Precision Measurements
(with electrons and neutrons)
Betsy Beise
University of Maryland
 parity violating electron scattering
 weak NN interactions
 neutron beta decay
 electric dipole moments (probably won’t cover)
June 2008
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Elizabeth Beise, U Md
caveats
• these topics reflect my personal expertise and
interest, and by no means the breadth of the
field
• there are many precision measurements; some
measurements I discuss are not “precise”. All
involve the weak interaction.
• I’m an experimentalist: my focus will be on
experimental techniques
June 2008
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Elizabeth Beise, U Md
some references
• General:
– “Subatomic Physics 3rd edition”, Henley & Garcia (2007)
– “Quarks & Leptons”, Halzen & Martin
– “Experimental Foundations of Particle Physics”, Cahn & Goldhaber
• PV electron scattering:
– M.J. Musolf et al., Phys. Rep. 239 (1994) 1.
– E. Beise, M.L. Pitt and D.T. Spayde, Prog. Part. Nucl. Phys. 54 (2005) 289.
– M. Pitt, NNPSS Lecture notes (Bar Harbor, ME)
• Neutrons:
– J. Nico and W.M. Snow, Ann. Rev. Nucl. Part. Sci. 55 (2005) 27.
– M.J. Ramsey-Musolf & S. Page, Ann. Rev. of Nucl. Part. Sci. 56 (2006) 1.
– NNPSS 2007 Lecture notes, G. Greene.
Thanks for slides, photos, etc.: B. Filippone, P. Mumm, J. Nico, K. Pashke, M.
Pitt, X. Zheng, P. Reimer, H. Gao, J. Martin, and many others
June 2008
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Elizabeth Beise, U Md
“The Fall of Parity”
from the NIST virtual museum
http://physics.nist.gov/GenInt/Parity/cover.html
T.D. Lee & C.N. Yang,
Phys. Rev. 104 (1956) 254.
(October 1956)
Co60Ni  e  e
60

C.-S. Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson,
Phys Rev 105 (1957) 1413. (January 1957)
June 2008
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Elizabeth Beise, U Md
The 60Co experiment
(go to the NIST museum!)
pseudoscalar
 
J  pe
<J> = average nuclear
polarization of 60Co
(in B-field and low T (0.01K)
anisotropy of g’s measures the
degree of pol’n of 60Ni (now used for
thermometry – see F. Pobell, “Matter
and Methods at Low Temperature”)
June 2008
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Elizabeth Beise, U Md
Weak interactions involving nucleons
(and electrons)
• electron scattering
• neutron decay
u
d
Z
e
d
n
N
use well-understood
target (nucleon) to
perform precision
test of EW physics
e
• weak NN interactions
Z,W
OR
use weak force to probe new aspects
of strong interaction
June 2008
W
u
d p
u
e
EXPERIMENTAL SIGNATURE IS
VIOLATION OF PARITY
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Elizabeth Beise, U Md
Neutral current e-quark interactions
PV
SM
L
GF

2
 C

 5
e
g
g
e
q
g
q

C
e
g
e
q
g
g q
1q
 5
2q


q
A x V
V x A
product of V and A violates parity
Ciq’s are “weak charges” of quarks (1=“vector”, 2=“axial”)
If scattering directly from quarks, can measure these, e.g.
1 4
C1u    sin 2 W
2 3
If scattering from a nucleon, have to take into account that quarks are
bound inside nucleon via strong interaction to disentangle the Ciq’s:
these are typically embedded in nucleon “form factors”
June 2008
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Elizabeth Beise, U Md
Elastic electron (electromagnetic) scattering from
nucleons
quark “charges” are buried in nucleon form factors
d
E 1
  Mott
 GE2 (Q 2 )   GM2 (Q 2 )
d
E  (1   )

G N
 e qg
q


e'
e

Q  ( , q )
qN
4 u 1 d 1 s
G  GE  GE  GE
3
3
3
2 u 2 d 1 s
n
GE  GE  GE  GE
3
3
3
p
E
 = Q2/4M2
= [1+2(1+)tan2(/2)]-1
contributions to charge,
similar for magnetism
GE and GM for both proton and neutron are now very well known
via precision electron scattering exps. But not enough information to
disentangle individual quark pieces from these alone.
June 2008
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Elizabeth Beise, U Md
Weak nucleon form factors
point-like fermions:

ieQ f g 
EM :
Qf
0

gM Z
Weak : i
g  gVf  g Af g 5
4M W

gVf
gAf
1
1
e,
1
u,c,t
+2/3
1  8/3 sin2W 1
d,s,b 1/3
1 + 4/3 sin2W +1
1 + 4 sin2W
+1
nucleons:


 2  4 sin  G
 1  4 sin  G
G uE ,,Mp  3  4 sin 2 W G gE ,,Mp  G EZ,,Mp
G Ed,,Mp
G Es ,,Mp
J
2
W
W
June 2008
E ,M
g ,p
2
Z
5
g ,p
E ,M
 G gE ,,Mn  G EZ,,Mp
 G gE ,,Mn  G EZ,,Mp
 Z 2

1
2
 u N GA (q )g  
GP (q )q g 5u N
MN


combine weak and
EM information to
separate (u,d,s)
contributions to
proton’s charge and
magnetism
axial part is
related to proton’s spin
ignore
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Elizabeth Beise, U Md
Parity Violating Electron-Proton Scattering
g
polarized electrons, unpolarized target
R L
A
R L
e
 few parts per million

p e
g
p
2
and want to measure to ~ 5%
Statistics : 1013-1014 counts for precision measurement
high beam intensity, very thick targets, large acceptance detector
(sometimes can’t count individually scattered particles....)
Systematic Effects:
High quality beam (intensity, position, energy) over a long time
Minimize systematic effects in detection that depend on initial beam state
(feedback on energy, position, angle and intensity of incident beam)
June 2008
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Elizabeth Beise, U Md
Deep inelastic e-D scattering
C.Y. Prescott, et al, PL 77B (1978) 347
SLAC 1978:
e + d (DIS) :
led to correct description of neutral weak force:
reverse beam via
g-2 precession
reverse handedness of
electron beam
June 2008
A ~ 60 ± 10 ppm
sin2 θW = 0.20 ± 0.03
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Elizabeth Beise, U Md
Polarized Electrons
D.T. Pierce et al., Phys. Lett. 51A (1975) 465.
Electron retains circular polarization of
laser beam: Pe ~ 85%
Reverse pol’n of beam
at rate of 30 Hz
Feedback on laser intensity
and position at high rate
See also Physics Today, Dec 2007
June 2008
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Elizabeth Beise, U Md
False asymmetries from the beam
Ameas  Aphys
1  Y 
 ΔPi
  
i 1 2Y  Pi 
N
At the injector:
Dx < 1 micron
June 2008
detector sensitivity
beam property
(x,,E,intensity)
Measurement at > 1 km:
Dx ~5 – 10 nm
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Elizabeth Beise, U Md
JLab polarized beam : 2005
G0 beam:
• strained GaAs (PB ~ 73%)
• 32 ns pulse spacing
• 40 A beam current
Beam Parameter
HAPPEX-II beam:
• superlattice (PB > 85%)
• 2 ns pulse spacing
• 35 A beam current
G0 beam
HAPPEX beam
Charge asymmetry
0.14 ± 0.32 ppm
2.6 ± 0.15 ppm
Position difference
4 ± 4 nm
8 ± 3 nm
Angle difference
1.5 ± 1 nrad
4 ± 2 nrad
Energy difference
29 ± 4 eV
66 ± 3 eV
Total correction to
Asymmetry
0.02 ± 0.01 ppm
0.08 ± 0.03 ppm
June 2008
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Elizabeth Beise, U Md
Electron accelerators (for nuclear physics)
Jefferson Lab,
Newport News, VA
Mainz Microtron
(Germany)
PVA4
MIT-Bates Lab,
Middleton, MA
JLab: 6 GeV beam, getting ready
for upgrade to 12 GeV
Also, parity-violating Moller scattering
(E158) carried out at SLAC….
now the MIT R&E center
June 2008
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Elizabeth Beise, U Md
Parity Violation at JLab: “HAPPEX”
High precision
measurement
at selected kinematics
H target data
A. Acha, etal PRL 98 (2007) 032031
Araw = 0.95  0.20 ppm
Using H and 4He targets, get
precise determination of s-quark
contributions to proton’s charge
radius and magnetic moment
June 2008
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Elizabeth Beise, U Md
Parity Violation at JLab: “G0”
D.S. Armstrong etal, PRL 95 (2005) 092001
smaller distance
• H and D targets, wide range of distance scales
• Independently determine neutral weak charge, magnetism
and axial form factors
June 2008
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Elizabeth Beise, U Md
PV-A4 at Mainz
PbF2 Calorimeter
phases I &II:
forward angles, H target
I : Aexp  5.6  0.6  0.2 ppm
II : Aexp  1.36  0.29  0.13 ppm
F. Maas et al.,
PRL 93 (2004) 022002,
PRL 94 (2005) 152001
phase III:
backward angles,
H and D targets
June 2008
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Elizabeth Beise, U Md
Strange quarks, charge and magnetism
Dashed ellipse:
R. Young et al.,
PRL 97, 102002 (2006)
Solid ellipse:
K. Pashke, private comm,
(agrees w/ J. Liu et al.,
PRC 76, 025202 (2007))
strange quark contribution:
to proton’s magnetism ≤ 5%
to charge radius ≤ 1%
s
GE,M
 1
% contrib  p     100
GE,M  3 


G Es ,M  1  4 sin 2W GgE ,,Mp  GgE ,,Mn  G EZ,,Mp
June 2008
thanks to K. Pashke and R. Young
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Elizabeth Beise, U Md
Precision tests of TeV-scale Electroweak Physics
Indirect searches for extensions to Standard Model of particle physics
using precision measurements of sin2W at several energy scales are
complementary to experiments at the LHC.
Case 1: electron-electron scattering (Moller)  SLAC E158
Ameas = 131 ± 14 ± 10
parts per billion
P. Anthony et al.,
PRL 95 (2005) 081601
Limits existence of new
e-e interactions on scale
of 7-16 TeV
June 2008
from M. Pitt
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Elizabeth Beise, U Md
Precision tests of TeV-scale Electroweak Physics
Case 2: e-p scattering: Weak Charge of the Proton
LL
PV
SM
L
PV
NEW
GF
g2

q


e g  g 5e C1q q g q 
e
g
g
e
h
q
g
q
 5  V
2
4
2
q
q
precise measurement
of known e-q interactions
(C1u, C1d) can place limits
on existence of new
ones on scale of a few
TeV
June 2008
from M. Pitt
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Elizabeth Beise, U Md
QWeak: Parity violating e-p scattering
R. Carlini, etal
QWeak toroidal magnet at MIT-Bates lab
combined existing PV electron
scattering data provide most precise
experimental limit on proton’s
weak charges
expected asymmetry ~ 0.3 ppm
measure to few %.
R. Young et al., PRL 99 (2007) 122003
June 2008
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Elizabeth Beise, U Md
Precision tests of TeV-scale Electroweak Physics
Case 3: Direct electron-quark scattering
A
C1i
high precision return to SLAC 1978 measurement
V
V
C2i
y
APV
E  E'
E
A
GF Q2

a(x)  f (y)b(x)
2
b( x)  2C2u  C2 d 
expected error band w/ planned
6 GeV measurement at JLab

X. Zheng, U Va & P.Reimer, ANL
June 2008
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Elizabeth Beise, U Md
Weak interactions between nucleons
(a brief interlude)
from P. Mumm, NIST
1 fm
Fundamental interaction is q-q weak interactions, but these are
buried inside strongly bound systems
 typical size of effect < 10-7
5 leading contributions
H. Griesshammer, coming soon...
• need a minimum of 5 experiments (more is better, and preferably
without too many nucleons)
• need a theoretical framework to interpret the results  chiral EFT
June 2008
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Elizabeth Beise, U Md
Experimental program summary


• Asymmetry in p-p and p-4He scattering
p-p measurement from TRIUMF


• spatial g asymmetry in n+p d+g and n+d  t+g
n+p d+g underway, moving from LANSCE to SNS
• circular pol’n of g in n+p  d+g

• spin rotation of n in
4He

and n in p
underway at NIST
• Also: laser spectroscopy on heavy rare isotopes (e.g., Francium)
June 2008
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Elizabeth Beise, U Md
neutron spin rotation in 4He
(at NIST)
fPV from •p term in forward scattering amplitude (P odd)
df
~ 10 7 rad/m
dz
June 2008
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Elizabeth Beise, U Md
NIST n-4He spin rotation
June 2008
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Elizabeth Beise, U Md
Polarized neutron beta decay
GF from -decay, apparently the same for
quarks and leptons
g  GFVud 
define  
u
d
n
p
e
d
2
2
V
u
d
u
W
e
gA
 1.2695  0.0029
gV
decay rate
 


 


p p
p
p
p p
m 
dW  gV2  3g A2 F ( Ee ) 1  a e   b   n   A e  B   D e 
Ee E
Ee
E
Ee E
 Ee



measurable and non-zero



T odd, P even
neutron lifetime
measures this piece
June 2008
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Elizabeth Beise, U Md
neutron lifetime experiments
Historically two techniques:
1) beam: send “cold” neutrons through a well-defined volume, determine
the “loss rate” by counting decay products:
decay product
rate into a
detector
dN
N

dt
n
# neutrons incident on the
defined volume
2) “bottle” or “storage”: trap neutrons, measure decay products vs time.
 t
N (t )  N (0) exp  
 
 exp




1
 exp

1

1
 n  loss
other loss mechanisms
June 2008
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Elizabeth Beise, U Md
“cold” and “ultracold” neutrons
“cold”:
temperatur e 
K .E.
 300 K  36 K
1 eV
40
2  K .E.

 v  760 m/s
mn

Produce by moderating thermal
neutrons from reactor or spallation
source
Can be transported long distances
by total internal reflection
h
 0.5 nm
p
“ultra-cold”:
v  5 m/s
T ~ few mK
 ~ 50 nm
Scatter cold neutrons from
solids, liquids, etc., that can
absorb bulk of neutron KE
into a “phonon” resonance
Can be trapped:
material walls
magnetic fields
gravity!
June 2008
Golub & Pendelbury, 1977
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Elizabeth Beise, U Md
NIST Center for Neutron Research
20 MW reactor is source of “cold” neutrons: Tn ~10-5-10-2 eV
reactor core
June 2008
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Elizabeth Beise, U Md
NIST Center for Cold Neutron Research
June 2008
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Elizabeth Beise, U Md
NIST lifetime experiment (beam type)
n 
June 2008
N t
  t

p
Np

L
v
33
Elizabeth Beise, U Md
neutron lifetime measurement at ILL (Grenoble, FR)
57 MW research reactor
UCN “gravitational”
trap neutrons
A.P. Serebrov, etal
cryostat
ultracold neutrons in
June 2008
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Elizabeth Beise, U Md
recent neutron lifetime measurements
neutron lifetime measurements
lifetime (s)
1000.0
beam
storage
950.0
Serebrov, etal,
Phys. Lett. B 605, 72 (2005)
(878.5 ± 0.7 ± 0.3) seconds
900.0
850.0
800.0
0
5
10
15
20
exp number (time-ordered)
NIST
2004 PDG average:
(885 ± 0.8) seconds
figure from J. Nico et al.,
PRC 71 (2005) 055502
June 2008
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Elizabeth Beise, U Md
NIST UCN lifetime experiment
June 2008
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Elizabeth Beise, U Md
T-odd effects in polarized neutron decay
D-coefficient
measurement,
emiT
Slide from P. Mumm, NIST, 2007
June 2008
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Elizabeth Beise, U Md
T-odd  CP odd: neutron EDM
CP-violation is important to
understanding of original of matter (vs
antimatter)
- +
d•E
s = 1/2
If neutron possesses has an electric
dipole moment, then, in an electric
field:
 
H E  d n  E
will reverse sign under reversal of E.
This is CP-odd observable.
many EDM experiments underway
or proposed, in many systems.
New U.S. experiment planned using
ultracold neutrons at SNS –Oak Ridge
http://p25ext.lanl.gov/edm/edm.html
June 2008
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Elizabeth Beise, U Md
Thanks
and have fun on the tour
June 2008
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Elizabeth Beise, U Md