th state might hide around 3200 MeV.

    Based on the quark content (charm strange strange), all those particles are excited ##\Omega_c^0## states. The ground state and one excited state have been known before.

    News at the LHCb website
    Publication at arXiv

    [​IMG]
     
  2. jcsd
  3. Mar 19, 2017 #2
    Can the excited hadrons be used to constrain BSM models or similar in any useful way, or is it mainly just dotting the i's?
     
  4. Mar 19, 2017 #3

    mfb

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    It can help to understand QCD at low energies better. The predictions for excited states are often quite inaccurate. No BSM physics.

    Here is an example for excited charmonium states. See the mass vs. JPC diagram: Yellow are predicted and discovered states, nearly all are below the threshold to decay to two charmed mesons. Grey are predicted but undiscovered states - most predicted states at higher energies have not been found. Red are discovered but unpredicted states which don't fit in the predicted pattern. And finally there are the tetraquarks in purple which also don't fit in.
     
  5. Mar 19, 2017 #4

    Vanadium 50

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    From a low-energy QCD perspective, this is very interesting: these particles are protons, but with every light quark replaced by a heavy one. That lets one separate effects due to quark mass and effects not due to quark mass. For this, the mass differences are more important than the absolute masses, and I would hope LHCb would make these public soon.
     
  6. Mar 19, 2017 #5

    mfb

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    You get a conservative uncertainty if you assume that both statistical and systematic uncertainty are uncorrelated, and treat the uncertainty from the ##\Xi## mass as fully correlated (because it is, obviously). The uncertainties on the masses are between 0.15 MeV and 0.9 MeV, the mass differences are between 15 and 120 MeV. A ~2% uncertainty on the mass differences with conservative assumptions. Is that too large?
     
  7. Mar 19, 2017 #6

    ChrisVer

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    interesting again... I envy LHCb a little (it always discovers something- of course not 100% revolutionary)...
     
  8. Mar 19, 2017 #7

    Vanadium 50

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    I don't do that kind of physics, but those who do tell me that they want the errors as small as possible. How much can they do with a 2% error that they couldn't do with a 3% error? Hard to tell. That said, it should be relatively easy for LHCb to do this: instead of m1..m5 being the fit variables, fot m1, (m2-m1), (m3-m2), etc. and repropagate the systematics.
     
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