K-MESONS: HISTORY AND
BASIC THEORY
Francisco Antonio
Physics 129
November 23, 2010
Li Zhengdao and Yang Zhenning, 1953
Overview:
 Experimental History of Kaons
 Evolving Theory of Kaons with respect to CP
 Current Technologies, Directions for Kaon
Science
Pre-Gellmann and Pais
 The state of the yet to be conceived



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“Standard Model”
The Neutral  Particle, one among the many
particles of the 1953 particle extravaganza
Late 1950’s: The Intermediate Vector Bosons,
courtesy of Yukawa
1964: The quark.
1960-1980’s: Parallel development of Kaons’
and the Standard Model’s main concepts.
The Gellmann-Pais Paper
 Theoretical
proposition that
there be a property
of strangeness to
allow for K0 < -- > K0
(Eigenstates of
Mass)
 CP|K0> = -|K0>
 By CP
conservation:K1 and
K2 were restricted
to 2 and 3 decays,
respectively
Initially thought to be equivalent to KS and KL
Later known as the Eigenstates of Weak Interaction
First Results:
 Lederman, 1956.
 1  10^-10 sec.
 2  10^-8 sec.
 Goal: Kinematically
confirming the events
were related to a
neutral secondary
particle.
36 “ Magnet Cloud Chamber Design with Proton Beam incident on target
(Brookhaven Bevatron)
Data On the Vo Events
Ruling Out the Possibilities
CP Violation After All
 Cronin and Fitch,
1964.
 First evidence for
Indirect CP
Violation.
 Goal: Measured
uncharacteristic
2 decays where
only KL = K1
should have
existed.
Again at the Brookhaven Bevatron, this time with
spark chamber detectors
CP Violation: The Contenders
 Direct CP Violation – Depends on decay process itself,
penguin diagrams
 Indirect CP Violation – related to mixing the
eigenstates of mass to form the eigenstates of decay.
Weak interactions of ΔS = 1 and superweak
interactions of ΔS = 2.
The Weak Interaction
 Asymmetry in the K0/K0 mixing that created KL and KS 
no longer equivalent to K2 and K1 respectively. Instead a
mixture characterized by  of K1 and K2 describe KL and
KS in the eigenstate of weak interaction.
 Is a measure of CP violation related to the mixing of K0/K0
 Result: Time/Space Dependent Oscillation between KL
and KS
The Kobayashi-Maskawa Model
of Weak Interaction
Instead of a diagonal matrix relating the mass
eigenstates to the weak interaction eigenstates, the off
diagonals are present, allowing a small amount of
mixing
The amount of mixing is parametrized by three angles ij and phase shift 
Weak Interaction Eigenstates
Mass Eigenstates
Superweak Interactions
 Asserts that the ONLY CP violation in the
neutral Kaon system is due to K0/K0mixing
 Would accomplish this by allowing ΔS = 2
so that  (for the superweak) is much larger
than  (from the second order KM weak
interaction).
Direct CP Violation
Measured!
 Alavi et. al. 1999,
 Data: from 96-97 Fermilab Run.
 Goal: To measure a non-zero Re(/) to a high
degree of accuracy
Note that in the case of Superweak models Re(/) = 0
Alavi Experiment, Fermilab.
KTeV Experiment, E832, First to use Dual Kaon Beams for great suppression of
unwanted channels
The Modern Kaon Experimental
Setup
 Beam – Dual Kaon Beams
 Regenerator – Takes advantage of eigenstate
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oscillations
Photon Veto – Rejects 2 and 2 decays.
EM Calorimeter – particle “shower”. Reconstruction
Magnetic Spectrometer- Phase space
measurements (Intensity vs. Momentum)
Collimators – maximize neutral beam acceptance.
Is The Superweak Theory
disproven?
 At least in the neutral Kaon system it has
been found to be “weak”-sauce.
 B Systems are currently being tested for
similar ratios of Re(/) with results pending.
Kaon Science Today
 Direct vs. Indirect CP violation (only Kaons
have the former)
 The Strong CP Problem (QCD doesn’t break
CP Symmetry)
 Matter/Antimatter existence due to
preferential decays that violate CP
 The Standard Model’s take on CP Violation Insufficient
References

Gell-Mann, Pais. “Behavior of Neutral Particles Under Charge Conjugation”. Phys Rev. 97. 1387.

Alavi et al. “Observation of Direct CP Violation in Ks,l  2Pi”. Phys Rev. Letters. 83. 22.

Cronin et al. “Evidence of the 2Pi Decay of the K2 Meson”. Physical Review Letters. 13. 138.

Lande et al. “Observation of Long-Lived Neutral V Particles”. Physical Review. 103. 1901.

Kobayashi, Maskawa. “CP-Violation in the Renormalizable Theory of Weak Interaction”. Progress
of Theoretical Physics. 49. 652

Biino et al. “The Simultaneous Long and Short lived Neutral Kaon Beams for Experiment NA48”.
European Organization for Nuclear Research CERN—SL Division.

Winstein. “The Search for Direct CP Violation”. Reviews of Modern Physics. 65. 1113