1 UNIVERSITA’ DEL SALENTO Facoltà di Scienze MM.FF.NN TIME MEASUREMENTS WITH THE ARGO-YBJ DETECTOR Dott. Anna Karen Calabrese Melcarne Dottorato di Ricerca in Fisica XIX ciclo Settore scientifico FIS/04

2 OUTLINE  ARGO-YBJ as a ground-based detector  Timing calibration in EAS experiments (Characteristic Plane Method)  Characteristic Plane (CP) correction applied to ARGO-YBJ data  Physics results after calibration

3 Cosmic Ray Spectrum

4 Observation of Extensive Air Showers produced in the atmosphere by primary  ’s and nuclei

5 High Altitude Cosmic Ray YangBaJing Site Altitude: 4300 m a.s.l., ~ 600 g/cm 2 Site Coordinates: longitude 90° 31’ 50” E, latitude 30° 06’ 38” N

6   Cosmic ray physics anti-p / p ratio at TeV energy spectrum and composition (E th few TeV) study of the shower space-time structure  VHE  -Ray Astronomy Search for point-like (and diffuse) galactic and extra-galactic sources at few hundreds GeV energy threshold  Search for GRB’s (full GeV / TeV energy range)  Sun and Heliosphere physics (E th  few GeV) Main Physics Goals

7 Layer (  92% active surface) of Resistive Plate Chambers (RPC), covering a large area (5600 m 2 ) + sampling guard ring cm lead converter time resolution ~1 ns space resolution = strip 10 Pads (56 x 62 cm 2 ) for each RPC 1 CLUSTER = 12 RPC 78 m 111 m 99 m74 m BIG PAD ADC RPC (  43 m 2 ) ARGO-YBJ layout

8 RPC is suited to be used as element of a surface detector RPC PAD Resistive Plate Chamber Low cost, high efficiency, high space & time resolution (<1ns), easy access to any part of detector, robust assembling, easy to achieve >90% coverage, mounting without mechanical supports. 2850x1258mm 2

9 Detector performances  good pointing accuracy (less than 0.5°)  detailed space-time image of the shower front  capability of small shower detection (  low E threshold)  large FoV (  2  ) and high “duty-cycle” (  100%)  continuous monitoring of the sky (-10°<  <70°) Impossible for Atmospheric Cherenkov telescopes

10  Full space-time reconstruction  Shower topology  Structure of the shower front A unique way to study EAS 74 m 60 m 90 m 150 ns 50 m

11 Study of the EAS space-time structure The High space-time granularity of the ARGO-YBJ detector allows a deep study of shower phenomenology with unique performance Example 1: Very energetic shower

12 Arrival Direction Reconstruction Conical Fit Planar Fit In EAS experiments for an event E the time t EP can be measured on each fired detector unit P, whose position (x P, y P ) is well known Primary direction cosines This quantity is not a proper    Indeed the measurement unit is ns 2

13 Timing Calibration  P = residual correction + systematic correction Residuals correction reduces the differences between fit time and measured time Systematic correction guarantee the removal of the complete offset Taking into account the time offset  P typical of the detector unit Plane-equation

14 The air shower arrival direction have the following distribution: The systematic offset introduces a quasi-sinusoidal modulation in azimuth distribution sin  0 cos  0 and sin  0 cos  0 were subtracted from the original direction cosines

15 Characteristic Plane (CP) Definition Fake Plane (FP) Real Plane (RP) On average Assuming uniform azimuth distribution

16 CP Method Checks (Fast MC simulation) Azimuth distribution before calibrationAzimuth distribution after calibration Time offsets introduced in the time measurement CP correction removes the time offsets

17 CP method works also when a pre-modulation on primary azimuth angle is present The CP method annulls and leaving a sinusoidal modulation on the distribution of the new  ’’ azimuth angle

18 Residual correction has been applied twice and systematic correction has been applied according to the values: A Gaussian fit is applied in the range ±10 ns around the bin with maximum number of entries ARGO-YBJ DATA (ARGO-42, ARGO-104, ARGO-130)

19 Correction Residuals after correction

20 Effect of conical shape of the shower front planar fit Conical shape FULL SIMULATION Corsika+ARGOG codes

21 CP method with conical correction Planar residual after CP conical correction Conical residual after CP conical correction

22 Geomagnetic field effect In the geomagnetic field, the secondary charged particles generated in EAS are stretched by the Lorentz force Average shift in the shower plane for a secondary electron

23 YBJ - the geomagnetic effect is stronger for showers from North than for showers from South This difference is more evident for larger zenith angles  H = 45° at ARGO-YBJ 15 ° 35 ° 45 ° 55 °  = North South  =

24 Estimate of South-North asymmetry: MC N events from North (161.5 º < Φ < º ) S events from South (161.5 º >Φ and Φ >341.5 º ) Tibet AS  estimate 2.5% higher rate from South direction with respect to North direction (geomagnetic field effect + slope of the hill where the array is located)

25 Estimate of South-North asymmetry: Data As expected CP method annulls the mean values of the primary direction cosines but a small sinusoidal modulation is still present in azimuth distribution The mean values of direction cosines after CP correction are 1.0%0.9%

26 TDC peaks distribution Before correction After correction

27 TDC method to update the calibration TDC peak distribution after calibration has a regular concave shape Without hardware change and with the same trigger, the concave surface should remain unvaried On the other hand ….

28 TDC peak dependence on temperature (night-day difference) A collective shift (~3 ns) is observed. Method odd-even events The main effect of the TDC dependence on temperature is a shift of all TDC peaks, negligible for calibration and a minor effect is present but it is of the order of 0.2 ns

29 TDC dependence on offline CLUSTERs The effect of offline CLUSTERs is visible only in peculiar conditions, thus this effect on the TDC calibration is negligible

30 Angular Resolution MC/data Chess board method  72 parameter is the range in the angular distribution which contains 72 % of the events The residual correction improves the angular resolution

31 Moon shadow: absolute pointing The systematical correction improves the absolute pointing Significance of ARGO-130 Moon shadow for showers with  <50 °. The color scale indicates the significance of the deficit on a 0.9 ° search window centered on the 0.1 ° x0.1 °

32 Time structure of EAS front  The curvature (T S ) of the shower as deviation from planar fit of the shower front  The shower thickness (T d ) as RMS of time residuals (conical fit) at different distance to core

33 COMPARISON DATA-simulation SIMULATION COMPARISON proton-photon

34 Conclusions  Characteristic Plane calibration has been defined and studied  Calibration with planar and conical fit for ARGO-42, ARGO-104 and ARGO-130  Fast TDC calibration  South-North azimuthal asymmetry studied with full simulation  Improvements in the angular resolution and absolute pointing  Study on time structure of the shower front

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37 Another paper in progress on the ARGO-YBJ calibration