Search for Cosmic Ray Anisotropy with the Alpha Magnetic Spectrometer on the International Space Station G. LA VACCA University of Milano-Bicocca INFN Milano-Bicocca on behalf of the AMS-02 collaboration 5-Sep-2016

Motivation for CR anisotropy search with AMS Sez. Milano-Bicocca Motivation for CR anisotropy search with AMS AMS-02 observes structures in the spectra of e+, e-, p, He that cannot be fully explained within the current physical knowledge. These features may be connected to new phenomena which could induce some degree of anisotropy in their fluxes: Local environment (galactic magnetic field, effects due to the solar activity at low rigidity) Local sources (pulsars for e+ and e-, local SNRs or Wolf-Rayet stars for p and He) AMS02 Proton Flux PRL 114, 171103 (2014) http://phys.org/news/2015-11-cosmic-rays-two-million-year-old-supernova.html AMS02 Positron Fraction PRL 113, 121101 (2014)

Sez. Milano-Bicocca AMS-02: the detector AMS: A TeV precision, multipurpose particle physics spectrometer in space. TRD: to identify e+ and e- TOF: measure Z, E Tracker: measure Z, P, incoming direction ECAL: measure lepton energy Proton background is reduced to the per mil level with a cut based selection on the TRD and ECAL estimators Selected particles for this analisys in [16:350]GeV, from 05/2011 to 11/2015: 70.000 positrons 920.000 electrons 60.000.000 protons

Sez. Milano-Bicocca AMS-02: operation Since May 2011, AMS-02 is operating 24/7 on board of ISS: 400Km altitude orbit inclination 51.6° relative to the Earth Equator South Geographic Pole AMS-02 exposure in Galactic cordinates BEING IN SPACE: A long term, (nearly) full-sky observation and a three-dimensional measurement of the tiny CR anisotropy signals; Primary CR detected before interacting with the atmosphere; In Low Earth Orbits, the presence of the magnetosphere: CR directions after trajectory reconstruction in magnetosphere, rigidity cut-off; Not uniform exposure.

Methodology for anisotropy search Sez. Milano-Bicocca Methodology for anisotropy search RELATIVE ANISOTROPIES: other CR species as reference for detector exposure: other cosmic ray species (e.g. p used for leptons, also at magnetosphere border) same cosmic ray species (at different energy) A likelihood fit procedure has been set up to compare the species under study to the reference sky map. It takes into account the differences in the exposure for different rigidities. A spherical harmonics expansion of the relative anisotropy is obtained: φ 𝑏,ℓ = 𝑙=0 ∞ 𝑚=−𝑙 +𝑙 𝑎 𝑙𝑚 𝑌 𝑙𝑚 (𝑏,ℓ) For the three dipole components (𝑙=1): North-South Forward-Backward East-West 𝜌 𝑁𝑆 = 3 4𝜋 𝑎 10 𝜌 𝐹𝐵 = 3 4𝜋 𝑎 11 𝜌 𝐸𝑊 = 3 4𝜋 𝑎 1−1 𝛿= 𝜌 𝑁𝑆 2 + 𝜌 𝐹𝐵 2 + 𝜌 𝐸𝑊 2 Dipole amplitude:

Positrons vs. electrons Sez. Milano-Bicocca Positrons vs. electrons Dipole components in galactic coordinates No significant deviation from isotropy is found

Positrons vs. electrons Sez. Milano-Bicocca Positrons vs. electrons Dipole upper limit in galactic coordinates Upper limit ẟ scaled to the statistics No significant deviation from isotropy is found 𝛿 𝑒 + 𝑒 − (>16GeV) < 2% at 95% C.L. Significance map

Back-tracing in magnetosphere Sez. Milano-Bicocca Back-tracing in magnetosphere The back-tracing allows to reconstruct the trajectories of the cosmic rays detected by AMS02 at ISS altitude in a deterministic way up to the border of the magnetosphere.

Back-tracing in magnetosphere Sez. Milano-Bicocca Back-tracing in magnetosphere Model description Internal Field: IGRF-12 model is a standard mathematic description of the Earth main magnetic field and its annual rate of change (secular variation) External Field: Tsyganenko 2005 model: includes all external currents contributions to external magnetic field. It describes the Earth magnetosphere during both quiet and active solar periods. Magnetosphere border in the day-side, variable from 15 to 10 Earth Radii, according to the solar wind pressure in the night-side, fixed to 25 Earth Radii Back-tracing Protons in [16:350]GV Focusing towards the equator At ISS altitude At magnetosphere border

Back-tracing in magnetosphere Sez. Milano-Bicocca Back-tracing in magnetosphere The reference sky map The magnetosphere introduces a displacement with respect the asymptotic direction. This displacement decreases with rigidity. M. Ackermann et al. 2012 The sky observed outside the magnetosphere depends on the considered rigidity and on the particle charge. Sky maps from opposite charge particles cannot be compared at magnetosphere border: e+ vs. p and e- vs. p* (*negative charge BT p)

No significant deviation from isotropy is found Sez. Milano-Bicocca Positrons vs. protons Dipole components in galactic coordinates No significant deviation from isotropy is found

No significant deviation from isotropy is found Sez. Milano-Bicocca Positrons vs. protons Dipole upper limit in galactic coordinates Upper limit ẟ scaled to the statistics No significant deviation from isotropy is found 𝛿 𝑒 + 𝑝 (>16GeV) < 2% at 95% C.L. Significance map

No significant deviation from isotropy is found Sez. Milano-Bicocca Electrons vs. protons* Dipole components in galactic coordinates No significant deviation from isotropy is found *Protons back-traced with negative charge

No significant deviation from isotropy is found Sez. Milano-Bicocca Electrons vs. protons* Dipole upper limit in galactic coordinates Upper limit ẟ scaled to the statistics No significant deviation from isotropy is found 𝛿 𝑒 − 𝑝 (>16GeV) < 0.6% at 95% C.L. Significance map *Protons back-traced with negative charge

Protons vs. protons Original idea: look for directional Sez. Milano-Bicocca Protons vs. protons Original idea: look for directional dependence of energetic protons. Use low energy protons as reference map, well above geomagnetic cut-off, low solar modulation effects. Reference sample High energy proton map Proton sample above 80GV

No significant deviation from isotropy is found Sez. Milano-Bicocca Protons vs. protons Dipole components in galactic coordinates No significant deviation from isotropy is found

No significant deviation from isotropy is found Sez. Milano-Bicocca Protons vs. protons Dipole upper limit in galactic coordinates Upper limit ẟ scaled to the statistics No significant deviation from isotropy is found 𝛿 𝑝 𝐻 𝑝 𝐿 (>80GV) < 0.3% at 95% C.L. Significance map

GSE – Geocentric Solar Ecliptic coordinate system Sez. Milano-Bicocca Seasonal variation Seasonal variation of the relative anisotropies is studied by dividing the period of analysis (May 2011 – Nov 2015) in seasons (4 seasons per year) Check for possible time dependence of the signal; Detect possible signal dependence as function of position in the heliosphere or correlation with solar activity and solar events. GSE – Geocentric Solar Ecliptic coordinate system

No significant deviation from isotropy is found Sez. Milano-Bicocca Seasonal variation No significant deviation from isotropy is found Results for electrons and protons Electrons vs. protons [16:350]GV Upper limit ẟ scaled to the statistics Nov-11 May-12 Feb-13 Aug-13 May-14 Feb-15 Aug-15 Protons vs. protons >80GV Nov-11 May-12 Feb-13 Aug-13 May-14 Feb-15 Aug-15

Sez. Milano-Bicocca Conclusions Positron and electron angular distributions are compatible with isotropy, at all energies and both at ISS location and at magnetosphere border. Upper limits are set on the dipole anisotropy strenght: 𝛿 𝑒 + 𝑒 − >16GV <2% at 95% C.L. 𝛿 𝑒 + 𝑝 >16GV <2% at 95% C.L. 𝛿 𝑒 − 𝑝 >16GV <0.6% at 95% C.L. Proton sky at high rigidity is compatible with the proton distribution at low rigidity. The upper limit on dipole anisotropy is 𝛿 𝑝 𝐻 𝑝 𝐿 >80GV <0.3% at 95% C.L. No significant seasonal variation observed in all observables.

Conclusions Projection of 95% upper limits on ẟ to 2024 Sez. Milano-Bicocca Conclusions Projection of 95% upper limits on ẟ to 2024 Current limit in e+/e-with 4.5 years of data 16 < E < 350 GeV