parity-violating electron scattering at jlab

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

MENU 201310/2/2013

16

Reduced Asymmetry

Hadronic part extracted through global fit of PVES data.

in the forward-angle limit (θ=0)4% of total data

MENU 201310/1/2013 MENU 2013

17

The C1q & the neutron’s weak charge

MENU 201310/1/2013

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)