The Q weak Experiment at JLab A search for parity violating new physics at the TeV scale by measurement of the Proton’s weak charge. Roger D. Carlini Jefferson 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. Carlini 1 (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. Kowalski 1, 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. Page 1, J. Pan, K. Paschke, S. Phillips, M. Pitt, B. (Matt) Poelker, Y. Prok, W. D. Ramsay, M. Ramsey-Musolf, J. Roche, N. Simicevic, G. Smith 2, 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 1 Spokespersons 2 Project 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 C Larry 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 Interference with EM amplitude makes Neutral Current (NC) amplitude accessible Small (~10 -6 ) cross section asymmetry isolates weak interaction Interference:  ~ |M EM | 2 + |M NC | 2 + 2Re(M EM* )M NC First discussed: Ya. B Zel’dovich JETP 36 (1959) scatter electrons of opposite helicities from unpolarized target

Q w n = -10 Neutron udd -2 C 1d = /3 sin 2  W -1/3d Q w p = sin 2  W  C 1u = + 1 – 8/3 sin 2  W Weak (vector) +1 Proton uud +2/3u Electric Charge Particle Weak Charges Note “accidental” suppression of Q w p  sensitivity to new physics Govern strength of neutral current interaction with fermion Q p weak is a well-defined experimental observable. Q p weak has a definite prediction in the electroweak Standard Model. Q e weak : electron’s weak charge was measured in PV Møller scattering (E158).

All Data & Fits Plotted at 1  Isoscalar weak charge Isovector weak charge Standard Model Prediction

 QED ss Measured Charges Depend on Distance (running of the coupling constants) 1/137 1/128 Electromagnetic coupling is stronger close to the bare charge Strong coupling is weaker close to the bare charge far close “screening”“anti-screening”

Running of sin 2  w Plot (updated) arXiv: v1 [nucl-th] 24 Aug Bentz, Cloet, Londergan & Thomas Qweak increasing Qweak decreasing 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 Q p weak & the data to extract the hadronic corrections for Qweak) SAMPLE (MIT/Bates) Q 2 = 0.1 HAPPEx-I (JLab/Hall A) Q 2 = 0.48 PV-A4 (MAMI) Q 2 = 0.23, 0.11 G0 (JLab/Hall C) Q 2 = 0.12  1.0 HAPPEx-II/helium (JLab/Hall A) Q 2  0.1 PV-A4 (backward) PRL 102, (2009) Q 2 = 0.22 G0 (backward)*Q 2 = 0.23, 0.63 HAPPEx-III (forward – Aug-Oct, 2009) Q 2 = 0.63 time

(at tree level) Q p Weak : Extract from Parity-Violating Electron Scattering measures Q p – proton’s electric charge measures Q p Weak – proton’s weak charge M EM M NC As Q 2  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 Q 2 = 0 (Young, Carlini, Thomas & Roche, PRL 99, (2007) ) Q weak 1σ bound from global fit to all PVES data (as/of 2007) PDG Q p weak SM PDG

All Data & Fits Plotted at 1  Isovector weak charge Isoscalar weak charge Standard Model Prediction Young, Carlini, Thomas & Roche, PRL HAPPEx: H, He G 0 (forward): H, PVA4: H SAMPLE: H, D

All Data & Fits Plotted at 1  Isovector weak charge Isoscalar weak charge Standard Model Prediction Young, Carlini, Thomas & Roche, PRL HAPPEx: H, He G 0 (forward): H, PVA4: H SAMPLE: 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, (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 TeV Analysis 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) 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 A z = (  + -  - )/(  + +  - )  -0.3 ppm   5 x 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 8.4 o beam dump 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  Q 2 = (GeV/c) 2 Want : 6 ppb (2%) on A z (  4% on Q w  0.3% on sin 2  W ) 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 LH 2 target Helicity-correlated beam properties: intensity < 0.1 ppm position < 2 nm angle < 30 nrad diameter < 0.7  m energy  E/E < % precision on electron beam polarization

Qweak’s Relative “difficulty factor” Figure by K. Paschke Statistical Errors : Higher beam currents Higher polarizations Denser targets Additive systematic errors: improved control of helicity correlated beam properties Normalization/systematic errors: Polarimetry Q 2 measurements

Error Budget 2% on A PV  4% on Q w  0.3% on sin 2  W Uncertainty  A PV /A PV  Q w /Q w Statistical (2,544 hours at 180  A) 2.1% 3.2% Systematic: 2.6% Hadronic structure uncertainties % Beam polarimetry 1.0% 1.5% Absolute Q 2 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  sin 2  W / sin 2  W includes QCD uncertainties (1-loop) in calculation of the running 0.2%  0.3%.

Estimates of 2 Boson Exchange effects on Q p Weak at our Kinematics TBE (Gorchtein & Horowitz) ~6% to 7% Correction Phys. Rev. Lett. 102, (2009) +1.5% TBE (Sibirtsev, Melnitchouk, Thomas) ~6% - 0.5% This correction is still ~4% at a Q 2 of 0.1 GeV 2 – so we already remove part of it with the “B” term determination from other measurements! SourceQ p Weak Uncertainty  sin  W (M Z )± Z  box±  sin  W (Q) hadronic ± WW, ZZ box - pQCD± Charge symmetry 0 Total± Electroweak Radiative Corrections Erler et al., PRD 68(2003) Q p Weak Standard Model (Q 2 = 0) ± Q p Weak Global Fit Value (Young, et.al.) ± Q p Weak Experiment anticipated uncertainty 0.0XXX ± 0.003

Schematic of the Qweak Experiment Polarized Electron Beam, GeV, 150 µA, P ~ 85% 35 cm Liquid Hydrogen Target Primary Collimator with 8 openings Region I GEM Detectors Region II Drift Chambers Toroidal Magnet Region III Drift 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 Q 2 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 2000 Rad-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 (I k = 3 nA) Current mode readout (I a = 6 μA) Elastic focus – blue Inelastics - red Toroidal Spectrometer Produces 8 Beam Spots Each focus is ~2 meters long

Custom 18 bit ADC Light Output Uniformity of Quartz Bar

Tracking System to Precisely Determine Q 2 Acceptance Why a Tracking System? easy answer  Goal: 0.5% on Q 2 Subtleties: Moments of Q 2 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 LH 2 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 (LH 2 ) 2.5 kW cooling power required Designed using CFD This will be the highest power cryotarget in the world! Jlab: ~2.5 kW LH 2 Cryotarget System Flow Space only LH 2 flow beam direction

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 Q p Weak happens to almost cancel, it is very sensitive to the value of sin 2 θ W, 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 g e-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

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