Bettini - PowerPoint Presentation

Slide1

Bettini Homework Problem 6.9 (QCD Chapter)

“Collisions take place between 28 GeV electrons circulating in one direction and 820 Gev protons circulating in the other. The cms energy squared is 4E1E2 ~ 90,000 GeV2. To achieve a similar energy in a fixed target experiment would require a ~50 TeV lepton beam incident on a stationary proton target. The results from HERA show a enormous increase of the quark density at very small values of x…..Feynman alluded to this as ‘the secrets of wee x’ “ Donald Perkins, 5th edition.

Inside the HERA tunnel, proton ring to the far right (superconducting), electron ring below.Slide2

US Nuclear Physicists: “A new electron-ion collider (EIC) could rise to the challenge, bending time and launching light-speed probes

to unravel the mysteries of the glue.” Two possible sites for such a facility “At Brookhaven (BNL) on Long Island, NY or Newport News, VA (JLAB).Highest future priority of US nuclear physics (NSAC 2015). Cost 1-1.5 billion dollars. Also being discussed in Europe.EIC = Electron Ion ColliderSlide3

Today’s plan

Collect homeworkWeak interactions (Chapter 7 continued):The τ-θ (tau-theta) puzzleThe Co60 experiment of Madame WuHelicity and ChiralityParity violationFCNC, GIM mechanismFriday: Quiz on QCD Slide4

The τ-θ puzzle

There are two particles called the τ and θ, which have the same mass and lifetime. (Examples on the right).

Question:

What are the possible spin-parities of the

θ

and the

τ

particles ?

(Note that studies of the

τ

Dalitz

plot show that it is flat and the

pions

are in a s-wave. The decay of the

θ

is also uniform)

Hint: The pion is a

pseudoscalar

particle

Ans

: The

θ

could be a 0+ and the τ would be 0-. Still pretty odd.

From emulsion experiments.Slide5

Reminder: From Bettini Chapter 4

Folded Dalitz plot forOrear, J. et al., (1956); Phys. Rev. 102 1676Slide6

The τ-θ puzzle

“To decide unequivocally whether parity is conserved in weak interactions, one must perform an experiment to determine whether weak interactions differentiate the right from the left.” T.D. Lee and C.N. Yang (1956)Marty Block and Dick Feynman were roommates at the 1956 Rochester conference on high energy physics. Brainstorming on the first evening of the conference, Marty suggested that the τ and

θ could be the same particle if parity were violated in the weak interaction. Marty thought this was too crazy to bring up at the meeting.

Feynman asked C.N. Yang a question that Marty had brought up:

“Is it possible that parity is not conserved ?”

On the train back to NYC after the meeting, Lee and Yang wrote their seminal paper based on this idea.Slide7

Madame Wu’s experiment

Need cryogenic temperatures (millikelvin) to line up nuclear spins. Spins must be in a vacuum (no leaks).Came to Berkeley from Shanghai in 1936To detect electrons

To detect photons

To detect photonsSlide8

Cobalt 60 decay scheme

Question: How do

P

N



P

e

and

J

N



P

e

behave when r-r ?

Ans

:

P

N



P

e

is unchanged but JN

Pe is parity violating. Need all the spins lined up and cryogenic operation.Slide9

Data from Madame Wu’s experiment

Adapted from Wu, C. S. et al. (1957); Phys. Rev. 105 1413 and ibid. 106 (1957) 1361

Experiment performed during Christmas break 1956 at the NBS (National Bureau of Standards) in Washington, D.C. C.S. Wu cancelled her vacation.

Turn on the B field to polarize the spins. Then turn it off and observe the result.Slide10

Find α≈1 or pure V-A for the weak interaction.

Notice the directions of the electron and neutrino spins relative to their direction of motion.Slide11

Reaction from Wolfgang Pauli

“That’s total nonsense. God is not a weak left-hander”Pauli assumed the result of C.S. Wu’s experiment was wrong. However, the result was confirmed by many other groups. Maximal parity violation is a characteristic of the weak interaction.Later Pauli reluctantly agreed that the result is correct. “Maybe God is a little left-handed.”Madame Wu was not included in the Nobel Prize for parity violation, which went to the theorists T.D. Lee and C.N. Yang.Slide12

One

spinor is a particle; the other is an anti-particle.

Let’s apply the spin projection operator to two Dirac

spinors

Question

: What do we obtain ?

Ans

: +1/2 and -1/2, respectively

whereSlide13

Helicity

The helicity operator is Helicity eigenstates are two component spinors

Question: What are the helicity

eigenvalues ?

Ans

: +1/2, -1/2

Two notes:

Helicity

is

not

Lorentz invariant. If the particle is not massless, can boost to another frame in which the particle is moving in the opposite direction.

Helicity

is not the same as chirality.Slide14

Chirality

If ψ is a solution to the Dirac equation

The charge conjugated states are

Question:

Verify that the operators ½ (1-γ

5

) ½(1+γ

5

)

a

re projectors i.e. applying twice gives the same result as applying once and applying both gives a null state.

Hint: What is γ

5

2

p.78-79 of

BettiniSlide15

Helicity of the neutrino

Photo of L. Grodznis Experiment done by Goldhaber, Grodznis and Sunyar in 1958.Slide16

1) Need recoil-less emission and absorption of a photon

2) Need β decay (K-capture) and transfer of neutrino helicity (hν) to a gamma/photon and measurement of the helicity of the latter. Sm

* axis is opposite to neutrino direction

Question:

Doesn’t nuclear recoil make resonant absorption impossible ?

In the Samarium slab in a B field, nuclei are aligned and can absorb only one photon

helicity

.

Ans

: No, use

Sm

* photons in the forward direction which are blue shifted.Slide17

Perkins:Slide18

Helicity of electrons versus speed

Adapted from Koks, F. & van Klinken, J. (1976); Nucl. Phys A272 61 Slide19

1957

CN Yang

TD Lee

P is violated maximally

in the weak interactions,

C(charge conjugation) is also violated maximally

but

CP

should be invariant

(also postulated

by the Russian theorists

Okun

and Landau)Slide20

C

P

CP ok

Parity Inversion

Spatial

mirror

Charge Inversion

Particle-antiparticle

mirror

LH

ν

RH

ν

LH anti

-

ν

RH anti

-

νSlide21

Needed a powerful apparatus to find weak neutral currents (neutrino beam from the SPS and the Gargamelle

bubble chamber)Can also have a hadronic vertex.Slide22

One of the first weak neutral current events

A neutral current event in Gargamelle. © CERN Gargamelle – a large bubble chamber with 15 tons of liquid Freon (Andre Lagarrigue et al)Slide23

Five of the collaboration members would not sign this paper because of worries about neutron background.Slide24

Question:

Which of these weak interaction couplings are allowed and exist ?Question: What does FCNC mean ?

✔

✔

Ans

: Flavor Changing Neutral Currents.

(very important)

Usually suppressed or forbidden.