J.P. Gómez-González. Outline Introduction: The ANTARES neutrino telescope Time calibration Muon track residuals based method: Description and implementation

Calibración temporal del telescopio de neutr inos ANTARES mediante trazas de muones atmosféricos

J.P. Gómez-González

Outline

• Introduction:• The ANTARES neutrino telescope • Time calibration

• Muon track residuals based method:• Description and implementation• Inter-line calibration• Intra-line calibration

• Cross-check with the OB system • Detector performance

• Summary/Conclusions

1 Calibración temporal en ANTARESBienal de Física 2013

Neutrino astrophysics


p

γν

n

• Astroparticle physics: Point-like sourcesDiffuse flux, galactic plane neutrino fluxFlaring blazars / microquasarsGRBsFermi Bubbles

• Particle physics:Neutrino oscillationAtmospheric neutrino and muon fluxCosmic ray anisotropy / compositionShower reconstructionElectromagnetic showers

• Detector related:Timing / PositioningMoon shadowWater optical properties, group velocity of lightAcoustics, bioluminescence

• Searches: Dark matter annihilation,magnetic monopoles, nuclearitesMulti-messenger astronomy: Gravitational waves, Auger CRs, optical telescopes

Detection principle


m

42°

interaction

Earth’s crust

Cherenkov light

from m

nm

Detection principle

“We propose getting up an apparatus in anunderground lake or deep in the oceanin order to determine the location of charged particles with the help of Cherenkov radiation” M. Markov, 1960

Detector:3D array of

photomultipliers

At high energies the neutrino and the muon are almost collinear Astronomy

Physical background


nm

p

nm

nm

mp, a

• Huge (downgoing) atmospheric muon contamination• Irreducible contribution from atmospheric neutrinos

Downgoing muon flux is about 6 orders of magnitude higher than neutrino flux

The ANTARES detectorANTARES is a submarine neutrino telescope made up by:


885 10” PMTs distributed in triplets of storeys along 12 strings of 450 meters longitude placed 2475 depth following an octagonal layout.

Muon track reconstructionTime and position information of the hits registered by the PMTs is used for track reconstruction Offline strategy based on the time residual; difference between the measured hit time and the time expected from the fitted muon (given a certain set of track parameters)


Time residualsThe reconstruction algorithm is a multi-step approach which final step consists on the minimization of the time residuals PDF


Time residual distribution for data events

Tim

e r

esi

duals

PD

F

Distribution of the quality of the reconstruction parameter for data and MC events

Time calibration

A good timing of the OMs is crucial to guarantee the expected performance of the telescope.

ANTARES is designed achive an angular resolution better than 0.5º at high energies (~10 TeV)

Relative time calibrationIntra-line: synchronize all the OMs within a line with respect to a common reference (one OM in the first storey)Inter-line: calibrate all the OMs with respect to a unique (global) reference


Method description

Using physics events, time offsets are computed iteratively:

1) A probe line is (randomly) selected which hits are not used in the track reconstruction

2) Fit muon trajectory using the remaining hits3) Compute time residuals using the fitted track*

parameters4) Distribution of these residuals are histogramed and fitted

to a Gaussian (around peak) which mean value is interpreted as the line offset

5) Time offsets measured in the previous step are accounted for in a new iteration

The full process is repeated until the size of the correction is sufficiently small


*Only rather good reconstructed events (Λ>-5.6) are used

Inter-line offsets Time residual distributions for different iterations of

the procedure


Iteration 1 Iteration 5

Inter-line offsets Time offsets obtained for the 12 detection lines


Cumulative corrections as a function of the number of iterations of the procedure

Inter-line time offsets

Time offsets measured using physics runs with about 5 days livetime


L1 L2

L5 L3 L4 L6

L9 L7 L8 L10

L11 L12

Detection lines layout

Largest offsets is found for Line 8 (where laser beacon is located)

Impact on the reconstructionCorrecting the inter-line timing translates in an increase of the

number of events better reconstructed(better data/MC agreement)


Time residual distribution fro data events

Distributions of the quality of the reconstruction parameter for vertical (left) and inclined (right) events

Effect on the performanceWhy if we don’t correct the inter-line timing? By mis-calibrating the lines, and using MC simulation, we check the effect on the angular resolution


Angular error between the reconstructed muon trajectory and the true neutrino direction: a wrong inter-line timing results in a ~40% degraded resolution

Cross-check with the LBUsing the laser beacon placed at the bottom of line 8 we have cross-checked the results obtained using atmospheric muons


Difference of the time offsets measured with these two methods

Several calibration runs are used to compute the time offset (fit to the green point)

Cross-check with the LBUsing the calibration constants provided by the method using atmospheric muon data improves (slightly) the reconstruction


Distributions of the quality of the reconstruction parameter for vertical (left) and inclined (right) events

Time offsets stabilityNo variations are expected unless PMT HV tuning or

replacement of a line reference OM is performed


Intra-line timingReconstruction uses the most updated calibration constants stored in the data-base check correction to the T0’s timing first obtained using the LED Beacon system

Similar iterative procedure, but optimized to reduce the number of runs needed to accumulate good statistics


Time residual distribution for one OM

Intra-line timingReconstruction uses the most updated calibration constanst stored in the data-base check correction to the T0’s timing first obtained using the LED Beacon system


90% of correction are smaller than 1ns

Summary/Conclusions

• Time calibration is crucial to achieve the best performance of the telescope:

• A new method using atmospheric muon data has been implemented which can work out the parameters of the relative timing calibration without stopping the physics data acquisition

• Results on the inter-line timing:• Correcting the time offsets translates in up to a factor 2

enhancement of the number of good reconstructed events traversing diagonally the detector (improved data/MC agreement)

• Improvement (40%) on the angular resolution is expected• Cross-chek of the laser beacon results.• Provides the official calibration constants used for data

processing

• Ongoing progress for the T0s calibration:• Cross-check of the results with the LED beacon


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Documents

J.P. Gómez-González. Outline Introduction: The ANTARES neutrino telescope Time calibration Muon track residuals based method: Description and implementation