UCB: Experiment tests underpinnings of quantum field theory

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UCB: Experiment tests underpinnings of quantum field theory

Post by bystander » Fri Jun 25, 2010 2:12 pm

Experiment tests underpinnings of quantum field theory, Bose-Einstein statistics of photons
University of California at Berkeley | 24 June 2010
Of all the assumptions underlying quantum mechanics and the theory that describes how particles interact at the most elementary level, perhaps the most basic is that particles are either bosons or fermions. Bosons, such as the particles of light called photons, play by one set of rules; fermions, including electrons, play by another.

Seven years ago, University of California, Berkeley, physicists asked a fundamental and potentially disturbing question: Do bosons sometimes play by fermion rules? Specifically, do photons act like bosons all the time, or could they sometimes act like fermions?

Based on the results of their experiment to test this possibility, published June 25 in the journal Physical Review Letters, the answer is a solid “no.”

The theories of physics – including the most comprehensive theory of elementary particles, Quantum Field Theory, which explains nature's electromagnetic, weak and strong nuclear forces (but not gravity) – rest on fundamental assumptions, said Dmitry Budker, UC Berkeley professor of physics. These assumptions are based on how the real world works, and often produce amazingly precise predictions. But some physicists would like to see them more rigorously tested.

"Tests of (these assumptions) are very important," said Budker. "Our experiment is distinguished from most other experimental searches for new physics in that others can usually be incorporated into the existing framework of the standard model of particles and forces. What we are testing are some of the fundamental assumptions on which the whole standard model is based."

Among these assumptions is the boson/fermion dichotomy, which is mandated by the Spin-Statistics Theorem of quantum field theory. Bosons, which are governed by Bose-Einstein statistics, are particles with an intrinsic spin of 0, 1, 2 or another integer, and include photons, W and Z bosons, and gluons. The fermions, governed by Fermi-Dirac statistics, are all particles with odd-half-integer spins – 1/2, 3/2, 5/2, etc. – and include the electron, neutrinos, muons and all the quarks, the fundamental particles that make up protons and neutrons.

Bosons can pile on top of one another without limit, all occupying the same quantum state. At low temperatures, this causes such strange phenomena as superconductivity, superfluidity and Bose-Einstein condensation. It also allows photons of the same frequency to form coherent laser beams. Fermions, on the other hand, avoid one another. Electrons around a nucleus stack into shells instead of collapsing into a condensed cloud, giving rise to atoms with a great range of chemical properties.
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The experiment bombarded barium atoms with photons in two identical laser beams and looked for evidence that the barium had absorbed two photons of the same energy at once, thereby kicking an electron into a higher energy state. The particular two-photon transition the scientists focused on was forbidden only by the Bose-Einstein statistics of photons. If photons were fermions, the transition would “go like gang-busters,” said English.

The experiment detected no such "fermionic" photons, establishing the distinctness of bosons and fermions, and validating the assumptions underlying Bose-Einstein statistics and Quantum Field Theory.
Testing the Best-Yet Theory of Nature
Lawrence Berkeley National Laboratory | 24 June 2010
With a confidence level of one hundred billion to one, the most sensitive test yet of one of the pillars of modern physics—the spin-statistics theorem that explains why fermions are different from bosons—shows that the theorem really holds up.

The best theory for explaining the subatomic world got its start in 1928 when theorist Paul Dirac combined quantum mechanics with special relativity to explain the behavior of the electron. The result was relativistic quantum mechanics, which became a major ingredient in quantum field theory. With a few assumptions and ad hoc adjustments, quantum field theory has proven powerful enough to form the basis of the Standard Model of particles and forces.
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The spin-statistics theorem dictates that all fundamental particles must be classified into one of two types, fermions or bosons. (The names come from the statistics, Fermi-Dirac statistics and Bose-Einstein statistics, that explain their respective behaviors.)

No two electrons can be in the same quantum state. For example, no two electrons in an atom can have identical sets of quantum numbers. Any number of bosons can occupy the same quantum state, however; among other phenomena, this is what makes laser beams possible.

Electrons, neutrons, protons, and many other particles of matter are fermions. Bosons are a decidedly mixed bunch that includes the photons of electromagnetic force, the W and Z bosons of the weak force, and such matter particles as deuterium nuclei, pi mesons, and a raft of others. Given the pandemonium in this particle zoo, it takes the spin-statistics theorem to tell what’s a fermion and what’s a boson.

The way to tell them apart is by their spin – not the classical spin of a whirling top but intrinsic angular momentum, a quantum concept. Quantum spin is either integer (0, 1, 2…) or half integer, an odd number of halves (1/2, 3/2…). Bosons have integer spin. Fermions have half integer spin.

... “If we were to knock down the spin-statistics theorem, the whole edifice of quantum field theory would come crashing down with it. The consequences would be far-reaching, affecting our assumptions about the structure of spacetime and even causality itself.” ...

The experiment starts with a stream of barium atoms; two lasers are aimed at it from opposite sides to prevent unwanted effects associated with atomic recoil. The lasers are tuned to the same frequency but have opposite polarization, which is necessary to preserve angular momentum. If forbidden transitions were caused by two same-wavelength photons from the two lasers, they would be detected when the atoms emit a particular color of fluorescent light.

The researchers carefully and repeatedly tuned through the region where forbidden two-photon transitions, if any were to occur, would reveal themselves. They detected nothing. These stringent results limit the probability that any two photons could violate the spin-statistics theorem: the chances that two photons are in a fermionic state are no better than one in a hundred billion – by far the most sensitive test yet at low energies, which may well be more sensitive than similar evidence from high-energy particle colliders.
Spectroscopic test of Bose-Einstein statistics for photons

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