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

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. PAMELA P ayload for A ntimatter/ M atter E xploration and L ight-nuclei A strophysics.

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

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  1. 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

  2. PAMELAPayload for Antimatter/Matter Exploration and Light-nuclei Astrophysics  Direct detection of CRs in space  Main focus on antimatter component Roberta SparvoliTEVO8

  3. 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

  4. Charge-dependent solar modulation Solar polarity reversal 1999/2000 Asaoka Y. Et al. 2002 Positron excess? ? ? ¯ + CR + ISM  p-bar + … kinematic treshold: 5.6 GeV for the reaction CR antimatter Present status Positrons Antiprotons ___ Moskalenko & Strong 1998 CR + ISM  p± + x m± + x  e± + x CR + ISM  p0 + x gg e± Roberta SparvoliTEVO8

  5. 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 • plastic scintillators + PMT: • High-energy e/h discrimination 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

  6. 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

  7. track average 4He B,C 3He Be d (saturation) p Li e± 1st plane Principle of operation Z measurement Bethe Bloch ionization energy-loss of heavy (M>>me) charged particles Roberta SparvoliTEVO8

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

  9. 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

  10. PAMELA design performance Maximum detectable rigidity (MDR) energy rangeparticles in 3 years Antiprotons80 MeV ÷190 GeV O(104) Positrons50 MeV ÷ 270 GeV O(105) Electrons up to 400 GeV O(106) Protonsup to 700 GeV O(108) Electrons+positronsup to 2 TeV (from calorimeter) LightNuclei up to 200 GeV/n He/Be/C: O(107/4/5) Anti-Nuclei searchsensitivity of 3x10-8 in anti-He/He Magnetic curvature & trigger spillover shower containment • 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 Roberta SparvoliTEVO8

  11. 1 GV 5 GV 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 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

  12. 10 GV 50 GV High-energy antiproton selection p p-bar “spillover” p R < MDR/10 MDR = 1/sh (evaluated event-by-event by the fitting routine) • 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

  13. Antiproton-to-proton ratio1 – 100 GeV Submitted to PRL on 13/09/2008 Roberta SparvoliTEVO8

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

  15. Positron identification • The main difficulty for the positron measurement is the interacting-proton bkg: • fluctuations in hadronic shower development  p0 ggmight 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

  16. Proton background suppression Fraction of charge released along the calorimeter track (left, hit, right) Z=-1 e- Rigidity: 20-30 GV p-bar (non-int) p-bar (int) NB! Z=+1 p (non-int) (e+) p (int) Roberta SparvoliTEVO8

  17. Proton background suppression Fraction of charge released along the calorimeter track (left, hit, right) Z=-1 e- Rigidity: 20-30 GV + Constraints on: p-bar Energy-rigidity match Z=+1 e+ p Roberta SparvoliTEVO8

  18. Proton background suppression Fraction of charge released along the calorimeter track (left, hit, right) Z=-1 e- Rigidity: 20-30 GV + Constraints on: Energy-momentum match Shower starting-point Z=+1 Longitudinal profile e+ p Roberta SparvoliTEVO8

  19. 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

  20. 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

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

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

  23. 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

  24. 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

  25. Power-law fit: ~ E-g g ~ 2.76 LBM NB! still large discrepancies among different primary flux measurements Proton flux * E2.75 Preliminary!! (statistical errors only) • 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 Roberta SparvoliTEVO8

  26. LBM Secondary nuclei Preliminary!! (statistical errors only) PAMELA • 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

  27. 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

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