That’s Strange…Conservation
of Strangeness
Since
I’m spending this summer dealing with particle physics, I thought I’d post on
one of my favorite elementary particles, the strange quark, and the concept of
conservation of strangeness. I suppose first
I should give a BRIEF introduction to modern particle physics, although perhaps
someday I’ll have a longer post on it.
Basically, according to contemporary particle physics, all matter and
antimatter in the universe is made of two types of particles: hadrons
(examples: protons and neutrons) and leptons (example: electrons). Leptons are fundamental, in that they aren’t
made up of smaller even smaller things, but hadrons are combinations of quarks,
of which there are six types: Up, Down, Strange, Charm, Top, and Bottom. There are sorts of rules about how they
combine to form hadrons, but I won’t bore you with those. For example, a proton is made of two up
quarks and one down quark, and a neutron is two down and one up quark. There are 4 fundamental forces: Gravity,
Electromagnetism, Weak, and Strong. The
latter 3 forces are carried by different particles (for example, photons and
gluons), and typically gravity is considered to be carried by the graviton,
although it’s not incorporated into theory very well yet. In contemporary theories, some of these
forces have been unified into something more “fundamental”, as in the
electroweak force, and another combines the electroweak and strong forces, but
for the moment I’ll neglect those.
So
how do we get to something as crazy as conservation of strangeness? In the early days of particle physics,
experimenters crashed various hadrons into each other to see what particles
were created. The statement of what
happened was called a reaction, like in chemistry, and how they reacted was
known as an interaction- for example, the strong interaction (involves the
strong force) and the weak interaction (involves the weak force). Here’s an example reaction:
Here, a proton and a pion are
reacting to form a lambda and a kaon.
The cross section (a measure of the likelihood of an interaction
occurring) of this reaction is high, which means that it occurs via the strong
interaction. Looking at the resultant
particles, the lambda and kaon, physicists had a problem. From theory they had developed at the time, which
only had up and down quarks, they expected the lambda and kaon to decay by the
strong interaction. Instead, the
experimenters measured that it takes a long time for them to decay (10
nanoseconds), which is characteristic of the weak interaction. Decay by the strong interaction is 13 orders
of magnitude smaller! Other experiments
had similar results, and the confused experimenters started to call these particles
“strange particles”, because they seemed to have properties of the weak
interaction even though they were expected to decay via the strong. This was quite a problem for physicists,
although since the theory of particle physics was still being developed they
didn’t dismiss it entirely.
Later,
as they thought more about quarks, they discovered that these strange particles
have a different quark in them, instead of the normal up and down quarks that
protons and neutrons are made of, and thus they named the new quark the strange
quark. How does this answer the
problem? Well, these are the lightest
particles that contain the strange quark, and so they cannot decay according to
the strong interaction because there’s nothing for them to decay into. The strong interaction involves the trading of
quarks, and these particles cannot trade the quark to become something else
that decays more quickly. Thus, the
particles have to decay through the weak interaction, which takes longer. The existence of this extra quark has since
garnered more experimental evidence in favor of it, and is well accepted.
Returning
to the beginning era of particle physics, experimenters soon noticed other
rules these particles obeyed. For
example, they are always made in pairs, and always in certain pairs. Even when all other conservation laws are met
(which in particle physics you get some strange ones, like conservation of
lepton number), the experimenters couldn’t get any reaction that would produce
just one of these strange particles to occur.
This is conclusive evidence of a new conservation law, because the reason
why a reaction cannot produce just one is that something wouldn’t be conserved
then. Consider chemistry reactions- if
you break apart water (H2O), you absolutely must get two hydrogen particles
(although they may be bonded together)- if not, then the hydrogen is not
conserved. Physicists describe conservation
laws this by describing them numerically- like momentum. In particle physics, this is done by
assigning one particle the number of -1, the other 1, and all the rest 0, so
that in the interaction the total sum of this number is 0 on both sides of the
reaction. So, a number known as
“strangeness” was created, and the kaon was assigned the value 1. Thus, the lambda was assigned -1, and all the
particles were given a strangeness number.
Thus, conservation of strangeness was discovered.
As
if this wasn’t weird enough, as physicists further refined their understanding
of particle physics and how the interactions occur, they discovered that when
particles react via the strong interaction, strangeness is perfectly conserved
(as expected, from above), but when particles interact via the weak
interaction, strangeness can change by +/- 1!
So, strangeness is only partially conserved for the weak
interaction! A similar thing happens
with conservation of charm, which is associated with the charm quark- it is
only completely conserved for reactions involving the strong interaction; it’s
partially conserved in weak interactions.
All of this just proves that it is indeed a strange world that we live
in.