OPTIMIZATION OF EXTINCTION EFFICIENCY IN THE 8-GEV MU2E BEAM LINE

A muon-to-electron conversion experiment at Fermilab, Mu2e, is being designed to probe for new physics beyond the standard model at mass scales up to 10 4 TeV [1]. For this experiment, the advance in experimental sensitivity will be four orders of magnitude when compared to existing data on charged lepton flavor violation. The muon beam will be produced by delivering a proton beam contained in short 100-ns bunches onto a muon production target, with an inter-bunch separation of about 1700 ns. A critical requirement of the experiment is to ensure a low level of background at the muon detector consistent with the required sensitivity. To meet the sensitivity requirement, protons that reach the target between bunches must be suppressed by an enormous factor, so that an extinction factor, defined as a number of background protons between main bunches per proton in such a bunch, should not exceed 10 −9 . This paper describes the advanced beam optics and results of numerical modeling with STRUCT [2] and MARS [3] codes for a beam line with a collimation system that allows us to achieve the experimental extinction factor of one per billion. BEAM LINE DESIGN In order to eliminate backgrounds, inter-bunch or out-oftime protons that would nominally strike the primary target between the 100-ns proton bunches will be swept off the central beam trajectory in the horizontal plane using a 294 kHz sinusoidal waveform AC dipole. The proton bunches are centered on the zero-field crossing point. Three optional solutions are shown in Fig. 1: a first harmonic sinusoidal waveform, a composition of first harmonic and the 17th harmonic (5 MHz) with the amplitude of 1/14 of the main one, and MECO [4] type waveform of the AC dipole. The main bunch length is 6σ = 0.185 rad or 0.1 μsec. Two possible distributions of the DC beam are considered in the simulations: a uniform distribution of background protons and distribution with increased population nearer in time with the main bunches. Beta-functions in the extinction beam line are presented in Fig. 2. The operational characteristics of the AC dipole are critical to the extinction system. In order to decrease the required strength of the AC dipole for technical reasons, it is placed in a high horizontal and small vertical betafunctions region. Five collimators (CH1-CH5) are located downstream of the AC dipole to intercept swept protons (Fig. 3). The horizontal jaws of first three collimators are