Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE

1

Detection and tracking of muons in the ATLAS experiment at LHC:

study for an online Z→μμ selection

Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi”Università Roma TRE

• Physics program at the Large Hadron Collider

• The ATLAS experiment at the LHC

• The muon spectrometer

• MDT : Operating principles

• MDT Chambers :

• Tracking in the experiment

• Conclusions

production and test tracking, autocalibratiom, resolution

fast tracking and momentum measurement Z→μμ selection and luminosity measurement

Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 2

Physics program at the Large Hadron Collider (LHC)

The Higgs mechanism of electroweak symmetry breaking (particle masses) has to be observed experimentally.

Search for Higgs boson in the mass range 114 GeV < mH < 1 TeV. Lower limit set by direct search in previous experiments, upper limit set by the stability of the theory. Present data suggest mH < 200 GeV.

Experimental behaviour of the coupling constants suggest a possible unification (GUT) at an energy scale ΛGUT = 1014 – 1016 GeV.

Higgs mass diverges quadratically with Λ (naturalness problem). → supersymmetric theories (MSSM) Search supersymmetric particles (Msusy > 100 GeV) and in particular study the

Higgs sector in the MSSM

LHC

The Standard Model describes accurately present data, but:

pp collider CM energy : 14 TeV luminosity : 1034cm-2s-1 bunch crossing period : 25 ns.

The ATLAS detector has been planned to fully exploit LHC potential.

Fabrizio Petrucci – Dottorando XV ciclo – Università Roma TRE 3

Higgs boson search: Low mass range (mH < 130 GeV): H → bb BR ~ 100% b-jet tagging and invariant mass resolution H → BR ~ 10-3

energy and direction measurementHigh mass range (mH > 130 GeV): H → WW(*) , ZZ(*)

(Z → ee, , jet - jet ) (W → e, , jet - jet ) and e p , E measurement; leptonic decay to detect signal

Higgs sector in the MSSM5 bosons (h, A, H0, H±)

A, H →

h, H → bb ,

→ ZZ → 4l

Supersymmetric particles:

Unknown masses, decay chain to the LSP:

Missing energy

W e Z boson production excess.

H(130Gev)ZZ* 4e

General requirements: • Particle identification: e/ – jets – – missing energy • Leptonic decays and high transverse momentum particles to detect signal above background• p , E measurement→

→

4 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE

ATLAS detector

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Muon SpectrometerRequirements :

1) Good momentum resolution in the range 6GeV-1TeV

Solutions :

Monitored Drift Tube (MDT) + Cathode Stip Chamber (CSC) : precision chambers

Resistive Plate Chamber (RPC) + Thin Gap Chamber (TGC) : trigger chambers

2) coverage up to ||~2.73) Trigger capability on single or double muons with programmable pt thresholds.

4) Must operate reliably for many years in an high rate and high background environment expecially in the forward regions.

Detector segmentation (low occupancy & pattern recognition) Low gas-gain (reduce ageing)

Air-core toroidal spectrometer 3 measurement stations Single point resolution

Dedicated trigger chambers

Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE

Monitored Drift Tube (MDT) :

Proportional drift tubes of 3cm diameter and of variable length (1.8-5.2 m).

Assembled in 2 multilayers of 3 or 4 tubes. Internal laser alignement system. Single point resolution ~ 80 m.

Maximum drift time ~ 700 ns.

Resistive Plate Chamber (RPC) : ionizanition chambers built with two resistive plates and read-

out in both coordinates with cathodic strips. Space resolution ~ 1 cm. Time resolution ~ 2 ns.

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RPCMDT


calorimeter

Tracking detectors

Spectrometer superconducting coil

Solenoid superconducting coil

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Gas Mixture : Argon (93%, high primary ionization density) - CO2(7%) Pressure : 3 bar (High pressure reduces diffusion effects)Gas gain : 2*104 (HV=3080V)Discriminator threshold : 20 primary e (3mV/e → 60mV)

Working conditions :

~ 100 ep/cmproduced

electrons drift time

Aluminium tube, diameter=3cm, thickness=400 m

start

stop

tungsten wire, 50 m

pressurized Ar·CO2 gas mixture tdc


MDT (Monitored Drift Tube)

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MDT Chamber : test site


Chambers equipped with gas system, HV connection, read-out electronics and tested with cosmics before shipping to CERN.

Tubes are individually tested and assembled before arriving in Roma Tre. Cosmic-ray hodoscope

in Roma TRE

• 4 tubes per multilayer• 2*144 = 288 tubes per chamber (270 cm)• Total volume : 2*275 l = 550 l

BIL chamber:

RPC planes

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Assembly and test sequence :1) Gas distribution system assembly and test2) Gas distribution mounted on the chamber3) Test for gas tightness4) High Voltage distribution boards 5) Test of the electrical properties (current drawn by the chamber) 6) Read-out electronics7) Tube maps and noise level8) Cosmic data analysis9) Chamber response check


MDT Chamber tests

Chambers have to fulfil specific requirements concerning mechanical precision, gas tightness, electrical properties, noise level and uniformity of response.

Elapsed time (hour)

Elapsed time (hour)

Pressure drop = 2 mbar/day

Tem

pera

ture

(de

g)P

ress

ure

drop

(m

bar)

before electronic optimizationafter electronic optimization

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MDT Chamber :test beam

Muon beam at the CERN SPSp = 10-180 GeV


Systems test and systems integration

•Reduced multiple scattering •High events rate → large data sample in the same working conditions

2002 H8 test beam set-up

2001 H8 test beam set-up


Tracking

1) list of hit tubes in the event : tube identifiers (position) and drift time (tdc measurement).

2) group aligned tubes in a multilayer to form a candidate track (only geometrical informations).

3) drift time to drift distance using the proper r-t relation.• fit a line to the drift circles and eventually drop hits with an

high contribution to the χ2.• track points definition and track parameters calculation.• Track can be extended to two multilayers

1)

2)

3)

track segment

track point

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Autocalibration : finding r-t relation• Iterative procedure.• Straight line computed fitting drift circles obtained with a seed r-t relation.• Residuals are computed.• The mean value of residual’s distribution is computed in different drift time slices.• It is used as the correction to the r-t relation.


Reconstructed Track

Drift circle

residual

resi

du

als (m

m)

time (ns)

Residual’s mean

value in the slice

r-t relation correctionReconstructed

Track

Drift circle

residualDrift Time (ns)

Dri

ft d

ista

nce

(m

m)

H8 2001 BIL chamber

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Time (ns)

Time (ns)

RM07 ml 12

RM07 ml 11

RM01 ml 12

RM01 ml 11

RM07 ml 12RM07 ml 11RM01 ml 12RM01 ml 11

Effects due to variations of temperature, pressure and gas composition change the r-t relation.

Different chamber can have different r-t relations.

Systematic uncertainty in r-t relation are of the order of 10 μm

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•Selection of “good” events (single track, 8 hits, good ).•Residual for each tube and its extrapolation error are computed with the track obtained with n-1 points.•Residual’s distribution width is given by:

r)= [Resolution(r)]2+ [<extrapolation error>(r)]2

r)

r (mm)

resi

dual

s (mm

)

Resolution(r) [r)]2 - [<extrapolation error>(r)]2

Tube Resolution


tube not included in the

track

Track fitted with n-1 points

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The resolution on different layers 4 layers average resolution

Run 2011 - BIL = 6Nominal conditions

reso

luti

on

(m

m)

reso

luti

on

(m

m)

Signed radius (mm)

Signed radius (mm)


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Fast tracking in the spectrometer

• A fast tracking procedure in the spectrometer is needed for calibration purpose and detector response monitoring. • Montecarlo simulation has been used: - physic processes included: multiple scattering, energy loss, δ-ray production - detailed geometry, material and magnetic field description - tube response is simulated using realistic r-t relation, resolution and efficiency.


• MDT measure only in the banding plane (R-φ plane): second coordinate from RPC hits to properly account for the magnetic field.• Track fit in each chamber: parameters of the segment, track points.• Comparison in both projections of segment parameters to form a track.• Fast tracking : assume circular trajectory Look for the circle best fit to all track points. Radius of curvature and error matrix computed analitically. Fast computation (150 μs).

Middle station

Inner station

Outer station

P(GeV)=0.3·B(Tesla)·Rcurv(m)

Fabrizio Petrucci – Università Roma TRE e INFN 17

Large sector

reso

luti

on (

%)

pgen (GeV)

Small sector

reso

luti

on (

%)

pgen (GeV)

From TDR. Full tracking used

Fast Tracking performance


Z→μμ

Z boson production and decay in muons is a clean and unambiguous signal. Can be used for the calibration of the detector response and for luminosity measurement.• σ pp→Z · Bz→ ll = 1.8 nb

• δ(σ pp→Z ) = 5% at the LHC energy

(αS, parton distribution functions, normalization of data sets)

• Bz→ ll very well known

Physics event Montecarlo generator and detector simulation

~0.1 events with both muons in the barrel all muons muons in the barrel


Z→μμ reconstruction

cos12 212 ppM

212121 coscoscossinsin1cos1 212121 sinsinsincos1

%8.2

2

1

cos1

cos1

2

1 2

122

2

2

2

1

1

p

p

p

p

p

p

M

M

• Only muon spectrometer used• Muon pair invariant mass to select events

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Background

Muon pairs with an invariant mass close to

that of the Z boson.

Main sources: heavy quarks semileptonic decays

pp→qq+X→μμ+X (q=c,b,t)


cc

bb tt

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Z→μμ selection and luminosity measurement

Range around the Z peak : ±10 GeV (±15 GeV)

Selection efficiency : 84% (91%) → 156 pb (169 pb)

Background contamination : 1.4 pb (2.2 pb)


L=σ/N Luminosity can be measured using a process with a small theoretical error on the production cross section.

δ(σ pp→Z ) = 5%

To keep statistical uncertainty below theoretical uncertainty at least 103 Z needed

σ pp→Z =160 pb → 103 Z = 6 pb-1 integrated luminosity → 20 minutes (3 h) of data taking at nominal (low) luminosity.

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MDT BIL chambers construction and test:• Setting up of the cosmic-ray hodoscope.• Definition of the procedures for chambers assembly and test.• The read-out software has been written and the prototype electronics has been

exploited.• 9 chambers produced and tested.• Chambers performance tested both at the test site and at the test-beam

showing the desired construction quality.

- Single point resolution: from 250 μm close to the wire down to 60 μm at the

maximum drift distance.

- Average single tube efficiency: >97 % over the full drift path.

- Autocalibration : r-t relation systematics lower than 10 μm.


Conclusions

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Fast tracking and momentum measurement method in the barrel spectrometer:• Mean resolution varies from 3.5 % at 25 GeV to 10 % at 1 TeV.• No bias in the momentum measurement up to 200 GeV.• Processing time is less than 10 ms on a 600 MHz processor.

Reconstruction and selection of Z→μμ events:• About 10 % of pp → Z + X →μμ + X events with both muons in the barrel.• Resolution of 3 % in Z mass measurement.• Background due to heavy quarks semileptonic decay has been studied and

accounts for less than 2 % in Z counting.• A statistical uncertainty of 3% can be obtained in 20 min. (3 h) at nominal

(low) luminosity.


Conclusions (2)

Backup slides

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LHC: parametri e condizioni di misura

Parametri di LHC

Luminosita’ 1034cm-2s-1

Energia nel CDM √s=14 TeV

Periodo di incrocio dei fasci 25 ns

protoni per bunch 1011

numero dei bunch 3600

tot(pp) = 70mb → 109 eventi/s

(~25 eventi ogni incrocio dei fasci)

H ~ 10 pb → 10-1 eventi/s

il fondo e’ 10 ordini di grandezza maggiore

↓

fondamentale la selezione (trigger) in impulso trasverso delle particelle


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ATLAS : il tracciatore interno Misura dell’impulso delle particelle cariche ed identificazione di vertici secondari•Capacita’ di tracciamento fino a |η|<2.5•Risoluzione : ΔpT/pT <30% (50%) per |η|<2 (2<|η|<2.5)•Efficienza : ε > 95% su tutto Ω per pT > 5 GeV

MSGC (Micro Strip Gas Chamber) : camere a guadagno moderato con elettrodi di lettura segmentati a strisce σ~35 μm

SCT (SemiConductor Tracker) : rivelatore al silicio (pixel + strisce); ulteriore strato vicino al vertice per la misura di vertici secondari. Risoluzione sul singolo punto σ~13 μm.

TRT (Transition Radiation Tracker) : straw tubes con σ~170 μm (identificazione degli elettroni tramite i γ generati)

6 punti di precisione + 36 negli straw tubes


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ATLAS : il calorimetro

Identificare e misurare elettroni, fotoni, getti adronici e energia mancante (copertura fino a |η|=4.5, profondita’ 10λ)

Calorimetro adronico : a campionamento ferro e scintillatore nel barrel (TILE) σE/E=50%/√(E(GeV))+3%

Calorimetro adronico : a campionamento rame e Argon liquido nelle zone in avanti σE/E=100%/√(E(GeV))+10%

Calorimetro elettromagnetico : geometria accordion, piombo e Argon liquido (2.5 mm, 4 mm) σE/E=10%/√(E(GeV))+1%

Calorimetri in avanti ad Argon liquido


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MDT (Monitored Drift Tube)

Good resolution on single point measurement

Gas mixture : Argon (high primary ionization density) + CO2

High pressure (reduced diffusion effects)Limits on gas gain Small signals to the read-out electronics

Gas Mixture : Argon (93%) - CO2(7%)Pressure : 3 barGas gain : 2*104 (HV=3080V)Discriminator threshold : 20 primary e (3mV/e → 60mV)

Working conditions :

electrons drift time

~ 100 ep/cmproduced

Aluminium tube, diameter=3cm,thickness=400 m thick

start

stop

tungsten wire, 50 m

pressurized Ar·CO2 gas mixture tdc


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DAQ and read-out electronics

Mezzanini : schede di front-end per la lettura di 6*4 tubi. Contengono un chip ASD (Amplificatore, Shaper, Discriminatore) e un TDC

Si utilizzano prototipi dell’elettronica finale per l’esperimento.Il software per il DAQ e’ stato sviluppato a Roma Tre.

Chamber Service Module (CSM) : raccoglie dati da 18 mezzanini tramite un adattatore ed e’ letto da una CPU via un bus VME.

Trigger esterno (ad esempio dal telescopio)



New hardware setup

Final mezzanine +10 K test site electronics.

data link

jtag in

jtag out

CSM0

VME

final mezzanine (AMT2)

Adapter

CPU

One more “adapterino”is needed (noise source)

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Drift time distribution

Tdrift = tMax - t0

Td

rift

(TD

C c

ounts

)

Two effects take place when temperature

increases at constant pressure and interplay:

• Gas is less dense less charge per unit path AND Chamber GAIN modifications

• Drift velocity is larger

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Efficiency1) Tracks that cross the tube under analysis are fitted excluding that tube.

2) Check the hit in the tube : - Hit not present - High contribution to the

tube not included in

the track

Track fitted with n-1 points

tube not efficient

~high 2 hits

Resi

duals

(m

m)

Radius (mm)

Hits due to rays can “hide” track hits.Effect grows with radius.

Total missing hits ~ 0.1%

Radius (mm)

Resi

du

als

(m

m)

“Good” hits (~efficient hits)


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Radius of curvature

We look for the circle which better fits all the track points.

χ2 minimization with respect to R2 instead of R.

χ2 = Σ( f(xc,yc) - Ri2 ) 2 / σ2

R f(xc,yc) = (x-xc)2+(y-yc)2

Impose that the first track point (x1,y1) belongs to the track:

(x1-xc)2+(y1-yc)2-Rc2 = 0 (*)

Use (x1,y1) as origin for other points:

Xi = xi - x1 ; Yi = yi - y1

f(xc,yc) - Ri2 = Xi

2 + Yi2 +2Xi (x1-xc) + 2Yi (y1-yc) (Ri

2 ~ Rc2)

It’s possible to find the point (xc,yc) which minimize the χ2 analitically. Also the error matrix is computable exactely.

The curvature radius is the obtained from (*)

The computation is fast (150 μs).

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Fast tracking • G4 spectrometer simulation• Track segments in the single chambers.• Second coordinate from RPC hits with a proper smearing (digitization not ready)• Comparison of fitted tracks parameters to match tracks.• Fast tracking : circular trajectories (radius of curvature computation →)

2 track segments

3 track segments

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p rec (

GeV

)

4 + 1 parameters needed (no η and no momentum dependence)

corrections needed

Momentum measurement

φ

φ

Rad

ius

(m)

Approximations not accurate expecially in small sectors

Large sector

Small sector

P(GeV)=0.3·Bl(Tesla)·Rcurv(m)

25 GeV muons



Performance (II)

Large sector

pgen (GeV)

(pge

n-p r

ec)/

p gen Small sector

(pge

n-p r

ec)/

p gen

pgen (GeV)


Resolution effects

Large sector

reso

luti

on (

%)

pgen (GeV)

Small sector

reso

luti

on (

%)

pgen (GeV)


R-t relation effect (I)

Tubes with different r-t relation. Example from H8 test beam analysis : triplet of tubes in the same multilayer with different max drift time. Effect simulated in digitization. Events reconstructed using a mean r-t relation (the same for all tubes).

pgen (GeV)

Small sector

reso

luti

on (

%)Large sector

reso

luti

on (

%)

pgen (GeV)

(pge

n-p r

ec)/

p gen

Large sector

pgen (GeV)


R-t relation effect (II)

pgen (GeV)

Small sector

(pge

n-p r

ec)/

p gen

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Trigger

3 livelli di trigger in cascata, riduzione della rate del fondo ed elevata efficienza per eventi di segnale.

selezione degli eventi

Sezione d’urto differenziale di produzione di

requisiti di trigger

Criteri utilizzati:

Tagli in impulso trasverso, richiesta di isolamento


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Trigger μ

Calcolo dell’impulso al

2o lvl di trigger

Schema del 1o lvl di trigger


Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE

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Transcript of Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE