karol buńkowski warsaw university the rpc based muon trigger of the cms experiment xi workshop on...

Karol BuńkowskiWarsaw University

The RPC based muon trigger of the CMS Experiment

XI Workshop on Resistive Plate Chambers and Related Detectors, 5-10 Feb 2012, Laboratori Nazionali di Frascati dell'INFN, Frascati (Italy)

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CMS detector

Karol Buńkowski, UWRPC2012, 5-10 February 2012

The Trigger and DAta Acquisition system at CMS

Readout buffers 128 events = 3.2 s

Level 1 Trigger

Dedicated electronics (ASICs, FPGAs) @ 40 MHz, only logic functions

Analyses every event (bunch crossing, BX) pipeline processing; latency 3.2 s, including ~2 s for data transmission between the detector and counting room, dead time free operation

Output ≤ 100 kHz

High Level Trigger (HLT)Computer Farm: 1008 nodes, 9216 cores, 16 TB memory

runs the software events selection algorithms

A few hundreds of Hz recorded on the magnetic tapes

Event Builder- switching network.Gathers the data from

one event into one HLT computer

Coarse data

Detector

keepreject

DAQ: readouts the data for the selected events, the events

are fragmented


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Level 1 trigger system

`

4 m4 m4+4 m

4 m

MIP+ISO bits

L1A (trigger)

40

MH

z p

ipe

lin

e

ECALTrigger

Primitives

HCALTrigger

Primitives

RegionalCalorimeter

Trigger

GlobalCalorimeter

Trigger

RPC hits CSC hits DT hits

Segment finder

Track finder

Pattern Comparator

Segment finder

Track finder

Global Muon Trigger

Global Trigger

TTC system

DAQ

TTS system

Detectors Frontend

Status

Link system

32 partitions

Muon TriggerCalorimeter Trigger

e/, J, ET, HT, ETmiss


Trigger subsystems: identify, measure and sort the trigger objects

Global Trigger apply cuts: single or multi-objects, topological correlations

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Counting roomDetector

FEB

FEB

FEB

RPC PAC muon trigger


Trigger Board

PAC

PAC

PAC

Resistive Plate ChambersUp to 6 layers of detectors.480 chambers in barrel, 504 in endcaps

FEB

FEB

FEB

Control & diagnostic

Ghost Buster &

Sorter

RMB

To the Global Muon Trigger

Link BoardLink BoardLink Board

Synchronization Unit & LMUX

Optic Links 90 m @ 1.6 GHz

1104 fibers

LVDS cables

To Data Acquisition

GB

&

Sorter

Data Concentrator

Card

1232 Link Boards in 96 Boxes,

Steered by Control Boards

84 Trigger Boardsin 12 Trigger Crates

Data transmission @ 320 MHz

SYNCH. &

LDMUX

* Numbers of elements for the staged version of the system

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Geometry of the RPC detector and PAC trigger segmentation


6 concentric layers of chambers in the barrel region, and 3 disc layers in each endcap (currently to || < 1.6,the endcap detector is staged, the 4th endcap station will be added in 2013/2014)

in phi plane: 1152 strips in each layer one strip = 0.3125˚

the detector is segmented in the eta plane into the trigger towers (~0.1-0.2 eta unit each)A tower comprise from 3 to 6 chamber layers

7 Karol Buńkowski, UW

Trigger Algorithm: Pattern Comparator (PAC)

RPC2012, 5-10 February 2012

The candidate is generated even though not all planes have hits. The minimum required number of fired planes is 3 (out of 3, 4, 5 or 6 planes available – depending on a tower). In this way the trigger efficiency is not suffering from the limited geometrical acceptance and inefficiency of the chambers.

The number of fired planes defines the candidate quality. The quality is used for the candidates sorting and “ghost busting” (cancelation of duplicated candidates).3/4

RPC layers

A pattern is a set of AND gates connected to selected strips

strips

The chamber signals (fired strips) are compared with the predefined set of patterns. Each pattern has assigned the pT and sign (depending on the track banding by the magnetic filed).

Muon candidate is recognized if the hits fit to the pattern and are in the same clock period (BX)

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Implementation of the PAC algorithm in the FPGAs

The trigger algorithm is implemented in the FPGA devices - Altera Stratix 2, 300 chips are

needed to cover full detector.

Each PAC comprises max 576 chamber strips and contains 3 000 – 14 000 patterns (most of them

low pT).

The patterns are built-in the firmware logic.

The patterns are generated based on the simulated muon track. Advanced algorithms are used to

create the patters from the simulated chamber hits, assign the pT, and then select optimal set of

patterns.

The goal is to achieve best possible trigger efficiency and purity with a patterns set that can be fit into

the PAC FPGAs.

Since each PAC contains different patterns, for each chip separate compilation is needed. The

software framework for patterns generation and firmware compilation on the computer cluster was

created. One iteration takes ~24 hours.

As the PAC algorithm is implemented in the reprogrammable FPGAs, it can be easy changed,

e.g. to correct bugs, improve performance, or implement new features.Karol Buńkowski, UWRPC2012, 5-10 February 2012

Synchronization of the trigger system (1)

4.2m = 14ns

14m = 42ns

Seminarium Oddziału Fizyki i Astrofizyki Cząstek IFJ, 26 maja 2009

Karol Buńkowski, UW

• The time of muon flight from the interaction point to the different chambers varies from 14 to 42 ns - more than 1 BX

• The time of signal propagation from the chambers to the Link Boards varies from 33 to 107 ns (due to differences in the cables lengths)

The chamber hits must be in the coincidence (in the same BX, i.e. 25 ns clock period) on the PAC input to produce the muon candidate the system synchronization is crucial for its performance

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• The initial position of the synchronization window winOpeni and data delay didata was

calculated based on:– muon hits timing ti

hits which is a sum of the muon time of flight (know from the simulations) and

signal propagation time in the cables,

– known length of the fibers transmitting the clock (clock phase difference iTTC ):

winOpeni = (tihits + i

TTC + offset) % 25 ns

didata = a – int[(ti

hits + offset)/25ns] + bi - (1*) + ciwin + (2SM)

• Then the synchronization was corrected based on the collected collision data

Synchronization of the trigger system (2)

Karol Buńkowski, UW

25ns

collision

LB1

LB2

LB3

delay

Synchronized signals

Time offlight

propagationin cables

time

Synchronization window

RPC2012, 5-10 February 2012

The chamber hits are synchronized to the 40 MHz LHC clock in the Link Boards. The hits are “quantized” to the full BX (i.e. the timing is measured with the 25 ns precision) with used of the “synchronization window”. Its position can be adjusts with 0.1 ns accuracy.

Then the hits are aligned between the Link Bards by applying full BX delays.

The goal is to have all hits of all muons fromgiven event within 25 ns on all LBs.

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Calculation of the timing correction from hits BX distribution

After the initial synchronization in most of the LBs the chamber hits were concentrated in ONE or TWO neighboring BXs: -1, 0, 1

Karol Buńkowski, CERN, UW, L1 DPG, 22 April 2010

BX =0 BX =1BX = -1window

#hitsTiming correction

winOpen

time

From the data we know only the distribution of the hits in the BXes (w.r.t. the correct BX of the event). From this the value of the timing correction must be obtained.

We have not measured the hits timing distribution from the collisions because it would required time consuming scanning. We utilized the hits timing distribution obtained from the simulations

Assuming that the mean hits BX (from data) corresponds to the cumulative distribution function of the simulated timing of the hits, the timing correction can be calculated:

Muon hits timing from simulations [ns]

Hit

s d

istr

ibu

tion

Cu

mu

lati

ve d

istr

ibu

tion

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Synchronization of the trigger system - results


Distribution of chamber hits BX w.r.t. the event BX

Distribution of the hits mean timing and spread (rms) for individual Link Boards

99.98% of hits associated to the muon tracks are in the correct BX=0

Since the start of the collisions in the April 2010 the synchronization was corrected 7 times.

In ??? of the 99.9??? % of hits is in the correct BXBad timing on a few LBs due to problems in chambers or signal cables

Data selection!!!!!!!!!!!!!!!!!

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Trigger on HSCPs• Some supersymetry and models foresee Heavy Stable

Charged Particles (HSCPs), e.g. stop, gluino, stau. They mass could range from ??? To ??? GeV, thus if produced at LHC they velocity would be ~0.2 – 0.9 c.

• In the CMS they will look like “slow muons”: the hits in the muon chambers (all or outermost) can be up to 1 BX later than the hits of the muons – they will not produced the muon trigger at all (hits not in

coincidence in one BX) or – the trigger will be 1 BX to late the tracker hits will not be

recorded (pixel detector stores only the hits from one BX/event).


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Trigger on HSCPs PAC modification

Karol Buńkowski, CERN, UW, Trigger Meeting, 28 June 2011

layer 6layer 5layer 4

Chamber hits (PAC input)

extended hits (in the PAC)

BXMuon

candidate

normal muon


L1AMasked by BPTX veto

In the PAC trigger we found a way to trigger on the HSCPs:• In the PACs the detector signal are extend to 2 BX and • On the GMT input the PAC candidates delay is reduced by

1 BX (w.r.t. the DT and CSC candidates)Þ the hits of the “late particle” generate the trigger in the

proper BX!Þ for in-time muons candidates in 2 BX appear - the first

candidate is too early, but he second is in the proper BX.• The first candidate is masked on the GT by the BPTX veto –

signal synchronous to collision, but advanced 1 BX (used for all trigger to eliminate the pretriggering).


BX

late particle


L1A

Muon candidate

Chamber hits (PAC input)

extended hits (in the PAC)

Significant increase of the efficiency to trigger on lower momentum, slower moving HSCPs e.g. for gluino 800 GeV from 24 to 32%

Guino 800 GeVMonteCarlo

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Timing of the RPC PAC candidates results


99.9??% of PAC candidates are in the correct BX=0

Data selection!!!!!!!!!!!!!!!!!

The candidates corresponding to the muons from the collisions are duplicated in the BXs -1 and 0

To early or to late candidates (~10-4) are mostly from the cosmic muons

BX of the RPC candidates w.r.t. the L1 trigger BX

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Efficiency of the RPC detector and PAC trigger vs.


Data selection and method !!!!!!!!!!!!!!!!!

The efficiency of RPC PAC trigger for identifying muons is a convolution of:• εacceptance – geometrical acceptance of

the RPC detector (probability that muon crosses at least 3 chambers),

• εchambers – chambers intrinsic efficiency,

• εpatterns – patterns efficiency i.e. probability that the chamber hits of a “triggerable” muon fit to any pattern;

“triggerable” muon – hits in at least 3 RPC layers inside the eta-phi cone covered by one PAC unit and in the same BXεtriggerable muon = εacceptance εchambers

Detector acceptance

Triggerable muons

RPC PAC trigger eff.

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RPC PAC efficiency- turn on curves

• From tag and probe???


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RPC chambers monitoring via the PAC trigger hardware


Time [s] strips

Hits

rat

e [H

z]

In the Link Boards firmware the multichannel counters allowing to measure the signal rate for each strip individually are implemented: All hits are counted: no bias from trigger (unlike in the DAQ data), big statistic, The signals for all strips are counted, even those masked, The data are stored for the offline analysis: neutron background, chamber noise

noisy strips masking, Front-end thresholds tuning (see talk by ?????).

The basic plots (rate v.s. time for each chamber, average and maximal rate per strip) are produced in the real time by the software controlling the hardware – the chambers performance can be evaluated online, the problems (noisy strips, dead chambers) can be noticed promptly.

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2011 performance summary• Only a few minor hardware failures, promptly repaired.

Beside that 100% of the trigger hardware operational and working correctly.

• CMS down time (dead time?) due to the RPC PAC trigger during 2011 collisions only !!!%

• Excellent synchronization of the system: 99.98% of muon chamber hits in the correct BX 99.99??% of the PAC candidates in the correct BX.

• The only trigger subsystem capable to trigger on HSCP.• Average RPC PAC trigger efficiency 80???% .

Continuous work on the patterns optimization, according the CMS requirements (efficiency – rate tradeoff)


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21 Karol Buńkowski, UWRPC2012, 5-10 February 2012

karol buńkowski warsaw university the rpc based muon trigger of the cms experiment xi workshop on...

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