1 detection and tracking of muons in the atlas experiment at lhc: study for an online zμμ...
TRANSCRIPT
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
MDT (Monitored Drift Tube)
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MDT Chamber : test site
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
11 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
• 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
18 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
19 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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.
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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
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
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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)
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
Fabrizio Petrucci – Università Roma TRE e INFN 30
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)
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 / 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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
Fabrizio Petrucci – Università Roma TRE e INFN 36
Performance (II)
Large sector
pgen (GeV)
(pge
n-p r
ec)/
p gen Small sector
(pge
n-p r
ec)/
p gen
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN 37
Resolution effects
Large sector
reso
luti
on (
%)
pgen (GeV)
Small sector
reso
luti
on (
%)
pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN 38
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)
Fabrizio Petrucci – Università Roma TRE e INFN 39
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
Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE
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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