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18 June 2005 PVES & CSB
Parity-Violating Electron Scattering & Charge Symmetry Breaking
Krishna KumarUniversity 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 BreakingTrento, June18, 2005
In collaboration with: Kent Paschke (UMass)Paul Souder (Syracuse)Robert Michaels (Jlab)
18 June 2005 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
€
10−4 ⋅Q2
€
10−5 ⋅Q2
to
For electrons scattering off nuclei or nucleons:
Z couplings provide access to different linear combination of underlying quark substructure
(gegT)
18 June 2005 PVES & CSB
2005: MeV to TeV Physics1970s
1980s
1990s
Recent Results
FutureResult Today!
• Steady progress in technology• part per billion systematic control• 1% normalization control
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?
GeV Physics
TeV Physics
MeV Physics•Valence quark structure of the nucleon: Jlab at 12 GeV
18 June 2005 PVES & CSB
Electroweak Physics at Low Q2
LEPII, Tevatron, LHC access scales greater than ~ 10 TeV
Logical to push to higher energies, away from the Z resonance
Parity Conserving Contact Interactions
Parity Violating Contact Interactions
Q2 << scale of EW symmetry breaking
18 June 2005 PVES & CSB
Weak Neutral Current at low Q2
Purely leptonic reaction Qe
W ~ 1 - 4sin2WZ0
Fixed Target Møller Scattering
E158 at Stanford Linear Accelerator Center
InstitutionsCaltech SyracusePrinceton Jefferson LabSLAC UC BerkeleyCEA Saclay UMass AmherstSmith College U. of Virginia
60 physicists, 7 Ph.D. students
18 June 2005 PVES & CSB
3%
sin2eff = 0.2397 ± 0.0010 ± 0.0008
Czarnecki and MarcianoErler and Ramsey-MusolfSirlin et. al.Zykonov
SLAC E158 Final Result
6
sin2WMS(MZ)
hep-ex/0504049, To be published in PRL
* Limit on LL ~ 7 or 16 TeV
* Limit on SO(10) Z’ ~ 1.0 TeV* Limit on lepton flavor violating coupling ~ 0.01GF
(95% confidence level)
18 June 2005 PVES & CSB
Future Possibilities (Leptonic)-e in reactorSLAC E158
Kurylov, Ramsey-Musolf, Su
95% C.L.JLab 12 GeVMøller
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
can test neutrino coupling: sin2W to ± 0.002
Møller at 11 GeV at Jlab sin2W to ± 0.00025!ee ~ 25 TeV reach!Higher luminosity and acceptance
Longstanding discrepancy between hadronic and leptonic Z asymmetries:Z pole asymmetries
18 June 2005 PVES & CSB
Future Possiblities (Semileptonic)Qweak at Jlab
• measure sin2W to ± 0.0007• nail down fundamental low energy lepton-quark WNC couplings
NuTeV
€
C1i ≡ 2gAe gV
i
€
C2i ≡ 2gVe gA
i
A
V
V
A
• C2i’s small & poorly known: difficult to measure in elastic scattering• PV Deep inelastic scattering experiment with high luminosity 11 GeV beam
APV in elastic e-p scattering
(2008)
18 June 2005 PVES & CSB
Lepton Deep Inelastic Scattering
b ~ 0.2 fm/sqrt(t) t=(p-p‘)2
r ~ 0.2 fm/Q (0.02 – 0.2 fm for 100>Q2>1 GeV2) transverse size of probe*
ct
b
ct ~ 0.2 fm (1/2x) (<1 fm to 1000‘s fm) – scale over which photon fluctuations survive. For x~1: valence quark structure
€
x ≡ xBjorken =Q2
2M(E − ′ E )
Fraction of proton momentum carried by struck quark
r
Cross-sections fall steeply with (1-x)nNeed high luminosity at high energy
Jefferson Lab upgraded to 11 GeV, 90 µA CW beam
Courtesey A. Caldwell
Parton distribution functions fi(x) poorly measured at high x
Lacking some fundamental knowledge of proton structure
18 June 2005 PVES & CSB
Parity Violating Electron DIS
€
APV =GFQ2
2παa(x) + f (y)b(x)[ ]
€
a(x) =
C1iQi f i(x)i
∑
Qi2 f i(x)
i
∑
€
fi(x) are quark distribution functions
e-
N X
e-
Z* *
For an isoscalar target like 2H, structure functions largely cancel in the ratio:
€
a(x) =3
10(2C1u − C1d )[ ] +L
€
b(x) =3
10(2C2u − C2d )
uv (x) + dv (x)
u(x) + d(x)
⎡
⎣ ⎢ ⎤
⎦ ⎥+L
Provided Q2 >> 1 GeV2 and W2 >> 4 GeV2 and x ~ 0.2 - 0.4
Must measure APV to fractional accuracy better than 1%
• 11 GeV at high luminosity makes very high precision feasible• JLab is uniquely capable of providing beam of extraordinary stability• Systematic control of normalization errors being developed at 6 GeV
€
y ≡1− ′ E / E
€
b(x) =
C2iQi f i(x)i
∑
Qi2 f i(x)
i
∑
€
x ≡ xBjorken
18 June 2005 PVES & CSB
2H Experiment at 11 GeV
E’: 5.0 GeV ± 10% lab = 12.5o
APV = 217 ppm
Ibeam = 90 µA 60 cm LD2 target
• Use both HMS and SHMS to increase solid angle• ~2 MHz DIS rate, π/e ~ 2-3
xBj ~ 0.235, Q2 ~ 2.6 GeV2, W2 ~ 9.5 GeV2
1000 hours
(APV)=0.65 ppm
(2C2u-C2d)=±0.0086±0.0080
PDG (2004): -0.08 ± 0.24 Theory: +0.0986
Advantages over 6 GeV:•Higher Q2, W2, f(y)•Lower rate, better π/e•Better systematics: 0.7%
18 June 2005 PVES & CSB
Physics Implications
Examples:•1 TeV extra gauge bosons (model dependent)•TeV scale leptoquarks with specific chiral couplings
Unique, unmatched constraints on axial-vector quark couplings:Complementary to LHC direct searches
(2C2u-C2d)=0.012
(sin2W)=0.0009
18 June 2005 PVES & CSB
PV DIS and Nucleon Structure• Analysis assumed control of QCD uncertainties
– Higher twist effects– Charge Symmetry Violation (CSV)– d/u at high x
• NuTeV provides perspective– Result is 3 from theory prediction– Generated a lively theoretical debate– Raised very interesting nucleon structure issues:
cannot be addressed by NuTeV• JLab at 11 GeV offers new opportunities
– 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
Sensitivity will be further enhanced if u+d falls off more rapidly than u-d as x 1
•measure or constrain higher twist effects at x ~ 0.5-0.6•precision measurement of APV at x ~ 0.7 to search for CSV
Strategy:
•u-d mass difference•electromagnetic effects
•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€
up (x) = dn (x)?
d p (x) = un (x)?
For APV in electron-2H DIS:
€
APV
APV
= 0.28δu −δd
u + d
€
u(x) = up (x) − dn (x)
δd(x) = d p (x) − un (x)
18 June 2005 PVES & CSB
Potential Sensitivity
18 June 2005 PVES & CSB
11
110
)(32)(
31
)(31)(
181)(
21
SS
SSS
uuduud
uduuduudup
d(x)/u(x) as x1
Proton Wavefunction (Spin and Flavor Symmetric)
SU(6):d/u~1/2
Valence Quark: d/u~0
Perturbative QCD: d/u~1/5
€
APV =GFQ2
2παa(x) + f (y)b(x)[ ]
€
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)
PV-DIS offhydrogen
Longstanding issue in proton structure
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
•CW 90 µA at 11 GeV•40 cm liquid H2 and D2 targets•Luminosity > 1038/cm2/s
•solid angle > 200 msr•Count at 100 kHz• online pion rejection of 102 to 103
18 June 2005 PVES & CSB
Dynamics of Low Energy QCD( 0.2 fm)
Profound qualitative and quantitative implications!
18 June 2005 PVES & CSB
Strangeness in Nucleons
?
Breaking of SU(3) flavor symmetry introduces uncertainties
€
Δs ~ N s γ μγ 5s N
€
N s γ μ s N ≠ 0?
Kaplan & Manohar (1988)McKeown (1990)
GEs(Q2), GM
s(Q2)
18 June 2005 PVES & CSB
NuuNJ qqq
qEM
μμ ∑= Q
€
M EM =4πα
Q2QEM l μ Jμ
EM [ ][ ]NCNCAV
FNC JJggG
M 55
22μμ
μμ ++= ll
[ ]NCV
NCA
FNCPV JgJg
GM 5
5
22μ
μμ
μ ll +=
NuuNJ qqq
NCμμ ∑= q
Vg
Parity Violating Amplitude dominated by NC terms:
NC vector current probes same hadronic flavor structure, with different couplings:
Electron Elastic Electroweak Scattering
18 June 2005 PVES & CSB
Q gV gA
e −1 −1 + 4 sin2W +1
u +2/3 1 − 8/3 sin2W −1
d,s −1/3 −1 + 4/3 sin2W −1
NFM
qiFNNuuNJ
Nqq
EM
⎥⎥⎦
⎤
⎢⎢⎣
⎡+==∑
μ
μμμ
21 2
Q
τ 2121 FFGFFG ME +=+=commonly used Sachs FF:
sME
dME
uMEME GGGG //// 3
1
3
1
3
2−−=
Decompose by Quark Flavor:
sMEW
dMEW
uMEW
ZME GGGG /
2/
2/
2/ sin
3
41sin
3
41sin
3
81 ⎟
⎠
⎞⎜⎝
⎛ −−⎟⎠
⎞⎜⎝
⎛ −−⎟⎠
⎞⎜⎝
⎛ −=
Flavor Decomposed Form Factors
18 June 2005 PVES & CSB
Charge symmetry: u-quark distribution in the proton is the same as the d-quark distribution in the neutron
snME
spME
unME
dpME
dnME
upME GGGGGG ,
/,/
,/
,/
,/
,/ ,, ===
GpE,M
GsE,M
GuE,M
GdE,MGn
E,MIsospin
symmetry
GpE,M
<N| sμ s |N>
GnE,M
GpE,M
GsE,M
Pick ‘n Choose
Well Measured
sME
dME
uME
pME GGGG ///
,/ 3
1
3
1
3
2−−=
sME
uME
dME
nME GGGG ///
,/ 3
1
3
1
3
2−−=
Charge Symmetry
18 June 2005 PVES & CSB
How Big?
Various theoretical approaches:
•Quark models•Dispersion Relations•Lattice Gauge theory•Skyrme models
)0( 2 =≡μ QGsMs
rs,M2 =
6π
dtImGM
s (t)t2
4mK2
∞
∫
?
€
€
K +
s s
HammerRamsey-Musolf
Dispersion Theory
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: GE
n(Q2=0.2) ~ 0.05, μ ~ -0.6SAMPLE
GM
s
GA(T=1)
GEs + (Q2) GM
s
Q2 (GeV/c)2
•Cancellations? •Q2 dependence? •Accuracy?
HAPPEX: Second Generation E=3 GeV, =6 deg, Q2=0.1 (GeV/c)2
1H: APV=-1.4 ppm, goal ± 0.08 (stat.)
(GsE+0.08Gs
M ) = 0.010 (GsE ) = 0.014
4He: APV=+7.6 ppm, goal ± 0.18 (stat.)
18 June 2005 PVES & CSB
HAPPEX at JLab
The HAPPEX CollaborationCalifornia State University, Los Angeles -
Syracuse University -DSM/DAPNIA/SPhN CEA Saclay -
Thomas Jefferson National Accelerator Facility- INFN, Rome - INFN, Bari -
Massachusetts Institute of Technology - Harvard University – Temple University –
Smith College - University of Virginia - University of Massachusetts – College of William and Mary
Hall A Proton Parity EXperiment
1998-99: Q2=0.5 GeV2, 1H2004-05: Q2=0.1 GeV2, 1H, 4He
18 June 2005 PVES & CSB
Spectrometers & Detectors
Beam diagnostics
Quadrupole magnets
Bending magnet
Target
Septum magnets
Comptonpolarimeter
Cherenkovcones
PMT
12 m dispersion sweeps away
inelastic events
18 June 2005 PVES & CSB
4He Asymmetry Result
Q2 = 0.091 (GeV/c)2
Araw = 5.63 ppm 0.71 ppm (stat)
Araw correction < 0.2 ppm
Helicity Window Pair Asymmetry
3.3 M pairs, total width ~1300 ppm
June 8-22, 2004
APV = 6.720.84(stat)0.21(syst) ppm
Submitted to PRL: nucl/ex-0506010
Data: (after all corrections)
A(Gs=0) = +7.51 0.08 ppmTheory
18 June 2005 PVES & CSB
1H Asymmetry Result
Q2 = 0.099 (GeV/c)2
Araw = -0.95 ppm 0.20 ppm (stat)
Helicity Window Pair Asymmetry
9.5 M pairs, total width ~620 ppm
Araw correction ~ 0.06 ppm
June 24-July 25, 2004
APV = -1.140.24(stat)0.06(stat)ppmSubmitted to PRL: nucl/ex-0506011
Data: (after all corrections)
A(Gs=0) = -1.44 ppm 0.11 ppm
Theory
18 June 2005 PVES & CSB
(S.L.Zhu et. al.)
Summary of 1H and 4He Results Gs
E = -0.039 0.041(stat) 0.010(syst) 0.004(FF)
GsE + 0.08 Gs
M = 0.032 0.026(stat) 0.007(syst) 0.011(FF)
18 June 2005 PVES & CSB
GEs + (Q2) GM
s
Q2 [GeV2]
Current Status and Prospects
•Each experiment consistent with Gs=0•Some indication of positive GM
s
•G0 data consistent and provocative!•Cancellations might play a role
Let us revisit the charge symmetry assumption
18 June 2005 PVES & CSB
Charge Symmetry Violation EstimatesSimple estimate on GM
s C. Horowitz
leads to different magnetic moments
Assume mu different from md is only source
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?
Pion cloud effects small and calculable
Chiral Perturbation Theory Lewis and Mobed 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?
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?
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?
95% C.L.
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