The Second Generation HAPPEX Experiments

17 June 2004 HAPPEX-II Experiments K. Paschke

The Second Generation HAPPEX Experiments

Kent Paschkefor

The HAPPEX Collaboration

California State University, Los Angeles - Syracuse University -DSM/DAPNIA/SPhN CEA Saclay - Thomas Jefferson National Accelerator

Facility - INFN, Rome - INFN, Bari - Harvard - Indiana University -University of Virginia - University of Massachusetts - Florida

International University - University of New Hampshire - MassachusettsInstitute of Technology - College of William and Mary in Virginia

(Thanks to Rich Holmes for .ppt from PAVI ’04)


● Spin

● Longitudinal momentum

● Strange mass

● Strange vector FF

Strangeness in the Nucleon


PVES

Leading contribution to parity-violating scattering asymmetry is from interference of EM and weak exchange amplitudes


PVES and strange form factorsFor hydrogen:

=> Measurement of APV yields linear combination of Gs

E, Gs

M

Related at Q2=0 to s,

s.


The HAPPEX Experiments

● A look back: HAPPEX, ep Q2=0.5 (GeV/c)2

● HAPPEX-H: ep, Q2=0.1 (GeV/c)2

● HAPPEX-He: e 4He, Q2=0.1 (GeV/c)2

● PREX: ePb, Q2=0.01 (GeV/c)2


A look back: HAPPEX

Hall A Proton Parity Experiment (E91-010)

ep at Q2=0.5 (GeV/c)2, 12.3 degrees

GsE + 0.392Gs

M = 0.014 ± 0.020 (exp) ± 0.010 (FF)

Phys. Rev. Lett. 82:1096-1100,1999;Phys. Lett. B509:211-216,2001;

Detailed paper accepted for PRC - arXiv nucl-ex/0402004


HAPPEX results

APV = -14.92 ppm ± 0.98 (stat) ppm ± 0.56 (syst) ppm


The next step

● Increase sensitivity● Choose different Q2 range

● Separate GsE , Gs

M

=> Two new experiments at smaller Q2


HAPPEX-H (JLAB E99-115)

● Polarized e- on 1H● Q2 = 0.1 (GeV/c)2, LAB = 6● APV = 1.6 ppm

● A = 5% (stat) + 2.5% (syst)

=> 80 ppb (stat) + 40 ppb (syst)


Helium and strange form factors

For helium:

=> APV sensitive only to GsE


HAPPEX-He (JLAB E00-114)

● Polarized e- on 4He● Q2 = 0.1 (GeV/c)2, LAB=6● APV = 8.4 ppm

● A = 2.2% (stat) + 2.1% (syst)

=> .18 ppm (stat) + .18 ppm (syst)


Experimental impact


Experimental impact


Tough new measurement: How do you do it?

•Small forward angle => new Septum magnets

•High statistical precision => Thick new targets, high current, rad-hard integrating detectors, improved DAQ, new photocathode

•High relative accuracy => improved polarimetry, new integrating focal plane profile scanner

•High systematic accuracy => improved polarized source, close attention to beam optics, lumi monitor.


Overview

Hall A

CEBAF

InjectorPolarized source


Septum magnets

● Minimum scattering angle 12.5° -> 6.0°● Installed and commissioned 2003-2004


100 x 600 mm

12 meter dispersion sweeps away inelastic events

‘L’ geometry design Hydrogen geometryCerenkov detectors overlap the elastic

line above the focal plane :

HAPPEX-H Detector geometry

He detector = (H detector) / 2 !


Brass-quartz stack

Light guide Filter box

PMT

Assembly at Saclay


Luminosity monitors● Target boiling● Beam parameters● Electronics noise

Tested to ~200 ppm resolution,expecting about ~100 ppm

Current A

pp

m


Target cells"Beer can" – 15 cm, worked well for HAPPEX-I

New "race track" design – 20 cm, boiling untested


Luminosity Fluctuations● Beer can cells good enough, as measured in HAPPEX● “Racetrack” cells never tried (Transverse flow good,

curvature of window bad)● Cold (6.6K), dense (230 psi) cryogenic 4He gas target…

“boils”!

Widths go down with increasing raster…

… and UP with increasingcurrent!


● Møller: Main uncertainty is foil polarization; p/p = 3 – 3.4% expected

● Compton: Added electron recoil detector since HAPPEX-I; p/p = 2% in ~1 hour seen – 1.3% probably achievable at 3 GeV

Polarimetry

● Big challenge for PREX – high current, low energy. Plan to upgrade Compton with green laser => 1% in ~16 hours

Compton GEANT4 monte carlo


Main Challenge: extreme requirements on halo/tail are necessary to reduce background

100 Hz / A at 5mm from the beam centroid (10-10)!

Compton Polarimeter

Photon detector Electron detector


Profile scannersQ2 measured at low current with VDCs...

... verified and monitored at high current with scanners.


Beam Requirements

Property Nominal Jitter (30 Hz) Hel corr (run)Energy 3.2 GeV < 80 ppm < 13 ppbCurrent 100 A < 1000 ppm < 600 ppbPosition 0 < 12 m < 2 nmAngle 0 < 12 rad < 2 nradHalo <100 Hz/A @ 5 mm

Specifications driven by sensitivities and estimates of quality of corrections.

CASA and EGG have worked closely with HAPPEX to meet these requirements


Polarized source● Pockels cell voltage (PITA) used to tune AQ, x● Intensity Attenuator (IA)● PZT mirror● IHWP for slow helicity reversal● RHWP for control of position/intensity asymmetries● Superlattice photocathode: >80% polarization, 100 A


Controlling Position Differences

Identify and control sources of position differences● Intrinsic birefringence gradient in the Pockels cell● Steering from distortions due to piezo-electric

deformation of the Pockels cell ● Analyzing power gradients

Lisa Kaufman, Brian Humensky, Gordon Cates, Ryan Snyder, Kent

Paschke, and the EGG.


ITS Laser Room

Studies in the laser room conducted over the past year have been crucial in developing our understanding of the sources of position differences.

● Characterization of Pockels cells on parameters relevant to sources of position differences

● Identification and characterization of the history effect in optical properties of the Pockels cells

● Development of alignment techniques to reduce effects from Pockels cell steering


Injector Beam Studies● Electron beam studies allows studies of

effects from the photocathode, as well as other idiosyncratic elements such as vacuum windows.

● Set points for source elements (PC voltages, rotating waveplate angle) can only be determined from beam data.

● Injector transmission becomes an important issue in getting the well-tuned beam to the Hall.


Phase Trombone• Goal: vary betatron phase while preserving the shape and orientation of the phase space ellipse

• implemented with eight existing quads at the beginning of the Hall A arc• Allows for independent betatron phase control in horizontal and vertical planes

• Uses:• Allows one to trade off position and angle differences• Periodic phase changes can be used to randomize or reverse the sign of position differences

Phase TromboneSetpoint (x , y)

x (m)0.3 m

y (m)0.3 m

x(rad)0.01 rad

y (rad)0.02 rad

(0o,0o) 2.9 2.0 -0.08 -0.19

(30o,0o) 2.7 1.2 -0.07 -0.22

(-30o,0o) 2.8 3.2 -0.07 -0.16

(30o,30o) 1.0 1.2 -0.12 -0.21


HAPPEX Has Started

● Installation started June 4, optics commissioning done June 9

● Luminosity limited by septum heating, improvement possible

● Progress on beam conditions (Helicity-correlated, Compton halo), spectrometers, detectors, target

The Second Generation HAPPEX Experiments

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