New LHC data has more of the same but could something be in the offing?

Dijet mass (TeV) v. no. of events. SOurce: ATLAS/CERN
Dijet mass (TeV) v. no. of events. Source: ATLAS/CERN

Looks intimidating, doesn’t it? It’s also very interesting because it contains an important result acquired at the Large Hadron Collider (LHC) this year, a result that could disappoint many physicists.

The LHC reopened earlier this year after receiving multiple performance-boosting upgrades over the 18 months before. In its new avatar, the particle-smasher explores nature’s fundamental constituents at the highest energies yet, almost twice as high as they were in its first run. By Albert Einstein’s mass-energy equivalence (E = mc2), the proton’s mass corresponds to an energy of almost 1 GeV (giga-electron-volt). The LHC’s beam energy to compare was 3,500 GeV and is now 6,500 GeV.

At the start of December, it concluded data-taking for 2015. That data is being steadily processed, interpreted and published by the multiple topical collaborations working on the LHC. Two collaborations in particular, ATLAS and CMS, were responsible for plots like the one shown above.

This is CMS’s plot showing the same result:

Source: CMS/CERN
Source: CMS/CERN

When protons are smashed together at the LHC, a host of particles erupt and fly off in different directions, showing up as streaks in the detectors. These streaks are called jets. The plots above look particularly at pairs of particles called quarks, anti-quarks or gluons that are produced in the proton-proton collisions (they’re in fact the smaller particles that make up protons).

The sequence of black dots in the ATLAS plot shows the number of jets (i.e. pairs of particles) observed at different energies. The red line shows the predicted number of events. They both match, which is good… to some extent.

One of the biggest, and certainly among the most annoying, problems in particle physics right now is that the prevailing theory that explains it all is unsatisfactory – mostly because it has some really clunky explanations for some things. The theory is called the Standard Model and physicists would like to see it disproved, broken in some way.

In fact, those physicists will have gone to work today to be proved wrong – and be sad at the end of the day if they weren’t.

Maintenance work underway at the CMS detector, the largest of the five that straddle the LHC. Credit: CERN
Maintenance work underway at the CMS detector, the largest of the five that straddle the LHC. Credit: CERN

The annoying problem at its heart

The LHC chips in providing two kinds of opportunities: extremely sensitive particle-detectors that can provide precise measurements of fleeting readings, and extremely high collision energies so physicists can explore how some particles behave in thousands of scenarios in search of a surprising result.

So, the plots above show three things. First, the predicted event-count and the observed event-count are a match, which is disappointing. Second, the biggest deviation from the predicted count is highlighted in the ATLAS plot (look at the red columns at the bottom between the two blue lines). It’s small, corresponding to two standard deviations (symbol: σ) from the normal. Physicists need at least three standard deviations () from the normal for license to be excited.

But this is the most important result (an extension to the first): The predicted event-count and the observed event-count are a match across 6,000 GeV. In other words: physicists are seeing no cause for joy, and all cause for revalidating a section of the Standard Model, in a wide swath of scenarios.

The section in particular is called quantum chromodynamics (QCD), which deals with how quarks, antiquarks and gluons interact with each other. As theoretical physicist Matt Strassler explains on his blog,

… from the point of view of the highest energies available [at the LHC], all particles in the Standard Model have almost negligible rest masses. QCD itself is associated with the rest mass scale of the proton, with mass-energy of about 1 GeV, again essentially zero from the TeV point of view. And the structure of the proton is simple and smooth. So QCD’s prediction is this: the physics we are currently probing is essential scale-invariant.

Scale-invariance is the idea that two particles will interact the same way no matter how energetic they are. To be sure, the ATLAS/CMS results suggest QCD is scale-invariant in the 0-6,000 GeV range. There’s a long way to go – in terms of energy levels and future opportunities.

Something in the valley

The folks analysing the data are helped along by previous results at the LHC as well. For example, with the collision energy having been ramped up, one would expect to see particles of higher energies manifesting in the data. However, the heavier the particle, the wider the bump in the plot and more the focusing that’ll be necessary to really tease out the peak. This is one of the plots that led to the discovery of the Higgs boson:



That bump between 125 and 130 GeV is what was found to be the Higgs, and you can see it’s more of a smear than a spike. For heavier particles, that smear’s going to be wider with longer tails on the site. So any particle that weighs a lot – a few thousand GeV – and is expected to be found at the LHC would have a tail showing in the lower energy LHC data. But no such tails have been found, ruling out heavier stuff.

And because many replacement theories for the Standard Model involve the discovery of new particles, analysts will tend to focus on particles that could weigh less than about 2,000 GeV.

In fact that’s what’s riveted the particle physics community at the moment: rumours of a possible new particle in the range 1,900-2,000 GeV. A paper uploaded to the arXiv preprint server on December 10 shows a combination of ATLAS and CMS data logged in 2012, and highlights a deviation from the normal that physicists haven’t been able to explain using information they already have. This is the relevant plot:

Source: arXiv:1512.03371v1
Source: arXiv:1512.03371v1


The one on the middle and right are particularly relevant. They each show the probability of the occurrence of an event (observed as a bump in the data, not shown here) of some heavier mass of energy decaying into two different final states: of W and Z bosons (WZ), and of two Z bosons (ZZ). Bosons make a type of fundamental particle and carry forces.

The middle chart implies that the mysterious event is at least 1,000-times less likelier to occur than normally and the one on the left implies the event is at least 10,000-times less likelier to occur than normally. And both readings are at more than 3σ significance, so people are excited.

The authors of the paper write: “Out of all benchmark models considered, the combination favours the hypothesis of a [particle or its excitations] with mass 1.9-2.0 [thousands of GeV] … as long as the resonance does not decay exclusively to WW final states.”

But as physicist Tommaso Dorigo points out, these blips could also be a fluctuation in the data, which does happen.

Although the fact that the two experiments see the same effect … is suggestive, that’s no cigar yet. For CMS and ATLAS have studied dozens of different mass distributions, and a bump could have appeared in a thousand places. I believe the bump is just a fluctuation – the best fluctuation we have in CERN data so far, but still a fluke.

There’s a seminar due to happen today at the LHC Physics Centre at CERN where data from the upgraded run is due to be presented. If something really did happen in those ‘valleys’, which were filtered out of a collision energy of 8,000 GeV (basically twice the beam energy, where each beam is a train of protons), then those events would’ve happened in larger quantities during the upgraded run and so been more visible. The results will be presented at 1930 IST. Watch this space.

Featured image: Inside one of the control centres of the collaborations working on the LHC at CERN. Each collaboration handles an experiment, or detector, stationed around the LHC tunnel. Credit: CERN.