When hundreds of physicists gathered this week in La Thuile, an old mining town in the heart of the Italian alps, one short and simple question hung in the cool, crisp air: is it real?
The source of their fascination, and no little excitement, was light. Not the sunlight that made the snow glint on the mountains in the Aosta valley, but light inside the Large Hadron Collider (LHC) across the border near Geneva. The machine had detected more photons than expected as it smashed particles beneath the quiet Swiss countryside. The brief flashes of light might be the first glimpse of the next big discovery.
Or it may be nothing. The LHC hunts for signs of new physics by slamming particles together and capturing the debris in giant detectors. It is a world where quantum weirdness rules, and random blips in the data are a daily nuisance. But what if this latest bump in the data has solid foundations? Enter a new era of physics, and a world of hitherto unknown particles and forces.
Speculation is rife. Some physicists suspect that the blip may be a heavier cousin of the Higgs boson, the mass-giving particle the LHC discovered in 2012. Alternatively, it could mean the Higgs itself is made up of a bunch of smaller particles. Others wonder if the bump might be a graviton, a particle that transmits gravity. That would be truly remarkable: so far, gravity has proved impossible to reconcile with theories of other particles and forces.
“If this thing turns out to be real, it’s a ten on the Richter scale of particle physics,” says John Ellis, professor of physics at King’s College London, and the former head of theory at Cern. “One’s excitometer gets totally broken.” That if, though, is a big one.
“I would love for it to persist, but I’ve seen so many effects come and go that I have to say in my heart of hearts I’m not very optimistic. It would be such a fantastic discovery if it were true, precisely because it’s unexpected, and because it would be the tip of an iceberg of new forms of matter,” Ellis says.
His caution is echoed by Frank Wilczek, who won the Nobel prize for physics in 2004. “It’s not what the doctor ordered to solve any specific problem that I know about. But I think there may be an attractive way to accommodate it, if it exists,” he says. Wilczek likes the sound of a particle made from new types of quarks (the constituents of protons and neutrons), bound together by a new super-strong force. “That said, I’m afraid the most likely resolution is that what’s been seen is a statistical fluke.”
The first hints of the intriguing blips emerged in December, when researchers on the LHC’s two main detectors, Atlas and CMS, revealed that they had both seen small bumps in their data. They showed that collisions of protons inside the huge detectors had produced slightly more high-energy photons, or particles of light, than our best theories predict.
Normally, such bumps barely draw comment. Too many come and go, never to be seen again. But what made physicists raise a collective eyebrow was that two teams, working independently, in competition, and on completely different detectors, had bumps in the same place: the energies of the extra photons matched. Both hinted at a hefty new particle 15 times more massive than an iron atom. If real, the mystery particle had burst into existence and promptly vanished, releasing a burst of light as a death throe.
Tiziano Camporesi, head of the CMS group, has offered colleagues 20:1 against the particle’s existence, with payment in bottles of decent French wine. So far, no takers. “I’m not finding anyone among my colleagues who wants to accept my bet,” he says. “I am betting against it, but I would gladly lose.”
“The point is, when you look at the difficulties something like this raises in terms of interpretation, you wonder whether you are not seeing some sort of extraordinary example of a coincidence in both experiments,” he adds.
It was measurements like these that led to the discovery of the Higgs boson in July 2012. The particle, first postulated in 1964, was predicted to appear in LHC collisions and immediately disintegrate into other, less massive particles. The cleanest death for the Higgs was to decay into pairs of photons, and it was counting these extra flickers of light that nailed the discovery.
The meeting at La Thuile, a key event in the particle physics calendar, was the first chance LHC scientists had to unveil their latest analyses of the new bumps. They had a captive audience. Since the LHC groups first announced the blips in December, theorists have churned out more than 200 papers proposing all manner of explanations for this mysterious - and still highly tentative - particle. The eagerness of theorists to jump on every surprise result from any experiment going has earned them the collective title of “ambulance chasers”. To be fair, building theories around new results is what they are meant to do.
In the months after Christmas, the CMS team worked hard to sharpen up their data. They compensated for technical problems that affected scores of collisions, and then combined their 2015 data with that collected during the 2011 to 2012 run that discovered the Higgs boson. Presenting in La Thuile on Thursday, the CMS team revealed that the new bump had grown, and now had a statistical strength of 3.4 sigma. After technical corrections though, which account for the fact that physicists look for bumps in lots of places and not just one, the strength fell to 1.6 sigma. It’s the same reason that the finding £20 in an ATM is less of a fluke if you check all the cashpoints in the neighbourhood, and not just the one in your office. The chance of a 1.6 sigma effect being a fluke is the same as flipping a few heads in a row. That’s far from impossible, especially in a machine that records billions of collisions a second.
As for the Atlas team, the latest analyses unveiled at La Thuile has their bump at 3.6 sigma, or about 2 sigma after corrections, the equivalent of tossing five heads in a row. Physicists can only claim a bona fide discovery once their signals reach a statistical significance of five sigma. The chances of a such a signal being a random fluke is less than one in 3 million. That is the same as tossing 21 heads in a row.
“If this particle is real, it is unavoidable that it’s a telltale signal of something new both in terms of states of matter and fundamental forces. But for the moment it is just a fluctuation,” says Camporesi. “Only more data will tell.”
That will soon be forthcoming. The LHC shuts down every winter for its annual check-up. But over the next week, the machine will gradually be brought back to life, and prepared for what Mike Lamont, operations group leader, calls “first beams” over the Easter weekend. Towards the end of April, the collider should start crashing particles again, and Atlas and CMS can start to collect the data they need.
If the machine behaves well and goes smoothly about its business, physicists could have their answer, one way or another, in time for the next major conference, in August.
“It’s intriguing, but it really is this year’s data that is going to tell us much more,” says Dave Charlton, head of the Atlas group. “Things like this do come and go. That doesn’t mean they’re not exciting, but it does mean you shouldn’t start rewriting the textbooks.”
Potential new discovery: what you need to know
How does the Large Hadron Collider find new particles?
The machine accelerates two beams of protons around a 27km loop at close to the speed of light. The beams go in different directions and are crossed at four points where the protons slam into one another inside giant detectors. The intense energy of the collisions is converted into all manner particles, including photons (particles of light), electrons and quarks. New particles are unstable, and the moment they are made they disintegrate into other more common particles. This creates unexpected patterns in the LHC data which reveal the particle’s presence. For example, the Higgs boson was discovered because it decayed into pairs of photons.
Why are physicists excited?
Scientists from two independent LHC teams, Atlas and CMS, have seen bumps in their data that might be caused by a new, unknown particle. Both have detected more high-energy photons in their collisions, and in both cases, they point to a new particle six times more massive than the Higgs boson. If the particle is real, physicists will be stunned. It would be the tip of an iceberg of new particles and forces.
What could the new particle be?
Theorists have come up with plenty of ideas. It could be a heavier cousin of the Higgs boson, or perhaps a type of graviton, a particle that transmits the force of gravity. Or it may be a heavy version of the neutrino born from a theory called supersymmetry, which calls for every known type of particle to have a more massive twin.
How likely is the particle to be real?
The evidence for a new particle is weak at the moment. While both experiments have similar blips in their data, random fluctuations happen all the time. These can look like new particles, but vanish as more data is collected. Scientists should know by July whether the particle is real or not. By that time, if the LHC performs well, it will have gathered twice as much data as the scientists have now.
