1. A. Bruno and F. Cafagna on behalf of the PAMELA collaboration
First detection of geomagnetically trapped antiprotons by the PAMELA experiment
A. Bruno and F. Cafagna on behalf of the PAMELA collaboration

2. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
PAMELA Collaboration
Italy:
Bari
Florence
Frascati
Trieste
Naples
Rome
CNR, Florence
Moscow / St. Petersburg
Russia:
Sweden:
KTH, Stockholm
Germany:
Siegen
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011 2

3. Spectrometer microstrip silicon tracking system+permanent magnet
PAMELA detectors
Main requirements  high-sensitivity antiparticle identification and 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, 0.6λI)
Discrimination e+/ p, pbar/e- (shower topology)
Direct E measurement for e-
Neutron detector &
Shower-tail catcher (S4):
High-energy e/h discrimination
Overview of the apparatus
GF: cm2 sr
Mass: 470 kg
Size: 130x70x70 cm3
Power Budget: 360W
Spectrometer microstrip silicon tracking system+permanent magnet
Magnetic rigidity (R = pc/Ze)
Charge sign
Charge value from dE/dx
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011 3

4. Antiproton sources in the atmosphere
Antiprotons are also created in pair production processes in reactions of energetic CRs with Earth’s atmosphere.
By the same mechanisms of the so-called CRAND (Cosmic Ray Albedo Neutron Decay) process (albedo neutrons decay into protons, a fraction of which become trapped in the geomagnetic field), albedo antineutrons are created, and decaying would also lead to trapped antiprotons (CRANbarD).
Part of the secondary antiprotons produced within the confinement region of the Earth’s magnetosphere are trapped by the geomagnetic field, creating an antiproton radiation belt around the Earth.
Small contribution from diffusive penetration of the galactic CR antiprotons (including tertiary contribution) from the ISM into the geomagnetic field.
The trapped antiprotons move adiabatically, drifting around the Earth and remaining on the same L-shell where they were born, being accumulated in the magnetic cavity until ionization losses and nuclear interactions eventually remove them (especially at low altitudes).
They could be held captive for a very long time in the absence of those losses.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

5. Geomagnetically trapped antiprotons
This population is expected to be distributed over a wide range of radial distance (up to 2000km), mainly in the inner radiation belt.
At this L-shell the trapped antiproton flux should exhibit a soft spectrum, which sharply falls to zero above E~1 GeV, while it should dominate the interstellar one at lower energies.
Although the antiproton source production is confined to a very narrow L-shell region, the trapped antiproton fluxes of the radiation belt are expected to disperse due to diffusion, that constantly influences the charged particles in the magnetosphere, into a wider L-shell region (slown process).
The existence of a significant flux of antiprotons in the Earth magnetosphere is considered theoretically in several works
Large uncertainties about contribution from albedo antineutrons (CRAND).
The CRANbarD mechanism is expected to provide the largest contribution to the trapped antiproton population
[Selesnick et al.]
Fuki, M., Int. J. Mod. Phys. A, 20, 6739 (2005)
Gusev, A. et al., Adv. Space Res., 42, 1550 (2008)
Selesnick, R. S et a., Geophys. Res. Lett., 34, 20 (2007)
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

6. Antiproton identification
Good track reconstruction for reliable charge sign separation and a precise estimate of rigidity.
The b by the ToF and dE/dx, in tracker and ToF, used to discard e- and secondary particles, p−, produced by CRs interacting in the aluminum vessel or in the upper parts of the support structures.
No activity in the anticoincidence systems and constrains on the ToF and tracker hit number and position.
Selections based on the interaction topology and energy loss in the calorimeter allow pbar/e- discrimination. Antiprotons in the selected energy range are likely to annihilate inside the calorimeter, thus leaving a clear signature.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

7. Antiproton identification
The residual contamination was estimated with simulations to be about 10% in the rigidity range 1–3 GV and negligible elsewhere.
Correction applied to account for losses due to ionization and multiple scattering in the upper part of the apparatus (Montecarlo, Fluka+GEANT3).
Selection and detector efficiencies were determined using flight data. Test beam and simulation data were used to support and cross-check these measurements.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

8. Stat. & Sys. Errors plotted
Antiproton Flux
Stat. & Sys. Errors plotted
Adriani et a., Phys. Rev. Lett. 105, (Published September 13, 2010)
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

9. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
Data set
During about 850 days of data acquisition (from 2006 July to 2008 December), 28 trapped antiprotons were identified within the kinetic energy range 60–750 MeV.
Events with geomagnetic McIlwain coordinates in the range 1.1< L < 1.3 and B < G were selected, corresponding to the SAA (IGRF2010 model).
The fractional livetime spent by PAMELA in this region amounts to the 1.7% (~4.6 × 109 s).
Under-cutoff proton candidates distribution as a function of L-shell and geomagnetic field intensity B [G].
SAA
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

10. Flux anisotropy and apparatus response
The apparatus gathering power,i.e. the factor of proportionality between flux and the number of candidates (corrected for efficiencies and acquisition time), depends both on the angular distribution of the flux and the detector geometry.
In presence of an isotropic particle flux, the gathering power depends only on the detector design, and it is usually called the geometrical factor.
In radiation belts, fluxes present significant anisotropy due to the particles motion along the field lines. This results in a well-defined pitch-angle distribution.
The flux angular distributions, needed for the estimate of the apparatus gathering power, were evaluated using a trapped antiproton model (Selesnick et al. 2007).
The calculation was performed using simulations according to the method described in Sullivan NIM, 95, 5 (1971), taking into account the dependency of the directional response function on the satellite orbital position and on its orientation relative to the geomagnetic field lines:
Pitch angle distributions were evaluated at more than 300 points along the orbit in the SAA and for most probable orientations of PAMELA relative to the magnetic field lines.
A mean gathering power, averaged over all PAMELA orbital positions and orientations, was derived.
The dependence of the instrument response on particle rigidity was studied by estimating the gathering power at 10 rigidity values in the range of interest.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

11. Antiproton back-tracing
The propagation of each antiproton candidate was checked by means of a simulation tool (by Clem, J.,&Owocki, K. 2010, thanks a lot to J.Clem for modifying it for this work) allowing to back-trace particle trajectories through the Earth's magnetosphere (pathlenght ray tracing extended up to 900 REarth).
The results of simulations show that all candidates gyrate around field lines while moving along them, bouncing back and forth between magnetic poles, with also a slow longitudinal drift around the Earth.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

12. Calculated for for the PAMELA orbit
ApJL, 737:L29, 2011 August 20
Sub-cutoff (B > 0.23 G), R< 0.8xSVC. Nearly isotropic flux distribution was assumed.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

13. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
ApJL, 737:L29, 2011 August 20
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

14. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
Conclusions
Antiprotons trapped in Earth’s inner radiation belt have been observed for the first time by the PAMELA experiment, confirming the existence of a significant antiproton flux in the SAA below ~1 GeV in kinetic energy.
The flux exceeds the galactic CR antiproton flux by three orders of magnitude at the current solar minimum, thereby constituting the most abundant antiproton source near the Earth.
A measurement of the sub-cutoff antiproton spectrum outside the SAA region is also presented.
PAMELA results allow CR transport models to be tested in the terrestrial atmosphere and significantly constrain predictions from trapped antiproton models, reducing uncertainties concerning the antiproton production spectrum in Earth’s magnetosphere.
ApJL, 737:L29, 2011 August 20
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

15. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
Backup slides
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

16. Antiproton sources in the atmosphere
In the so-called CRAND (Cosmic Ray Albedo Neutron Decay) process, albedo neutrons decay into protons, a fraction of which become trapped in the geomagnetic field. By the same mechanisms, albedo antineutrons are created, and decaying would also lead to trapped antiprotons (CRANbarD).
Antineutrons are produced by reactions such as: ppppnn̅, resulting in significant albedo only at energies >100MeV.
The high energy thresholds result in the antineutrons directed nearly parallel to the CR arrival directions (or can be backscattered after interacting with the atmosphere).
Therefore albedo antineutrons are primarily at high zenith angles of 60° to 90° (near the horizon) having being produced by similarly directed primary CR.
The CRANbarD mechanism is expected to provide the largest contribution to the trapped antiproton population
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

17. A trapped antiproton candidate
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

18. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
Flux anisotropy
Particle fluxes in the SAA region are highly anisotropic due to the interaction with the Earth’s atmosphere.
As a consequence, the gyro-motion along magnetic field lines and bouncing between two conjugate mirror points result in a defined pitch angle distribution.
Particles whose mirror points are located at an altitude where the atmosphere is dense enough, are likely to be lost in collisions with atmospheric nuclei.
This defines a range of pitch angles for which magnetic trapping does not occur, that is known as the loss cone.
Another source of anisotropy is related to the so called East-West effect:
gyroradius varies from  50 km for a 60 MeV proton, up to  700 km for a 3 GeV proton.
the effect is reduced since the satellite axis is mainly pointing towards the zenith, and due to the finite PAMELA aperture ( 20deg).
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011 18

19. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
The loss cone
The loss cone angle can be expressed as: where Beq is the magnetic field intensity at equator, and Batm corresponds to the field line intersection with the absorbing atmosphere (typically  100 km).
Since Batm on a given field line can be different at the two hemispheres, due to the north-south asymmetry in the geomagnetic field, the lower value of Batm was used.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011 19

20. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
Under-cutoff proton candidates distribution as a function of equatorial pitch angle and L-shell.
The region between superimposed black lines corresponds to the pitch angle range outside the calculated loss cone.
Since only trapped particles whose mirror points are located below PAMELA altitude can be registered, pitch angles near 90° can be observed only at low L-shells (below L ~1.17); conversely, pitch angles far from 90° can be detected at higher L values.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011 20

21. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
The livetime spent by PAMELA at each different satellite orientation:  = (B, B) where B and B denote respectively the zenith and the azimuth angles of geomagnetic field in the PAMELA reference frame.
As a consequence of the flux anysotropy, measured intensities depend on the orientation of the PAMELA axis respect to the local magnetic field B.
Since measurements were performed mainly in the southern magnetic hemisphere, the mean zenith angle B results to be ~70° and, consequently, the range of observable pitch angles is asymmetric respect to eq=90°.
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011 21

22. East-West effect B pitch angle < 90 (directed to North) West East
pitch angle > 90 (directed to South)
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011

23. A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011
East-West effect
A. Bruno & F. Cafagna, 32nd ICRC, Beijing 2011