HBD Update

1. HBD Update Itzhak Tserruya DC meeting, May 7, 2008

2. HBD status and plans • HBD west is closed inside the glove box with CF4 in recirculation mode through the LEGS gas system. • - We did not install the light cube and the large shades as originally planned. Better design will be done for the East arm. • - All 30 GEMs were tested up to 400 V prior of closing the detector. 29 passed the test, one GEM (WN1 – bottom) was found with a dead short even before applying HV. • The HBD was closed inside the glove box a few days ago, with N2 flow in recirculation mode at a flow of 2 lpm. • Initial reading of 22 ppm water, dropped to 17.4 ppm within 1h and to 11 ppm overnight. • Shift to CF4 on Tuesday morning • LEGS system is working very stably. Glove box is at 4T overpressure wrt to outside pressure, LEGS gas system is running at 1T overpressure wrt to glove box. • HV tests to start on Thursday

3. HBD background rejection in run-7

4. Data selection and electron ID • EWG nanoDST files, 331 runs, there are only 49 runs left. • Vertex cut: +/- 20 cm. • Centrality > 50%. • 1682.7M events before vertex cut • 1194.1M events after vertex • 546.2M events after centrality cut. • DC quality cut: 31 or 63. • 3σ matching cuts on Δφ and Δz to EMC. • RICH: n0>2, 2/npe<10, disp<5. • EMC: 0.6 < E/p < 1.2. • For pairing modified PairObj has been used. • Then PairAnalysis to analyse the pairs and fill N-tuple.

5. Rejection step 0: fully reconstructed conversions • This rejection is done using the well known phiV (orientation angle of the pair in the magnetic field) cut. • The phiV distribution in Run-7 looks different (it is much broader) than in Run-4 presumably due to multiple scattering in the HBD (resolution of the phiV becomes worse) and different magnetic field. • Usually a phiV cut of <0.14 || > 3.0 (depending on the magnetic field polarity) is used. • Here we use the same cuts although they remove only a fraction of the conversions because (i) if we use a stronger cut (as this plot suggests) we will significantly cut fully reconstructed Dalitz pairs which are part of our signal and (ii) the HBD removes these conversions as will be shown later.

6. Rejection step I: matching to the HBD • Step I is just 3σ cut on the matching distributions between the DC track projections and the closest cluster in the HBD. • Matching cuts to HBD: |Δφ|<0.05 rad and |ΔZ|<4.0 cm.

7. Conversions • There are three main sources of conversions: beam pipe (~0.3%), CF4 gas (~0.6%) and backplane of the HBD (~2.5%). • The soft phiV cut <0.14 || >3.0 of step 0 removes only part of the conversions. The remaining conversions are removed by the matching cuts to the HBD (step I).

8. Single vs double e response • Select pairs with m<0.15 GeV/c2 . • Divide them into two classes: V-pairs and open pairs, both are Dalitz pairs. • The 1st row plots are for the V-pairs. • The 2nd row plots are for the open pairs. • Cut on 150 ADC counts to separate singles from doubles.

9. Rejection step II: double cluster in HBD • If one of the tracks is matched to a double HBD cluster the track (pair) is removed from the list. • Definition for a double cluster: pulse height A>150 ADC counts. • This step rejects a significant fraction of the combinatorial background originating from conversions and Dalitz pairs. It works both for the fully and partially reconstructed conversions and Dalitz pairs.

10. Rejection step III: conversions in CF4 • There is still a remaining fraction of tracks that originate from conversions in the CF4 gas but create a single cluster. This happens when the photon converts approximately in the second half of the radiator, towards the GEM detectors. • Pairs with a V pattern and single cluster are rejected as fully reconstructed conversions originating in the CF4 gas.

11. Rejection step IV: close hit cut • If the HBD cluster associated to a central arm electron has a close hit that does not belong to any track, the central arm electron is rejected. • Close hit definition: - distance < 10 cm (~200 mrad) - # of pads/cluster > 1 - charge < 100 ADC

12. Results of the rejection steps Red: unlike, same event Blue: unlike, mixed events Subtracted • The 1st row is the mass spectrum after step 0 (conversions rejection with soft phiV cut). • The 2nd row is the spectrum after step I (3σ matching cuts to HBD). • The 3rd row is after step II, (rejection of conversions cutting on the doubles). • The 4th row is after step III (rejection of photon conversions in the CF4 gas) . • The 5th row is the final result after step IV (close hit cut). • Combined amplitude-size cut is not yet implemented. It was successfully used in HBD PISA simulations using HIJING events.

13. Before and after Step IV Signal Step 0

14. Let’s look at the numbers • Step 0: m>0.15 GeV/c2, S=929 +/- 223 S/B=0.04 m<0.15 GeV/c2, S=187220 +/- 460, S/B=15.2 • Step I: m>0.15 GeV/c2, S=232 +/- 90, S/B=0.06 m<0.15 GeV/c2, S=87977 +/- 304, S/B=40.1 • Step II: m>0.15 GeV/c2, S=431 +/- 59, S/B=0.28 m<0.15 GeV/c2, S=87338 +/- 298, S/B=104.4 • Step III: m>0.15 GeV/c2, S=523 +/- 55, S/B=0.42 m<0.15 GeV/c2, S=87333 +/- 298, S/B=128.5 • Step IV: m>0.15 GeV/c2, S=519 +/- 43, S/B=0.80 m<0.15 GeV/c2, S=86887 +/- 296, S/B=242.9 Seff increased from ~17 after step 0 to 148 after step IV.