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Recent results on antiparticles in cosmic rays from PAMELA experiment

Recent results on antiparticles in cosmic rays from PAMELA experiment. Sergio Ricciarini INFN – Florence, Italy On behalf of the PAMELA collaboration. Summary. The PAMELA experiment: short introduction. Discussion of recent results. (1) Antiproton/proton ratio at high energies

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Recent results on antiparticles in cosmic rays from PAMELA experiment

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  1. Recent results on antiparticlesin cosmic raysfrom PAMELA experiment Sergio Ricciarini INFN – Florence, Italy On behalf of the PAMELA collaboration

  2. Summary The PAMELA experiment: short introduction. Discussion of recent results. (1) Antiproton/proton ratio at high energies (submitted to Phys. Rev. Lett.). (2) Positron fraction at high energies (submitted to Nature). Conclusion and prospects. S. Ricciarini GDR-SUSY 08

  3. The PAMELA collaboration Bari Frascati Naples Rome Trieste Italy Florence Russia Moscow, St. Petersburg Germany Sweden Siegen Stockholm S. Ricciarini GDR-SUSY 08

  4. PAMELA scientific objectives Study antiparticles in cosmic rays. Search for dark matter annihilation(e+ and p-bar spectra). Study cosmic-ray production and propagation. Study composition and spectra of cosmic rays (including light nuclei). Search for anti-He (primordial antimatter). Study solar physics and solar modulation. Study of terrestrial magnetosphere and radiation belts. S. Ricciarini GDR-SUSY 08

  5. PAMELA nominal capabilities Energy range (with 3 years statistics) Antiprotons 80 MeV - 190 GeV Positrons 50 MeV - 270 GeV Protons up to 700 GeV Electrons up to 400 GeV Electrons+positrons up to 2 TeV (from calorimeter) Light Nuclei up to 200 GeV/n (He/Be/C) AntiNuclei search • Simultaneous measurement of many cosmic-ray species. • New energy range. • Unprecedented statistics. S. Ricciarini GDR-SUSY 08

  6. PAMELA detectors Main requirements:high-sensitivity antiparticle identification and precise momentum measurement + - • Time-Of-Flight (TOF) • plastic scintillators + PMT: • Trigger • Albedo rejection • Mass identification up to 1 GeV • - Charge value from dE/dL • Electromagnetic calorimeter • W/Si sampling (16.3 X0, 0.6 λI) • Discrimination e+ / p, p-bar / e- • (shower topology) • Direct E measurement for e-/e+ • Neutron detector • polyethylene + 3He counters: • High-energy e/h discrimination GF: 21.6 cm2 sr Mass: 470 kg Size: 130x70x70 cm3 Power Budget: 360 W Spectrometer microstrip Si tracking system (TRK)+ permanent magnet - Magnetic rigidity R = pc/Ze (GV); magnetic deflectionη=1/R (GV-1) - Charge sign, momentum - Charge value from dE/dL S. Ricciarini GDR-SUSY 08

  7. Satellite and orbit 350 km 70o SAA 610 km orbit period ~90 min 350 km 70° 610 km PAMELA • PAMELA mounted on Russian satellite Resurs-DK1, inside a pressurized container. • Minimum lifetime 3 years starting from June 2006. • Quasi-polar low-earth elliptical orbit (70.0°, 350 - 610 km). • Traverses and operates in the South Atlantic Anomaly. • Crosses the outer (electron) Van Allen belt at south pole. S. Ricciarini GDR-SUSY 08

  8. Antiproton/proton ratioat high energies(kinetic energy 1.5 - 100 GeV) S. Ricciarini GDR-SUSY 08

  9. High-energy antiproton/proton analysis Results discussed here have been submitted to Phys. Rev. Lett. Analyzed data: July 2006 - February 2008. Total acquisition time ~ 500 days. Collected ~ 1x109 triggers (~ 8.8TB of data). Identified ~10 x 106 protons and ~1 x 103 antiprotons with kinetic energy between 1.5 and 100 GeV. • Collected 100 antiprotons above 20 GeV. S. Ricciarini GDR-SUSY 08

  10. Antiproton and proton: basic cuts • All requirements in the p-bar/p analysis are applied for both charge signs. • Clean event pattern (reject spurious events): • single track in TRK; • no activity in CARD+CAT; • no multiple hits in S1+S2 (segmented). • MIP |Z|=1 particle. • TRK+S1+S2 dE/dL < 3 MIP. • Galactic particle (reject albedo, reentrant, East-West effect): • downward-going particle (300 ps TOF resolution over 3 ns flight time); • measured rigidity R > 1.3 vertical geomagnetic cutoff. S1 CARD CAT S2 . TOF TRK CAS S3 CALO S4 ND S. Ricciarini GDR-SUSY 08

  11. Electron/hadron separation with CALO • Contamination from e- on p-bar sample is reduced to a negligible amount. • e- are easily identified in CALO from interaction topology (rejection factor >104): they interact in the first CALO layers and give well contained and compact EM showers; • on the other hand, most hadrons interact well deep in the CALO or do not interact at all. hadron (R=19GV) electron (R=17GV) 22 modules (Y Si-strip + W layer + X Si-strip) Total depth: 16.3 X0 or 0.6 λI S. Ricciarini GDR-SUSY 08

  12. Momentum and charge sign with TRK • Minimal track requirements for good rigidity measurement: • at least 4 X (bending view) + 3 Y hits; • energy-dependent cut on track c2 (~95% total efficiency); • consistent TRK+TOF+CALO spatial information. Magnetic rigidity R = pc/Ze (GV) Magnetic deflectionη = 1/R (GV-1) MDR (Maximum Detectable Rigidity): Def.: |R|=MDR  σR=|R| MDR=1/ση (ση spectrometer deflection resolution) MDR depends on event characteristics and is evaluated event-by-event with the fitting routine: - number and distribution of fitted points along the track; - spatial resolution of the single position measurements (varies with track inclination and strip noise); - magnetic field intensity along the track. S. Ricciarini GDR-SUSY 08

  13. Rejection of p “spillover” background • Main difficulty here is the background from “spillover” protons in the p-bar sample at high energies (protons with wrong charge sign): • finite MDR limits the precision of η (R) measurement; • high (~ 104) p/p-bar ratio in cosmic rays. Minimal track requirements plus: MDR > 850 GV (high-precision subsample). • Defined additional optimized track requirements to improve MDR: • - stronger constraints on χ2 at high energies (~75% efficiency); • - rejected tracks with low-spatial-resolution clusters along the trajectory: • - faulty strips (high noise); • - δ-rays (high signal and multiplicity). Protons and spillover R = - 10 GV R = - 50 GV Antiprotons S. Ricciarini GDR-SUSY 08

  14. Rejection of p “spillover” background R= - 50 GV Preliminary!! • Rigidity-dependent cut to reject residual spillover: MDR > 10 ∙ |R| • This cut is equivalent to:|η| > 10 ∙ ση • This conservative rejection cut reduces residual spillover contamination to a negligible amount. p-bar subsample with MDR > 850 GV  Protons and spillover MDR > 10 ∙ |R|  R = - 10 GV S. Ricciarini GDR-SUSY 08

  15. Antiproton/proton ratio • Excellent agreement with recent data from other experiments. • One order of magnitude improvement in statistics. • Most extended energy range ever achieved. • Expected further improvements with new data. • Correction factors are included and ~ one order of magnitude less than statistical error. • CALO efficiency (different for p-bar and p); • loss of particles for interactions. S. Ricciarini GDR-SUSY 08

  16. PAMELA p-bar/p ratio and theory • Ratio increases smoothly with energy from 4 x 10-5 and levels off at ~ 1 x 10-4. • Our results are enough precise to place tight constraints on parameters relevant for secondary production calculations. • Our data above 10 GeV place limits on contributions from exotic sources, e.g. dark matter particle annihilations. S. Ricciarini GDR-SUSY 08

  17. Positron fractionat high energies(energy 1.5 - 100 GeV) S. Ricciarini GDR-SUSY 08

  18. High-energy positron fraction analysis Results discussed here have been submitted to Nature. Analyzed data: July 2006 - February 2008. Total acquisition time ~ 500 days. ~ 1x109 triggers (~ 8.8TB of data). Identified ~150 x 103 electrons and ~9 x 103 positrons with energy between 1.5 and 100 GeV. • Collected 180 positrons above 20 GeV. S. Ricciarini GDR-SUSY 08

  19. Distribution of fraction F before CALO cuts Preliminary!! Rigidity: 20-30 GV Fraction F of energy released in CALO along the track in a cylinder of radius 0.3 rMolière (central + 2 lateral Si strips) Z = -1 e- p-bar (non-int) p-bar (int) after basic event cuts Z = +1 p (non-int) (e+) p (int) S. Ricciarini GDR-SUSY 08

  20. Cut on “energy-rigidity match” • Consider the ratio between total energy measured by CALO and rigidity measured by TRK. • For electrons (positrons) ratio is constant over rigidity. e- e+ int. p total energy measured in CALO/ rigidity measured in TRK (MIP/GV)  ‘electron cut’ non-int. + int. protons non-int. p-bar S. Ricciarini GDR-SUSY 08

  21. Cut on “energy-rigidity match” Preliminary!! Rigidity: 20-30 GV Fraction F of energy released in CALO along the track Z = -1 e- + Constraints on: p-bar Energy-rigidity match Z = +1 all non-interacting and most interacting protons are rejected e+ p S. Ricciarini GDR-SUSY 08

  22. e+ and e- identification in calorimeter • Less than 1 proton out of 105 survives the complete set of CALO cuts, with e+ efficiency 80%. e- e+ p S. Ricciarini GDR-SUSY 08

  23. Cut on shower starting point Constraints on: • Proton background was also characterized at beam tests. Energy-rigidity match Shower starting point Flight data. Rigidity: 20-30 GV Beam-test data after same cuts are applied. Rigidity: 50 GV Z = -1 e- e- e- e+ p Z = +1 e+ p p S. Ricciarini GDR-SUSY 08

  24. Cross-check with ND flight data • Cross-check with flight data from neutron detector to validate the selection procedure. Fraction F Rigidity: 20-30 GV Neutrons detected by ND Z = -1 e- e- Z = +1 e+ e+ p p Residual p background S. Ricciarini GDR-SUSY 08

  25. Cut on longitudinal profile Flight data: 51 GV positron S. Ricciarini GDR-SUSY 08

  26. Cut on longitudinal profile Preliminary!! Fraction F of energy released in CALO along the track Rigidity: 20-30 GV + Constraints on: Z = -1 Energy-rigidity match Shower starting point Longitudinal profile Z = +1 S. Ricciarini GDR-SUSY 08

  27. Cross-check with energy loss in TRK • Top: proton and electron samples, identified with TRK only (charge sign). Rigidity: 10-15 GV Rigidity: 15-20 GV p e- p e- p e+ p e+ • Bottom: proton and positron (+ residual p background) samples,identified with present CALO requirements. S. Ricciarini GDR-SUSY 08

  28. Proton contamination • Proton contamination obtained directly from flight data (no simulation involved) and subtracted with statistical “bootstrap” analysis. • Considered three F distributions in a reduced calorimeter after applying all CALO cuts: • (a) electrons and (c) e+ with residual p background: selected in upper CALO. • (b) protons: “pre-sampled” in first 2 modules and then selected in lower CALO. e- Rigidity: 28-42 GV electron selection reduced CALO (20 out of 22 modules) p proton selection p e+ positron + residual p background selection S. Ricciarini GDR-SUSY 08

  29. Positron fraction at high energies • One order of magnitude improvement in statistics over previous measurements. • Most extended energy range ever achieved. • Expected further improvements with new data. • At high energies our data show a significant increase with energy. • This cannot be explained by standard models of secondary production of cosmic rays. • Either a significant change in the acceleration or propagation models is needed; • or a primary component is present. • Among primary-component candidates: • annihilation of dark matter in the vicinity of our galaxy; • near-by astrophysical sources, like pulsars. line: secondary production, Moskalenko and Strong, Astrophys. J. 493 (1998) S. Ricciarini GDR-SUSY 08

  30. Positron fraction at low energies • At low energies our results are systematically lower than data collected in 1990’s. • Clem 2007 (with much lower statistics) is consistent with PAMELA. • This is interpreted as effect of charge-sign dependent solar modulation. • our data are enough precise to allow tuning of models of the heliosphere. • Ruled out as negligible a possible combined effect of: • asymmetry of spectrometer magnetic field; • East-West effect or reentrant albedo particles. S. Ricciarini GDR-SUSY 08

  31. Low-energy e+ fraction and solar modulation • Solar modulation (through solar wind) of cosmic ray fluxes depends on: • amount of solar activity; • polarity of solar magnetic field; • cosmic-ray energy and mass; • charge sign of cosmic ray. 2000: inversion of solar magnetic field Neutron intensity (Rome monitor) PAMELA Clem now: solar minimum low-energy p-bar/p ratio (BESS) low-energy e+ fraction (Caprice, MASS, HEAT, AMS98...) year A- A+ A- A+ S. Ricciarini GDR-SUSY 08

  32. Charge-sign dependence of solar modulation A > 0 Positive particles A < 0 Preliminary!! (Preliminary!) A>0 A<0 A<0 “drift” component of solar modulation is enhanced during solar minimum for low-mass particles [Potgieter et al., Space Sci. Rev. 97 (2001)] A>0 S. Ricciarini GDR-SUSY 08

  33. Conclusion and prospects • Precise measurements of p-bar/p ratio and of positron fraction over a wide energy range have been presented and discussed. • PAMELA is expected to collect data until at least December 2009. • increase in statistics will allow to extend energy range for p-bar/p ratio and positron fraction to the design limits. • Several other items are currently under analysis: • p-bar/p ratio and positron fraction in the energy range 100 MeV - 1 GeV; • absolute differential spectra of |Z|=1 cosmic rays; • nuclei (up to Z = 8); • spectra of high-energy Solar Energetic Particles (SEP); • radiation belts: morphology and energy spectrum; • search for anti-He; • study of isotope composition (d, 3He). S. Ricciarini GDR-SUSY 08

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