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ELENA VANNUCCINI ON BEHALF OF PAMELA COLLABORATION Measurement of the Hydrogen and Helium absolute fluxes with the PAMELA experiment

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Direct detection of CRs in space Main focus on antiparticles (antiprotons and positrons) Launch from Baykonur PAMELA on board of Russian satellite Resurs DK1 Orbital parameters: - inclination ~70 o ( low energy) - altitude ~ 360-600 km (elliptical) - active life >3 years ( high statistics) Launched on 15th June 2006 PAMELA in continuous data-taking mode since then!

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Main requirements: - high-sensitivity antiparticle identification - precise momentum measure GF: 21.58 cm 2 sr Mass: 470 kg Size: 130x70x70 cm 3 Power Budget: 360W Spectrometer microstrip silicon tracking system + permanent magnet It provides: - Magnetic rigidity R = pc/Ze = 1/| - Charge sign sign of - Charge value from dE/dx MDR* up to 1400GV *MDR = Maximum Detectable Rigidity R/R=100% Time-Of-Flight plastic scintillators + PMT: - Trigger - Albedo rejection; - Mass identification up to 1 GeV; - Charge identification from dE/dX. Electromagnetic calorimeter W/Si sampling (16.3 X0, 0.6 λI) - Discrimination e+ / p, anti-p / e - (shower topology) - Direct E measurement for e - Neutron detector plastic scintillators + PMT: - High-energy e/h discrimination + -

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Single good-quality track in the spectrometer Particle rigidity (R = pc/Ze ) Downward-going ( >0) & positive-curvature (R>0) trajectory Positive-charge particle from above Clean pattern through the apparatus Not an interaction product Energy deposits in the tracking system consistent with H and He nuclei He H High-statistic (~10 8 ) sample of H and He (no isotope separation) Negligible bk of -interaction products -misidentified particles

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Galactic particles selected by requiring: R >1.3 C C = vert. Störmer cutoff H flux Polar regions Equator

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Selection cuts R

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Protons 10% Bayesian unfolding Spectrometer response matrix from MC

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Possibility of residual coherent misalignment (distortion) of the tracking system Evaluated from in-flight electron/positron data by comparing the spectrometer momentum with the calorimeter energy Upper limit set by positron statistics: sys ~10 -4 GV -1 ~ 10 -3 GV -1 A systematic deflection shift causes an offset between e - and e + distribution e+e+ e-e- sys

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At low R selection- efficiency uncertainties dominate Above 500GV tracking- system (coherent) misalignment dominates selection-efficiency uncertainties spectrometer systematic error

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Fluxes evaluated by varying the selection conditions: Total vs time Total vs polar/equatorial Total vs reduced acceptance Total vs different tracking conditions ( different response matrix) … Time interval (2 months) Integral proton flux (>50GV) 3%

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First high-statistics and high-precision measurement over three decades in energy Low energy minimum solar activity ( 450÷550 GV) High-energy (>30GV) a complex structure of the spectra emerges… Adriani et al. - Science - 332 (2011) 6025 PAMELA data Jul 2006 ÷ Mar 2008

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Deviations from single power law (SPL): Spectra gradually soften in the range 30÷230GV Spectral hardening @ R~235GV ~0.2÷0.3 SPL hp rejected at 98% CL Origin of the structures? - At the sources: multi- populations, non-linear DSA - Propagation effects Solar modulation 2.85 2.67 232 GV Spectral index 2.77 2.48 243 GV H He

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Power-law fit ( 2 ~1.3) He - p = 0.078 ±0.008

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Very large statistics collected Precise spectral measurement down to 400MV Detailed study of solar modulation effect Protons

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Summary and conclusions PAMELA has been in orbit and studying cosmic rays for ~4.5 years. >10 9 triggers registered H and He absolute fluxes up to 1.2TV Most precise measurement so far. Complex spectral structures observed (spectral hardening at ~200GV!) Step forward in understanding galactic CR origin and propagation! Forthcoming results on long-term flux variations down to few hundred MV Step forward in understanding propagation in the Solar System!

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Putze et al. A&A 526 (2011) A101

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