On your marks, steady, go: The race to discover new physics returns today when the Large Hadron Collider (LHC) is reignited and slammed into heavy ion particles at 99.99% the speed of light to restore a state of pristine matter that has not been visible since shortly after the Big Bang.
The Large Hadron Collider is the world’s longest and most powerful particle accelerator, firing beams of subatomic particles around a 27-kilometer loop underground near Geneva on the French-Swiss border. Since the LHC originally went online in 2010, its experiments have spawned 3,000 scientific papers, with a range of results including the most famous of all: the discovery of the Higgs boson.
“It’s really true to say that we’ve been making discoveries weekly,” said Chris Parkes, a spokesman for the LHCb experiment, at a news conference in late June.
Related: 10 years after the discovery of the Higgs boson, physicists still can’t get enough of the “God particle”
The particle accelerator has spent the last three and a half years receiving major technological upgrades that will allow it to smash particle beams with record-breaking energies 6.8 trillion electron volts (TeV) in collisions that will total an unprecedented 13.6 TeV. This is 4.6% more than in October 2018.
An increased rate of particle collisions, an improved ability to collect more data than ever before, and brand new experiments will pave the way for researchers to push science beyond the Higgs boson and perhaps even beyond the current Standard Model of particle physics.
In 2020, a new device, the Linear Accelerator (Linac) 4, was installed at the LHC. Instead of injecting protons into the system like before, Linac 4 amplifies negatively charged hydrogen ions, which are protons accompanied by two electrons. As the ions move through linac 4, the electrons are stripped off to leave only the protons, and the entanglement of these ions allows for the formation of tighter bundles of protons. This causes narrower beams of protons to be shot through the collider, increasing the collision rate.
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Perhaps the most important technological upgrade, however, is the system that triggers experiments at the LHC to begin collecting data.
With scientific research now firmly in the age of Big Data, the question of what data is worth recording and analyzing becomes an even bigger problem. “We have 14 million beam crossings per second,” Parkes said. At each beam crossing, particle bundles collide.
Previously, gleaning the useful information from all these collisions was left to conventional hardware and the intuition of human researchers, resulting in only 10% of collisions inside the LHC being recorded. The new triggering system uses machine learning to analyze the situation faster and be more efficient in what data to collect for later analysis. With this upgrade, for example, the LHCb will triple its sampling rate, while the ALICE instrument (A Large Ion Collider Experiment) will increase the number of recorded events by a factor of 50.
“This is clearly a big deal,” Luciano Musa, a spokesman for ALICE, said at the press conference.
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While there is still much to be done to learn more about the Higgs boson, the LHC is equipped for much more.
“We have this ambition to put the Higgs boson in a broader context, and that just can’t be summed up in one or two questions,” said Gian Guidice, head of CERN’s Department of Theoretical Physics, during the press conference. “So we have a very broad program that addresses many issues in particle physics.”
Two new detectors installed during the recent LHC shutdown are FASER, the Forward Search Experiment, and SND, the Scattering and Neutrino Detector. FASER will search for light and weakly interacting particles, including neutrinos and possible Dark matterwhile SND focuses exclusively on neutrinos.
Neutrinos are elusive, ghostly particles that barely interact with anything else around them – a pencil a light year thick would only prevent half of the neutrinos from passing through – and trillions of them harmlessly pass through your body every second. Given this, and although scientists know that the collisions inside the LHC should regularly produce neutrinos, no neutrino produced in a particle accelerator has ever been detected (the neutrinos observed by previous neutrino detectors are mainly from The sun). However, this is set to change as FASER and SND are expected to detect nearly 7,000 neutrino events over the next four years.
At first glance, FASER and SND do not look like neutrino detectors. These are usually enormous, like the stainless steel tank of the Super Kamiokande detector in Japan, which holds 50,000 tons of pure water, or the IceCube neutrino observatory at the South Pole, where sensors are placed in 0.6 cubic miles (1 cubic kilometer) of ice below the surface. Instead, FASER is only 5 feet (1.5 meters) long and SND is only slightly larger at 8 feet (2.4 meters). Instead of huge amounts of liquid or ice, they have simple tungsten detectors and emulsion film not unlike old photographic film.
FASER and SND get away with being so small because “the LHC produces a large number of neutrinos, so it takes less mass in the detector to get some of them to interact, and these are the neutrinos produced in the LHC’s collisions extremely high energy, and the likelihood of interaction increases with energy,” Jamie Boyd, a spokesman for FASER, told Space.com.
FASER is located 1,500 feet (480 meters) downstream of the ATLAS experiment in disused tunnels that were once part of the LHC’s predecessor, the Large Electron-Positron Collider. The FASER and SND experiments complement each other – FASER is right on the beamline while SND is at an angle. In this way, they can detect neutrinos of different energies originating from different particle collisions. Most neutrinos pass through the two experiments unnoticed, but a small number interact with the atoms in the dense layers of tungsten, causing the neutrinos to decay and produce daughter particles that leave traces in the emulsion, called vertices, pointing to the position of the interaction. Every three to four months, the emulsion film is removed and sent to a laboratory in Japan for inspection. A small prototype has already been discovered neutrino candidatesbut the prototype was too small to confirm the measurements.
“The main outcome we’re looking for is what we call the cross-section,” Boyd said. “This describes how the three types of neutrinos – electron, muon and tau neutrinos – interact depending on their energy.”
These different types or “flavors” of neutrinos can oscillate into each other as they travel large distances. For example, a neutrino might start out as a muon neutrino before oscillating into an electron neutrino. In the LHC, the distance between the neutrino detectors and the source of the collisions in the LHC is too small to expect oscillations, unless it is a new particle.
“If we saw more electron neutrinos and fewer muon neutrinos than expected, this could indicate that there is an additional type of neutrino called a sterile neutrinothat’s what causes these oscillations to happen,” Boyd said. For now, sterile neutrinos remain hypothetical, and finding evidence for them would be a great discovery.
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Speaking of discoveries, while the LHC has been shut down for its recent upgrades, data analysis from the old Tevatron particle accelerator at Fermilab in the US, which was shut down in 2011, has revealed a tantalizing hint that physics is working beyond the Standard Model. In particular, the Tevatron found evidence that the W boson particle involved in mediating the weak force that controls radioactivity may be more massive than the Standard Model predicts. Meanwhile, there have been strange readings from the LHC and the Tevatron about the behavior of electrons and muons this, if true, could contradict Standard Model predictions. The responsibility now lies with the LHC to investigate further.
However, scientists at the LHC are not ready to jump to conclusions about these or other deviations from the Standard Model. Instead, they prefer to remain agnostic when it comes to different theories about what the LHC is observing, to avoid biasing the results.
“We’re not chasing theory,” said Fabiola Gianotti, Director General of CERN, at the press conference. “I think our goal is to understand how nature works at the most basic level. Our goal is not to look for specific theories.”
Chris Parkes is optimistic that the LHC can get to the bottom of these discrepancies one way or another. “We really expect that with the new data that we’re collecting, we can really examine these interesting leads that we have and see if they show any violations of the Standard Model,” he said.
There is no rush. Following this new four-year observing run of the LHC, there will be another shutdown for further upgrades that will result in what is being dubbed the High Luminosity LHC. This will go into operation around 2029 and detect more than 15 million Higgs bosons per year at collision energies of 14 TeV. Alongside the LHC, plans are underway for a brand new accelerator at CERN called the Future Circular Collider (FCC), which will be powerful enough to reach energies of 100 TeV when it starts operating around 2040. The FCC would be much larger than the LHC, with a 62-mile (100 km) tunnel, although the concept recently sparked controversy with some physicists claiming that its potential $100 billion price tag would not be worth the benefits of construction and that the money could be wiser spent on smaller, more focused projects.
That’s all in the future. In the here and now, the LHC still has to generate Higgs bosons, discover neutrinos, find new particles and put theories to the test. What new discoveries will we be talking about in four years?
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