Occasional Musings of a Particle Physicist

Is it a Higgs?

with 6 comments

Here’s a quick relatively non-technical FAQ on the Higgs results from yesterday. If you have another question send me a tweet or leave a comment below and I’ll answer it if I get time.

Has the Higgs been discovered?

Maybe! Usually ‘the’ Higgs refers to the Standard Model Higgs but many other theories (e.g., Supersymmetry) have a Higgs sector. The searches at ATLAS and CMS are designed to look for the Standard Model Higgs. This means we can get a very good idea of how consistent the data are with what we expect the Standard Model to look like. The current results are largely consistent with these expectations.

Have we reached the 5 sigma benchmark for observation?

Yes and no. The Standard Model Higgs, once its mass is known, has well predicted decay rates. At the LHC we reconstruct 5 decay channels:

  • Higgs decays to 2 photons
  • Higgs decays to 4 charged leptons via ZZ(*)
  • Higgs decays to 2 charged leptons and 2 neutrinos via WW(*)
  • Higgs decays to 2 tau leptons
  • Higgs decays to a b quark and an anti-b quark

None of these individual channels have been measured at the 5 sigma level, instead when statistically combining the data from each decay channel the individual – smaller – excesses over background reinforce each other and push the combined significance over the 5 sigma level. The goal in the next months will be to see if the 5 sigma measurement can be made in each decay channel. The current measurement is mainly dominated by the 2 photon channel followed by the ZZ channel.

Is there any inconsistency with a Standard Model Higgs?

Possibly. There are not enough data to draw firm conclusions but both experiments see a slightly higher than expected rate in the 2 photon decay channel. Perhaps more interesting and in need of further investigation is the CMS result for the 2 tau lepton decay channel. Currently this updated result comes very close to excluding a 125GeV Standard Model Higgs at the 95% confidence level. ATLAS does not have an updated result yet but if it reinforces the CMS result this would be a very compelling indication that we’re not dealing with a Standard Model Higgs. The next few months could be very interesting!

What can we say about the new particle?

It decays to 2 photons and 4 charged leptons via ZZ and possibly to 2 charged leptons and 2 neutrinos via WW. All of these decays are at a rate close to the expectation from a Standard Model Higgs. The particle has a mass somewhere around 126GeV and the observed decay channels mean it must be a boson with even spin (0 or 2); we expect the Standard Model Higgs to have spin 0. Tevatron and CMS data hints that it also decays to a b quark and an anti-b quark but there is no evidence at the moment that it decays to two tau leptons.

Written by Mark

05/07/2012 at 2:27 am

Posted in Uncategorized

6 Responses

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  1. Good post. Does the Higgs production channels have any relation to the decay channels cited above?


    06/07/2012 at 5:09 am

    • Hi Daniel,

      The production channels help experimentally. For example vector boson fusion can give H->2photons + jets which has lower background that gluon fusion production. This production does contribute to both the CMS and ATLAS ~4sigma measurement in the 2 photon decay channel. Also for the H->b anti-b channel the cleanest way to see this in the detector is associated production with a vector boson (W or Z) because the background is much much lower relative to the signal. The figures here show the production channels.


      06/07/2012 at 6:52 am

      • Thanks a lot Mark


        06/07/2012 at 6:34 pm

  2. I read about a physicist who claims that the new particle must be something else and suggests a test.
    You can read it here:

    To make things short – he says that according to previous physical experiments, only a composite of spin-1/2 particle and its antiparticle, may decay into 2 photons. He provides several examples. Then he claims that the new particle might be tt-bar meson. Then he suggests to count also decays to photon+electron+positron or photon+muon+antimuon. These decays must appear in tt-bar and should not appear in Higgs.

    Do you have an opinion about it?

    Thanks in advance!


    17/07/2012 at 6:14 pm

    • Hi Valery,

      The only work I know on ttbar meson states is quite old but for top mass of 175GeV it shows that the cross-section of forming that state at the LHC is essentially negligible. The paper is here:


      The reason is because of its high mass the top quark lifetime is very short (how long it survives before it decays to a b and a real W boson). This means one of the two top quarks will decay before an orbit of the bound state is completed. If the top quark had a mass less than 130GeV then the state could exist but QCD predictions suggest it would have mass greater than 200GeV which is much larger than the ~125GeV resonance now seen at the LHC.

      However, it is not wrong to say that the resonance could be a bound state of some *new* fundamental particles and in fact this has already been discussed by theorists. The bottom line is that over the coming months and years all possible measurements of the resonance will be made to see if it is the Standard Model Higgs or something more exotic.

      For the final state you suggest (lepton, antilepton, photon) measurements are already being made in the so called Z+photon cross-section measurements, see a recent result from ATLAS here:


      Hope this helps!



      19/07/2012 at 11:24 am

      • Thank you!


        19/07/2012 at 9:36 pm

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