As the Large Hadron Collider ramps up, physicists’ hopes soar

In April, scientists at the European Center for Nuclear Research, or CERN, near Geneva, again fired their cosmic cannon, the Large Hadron Collider. After a three-year shutdown for repairs and upgrades, the collider has resumed firing protons — the bare innards of hydrogen atoms — around its 17-mile underground electromagnetic racetrack. In early July, the collider will begin to smash these particles together to create sparks of primordial energy.

And so the great game of chasing the secret of the universe is about to resume, amid new developments and renewed hopes from particle physicists. Even before its renovation, the collider had hinted that nature might hide something spectacular. Mitesh Patel, a particle physicist at Imperial College London who is conducting an experiment at CERN, described the data from his previous experiments as “the most exciting set of results I have seen in my professional life”.

Ten years ago, physicists at CERN made world headlines with the discovery of the long-sought Higgs boson, a particle that gives mass to all other particles in the universe. What’s left to find? Almost everything, say optimistic physicists.

When the CERN collider was first turned on in 2010, the universe was up for grabs. The largest and most powerful machine ever built was designed to find the Higgs boson. This particle is the keystone of the Standard Model, a set of equations that explains everything scientists have been able to measure about the subatomic world.

But there are deeper questions about the universe that the standard model doesn’t explain: where did the universe come from? Why is it made of matter rather than antimatter? What is the “dark matter” that permeates the cosmos? How does the Higgs particle itself have mass?

Physicists hoped that some answers would materialize in 2010 when the Large Collider was first turned on. Nothing appeared except the Higgs – in particular, no new particles that could explain the nature of dark matter. Frustratingly, the standard pattern remained unwavering.

The collider was shut down in late 2018 for major upgrades and repairs. Under the current schedule, the collider will operate until 2025, then shut down for another two years for other major upgrades to be installed. Among this set of upgrades are improvements to the giant detectors that sit at the four points where proton beams collide and analyze collision debris. From July, these detectors will have their work cut out for them. Proton beams have been compressed to make them more intense, increasing the chance of proton collisions at crossing points – but confusing detectors and computers in the form of multiple sprays of particles that need to be distinguished from each other. each other.

“The data is going to come in at a much faster rate than we were used to,” Dr Patel said. Where once only a few collisions occurred with each beam crossing, there would now be more than five.

“It makes our lives more difficult in a certain sense because we have to be able to find the things that interest us among all these different interactions,” he said. “But that means there’s a higher likelihood of seeing the thing you’re looking for.”

Meanwhile, a variety of experiments have revealed possible cracks in the Standard Model – and hinted at a broader, deeper theory of the universe. These findings imply rare behaviors of subatomic particles whose names are unfamiliar to most of us in the cosmic bleachers.

Take the muon, a subatomic particle that briefly rose to fame last year. Muons are often called fat electrons; they have the same negative electrical charge but are 207 times more massive. “Who ordered this? said physicist Isador Rabi when he discovered muons in 1936.

Nobody knows where the muons are in the grand scheme of things. They are created by cosmic ray collisions – and in collider events – and they radioactively decay within microseconds into a fizzle of electrons and ghostly particles called neutrinos.

Last year, a team of some 200 physicists associated with the Fermi National Accelerator Laboratory in Illinois reported that muons spinning in a magnetic field oscillated much faster than predicted by the Standard Model.

The discrepancy with theoretical predictions came in the eighth decimal of the value of a parameter called g-2, which describes how the particle responds to a magnetic field.

The scientists attributed the fractional but real difference to the quantum murmur of as-yet-unknown particles that would briefly materialize around the muon and affect its properties. Confirming the existence of the particles would finally break the Standard Model.

But two groups of theorists are still working to reconcile their predictions of what g-2 should be, pending further data from the Fermilab experiment.

“The g-2 anomaly is still very much alive,” said Aida X. El-Khadra, a University of Illinois physicist who helped lead a three-year effort called the Muon g-2 Theory Initiative to make a consensus prediction. “Personally, I’m optimistic that the standard model cracks will turn into an earthquake. However, the exact position of the cracks can still be a moving target.

The muon also features in another anomaly. The main character, or perhaps the villain, of this drama is a particle called the B quark, one of six varieties of quarks that make up heavier particles like protons and neutrons. B stands for low or, perhaps, beauty. These quarks are found in two-quark particles called B mesons. But these quarks are unstable and tend to collapse in ways that appear to violate the Standard Model.

Some rare decays of a B quark involve a daisy chain of reactions, ending in a different, lighter type of quark and a pair of light particles called leptons, either electrons or their plump cousins, muons. The Standard Model holds that electrons and muons are equally likely to appear in this reaction. (There is a third, heavier lepton called the tau, but it decays too quickly to observe.) But Dr. Patel and his colleagues found more electron pairs than muon pairs, violating a principle called the universality of leptons.

“It could be a Standard Model killer,” said Dr Patel, whose team studied B quarks with one of the large detectors at the Large Hadron Collider, LHCb. This anomaly, like the magnetic muon anomaly, alludes to an unknown “influencer” — a particle or force interfering with the reaction.

One of the most dramatic possibilities, if this data holds up in the collider’s next run, Dr. Patel says, is a subatomic speculation called a leptoquark. If the particle exists, it could bridge the gap between two classes of particles that make up the material universe: light leptons – electrons, muons and also neutrinos – and heavier particles like protons and neutrons, which are made up of quarks. . Curiously, there are six types of quarks and six types of leptons.

“We enter this race with more optimism that there could be a revolution ahead,” Dr Patel said. “Crossed fingers.”

There is yet another strangely behaving particle in this zoo: the W boson, which carries the so-called weak force responsible for radioactive decay. In May, physicists at Fermilab’s Collider Detector, or CDF, reported on a 10-year effort to measure the mass of this particle, based on some 4 million W bosons collected from collisions at Fermilab’s Tevatron. , which was the most powerful collider in the world. until the construction of the Large Hadron Collider.

According to the Standard Model and previous mass measurements, the W boson should weigh about 80.357 billion electron-volts, the mass-energy unit favored by physicists. By comparison, the Higgs boson weighs 125 billion electron volts, about as much as an iodine atom. But the CDF measurement of W, the most accurate ever, was higher than expected at 80.433 billion. The experimenters calculated that there was only a 1 in 2 trillion chance – 7 sigma, in physics jargon – that this discrepancy was a statistical fluke.

The mass of the W boson is related to the masses of other particles, including the infamous Higgs. So this new gap, if it holds, could be another crack in the standard model.

Yet the three anomalies and the theorists’ hopes for revolution could evaporate with more data. But for optimists, all three point in the same encouraging direction to hidden particles or forces interfering with “known” physics.

“So a new particle that could explain both g-2 and the mass W could be within reach at the LHC,” said Kyle Cranmer, a physicist at the University of Wisconsin who works on other experiments at CERN. .

John Ellis, a theorist at CERN and Kings College London, noted that at least 70 papers have been published suggesting explanations for the new mass difference W.

“Many of these explanations also require new particles that could be accessible to the LHC,” he said. “Did I mention dark matter? So many things to watch out for!

Of the upcoming race, Dr Patel said: “It will be exciting. It’s going to be hard work, but we’re really looking forward to seeing what we have and if there’s anything really exciting in the data.

He added: ‘You could make a career out of science and not be able to say it once. So it’s a privilege. »

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