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Muon Identification and Reconstruction. Stefano Rosati INFN – Roma 1. Muon Detectors for LHC. Aspects of central relevance: Trigger: reduce the event rate from the initial 40 MHz to the ~200 Hz affordable by the event storage system

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Muon identification and reconstruction
Muon Identification and Reconstruction

Stefano Rosati

INFN – Roma 1

S. Rosati - MC Workshop

Muon detectors for lhc
Muon Detectors for LHC

Aspects of central relevance:

  • Trigger: reduce the event rate from the initial 40 MHz to the ~200 Hz affordable by the event storage system

    • Organized over more levels, the first one has to operate a fast (<10 ns) choice and identification of the Region of Interest

    • Following levels process a limited subset of data (only from the RoI) with higher resolution and detail

    • Final level very close to offline reconstruction, running online on RoI data.

  • Offline reconstruction: provide optimal muon identification and momentum resolution over the pT range 5-1000 GeV

    • Standalone reconstruction can exploit the cleanerenvironment of the muon system

    • Combination with inner tracking detectors to improve resolution

S. Rosati - MC Workshop

Atlas and cms experiements
ATLAS and CMS Experiements

Two approaches for the two experiments:

  • ATLAS:

    • 3 Air-core Toroids (one barrel, two endcaps), mean field 0.6 T with excellent standalone capabilities – complemented by a 2T Central Solenoid)

    • Different bending planes for Inner Detector and Muon Spectrometer (f and h)

    • Stringent requirements on tracking detectors resolution, calibration and alignment

    • Combined reconstruction gives optimal resolution in a certain momentum range

  • CMS:

    • Muon Detectors in the return yoke of the 4 T inner solenoidal field

    • Resolution dominated by Multiple Scatteringup to ~200 GeV pT

    • Combined reconstruction neededto achieve optimal resolution

    • Less stringent requirements on muon tracking detectors resolution, and on their calibration and alignment

S. Rosati - MC Workshop

Atlas muon trigger lvl1
ATLAS Muon Trigger – LVL1

Barrel Trigger

Uses dedicated detector system based on RPCs and TGCs

Selection of events with muons above a given pT threshold (up to six programmable thresholds)

Coincidence of hits in space (both h and f) and time within geometrical windows in different trigger detector layers

S. Rosati - MC Workshop

Atlas level 1 trigger
ATLAS – Level 1 Trigger

Endcap Efficiency vs pT Threshold – acceptance up to |h|<2.4

Example trigger menus and final rates, after also LVL2 and Event Filter (for L=2•1033 cm-2s-1):1m 20 GeV, 2m 10 GeV (40 Hz)2m 6 GeV (25 Hz)

Valid for both Barrel and Endcap

S. Rosati - MC Workshop

Muon reconstruction in atlas

3 points

Initial layout










Muon Reconstruction in ATLAS

S. Rosati - MC Workshop

Atlas combined reconstruction

Initial layout

Detector acceptance

ATLAS - Combined Reconstruction

  • Tracks are back-extrapolated to the IP

  • Parameters corrected for energy losses and multiple scattering

  • Energy loss ~3 GeV at h=0

  • Look for match with tracks reconstructed in the ID

    • Combined refit of the two tracks

    • or: statistical combination oftrack parameters

  • Inner Detector in a Solenoidal Field of 2 T.

Combined reco efficiency

S. Rosati - MC Workshop

Atlas p t resolution
ATLAS – pT Resolution

Resolution vs pT

  • m-Spectrometer Standalone:~10%*pT 2 to 3% (pT in TeV)

    150 X0 Calo Material:non-gaussian tails when back-extrapolated

  • Inner Detector Standalone:~40%*pT 1.5 % (|h|<1.9)~200%*pT 3% (|h|=2.5)(pT in TeV)

  • Combination dominated by the Inner Detector below the cross-over point~40 to 80 GeV (20 GeV in forward region)

S. Rosati - MC Workshop

Atlas p t resolution1
ATLAS – pT Resolution

  • Muon Standalone reconstruction in brief: - 10% resolution up to 1 TeV requires 50 mm sagitta resolution

    - Single point resolution ~80mm(MDT tracker – r-t calibration needed) - ~25 measurement points over the 3 stations

  • Alignment and calibration contribution becomes relevant above ~200 GeV

  • Alignment through optical system + alignment with tracks(e.g. data with field off/on)

    required ~20 mm alignment precision obtained during TB of a full-scale slice

Contributions to the standalone resolution

S. Rosati - MC Workshop

Low p t muon reconstruction

pT (MeV)

Low pT MuonReconstruction

Low pT muons (pT5 GeV) do not reach the outer muon stations

Extrapolate ID tracks and match with patterns of hits in the muon chambers

s=40 MeV


S. Rosati - MC Workshop

Atlas muon isolation







ATLAS - Muon Isolation

Calorimeter Isolation - transverse energy

ID Isolation, SpT

  • Isolation energies in a DR = 0.2 cone

  • Correlation between Inner Detector and Calo isolation

ID vs Calo isolation

S. Rosati - MC Workshop

Atlas muon isolation1
ATLAS - Muon Isolation

Mean value of the transverse EM energy vs cone size

Low and High Luminosity Pileup

S. Rosati - MC Workshop

Impact parameter




Impact Parameter


d0 significance in HZZ*4l event selection

Reject Zbb and tt backgrounds

d0 w.r.t. primary interaction vertex fitted

s=13 mm

Highest significance

2nd Highest

S. Rosati - MC Workshop

Atlas cavern background
ATLAS - Cavern Background

  • High background level expected in the ATLAS experimental hall

  • Background particles originating from p+phadrons + interactions in:

    • ATLAS shielding, forward detectors, machine elements

  • Relevant for trigger (fake coincidences), reconstruction (pattern recognition), detectors ageing (~0.7 C/cm after 10 years LHC on MDT wires)

Cavern background composition


S. Rosati - MC Workshop

Cavern background

10 keV

Cavern Background

Tracking detectors sensitivities to neutral particles- photons ~1% - neutrons ~0.1%

Safety factors included in simulations to account for model uncertainties

High rates of uncorrelated hits:e.g. at L=1034cm-2s-1, safety factor 5,30K hits in MDT chambers(~10% occupancy)

Forward processes critical for the correct estimation of background production

Propagation of low-energy g and n

Energy distribution

S. Rosati - MC Workshop

Atlas performance


Muon Standalones=3.0 GeV

ATLAS - Performance

Mass resolutions for benchmark physics processes

Zmm fundamental to determine the detector mass scale with the first data, MS and MS-ID data

HZZ*4m(M=130 GeV)s=1.9 GeV




s=2.5 GeV

S. Rosati - MC Workshop

Cms muon system
CMS Muon System

4 measurement stations interleaved with the iron yoke slabs

4T field in the Solenoid

Drift Tubes and RPC in the Barrel

CSC and RPC in endcap, RPC coverage up to |h|=1.6

S. Rosati - MC Workshop

Cms lvl1 trigger
CMS LVL1 Trigger

Two independent and redundant systemsDT+CSC or RPC, can be combined, together with calorimeters in a global trigger (GMT)

Trigger coverage for single muons up to |h|=2.1

RPC Trigger will cover up to |h|=1.6 at the startup

S. Rosati - MC Workshop

Cms muon reconstruction
CMS Muon Reconstruction

Tracks are reconstructed in the muon spectrometer and back-extrapolated to the inner silicon tracker

GEANE package for the propagation through calo and coil material

Combined refit with vertex constraint

S. Rosati - MC Workshop

Muon identification and reconstruction

CMS Muon Identification

  • Muon Compatibility Values for two algs:

    • matching tracks with deposits in outer hadron calo

    • matching tracks with patterns in the inner muon chambers, not used for a standalone track fit

  • Cuts on discriminating values tunable for efficiency/purity

Calorimeter Match

Muon Detectors Match

S. Rosati - MC Workshop

Muon identification and reconstruction

CMS Muon Identification

Reconstruction+identification efficiency for muons in b-jets (pT>5 GeV)

Outside-in approach

Inside-out approach(track in Inner Detectormatched with muon hits)

S. Rosati - MC Workshop

Cms p t resolution
CMS - pT Resolution

s(q/pT) for various momenta

Combined reconstruction

Standalone reconstruction

S. Rosati - MC Workshop

Cms p t resolution1
CMS pT Resolution

Dp/p resolution in barrel and endcap

S. Rosati - MC Workshop

Cms muon isolation
CMS Muon Isolation

b-jet muon rejection vs efficiency for Wmn identification

Three independent isolation criteria:- Energy deposits in calorimeters- Hits in pixel detector- Tracks reconstructed in inner tracker

S. Rosati - MC Workshop

Cms performance
CMS - Performance

S. Rosati - MC Workshop

Cms performance1
CMS - Performance

Zmm , reconstructed mass - 1 day of data taking at L=2•1033 cm-2s-1 - QCD background and pileup included

Z’(1 TeV)mmin three scenarios: - Ideal geometry - First data misalignment - Long term misalignment

Alignment exploiting inclusive single muons with pT>40 GeV and Zmm

S. Rosati - MC Workshop

In conclusione competenze italiane
In conclusione: competenze italiane

  • ATLAS-Muon (Bologna, Cosenza, Frascati, Lecce, Napoli, Pavia, Roma 1, Roma 2, Roma 3)

    • Trigger (Livello 1 barrel, Livello 2, Event Filter)

    • Calibrazione ed allineamento MDT

    • Simulazione del rivelatore, studi sul fondo di caverna

    • Ricostruzione standalone e combinata, online e offline, Analysis Software Framework

    • Analisi (Z+jets, HZZ*4l, A/hmm, Susy searches )

  • CMS-Muon (Bari, Bologna, Napoli, Padova, Torino)

    • Trigger di Livello 1 con I DT

    • Simulazione/digitizzazione, trigger RPC

    • Ricostruzione, High Level Trigger, Analysis Software Framework

    • Analisi (HWW2m2n, HZZ2e2m, hmm, WW scattering)

  • Grazie a Ugo Gasparini per tutta la documentazione su CMS

S. Rosati - MC Workshop