Re: [Phys-l] Help with particle physics preparations...

[John Denker <jsd@av8n.com>]

On 04/12/2007 10:56 PM, Jeff Loats wrote:

> The strange, charm, top and bottom quarks all have their own quantum 
> numbers, while the up and down quark do not. 

That's quite a deep question.

> It is quite possible that 
> there is no "why" here, since all of these conservation laws are 
> empirically derived from what we observe in particle reactions and 
> decays. 

I wouldn't say that.  Conservation laws have deep theoretical
underpinnings.  Conservation laws are related to symmetries
via Noether's theorem.  They're a long way from being purely
empirical.

> Is there any other way to describe why this is?

Yes, there is.

I'm not an expert, but according to my limited understanding,
we are free to define two more quantum numbers, upness and
downness.  That gives us six quantum numbers to go with the
six quarks, e.g. the table on page 16 of
  http://theory.uchicago.edu/~smaria/compton/lectures/l7/clec7.pdf
which is much more symmetric-looking than the table at the
top of:
  http://hyperphysics.phy-astr.gsu.edu/hbase/particles/quark.html

However, that's not usually done.  It's harmless but purely
cosmetic;  it doesn't tell us much beyond what we already know.

At the next level of detail, you may ask why not.  Well, that's
related to the /degeneracy/ of the up and down quarks.  They
have nearly the same mass ... in contrast to the other doublets
(C/S) and (T/B), where the members of each doublet have wildly
different masses.  There's a symmetry for (U/D) and a broken
symmetry for (C/S) and (T/B).

For details on this, try:
  http://www.google.com/search?q=quarks+upness
which leads to things like the "upness" discussion here:
  http://www.physicsforums.com/archive/index.php/t-10124.html


Pursuing this to yet another level, you may ask why flavor
symmetry is broken for (C/S) and (T/B) but unbroken for (U/D).
This is waaay outside my bailiwick.  I reckon it is like asking
why Ni is ferromagnetic at room temperature, while Dy is not.
   http://hyperphysics.phy-astr.gsu.edu/hbase/tables/curie.html
You could explain it in more fundamental terms, but it would
require diving into a sea of details.