Mu2e : A proposal to extend the sensitivity to charged lepton flavour violation by 4 orders of magnitude.

The electron is the lightest, stable charged particle & its properties are extremely well measured & underpin life through its role in chemical reactions. In 1937 a similar but heavier charged particle, the muon, was discovered in cosmic rays. The muon has been studied for the past 80 years & it seems to behave like a heavier version of the electron with its properties only modified by virtue of it being approximately 220 times heavier. It appears, like the electron, to have no structure & is not an excited state of the electron but a distinct fundamental particle. This distinction is embodied in a property called lepton-flavour: both the electron & muon (& tau) are charged leptons & we say that the electron is a charged lepton with electron-flavour & the muon, a charged lepton with muon-flavour. It is a similar case for the neutral leptons: the neutrinos. They also appear to come in three distinct flavours: electron, muon & tau. The 2015 Nobel Prize was awarded for the observation that neutrinos change flavour as they move through space: a neutrino of electron flavour (so called electron-neutrino) changes into one with muon-flavour (a muon-neutrino). This illustrates that the quantity of lepton flavour is not sacrosanct i.e. that it's not always conserved: one can start with 1 unit of electron-flavour & finish with zero but instead 1 unit of muon-flavour. The larger mass of the muon compared to the electron means it is unstable & decays with a lifetime of 2 x 1/millionth of a sec. To date we have only seen the muon decay in one way: to electrons (& positrons) & neutrinos & anti-neutrinos (with occasionally an additional photon). In each of these decays when one considers the combined lepton flavour of the decay particles it is always one unit of muon-flavour just like the initial decaying muon. The lepton-flavours of the neutrinos, electrons & positrons always cancel out. Given that this isn't the case for the neutrinos, we expect it will not always be the case for charged leptons & we expect some muons to decay in a way that does not conserve lepton-flavour. We have been searching for such decays since 1947 ! With the known particles in the Standard Model of particle physics it is possible for the process to occur, but only once for every 10^50 (1 with 50 zeros) muons: to put this into context 10^50 is approximately the number of atoms on our entire planet & so observing such an unlikely occurrence is impossible. We are instead trying to observe this decay at rate of one anomalous decay per 10^17 muons which is much easier: it is only the equivalent of observing one of the earth's grains of sand across all its deserts & beaches behave strangely! However this is now possible with muons thanks to technological advances that allow us to produce muons in huge quantities: approximately a billion every second. If one of these anomalous decays of the muon is observed it would signal that there are additional new particles or interactions that are not embodied in our current theory. Our current theory is sadly inadequate: it fails to describe gravity on the atomic scale, cannot explain the existence of dark-matter nor why our universe is dominated by matter & has very little anti-matter. To explain this requires there to be new particles or interactions & the observation of the anomalous decay of the muon would prove that such new particles & interactions do exist. Three UK institutes (Liverpool, Manchester & UCL) will be making a key contribution to the search for this anomalous muon decay. We will be building a detector for the Mu2e experiment at Fermilab that will measure the number of muons being produced in the experiment such that if an anomalous decay is observed we can determine its rate. Without this detector we have no normalization for the measurement. We will build the detector in the next 3 years & start examining the muon decays in 2020 & hope then to answer whether there are indeed new, previously unseen particles.

See the Pathways to Impact document submitted as an attachment.