Parity-Violating Electron Scattering
& Charge Symmetry Breaking
Krishna Kumar
University of Massachusetts
thanks to:
C. Horowitz, T. Londergan, W. Marciano, G. Miller, M.J. Ramsey-Musolf,
A. Thomas, B. van Kolck
Workshop on Charge Symmetry Breaking
Trento, June18, 2005
In collaboration with:
18 June 2005
Kent Paschke (UMass)
Paul Souder (Syracuse)
Robert Michaels (Jlab)
PVES & CSB
Outline
• Parity-Violating Electron Scattering
• Deep Inelastic Scattering
• Physics with 11 GeV Electrons
– Beyond the Standard Model
– Search for Charge Symmetry Violation
– d/u
• Towards a 11 GeV Program
• Elastic Electron-Nucleon Scattering
– Strangeness in Nucleons
– Recent Results
– Charge Symmetry Assumption
• Outlook
18 June 2005
PVES & CSB
PV Asymmetries
5
10  Q
2
to

104  Q2
(gegT)

For electrons scattering off nuclei or nucleons:
Z couplings provide access to different linear
combination of underlying quark substructure
18 June 2005
PVES & CSB
2005: MeV to TeV Physics
1970s
1980s
• Steady progress in technology
1990s
• part per billion systematic control
• 1% normalization
control
Recent
Results
Result Today!
Future
Three different avenues of investigation:
•Are there new interactions at very high energy?
•Do strange quarks in the nucleon affect its
charge and magnetization distributions?
•How thick is the neutron skin in a heavy nucleus?
TeV Physics
GeV Physics
MeV Physics
•Valence quark structure of the nucleon: Jlab at 12 GeV
18 June 2005
PVES & CSB
Electroweak Physics at Low Q2
Q2 << scale of EW symmetry breaking
Logical to push to higher energies, away from the Z resonance
LEPII, Tevatron, LHC access scales greater than L ~ 10 TeV
18 June 2005
PVES & CSB
Weak Neutral Current at low Q2
Fixed Target Møller Scattering
Z0
Purely leptonic reaction
QeW ~ 1 - 4sin2W
E158 at Stanford Linear Accelerator Center
Institutions
60 physicists, 7 Ph.D. students
18 June 2005
Caltech
Princeton
SLAC
CEA Saclay
Smith College
PVES & CSB
Syracuse
Jefferson Lab
UC Berkeley
UMass Amherst
U. of Virginia
SLAC E158 Final Result
sin2eff = 0.2397 ± 0.0010 ± 0.0008
sin2WMS(MZ)
Czarnecki and Marciano
Erler and Ramsey-Musolf
Sirlin et. al.
Zykonov
3%
6s
(95% confidence level)
hep-ex/0504049,
* Limit on LLL ~ 7 or 16 TeV
To be published in PRL
* Limit on SO(10) Z’ ~ 1.0 TeV
* Limit on lepton flavor violating coupling ~ 0.01GF
18 June 2005
PVES & CSB
Future Possibilities (Leptonic)
SLAC E158
-e in reactor
can test neutrino coupling: sin2W to ± 0.002
Møller at 11 GeV at Jlab
sin2W to ± 0.00025!
Higher luminosity and acceptance
Lee ~ 25 TeV reach!
Longstanding discrepancy between hadronic and leptonic Z asymmetries:
Z pole asymmetries
Kurylov, Ramsey-Musolf, Su
95% C.L.
JLab 12 GeV
Møller
Does Supersymmetry (SUSY) provide a candidate for dark matter?
•Neutralino is stable if baryon (B) and lepton (L) numbers are conserved
•B and L need not be conserved (RPV): neutralino decay
18 June 2005
PVES & CSB
Future Possiblities (Semileptonic)
NuTeV
Qweak at Jlab
APV in elastic e-p scattering
• measure sin2W to ± 0.0007
• nail down fundamental low energy lepton-quark WNC couplings
A
V
V
(2008)
A
C1i  2g g
C2i  2gVe gAi
e i
A V

• C2i’s small & poorly known: difficult to measure in elastic scattering
• PV Deep inelastic scattering experiment with high luminosity 11 GeV beam
18 June 2005
PVES & CSB
Lepton Deep Inelastic Scattering
ct
*
r
b
r ~ 0.2 fm/Q (0.02 – 0.2 fm
for 100>Q2>1 GeV2)
transverse size of probe
Courtesey A. Caldwell
Q2
x  x Bjorken 
2M(E  E )
b ~ 0.2 fm/sqrt(t)
t=(p-p‘)2
Fraction of proton momentum
carried by struck quark
ct ~ 0.2 fm (1/2x) (<1 fm to 1000‘s fm) – scale over which photon
fluctuations survive. For x~1: valence quark structure
Parton distribution functions fi(x) poorly measured at high x
Lacking some fundamental knowledge of proton structure
Cross-sections fall steeply with (1-x)n
Need high luminosity at high energy
Jefferson Lab upgraded to 11 GeV, 90 µA CW beam
18 June 2005
PVES & CSB
Parity Violating Electron DIS
e-
APV
eZ*
*
X
N

GF Q2

a(x)  f (y)b(x)
2
 C Q f (x)
a(x) 
 Q f (x)
1i
i i
i
2
i i
 C Q f (x)
b(x) 
 Q f (x)
2i
i i
i
x  x Bjorken
y 1 E / E
2
i i
i
fi(x) are quark distribution
functions

i
2H, structure functions largely
 cancel in the ratio:
For an isoscalar
target
like


Provided Q2 >> 1 GeV2 and W2 >> 4 GeV2 and x ~ 0.2 - 0.4
3
a(x)  (2C1u  C1d ) 
10
3 
uv (x)  dv (x) 
b(x)  (2C2u  C2d )

10 
u(x)  d(x) 
Must measure APV to fractional
accuracy better than 1%

• 11 GeV at high luminosity makes very high precision feasible
 of providing beam of extraordinary stability
• JLab is uniquely capable
• Systematic control of normalization errors being developed at 6 GeV
18 June 2005
PVES & CSB
2H
Experiment at 11 GeV
lab = 12.5o
E’: 5.0 GeV ± 10%
60 cm LD2 target
Ibeam = 90 µA
• Use both HMS and SHMS to increase solid angle
• ~2 MHz DIS rate, π/e ~ 2-3
APV = 217 ppm
xBj ~ 0.235, Q2 ~ 2.6 GeV2, W2 ~ 9.5 GeV2
Advantages over 6 GeV:
•Higher Q2, W2, f(y)
•Lower rate, better π/e
•Better systematics: 0.7%
(APV)=0.65 ppm
(2C2u-C2d)=±0.0086±0.0080
Theory: +0.0986
18 June 2005
1000 hours
PVES & CSB
PDG (2004): -0.08 ± 0.24
Physics Implications
(2C2u-C2d)=0.012
(sin2W)=0.0009
Unique, unmatched constraints on axial-vector quark couplings:
Complementary to LHC direct searches
Examples:
18 June 2005
•1 TeV extra gauge bosons (model dependent)
•TeV scale leptoquarks with specific chiral couplings
PVES & CSB
PV DIS and Nucleon Structure
•
Analysis assumed control of QCD uncertainties
•
NuTeV provides perspective
•
JLab at 11 GeV offers new opportunities
– Higher twist effects
– Charge Symmetry Violation (CSV)
– d/u at high x
– Result is 3s from theory prediction
– Generated a lively theoretical debate
– Raised very interesting nucleon structure issues:
cannot be addressed by NuTeV
– PV DIS can address issues directly
• Luminosity and kinematic coverage
• Outstanding opportunities for new discoveries
• Provide confidence in electroweak measurement
18 June 2005
PVES & CSB
Search for CSV in PV DIS
u (x)  d (x)?
p
n
d (x)  u (x)?
p
n
u(x)  u p (x)  d n (x)
•u-d mass difference
•electromagnetic effects
d(x)  d p (x)  u n (x)
•Direct observation of parton-level CSV would be very interesting
•Important implications for high energy collider pdfs

•Could explain significant portion of the NuTeV anomaly
For APV in
electron-2H
DIS:
APV
APV
 0.28
u  d
u d
Sensitivity will be further enhanced if u+d falls off more rapidly than u-d as x  1
Strategy:

•measure or constrain higher twist effects at x ~ 0.5-0.6
•precision measurement of APV at x ~ 0.7 to search for CSV
18 June 2005
PVES & CSB
Potential Sensitivity
18 June 2005
PVES & CSB
d(x)/u(x) as x1
Proton Wavefunction (Spin and Flavor Symmetric)
p
 1 u (ud)S0  1 u (ud)S 1  1 u  (ud)S 1
3
2
18
 1 d (uu)S1  2 d  (uu)S1
3
3
Perturbative QCD: d/u~1/5
SU(6):
d/u~1/2
Valence Quark: d/u~0
Longstanding issue in proton structure
PV-DIS off
hydrogen

APV
GF Q2

a(x)  f (y)b(x)
2
a(x) 
u(x)  0.91d(x)
u(x)  0.25d(x)
•Allows d/u measurement on a single proton!
•Vector quark current! (electron is axial-vector)

18 June 2005
PVES & CSB
A New APV DIS Program
•2 to 3.5 GeV scattered electrons
•20 to 40 degrees
•Factor of 2 in Q2 range
•High statistics at x=0.7, with W>2
18 June 2005
PVES & CSB
A PV DIS Program with upgraded JLab
•
•
•
•
•
•
Hydrogen and Deuterium targets
Better than 2% errors
– It is unlikely that any effects are
larger than 10%
x-range 0.25-0.75
W2 well over 4 GeV2
Q2 range a factor of 2 for each x point
– (Except x~0.7)
Moderate running times
•solid angle > 200 msr
•Count at 100 kHz
• online pion rejection of 102 to 103
18 June 2005
PVES & CSB
•CW 90 µA at 11 GeV
•40 cm liquid H2 and D2 targets
•Luminosity > 1038/cm2/s
Dynamics of Low Energy QCD
( 0.2 fm)
Profound qualitative and quantitative implications!
18 June 2005
PVES & CSB
Strangeness in Nucleons
s ~ N s   5s N

?
Breaking of SU(3)
flavor symmetry
introduces
uncertainties
N s   s N  0?
Kaplan & Manohar (1988)
McKeown (1990)
GEs(Q2), GMs(Q2)
18 June 2005
PVES & CSB
Electron Elastic Electroweak Scattering
M EM 
4 
Q EM
2
Q

J EM
Parity Violating Amplitude
dominated by NC terms:
NC vector current probes
same hadronic flavor
structure, with different
couplings:
18 June 2005
M NC 
M
NC
PV


GF
gV    g A  5 J NC  J NC5
2 2

GF

g A  5 J NC  gV   J NC5
2 2
J EM   Qq N uq  uq N
q
J NC   g qV N uq  uq N
q
PVES & CSB


Flavor Decomposed Form Factors
J EM



i
s
q



  Q q N uq  uq N  N   F1 
F2  N
2M N
q


commonly used Sachs FF:
GE  F1  F2
Decompose by Quark Flavor:
GE / M 
2 u
1
1
GE / M  GEd / M  GEs / M
3
3
3
e
Q
1
u
+2/3
d,s
1/3
GM  F1  F2
gV
1 + 4 sin2W
1  8/3 sin2W
gA
+1
1
1 + 4/3 sin2W 1
 8

 4

 4

GEZ / M  1  sin 2 W GEu / M  1  sin 2 W GEd / M  1  sin 2 W GEs / M
 3

 3

 3

18 June 2005
PVES & CSB
Charge Symmetry
Charge symmetry: u-quark distribution in the proton
is the same as the d-quark distribution in the neutron
GEp/,uM  GEn,/dM ,
GE ,/ pM 
GEp/,dM  GEn ,/uM ,
GEp/, sM  GEn ,/sM
2 u
1
1
GE / M  GEd / M  GEs / M
3
3
3
GE ,/nM 
G,pE,M
G,nE,M
GZ,pE,M
Isospin
symmetry
GuE,M
GdE,M
GsE,M
2 d
1
1
GE / M  GEu / M  GEs / M
3
3
3
Pick ‘n
Choose
<N| ss |N>
18 June 2005
PVES & CSB
GpE,M
GnE,M
GsE,M
Well
Measured
How Big?
Various theoretical
approaches:
•Quark models
•Dispersion Relations
•Lattice Gauge theory
•Skyrme models
Dispersion Theory
6
r 

2
s, M

s
M
2
Im G (t)
 2 dt t
4m
K
s s
s
 s  GM
(Q 2  0)
K
L

Hammer

Ramsey-Musolf
?
18 June 2005
PVES & CSB
Elastic Electroweak Measurements
•APV measurements ranging 0.1 < Q2 < 1
•forward and backward angles
•Hydrogen, Deuterium and Helium targets
•Statistical and systematic errors < 10%
Rough guide: GEn(Q2=0.2) ~ 0.05, L ~ -0.6
SAMPLE
GEs + (Q2) GMs
GMs
•Cancellations?
•Q2 dependence?
•Accuracy?
Q2 (GeV/c)2
GA(T=1)
HAPPEX: Second Generation
1H:
APV=-1.4 ppm, goal ± 0.08 (stat.)
 (GsE+0.08GsM ) = 0.010
18 June 2005
E=3 GeV, =6 deg, Q2=0.1 (GeV/c)2
4He:
APV=+7.6 ppm, goal ± 0.18 (stat.)
 (GsE ) = 0.014
PVES & CSB
HAPPEX
at
JLab
Hall A Proton Parity EXperiment
1998-99: Q2=0.5 GeV2, 1H
2004-05: Q2=0.1 GeV2, 1H, 4He
The HAPPEX Collaboration
California State University, Los Angeles Syracuse University DSM/DAPNIA/SPhN CEA Saclay Thomas Jefferson National Accelerator FacilityINFN, Rome - INFN, Bari Massachusetts Institute of Technology Harvard University – Temple University –
Smith College - University of Virginia University of Massachusetts –
College of William and Mary
18 June 2005
PVES & CSB
Spectrometers & Detectors
Cherenkov
cones
PMT
Compton
polarimeter
Target
12 m dispersion
sweeps away
Beam diagnostics
inelastic events
Septum magnets
18 June 2005
Bending magnet
Quadrupole magnets
PVES & CSB
4He
Asymmetry Result
June 8-22, 2004
3.3 M pairs, total width ~1300 ppm
Araw correction < 0.2 ppm
Q2 = 0.091 (GeV/c)2
Araw = 5.63 ppm  0.71 ppm (stat)
Helicity Window Pair Asymmetry
Theory
A(Gs=0) = +7.51  0.08 ppm
APV
18 June 2005
PVES & CSB
Data: (after all corrections)
= 6.720.84(stat)0.21(syst) ppm
Submitted to PRL: nucl/ex-0506010
1H
Asymmetry Result
June 24-July 25, 2004
9.5 M pairs, total width ~620 ppm
Araw correction ~ 0.06 ppm
Q2 = 0.099 (GeV/c)2
Araw = -0.95 ppm  0.20 ppm (stat)
Helicity Window Pair Asymmetry
Theory
A(Gs=0) = -1.44 ppm  0.11 ppm
Data: (after all corrections)
APV = -1.140.24(stat)0.06(stat)ppm
18 June 2005
PVES & CSB
Submitted to PRL: nucl/ex-0506011
Summary of 1H and 4He Results
GsE = -0.039  0.041(stat)  0.010(syst)  0.004(FF)
GsE + 0.08 GsM = 0.032  0.026(stat)  0.007(syst)  0.011(FF)
(S.L.Zhu et. al.)
18 June 2005
PVES & CSB
Current Status and Prospects
GEs + (Q2) GMs
Q2 [GeV2]
•Each experiment consistent with Gs=0
•Some indication of positive GMs
•G0 data consistent and provocative!
•Cancellations might play a role
18 June 2005
Let us revisit the charge
symmetry assumption
PVES & CSB
Charge Symmetry Violation Estimates
Simple estimate on
GMs
C. Horowitz
Assume mu different
from md is only source
leads to different magnetic moments
Propagates to 0.007 n.m.
Non-relativistic constituent quark models
G. Miller
Account for mass difference, Coulomb and hyperfine interaction
GMs effects less than 1% over relevant range of Q2
GEs effects between 1 and 2% at Q2 ~ 0.1 GeV2
Are relativistic effects important?
Chiral Perturbation Theory
Lewis and Mobed
Pion cloud effects small and calculable
GMs effects similar
Unknown normalization at Q2=0?
GEs estimated at LO: NLO not parameter-free
Can one carry isospin violation effects in GE to NLO?
18 June 2005
PVES & CSB
Strange Quarks or CSV?
95% C.L.
We would like to be able to
claim non-trivial dynamics of
the sea in the static
properties of the nucleon: is
that viable?
Any other experimental CSV handles?
Need consensus on effects of CSV on GEs:
•Better quark models?
•Additional work on chiral perturbation theory?
•Handles from observed CSB effects in nucleon systems?
18 June 2005
PVES & CSB
Summary
• CSV in parton distributions at high x
– Parity violating DIS quite sensitive
– Could be part rich 11 GeV program
• CSV in nucleon form factors
– Parity violating elastic scattering increasingly
sensitive: strange quarks or CSV?
– Can theory help constrain effects in GE?
– Can existing data on CSV help?
– Are there new experimental handles on CSV
in the context of nucleon form factors?
– Critical to resolve the issue if strange quark
interpretation is to be clean
18 June 2005
PVES & CSB