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The PAMELA experiment: looking for antiparticles in Cosmic Rays

Waseda, 14 January 2009. The PAMELA experiment: looking for antiparticles in Cosmic Rays. Oscar Adriani University of Florence and INFN Florence. Outline. The PAMELA experiment: short review Results on cosmic-ray antimatter abundance: Antiprotons Positrons Other results:

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The PAMELA experiment: looking for antiparticles in Cosmic Rays

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  1. Waseda, 14 January 2009 The PAMELA experiment:looking for antiparticles in Cosmic Rays Oscar Adriani University of Florence and INFN Florence

  2. Outline • The PAMELA experiment: short review • Results on cosmic-ray antimatter abundance: • Antiprotons • Positrons • Other results: • Cosmic-ray galactic light nuclei (primaries & secondaries) • Solar physics • Terrestrial physics • Conclusions  Oscar Adriani  Waseda, January 14th, 2009 

  3. Italy CNR, Florence Bari Florence Frascati Naples Rome Trieste Russia Moscow St. Petersburg Germany Sweden Siegen KTH, Stockholm Tha PAMELA collaboration  Oscar Adriani  Waseda, January 14th, 2009 

  4. PAMELAPayload for Antimatter/Matter Exploration and Light-nuclei Astrophysics  Direct detection of CRs in space  Main focus on antimatter component  Oscar Adriani  Waseda, January 14th, 2009 

  5. 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 + …  Oscar Adriani  Waseda, January 14th, 2009 

  6. 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 (c), the lightest SUSY Particle (LSP). Most likely processes: cc  qq  hadrons anti-p, e+,… cc  W+W-,Z0Z0,…  e+,…  positron peak Ee+~Mc/2  positron continuum Ee+~Mc/20 Another possible candidate is the lightest Kalusa-Klein Particle (LKP): B(1) Fermionic final states no longer suppressed: B(1)B(1)  e+e- direct dicay  positron peak Ee+ ~ MB(1) Cosmic-ray Antimatter from Dark Matter annihilation Halo You are here Milky Way p-bar, e+ c c  Oscar Adriani  Waseda, January 14th, 2009 

  7. Charge-dependent solar modulation Solar polarity reversal 1999/2000 Asaoka Y. Et al. 2002 Positron excess? ? ? ¯ + • CR + ISM  p-bar + … • Propagation dominated by nuclear interactions • Kinematical threshold: Eth~5.6 for the reaction CR antimatter Experimental scenario before PAMELA Positrons Antiprotons ___ Moskalenko & Strong 1998 • CR + ISM  p± + x m± + x  e± + x • CR + ISM  p0 + x gg e± • Propagation dominated by energy losses • (inverse Compton & synchrotron radiation) • Local origin (@100GeV 90% from <2kpc)  Oscar Adriani  Waseda, January 14th, 2009 

  8. BESS-polar (long-duration) “Standard” balloon-borne experiments • low exposure (~20h) •  large statistical errors • atmospheric secondaries (~5g/cm2) •  additional systematic uncertainty @low-energy CR antimatter: available data Why in space? Positrons Antiprotons ___ Moskalenko & Strong 1998  Oscar Adriani  Waseda, January 14th, 2009 

  9. 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  Oscar Adriani  Waseda, January 14th, 2009 

  10. 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 Principle of operation  Oscar Adriani  Waseda, January 14th, 2009 

  11. track average Z measurement 4He B,C 3He Be Bethe Bloch ionization energy-loss of heavy (M>>me) charged particles d (saturation) p Li e± 1st plane Principle of operation  Oscar Adriani  Waseda, January 14th, 2009 

  12. Velocity measurement • Particle identification @ low energy • Identify albedo (up-ward going particles b < 0 ) •  NB! They mimic antimatter! Principle of operation  Oscar Adriani  Waseda, January 14th, 2009 

  13. Electron/hadron separation • Interaction topology • e/h separation • Energy measurement of electrons and positrons • (~full shower containment) hadron (19GV) electron (17GV) Principle of operation + NEUTRONS!!  Oscar Adriani  Waseda, January 14th, 2009 

  14. Multi-spectral remote sensing of earth’s surface near-real-time high-quality images Built by the Space factory TsSKB Progress in Samara (Russia) Operational orbit parameters: inclination ~70o altitude ~ 360-600 km (elliptical) Active life >3 years Data transmitted via Very high-speed Radio Link (VRL) The Resurs DK-1 spacecraft Mass: 6.7 tons Height: 7.4 m Solar array area: 36 m2 • PAMELA mounted inside apressurized container • moved from parking to data-taking position few times/year  Oscar Adriani  Waseda, January 14th, 2009 

  15. 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  Oscar Adriani  Waseda, January 14th, 2009 

  16. Launch from Baikonur June 15th 2006, 0800 UTC. ‘First light’ June 21st 2006, 0300 UTC. • Detectors operated as expected after launch • Different trigger and hardware configurations evaluated PAMELA in continuous data-taking mode since commissioning phase ended on July 11th 2006 PAMELA milestones Main antenna in NTsOMZ Trigger rate* ~25Hz Fraction of live time* ~ 75% Event size (compressed mode) ~ 5kB 25 Hz x 5 kB/ev ~ 10 GB/day (*outside radiation belts) Till December 2008: ~800 days of data taking ~16 TByte of raw data downlinked ~16•108 triggers recorded and analysed (Data from May till now under analysis)  Oscar Adriani  Waseda, January 14th, 2009 

  17. Antiprotons

  18. High-energy antiproton analysis • Analyzed data July 2006 – February 2008 (~500 days) • Collected triggers ~108 • Identified ~ 10 106 protons and ~ 1 103 antiprotons between 1.5 and 100 GeV ( 100 p-bar above 20GeV ) • Antiproton/proton identification: • rigidity (R)  SPE • |Z|=1 (dE/dx vs R)  SPE&ToF • b vs R consistent with MpToF • p-bar/p separation (charge sign)  SPE • p-bar/e- (and p/e+ ) separation CALO • Dominant background spillover protons: • finite deflection resolution of the SPE  wrong assignment of charge-sign @ high energy • proton spectrum harder than positron  p/p-bar increase for increasing energy (103 @1GV 104 @100GV) •  Required strong SPE selection  Oscar Adriani  Waseda, January 14th, 2009 

  19. 1 GV 5 GV Antiproton identification Preliminary!! -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 ( For |Z|=1, deflection=1/p )  Oscar Adriani  Waseda, January 14th, 2009 

  20. Proton-spillover background MDR = 1/sh (evaluated event-by-event by the fitting routine) Minimal track requirements MDR > 850GV • Strong track requirements: • strict constraints on c2 (~75% efficiency) • rejected tracks with low-resolution clusters along the trajectory • - faulty strips (high noise) • - d-rays (high signal and multiplicity)  Oscar Adriani  Waseda, January 14th, 2009 

  21. R < MDR/10 10 GV 50 GV Proton-spillover background MDR = 1/sh (evaluated event-by-event by the fitting routine) p p-bar “spillover” p • 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  Oscar Adriani  Waseda, January 14th, 2009 

  22. Antiproton-to-proton ratio *preliminary* astro-ph 0810.4994  In press by PRL (Petter Hofverberg’s PhD Thesis)  Oscar Adriani  Waseda, January 14th, 2009 

  23. Positrons

  24. S1 CARD CAT S2 TOF SPE CAS S3 CALO S4 ND High-energy positron analysis • Analyzed data July 2006 – February 2008 (~500 days) • Collected triggers ~108 • Identified ~ 150 103 electrons and ~ 9 103 positrons between 1.5 and 100 GeV (180 positrons above 20GeV) • Electron/positron identification: • rigidity (R)  SPE • |Z|=1 (dE/dx=MIP)  SPE&ToF • b=1 ToF • e-/e+ separation (charge sign)  SPE • e+/p (and e-/p-bar) separation CALO • Dominant background interacting protons: • fluctuations in hadronic shower development  p0 ggmight mimic pure em showers • proton spectrum harder than positron  p/e+ increase for increasing energy (103 @1GV 104 @100GV) •  Required strong CALO selection  Oscar Adriani  Waseda, January 14th, 2009 

  25. Positron identification with CALO 51 GV positron • Identification based on: • Shower topology (lateral and longitudinal profile, shower starting point) • Total detected energy (energy-rigidity match) • Analysis key points: • Tuning/check of selection criteria with: • test-beam data • simulation • flight data  dE/dx from SPE & neutron yield from ND • Selection of pure proton sample from flight data (“pre-sampler” method): • Background-suppression method • Background-estimation method 80 GV proton Final results make NON USE of test-beam and/or simulation calibrations. The measurement is based only on flight data with the background-estimation method  Oscar Adriani  Waseda, January 14th, 2009 

  26. Z=-1 e- Rigidity: 20-30 GV p-bar (non-int) p-bar (int) NB! Z=+1 0.6 RM p (non-int) LEFT HIT RIGHT planes (e+) p (int) strips Positron identification Fraction of charge released along the calorimeter track  Oscar Adriani  Waseda, January 14th, 2009 

  27. Positron identification Energy-momentum match Energy measured in Calo/ Deflection in Tracker (MIP/GV) e- ( e+ )  e  h p-bar p  Oscar Adriani  Waseda, January 14th, 2009 

  28. Z=-1 Z=-1 e- e- Rigidity: 20-30 GV Rigidity: 20-30 GV + Constraints on: p-bar (non-int) p-bar (int) p-bar Energy-momentum match NB! Z=+1 Z=+1 p (non-int) e+ (e+) p (int) p Positron identification Fraction of charge released along the calorimeter track  Oscar Adriani  Waseda, January 14th, 2009 

  29. Shower starting-point Longitudinal profile Positron identification 51 GV positron 80 GV proton  Oscar Adriani  Waseda, January 14th, 2009 

  30. Z=-1 Z=-1 e- e- Rigidity: 20-30 GV Rigidity: 20-30 GV p-bar Shower starting-point Z=+1 Z=+1 Longitudinal profile Lateral profile e+ e+ p BK-suppression method p Positron identification Fraction of charge released along the calorimeter track + Constraints on: Energy-momentum match  Oscar Adriani  Waseda, January 14th, 2009 

  31. Check of calorimeter selection Flight data Rigidity: 20-30 GV Test beam data Momentum: 50GeV/c Fraction of charge released along the calorimeter track e- e- + Constraints on: Energy-momentum match e+ p Shower starting-point p  Oscar Adriani  Waseda, January 14th, 2009 

  32. Check of calorimeter selection Flight data Rigidity: 20-30 GV Flight data Neutron yield in ND Fraction of charge released along the calorimeter track e- e- + Constraints on: Energy-momentum match e+ e+ Shower starting-point p p  Oscar Adriani  Waseda, January 14th, 2009 

  33. Check of calorimeter selection Energy loss in silicon tracker detectors: • Top: positive (mostly p) and negative events (mostly e-) • Bottom: positive events identified as p and e+ by trasversal profile method Relativistic rise Rigidity: 10-15 GV Rigidity: 15-20 GV neg (e-) pos (p) neg (e-) pos (p) p e+ p e+  Oscar Adriani  Waseda, January 14th, 2009 

  34. The “pre-sampler” method Selection of a pure sample of protons from flight data CALORIMETER: 22 W planes: 16.3 X0 2 W planes: ≈1.5 X0 20 W planes: ≈15 X0  Oscar Adriani  Waseda, January 14th, 2009 

  35. Proton background evaluation Rigidity: 20-28 GV e- Fraction of charge released along the calorimeter track (left, hit, right) + Constraints on: p (pre-sampler) Energy-momentum match Shower starting-point e+ p  Oscar Adriani  Waseda, January 14th, 2009 

  36. Proton background evaluation Rigidity: 28-42 GV e- Fraction of charge released along the calorimeter track (left, hit, right) + Constraints on: p (pre-sampler) Energy-momentum match Shower starting-point e+ p  Oscar Adriani  Waseda, January 14th, 2009 

  37. Positron fraction astro-ph 0810.4995  Oscar Adriani  Waseda, January 14th, 2009 

  38. (( Do we have any antimatter excess in CRs? ))  Oscar Adriani  Waseda, January 14th, 2009 

  39. Antiproton-to-proton ratioSecondary Production Models CR + ISM  p-bar + … • (Moskalenko et al. 2006) GALPROP code • Plain diffusion model • Solar modulation: drift model ( A<0, a=15o ) • (Donato et al. 2001) • Diffusion model with convection and reacceleration • Solar modulation: spherical model (f=500MV ) •  Uncertainty band related to propagation parameters (~10% @10GeV) •  Additional uncertainty of ~25% due to production cs should be considered !! • (Ptuskin et al. 2006) GALPROP code • Plain diffusion model • Solar modulation: spherical model ( f=550MV ) No evidence for any antiproton excess  Oscar Adriani  Waseda, January 14th, 2009 

  40. Positron fractionSecondary Production Models CR + ISM  p± + … m± + …  e± + … CR + ISM  p0 + … gg e± • (Moskalenko & Strong 1998) • GALPROP code • Plain diffusion model • Interstellar spectra  Oscar Adriani  Waseda, January 14th, 2009 

  41. Positron fractionSecondary Production Models CR + ISM  p± + … m± + …  e± + … CR + ISM  p0 + … gg e± • (Moskalenko & Strong 1998) • GALPROP code • Plain diffusion model • Interstellar spectra Soft (g ‘big’) Hard (g ‘small’) • (Delahaye et al. 2008) • Plain diffusion model • Solar modulation: spherical model (f=600MV) • Uncertainty band related to e- spectral index (ge= 3.44±0.1 (3s) MIN÷MAX) • Additional uncertainty due to propagation parameters should be considered (factor ~6 @1GeV ~4 @high-energy)  Oscar Adriani  Waseda, January 14th, 2009 

  42. Preliminary!! fe+ ~E-3.5 expected from pure secondary models Electron spectrum • NB!! • Flux in the spectrometer • No solar demodulation Preliminary!! Geom.cutoff Hard, g small! ~ E- 3.24±0.04  Oscar Adriani  Waseda, January 14th, 2009 

  43. Positron fractionSecondary Production Models soft hard Quite robust evidence for a positron excess  Oscar Adriani  Waseda, January 14th, 2009 

  44. Primary positron sources Dark Matter • e+ yield depend on the dominant decay channel • LSPs seem disfavored due to suppression of e+e- final states • low yield (relative to p-bar) • soft spectrum from cascade decays • LKPs seem favored because can annihilate directly in e+e- • high yield (relative to p-bar) • hard spectrum with pronounced cutoff @ MLKP (>300 GeV) • Boost factor required to have a sizable e+ signal • NB: constraints from p-bar data!! LKP -- M= 300 GeV (Hooper & Profumo 2007)  Oscar Adriani  Waseda, January 14th, 2009 

  45. Primary positron sources Astrophysical processes • Local pulsars are well-known sites of e+e- pair production:  they can individually and/or coherently contribute to the e+e- galactic flux and explain the PAMELA e+ excess (both spectral feature and intensity) • No fine tuning required • if one or few nearby pulsars dominate, anisotropy could be detected in the angular distribution • possibility to discriminate between pulsar and DM origin of e+ excess All pulsars (rate = 3.3 / 100 years) (Hooper, Blasi, Seprico 2008)  Oscar Adriani  Waseda, January 14th, 2009 

  46. (Chang et al 2008) PAMELA positron excess might be connected with ATIC electron+positron structures?  Oscar Adriani  Waseda, January 14th, 2009 

  47. Primary positron sources PAMELA positron fraction alone insufficient to understand the origin of positron excess Additional experimental data will be provided by PAMELA: • e+ fraction @ higher energy (up to 300 GeV) • individual e- e+ spectra • anisotropy (…maybe) • high energy e++e- spectrum (up to 2 TV) Complementary information from: • gamma rays • neutrinos  Oscar Adriani  Waseda, January 14th, 2009 

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