Parity Violation in Deep Inelastic
Scattering
Kent Paschke
Figures and slides from E. Chudakov, K.Kumar, T. Londergan, P. Souder, X.Zheng
Kent Paschke SPIN ’08 October 9, 2008
• The couplings gT depend on electroweak physics as well as on the weak vector and axial-vector hadronic current • For PVDIS, both new physics at high energy scales as well as interesting features of hadronic structure come into play• A program with a broad kinematic range can untangle the physics
Weak Neutral Current (WNC) Interactions at Q2 << MZ2
Longitudinally Polarized Electron Scattering off
Unpolarized Fixed Targets
PV Asymmetries
! = |M! + MZ |2
APV =!L ! !R
!L + !R" MZ
M!" GF Q2
4 " #
!ge
AgTV + $ge
V gTA
"
Kent Paschke SPIN ’08 October 9, 2008
e-
NX
e-
Z* γ*
fi(x) are quark distribution functions
PV in Deep Inelastic Scattering
A
V
V
A
Kent Paschke SPIN ’08 October 9, 2008
e-
NX
e-
Z* γ*
fi(x) are quark distribution functions
For an isoscalar target like 2H, structure functions largely cancel in the ratio
Above x~0.5, APV/Q2 becomes independent of x, W, and depends only on y
Deuterium PVDIS
a(x) =310
!2C1u
"1 +
2c+
u+ + d+
#!C1d
"1 +
2s+
u+ + d+
#+ · · ·
$
b(x) =310
[2C2u !C2d]!
u! ! d!
u+ + d+
"+ · · ·
Kent Paschke SPIN ’08 October 9, 2008
Fundamental Parameters in PVDISAPV !
3GF Q2
10"
2!"[(2C1u #C1d) + f(y)(2C2u #C2d)]
2C2u !C2d
2C1u !C1d" 0.136
Isovector weak charge
Isos
cala
r wea
k ch
arge
Young et. al
C1u and C1d will be determined to high
precision by Qweak, Cs
C2u and C2d are small and poorly known,combination accessible in PV DIS
Useful precision on C2‘s requires dA/A~0.5%
X. Zheng, 2008 Hall C Summer Workshop
Current Knowledge on C1,2q
MIT/ Bates
SLAC/Prescott
R. Young (PVES)
R. Young (combined)
PDG best fit
Cs APV
PDG 2006 fitSAMPLE
SLAC/ Prescott
R. Young (combined)
all are 1 ! limit
Best: PDG2002 "(2C2u-C2d) = 0.24
C2u
+C2d
1.25
1.5
1.75
1.0
0.75
0.5
0.25
0
0.25
-0.5
-0.75
C2u-C2d
- 0.2 0.40.20- 0.4
0.10
0.125
0.15
0.175
C1u-C1d
- 0.4- 0.6- 0.8
C1u
+C1d
Tl APV
Qweak (expected)
Figure from X.Zheng
Kent Paschke SPIN ’08 October 9, 2008
Comprehensive Search for New Neutral Current Interactions
�
Lf1 f2= 4π
Λ ij2 ηij
i, j= L ,R∑ f 1iγµ f1i f 2 jγ
µ f2 j
Consider
�
f1 f 1 → f2 f 2
�
f1 f2 → f1 f2or Λ’s for all f1f2 combinations and L,R combinations
Eichten, Lane and Peskin, PRL50 (1983)
Many new physics models give rise to neutral ‘contact’ (4-Fermi) interactions: Heavy Z’s, compositeness, extra dimensions…
Important component of indirect signatures of “new physics”
One goal of neutral current measurements at low energy AND colliders: Access Λ > 10 TeV for as many f1f2 and L,R combinations as possible
Covers a unique corner of parameter space, and is a
crucial component of a comprehensive search for
new physics
RPC SUSY
RPV SUSY 12 GeV
Møller
PVDIS
Kent Paschke SPIN ’08 October 9, 2008
Sensitivity to QCD and Nucleon Structure
NuTeV result on Paschos-Wolfenstein ratio deviates from SM expectation; suspicion focuses on hadronic and
nuclear effects
Electroweak studies in nuclear systems require control of QCD effects
Sensitivities to QCD or hadronic/nuclear structure provide an opportunity to study:
• partonic charge symmetry violation• higher twist
The well-defined kinematics of electron scattering provide an advantage
Kent Paschke SPIN ’08 October 9, 2008
Sensitivity will be enhanced if u+d falls off more rapidly than δu-δd as x → 1
• u-d mass difference• electromagnetic effects
• Direct observation of parton-level CSV would be very exciting• Important implications for high energy collider pdfs• Could explain significant portion of the NuTeV anomaly
For APV in electron-2H DIS:
Search for CSV in PV DIS
!APV
APV! 0.28
!u" !du + d
Kent Paschke SPIN ’08 October 9, 2008
MRST Phenomenological PDFs (2004)include CSVMartin, Roberts, Stirling, Thorne (03):
Chose restricted form for parton CSV:
Best fit: κ = -0.2, large uncertainty ! Best fit remarkably similar to model
CSV predictions
MRST ADEL
[f(x): 0 integral; matches to valence at small, large x]
90% conf limit (κ)
Bag model
Phenomenological Parton CSV PDFs
Slide from T.Londergan
Kent Paschke SPIN ’08 October 9, 2008
Effects at high-x are predicted to be very large, but Q2 evolution not predicted
from Londergan, using MRST fit
RC
SV [
%]
CSV and PV-DIS
x
RC
SV
x
δd
δu
bag model (solid) Radionov et al.QED splitting (dashed) Glueck et al.
x
Kent Paschke SPIN ’08 October 9, 2008
(Does not Evolve)
Evolves accordingTo DGLAP equations
Higher Twist iswhat is left over
Higher Twist is anyQ2-dependent deviationFrom the SM prediction
Higher Twist Coefficients
Kent Paschke SPIN ’08 October 9, 2008
x D(x) D(x) Q2min D/Q2
min(%) D/Q2min(%)
LO NNNLO LO NNNLO
0.1-0.2 -0.007 0.001 0.5 -14 2
0.2-0.3 -0.11 0.003 1.0 -11 0.0
0.3-0.4 -0.06 -0.001 1.7 -3.5 -0.5
0.4-0.5 0.22 0.11 2.6 8 4
0.5-0.6 0.85 0.39 3.8 22 10
0.6-0.7 2.6 1.4 5.8 45 24
0.7-0.8 7.3 4.4 9.4 78 47
F2(x,Q2)=F2(x)(1+D(x)/Q2) Q2=(W2-M2)/(1/x-1) Q2min=Q2(W=2)
PV-DIS might show higher twist at high x without needing QCD evolution.
MRST, PLB582, 222 (04)
Higher Twist Coefficients Large at High x
Kent Paschke SPIN ’08 October 9, 2008
Observing a clean higher twist operatormay become very interesting.
Interpretation of Higher Twist
• Many higher twist effects will cancel in ratio • APV sensitive to diquarks: ratio of weak to electromagnetic
charge depends on amount of coherence• Do diquarks have twice the x of single quarks?
≠+
+
Kent Paschke SPIN ’08 October 9, 2008
To achieve a high precision constraint on axial-vector quark couplings, first study
hadronic physics
• Measure AD in NARROW bins of x, Q2 with 1% precision
• Cover broad Q2 range for x in [0.3,0.6] to constrain HT• Search for CSV with x dependence of AD at high x
• Use x>0.4, high Q2, and Y dependence to measure C2q’s
Requires precise kinematics and broad range
x y Q2
New Physics no yes no
CSV yes no no
Higher Twist yes no yes
Coherent Program of PV-DIS Study
Kent Paschke SPIN ’08 October 9, 2008
PVDIS w/ Base Equipment
Prescott et al(SLAC)
PDG Best Fit
Young Full Fit
Sample
6 GeV PVDIS 3% Ad measurement:Bands correspond to central values of either PDG
best fit or Young et al.’s best fit.
StandardModel
SHMS
HMS
Proposal approved for 11 GeV:Factor of ~ 2 to 3 improvement
E08-011: PVDIS off 2H at 6 GeV
• 08-011 provides first look, at x~0.25-0.3
• Insensitive to CSV, HT, but possibly sensitive to the quark sea?
• 11 GeV, allows greater precision at higher x, but doesn’t provide lever arm to fully separate QCD effects
Kent Paschke SPIN ’08 October 9, 2008
• High luminosity with thick (40 cm) cryotargets• Better than 1% errors• x-range 0.25-0.75• Q2 range a factor of 2 for each x• W2 well over 4 GeV2
• Moderate running times
A Design for Precision PV DIS Physics
A large solenoidal spectrometer works •need BaBar, CDF or CLEOII Solenoid•fast tracking, particle ID and “parity” counting electronics•polarimetry ~ 0.5%
Kent Paschke SPIN ’08 October 9, 2008
• 20o - 35o, E’~ 1.5 - 5 GeV• δp/p ~ 2% • some regions 10’s of kHz/mm2
• Pion rejection with Cerenkov + segmented calorimeter.• Asymmetric yoke, detectors in magnetic field
Design for PVDIS Physics
Figure from E.Chudakov
trackingE-calgas Cerenkovcollimator
Kent Paschke SPIN ’08 October 9, 2008
Reconstruction
For an idealized solenoid (constant field within, zero field without), two points outside magnet and beam position give analytic expressions for θ, E, zvertex, φ
y
x
End-on view
Beam position Magnet-exit tracker
Last trackerSimulation•Regression training for tracking parameters•radiation in target•slit scattering•simple “chamber resolution” smearing is added
Δp
/p
Δθ/θ
θ, deg θ, deg
Kent Paschke SPIN ’08 October 9, 2008
Good Resolution Prevents Bin Migration
simulatedSimulated x,
reconstructed Q2, W2
Kent Paschke SPIN ’08 October 9, 2008
Broad Kinematic Acceptance
Q2 Q2
x x
11 GeV 8.8 GeV
Q2
x
• 11 GeV Solenoid• 8.8 GeV Solenoid• 11 GeV HMS/SHMS
35 days at each energy,
85% pol
Error bars in each
[x,Q2] bin
Kent Paschke SPIN ’08 October 9, 2008
•Allows d/u measurement on a single proton!•Vector quark current (electron is axial-vector)
+ small corrections
APV in DIS on 1H
Kent Paschke SPIN ’08 October 9, 2008
Uncertainty in d/u at High x
•LD2 studies can control higher twist, SM coupling
• 35 days on proton can pin d/u at x~0.75
• No nuclear corrections!
Kent Paschke SPIN ’08 October 9, 2008
OutlookProposed design provides sufficient acceptance, resolution, and shielding for broad PVDIS program• charge symmetry violation• C2q’s and new physics• higher twist and quark correlations• d/u• new structure functions• PV analog of EMC• SIDIS, spin structure fcns
Proposal planned for
Jan ‘09
Fall ’08: INT workshop, Seattle
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