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 1 Calibración temporal en ANTARESBienal de Física 2013 Astroparticle physics: Point-like sources Diffuse flux, galactic plane neutrino flux Flaring blazars / microquasars GRBs Fermi Bubbles Particle physics: Neutrino oscillation Atmospheric neutrino and muon flux Cosmic ray anisotropy / composition Shower reconstruction Electromagnetic showers Detector related: Timing / Positioning Moon shadow Water optical properties, group velocity of light Acoustics, bioluminescence Searches: Dark matter annihilation,magnetic monopoles, nuclearites Multi-messenger astronomy: Gravitational waves, Auger CRs, optical telescopes

Detection principle 1 Calibración temporal en ANTARESBienal de Física 2013  42 ° interaction Earth’s crust Cherenkov light from   Detection principle “We propose getting up an apparatus in an underground lake or deep in the ocean in order to determine the location of charged particles with the help of Cherenkov radiation” M. Markov, 1960 Detector: 3D array of photomultiplie rs At high energies the neutrino and the muon are almost collinear  Astronomy

Physical background 1 Calibración temporal en ANTARESBienal de Física 2013  p    p,  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 detector ANTARES is a submarine neutrino telescope made up by: 1 Calibración temporal en ANTARESBienal de Física ” PMTs distributed in triplets of storeys along 12 strings of 450 meters longitude placed 2475 depth following an octagonal layout.

Muon track reconstruction Time 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) 1 Calibración temporal en ANTARESBienal de Física 2013

Time residuals The reconstruction algorithm is a multi-step approach which final step consists on the minimization of the time residuals PDF 1 Calibración temporal en ANTARESBienal de Física 2013 Time residual distribution for data events Time residuals PDF 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 calibration Intra-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 1 Calibración temporal en ANTARESBienal de Física 2013

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 hits 3)Compute time residuals using the fitted track* parameters 4)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 1 Calibración temporal en ANTARESBienal de Física 2013 *Only rather good reconstructed events (Λ>-5.6) are used

Inter-line offsets Time residual distributions for different iterations of the procedure 1 Calibración temporal en ANTARESBienal de Física 2013 Iteration 1Iteration 5

Inter-line offsets Time offsets obtained for the 12 detection lines 1 Calibración temporal en ANTARESBienal de Física 2013 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 1 Calibración temporal en ANTARESBienal de Física 2013 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 reconstruction Correcting the inter-line timing translates in an increase of the number of events better reconstructed(better data/MC agreement) 1 Calibración temporal en ANTARESBienal de Física 2013 Time residual distribution fro data events Distributions of the quality of the reconstruction parameter for vertical (left) and inclined (right) events

Effect on the performance Why 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 1 Calibración temporal en ANTARESBienal de Física 2013 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 LB Using the laser beacon placed at the bottom of line 8 we have cross- checked the results obtained using atmospheric muons 1 Calibración temporal en ANTARESBienal de Física 2013 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 LB Using the calibration constants provided by the method using atmospheric muon data improves (slightly) the reconstruction 1 Calibración temporal en ANTARESBienal de Física 2013 Distributions of the quality of the reconstruction parameter for vertical (left) and inclined (right) events

Time offsets stability No variations are expected unless PMT HV tuning or replacement of a line reference OM is performed 1 Calibración temporal en ANTARESBienal de Física 2013

Intra-line timing Reconstruction 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 1 Calibración temporal en ANTARESBienal de Física 2013 Time residual distribution for one OM

Intra-line timing Reconstruction 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 1 Calibración temporal en ANTARESBienal de Física % 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 1 Calibración temporal en ANTARESBienal de Física 2013

Probe line selection 1 Calibración temporal en ANTARESBienal de Física 2013 b  residual = t_measured – t_th

Optical beacons calibration 1 Calibración temporal en ANTARESBienal de Física 2013 T 0 in the same line LED OB Early-photon region T 0 in the same line LED OB Early-photon region Intraline calibration time offsets between lines Laser OB Photo-electron level region time offsets between lines Laser OB Photo-electron level region Interline calibration

Method description Before deployment of the lines, a full intra-line calibration is performed using a laser sending light through optical fibre. Inter-line timing  measurement of the time offsets between the 12 reference OMs has to be performed in-situ. Iterative procedure based on atmospheric muon data 1 Calibración temporal en ANTARESBienal de Física 2013 Atmospheric muons are reconstructed in ANTARES at a rate of 5-10 Hz