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Can someone explain Psi and Upsilon resonance?

  1. Jul 26, 2012 #1

    I am an undergrad doing research with CERN and in examining data from ATLAS. I was looking for instances of Z bosons decaying into muons by taking systems with two energetic, oppositely charged muons, adding their four vectors, and getting the invariant mass of the result. There was an expected local peak at ~90 GeV (the z boson mass), but there was an even higher peak at about 3 GeV. There were also peaks at ~0.75, ~1.1, ~3.75, and 9 GeV. My professor said it was probably caused by Psi and Upsilon Resonances.

    Could someone explain what exactly these are to me? I haven't found any direct explanations by searching, mostly just very technical explanations and specific papers not about these phenomenon in general.
  2. jcsd
  3. Jul 26, 2012 #2
    As almost always these days, wikipedia has the answers:


    The J/psi is a meson, i.e. quark/anti-quark bound state, in particular it is one made of charm quarks ("charmonium"). There are various such charmonium states, with different orbital angular momentum of the quarks:


    J/psi is the first excited state, with a mass about 3 GeV. The upsilon is the same sort of deal, just made out of bottom/anti-bottom quarks ("bottomonium"). I don't know which excitation of bottomonium it is, probably it is the ground state since b quarks are pretty heavy.
  4. Jul 26, 2012 #3
    Thanks, but I already read those.
    I guess my question should have been, What is the resonance part. I remember a bit about resonance from classical mechanics and waves but my knowledge of resonance in the context of particle and quantum physics is very limited. I assume it's linked to excitation because I didn't just see peaks at 3 GeV & 9 GeV, I also saw them at higher energies (there were also very slight bumps above 9 GeV which were obscured by a larger bump from some other process).
  5. Jul 26, 2012 #4
    Like, can someone explain/elaborate on this page http://en.wikipedia.org/wiki/Resonance_(particle_physics [Broken])
    Last edited by a moderator: May 6, 2017
  6. Jul 27, 2012 #5


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    A resonance is like a very short-living particle, and there is no clear border between "particle" and "resonance".

    I found two nice dimuon spectra here, and LHCb has similar graphs (probably ALICE, too). You can clearly the see J/psi particle at its mass of ~3 GeV and the Z at ~90 GeV, together with several other particles. The CMS plot has a zoomed in version of the curve at ~10 GeV, and you see the different excitations of bottomium.
    The widths of those peaks are determined by the natural widths of the particles and the detector resolution.
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