From: sichase@csa1.lbl.gov (SCOTT I CHASE)
Newsgroups: sci.physics
Subject: Re: Why do protons outnumber antiprotons?
Keywords: electron positron proton antiproton subatomic particle theory GUT unified
Date: 1 Sep 92 21:02:40 GMT
Reply-To: sichase@csa1.lbl.gov
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In article <1992Sep01.152644.18241@CS.ORST.EDU>, cavenej@prism.CS.ORST.EDU (Jeff Cavener) writes...
>And for that matter (NPI;-) why do electrons outnumber positrons?

A chocolate fish for the man with the good question.

The best explanation I have seen is in Kolb and Turner's _The Early Universe_
from which I crib heavily below.

(I) How do we really *know* that the universe is not matter-antimatter 
symmetric?  

(a) The Moon:  Neil Armstrong did not annihilate, therefore the moon
is made of matter.
(b) The Sun:  Solar cosmic rays are matter, not antimatter.
(c) The other Planets:  We have sent probes to almost all.  Their survival 
demonstrates that the solar system is made of matter.
(d) The Milky Way:  Cosmic rays sample material from the entire galaxy.
In cosmic rays, protons outnumber antiprotons 10^4 to 1.
(e) The Universe at large: This is tougher.  If there were antimatter
galaxies then we should see gamma emmisions from annihilation.  Its absence
is strong evidence that at least the nearby clusters of galaxies (e.g., Virgo)
are matter-dominated.	At larger scales there is little proof.
    However,  there is a problem, called the "annihilation catastrophe"
which probably eliminates the possibility of a matter-antimatter symmetric
universe.  Essentially, causality prevents the separation of large chucks
of antimatter from matter fast enough to prevent their mutual annihilation
in in the early universe.  So the Universe is most likely matter dominated.

(II) How did it get that way?  

	Annihilation has made the asymmetry much greater today than in the
early universe.  At the high temperature of the first microsond, there were
large numbers of thermal quark-antiquark pairs.  K&T estimate 30 million
antiquarks for every 30 million and 1 quarks during this epoch.  That's a 
tiny asymmetry.  Over time most of the antimatter has annihilated with matter,
leaving the very small initial excess of matter to dominate the Universe.
	
	There are two possibilities for why we are matter dominated today:
(a) The Universe just started that way, and (b) processes since the Big
Bang have produced the matter excess.  (a) is a rather sterile hypothesis.
Let's consider if current physics (i.e., the Standard Model) can account 
for (b).

	You need three conditions (first enumerated by Sakharov) to generate
non-zero baryon number in an initially symmetric universe, assuming the
Standard Model.  (Baryon number is the total count of baryons minus antibaryons 
in the Universe.)

	(1)  Baryon number violation.  If baryon number is conserved in all
reactions, then the present baryon asymmetry can only reflect asymmetric
initial conditions, and we are back to case (a), above.
	(2) C and CP violation.  Even in the presense of B-violating reactions,
without a preference for matter over antimatter the B-violation will take
place at the same rate in both directions, leaving no excess.
	(3) Thermodynamic Nonequilibrium.  Because CPT guarantees equal
masses for baryons and antibaryons, chemical equilibrium would drive the 
necessary reactions to correct for any developing asymmetry.  

	Condition (2) has been experimentally observed.  Condition (1) is
believed to hold at high temperature. Condition (3) is valid in the rapidly 
expanding early universe. 

	This explanation hinges on whether (1) is really true.  There is no
baryon-number (B) violation in the Standard Model by itself.  Only in the
framework of grand unified theories (GUTs) is does B violation become a part of
the extended SM.  And for a long time, it was believed that B violation is 
so suppressed by the large mass scale involved that it could never be enough
to do the trick.  In the last couple of years (see the work of McLerran 
others) it has become clear that at suffiently high temperature that B violation
becomes unsuppressed, and can be enough to produce the observed 
matter-antimatter asymmetry.

-Scott

--------------------
Scott I. Chase			"The question seems to be of such a character
SICHASE@CSA2.LBL.GOV		that if I should come to life after my death
				and some mathematician were to tell me that it
				had been definitely settled, I think I would
				immediately drop dead again."      - Vandiver