[Physics] Did the researchers at Fermilab find a fifth force

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Please consider the publication

Invariant Mass Distribution of Jet Pairs Produced in Association with a W boson in $p\bar{p}$ Collisions at $\sqrt{s} = 1.96$ TeV

by the CDF-Collaboration, already with huge media attention. I must admit that I am puzzled, astonished and excited at the same time. Not being an expert, it would be really nice if someone could shed light on the following question they posed:

Did we find a fifth fundamental force that cannot be explained by the standard model?

What do you think?

PS.: A part of figure 1 in the publication that is re-printed over and over again:

Fig 1

Best Answer

Let me add an experimentalist's opinion. From the plots shown by Lubos above, one sees that two distributions are being subtracted in order to bring up the signal. A Monte Carlo background from expected interactions with a number like 500 events/8GeV, and a similar number for the experimental data. As the Monte Carlo background has no error bars, I presume the statistics are much higher and the histogram is just normalized to the number of events in the data.

The statistical error in a number of events for each data bin is about $\sqrt{500}$=22.4 events. If I measure the error in the difference plot you show above, it is not larger than this number for each bin. This means they have not included systematic errors in their error estimates. One such is the error produced by the shift in energy as discussed by Lubos. This should have been added to the errors by varying the Monte Carlo background according to the 1-sigma error of the energy of the jet and added to the error bars. There are other systematic errors one can think about in subtracting data from Monte Carlo events and in cut decisions. The effect of each cut should be in a systematic error. Each variable used in the cuts (including the 8 Gev binning above) should be varied in the Monte Carlo within the error bars of the variable and the error estimate.

Note that systematic errors are added linearly and not in quadrature.

If this bump is not a statistical fluctuation but a result of underestimation of systematic errors, even if CDF doubles the statistics the problem will remain. It is independent experiments that will inform us of whether it is a statistical fluctuation, an analysis artifact or a true signal.

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