1. Signatures of dark matter in cosmic antimatter fluxes: results from the PAMELA experiment
Roberta Sparvoli
University of Rome “Tor Vergata” and INFN Rome, Italy
On behalf of the PAMELA collaboration
 Roberta Sparvoli  TEVO8 

2.  Roberta Sparvoli  TEVO8 
PAMELA Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics
 Direct detection of CRs in space
 Main focus on antimatter component
 Roberta Sparvoli  TEVO8 

3. Why CR antimatter? Evaporation of primordial black holes
Anti-nucleosyntesis
First historical measurements of p-bar/p ratio
WIMP dark-matter annihilation in the galactic halo
Background:
CR interaction with ISM
CR + ISM  p-bar + …
 Roberta Sparvoli  TEVO8 

4. Charge-dependent solar modulation
Solar polarity reversal 1999/2000
Asaoka Y. Et al. 2002 +
CR antimatter
Present status
Antiprotons
Positrons
Positron excess? ? ?
___ Moskalenko & Strong 1998
CR + ISM  p-bar + …
kinematic treshold:
5.6 GeV for the reaction
CR + ISM  p± + x  m± + x  e± + x
CR + ISM  p0 + x  gg  e±
 Roberta Sparvoli  TEVO8 

5. CR antimatter: available data
Why in space?
Antiprotons
Positrons
___ Moskalenko & Strong 1998
BESS-polar
(long-duration)
“Standard” balloon-borne experiments
low exposure (~20h)
 large statistical errors
atmospheric secondaries (~5g/cm2)
 additional systematic
 Roberta Sparvoli  TEVO8 

6.  Roberta Sparvoli  TEVO8 
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, 0.6 λI)
Discrimination e+ / p, anti-p / e-
(shower topology)
Direct E measurement for e-
Neutron detector
High-energy e/h discrimination
Overview of the apparatus
GF: 21.5 cm2 sr Mass: 470 kg
Size: 130x70x70 cm3
Power Budget: 360W
Spectrometer
microstrip silicon tracking system permanent magnet
It provides:
- Magnetic rigidity  R = pc/Ze
Charge sign
Charge value from dE/dx
 Roberta Sparvoli  TEVO8 

7. Principle of operation
Track reconstruction
with protons of known momentum
 MDR~1TV
Cross-check in flight with protons (alignment) and electrons (energy from calorimeter)
Iterative c2 minimization as a function of track state-vector components a
Magnetic deflection
|η| = 1/R
R = pc/Ze  magnetic rigidity
sR/R = sh/h
Maximum Detectable Rigidity (MDR)
R=MDR  sR/R=1
MDR = 1/sh
 Roberta Sparvoli  TEVO8 

8. Principle of operation
Z measurement p d
3He
4He Li Be
B,C
track average e±
(saturation)
Bethe Bloch
ionization energy-loss of heavy (M>>me) charged particles
1st plane
 Roberta Sparvoli  TEVO8 

9. Principle of operation
Velocity measurement
Particle low energy
Identify albedo (up-ward going particles b < 0 )
 NB! They mimic antimatter!
 Roberta Sparvoli  TEVO8 

10. Principle of operation
Electron/hadron separation
Interaction topology
e/h separation
Energy measurement of electrons and positrons
(~full shower containment)
hadron (19GV)
electron (17GV)
+ NEUTRONS!!
 Roberta Sparvoli  TEVO8 

11. Antiproton identification
-1  Z  +1
p (+ e+) p
e- (+ p-bar)
proton-consistency cuts
(dE/dx vs R and b vs R)
“spillover” p
p-bar
electron-rejection cuts based on calorimeter-pattern topology
5 GV
1 GV
The main difficulty for the antiproton measurement is the spillover proton bkg @ high energy:
 finite deflection resolution of the spectrometer
 p/p-bar ~ 104 !!
 Roberta Sparvoli  TEVO8 

12. High-energy antiproton selection
p-bar
“spillover” p p
R < MDR/10
MDR = 1/sh
(evaluated event-by-event by the fitting routine)
10 GV
50 GV
MDR depends on:
number and distribution of fitted points along the trajectory
spatial resolution of the single position measurements
magnetic field intensity along the trajectory
 Roberta Sparvoli  TEVO8 

13. Antiproton-to-proton ratio 1 – 100 GeV
Submitted to PRL on 13/09/2008
 Roberta Sparvoli  TEVO8 

14. Antiproton-to-proton ratio
(statistical errors only)
 Roberta Sparvoli  TEVO8 

15. Antiproton-to-proton ratio
(statistical errors only)
 Roberta Sparvoli  TEVO8 

16. Antiproton-to-proton ratio
Preliminary!!
(statistical errors only)
 Roberta Sparvoli  TEVO8 

17. Positron identification
The main difficulty for the positron measurement is the interacting-proton bkg:
fluctuations in hadronic shower development  p0 gg might mimic pure em showers
proton spectrum harder than positron  p/e+ increase for increasing energy
Energy-rigidity match e-
( e+ )
 e
 h
p-bar p
 Roberta Sparvoli  TEVO8 

18. Proton background suppression
Z=-1
Fraction of charge released along the calorimeter track (left, hit, right)
Rigidity: GV e-
p-bar (non-int)
p-bar (int)
NB!
Z=+1
Total negative  3067
Total positive 
Selected electrons  2032
Selected positrons  322
p (non-int)
(e+)
p (int)
 Roberta Sparvoli  TEVO8  18

19. Proton background suppression
Z=-1
Fraction of charge released along the calorimeter track (left, hit, right)
Rigidity: GV e- +
Constraints on:
p-bar
Energy-rigidity
match
Z=+1
Total negative  3067
Total positive 
Selected electrons  2032
Selected positrons  322 e+ p
 Roberta Sparvoli  TEVO8  19

20. Proton background suppression
Z=-1
Fraction of charge released along the calorimeter track (left, hit, right)
Rigidity: GV e- +
Constraints on:
Energy-momentum match
Shower starting-point
Z=+1
Longitudinal profile
Total negative  3067
Total positive 
Selected electrons  2032
Selected positrons  322 e+ p
 Roberta Sparvoli  TEVO8  20

21. Proton background evaluation
Preliminary!!
Rigidity: 6-8 GV e-
Fraction of charge released along the calorimeter track (left, hit, right) +
Constraints on: e+
Energy-momentum match p
Shower starting-point
Total negative  3067
Total positive 
Selected electrons  2032
Selected positrons  322
Longitudinal profile p
 Roberta Sparvoli  TEVO8  21

22. Tests on the positron selection
Fraction of charge released along the calorimeter track (left, hit, right)
Flight data:
rigidity: GV
Test beam data
Momentum: 50 GeV/c e- e- e- p e+ p e+ p
Energy-rigidity match
Starting point of shower
 Roberta Sparvoli  TEVO8 

23. Tests on the positron selection
Rigidity: GV
Fraction of charge released along the calorimeter track (left, hit, right)
Neutrons detected by ND e- e- p e+ e+ p
Energy-rigidity match
Starting point of shower
 Roberta Sparvoli  TEVO8 

24.  Roberta Sparvoli  TEVO8 
Positron charge ratio
Preliminary!!
statistical errors only
energy in the spectrometer
___ Moskalenko & Strong 1998
Currently
under
embargo
 Roberta Sparvoli  TEVO8 

25. Charge dependent solar modulation
Preliminary!! + +
Clem J. & Evenson 2007
--- Clem 1995
--- Bibier 1999
Drift Model
A > 0
Positive particles
A < 0
 Roberta Sparvoli  TEVO8 

26. High-energy positrons
Pulsars
LSP (Neutralino) annihilation
statistical errors only
energy in the spectrometer
(Strong-Moskalenko-Ptuskin 2007)
___ Moskalenko & Strong 1998
Contribution from pulsars ?
Contribution from WIMP annihilation in the galactic halo
 Roberta Sparvoli  TEVO8 

27. Galactic H and He spectra
Preliminary!!
(statistical errors only)
Very high statistics over a wide energy range
 Precise measurement of spectral shape
 Possibility to study time variations and transient phenomena
 Roberta Sparvoli  TEVO8 

28.  Roberta Sparvoli  TEVO8 
Solar modulation
Interstellar spectrum
Preliminary!!
(statistical errors only)
Increasing
GCR flux
July 2006
August 2007
February 2008
solar activity
Decreasing
sun-spot number
Ground neutron monitor
PAMELA
 Roberta Sparvoli  TEVO8 

29.  Roberta Sparvoli  TEVO8 
Proton flux * E2.75
Preliminary!!
(statistical errors only)
Power-law fit: ~ E-g
g ~ 2.76
Proton of primary origin
Diffusive shock-wave acceleration in SNRs
Local spectrum:
injection spectrum  galactic propagation
Local primary spectral shape:
study of particle acceleration mechanism
LBM
 Roberta Sparvoli  TEVO8 

30.  Roberta Sparvoli  TEVO8 
Proton flux * E2.75
Preliminary!!
(statistical errors only)
Power-law fit: ~ E-g
g ~ 2.76
Proton of primary origin
Diffusive shock-wave acceleration in SNRs
Local spectrum:
injection spectrum  galactic propagation
Local primary spectral shape:
study of particle acceleration mechanism
LBM
NB! still large discrepancies among different primary flux measurements
 Roberta Sparvoli  TEVO8 

31.  Roberta Sparvoli  TEVO8 
Secondary nuclei
Preliminary!!
(statistical errors only)
PAMELA
LBM
B nuclei of secondary origin:
CNO + ISM  B + …
Local secondary/primary ratio sensitive to average amount of traversed matter (lesc) from the source to the solar system
Local secondary abundance:
study of galactic CR propagation
(B/C used for tuning of propagation models)
 Roberta Sparvoli  TEVO8 

32. Conclusions PAMELA is taking data since July 2006
(lot of fun analyzing the data!)
Presented preliminary results from ~600 days of data:
Antiproton charge ratio (~ 100 MeV ÷ 100 GeV)
no evident deviations from secondary expectations
more high energy data to come (up to ~150 GeV)
Positron charge ratio (~ 400 MeV ÷ 10 GeV)
indicates charge dependent modulation effects
more data to come at lower and higher energies (up to ~ 200 GeV)
Galactic primary proton spectra
primary spectra up to Z=8 to come
Galactic secondary-to-primary ratio (B/C)
abundance of other secondary elements (Li,Be) and isotopes (d,3He) to come
 PAMELA is providing significant experimental results for dark matter searches and for understanding CR origin and propagation
Please attend the talk of prof. P. Picozza
for more details (Friday, 10:45)
 Roberta Sparvoli  TEVO8 