I'm newbie here, no background in physics, just trying to understand a bit about the Higgs Boson.

I guess the key moment of this video (https://www.youtube.com/watch?v=RC-apKhWNDE) is when Fabiola Gianotti, Ph.D. shows the plots about the combined results (1:43-1:54).

where does the graphics came from? what is the thick and thin line? which units are used for the axes?

Thanks a lot.


1 Answer 1


The plots are "expected from background" (thin line) vs "observed" (thick line); the horizontal axis is energy (in GeV), with a peak at 125 GeV. On the left is the raw data - the frequency with which certain energies were observed (note it's a log axis); the plot on the right is the "statistical significance" in standard deviations. The peak is at 5 sigma - the probability of such a peak occurring "anywhere" by chance is tiny (about $3\cdot10^{-7}$ for a five sigma peak); the probability of it occurring exactly "where it was expected" is even smaller.

  • $\begingroup$ Great answer @floris, One last question: How does scientist measure the "expected from background" energy? $\endgroup$ Aug 19, 2014 at 17:15
  • $\begingroup$ @akim - I am not sure. I guess that if you create a beam with no collisions, then you have just the background. But I am just guessing. Hoping that someone from CERN has a more authoritative answer for you. $\endgroup$
    – Floris
    Aug 19, 2014 at 18:30
  • 2
    $\begingroup$ "Background" are events that are not caused by Higgs decays, and are being produced in a number of ways: "irreducible background" are real (non-Higgs) particle events that look exactly like a Higgs decay would look like. They can not be eliminated by even the most perfect detector. In addition, there are events caused by the limits of the real detector, that a perfect detector would not produce. They can be caused by misidentification of particle type, energy and momentum or even by electrical noise in the system. The total "background" is found by very careful simulation and calibration. $\endgroup$
    – CuriousOne
    Aug 20, 2014 at 5:01

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