Ready, calmly: the race to discover new physics returns today when the Large Hadron Collider (LHC) ignites again, firing heavy ion particles at each other at 99.99% of the speed of the light to recreate a state of primordial matter that is not seen. shortly after the Big Bang.
The Large Hadron Collider is the longest and most powerful particle accelerator in the world, firing rays of subatomic particles around a 17-mile (27-kilometer) underground loop near Geneva, in the Franco-Swiss border. Since the LHC went online in 2010, its experiments have produced 3,000 scientific articles, with a series of findings, including the most famous of all: the discovery of the Higgs boson.
“It’s really true to say we’ve been making discoveries weekly,” Chris Parkes, a 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 don’t have enough of the “God particle”
New technology
The particle accelerator has spent the last three and a half years receiving vital technological upgrades that will allow it to crush beams of particles with a record energy of 6.8 trillion electron volts (TeV) in collisions that will account for an unprecedented 13.6 TeV total. This is 4.6% more than where it was left in October 2018.
An increase in the rate of particle collisions, an improved ability to collect more data than ever before, and new experiments will pave the way for researchers to do science beyond the Higgs boson and perhaps even beyond the standard model. current particle physics.
In 2020, a new device, the Linear Accelerator (Linac) 4, was installed at the LHC. Instead of injecting protons into the system as before, Linac 4 will increase the negatively charged hydrogen ions, which are protons accompanied by two electrons. As the ions move through the Linac 4, the electrons are removed to leave only the protons, and the entanglement of these ions allows for closer groups of protons to form. This causes the narrower proton beams to fire through the collider, increasing the collision rate.
A representation of a proton-proton collision observed by the LHCb experiment. A new trigger system that uses artificial intelligence will be able to better choose which collisions to record. (Image credit: CERN / Saverio Mariani) (opens in a new tab)
However, perhaps the most important technological update is the system that triggers experiments at the LHC to start collecting data.
With scientific research in the big data era, how to discern what data is worth recording and analyzing becomes an even bigger problem. “We have 14 million beam crossings per second,” Parkes said. Each intersection of the beam sees how bundles of particles collide with each other.
Previously, choosing the useful information from all those collisions was left to the conventional hardware and the intuition of human researchers, resulting in only 10% of collisions being recorded within the LHC. The new activation system uses machine learning to analyze the situation more quickly and be more efficient in what data to collect for later analysis. This update, for example, will cause the LHCb to triple its sampling frequency, while the ALICE (A Large Ion Collider Experiment) instrument will increase its number of recorded events by a factor of 50.
“This is clearly a big problem,” ALICE spokesman Luciano Musa told a news conference.
The giant ALICE experiment is back in operation this summer. (Image credit: CERN / Maximilien Brice) (opens in a new tab)
New experiments
While there is still work to be done to learn 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 broader context, and that simply cannot be summed up in one or two questions,” Gian Guidice, head of CERN’s theoretical physics department, said during the press conference. “So we have a very broad program that addresses a lot of issues in particle physics.”
Two new detectors installed during the recent LHC shutdown are FASER, the forward search experiment, and SND, the dispersion and neutrino detector. FASER will look for light and weakly interacting particles, including neutrinos and possible dark matterwhile SND will focus exclusively on neutrinos.
Neutrinos are elusive, ghost-like particles that barely interact with anything else around them: a lead bar. light year thick would only stop half of the neutrinos passing through it, and billions of them pass harmlessly through your body every second. With this in mind, and although scientists know that collisions within the LHC should regularly produce neutrinos, no neutrinos created in a particle accelerator (neutrinos observed by neutrino detectors) have ever been detected. above come mostly from the sun). However, this is expected to change, with FASER and SND expected to detect close to 7,000 neutrino events between them over the next four years.
At first glance, FASER and SND do not look like neutrino detectors. These are usually huge, such as the Super Kamiokande detector stainless steel tank in Japan that contains 50,000 metric tons of pure water, or the IceCube neutrino observatories at the South Pole, which has sensors placed on 0.6 cubic miles (one cubic kilometer) of ice below the surface. In contrast, FASER is only 5 feet (1.5 meters) long, and SND is only slightly larger at 8 feet (2.4 meters). Instead of huge volumes of fluid or gel, they feature simple tungsten detectors and emulsion films, no different from old photographic films.
FASER and SND are able to come out with their small size because “the LHC produces a lot of neutrinos, so you need less mass in the detector to get some of them to interact, and the neutrinos produced in the col LHC’s decisions are 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 previously part of the LHC’s predecessor, the Large Electron-Positron Collider. The FASER and SND experiments are complementary: FASER is on the light line, while SND is at an angle. In this way, they are able to detect neutrinos of different energies from collisions of different particles. Most neutrinos will go unnoticed by the two 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 traces in the emulsion called vertices that point to the position of the emulsion. interaction. Every three or four months the emulsion film is removed and sent to a laboratory 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, depending on their energy, the three types of neutrinos (electrons, muons, and tau) interact.”
These different types, or “flavors,” of neutrinos, are able to oscillate with each other as they travel long distances. For example, a neutrino could start as a muon neutrino before oscillating into an electron neutrino. At the LHC, the distance between the neutrino detectors and the source of the collisions at the LHC is too small to wait for oscillations to occur unless a new particle is involved.
“If we saw more electron neutrinos and fewer muon neutrinos than we expected, this could indicate that there is an additional type of neutrino, called sterile neutrinosthat causes these oscillations to occur, “Boyd said. For now, sterile neutrinos remain hypothetical, and finding evidence for them would be an important discovery.
New theories
The Future Circular Collider will eclipse the Large Hadron Collider. (Image credit: CERN / Panagiotis Charitos) (opens in a new tab)
Speaking of discoveries, while the LHC shut down for its most recent updates, data analysis of the former Tevatron particle accelerator at Fermilab in the United States that closed in 2011 has given a tempting indication of the physics that works beyond the standard model. Specifically, the Tevatron found evidence that the W boson particle, which is involved in mediating the weak force governing radioactivity, could be more massive than predicted by the standard model. Meanwhile, there have been curious readings from the LHC and Tevatron of the behavior of electrons and muons which, if true, could challenge the predictions of the standard model. Now the responsibility lies with the LHC to investigate further.
However, LHC scientists are unwilling to draw conclusions about this or any other discrepancy from the standard model. Instead, they prefer to remain agnostic when it comes to various theories about what the LHC is observing, to avoid biasing the results.
“We are not pursuing the theory,” Fabiola Gianotti, director general of CERN, told a news 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 get to the bottom of these discrepancies, one way or another. “We really hope that from the new data we collect, we can really investigate these interesting clues we have and see if they show any violation of the standard model,” he said.
There’s no hurry. Following this new four-year observation by the LHC, there will be another closure for further upgrades that will result in what is called the high-brightness LHC. This will start working by 2029, detecting more than 15 million Higgs bosons a year from 14 TeV collision energies. Beyond the LHC, there are plans for a new accelerator at CERN called Future Circular Collider (FCC), …