Just a week ago I hailed the new king, and already there was an assassination attempt. A new paper claims that the statistical significance of the 750 GeV diphoton excess is merely 2 sigma local. The story is being widely discussed in the corridors and comment sections because we all like to watch things die... The assassins used this plot:
The Standard Model prediction for the diphoton background at the LHC is difficult to calculate from first principles. Therefore, the ATLAS collaboration assumes a theoretically motivated functional form for this background as a function of the diphoton invariant mass. The ansatz contains a number of free parameters, which are then fitted using the data in the entire analyzed range of invariant masses. This procedure leads to the prediction represented by the dashed line in the plot (but see later). The new paper assumes a slightly more complicated functional form with more free parameters, such that the slope of the background is allowed to change. The authors argue that their more general ansatz provides a better fit to the entire diphoton spectrum, and moreover predicts a larger background for the large invariant masses. As a result, the significance of the 750 GeV excess decreases to an insignificant value of 2 sigma.
There are several problems with this claim. First, I'm confused why the blue line is described as the ATLAS fit, since it is clearly different than the background curve in the money-plot provided by ATLAS (Fig. 1 in ATLAS-CONF-2015-081). The true ATLAS background is above the blue line, and much closer to the black line in the peak region (edit: it seems now that the background curve plotted by ATLAS corresponds to a1=0 and one more free parameter for an overall normalization, while the paper assumes fixed normalization). Second, I cannot reproduce the significance quoted in the paper. Taking the two ATLAS bins around 750 GeV, I find 3.2 sigma excess using the true ATLAS background, and 2.6 sigma using the black line (edit: this is because my estimate is too simplistic, and the paper also takes into account the uncertainty on the background curve). Third, the postulated change of slope is difficult to justify theoretically. It would mean there is a new background component kicking in at ~500 GeV, but this does not seem to be the case in this analysis.
Finally, the problem with the black line is that it grossly overshoots the high mass tail, which is visible even to a naked eye. To be more quantitative, in the range 790-1590 GeV there are 17 diphoton events observed by ATLAS, the true ATLAS backgrounds predicts 19 events, and the black line predicts 33 events. Therefore, the background shape proposed in the paper is inconsistent with the tail at the 3 sigma level! While the alternative background choice decreases the significance at the 750 GeV peak, it simply moves (and amplifies) the tension to another place.
So, I think the plot is foiled and the claim does not stand scrutiny. The 750 GeV peak may well be just a statistical fluctuation that will go away when more data is collected, but it's unlikely to be a stupid error on the part of ATLAS. The king will live at least until summer.