Muon Identification in LHCb

CBM workshop on Muon ID 17.10.2006

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Muon Identification in LHCb

Workshop on Muon Detection in the CBM Experiment

Outline: • Introduction

– Physics requirements– Background conditions

• Overview of the Muon System• Physics Performance

– Muon trigger– Muon identification

• MWPC Detector– Performance and aging studies– Detector construction

• Triple Gem Detector – Performance and aging studies


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Introduction

Physics Goals:• The Muon system of LHCb is primarily used to trigger on muons

produced in the decay of b-hadrons: b X ;In particular: B J/(+-) Ks ; Bs J/(+-) ; Bs +-

• The muon momentum is measured precisely in the tracking system• The muon system identifies muons from tracks in the tracking system

Requirements:

• Modest momentum resolution (~20%) for a robust PT -selective trigger

• Good time resolution (a few ns) for reliable bunch-crossing identification

• Good muon identification (~90%); small pion-misidentification (~1%)

0 00


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IntroductionBackground sources in the LHC environment:

,K X decays – main background for L0 muon trigger

• Shower particles– hadron punch-through including shower muons

• Low-energy background induced by n- processes – contributes significant to chamber hit rate

• Machine background, in particular high energy beam-halo muons

Requirements:• High rate capability of chambers • Good ageing properties of detector components• Detector instrumentation with sufficient redundancy


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Overview of the LHCb Muon Detector

• 435m2 of detector area with 1380 chambers

• Hadron Absorber of 20 thickness in total

• 5 Muon stations, with 2 independent layers/station -> redundancy Layers are logically ORed -> high station efficiency

M1 M2 M3 M4 M5

-> Projectivity to interaction point


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Detector status: snapshot from the pit

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Calo

rim

ete

rs

RIC

H2

Mag

net

Tra

ckin

g s

yste

m

Vert

ex

Dete

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r

RIC

H1

In the pit

To be installed


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Muon Detector Layout

• Space point measurement

• 4 Regions with granularity variable with distance from beam axis

- 0.6cm x 3.1cm in Region 1,- 5cm x 25cm in Region4

• Particle flux varies between - 0.5 MHz/cm2 (M1 Region 1) - 0.2 kHz/cm2 (M5 Region 4)

Chamber

Detector Technology : - MWPC will be used in most regions - triple GEMs are used for M1R1Electronics: - System has 126k FE-channels, reduced to 26k readout channels


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Muons in LHCb


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Muon Trigger AlgorithmLevel 0 Muon Trigger:

Muon track finding: • Find seed pad in station M3 • Find pads within opened search windows (FOI) in stations M2, M4 and M5• Use pads found in M2 and M3 to extrapolate to M1 and find pad in M1 within FOI• Stations M1 and M2 are used for the PT-measurement

-> Muon Trigger exploits multiple scattering in the muon shield by applying tight search windows


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Background Simulation


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Particle Rates in the LHCb Muon System


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Level 0 Muon TriggerTrigger Performance:

• Muon system includes realistic chamber geometry and detector response

->Muon System is robust


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Level 0 Muon Trigger

Beam halo muons:• Distribution of energy and radial

position of halo muons 1m upstream of IP travelling in the direction of the muon system

• Muons entering the experimental hall behind M5 give hits in different BX in the muon stations

->No significant effect

• Halo muons are present in ~1.5% of the bunch crossings

• About 0.1% of them cause a L0 muon trigger


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Muon IdentificationAlgorithm:

• Extrapolate reconstructed tracks with p > 3GeV/c and first hits in Velo from T10 to the muon system (M2 etc.)

• Define a field of interest (FOI) around extrapolation point and

• Define minimum number of stations with hits in FOIs

• M2+M3 for 3 p 6 GeV/c• M2+M3+(M4 or M5)

for 6 p 10 GeV/c • M2+M3+M4+M5 for p 10 GeV/c


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Muon IdentificationPerformance:

Nominal Maximal background background

p>6GeV/c

Sx<0.053

94.00.3 94.30.3 90.00.6Me 0.780.09 3.50.2 0.60.1 M 1.500.03 4.000.05 1.20.05 MK 1.650.09 3.80.1 1.20.1 MP 0.360.05 2.30.1 0.30.1

Additional cuts on slope difference Sx

between tracking and muon system and p are required in case of large bkg.

-> M ~ 1% ~ 90%


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Muonic Final States

B0 J/(+-) Ks :• Well established CP-violating decay from which angle in the unitary

triangle can be determined. • J/ (+-) reconstruction:

- oppositely charged tracks identified as muons. - Mass of dimuon pair consistent with J/ mass-> More than 100k ev./year expected in LHCb

Bs +-: • Decay involves FCNC and is strongly

suppressed in the Standard Model-> BO mass resolution 18 MeV/c2

-> ~10 signal events over 3 bkg expected per year

LO performance for both decays:• L0 trigger acceptance of fully

reconstructed events is 98%.• L0 muon acceptance is 95% with >70% triggered by muon trigger

alone.

0


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MWPC Readout Types

Region 4:Anode wire readout

Region 3:Cathode pad readout

Region 1+2:(in stations M2+M3) Combined anode and cathode readout

-> Different readouts and variations in input capacitance pose special requirements on FE-chip


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MWPC Requirements and Layout

2 independent ASDORed on FE Board

(similar ORing schemealso applies to cathodepads)

HV

HV

HV

HV

Requirements:• High rate capability of chambers -> up to 0.2 MHz/cm2

• Good ageing properties of detector components: -> up to 1.6 C/cm2 on cathodes and 0.3 C/cm on wires

• Detector instrumentation with good redundancy and time resolution -> each chamber has (generally) 4 layers-> 99% station efficiency in 20ns time window

Structuralpanel

Cathode padsPCB

4 separateHV suppliesto anode wires


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Geometry and FieldsV

olt

/cm

Pitch: 2 mmGap: 5 mm Wire: 30 µm HV: 2650 VCathode field: 6.2 kV/cmWire field: 262 kV/cmGain: 105

Total avalanche charge:0.74pC

cm


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Gas Properties

Drift velocity of 90 µm/ns is ‘saturated’ meaning that a small change in electric field does not affect the time resolution.

In Ar/CO2/CF4 40/40/20 we expect

for 10 GeV muons about: ~ 42 clusters/cm ~ 2.35 e-/cluster,-> 99 e-/cm


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Front-End Electronics3x4mm prototype, (2.5x3mm final)

FE-chip ‘CARIOCA’:– 0.25m CMOS technology– 8 channel ASD – fully differential– 10ns peaking time– 30mW/channel

UFRJ Rio, CERN

FE-chip specifications:• Peaking time ~ 10ns• Rin: < 50 • Cdet : 40-250pF• Noise: <2fC for Cdet=250pF• Rate: up to 1MHz• Pulse width: < 50ns• Dose: up to 1Mrad


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Chamber Performance: Time resolution

• Optimum amplifier peaking time ~10ns• Intrinsic time resolution is about 3ns

20ns


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Chamber Performance: Efficiency Cathode Efficiency: Anode Efficiency:

WPWP

-> Plateau length is about 350 – 450 V


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High rate behaviour

1.2 MHz/wire pad, 40kHz/cm2

• Time RMS is unchanged - no space charge effect, as expected • 4% efficiency loss at 1MHz due to signal pileup, as expected


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Chamber Performance High rate performance: (old result obtained with 1.5mm pitch)

Time resolution stable (no space charge

effects)

Small Efficiency drop due to pile up


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Comparison with Simulation

Double gap chamber: •95% efficiency if the threshold is set to 30% of the average signal.

• 99% efficiency if the threshold is set to 20% of the average signal.

• In order to have a double gap chamber well within the plateau we want to be able to use a threshold of ~15% of the average signal.

-> Good agreement, we understand our detector

Full simulation:- Primary ionization (HEED)- Drift, Diffusion (MAGBOLTZ)- Induced Signals (GARFIELD)


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Chamber Specifications Gas Gain variations:• Working point should not move out of the plateau

• The gas gain should be for 95% of the area within 25% of G0

• For the remaining 5% of the area the gas gain should be within 50%

of G0

• A gain change of a factor 1.25 (1.5) corresponds to a voltage change of ±34 (62)V around 2650 V, corresponding to ±1.25 (2.25)%.-> The gas gain changes by a factor 1.25 (1.5) if the wire surface field changes by ±1.25 (2.25)%.

Resulting chamber specifications:• Gap: 95% in 90m 1% 5% in 180m 2%• Pitch: 95% in 50m 0.35% 5% in 100m0.7%• Wire y-offset: 95% in 100m 0.1% 5% in 200m0.4%• Wire plane y-offset:95% in 100m 0.1% 5% in 200m0.4%-> Added in squares : 1.07%2.2%


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Chamber Construction

Production Program:• Within two years ~1500 chambers had to be build in 6 production centres. • Quality assurances (QA) is a key issue

Example for QA:• Measurement of wire pitch with CCD cameras using, telecentric lenses


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Wire Pitch Measurement

Small pitch fluctuations mainly due to precision of combs -> 99% of wires within (2.00 ±0.05)mm

Quality check of method: Reproducibility of result within 1.5m (RMS) on average

-> Method well adapted for reliable QA

2.1

2.0

1.90 75 150 225 300 375 450 525 600


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Example for Gas Gain Uniformity

2.1

2.0

1.90 75 150 225 300 375 450 525 600


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MWPC Aging studies

Two long term aging tests have been performed: 1st test at GIF in 2001, where charges of 0.25 C/cm have been

accumulated in 6 months.

Open loop gas system has been used

Gas mixture: Ar / CO2 / CF4 (40%, 50%, 10%)

2nd test at ENEA Cassacia in June 2003, where charges of 0.5 C/cm have been accumulated in 1 month (using the 25kCi (!) Co source).

One chamber has been in open loop, one in closed loop

Gas mixture: Ar / CO2 / CF4 (40%, 40%, 20%)

Parameters controlled :• Relative gas gain: In both tests one gap was used as reference gap to avoid complicated corrections for P and T variations. The reference gap was only for very short periods per day under HV and always flashed with the fresh gas.

• Dark currents, including self-sustaining rest current following the beam off. The dark currents were measured with current monitors with a resolution of 1 nA.


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No signs for aging from the measured parameters:

Relative currents did not change.

Malter currents were not observed.

No variations in current ratios after/before Casaccia from measurements with GIF and with an Am-source.

Summary of Results from Casaccia

Chamber Gap IC (C/cm)

M3R1 A,C,D 0.43

S/A S1 0.52

S/A S2 0.42

S/A A2 0.38

Accumulated charges:

Analysis of Surfaces

Strong FR4 etching of open surfaces

Bubbles under Kapton foil

1st the wires . . .

CF4 is a

phantastic

gas !

CF4 is a

terrible

gas !

Detaching of the ground grid between the pads

(due to FR4 etching)


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Conclusions

Conclusions: Although no drop in gas gain was visible after the Casaccia aging

test, the materials exposed to CF4 show strong surface etching.

The effects are very similar for the chambers in the two gas system used in Casaccia (vented and re-circulation gas-system).

The effect has been stronger the 2nd aging test, where the gas mixture contained 20% CF4, than in 1st test, where 10% CF4 have been used.

Plans: Repeat the test at GIF in the coming months, where the test takes

however 5 times longer than in Casaccia (source strength 25kCi).

New mixture with less/no CF4 .

Both gas loops will be used.

Control of O2 and H2O and contamination is foreseen.


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Muon Identification in LHCb