Weird Muons May Point to New Particles and Forces of Nature

by Msnbctv news staff

After leaving the European Group for Nuclear Analysis (CERN) physics laboratory years in the past, I crossed the Swiss-German border by high-speed prepare. Searching the window of the carriage, I used to be enthralled by the scenes flashing by: a younger couple embracing on an in any other case abandoned platform, an outdated man standing by a rusty wagon with a lacking wheel, two ladies wading right into a reedy pond. Every was only a few flickering frames, gone within the blink of a watch, however sufficient for my creativeness to fill in a narrative.

I had simply completed writing up some theoretical work on muon particles—heavier cousins to electrons—and it was out for the scrutiny of my particle physics colleagues throughout peer assessment. There was a symmetry between my ideas as I seemed out the prepare window that day and the analysis I had been engaged on. I had been analyzing the flickering results of unseen “digital” particles on muons, aiming to make use of the clues from these interactions to piece collectively a fuller image of our quantum universe. As a younger theorist simply launching my profession, I had heard about proposed experiments to measure the tiny wobbles of muons to assemble such clues. I had simply spent my previous couple of months at CERN engaged on an thought that might relate these wobbling muons to the identification of the lacking darkish matter that dominates our universe and different mysteries. My thoughts fast-forwarding, I believed, “Nice—now I simply have to attend for the experiments to type issues out.” Little did I think that I’d find yourself ready for 1 / 4 of a century.

Lastly, this previous April, I tuned in to a Webcast from my dwelling establishment, Fermi Nationwide Accelerator Laboratory (Fermilab) close to Chicago, the place scientists had been reporting findings from the Muon g-2 (“g minus two”) experiment. 1000’s of individuals world wide watched to see if the legal guidelines of physics would quickly must be rewritten. The Fermilab mission was following up on a 2001 experiment that discovered tantalizing hints of the muon wobble impact I had been hoping for. That trial didn’t produce sufficient knowledge to be definitive. However now Muon g-2 co-spokesperson Chris Polly was unveiling the long-awaited outcomes from the experiment’s first run. I watched with pleasure as he confirmed a group of latest proof that agreed with the sooner trial, each suggesting that muons should not appearing as present concept prescribes. With the proof from these two experiments, we are actually very close to the rigorous statistical threshold physicists require to say a “discovery.”

What is that this wobble impact that has me and different scientists so intrigued? It has to do with the way in which a muon spins when it travels via a magnetic area. This variation in spin route will be affected by digital particles that seem and disappear in empty area in line with the bizarre guidelines of quantum mechanics. If there are extra particles within the universe past those we learn about, they, too, will present up as digital particles and exert an affect on a muon’s spin in our experiments. And this appears to be what we’re seeing. The Fermilab experiment and its precursor measured a stronger wobble in muons’ spins than what we anticipate based mostly on simply the recognized particles. If the present discrepancy holds up, this would be the largest breakthrough in particle physics because the discovery of the Higgs boson—the newest novel particle found. We is likely to be observing the consequences of particles that might assist unveil the identification of darkish matter and even reveal a brand new drive of nature.

The Normal Mannequin

My romance with physics started once I was a toddler, gazing in amazement on the Through Lactea (the Milky Manner) within the deep darkish sky of Argentina’s Pampas the place I grew up. The identical surprise fills me now. It’s my job as a particle physicist to analyze what the universe is fabricated from, the way it works and the way it started.

Scientists imagine there’s a easy but elegant mathematical construction, based mostly on symmetries of nature, that describes the way in which microscopic elementary particles work together with each other via the electromagnetic, weak and powerful forces; that is the miracle of particle physics that scientists prosaically name the Normal Mannequin. The distant stars are fabricated from the identical three elementary matter particles as our our bodies: the electron and the “up” and “down” quarks, the 2 latter of which kind protons and neutrons. Starlight is the results of the electromagnetic drive appearing between the charged protons and electrons, liberating gentle power on the sizzling floor of the star. The warmth supply of those stars, together with our solar, is the sturdy drive, which acts on the protons and neutrons to provide nuclear fusion. And the weak drive, which operates on each the quarks and the electrons, turns protons into neutrons and positively charged electrons and controls the speed of step one within the fusion course of. (The fourth drive of nature, gravity, shouldn’t be a part of the Normal Mannequin, though integrating it with the opposite forces is a serious purpose.)

Physicists assembled the Normal Mannequin piece by piece over the course of a long time. At particle accelerators world wide, we’ve got been in a position to create and observe all the particles that the mathematical construction requires. The final to be discovered, the Higgs boson, was found nearly a decade in the past at CERN’s Massive Hadron Collider (LHC). But we all know the Normal Mannequin shouldn’t be full. It doesn’t clarify, for instance, the 85 % of the matter within the universe—darkish matter—that holds the cosmos collectively, making galaxies resembling our Milky Manner attainable. The Normal Mannequin falls in need of answering why, at some early time in our universe’s historical past, matter prevailed over antimatter, enabling our existence. And the Muon g-2 experiment at Fermilab could now be exhibiting that the Normal Mannequin, as splendid as it’s, describes simply part of a richer subatomic world.

Spinning Muons: Particles circle round this 50-foot-diameter ring within the Muon g-2 experiment. Credit score: Reidar Hahn Fermi Nationwide Accelerator Laboratory

The topic of the experiment—muons—are produced in abundance by cosmic rays in Earth’s ambiance; greater than 10,000 of them cross via our our bodies each minute. These particles have the identical bodily properties because the acquainted electron, however they’re 200 instances heavier. The additional mass makes them higher probes for brand spanking new phenomena in high-precision laboratories as a result of any deviations from their anticipated conduct will probably be extra noticeable. At Fermilab, a 50-foot-diameter ring of highly effective magnets shops muons created below managed circumstances by smashing a beam of protons from a particle accelerator right into a goal of largely nickel. This course of produces pions, unstable composite particles that then decay into neutrinos and muons via weak drive results. At this level, the muons enter a hoop crammed with the vacuum of “empty” area.

Like electrons, muons have electrical cost and a property we name spin, which makes them behave as little magnets. Due to the way in which they had been created, when negatively charged muons enter the ring their spins level in the identical route as their movement, whereas for positively charged muons (used within the Fermilab experiment) the spins level in the wrong way of their movement. An exterior magnetic area makes the electrically charged muons orbit across the ring at nearly the pace of sunshine. On the identical time, this magnetic area causes the spin of the muons to precess easily like a gyroscope, because the particles journey across the ring, however with a small wobble.

The speed of precession depends upon the power of the muon’s inner magnet and is proportional to an element that we name g. The way in which the equations of the Normal Mannequin are written, if the muon didn’t wobble in any respect, the worth of g can be 2. If that had been the case, the muon’s route of movement and route of spin would at all times be the identical with respect to one another, and g-2 can be zero. In that case, scientists would measure no wobble of the muon. This case is strictly what we might anticipate with out contemplating the properties of the vacuum.

However quantum physics tells us that the nothingness of empty area is essentially the most mysterious substance within the universe. It’s because empty area accommodates digital particles—short-lived objects whose bodily results are very actual. All of the Normal Mannequin particles we all know of can behave as digital particles because of the uncertainty precept, a component of quantum concept that limits the precision with which we will carry out measurements. Consequently, it’s attainable that for a really quick time the uncertainty within the power of a particle will be so giant {that a} particle can spring into existence from empty area. This mind-blowing function of the quantum world performs a vital function in particle physics experiments; certainly, the invention of the Higgs boson was enabled by digital particle results on the LHC.

Digital particles additionally work together with the muons within the Fermilab ring and alter the worth of g. You possibly can think about the digital particles as ephemeral companions {that a} muon emits and instantly reabsorbs—they observe it round like somewhat cloud, altering its magnetic properties and thus its spin precession. Due to this fact, scientists at all times knew that g wouldn’t be precisely 2 and that there can be some wobble as muons spin across the ring. But when the Normal Mannequin shouldn’t be the entire story, then different particles that we’ve got not but found might also be present in that cloud, altering the worth of g in ways in which the Normal Mannequin can not predict.

Muons themselves are unstable particles, however they reside lengthy sufficient contained in the Muon g-2 experiment for physicists to measure their spin route. Physicists do that by monitoring one of many decay particles they create: electrons, from decays of negatively charged muons, or positrons—the antiparticle model of electrons—from decays of positively charged muons. By figuring out the power and arrival time of the electrons or positrons, scientists can deduce the spin route of the mum or dad muon. A crew of about 200 physicists from 35 universities and labs in seven international locations developed methods for measuring the muon g-2 property with unprecedented accuracy.

Graphic shows setup of Muon g-2 experiment and particle interactions that may cause muons’ spins to wobble so much.

Credit score: Jen Christiansen

A Affirmation

The primary experiments to measure the muon g-2 befell at CERN, and by the late Nineteen Seventies that they had produced outcomes that, inside their spectacular however restricted precision, agreed with commonplace concept. Within the late Nineteen Nineties the E821 Muon g-2 experiment at Brookhaven Nationwide Laboratory began taking knowledge, with an identical setup to that at CERN. It ran till 2001 and received spectacular outcomes exhibiting an intriguing discrepancy from the Normal Mannequin calculations. It collected solely sufficient knowledge to ascertain a three-sigma deviation from the Normal Mannequin—effectively in need of the five-sigma statistical significance physicists require for a “discovery.”

A decade later Fermilab acquired the unique Brookhaven muon ring, shipped the 50-ton equipment from Lengthy Island to Chicago through highways, rivers and an ocean, and began the following technology of the Muon g-2 experiment. Practically a decade after that, Fermilab introduced a measurement of muon wobble with an uncertainty of lower than half a component in one million. This spectacular accuracy, achieved with simply the primary 6 % of the anticipated knowledge from the experiment, is corresponding to the end result from the complete run of the Brookhaven trial. Most vital, the brand new Fermilab outcomes are in hanging settlement with the E821 values, confirming that the Brookhaven findings weren’t a fluke.

To verify this 12 months’s outcomes, we’d like not simply extra experimental knowledge but additionally a greater understanding of what precisely our theories predict. Over the previous twenty years we’ve got been refining the Normal Mannequin predictions. Most just lately, greater than 100 physicists engaged on the Muon g-2 Idea Initiative, began by Aida El-Khadra of the College of Illinois, have strived to enhance the accuracy of the Normal Mannequin’s worth for the muon g-2 issue. Advances in mathematical strategies and com putational energy have enabled essentially the most correct theoretical calculation of g but, making an allowance for the consequences from all digital Normal Mannequin particles that work together with muons via the electromagnetic, weak and powerful forces. Simply months earlier than Fermilab revealed its newest experimental measurements, the speculation initiative unveiled their new calculation. The quantity disagrees with the experimental end result by 4.2 sigma, which signifies that the probabilities that the discrepancy is solely a statistical fluctuation are about one in 40,000.

Nonetheless, the most recent theoretical calculation shouldn’t be iron-clad. The contributions to the g-2 issue ruled by results from the sturdy drive are extraordinarily tough to compute. The Muon g-2 Idea Initiative used enter from twenty years of judiciously measured knowledge in associated experiments with electrons to guage these results. One other approach, although, is to attempt to calculate the scale of the consequences straight from theoretical rules. This calculation is means too advanced to resolve precisely, however physicists could make approximations utilizing a mathematical trick that discretizes our world right into a gridlike lattice of area and time. These methods have yielded extremely correct outcomes for different computations the place sturdy forces play a dominant function.

Groups world wide are tackling the lattice calculations for the muon g-2 issue. Up to now just one crew has claimed to have a results of comparable accuracy to these based mostly on experimental knowledge from electron collisions. This end result occurs to dilute the discrepancy between the experimental and Normal Mannequin expectations—whether it is appropriate, there is probably not proof of extra particles tugging on the muon in any case. But this lattice end result, if confirmed by different teams, would itself battle with experimental electron knowledge—the puzzle then can be our understanding of electron collisions. And it could be onerous to seek out theoretical results that might clarify such a end result as a result of electron collisions have been so totally studied.

Muon g-2 ring being transported.
By boat and massive rig: Getting the Muon g-2 ring to Fermilab from Brookhaven required a barge and a specialised truck. Credit score: Reidar Hahn Fermi Nationwide Accelerator Laboratory

A Message from the Void

If the mismatch between Fermilab’s measurements and concept persists, we could also be glimpsing an uncharted world of unfamiliar forces, novel symmetries of nature and new particles. Within the analysis I printed 25 years in the past trying to find clues in regards to the muon’s wobble, my collaborators and I thought of a proposed property of nature referred to as supersymmetry. This concept bridges two classes of particles—bosons, which will be packed collectively in giant numbers, and fermions, that are delinquent and can share area solely with particles of reverse spin. Supersymmetry postulates that every fermion matter particle of the Normal Mannequin has a but to be found boson particle superpartner, and every Normal Mannequin boson particle additionally has an undiscovered fermion superpartner. Supersymmetry guarantees to unify the three Normal Mannequin forces and provides pure explanations for darkish matter and the victory of matter over antimatter. It might additionally clarify the hanging Muon g-2 outcomes.

Simply after the Fermilab collaboration introduced its measurement, my colleagues Sebastian Baum, Nausheen Shah, Carlos Wagner and I posted a paper to a preprint server investigating this intriguing notion. Our calculations confirmed that digital superparticles within the vacuum may make the muons wobble quicker than the Normal Mannequin predicts, simply because the experiment noticed. Much more exhilarating, a kind of new particles—referred to as a neutralino—is a candidate for darkish matter. Supersymmetry can take quite a few kinds, lots of them already dominated out by knowledge from the LHC and different experiments—however loads of variations are nonetheless viable theories of nature.

The paper my crew submitted was simply one in every of greater than 100 which have appeared proposing attainable explanations for the Muon g-2 end result because it was introduced. Most of those papers counsel new particles that fall into one in every of two camps: both “gentle and feeble” or “heavy and powerful.” The primary class consists of new particles which have plenty corresponding to or smaller than the muon and that work together with muons with a power tens of millions of instances weaker than the electromagnetic drive. The best theoretical fashions of this sort contain new, lighter cousins of the Higgs boson or particles associated to new forces of nature that act on muons. These new gentle particles and feeble forces could possibly be onerous to detect in terrestrial experiments apart from Muon g-2, however they could have left clues within the cosmos. These gentle particles would have been produced in enormous numbers after the massive bang and might need had a measurable impact on cosmic enlargement. The identical thought—that gentle particles and feeble forces wrote a chapter lacking from our present historical past of the universe—has additionally been proposed to clarify discrepancies in observations of the enlargement charge of area, the so-called Hubble fixed disaster.

The second class of explanations for the muon outcomes—heavy and powerful—includes particles with plenty about as heavy because the Higgs boson (roughly 125 instances the mass of a proton) to as much as 100 instances heavier. These particles may work together with muons with a power corresponding to the electromagnetic and weak interactions. Such heavy particles is likely to be cousins of the Higgs boson, or unique matter particles, or they is likely to be carriers of a brand new drive of nature that works over a brief vary. Supersymmetry provides some fashions of this sort, so my youthful speculations at CERN are nonetheless within the operating. One other risk is a brand new sort of particle referred to as a leptoquark—a wierd type of boson that shares properties with quarks in addition to leptons such because the muon. Relying on how heavy the brand new particles are and the power of their interactions with Normal Mannequin particles, they is likely to be detectable in upcoming runs of the LHC.

Some latest LHC knowledge already level towards uncommon conduct involving muons. Just lately, as an illustration, LHCb (one of many experiments on the LHC) measured the decays of sure unstable composite particles much like pions that produce both muons or electrons. If muons are simply heavier cousins of the electron, because the Normal Mannequin claims, then we will exactly predict what fraction of those decays ought to produce muons versus electrons. However LHCb knowledge present a persistent three-sigma discrepancy from this prediction, maybe indicating that muons are extra completely different from electrons than the Normal Mannequin permits. It’s affordable to wonder if the outcomes from LHCb and Muon g-2 are completely different, flickering frames of the identical story.

One Puzzle Piece

The Muon g-2 experiment could also be telling us one thing new, with implications far past the muons themselves. Theorists can engineer situations the place new particles and forces clarify each the muons’ humorous wobbling and remedy different excellent mysteries, resembling the character of darkish matter or, much more daring, why matter dominates over antimatter. The Fermilab experiment has given us a primary glimpse of what’s going on, however I anticipate it would take many extra experiments, each ongoing and but to be conceived, earlier than we will confidently end the story. If supersymmetry is a part of the reply, we’ve got a good likelihood of observing a few of the superparticles on the LHC. We hope to see proof of darkish matter particles there or in deep underground labs searching for them. We will additionally take a look at the conduct of muons in numerous sorts of experiments, resembling LHCb.

All of those experiments will preserve operating. Muon g-2 ought to finally produce outcomes with almost 20 instances extra knowledge. I think, nevertheless, that the ultimate measured worth of the g-2 issue won’t considerably change. There’s nonetheless a shadow of doubt on the speculation aspect that will probably be clarified within the subsequent few years, as lattice computations utilizing the world’s strongest supercomputers obtain larger precision and as impartial groups converge on a closing verdict for the Normal Mannequin prediction of the g-2 issue. If a giant mismatch between the prediction and the measurement persists, it would shake the foundations of physics.

Muons have at all times been stuffed with surprises. Their very existence prompted physicist I. I. Rabi to complain, “Who ordered that?” after they had been first found in 1936. Practically a century later they’re nonetheless superb us. Now it appears muons stands out as the messengers of a brand new order within the cosmos and, for me personally, a dream come true.

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