Monday, July 7, 2014

Still No Sign of CPT Symmetry Violations


There is no experimental evidence for CPT symmetry violations to date. CPT symmetry widely believed to be a perfect symmetry, unlike CP symmetry.  New experimental data tightens the experimental boundaries on any potential CPT symmetry violations by a factor of up to 100.


What is CP Symmetry Violation?

CP violation is a rare "arrow of time" in the laws of physics.  In certain neutral mesons, decays of matter and anti-matter version of the neutral mesons do not decay at the same rates, and the forward and backward versions of these decays occur at different rates.

CP symmetry is violated in a few esoteric circumstances driven in the Standard Model entirely by (1) the CP violation phase in the CKM matrix, and (2) the CP violation phase in the PMNS matrix, if any, which governs transitions between neutrino mass states.  The latest neutrino physics data favor CP violation in neutrino oscillation, but aren't yet definitive.

Experimental evidence of CP violation is restricted to electrically neutral mesons (originally, neutral kaons where it was first observed in 1964; CP violation in other mesons was not observed until 2001), even though, in principle, it is present at tiny levels too small to be detected by current experiments in all weak force interactions of quarks.

What is CPT Symmetry Violation?

CPT violation says that (1) if any physical process proceeds differently starting at one point and ending at another, from the way it would if it started at that end point and ended at that starting point (i.e. reversed in the time coordinate), then (2) the process is identical except that the charge and parity of the interaction is reversed when the direction in time of the process is reversed.

CPT symmetry violation also implies a violation of Lorentz invariance. Lorentz invariance means that the laws of special relativity is perfectly observed. Special relativity, in turn, implements to law of nature that says nothing can exceed the speed of light in a particularly elegant way. For example, special relativity modifies Newton's laws of motion because momentum does not translate linearly into acceleration, so that it takes more momentum to produce the same increment of additional velocity as one approaches the speed of light. Special relativity also provides that time slows down in accelerating objects relative to objects at rest with time standing still in the limit of the reference frame of a photon moving at the speed of light.

The New Data

New experimental data from the Large Hadron Collider (LHC) from the decays of electrically neutral B (made of a b quark and an anti-down quark, or their respective anti-particles), Bs (made of a b quark and an anti-strange quark, or their respective anti-particles) and D mesons (made of a charm quark and an anti-down quark, or their respective anti-particles) tightens already strict experimental constraints on CPT violation.

The old constraints on CPT violation are expressed in a formalism where the CPT violation parameter delta alpha sub mu is expressed in units of GeV/c2 were on the order of 10-12 to 10-14.  A smaller parameter means a tighter constraint on any possible CPT violations.

Current experimental constraints in neutral kaons (which are made of a strange quark and an anti-up quark, or their respective anti-particles), are much more strict, with limits on the CPT violation parameter delta alpha sub mu of about 2*10-18 and limits on a related parameter which is of the same order of magnitude in B and Bs mesons of less than 5*10-21.

The improvement in the B and Bs meson cases is two orders of magnitude (i.e. a factor of 100), and in  the D meson case by about 1 order of magnitude (i.e. a factor of 10), to new constraints on the order of 10-14 for D and Bs mesons and 10-15 for B mesons without strange quarks. The conclusion to the paper states (with internal cross references omitted):
We have presented new results on CPT violation in B0 and Bs0 mixing in both the classical and SME [Standard Model Extension] approach, derived from existing BaBar, Belle and LHCb data. In both approaches there is a significant improvement over previous results. LHCb will be able to further improve these numbers in the B0 and Bs0 systems, as well as in the D0 system, with dedicated analyses. In most cases these possible LHCb measurements would improve the current best values by orders of magnitude and the corresponding precision on [CPT violation parameter delta alpha sub mu] approaching the interesting region of m2/MPl.
MPl is the Planck mass, which is about 1.2209×1019 GeV/c2 the Planck mass is significant in physics because it is related to the point at which the Heisenberg uncertainty principle applies, and quantum rather than classical physics is the only regime that can meaningfully analyze phenomena.


Very few physics seriously expect there to be CPT violations in meson decays.  But, the new experimental evidence confirms this expectation experimentally to unprecedented precision, securing a bedrock principle of fundamental physics.

Lorentz invariance violations, where they are theoretically proposed at all, are detectible only over very long distances as a result of quantum gravity theories that propose that space-time is not perfectly continuous and local.

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