parity-violating electron scattering at jlab
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Parity-Violating Electron Scattering at JLab. David S. Armstrong College of William & Mary. (replacing Juliette Mammei , who kindly provided most of the slides). MENU 2013 Rome, Italy Oct 2 2013. Parity-Violating Electron Scattering at JLab. Electroweak interaction (neutral current) - PowerPoint PPT PresentationTRANSCRIPT
Parity-Violating Electron Scattering at JLab
David S. ArmstrongCollege of William & Mary
MENU 2013 Rome, ItalyOct 2 2013
(replacing Juliette Mammei, who kindly provided most of the slides)
Parity-Violating Electron Scattering at JLab
Electroweak interaction (neutral current) - not one of the canonical probes for Hadron Physics (i.e. strong or electromagnetic)
- nevertheless, much of the program should be of interest to this community
- Focus today: - new results since MENU2010 - plans for 12 GeV era at Jefferson Lab
Outline
1) Intro to Parity-Violating Electron Scattering (PVES)
2) The Vector Strange Form Factors a ≈ completed program
3) Qweak: first results on the proton’s weak charge
4) Neutron radius in Heavy Nuclei
5) Standard Model Tests with PVES: plans at JLab-12 GeV
MENU 2013 4
1) Search for physics Beyond the Standard Model– Low energy (Q2 <<M2) precision tests complementary
to high energy measurements
10/2/2013
• Neutrino mass and their role in the early universe 0νββ decay, θ13 , β decay,…• Matter-antimatter asymmetry in the present universe EDM, DM, LFV, 0νββ, θ13
• Unseen Forces of the Early Universe Weak decays, PVES, gμ-2,…
LHC new physics signals likely will need additional indirect evidence to pindown its nature
• Neutrons: Lifetime, P- & T-Violating Asymmetries (LANSCE, NIST, SNS...)• Muons: Lifetime, Michel parameters, g-2, Mu2e (PSI, TRIUMF, FNAL, J-PARC...)• PVES: Low-energy weak neutral current couplings, precision weak mixing angle
(SLAC, Jefferson Lab, Mainz) new since MENU2010: first result from QWeak
2) Study nucleon and nuclear properties– Strange quark content of nucleon new since MENU2010: HAPPEX-III– Neutron radius of heavy nuclei new since MENU2010 – first PREx results
MENU 2013 5
A brief history of parity violation
R
R
L
L
1930s – weak interaction needed to explain nuclear β decay
1950s – parity violation in weak interaction; V-A theory to describe 60Co decay
1970s – neutral weak current events at Gargamelle
late 1970s – parity violation observed in electron scattering - SLAC E122
10/2/2013
MENU 2013 6
Parity-violating electron scattering
10/2/2013
PVA
APV ∝
G FQ2
4πα 2gAe gV
T βgVe gA
T( )~10−4Q2 GeV 2⎡⎣ ⎤⎦
2gM
M Z
Electroweak interference
MENU 2013 7
SLAC Experiments
10/2/2013
SLAC E122 – crucial confirmation of WSG electroweak model
• Electron-deuteron deep inelastic scattering• High luminosity: photoemission from NEA GaAs cathode• Rapid helicity-flip (sign of e- polarization)• Polarimetry to determine beam polarization• Magnetic spectrometer: backgrounds and kinematic
separation
APV ~ 100 ± 10 ppm
SLAC E158 – 1999• electron-electron scattering - purely leptonic interaction• electron-electron weak attractive force had never been
measured!APV ~ -131 ± 14 ± 10 ppb
sin2θW=0.20±0.03
sin2θW=0.2403±0.0013
Genesis of a Strange Idea
Puzzle: Initial DIS measurements of spin-structure of nucleon (EMC): valence quarks contribute unexpectedly low fraction to total spin - “Spin Crisis”
Possible reconciliation: large fraction of spin from ? eg. D. B. Kaplan and A. Manohar, Nucl. Phys. B310, 527 (1988).
Theoretical realization: not only did many available nucleon model calculations allow this, but they also allowed (and in some cases favored) large strange quark contributions to other properties of nucleon.
Consternation and excitement: at the time, data gave no constraint on strange contributions to charge distribution and magnetic moment!
Challenge: how to isolate strange vector form factors?
Answer: exploit the weak neutral current as a probe
ss
Puuduu dd ss g +.....
Qg QZ
u +2/3 1 8/3 sin2W
d 1/3 1 + 4/3 sin2W
s 1/3 1 + 4/3 sin2W
strange quark contribution
Define the nucleon form factors associated with a given quark currentq as:
N | qgμ |N ψN F1gμ F2
iμν
ν
2MN
⎛⎝⎜
⎞⎠⎟ ψN
sME
dME
uME
Zs
Zd
Zu
pZME
nME
pME
GGG
QQQGGG
,
,
,
,,
,,
,,
31
32
31
31
31
32
g
g
Assume isospin symmetry, and we have
this
and this
are well known
what about this?
(Assume neutral weak charges are known)
qqqE FFG 21
qqqM FFG 21
MENU 201310/2/2013
Nucleon Form Factors
sME
dME
uMEME GGGG //// 3
131
32
g
sMEW
dMEW
uMEW
ZME GGGG /
2/
2/
2/ sin
341sin
341sin
381
NC and EM probe same hadronic flavor structure, with different couplings:
Assume Charge Symmetry:
snME
spME
unME
dpME
dnME
upME GGGGGG ,
/,/
,/
,/
,/
,/ ,,
MENU 2013 11
Strange Form Factors
10/2/2013
Qweak: Proton’s weak charge
12
The Standard Model makes a firm prediction of
EM Charge Weak Charge
u 2/3
d -1/3
P (uud) +1N (udd) 0 -1
“Accidental suppression”sensitivity to new physics
Note:
Q-weak is particularly sensitive to the quark vector couplings ( and ) .
𝑄𝑊𝑛 =−2(𝐶1𝑢+2𝐶1𝑑)
-Neutral current analog of electric charge:
10/1/2013 MENU 2013
Qweak: Proton’s weak charge
Use four-fermion contact interaction to parameterize the effective PV electron-quark couplings (mass scale and coupling)
Large θSmall θ
Erler, Kurylov, and Ramsey-Musolf, PRD 68, 016006 2003
Planned 4% measurement of proton’s weak charge - probes TeV scale new physics
New physics:
new Z', leptoquarks, SUSY ...
For electron-quark scattering:
MENU 201310/1/2013
SM
Qweakkinematics Hadronic term extracted from fit
Extracting the weak charge
14
𝐴𝑃𝑉=−𝐺𝐹𝑄2
4𝜋𝛼 √2[𝑄𝑤
𝑝 +𝐵 (𝜃 ,𝑄2 )𝑄2 ]Hadron structure enters here: electromagnetic and
electroweak form factors…
The previous strange form factor program (experiments at MIT/Bates, JLab and MAMI) allow us to subtract our hadronic contribution
𝐴0=−𝐺𝐹𝑄2
4𝜋𝛼 √ 2
One must extrapolate to .We measure
at .
Reduced asymmetry more convenient: Data rotated to
10/1/2013 MENU 2013
15
The Qweak ApparatusMain detectors
8-fold symmetry
Vertical drift chambers (VDC)
(rotate)
Trigger scintillator
Shield wall
Lintels
QTOR
Lead collar
Cleanup collimators
Target
Tungsten plug Horizontal drift
chambers (HDC)(rotate)
e- beam
e- beam
Acceptance defining
collimator
QTOR
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16
Reduced Asymmetry
Hadronic part extracted through global fit of PVES data.
in the forward-angle limit (θ=0)4% of total data
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17
The C1q & the neutron’s weak charge
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18
The C1q & the neutron’s weak charge
MENU 201310/1/2013
Combining this result with the most precise atomic parity violation experimentwe can also extract, for the first time, the neutron’s weak charge:
Qweak – first result
19
ppb ⟨𝑄2 ⟩=0.0250±0.0006𝐺𝑒𝑉 2
10/1/2013 MENU 2013
First result (4% of data set):
Lots of work to push down systematic errors, but no show-stoppers found….
Expect final result in 12-18 months time.
More details: Fundamental Symmetries 3, this afternoon
A of Auxiliary Measurements
2010/1/2013 MENU 2013
Qweak has data (under analysis) on a variety of observables of potential interest for Hadron physics:
• Beam normal single-spin asymmetry* for elastic scattering on proton• Beam normal single-spin asymmetry for elastic scattering on 27Al• PV asymmetry in the region.• Beam normal single-spin asymmetry in the region.• Beam normal single-spin asymmetry near W= 2.5 GeV • Beam normal single-spin asymmetry in pion photoproduction• PV asymmetry in inelastic region near W=2.5 GeV (related to box diagrams) • PV asymmetry for elastic/quasielastic from 27Al• PV asymmetry in pion photoproduction
*: aka vector analyzing power aka transverse asymmetry; generated by imaginary part of two-photon exchange amplitude (pace Wim van Oers)
N→ Δ
N→ Δ
gZ
208Pb208Pb
208Pb
2
Neutron skin – PREx
)()(
sin4122 2
22
2
QFQFQG
p
nW
F πα
)()(41)( ,0
32, rqrjrdQF pnpn
π
0
dd
dd
dd
dd
APV
MENU 201310/2/2013
Exploit the large weak charge of neutron to extract radius of neutron distribution in heavy nucleus; theoretically clean probe.
First result of PREx experiment at JLab: PRL 108(2012)112502
APV = 0.656 ± 0.060 (stat) ± 0.014 (syst) ppm
PREX-II
(proposed)
PREX, PREX II and CREX
Theory from P. Ring et al. Nucl. Phys. A 624 (1997) 349
208Pb more closely approximates infinite nuclear matterThe 48Ca nucleus is smaller, so APV can be measured at a Q2 where the figure of merit is higher
and are expected to be correlated, but the correlation depends on the correctness of the models
The structure of 48Ca can be addressed in detailed microscopic models
Measure both and - test nuclear structure models over a large range of A
208nR
48nR
208nR
48nR
MENU 2013
More info: yesterday’s talks by G. Urcioli and M. Thiel in Fundamental Symmetries 2
MENU 2013 23
Future: PVES at JLab in 12 GeV era
10/2/2013
MOLLER - precision Standard Model test by measuring weak charge of electron in PV electron-electron scattering
(revisit SLAC E158)
SOLID - precision Standard Model test by measuring PV DIS on deuteron: access the quark weak axial couplings C2q
(also – a similar measurement was made at 6 GeV in Hall A at Jefferson Lab, X. Zheng et al., being readied for publication, but I’m not authorized to show the results; stay tuned…)
Large kinematic coverage: disentangle CSV and higher-twist effects
ℒ𝑁𝐸𝑊𝑃𝑉 = ∑
𝑖 , 𝑗=𝐿 ,𝑅
𝑔𝑖𝑗2
2 Λ𝑖𝑗2 𝑒𝑖𝛾𝑖𝑒𝑖𝑒 𝑗𝛾❑
𝜇 𝑒 𝑗 𝑒𝐿 ,𝑅=12 (1∓𝛾5 )𝜓𝑒
𝑔𝑖𝑗=𝑔𝑖𝑗∗
𝑔𝐿𝑅=𝑔𝑅𝐿 ¿Λ
√√2𝐺𝐹|Δ𝑄𝑊𝑒 | →𝟕 .𝟓𝑻𝒆𝑽
2.3% MOLLER uncertainty
Coupling constants
Mass scaleΛ
√|𝑔𝐿𝐿2 −𝑔𝑅𝑅
2 |
Lepton compositeness – strong coupling – 47 TeV
LEP2 (gLR and sum) mass scale sensitivity ~5.2 and 4.4 TeV
e- e-
e- e-
MOLLER at 12 GeV
MENU 201310/2/2013
Doubly-charged scalar, heavy Z’, SUSY, dark Z…
MENU 2013 25
MOLLER and weak mixing angle
10/2/2013
Reminder: at tree-level QW
e = (1 – 4 sin2θW)
Higgs discovery at LHC allows firm prediction of MOLLER asymmetry in Standard Model
MENU 2013 26
MOLLER and weak mixing angle
10/2/2013
Detector Array
Scattering Chamber
Target
Hybrid Torus
Upstream Torus
Incoming BeamCollim
ators28 m
The MOLLER Experiment
MENU 201310/2/2013
MENU 2013 28
SOLID – accessing the C2q’s
10/2/2013
XN
e-
Z* γ*
e-
Cahn and Gilman, PRD 17 1313 (1978) polarized electrons on deuterium
MENU 2013 2910/2/2013
Large θSmall θ
Red ellipses are PDG fits
Blue bands represent expected data: Qweak (left) and PVDIS- 6GeV (right)
Green bands are proposed SOLID PVDIS
MENU 2013 30
SOLID – Large Acceptance Device
10/2/2013
MENU 2013 31
SOLID – Parity-Conserving Physics
10/2/2013
- SIDIS with Transversely Polarized 3He approved 90 days
- SIDIS with Longitudinally Polarized 3He approved 35 days
- SIDIS with Transversely Polarized Proton approved 120 days
- Near Threshold Electroproduction of J/Psi approved 60 days
PVDIS approved for 169 days (half of full request)
MENU 2013 32
PVES Experiment Summary
10/2/2013
MENU 2013 33
Summary
10/2/2013
Grazie to the MENU 2013 organizers for inviting Juliette Mammei to give this talk, and for accepting me as a poor substitute….
• Strange vector form factors of proton – small, consistent with zero.
• Qweak: First measurement of proton’s weak charge, consistent with Standard Model, 25x more data on tape
• PREx: two-sigma evidence for neutron “skin” of 208Pb; will improve after JLab comes online after 12 GeV upgrade, and will extend to 48Ca
• MOLLER and SOLID: major programs after JLab upgradetwo complementary Standard Model tests.
MENU 2013 34
Extra slides
10/2/2013
MENU 2013 35
Qweak and weak mixing angle
10/2/2013
MENU 2013 36
Magnetic spectrometerBackground and
kinematic separation
SLAC Experiment E122
10/2/2013
Polarimetry
Integrating detectors
• High luminosity from photoemission from NEA GaAs cathode• Rapid helicity-flip (sign of e- polarization)
Huge achievement!Highest P2I ever, by far. Developed for this
experiment at SLAC and used ever since
MENU 2013 37
SLAC Experiment E122
10/2/2013
sin2θW=0.20±0.03
GWS -- Nobel Prize 1979‐
Parity Non-Conservation in Inelastic Electron Scattering, C.Y. Prescott et. al, 1978
Deep inelastic scattering:Y dependence reflects quark axial/electron vector coupling strength
( ) ( ) YxbxaQGA FPV
πα2103 2
At high x ( ) eA
dV
eA
uV ggggxa 2
( ) eV
dA
eV
uA ggggxb 2
Left Right
γ Charge 0, ±1, ±1/3, ±2/3 0, ±1, ±1/3, ±2/3W Charge T=±1/2 0
Z Charge T-qsin2θW -qsin2θW
APV ~ 100 ± 10 ppm
COMPLEMENTARY TO THE LHC - Z΄
𝛼=0→𝐸 6𝑚𝑜𝑑𝑒𝑙𝑠 , α ≠0describes kineticmixing𝛽=0→𝑆𝑂 (10 ) (𝑖𝑛𝑐𝑙𝑢𝑑𝑖𝑛𝑔 h𝑡 𝑜𝑠𝑒𝑏𝑎𝑠𝑒𝑑𝑜𝑛𝐿𝑅𝑠𝑦𝑚𝑚𝑒𝑡𝑟𝑦 )
Assume LHC
discovers a new spin
1 gauge boson with
M =1.2 TeV
MOLLER can
distinguish between
models Erler and Rojas
If the SM value
is measured
Half-way between SM and
E158 central value
MENU 201310/2/2013
39
Isolating strange form factors
p
AMEF AAAQGAπα
24
2
~ few parts per million
For a proton:
For 4He: GEs alone
Forward angle Backward angle
)(2
sin2
22
nE
pE
sE
WF
PV GGGQGA
πα
For deuteron: enhanced GA
e sensitivity
d
nnppd
AAA
( ) eA
pMWA
pZM
pMM
pZE
pEE GGAGGAGGA ,'2,,,, sin41 , , ggg ee
)1)(1(
2tan)1(21
4
2
12
2
ee
e
e
pMQ
MENU 201310/2/2013
Electroweak Radiative Corrections
40
Electroweak Radiative Corrections
41
Normal production running: 89% longitudinal beam polarizationSmall amount of transverse polarization
• Large parity conserving asymmetry (~5ppm)• Leaks into the experimental asymmetry through broken azimuthal symmetry
Measurements of the Beam Normal Single Spin Asymmetry (BNSSA) provide:• Direct access to imaginary part of two-photon exchange
We need to correct for this…
Transverse Asymmetry Measurement
42
Horizontal and vertical components measured separately
𝐴𝐵𝑁𝑆𝑆 (𝜙𝑑𝑒𝑡 )=𝐵𝑛|𝑃𝑇|sin (𝜙𝑑𝑒𝑡−𝜙𝑠)
Detector number
Normal production running: 89% longitudinal beam polarizationSmall amount of transverse polarization
• Large parity conserving asymmetry (~5ppm)• Leaks into the experimental asymmetry through broken azimuthal symmetry
Transverse Asymmetry Measurement
43
This is the most precise measurement of Beam Normal Single Spin Asymmetry to date.
ppm
Source Preliminary Anticipated
Polarization 2.2% ~1.0%
Statistics 1.3% ~1.3%
1.2% ~0.5%
Non-linearity 1.0% ~0.2%
Regression 0.9% ~0.9%
Backgrounds 0.3% ~0.3%
PhD thesis of Buddhini Waidyawansa; being prepared for publication
44
Global PVES Fit Details• 5 free parameters ala Young, et al. PRL 99, 122003 (2007):
• Employs all PVES data up to Q2=0.63 (GeV/c)2 • On p, d, & 4He targets, forward and back-angle data
• SAMPLE, HAPPEX, G0, PVA4
• Uses constraints on isoscalar axial FF • Zhu, et al., PRD 62, 033008 (2000)
• All data corrected for E & Q2 dependence of• Hall et al., arXiv:1304.7877 (2013) & Gorchtein et al., PRC84, 015502 (2011)
• Effects of varying Q2, θ, & λ studied, found to be small
GAZ
□γZ RC
MENU 2013 4510/2/2013
46
𝐴𝑒𝑝=𝑅𝑅𝐶𝑅𝐷𝑒𝑡 𝑅𝐵𝑖𝑛𝑅𝑄2❑
𝐴𝑚𝑠𝑟
𝑃 −∑𝑖=1
4
𝑓 𝑖 𝐴𝑖
1−∑𝑖𝑓 𝑖
Qweak “Run 0” corrections and their corresponding sizes
Target aluminum windows are our largest (~30%) correction
Beam line background is currently our largest uncertainty
MOLLER: if Z‘ seen at LHC...
•Virtually all GUT models predict new Z’s (E6, SO(10)…): LHC reach ~ 5 TeV•With high luminosity at LHC, 1-2 TeV Z’ properties can be extracted
Suppose a 1 to 2 TeV heavy Z’ is discovered at the LHC…
LHC data can extract the mass, width and AFB(s)
MOLLER: resolve signs on eR, eL
MOLLER
F.Petrielloetal,PRD80(2009)
SLHC,1ab-1
MOLLER: if SUSY seen at LHC...
MSSM sensitivity if light super-partners and large tan β
RPVSUSY
MSSM
MOLLER
Ramsey-MusolfandSu,Phys.Rep.456(2008)