Quantum gravity corrects QED

【转自Physics World】

This week's issue of Nature
includes a paper that's remarkable for two reasons: it is about quantum gravity – a topic usually not covered in the journal – and it is written by just one person. Now, after a little digging, physicsworld.com can answer all of the important questions about this paper.

So, whose citation index ranking is about to go into the stratosphere?

The paper was written by David Toms, a Canadian mathematical physicist and lecturer at Newcastle University in the UK.

What has Toms done?

He has shown that interactions between quantum gravity and quantum electrodynamics (QED) cause electric charge to vanish at very high energies (above about 10¹⁵ GeV). He told physicsworld.com that his technique can be generalized to apply to the two other "gauge couplings", which define the strong and weak forces.

Why should electric charge vanish at high energies?

A major problem with QED, which describes the interaction between charged particles and photons, is that electric charge increases at higher interaction energies. This is a result of vacuum polarization, whereby the spontaneous creation of electron–positron pairs tends to screen the electric charge of a particle at low energies. At higher energies, however, the screening is much reduced and the effective charge increases – and this cannot be correct.

Can you explain?

Physicists already know that the strong force – which binds together quarks within hadrons – goes to zero at extremely high energies. This property is called asymptotic freedom and its discovery earned Frank Wilczek, David Gross and David Politzer the 2004 Nobel Prize for Physics. If it can be proved that quantum gravity makes QED asymptotically free then it could stand as a viable theory on its own.

Can you elaborate slightly?

The main reason why QED was viewed as incomplete, prior to Gross et al, was that without asymptotic freedom the electric charge becomes infinitely large at some energy scale and the theory is no longer reliable. For their calculations to be reliable at high energies, physicists expect the strong, weak and electromagnetic forces to become unified and become asymptotically free.

Hold on, didn't Frank Wilczek and Sean Robinson establish gravity-induced asymptotic freedom of charge in 2006?

Yes, sort of. Robinson and Wilczek came up with the idea of gravity-driven asymptotic freedom and worked out that it applied to all three gauge couplings (Phys. Rev. Lett.96 231601). It was later pointed out, however, that there were errors in their calculations. This caused a flurry of activity as other physicists tried
and failed to do the calculation using different approaches.

Now, Toms has worked out a way of avoiding these errors by performing a set of careful checks to guarantee that the calculation meets certain mathematical and physical criteria. In doing so, he has shown that Robinson and Wilczek's idea was correct all along.

So what do they have to say?

"Toms' work is important equally as much because of the way in which he did the calculation as the result itself," said Robinson who is a lecturer at Massachusetts Institute of Technology. He said that an important feature of the technique is that it is "demonstrably flawless". He also pointed out that while Toms' paper was under review at Nature, an independent group of physicists at Tsinghua University in China posted a preprint (arXiv:1008.1839) using a similar "flawless" technique but a different set of cross-checks. The Tsinghua team obtained essentially the same result as Toms, illustrating the power of the technique.

That must be good news for physicists working on unification?

Sort of. Toms has shown that quantum gravity causes asymptotic freedom in all the gauge couplings. This is handy if you want to show that all forces unify in a single (very weak) force at very high energies. However, he treated quantum gravity by simply quantizing Einstein's general theory of relativity. This approach breaks down at the very energies that unification is expected to occur. To take things further, physicists would need to integrate more exotic aspects of quantum gravity such as additional dimensions and supersymmetry.

Where can I find out more?

• Nature paper
• independent work on arXiv
• Robinson and Wilczek paper (2006)

About the author

Hamish Johnston
is editor of physicsworld.com

A fourth flavor of neutrino?

【转自University of Michigan News Service，图片来自BooNE】

Physics experiment suggests existence of new particle

ANN ARBOR, Mich.—The results of a high-profile Fermilab physics experiment involving a University of Michigan professor appear to confirm strange 20-year-old findings that poke holes in the standard model, suggesting the existence of a new elementary particle: a fourth flavor of neutrino.

The new results go further to describe a violation of a fundamental symmetry of the universe asserting that particles of antimatter behave in the same way as their matter counterparts.

Neutrinos are neutral elementary particles born in the radioactive decay of other particles. The known "flavors" of neutrinos are the neutral counterparts of electrons and their heavier cousins, muons and taus. Regardless of a neutrino's original flavor, the particles constantly flip from one type to another in a phenomenon called "neutrino flavor oscillation."

An electron neutrino might become a muon neutrino, and then later an electron neutrino again. Scientists previously believed three flavors of neutrino exist. In this Mini Booster Neutrino Experiment, dubbed MiniBooNE, researchers detected more oscillations than would be possible if there were only three flavors.

"These results imply that there are either new particles or forces we had not previously imagined," said Byron Roe, professor emeritus in the Department of Physics, and an author of a paper on the results newly published online in Physical Review Letters.

"The simplest explanation involves adding new neutrino-like particles, or sterile neutrinos, which do not have the normal weak interactions."

The three known types of neutrino interact with matter primarily through the weak nuclear force, which makes them difficult to detect. It is hypothesized that this fourth flavor would not interact through the weak force, making it even harder to find.

The existence of sterile neutrinos could help explain the composition of the universe, said William Louis, a scientist at Los Alamos National Laboratory who was a doctoral student of Roe's at U-M and is involved in the MiniBooNE experiment.

"Physicists and astronomers are looking for sterile neutrinos because they could explain some or even all of the dark matter of the universe," Louis said. "Sterile neutrinos could also possibly help explain the matter asymmetry of the universe, or why the universe is primarily composed of matter, rather than antimatter."

The MiniBooNE experiment, a collaboration among some 60 researchers at several institutions, was conducted at Fermilab to check the results of the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos National Laboratory, which started in 1990. The LSND was the first to detect more neutrino oscillations than the standard model predicted.

MiniBooNE's initial results several years ago, based on data from a neutrino beam (as opposed to an antineutrino beam), did not support the LSND results. The LSND experiment was conducted using an antineutrino beam, though, so that was the next step for MiniBooNE.

These new results are based on the first three years of data from an antineutrino beam, and they tell a different story than the earlier results. MiniBooNE's antineutrino beam data does support the LSND findings. And the fact that the MiniBooNE experiments produced different results for antineutrinos than for neutrinos especially astounds physicists.

"The fact that we see this effect in antineutrinos and not in neutrinos makes it even more strange," Roe said. "This result means even more serious additions to our standard model would be necessary than had been thought from the first LSND result."

The result seems to violate the "charge-parity symmetry" of the universe, which asserts that the laws of physics apply in the same ways to particles and their counterpart antiparticles. Violations of this symmetry have been seen in some rare decays, but not with neutrinos, Roe said.

While these results are statistically significant and do support the LSND findings, the researchers caution that they need results over longer periods of time, or additional experiments before physicists can rule out the predictions of the standard model.

The paper is called "Event Excess in the MiniBooNE Search for ν̅ _μ→ν̅ _e Oscillations." It will be published in an upcoming edition of Physical Review Letters.

This research is funded by Fermilab, the Department of Energy and the National Science Foundation.