22
$\begingroup$

A few months ago, there were press releases about a possible second Higgs mode unveiled by LHC experiments. Or perhaps a new particle even more surprising like a graviton ... There was no certainty about this new particle because experiments didn't get the 5-sigma precision on the measures.

I'd like to know whether this new particle is now officially out or not, and a bit more information about it (its nature, mass, etc. if known).

$\endgroup$
  • 5
    $\begingroup$ Multiple related posts: physics.stackexchange.com/search?q=750+GeV. Part of your problem is trying to find the actual announcement in that flood of 'for the general public' verbiage which is neither precise nor terse. Short-short version: despite the initial situation (which looked hopeful) the significance has decreased and people are less excited. $\endgroup$ – dmckee Oct 9 '16 at 16:53
  • 1
    $\begingroup$ @dmckee Thank you for the other posts. I think this question is not really a duplicate, since I've learned from colleagues that the enthusiasm about this particle (indeed, it was around 750 GeV, thanks) indeed decreased recently. But I completely misunderstood why, so if anyone could explain this, I'd be glad to learn a bit more details. $\endgroup$ – FraSchelle Oct 9 '16 at 16:57
  • 2
    $\begingroup$ Hmmm ... more specific links: physics.stackexchange.com/q/268880 physics.stackexchange.com/q/227353 it's not actually an easy topic to search on the site. But in any case I wasn't proposing a duplicate, I was suggesting that you look to see if your question has been addressed. $\endgroup$ – dmckee Oct 9 '16 at 16:59
  • $\begingroup$ If you want an inside perspective on particle physics this is the blog to follow resonaances.blogspot.se $\endgroup$ – Robin Ekman Oct 9 '16 at 18:57
  • 1
    $\begingroup$ This is all on the wiki en.wikipedia.org/wiki/750_GeV_diphoton_excess $\endgroup$ – innisfree Oct 10 '16 at 12:06
36
$\begingroup$

One of the searches performed at the LHC consists in selecting events in which two high energy photons are produced ($\gamma\gamma$ channel) and in computing their invariant mass - the energy of the photon pair in its rest frame $m_{\gamma\gamma}$ - distribution. The standard-model predicts these events to be fairly common (mainly produced by direct QCD processes), and the SM prediction is called the "background". The $m_{\gamma\gamma}$ distribution of this background is quite smooth and if a new particle were to decay into two photons, there might appear a clean, significant peak around the particle mass in the data on top of the background.

(There are many other ways new physics could alter the distribution shape, but this is one of the easiest, and also more or less what happens in the Higgs case, who can decay into two photons).

In 2015, the ATLAS and CMS experiments at the LHC collected around 3 $\text{fb}^{-1}$ of data at a center of mass energy of 13 TeV. Their independent analyses both revealed a mild excess of events above background around a mass of 750 GeV, for which one of the simple explanations was a new spin-0 or spin-2 particle decaying into two photons.

2015 data (Source: http://resonaances.blogspot.fr/)

However, because of the finite amount of events, it was very well plausible that this excess was nothing but a statistical fluctuations - we had been 'lucky' and observed more events then there should be on average for $m_{\gamma\gamma}$ around 750 GeV — and the analysis uses a frequentist approach to evaluate the probability to observe more extreme fluctuations that the excesses observed. This probability is required to be higher than approximately $10^{-7}$ (5 'sigmas') but it was closer to 3 sigmas (corresponding to a probability $\sim 10^{-4}$). Moreover, since a large mass range was probed, it was not so unlikely to get at least one excess of this amplitude. This effect is called the Look-Elsewhere Effect. Once accounted for, it reduced the significance down to 1.9 sigma if I recall correctly in the case ATLAS' spin-0 search.

For these reasons, more data needed to be collected in order to conclude about the nature of this excess (true signal of new physics or just a statistical fluctuation).

The LHC collected much more data this year and in August ATLAS and CMS experiments published their new results at ICHEP based on roughly 15 $fb^{-1}$ of data. It appears that the excess is now gone.

2016 data (Source: http://resonaances.blogspot.fr/)

$\endgroup$
  • 2
    $\begingroup$ Good answer, but would you mind adding references for the figures, please? $\endgroup$ – Fato39 Oct 10 '16 at 10:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.