19 May 2010

New CP Violation Physics Probably Necessary

One of the odd little bits of particle physics is that there is a symmetry, called "charge-parity" (CP for short) that is almost always observed in quantum physics events, except for a handful of odd little exceptions like the decay of the B meson (a meson is a particle made of two quarks, rather than the three found in a proton or neutron). If CP symmetry were observed, this decay would produce identical quantities of matter and anti-matter. But, the decays don't do that. Instead, they produce 1% more matter than anti-matter. (Study pre-print here).

This is "50 times larger than the asymmetry between matter and antimatter predicted for B meson decays by the standard model of particle physics."

[S]pokesperson Stefan Söldner-Rembold of the University of Manchester in England [said] “We were very excited because it means there’s some new physics beyond the standard model that has to be within our reach for the asymmetry to be so large.”

The odds that this is just random chance are one in a thousand based on this experiment alone, and is really more rare, because the experiment was set up to replicate an earlier less statistically significant result.

What possibilities are being considered:

Theories that might account for the DZero observations include supersymmetry, which assumes that each elementary particle in the standard model of particle physics has an as-yet-undiscovered heavier superpartner, notes theorist Marcela Carena of Fermilab, who is not a member of the discovery team. Other possible theories, she notes, include a model in which gravity and other forces operate in extra, hidden dimensions, and the notion that there’s an additional, fourth family of quarks beyond the three generations (up and down, strange and charm, and top and bottom) that serve as the building blocks of atomic nuclei and some other particles.

In models with a fourth quark family, the presence of new, heavy quarks and their interaction with the three known families could lead to a larger imbalance between matter and antimatter than found in the standard model, Carena notes. In supersymmetry theory, heavy superpartners would play a role similar to that of the heavy quark in creating interactions that might slightly favor the production of matter over antimatter, she adds.

And in theories with extra dimensions, new “messenger” particles — carriers of previously unknown forces — would move in hidden dimensions. These carriers could alter the charge and another property, called flavor, of elementary particles, causing the additional imbalance between matter and antimatter.

“Still, it is difficult to find a theory that can generate this asymmetry without contradicting other experimental results,” Carena adds.

Of the theories offered, a fourth generation quark seems most plausible, but it is surprising that we would see evidence of a fourth generation quark before seeing evidence as a presumably much lighter (and hence easier to produce experimentally) fourth generation electron.

Background here about an earlier B meson decay experiment.

The B meson has a mass of about 5.279 giga-electron volts (an electron volt is a unit of mass based upon Einstein's E=mc^2 relationship commonly used for fundamental physical particles). The experimental results point to a particle that would be somewhat heavier than a B meson. Particles that heavy are also always very short lived, lasting only a tiny fraction of a second before decaying. . . .

By comparison, the heaviest weight quarks (top and bottom) have masses of 171.2 GeV and 4.2 GeV respectively, the heaviest neutrino weighs less than 15.5 MeV, the heaviest version of the electron (the Tau) weighs 1.777 GeV, the W particle weighs 80.4 GeV, and the Z particle weighs 91.2 GeV. Gluons and photons are massless.

Another interesting (just stupid me) idea that popped into my head comes from some of the last post I did on this subject. The top quark mass is much bigger than bottom quark mass. Due to its very high mass and hence very rapid decay, the top quark should theoretically have almost no impact on B meson decay. But, we don't know much about the top quark with new breakthrough research from the same lab on its basic properties being published as recently as last year, and it was first definitively observed in 1995 around the same time as B meson decay. Its decay rate has been inferred from the standard model to be very short, but has not been experimentally confirmed independently.

One possibility that would be quite modest in terms of new physics, is that the top quark is simply a lot more stable than the standard model would predict, perhaps due to something similar in some way to the island of stability phenomena seen in the periodic table. A longer lived top quark would increase the amount of CP violation we'd expect to see in B meson decay, while still remaining too short to observe directly, and it would be calculate, and then it would be possible to test this possibility by some other means. The results observed might also be the result of an exotic mode of top quark decay, predicted by the standard model but not observed.

Skeptical blog coverage here. Motls thinks the significance value is for practical purposes lower than it seems due to publication bias. I'm less skeptical.


Andrew Oh-Willeke said...

This blog suggests a mystery particle closer to 100TeV mass in the decay chain, which is several hundred times the mass of the t quark.

Andrew Oh-Willeke said...

Lattice QCD perspective here. Key observation:

"What I find interesting is that all of the evidence of flavour physics beyond the Standard Model comes from particles containing a strange (rather than an up or down) quark besides a heavy flavour. The contribution to the charge asymmetry from B0d decays is well constrained by other experiments, so most of the D0 result would appear to be coming from the B0s system. I'm not a BSM phenomenologist, but I could imagine this to be relevant input for an understanding of possible BSM physics.

The Standard Model predictions rely on hadronic quantities such as decay constants, form factors and mixing parameters of the B meson, which must be determined nonperturbatively in lattice QCD. Better accuracy here could have real impact on the most stringent tests of the Standard Model that we have so far, and this is an area where significant progress is being made."

Andrew Oh-Willeke said...

A survey of the anomolous QCD experimental results generally can be found here.

A post there lead to various non-mainstream speculations about non-standard model physics by a Finnish physicist who calls his approach (notably feature the same kind of quantum color features in electrons as those found in quarks) called Topological Geometrodynamics (TGM for short), for example, who discusses experiments in 2008 and one set of experiments after that which suggest that there might be a 450 GeV fourth generation version of a top quark (far lower than one would expect given the mass ratios from one generation to the next of other known particles).

Andrew Oh-Willeke said...

Expert commentary here accompanying the publication of the results. The commentary basically explains the methodological reasons to infer that the result, which is 3.2 standard deviations from the Standard Model expectation is not a fluke.