The Qweak Experiment at JLab

A search for parity violating new physics at the TeV scale by measurement of the Proton’s weak charge.

Roger D. CarliniJefferson Laboratory

• Scatter longitudinally polarized electrons from liquid hydrogen

• Flip the electron spin and see how much the scattered fraction changes

• The difference is proportional to the weak charge of the proton

• Hadronic structure effects determined from global PVES measurements.

D. Androic, D. Armstrong, A. Asaturyan, T. Averett, R. Beminiwattha, J. Benesch, J. Birchall, P. Bosted, C. Capuano, R. D. Carlini1 (PI), G. Cates, S. Covrig, M Dalton, C. A. Davis, W. Deconinck, K. Dow, J. Dunne, D. Dutta, R. Ent, J. Erler,

W. Falk, H. Fenker, J.M. Finn, T. A. Forest, W. Franklin, M. Furic, D. Gaskell, M. Gericke, J. Grames, K. Grimm, D. Higinbotham, M. Holtrop, J.R. Hoskins, K. Johnston, E. Ihloff, M. Jones, R. Jones, K. Joo, J. Kelsey, C. Keppel, M. Khol,

P. King, E. Korkmaz, S. Kowalski1, J. Leacock, J.P. Leckey, J. H. Lee, L. Lee, A. Lung, D. Mack, J. Mammei, J. Martin, D. Meekins, A. Micherdzinska, A. Mkrtchyan, H. Mkrtchyan, N. Morgan, K. E. Myers, A. Narayan, Nuruzzaman,

A. K. Opper, S. A. Page1, J. Pan, K. Paschke, S. Phillips, M. Pitt, B. (Matt) Poelker, Y. Prok, W. D. Ramsay, M. Ramsey-Musolf, J. Roche, N. Simicevic, G. Smith2, T. Smith, P. Souder, D. Spayde, R. Suleiman, B. Sawatzky,E. Tsentalovich, W.T.H. van Oers, B. Waidyawansa, W. Vulcan, P. Wang, S. Wells, S. A. Wood, S. Yang, R. Young,

H. Zhu, C. Zorn

1Spokespersons2Project Manager

College of William and Mary, University of Connecticut, Instituto de Fisica, Universidad Nacional Autonoma de Mexico, University of Wisconsin, Hendrex College, Louisiana Tech University, University of Manitoba, Massachusetts Institute of

Technology, Thomas Jefferson National Accelerator Facility, Virginia Polytechnic Institute & State University, TRIUMF, University of New Hampshire, Yerevan Physics Institute, Mississippi State University,

University of Northern British Columbia, Ohio University, Hampton University, University of Winnipeg, University of Virginia, George Washington University, Syracuse University,

Idaho State University, University of Connecticut, Christopher Newport University, University of Zagreb

The Qweak Collaboration

A search for parity violating new physics at the TeV scale by measurement of the Proton’s weak charge.

(Funded by DOE, NSF, NSERC and the State of Va)Installation nearing completion Runs June 2010 to May 2012

Qweak in Hall CLarry Lee

Accelerator at Jefferson Lab

Precision Tests of the Standard Model

• Standard Model is known to be the effective low-energy theory of a more fundamental underlying structure.

• Finding new physics: Two complementary approaches:

– Energy Frontier (direct) : eg. Tevatron, LHC– Precision Frontier (indirect) : (aka Intensity Frontier)

• (g-2) , EDM, decay, e , e, K, etc.• - oscillations• Atomic Parity violation• Parity-violating electron scattering

Hallmark of the Precision Frontier: Choose observables that are “precisely predicted” or “suppressed” in Standard Model.

If new physics is found in direct measurements, precision measurements useful to determine e.g. couplings…

Often at modest or low energy…

Parity Violating Electron Scattering: Weak Neutral Current Amplitudes

EMEM JQ

M lQ

24

€

MPVNC =

GF2 2

gAlμ 5Jμ

NC + gV l μJμ 5NC

[ ]

Interference with EM amplitude makes Neutral Current (NC) amplitude accessible

22

~~Z

EM

NCPV

LR

LRPV

M

Q

M

MA

Small (~10-6) cross section asymmetry isolates weak interaction

Interference: ~ |MEM |2 + |MNC |2 + 2Re(MEM*)MNC

First discussed: Ya. B Zel’dovich JETP 36 (1959)

scatter electrons of opposite helicities from unpolarized target

Qwn = -10

Neutron

udd

-2 C1d = -1 + 4/3 sin2W-1/3d

Qwp = 1 - 4 sin2W 0.07

-2 C1u = + 1 – 8/3 sin2W

Weak (vector)

+1

Proton

uud

+2/3u

Electric

Charge

Particle

Weak Charges

Note “accidental” suppression of Qwp sensitivity to new physics

Govern strength of neutral current interaction with fermion

• Qpweak is a well-defined experimental observable.

• Qpweak has a definite prediction in the electroweak Standard Model.

• Qeweak : electron’s weak charge was measured in PV Møller scattering (E158).

All Data & Fits Plotted at 1

Iso

scal

ar w

eak

char

ge

Isovector weak charge

Standard Model Prediction

QED s

Measured Charges Depend on Distance(running of the coupling constants)

1/137

1/128

Electromagnetic coupling isstronger close to the bare charge

Strong coupling isweaker close to the bare charge

far close far close

“screening” “anti-screening”

Running of sin2 qw Plot (updated)

arXiv:0908.3198v1 [nucl-th] 24 Aug 2009 - Bentz, Cloet, Londergan & Thomas

Qweakincreasing

Qweakdecreasing

far close

PDG 2008: “Electroweak and constraints on New Physics Model” J. Erler & P. Langacker

PVES and Hadronic Structure Effects

Neutral-weak form factors

assume charge symmetry:

Proton weak charge(tree level)

Strangeness

Axial form factor

(Now measured to be relatively small!)

Note: Parity-violating asymmetry is sensitive to both weak charges and to hadron structure.

Two Decades of Strange Form Factor Searches

(Provides the best present measurement of Qpweak

& the data to extract the hadronic corrections for Qweak)

• SAMPLE (MIT/Bates) Q2 = 0.1• HAPPEx-I (JLab/Hall A) Q2 = 0.48• PV-A4 (MAMI) Q2 = 0.23, 0.11• G0 (JLab/Hall C) Q2 = 0.121.0• HAPPEx-II/helium (JLab/Hall A) Q2 0.1 • PV-A4 (backward) PRL 102, 151803 (2009) Q2 = 0.22• G0 (backward)* Q2 = 0.23, 0.63

• HAPPEx-III (forward – Aug-Oct, 2009) Q2 = 0.63

tim

e

Qweakp 1 4sin2 W ~ 0.072 (at tree level)

A 2MNC

MEM

GF

4 2

Q2 Qweakp F p Q2 ,

Q2 0 0

GF

4 2

Q2 Qweakp Q4B Q2

ZMEME GG ,, and contains

QpWeak : Extract from Parity-Violating Electron Scattering

measures Qp – proton’s electric charge measures QpWeak

– proton’s weak charge

MEM MNC

As Q2 0

Correction involving hadronic form factors.Exp determined using global analysis of recently completed PVES experiments.

The lower the momentum transfer, Q, the more the proton looks like a point and the less important are the form factor corrections.

Parity-Violating Asymmetry Extrapolated to Q2 = 0(Young, Carlini, Thomas & Roche, PRL 99, 122003 (2007) )

Qweak

1σ bound from global fit to all PVES data (as/of 2007)

PDG

Qpweak

SM

PDG

All Data & Fits Plotted at 1

Isovector weak charge

Iso

scal

ar w

eak

char

ge

Standard Model Prediction

Young, Carlini, Thomas & Roche, PRL

HAPPEx: H, HeG0 (forward): H, PVA4: HSAMPLE: H, D

All Data & Fits Plotted at 1

Isovector weak charge

Iso

scal

ar w

eak

char

ge

Standard Model Prediction

Young, Carlini, Thomas & Roche, PRL

HAPPEx: H, HeG0 (forward): H, PVA4: HSAMPLE: H, D

(anticipated uncertainty assuming SM)

Energy Scale of an Indirect Search

• Estimate sensitivity to new physics Mass/Coupling ratio add new contact term to the electron-quark Lagrangian: Erler et al. PRD 68, 016006

(2003)

Λ = mass g = coupling

TeV scale can be reached with a 4% Qweak experiment. If the weak charge didn’t happen to be suppressed, would have to do a 0.4% measurement to reach the TeV-scale.

95% CL

Atomic only

with PVES

Qweak (4%)

Qweak constrains new PV physics to beyond 2 TeVAnalysis by R.D.Young et al.

Lower Bound for “Parity Violating” New Physics

New Physics: Examples

• Extra neutral gauge bosons: Z’ eg. E6 SO(10)U(1) GUT,

SUSY, left/right symmetric models, technicolor, string theories…

• Composite fermions

• Leptoquarks (scalar LQs can arise in R-parity violating SUSY)

M.J. Ramsey-Musolf PRC 60(1999)015501; PRD62(2000)056009 J. Erler, A. Kurylov, M.J. Ramsey-Musolf PRD 68(2003)016006

Slightly more likely to “hit a proton” if the electron is spinning to the left (parity violation)

Expect Az = (+ - -)/(+ + -) -0.3 ppm

5 x 10-9 statistics in ~2200 hours

Use eight detectors at 900 MHz each and in “current integrating mode”

LH2 target

Longitudinally polarized (rapid helicity flip)electron beam (85%) spectrometer

8.4o

beam dump

1.165 GeV, 180 A

35 cm

The Qweak Measurement(ep parity violation)

Qweak Experiment Overview

• Forward-angle elastic scattering 1.16 GeV e’s from proton at mean 8.4 in the lab frame Q2 = 0.026 (GeV/c)2

• Want: 6 ppb (2%) on Az ( 4% on Qw 0.3% on sin2W )

• Expand on success/techniques of JLab PVES program

• Runs June 2010 to May 2012

– Final expt. in Hall C before 12 GeV upgrade

Some Challenges:

• 6.5 GHz rate rad-hard detectors • 2.5 kW cryogenic LH2 target• Helicity-correlated beam properties:

intensity < 0.1 ppm position < 2 nm angle < 30 nrad diameter < 0.7 m energy E/E < 10-9

• 1% precision on electron beam polarization

Qweak’s Relative “difficulty factor”

Figure byK. Paschke

Statistical Errors:

• Higher beam currents• Higher polarizations• Denser targets

Additive systematic errors: improved control of helicity correlated beam properties

Normalization/systematic errors:• Polarimetry• Q2 measurements

Error Budget

2% on APV 4% on Qw 0.3% on sin2W

Uncertainty APV/APV Qw/Qw

Statistical (2,544 hours at 180 A) 2.1% 3.2%

Systematic: 2.6% Hadronic structure uncertainties --- 1.5% Beam polarimetry 1.0% 1.5% Absolute Q2 determination 0.5% 1.0% Backgrounds 0.5% 0.7% Helicity correlated beam properties 0.5% 0.7%

Total: 2.5% 4.1%

Final error on sin2W / sin2W includes QCD uncertainties (1-loop) in calculation of the running 0.2% 0.3%.

Estimates of 2 Boson Exchange effects on QpWeak at our Kinematics

TBE (Gorchtein & Horowitz) ~6% to 7% Correction Phys. Rev. Lett. 102, 091806 (2009)

+1.5%

TBE (Sibirtsev, Melnitchouk, Thomas) ~6% - 0.5%

This correction is still ~4% at a Q2 of 0.1 GeV2 – so we already remove part of it with the “B” term determination from other measurements!

Source QpWeak Uncertainty

sin W (MZ) ±0.0006Z box ±0.0005sin W (Q)hadronic ±0.0003WW, ZZ box - pQCD ±0.0001Charge symmetry 0

Total ±0.0008

Electroweak Radiative Corrections

Erler et al., PRD 68(2003)016006.

QpWeak Standard Model (Q2 = 0) 0.0713 ± 0.0008

QpWeak Global Fit Value (Young, et.al.) 0.055 ± 0.017

QpWeak Experiment anticipated uncertainty 0.0XXX ± 0.003

Schematic of the Qweak Experiment

35 cm Liquid Hydrogen Target

Polarized Electron Beam

Collimator With Eight Openings = 9 ± 2°

Toroidal Magnet

Eight Fused Silica (quartz)Cerenkov Detectors

5 inch PMT in Low GainIntegrating Mode on Each

End of Quartz Bar

Elastically Scattered Electrons

325 cm

580 cm

LuninosityMonitor

Region 3Drift Chambers

Region 2Drift Chambers

Region 1GEM Detectors

Polarized Electron Beam, 1.165 GeV, 150 µA, P ~ 85%

35 cm Liquid Hydrogen Target

Primary Collimator with 8 openings

Region IGEM Detectors

Region IIDrift Chambers

Toroidal Magnet

Region IIIDrift Chambers

Elastically Scattered Electron

Eight Fused Silica (quartz) Čerenkov Detectors - Integrating Mode

Luminosity Monitors

~3.2 m

Region I, II and III detectors are for Q2 measurements at low beam current

The Qweak Apparatus (shielding not shown)

Two opposing octants instrumented, rotator system for each region to cover all octants and to move to “parked” position for asymmetry measurement.Periodic tracking measurements at sub-nA beam current.

Qweak – (partial shielding shown)

QTOR Magnet at JLab

Attaching a Coil to the Support Structure

Partially Assembled Magnet at JLab

Main Detector System

• 8 fused silica radiators 200 cm x 18 cm x 1.25 cm

• Spectrosil 2000Rad-hard, low luminescence (expensive)

• 900 MHz e- per bar• Light collection by TIR

5 Angstroms rms polish (even more expensive)

• 5” PMTs with gain = 2000

• S20 photocathodes (Ik = 3 nA)

• Current mode readout (Ia = 6 μA)

Elastic focus – blue Inelastics - redToroidal Spectrometer Produces 8 Beam SpotsEach focus is ~2 meters long

Custom 18 bit ADC

Light Output Uniformity of Quartz Bar

Tracking System to Precisely Determine Q2 Acceptance

35 cm Liquid Hydrogen Target

Polarized Electron Beam

Collimator With Eight Openings = 9 ± 2°

Toroidal Magnet

Eight Fused Silica (quartz)Cerenkov Detectors

5 inch PMT in Low GainIntegrating Mode on Each

End of Quartz Bar

Elastically Scattered Electrons

325 cm

580 cm

LuninosityMonitor

Region 3Drift Chambers

Region 2Drift Chambers

Region 1GEM Detectors

Why a Tracking System?

easy answer

Goal: 0.5% on Q2

Subtleties:• Moments of Q2 acceptance

• Light-weighted response of Cerenkov • Check on collimators, toroid field• Diagnostics on backgrounds• Radiative corrections

Region I - GEMs

Cosmic Ray: Trigger out

Cosmic Ray: ADC spectrum

Region I - GEMs

Region II - HDCs

All chambers completed

Pairs of 6-layer Horizontal Drift Chambers per tracking octant x, x‘ 39 sense wires each u,u',v,v' (45) 55 sense wires each 1192 electronic channels F1TDC readout

Region III - VDCs

Pair of Vertical Drift chambers per tracking octant; each has U and V planes 8 G10 frames + 2 Al frames/VDC 280 wires/plane 4 chambers + 1 spare

Assembled VDC

2.5 kw LH2 Cryotarget

• First use of a Fluid dynamics simulation used in the design to minimize “boiling noise” risk - in the liquid or at the windows.

• Additional safeguards: large raster size ~(5mm x 5mm), faster pump speed, and more cooling directed onto windows....

• Faster helicity reversal – 1 ms flip rate. Common mode rejection of “boiling” noise, line noise and undesirable beam properties increases significantly as Helicity reversal/readout rate is raised.

35 cm long liquid hydrogen (LH2)

2.5 kW cooling power required Designed using CFD

This will be the highest power cryotarget in the world!

Jlab: ~2.5 kW LH2 Cryotarget System

Flow Space only

LH2

flow

beamdirection

New Hall-C Compton Polarimeter

Compton polarimeter can run all the time (unlike the Møller).

Hall C has ~1% precision Møller polarimeter based on magnetized foil in several Tesla field. This technique needs dedicated runs.

< 1% precision with the Compton at is anticipated via cross-calibration. against existing Møller polarimeter.

• Interpretable precision measurement of the proton’s weak charge in the simplest system. Most hadronic structure effects determined from global PVES measurements. Other theoretical uncertainties calculated to be small.

• Because QpWeak happens to almost cancel, it is very sensitive to the

value of sin2θW, making possible a ~10 σ test of the running.

• Sensitive search for parity violating new physics with CL of 95% at the ~2.3 TeV scale.

• If the LHC observes a new neutral boson with mass Λ, our results could help identify it by constraining the magnitude and sign of the coupling-to-mass ratio ge-p/Λ. • Measurement unique to Jlab that is unlikely to be repeated in the foreseeable future so we need to push the precision envelope.

• This experiment builds the scientific and technical foundation for a next generation Møller measurement at a 12 GeV JLab.

Summary

END