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HBD Towards Run-9

Itzhak Tserruya

H/L meeting, April 3, 2008

April 3, 2008

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Is HBD ready for run-9 ?

Request from Raphael (H/L conveners):“In the light of defining our beam use proposal, we need some input on the HBD. Would it be possible to have a presentation on the subject? In particular, we'd like to hear :-a quick summary of the run7 problems (HV, scintillation...)

-the status of the analysis (electron id up to which centrality?)

-the honest status of the repairs, and the likelihood of the readiness for the run 9.”

Request from Tony (Analysis coordinator):“I would like to try to understand the status and plans for HBD offline software with a view towards Run 9… We will need, by the start of Run 9:

1) The ability to reconstruct HBD data within 2 weeks of physics collisions, earlier if possible. This will require that we have all necessary calibrations in place, and have working reconstruction code in place. It is not necessary that the reconstructed HBD data be final physics quality for this purpose, but it has to be good enough for meaningful conclusions to be drawn about performance and yields.

2) The ability to match reconstructed tracks in the central arm to HBD hits and add the appropriate physics parameters to PHCentralTracks.

3) Analysis code in place that extracts meaningful physics data from the HBD-matched central arm tracks, so that performance and yields can be evaluated during the run.”

April 3, 2008

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Executive summary

A. HARDWARE We have understood the origin of, and solved, the HV problem

encountered in run-7. We have considerably improved the protection against discharges. HBD west is fully refurbished and shall undergo lab test soon. We are on track for installation of the entire HBD well before the start of

run-9.

B. SOFTWARE Full chain reconstruction code for HBD data exists. Improvement and additional work (new cluster algorithm) are needed to

deal with centraI events, that could not be done up to now since our highest priority has been on the hardware front, but will be done prior of run-9.

We have learned a lot from run-7 data. There is more to be learned. Even with a relatively poor performance, as achieved in run-7, the HBD

can improve considerably the quality of the low-mass pair measurement (see Zvi’s talk).

April 3, 2008

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The run-7 HV problem

• A lot of work went on improving the resistive chain and modifying the HV PS to minimize GEM damage created by discharges.

• When we started GEM assembly, excessive spark occurred while trying to reach the operational gain of 5000 in CF4. It appeared that we were having the same basic problem as in run-7, namely the GEMs did not hold the required HV in CF4. The problem appeared during the GEM tests performed in the stainless steel box prior to installation into the HBD whereas in run-7 it was found in the HBD vessel at the IR (no CF4 tests were done prior of installation).

• We pursued two venues in parallel: - find the problem and fix it. - modify the design and/or the operating conditions so as to be able to operate

the GEMs as they are.

• The problem was found to be dust and was fixed by thorough cleaning and monitoring of the glove box (dust meter, ULPA vacuum cleaner). GEMs hold much higher voltages than before and reach much higher gains in a stable mode.

April 3, 2008

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QA assurance

We adopted a new protocol of assembly and QA assurance:

1. Each GEM is HV tested in air to 550V and then washed with de-ionized water. A considerable

fraction of damaged GEMs have been recuperated by this washing.

2. Washing with alcohol and drying with Argon in the tent under the laminar table.

3. HV test for every GEM. (If GEM does not pass HV test after second wash it is declared bad.

4. Good GEMs are stored in vacuum

5. Assemble triple GEM stack in the test box , operate in CF4 and measure gain curve.

6. Store stack in the vacuum storage.

7. As soon as 4 stacks are ready evaporate .

8. Assemble stack in the SS vessel inside the glove box, test each GEM individually and the stack in

CF4. Require at least 24h stable operation

9. Install this stack into HBD vessel.

April 3, 2008

April 3, 2008 6

Gain vs time: installed stacks

VGEM = 511 V

Typical GEM failure rate: 2/39

April 3, 2008

CsI evaporation CsI is evaporated onto 4 GEMs in one shot. For each GEM 3 measurements are taken at 160 nm across X axis. QE looks great

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HBD West arm status: March 12, 2008.

Tests of the entire vessel with CF4 will start soon.April 3, 2008

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Hadron Blindness & e-h separationElectron - hadron separation (RB)

Hadron rejection factor

Hadron suppression illustrated by comparing hadron spectra in FB and RB

(same number of central tracks)

Results very similar to those obtained in previous lab or

beam tests

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Single vs double electron separation

For a better comparison should restrict the East arm mass spectrum to the same acceptance as the HBD.

HBD response is a mixture of single, double electrons and a “continuum” (from pad threshold to double electron response) due to conversions in CF4.

Hint of single vs. double hit separation?

Our best estimate of the nr of p.e. Is 15, on the low side but not too far from the theoretical limit (36 p.e.) considering the known losses: pad threshold ~10% reverse bias ~15-20% gas transmission ~15% Collection efficiency?

Much more on Zvi’s presentation

e+e- pair spectrum in the central East armInclusive Matched to HBD-E

Pulse height of matched hits in HBD-E

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Single electron spectra

April 3, 2008

West East

• Analysis restricted to the HBD working modules acceptance, namely WS5, WN2, WN3, WN5 in the West Arm and ES1, ES2, ES3, EN3, EN4, EN5 in the East arm. • 40% most peripheral events are selected. • This analysis clearly demonstrates that HBD rejects not only most of the background it generates but also most of the background electron tracks in the Central Arms.

1

3

2

1/3

3/21/2

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Reconstruction software

The HBD reconstruction software involves four main elements: Gain equilibration

Cluster algorithm

Tracking to central arm

Close pair rejection

April 3, 2008

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Gain equilibration

• is in excellent shape.

• exploits the scintillation signals

• has been used in run-7 off-line analysis

• software exists.

• a short time after the first collisions in run-9 we should be able to have the gain equilibration module ready.

April 3, 2008

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Gain derived from scintillation (I)

Scintillation hit identification:• single pad hits not belonging to any track in peripheral events

Gain determination:• Fit the range (10-50) ADC channels with an exponential function

• 1/slope increase with event multiplicity

• 1/slope = Gain . <m>

(where <m> = avrg nr of scintillation photons in a fired pad)

Assuming the nr of scintillation photons per pad follows a Poisson distribution:

A fired pad measures:

<m> =

P(0) = probability to have no hit in a pad =

<m> =

)0(1)(

)(

1

1

PnP

nnP

n

n

n!eμ μn

P(n)

e

2/)]0(ln[12/11

Pe

1

/slo

pe

April 3, 2008

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Gain derived from scintillation (II)

P(0) is not measured

Determine the probability P(0,th) of not firing a pad for a given threshold and extrapolate to a zero threshold

P(0,th) = 1 – [nr of fired pads (A>th)] / [total nr of pads]*

* The large pads are excluded in this analysis i.e. the total nr of pads = 93 or 94.

1/slope

April 3, 2008

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Gain equilibration works

April 3, 2008

Raw data show large local (pad to pad) gain variations. The corrections have been derived for the run with nominal HV (230669) and +100V (237092)

Before After

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Cluster algorithm

• Purpose: determine the location, size (number of pads) and total signal of each hit in the HBD.

• A simple algorithm exists that has been used in the off-line analysis of run-7 Au+Au events with centrality > 50%.

• Needs a lot of work: of optimization, cluster splitting and also conceptually in order to deal with more central events.

April 3, 2008

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Tracking to central arm

• Software exists, has been used in run-7 off-line analysis, in excellent shape.

Hadrons selected in central arm:Vertex +/- 20 cm< 50 tracks3 matching to PC3 and EMCaln0 < 0EMC energy < 0.5

Projected onto HBD:Z in HBD +/- 2 cm in HBD +/- 25 mrad

• Position resolution:

z ≈ ≈ 1 cm

Dictated by pad size: hexagon a = 1.55 cm(2a/√12 = 0.9 cm) April 3, 2008

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Run-9 reconstruction

• I believe we can have the HBD reconstruction code in good shape prior of run-9.

• We will quantify the number of events and the resources (disk space and computing power) needed for:• Gain equilibration• Performance assessment

April 3, 2008