Matter and Antimatter, Dancing to Different Drummers?kicp.uchicago.edu/~odom/compton_files/compton_lecture_07.pdf · • Matter and antimatter dance to different drummers.Only slightly

Lecture 7: Matter and Antimatter, Dancing to Different Drummers?

The 64th Compton Lecture Series

Unsolved Mysteries of the Universe:Looking for Clues in Surprising Places

THE ENRICO FERMI INSTITUTE

Brian Odom

http://kicp.uchicago.edu/~odom/compton.htm

Nov. 4, 2006 Brian Odom Compton Lecture 7

Antimatter—It’s Not Just Theoretical

Fundamental anti-matter particles

Composite antimatter

http://pdg.web.cern.ch/pdg/cpep/antimatter.html

Matter-antimatter annihilation


The Bottom Line

• As a result, the observed asymmetry between matter and antimatter in the universe is difficult to explain.

• In other words, we have a hard time explaining the fact that we exist at all.

image from A. Weinstein, Caltech course

• It was extremely difficult to discover that nature treats matter and antimatter differently. A world of antiatoms would look very much like our own.

(Note, “dark matter”and “antimatter” are completely different classifications for particles.)


Continuous & Discrete Symmetries

• Symmetry is one of the most important concepts in physics.

• Rotational symmetry and time-translation symmetry are “continuous” symmetries which appear to be good symmetries of nature.

• C (charge conjugation), P (parity inversion), and T (time reversal) are “discrete” symmetries. It took a while to find out, but none of these individually are good symmetries of nature.


Parity (P) Transformation

Quinn and Witherell, Scientific American 2003

Parity transformation is accomplished by looking in a mirror.Technically, you also have to rotate 180o, but since rotations are harmless, this is not an important distinction.

Under P, right hands go to left hands


Charge-Reversal (C) Tranformation

Charge conjugation replaces all particles with their antimatter counterparts. The name is a little confusing because many neutral particles are not their own antiparticles (e.g. the neutron and the antineutron).

A “CP-mirror” changes both the charge (electron goes to positron) and handedness (seen here by text on the particle)


Time-Reversal (T) Transformation

• At first glance, time-reversal seems to be a very poor symmetry of nature

• Under T, we play a movie backwards (that is the “T-mirror”)

• If T is a good symmetry, events in the movie will look like they are obeying the usual laws of physics

• However, except in extremely special cases, time-reversed movies really do follow the normal microscopic physical laws

• The confusion is about entropy, or statistical likelihood

• If we reversed momenta of the particles, we could actually reproduce electromagnetic processes like glass unbreaking


P-Transformation of Spin


• Normal vectors, like position or momentum, are reversed under a P-transformation

• But, spin is a product of two normal vectors

• -1 x -1 = 1, so under P spin remains unchanged

• And under P, a particle’s handedness reverses


Neutron Beta Decay

www.astro.iag.usp.br/

A free neutrons decays into an electron, a proton, and an antineutrino

Two problems for C and P symmetries:• The electron tends to come off in the direction of the neutron spin• The antineutrino is always right-handed

electron

proton

RH antineutrino

spin-up neutron


P-Transformed Neutron Decay

Two things look wrong when we watch neutron beta decay in a mirror:

1. The electron & proton go the wrong ways2. The antineutrino has the wrong handedness

P-mirror

Actual beta decay Beta decay seen in P-mirror

electron

proton

RH antineutrino

spin-up neutronspin-up neutron

proton

electron

LH antineutrino


P-Violation in Neutron DecaySo, viewing beta decay in a mirror looks funny. The laws of physics we know are not followed in the image. So P is not a good symmetry of nature. Specifically, P is maximally violated by the weak interactions.

P-mirror

Actual beta decay Beta decay seen in P-mirror

electron

proton

RH antineutrino

spin-up neutronspin-up neutron

proton

electron

LH antineutrino


C-Transformed Neutron Decay

C-mirror

C also has big trouble with the weak interactions

C-reversed neutron beta decayActual beta decay

electron

proton

spin-up neutron

positron

anti-proton

spin-up anti-neutron

RH antineutrino RH neutrino


C-Transformed Neutron Decay

The neutron beta decay viewed through a “C-mirror”does not look like the correct behavior for an antineutron. Thus, C is not a good symmetry of nature.

C-reversed neutron beta decay Actual antineutron beta decay

positron

anti-proton


RH neutrino


positron

LH neutrino

anti-proton


CP Saves the Day

CP-mirror

Actual neutron decay

electron

proton

spin-up neutron

RH antineutrino

CP-reversed neutron decay


positron

LH neutrino

anti-proton

Things in the CP mirror look like they are obeying the right laws of physics! CP symmetry is preserved in this process.


Does Nature Know its Left from Right?

Imagine trying to communicate information about humans to distant aliens? (based on an example from Feynman)

• Describe human height using wavelengths of light• Describe typical human age by an atomic clock• But, how can you communicate the Western convention of a right-handed handshake? P-violation gets you started.

www.physi.uni-heidelberg.de

You could tell the alien to shake hands using the right hand, as defined by the handedness of the lightest particle coming from beta decays (we call them antineutrinos)


Does Nature Know its Left from Right?

But if the alien is made of antimatter, you end up with a big problem. He extends his left hand for a shake and annihilates your right hand.

How did this happen? He thought you were telling him to define “right” using neutrinos coming from antineutrons, the neutral particles found in his local world.

• With just P-violation, there is no deep sense in which nature knows its left from its right.

• Similarly, with just C-violation there is no fundamental distinction between matter and antimatter


Special Mesons for CP Violation


Cross-generational mesons, such as the K-mesons (kaons) and B-mesons provide a special laboratory to explore CP symmetry.


CP-Violation in Kaons

02K ds sd= +

These particles are each unchanged under C and under CP—they are their own antiparticles

01K ds sd= −

But, once again, weak interactions cause trouble. They cause the K2

0 to evolve into KL0:

0 0 01 2LK K Kε= +

This is particle is just a bit different from its antiparticle. It does not transform into itself under CP. Thus, we expect the decay products to have a particle-antiparticle asymmetry.



Indeed, there are two particular decays of the KL0

which make the problem very obvious.

0L eK eπ ν+ −→ + +

We cannot analyze the matter-antimatter asymmetry in terms of only C-tranformations, since weak decays maximally violate C. As we saw in neutron decay, the only hope for particle-antiparticle symmetry comes from thinking about CP, not C by itself. (For instance, transforming νe by C gives a non-physical right-handed νe .)

0L eK eπ ν− +→ + +

and



0L eK eπ ν+ −→ + +

The only thing to do is look into our CP mirror. We find that the two decay products of

0L eK eπ ν− +→ + +

and

transform into each other by CP:

( )e eCP e eπ ν π ν+ − − ++ + → + +



0L eK eπ ν+ −→ + +

If we look into the CP mirror these decay products transform into eacy other. So, for CP symmetry to hold, the KL

0 must decay into these products with equal probability.

0L eK eπ ν− +→ + +

x %

Unfortunately for CP symmetry, this is not the case. The 2nd decay happens 0.3% more often than the first.

y %

x = y ?



02 eK eπ ν+ −→ + +

02 eK eπ ν− +→ + +

Our world

CP-mirror

CP Image

1000 times

1003 times02 eK eπ ν+ −→ + +

02 eK eπ ν− +→ + +

1000 times

1003 times

The CP image looks almost right. But not quite. The real world favors positron production. CP violation is more pronounced in some other processes.


The Kaon Punchline

• The punchline is this. We start with a particle, the K20,

which is its own antiparticle. We expected things to be bad under just a C transformation. But, even viewed in the CP-mirror, things look funny.

• Said a different way, we start with a particle, the K20,

which is its own antiparticle. The end result is that we get more positrons than electrons, by a small amount. In this particular decay, nature favors antimatter over matter! (Other reactions favor matter over antimatter.)

• Matter and antimatter dance to different drummers. Only slightly different, but still different.


Back to the Aliens

Now, we can avoid the catastrophy of losing our hand when greeting an antimatter alien. CP violation allows us to unambiguously define:

1. What is matter and what is antimatter (anti-electrons are favored in K2

0 decay)

2. What is positive and what is negative charge (positive leptons are favored in K2

0 decay)

3. What is right-handed and what is left-handed (left-handed neutrinos are favored in K2

0 decay)

The universe draws a fundamental distinction between matter/antimatter, positive/negative, right-handed/left-handed.


CPT Saves the Day

• Given a few assumptions, CPT can be proved to be a good symmetry of quantum field theories.

• When we view the kaon processes through a CPT-mirror, the physics again looks correct.

• So, if CP is violated, then so is T. There are actually some reactions which run different in the time-reversed movie:

( ) ( )P A B C D P C D A B+ → + ≠ + → +• After lots of looking, this asymmetry has actually been observed in the kaon system. It was not easy, though!

• Next week, we’ll talk about searches for fundamental electric dipole moments, which also violate T


Is CPT Really True?• String theory actually breaks one of those assumptions (locality) that lead to CPT symmetry, so CPT might not hold.

• CPT symmetry dictates that, while in some ways matter might be fundamentally different from antimatter, in other ways symmetry must be maintained.

• Particles and antiparticles must have the same mass, opposite charge, the same lifetimes, etc.

Comparing atoms to antiatoms, e.g. their absorption spectra, tests CPT


Next Week

• Is the distinction between matter and antimatter (i.e. CP violation) seen in kaons enough to explain why we are made of matter that didn’t get annihilated a long time ago?

• No. We’ll talk about this next week.

• Also, we will talk about experiments which are attempting to find new physics to explain this dilemma.

http://pdg.web.cern.ch/pdg/cpep/antimatter.html

image from A. Weinstein, Caltech course

Documents

Matter and Antimatter, Dancing to Different Drummers?kicp.uchicago.edu/~odom/compton_files/compton_lecture_07.pdf · • Matter and antimatter dance to different drummers.Only slightly