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Because the Large Hadron Collider Revs Up, Physicists’ Hopes Soar


In April, scientists on the European Center for Nuclear Research, or CERN, outside Geneva, once more fired up their cosmic gun, the Large Hadron Collider. After a three-year shutdown for repairs and upgrades, the collider has resumed shooting protons — the naked guts of hydrogen atoms — around its 17-mile electromagnetic underground racetrack. In early July, the collider will begin crashing these particles together to create sparks of primordial energy.

And so the nice game of looking for the key of the universe is about to be on again, amid recent developments and the refreshed hopes of particle physicists. Even before its renovation, the collider had been producing hints that nature may very well be hiding something spectacular. Mitesh Patel, a particle physicist at Imperial College London who conducts an experiment at CERN, described data from his previous runs as “essentially the most exciting set of results I’ve seen in my skilled lifetime.”

A decade ago, CERN physicists made global headlines with the invention of the Higgs boson, a long-sought particle, which imparts mass to all the opposite particles within the universe. What’s left to seek out? Almost all the things, optimistic physicists say.

When the CERN collider was first turned on in 2010, the universe was up for grabs. The machine, the most important and strongest ever built, was designed to seek out the Higgs boson. That particle is the keystone of the Standard Model, a set of equations that explains all the things scientists have been in a position to measure in regards to the subatomic world.

But there are deeper questions on the universe that the Standard Model doesn’t explain: Where did the universe come from? Why is it made from matter slightly than antimatter? What’s the “dark matter” that suffuses the cosmos? How does the Higgs particle itself have mass?

Physicists hoped that some answers would materialize in 2010 when the big collider was first turned on. Nothing showed up except the Higgs — particularly, no recent particle that may explain the character of dark matter. Frustratingly, the Standard Model remained unshaken.

The collider was shut down at the top of 2018 for extensive upgrades and repairs. In accordance with the present schedule, the collider will run until 2025 after which shut down for 2 more years for other extensive upgrades to be installed. Amongst this set of upgrades are improvements to the large detectors that sit on the 4 points where the proton beams collide and analyze the collision debris. Starting in July, those detectors can have their work cut out for them. The proton beams have been squeezed to make them more intense, increasing the possibilities of protons colliding on the crossing points — but creating confusion for the detectors and computers in the shape of multiple sprays of particles that should be distinguished from each other.

“Data’s going to be coming in at a much faster rate than we’ve been used to,” Dr. Patel said. Where once only a few collisions occurred at each beam crossing, now there can be more like five.

“That makes our lives harder in some sense because we’ve got to have the ability to seek out the things we’re desirous about amongst all those different interactions,” he said. “But it surely means there’s a much bigger probability of seeing the thing you might be on the lookout for.”

Meanwhile, a wide range of experiments have revealed possible cracks within the Standard Model — and have hinted to a broader, more profound theory of the universe. These results involve rare behaviors of subatomic particles whose names are unfamiliar to most of us within the cosmic bleachers.

Take the muon, a subatomic particle that became briefly famous last yr. Muons are also known as fat electrons; they’ve the identical negative electrical charge but are 207 times as massive. “Who ordered that?” the physicist Isador Rabi said when muons were discovered in 1936.

No one knows where muons fit within the grand scheme of things. They’re created by cosmic ray collisions — and in collider events — they usually decay radioactively in microseconds right into a fizz of electrons and the ghostly particles called neutrinos.

Last yr, a team of some 200 physicists related to the Fermi National Accelerator Laboratory in Illinois reported that muons spinning in a magnetic field had wobbled significantly faster than predicted by the Standard Model.

The discrepancy with theoretical predictions got here within the eighth decimal place of the worth of a parameter called g-2, which described how the particle responds to a magnetic field.

Scientists ascribed the fractional but real difference to the quantum whisper of as-yet-unknown particles that will materialize briefly across the muon and would 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 needs to be, while they wait for more data from the Fermilab experiment.

“The g-2 anomaly continues to be very much alive,” said Aida X. El-Khadra, a physicist on the University of Illinois who helped lead a three-year effort called the Muon g-2 Theory Initiative to determine a consensus prediction. “Personally, I’m optimistic that the cracks within the Standard Model will add as much as an earthquake. Nonetheless, the precise position of the cracks should still be a moving goal.”

The muon also figures in one other anomaly. The most important character, or perhaps villain, on this drama is a particle called a B quark, certainly one of six varieties of quark that compose heavier particles like protons and neutrons. B stands for bottom or, perhaps, beauty. Such quarks occur in two-quark particles often called B mesons. But these quarks are unstable and are susceptible to disintegrate in ways in which appear to violate the Standard Model.

Some rare decays of a B quark involve a daisy chain of reactions, ending in a special, lighter form of quark and a pair of lightweight particles called leptons, either electrons or their plump cousins, muons. The Standard Model holds that electrons and muons are equally prone to appear on this response. (There may be a 3rd, heavier lepton called the tau, nevertheless it decays too fast to be observed.) But Dr. Patel and his colleagues have found more electron pairs than muon pairs, violating a principle called lepton universality.

“This may very well be a Standard Model killer,” said Dr. Patel, whose team has been investigating the B quarks with certainly one of the Large Hadron Collider’s big detectors, LHCb. This anomaly, just like the muon’s magnetic anomaly, hints at an unknown “influencer” — a particle or force interfering with the response.

Probably the most dramatic possibilities, if this data holds up within the upcoming collider run, Dr. Patel says, is a subatomic speculation called a leptoquark. If the particle exists, it could bridge the gap between two classes of particle that make up the fabric universe: lightweight leptons — electrons, muons and likewise neutrinos — and heavier particles like protons and neutrons, that are made from quarks. Tantalizingly, there are six sorts of quarks and 6 sorts of leptons.

“We’re going into this run with more optimism that there may very well be a revolution coming,” Dr. Patel said. “Fingers crossed.”

There may be one more particle on this zoo behaving strangely: the W boson, which conveys the so-called weak force accountable for radioactive decay. In May, physicists with the Collider Detector at Fermilab, or C.D.F., reported on a 10-year effort to measure the mass of this particle, based on some 4 million W bosons harvested from collisions in Fermilab’s Tevatron, which was the world’s strongest collider until the Large Hadron Collider was built.

In accordance with the Standard Model and former mass measurements, the W boson should weigh about 80.357 billion electron volts, the unit of mass-energy favored by physicists. By comparison the Higgs boson weighs 125 billion electron volts, about as much as an iodine atom. However the C.D.F. measurement of the W, essentially the most precise ever done, got here in higher than predicted at 80.433 billion. The experimenters calculated that there was just one likelihood in 2 trillion — 7-sigma, in physics jargon — that this discrepancy was a statistical fluke.

The mass of the W boson is connected to the masses of other particles, including the infamous Higgs. So this recent discrepancy, if it holds up, may very well be one other crack within the Standard Model.

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

“So a recent particle that may explain each g-2 and the W mass is perhaps nearby on the L.H.C.,” said Kyle Cranmer, a physicist on the University of Wisconsin who works on other experiments at CERN.

John Ellis, a theoretician at CERN and Kings College London, noted that at the very least 70 papers have been published suggesting explanations for the brand new W-mass discrepancy.

“Lots of these explanations also require recent particles that could be accessible to the L.H.C.,” he said. “Did I mention dark matter? So, loads of things to observe out for!”

Of the upcoming run Dr. Patel said: “It’ll be exciting. It’ll be exertions, but we’re really keen to see what we’ve got and whether there’s something genuinely exciting in the info.”

He added: “You may undergo a scientific profession and never have the ability to say that when. So it appears like a privilege.”

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