3. A Higgs of Simplest Type?

Matt Strassler [January 17, 2013]

Might the New Particle be a Higgs of the Simplest Possible Type (the Standard Model Higgs)?

Since the new particle appears to be a Higgs particle but not a composite one, it is more likely elementary — and therefore it might be a Higgs of the simplest possible type, the “Standard Model Higgs”. Do we have evidence in favor or against this possibility?

Fig. 1: The data on the new particle and its decay, compared to Standard Model prediction for a Higgs of mass 125 GeV/c^2.

Fig. 1: The data on the new particle and its decay, compared to Standard Model prediction for a Higgs of mass 125 GeV/c^2. The parameter “a” quantifies how the new particle interacts with W and Z partlcles, while “c” quantifies how it interacts with matter particles; the Standard Model predicts both a and c equal to 1, marked with a star. The light regions, one of which includes the Standard Model, are those allowed by current data. Various alternatives, marked in yellow lines, are mostly excluded by the data.

So far, all the evidence is consistent with this possibility; there is no strong evidence against it. You can get a sense for this from Figure 1, from Ellis’s talk. It shows his calculation of how well the data (from one point of view) accords with the expectations of the Standard Model — the data prefers the light colored regions, and the Standard Model prediction, the black star, lies within that one of those regions. These types of calculations have been done by other physicists too, and basically everyone agrees; the black star of the Standard Model lies in a region favored by the data. But of course, the data isn’t precise enough to zero in on the star; indeed, there’s even a second region (though not a very plausible one, if you study it in detail) far from the Standard Model prediction, in the lower half of the plot.

So far (and the data is still scanty), there’s only one thing slightly amiss: a hint that the new particle decays to photons more often than expected. We’ve had that hint since the new particle was discovered; indeed, it was discovered earlier than was expected for a Standard Model Higgs of this same mass. That decay is due to an indirect effect involving mainly W and top-quark “virtual particles” (i.e. general disturbances in the W and top-quark fields), and so it is sensitive to the presence of unknown fields in nature — fields that we haven’t yet discovered, but which interact both with Higgs particles and with photons, just as W fields do. At the moment this hint is not very convincing, so to find out whether it is true or not, we’ll have to wait a while, until we have a lot more data — probably 3 to 5 years from now, after the 2013-2014 shutdown is over and the machine is running again.

Fig. 2: Current ATLAS and CMS data, showing the rate for the new particle to be observed decaying in the five ways that can currently be measured.  The data is the black dots; the horizontal lines are the uncertainties (one "standard deviation")  The dashed lines are the Standard Model prediction; the solid lines are the best estimate of the overall rate relative to what the Standard Model predicts.  A discrepancy between data and theory requires that the horizontal lines, when doubled or tripled in length, not overlap the dashed vertical line.  Clearly there is no striking discrepancy yet, but it is interesting that both experiments see more H → γγ events than expected.

Fig. 2: Current ATLAS and CMS data, showing the rate at which the new particle is observed decaying in the five ways that can currently be measured. The data are the black dots; the horizontal lines are the uncertainties (one “standard deviation”). The dashed vertical lines at “1″ show the Standard Model prediction; the solid vertical black lines are the current best estimate of the overall rate relative to what the Standard Model predicts. A serious discrepancy between data and theory would require that the horizontal lines, when doubled or tripled in length, not overlap the dashed vertical line. Clearly there is no striking discrepancy yet. But it is interesting that both experiments see more H → γγ events than expected.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s