ATLAS Luminosity Measurements Kristof Kreutzfeldt Justus-Liebig-Universität Gießen On behalf of the ATLAS Collaboration Latsis Symposium Zürich 03. June – 06. June 2013
Luminosity in 2011 and 2012 2012 2011 ● First LHC running period concluded in 2013 with RECORDS in luminosity ● Delivered Luminosity in 2011: L dt = (5.61 ± 0.10) fb -1 ∫ ● Delivered Luminosity in 2012: L dt = (23.3 ± 0.84) fb -1 (preliminary) ∫ ● Luminosity measurements played a major role in the Higgs boson discovery https://twiki.cern.ch/twiki/bin/view/AtlasPublic/LuminosityPublicResults Kristof Kreutzfeldt, U. Gießen 2
Luminosity Measurement Observed average number of inelastic ℒ= R inel μ vis interactions per bunch crossing σ inel = n b f r σ vis visible σ inel calibration of → absolute luminosity scale n b : Number of colliding bunch pairs f r : Revolution frequency (f LHC = 11245.5 Hz) OR )= N OR Strategy: OR ) P EventOR (μ vis = 1 − exp (−μ vis N BC ● Several detectors and algorithms to measure μ vis via inelastic rate ● Calibration of absolute luminosity scale by determining σ vis beam separation scans → ● Consistency of algorithms systematic uncertainties → Kristof Kreutzfeldt, U. Gießen 3
Luminosity Detectors Bunch-by-bunch luminosity: ● LUCID ● Dedicated Luminosity Monitor (5.6 < | | < 6) η BCM ● Beam Condition Monitor (BCM) ● Diamond sensors (| | = 4.2) η LUCID ● Horizontal and vertical pairs ● Inner detector system ● Primary vertex counting BCM (| | < 2.5) η ● Special conditions needed Bunch-blind luminosity: ● Calorimeter currents ● TileCal PMT (| | < 1.7) η ● FCal HV (3.2 < | | < 4.9) η Diamond detectors Kristof Kreutzfeldt, U. Gießen 4
Beam separation scans ρ 2 (x,y) ρ 1 (x,y) Bunch 2 Bunch 1 n 2 n 1 ℒ= n b f r n 1 n 2 ∬ ρ 1 ( x , y )ρ 2 ( x , y ) dxdy = n b f r n 1 n 2 2 πΣ x Σ y ● Proposed by van der Meer Σ x , Σ y : convolved beam widths ● Measure specific interaction rate n 1 n 2 : bunch population product ρ 1 , ρ 2 : normalized particle density for several beam separations in transverse plane Kristof Kreutzfeldt, U. Gießen 5 S. van der Meer, CERN-ISR-PO-68-31 (1968)
Separation scans in practice Luminosity from scan and rate: MAX 2 π Σ x Σ y ⇒σ vis =μ vis n 1 n 2 ● From scan data: ● Convolved beam widths (if gaussian → RMS ) ● Peak interaction rate ● Bunch population product from external beam current measurement (LHC group) ● Conditions with relative low number of bunches and peak rate ● Stability of measured σ vis with BCID and different scans assess uncertainties → Kristof Kreutzfeldt, U. Gießen 6
Scan stability ● L spec = L/(n b n 1 n 2 ) ● Emittance growth between scans ● Up to ≈ 10% variation by colliding ● Uncertainty: variation between bunch pairs (BCID) due to transverse BCIDs and scans ● Good algorithm consistency emittance (yellow band) Kristof Kreutzfeldt, U. Gießen 7
Extrapolation I-III VII-IX I II-III IV-V X-XV IV-IX ● Calibration for whole data taking periods → long term stability, highest rates, different bunch structure ● (Online) Luminosity from BCMV_EventOR algorithm ● Consistency of algorithms provides data driven uncertainties Kristof Kreutzfeldt, U. Gießen 8
Long term stability 2011 2012 Reference Reference ● Calibration of σ vis by only few vdM ● Very small variations in BCM scans assume stable σ vis over data → algorithms ● Slow drifts in TileCal and FCal taking period ● < >: average number of interactions ● Larger variations in 2012 than μ of one ATLAS run 2011 Kristof Kreutzfeldt, U. Gießen 9
dependence μ ● Pile-up effects increase at larger rates → linear measurements up to highest ? μ ● apparent < > dependence actually time dependence from “ramp up” μ ● Variation in FCal: systematic non-linear dependence on total luminosity Kristof Kreutzfeldt, U. Gießen 10
Systematic uncertainties Uncertainty source δ L / L 2010 2011 2012 Bunch population product 3.1% 0.5% vdM calibration Other vdM calibration 1.3% 1.4% Preliminary uncertainties Afterglow correction 0.2% BCM stability 0.2% extrapolation Long-Term stability 0.5% 0.7% μ dependence 0.5% 0.5% Total 3.4% 1.8% 2.8% ● Uncertainty of bunch population product reduced significantly ● 2012 analysis ongoing preliminary result as input for winter conferences → Kristof Kreutzfeldt, U. Gießen CERN-PH-EP-2013-026 11
Outlook ● Accuracy of luminosity calibration exceeded predictions ● Several luminosity calibrations from first LHC run still ongoing: ● 2012 p-p; 2011 Pb-Pb; 2013 p-Pb ● New challenges after LS1 with higher energies and interaction rates Additional calibration method beside vdM scans for the future: ● Measure small angle elastic p-p scattering in the Coulomb-Nuclear interference region with the ALFA detector (special beam optics needed) ALFA station Kristof Kreutzfeldt, U. Gießen 12
Backup Kristof Kreutzfeldt, U. Gießen 13
ATLAS detector Tile Calorimeter FCal ZDC @ 140 m ALFA @ 240 m LUCID MBTS BCM Inner detector Kristof Kreutzfeldt, U. Gießen 14
Bunch population product DC Current Transformer Fast Beam Current Transformer DCCT FBCT ● total current measurement ● bunch-by-bunch current with high accuracy measurement ● two in each beam ● two in each beam ● Relative fraction of total current in each BCID from FBCT CERN-ATS-Note-2012-026 ● Normalization to overall current scale provided by DCCT CERN-ATS-Note-2012-028 CERN-ATS-Note-2012-029 Kristof Kreutzfeldt, U. Gießen 15
Afterglow ● Likely caused by photons from nuclear de-excitation ● Luminosity background in bunch trains ● Corrected by subtracting luminosity in previous BCID Kristof Kreutzfeldt, U. Gießen 16
BCM calibration shifts ● Diamond sensors ● Luminosity scale varies up to 1% right after extended period without beam ● Stable value after several hours of exposure Physics ( L dt ≈ 5 x 10 36 cm -2 ) ∫ vdM ● BCMH calibration corrected for this drift ● No net drift for BCMV after several hours ● Additional systematic uncertainty applied Kristof Kreutzfeldt, U. Gießen 17
Single run dependence μ Single run from 2011 ● Shifts of algorithms result from long term stability variations ● Linear response with variations up to 0.5% level Kristof Kreutzfeldt, U. Gießen 18
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