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High Level Trigger of M uon Spectrometer. Indranil Das Saha Institute of Nuclear Physics. Description of the problem. Signal. Background. y. L0 Trigger by Muon Trigger. J/ Ψ  μ + μ - Y  μ + μ - B  μ + / μ - + X D  μ + / μ - + X. Muons from π and K. Z.

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High level trigger of m uon spectrometer

High Level Trigger of Muon Spectrometer

Indranil Das

Saha Institute of Nuclear Physics

Description of the problem
Description of the problem




L0 Trigger by Muon Trigger

J/Ψ μ+ μ-

Y  μ+ μ-

B  μ+/μ- + X

D  μ+/μ- + X

Muons from π and K


(for hadronic background)

Continue hardware and software trigger…..

P2 = (P1 + P2)2

Description of the problem1
Description of the problem

  • For ALICE Muon Spectrometer

    L0 (hardware trigger)

    == Hits in trigger station.


    Track pointing to origin


    Option for transverse momentum (pT) selection.

  • Muons coming from Pions and Kaons decay have low pT .

  • Thus to reduce the background two pTcuts of 1 GeV or 2 GeV has been planned to clean invariant mass spectrum of J/Ψ and Y.






Software cut improves the result, so why not online software cut ?

Description of the problem2
Description of the problem

HLT : Thus, on top of the hardware trigger a software trigger is applied, which is called High Level Trigger (HLT). The HLT is an online system that provides the common facilities (i.e. Input/Output Link, PC farm for analysis) and executes the triggering algorithms (full reconstruction) developed for different detectors.

Main motivation of Dimuon HLT: In case of Muon Spectrometer, HLT algorithms are supposed to improve the pTcuts (1 GeV or 2 GeV at most, depending on beam luminosity) as defined by Muon Trigger Station (L0 Trigger), which are important remove the combinatorial background of low momentum particles to clear the J/Ψ and Y signals. A typical processing rate of 1 kHz is the design requirement of muon HLT for heavy ion Pb-Pb collisions.

How do we deal online data in reality ?

Alice data flow in online mode
ALICE data flow in online mode


Rawdata (GRID)









Physics Trigger

{Level 0 [L0]}


Event Display


But then it is very straight forward right ?

Description of the problem3
Description of the problem

  • One simulated Pb-Pb event needs few seconds for offline reconstruction (official physics analysis software) and therefore this can not be used for online analysis.

  • Thus, a new algorithm has been developed which will perform online analysis of data and satisfies,

    • A online processing component has to be robust (24x7)

    • It is to be fault tolerant against input data stream

    • It has to provide results of appreciable quality

    • It has to be fast enough so that the rest of experimental component does not wait for HLT

      If any of above conditions are violated dimuon part of HLT is excluded from the experimental run.

Data rate of 500 MB/s for 1 kHz trigger rate with hit multiplicity ~ 300

Accuracy as good as offline reconstruction within the time limit of 1 ms


  • Development of the Algorithm

    • Hit Reconstruction (Thesis work)

    • Trigger Reconstruction (UCT + Cagliary)

    • Track Finding (Thesis work)

      • Cellular Automata

      • Kalman Filtering

  • Validation with the simulated data (Thesis work)

  • Implementation in HLT PC-Cluster and validation with cosmic data (Thesis work)

  • Online display of LHC p-p data (Thesis work)

  • Ongoing Analysis of p-p data (Thesis work)

At first the Hit Reconstruction algorithm…..

A fast algorithm
A fast Algorithm

  • The charges are found to spread over 2 or 3 pads along a direction in most of the cases => one cluster.

  • Each cluster is characterized by one Central Pad (Pad with maximum charge for the given cluster).

  • Thus, to generate reconstructed hits, it is not essential to make clusters but only identification of Central Pad is sufficient.

  • Once the central pad is found the reconstructed Y hits in bending plane is calculated using centre of gravity method over three pads around the central pad.

  • Above method is repeated for non bending plane to find out reconstructed X .

  • Finally the bending and non bending hits are merged to form reconstructed X and Y.

Comparison with offline…

Comparison of simulation results
Comparison of Simulation Results

Fine !!! What about tracking ?

Charged particle track
Charged particle track



MCS and E.Loss Correction

Cellular Automata

St. line track

Kalmanχ2 match

Trigger Seed

Full tracker scheme
Full Tracker Scheme

Linear Extrapolation in St4/5

A trigger tracklet can be formed

with the trigger seed, which is

extrapolated to find the ROI in St4/5.

the hits inside those ROI are

checked for the alignment with the

trigger tracks and added to extrapolate

the tracks.

Trigger Seed

The Trigger chamber issues a

Trigger when at least three out of four

planes are fired along linear direction

Kalman Chi2Test

The tracksegs in the St4/5 are

extrapolated through the magnetic field

using Kalman filter to meet the trackseg

In St1/2.

CA in St1 and St2

At First the small tracklets between the

two chambers of a given station are

formed. Then the tracklets from different

stations are collected to make a

track segment in the forward half of the



The tracks are then extrapolated to

vertex to incorporate energy loss and

MCS (multiple Coulomb Scattering)

in the Muon absorber .

What is Cellular Automata ?

Cellular automaton
Cellular Automaton

Cell Creation

Cell is a small tracklet between the

two chambers of a given station.

The tracklet object contains three

pointers, one corresponds to its rank

in the track and the rest two points

towards each other

Connect Tracks

The cells from different stations are

collected into a track from the Muon

Trigger stations towards

station1 with certain strict boundary


How does Kalman Filter Works ?

Kalman filtering process
Kalman Filtering Process

Initial Estimate


Kalman Gain

Project to k+1

Update Estimate

Update Covariance

Projected Estimate

Result on simulated data….

Transverse momentum reconstruction at a given p t
Transverse Momentum Reconstruction at a given PT

For other delta PT values…..

Comparison of different tracking approach
Comparison of Different Tracking Approach

How does it fit to the PTefficiency plot ?

P t cut efficiency
PT Cut Efficiency

Pause and discuss….

Offline testing is not same as online implementation….

First track in muon spectrometer of alice was detected by sinp group
First Track in Muon Spectrometer of ALICE was detected by SINP group

Run 24841

March 3, 2008

And many more…..

Event snapshots of alice control room
Event Snapshots of ALICE Control Room

ALICE Control Room (ACR)

First online snapshot of muon track in 7 TeV(highest energy) pp collision beam at record luminosity (~ 6 × 1028 cm-2 s-1), processing at a rate of 700 Hz

Run 119842

Not only display but quality assurance qa as well
Not Only Display, but Quality Assurance(QA) as well……….

Cluster Dist. Attached to the tracks

PT Distribution


Real Time Vertexing !!!!!!

Online Inv. Mass Distribution

Analysis of the quality of the events
Analysis of the quality of the events….. well……….

Total ADC Charge per Data Link

Number of Clusters per Data Link

Trigger Type as declared by Trigger Chamber

Charge Ratio in the Two Planes of the Detectors

Monitoring even deeper
Monitoring even deeper….. well……….

Cluster size distribution

Cluster Charge Distribution

Offline Comparison

Ongoing analysis of p p data
Ongoing Analysis well……….of p-p data

The performance plots are shown for the Full-Tracker on data from 54 runs from LHC10d:

126097 12608 126087 126081 126078 126073 126008 126007 126004 125855 125848 125847 125844 125843 125842 125632 125630 125628 125296 125292 125186 125156 125139 125134 125131 125101 125100 125097 125085 125083 124608 124607 124606 124605 124604124603 124600 124388 124380 124378 124374 124371 124367124364 124362 124360 124359 124358 124355 124187 124186 124183 122375 122374



How does L0, Offline, HLT triggered mass spectrum look like …. ?

Any track passes 1 0 gev cut
Any track passes 1.0 well……….GeV cut

Summary and future plan
Summary and Future Plan well……….

Successful development, simulation testing, onlineimplementation, real-time execution and validation by physics analysis of the Hit Reconstruction and Track Reconstruction algorithms

Histogram incorporation to QA for global run characteristics.

Continue Validation test with p-p and Pb-Pb collision.