The high-energy antiproton-to-proton flux ratio with the PAMELA experiment Massimo Bongi INFN - Florence Payload for Antimatter/Matter Exploration and Light-nuclei Astrophysics

Massimo Bongi – 31 st ICRC – Lodz – 11 July Moscow, St. Petersburg Russia Sweden KTH, Stockholm Germany University of Siegen Italy BariFlorenceFrascatiTriesteNaplesRome CNR, Florence PAMELA collaboration

Massimo Bongi – 31 st ICRC – Lodz – 11 July Resurs-DK1 satellite: multi- spectral imaging of the Earth’s surface PAMELA is mounted inside a pressurized container Quasi-polar and elliptical orbit (70.0°, 350 km km, 90 min) Launch from Baikonur on June 15 th 2006, 0800 UTC, expected lifetime: 3 years PAMELA is in data-taking mode since July 11 th 2006 Data transmission to NTsOMZ (Moscow) via high-speed radio downlink, ~16 GB/day Resurs-DK1 Mass: 6.7 tons Height: 7.4 m Solar array area: 36 m 2 PAMELA PAMELA space mission Extended till December 2011!

Massimo Bongi – 31 st ICRC – Lodz – 11 July First historical measurements of p-bar/p ratio Anti-nucleosynthesis WIMP dark-matter annihilation in the galactic halo Background: CR interaction with ISM CR + ISM  p-bar + … Evaporation of primordial black holes Why CR antimatter? UNEXPLORED REGION

Massimo Bongi – 31 st ICRC – Lodz – 11 July 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 (  ), the Lightest SUSY Particle (LSP) Most likely processes:   qq  hadrons  p-bar, e +,…   W + W -,Z 0 Z 0,…  e +,… Another possible candidate is the Lightest Kaluza-Klein Particle (LKP): B (1) Fermionic final states no longer suppressed: B (1) B (1)  e + e -   p-bar, e + You are here Milky Way Halo

Massimo Bongi – 31 st ICRC – Lodz – 11 July Time-Of-Flight system plastic scintillators + PMT Electromagnetic calorimeter W/Si sampling (16.3 X 0, 0.6 λ I ) Neutron detector 3 He counters PAMELA detectors GF: 21.6 cm 2 sr Mass: 470 kg Size: 130x70x70 cm 3 Power Budget: 360 W Magnetic spectrometer microstrip Si tracking system (  x ~3 μm), permanent magnet (0.4 T) Main requirements: high-sensitivity antiparticle identification and precise momentum measurement Anticoincidence shields plastic scintillators + PMT Bottom scintillator plastic scintillator + PMT

Massimo Bongi – 31 st ICRC – Lodz – 11 July Antiproton/positron identification Time-of-flight measurement: trigger, albedo rejection, mass determination (up to 1 GeV) Bending in the magnetic field: sign of charge, magnetic deflection Ionization energy loss (dE/dx): magnitude of charge Interaction pattern inside the calorimeter: electron-like or proton-like interaction, energy (in case of electrons and positrons) AntiprotonPositron

Massimo Bongi – 31 st ICRC – Lodz – 11 July Principle of operation Measured at 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) R=MDR   R /R=1 MDR = 1/   Track reconstruction

Massimo Bongi – 31 st ICRC – Lodz – 11 July High-energy antiproton analysis This analysis: data from July 2006 to December 2008 (~750 days) Collected triggers: ~1.5 x 10 9 Antiproton/proton identification: single, good track (N x, N y,  2, no AC activity) no albedo, above cut-off |Z|=1 (dE/dx vs R)  vs R consistent with m p p-bar/e - (and p/e + ) separation p-bar/p separation (charge sign) Dominant background  spillover protons: finite deflection resolution of the SPE  wrong assignment of the sign of the charge at high energy proton – antiproton flux ratio ~ 10 4 at high energy  strong SPE selection is required

Massimo Bongi – 31 st ICRC – Lodz – 11 July Antiproton identification e - (and p-bar) p-bar p -1  Z  +1 spillover protons p (and e + ) |Z|=1 particle cuts (dE/dx vs R and  vs R) electron-rejection cuts based on calorimeter-pattern topology 1 GV5 GV electron antiproton

Massimo Bongi – 31 st ICRC – Lodz – 11 July 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 p-bar p 10 GV50 GV Proton-spillover background MDR = 1/   (evaluated event-by-event by the fitting routine) spillover protons

Massimo Bongi – 31 st ICRC – Lodz – 11 July MDR > 850 GV Minimal track requirements Strong track requirements: strict constraints on  2 reject tracks with low-resolution clusters - bad strips (high noise) -  -rays Proton-spillover background p-bar p 50 GV 10 GV

Massimo Bongi – 31 st ICRC – Lodz – 11 July p-bar p Proton-spillover background R 10/) 50 GV 10 GV

Massimo Bongi – 31 st ICRC – Lodz – 11 July Antiproton-to-proton ratio PRL 2009

Massimo Bongi – 31 st ICRC – Lodz – 11 July Antiproton-to-proton ratio PRELIMINARY

Massimo Bongi – 31 st ICRC – Lodz – 11 July Antiproton-to-proton ratio PRELIMINARY (preliminary)

Massimo Bongi – 31 st ICRC – Lodz – 11 July SUMMARY Analysis of the antiproton-to-proton flux ratio with PAMELA from July 2006 to December 2008: – improved statistics – extended kinetic energy range up to 180 GeV – estimation of the proton spillover for the highest energy bin is in progress No evidence of primary or exotic components of antiprotons: – constraints on dark matter models – useful parameters for secondary production calculations

Massimo Bongi – 31 st ICRC – Lodz – 11 July SPARE SLIDES

Massimo Bongi – 31 st ICRC – Lodz – 11 July PAMELA design performances energy range particles in 3 years Antiprotons 80 MeV ÷190 GeV O(10 4 ) Positrons 50 MeV ÷ 270 GeV O(10 5 ) Electrons up to 400 GeVO(10 6 ) Protons up to 700 GeVO(10 8 ) Electrons+positrons up to 2 TeV (from calorimeter) Light Nuclei up to 200 GeV/n He/Be/C:O(10 7/4/5 ) 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 shower containment Maximum detectable rigidity (MDR)

Massimo Bongi – 31 st ICRC – Lodz – 11 July