Signatures of dark matter in cosmic antimatter fluxes: results from the PAMELA experiment

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


PAMELAPayload for Antimatter/Matter Exploration and Light-

nuclei Astrophysics

Direct detection of CRs in space Main focus on antimatter

component


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?


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


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+ -


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


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)



• Particle identification @ low energy• Identify albedo (up-ward going particles < 0 ) NB! They mimic antimatter!

Velocity measurement



• 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!!


PAMELA design performance

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+positronsup 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

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

Magnetic curvature & trigger spillover showercontainment

Maximum detectable rigidity (MDR)


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 !!


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)


Antiproton-to-proton ratio1 – 100 GeV

Submitted to PRL on 13/09/2008


Antiproton-to-proton ratio

(statistical errors only)


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


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


e+

p

p-bar

e-

Proton background suppressionZ=-1

Z=+1

Fraction of charge released along the


+Constraints on:

Energy-rigidity match

Rigidity: 20-30 GV


e+

p

e-Rigidity: 20-30 GV

Proton background suppressionFraction of charge released along the


+Constraints on:

Energy-momentum match

Shower starting-point

Longitudinal profile

Z=-1

Z=+1


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


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


Positron charge ratio

___ Moskalenko & Strong 1998statistical errors onlyenergy in the spectrometer

Currentlyunder

embargo


Charge dependent solar modulation

A > 0 Positive particles

A < 0 Clem J. & Evenson 2007

--- Clem 1995--- Bibier 1999 Drift Model

¯

+

¯

+


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


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



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



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


PAMELA


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)

Documents

Signatures of dark matter in cosmic antimatter fluxes: results from the PAMELA experiment