http://hep.uchicago.edu/compton 1

INVASIONSIN

PARTICLE PHYSICS

Compton Lectures Autumn 2001Lecture 4

Oct. 27 2001

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LECTURE 4 LECTURE 4

Neutrino - The weak force -Symmetries in the physical lawsSymmetries in the physical laws

Parity and Strangeness

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Neutron(n)

Proton (p)

Electron (e)

Antineutrino (νννν, nu-bar)

A neutron decays spontaneously with a half-life of 10 minutesinto a proton and an electron. Can it be due to the strong force?Can it be due to the electromagnetic force?

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1931 - A hypothetical particle ispredicted by the theoristWolfgang Pauli. Pauli basedhis prediction on the fact thatenergy and momentum did notappear to be conserved incertain radioactive decays.Pauli suggested that thismissing energy might be carriedoff, unseen, by a neutral particlewhich was escaping detection.

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In 1924 Enrico Fermi develops acomprehensive theory ofradioactive decays, includingPauli's hypothetical particle,which Fermi names “theneutrino” (Italian: "little neutralone"). With inclusion of theneutrino, Fermi's theoryaccurately explains manyexperimentally observed results.

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+ -

+ -

+ - How do you know there are two neutrinos?

Neutron(n)

Proton (p)

Electron (e)

Antineutrino (νννν nu-bar)

How do you know thisis antineutrino?

If it is neutral maybe it is the same as the neutrino

Is this the same neutrino as theothers??

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+ -

+ -

+ -From the energy spectrumand the conservation of angular momentum

Neutron(n)

Proton (p)

Electron (e)

Antineutrino (νννν nu-bar)

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Neutron(n)

Proton (p)

Electron (e)

Antineutrino (νννν nu-bar)

in out n

e+

Inverse beta decayp

nu-bar

n

e+

p nu

Neutron beta decay

proton beta “decay” p+νννν ! e+ + n e+ + n ! p+νννν

n+νννν ! e- + p n+ νννν ! p+ e-

Neutrino ! Antineutrino

(1959 Davis,Harmer)

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n+νννν ! e- + p n+ νννν ! p+ e-

To explain the experimental result a new conserved quantityis declared, namely the lepton number L which is 1 forelectrons and neutrinos and -1 for positrons and antineutrinosand 0 for non leptons

Lepton number conservation 0+1 ! 1+0 Lepton number violation 0+(-1) ! 1+0

p+ νννν ! µµµµ + n p+ νννν ! e +n

They got thesefrom pion ! muon

decayThere is a “muon-ness” and an“electron-ness” ;a muon-neutrino and an electron-neutrino

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The “weak” nuclear force

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" A new “super-short range” force responsible forthe disintegration of the neutron.

" It acts over a microscopic range (<10-18 m)

" If the strength of the “strong force” is 1, thestrength of the electromagnetic force is 1/137, thestrength of the weak force is 10-5 and the strength ofgravity is 10-39

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SYMMETRY OPERATIONS

Translation in spaceTranslation in timeRotation through a fixed angleUniform velocity in a straight lineReversal of timeReflection in spaceInterchange of identical atoms or particlesQuantum mechanical phaseMatter-Antimatter (Charge conjugation)

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e.g.Translation in space ! Momentum ConservationInvariance under rotation ! Angular Momentum ConservationShift of the quantum mechanical phase ! Charge Conservation

For every symmetry rule, there exist a corresponding conservation law. For every conservation law, there exists a corresponding symmetry rule.

Emily Noether

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Can we distinguish right from left by means of the physical laws?

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Can we say north from south??

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# The assumption was that at the fundamental levelnature does not have a preferred handedness.

# Until 1957 physicists believed this symmetry to hold forall physical processes. A mirror image of any reactionshould be identical in every way to the actual reaction.

#This idea was intuitive to the physicist --- what could itmean if nature preferred left over right or vice-versa?

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To describe more precisely the symmetry between left andright, physicists used the concept of parity. Parityoriginated with the development of quantum mechanics.• In 1924 Laporte classified the wavefunctions of an atomas either even or odd, depending upon the symmetry of thewavefunction.• Laporte discovered that when an atom transitions fromone state to another and a photon is emitted, thewavefunction changes from even to odd or vice-versa, butnever remains the same.

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# Even functions were defined to have parity of +1 and oddfunctions have parity of -1. In addition, the emitted photonwas defined to have parity of -1. Laporte's rule could then bestated as a conservation law, the conservation of parity.# For mathematical reasons, the parity of any system is theproduct of the parities of the individual components.# Parity is conserved in atomic transitions. If the initialwavefunction was odd (-1 parity), Laporte's rule asserts thefinal wavefunction must be even (+1 parity). Since the initialsystem has -1 parity and the final system has as its parity theproduct of parities of the final wavefunction and the parity ofthe emitted photon, (+1)(-1) = -1, parity is conserved in thetransition.

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! Wigner proved that Laporte's rule was a consequence ofright-left symmetry (or mirror image symmetry) of theelectromagnetic force.!Conservation of parity rested upon Maxwell's equationsdescribing electromagnetism, but more important, the intuitiveidea that nature should be left-right symmetric had beenestablished on the quantum level.! When in 1949, the weak force was postulated to explaindisintegration of elementary particles, it was inconceivable thatparity conservation would not hold for reactions involving theweak force.! However that there was no direct evidence for the extensionof this law to the fourth force of nature

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# In 1949 Powell (who discovered the pi meson (pion)) identified acosmic ray particle which disintegrated into three pions. He dubbed thisnew particle “tau meson”.# Another particle called the “theta meson” was also discovered. Itdisintegrated into two pions. Both particles disintegrated via the weakforce.# A problem arose when the masses and the lifetimes of the tau andtheta particles were considered. The two particles turned out to beindistinguishable other than their mode of decay. Their masses andlifetimes were identical, within the experimental uncertainties. Were theyin fact the same particle?# The problem itself was not that the tau and theta, if indeed they werethe same particle, decayed in two different modes, one by two pions, theother by three pions. The problem dealt with the more fundamental parityconservation law.

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# In 1953 R. H. Dalitz argued that since the pion has parity of -1,two pions would combine to produce a net parity of (-1)(-1) = +1,and three pions would combine to have total parity of (-1)(-1)(-1)= -1. Hence, if conservation of parity holds, the “theta” shouldhave parity of +1, and the “tau” of -1: they could not be the sameparticle. This was the “theta-tau puzzle”. It's resolution wouldinvolve an almost unacceptable proposition to the physicists of thetime.# Lee and Yang's in 1956 published a paper titled “Question ofParity Conservation in Weak Interactions”. Their idea was thatcertain kinds of elementary particles occur in two forms withdifferent parities.

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Lee and Yang stated: "To decide unequivocally whether parity is conserved in weakinteractions, one must perform an experiment to determinewhether weak interactions differentiate the right from the left."

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"I thought the idea (of parity violation) unlikely, butpossible, and a very exciting possibility."

Feynman

“The fact that parity conservation in the weak interactions wasbelieved for so long without experimental support was verystartling. But what was more startling was the prospect that aspace-time symmetry law which the physicists have learned sowell may be violated. This prospect did not appeal to us.”

Yang

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Madame Chien-Shiung Wu

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! Put cobalt nuclei in a strong magnetic field so that their spinsare aligned in the same direction.! Beta rays (electrons) are emitted at the poles of the nuclei.! A mirror image of the system would also show beta rays beingemitted from the poles of the mirror cobalt nuclei, the onlydifference being that the north and south poles of the mirror nucleiwould be reversed since they spin in opposite direction of theirreal counterparts.! Parity conservation demands that the emitted beta rays beequally distributed between the two poles. If more beta particlesemerged from one pole than the other, it would be possible todistinguish the mirror image nuclei from the real ones. Ananisotropy in the emitted beta rays would be tantamount to parityviolation.

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Idealized drawing shows polarization of cobalt-60 nuclei in amagnetic field at a temperature near absolute zero. The emissionof electrons in beta decay of cobalt-60 nuclei is greater in thedirection of the south pole of the nucleus (pointing toward thenorth pole of the magnet), as indicated in the drawing.

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The south pole of a magnet is of such kind that the electrons in a beta disintegration tend to go away from it.

This distinguishes in a physical way the north pole from thesouth pole and left from right.

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Strange Particles

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#They were always produced in pairs.#They were all produced from reactions where the initial statecontained protons or neutrons, but in some decays there would beno protons or neutrons in the final state.#All of the strange particles were unstable.#All strange particles were produced via strong interactions, butoften decay via weak interactions.#Their origin and purpose was an entire mystery.#There were two kinds of them: one kind whose decay productsincluded always a proton were called hyperons and one kindwhose decay products only consisted of mesons were called K-mesons or kaons.

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Isotopic Spin (a parenthesis on the nucleons and pions)

# Protons and neutrons differ slightly in their masses and havedifferent electric charges# The pions also differ slightly in their masses and have differentelectric charges# As far as the strong force is concerned there is one nucleon andone pion# In 1932 Heisenberg described this mathematically byintroducing the isotopic spin or isospin# The isospin concept is a prototype of the elementary particleclassification schemes# In an abstract charge space different orientations of the thirdcomponent of the isotopic spin is a mathematically convenient wayto represent the charged states within a family in the presence ofelectromagnetism

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# To categorize the observed particle’s decay patterns physicistswere awarding particles different values of hypothetical quantumnumbers# To explain the production and decay of strange particles a newquantum number was introduced called “strangeness”# The production of the strange particles was due to the strongforce that respects “strangeness”

ππππ-+p!ΛΛΛΛ0+K0

0 0 -1 +1 (strangeness number)# The decay of strange particles into non-strange particles cannotproceed via the strong interactions

ΛΛΛΛ0 ! ππππ-+p (10-10 sec)-1 0 0 (strangeness number)

# For hyperons we use the Hypercharge as the distinguishingquantum number which is the sum of the baryon number andstrangeness

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Cataloguing mesons

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Cataloguing baryons

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Cataloguing baryons

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Forces Review