A First Analysis of the 3He Test Beamtime

CB Meeting Mainz, March 9, 2010

Patricia Aguar BartoloméAlexander 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 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 M1 – Ethr ~ 20 MeV TAPS 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

• APROX. 110 HOURS OF BEAMTIME

• NO EVENT SELECTION • ONLY TRUE - RANDOM SUBTRACTION for the tagger TDCs

A. Fix Model for 3He X

MAIN LIMITING FACTORS

1) Target relaxation time ≈ 4 h.

2) Large beam halo induced huge background in our detector.

AP

Beam Halo ProblemE= 855 MeV E = 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

AP

ONLY TAPS (WITHOUT CB) TRIGGER

Polarised Total Inclusive Cross Section

only for TAPS M1 Events - 855 MeV

• Count Rate TAPS trigger ≈ CB trigger.

• Error bars as large as before.

AP

Polarised Total Inclusive Cross Section for CB+Cuts -

855 MeV

• CB events rejected when all clusters satisfied the conditions:

• Ecluster < 50 MeV. AND.

• <300 or >1400

(NEGLIGIBLE LOSS OF EFFICIENCY )

ERROR BARS DECREASED BY 1.5

ONLY CB or CB.AND.TAPS TRIGGERS

AP

Monte Carlo Detection & Geometrical Efficiency Simulation – 525 MeV

Efficiency for p ≈ 90 - 95 % Efficiency for n0 ≈ 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

• 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 3He X

ERROR BARS SIGNIFICANTLY REDUCED

ONLY CB or CB.AND.TAPS TRIGGERS

AP

Polarised Total Inclusive Cross Section for CB+Cuts -

525 MeV

• CB events rejected when all clusters satisfied the conditions:

• Ecluster < 50 MeV. AND.

• <300 or >1400

(NEGLIGIBLE LOSS OF EFFICIENCY )

No efficiency correction applied (points will slightly decrease, especially below 300 MeV)

ONLY CB or CB.AND.TAPS TRIGGERS

AP

A. Fix Model for 3He X

Next Analysis Steps Improved CB energy cuts.

Analysis of TAPS (without CB) events. Separation between no 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 3He 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 (n0, no, 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).