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Higgs to photon decay + neutrino

  1. Jan 1, 2014 #1

    I'm currently in my last year of high school and I'm doing a project about the higgs particle decaying to two photons. I am using HYPATIA to analyse ATLAS events. When a higgs boson decays into two photons, you can see activity in the electromagnetic calorimeter without seeing a track pointing to it in the tracker (because photons don't have charge). To mark an event as a Higgs particle decaying into two photons requires some criteria.
    Can you mark an event as a higgs particle decaying into two photons if there are also neutrino's? Is it possible that a higgs particle decays into two photons and that the neutrino's are just some kind of noise?

    I'm very sorry if I'm being vague, but I'm not really good in explaining my question.

    Thanks in advance!
  2. jcsd
  3. Jan 1, 2014 #2


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    I don't know this stuff so can't answer your question but one thing you should be aware of, just in case you are not, is that neutrinos are practically impossible to detect so the odds of being able to detect their presence for any given collision are zero for all practical purposes.
  4. Jan 1, 2014 #3
    Thank you, I am aware of that, but HYPATIA knows if there is momentum and energy missing because of the conservation of energy and momentum. The missing energy and momentum is displayed as a neutrino, because it probably is one. All other particles can be detected by the detector. The neutrino is not really detected, but the missing energy and momentum is.
  5. Jan 1, 2014 #4

    Vanadium 50

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    How much missing energy is in the event? Is this more likely due to a neutrino or to mismeasurement?
  6. Jan 1, 2014 #5


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  7. Jan 1, 2014 #6
    Yeah it can happen like that. Here are the most common ways Higgses can be produced:


    The purple diagram shows the Higgs being produced along with vector bosons. If the vector boson is a Z, then the Z decays like 20% of the time (or some such) into a neutrino-antineutrino pair, which escapes the detector and gives you your missing energy. I don't know details like how much, I'm not an experimentalist :p.

    You can see from these diagrams that there is a bunch of junk you can get along with your Higgs; jets (from the quarks), extra leptons, missing energy. There is also lots of junk that can come from higher-order diagrams than those four. It gets tricky.

    I know it's kind of rough reading for high-school level, but do you know that the ATLAS public results are all here?: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults

    A recent diphoton analysis is here: http://cds.cern.ch/record/1523698/files/ATLAS-CONF-2013-012.pdf

    In the analysis they describe all the different kinds of events they count, though it is not light reading and there is quite a bit of jargon.

    In figure 1 you can see a bit of an overview of the event selection process, and you see here they do consider events with Z's going to neutrinos in there. All the details of the cuts they make are in there, but there are a lot of them on all sorts of different properties of the events, it's not very nice to dig through.

    For whatever simplified thing you are doing, though, you can probably ignore a lot of that. Most higgses are produced by gluon-gluon fusion (like 95% or something) and if you just count events with two photons (with high enough transverse momenta) and no other junk then there might be a visible signal in there these days. In fact I'm pretty certain there is.
    Last edited: Jan 1, 2014
  8. Jan 1, 2014 #7


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    Calculating those cross-sections is theory :p.

    I guess if he is analyzing ATLAS data, he as access to non-public documents as well.

    I did not see a plot with such a simple selection, but I would expect without additional cuts (like isolation - "not too much junk close to the photons", or the direction of the photons) you don't see a real peak, you would get way too much background.
  9. Jan 1, 2014 #8
    Yeah that's probably necessary. For that analysis it looks like the main cuts are:

    "The transverse energies for the leading and sub-leading photons are required to be larger than 40 GeV and 30 GeV, respectively, and both need to be within the fiducial calorimeter region of |η| < 2.37"

    "…the scalar sum of the transverse momenta of all tracks with pT > 1 GeV in a cone of size ∆R = 0.2 around each photon is required to be less than 2.6 GeV."

    "…the transverse energy sum of positive-energy topological clusters deposited
    in the calorimeter around each photon in a cone of ∆R = 0.4 is required to be less than 6 GeV."

    There is then some tricky stuff about only applying these cuts to tracks originating from the same vertex as the photons, with some neural net magic to match the photons to particular vertices. Hopefully the OP already has the events separated out into stuff believed to originate from a single vertex though... anyway I guess they already have some of that sorted out if they are thinking about this analysis at all.
  10. Jan 1, 2014 #9

    Vanadium 50

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    Can we possibly take this over the poster's head any faster? High school, people! High school!

    HYPATIA is a stripped down analysis framework intended for high school students to learn about ATLAS events. You can find out about it here: http://hypatia.phys.uoa.gr/ Many QuarkNet teams use it.

    The first thing to check - before deciding that there are additional interesting particles in the event (just like the real scientists do) is to determine if this missing energy is real. It is entirely possible that it's just measurement noise. That's why I asked how much there was. If it's 1 MeV, won't you feel a little silly?
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