1. Recent results on antiparticles in cosmic rays from PAMELA experiment
Sergio Ricciarini
INFN – Florence, Italy
On behalf of the PAMELA collaboration

2. Summary The PAMELA experiment: short introduction.
Discussion of recent results.
(1) Antiproton/proton ratio at high energies
(submitted to Phys. Rev. Lett.).
(2) Positron fraction at high energies
(submitted to Nature).
Conclusion and prospects.
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3. The PAMELA collaboration
Italy
Bari
Naples
Rome
Trieste
Frascati
Florence
Russia
Moscow,
St. Petersburg
Germany
Sweden
Siegen
Stockholm
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4. PAMELA scientific objectives
Study antiparticles in cosmic rays.
Search for dark matter annihilation (e+ and p-bar spectra).
Study cosmic-ray production and propagation.
Study composition and spectra of cosmic rays (including light nuclei).
Search for anti-He (primordial antimatter).
Study solar physics and solar modulation.
Study of terrestrial magnetosphere and radiation belts.
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5. PAMELA nominal capabilities
Energy range (with 3 years statistics)
Antiprotons 80 MeV GeV
Positrons MeV GeV
Protons up to 700 GeV
Electrons up to 400 GeV
Electrons+positrons up to 2 TeV (from calorimeter)
Light Nuclei up to 200 GeV/n (He/Be/C)
AntiNuclei search
Simultaneous measurement of many cosmic-ray species.
New energy range.
Unprecedented statistics.
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6. PAMELA detectors
Main requirements: high-sensitivity antiparticle identification and precise momentum measurement
Time-Of-Flight (TOF)
plastic scintillators + PMT:
Trigger
Albedo rejection
Mass identification up to 1 GeV
- Charge value from dE/dL
Electromagnetic calorimeter
W/Si sampling (16.3 X0, 0.6 λI)
Discrimination e+ / p, p-bar / e-
(shower topology)
Direct E measurement for e-/e+
Neutron detector
polyethylene + 3He counters:
High-energy e/h discrimination
GF: 21.6 cm2 sr Mass: 470 kg
Size: 130x70x70 cm3
Power Budget: 360 W
Spectrometer
microstrip Si tracking system (TRK) + permanent magnet
- Magnetic rigidity R = pc/Ze (GV); magnetic deflection η=1/R (GV-1)
- Charge sign, momentum
- Charge value from dE/dL
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7. Satellite and orbit
PAMELA mounted on Russian satellite Resurs-DK1, inside a pressurized container.
Minimum lifetime 3 years starting from June 2006.
Quasi-polar low-earth elliptical orbit (70.0°, km).
Traverses and operates in the South Atlantic Anomaly.
Crosses the outer (electron) Van Allen belt at south pole.
PAMELA
350 km
610 km
70o
SAA
orbit period ~90 min
70°
610 km
350 km
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8. Antiproton/proton ratio at high energies (kinetic energy 1
Antiproton/proton ratio at high energies (kinetic energy GeV)
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9. High-energy antiproton/proton analysis
Results discussed here have been submitted to Phys. Rev. Lett.
Analyzed data: July February 2008.
Total acquisition time ~ 500 days.
Collected ~ 1x109 triggers (~ 8.8TB of data).
Identified ~10 x 106 protons and ~1 x 103 antiprotons with kinetic energy between 1.5 and 100 GeV.
Collected 100 antiprotons above 20 GeV.
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10. Antiproton and proton: basic cuts
All requirements in the p-bar/p analysis are applied for both charge signs.
Clean event pattern (reject spurious events):
single track in TRK;
no activity in CARD+CAT;
no multiple hits in S1+S2 (segmented).
MIP |Z|=1 particle.
TRK+S1+S2 dE/dL < 3 MIP.
Galactic particle (reject albedo, reentrant, East-West effect):
downward-going particle (300 ps TOF resolution over 3 ns flight time);
measured rigidity R > 1.3 vertical geomagnetic cutoff. S1
CARD
CAT S2 .
TOF
TRK
CAS S3
CALO S4 ND
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11. Electron/hadron separation with CALO
Contamination from e- on p-bar sample is reduced to a negligible amount.
e- are easily identified in CALO from interaction topology (rejection factor >104): they interact in the first CALO layers and give well contained and compact EM showers;
on the other hand, most hadrons interact well deep in the CALO or do not interact at all.
electron (R=17GV)
hadron (R=19GV)
22 modules (Y Si-strip + W layer + X Si-strip)
Total depth: 16.3 X0 or 0.6 λI
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12. Momentum and charge sign with TRK
Minimal track requirements for good rigidity measurement:
at least 4 X (bending view) + 3 Y hits;
energy-dependent cut on track c2 (~95% total efficiency);
consistent TRK+TOF+CALO spatial information.
Magnetic rigidity R = pc/Ze (GV)
Magnetic deflection η = 1/R (GV-1)
MDR (Maximum Detectable Rigidity):
Def.: |R|=MDR  σR=|R|
MDR=1/ση (ση spectrometer deflection resolution)
MDR depends on event characteristics and is evaluated event-by-event with the fitting routine:
- number and distribution of fitted points along the track;
- spatial resolution of the single position measurements (varies with track inclination and strip noise);
- magnetic field intensity along the track.
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13. Rejection of p “spillover” background
Main difficulty here is the background from “spillover” protons in the p-bar sample at high energies (protons with wrong charge sign):
finite MDR limits the precision of η (R) measurement;
high (~ 104) p/p-bar ratio in cosmic rays.
Minimal track requirements plus: MDR > 850 GV (high-precision subsample).
Defined additional optimized track requirements to improve MDR:
- stronger constraints on χ2 at high energies (~75% efficiency);
- rejected tracks with low-spatial-resolution clusters along the trajectory:
- faulty strips (high noise);
- δ-rays (high signal and multiplicity).
Protons
and spillover
R = - 10 GV
R = - 50 GV
Antiprotons
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14. Rejection of p “spillover” background
Preliminary!!
Rigidity-dependent cut to reject residual spillover: MDR > 10 ∙ |R|
This cut is equivalent to: |η| > 10 ∙ ση
This conservative rejection cut reduces residual spillover contamination to a negligible amount.
p-bar
subsample with
MDR > 850 GV 
Protons and spillover
MDR > 10 ∙ |R| 
R= - 50 GV
R = - 10 GV
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15. Antiproton/proton ratio
Excellent agreement with recent data from other experiments.
One order of magnitude improvement in statistics.
Most extended energy range ever achieved.
Expected further improvements with new data.
Correction factors are included and ~ one order of magnitude less than statistical error.
CALO efficiency (different for p-bar and p);
loss of particles for interactions.
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16. PAMELA p-bar/p ratio and theory
Ratio increases smoothly with energy from 4 x 10-5 and levels off at ~ 1 x 10-4.
Our results are enough precise to place tight constraints on parameters relevant for secondary production calculations.
Our data above 10 GeV place limits on contributions from exotic sources, e.g. dark matter particle annihilations.
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17. Positron fraction at high energies (energy 1.5 - 100 GeV)
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18. High-energy positron fraction analysis
Results discussed here have been submitted to Nature.
Analyzed data: July February 2008.
Total acquisition time ~ 500 days.
~ 1x109 triggers (~ 8.8TB of data).
Identified ~150 x 103 electrons and ~9 x 103 positrons with energy between 1.5 and 100 GeV.
Collected 180 positrons above 20 GeV.
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19. Distribution of fraction F before CALO cuts
Preliminary!!
Rigidity: GV
Z = -1
Fraction F of energy released in CALO along the track in a cylinder of radius 0.3 rMolière
(central + 2 lateral Si strips) e-
p-bar (non-int)
p-bar (int)
after basic event cuts
Z = +1
p (non-int)
(e+)
p (int)
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20. Cut on “energy-rigidity match”
Consider the ratio between total energy measured by CALO and rigidity measured by TRK.
For electrons (positrons) ratio is constant over rigidity.
rigidity measured in TRK (MIP/GV)
total energy measured in CALO/ e- e+
int. p
 ‘electron cut’
non-int. + int. protons
non-int. p-bar
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21. Cut on “energy-rigidity match”
Preliminary!!
Rigidity: GV
Z = -1
Fraction F of energy released in CALO along the track e- +
Constraints on:
p-bar
Energy-rigidity match
Z = +1
all non-interacting and most interacting protons are rejected e+ p
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22. e+ and e- identification in calorimeter
Less than 1 proton out of 105 survives the complete set of CALO cuts, with e+ efficiency 80%. e- e+ p
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23. Cut on shower starting point
Constraints on:
Proton background was also characterized at beam tests.
Energy-rigidity match
Shower starting point
Flight data.
Rigidity: GV
Beam-test data after same cuts are applied.
Rigidity: 50 GV
Z = -1 e- e- e- e+ p
Z = +1 p e+ p
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24. Cross-check with ND flight data
Cross-check with flight data from neutron detector to validate the selection procedure.
Fraction F
Rigidity: GV
Neutrons detected by ND
Z = -1 e- e-
Z = +1 e+ e+ p p
Residual p background
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25. Cut on longitudinal profile
Flight data:
51 GV positron
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26. Cut on longitudinal profile
Preliminary!!
Fraction F of energy released in CALO along the track
Rigidity: GV
Z = -1 +
Constraints on:
Energy-rigidity match
Shower starting point
Z = +1
Longitudinal profile
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27. Cross-check with energy loss in TRK
Top: proton and electron samples, identified with TRK only (charge sign).
Rigidity: GV
Rigidity: GV p e- p e- p e+ p e+
Bottom: proton and positron (+ residual p background) samples, identified with present CALO requirements.
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28. Proton contamination
Proton contamination obtained directly from flight data (no simulation involved) and subtracted with statistical “bootstrap” analysis.
Considered three F distributions in a reduced calorimeter after applying all CALO cuts:
(a) electrons and (c) e+ with residual p background: selected in upper CALO.
(b) protons: “pre-sampled” in first 2 modules and then selected in lower CALO. e-
Rigidity: GV
electron selection
reduced CALO (20 out of 22 modules) p
proton selection p e+
positron + residual p background selection
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29. Positron fraction at high energies
At high energies our data show a significant increase with energy.
This cannot be explained by standard models of secondary production of cosmic rays.
Either a significant change in the acceleration or propagation models is needed;
or a primary component is present.
Among primary-component candidates:
annihilation of dark matter in the vicinity of our galaxy;
near-by astrophysical sources, like pulsars.
One order of magnitude improvement in statistics over previous measurements.
Most extended energy range ever achieved.
Expected further improvements with new data.
line: secondary production,
Moskalenko and Strong,
Astrophys. J. 493 (1998)
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30. Positron fraction at low energies
At low energies our results are systematically lower than data collected in 1990’s.
Clem 2007 (with much lower statistics) is consistent with PAMELA.
This is interpreted as effect of charge-sign dependent solar modulation.
our data are enough precise to allow tuning of models of the heliosphere.
Ruled out as negligible a possible combined effect of:
asymmetry of spectrometer magnetic field;
East-West effect or reentrant albedo particles.
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31. Low-energy e+ fraction and solar modulation
Solar modulation (through solar wind) of cosmic ray fluxes depends on:
amount of solar activity;
polarity of solar magnetic field;
cosmic-ray energy and mass;
charge sign of cosmic ray.
2000: inversion of solar magnetic field
Neutron intensity (Rome monitor)
PAMELA
Clem
now: solar minimum
low-energy p-bar/p ratio
(BESS)
low-energy e+ fraction (Caprice, MASS, HEAT, AMS98...)
year A- A+ A- A+
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32. Charge-sign dependence of solar modulation
Preliminary!!
(Preliminary!)
A>0
A<0
A<0
“drift” component of solar modulation
is enhanced during solar minimum
for low-mass particles
[Potgieter et al., Space Sci. Rev. 97 (2001)]
A>0
A > 0
Positive particles
A < 0
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33. Conclusion and prospects
Precise measurements of p-bar/p ratio and of positron fraction over a wide energy range have been presented and discussed.
PAMELA is expected to collect data until at least December 2009.
increase in statistics will allow to extend energy range for p-bar/p ratio and positron fraction to the design limits.
Several other items are currently under analysis:
p-bar/p ratio and positron fraction in the energy range 100 MeV - 1 GeV;
absolute differential spectra of |Z|=1 cosmic rays;
nuclei (up to Z = 8);
spectra of high-energy Solar Energetic Particles (SEP);
radiation belts: morphology and energy spectrum;
search for anti-He;
study of isotope composition (d, 3He).
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