roberta sparvoli tevo8 signatures of dark matter in cosmic antimatter fluxes: results from the...
TRANSCRIPT
Roberta Sparvoli TEVO8
Signatures of dark matter in cosmic antimatter fluxes: results from the PAMELA
experiment
Roberta SparvoliUniversity of Rome “Tor Vergata”
and INFN Rome, Italy
On behalf of the PAMELA
collaboration
Roberta Sparvoli TEVO8
PAMELAPayload for Antimatter/Matter Exploration and Light-
nuclei Astrophysics
Direct detection of CRs in space Main focus on antimatter
component
Roberta Sparvoli TEVO8
First historical measurements of p-bar/p ratio
Anti-nucleosyntesis
WIMP dark-matter
annihilation in the galactic
halo
Background:CR interaction with ISMCR + ISM p-bar + …
Evaporation of primordial black holes
Why CR antimatter?
Roberta Sparvoli TEVO8
CR antimatter
Antiprotons Positrons
CR + ISM ± + x ± + x e± + x CR + ISM 0 + x e±
___ Moskalenko & Strong 1998 Positron excess?
Charge-dependent solar modulation
Solar polarity reversal 1999/2000
Asaoka Y. Et al. 2002
¯
+
CR + ISM p-bar + …kinematic treshold: 5.6 GeV for the reaction
pppppp
Present status
Roberta Sparvoli TEVO8
PAMELA detectors
GF: 21.5 cm2 sr Mass: 470 kgSize: 130x70x70 cm3
Power Budget: 360W Spectrometer microstrip silicon tracking system + permanent magnetIt provides:
- Magnetic rigidity R = pc/Ze- Charge sign- Charge value from dE/dx
Time-Of-Flightplastic scintillators + PMT:- Trigger- Albedo rejection;- Mass identification up to 1 GeV;- Charge identification from dE/dX.
Electromagnetic calorimeterW/Si sampling (16.3 X0, 0.6 λI) - Discrimination e+ / p, anti-p / e- (shower topology)- Direct E measurement for e-
Neutron detectorplastic scintillators + PMT:- High-energy e/h discrimination
Main requirements high-sensitivity antiparticle identification and precise momentum measure+ -
Roberta Sparvoli TEVO8
Principle of operation
•Measured @ground with protons of known momentum
MDR~1TV•Cross-check in flight with protons (alignment) and electrons (energy from calorimeter)
Iterative 2 minimization as a function of track state-vector components
Magnetic deflection|η| = 1/R R = pc/Ze magnetic rigidity
R/R =
Maximum Detectable Rigidity (MDR)def: @ R=MDR R/R=1
MDR = 1/
Track reconstruction
Roberta Sparvoli TEVO8
Principle of operationZ measurement
Bethe Bloch ionization energy-loss of heavy (M>>me) charged particles
1st plane
pd
3He
4He
Li
Be
B,C
track average
e±
(saturation)
Roberta Sparvoli TEVO8
Principle of operation
• Particle identification @ low energy• Identify albedo (up-ward going particles < 0 ) NB! They mimic antimatter!
Velocity measurement
Roberta Sparvoli TEVO8
Principle of operation
• Interaction topologye/h separation
• Energy measurement of electrons and positrons (~full shower containment)
electron (17GV)hadron (19GV)
Electron/hadron separation
5%aE
ba
E
σE
+ NEUTRONS!!
Roberta Sparvoli TEVO8
Antiproton identification
e- (+ p-bar)
p-bar
p
-1 Z +1
“spillover” p
p (+ e+)
proton-consistency cuts(dE/dx vs R and vs R)
electron-rejection cuts based on calorimeter-pattern topology
1 GV5 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
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
High-energy antiproton selection
R < MDR/10
p-bar p
10 GV 50 GV
“spillover” p
MDR = 1/(evaluated event-by-
event by the fitting
routine)
Roberta Sparvoli TEVO8
Antiproton-to-proton ratio1 – 100 GeV
Submitted to PRL on 13/09/2008
Roberta Sparvoli TEVO8
Antiproton-to-proton ratio
(statistical errors only)
Roberta Sparvoli TEVO8
Positron identification
e- ( e+ )
pp-bar
The main difficulty for the positron measurement is the interacting-proton bkg:• fluctuations in hadronic shower development might mimic pure em showers• proton spectrum harder than positron p/e+ increase for increasing energy Energy-rigidity match
e
h
Roberta Sparvoli TEVO8
Proton background suppression
(e+)
p (non-int)
p (int)
NB!
p-bar (int)
e-
p-bar (non-int)
Z=-1
Z=+1
Fraction of charge released along the
calorimeter track (left, hit, right)
Rigidity: 20-30 GV
Roberta Sparvoli TEVO8
e+
p
p-bar
e-
Proton background suppressionZ=-1
Z=+1
Fraction of charge released along the
calorimeter track (left, hit, right)
+Constraints on:
Energy-rigidity match
Rigidity: 20-30 GV
Roberta Sparvoli TEVO8
e+
p
e-Rigidity: 20-30 GV
Proton background suppressionFraction of charge released along the
calorimeter track (left, hit, right)
+Constraints on:
Energy-momentum match
Shower starting-point
Longitudinal profile
Z=-1
Z=+1
Roberta Sparvoli TEVO8
Tests on the positron selection
pp
ee--
ee++
pp
Flight data:rigidity: 20-30 GV
Fraction of charge released along the calorimeter track (left, hit, right)
Test beam dataMomentum: 50 GeV/c
ee--ee--
ee++
• Energy-rigidity match• Starting point of shower
p
Roberta Sparvoli TEVO8
Tests on the positron selection
ee--
pp
ee--
ee++
pp
Neutrons detected by ND
Rigidity: 20-30 GVFraction of charge released along the calorimeter track (left, hit, right)
ee++
• Energy-rigidity match• Starting point of shower
Roberta Sparvoli TEVO8
Positron charge ratio
___ Moskalenko & Strong 1998statistical errors onlyenergy in the spectrometer
Currentlyunder
embargo
Roberta Sparvoli TEVO8
Charge dependent solar modulation
A > 0 Positive particles
A < 0 Clem J. & Evenson 2007
--- Clem 1995--- Bibier 1999 Drift Model
¯
+
¯
+
Roberta Sparvoli TEVO8
High-energy positrons
Contribution from pulsars
PulsarsLSP (Neutralino) annihilation
Contribution from WIMP
annihilation in the galactic halo
___ Moskalenko & Strong 1998
(Strong-Moskalenko-Ptuskin 2007)
statistical errors onlyenergy in the spectrometer
Roberta Sparvoli TEVO8
Galactic H and He spectra
Very high statistics over a wide energy range Precise measurement of spectral shape Possibility to study time variations and transient phenomena
(statistical errors only)
Roberta Sparvoli TEVO8
Proton flux * E2.75
• 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
Power-law fit: ~ E-
~ 2.76
NB! still large discrepancies among different primary flux measurements
escPP λQN LBM
(statistical errors only)
Roberta Sparvoli TEVO8
Secondary nuclei
• B nuclei of secondary origin: CNO + ISM B + …
• Local secondary/primary ratio sensitive to average amount of traversed matter (esc) from the source to the solar system
Local secondary abundance: study of galactic CR propagation
(B/C used for tuning of propagation models)
SPescP
S σλNN
LBM
(statistical errors only)
PAMELA
Roberta Sparvoli TEVO8
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. Picozzafor more details (Friday, 10:45)