1. Roberta Sparvoli University of Rome ”Tor Vergata” and INFN
Indirect searches of Dark Matter in space: results from the PAMELA space experiment
Roberta Sparvoli
University of Rome ”Tor Vergata”
and INFN

2. Everything starts with …
 Roberta Sparvoli  July 17th, 2009  Paris

3. and various ideas of theoretical interpretations
Anti-nucleosyntesis
WIMP dark-matter annihilation in the galactic halo
Background:
CR interaction with ISM
CR + ISM  p-bar + …
Evaporation of primordial black holes
 Roberta Sparvoli  July 17th, 2009  Paris

4.  Roberta Sparvoli  July 17th, 2009  Paris

5.  Roberta Sparvoli  July 17th, 2009  Paris

6. Charge-dependent solar modulation
Solar polarity reversal 1999/2000
Asaoka Y. Et al. 2002 +
CR antimatter
Experimental scenario before PAMELA
WHAT DO WE NEED TO BETTER UNDERSTAND ?
Measurements at higher energies
Better knowledge of background
High statistics
Continuous monitoring of solar modulation
Long Duration Flights
Antiprotons
Positrons
Positron excess? ? ?
___ Moskalenko & Strong 1998
CR + ISM  p± + x  m± + x  e± + x
CR + ISM  p0 + x  gg  e±
Propagation dominated by energy losses
(inverse Compton & synchrotron radiation)
Local origin 90% from <2kpc)
CR + ISM  p-bar + …
Propagation dominated by nuclear interactions
Kinematical threshold: Eth~5.6 for the reaction
 Roberta Sparvoli  July 17th, 2009  Paris

7.  Roberta Sparvoli  July 17th, 2009  Paris
PAMELA pre-history
 Roberta Sparvoli  July 17th, 2009  Paris

8. The PAMELA collaboration
Italy
Bari
Florence
Frascati
Trieste
Naples
Rome
CNR, Florence
Moscow
St. Petersburg
Russia
Germany
Siegen
Sweden
KTH, Stockholm
 Roberta Sparvoli  July 17th, 2009  Paris

9. PAMELA Scientific goals
Halo
p-bar, e+
 Roberta Sparvoli  July 17th, 2009  Paris

10. PAMELA design performance
Maximum detectable rigidity (MDR)
Magnetic curvature & trigger
spillover
shower
containment
energy range particles in 3 years
Antiprotons 80 MeV ÷190 GeV O(104)
Positrons 50 MeV ÷ 270 GeV O(105)
Electrons up to 400 GeV O(106)
Protons up to 700 GeV O(108)
Electrons+positrons up to 2 TeV (from calorimeter)
Light Nuclei up to 200 GeV/n He/Be/C: O(107/4/5)
Anti-Nuclei search sensitivity of 3x10-8 in anti-He/He
 Roberta Sparvoli  July 17th, 2009  Paris

11.  Roberta Sparvoli  July 17th, 2009  Paris
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
36 He3 counters
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  July 17th, 2009  Paris

12. The Resurs DK-1 spacecraft
Characteristic of the satellite
 Roberta Sparvoli  July 17th, 2009  Paris

13.  Roberta Sparvoli  July 17th, 2009  Paris

14.  Roberta Sparvoli  July 17th, 2009  Paris

15.  Roberta Sparvoli  July 17th, 2009  Paris
PAMELA milestones
Launch from Baikonur  June 15th 2006, 0800 UTC.
‘First light’  June 21st 2006, 0300 UTC.
• Detectors operated as expected after launch
• Different trigger and hardware configurations evaluated
 PAMELA in continuous data-taking mode since
commissioning phase ended on July 11th 2006
Main antenna in NTsOMZ
Trigger rate* ~ 25 Hz
Fraction of live time* ~ 75%
Event size (compressed mode) ~ 5kB
25 Hz x 5 kB/ev  ~ 10 GB/day
(*outside radiation belts)
Today 1129 days in flight
>13 TBytes of raw data downlinked
>109 triggers recorded and under analysis
 Roberta Sparvoli  July 17th, 2009  Paris

16. Antiproton/positron discrimination
energy measurement
 Roberta Sparvoli  July 17th, 2009  Paris

17. Antiparticle selection
p, d p
‘Electron’
‘Hadron’
 Roberta Sparvoli  July 17th, 2009  Paris

18.  Roberta Sparvoli  July 17th, 2009  Paris
Antiprotons
 Roberta Sparvoli  July 17th, 2009  Paris

19. High-energy antiproton analysis
Analyzed data July 2006 – February 2008 (~500 days)
Collected triggers ~108
Identified ~ protons and ~ antiprotons between 1.5 and 100 GeV ( 100 p-bar above 20 GeV )
Antiproton/proton identification:
rigidity (R)  SPE
|Z|=1 (dE/dx vs R)  SPE&ToF
b vs R consistent with Mp  ToF
p-bar/p separation (charge sign)  SPE
p-bar/e- (and p/e+ ) separation  CALO
Dominant background  spillover protons:
finite deflection resolution of the SPE  wrong assignment of high energy
proton spectrum harder than positron  p/p-bar increase for increasing energy
 Required strong SPE selection
 Roberta Sparvoli  July 17th, 2009  Paris

20. Proton-spillover background
Electrons: efficiently removed by CALO
Pions (from interactions in dome) : about 3% in the pbar sample
 Roberta Sparvoli  July 17th, 2009  Paris

21. PAMELA: Antiproton-to-proton ratio
PRL 102, (2009)
 Roberta Sparvoli  July 17th, 2009  Paris

22. PAMELA: Antiproton-to-proton ratio
PRL 102, (2009)
 Roberta Sparvoli  July 17th, 2009  Paris

23. PAMELA: Antiproton-to-proton ratio New (preliminary) data !
Full statistics
 Roberta Sparvoli  July 17th, 2009  Paris

24. PAMELA: Antiproton flux preliminary
 Roberta Sparvoli  July 17th, 2009  Paris

25.  Roberta Sparvoli  July 17th, 2009  Paris
Positrons
 Roberta Sparvoli  July 17th, 2009  Paris

26. High-energy positron analysis
Analyzed data July 2006 – February 2008 (~500 days)
Collected triggers ~108
Identified ~ electrons and ~ positrons between 1.5 and 100 GeV (180 positrons above 20 GeV )
Electron/positron identification:
rigidity (R)  SPE
|Z|=1 (dE/dx=MIP)  SPE&ToF
b=1  ToF
e-/e+ separation (charge sign)  SPE
e+/p (and e-/p-bar) separation  CALO
Dominant background  interacting protons:
fluctuations in hadronic shower development  p0 gg might mimic pure em showers
proton spectrum harder than positron  p/e+ increase for increasing energy
 Required strong CALO selection S1 S2
CALO S4
CARD
CAS
CAT
TOF
SPE S3 ND
 Roberta Sparvoli  July 17th, 2009  Paris

27. Positron identification with CALO
Identification based on:
Shower topology (lateral and longitudinal profile, shower starting point)
Total detected energy (energy-rigidity match)
Analysis key points:
Tuning/check of selection criteria with:
test-beam data
simulation
flight data  dE/dx from SPE & neutron yield from ND
Selection of pure proton sample from flight data (“pre-sampler” method):
Background-suppression method
Background-estimation method
51 GV positron
80 GV proton
Final results make NO USE of test-beam and/or simulation calibrations.
The measurement is based only on flight data
with the background-estimation method
 Roberta Sparvoli  July 17th, 2009  Paris

28.  Roberta Sparvoli  July 17th, 2009  Paris
Particle selection after total energy cut and starting point of the shower
e - e+
(anti p) p - +
 Roberta Sparvoli  July 17th, 2009  Paris

29. PAMELA: Positron fraction
NATURE 458, 697, 2009
(Moskalenko & Strong 1998)
GALPROP code
Plain diffusion model
Interstellar spectra
Solar modulation effects
Anomalous increasing ?
 Roberta Sparvoli  July 17th, 2009  Paris

30. PAMELA: Positron fraction
NATURE 458, 697, 2009
 Roberta Sparvoli  July 17th, 2009  Paris

31. Low energy e+: solar modulation effects
 Roberta Sparvoli  July 17th, 2009  Paris

32. PAMELA antimatter data:
Do we have any antimatter excess in CRs?
 Roberta Sparvoli  July 17th, 2009  Paris

33. Antiproton-to-proton ratio Secondary Production Models
CR + ISM  p-bar + …
~20%
 Uncertainty band related to propagation parameters GeV)
 Additional uncertainty of ~25% due to production cs should be considered !!
NB!
Solar modulation
Nuclear cross-section …
No evidence for any antiproton excess
 Roberta Sparvoli  July 17th, 2009  Paris

34. Positron fraction Secondary Production Models
CR + ISM  p± + …  m± + …  e± + …
CR + ISM  p0 + …  gg  e±
Increasing positron fraction only if
ge- gp > 0.6
unlikely
(Serpico 2008)
Quite robust evidence for a positron excess
 Roberta Sparvoli  July 17th, 2009  Paris

35. Primary positron sources
Dark Matter
e+ yield depend on the dominant decay channel
LSPs (SUSY) seem disfavored due to suppression of e+e- final states
low yield (relative to p-bar)
soft spectrum from cascade decays
LKPs seem favored because can annihilate directly in e+e-
high yield (relative to p-bar)
hard spectrum with pronounced MLKP (>300 GeV)
Boost factor required to have a sizable e+ signal
NB: constraints from p-bar data!!
Other hypothesys possible and under study (i.e. Minimal DM Model, decaying DM, new gauge bosons, …)
LKP -- M= 300 GeV
(Hooper & Profumo 2007)
More than 150 articles
claim DM is discovered !
 Roberta Sparvoli  July 17th, 2009  Paris

36.  Roberta Sparvoli  July 17th, 2009  Paris
Example: dark matter
 Roberta Sparvoli  July 17th, 2009  Paris

37. Primary positron sources
Astrophysical processes
Local pulsars are well-known sites of e+e- pair production (the spinning B of the pulsars strips e- that emit gammas then converting to pairs trapped in the cloud, accelerated and then escaping at the Poles) :
 they can individually and/or coherently contribute to the e+e- galactic flux and explain the PAMELA e+ excess (both spectral feature and intensity)
No fine tuning required
if one or few nearby pulsars dominate, anisotropy could be detected in the angular distribution
possibility to discriminate between pulsar and DM origin of e+ excess
All pulsars (rate = 3.3 / 100 years)
(Hooper, Blasi, Seprico 2008)
 Roberta Sparvoli  July 17th, 2009  Paris

38.  Roberta Sparvoli  July 17th, 2009  Paris
Example: pulsars
 Roberta Sparvoli  July 17th, 2009  Paris

39.  Roberta Sparvoli  July 17th, 2009  Paris
Explanation with supernovae remnants Shaviz and al. astro-ph.HE
 Roberta Sparvoli  July 17th, 2009  Paris

40. Positrons from old SNR’s P. Blasi 0903.2794
 Roberta Sparvoli  July 17th, 2009  Paris

41. Standard Positron Fraction
Theoretical Uncertainties
γ = 3.54
γ = 3.34
Need for high statistics CR measurements
T. Delahaye et al., arXiv: v3
 Roberta Sparvoli  July 17th, 2009  Paris

42. Protons, electrons, nuclei, …
 Roberta Sparvoli  July 17th, 2009  Paris

43.  Roberta Sparvoli  July 17th, 2009  Paris
PAMELA electron flux
preliminary
 Roberta Sparvoli  July 17th, 2009  Paris

44.  Roberta Sparvoli  July 17th, 2009  Paris
PAMELA electron flux
 Roberta Sparvoli  July 17th, 2009  Paris

45. PAMELA proton and helium fluxes
preliminary
See next talk by M. Casolino
 Roberta Sparvoli  July 17th, 2009  Paris

46. PAMELA secondary/primary nuclei ratios
preliminary
 Roberta Sparvoli  July 17th, 2009  Paris

47. Additional info in talk by M. Casolino
Summary
~37 13
Additional info in talk by M. Casolino
 Roberta Sparvoli  July 17th, 2009  Paris