FMS review, Sep-1-2009

FPD/FMS: calibrations and offline reconstruction

Measurements of inclusive 0 productionReconstruction algorithm - clustering - shower shape function - fittingCalibration - cell-by-cell gains - energy dependent corrections - run dependent corrections and LED monitoring Run9 analysis

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Measurements of 0 inclusive production with the FPD/FMS

Important physics results:

• cross-section is consistent with NLO pQCD calculations

PRL 92, 171801 (2004)PRL 97, 152302 (2006)

• precision AN measurements allow for

a quantitative comparison with theoretical models [PRL 101, 222001 (2008)]

• transverse spin asymmetries found at lower energies persist to √s=200 GeV

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Measurements of 0 inclusive production (2) Important physics results:

• azimuthal dependence appears to be as expected• AN is comparable to prior measurements with the FPD

0 reconstruction is a powerful tool for detector calibrations and monitoring

Measurements of 0 AN at √s=500 GeV are planned (Run 11?)

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Clustering - definitions• Start with a cell with maximum energy deposition in the matrix , adding adjacent cells with non-zero energy

• Define cluster parameters:

• Require that cluster energy Ec> 2 GeV

• Perform moment analysis to make an “educated guess” whether the cluster contains one or two photons

provide information about size and orientationof the cluster

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Clustering - categorizationSimulations of 0→

On the Ecmax–Ec plane, there are distinct event loci when the two photons maketwo different (1) clusters or a single (2) cluster, and the overlapping region => these parameters can be used to distinguish between one- and two-photon clusters

FPD data

type=11 clusters

type=0

type=22 clusters

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Shower shape functionTransverse shower profile was measured in the detector of lead-glass cells (the same as used in the FPD and in the FMS inner matrix) with a 10 GeV electron beam, and fitted by the function:

where d is a cell size and parameters are as follows: a1=0.8, a2=0.3, a3=-0.1 (a1+a2+a3=1),b1=0.8, b2=0.2, b3=7.6

For the “large” cells (outer matrix of the FMS) therewas no dedicated studies of shower shape; the samefunction used given that ratio of Molière radius to the cell size is close to that for the “small” cells.

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Fitting

• 1-fit - three parameters: photon coordinates x and y, and energy

• 2-fit - six parameters: pion coordinates x and y, polar angle of the line that connects the two photons in the detector local coordinate system, distance between the photons d, energy sharing z=(E1-E2)/(E1+E2), and summed energy of the two photons E=E1+E2

Each cluster is fitted with the shower shape function:• type=1 – only 1-fit is tried• type=2 – 2-fit is tried, but required that 2 is less than a preset value• type=0 – both fits are tried, decision made based on 2

Outcome of the reconstruction – list of photons with their coordinates and energies

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Calibrations - overview

• Cell-by-cell gains – using reconstruction gi = ci b, b – “basic” gain [MeV/ADC count], the same for all cells in a module; ci – correction factor for each cell

• Software correction factors ci then used for “online” calibration on hardware level:

- effective gains through LUT - adjusting HV for the PMTs need to know gain curve for an individual channel; data were obtained for the FMS in Run9

• Offline corrections: - energy dependent - run (time) dependent

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Cell-by-cell correction factors

• The Pb-glass detectors of the FPD/FMS are calibrated by associating gaussian centroid of the 0 peak seen in the di-photon invariant mass spectrum with the "high-tower" in the module

• The absolute gain of the cell is scaled to put the 0 peak at its known position, and the procedure is iterated until convergence

Run9, day=154-160 (200 GeV, “far” position)

• The calibration methodologies employed for the FPD have been successfully adapted to the FMS

Run8, FMS, WS-sml-top

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Energy dependent corrections

• Position of the 0 peak in the invariant mass distribution increased as a function of energy

• Dedicated MC study (with full Čerenkov light simulation) showed that there are three possible sources of this dependence: - missing energy due to longitudinal shower profile- transverse leakages- ADC granularity

Run9, day=154-160 (200 GeV, “far” position)

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Energy dependent corrections (2)

Correction works well forthe energy range where it wasdetermined, but extrapolation does not work if we go below ~10 GeV or above ~65 GeV =>

Energy dependent corrections must be determined for the whole energy range, where physics results are to be obtained(especially critical for data at √s=500 GeV)

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Energy dependent corrections (3)

.,., gensim EEE

• GEANT simulations and association analysis: comparison of generated quantities

to reconstructed values

Eliminating energy dependence in mass peak position gives the correct

neutral pion energy

uncorrected corrected

Calibration on s gets mass peak for heavier mesons correct

→ fit by Gaussian+p3μ=0.784 ± 0.008 GeV/c2

J/→e+e-

fit by Gaussian+Offsetμ = 3.080 ± 0.020 GeV/c2

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Run dependent correctionsFPD/FMS responses vary with time and beam conditions Run8, FMS

Run6, FPD

Up to ~10% variation; corrections were applied per module

The detector was stable within a few percent;no corrections applied

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LED monitoring system

LED system: critical calibration tool Run9, FMS – variations withtime in individual channels

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Run9 - first data at √s=500 GeVevent reconstruction in the FPD:

• using matrix+preshower (no SMD data)

• 20 GeV < Etotal < 80 GeV

• fixed vertex (z=0), no minbias condition

• N=2

FPD measures energy up to ~200 GeV ═>

SMD information is required to reconstruct pions above ~60 GeV

Example of 2-photon event when two clusters Significantly overlap in the matrix, but are clearly separated in the SMD

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Run9 data analysis

• Energy dependence is much stronger than it was for the FPD in all previous runs (comparable to FMS in Run8) –not yet fully understood

• slopes of the corrections in the data and simulations ??? • linear corrections do not work above ~65 GeV

FPD, day=101-103

PYTHIA+GSTAR simulations