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Back in December 2015, there was found a 750 GeV diphoton excess in both CMS and ATLAS at the same location with a significance well above $3\sigma$; a 0.13% chance of being false.

However, there hasn't been any recent rumours on a well known blog confirming that this diphoton excess is increasing with 2016 data, currently $13.5 fb^{-1}$ from both ATLAS and CMS. In my opinion, it's now looking likely that the 750Gev diphoton excess is disappearing in both detectors, despite both signals being seen at the same location, and having a local 0.13% chance of being false!

The Look Else Where Effect has the effect of making it likely that a statistically insignificant local event is likely to occur somewhere over a large parameter space. I would like to know: what are the chances that a false $3\sigma$ diphoton excess will be detected at the LHC?

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    $\begingroup$ This is, of course, exactly why five sigma is considered a minimum for claiming anything solid from a simple bump-hunting analysis. To answer the question you need to do some Monte Carlo, and you need to know how they typically bin their data. You could look at the CMS data dump from the 2011 run for some hints. $\endgroup$ – dmckee♦ Jul 17 '16 at 22:21
  • $\begingroup$ You are reading the wrong blogs :) Jokes aside, the mentioned one is a very heavily opinion-based blog that a large percentage of physicists view with caution. $\endgroup$ – Helen - down with PCorrectness May 20 at 14:21
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The exact answer depends on the specifics of the analyses, and also on the overall number of analyses performed.

Speaking however about the first time that the specific diphoton excess was announced by both experiments, you can find here the values for the significance before and after the look elsewhere effect being taken into account (and also links to slides with details).

To quote, the values before/after were 3.6 / 1.9 sigma for ATLAS and 2.6 / 1.2 sigma for CMS.

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