Post on 14-Dec-2015
transcript
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!