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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