On your marks, set, go: The race to discover new physics returns today as the Large Hadron Collider (LHC) is reignited, blasting heavy ion particles against each other at 99, 99% the speed of light to recreate a state of primordial matter not seen since just after the Big Bang.
The Large Hadron Collider is the world’s longest and most powerful particle accelerator, shooting beams of subatomic particles around a 27 kilometer loop underground near Geneva, on the Franco-Swiss border. Since the LHC’s initial commissioning in 2010, its experiments have produced 3,000 scientific papers, with a series of discoveries including the most famous of all: the discovery of the the Higgs boson.
“It’s really true to say that we make discoveries every week,” said Chris Parkes, spokesman for the LHCb experiment, at a press 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 past three and a half years receiving vital technological upgrades that will allow it to smash particle beams with record energy of 6.8 trillion electron volts (TeV) in collisions that will reach an unprecedented total of 13.6 TeV. That’s 4.6% more than where it left off in October 2018.
An increased rate of particle collisions, an improved ability to collect more data than ever before, and all-new experiments will pave the way for researchers to conduct research beyond the Higgs boson and, perhaps, beyond. of the current standard model of particle physics.
In 2020, a new device, Linear Accelerator (Linac) 4, was installed in the LHC. Rather than injecting protons into the system as before, Linac 4 will stimulate negatively charged hydrogen ions, which are protons accompanied by two electrons. As the ions pass through Linac 4, the electrons are removed leaving only the protons, and the intertwining of these ions allows the formation of tighter bunches of protons. This results in narrower proton beams being shot through the collider, increasing the collision rate.
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However, perhaps the most significant technology upgrade is the system that triggers the experiments in the LHC to start collecting data.
With scientific research now firmly entrenched in the era of big data, how to discern which data is worth recording and analyzing becomes an even bigger problem. “We have 14 million beam crossings per second,” Parkes said. Each beam crossing sees particle beams crashing into each other.
Previously, the selection of useful information from all these collisions was left to conventional hardware and the intuition of human researchers, resulting in only 10% of collisions being recorded inside the LHC. The new triggering system uses machine learning to analyze the situation faster and be more efficient at collecting data for later analysis. This upgrade will see, for example, the LHCb triple its sampling frequency, while the ALICE instrument (A Large Ion Collider Experiment) will increase the number of events recorded by 50.
“It’s clearly a big deal,” ALICE spokesperson Luciano Musa said at the press conference.
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Although there is still work to be done to learn more about the Higgs boson, the LHC is equipped to do much more than that.
“We have this ambition to put the Higgs boson in a bigger context, and it just can’t be boiled down to one or two questions,” said Gian Guidice, head of CERN’s theoretical physics department, during the press conference. “So we have a very broad program that addresses many questions in particle physics.”
Two new detectors installed during the recent LHC shutdown are FASER, the direct search experiment, and SND, the scattering and neutrino detector. FASER will search for light and weakly interacting particles, including neutrinos and possible black matterwhile SND will focus exclusively on neutrinos.
Neutrinos are elusive, ghostly particles that barely interact with anything else around them – a lead bar an light year thick would only prevent half of the neutrinos from passing through it – and billions of them pass harmlessly through your body every second. That said, and although scientists know that collisions inside the LHC should regularly produce neutrinos, no neutrino created in a particle accelerator has ever been detected (the neutrinos observed by previous neutrino detectors come mainly from of the sun). However, that is about to change, with FASER and SND set to detect nearly 7,000 neutrino events between them over the next four years.
At first glance, FASER and SND do not look like neutrino detectors. These tend to be huge, 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, which has sensors placed in 0.6 cubic miles (one cubic kilometer) of ice below the surface. Instead, FASER is only 5 feet (1.5 meters) long and SND is only a bit taller at 8 feet (2.4 meters). Rather than huge volumes of fluid or ice, they feature simple tungsten detectors and emulsion films, similar to old photographic film.
FASER and SND can get away with being so small because “the LHC produces a large number of neutrinos, so less mass is needed in the detector to make some of them interact, and the neutrinos produced in the collisions of the LHC are extremely high in energy, and the probability of interaction increases with energy,” FASER spokesperson Jamie Boyd told Space.com.
FASER is located 480 meters (1,500 feet) 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 are complementary – FASER is right on the beamline, while SND is at an angle. In this way, they are able to detect neutrinos of different energies from different particle collisions. Most neutrinos will go unnoticed in both experiments, but a small number will interact with the atoms in the dense tungsten layers, causing the neutrinos to decay and produce daughter particles that leave trails in the emulsion called vertices that point towards the position of the interaction. Every three or four months, the emulsion film is removed and sent to a laboratory in Japan for inspection. Already, a small prototype has detected neutrino candidatesbut the prototype was too small to be able to confirm the measurements.
“The main result we’re looking for is what we call the cross section,” Boyd said. “It describes how, depending on their energy, the three types of neutrinos – electrons, muons and tau neutrinos – interact.”
These different types, or “flavors”, of neutrinos are able to oscillate into each other as they travel great 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 to occur unless a new particle is involved.
“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 causes these oscillations,” Boyd said. For now, sterile neutrinos remain hypothetical, and finding proof would be a major discovery.
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Speaking of discoveries, as the LHC was powered down for its latest upgrades, analysis of data from Fermilab’s former Tevatron particle accelerator in the US, which was shut down in 2011, revealed a tantalizing hint of physics working beyond the standard model. Specifically, the Tevatron has found evidence that the W boson particle, involved in mediating the weak force that governs radioactivity, may be more massive than predicted by the Standard Model. Meanwhile, there have been curious readings from the LHC and the Tevatron of the behavior of electrons and muons this, if true, could defy the predictions of the Standard Model. It is now up to the LHC to do more research.
However, LHC scientists are not willing to jump to conclusions about this or any other deviation from the Standard Model. Instead, they prefer to remain agnostic when it comes to various theories of what the LHC is observing, to avoid biasing the results.
“We are not chasing after theory,” Fabiola Gianotti, Director General of CERN, said at the press conference. “I think our goal is to understand how nature works at the most fundamental level. Our goal is not to seek particular theories.”
Chris Parkes is optimistic that the LHC will be able to resolve these discrepancies, one way or another. “We’re very much looking forward to the new data that we’re collecting that we can really probe these interesting clues that we have and see if they show violations of the Standard Model,” he said.
There is no emergency. Following this new four-year observation period led by the LHC, there will be another stop for further upgrades that will culminate in what is known as the High-Luminosity LHC. This will start operating around 2029, detecting more than 15 million Higgs bosons per year from collision energies of 14 TeV. Beyond 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 tunnel 62 miles (100 km) long, although the concept has recently sparked controversy with some physicists claiming that its possible $100 billion price tag would not be worth the benefits of building it and that the money could be spent more wisely on smaller, more targeted projects.
All of this is still in the future. Here and now, the LHC still has Higgs bosons to create, neutrinos to detect, new particles to find and theories to test. What new discoveries will we be talking about in four years?
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