Transcript Document

Firenze, 25 February 2009
The PAMELA experiment:
looking for antiparticles in
Cosmic Rays
Oscar Adriani
University of Florence and
INFN Florence
Outline
• The PAMELA experiment: short review
• Results on cosmic-ray antimatter abundance:
– Antiprotons
– Positrons
• Other results:
– Cosmic-ray galactic light nuclei (primaries & secondaries)
– Solar physics
– Terrestrial physics
• Conclusions
 Oscar Adriani  Florence, February 25th, 2009 
Tha PAMELA collaboration
Italy
Bari
Florence Frascati
Naples
Rome
Trieste CNR, Florence
Russia
Moscow
St. Petersburg
Germany
Sweden
Siegen
KTH, Stockholm
 Oscar Adriani  Florence, February 25th, 2009 
Evaporation of
primordial black
holes
Why CR antimatter?
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 + …
 Oscar Adriani  Florence, February 25th, 2009 
Cosmic-ray Antimatter from Dark Matter annihilation
Annihilation of relic Weakly Interacting
Massive Particles (WIMPs) gravitationally
confined in the galactic halo
 Distortion of antiproton and positron
spectra from purely secondary production
• A plausible dark matter candidate is
neutralino (c), the lightest SUSY
Particle (LSP).
Most likely processes:
Halo
You are here
p-bar, e+
c
• cc  qq  hadrons anti-p, e+,…
• cc  W+W-,Z0Z0,…  e+,…
 positron peak Ee+~Mc/2
 positron continuum Ee+~Mc/20
• Another possible candidate is the lightest
Kaluza-Klein Particle (LKP): B(1)
Fermionic final states no longer suppressed:
• B(1)B(1)  e+e-
direct dicay  positron peak Ee+ ~ MB(1)
 Oscar Adriani  Florence, February 25th, 2009 
c
Milky Way
Charge-dependent
solar modulation
Asaoka Y. Et al. 2002
CR antimatter
Experimental scenario before PAMELA
Antiprotons
Positrons
___ Moskalenko & Strong 1998 Positron excess?
Solar polarity reversal
1999/2000
¯
+
CR + ISM  p-bar + …
•Propagation dominated by nuclear interactions
•Kinematical threshold: Eth~5.6 for the reaction
pp  pppp
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 (@100GeV 90% from <2kpc)
 Oscar Adriani  Florence, February 25th, 2009 
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-
GF: 21.5 cm2 sr
Mass: 470 kg
Size: 130x70x70 cm3
Power Budget: 360W
Neutron detector
plastic scintillators + PMT:
- High-energy e/h discrimination
Spectrometer
microstrip silicon tracking system + permanent magnet
It provides:
- Magnetic rigidity  R = pc/Ze
- Charge sign
- Charge value from dE/dx
 Oscar Adriani  Florence, February 25th, 2009 
Principle of operation
Track reconstruction
Iterative c2
minimization as a
function of track statevector components a
Magnetic deflection
|η| = 1/R
R = pc/Ze  magnetic rigidity
sR/R = sh/h
Maximum Detectable Rigidity (MDR)
def: @ R=MDR  sR/R=1
MDR = 1/sh
•Measured @ground with protons of known momentum
 MDR~1TV
•Cross-check in flight with protons (alignment) and
electrons (energy from calorimeter)
 Oscar Adriani  Florence, February 25th, 2009 
Principle of operation
Z measurement
track average
Bethe Bloch
ionization energy-loss of
heavy (M>>me)
charged particles
(saturation)
4He
3He
d
p
1st plane
B,C
e±
 Oscar Adriani  Florence, February 25th, 2009 
Be
Li
Principle of operation
Velocity measurement
• Particle identification @ low energy
• Identify albedo (up-ward going particles b < 0 )
 NB! They mimic antimatter!
 Oscar Adriani  Florence, February 25th, 2009 
Principle of operation
Electron/hadron separation
• Interaction topology
e/h separation
hadron (19GV)
electron (17GV)
+ NEUTRONS!!
• Energy measurement of electrons and positrons
(~full shower containment)
σE
b
a
E
E
 a  5%
 Oscar Adriani  Florence, February 25th, 2009 
The Resurs DK-1 spacecraft
• Multi-spectral remote sensing of earth’s surface
-near-real-time high-quality images
• Built by the Space factory TsSKB Progress in Samara
(Russia)
• Operational orbit parameters:
-inclination ~70o
-altitude ~ 360-600 km (elliptical)
• Active life >3 years
• Data transmitted via Very high-speed Radio Link
(VRL)
Mass: 6.7 tons
Height: 7.4 m
Solar array area: 36 m2
• PAMELA mounted
inside a pressurized
container
• moved from parking
to data-taking position
few times/year
 Oscar Adriani  Florence, February 25th, 2009 
PAMELA design performance
Magnetic curvature & trigger
spillover
energy range
Antiprotons
Positrons
Electrons
Protons
Electrons+positrons
Light Nuclei
Anti-Nuclei search
shower
containment
Maximum detectable
rigidity (MDR)
particles in 3 years
80 MeV ÷190 GeV
O(104)
50 MeV ÷ 270 GeV
O(105)
up to 400 GeV
O(106)
up to 700 GeV
O(108)
up to 2 TeV
(from calorimeter)
up to 200 GeV/n
He/Be/C:
O(107/4/5)
sensitivity of 3x10-8 in anti-He/He
 Unprecedented statistics and new energy range for cosmic ray physics
(e.g. contemporary antiproton and positron maximum energy ~ 40 GeV)
 Simultaneous measurements of many species
 Oscar Adriani  Florence, February 25th, 2009 
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* ~25Hz
Fraction of live time* ~ 75%
Event size (compressed mode) ~ 5kB
25 Hz x 5 kB/ev  ~ 10 GB/day
(*outside radiation belts)
Till December 2008:
~800 days of data taking
~16 TByte of raw data downlinked
~16•108 triggers recorded and analysed
(Data from
May till now under analysis)
th
 Oscar Adriani  Florence, February 25 , 2009 
Antiprotons
High-energy antiproton analysis
• Analyzed data July 2006 – February 2008 (~500 days)
• Collected triggers ~108
• Identified ~ 10 106 protons and ~ 1 103 antiprotons between 1.5
and 100 GeV ( 100 p-bar above 20GeV )
• 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 charge-sign @ high energy
• proton spectrum harder than positron  p/p-bar
increase for increasing energy (103 @1GV 104 @100GV)
 Required strong SPE selection
 Oscar Adriani  Florence, February 25th, 2009 
Antiproton identification
-1  Z  +1
p (+ e+)
p
e- (+ p-bar)
proton-consistency cuts
(dE/dx vs R and b vs R)
“spillover” p
electron-rejection cuts based on
calorimeter-pattern topology
p-bar
5 GV
1 GV
( For |Z|=1, deflection=1/p )
 Oscar Adriani  Florence, February 25th, 2009 
Proton-spillover background
Minimal track requirements
MDR = 1/sh
(evaluated event-by-event by
the fitting routine)
MDR > 850 GV
Strong track requirements:
•strict constraints on c2 (~75% efficiency)
•rejected tracks with low-resolution
clusters along the trajectory
- faulty strips (high noise)
- d-rays (high signal and multiplicity)
 Oscar Adriani  Florence, February 25th, 2009 
Proton-spillover background
MDR = 1/sh
(evaluated event-by-event by
the fitting routine)
p-bar
“spillover” p
R < MDR/10
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
 Oscar Adriani  Florence, February 25th, 2009 
p
Antiproton-to-proton ratio
*preliminary*
(Petter Hofverberg’s PhD Thesis)
PRL 102, 051101 (2009)
 Oscar Adriani  Florence, February 25th, 2009 
Positrons
High-energy positron analysis
• Analyzed data July 2006 – February 2008 (~500 days)
• Collected triggers ~108
• Identified ~ 150 103 electrons and ~ 9 103 positrons between
1.5 and 100 GeV (180 positrons above 20GeV )
S1
CAT
• Electron/positron identification:
TOF
SPE
S3
• 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 (103 @1GV 104 @100GV)
 Required strong CALO selection
 Oscar Adriani  Florence, February 25th, 2009 
CAS
• 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
S2
CALO
S4
ND
Positron identification with CALO
• Identification based on:
51 GV positron
– 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
80 GV proton
– Selection of pure proton sample from flight data
(“pre-sampler” method):
• Background-suppression method
• Background-estimation method
Final results make NON USE of test-beam and/or simulation calibrations.
The measurement is based only on flight data
with the background-estimation method
 Oscar Adriani  Florence, February 25th, 2009 
Positron identification
Fraction of charge
released along the
calorimeter track
Z=-1
Rigidity: 20-30 GV
ep-bar (non-int)
p-bar (int)
NB!
Z=+1
p (non-int)
p (int)
(e+)
 Oscar Adriani  Florence, February 25th, 2009 
planes
0.6 RM
LEFT
HIT
RIGHT
strips
Energy measured in Calo/
Deflection in Tracker (MIP/GV)
Positron identification
Energy-momentum
match
e-
( e+ )
e
h
p-bar
p
 Oscar Adriani  Florence, February 25th, 2009 
Positron identification
Fraction of charge
released along the
calorimeter track
Z=-1
Rigidity: 20-30 GV
ep-bar (non-int)
p-bar
p-bar
(int)
Constraints on:
Energy-momentum
match
NB!
Z=+1
e+
p (int)
p
+
p (non-int)
(e+)
 Oscar Adriani  Florence, February 25th, 2009 
Positron identification
51 GV positron
Shower starting-point
Longitudinal profile
80 GV proton
 Oscar Adriani  Florence, February 25th, 2009 
Positron identification
Z=-1
e-
Rigidity: 20-30 GV
Fraction of charge
released along the
calorimeter track
+
Constraints on:
p-bar
Energy-momentum
match
Shower starting-point
Longitudinal profile
Z=+1
Lateral profile
ee++
p
p
 Oscar Adriani  Florence, February 25th, 2009 
BK-suppression
method
The “pre-sampler” method
Selection of a pure sample of protons from flight data
CALORIMETER: 22 W planes: 16.3 X0
2 W planes: ≈1.5 X0
20 W planes: ≈15 X0
 Oscar Adriani  Florence, February 25th, 2009 
Proton background evaluation
Rigidity: 20-28 GV
eFraction of charge
released along the
calorimeter track (left,
hit, right)
+
Constraints on:
p (pre-sampler)
Energy-momentum
match
Shower starting-point
e+
p
 Oscar Adriani  Florence, February 25th, 2009 
Positron fraction
astro-ph 0810.4995
Accepted by Nature
 Oscar Adriani  Florence, February 25th, 2009 
Do we have any antimatter excess in CRs?
 Oscar Adriani  Florence, February 25th, 2009 
Antiproton-to-proton ratio
Secondary Production Models
CR + ISM  p-bar + …
(Moskalenko et al. 2006) GALPROP code
• Plain diffusion model
• Solar modulation: drift model ( A<0, a=15o )
(Donato et al. 2001)
• Diffusion model with convection and reacceleration
• Solar modulation: spherical model (f=500MV )
 Uncertainty band related to propagation parameters (~10%
@10GeV)
 Additional uncertainty of ~25% due to production cs should
be considered !!
(Ptuskin et al. 2006) GALPROP code
• Plain diffusion model
• Solar modulation: spherical model ( f=550MV )
No evidence for any antiproton excess
 Oscar Adriani  Florence, February 25th, 2009 
Positron fraction
Secondary Production Models
CR + ISM  p± + …  m± + …  e± + …
CR + ISM  p0 + …  gg  e±
(Moskalenko &
Strong 1998)
GALPROP code
• Plain diffusion
model
• Interstellar spectra
 Oscar Adriani  Florence, February 25th, 2009 
Charge dependent solar modulation
+
¯
¯
+
(Clem & Evenson 2007)
Drift model
A>0
A<0
Positive particles
 Oscar Adriani  Florence, February 25th, 2009 
Positron fraction
CR + ISM  p± + …  m± + …  e± + …
CR + ISM  p0 + …  gg  e±
Secondary Production Models
(Moskalenko &
Strong 1998)
GALPROP code
• Plain diffusion
model
• Interstellar spectra
The positron fraction
depends on the primary
(+secondary)
electron spectrum
Soft electron spectrum (g = 3.54)
Hard electron spectrum (g = 3.34)
(Delahaye et al. 2008)
• Plain diffusion model
• Solar modulation: spherical model (f=600MV)
 Uncertainty band related to e- spectral index
(ge = 3.44±0.1  MIN÷MAX)
 Additional uncertainty due to propagation
parameters should be considered (factor ~6
@1GeV ~4 @high-energy)
 Oscar Adriani  Florence, February 25th, 2009 
Positron fraction
Secondary Production Models
Preferred by Pamela
electron data!
Quite robust evidence for a positron excess
 Oscar Adriani  Florence, February 25th, 2009 
Primary positron sources
Dark Matter
•
e+
yield depend on the dominant decay
channel
LKP -- M= 300 GeV
(Hooper & Profumo 2007)
 LSPs 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 cutoff @
MLKP (>300 GeV)
• Boost factor required to have a sizable e+
signal
NB: constraints from p-bar data!!
 Oscar Adriani  Florence, February 25th, 2009 
Primary positron sources
Astrophysical processes
• Local pulsars are well-known sites of
e+e- pair production:
All pulsars (rate = 3.3 / 100 years)
(Hooper, Blasi, Seprico 2008)
 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
~80 theoretical paper on Pamela data since our ArXiv publication!!!!!
 Oscar Adriani  Florence, February 25th, 2009 
PAMELA positron excess might be connected with
ATIC electron+positron structures?
(Chang et al 2008)
 Oscar Adriani  Florence, February 25th, 2009 
Primary positron sources
PAMELA positron fraction alone
insufficient to understand the
origin of positron excess
Additional experimental data will be provided
by PAMELA:
– e+ fraction @ higher energy (up to 300 GeV)
– individual e- e+ spectra
– anisotropy (…maybe)
– high energy e++e- spectrum (up to 2 TV)
Complementary information from:
– gamma rays
– neutrinos
 Oscar Adriani  Florence, February 25th, 2009 
Galactic cosmic-ray origin &
propagation
H and He spectra
(statistical errors only)
NP  QP λ esc
• 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
Very high statistics over a wide energy range
 Precise measurement of spectral shape
 Possibility to study time variations and transient
phenomena
 Oscar Adriani  Florence, February 25th, 2009 
Secondary nuclei
• 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)
NS
 λ esc  σ PS
NP
 Oscar Adriani  Florence, February 25th, 2009 
Solar physics
Solar modulation
Solar Energetic Particle events (SEPs)
Solar modulation
(statistical errors only)
Interstellar spectrum
PAMELA
Ground neutron monitor
sun-spot number
 Oscar Adriani  Florence, February 25th, 2009 
Decreasing
solar activity
Increasing
GCR flux
July 2006
August 2007
February 2008
December 2006 CME/SEP events
SOHO/LASCO
SOHO/EIT
Coronal Mass Ejection (CME)
X-ray flares
Solar Energetic Particles (SEPs)
protons: 1÷100 MeV
alphas: 150÷500 MeV
 Oscar Adriani  Florence, February 25th, 2009 
December 13th 2006 event
from 2006-12-1 to 2006-12-4
Increase of low
energy component
 Oscar Adriani  Florence, February 25th, 2009 
December 13th 2006 event
from 2006-12-1 to 2006-12-4
from 2006-12-13 00:23:02 to 2006-12-13 02:57:46
Increase of low
energy component
 Oscar Adriani  Florence, February 25th, 2009 
December 13th 2006 event
from 2006-12-1 to 2006-12-4
from 2006-12-13 00:23:02 to 2006-12-13 02:57:46
from 2006-12-13 02:57:46 to 2006-12-13 03:49:09
Increase of low
energy component
 Oscar Adriani  Florence, February 25th, 2009 
December 13th 2006 event
from 2006-12-1 to 2006-12-4
from 2006-12-13 00:23:02 to 2006-12-13 02:57:46
from 2006-12-13 02:57:46 to 2006-12-13 03:49:09
from 2006-12-13 03:49:09 to 2006-12-13 04:32:56
Decrease of high
energy component
Increase of low
energy component
Increase of low
energy component
 Oscar Adriani  Florence, February 25th, 2009 
December 13th 2006 event
from 2006-12-1 to 2006-12-4
from 2006-12-13 00:23:02 to 2006-12-13 02:57:46
from 2006-12-13 02:57:46 to 2006-12-13 03:49:09
from 2006-12-13 03:49:09 to 2006-12-13 04:32:56
from 2006-12-13 04:32:56 to 2006-12-13 04:59:16
Increase of low
energy component
 Oscar Adriani  Florence, February 25th, 2009 
December 13th 2006 event
from 2006-12-1 to 2006-12-4
from 2006-12-13 00:23:02 to 2006-12-13 02:57:46
from 2006-12-13 02:57:46 to 2006-12-13 03:49:09
from 2006-12-13 03:49:09 to 2006-12-13 04:32:56
from 2006-12-13 04:32:56 to 2006-12-13 04:59:16
from 2006-12-13 08:17:54 to 2006-12-13 09:17:34
SEPs accelerated during CMEs
SEP spectral shape and time evolution
 study of particle acceleration
mechanisms in CME
Additional information will be provided by:
• electrons & positrons
• nuclear and isotopic (3He, 4He)
Increase
of low
composition
energy component
 Oscar Adriani  Florence, February 25th, 2009 
Terrestrial physics
Magnetosphere
Radiation belts & SAA
Interactions of CRs with the atmosphere
Analysis of a PAMELA orbit
S1
Low energy particles
S2 stops inside the apparatus
 Counting rates:
S1>S2>S3
S3
SAA
Outer
radiation belt
Download @orbit 3754 – 15/02/2007 07:35:00 MWT
NP
SP
S1
Inner radiation
belt
(SAA)
Ratemeters
Independent
from trigger
95 min
EQ
orbit 3751
orbit 3752
S2
S3
orbit 3753
 Oscar Adriani  Florence, February 25th, 2009 
EQ
Trigger rate
SAA
trapped
 Oscar Adriani  Florence, February 25th, 2009 
Trapped-particle spectrum
(statistical errors only)
Trapped
Galactic
B > 0.30 G
0.22 G ≤ B < 0.23 G
0.21 G ≤ B < 0.22 G
0.20 G ≤ B < 0.21 G
0.19 G ≤ B < 0.20 G
B < 0.19 G
Always: 10 GV < cutoff < 11
 Oscar Adriani  Florence, February 25th, 2009 
Geomagnetic cutoff
(statistical errors only)
Magnetic poles
( galactic protons)
Secondary reentrant-albedo
protons
Geomagnetic
cutoff (GV/c)
0.4 to 0.5
1.0 to 1.5
1.5 to 2.0
2 to 4
4 to 7
7 to 10
10 to 14
> 14
--- M. Honda, 2008
Magnetic equator
•Up-ward going albedo excluded
•SAA excluded
Atmospheric neutrino contribution
Astronaut dose
Indirect measurement of cs in the
atmosphere
 Agile e Glast background estimation
 Oscar Adriani  Florence, February 25th, 2009 
Conclusions
• PAMELA is the first space experiment which is measuring the
antiproton and positron cosmic-ray components to the high
energies (>100GeV) with an unprecedented statistical precision
 search for evidence of DM candidates
 “direct” measurement of particle acceleration in astrophysical sources
(pulsars?)
• Furthermore:
– PAMELA is providing measurements on low-mass elemental (and isotopical)
spectra with an unprecedented statistical precision
 study of particle origin and propagation in the interstellar medium
– PAMELA is able to measure the high energy tail of solar particles.
– PAMELA is measuring composition and spectra of trapped and re-entrant albedo
particles in the Earth magnetosphere
 Oscar Adriani  Florence, February 25th, 2009 