FOR a few heady months last year, the door to an unknown world was nudged ajar. An anomaly in data from the Large Hadron Collider, based at CERN near Geneva, Switzerland, indicated the presence of a peculiar new particle, a visitor so unexpected that it promised to transform our picture of how nature works. Then, with more data, the anomaly disappeared. The door creaked shut again.
It was a massive letdown. When the LHC found the Higgs boson in 2012, it marked the completion of the standard model, our best theory of matter and its workings. But there were no answers to the standard model’s many mysteries: nothing new that might explain gravity, or the nature of the dark matter that dominates galaxies, or why particles such as the Higgs have the masses they do.
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“The standard model is incomplete, so there must be something,” says Lee Roberts, a particle physicist at Boston University. “But it seems like it’s hard for them to see anything right now.”
Pessimism is beginning to spread. A recent survey of 54 physicists at a workshop in Madrid revealed that 29 per cent of them think the LHC will not see anything new. Given that building an even bigger machine looks impractical, if not impossible, what now?
Go for precision, not power. That’s the rationale behind a slew of experiments examining familiar particles for infinitesimal signs of deviant behaviour – weirdness that may betray the influence of new phenomena. It won’t be easy. But by thinking small, the efforts described here might just beat the big beasts to the punch.
Shape-shifting neutrinos
Coy characters hint at hidden forces
Perhaps the most notorious known deviants from the standard model are neutrinos.
In the theory’s original incarnation, these shy, shifty particles weren’t supposed to have any mass. But in 1998, a 50,000-tonne tank of water under a mountain in Japan showed otherwise. Neutrinos come in three different “flavours” – electron, muon and tau – and the Super-Kamiokande detector caught muon neutrinos from the upper atmosphere morphing into the other two.
“Neutrino oscillations point to something beyond what we know, but what?”
These “neutrino oscillations” are only possible if the three flavours have different masses – implying that they all do have mass, albeit tiny ones. The discovery earned the 2015 Nobel prize in physics. But while neutrino oscillations definitely point to something beyond the standard model, it’s not clear what.
“There is more than one way to tweak the standard model to include it,” says at Boston University, a member of the Super-Kamiokande collaboration. “We are in the business now of putting together all the things we think we can measure about neutrinos and holding them up to the light to see if there’s anything shining through a crack.”
One possible ray of light would be if neutrinos and their antimatter siblings, antineutrinos, oscillate differently – that is, if they violate a rule known as CP symmetry.
Standard theories say that the big bang created an equal amount of matter and antimatter, and that the laws of physics treat both identically. This can’t be absolutely true. Matter and antimatter destroy each other whenever they meet, so the fact that we live in a matter-dominated cosmos means CP symmetry must be violated in some way.
In fact, we know that the weak nuclear force, one of the four fundamental forces of nature, treats matter and antimatter differently – just not nearly enough to explain matter’s dominance. That suggests some unknown force is responsible for the rest of the discrepancy. And because neutrinos are so shy of interacting through known forces, if they feel this new force it should be easier to spot.
Super-Kamiokande’s water tank also detects neutrinos and antineutrinos that come from a particle accelerator in Tokai, Japan, 295 kilometres away. In July, this experiment, called T2K, saw signs that neutrinos oscillate differently from antineutrinos. “That’s basically the definition of CP violation,” Kearns says. “If it’s there, you can seriously take into consideration that the neutrino sector is responsible for the matter-antimatter asymmetry of the universe.”
The case is not closed yet, though. Out of the millions of muon neutrinos it has detected, T2K spotted 32 shifting into the electron flavour and just 4 muon antineutrinos doing the antimatter equivalent. That’s not sufficient to say for sure that CP symmetry is violated. T2K is still gathering data and a similar experiment in the US, called NOvA, should help firm up the results.
There are other ways that neutrinos could show us new physics. The T2K result notwithstanding, they might surprise us by being their own antiparticles, a feature that would show up in a rare form of radioactive decay. Or there could prove to be a fourth, even more antisocial “sterile” neutrino, far heavier than anything the LHC can produce – and a strong candidate for dark matter.
Electric squeeze
Inspecting the electron’s flawless figure
Given we have poked and prodded them for so long, electrons and neutrons should hold no secrets. So any surprises these humdrum particles do hold would have big implications.
The standard model predicts they should be perfectly round. But any exotic unknown particles could have some subtle effects on these ordinary ones, squishing or stretching them out of their spherical shape. Specifically, they would induce an electric dipole moment (EDM): a slight separation between positive and negative charges inside the particle.
“If you see an EDM, you know without a shadow of a doubt that that’s new physics,” says at the University of Washington in Seattle. That makes it an attractive target, particularly for budget-conscious physicists, because experiments searching for EDMs tend to be relatively small-scale and inexpensive.
The trick is to look very, very closely at a property called spin. Just as a spinning top wobbles slightly as it slows thanks to the torque applied by gravity, so a particle will wobble in an electric field – that is, if it has an EDM. The trouble is that any such wobble will be incredibly small and therefore ridiculously hard to spot.
“Imagine trying to spot a sliver 10 nanometres wide shaved off the top of Earth”
Using supercooled thorium oxide molecules to amplify any deformations, the Advanced Cold Molecule Electron EDM experiment (ACME) at Harvard University has made the most precise measurement of the electron’s spherical figure to date. In 2013, it found that any , a measure of the distance between the positive and negative charges – that’s a decimal point followed by 27 zeros before the 1.
at Yale University, who works on ACME, puts it another way: if the electron were the size of Earth, its deviation from perfectly spherical must be equivalent to shaving a sliver less than 10 nanometres wide from the top and slapping it on the bottom. The team is now tweaking the experiment to increase that sensitivity.
Meanwhile, the experiment at Oak Ridge National Lab in Tennessee is probing neutrons. Previous experiments showed that the neutron is round to 1 part per trillion. nEDM is trying to improve that precision by another factor of 100 by embedding the experiment in superfluid helium. This will let the team increase the strength of the electric field acting on the neutrons and slow them down, boosting their chances of seeing a deformation – if there is one.
Over in Seattle, Graner’s experiment is scrutinising the mercury atom. Others have even suggested trying to spot EDMs in protons to search for a hypothetical dark matter particle called the axion.
All these experiments are sensitive to the effects of particles that the LHC may not be capable of seeing directly. The LHC relies on smashing protons together, which briefly spawns other massive particles that can be identified by sifting through the debris. The higher the energy, the more massive the particles it can create. Even at its maximum design energy, however, the heaviest particles the LHC can find will be about 4 or 5 teraelectronvolts (TeV), says DeMille.
By contrast, with its current sensitivity, ACME would have sensed particles at 7 or 8 TeV, if they existed. The group’s proposed improvements would push that limit, which should make it possible to see the influence of particles up 40 TeV. Further tweaks could feasibly get them as high as 100 TeV.
“By doing these very precise measurements, it’s possible to be sensitive to the existence of certain kinds of new particles that have masses beyond the reach of any accelerator,” says DeMille. “Certainly anything that’s running now, but probably anything that’s been conceived.”
Magnetic misbehaviour
The muon is preparing for its moment
The electron’s lesser-known cousin, the muon, has been behaving badly for 15 years. Next year we might finally find out what’s behind its disobedience.
Both particles are essentially spinning balls of charge, so they generate a magnetic moment – to you and me, a north and south pole. In 1928, physicist Paul Dirac calculated that a quantity related to this magnetic moment, called the g-factor, should be exactly 2 for electrons and muons. But when we measured the electron’s magnetic moment in the 1940s, it turned out to be slightly larger: more like 2.002.
We later discovered that was because so-called virtual particles, which constantly pop in and out of the vacuum according to the rules of quantum mechanics, nudge the electron’s magnetic moment off-kilter. “It’s like it has a dance partner that appears out of the vacuum, grabs its hand and spins it around,” says at Fermilab in Batavia, Illinois – and the effect is even larger on the muon, which has 207 times the mass of its cousin. “Other particles seem to come out even more prolifically when they’re around the muon.” That makes it a particularly good candidate for finding the heavy particles predicted by a popular extension of the standard model called supersymmetry, which the LHC has so far failed to detect.
Most of the observed difference in magnetic moment comes from the influence of ordinary particles like electrons and positrons, plus a bit more from things that were unknown to Dirac but ended up forming the basis of the standard model: quarks, W and Z bosons and the Higgs boson. However, in 2001 the E821 experiment at Brookhaven National Laboratory in Upton, New York, showed that the muon’s magnetic moment is more deviant, being roughly 1 part in 4 billion bigger than it should be according to the standard model. The anomaly wasn’t statistically significant enough to count as a discovery, and the experiment shut down before the team could get what they needed to make the result stick. But a new experiment called Muon g-2 (“gee minus-two”) gives us another chance.
In 2013, the detector used in E821 – a gigantic ring of superconducting magnets 15 metres in diameter – was shipped on a barge from New York to Chicago (the picture below shows it in transit). There it will run again using Fermilab’s muon beam. It should fire up sometime next spring, and if all goes well it will be taking high-quality data by next October. The physicists running the experiment hope that in 2018 they will be able to publish the first result verifying that Brookhaven was right.
For Polly, it’s personal. He did his PhD research on the Brookhaven data, and has been Muon g-2’s project manager since its inception. “I’ve been anxiously wondering whether that 15-year-old result was real or not, if it was something new and interesting,” he says. “It’s great for me to get to work on that here.”
The immortal particle
Protons never die – or do they?
The proton, that bedrock of the atomic nucleus, is supposed to be absolutely stable, meaning it never disintegrates. If it isn’t, there must be some new force overseeing its demise. But no one has ever seen a proton fall apart – and it’s not as if we haven’t been looking.
That brings us back to the giant tub of water in the mountains of Japan. Although Super-Kamiokande is best known for its neutrino experiments, its detectors have been waiting for a proton to fall apart for the entirety of its 20-year run. It is looking for a particular kind of flash: unlike a neutrino strike, which sends a burst of blue light forward in the same direction it was travelling, a decaying proton would reveal itself in light beams shooting off in opposite directions.
“We have to sift through our data among things that are mostly going in one direction, and find things that are going back to back,” says Kearns. “We’re just waiting. We can’t do anything to speed it up.”
“No one has ever seen a proton fall apart, and it’s not as if we haven’t looked”
There are several different ways a proton could disintegrate, and each gives a different estimate for how long you might have to wait to see it happen. Super-Kamiokande’s best limit on the one favoured by most physicists was published in 2014: . That’s 24 orders of magnitude greater than the lifetime of our universe, which has been around for 13 billion years – so we have no right to expect such an unlikely event to occur right when we happen to be watching.
Then again, some “grand unified theories”, which posit that all the forces merge into one at extremely high energies, suggest that the proton should live between 1030 and 1035 years – in which case there’s a chance we might spot one dying among those passing through Super-Kamiokande. “It could be around the corner, it could be out of reach of even the next-generation experiment,” Kearns says. “That’s just what the universe has handed us.”
Smashing comeback
Why the LHC could surprise us yet
The hunt for new physics on the “precision frontier”, as physicists refer to their exquisitely sensitive tests, is full of promise. But as indirect experiments. which focus on the effects of new particles rather than trying to make them, they have drawbacks: for one, they can’t tell you what that new physics is.
“That’s the rub,” says former LHC researcher , now at Valparaiso University, Indiana. “If you make these things at colliders you have a better chance of telling what it is.”
It may be too early to write off the LHC’s chances. The collider recently restarted following a major upgrade in 2015, and has a way to go before it reaches its maximum design energy. So although the first run failed to turn up things theorists did expect, later runs could always find things they didn’t. And even if not, the LHC itself will probably run precise tests on particles it can already detect to see if they continue to adhere to the standard model, says Gibson.
Wherever it shows, it would be strange if nothing new turns up, says DeMille. “Everyone believes there must be some new particles. It would be truly bizarre if there’s not.”
This article appeared in print under the headline “A bit eccentric”
