Simulation of the ATLAS Inner Detector

Laura Gilbert: Graduate Seminars 04/03/04

Contents

• Introduction

• The Inner Detector

• Event Generation

• Detector Simulation

• Event Reconstruction

• Summary

Introduction

Worldwide more than 1800 physicists from 35 countries are working on one of the biggest experimental set-ups in physics, the ATLAS detector at the CERN laboratory. ATLAS will be put into action with the most powerful accelerator of the world, the Large Hadron Collider (LHC). The LHC should be operational in 2007 and will create conditions like those a fraction of a second after the big bang.

The simulation of the detector is important during the design phase to develop a detector with an optimal discovery potential within the constraints from technology, survivability and finances. When the final detector is taking data the simulations become important for calibration and understanding of the data.

The Inner Detector

The Inner Detector

• The ATLAS Inner Detector sits within the Central Solenoid (around 2T).

• Combines high-resolution pixel detectors at the inner radii with continuous tracking elements (SemiConductor Tracker, Transition Radiation Tracker) at the outer radii.

• High-precision detectors are arranged in concentric cylinders around the beam axis.

• End-cap detectors mounted perpendicular to the beam axis.

• Relative precision of detector measurements well matched, no single measurement dominates the momentum resolution.

1.15m0.5m

7m

The Inner Detector

• Semiconductor Pixel Detector: Very high granularity set of high precision measurements close to interaction point :- short-lived particles.

• Silicon Tracker: Eight precision measurements per track in the intermediate radial range :- momentum, impact parameter, vertex position.

• Typically pixel detector contributes three and SCT four space points per track.

• Transition Radiation Tracker: “straw tube” tracker - straw detectors running parallel to beam direction

• Operates at high rates due to small diameter and isolation of sense wires within individual gas volumes.

• Provides typically 36 tracking points. 1.15m0.5m

7m

Simulation

• Event Generation– Monte Carlo packages

• Detector Simulation– The Geant Toolkit

• Event Reconstruction– Pattern Recognition and Track Reconstruction– H→bb

Event Generation• Simulation of pp collisions and subsequent hadronisation according to models

such as QCD and QED. • Attempts to simulate the complete process from given initial state to a proper

sampling of possible final states, using Monte Carlo method.• Method:

– Leading order calculation of hard parton-parton matrix elements– Treatment of initial state (inc gluon radiation) before hard scatter– Treatment of final state with more gluon radiation and hadronisation

• Monte Carlo packages used to generate events include (soft gluon emission and fragmentation):– HERWIG: simulating Hadron Emission Reactions With Interfering Gluons– ISAWIG includes supersymmetry in a “completely general way”.– PYTHIA + JETSET: Uses the “symmetric string” Lund Model.– ISAJET: Uses “independent string fragmentation” model for p-p, pbar-p,

and e-e interactions at high energies. pQCD + phenomenological models. • Output of event generator is a set of 4-vectors describing “stable” final state

particles (τ>10-8).

ATLAS Simulation• Every detector simulation has to start with a description of the detector geometry.

For the Inner Detector the most important physics processes simulated are – multiple scattering, – continuous energy loss, – bremsstrahlung for electrons, – conversions for photons and – nuclear interactions for the hadrons.

• For the ATLAS detector almost every detail is included in the detector description. Eg. TRT:

– carbon fibre shells of the modules– straw walls– gas composition inside and outside the straws– wires

• Some simplifications used in materials not acting as active detecting elements: – For support structure, cooling, electronics: materials assumed to have a flat distribution

in the φ coordinate.– Misalignments of detector elements not considered.

• After particle generation, the particles must be followed through the rest of the virtual detector apparatus.

→ GEANT

The Geant ToolkitGEANT: Detector Description and Simulation Tool

• Program describing the passage of elementary particles through the matter. Used in HEP and accelerator design (eg. LHC, BaBar, CMS), astrophysics, nuclear and medical physics.

• Includes a complete range of tools for all the domains of detector simulation: - Geometry - Tracking - Run, Event, Track management - Detector Response - Visualisation - User Interface.

Also includes interfaces to event generators mentioned earlier. • Offers many choices of physics models, eg. EM, hadronic and optical processes.• Large set of long-lived particles, materials and elements, wide energy range (~250

eV - TeV).• To get accurate results need the correct distribution and composition of the

materials. • Digitisation phase: For each sub-detector there is a model of how the ionisation in

the active detector element is changed into the digital output of the readout electronics, inc. simulation of noisy and dead channels .

• Simulated data should equivalent in format to the “real” data.

• Reconstruction of the kinematic information and particle identification.

• For the Inner Detector (ID) the problem is to find all the tracks – pattern recognition (algorithms).

• Must consider or define what makes a “hit”: occupancy, efficiency, noise level and shared hits.

• Optimal fit is then made with positions of all hits on a track to extract the track parameters – track reconstruction.

• b-quark jets recognisable in TRT but not in silicon detectors:

– Small number of hits – presentation poor for human eye

• Errors: wrong hits, double counting, reconstruction of fake tracks.

• All detector information combined to obtain kinematic information -individual hits in ID and muon system, clusters in calorimeter cells.

An event display of the barrel part of the Inner Detector with the decay of a Higgs particle to b-quarks at high luminosity.

Barrel TRT

Silicon detectors

Reconstructed tracks

Event ReconstructionExample: H→bb Simulation in the Inner Detector

•Point out pixel dets 3 points, sct 4

Pattern Recognition Algorithms

iPatRec • Initiate track-finding from space-points in the outer pixel

and SCT tracking layers: excellent resolution, low occupancy.

• Reconstructed tracks in solenoidal field described by helix (5 parameters): define narrow road connecting EM cluster seed and primary vertex.

• Innermost (“B”) pixel layer and TRT drift straws used to confirm tracks and improve precision of reconstructed quantities (historgramming method).

• For each track candidate, track is propagated into the remaining detectors. Fitted tracks passing quality cuts on no. silicon detector hits and χ2 are accepted.

XKalman• Starts with Hough transform of all hits in the TRT.• Kalman filtering method is used to propagate the track

candidate parameters through the silicon detectors towards the primary vertex – full track info including error parameters. (PR and TF in one step.)

• Multiple scattering effects + possible kinks on the track from hard bremsstrahlung photons emission.

Several different reconstruction methods have been developed for the pattern recognition based on different philosophies. Examples:

parameterising curves into straight lines, polynomials, circles etc in parameter space

mathematical equations that provides an efficient computational (recursive) means to estimate the state of a process, in a way that minimizes the mean of the squared error

CalorimeteriPatRec

XKalman

Summary

Sources of further information

• Atlas Homepage: http://atlas.web.cern.ch/Atlas/

• Atlas detector overview: http://atlas.web.cern.ch/Atlas/SUB_DETECTORS/DetStatus/DetStatus.html

• “Physics Reference Manual” Physics of particle decays (GEANT): http://wwwasd.web.cern.ch/wwwasd/geant4/G4UsersDocuments/UsersGuides/PhysicsReferenceManual/html/PhysicsReferenceManual.html

• Thesis by Ulrik Egede (“The search for a standard model Higgs at the LHC and electron identification using transition radiation in the ATLAS tracker”): http://www.quark.lu.se/~atlas/thesis/egede/thesis.html