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### Mu2e: A little background (the nasty kind)

In experimental particle physics, the term “background” refers to events that can be easily confused for signal.  In my last post , I introduced the Mu2e experiment and pointed out that this experiment needs a huge amount of muons (1 million trillion, 1018,  or more) and hopes to be sensitive to even one muon decaying directly into an electron.  To achieve such a single-event sensitivity the sources of backgrounds must be minimized and/or understood extremely well.

So, what is so difficult about that?  Mu2e must have a striking experimental signature that is extremely hard to fake, right?  Not exactly!  The signal for the Mu2e experiment is just a single electron!  Hmmm… That sounds like it could be a problem because every ordinary atom making up the experiment, the building housing the experiment and planet Earth that it sits on is made up of electrons!

The figure shows the muon-electron conversion energy distribution in light green and the energy distribution for electrons from one of the backgrounds in red. The signal energy is spread out due to the limited resolution of the Mu2e detector (not all of the signal events are measured to have the exact energy produced in the decay). The source of the background shown in red is from muons that decay in orbit (DIO) into an electron and neutrinos. This decay is allowed in the Standard Model. Because of the extra neutrinos produced in the final state, the electron carries less energy than the signal events where the muon decays only to electrons since no neutrinos are involved to take away some of the energy.

So, let’s state the problem again:  the Mu2e experiment wants to stop 1018 muons on a target nucleus, and then be sensitive to even one event in which the muon decays directly into an electron.  It isn’t easy! In fact, the experiment is carefully designed to minimize all potential sources of background events.

Luckily, the electrons produced from the direct muon-to-electron conversion are special in that their energy will always have the same value 105 megaelectron volts, or 105 MeV.  This is an important point, because now, assuming that we can measure the energy of the electron well, our background has been reduced from “all electrons” to “electrons that have an energy close to 105 MeV” (see the figure at right).  In the case of the Mu2e experiment, this means that we can reduce our total background to less than one event expected over the total running time of the experiment!

Taking this into account, it is clear that the amount of background will depend on how well the experiment can measure the energy of the 105 MeV electron.  In other words, the sensitivity of the experiment depends critically on its ability to resolve the energy of an electron.

Future posts will include a series of “tricks” used by the experiment to control each of the major background sources.

— Craig Group