Everything about Cp-violation totally explained
In
particle physics,
CP violation is a violation of the postulated
CP symmetry of the laws of physics. It plays an important role in theories of
cosmology that attempt to explain the dominance of
matter over
antimatter in the present
Universe. The discovery of
CP violation in 1964 in the decays of neutral
kaons resulted in the
Nobel Prize in Physics in
1980 for its discoverers
James Cronin and
Val Fitch. The study of
CP violation remains a vibrant area of theoretical and experimental work today.
What is CP?
CP is the product of two
symmetries: C for
charge conjugation, which transforms a particle into its
antiparticle, and P for
parity, which creates the mirror image of a physical system. The
strong interaction and
electromagnetic interaction seem to be invariant under the combined CP transformation operation, but this symmetry is slightly violated during certain types of
weak decay. Historically, CP-symmetry was proposed to restore order after the discovery of
parity violation in the 1950s.
The idea behind
parity symmetry is that the equations of particle physics are invariant under mirror inversion. This leads to the prediction that the mirror image of a reaction (such as a
chemical reaction or
radioactive decay) occurs at the same rate as the original reaction. Parity symmetry appears to be valid for all reactions involving
electromagnetism and
strong interactions. Until 1956, parity conservation was believed to be one of the fundamental geometric conservation laws (along with
conservation of energy and
conservation of momentum). However, in 1956 a careful critical review of the existing experimental data by theoretical physicists
Tsung-Dao Lee and
Chen Ning Yang revealed that while parity conservation had been verified in decays by the strong or electromagnetic interactions, it was untested in the weak interaction. They proposed several possible direct experimental tests. The first test based on
beta decay of
Cobalt-60 nuclei was carried out in 1956 by a group led by
Chien-Shiung Wu, and demonstrated conclusively that weak interactions violate the P symmetry or, as the analogy goes, some reactions didn't occur as often as their mirror image.
Overall, the symmetry of a
quantum mechanical system can be restored if another symmetry
S can be found such that the combined symmetry
PS remains unbroken. This rather subtle point about the structure of
Hilbert space was realized shortly after the discovery of
P violation, and it was proposed that charge conjugation was the desired symmetry to restore order.
Simply speaking, charge conjugation is a simple symmetry between particles and antiparticles, and so CP symmetry was proposed in 1957 by
Lev Landau as the true symmetry between matter and antimatter.
In other words a process in which all particles are exchanged with their
antiparticles was assumed to be equivalent to the mirror image of the original process.
Experimental status
In 1964,
James Cronin,
Val Fitch with co-workers provided clear evidence (which was first announced at the 12th
ICHEP conference in
Dubna) that CP symmetry could be broken, too, winning them the 1980
Nobel Prize. This discovery showed that weak interactions violate not only the charge-conjugation symmetry C between particles and antiparticles and the P or parity, but also their combination. The discovery shocked particle physics and opened the door to questions still at the core of particle physics and of cosmology today. The lack of an exact CP symmetry, but also the fact that it's so nearly a symmetry created a great puzzle.
Only a weaker version of the symmetry could be preserved by physical phenomena, which was
CPT-symmetry. Besides C and P, there's a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time reversal implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. The combination of CPT is thought to constitute an exact symmetry of all types of fundamental interactions. Because of the CPT-symmetry, a violation of the CP-symmetry is equivalent to a violation of the T-symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of
quantum field theory, charge conjugation, parity, and time reversal are applied together.
The kind of CP violation discovered in 1964 was linked to the fact that neutral
kaons can transform into their
antiparticles (in which each
quark is replaced with its antiquark) and vice versa, but such transformation doesn't occur with exactly the same probability in both directions; this is called
indirect CP violation.
Despite many searches, no other manifestation of CP violation was discovered until the '90s, when the NA31 experiment at
CERN suggested evidence for CP violation in the decay process of the very same neutral kaons, so-called
direct CP violation. The observation was somehow controversial, and final proof for it came in 1999 from the KTeV experiment at
Fermilab and the
NA48 experiment at
CERN.
In 2001, a new generation of experiments, including the
BaBar Experiment at the Stanford Linear Accelerator Center (
SLAC) and the
Belle Experiment at the High Energy Accelerator Research Organisation (
KEK) in
Japan, observed CP violation in a different sector of particle physics, namely in decays of the B
mesons
(External Link
). By now a large number of CP violation processes in B-meson decays have been discovered. Before these "B-factory" experiments, it was a logical possibility that all CP violation was confined to kaon physics. However, this raised the question of why it's
not extended to the strong force, and furthermore, why this isn't predicted in the unextended
Standard Model, despite the model being undeniably accurate with "normal" phenomena.
The CP violation is incorporated in the Standard model by including a complex phase in the
CKM matrix describing
quark mixing. In such scheme a necessary condition for the appearance of the complex phase, and thus for CP-violation, is the presence of at least three generations of
quarks.
There is no experimentally known violation of the CP-symmetry in
quantum chromodynamics which presents the
strong CP problem.
Strong CP problem
In
particle physics, the
strong CP problem is the puzzling question why
quantum chromodynamics (QCD) doesn't seem to break the
CP-symmetry.
QCD doesn't violate the CP-symmetry as easily as the
electroweak theory; unlike the electroweak theory in which the gauge fields couple to
chiral currents constructed from the
fermionic fields, the gluons couple to vector currents. Experiments don't indicate any CP violation in the QCD sector. For example, a generic CP-violation in the strongly interacting sector would create the
electric dipole moment of the
neutron which would be comparable to
For a nonzero choice of the QCD
-angle and the chiral quark mass phase
one expects the CP-symmetry to be violated. One usually assumes that the chiral quark mass phase can be converted to a contribution to the total effective
-angle, but it remains to be explained why Nature chooses an unbelievably small value of this angle instead of an angle of order one; the special choice of the
-angle that must be very close to zero (in this case) is an example of
fine-tuning in physics.
There are several proposed solutions to solve the strong CP problem. The most well-known is
Peccei-Quinn theory, involving new
scalar particles called
axions. A newer, more radical approach not requiring the axion is a theory involving
two-time dimensions first proposed in 1998 by Bars, Deliduman, and Andreev.
(External Link)
CP violation and the matter-antimatter imbalance
Sakharov conditions must be satisfied, one of which is the existence of CP violation during the extreme conditions of the first seconds after the
Big Bang. Explanations which don't involve CP violation are less plausible, since they rely on the assumption that the matter-antimatter imbalance was present at the beginning, or on other admittedly
exotic assumptions.
The Big Bang should have produced equal amounts of matter and anti-matter if CP-symmetry was preserved; as such, there should have been total cancellation of both. In other words,
protons should have cancelled with
anti-protons,
electrons with
positrons,
neutrons with
anti-neutrons, and so on for all elementary particles. This would have resulted in a sea of photons in the universe with no matter. Since this is quite evidently not the case, after the Big Bang, physical laws must have acted differently for matter and antimatter, for example violating CP symmetry.
The
Standard Model contains only two ways to break CP symmetry. The first of these, discussed above, is in the QCD
Lagrangian, and hasn't been found experimentally; but one would expect this to lead to either no CP violation or a CP violation that's many, many orders of magnitude too large. The second of these, involving the weak force, has been experimentally verified, but can account for only a small portion of CP-violation. It is predicted to be sufficient for a net mass of normal matter equivalent to only a single galaxy in the known universe.
Since the Standard Model doesn't accurately predict this discrepancy, it would seem that the current Standard Model has gaps (other than the obvious one of gravity and related matters) or physics is otherwise in error. Moreover, experiments to probe these CP-related gaps may not require the practically impossible-to-obtain energies that may be necessary to probe the gravity-related gaps (see
Planck mass).
Further Information
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