Transcript PAMELA - Istituto Nazionale di Fisica Nucleare

Measurement of the Hydrogen and Helium
absolute fluxes with the PAMELA experiment
ELENA VANNUCCINI
ON BEHALF OF
PAMELA COLLABORATION
PAMELA
Payload for Matter/antimatter Exploration and Lightnuclei 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)
Launch from Baykonur
 Launched on 15th June 2006
 PAMELA in continuous data-taking mode since then!
+
PAMELA
detectors
Main requirements:
- high-sensitivity
antiparticle
identification
- precise momentum
measure
-
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 eNeutron detector
plastic scintillators + PMT:
- High-energy e/h discrimination
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
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)
Negligible bk of
-interaction products
-misidentified particles
He
H
H-flux
vs L-shell
Polar regions
Galactic particles selected
by requiring:
R >1.3  C
C = vert. Störmer cutoff
Equator
H flux
Protons
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
R<MDR
(MDR = 200÷1400GV)
Selection cuts
Fiducial acceptance
±4%
Spectrum
unfolding
Protons
Real-energy spectrum (R)
• Physical effects 
ionization & m.scattering
• Instrumental effects 
spatial resolution &
alignment uncertainties
10%
Measured-energy
spectrum (Rm=R±R)
@ high energy:
R =
R2 /MDR
•Bayesian unfolding
•Spectrometer response matrix from MC
Spectrometer
systematic
uncertainty
 Possibility of residual
coherent misalignment
(distortion) of the tracking
system
 Evaluated from

e+
hsys
Ptrk
1

E cal sys E cal 1  ε sys  η trk  Δηsys
in-flight electron/positron
data by comparing the
spectrometer momentum
with the calorimeter energy
 Upper limit set by positron
statistics:
hsys ~10-4 GV-1
h~ 10-3 GV-1
A systematic
deflection shift
causes an offset
between e- and e+
distribution
e-
Overall
systematic
uncertainties
 At low R selection-
efficiency uncertainties
dominate
spectrometer
systematic
error
 Above 500GV tracking-
system (coherent)
misalignment dominates
selection-efficiency
uncertainties
Check of
systematics
Fluxes evaluated by varying
the selection conditions:
Integral proton flux (>50GV)
3%
• Total vs time
• Total vs polar/equatorial
• Total vs reduced
acceptance
• Total vs different tracking
conditions ( different
response matrix)
•…
Time interval (2 months)
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
H & He
absolute fluxes
@ high energy
2.85
Spectral index
2.77
2.67
232 GV
243 GV
Deviations from single
power law (SPL):
 Spectra gradually soften
in the range 30÷230GV
Origin of the structures?
- At the sources: multipopulations, non-linear
DSA
- Propagation effects
Solar modulation
SPL hp rejected at 98% CL
H
Solar modulation
 Spectral hardening
@ R~235GV
~0.2÷0.3
2.48
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!