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1 Detection and tracking of muons in the ATLAS experiment at LHC: study for an online Zμμ selection Fabrizio Petrucci Dipartimento di Fisica E.Amaldi Università

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Presentation on theme: "1 Detection and tracking of muons in the ATLAS experiment at LHC: study for an online Zμμ selection Fabrizio Petrucci Dipartimento di Fisica E.Amaldi Università"— Presentation transcript:

1 1 Detection and tracking of muons in the ATLAS experiment at LHC: study for an online Zμμ 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 Zμμ selection and luminosity measurement

2 Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE2 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 < m H < 1 TeV. Lower limit set by direct search in previous experiments, upper limit set by the stability of the theory. Present data suggest m H < 200 GeV. Experimental behaviour of the coupling constants suggest a possible unification (GUT) at an energy scale Λ GUT = 10 14 – 10 16 GeV. Higgs mass diverges quadratically with Λ (naturalness problem). supersymmetric theories (MSSM) Search supersymmetric particles (M susy > 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 : 10 34 cm -2 s -1 bunch crossing period : 25 ns. The ATLAS detector has been planned to fully exploit LHC potential.

3 Fabrizio Petrucci – Dottorando XV ciclo – Università Roma TRE3 Higgs boson search: Low mass range (m H < 130 GeV): H bb BR ~ 100% b-jet tagging and invariant mass resolution H BR ~ 10 -3 energy and direction measurement High mass range (m H > 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 MSSM 5 bosons (h, A, H 0, 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 4Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE ATLAS detector

5 5 Muon Spectrometer Requirements : 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 | |~2.7 3) Trigger capability on single or double muons with programmable p t 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

6 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. 6 RPC MDT Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE calorimeter Tracking detectors Spectrometer superconducting coil Solenoid superconducting coil

7 7 Gas Mixture : Argon (93%, high primary ionization density) - CO 2 (7%) Pressure : 3 bar ( High pressure reduces diffusion effects) Gas gain : 2*10 4 (HV=3080V) Discriminator threshold : 20 primary e (3mV/e 60mV) Working conditions : ~ 100 e p /cm produced electrons drift time Aluminium tube, diameter=3cm, thickness=400 m start stop tungsten wire, 50 m pressurized Ar·CO 2 gas mixture tdctdc Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE MDT (Monitored Drift Tube)

8 8 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

9 9 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 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) Pressure drop = 2 mbar/day Temperature (deg) Pressure drop (mbar) before electronic optimization after electronic optimization

10 10 MDT Chamber : test beam Muon beam at the CERN SPS p = 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 11Fabrizio 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 χ 2. track points definition and track parameters calculation. Track can be extended to two multilayers 1) 2) 3) track segment track point

12 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 residuals 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 residuals (mm) time (ns) Residuals mean value in the slice r-t relation correction Reconstructed Track Drift circle residual Drift Time (ns) Drift distance (mm) H8 2001 BIL chamber

13 13 Time (ns) RM07 ml 12 RM07 ml 11 RM01 ml 12 RM01 ml 11 RM07 ml 12 RM07 ml 11 RM01 ml 12 RM01 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

14 14 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. Residuals distribution width is given by: r)= [Resolution(r)] 2 + [ (r)] 2 r) r (mm) residuals (mm) Resolution(r) [ r)] 2 - [ (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

15 15 The resolution on different layers4 layers average resolution Run 2011 - BIL = 6 Nominal conditions resolution (mm) Signed radius (mm) Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE

16 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. 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)·R curv (m)

17 Fabrizio Petrucci – Università Roma TRE e INFN17 Large sector resolution (%) p gen (GeV) Small sector resolution (%) p gen (GeV) From TDR. Full tracking used Fast Tracking performance

18 18Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE Zμμ 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 ppZ · B z ll = 1.8 nb δ(σ ppZ ) = 5% at the LHC energy (α S, parton distribution functions, normalization of data sets) B z 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 19Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE Zμμ reconstruction Only muon spectrometer used Muon pair invariant mass to select events

20 20 Background Muon pairs with an invariant mass close to that of the Z boson. Main sources: heavy quarks semileptonic decays ppqq+Xμμ+X (q=c,b,t) Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE

21 21 Zμμ 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 ppZ ) = 5% To keep statistical uncertainty below theoretical uncertainty at least 10 3 Z needed σ ppZ =160 pb 10 3 Z = 6 pb -1 integrated luminosity 20 minutes (3 h) of data taking at nominal (low) luminosity.

22 22 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

23 23 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 Zμμ 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)

24 Backup slides

25 25 LHC: parametri e condizioni di misura Parametri di LHC Luminosita10 34 cm -2 s -1 Energia nel CDMs=14 TeV Periodo di incrocio dei fasci25 ns protoni per bunch10 11 numero dei bunch3600 tot (pp) = 70mb 10 9 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

26 26 ATLAS : il tracciatore interno Misura dellimpulso delle particelle cariche ed identificazione di vertici secondari Capacita di tracciamento fino a |η|<2.5 Risoluzione : Δp T /p T <30% (50%) per |η|<2 (2<|η|<2.5) Efficienza : ε > 95% su tutto Ω per p T > 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

27 27 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) σ E /E=50%/(E(GeV))+3% Calorimetro adronico : a campionamento rame e Argon liquido nelle zone in avanti σ E /E=100%/(E(GeV))+10% Calorimetro elettromagnetico : geometria accordion, piombo e Argon liquido (2.5 mm, 4 mm) σ E /E=10%/(E(GeV))+1% Calorimetri in avanti ad Argon liquido Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE

28 28 MDT (Monitored Drift Tube) Good resolution on single point measurement Gas mixture : Argon (high primary ionization density) + CO 2 High pressure (reduced diffusion effects) Limits on gas gain Small signals to the read-out electronics Gas Mixture : Argon (93%) - CO 2 (7%) Pressure : 3 bar Gas gain : 2*10 4 (HV=3080V) Discriminator threshold : 20 primary e (3mV/e 60mV) Working conditions : electrons drift time ~ 100 e p /cm produced Aluminium tube, diameter=3cm,thickness=400 m thick start stop tungsten wire, 50 m pressurized Ar·CO 2 gas mixture tdctdc Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE

29 29 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 dellelettronica finale per lesperimento. 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

30 Fabrizio Petrucci – Università Roma TRE e INFN30 New hardware setup Final mezzanine + 10 K test site electronics. data link jtag in jtag out CSM0CSM0 VME final mezzanine (AMT2) Adapter CPUCPU One more adapterino is needed (noise source)

31 31 Drift time distribution T drift = t Max - t 0 T drift (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

32 32 Efficiency 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 tube not included in the track Track fitted with n-1 points tube not efficient ~high 2 hits Residuals (mm) Radius (mm) Hits due to rays can hide track hits. Effect grows with radius. Total missing hits ~ 0.1% Radius (mm) Residuals (mm) Good hits (~efficient hits) Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE

33 33 Radius of curvature We look for the circle which better fits all the track points. χ 2 minimization with respect to R 2 instead of R. χ 2 = Σ( f(x c,y c ) - R i 2 ) 2 / σ 2 R f(x c,y c ) = (x-x c ) 2 +(y-y c ) 2 Impose that the first track point (x 1,y 1 ) belongs to the track: (x 1 -x c ) 2 +(y 1 -y c ) 2 -R c 2 = 0 (*) Use (x 1,y 1 ) as origin for other points: X i = x i - x 1 ; Y i = y i - y 1 f(x c,y c ) - R i 2 = X i 2 + Y i 2 +2X i (x 1 -x c ) + 2Y i (y 1 -y c ) (R i 2 ~ R c 2 ) Its possible to find the point (x c,y c ) which minimize the χ 2 analitically. Also the error matrix is computable exactely. The curvature radius is the obtained from (*) The computation is fast (150 μs).

34 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

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

36 Fabrizio Petrucci – Università Roma TRE e INFN36 Performance (II) Large sector p gen (GeV) (p gen -p rec )/p gen Small sector (p gen -p rec )/p gen p gen (GeV)

37 Fabrizio Petrucci – Università Roma TRE e INFN37 Resolution effects Large sector resolution (%) p gen (GeV) Small sector resolution (%) p gen (GeV)

38 Fabrizio Petrucci – Università Roma TRE e INFN38 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). p gen (GeV) Small sector resolution (%) Large sector resolution (%) p gen (GeV)

39 (p gen -p rec )/p gen Large sector p gen (GeV) Fabrizio Petrucci – Università Roma TRE e INFN39 R-t relation effect (II) p gen (GeV) Small sector (p gen -p rec )/p gen

40 40 Trigger 3 livelli di trigger in cascata, riduzione della rate del fondo ed elevata efficienza per eventi di segnale. selezione degli eventi Sezione durto 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

41 41 Trigger μ Calcolo dellimpulso al 2 o lvl di trigger Schema del 1 o lvl di trigger Fabrizio Petrucci – Dipartimento di Fisica E.Amaldi - Università Roma TRE


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