Does the particle number in the universe need to increase? Can someone give me an example of a physical process - some (realistic) scattering process - in which one can observe that the particle number has decreased?
I was wondering, because it is relatively easy to create antiparticle-particle pairs, but is there a process that in some sense makes up for that, or does the overall number of (fundamental) particles (say in the universe) just keep increasing?
Edit: (I have also changed the title of my question, sorry) So far the photoelectric effect and the gluon + gluon --> Higgs process have been mentioned. So interestingly the number of gauge bosons can decrease. But are there processes for leptons or fermions where the particle number decreases or is it true that these numbers keep increasing in the universe?
 A: The laws of physics are time reversal invariant, so for every reaction there is a time reversed version of it which does the opposite. However, reactions which decrease the number of particles, generically things like $X+Y+Z\to A+B$ etc., are rarely seen in nature because it is much harder to get three particles $X,Y,Z$ together at the same place and time compared to two particles $A,B$. For it to work you need to have an incredibly dense medium, or some intermediate process like $X+Y+Z \to A^\star+Z \to A+B$ which ultimately involves only a series of two body interactions.
So increasing the number of particles is an entropic thing, like scrambling an egg. There is no law saying that unscrambling an egg is impossible. Indeed, if you carefully reversed the velocity of every particle involved in the egg scrambling you would get it to unscramble. But you never see it reversed because it is hard to get the conditions just right.
A: One of the Higgs production channels is the fusion of two gluons to create one Higgs. See the Résonaances blog for a description of this.
