Muon Physics T.E. Coan and J. Ye v090326.02Muon Physics Contents INTRODUCTION ................................................................................................................ 3 OUR MUON SOURCE ........................................................................................................ 3 MUON DECAY TIME DISTRIBUTION ................................................................................. 5 DETECTOR PHYSICS ......................................................................................................... 5 INTERACTION OF µ−’S WITH MATTER ................................................................................ 7 µ+/µ− CHARGE RATIO AT GROUND LEVEL ....................................................................... 9 BACKGROUNDS .............................................................................................................. 10 FERMI COUPLING CONSTANT GF .................................................................................... 10 TIME DILATION EFFECT ................................................................................................. 11 ELECTRONICS ................................................................................................................. 15 SOFTWARE AND USER INTERFACE ................................................................................. 19 Control ...................................................................................................................... 21 Data file format ......................................................................................................... 23 Monitor ..................................................................................................................... 24 Rate Meter ................................................................................................................. 24 Muons through Detector ........................................................................................... 24 Muon Decay Time Histogram ................................................................................... 24 THE LIFETIME FITTER .................................................................................................... 25 MUON DECAY SIMULATION ........................................................................................... 25 UTILITY SOFTWARE ....................................................................................................... 26 REFERENCES .................................................................................................................. 27 Instrumentation and Technique ................................................................................ 27 Cosmic Rays .............................................................................................................. 27 General ..................................................................................................................... 28 Muon Lifetime in Matter ........................................................................................... 28 GETTING STARTED ....................................................................................................... 29 SUGGESTED STUDENT EXERCISES .................................................................................. 31 HOW TO GET HELP .......................................................................................................... 33 3Muon Physics Introduction The muon is one of nature’s fundamental “building blocks of matter” and acts in many ways as if it were an unstable heavy electron, for reasons no one fully understands. Discovered in 1937 by C.W. Anderson and S.H. Neddermeyer when they exposed a cloud chamber to cosmic rays, its finite lifetime was first demonstrated in 1941 by F. Rasetti. The instrument described in this manual permits you to measure the charge averaged mean muon lifetime in plastic scintillator, to measure the relative flux of muons as a function of height above sea-level and to demonstrate the time dilation effect of special relativity. The instrument also provides a source of genuinely random numbers that can be used for experimental tests of standard probability distributions. Our Muon Source The top of earth's atmosphere is bombarded by a flux of high energy charged particles produced in other parts of the universe by mechanisms that are not yet fully understood. The composition of these "primary cosmic rays" is somewhat energy dependent but a useful approximation is that 98% of these particles are protons or heavier nuclei and 2% are electrons. Of the protons and nuclei, about 87% are protons, 12% helium nuclei and the balance are still heavier nuclei that are the end products of stellar nucleosynthesis. See Simpson in the reference section for more details. The primary cosmic rays collide with the nuclei of air molecules and produce a shower of particles that include protons, neutrons, pions (both charged and neutral), kaons, photons, electrons and positrons. These secondary particles then undergo electromagnetic and nuclear interactions to produce yet additional particles in a cascade process. Figure 1 indicates the general idea. Of particular interest is the fate of the charged pions produced in the cascade. Some of these will interact via the strong force with air molecule nuclei but others will spontaneously decay (indicated by the arrow) via the weak force into a muon plus a neutrino or antineutrino: π+ → µ+ νµ π−→ µ− νµ The muon does not interact with matter via the strong force but only through the weak and electromagnetic forces. It travels a relatively long instance while losing its kinetic energy and decays by the weak force into an electron plus a neutrino and antineutrino. We will detect the decays of some of the muons produced in the cascade. (Our detection efficiency for the neutrinos and antineutrinos is utterly negligible.)4Muon Physics Figure 1. Cosmic ray cascade induced by a cosmic ray proton striking an air molecule nucleus. Not all of the particles produced in the cascade in the upper atmosphere survive down to sea-level due to their interaction with atmospheric nuclei and their own spontaneous decay. The flux of sea-level muons is approximately 1 per minute per cm2 (see http://pdg.lbl.gov for more precise numbers) with a mean kinetic energy of about 4 GeV.