Introduction to

Helicity

Particle Physics of μSR

The World's μSR Facilities

Basic Techniques of μSR

Applications of μSR

Jess H. Brewer

15 May 2004

Visit our Web site! http://musr.org

Suppose you sign up your home computer for the SETI search

and the data they give you to analyze contains this message:

We of Barnard's Star II have been studying your Earth civilization

for decades and have now deciphered your language, units of

measurement and cultural conventions by watching your TV

broadcasts. We would like to begin trading with Earth, but we

wish to negotiate exclusively through the first person to receive

this message.

Our specialty is manufacturing metal fasteners (nuts and bolts)

and we will send you a shipment to sell for us at 10% of standard

Earth prices as soon as we receive your message explaining the

difference between right-handed and left-handed threads.

What message would you send to land this entreprenuerial plum?

“left handed” = negative helicity

“right handed” = positive helicity

Pion Decay:

Conservation of Linear Momentum: The + is emitted with momentum equal & opposite to that of the  .

+  + + 

Conservation of Angular Momentum: + &  have equal & opposite spin.

Some pions stop in the “skin” of the primary production target.

They have zero linear momentum and zero angular momentum.

Weak Interaction:

Only “left-handed” 's

are created.

mirror

X

Thus the emerging +

has its spin pointing

antiparallel to its

momentum

direction.

What's a PION ?

An unstable elementary particle

(mean lifetime tp ≈ 26 ns)

made when protons hit nuclei.

“Nuclear glue” (Yukawa, 1937)

Mass intermediate between electron and proton, hence

called a “meson”.

No spin.

Three types: p+, p0, p-

+  + + 

What's a MUON ?

A slightly more stable particle

(mean lifetime t ≈ 2.2 µs)

µ- = “heavy electron”

(mµ ≈ 207 me)

µ+ = “light proton”

(mµ ≈ mp/9)

Spin precesses in a magnetic field:

µ+  e+ + ne+ nμ (asymmetrically)

TRIUMF

Lower half of

TRIUMF

cyclotron

magnet

1972

Inside

TRIUMF

cyclotron

today

4.1 MeV

 = 2.2 s

500 MeV

 = 26 ns

Primary

Production

Target

Pion Decay:

Conservation of Linear Momentum: The + is emitted with momentum equal & opposite to that of the  .

+  + + 

Conservation of Angular Momentum: + &  have equal & opposite spin.

A pion stops in the “skin” of the primary production target.

It has zero linear momentum and zero angular momentum.

Weak Interaction:

Only “left-handed” 's

are created.

Thus the emerging +

has its spin pointing

antiparallel to its

momentum direction,

giving

X

mirror

spin

polarized

muons.

Muon Decay

Neutrinos have negative helicity, antineutrinos positive.

An ultrarelativistic positron behaves like an antineutrino.

Thus the positron tends to be emitted along the muon spin

when e and μ go off together (highest energy e+ ).

+-Decay Asymmetry

Angular distribution of positrons from +-decay. The asymmetry is

a = 1/3 when all positron energies are detected with equal probability.

Spin Precession

magnetic

field

What happens when you try to tip a top?

Instead of tipping over the way you twist,

it moves sideways! This is how you steer

a bicycle, though you never think about it.

If gravity keeps trying to tip over the top,

it keeps slipping away sideways.

This is called precession.

Many elementary particles

(like protons, electrons & muons)

have an intrinsic spin like a little

top that never needs winding.

When a magnetic field tries to

tip them over, guess what we get!

The World's μSR Facilities

Basic Techniques of μSR

Applications of μSR

Visit our Web site! http://musr.org

Where in the World is μSR?

Transverse Field (TF)-µSR

Typical time spectrum (histogram)

Zero Field (ZF)-µSR

Typical asymmetry spectrum

( B – F )

──────

( B + F )

Brewer's List of μSR Acronyms

Transverse

Field

Zero Field

Longitudinal

Field

Avoided

Level

Crossing

Resonance

Muon

Spin

Echo

Muon

Spin

Resonance

Fourier

Transform

µSR

“Themes” in µSR

Muonium as light Hydrogen

(Mu = µ+e-) (H = p+e-)

Mu vs. H atom Chemistry:

- gases, liquids & solids

- Best test of reaction rate theories.

- Study “unobservable” H atom rxns.

- Discover new radical species.

Mu vs. H in Semiconductors:

- Until recently, µ +SR → only data on

metastable H states in semiconductors!

The Muon as a Probe

Quantum Diffusion: µ + in metals (compare H+); Mu in nonmetals (compare H).

Probing Magnetism: unequalled sensitivity

- Local fields: electronic structure; ordering

- Dynamics: electronic, nuclear spins

Probing Superconductivity: (esp. HTcSC)

- Coexistence of SC & Magnetism

- Magnetic Penetration Depth λ

- Coherence Length ξ

Magnetic Field Distribution

of a Vortex Lattice

NbSe2

Time (s)

APx(t), APy(t)

Frequency (MHz)

Real Amplitude (10-2)

ab in the Meissner & Vortex States

H

c

ab in the Meissner & Vortex States

PowerPoint Presentation

Jeff E. Sonier

2002-03-19T14:03:53