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Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi”

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Presentation on theme: "Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi”"— Presentation 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 1

2 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 pp collider CM energy : 14 TeV luminosity : 1034cm-2s-1 bunch crossing period : 25 ns. LHC The ATLAS detector has been planned to fully exploit LHC potential. Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 2

3 Higgs sector in the MSSM 5 bosons (h, A, H0, H±)
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 H(130Gev)ZZ*  4e Higgs sector in the MSSM 5 bosons (h, A, H0, H±) A, H → tt h, H → bb , gg H → ZZ → 4l Supersymmetric particles: Unknown masses, decay chain to the LSP: Missing energy 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 3

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

5 Muon Spectrometer Detector segmentation Requirements :
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 5

6 Monitored Drift Tube (MDT) :
RPC MDT calorimeter Tracking detectors Spectrometer superconducting coil Solenoid superconducting coil Monitored Drift Tube (MDT) : Proportional drift tubes of 3cm diameter and of variable length ( m). Assembled in 2 multilayers of 3 or 4 tubes. Internal laser alignement system. Single point resolution ~ 80 mm. 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. Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 6

7 MDT (Monitored Drift Tube)
~ 100 ep/cm produced Aluminium tube, diameter=3cm, thickness=400 mm start stop tungsten wire, 50 mm pressurized Ar·CO2 gas mixture t dc electrons drift time 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 7

8 MDT Chamber : test site Tubes are individually tested and assembled before arriving in Roma Tre. Cosmic-ray hodoscope in Roma TRE Chambers equipped with gas system, HV connection, read-out electronics and tested with cosmics before shipping to CERN. RPC planes Nota!!!!!! 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 8

9 MDT Chamber tests Assembly and test sequence : Gas distribution system
Chambers have to fulfil specific requirements concerning mechanical precision, gas tightness, electrical properties, noise level and uniformity of response. before electronic optimization after electronic optimization Elapsed time (hour) Pressure drop = 2 mbar/day Temperature (deg) Pressure drop (mbar) Assembly and test sequence : 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 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 9

10 MDT Chamber : test beam 2001 H8 test beam set-up
Muon beam at the CERN SPS p = 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 10

11 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 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 2) 3) track segment track point Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 11

12 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 residuals (mm) time (ns) Residual’s mean value in the slice r-t relation correction Reconstructed Track Drift circle residual Drift Time (ns) Drift distance (mm) H BIL chamber Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 12

13 Different chamber can have different r-t relations.
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 RM07 ml 12 RM07 ml 11 RM01 ml 12 RM01 ml 11 Time (ns) Systematic uncertainty in r-t relation are of the order of 10 μm Time (ns) 13

14 Tube Resolution tube not included in the track 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: s(r)=  [Resolution(r)]2+ [<extrapolation error>(r)]2 s(r) r (mm) residuals (mm) Resolution(r) =  [s(r)]2 - [<extrapolation error>(r)]2 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 14

15 The resolution on different layers 4 layers average resolution
Run QBIL = 6 Nominal conditions The resolution on different layers 4 layers average resolution resolution (mm) resolution (mm) Signed radius (mm) Signed radius (mm) Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 15

16 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 16

17 Fast Tracking performance
From TDR. Full tracking used Large sector resolution (%) pgen (GeV) Small sector resolution (%) pgen (GeV) Fabrizio Petrucci – Università Roma TRE e INFN 17

18 ~0.1 events with both muons in the barrel
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 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 18

19 Z→μμ reconstruction Only muon spectrometer used
Muon pair invariant mass to select events Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 19

20 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 20

21 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 21

22 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 22

23 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 23

24 Backup slides

25 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 25

26 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 26

27 ATLAS : il calorimetro Calorimetro elettromagnetico : geometria accordion, piombo e Argon liquido (2.5 mm, 4 mm) σE/E=10%/√(E(GeV))+1% Calorimetro adronico : a campionamento ferro e scintillatore nel barrel (TILE) σE/E=50%/√(E(GeV))+3% Identificare e misurare elettroni, fotoni, getti adronici e energia mancante (copertura fino a |η|=4.5, profondita’ 10λ) Calorimetri in avanti ad Argon liquido Calorimetro adronico : a campionamento rame e Argon liquido nelle zone in avanti σE/E=100%/√(E(GeV))+10% Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 27

28 MDT (Monitored Drift Tube)
~ 100 ep/cm produced Aluminium tube, diameter=3cm,thickness=400 mm thick start stop tungsten wire, 50 mm pressurized Ar·CO2 gas mixture t dc electrons drift time Gas mixture : Argon (high primary ionization density) + CO2 High pressure (reduced diffusion effects) Good resolution on single point measurement Limits on gas gain Small signals to the read-out electronics Gas Mixture : Argon (93%) - CO2(7%) Pressure : 3 bar Gas gain : 2*104 (HV=3080V) Discriminator threshold : 20 primary e (3mV/e → 60mV) Working conditions : Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 28

29 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 front-end 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 29

30 10 K test site electronics.
New hardware setup data link final mezzanine (AMT2) jtag in Adapter Final mezzanine + 10 K test site electronics. jtag out C S M C P U VME One more “adapterino” is needed (noise source) Fabrizio Petrucci – Università Roma TRE e INFN 30

31 Drift time distribution
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 Tdrift (TDC counts) 31

32 Efficiency tube not included in the track Track fitted with n-1 points 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 tube not efficient Total missing hits ~ 0.1% Radius (mm) Residuals (mm) “Good” hits (~efficient hits) Hits due to d rays can “hide” track hits. Effect grows with radius. Residuals (mm) ~high C2 hits Radius (mm) Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 32

33 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 = (*) 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

34 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

35 P(GeV)=0.3·Bl(Tesla)·Rcurv(m)
φ Radius (m) Momentum measurement prec (GeV) P(GeV)=0.3·Bl(Tesla)·Rcurv(m) Large sector Small sector 25 GeV muons Approximations not accurate expecially in small sectors corrections needed 4 + 1 parameters needed (no η and no momentum dependence) φ Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 35

36 Performance (II) Large sector Small sector (pgen-prec)/pgen
pgen (GeV) (pgen-prec)/pgen Small sector (pgen-prec)/pgen pgen (GeV) Fabrizio Petrucci – Università Roma TRE e INFN 36

37 Resolution effects Large sector Small sector resolution (%) pgen (GeV)
Fabrizio Petrucci – Università Roma TRE e INFN 37

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). Large sector resolution (%) pgen (GeV) pgen (GeV) Small sector resolution (%) Fabrizio Petrucci – Università Roma TRE e INFN 38

39 R-t relation effect (II)
(pgen-prec)/pgen Large sector pgen (GeV) pgen (GeV) Small sector (pgen-prec)/pgen Fabrizio Petrucci – Università Roma TRE e INFN 39

40 Sezione d’urto differenziale di produzione di m
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 40

41 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 41


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