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CP Violation

There are symmetries in our universe. You may be familiar with symmetries such as the symmetry of a circle, or the symmetry of our world and the reflected world we see in a mirror.

In physics, symmetry takes a slightly different meaning: symmetry exists when a transformation can be applied without changing the laws of physics.

One basic type is translational symmetry: no matter where you translate (i.e. move) an object, it will still follow the same laws of physics. For instance, regardless of where you may drop a ball (in New York, in Hong Kong, on Mars, etc.), the ball may fall at different speeds, but it will always fall according to the law of gravity. Another type of symmetry is in time: no matter what time it is, the laws of physics are still the same.

Physicists like symmetry in their theories, and whenever an experiment reveals that an accepted symmetry is violated, it is big news in the scientific community.

For a while, physicists thought that something called CP symmetry existed, but then CP violation was discovered. CP violation, the lack of CP symmetry, has interesting implications, and is the subject of study for the DAEδALUS experiment.

C, P, and CP Symmetries

CP symmetry is a composite of C and P symmetries, two symmetries previously thought to be absolute symmetries.

  • C-symmetry is charge conjugation symmetry: it transforms a particle into its antiparticle or vice versa.
  • P-symmetry is parity symmetry: it transforms everything into its mirror image, so left becomes right and vice versa.

For a while, C and P symmetry were accepted to be real symmetries. No experimental evidence seemed to disprove them, and the two symmetries also seemed naturally right. It seemed natural for particles and antiparticles to share the same physics, and for laws of physics to remain the same even when mirrored. However, the weak interaction, and thus neutrinos, shattered this belief.

Most of nature seems to follow both C-symmetry and P-symmetry. However, the weak force follows neither C- nor P-symmetry.

Neutrinos' signature force is the weak force, though they also feel the gravitational force. Due to neutrinos' strong association with the weak force, they never follow either C-symmetry or P-symmetry. The violations of those two symmetries were first discovered in experiments that involved neutrinos and their spin.

Neutrinos, like all other elementary particles, have a spin. As long as the spin is not zero, a particle's spin is characterized as either being left-handed or right-handed. The handedness of spin is determined by using your left or right hand: if your thumb points in the direction the particle is traveling, your fingers should curl in the direction of the particle's spin. If the particle has a left-handed spin, then your left hand gives the correct orientations. If the particle has a right-handed spin, then your right hand gives the correct orientation.

Neutrinos Violate P-Symmetry

If you flip, i.e. mirror, a left-handed neutrino, you get a right-handed neutrino. The problem is, while we have seen plenty of left-handed neutrinos, no right-handed neutrinos have ever been observed. Here, mirroring causes the laws of physics to change. This violates P-symmetry.

Neutrinos Violate C-Symmetry

If you take the charge conjugation of a left-handed neutrino, you get a left-handed antineutrino. The problem here is that left-handed antineutrinos have never been observed either... and so C-symmetry is violated here too!

Neutrinos and CP Symmetry

The realization that C- and P-symmetry are both violated caused both excitement and dismay in the physics community. (It was very groundbreaking news, really.)

Some order was reinstated when physicists came up with the idea of CP symmetry, a combination of C- and P-symmetry. The reasoning was this: maybe C and P were just the wrong ways of defining a symmetry and CP is actually the true symmetry. For a while, CP did seem to fix the now more-symmetrical world of neutrinos (and the weak force):

Using CP symmetry, if you take both the mirror and charge conjugation of a left-handed neutrino, you get a right-handed antineutrino, which does exist!

CP symmetry patched up the broken C and P symmetries. Particles behaved according to CP symmetry as far as physicists could tell. All seemed right and symmetrical again. There were no surprises--until the violation of CP symmetry was discovered.

CP Violation

CP Violation was first discovered in the decay patterns of kaons, a specific group of composite particles.

Two of the decay paths of the K-long kaon are particularly striking. The products of the two decays are entirely CP-symmetric versions of one another. For example, one involved an electron antineutrino and the other an electron neutrino, and all the other corresponding particles of the two decays were also entirely CP-symmetric.

Despite the products of the two decays being CP-symmetric, they weren't produced at equal rates. If CP symmetry were absolutely true, then K-long kaons would decay via those two paths in equal amounts. However, in experiments, the K-long was more likely to decay into the path involving the electron neutrino by a very small fraction. It is this fractional difference that demonstrates clear CP violation and so provides the undeniable proof that physics does distinguish between matter and antimatter. Click here to learn more about antimatter.

The light bulb then went off: if CP violation draws a clear distinction between matter and antimatter, then CP violation may be what caused our universe to be matter-dominated!

Detecting CP Violation in Neutrinos

Since this discovery, CP symmetry violation has already been observed in quark oscillations and incorporated into quark mixing theory. (Quarks are elementary particles that make up protons, neutrons, kaons, and more.) However, more CP violation must be observed in order to explain the universe's matter dominance. Therefore, physicists are looking to find CP violation in other particles, such as neutrinos.

CP violation has yet to be observed in neutrinos, but physicists have a hunch of where to look: in neutrino oscillations. Hidden within the patterns of how neutrinos oscillate may be the hint of CP violation that DAEδALUS is searching for. CP violation has already been worked into neutrino oscillation theory through the neutrino mixing model. What's next is to observe if the theory is experimentally true.

The parameter that describes CP violation in neutrino oscillations is the angle δcp (hence the δ in DAEδALUS). If δcp is not equal to 0 (or 180 degrees), then CP violation exists in neutrino oscillation. The value of δcp signifies how much CP is violated, and affects how the neutrinos oscillate between the three flavor states. If we can find δcp, we can determine whether or not neutrinos violate CP symmetry, and if so, to what extent.

Finding δcp is DAEdALUS's goal.