A First Analysis of the 3 He Test Beamtime CB Meeting Mainz, March 9, 2010 Patricia Aguar Bartolomé Alexander Mushkarenkov

Physics Motivations  Experimental verification of the GDH sum rule on the neutron (measurement of a total inclusive helicity dependent cross section)  Helicity dependent cross section on some (N  )  channels  Polarised 3 He is the best substitute of a polarised free neutron  =>First double polarisation experiment on 3 He !! in the test, measured photon range was [ MeV]

Setup  Cell transport system worked fine.  Minimal loss of polarisation during transport.  Setup for the cell polarisation measurement worked well.

Experimental Conditions  2 different MAMI energies (855, 525 MeV) with circularly polarised (≈ 80%) electrons  3 He gas target 20 cm long at ~ 4 bar  Open hadronic trigger (inclusive measurement) (CB.or.TAPS): CB M1 – E thr ~ 20 MeV TAPS M1 – E thr ~ 40 MeV with Cherenkov on-line Veto

Beamtime in July 2009 Approx. 380 hours of beamtime E e- = 855MeV => 110 h of data on tape E e- = 525MeV => 115 h of data on tape Tagg. eff. => 16 h Exchange of the cell two times per day ( minutes) More than 1/3 of the beam time “lost” for several different problems (Acqu, MAMI, …) 241 h

Polarised Total Inclusive Cross Section for 855 MeV Runs APROX. 110 HOURS OF BEAMTIME NO EVENT SELECTION ONLY TRUE - RANDOM SUBTRACTION for the tagger TDCs A. Fix Model for  3 He  X MAIN LIMITING FACTORS 1) Target relaxation time ≈ 4 h. 2) Large beam halo induced huge background in our detector.

Beam Halo Problem E  = 855 MeVE  = 525 MeV LESS BACKGROUND FROM BEAM HALO AT 525 MeV

Polarised Total Inclusive Cross Section only for CB Events – 855 MeV ONLY CB or CB.AND.TAPS TRIGGERS

ONLY TAPS (WITHOUT CB) TRIGGER Polarised Total Inclusive Cross Section only for TAPS M1 Events MeV Count Rate TAPS trigger ≈ CB trigger. Error bars as large as before.

Polarised Total Inclusive Cross Section for CB+Cuts MeV CB events rejected when all clusters satisfied the conditions: E cluster < 50 MeV. AND.  (NEGLIGIBLE LOSS OF EFFICIENCY ) ERROR BARS DECREASED BY 1.5 ONLY CB or CB.AND.TAPS TRIGGERS

Monte Carlo Detection & Geometrical Efficiency Simulation – 525 MeV Efficiency for p   ≈ %Efficiency for n  0 ≈ 100 % SIMULATED REACTIONS ON THE FREE NEUTRON (quasi-free generation in GEANT4 did not work) In a first approximation, TAPS (without CB) events can be neglected for the evaluation of a total inclusive cross section (small correction factor)

Polarised Total Inclusive Cross Section for CB MeV Runs APROX. 110 HOURS OF BEAMTIME NO EVENT SELECTION ONLY TRUE - RANDOM SUBTRACTION for the tagger TDCs LONGER TARGET RELAXATION TIME (10-15 hours) REDUCED BEAM HALO A. Fix Model for  3 He  X ERROR BARS SIGNIFICANTLY REDUCED ONLY CB or CB.AND.TAPS TRIGGERS

Polarised Total Inclusive Cross Section for CB+Cuts MeV CB events rejected when all clusters satisfied the conditions: E cluster < 50 MeV. AND.  (NEGLIGIBLE LOSS OF EFFICIENCY ) No efficiency correction applied (points will slightly decrease, especially below 300 MeV) ONLY CB or CB.AND.TAPS TRIGGERS A. Fix Model for  3 He  X

Next Analysis Steps  Improved CB energy cuts.  Analysis of TAPS (without CB) events.  Separation between n  o and p  - using the MWPCs information.  Evaluation of the detection efficiency.

Conclusions  Despite several problems (ACQU, PID, Beam halo...) a good result was obtained from the test, especially at 525 MeV.  With improved beam and/or trigger conditions (lead wall in front of CB, most upstream crystals out of CB trigger, harder trigger,...) luminosity can be improved by a factor 2 to 10.  We have proved that a polarised gas 3 He target can be used for the check of the GDH sum rule on the neutron.  As a by-product, the  0 polarisability of the neutron can also be determined.  The helicity dependence of some partial channels (n  0, n  o  , n  ) can also be measured (determination of the resonance properties). Advantage with respect to the polarised deuterated butanol target: no need to detect the recoil neutron to identify these partial channels (effects due to polarised protons are small).