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Measurement of the Cosmic Ray energy spectra by the ARGO-YBJ experiment A. Surdo

ARGO-YBJ. Measurement of the Cosmic Ray energy spectra by the ARGO-YBJ experiment A. Surdo Istituto Nazionale di Fisica Nucleare Sezione di Lecce, Italy (on behalf of the ARGO-YBJ Collaboration). The ARGO-YBJ experiment. ARGO-YBJ. A strophysical R adiation with G round-based

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Measurement of the Cosmic Ray energy spectra by the ARGO-YBJ experiment A. Surdo

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  1. ARGO-YBJ Measurement of the Cosmic Ray energy spectra by the ARGO-YBJ experiment • A. Surdo IstitutoNazionale di Fisica Nucleare Sezione di Lecce, Italy (on behalf of the ARGO-YBJ Collaboration)

  2. The ARGO-YBJ experiment ARGO-YBJ Astrophysical Radiation with Ground-based Observatory at YangBaJing ARGO-YBJ High Altitude Cosmic Ray Observatory @ YangBaJing,Tibet, China Site Altitude: 4,300 m a.s.l. , ~606 g/cm2 1/20

  3. ARGO-YBJ physics ARGO-YBJ • VHE g-Ray Astronomy: study of point-like (and diffuse) galactic and extra-galactic sources with few hundreds GeV energy threshold • Cosmic ray physics: • energy spectrum and composition • study of the shower space-time structure • flux anisotropies at different angular scales • p-Air cross section measurement • hadronicinteraction studies • anti-p / p ratio at TeV energies, • geomagnetic effects on EAS ….. • Search for GRB’s (full GeV / TeV energy range) • … throughthe… Observation of Extensive Air Showers produced in the atmosphere by primary g’s and nuclei NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 2/20

  4. ARGO-YBJ detector ARGO-YBJ RPC NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

  5. ARGO-YBJ detector BP Amplitude : mV to many Volts ARGO-YBJ Detector layout (5,800 m2) 74 m 99 m 8 Strips (6.5 x 62 cm2) for each Pad 10 Pads (56 x 62 cm2) for each RPC 1 CLUSTER = 12 RPC (43 m2) 78 m 111 m RPC BigPad Strip = spatial pixel Pad = time pixel Time resolution ~1.8 ns BigPad =CHARGE readout PIXEL, 123 x 139 cm2, 3120 (central carpet) + Analog RPC charge read-out NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

  6. EAS reconstructionbydigitalreadout ARGO-YBJ Data takingwith full configuration: November 2007- February 2013 Event Rate ~ 3.5 kHz for Nhit≥20 - Duty cycle ~ 86% - 1011evts/yr duty-cycle 86 % Space/time granularity + full coverage + high altitude event imaging and EAS space/time structure study with unprecedented details NumberoffiredStrips NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

  7. The RPC analog readout (I) ARGO-YBJ • Analogreadout pixel = BigPad (half RPC chamber) • Extended energyrange (above 100 TeV) • Access to the LDF down to the showercore • (particledensities up to ~ 2x104/m2) • Sensitivity toCR primarymass • Info/checks on Hadronic Interactions Saturated digital signal Same event in analogview NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 5/20

  8. The RPC analog readout (II) ARGO-YBJ • Eightgain scales (G0, G1, … G7) ensuregoodlinearity up toabout • 2 x 104 particles/m2 • G7 data overlap the digital-mode linearity range, and have been • used for intercalibration and cross checks • G0 allowsto cover the energyrangeuptto 10 – 15 PeV • importantregionfor the estimationofatmosfericnflux • Here we use G4and G1scales • to cover the 50 TeV – 5 PeV • range with high efficiency • and without saturation • Eventselection: • Corereconstructed in a central • detector fiducial area • - Reconstructedzenith angle <15o trigger effect Log (particle maximum density/(part/m2)) NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

  9. The truncatedsizeNp8as (mass dependent) energyestimator ARGO-YBJ • Np8(number of particles within 8m from the core): • well correlated with primary energy • not biased by finite detector size effects • weakly affected by shower fluctuations Onlyeventswithzenith angle lessthan15 degrees in thisanalysis Look for information on the showerage in ordertohave a mass independentenergyestimator P He CNO Fe NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 7/20

  10. Lateral Distribution Function (LDF)and shower age ARGO-YBJ With the analog data the LDFcan be studied nearthe core withoutsaturation • LDF is well fitted by • a modified NKG function The LDF slope (s’) is related to the shower age and does not depend on the primarymass (‘universalityproperty’) Average Xmax (g/cm2) Assume anexponentialabsorptionafter the showermaximum. Get the correctsignal at maximum (Np8max) byusing Np8 and s’ (fitparameter) measurementforeachevent Checks in progress with Gaisser-Hillas profile NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

  11. Mass independent Energy reconstruction ARGO-YBJ MC sample followingHorandelmodelspectra and composition SimilarresultswithGaisser-Stanev-Tilav (GST) model In excellent agreement withtotal-size-vs-Etheoretical plot. The shiftissimply due to the factthatwe are using the truncatedsize Measurement energy range The measurementof Np8 and the (agecorrelated) LDF slope allowsestimating the truncatedsize at the showermaximum. Thisensures a mass independent Energy determination. NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 9/20

  12. Trigger and eventselectionefficienciesfor the allparticlespectrum ARGO-YBJ G4 U G1 in full efficiencyforallspeciesfrom 300 TeVto 5 PeV Energy range for the spectrum measurement G4 P He Fe CNO Average (Hoerandel) Average (GST) Low MC statistics. Fit with Fermi functions. Enlarged MC data sample on the way. Energy range for the spectrum measurement G1 NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 10/20

  13. The allparticlespectrum ARGO-YBJ • Picture consistentwithmodels and previousmeasurements • Niceoverlapbetweenthe twogainscales (differentdata samples, …) • Resultssuggestspectralindex-2.6 below 1 PeVand -2.8 from 1 to 5 PeV • G0 wouldextend the energyrange up to 10-15 PeV • About a factor 5 shouldbegainedbyconsideringinclinedevents • The extensionstohigherenergywouldbe the subjectof a future work Preliminary Extent of the possible shift due to the energy scale uncertainty NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 11/20

  14. p and Heselection(MC Hoerandelspectra and normalizations) ARGO-YBJ s’ vs Log(Np8) He protons CNO Fe ~ 15% contaminationfrom CNO and heavier elements NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

  15. The p+Hespectrum (1st) ARGO-YBJ • Overlap of the points with different gain scales • Overlap with direct measurements at low energy • Gradual change of the slope starting around 700 TeV: possible (p+He) knee !?! • Consistent with previous hints (MACRO, CASA-MIA, Chacaltaya, EAS-TOP, …) • and YAC-Tibet spectrum • Flux systematics + CNO contamination  Overall uncertainty < 20 % Preliminary Extent of the possible shift due to the energy scale uncertainty NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 13/20

  16. p+Hespectrum (2nd):Bayesian unfolding of analog data ARGO-YBJ Phys. Rev. D 85, 092005 (2012) Bayesian analysis for ARGO-YBJ digital data NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 14/20

  17. p+Hespectrum (2nd):Bayesian analysis of analog data ARGO-YBJ • The results are consistent with previous analysis • The approach is fully Bayesian • Different fiducial cuts, also more inclined events (qzenith<35°) Preliminary NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 15/20

  18. p+He hybrid measurement (3rd):Cerenkov telescope + ARGO-YBJ array ARGO-YBJ Wide Field of view Cerenkov Telescope Array (WFCTA) 5 m2spherical mirror 16×16 PMT array Field of View: 14° × 16° Elevation angle: 60° Chinese Phys. C 38 (2014) 045001 Hybrid analysis for lower energy showers Cerenkov signal: energy measurement Hillas parameters ARGO-YBJ analog data: core position particle number at maximum shower direction Light elements are selected according to particle density near the core and shape of the Cerenkov image (L, W) 2L 2W NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 16/20

  19. Hybrid measurement (3rd):Cerenkov telescope + ARGO-YBJ array ARGO-YBJ • The results are consistent with previous analyses • Possible shape difference • Different data sample and introduction of another detector • Different analysis cuts (also inclined events) Preliminary NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 17/20

  20. ARGO-YBJ (+ WFCTA): p+He spectrum ARGO-YBJ Published ARGO-YBJ (strip) spectrum Preliminary Preliminary • The resultsare consistent withARGO-YBJ measureat lowerenergies (strip data) • The 3 new analyses are consistent within systematics • (further cross-checks in progress) NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 18/20

  21. Comparisonwith other p+He measurements ARGO-YBJ Preliminary Preliminary These results are consistent with direct (i.e. below 200 TeV) and YAC-Tibet measurements NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 19/20

  22. Summary and Outlook ARGO-YBJ • ARGO-YBJ measured the CR spectrum in the TeV – 5 PeV range. All-particle spectrum consistent with other experiments • Evidence for a bending in the p+He spectrumbelow 1 PeV (two different analyses of ARGO-YBJ data in agreement within quoted uncertainties) • Consistent results from a third independenthybridanalysis (RPC + Cerenkov signal) • Further cross checks, larger data and MC statisticsforfinalresults • Clueforanimprovedestimationofatmosphericnflux (spectrumofnucleonsdeterminesfluxesofatmosphericn and m) NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ 20/20

  23. Backup slides

  24. ARGO-YBJ Bruno Rossi conceptual EAS detector 4000 3500 3000 2500 2000 Analog view of a shower 1500 1000 500 3-D view of a shower detected in ARGO-YBJ

  25. ARGO-YBJ

  26. The analog readout system ARGO-YBJ Eight gain scales (G0, G1, … G7) ensure good linearity up to about 2 x 104 particles/m2 G7 data overlap the digital-mode linearity range, and have been used for intercalibration and cross checks Here we use G4 and G1 scales to cover the 50 TeV – 5 PeV range with high efficiency and without saturation Np8 = how many particles within 8 m from the core Lateral Distribution Function (LDF) G1 scale G4 scale Linearity range 5x104 < Np8 < 105 5x103 < Np8 < 104 Linearity range Sensitivity threshold Sensitivity threshold

  27. Truncated particle size ARGO-YBJ Np8:particle size truncated at 8m of core distance Not affected by possible saturation of Analog System Log(Np8) distributions for DATA from G4 and G1 scales

  28. MC simulation ARGO-YBJ • Simulated air shower samples: • (a) p showers (1- 10,000)TeV, Theta<45° • (b) He showers “ “ • (c) CNO showers “ “ • (d) Fe showers “ “ • produced using CORSIKA code (QGSJET-II.03 + Fluka) • Also p and He showers (1- 3000)TeV, Theta<45° • produced using a different hadronic model: SIBYLL-2.1 (+ Fluka) • Simulated showers (sampled on large areas) given in input to the ARGO MC (based on Geant-3) fully simulating the detector response (analog charge trigger and readout system included) • MC data reconstructed by using the same program as for real data. • Event selection: core inside a fiducial area Afid = (64 x 64) m2 • (qzen < 15° used in this analysis)

  29. ARGO-YBJ data: LDF fits ARGO-YBJ DLog(Np8) = (4.7 - 5.0) Fit with r’NKG ±10% band Comparison of residuals from different function fits:

  30. The all-particle spectrum (II) ARGO-YBJ • Picture consistent with models and previous measurements • Nice overlap with the two gain scales (and different data) • The plot suggests spectral index -2.6 below 1 PeV and -2.8 from 1 to 5 PeV • G0 would extend the energy range up to ~15 PeV • About a factor 5 should be gained by considering inclined events • The higher energies would be the subject of a future work Preliminary possible shift due to energy uncertainty

  31. Efficiency of trigger and event selectionfor the p+He spectrum ARGO-YBJ range for p+He spectrum measurent Efficiency plateau above 200 TeV range for p+He spectrum measurement • H • He • Fe • CNO • Average (Hörandel) • Average (GST)

  32. Finding the best labsparameter ARGO-YBJ Furtherimprovements in progress LogE resolution at 270 TeV vs labs • p • He • CNO • Fe • Total Correction with l=120 g/cm2 1 s of Log(Erec/Etrue) l=120 g/cm2 Small residual shift with LogA as foreseen by theory l→∞ • p • He • CNO • Fe No correction ( l→∞ )

  33. Energy reconstruction:bias and resolution ARGO-YBJ The response function is Gaussian in LogE. The spectra are then given in LogE bins, much larger than the estimated bias and well above the LogE resolution, in the considered energy range. Bin size chosen for the energy spectrum Bias < +/- 0.05 Measurement energy range Measurement energy range

  34. MC Energy distributions ARGO-YBJ MC sample following Horandel model spectra and composition Similar results with Gaisser-Stanev-Tilav (GST) model Measurement energy ranges MC true energy MC reconstructed energy

  35. Systematic uncertainty evaluations ARGO-YBJ • Flux • Geometrical aperture : (5 % in/out contamination)  (2.5% angular contamination) = 5.6 % • Efficiency: (5% from MC samples)  (<10% efficiency estimation of the mixture) = 5.0-11.2 % • Unfolding: 3 % • Hadronic interaction model < 5 % • TOTAL: 8.1 % - 13.8 % TOTAL (conservative) = 14% • Energy scale • Gain of the analog system: 3.7 % • Energy calibration: 0.03 in LogE = 6.9 % • Hadronic interaction model: 5 % • TOTAL: 9.3 % TOTAL (conservative) = 10% In the flux plots an over-conservative ± 14% shaded area has been temporarily drawn on the flux measurements Error bars show the statistical uncertainties

  36. Systematics from the hadronic interaction models ARGO-YBJ The dependence on the adopted hadronic interaction model is small. The differences among the QGSJET-II.03 and Sibyll-2.1 are within few percent in the explored energy range (no bias due to muon number). All further results shown here were obtained with QGSJET-II.03.

  37. ARGO-YBJ

  38. Mass selection for the hybrid measurement ARGO-YBJ 2L Log(RPCmax) – 1.44 Log(E) 2W L/W – 0.0091 Rp – 0.14 Log(E) Rp – shower impact parameter E – reconstructed energy RPCmax – maximum RPC signal

  39. ARGO-YBJ

  40. The ARGO-YBJ measurements of the p+He spectrum Results also consistent with measurement at lower energies, done with the strip data. Consistent picture within systematics. Further cross-checking still ongoing. Preliminary 3 TeV – 3 PeV Update of the analysis published in PRD 85 (2012) 092005 Gaisser- Stanev- Tilav p+He Hoerandel p+He with Ec= Z PeV Hoerandel p+He I. De Mitri: Measurement of CR energy spectra with ARGO-YBJ

  41. The p+Hespectrumindex Preliminary ARGO-YBJ analogdata (G4and G1) CREAM p+He Preliminary p+Hespectralindex Direct measurements combination Log(E/TeV)

  42. Comparison with other p+He measurements Consistent results with direct measurements (i.e. below 200 TeV) and YAC-Tibet Preliminary Gaisser- Stanev- Tilav p+He Hoerandel p+He with Ec= Z PeV Hoerandel p+He I. De Mitri: Measurement of CR energy spectra with ARGO-YBJ

  43. The overall picture Preliminary I. De Mitri: Measurement of CR energy spectra with ARGO-YBJ

  44. CREAM1.09 x 1.95 x 10-11 (E/400 TeV)-2.62 • ARGO-YBJ 1.95 x 10-11 (E/400 TeV)-2.61 • Hybrid 0.92 x 1.95 x 10-11 (E/400 TeV)-2.63

  45. The overall picture ARGO-YBJ Preliminary Preliminary

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