Online Measurement of LHC Beam Parameters with the ATLAS High Level Trigger David W. Miller on behalf of the ATLAS Collaboration 27 May th Real-Time Conference Lisbon, Portugal 27 May, ATLAS Online Beam Parameter Measurment - RealTime 2010

The Inner Tracking Detectors ATLAS Online Beam Parameter Measurment - RealTime May, 2010 Silicon Strips –4 barrel layers + 2 x 9 end-cap disks –σ r ϕ ~ 17μm; σ Z ~580μm –6.3 million channels Silicon Pixels –3 barrel layers + 2 x 3 end-cap disks –σ r ϕ ~ 10μm; σ Z ~115μm –80 million channels Transition Radiation Drift Tubes –73 barrel straws + 2 x 160 end-cap disks –σ r ~ 130μm –350,000 channels TRT SCT PIX

The ATLAS Trigger System ATLAS Online Beam Parameter Measurment - RealTime May, 2010 Hardware Level ‐ 1 Trigger Calorimeter Muon System Hardware based Coarse granularity 2.5us 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 ATLAS Online Beam Parameter Measurment - RealTime May, 2010 Param900 GeV7 TeV14 TeV Spot size [μm]20774 / 3212 Length [cm] 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 (σ x, σ y, σ z ) 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 ATLAS Online Beam Parameter Measurment - RealTime May, 2010 Estimate the vector R (vertex position) using the measurements at the reference surface

ATLAS Online Beam Parameter Measurment - RealTime May, 2010 The LHC came online in record time

ATLAS Online Beam Parameter Measurment - RealTime May, 2010 declare stable beams Before we can safely turn on the silicon tracking detectors to see beam, LHC operators must “ declare 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 ATLAS Online Beam Parameter Measurment - RealTime May, 2010 See PDAQ-28 from I. Christidi See PDAQ-28 from I. Christidi

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 ATLAS Online Beam Parameter Measurment - RealTime 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 ATLAS Online Beam Parameter Measurment - RealTime 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! ATLAS Online Beam Parameter Measurment - RealTime May, 2010 First LHC fill with > 4 colliding bunches Only 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 ATLAS Online Beam Parameter Measurment - RealTime May, 2010

Measuring the luminous region at 7 TeV ATLAS Online Beam Parameter Measurment - RealTime May, 2010

Luminous region tilt ATLAS Online Beam Parameter Measurment - RealTime May, 2010 “Real-time” interaction point characterization Bunch-to-bunch position Time evolution Independent track-only fitRelative Luminosity Beam 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, s MBs of configuration data –Configuration data cached in proxy tree ATLAS Online Beam Parameter Measurment - RealTime 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 ATLAS Online Beam Parameter Measurment - RealTime May, 2010 Gather data from nodes Gather data from nodes Process result (fit beamspot position, update DB) Process result (fit beamspot position, update DB) Feedback to L2 nodes and algos Feedback to L2 nodes and 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 ATLAS Online Beam Parameter Measurment - RealTime May, 2010