Muon detection in the CBM experiment at FAIR

Andrey Lebedev1,2

Claudia Höhne1

Ivan Kisel1Anna Kiseleva3

Gennady Ososkov2

1 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany2 Laboratory of Information Technologies, Joint Institute for Nuclear Research, Dubna, Russia3 Frankfurt University, Germany

DPG 2010March 15-19, 2010 Bonn

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The CBM experiment at FAIR

• exploration of the QCD phase diagram in regions of high baryon densities and moderate temperatures• comprehensive measurement of hadron and lepton (vector mesons μ+μ-) production in pp, pA and AA collisions for 8-45 AGeV beam energy• fixed target experiment

beam

STS track, vertex and momentum reconstruction

MUCH muon identification

TRD in case of muons only tracking

TOF time of flight measurement

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Challenges for μ identificationPeculiarities for CBM:• large track multiplicities

‒ up to 800 charged particles per reaction in +/- 25˚

• high reaction rate‒ up to 10 MHz

• reconstruction of low momentum muons (p> 1 GeV/c)• measurement of rare probes fast and efficient trigger and tracking algorithms• major background of muons from pion and kaon decay• punch through of hadrons, track mismatches• huge amount of data: foreseen are currently 1Gb/s archiving rate (600Tb/week) Central Au+Au collision at 25

AGeV (UrQMD + GEANT3)→ clean μ identification→ absorber-detector layers for continues tracking and

variable μ ID for different momentum ranges→ fast tracking algorithms are essential

standard: muon identification by absorber technique

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The Muon detector (MUCH)

3 detector stations between the absorbers

Low mass vector mesons 5 Fe absorbers (125 cm)7.5 I, p> 1 GeV/cCharmonium 6 Fe absorbers (225 cm)13.5 I, p> 2.8 GeV/c

Measurements of:

Alternating detector-absorber layout for continuous tracking of muons through the absorber

Detector challenges:• High hit density (up to 1 hit per cm2 per event)• High event rates (up to 107 events/s)• Position resolution < 300 μm→ consider GEMs (minimum pad size 2.8x2.8 mm2) for first stations→ for the last stations straw tubes are under investigation

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Track reconstruction algorithm

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Two main steps:– Tracking– Global track selection

Track propagation• Inhomogeneous magnetic field:

‒ solution of the equation of motion with the 4th order Runge-Kutta method• Large material budget: ‒ Energy loss (ionization: Bethe-Bloch, bremsstrahlung: Bethe-Heitler, pair production) ‒ Multiple scattering (Gaussian approximation)

Tracking is based on• Track following• Initial seeds are tracks reconstructed

in the STS (fast Cellular Automaton (CA), I.Kisel)

• Kalman Filter• Validation gate• Hit-to-track association techniques

‒ nearest neighbor: attaches the closest hit from the validation gate

Global track selection• aim: remove clone and ghost tracks• tracks are sorted by their quality,

obtained by chi-square and track length

• Check for shared hits

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Optimization and parallelism

Parallel programming is mainstream!• SIMD: Single Instruction Multiple Data• Multithreading make use of many core CPUs

Algorithm optimization• Minimize access to global memory: approximation of the MF map• Simplification of the detector geometry• Computational optimization of the Kalman Filter• float precision

Time [ms/event] SpeedupInitial 730 -Optimization 7.2 101SIMDization 4.8 1.5Multithreading 1.5 3.3Final 1.5 487

Computer with 2xCPUs Intel Core i7 (8 cores in total) at 2.67 GHz

Parallel tracking performance• Minor loss in tracking efficiency: 94.7% 94.0%• Significant speedup of the track finder:

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Tracking performanceSimulation: UrQMD events at 25 AGeV AuAu collisions + 5 μ+ and 5 μ-

Tracking efficiency vs. momentum

225 cm Fe125 cm Fe

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Reconstructed background percentral Au+Au collision at 25 AGeV

beam energy

• select primary vertex tracks (impact parameter)• cut on 2 of reconstructed MUCH tracks

Cuts:Muon identification

Rejection of muons from pion and kaon decays in STS applying

different cuts

125 cm Fe

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Physics feasibility study for low-mass vector mesons and J/ reconstruction in central Au+Au collisions at 25 AGeV beam energy

signals J/ψ Ψ'

background

signals ρ ω φ η ηDalitz

background

Signal-to-Background (S/B) ratio

Efficiency (%)

Mass resolution

(MeV)ωω 0.09 2 10φφ 0.03 4 12

J/ψJ/ψ 18 13 21Ψ'Ψ' 0.8 16 27

Muon identification

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Summary• Succesful demonstration of track reconstruction in the

proposed muon detector:– 95% tracking efficiency for muons passing the absorber; even in the high

track density environment– low rate of ghosts, clones and track mismatches

• Physics feasibility studies on J/ and low-mass vector meson reconstruction promising

• Preparation of fast reconstruction algorithms‒ Optimization and parallelization of algorithms: speedup factor of 2400 for

track fit; factor 487 for track finder

→ Develop fast trigger algorithms→ Use established tracking routines for layout optimization→ Investigate new parallel languages (CUDA, OpenCL)

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Backup

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Acceptance for vector mesons

Rho acceptance J/psi acceptance

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Absorption of particlesAbsorption of particles vs. absorber length

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Efficiency

Rho efficiency LMVM: Pt vs. invariant mass