Higgs couplings to fermions and massive vector bosons

November 23, 2012 at 10:24 am 2 comments

Let’s review the tests of the Higgs couplings to fermions and massive vector bosons completed by CMS and ATLAS.

The Higgs mechanism generates mass terms in the standard model Lagrangian. Electroweak symmetry is broken at the same time that the W and Z bosons get their mass. Fermions masses, on the other hand, are generated via Yukawa terms, and have nothing to do with electroweak symmetry breaking. The Higgs coupling to the electroweak gauge bosons go as MV2/v, while the Higgs coupling to the fermions goes as Mf/v, where v is the Higgs vacuum expectation value (v = 246 GeV).

Could there be a new physics effect that modifies these tree-level couplings? CMS and ATLAS have taken all their Higgs data and performed a fit with two free scale factors: κV for the vector boson couplings and κF for the fermions. The effective couplings for the Higgs boson to gluon pairs and photon pairs are expressed as their standard model loops modified by κV and κF as appropriate.

The CMS result is here. The contours centered on the black cross are the constraints from CMS data, and the yellow diamond is the standard model expected values, that fall within the 1σ CMS curve. While best value for κF is less than the SM value, the best value for κV agrees perfectly with the SM. There is a second local minimum with κF < 0 but that one is not favored by the data. (Source: CMS public Higgs page)

This plot shows contours from ATLAS data including the solution with κF < 0. The best value is marked by the X and the SM value is marked by the blue cross. The ATLAS data agree with the SM for κF and are a bit above the SM for κV. (Source: ATLAS Higgs page)

I have tried to put the two curves on the same grid:

CMS and ATLAS 68% CL contours

This plot is not very accurate: I read a few points from the CMS and ATLAS plots and typed them into a root macro. Nonetheless, one sees that the both results are consistent with the SM, yet they do not have a large overlap (these are 68% contours). Certainly we cannot say that there is any hint of a deviation from the SM, and we will have to wait to see how these contours evolve with more data.


Entry filed under: Particle Physics.

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2 Comments Add your own

  • 1. outstanding  |  May 13, 2013 at 6:14 pm

    I’m a writer from Sandford, Great Britain just forwarded this onto a coworker who was conducting a bit of research on this. And she in fact bought me lunch just because I stumbled upon it for her… lol. Actually, allow me to paraphrase this…. Thanks for the food… But anyhow, thanks for spending some time to discuss this subject here on your web site.

  • 2. jackykumar  |  June 18, 2013 at 9:43 am

    For my work in Higgs physics, i need the best fit
    numerical values , one sigma errors and correlation corresponding
    to Figure 2 in the paper ATLAS-CONF-2013-034, which is not mentioned in the papers itself.

    Can you please help me, from where i can get these values?
    and also if possible in other papers like ATLAS-CONF-2013-012,014,013,030
    As you said you picked up the few points from the graph and then
    put into root macro, how did you picked up the points from the graph?


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