Ready, stable, go: The race to discover new physics returns today as the Large Hadron Collider (LHC) is re-ignited, firing heavy ion particles at each other at 99.99% the speed of light to create a state of primordial matter to create that has not been seen since just 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 17-mile (27-kilometer) loop underground near Geneva, on the French-Swiss border. Since the LHC originally came online in 2010, its experiments have yielded 3,000 scientific papers, detailing a range of findings, including the most famous: the discovery of the Higgs boson†
“It’s true that we make discoveries every week,” Chris Parkes, spokesman for the LHCb experiment, said 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 received critical technological upgrades over the past three and a half years that will enable it to crush beams of particles with a record energy of 6.8 trillion electron volts (TeV) in collisions that will yield an unprecedented 13.6 TeV in total. This is 4.6% higher than where it ended in October 2018.
An increased speed of particle collisions, an improved ability to collect more data than ever before, and brand new experiments will pave the way for researchers to pursue 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 in the LHC. Instead of injecting protons into the system as before, Linac will stimulate 4 negatively charged hydrogen ions, which are protons accompanied by two electrons† As the ions move through Linac 4, the electrons are stripped away to leave only the protons, and by interweaving these ions, tighter beams of protons can be formed. This results in narrower beams of protons fired by the collider, increasing the speed of collisions.
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Perhaps the most significant technological upgrade, however, is the system that activates the experiments in the LHC to collect data.
Now that scientific research is firmly in the age of big data, it becomes an even bigger problem how to distinguish which data is worth recording and analyzing. “We have 14 million beam transitions per second,” Parkes said. Any beam that crosses the beam will see beams of particles collide with each other.
Previously, figuring out the useful information from all those collisions was left to conventional hardware and the intuition of human researchers, with only 10% of the collisions being recorded in the LHC. The new trigger system uses machine learning to analyze the situation faster and be more efficient in collecting data for later analysis. For example, this upgrade will cause the LHCb to triple its sampling rate, while the ALICE (A Large Ion Collider Experiment) instrument will increase the number of recorded events by a factor of 50.
“This is clearly a big problem,” Luciano Musa, an ALICE spokesperson, said at the press conference.
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While there is still work to be done to learn more about the Higgs boson, the LHC is equipped to do a lot more.
“We have the 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 a lot of questions in particle physics.”
Two new detectors installed during the recent closure of the LHC are FASER, the Forward Search Experiment, and SND, the Scattering and Neutrino Detector. FASER looks for light and weakly interacting particles, including neutrinos and possibly dark matterwhile SND will focus solely on neutrinos.
Neutrinos are elusive, ghostly particles that barely interact with anything else around them – a rod of lead a light year thick would stop only half of the neutrinos passing through — and trillions of them pass harmlessly through your body every second. Given this, and despite the fact that scientists know that the collisions in the LHC should regularly produce neutrinos, a neutrino made in a particle accelerator has never been detected (the neutrinos observed by previous neutrino detectors usually come from the sun† This will change, however, with FASER and SND expected to detect nearly 7,000 neutrino events over the next four years.
At first glance, FASER and SND do not resemble neutrino detectors. These are usually huge, like the stainless steel tank of the Super Kamiokande detector in Japan that 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 (one cubic kilometer) of ice below the surface. Instead, FASER is only 1.5 meters long and SND is only slightly larger at 2.4 meters (8 feet). Instead of massive amounts of liquid or ice, they feature simple tungsten detectors and emulsion films, which are no different from old photographic film.
FASER and SND can get away with being so small because “the LHC produces a huge number of neutrinos, so you need less mass in the detector to get some of them to interact, and the neutrinos produced in the collisions of the LHC are extremely high energy, and the probability of interaction increases with energy,” Jamie Boyd, a spokesperson for FASER, told Space.com.
FASER is located 480 meters downstream from 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 bang on the beamline, while SND is at an angle. In this way, they can detect neutrinos of different energies coming from different particle collisions. Most of the neutrinos will pass through the two experiments undetected, 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 back to the position of the interaction. Every three or four months, the emulsion film is removed and sent to a lab in Japan for inspection. A small prototype has already been detected neutrino candidatesbut the prototype was too small to confirm the measurements.
“The main result we’re looking for is what we call the cross section,” Boyd said. “This describes how, as a function of their energy, the three types of neutrino – electron, muon and tau neutrinos – interact.”
These different types, or “flavors,” of neutrino can oscillate within each other as they travel great distances. For example, a neutrino may 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 a new particle is involved.
“If we were to see more electron neutrinos and fewer muon neutrinos than we expect, that could indicate that there is an additional type of neutrino, a sterile neutrinothat causes these oscillations to take place,” Boyd said. For now, sterile neutrinos remain hypothetical and finding evidence for them would be an important discovery.
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Speaking of discoveries, while the LHC was shut down for its most recent upgrades, data analysis of the old Tevatron particle accelerator at Fermilab in the US that was shut down in 2011 has provided a tantalizing hint of physics beyond the Standard Model. In particular, the Tevatron found evidence that the W boson particle, which is involved in mediating the weak force that controls radioactivity, could be more massive than the Standard Model predicts. Meanwhile, there are curious readings from the LHC and the Tevatron of the behavior of electrons and muons that, if true, could defy the predictions of the Standard Model. It is now up to the LHC to conduct further investigations.
However, scientists at the LHC are unwilling to draw any conclusions about this or any other discrepancy from the Standard Model. Instead, they prefer to remain agnostic about different theories about what the LHC is observing, to avoid biasing the results.
“We are not running after theory,” Fabiola Gianotti, CERN’s director general, said during the press conference. “I think our goal is to understand how nature works at the most fundamental level. Our goal is not to look for particular theories.”
Chris Parkes is optimistic that the LHC can somehow fathom these discrepancies. “We very much expect that based on the new data that we are collecting, we can really investigate these interesting hints that we have and see if they show violations of the Standard Model,” he said.
There is no rush. Following this new four-year observation of the LHC, there will be another shutdown for further upgrades that will result in what will be referred to as the High Luminosity LHC. It will become operational around 2029 and detect more than 15 million Higgs bosons per year from collision energies of 14 TeV. In addition to the LHC, there are plans 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 is said to be much larger than the LHC, with a 100km tunnel, although the concept has recently sparked controversy with some physicists arguing that the potential $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 focused projects.
That is all still in the future. In the here and now, the LHC still has Higgs bosons to create, neutrinos to detect, new particles to find, and theories to put to the test. What new discoveries are we talking about in four years?
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