INVASIONS IN PARTICLE PHYSICS Compton Lectures Autumn 2001 Lecture 3 Oct. 20 2001 http://hep.uchicago.edu/compton 1
LECTURE 2 Cosmic Invasion The muon and the pion Previous Lecture 2
3
Orbital n=4 l=3 m=0 Orbital n=4, l=2 m=0 Orbital n=5 l=3 m=0 Orbital n=5, l=4, m=4 4
Rutherford Scattering Most of the atom is empty space. The positive charge of the atom is concentrated in a tiny nucleus. The tiny nucleus has a huge mass. 5 Rutherford named the Hydrogen nucleus proton(p)
THE NEUTRON (n)! Originally it was thought that protons and electrons make up the atomic nucleus, because these were the only known particles and because some unstable nuclei were known to emit electrons.! The Nitrogen nucleus was measured to have spin 1. It has +7 charge, so for the atom to be neutral it would have to have +14(protons)-7(electrons)=+7 charge in the nucleus and 14+7=21 particles (each with spin ½ (up or down)) in the nucleus. There is no way the total spin can be 1. 6
! The idea in the 30s was that there existed a neutral object of ½ spin in the nucleus (Rutherford s idea) and for example the Nitrogen nucleus is (7 protons+7 neutrons)! Chadwick found the neutron (1932) 7
" Alpha (α) " beta (β) " gamma (γ) 8
ALPHA DECAY 9
BETA DECAY 10
GAMMA RADIATION excited nucleus # unexcited (same) nucleus + gamma 11
12
Gravity too weak Electromagnetism wrong sign 13
The size of the nucleus is 10-15 m gold nucleous 1ft 3.3 miles outer electron 1ft 215 ft Earth 1.6 miles Pluto sun 14
A short range (10-15 m) force is needed to hold the nuclear pieces together What makes a force short range?? 15
BACK TO THE PHOTON: The photon which has zero mass is the messenger of the electromagnetic force. The range of the electromagnetic (EM) force is infinite -The EM force can be attractive or repulsive. 16
$ The nuclear force that keeps the nuclear pieces together must act at very small distances (the size of the nucleus). $ It must be attractive between proton and proton proton and neutron neutron and neutron which is to say independent of the charge 17
$ A clue for the short range of this force: bigger heavier nuclei decay into smaller more stable nuclei. $ This force is gluing the closest neighboring nucleons; Too many protons in a nucleus will cause it to break up -- the nuclear stability is described in part by the repulsive electrical forces between the protons and in part by the new short range strong nuclear force between the nucleons. $ Why is it not that we can dilute the repulsive effects of the protons electric charges by adding arbitrary number of neutrons in the nucleus? 18
$Who is the messenger? What is the mass of the messenger? $ From the size of the nucleus and the uncertainty principle it was figured that a particle of about 200 MeV (1/5 of the mass of the proton and 400 times the mass of the electron) should be the messenger of this nuclear force. $ H. Yukawa in 1934 predicted this particle which he called a meson (meson because it was middle-weight) 19
Anderson had already discovered the positron in a cloud chamber in 1930. In 1937 a middle-weight particle was discovered in cloud chamber experiments by two groups of scientists. (Carl D. Anderson and Seth H. Neddermeyer of California Institute of Technology and Jabez C. Street and Edward C. Stevenson of Harvard.) The new particle s mass was close to Yukawa s predicted pimeson (pion, π) mass so Andreson et al. thought that was it! 20
POSITRON IN CLOUD CHAMBER 21
Ionizing radiation that does not originate from the earth :cosmic rays. In 1905 Victor Hess performed a series of high-altitude balloon experiments and concluded that the origin of this radiation is beyond the Earth s atmosphere. Cosmic rays were the only source of high energy particles to study until accelerators were developed. 22
23
Simulation by Miguel F. Morales (Milagro) 24
25
Rings:two jets of highenergy particles created from matter from the supernovae remnant falling towards the black hole. The picture is a very nice illustration of one possible source of highenergy protons 26
$ The alleged pi-meson that Anderson et al. discovered, seemed very reluctant to interact with the atomic nuclei: it penetrated Earth s atmosphere and reached the cloud chamber at ground level! $ For a particle that was expected to be the carrier of the nuclear force that behavior was unacceptable. $ WHAT WAS IT? µ 27
$ A particle that had no purpose of existence. $ A heavier version of the electron (about 200 times heavier) that decays into and electron in about 2 microseconds. $ I.I. Rabi : Who ordered that? $ The muon is not a meson at all; like the electron is a lepton. $ Why this repetition? It is still not obvious today. 28
The pion was discovered by Cecil Powel and Giuseppe Occhialini in 1947 using photographic emulsions at the Pic du Midi, high in the French Pyrrenees. 29
The picture shows the particle tracks left in a photographic emulsion during the decay of a pion. The pion enters moving upwards from the bottom left, and comes to rest. It decays to produce a muon, which travels to the right. The muon track becomes increasingly dense as the muon slows down, and eventually stops. The muon then decays to an electron, producing the final track leaving at the top right. 30
+ - + - (200 times heavier that the electron) + - (273 times heavier that the electron) 31
DECAY CONCEPTS $ For each particle there is a probability Γ that it will decay in a unit time. $ If we have N(0) particles at t=0 we will have N(t)=N(0)exp(-Γt) at a later time t. $ For a single particle it means that if the particle exists at t=0, it has a probability exp(-gt) to still be there at time t=t. 32
$We say that a particle has lifetime τ =1/Γ. $ We say that Γ is the width of the particle. $ m=γ (From E=mc 2 and Ε t=h/2π) $ A DECAYING PARTICLE DOES NOT HAVE A DEFINITE MASS BUT A SPREAD OF MASSES GIVEN BY Γ. $ There is a force that causes a particle to decay. If the force is weak the particle will live for a long time. If the force is strong it will live for short time. 33
34
$ nucleons : neutrons (n) and protons (p) $ hadrons : all particles affected by the strong nuclear force $ baryons : hadrons which are fermions (such as the nucleons) $ mesons : hadrons which are bosons (such as the pion) $ leptons : all particles not affected by the strong nuclear force (such as the electron and the muon) 35
36
NEXT More particles Symmetries Accelerators 37
POSTCRIPT ON ALPHA DECAY 38
POSTSCRIPT ON SOURCE USED IN THE COUD CHAMBER DEMO Polonium-210 is a low-melting, fairly volatile metal, 50% of which is vaporized in air in 45 hours at 55C. It is an alpha emitter with a half-life of 138.39 days. 39
POSTSCRIPT ON CLOUD CHAMBER The chamber creates a volume of supersaturated alcohol vapor that condenses on ions left in the wake of charged particles. This is accomplished by establishing a steep vertical temperature gradient with dry ice. Alcohol evaporates from the warm top side and diffuses toward the cold bottom. The gravitationally stable temperature distribution permits a layer of supersaturation near the chamber bottom. Tracks of alcohol droplets indicate trajectories of charged particles, since each ion becomes a nucleation site for droplet condensation. HOW TO MAKE A CLOUD CHAMBER AND OBSERVE COSMIC RAYS AT HOME 40