The world's biggest particle collider, CERN's Large Hadron Collider (LHC), suffered a disastrous failure during its initial startup. After basic repairs, scientists ran it cautiously for a few years, enough time to gather data to confirm the discovery of the Higgs boson.

With that success in the books, the whole assembly was taken apart and upgraded. There is no clear target to search for, so the LHC is being run at its full design energy (13 Tera-electonVolts) in the hope that new physics will pop out. With the end of proton collisions less than a month ago, CERN hosted a series of talks today in which researchers described the first analysis of the data from the new, high-energy run.

The talks reaffirmed that the leading candidate to replace physics' Standard Model, called Supersymmetry, was still not showing up in any of the data. But the two main detectors in the LHC, CMS and ATLAS, both see hints of a signal that could represent a new, heavy particle. The hints aren't very statistically significant and are likely to disappear into the noise as more data is gathered next year. But they'll undoubtedly get theorists very excited in the meantime, as they showed up in a search for theoretical particles predicted by a model developed using the approaches of string theory.

It’s all about energy

Colliders like the LHC generate new particles by smashing known ones—in this case, protons—together at high energies. Some of that energy gets converted to mass, via Einstein's famous E = mc2, allowing exotic particles to pop out of the debris of the collision. The more energy involved in the collision, the more mass you can create, allowing heavier particles to pop out.

The relationship between the energy of the collisions and the mass of the particles that are created, however, is rather complex. The protons being smashed by the LHC are composed of three quarks and some gluons; the quarks each carry a share of the proton's total energy (gluons are massless, so they can't pick up energy through acceleration). When a collision takes place, the quarks can experience anything from a glancing blow to a head-on smash.

As a result, only a fraction of the protons' energy gets converted into new particles—you can't ram two protons together at 6TeV and expect a 6TeV particle to pop out. As energy increases, heavier particles remain rare but become more and more probable.

(To give an example: Fermilab's Tevatron was the first collider with sufficient energy to regularly generate top quarks, with a mass of 175 Giga-electronVolts. But the Tevatron, as its name implies, smashed particles together with energies of nearly 2TeV—an order of magnitude higher.)

By shifting collision energies from 8TeV to 13TeV, two things can happen. Particles that would be out of reach at the lower energy start being created. But they will still be extremely rare, meaning that it will take years of data to identify them. However, it will also make particles that were rare at the lower energies relatively common. That means that even though we have three years of low-energy data, we might be able to find something new with just one year's worth of data at the new, higher energy.

Searching the wreckage

Marumi Kado, from France's Laboratoire de l’Accelerateur Lineaire, described the analysis performed by the team behind the ATLAS detector, while Princeton's Jim Olsen spoke on behalf of CMS. Olsen said CMS had nearly 20 inverse femtobarns of data (a standard measurement of collision counts) at low energies, but only 2.2 inverse femtobarns at the new energies, in part because it was limited by magnet trouble. ATLAS did a bit better, gathering about four inverse femtobarns at high energy.

Kado showed a slide indicating that at lower masses, the low-energy runs of the LHC were able to provide much more information about the presence or absence of new particles. But as the energies shifted upward, the new run provided much stricter limits despite having a fraction of collisions.

These collisions were then divided up into different channels, each characterized by specific items appearing in the debris—four electrons, for example, or two photons and a Z boson. The Standard Model predicts the appearance of these various particle combinations at different energies. An excess of the particles is an indication that both Standard Model processes and something else are producing them—a sign of new physics.

Both teams looked at a lot of potential new physics. Searches for dark matter particles rely on part of the energy of a collision going missing, as it ends up in a particle that doesn't interact with the detector. These searches turned up nothing. Olsen described computer models of what would happen if a black hole was created—"we have both semi-classical and quantum black holes simulated here"—but nothing in the collisions matched the predictions.

Going into the first run of the LHC, lots of people were excited about a Standard Model replacement called supersymmetry, which posited additional partners for all the known particles. It also gave them horrible names like neutralinos and squarks. None of these showed up at masses the low energy runs could search effectively; the new, high-energy runs exclude an even larger range of masses. So supersymmetry seems to be on life support.

ATLAS had also seen a couple of signs of potential new particles in its low energy searches, but the signal did not reach statistical significance. The new data shows that these were just random fluctuations, not signs of real particles.

Gravitons, anyone?

But there was some hope for theorists. Both detectors were doing searches for decays that produced two high-energy photons, called gamma-gamma signals. The gamma-gamma channel provided some of the data that helped identify the Higgs boson. But another source of gamma rays could come from the decay of a particle predicted by a model based on string theory.

The Standard Model unifies all the forces except gravity, which is 1032 times weaker than anything else. Nobody knows why, leaving physicists stuck with what's been termed a hierarchy problem—why are the forces arranged as they are? One stab at solving this problem, developed by Lisa Randall and Raman Sundrum, relied on ideas from string theory. Randall–Sundrum models posit extra dimensions and branes that represent particles in our Universe. One of those particles is a graviton, a theoretical particle that helps mediate gravitational force.

These gravitons are predicted to decay along a pathway that should release two gamma rays. And people at the LHC had been searching for signs of these decays. With the new data, both CMS and ATLAS see a weak hint that they might actually be taking place.

The hint comes from what's called bump hunting. Predict the number of two-photon signals you should see at a given energy based on known Standard Model processes, then look for energies that show more of a signal than you'd expect—a "bump" in the otherwise smooth match between prediction and experiment.

Both CMS and ATLAS see a weak bump in the area of 750GeV. If you simply ask "is there a signal at 750GeV?" then CMS says "yes," with a significance of 2.6 sigma; ATLAS, with more data, has a significance of 3.6 sigma. This is interesting, but discovery requires a significance of five sigma.

Things get worse when you consider the fact that the scientists were scanning a huge range of energies rather than just looking at 750GeV. This is called the "look elsewhere effect" (because you just as easily could have been looking at some other energy). When the look elsewhere effect is compensated for, the significance drops to less than 1.2 sigma for CMS, 1.9 sigma for ATLAS. This is far less interesting.

On the plus side, Olsen said that the CMS team went back and added data from the low-energy runs and found it actually increased the statistical significance, though not by much. All Kado would say about ATLAS is that the earlier runs have data that's consistent with the presence of the bump.

So what can we conclude? At the moment, not much. Signals like this do appear and then vanish as more data comes in—Kado presented some examples of this in his summary of ATLAS' data. But it is showing up in both detectors, and sometimes these results do get stronger with more data. Physicist Matt Strassler even commented that the bump "looks amusingly similar to the first hint of a Higgs bump from four years ago!"

If it does turn out to be real, it's important to emphasize that the source wouldn't necessarily be a graviton. The search was motivated by people suggesting that gravitons would show this decay pattern, but other particles, predicted or unknown, could also undergo similar decays. If it turns out that the particle's there, we will need to characterize its behavior and interactions with other particles in order to determine whether it actually acts like a graviton.

Aside from the implications of physics beyond the Standard Model, the possibility of a particle at that mass—nearly 800 times that of the proton that was sacrificed to create it—seems pretty exciting. Something like the Tevatron couldn't even produce it, and it would have been extremely rare in the earlier, low-energy runs of the LHC. It hints that further discoveries could be waiting for us if we do ever build any of the proposed follow-ups to the LHC.