# Any case of a particle seemingly decaying into copies of itself?

Is there any case reported that seems to resemble the following: there is a particle and at some moment, the particle seems to break down into two or more particles that are all identical to the original particle?

• additional information: I am not talking of antimatter/matter issues.
-
A related question physics.stackexchange.com/questions/13851 – Slaviks Mar 28 '12 at 10:58
Very loosely interpreted, isn't this how lasers work? One photon of a given frequency produces more photons of the same frequency. – Peter Shor Mar 28 '12 at 23:39
@PeterShor. No. The question is about decays, not interactions. Lazers are interacting photons and crystals/gas molecules and another story. – anna v Mar 29 '12 at 4:00
@annav is energy conserved in the case of lasers (interactions), then? As E=hf in quantum level, if a single photon can produce more photons of the same frequency, energy seems to be not conserved.... – user27515 Mar 29 '12 at 15:10
@user27515 In the laser phenomenon a single photon does not produce more photons of the same frequency in vacuum. The crystal/gas molecules are pumped up to a higher energy level by inputted energy. In lasing there is induced emission, i.e. a photon of the same frequency as the pumped up to a higher energy level molecules, induces a coherent relaxation to the ground state of the excited molecules and the laser beam is created.en.wikipedia.org/wiki/Laser . look at the "physics paragraph – anna v Mar 29 '12 at 16:00

In "normal" cases, no, this is not possible. You can easily understand why by considering this process in the center-of-mass frame (which is the rest frame of the original particle). In this frame, you would start with a single particle $X$ at rest, which has energy $m_Xc^2$, and wind up with 2 or more $X$, which will necessarily have an energy of at least $2m_Xc^2$. So energy conservation has to be violated by these sorts of reactions.
But consider a caveat: what do I mean by "normal"? Well, what we consider normal matter is made of massive particles. If you're looking at massless particles, on the other hand, the above argument doesn't apply because a massless particle doesn't have a rest frame. So you have to examine it from a lab frame (that is, any inertial frame). It should be easy to convince yourself that a massless particle of energy $E$ can decay into multiple instances of the same kind of particle with energies $\{E_1,\ldots,E_N\}$ such that $\sum_i E_i = E$ as long as all the products have momentum parallel to that of the original particle.