"While looking for the decay pi+ -> e+ nu_e, we focused all our attention on reducing backgrounds, since a prior experiment had set a limit at the level of 10^-6 on the branching ratio. When we heard that an experiment at CERN had seen a signal around 10^-4 I switched from delayed to prompt. The signal was right there, and could have been seen on the first day."
Burton Richter, quoted in J. Klein, A.Roodman, "Blind Analysis in Nuclear and Particle Physics"
A reader correctly mentions that it is not exactly trivial to understand the above quote without some explanation, so here it is.
Pions ordinarily decay to muons (within some nanoseconds), and only in a very rare fraction
do they go directly into electrons. Muons themselves decay to electrons, but do so
after 2.2 microseconds (on average).
A digression: why pions don't decay to electrons as frequently as to muons ?
Incidentally, while the lifetimes of pions and muons can only be understood by studying quantitatively the weak interaction, the strongly difference in relative frequency of pi->mu and pi->e is easy to figure out. Pions are spin-zero particles, and they produce two-body decays when they turn into a fermion-antifermion pair (muon/muon antineutrino, or electron/electron antineutrino). Fermions have spin 1/2. The spin of neutrinos, which are massless (to an excellent approximation), can only be aligned opposite to their direction of motion: they are left-handed, and antineutrinos are right-handed (their spin being always pointing in their direction of motion).
Now, for simplicity imagine that you are sitting in the center-of-mass frame of the decaying pion. You see a spin-zero particle decaying into two spin-1/2 particles, leaving the scene in opposite direction at high momentum. Given that the antinetrino wants to have its spin pointing in the direction of their motion, the charged fermion emitted in the opposite direction is bound to do the same, such that the two spin vectors cancel: conservation of angular momentum forces them to. But while the antineutrino is a anti-fermion, and thus wants to be right-handed (spin perfectly aligned with direction of motion, as we specified above), the charged fermion -be it a muon or an electron- is a fermion, and thus prefers to be left-handed. The charged fermion has mass, and this allows it to have its spin misaligned with its momentum vector; but the smaller the mass, the less probable is a non-aligned configuration.
Muons weigh 200 times as much as electrons do. This makes them much more willing to be "right-handed", when they travel at speeds smaller than the speed of light (in the ultra-relativistic limit they would also be exactly left-handed as the neutrinos) than electrons.
So this is the basic difference: the pion decay to fermion-antifermion pair forces the charged fermion to have the "wrong" alignment of spin and momentum. This is much easier to do for the muon because of its large mass. Therefore the decay of pions to electrons is highly suppressed, despite the fact that there would be a larger phase space for that decay if compared to the muon decay. Oh, I have not explained what phase space is: stuff for another post, but in a nutshell the larger the energy release not going into mass of the decay products, the faster is a decay.
Back to Richter's experiment
Now, if you search for pion decays to
electrons without the intermediate muon, you want to look for immediate
decays (when the pion enters your apparatus). But muons will always be
present as a background, so you need to estimate that.
studied muons by placing a delay between the timing of pion detection
and the timing of electron detection. He could thus study the
pi->mu->e reaction, to determine the background he would have when
at some point he would look at the immediate pi->e reactions. By
doing this he was doing things "blindly", to avoid involuntary biasing
himself. When he heard that a signal of the direct decay pi->e had
been seen elsewhere he realized this was not such a rare thing, and so
by removing the delay in his detection electronics he saw that there was
a ready signal of pi->e events which he could have seen on day one.
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