Why is the Standard Model considered incomplete?

The Standard Model does not describe gravity, nor does it explain dark matter and dark energy, which constitute about 95% of the universe. It also doesn't explain particle mass differences or the absence of antimatter in the everyday world, suggesting a larger, more complete theory is needed.

What is the 'g-factor' and why is it important for muons?

The g-factor measures how strongly a particle interacts with a magnetic field. For a point-like particle, quantum mechanics predicts g=2. However, interactions with 'virtual particles' that constantly pop in and out of existence cause g to be slightly different from 2. The muon's heavier mass makes it more sensitive to these virtual particle corrections, including contributions from potential new physics.

How do scientists measure the muon's spin precession?

Muons are injected into a magnetic storage ring where their spins precess. When a muon decays, it emits a positron. The energy of this positron is highest when emitted in the direction of the muon's spin. By counting positrons above an energy threshold, a 'wiggle plot' is created, showing oscillations whose frequency directly relates to the anomalous g-factor.

What was the significance of Brookhaven's previous g-2 result?

The Brookhaven experiment, conducted 20 years prior, measured a 3.7 sigma discrepancy between the muon's anomalous magnetic moment and the Standard Model prediction. While not a 'discovery' (which requires 5 sigma), it generated significant interest as a hint of new physics.

Why did Fermilab repeat the Muon g-2 experiment?

Fermilab aimed to achieve four times better precision than Brookhaven, delivering 20 times more muons and making numerous experimental improvements. The goal was to confirm Brookhaven's 3.7 sigma discrepancy and potentially reach the 5 sigma threshold for a new physics discovery.

How was the large muon storage ring transported to Fermilab?

The 50-foot diameter magnet, too fragile to break apart and too wide for typical roads, was moved on a custom-built frame. It traveled by barge down the Eastern Seaboard, around Florida, up the Gulf of Mexico, and into the Illinois river system, then was trucked the final 30 miles to Fermilab over three nights.

What is the 'blinding' process in the Muon g-2 experiment?

To prevent unconscious bias, the experiment was 'blinded.' A secret factor was applied to shift the data's time axis, preventing scientists from knowing the true g-factor result until all analyses were complete and unanimously approved. This ensures objectivity in the final reported value.

What was the first result from Fermilab's Muon g-2 experiment?

Fermilab's first result confirmed the Brookhaven finding, showing a value very close to the previous one. When combined with Brookhaven's data, the discrepancy with the Standard Model increased to 4.2 sigma, strengthening the evidence for new physics, though not yet a 5 sigma discovery.

How much more data will Fermilab collect for the Muon g-2 experiment?

The first result (Run 1) used only 6% of the total data planned. Fermilab has two more years of data (Run 2 and Run 3) in storage and is currently taking Run 4, with plans for Run 5 and potentially Run 6. They aim for 20 times the total data of Brookhaven, expecting to unblind Run 2 and 3 results in about a year.

Could the discrepancy be due to theoretical errors rather than new physics?

The theoretical prediction for g-2 is produced by a large collaboration that scrutinizes different calculation methods and assigns uncertainties. While new lattice QCD calculations are emerging that pull the theoretical value closer to the experimental one, these are very new and still under intense community scrutiny, requiring validation.

What does a '5 sigma' discovery mean in particle physics?

Five sigma is the gold standard for proclaiming a discovery in particle physics. It means there's a 1 in 1.7 million chance that the observed result is a mere statistical fluctuation if the true answer were the Standard Model prediction. This extremely low probability provides high confidence in a discovery.

What Can Wobbling Muons Tell Us About the Particles in our Universe?

Fermilab

Science & Technology4 min read102 min video

May 3, 2021|101,593 views|1,443|131

Fermilab Physics

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Key Moments

TL;DR

Muon g-2 experiment reveals a persistent discrepancy with the Standard Model, hinting at new physics.

Key Insights

The Muon g-2 experiment measures a fundamental property of muons, their magnetic dipole moment, with extreme precision.

The results show a significant difference between the experimental measurement and the theoretical prediction from the Standard Model, a discrepancy that has persisted from previous experiments.

Muons, heavier cousins of electrons, are used because their larger mass makes them more sensitive to potential new particles or forces not included in the Standard Model.

The experiment involves storing muons in a precisely controlled magnetic field and observing how their spin 'wobbles' and decays.

The persistence of this 4.2 sigma anomaly (currently not a 5-sigma discovery) suggests the possibility of undiscovered particles or forces influencing the muon's behavior.

Fermilab's Muon g-2 experiment, by reusing and improving upon the previous Brookhaven experiment's apparatus, has achieved higher precision and collected more data, strengthening the tension with the Standard Model.

THE MYSTERY OF THE MUON g-2 EXPERIMENT

The Muon g-2 experiment at Fermilab aims to answer fundamental questions about the universe's building blocks and forces. A key focus is the muon, a particle similar to an electron but much heavier and unstable. Physicists measure a property called the muon's magnetic dipole moment (g-factor). The Standard Model of particle physics predicts this value precisely. However, discrepancies between experimental measurements and theoretical predictions have persisted, suggesting that our current understanding of physics might be incomplete. This intriguing anomaly has generated significant excitement and further investigation.

THE STANDARD MODEL AND ITS LIMITATIONS

Particle physics has a highly successful framework called the Standard Model, which describes known fundamental particles like quarks and leptons (including muons and electrons), and their interactions via forces like electromagnetism and the weak nuclear force. It accounts for most observable phenomena. However, the Standard Model does not include gravity, dark matter, or dark energy, which constitute the vast majority of the universe. It also doesn't explain why particles have the masses they do. These limitations motivate the search for physics beyond the Standard Model.

MUONS AS PROBES OF NEW PHYSICS

Muons are crucial for this experiment because their larger mass makes them more sensitive to subtle quantum effects and potential interactions with undiscovered particles. When a muon spins in a magnetic field, its spin angle (g-factor) precesses at a frequency related to the field strength and the muon's properties. Tiny deviations from the Standard Model prediction for this 'wobble' can indicate the presence of 'virtual particles' – fleeting particles that pop in and out of existence – which are not accounted for in the simplest Standard Model calculations. The experiment's precision aims to reveal these deviations.

THE FERMILAB MUON g-2 EXPERIMENT SETUP

The Fermilab Muon g-2 experiment utilizes a large, superconducting storage ring, once housed at Brookhaven National Laboratory. Muons are created by smashing high-energy protons into a target, producing pions which then decay into muons and neutrinos. Crucially, these muons are produced with their spins aligned in a specific direction. They are then injected into the storage ring where magnets keep them circulating and provide the magnetic field. Detectors surrounding the ring capture the positrons emitted when muons decay, measuring their energy. This energy distribution, over time, reveals the muon's spin precession frequency, the 'wobble'.

MEASURING PRECISION AND UNCOVERING ANOMALIES

The experiment collects vast amounts of data – billions of positrons from millions of muon 'fills' of the ring. Sophisticated detectors and rigorous analysis are employed to measure the muon's anomalous magnetic moment, denoted as 'a', which represents the deviation from the Standard Model's predicted value. The Fermilab experiment has achieved unprecedented precision, significantly improving upon previous measurements. The team meticulously accounts for all known Standard Model contributions and potential experimental systematic errors. The recent results confirm the anomaly first seen at Brookhaven, showing the experimental value is statistically different from the Standard Model's prediction.

INTERPRETING THE RESULTS AND FUTURE PROSPECTS

The current discrepancy stands at 4.2 sigma, falling short of the 'five-sigma' gold standard for a discovery in particle physics, but it strongly suggests physics beyond the Standard Model. The experiment has much more data yet to be analyzed from additional runs (Runs 2, 3, 4, and possibly 5), which are expected to further reduce uncertainties and potentially reach the discovery threshold. While the experiment cannot pinpoint the exact nature of the new physics, it points towards theories like supersymmetry or new particles interacting with muons. Future experiments and theoretical advancements will be crucial to deciphering this profound cosmic mystery.

Mentioned in This Episode

●Products

●Tools

●Companies

●Organizations

●Studies Cited

●Concepts

●People Referenced

Common Questions

The experiment measures the anomalous magnetic moment of the muon, comparing it to theoretical predictions from the Standard Model. A discrepancy could indicate the presence of new, undiscovered particles or forces beyond the Standard Model, potentially answering fundamental questions about the universe.

Topics

Muon G-2 Quantum Electrodynamics Brookhaven National Laboratory Muons Magnetic Moment Spin Precession New Physics

Mentioned in this video

particleUp quark

A fundamental particle, along with the down quark and electron, that makes up everyday matter.

toolLarge Hadron Collider (LHC)

A high-energy particle collider at CERN, used to create and discover new particles.

personChris Polly

One of the spokespersons for the Muon g-2 experiment at Fermilab, who presented the first result.

particlePions

Particles made of two quarks, produced when high-energy protons hit a target, which then decay into muons and neutrinos.

toolTrackers

Devices with thin straws containing wire and gas, through which positrons pass, ionizing the gas. This allows scientists to deduce where muons decayed.

particleTau

An even heavier cousin of the electron and muon.

particleW boson

Heavy force-carrying particle for the weak force, involved in radioactive decay.

companyEmmer

The company contracted to move the large, fragile Muon g-2 storage ring from Brookhaven to Fermilab.

particleGluons

Force-carrying particles for the strong force, binding quarks within protons and neutrons.

productLead fluoride crystals

Material used in the calorimeters; positrons entering them rattle around, producing light which is then measured to determine energy.

experimentMu2e

Another experiment under construction at Fermilab, which will search for a very rare decay of the muon.

organizationMuon g-2 Collaboration

A group of over 200 people from 35 institutions and 7 countries working on the Muon g-2 experiment.

particlePhotons

Force-carrying particles for the electromagnetic force.

personFoley and Kutch

Conducted an experiment in the late 1940s that found the electron's g-factor to be slightly greater than two.

toolTevatron collider

A particle collider at Fermilab that was shut down in 2011, freeing up components for the g-2 experiment.

personSteve Herzberg

From the University of Washington, involved in the experiment and opened one of the sealed envelopes containing the blinding factor.

particleDown quark

A fundamental particle, along with the up quark and electron, that makes up everyday matter.

productNickel alloy cylinder

The target material into which trillions of protons are smashed to produce pions for the muon beam.

particleZ boson

Heavy force-carrying particle for the weak force, involved in radioactive decay.

conceptSupersymmetry

An extension to the Standard Model that predicts many new particles, which have been searched for at the Tevatron and LHC.

organizationMuon g-2 Theory Initiative

A collaboration of over a hundred theorists formed to reach a consensus on the Standard Model prediction for the g-factor.

personAdam Lyon

Experimental particle physicist at Fermilab, involved in data management, high performance computing, quantum computing, and a member of the g-2 collaboration since 2011.

organizationBrookhaven National Laboratory

The previous location of the g-2 experiment, which first measured a slight discrepancy with the Standard Model.

toolStorage ring

A magnetic ring used to keep muons in a circular path for a prolonged period, providing the magnetic field for spin precession.

locationMuon Campus

The area at Fermilab housing the g-2 experiment and the new Mu2e experiment, utilizing recycled components from the Tevatron anti-proton source.

particleElectron

A fundamental particle, an ingredient of everyday matter, and a lighter cousin of the muon and tau.

personLeon Lederman

Second director of Fermilab, led one of the g-2 experiments for the muon in the 1950s.

toolKicker plates

High-voltage plates that fire within 100 nanoseconds to kick muons onto the correct orbit inside the storage ring, preventing them from hitting the wall.

personJulian Schwinger

Received the Nobel Prize for first predicting the "anomalous" part of the g-factor, confirming that quantum corrections contribute to it.

toolCalorimeter

A detector used to catch positrons and measure their energy by converting their energy into light in lead fluoride crystals.

toolInflector

A special magnet that cancels out the main magnetic field of the ring where muons enter, preventing them from being immediately lost.

conceptLattice QCD

A technique used for calculating some of the complicated diagrams for the Standard Model prediction of g-2, currently producing results that are new and need scrutiny.

toolQuadrupole plates

Electrostatic plates that act as a lens to keep the muon beam vertically focused within the storage ring.