A First Analysis of the 3He
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 (Np)p channels
Polarised 3He is the best substitute of a polarised free
neutron
=>First double polarisation experiment on 3He !!
in the test, measured photon range was [200-800 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

3He
gas target 20 cm long at ~ 4 bar

Open hadronic trigger (inclusive measurement)
(CB.or.TAPS):
CB
TAPS
M1 – Ethr ~ 20 MeV
M1 – Ethr ~ 40 MeV
with Cherenkov on-line Veto
Beamtime in July 2009
•
Approx. 380 hours of beamtime
Ee- = 855MeV => 110 h of data on tape
Ee- = 525MeV => 115 h of data on tape
Tagg. eff.
=> 16 h
•
Exchange of the cell two times per day (30 - 60
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
   P   A
A. Fix Model for g3He
•
APROX. 110 HOURS OF
BEAMTIME
•
NO EVENT SELECTION
•
ONLY TRUE - RANDOM
SUBTRACTION for the
tagger TDCs
1) Target relaxation
time ≈ 4 h.
MAIN LIMITING FACTORS
pX
2) Large beam halo
induced huge
background
in our detector.
Beam Halo Problem
Eg = 855 MeV
Eg = 525 MeV
LESS BACKGROUND FROM BEAM HALO AT 525 MeV
Polarised Total Inclusive Cross Section
only for CB Events – 855 MeV
   P   A
ONLY CB or CB.AND.TAPS TRIGGERS
Polarised Total Inclusive Cross Section
only for TAPS M1 Events - 855 MeV
   P   A
• Count Rate
TAPS trigger ≈ CB trigger.
• Error bars as large as
before.
ONLY TAPS (WITHOUT CB) TRIGGER
Polarised Total Inclusive Cross Section
for CB+Cuts - 855 MeV
ONLY CB or CB.AND.TAPS TRIGGERS
   P   A
• CB events rejected
when all clusters
satisfied the conditions:
• Ecluster < 50 MeV. AND.
• q<300 or q>1400
(NEGLIGIBLE LOSS OF
EFFICIENCY )
ERROR BARS DECREASED BY 1.5
Monte Carlo Detection & Geometrical
Efficiency Simulation – 525 MeV
Efficiency for pp ≈ 90 - 95 %
Efficiency for np0 ≈ 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 - 525 MeV Runs
ONLY CB or CB.AND.TAPS TRIGGERS
   P   A
A. Fix Model for g3He
pX
• 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
ERROR BARS SIGNIFICANTLY REDUCED
Polarised Total Inclusive Cross Section
for CB+Cuts - 525 MeV
ONLY CB or CB.AND.TAPS TRIGGERS
   P   A
A. Fix Model for g3He
pX
• CB events rejected
when all clusters
satisfied the conditions:
• Ecluster < 50 MeV. AND.
• q<300 or q>1400
(NEGLIGIBLE LOSS OF
EFFICIENCY )
No efficiency correction applied (points will slightly decrease, especially
below 300 MeV)
Next Analysis Steps

Improved CB energy cuts.

Analysis of TAPS (without CB) events.


Separation between npo and pp- 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 3He target can be
used for the check of the GDH sum rule on the neutron.
As a by-product, the g0 polarisability of the neutron can also
be determined.
The helicity dependence of some partial channels (np0, npopo,
nh) 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).