Strange Quarks in the Proton Jeff Martin University of Winnipeg

The Proton Charge = e Spin = ½  = 2.79  N Mass = 938 MeV/c 2

The Proton u u d Naïve quark content: u+2/3e d-1/3e e Also does pretty well on the magnetic moment.

The Proton u u d Quarks are held together in a bag by gluons (carriers of the strong interaction) g g

The Proton u u d s s Zooming in, we begin to see sea quarks. In particular, STRANGE quarks Proton  uud + g … The ‘‘Sea’’

courtesy Dave Armstrong College of W&M.

Momentum: Spin: Mass: Charge and current: What role do Strange Quarks Play in the Properties of the Proton? Main goal of this program To determine the vector contribution of the strange quark sea (ss) to the electromagnetic properties of the nucleon ("strange form factors").

Elastic Electron Scattering Scattering rate depends on two “form factors” G E (Q 2 ), G M (Q 2 ). At small Q 2, form factors are Fourier transforms of spatial distributions of charge and magnetization densities in the proton.  e p k’ k q = k – k’ “4-momentum transfer” A useful variable:

The charge and magnetization are carried by quarks We can do the same experiment for the neutron (udd) Relationship to Quarks isospin symmetry

The Extra Handle: Z 0 scattering  e p SpeciesChargeWeak Charge u d s

vs So how can we see the effects of the weak force? The weak force doesn’t look the same when viewed in the mirror! I.e. it violates parity. To measure the weak force, do the same experiment with the helicity reversed.  e p  e p  ~ 1  ~ Parity Violation

Parity-Violating Elastic Scattering where: Scatter polarized electrons from unpolarized protons, and measure the asymmetry prevalent at forward angles prevalent at backward angles Note: asymmetry is of order 10 -6, measure in ppm

The Nucleon's e-N Axial Form Factor G A e Z 0 has axial, as well as vector couplings  we measure axial FF too G A Z : neutral weak axial form factor, determined from neutron  decay and neutrino scattering F A : nucleon’s anapole moment – parity-violating electromagnetic moment R e : electroweak radiative corrections to e-N scattering + +

Nucleon weak form factors  Extraction of, and A PV : linear combination of the Form Factors : Separation of strange contributions : At fixed Q 2 and different kinematics ,  and  depends on kinematics Q 2 = 0.5 (GeV/c) 2 A 0 (ppm)  (ppm)  (ppm)  (ppm) AFAF A B (LH 2 ) A B (LD 2 ) ) Proton and small  e  2) Proton and large  e  3) Deuteron and large  e 

Survey of Experiments Experiment e()e() Target Q 2 [(GeV/c) 2 ] Sensitivity SAMPLE140p0.1G M s, G A e (Bates Lab)140d0.1GAeGAe 140d0.04GAeGAe HAPPEX12p0.48G E s, G M s (JLab)6*p0.1G E s, G M s 6* 4 He0.1G E s, G M s A435p0.23G E s, G M s (MAMI)35p0.1G E s, G M s 145*p0.23G M s, G A e 145*d0.23GAeGAe G0G0 6-20p0.1 – 1.0G E s, G M s (JLab)110*p0.3, 0.5, 0.8G M s, G A e 110*d0.3, 0.5, 0.8GAeGAe * = data taking not yet complete

Forward-Angle Data HAPPEX constraints on G E s G M s - Aniol et al Phys. Rev. C69, (2004) data

Forward-Angle Data A4 constraints on G E s G M s - Maas et al, Phys. Rev. Lett. 93, (2004). Noted that overlap tends to give G E s > 0. Same for HAPPEX preliminary Q 2 = 0.1

Backward-Angle Data SAMPLE e-d scattering at backward angles New results suggest G A e is understood (at these energies) T. Ito et al (including JWM), Phys. Rev. Lett. 92, (2004) theory for G A e new data re-evaluated

Backward-Angle Data SAMPLE results for G M s and G A e for Q 2 = 0.1 (GeV/c) 2 Suggests G M s > 0 and therefore  s > 0 D. T. Spayde et al, Phys. Lett. B 583, 79 (2004) Zhu et al (theory) D 2 Experiment H 2 Experiment  S = 0.37 ± 0.36 Recall  p = 2.79,  n = -1.90

Experiment General requirements: High statistics ( ) events –high current, high polarization beam –large acceptance –high rate capability Systematics –control of helicity-correlated beam properties –background characterization and rejection

Experiment Caltech Carnegie-Mellon William+Mary Grinnell College Hampton IPN-Orsay ISN-Grenoble Kentucky LaTech NMSU JLab TRIUMF UConn U Illinois U Manitoba U Maryland U Mass UNBC VPI Yerevan and the University of Winnipeg

Forward-Angle Measurements Elastic proton detection toroidal focusing spectrometer Time-of-flight distinguishes pions and protons

G 0 beam monitoring girder superconducting magnet (SMS) detectors (Ferris wheel) cryogenic supply target service module G 0 Forward-Angle Configuration Beam

Target 20 cm LH 2 cell High circulation rate to minimize target density fluctuations 320 W heat load from beam Helium cell upstream of LH 2 cell to minimize sensitivity to helicity- correlated beam motion.

Focal-Plane Detectors “FPD’s” 16 Pairs of scintillator detectors shaped to the focal plane and acceptance of the spectrometer 16 “Detectors” or “Rings” Divided into 8 octants - 4 North American - 4 French same with the electronics

Experimental Sequence 1.Collide a bunch of electrons with the target. Start your stopwatch. 2.Wait. 3.If the detector was hit, stop your stopwatch. Add the result to a histogram. 4.Take another bunch. 5.At some point, record and clear the histogram. 6.Reverse electron helicity and repeat. pions inelastic protons elastic protons Det 8

The Electron Beam target

The Electron Beam target these experiments are the same to 1 part in 10 6

The Electron Beam target

The Electron Beam target symmetry in detector to minimize effect must have excellent control of all “helicity- correlated” beam properties use feedback to maintain these properties

Beam Requirements E = 3 GeV, I = 40  A electron beam, P = 80% polarization 32 ns between beam pulses for TOF reconstruction 31.5 MHz production laser tests spring < A meas < 50 ppm,  A/A) STAT  5% Beam PropertyNominal valuehelicity corr. in 30 days Energy3 GeV< 2.5 x Current40  A< 1 ppm Position< 20 nm Angle< 2 nrad All have been achieved Also use feedback on position and charge

Helicity - Correlated Beam Properties - Sensitivity O2 O4 O6 O8 O1 O5 O7 O Symmetry of apparatus  reduces sensitivity to some helicity-correlated beam properties Example: Sensitivity to vertical beam motion (y direction) Measured yield slopes (1/Y) dY/dy (%/mm)

Systematics: From raw asymmetry to physics results Form raw measured asymmetry from the detector yields: Correct for false asymmetries from helicity-correlated beam properties: Correct for background and its asymmetry: helicity-correlated beam properties deadtime corrections background dilution factor correction Correct for beam polarization and radiative corrections: electron beam polarization electromagnetic radiative corrections Correct for measured Q 2 and EM form factors: determination electromagnetic form factors

The Largest Systematic Effects Beam leakage Backgrounds

- leakage of beam from Hall A, B lasers into Hall C was observed with large asymmetry (40 nA leakage, 40 µA main beam; leakage asymmetry ~340 ppm) - Different beam time structure (499 MHz for Hall A,B beams and 32 MHz for Hall C)  Induced false asymmetry which depends on the TOF and the beam intensity Systematic uncertainty due leakage beam: ~ 0.1 ppm Beam Leakage Asymmetries were corrected for this effect  measured effect using “signal-free” region of TOF spectra;  performed studies with other lasers turned off, high-rate luminosity monitors  cross-checks with low-rate runs.

Backgrounds January 2003: Yield ~ % depending on detector Extensive program undertaken to reduce and characterize backgrounds: Detailed Monte Carlo model developed. indicated Al target windows partly responsible. Thinning of target exit window from 11 to 3 mil. reduced background to 8 – 16%. Al and empty targets asymmetry data. Extrapolation/Data/MC methods. Most work now: quantifying systematic uncertainty.

Estimate of background yield under the elastic peak is performed by using - data with full and empty (gas H 2 ) targets, different pressures - data with dummy entrance and exit windows (Al) - Data with W radiator and dummy windows (electro/photo production)  Unfold backgrounds from target windows and inelastic LH 2 processes pions elastic protons inelastics Fractional background in elastic cut Backgrounds

Yield asymmetry Detector 8 For smaller detectors, yields and asymmetries are measured on each side of the elastic peak and smooth interpolation is reasonable. For larger detectors, background asymmetry is large and varies across the elastic peak. Small increase of uncertainty in elastic asymmetry due to background for small detectors. Yield asymmetry Detector 13 Effect of backgrounds will likely dominate systematic uncertainty for larger detectors. Background Correction For preliminary result, used linear interpolation

Recent Progress Towards Improved Background Correction Constrain yield with Monte Carlo motivated fit. Constrain asymmetry by enforcing constant elastic asymmetry. ppm Yield – Ring 8Asymmetry – Ring 8

Forward angle asymmetries - full statistics - detectors 13 to 15 not shown. - preliminary background corr. - 25% blinding factor applied Asymmetry (ppm) Increasing Q 2 Detector number Preliminary Results First estimates of systematic uncertainties : - Deadtime corr2%  - Beam parameters corr0.01 ppm  - Leakage beam corr0.10 ppm  - Beam Polarization2%  - Background corrin progress - Q 2 determination1%  - Rad. Corr., EM FF’ssmall BLINDING FACTOR APPLIED

Backward-Angle Measurements Electron detection Add Cryostat Exit Detectors (“CED’s”) to define electron trajectory Add aerogel Čerenkov counter to reject  - Measurements on H and D to separate G M s, G A e E beam (MeV) Q 2 (GeV 2 ) beam target magnet FPD #1 FPD #16 CED #9 CED #1 Čerenkov inelastic e - or photo  - elastic e -

Additional Physics from Backward-Angle Runs Axial Form Factor G A e (Q 2 ) –related to electroweak radiative corrections. Two photon exchange –related to electromagnetic radiative corrections for EM form factor determination (also fwd angle). Parity-violation in the N  transition –Electro-excitation: Axial Form Factor –Photo-excitation: Mixing with Negative-Parity Resonances, electroweak radiative corrections.

Canadian Contributions to Backward-Angle Expt. Čerenkov counters built by: –U Manitoba, TRIUMF, UNBC –Grenoble (France) Čerenkov/CED support structure (TRIUMF) and CED construction. Electronics for Canadian Čerenkov counters: –Request to NSERC by U Winnipeg Acquisition and Analysis of Data –Pion asymmetries (U Winnipeg) First run on Dec. 3, 2005

Summary Forward angle production run complete. Analysis of forward angle data nearing completion. Preparations for back-angle run underway: –turn around complete. –additional scintillators, Čerenkov counters built. –new electronics being acquired and tested. Back angle running will occur in 2005 – 2007.

By 2008: Separated Form Factors Q 2 dependence of strange magnetic and electric form factors below Q 2 = 1 GeV 2

G 0 Beam G 0 beam requires unusual time structure: 31 MHz (32 nsec between pulses) (1/16 of usual CEBAF time structure of 499 MHz (2 nsec between pulses) Required new Ti:Sapphire laser in polarized electron gun Higher charge per bunch  space charge effects complicated beam transport in injector (challenging beam optics problem) Beam with most desired properties delivered for Jan Beam current 40  A Beam fluctuations at (30 Hz/4) ~  X,  Y < 20  m  I/I < 2000 ppm CEBAF polarized injector CEBAF polarized injector laser table

By 2008: Axial Form Factor From deuterium backward-angle measurement requires particle identification (Čerenkov)

Background Measurement Measurement of pion backgrounds in Hall C using Short-Orbit Spectrometer Beam energy 824 MeV angle 136  Good agreement with predictions

FPD/CED separation and pion backgrounds E beam (MeV)  /e ratio HD For deuterium, pion rejection required Elastics separated from inelastics, pions in FPD/CED space aerogel Čerenkov detector (Caltech, TRIUMF, Grenoble)

Requirements

Čerenkov Goals: –100:1 pion rejection at p = 350 MeV/c. –uniform efficiency, covering fiducial area of CED’s. –fast trigger, small dead time, fit in 32 ns beam structure. –fit into G 0.

Diffusely reflective Čerenkov detector concept Aerogel radiator n=1.03 white paper lining Four 5 inch photomultiplier tubes diffusion box  > 0.97 aerogel PMT

Design Considerations Light yield Pion rejection limitations –  -rays produced from material in front of box (e.g. CED’s) MeV –scintillation yield in air Timing/Interface to G 0 electronics PMT environment (radiation and magnetic field) Motivates prototype

Caltech Čerenkov Prototype n=1.03 aerogel white box four 5” PMT’s

Prototype Beam Tests performed in M11 beamline at TRIUMF use combination of scint, RF time-of-flight to separate  /  /e from 120 to 400 MeV/c beam

Electron Detection ADC sum spectrum for incident electrons, =5.5 Only 10% loss when ADC gate narrowed to 30 ns

Electron Efficiency Electron detection efficiency for various p.e. thresholds

Electron Efficiency p.e. yield and efficiency for 3.0 p.e. cut, as a function of distance from PMT’s average efficiency was 88 % for 3.0 p.e. threshold cut

Pion Rejection 100:1 achieved for 350 MeV/c pions for 3.0 p.e. threshold investigation of different trigger schemes (sum vs multiplicity)

New Geant4 simulation Model parameters PMT q.e. data Millipore diffuse reflective 96% scattering and absorption in aerogel from HERMES data No tuning electronsG 0 pions p (MeV/c) ee  PRF  G 0 pions Simulation results (3 p.e. sum trigger)

Prototype Summary = 5.5 p.e. efficiency = 88% pion rejection 140:1 expected for G 0 highest energy running timing faster than 10 ns trigger, live in 30 ns agreement with new Geant4 simulation

Čerenkov Progress Caltech: prototyping complete TRIUMF: –funding approved spring 2001 –prototyping with more detailed tests –design of support structure Grenoble: –funding approved April 2002 –aerogel counter design, LITRANI model –prototyping, triggering studies –testing magnetic shield design –electronics design

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Axial Transition Form Factor Q 2 (GeV 2 )  (1) = 2(1  sin 2  W ) = 1  (2) = non-resonant contrib. (small)  (3) = 2(1  4sin 2  W ) F(Q 2,s) at tree-level: First measurement in neutral current process sensitive to hadronic radiative corrections Goal: 5% determination of M A parameter (governs dipole behavior)

New developments Including Electroweak Radiative Corr  (3) = 2(1  4sin 2  W ) F(Q 2,s) (neutral current) +  (3) (anapole) +  (3) (Siegert) “Siegert” term dominates at low Q 2, giving potentially large theory uncertainty to G A extraction Based on model, potentially large and does not vanish in the photoproduction limit

PV   Photoproduction on the  -resonance d  is a low-energy constant characterizing the PV  coupling. Interesting, because: results from “Siegert’s theorem” term in electroproduction potentially large asymmetry, may be related to SU(3)-violating effects seen in hyperon decays Zhu et al, PRL 87 (2001)

Two Puzzles in Hyperon Decay 2. Hyperon weak radiative decays, e.g. Photon asymmetry   should be zero under SU(3) symmetry (Hara’s theorem) Chiral corrections should be of order 15% 1. Hyperon non-leptonic decays, e.g. S-wave and P-wave cannot be fit simultaneously in simple chiral fit: “S-wave/P-wave Puzzle” Only one model solves both problems and fits data…

Resonance Saturation Model Borasoy and Holstein PRD 59 (1999) introduce mixing with intermediate negative-parity resonances can be tested in  S=0 sector using PV  photoproduction d   (multiples of g  )Model parameters 1“natural” scale 10-25resonance saturation V ud /V us, neutral current

Resonance Saturation Model Resonances included: –nucleon octet (  =+) –S11(1535) octet (  =-) –  (1405) singlet (  =+) –Roper N(1440) Octet (  =+) ModeExptFit PV S-wave  p+   +  n+   -  n+   -  +  PC P-wave  p+   +  n+   -  n+   -  +  HWRD  +  p+    -  - +  1.0   0  0 +   *  0  +  0.43   n+  * didn’t include  (1405), could be fixed

Application to  Photoproduction Estimate amplitudes for transition based on fit of Bosaroy and Holstein Consider 1/2 - and 3/2 - intermediate states Decays of 1/2 - resonances to Delta not observed… exclude Decays R(3/2 - )  p  have been observed, use measured widths uncertainty in phase gives |d  | = g   S=0 amplitudes generally enhanced by V ud /V us and neutral current contributions factor enhancement  |d  | = g 

 Photoproduction Possibilities for G 0 G 0 must work hard to reject  - from photoproduction at backward angles for deuterium quasi-elastic measurement. Instead, use for physics! –First Back-Angle Physics run is at 799 MeV -- too high for . –Future Run at 424 MeV -- Very close to  ! –Two cases explored: 1. normal running at 424 MeV 2. Additional running with spectrometer tuned to , possibly with additional radiator to boost photoproduction rate.

Calculation of Pion Rate Contributions of virtual photons and Bremsstrahlung photons Pion photoproduction cross-section from MAID2000 (Drechsel et al) –calculation tested at 824 MeV,   =135  at test run in Hall C, Sept G 0 Monte Carlo Code in back-angle mode

Experimental Asymmetry Assumes non-resonant background has zero asymmetry Sources of background –Non-resonant –Multi-pion production f  estimated to be max 0.6, due to non-resonant background

Tuning for  scan of spectrometer magnet current rate and sensitivity to  peak at same magnet current, suggests method of tuning online

Backgrounds Multi-pion production –from kinematical arguments, must be small at 424 MeV Non-resonant background –not considered by Zhu et al –estimated to give maximum f  of 0.6 –asymmetry likely of order h  (< )

Parasitic to G 0 at 424 MeV Beam Energy424 MeV Beam Current40  A Beam Polarization80% Radiator thickness1/2 target+virtual photons 337 MeV =  P e 0.92  80% = 74% Target20 cm LD2 DetectorG 0 Backward Angle Magnet CurrentNominal Pion Rate6.8 MHz A   (ZMHRM)1.3  A meas  (ZMHRM)5.1   A meas  for 700 hours2.4  = 0.47 A meas  (ZMHRM)  A   for 700 hours6.1  = 0.47 A   (ZMHRM) dd 12 g  Requires alterations to planned electronics for backward angle (currently set to discriminate all pions) 47% measurement (12 g  )

Delta Run at 424 MeV Beam Energy424 MeV Beam Current40  A Beam Polarization80% Radiator thickness1/2 target+virtual photons 290 MeV =  P e 0.84  80% = 67% Target20 cm LD2 DetectorG 0 Backward Angle Magnet CurrentTuned to  resonance Pion Rate22 MHz A   (ZMHRM)1.3  A meas  (ZMHRM)4.8   A meas  for 700 hours1.3  = 0.27 A meas  (ZMHRM)  A   for 700 hours3.5  = 0.27 A   (ZMHRM) dd 7 g  Requires additional running 27% measurement (7 g  )

Addition of Radiator Place upstream of helium cell on target take 10% radiator boosts rate to 103 MHz 12% measurement (3 g  ) Multiple scattering 0.5  spot radius at He cell = r He /5 spot radius at dump = 0.5 ft

Conclusions d   (multiples of g  )Model parameters 1“natural” scale 10-25resonance saturation V ud /V us, neutral current New model suggests large asymmetry in  photoproduction “Best value” is 25 g , “reasonable range” is g  Possible parasitic measurement at 424 MeV would yield:  d  = 12 g  Better measurement needs  tune, additional radiator such simple improvements could give:  d  = 3 g 

theory ranges possibilities for G 0