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 :
 production and test
 tracking, autocalibratiom, resolution
• Tracking in the experiment
 fast tracking and momentum measurement
 Z→μμ selection and luminosity measurement
• Conclusions
1
Physics program at the Large Hadron Collider (LHC)
The Standard Model describes accurately present data, but:
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
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 – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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H(130Gev)ZZ*  4e
Higgs boson search:
Low mass range (mH < 130 GeV):
H → bb
BR ~ 100%
b-jet tagging and invariant mass resolution
H → gg
BR ~ 10-3
g energy and direction measurement
High mass range (mH > 130 GeV):
H → WW(*) , ZZ(*)
(Z → ee, mm, jet - jet )
(W → en, mn, jet - jet )
m and e →
p , E measurement;
leptonic decay to detect signal
Higgs sector in the MSSM
5 bosons (h, A, H0, H±)
Supersymmetric particles:
A, H → tt
Unknown masses, decay chain to the LSP:
h, H → bb , gg
Missing energy
H → ZZ → 4l
W e Z boson production excess.
General requirements:
• Particle identification: e/g – jets – m – missing energy
• Leptonic decays and high transverse momentum particles to detect
signal above background
•→
p , E measurement
Fabrizio Petrucci – Dottorando XV ciclo – Università Roma TRE
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ATLAS detector
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Muon Spectrometer
Requirements :
1) Good momentum resolution in the range
6GeV-1TeV
Air-core toroidal spectrometer
 3 measurement stations
 Single point resolution

2) h coverage up to |h|~2.7
3) Trigger capability on single or double muons with
programmable pt thresholds.
Dedicated trigger chambers
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)

Solutions :
Monitored Drift Tube (MDT) + Cathode Stip Chamber (CSC) : precision chambers
Resistive Plate Chamber (RPC) + Thin Gap Chamber (TGC) : trigger chambers
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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MDT
RPC
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 mm.
Maximum drift time ~ 700 ns.
Tracking detectors
Spectrometer superconducting coil
Resistive Plate Chamber (RPC) : ionizanition
chambers built with two resistive plates and readout in both coordinates with cathodic strips.
Space resolution ~ 1 cm.
Time resolution ~ 2 ns.
calorimeter
Solenoid superconducting coil
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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MDT (Monitored Drift Tube)
~ 100 ep/cm
produced
Aluminium tube,
diameter=3cm, thickness=400 mm
tungsten wire, 50 mm
pressurized Ar·CO2 gas mixture
start
t
d
c
electrons
drift time
stop
Working conditions :
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)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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MDT Chamber : test site
Tubes are individually tested and
assembled before arriving in Roma Tre.
Chambers equipped with gas system, HV
connection, read-out electronics and tested
with cosmics before shipping to CERN.
Cosmic-ray hodoscope
in Roma TRE
RPC
planes
BIL chamber:
4 tubes per multilayer
• 2*144 = 288 tubes per chamber (270 cm)
• Total volume :
2*275 l = 550 l
•
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Assembly and test sequence :
1) Gas distribution system
assembly and test
2) Gas distribution mounted
on the chamber
3) Test for gas tightness
4) High Voltage distribution boards
5) Test of the electrical properties
(current drawn by the chamber)
6) Read-out electronics
7) Tube maps and noise level
8) Cosmic data analysis
9) Chamber response check
before electronic optimization
after electronic optimization
Elapsed time (hour)
Pressure drop (mbar)
Chambers have to fulfil specific requirements
concerning mechanical precision, gas tightness,
electrical properties, noise level and uniformity
of response.
Temperature (deg)
MDT Chamber tests
Pressure drop = 2 mbar/day
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
Elapsed time (hour)
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MDT Chamber :
test beam
2001 H8 test
beam set-up
Muon beam at the CERN SPS
p = 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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Tracking
1)
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
2)
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.
3)
• Track can be extended to two multilayers
track segment
track point
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Autocalibration : finding r-t relation
•
•
Reconstructed
Track
Drift distance (mm)
residuals (mm)
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.
H8 2001 BIL chamber
value in the slice
r-t relation
correction
Drift
circle
residual
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
Residual’s mean
time (ns)
Drift Time (ns)
12
Effects due to variations of temperature, pressure
and gas composition change the r-t relation.
Different chamber can have different r-t relations.
RM07 ml 12
RM07 ml 11
RM01 ml 12
RM01 ml 11
Time (ns)
Time (ns)
RM07 ml 12
RM07 ml 11
RM01 ml 12
RM01 ml 11
Systematic uncertainty in
r-t relation are of the order
of 10 μm
13
Tube Resolution
Track fitted with
n-1 points
Selection of “good” events (single track, 8 hits, good c2).
•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:
•
tube not
included in the
track
residuals (mm)
s(r)=  [Resolution(r)]2+ [<extrapolation error>(r)]2
s(r)
r (mm)
Resolution(r) =  [s(r)]2 - [<extrapolation error>(r)]2
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Run 2011 - QBIL = 6
Nominal conditions
4 layers average resolution
resolution (mm)
resolution (mm)
The resolution on different layers
Signed radius (mm)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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).
Outer station
Middle station
P(GeV)=0.3·B(Tesla)·Rcurv(m)
Inner station
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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resolution (%)
resolution (%)
Fast Tracking
performance
Large sector
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN
From TDR. Full tracking used
Small sector
pgen (GeV)
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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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
muons in the barrel
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Z→μμ reconstruction
• Only muon spectrometer used
• Muon pair invariant mass to select events
2
M mm
= 2 pm1 pm 2 (1  cos  
(1  cos   = 1  sin 1 sin 2 cos(1   2   cos1 cos2
(1  cos  = sin 1 sin 2 sin (1  2 (1  2 
M mm
M mm
1  p1   p2   (1  cos  
 
 


2  p1   p2   (1  cos  

2
=
2
2
1
2

1  p 

  2.8%
 
p
2



Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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cc
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)
bb
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
tt
20
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Conclusions
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Conclusions (2)
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Backup slides
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
stot(pp) = 70mb → 109 eventi/s
(~25 eventi ogni incrocio dei fasci)
sH ~ 10 pb → 10-1 eventi/s
il fondo e’ 10 ordini di grandezza maggiore
↓
fondamentale la selezione (trigger) in
impulso trasverso delle particelle
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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
6 punti di precisione
+ 36 negli straw tubes
TRT (Transition Radiation
Tracker) : straw tubes con
σ~170 μm (identificazione degli
elettroni tramite i γ generati)
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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ATLAS : il calorimetro
Calorimetro adronico : a campionamento
ferro e scintillatore nel barrel (TILE)
σE/E=50%/√(E(GeV))+3%
Calorimetro elettromagnetico : geometria
accordion, piombo e Argon liquido (2.5
mm, 4 mm) σE/E=10%/√(E(GeV))+1%
Identificare e misurare
elettroni, fotoni, getti
adronici e energia
mancante (copertura fino
a |η|=4.5, profondita’ 10λ)
Calorimetro adronico : a campionamento
rame e Argon liquido nelle zone in avanti
σE/E=100%/√(E(GeV))+10%
Calorimetri in avanti ad
Argon liquido
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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MDT (Monitored Drift Tube)
~ 100 ep/cm
produced
Aluminium tube,
diameter=3cm,thickness=400 mm thick
tungsten wire, 50 mm
pressurized Ar·CO2 gas mixture
start
t
d
c
stop
electrons
drift time
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
Working conditions :
Gas Mixture : Argon (93%) - CO2(7%)
Pressure : 3 bar
Gas gain : 2*104 (HV=3080V)
Discriminator threshold : 20 primary e (3mV/e → 60mV)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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DAQ and read-out electronics
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)
Mezzanini : schede di frontend per la lettura di 6*4 tubi.
Contengono un chip ASD
(Amplificatore, Shaper,
Discriminatore) e un TDC
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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New hardware setup
data link
final mezzanine
(AMT2)
Final
mezzanine +
10 K test site
electronics.
One more “adapterino”
is needed (noise source)
Fabrizio Petrucci – Università Roma TRE e INFN
Adapter
jtag in
jtag out
C
P
U
VME
C
S
M
0
30
Drift time distribution
Tdrift (TDC counts)
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
Tdrift = tMax - t0
31
Total missing hits ~ 0.1%
Track fitted
with n-1
points
Efficiency
tube not
efficient
tube not
included in
the track
Residuals (mm)
1) 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 c2
Residuals (mm)
“Good” hits (~efficient hits)
Radius (mm)
Hits due to d rays can “hide” track hits.
Effect grows with radius.
~high C2 hits
Radius (mm)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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 / σ2R
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 = Xi2 + Yi2 +2Xi (x1-xc) + 2Yi (y1-yc)
(Ri2 ~ 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).
33
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
34
Radius (m)
prec (GeV)
Momentum measurement
P(GeV)=0.3·Bl(Tesla)·Rcurv(m)
Large sector
Small sector
25 GeV
muons
φ
Approximations not accurate
expecially in small sectors
φ
corrections
needed
4 + 1 parameters needed
(no η and no momentum dependence)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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(pgen-prec)/pgen
(pgen-prec)/pgen
Performance (II)
Large sector
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN
Small sector
pgen (GeV)
36
resolution (%)
resolution (%)
Resolution effects
Large sector
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN
Small sector
pgen (GeV)
37
R-t relation effect (I)
Large sector
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN
resolution (%)
resolution (%)
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).
Small sector
pgen (GeV)
38
Large sector
(pgen-prec)/pgen
(pgen-prec)/pgen
R-t relation effect (II)
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN
Small sector
pgen (GeV)
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Trigger
Sezione d’urto differenziale di produzione di m
3 livelli di trigger in cascata, riduzione
della rate del fondo ed elevata efficienza
per eventi di segnale.
Criteri utilizzati:
Tagli in impulso trasverso, richiesta di
isolamento
requisiti di trigger
selezione degli eventi
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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Trigger μ
Schema del 1o lvl di trigger m
Calcolo dell’impulso al
2o lvl di trigger
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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