Measurement of the Hydrogen and Helium absolute fluxes with the PAMELA experiment

1. Measurement of the Hydrogen and Helium absolute fluxes with the PAMELA experiment Elena Vannuccini On behalf of PAMELA collaboration

2. PAMELAPayload for Matter/antimatter Exploration and Light-nuclei Astrophysics • Direct detection of CRs in space • Main focus on antiparticles(antiprotons and positrons) • PAMELA on board of Russian satellite Resurs DK1 • Orbital parameters: • inclination ~70o ( 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! Launch from Baykonur

3. + - PAMELA detectors • 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 Main requirements: - high-sensitivity antiparticle identification - precise momentum measure • Spectrometer • microstrip silicon tracking system+ permanent magnet • It provides: • - Magnetic rigidity R = pc/Ze = 1/|h| • Charge sign  sign of h • Charge value from dE/dx • MDR* up to 1400GV • *MDR = Maximum Detectable Rigidity R/R=100% GF: 21.58 cm2 sr Mass: 470 kg Size: 130x70x70 cm3 Power Budget: 360W

4. Single good-quality track in the spectrometer  Particle rigidity (R = pc/Ze ) • Downward-going (b>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 H/He selection High-statistic (~108) sample of H and He (no isotope separation) Negligiblebk of -interaction products -misidentified particles He H

5. H-flux vs L-shell Polar regions Galactic particles selected by requiring: R >1.3 C C = vert. Störmercutoff Equator H flux

6. Protons Selection efficiencies R<MDR (MDR = 200÷1400GV) General approach: Efficiency evaluated from flight data  Real performances Cross-checks and corrections from MC simulation  Complete information  Test of measurement procedure Evaluated every 2 months Selection cuts ±4% Fiducial acceptance

7. Spectrum unfolding Protons Real-energy spectrum(R) Physical effects ionization & m.scattering Instrumental effects spatial resolution & alignment uncertainties Measured-energy spectrum (Rm=R±) @ high energy: 10% • Bayesian unfolding • Spectrometer response matrix from MC

8. Spectrometer systematic uncertainty  e+ e- sys 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: Dhsys ~10-4 GV-1 h~ 10-3 GV-1 A systematic deflection shift causes an offset between e- and e+ distribution

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

10. Check of systematics Integral proton flux (>50GV) 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) … 3% Time interval (2 months)

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

12. H& He absolute fluxes@ high energy 2.77 Spectral index 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 2.85 2.48 2.67 232 GV 243 GV Solar modulation Solar modulation He H

13. H/He ratio vs R Instrumental p.o.v. Systematic uncertainties partly cancel out Theoretical p.o.v. Solar modulation negligible  information about IS spectra down to GV region Propagation effects small above ~100GV  information about source spectra (eg. Putze et al.) • Power-law fit (c2~1.3) • aHe-ap= 0.078 ±0.008

14. Solar activity during PAMELA life PAMELA launched in 2006 @ beginning of last solar cycle. Data collected over 5 years around minimum solar activity

15. Long-term flux variations Preliminary!! Protons Very large statistics collected Precise spectral measurement down to 400MV  Detailed study of solar modulation effect

16. Summary and conclusions PAMELA has been in orbit and studying cosmic rays for ~4.5 years. >109 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!

17. Thanks!!