Hopes for a new particle at the LHC offset by call for more data

At a seminar at CERN on Tuesday, scientists working with the Large Hadron Collider provided the latest results from the particle-smasher at the end of its operations for 2015. The results make up the most detailed measurements of the properties of some fundamental particles made to date at the highest energy at which humankind has been able to study them.

The data discussed during the seminar originated from observations at two experiments: ATLAS and CMS. And while the numbers were consistent between them, neither experimental collaboration could confirm any of the hopeful rumours doing the rounds – that a new particle might have been found. However, they were able to keep the excitement going by not denying some of the rumours either. All they said was they needed to gather more data.

One rumour that was neither confirmed nor denied was the existence of a particle at an energy of about 750 GeV (that’s about 750x the mass of a proton). That’s a lot of mass for a single particle – the heaviest known elementary particle is the top quark, weighing 175 GeV. As a result, it’d be extremely short-lived (if it existed) and rapidly decay into a combination of lighter particles, which are then logged by the detectors.

When physicists find such groups of particles, they use statistical methods and simulations to reconstruct the properties of the particle that could’ve produced them in the first place. The reconstruction shows up as a bump in the data where otherwise there’d have been a smooth curve.

This is the ATLAS plot displaying said bump (look in the area over 750 GeV on the x-axis):

ATLAS result showing a small bump in the diphoton channel at 750 GeV in the run-2 data. Credit: CERN
ATLAS result showing a small bump in the diphoton channel at 750 GeV in the run-2 data. Credit: CERN

It was found in the diphoton channel – i.e. the heavier particle decayed into two energetic photons which then impinged on the ATLAS detector. So why aren’t physicists celebrating if they can see the bump?

Because it’s not a significant bump. Its local significance is 3.6σ (that’s 3.6 times more than the average size of a fluctuation) – which is pretty significant by itself. But the more important number is the global significance that accounts for the look-elsewhere effect. As experimental physicist Tommaso Dorigo explains neatly here,

… you looked in many places [in the data] for a possible [bump], and found a significant effect somewhere; the likelihood of finding something in the entire region you probed is greater than it would be if you had stated beforehand where the signal would be, because of the “probability boost” of looking in many places.

The global significance is calculated by subtracting the effect of this boost. In the case of the 750-GeV particle, the bump stood at a dismal 1.9σ. A minimum of 3 is required to claim evidence and 5 for a discovery.

A computer’s reconstruction of the diphoton event observed by the ATLAS detector. Credit: ATLAS/CERN
A computer’s reconstruction of the diphoton event observed by the ATLAS detector. Credit: ATLAS/CERN

Marumi Kado, the physicist who presented the ATLAS results, added that when the bump was studied across a 45 GeV swath (on the x-axis), its significance went up to 3.9σ local and 2.3σ global. Kado is affiliated with the Laboratoire de l’Accelerateur Lineaire, Orsay.

A similar result was reported by James Olsen, of Princeton University, speaking for the CMS team with a telltale bump at 745 GeV. However, the significance was only 2.6σ local and 1.2σ global. Olsen also said the CMS detector had only one-fourth the data that ATLAS had in the same channel.

Where all this leaves us is that the Standard Model, which is the prevailing theory + equations used to describe how fundamental particles behave, isn’t threatened yet. Physicists would much like it to be: though it’s been able to correctly predict the the existence of many particles and fundamental forces, it’s been equally unable to explain some findings (like dark matter). And finding a particle weighing ~750 GeV, which the model hasn’t predicted so far, could show physicists what could be broken about the model and pave the way for a ‘new physics’.

However, on the downside, some other new-physics hypotheses didn’t find validation. One of the more prominent among them is called supersymmetry, SUSY for short, and it requires the existence of some heavier fundamental particles. Kado and Olsen both reported that no signs of such particles have been observed, nor of heavier versions of the Higgs boson, whose discovery was announced mid-2012 at the LHC. Thankfully they also appended that the teams weren’t done with their searches and analyses yet.

So, more data FTW – as well as looking forward to the Rencontres de Moriond (conference) in March 2016.