ELENA VANNUCCINI

O N B E H A L F O F PAMELA COLLABORATION

Measurement of the Hydrogen and Helium absolute fluxes with the

PAMELA experiment

PAMELAPayload for Matter/antimatter Exploration and

Light-nuclei Astrophysics

•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 ~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!

PAMELA detectors

Main requirements:

- high-sensitivity antiparticle identification

- precise momentum measure

GF: 21.58 cm2 sr Mass: 470 kgSize: 130x70x70 cm3

Power Budget: 360W

Spectrometer microstrip silicon tracking system + permanent magnetIt 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%

Time-Of-Flightplastic scintillators + PMT:- Trigger- Albedo rejection;- Mass identification up to 1 GeV;- Charge identification from dE/dX.

Electromagnetic calorimeterW/Si sampling (16.3 X0, 0.6 λI) - Discrimination e+ / p, anti-p / e- (shower topology)- Direct E measurement for e-

Neutron detectorplastic scintillators + PMT:- High-energy e/h discrimination

+ -

H/He selection

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

He H

High-statistic (~108)sample of H and He (no isotope separation)Negligible bk of-interaction products-misidentified particles

H-flux vs L-shell

Galactic particles selected by requiring:

R >1.3 C

C = vert. Störmer cutoff

H flux

Polar regions

Equator

Selection efficiencies

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

R<MDR

Protons

±4%

(MDR = 200÷1400GV)

Fiducial acceptance

Spectrum unfolding

Protons

10%

Real-energy spectrum (R)

• Physical effects ionization & m.scattering• Instrumental effects spatial resolution & alignment uncertainties

Measured-energy spectrum (Rm=R±)@ high energy: •Bayesian unfolding

•Spectrometer response matrix from MC

Spectrometer systematic uncertainty

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

e+ e-

sys

systrksyscalcal

trk

Δηηε1E

1

E

P

sys

Overall systematic uncertainties

At low R selection-efficiency uncertainties dominate

Above 500GV tracking-system (coherent) misalignment dominates

selection-efficiency

uncertainties

spectrometer systematic

error

Check of systematics

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%

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…

Adriani et al. - Science - 332 (2011) 6025

PAMELA data Jul 2006 ÷ Mar 2008

H & He absolute fluxes@ high energy

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

Sola

r m

od

ula

tion

Sola

r m

od

ula

tion

2.852.67

232 GV

Spectral index 2.772.48

243 GV

H He

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

Solar activity during PAMELA life

PAMELA launched in 2006 @ beginning of last solar cycle.

Data collected over 5 years around minimum solar activity

Long-term flux variations

• 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. >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!

Thanks!!