1 / 33

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

vevina
Télécharger la présentation

Recent results on antiparticles in cosmic rays from PAMELA experiment

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related