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Physicists are getting antsy. Their most highly prized tool for studying the smallest bits of nature—the Large Hadron Collider (LHC) particle accelerator—has been shut down since the end of 2012 for $163 million worth of upgrades. But within two months it will be back with a vengeance, colliding protons at mind-numbing energies that have never been achieved in a man-made machine. Physicists hope that these energies will be enough to produce new particles or phenomena that expose secrets the universe has thus far been unwilling to give up. In particular, the upcoming run at the LHC could yield evidence for an idea called supersymmetry, which would be upheld if extra particles and dimensions of matter show up—and which would explain many puzzling facets of the cosmos.
The largest machine on Earth, the LHC comprises an underground loop with a circumference of 27 kilometers beneath France and Switzerland. Inside the ring, first opened in 2008, protons sent off in opposite directions accelerate to near light-speed then smash head-on into one another and explode. In the aftermath their energy is converted into mass in the form of particles—some of which are exotic species rarely seen in nature. One such, the Higgs boson, revealed itself at the collider in 2012 after theorists predicted its existence more than four decades earlier. Now scientists are hoping the LHC can reprise the feat and expose more new particles—perhaps even other, heavier versions of the Higgs boson.
LHC’s energy boost might make these new particles accessible. Its protons used to collide at energies of 8 trillion electron volts (TeV), but the machine’s electromagnetic fields will now inject them with more energy, causing them to crash together at 13 TeV. Particles will begin traveling around the loop at the end of March and if all goes well, the first collisions will start in May. To accommodate the energy uptick engineers made extensive improvements to the facility during the downtime. In particular, they enhanced the interconnections between the ring’s thousands of powerful magnets. The magnets keep the protons moving in a circle; when the protons become more energetic, they require stronger magnetic fields to keep them on track. The magnets that used to produce fields with a strength of 5.9 teslas will now create 7.7-tesla fields.
“We opened all the interconnections, we checked them and we completely redid one third of them,” says Frédérick Bordry, head the accelerator division at LHC’s home laboratory, CERN (the European Organization for Nuclear Research). “It was an interesting adventure.” Workers also did maintenance on thousands of other components of the machine and tested them thoroughly to make sure the collider is healthy. Bordry says he is confident the LHC will not see a repeat of the electrical glitch that caused major magnet damage just after the accelerator first opened seven years ago, delaying operations by 14 months.
The souped-up smasher will now have access to a whole new realm of particles and interactions, thanks to Einstein’s E = mc2 law. The equation shows that energy (E) is equivalent to mass (m) times the speed of light (c) squared. Thus, the energy that goes into a collision determines how massive the resulting particles can be. “If the energy is twice as high, it means we can produce particles that are twice as massive,” says Beate Heinemann, deputy head of the LHC’s Atlas experiment. “It also means that we can produce particles of a lower mass at a dramatically increased rate. Instead of producing, say, 10 in a second we would produce 1,000 in a second. We have a better chance to see them.”
Among the new particles that might appear are species predicted by supersymmetry, such as additional Higgs bosons as well as heavier “partner” particles to all the known particles in nature—such as “selectrons” to accompany the familiar electrons inside atoms. Supersymmetry is appealing because it explains some of the facets of the universe that the existing theory of particle physics, the standard model, does not. For example, the standard model cannot account for the invisible dark matter that astronomers think contributes most of the matter in the universe. Yet the extra particles predicted by supersymmetry seem like perfect candidates to make up dark matter. The standard model also has no explanation for why the cosmos is chiefly made of matter and not antimatter, which were thought to exist in equal measure when the universe was born. Supersymmetry’s partner particles, on the other hand, could explain this outcome because they might interfere with the way matter or antimatter particles decay, causing an asymmetry that could have allowed matter to predominate.
Despite the shortcomings of the standard model, however, everything seen at the LHC so far, including the Higgs boson, agrees with the theory. “We know the standard model can’t be a complete theory, can’t be the final answer, which is why it’s so frustrating that it’s behaved so well in run one,” says Tara Shears, a physicist at the University of Liverpool in England. “In run two we’re hoping to see cracks.”
Such cracks could appear not just in the form of never-before-seen particles but also in subtler differences in the way known particles behave. For example, the LHC beauty (LHCb) experiment monitors how often a certain particle called a B_s (pronounced B-sub-s) meson decays into another particle, a muon, and its antimatter counterpart, an antimuon. “It happens three times in every million decays,” says Shears, a member of the LHCb project. “If there’s new physics out there like supersymmetry, then these new particles can participate in this decay; they can speed it up and make you see it more often or slow it down. In [the first] run we made a measurement and unfortunately it’s consistent with the standard model, but we haven’t measured it precisely yet so there’s still plenty of room [for deviations].”
Hopes are high for the LHC’s second run because the energy levels it will now probe are capable of producing the masses expected for supersymmetric particles by many versions of the theory. “You’re exactly going to the energy where there should be supersymmetric particles,” Heinemann says. “The dark matter particle should also be approximately at this energy scale—it doesn’t have to be, but it could well be.”
Of course, everything that shows up during the LHC’s next run could also continue to obey all the standard model’s rules—thus revealing nothing about what causes dark matter or the antimatter asymmetry. “We’re hoping to get that quantum leap in our understanding that comes from the discovery of something unexpected—that’s what we live for,” Shears says. “But even if it doesn’t happen, we are going to learn more about the universe because we will have searched in this new area where we’ve never been able to study before.”