1

Timing Counter analysis

C.Voena - INFN Romafor the TC group

MEG Review MeetingFeb 17th 2010

2

TC before insertion in COBRA

3

TC configuration in 2009 run• Trigger thresholds lower than 2008: 20mV on single PMT, 70mV on sum of PMT(40/100mV in 2008)

• PMT gains re-equalized by adjusting HV

• Waveform digitization: - DRS3 for NIM pulse from Double Threshold Discriminator (DTD) for time measurement (like 2008) - DRS4 for PMT pulse

• Double-Threshold Discriminator thresholds revisited to optimize efficiency and time resolution 2009: 25/600 mV 2008:25/800mV

4

PMT gain equalization• Use mean of log(QPMT1)/QPMT0) in cosmic ray data at “center” of the bar

Change HV to have PMTs (inner vs outer) and bar equalization within 30%

(changed 10 PMT HV with respect to 2008)

PMT0

(inner)

PMT1

(outer)

TC bar

5

DTD thresholds scanLow threshold: as low as possible (first photoelectron) => Can improve time resolution

- Limited by noise- No improvement seen lowering this threshold (tried 10-17 mV) => left at 25mV which was already optimal value!

High threshold: select good pulses (tracks producing enough p.e., Landau peak)

- Lowered to 600mV (was 800mV) to have more acceptance on positron with lower pulses

800-25400-25250-25

Eloss (a.u.)

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TC time measurement

€

t0 = T +z

veff+ b0 +

c0

A0

e+

L

zPMT0

€

t1 = T +L − z

veff+ b1 +

c1

A1

.

t0,1= extracted with waveform template fits to NIM pulsesfrom Double Threshold Discriminator for PMT0,1

T : time of positron at the impact point on first hit bar (connected to the positron track from DCH)

z : impact point along bar length

effective velocity

PMT1

amplitude of PMT signal

7

TC calibrations

Time walk correction for each PMT

Improve single bar

time resolution

Michel nmult=3

Time offset between PMT of the same bar (z offset calibration)

Michel

(Cosmic in 2008)

Effective velocity for each bar (z scale)

DCH matched Michel positrons

Relative time offset between bars

Michel

(Boron in 2008)

Absolute time offset between positron and

photon

_ Dalitz 0

€

b0 −b1

L

2veffb0 b1

2

veff€

c0,1€

T =t0 + t1

2−L

2veff−b0 + b1

2

⎛

⎝ ⎜

⎞

⎠ ⎟−

1

2

c0

A0

+c1

A1

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

€

z −L

2=veff2t0 − t1( ) − b0 −b1( ) −

c0

A0

−c1

A1

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

8

As in 2008 TW corrections from triple-bars events in Michel Data

8

On events with three adjacent hit bars (triples) minimize the differences (for all the bars) of:

T TA TB

Time Walk calibration

|z|

bar

#

e+

1st bar

€

t0C + t0B2

− t0A −1

2

c0C

A0C

+c0B

A0B

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟−c0A

A0A

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥.

TB

TA

TC

Control sample: double-bars events

TA

TB

TA-TB (ns)

- no TW- with TW

9

2009 resolution2008 resolution

from double-bars events: upper limit on: TC intrinsic resolution + DRS resolution (~10ps)

TC Time Resolution

€

σΔT

2

Upper limit on timeresolution (σ) in 70-100 ps range in 2009 (exceptbar 21)

Slightly worse than2008: under study.Still adequate for MEG performances

10

Difference of PMTs electronic offsets- Needed for z measurement and to combine time measurement of adjacent hits- Use Michel matched positrons instead of previously used cosmic rays

z-offset calibration

Obtained by aligning mean of ztrack-zTC

ztrack= z predicted extrapolating track at TCzTC = z measured by TC

The procedure assumes φ-symmetry

11

Good alignment of bars as can be seen from hit-map

TC hit-map with z-offset calibration

trigger “MEG” data before calibration

trigger “MEG” data after calibration

12

Temporary calibration - Michel matched positrons

Use ztrack as estimate of z at TC - Eventually use fibers for z mesurement

z-scale:effective velocity

€

z =veff2t0 − t1( )

veff = 14.8 cm/ns

t0-t1(ns)

z tra

ck –

z ce

nte

r (c

m)

13

LIB

Double-bar events in Michel data

€

toff , j − toff ,0 = Tk −Tk−1( )mean −LIBc

⎡ ⎣ ⎢

⎤ ⎦ ⎥

k=1

j

∑

€

toff , j − toff ,15 = Tk −Tk−1( )mean −LIBc

⎡ ⎣ ⎢

⎤ ⎦ ⎥

k=16

j

∑

After that, Downstream bars are aligned with bar #0 and Upstream bars are aligned with bar#15 (there are not double-bar events which connect US and DS)

LIB = Inter-Bar path, taken from MC LIB/c ~200ps

Inter-bar time offset calibration

€

Tj = Ti +LIBc

Tj

Ti

Offsets for DS bars

Offsets for US bars

€

toff =L

2veff+b0 + b1

2

€

ΔT = Tj −Ti

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Mea

n of

T

γγ (

ns)

TC bar #

before calibration

after calibration €

Tγγ = TXEC −LγXEC

c

⎛

⎝ ⎜

⎞

⎠ ⎟− TTC −

LγTC

c

⎛

⎝ ⎜

⎞

⎠ ⎟

Boron events two photons: 4.4MeV (XEC) and 11.7MeV(TC)

Cuts for cosmic ray rejection

The Boron sample

Systematic effect of assumption on LIB/c (path between twobars) under study

RMS of residual offsets(after calibration) is~70ps

15

Monitoring TC stability

Single bar time resolution- different colors correspond to different weeks

Inter-bar time offset- Boron sample- different colors correspond to Oct/Nov/Dec

TC bar #

Mea

n of

Tγγ

(ns

)

Stability over time

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0 e(e ) Dalitz 0 events

– Same topology as signal

– Worse resolution due to LH2 target Do not look at time resolution!

Absolute XEC-TC time offsets

€

Teγ = TXEC −LγXEC

c

⎛

⎝ ⎜

⎞

⎠ ⎟− TTC −

LeTC

c

⎛

⎝ ⎜

⎞

⎠ ⎟

Teγ for reference bar

Center of blinding window

(for pre-selection)μ=24.9ns

- Dalitz data suffer from

hardware problem on

DCH side (DRS) that

may affect resolution

Teγ (ns)

17

DCH-TC match- Extrapolated track at bar surface - Reject bars with multiple hits- Reject pairs TC-DCH with bad ztrack-zTC , rtrack-rTC and bad χ2 of match (multiple turns taken into account)

Positron time :- If more than 1 TC hit in matched cluster: combine time measurement taking into account track length between bars- Correct ad-hoc for Te+ correlation with ztrack-zTC

TC-DCH match and Te+ algorithm

Plans: do systematic studies of the algorithms (Monte Carlo and Dalitz sample)

€

Te+ = TTC −LeTC

c

18

Check on radiative decay peak

Clearly see radiative decay peak in Eγ sideband Inter-bar calibration working well - Pre-selection window not well centered at this stage

€

Teγ = TXEC −LγXEC

c

⎛

⎝ ⎜

⎞

⎠ ⎟− TTC −

LeTC

c

⎛

⎝ ⎜

⎞

⎠ ⎟

Example : radiative peak for bar 19

Teγ (ns) TC bar #

Temporary track and photonselection, kinematic cut (m2

νν>0)

Mea

n of

Teγ

(ns

)

Position of radiative decay peak vs TC bar#

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Summary

• TC bars very stable during 2009 run - Ready for the incoming long data-taking

• Calibration strategy applied successfully to 2009 data

• TC intrinsic time resolution < 70-100ps (a little worse than 2008, investigating)

- one of the best Timing Counter detector

• Calibration methods and analysis algorithm constantly improving

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Backup

21

Electronic We checked the electronic contribution to the resolution by splitting PMT signal in two different electronic channels =>Same as 2008

Noise Same noise level as 2008 at DRS input for PMT signal (not sure of situation at DTD input)

tin-tout

PMT#

Possible causes of worse resolution

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Other possible causes: deterioration of PMT-bar coupling? Less scintillation light?

1) (relative) width of Landau peak: done but not conclusive, dominated by Eloss fluctuation

2) change of Λeff. Underway but may be not conclusive since we do not have precise measurement of veff

3) Tests in labs foreseen

Possible causes of worse resolution

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2008

2009

Width of CR landau distr.

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before calibrationz-offset calibration from Michel dataz-offset calibration from CR data

z1

z2

Second hit bar number

Mean of DZ=z2-z1in double bar events (cm)

Applying z-calibration to doubles

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Boron: High energy photonHigh energy photon in XECLow energy photon in XEC

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CR backgound in Boron Sample

Tγγ (ns)

Black: boron data Red: CR data taken with same Boron trg

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Resolution in Boron Sample

Tγγ resolution (ns) vs TC bar: 2009, 2008 (July processing)

Tγγ resolution vs Time

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Multiple hits of TC bars

|z|

Positron track

**

TC hit position: dz = |z2|-|z1| Bar 1 Bar 2

|z|

*

*

dz>0

dz< 0

1st bar with multiple hit

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Teγ vs ztrack-zTC on Dalitz

• No constraint for the track to come from muon target

TC US

Correlation is visible

Correction of 20ps/cm

(DZCorrection)

Same as in 2008