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

Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE. Detection and tracking of muons in the ATLAS experiment at LHC: study for an online Z →μμ selection. Physics program at the Large Hadron Collider The ATLAS experiment at the LHC The muon spectrometer

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

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  1. Fabrizio Petrucci Dipartimento di Fisica “E.Amaldi” Università Roma TRE Detection and tracking of muons in the ATLAS experiment at LHC: study for an online Z→μμ selection • 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. LHC 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. The ATLAS detector has been planned to fully exploit LHC potential. Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 2

  3. 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 genergy 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. Air-core toroidal spectrometer • 3 measurement stations • Single point resolution • Detector segmentation (low occupancy & pattern recognition) • Low gas-gain (reduce ageing) 4) Must operate reliably for many years in an high rate and high background environment expecially in the forward regions. Muon Spectrometer • Requirements : • Good momentum resolution in the range 6GeV-1TeV 2) h coverage up to |h|~2.7 3) Trigger capability on single or double muons with programmable pt thresholds. Dedicated trigger chambers 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. Tracking detectors Spectrometer superconducting coil calorimeter Solenoid superconducting coil RPC MDT 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. 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. start stop MDT (Monitored Drift Tube) ~ 100 ep/cm produced Aluminium tube, diameter=3cm, thickness=400 mm tungsten wire, 50 mm t dc pressurized Ar·CO2 gas mixture 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 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. before electronic optimization after electronic optimization Temperature (deg) Elapsed time (hour) Pressure drop = 2 mbar/day Pressure drop (mbar) Elapsed time (hour) MDT Chamber tests Chambers have to fulfil specific requirements concerning mechanical precision, gas tightness, electrical properties, noise level and uniformity of response. • 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. 2001 H8 test beam set-up MDT Chamber :test beam 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 10

  11. 1) 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 2) 3) track segment track point Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 11

  12. Residual’s mean value in the slice H8 2001 BIL chamber residuals(mm) Drift distance (mm) Drift circle Drift circle Reconstructed Track Reconstructed Track r-t relation correction residual residual time(ns) Drift Time (ns) 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 12

  13. 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. Track fitted with n-1 points tube not included in the track s(r) residuals(mm) r(mm) Tube Resolution • 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 Resolution(r) =  [s(r)]2 - [<extrapolation error>(r)]2 Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 14

  15. Run 2011 - 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. From TDR. Full tracking used Large sector resolution (%) resolution (%) Small sector pgen (GeV) pgen (GeV) Fast Tracking performance Fabrizio Petrucci – Università Roma TRE e INFN 17

  18. 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 muons in the barrel all muons 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 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. 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 ATLAS : il tracciatore interno MSGC (Micro Strip Gas Chamber) : camere a guadagno moderato con elettrodi di letturasegmentati 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 alvertice 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 scintillatorenel 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. start stop MDT (Monitored Drift Tube) ~ 100 ep/cm produced Aluminium tube, diameter=3cm,thickness=400 mm thick tungsten wire, 50 mm t dc pressurized Ar·CO2 gas mixture 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. C S M 0 New hardware setup data link final mezzanine (AMT2) jtag in Adapter Final mezzanine + 10 K test site electronics. jtag out C P U VME One more “adapterino” is needed (noise source) Fabrizio Petrucci – Università Roma TRE e INFN 30

  31. Tdrift (TDC counts) Tdrift = tMax - t0 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 31

  32. Track fitted with n-1 points tube not included in the track Residuals (mm) Total missing hits ~ 0.1% “Good” hits (~efficient hits) Radius (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 Efficiency tube not efficient 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 = 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

  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. Radius (m) prec (GeV) φ Momentum measurement 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+1parameters needed (no η and no momentum dependence) φ Fabrizio Petrucci – Dipartimento di Fisica “E.Amaldi” - Università Roma TRE 35

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

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

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

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

  40. requisiti di trigger selezione degli eventi Sezione d’urto differenziale di produzione di m Trigger 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 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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