1 The G 0 Experiment: Parity Violation in e-N Scattering CalTech, Carnegie-Mellon,William & Mary, Hendrix, IPN-Orsay, LPSC-Grenoble, JLab, LaTech, NMSU, Ohio University,TRIUMF, U Conn, UIUC, U Manitoba, U Maryland, U Mass, UNBC, U Winnipeg, VPI, Yerevan, Zagreb Colleen Ellis The University of Maryland The G 0 Collaboration: Hall C Meeting 18 January 2008

2 G 0 Graduate Students Carissa Capuano : W&M, USA Maud Versteegen: LPSC, France. Alexandre Coppens: Manitoba, Canada Mathew Muether: Illinois, USA Colleen Ellis : Maryland, USA John Schaub : NMSU, USA. Juliette Mammei Virginia Tech. USA.

3 Overview Physics Introduction G0 Forward Angle G0 Backward Angle--Elastic Electron Scattering –Experimental Set-up –Analysis Overview –Preliminary Data –Detector Performance Other Backward Angle Physics Topics –Inelastic e-p measurement to measure parity violation in N-  transition –Elastic e-p scattering with transverse beam polarization to investigate 2 photon exchange –PV pion photoproduction on the  resonance

4 Assume: Isospin symmetry Strange Form Factors Electron scattering involves EM and Weak interactions Known G 0 measures How does s quark contribute to electromagnetic properties of the nucleon?

5  can be varied between zero and unity for a fixed Q 2 by varying the beam energy and electron scattering angle. Two kinematics, two targets gives 3 linear combinations of EM and weak form factors Model Independent Form Factors PV asymmetries from EM and weak interference terms

6 G 0 Forward Angle Experiment Forward angle measurement completed May 04 LH2 target, detect recoil protons Q 2 = (GeV/c) 2, E=3.03GeV Spectrometer sorts protons by Q 2 in focal plane detectors (16 rings in total) Detector 16: “super-elastic”, crucial for measuring the background Beam bunches separated by 32 ns Time-of-flight separates protons from pions Results published in : D.S. Armstrong, et al., PRL 95, (2005)

7 G 0 Forward Angle Results Forward Angle –700 hrs of data taking –101 C. –18 Q 2 measurements –Good agreement with other experiments (HAPPEx and PVA4) Backward Angle –Two Q2 measurements 0.23 and 0.62 GeV 2 –Required for complete separation of and (GeV 2 )

8 G 0 Backward Angle Hydrogen and deuterium targets Electron beam energy of : –362 MeV : Q 2 =0.23 (GeV/c) 2 –687 MeV : Q 2 =0.62 (GeV/c) 2 Detection of scattered electrons ~ 108º Particle detection and identification : –16 Focal Plan Detectors –9 Cryostat Exit Detectors elastic and inelastic electron separation –Additional Cerenkov detectors electron and pion separation Backward Angle Configuration e - beam target CED + Cerenkov FPD e - beamline

9 G 0 Backangle Superconducting Magnet (SMS) Detectors: F erris Wheel (FPDs) Detectors: Mini-Ferris wheel (CEDs+Cerenkov) Target Service Module G0 Beam Monitoring

10 Collected Data Longitudinal –LH2 362MeV 90 C –LD2 362MeV 70 C –LH2 687MeV 120 C –LD2 687MeV 45 C Transverse –LH2 362MeV 3.6 C –LD2 362MeV 2.1 C –LH2 687MeV 1.0 C Special Runs Types pion matrix random matrix magnet scans

11 Blinding Factor Rate Corrections for Electronics -- Deadtime and Random Coincidences Helicity Correlated Beam Corrections Raw Yields and Blinded Asymmetries by target and Q 2 Raw Yields and Blinded Asymmetries by target and Q 2 Corrections from inelastic electrons Background from target walls Pion Asymmetry Contamination EM Radiative Corrections (via Simulation) EM Radiative Corrections (via Simulation) Beam Polarization Correction G0 Backangle Analysis Approach A phys Unblind Forward Angle Q 2 Determination

12 measure raw yield for each helicity state (+ or -) apply rate corrections (electronic deadtime and random coincidences): correct for beam correlated effects : form asymmetry : correct for background contribution : correct for beam polarization (P) Forming Asymmetry LH2 LD2 A false < 4 ppb A m ~ 10 ppm f b < 10 %

13 LH2, 687 MeV LD2, 687 MeV LH2, 362 MeV LD2, 362MeV Electron Yields (Hz/uA) 90 C 120 C 70 C 45 C Quasi Elastic Inelastic Elastic Inelastic Elastic Inelastic

14 LH2 362 LD2 362 IHWP IN OUT Elastic Electron Asymmetries PRELIMINARY RAW BLINDED

15 LH2 687 PRELIMINARY RAW BLINDED LD2 687 Elastic Electron Asymmetries

16 G 0 Backward Angle : Beam Specifications Beam ParameterAchieved (IN-OUT)/2“Specs” Charge asymmetry / ppm x position difference -19 +/- 340 nm y position difference -17 +/- 240 nm x angle difference / nrad y angle difference 0.0 +/ nrad Energy difference 2.5 +/ eV Beam halo (out 6 mm)< 0.3 x Beam parameters specifications were set to assure: Helicity correlated beam properties false asymmetry Correction : linear regression All Møller measurements during run) P= / (stat) +/-1.38 (sys) %

17 LD2 687 Field Scan (Octant 1) Ramped SMS from 1900A to 4900A Cell by cell fits made using a Gaussian (blue) for low momentum “background” and 2 Gaussians (with shared width) (red) for the elastic peak. A constant (lt. green) is also added to the fit to remove any field independent rate. Random subtracted Electron Yield vs SMS Current (2 sample cells) Cell by Cell dilutions extracted as:

18 Cerenkov Efficiencies Electron detection efficiency Determined using three different techniques Does not change asymmetry Four Cerenkov Detectors CED/FPD Coincidence electron pion

19 Measured Cerenkov Efficiencies

20 EM Radiative Effects Follow process of Tsai [SLAC=PUB-848] Compute asymmetry [ ] based on the kinematics at the reaction vertex after the radiative emission. This is compared to Born asymmetry calculation [ ] with Net effect is to reduce the energy of the scattered electron so elastic peak now has a low energy tail due to events which have “radiated” out of the peak.

21 LH2 687 RC Yield Simulations With RC Effects Without RC Effects Without RC Effects

22 Expected G0 Results

23 G0: N →  Measurement: Parity-violating asymmetry of electrons scattered inelastically A N Δ gives direct access to G A N Δ Directly measure the axial (intrinsic spin) response during N →Δ + transition First measurement in neutral current process Elastic Region: G0 Inelastic Region: N   BLINDED Asymmetry (ppm) vs Octant (LH 687MeV) Raw Asymmetry (averaged over inelastic region) IN OUT Octant Asymmetry (ppm) Data: Inelastic electrons Scattered from both LH2 and LD2, each at two energies (362MeV & 687MeV)

24 Transverse Polarization 2  -Exchange When a transversely polarized electron scatters from a proton, the scattering rate has an azimuthal dependence arising from two-photon exchange contributions This beam normal single spin asymmetry is of the same order of magnitude as the PV asymmetry; it can introduce a background asymmetry if the beam polarization has a transverse component

25 G MeV LH 2 Transverse Asymmetry BLINDED ---no corrections for helicity correlated beam parameters, deadtime, … Octant

26 LH2 687MeV Transverse Polarization-- Luminosity Detectors A long = ppm A trans = ppm sin  long = A long /A trans  long =  +/  longitudinal transverse LH  +/- 0.1  LD2 687 (fall)  +/  LD2 687 (3/1-9) 5.87  +/  LD2 687 (3/9-) 1.96  +/  LH  +/ 

27 Parity Violating Photoproduction of  - on the Delta Resonance PV asymmetry for pion photoproduction may be as large as  5 ppm (based on hyperon model) with several ppm statistical uncertainty Can access this from inclusive  - asymmetries at  kinematics. (Zhu et al, Phys. Rev. Lett.) Electroweak radiative corrections generate a non- zero asymmetry at Q 2 = 0. (Siegert’s theorem)

28 Pion Yield Measurement Rate corrections : f r ~15% (2/3 deadtime, 1/3 random coincidences) Longitudinal A  is small Analysis well underway FPD CED LD2, 687 MeV Pion Yields Hz/uA

29 Summary G0 Forward Angle and G0 Backward Angle Measurement allows model independent determination of Analysis underway; good progress Above specification beam and well-understood detector performance Other Backward Angle Physics Topics Analysis well underway

30

31 Nucleon’s e-N Axial Form Factor Z 0 has axial as well as vector couplings : neutral weak axial form factor, determined from neutron  decay and neutrino scattering : nucleon’s anapole moment-- PV electromagnetic moment : electroweak radiative corrections to e-N scattering

32 Electron Measured-Corrected Yield LD2-362 MeV (Hz/  A) AA Measured Corrected No Dead Time AA DT + Contam LD2-362 MeV (Hz/  A) Corrected Measured Electron Measured-Corrected Yield CED-FPD Coincidence Deadtime Cerenkov Trigger CFD MT Coincidence pion electron CED FPD -coincidence electronics dead time -contamination of e and pi matrices due to Cerenkov randoms and dead time -random coincidences.

33 Quartz PMTs Aerogel Cerenkov counters for  /e separation (LD2) boroscilicatequartz Note difference in vertical scales –4 - 5 in. PMTs each –boroscilicate glass very sensitive to neutrons –replace with quartz –beam current for LD2 limited by high real  rates  (neutron) Ch. accidentals quartz tubes allow increase in effective electron efficiency by ~x2 –current limits 20  A (35  A) at 687 (362) MeV final tubes installed over Xmas break

34 Rate Corrections LH2, 687 MeV LH2, 362 MeV LH2, 687 MeV, 60  A ~7% LH2, 362 MeV, 60  A ~6% LD2, 687 MeV, 20  A ~9% LD2, 362 MeV, 35  A ~13% Deadtime corrections –Simulated the complete electronics chain Deadtimes (%) Randoms corrections –LH2 randoms small –LD2 randoms significant higher pion rate –Direct (out-of-time) measurement Multihit corrections smaller –Arise because of trigger: (any CED) and (any FPD) Assessing uncertainty contribution in asymmetries

35 Lumin osity Monitors Asymmetries: Lumi = ppm Octant 1 = ppm Asymmetry widths: Lumi < 300 ppm Octant 1 = ppm beam Forward location, close to beam Small asymmetry, High rates

36 Upsweep at higher currents indicative of target boiling Boiling calculation: Assume no boiling at 19 uA W expected I beam = W 19uA * √19uA / √I beam W 2 boil = W 2 measure – W 2 expected W boil at 57uA => ppm for the 8 lumi’s LD2 Asymmetry Widths

37 Asymmetry Widths & Target Density Fluctuations for LH2 at 687MeV Asymmetries: Single Lumi = ppm Octant 1 = ppm Asymmetry widths: Single Lumi < 300 ppm Octant 1 = ppm W boil at 57 uA LH2 => ppm LD2 => ppm W boil at 57 uA LH2 => ppm LD2 => ppm

38 Transverse Asymmetry 2 photon exchange Inclusion of the real part of the 2 γ exchange in the cross section may account for the difference between measurements of G E /G M from unpolarized cross section and polarization transfer measurements It also tests the theoretical framework that calculates the contribution of γ Z and W + W - box diagrams that are important corrections to precision electroweak measurements