16.451 Lecture 15: Beta Decay 23/10/2003

eepn Neutron beta decay:

light particles or “leptons”, produced in association.

Neutrino presence is crucial to explain the shapeof the electron energy spectrum:

(otherwise, the electrons would be monoenergetic – 2 body final state! )

• “Neutrino” or “little neutral one” postulated in 1931 by Pauli (q = 0, m = 0, s = ½ )

• Only associated with the weak interaction – very difficult to detect

• First detected by Reines & Cowan, 1959 Nobel prize 1995

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Related processes:

eenp )1 “+ decay” in a nucleus, where energetically favourable, eg 25Al25Mg decay

enep )2 “Electron capture” or EC decay in a nucleus; innershell atomic electron is captured.

enpe)3 “Antineutrino capture”, used by Reines & Cowan to detect the antineutrino.

... and many more!!!

Notice: the electron and anti-neutrino appear together; the positron and neutrino appear together....

This suggests a new conserved quantity called “lepton number”, Le :

1have;1have

ee

ee

Le

Le

Empirical conservation law: Le = constant !!distinctareand ee

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More on lepton number:

There are actually three “generations” ofleptons that we know about (in order ofincreasing mass: e, , ) and each one hasits own distinct associated neutrino type witha separately conserved lepton number....

LLLe

eeExample: muon decay: two distinct neutrinos

are emitted, as proved by the spectrum shape

PDG listing:

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Back to the electron-type neutrinos:

enpe “Antineutrino capture” reaction used by Reines & Cowan to detect the antineutrino.

Nobel-prize winning experiment:

(Physical Review 117, p. 159, 1960)

http://www.nobel.se/physics/laureates/1995/illpres/neutrino.html

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Reines & Cowan Experiment (1953):

enpeintense beam from a nuclear reactor

protons in large water tanks

slow down by scatteringin the water; detect bycapture in dissolved Cd salt

detect via gamma raysfrom annihilation with e-

• Very low rate experiment: > 1013 incident antineutrinos/sec but only 3 events/hr! 5 months of data taking!

• No computer data acquisition! For each event, an automatically-triggered camera system took a photograph of analog oscilloscope traces!

• “Delayed coincidence” detection of both neutron and positron suppressed background

• Auxiliary measurements to determine detection efficiencies, etc.

• Absolute cross section measured was 1 x 10-43 cm2 (10-19 b), in agreement with theory!!

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Schematic of the experiment:

enpe

positron annihilation (instantaneous): )keV511(2 ee

delayed: neutronshave to slow down...

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2 water tanks, sandwiched between 3 scintillation counters to detect ’s

enpe

vertical height ~ 2 m; surrounded inPb shielding to reduce background...

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No computers! electronic coincidence pulses for e+ and n-capture ’s....

events captured onanalog oscilloscopedisplays....

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Raw data: oscilloscope photographs!

scintillation light from e+ annihilation first, neutron capture later ( 3 – 10 s)

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Data: coincidence event rate as a function of time delay

distribution indicatesslowing-down time ofneutrons in water –1/v Cd capture cross section is large at lowenergy!

Bottom line: first direct demonstration of the existence of antineutrinos!

2437.04.0 cm102.1

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Parity Violation in Beta Decay

Famous experiment carried out by C.S. Wu (1957) at the suggestion of Lee & Yang (1956, Nobel Prize 1957) demonstrated that the weak interaction violates parity

ee NiCo 6028

6027

Key observation: when cobalt nuclei were polarized in a magnetic field at low temperature, electrons were emitted preferentially in a direction opposite to the nuclear spin...

J

)5(Co6027

J

“before”

)4(Ni6028

J“after”

e

ep

eRp

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Data from Krane, Chapter 9: 12

A pseudoscalar observable:

epJ

~J

ep electron emission angle:

Under a parity transformation: rr

Angular momentum: Jdt

rdr

dt

rdrJ

~~

Linear momentum: pdt

rd

dt

rdp

~~

pJpJ

Observer using a parity-reversedcoordinate system deduces theopposite correlation of e- and J...but this is “crazy”.... ????

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Consider what a parity transformation does to a coordinate system:

x

y

z

x’ = -x

y’ = -y

z’ = -z

“Normal” RIGHT-handed Cartesian system: kji ˆˆˆ

Reverse of coordinate axes: x’ = -x, etc. the system is LEFT-handed: 'ˆ'ˆ'ˆ kji

rr

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Principle of parity conservation:

Laws of physics should be independent of coordinate system! In particular, a right-handed and left-handed choice of Cartesian coordinates should be completelyarbitrary. (We should get the same answer both ways.)

(True for gravity, strong, and electromagnetic interactions)

This is not true for the weak interaction:

has the opposite sign in LH and RH systems

by demonstrating a preferred correlation , beta-decay

“prefers” a LH coordinate system symmetry is broken!

pJ

pJ

In fact, the electron and antineutrino themselves show a similar correlation:

define “helicity” h: for a particle with spin s, and momentum p

Electrons emitted in -decay have h = -v/c “left handed”(positrons “ “ h = +v/c “right handed”)

Neutrinos have h = -1 (LH) and antineutrinos have h = +1 (RH) -- this is the onlyperceptible difference between them!!!!!

11,.

hps

psh

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Parity Nonconservation and the Standard Model:

Weak force carriers, W+, Z°, W- have spin – 1 (bosons) and are left-handed, i.e. they have h = -1 always (spin in the direction of motion, never opposite)

If this is the case, then parity violation inthe weak interaction is a “built-in” feature.

But nobody knows why....

Extensive searches for physics “Beyond theStandard Model” probe the existence of asymmetric set of right-handed force carriers.

None detected yet, but if they exist, theyare required to be extremely heavy!

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