AMS-02 in space: physics results, overview, and challenges
aa r X i v : . [ a s t r o - ph . H E ] O c t Nuclear Physics B Proceedings Supplement 00 (2018) 1–3
Nuclear Physics BProceedingsSupplement
AMS-02 in space: physics results, overview, and challenges
Nicola Tomassetti, on behalf of the AMS collaboration
LPSC, Universit´e Grenoble Alpes, IN2P3 / CNRS – Grenoble, France
Abstract
The Alpha Magnetic Spectrometer (AMS-02) is a state of the art particle detector measuring cosmic rays (CRs)on the International Space Station (ISS) since May 19th 2011. AMS-02 identifies CR leptons and nuclei in theenergy range from hundreds MeV to few TeV per nucleon. Several sub-detector systems allow for redundant particleidentification with unprecedented precision, a powerful lepton-hadron separation, and a high purity of the antimattersignal. The new AMS-02 leptonic data from 1 to 500 GeV are presented and discussed. These new data indicatethat new sources of CR leptons need to be included to describe the observed spectra at high energies. Explanationsof this anomaly may be found either in dark-matter particles annihilation or in the existence of nearby astrophysicalsources of e ± . Future data at higher energies and forthcoming measurements on the antiproton spectrum and theboron-to-carbon ratio will be crucial in providing the discrimination among the di ff erent scenario. Keywords:
Cosmic Rays; Antimatter; Positrons; Dark Matter;
1. Introduction
AMS-02 is a general purpose high-energy particledetector, capable of measuring CR leptons and nuclei,from hydrogen up to iron, from hundreds MeV up to ∼ ∼
20 years) of fundamental physicsresearch in space. The layout of the AMS-02 detector issketched in Fig. 1. It consists of nine planes of precisionSilicon Tracker, a Transition Radiation detector (TRD),four planes of Time of Flight counters (TOF), a per-manent magnet, an array of AntiCoincidence Counters(ACC), a Ring imaging Cherenkov detector (RICH),and an Electromagnetic Calorimeter (ECAL). The re-dundancy of the measurement ensures a correct parti-cle identification and allows for detection of interac-tions inside the detector [1, 2] The TRD [6] is built outof 5248 proportional chambers, filled with xenon andcarbon-dioxide, arranged in 20 layers with fiber–fleeceradiator between each layer, allowing for discrimina-tion between leptons and hadrons. The TOF [4] con-sists of four planes of scintillators, where two planes are located above the magnet case and two planes areplaced below, allowing for velocity measurements, trig-ger, and charge measurements. The Tracker is build ofnine planes of silicon micro-strip detectors [3], from L1to L9, distributed over the instrument. Planes from L2to L8 are assembled with the permanent magnet, thathas a magnetic field strength of 0.15 T. L1 is locatedon top of the TRD, and L9 is located between betweenRICH and ECAL. Each Tracker layer has a spatial res-olution of 10 µ m. The Tracker measures the rigidity R up to R ≈ R ≡ p / Z , momentum to charge ratio)for charge one particles, and charge measurement up to Z =
28. The RICH [5] is made of a radiator layer, aconical mirror, and a photomultiplier plane for detect-ing the Cherenkov light. A NaF (aerogel) radiator withrefractive index n = n = Nuclear Physics B Proceedings Supplement 00 (2018) 1–3
A boosted decision tree (BDT) classifier is used to iden-tify leptons based on their 3D shower shapes. Mountedon the ISS, AMS-02 is orbiting the Earth at an altitudeof about 400 km, with inclination of 51.6 ◦ . The averagetrigger rate is 600 Hz with event size of 2 kByte. Theminimal down-link bandwidth is 9 Mbit / s. The detec-tor is controlled from the AMS-02 Payload OperationsControl Center (POCC) at CERN, Geneva. From thePOCC, a continuous monitoring of the data flow anddetector systems is performed. This includes health andstatus of all sub-systems and the temperatures of theindividual components. Intensive time-dependent cal-ibrations have been performed to all sub-detectors. Nosignificant degradation of the sub-systems has been ob-served during 3 years of operation in the ISS.
2. New results
AMS-02 collects about 1.5 billion CRs during eachmonth of operation. The new results are based on thedata collected during the initial 3 years of operations onthe ISS, from May 2011 to May 2014. This constitutes ∼
16% of the expected AMS data sample. In the mea-surement of leptons, the TOF is used to select Z = Z = E / R ratio between ECAL energy and Trackerrigidity. This redundancy allowed to characterize theperformance of each discrimination method using thedata. Various analyses with di ff erent combinations ofcuts or template fits were also tested as a cross-check.Another important source of background comes from charge-confusion of electron events. It may arise fromto the finite rigidity resolution of the Tracker, or fromthe emission of secondary particles near the primarytrack. The charge-confusion contribution can be esti-mated from data and calculated with Monte Carlo sim-ulations. The relevant processes are well described bythe simulations [8]. A new measurement of the positronfraction, e + / ( e + + e − ), from 1 to 500 GeV of energy, hasbeen recently presented [9]. The results are shown inFig.2. The left figure shows the fraction up to 35 GeVof energy compared with the model prediction fromconventional calculations. In conventional models, CRelectrons are emitted from supernova remnants (SNRs),while secondary e ± arise from collisions of CR nucleiwith the interstellar matter (ISM). From this model, thepositron fraction must have a decreasing behavior withenergy. Below few GeV the AMS data decrease withenergy, as expected, although this energy region is af-fected by charge-dependent solar modulation. Above10 GeV, the data show a persistent rise up to 200 GeV, inclear contrast with the conventional picture, followed byan intriguing flattening at higher energies (right panel).Additional information can be obtained from the sin-gle spectra of e + and e − , that have been recently pub-lished [10]. The data show a slight spectral hardeningof both e − and e + component above ∼
30 GeV, where the e + spectrum experience a stronger hardening than the e − spectrum. Both spectra are substantially smooth overthe measured energy range. The data are also consis-tent with a new dedicated analysis of the total ( e − + e + )spectrum up to E = e + and e − in addi-tion to the conventional predictions based on secondaryproduction. Such a source can be interpreted either interms of dark matter (DM) particles annihilation or interms of nearby astrophysical sources of e ± . The firstclass of interpretation requires DM particles with massof the order of ∼ p / p ratio, depending on the DM–DM anni-hilation channels [12, 13]. The present antiproton datashow no clear evidence of such an excess within the un- Nuclear Physics B Proceedings Supplement 00 (2018) 1–3 certainties in the data and in the model predictions. Inthe second class, a recent work demonstrated that theAMS-02 data may be described if nearby pulsar windnebulae (PWNe) are accounted [14]. The PWN sce-nario gives no signatures in the antiproton channel. Itis also proposed that high-energy e ± can be producedinside old-SNRs via interactions of CR protons with thebackground medium [15, 16]. This mechanism Thismechanism predicts remarkable signatures in the ¯ p / p ratio [17] as well as in the B / C ratio [18], that maybe detectable by AMS-02 [19]. Understandig the back-ground due secondary production is therefore crucial fora multi-channel investigation of the CR data. Presentmodels are a ff ected by large astrophysical uncertainties[20, 21] that may be dramatically reduced with new dataon CR (anti)protons and light nuclei.
3. Conclusions
After one century from the discovery of CRs, thanksto AMS-02 we are eventually entering the era of pre-cision astroparticle physics and we can search for newphysics phenomena using CR data. High precision datafrom AMS-02 of the positron fraction and the electronand positron spectra point consistently to the existenceof a new source of high-energy leptons, that can be in-terpreted either by DM annihilation or by astrophysi-cal objects such as PWNe or old-SNRs. We emphasizethe crucial role of the CR propagation physics in un-derstanding the nature of such a new source. First, forhaving a robust estimate of the level of astrophysicalbackground from secondary production of e ± , which ispresently a ff ected by large uncertainties. Second, formodeling the propagation e ff ects on the spectral shapeof both signal and background of e ± . Third, for test-ing the di ff erent scenarios using CR nuclear data. Forinstance, high-energy measurements of CR antiprotons and boron-to-carbon ratio may provide a conclusive dis-crimination among the DM-scenarios (from which onemay expect an excess in the ¯ p / p ratio), old-SNR scenar-ios (which predict signatures on the B / C ratio) or PWNscenarios (from which no signatures are expected in thenuclear channels). Clearly, it is also crucial the behaviorof the positron fraction in the high energy region. Thebehavior between ∼
200 GeV and ∼
4. Acknowledgement
This work is supported by acknowledged persons andinstitutions in [8] and by the Labex grant
ENIGMASS . References [1] Tomassetti, N., & Oliva, A., 2013, 33 th ICRC, 896, Rio deJaneiro[2] Saouter, P., et al., 2013, 33 th ICRC, 789, Rio de Janeiro[3] Alpat, B., et al., 2010, NIM, A 613, 207[4] Basili, A., et al., 2013, NIM, A 707, 99[5] Aguilar, M., et al., 2010, NIM, A 614, 237[6] Kirn, T., et al., 2013, NIM, A 706, 43[7] Adlo ffff