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 

 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 

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 

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 

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 uncertainty @low-energy  Roberta Sparvoli  TEVO8 

 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 

Principle of operation Track reconstruction Measured @ground 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) def: @ R=MDR  sR/R=1 MDR = 1/sh  Roberta Sparvoli  TEVO8 

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 

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

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 

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 

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 

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

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

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

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

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 

Proton background suppression Z=-1 Fraction of charge released along the calorimeter track (left, hit, right) Rigidity: 20-30 GV e- p-bar (non-int) p-bar (int) NB! Z=+1 Total negative  3067 Total positive  189055 Selected electrons  2032 Selected positrons  322 p (non-int) (e+) p (int)  Roberta Sparvoli  TEVO8  18

Proton background suppression Z=-1 Fraction of charge released along the calorimeter track (left, hit, right) Rigidity: 20-30 GV e- + Constraints on: p-bar Energy-rigidity match Z=+1 Total negative  3067 Total positive  189055 Selected electrons  2032 Selected positrons  322 e+ p  Roberta Sparvoli  TEVO8  19

Proton background suppression Z=-1 Fraction of charge released along the calorimeter track (left, hit, right) Rigidity: 20-30 GV e- + Constraints on: Energy-momentum match Shower starting-point Z=+1 Longitudinal profile Total negative  3067 Total positive  189055 Selected electrons  2032 Selected positrons  322 e+ p  Roberta Sparvoli  TEVO8  20

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  189055 Selected electrons  2032 Selected positrons  322 Longitudinal profile p  Roberta Sparvoli  TEVO8  21

Tests on the positron selection Fraction of charge released along the calorimeter track (left, hit, right) Flight data: rigidity: 20-30 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 

Tests on the positron selection Rigidity: 20-30 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 

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

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

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 

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 

 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 

 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 

 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 

 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 

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 