online measurement of lhc beam parameters with the atlas high level trigger

Online Measurement of LHC Beam Parameters with the ATLAS High

Level Trigger

David W. Milleron behalf of the ATLAS Collaboration

27 May 201017th Real-Time Conference

Lisbon, Portugal

27 May, 2010 1ATLAS Online Beam Parameter Measurment - RealTime 2010

The Inner Tracking Detectors

ATLAS Online Beam Parameter Measurment - RealTime 2010

227 May, 2010

• Silicon Strips– 4 barrel layers + 2 x 9 end-cap disks– σrϕ~ 17μm; σZ~580μm– 6.3 million channels

• Silicon Pixels– 3 barrel layers + 2 x 3 end-cap disks– σrϕ~ 10μm; σZ~115μm– 80 million channels

• Transition Radiation Drift Tubes– 73 barrel straws + 2 x 160 end-cap disks– σr~ 130μm– 350,000 channels

TRTTRT

SCTSCT

PIXPIX

The ATLAS Trigger System


327 May, 2010

HardwareHardwareLevel‐1 TriggerCalorimeter

Muon SystemHardware based

Coarse granularity2.5us

Level‐2 Trigger RoI e/γ, μ, jet, ..

Full granularity in RoI~ 500 PC (multi‐core)

~40ms

Event Filter~1800 PC (multi‐core)High bandwidth data

network~4s

3-level trigger system– L1: Hardware/firmware algorithms– L2: Software algos: regions of interest– L3 (EF): Software: full detector

Access to inner tracking detectors– Level-2 is first opportunity to perform track

reconstruction– Limited to 40ms per algorithm– Can pull data from nearly 90 million

channels

The LHC Machine


427 May, 2010

Param 900 GeV 7 TeV 14 TeV

Spot size [μm] 207 74 / 32 12

Length [cm] 7.9 4.2 4.5

• It is not enough to simply collect data, we have to collect good data– Measure and monitor the LHC

beams inside of ATLAS every two minutes

• Optimal ATLAS and LHC performance depends on high beam quality and operational efficiency– Feedback information on beam

quality within ATLAS to LHC operators

Introduction to the online beam spot measurement

Motivation and Goals• Measure and monitor the

interaction point position (x, y, z) profile (σx, σy, σz) and tilt

• Communicate the “luminous region” parameters to the ATLAS and LHC control rooms

• Feedback to Level-2 (L2) algorithms (e.g. b-tag) for optimal performance

• Provide relative luminosity monitor via vertex counting

Design and Constraints• Robust L2 tracking algorithms with

Silicon-based pattern recognition– Full Tracking: ~100s ms (subset of evts)

• Fast L2 vertexing using decorrelating transformation

– Vertexing: ~0.2 ms (10-2 of time budget)

• Expect ~kHz rates into L2, run also on rejected events: factor >10 more stats

• Gather (“pull”) and sum data from 1000’s of processor nodes


527 May, 2010

Estimate the vector R (vertex position) using

the measurements at the reference surface


627 May, 2010

The LHC came online in record time


727 May, 2010

Before we can safely turn on the silicon tracking detectors to see beam, LHC operators must “declare stable beamsdeclare stable beams”…we

were very happy

First ATLAS Data with the HLT

• With first stable beams came the first opportunity to catch a glimpse of the LHC beams within ATLAS– Activate full HLT farm (= hundreds-thousands of nodes)– Pull data from Inner Detector read-out drivers– Perform full track pattern recognition and fitting– Use fast vertex-fitter to reconstruct individual event vertices

• All within the time budget of a ~40ms at L2


827 May, 2010

See PDAQ-28

from I. C

hristidi

See PDAQ-28

from I. C

hristidi

Routine online luminous region measurements

• Within days, the high-level trigger became a routine component of operations– Position measured every ~2 min.

• Online “beam spot” (luminous region) parameter determination based on massively parallel monitoring infrastructure


927 May, 2010

Complementing the beam instrumentation

• By measuring the longitudinal vertex position we can compare to (and calibrate) the LHC beam instrumentation

• The BPTX sensors provide precise ToF measurements of the Z-position• We calibrated the ToF (remove offsets) and provided feedback to the

LHC operators on the positioning of the interaction point in ATLAS


1027 May, 2010

BPTX: electrostatic sensors to provide time-of-flight measurements of the Z-position of individual proton bunches

(See talk by J. Lundberg)

Bunch-to-bunch Measurements

• Ultimate LHC design: 2808 colliding bunches per orbit– Crucial to understand if all bunches “look the same”– Monitor the bunch-to-bunch positions and vertex count– Provides estimate of background

• i.e. “do we see vertices where we shouldn’t?” --- Answer today: No!


1127 May, 2010

First LHC fill with > 4 colliding bunches

First LHC fill with > 4 colliding bunches

Only find vertices in 9 colliding bunchesOnly find vertices in 9 colliding bunches

Online luminosity monitoring

• By continuously monitoring the vertex count we obtain a direct measure of the relative luminosity

• Comparison with “standard” luminosity detectors indicates excellent shape agreement over large range of luminosity– Orthogonal acceptance ranges– Implies very little background


1227 May, 2010

Measuring the luminous region at 7 TeV


1327 May, 2010

Luminous region tilt


1427 May, 2010

“Real-time” interaction point characterization

Bunch-to-bunch position Time evolution

Independent track-only fit Relative LuminosityBeam instrumentation calibration

Full circle: feeding back measurements to the HLT

• Primary clients of online beam spot measurement:– Tracking (generally)– b-Tagging

• Precise knowledge of the LHC beams in ATLAS is crucial for optimal trigger performance

• But need to redistribute parameters determined in quasi-real time to thousands of running processes– Extremely challenging

• Real-time reconfiguration of HLT farm made possible via proxy-tree– ~810 nodes, 10-100s MBs of

configuration data– Configuration data cached in

proxy tree


1527 May, 2010

See PDAQ-15

for the details

See PDAQ-15

for the details

Real-time configuration changes

• Must ensure consistent and reproducible configuration across the entire HLT farm…

• …without incurring deadtime or disrupting data-taking

• Each proxy caches the result of DB queries

• Client applications are “notified” of conditions update and read new beam spot information


1627 May, 2010

Gatherdata from nodes

Gatherdata from nodes

Processresult

(fit beamspotposition,

update DB)

Processresult

(fit beamspotposition,

update DB)

Feedbackto L2 nodesand algos

Feedbackto L2 nodesand algos

See PDAQ-15

for the details

See PDAQ-15

for the details

Summary and Conclusions

• We have successfully deployed and utilized a set of algorithms for measuring and monitoring the LHC luminous region parameters in ATLAS in real-time

• Measurements at both 900 GeV and 7 TeV indicate that these algorithms are robust and crucial for optimal performance of L2 trigger algorithms

• The redistribution of these measurements to thousands of running processes within the L2 trigger farm has been successfully tested and will be used for real-time updates of the LHC parameters


1727 May, 2010

online measurement of lhc beam parameters with the atlas high level trigger

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