# 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?

• The photoelectric effect is an example. – Ben Crowell Jul 5 '13 at 15:05

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.