Baryosynthesis and Antimatter Generation 143the Solar System is matter. Since no gamma rays are produced in the interaction of the
solar wind with the local interstellar medium, we know that the interstellar medium,
and hence our Galaxy, is matter. The main evidence that other galaxies are not com-
posed of antimatter comes from cosmic rays. Our Galaxy contains free protons (cos-
mic rays) with a known velocity spectrum very near the speed of light. A fraction of
these particles have sufficiently high velocities to escape from the Galaxy. These pro-
tons would annihilate with antimatter cosmic rays in the intergalactic medium or in
collisions with antimatter galaxies if they existed, and would produce characteristic
gamma rays many orders of magnitude more frequently than have been seen. The
observed ratio is
푁B
푁B= 10 −^5 − 10 −^4 ,
depending on the kinetic energy. This small number is fully consistent with all the
observed anti-protons having been produced by energetic cosmic rays in the Earth’s
atmosphere, and it essentially rules out the possibility that other galaxies emit cos-
mic rays composed of antimatter. There are many other pieces of evidence against
antimatter, but the above arguments are the strongest.
We are then faced with two big questions. What caused the large value of휂?And
why does the Universe not contain antimatter, anti-quarks and positrons. The only
reasonable conclusion is that푁Band푁Bmust have started out slightly different while
they were in thermal equilibrium, by the amount
푁B−푁B≃휂푁훾. (6.104)Subsequently most anti-baryons were annihilated, and the small excess 휂푁훾 of
baryons is what remained. This idea is fine, but the basic problem has not been
removed; we have only pushed it further back to earlier times, to some early BB-
asymmetric phase transition.
Primeval Asymmetry Generation. Let us consider theories in which a BB-
asymmetry could arise. For this three conditions must be met.
First, the theory must contain reactions violating baryon number conservation.
Grand unified theories are obvious candidates for a reason we have already met in
Section 6.2. We noted there that GUTs are symmetric with respect to leptons and
quarks, because they are components of the same field and GUT forces do not see any
difference. Consequently, GUTs contain leptoquarks X, Y which transform quarks into
leptons. Reactions involving X, Y do explicitly violate both baryon number conserva-
tion and lepton number conservation since the quarks have퐵=^13 ,퐿푖=0, whereas lep-
tons have퐵=0,퐿푖=1, where푖=e,휇,휏. The baryon and lepton numbers then change,
as for instance in the decay reactions
푋→e−+d, 훥퐵=+^1
3,훥퐿e= 1 , (6.105)푋→u+u,훥퐵=−^2
3