1. Christopher Crawford Cosmic Lunch 2010-02-19
Parity Violation and the Neutron – Physics at the SNS, Oak Ridge Nat. Lab.
Christopher Crawford
Cosmic Lunch
Hi, I’m glad to be here and talk about the NPDGamma experiment which I have been working on.
I will start out explaining some of the theory pertinent to the experiment,
and then I want to take a detour and describe some properties of the neutron,
and some of the interesting things you can do with them.
Finally, I will return describe the our experiment in more detail.
As a quick overview, we are looking at the capture of a neutron and proton
forming a deuteron and radiating a 2.2 MeV gamma ray.
We are measuring the A_gamma, the correlation between the spin of the neutron and direction of the emitted gamma.
This is a parity violating observable, in the sense that it changes sign when you look at it in a mirror.
Now in this picture of there is no mirror,
but my twins, Spencer and Madison are not identical,
so this is also parity violation.
Interactions and Symmetries
Properties of the Neutron
Spallation Neutron Source (SNS)
Madison
Spencer

2. Standard Model of Particles
SPACE
TIME
Strong Interaction
E&M Interaction
The standard model consists of spin 1/2 quarks and leptons which interact through the interchange of gauge bosons.
The electromagnetic force is mediated by the massless photon, which is the only stable boson.
Thus the range is infinite.
The coupling is very small, and so the electromagnetic interaction can be calculated perturbatively to a very high precision.
The weak interaction is more like the electromagnetic than the strong interaction.
It also has a weak coupling, but the W and Z are very heavy, which limits its range to about .2% of the radius of the neutron.
The other property is that the W boson caries electric charge,
and thus can convert an ‘up’ quark into a ‘down’ or a ‘strange’,
and same for the leptons.
The strong interaction is mediated by gluons with three separate charges labeled by color.
The gluon does not interact with leptons, but does with quarks and even other gluons.
so what you end up with is that even the ground state
is a messy tangle of gluons and quark- anti-quark pairs called the ‘sea’.
This multiplies the force and strongly confines quarks and gluons inside of the nucleon.
So if both the strong and weak forces are short-range, how do neutrons interact?
By the exchange of quark - anti-quark pairs or mesons, which has the range of 1-2 fm.
The interesting physics happens at the vertex, which can involve weak or strong interactions.
That is what we will be discussing today.
Now, besides their obvious quantum mechanical differences fermions are a lot like cars,
so let me take you on an auto-mechanical tour of the standard model.
Weak Interaction
Hadronic Interaction
(residual nuclear force)

3. Standard Model of Automobiles
Annihilation
Higgs?
Degenerate Fermi Gas
Now, aside from the obvious quantum-mechanical differences, fermions are a lot like cars.
So let me take you on an auto-mechanical tour of the standard model.
Cars obey the Pauli exclusion principle, which is painfully obvious in any modern city around quitting time.
They decay, with different lifetimes; although some manufacturers make the immortal claim of natural stability.
They also undergo interactions; however almost exclusively inelastic.
For example, imagine an anti-car to be a driving in the same lane, but in the opposite direction.
Then we have a crude form of car anti-car annihilation.
And so forth…
Whatever the Higgs is, we know it is very heavy.
And I’m not sure what that is!
Particle Decay

4. Symmetries Continuous Symmetries
space-time translation
rotational invariance
Lorentz boosts
gauge invariance
Noether’s Theorem continuous symmetries correspond to conserved quantities
energy-momentum
angular momentum
center-of-momentum
electric charge
Discrete Symmetries
parity P : x  -x
time T : t  -t
charge C : q  -q
particle exchange P12: x1  x2
Discrete Theorems
spin-statistics theorem
CPT theorem
position symmetry
conserved momentum

5. Be careful, some idiot’s
Car Symmetries
U-turn Operator
CP (charge, parity)
Reverse Operator
T (time) R
Chunnel Operator
P (parity)
100
km/h
km/h
99.7 L R
I’m coming
home …
Be careful, some idiot’s
going the wrong way
on the freeway.
Only one? They’re
all over the place!
A little more seriously, let me talk about some discrete symmetries like parity in the context of cars.
Imagine two countries separated by water where cars drive on the opposite sides of the road.
So in one country we have left-sided cars, and in the other, right-sided cars.
Now if I drive through this tunnel, I change from a right-handed car to a left-handed car.
As a conscious law-abiding citizen, I ask myself: are the laws the same now?
For the strong force, electromagnetism, and gravity, I get the reassuring answer: yes.
But for the weak force, I get a complete shock: Right-handed cars are absolutely illegal.
The same goes for left-handed anti-cars.
We see that charge conjugation, going from a car to an anti-car in the same lane is also forbidden,
because then we could convert back to a right-handed car.
So neither charge conjugation nor parity have a symmetry are preserved by the law.
In the car world this makes sense and saves a lot of headaches
(and neck pain), but in the particle world, it makes us ask
uncomfortable questions, like why like this country and not that?
Is some other symmetry preserved? Maybe: I can go from a left-handed car to a right-handed anti-car.
It turns out that this symmetry is very closely respected, but not exactly.
Is all lost? There is one other symmetry: Time, which is represented as driving backwards.
But now you say: isn’t an anti-car just a negative energy car driving backwards in time? Why yes!
As far as I can tell, that is the jest of the CPT theorem, which is closely related to the Pauli exclusion principle.
With all these deep and natural symmetries, it’s no wonder that this poor driver was overcome with charge-confusion.
Now a disclaimer: Physics students, do not use this model to answer questions on your tests.
CPT theorem: ALL laws are invariant under CPT

6. Parity-violation in weak interaction (1956)
Parity-transformation (P) :
October 1, 1956 issue of the Physical Review

7. Madame C.S. Wu

8. Properties of the Neutron
spin 1/ isospin 1/2
mn = mp + me keV
n = ± 0.8 s
qn < 2 x e
dn < 3 x e cm
n = N
rm = fm
re2 = fm2
3 valence quarks + sea
exponential magnetization distribution
pion cloud: p n up
down
uud
udd
So what does the neutron look like?
That’s a loaded question, because you can’t “SEE” anything smaller than nanoscale because that’s the size of light waves.
The bottom line, neutrons break all the rules of conventional nuclear physics:
No particle tracking, No conventional magnetic spectrometers,
No conventional acceleration, No free neutron targets.
However, as I hope to show, after you get around this the study of neutrons is very rewarding.
(see A. Young, FNPSS slide 15)
r_m from Mergell
data from BLAST

9. Neutron sources - Reactors
ILL, Grenoble, France
reactors: ILL 50 MW, NIST 20 MW
high flux, monochromator crystals

10. Spallation Neutron Source (SNS)
Oak Ridge National Laboratory, Tennessee
spallation sources: LANL, SNS
pulsed -> TOF -> energy
LH2 moderator: cold neutrons
thermal equilibrium in ~30 interactions
$1.4B--1GeV protons at 2MW, ready in ~2007 (first neutrons recently!
Short (~1 usec) proton pulse– mainly for high TOF resolution
Will be brightest spallation neutron source (also

11. Spallation Neutron Source (SNS)
spallation sources: LANL, SNS
pulsed -> TOF -> energy
LH2 moderator: cold neutrons
thermal equilibrium in ~30 interactions

12. Beamline 13 Allocated for Nuclear Physics (FnPB)
11A - Powder Diffractometer
Commission 2007
7 - Engineering Diffractometer
IDT CFI Funded
Commission 2008
9 – VISION
12 - Single Crystal Diffractometer
Commission 2009
6 - SANS
Commission 2007
5 - Cold Neutron Chopper Spectrometer Commission 2007
13 - Fundamental Physics Beamline
Commission 2007
4B - Liquids Reflectometer
Commission 2006
14B - Hybrid Spectrometer
Commission 2011
4A - Magnetism Reflectometer
Commission 2006
15 – Spin Echo
3 - High Pressure Diffractometer
Commission 2008
17 - High Resolution Chopper Spectrometer
Commission 2008
18 - Wide Angle Chopper Spectrometer
Commission 2007
1B - Disordered Mat’ls
Commission 2010
2 - Backscattering Spectrometer Commission 2006

13. FnPB Cold & UCN Line

14. FnPB – Fundamental Neutron Physics Beamline
Choppers
Shutter
14 m guide, 10x12 cm2,
1 m from
moderator

15. What can we do with neutrons?
scattering / diffraction
complementary to X-ray Bragg diffraction
large penetration
large H,D cross section
life sciences
fuel cell research
oil exploration
fundamental tests of quantum mechanics
neutron interferometry
scattering lengths
neutron charge radius
spinor 4 periodicity
gravitational phase shift
quantum states in a gravitational potential
If you go into the experimental hall of the ILL or SNS, you will find 23 out the the 24 neutron beams with specialized instruments for neutron diffraction used to characterize other materials.
I’m interested in the one beam line devoted to the study of fundamental physics of the neutron itself.

16. What can we do with neutrons?
Electron
Proton
Neutrino
Neutron Spin A B C
fundamental symmetry tests of the standard model
neutron decay lifetime and correlations
PV: NPDGamma, 4He spin rotation
T reversal: electric dipole moment
p (or d)
d (or t) n γ
nEDM
gV and gA, CKM matrix unitarity
astrophysicists: nucleosynthesis: early universe had about 15 minutes to form D after the quark soup froze into n and p.

17. Neutron Traps
ultra cold neutrons: slow enough to be completely reflected by 58Ni optical potential
kinetic: m/s
thermal: mK
wavelength: 50 nm
nuclear: neV (58Ni)
magnetic: neV (1 T)
gravity: neV (1 m)
Before I continue, I would like to mention that people have come up with ingenious ways to trap other matter, such as atoms, or ..

18. Car Traps