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

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 ~ km (elliptical) - active life >3 years (  high statistics)  Launched on 15th June 2006  PAMELA in continuous data-taking mode since then!

Main requirements: - high-sensitivity antiparticle identification - precise momentum measure GF: 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 + -

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

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

Selection cuts R<MDR Protons ±4% (MDR = 200÷1400GV) Fiducial acceptance

Protons 10% Bayesian unfolding Spectrometer response matrix from MC

 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   ~ GV -1 A systematic deflection shift causes an offset between e - and e + distribution e+e+ e-e-   sys

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

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%

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 (2011) 6025 PAMELA data  Jul 2006 ÷ Mar 2008

Deviations from single power law (SPL):  Spectra gradually soften in the range 30÷230GV  Spectral 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 GV Spectral index GV H He

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

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

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!

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