Observation of the Energy Dependence of Primary and Secondary Cosmic Rays with the AMS Detector on the International Space Station

. Precision study of cosmic nuclei provides detailed knowledge on the origin and propagation of cosmic rays. AMS is a multi-purpose high energy particle detector designed to measure and identify cosmic ray nuclei with unprecedented precision. It is able to provide precision studies of nuclei simultaneously to multi-TeV energies. In 7 years on the Space Station, AMS has collected more than 120 billion both primary and secondary cosmic rays. Primary cosmic rays, such as p, He, C and O, are believed to be mainly produced and accelerated in supernova remnants, while secondary cosmic rays, such as Li, Be and B are thought to be produced by collisions of heavier nuclei with interstellar matter. Primary cosmic rays such as He, C, and O are found to have identical rigidity dependence, similarly to secondary cosmic rays (such as Li, Be and B) which share the same the same spectral shape. The peculiar case of Nitrogen being a mixture of a primary and secondary component will also be shown.


Introduction
Cosmic rays (CRs) are charged particles coming isotropically from outer space to the Earth atmosphere with an energy range spanning over several decades. It is still unclear where and how CRs are accelerated in the Galaxy, as well as how they propagate through the interstellar medium (ISM), and both these aspects are subjects of current research. Another fundamental aspect in CR physics is the search for any signal of new physics in the antimatter component of CRs that could be linked to the decay of dark matter (DM) particles or to the existence of primordial antimatter. The bulk of cosmic rays is made of protons (∼ 88%) and helium nuclei (∼ 9%), while the remaining fraction is composed mostly of electrons (∼ 2%) and heavier nuclei (∼ 1%) with small traces of antimatter (positrons and antiprotons). Depending on wheter a cosmic ray reaches the solar system after being accelerated at its source or after being created in a anelastic interaction of other CRs with the ISM, it is labelled a primary or a secondary cosmic ray. Examples of primary species are protons, 4 He nuclei, carbon and oxygen nuclei, and electrons, while typical examples of secondary species are 2 H, 3 He, lithium, beryllium and boron nuclei.
Being created directly during the propagation of primary CRs in the Galaxy, secondary CRs carry important information on how the transport process modifies the original CR energy spectrum after the a e-mail: valerio.formato@pg.infn.it acceleration at the source. This is important for the theoretical prediction of the amount of secondary antimatter produced by CR propagation in the Galaxy, since this represents the natural background in the search for new physics in the antimatter channel. Thus the hadronic component of CRs holds valuable information on how CRs are accelerated and on how their energy spectrum is affected by the propagation in the galactic magnetic field. By measuring also secondary species, important informations can be obtained about CR propagation, especially when the flux of a secondary species is divided by the flux of a primary species, preferably one of its progenitors. Not counting protons, the most abundant species in the hadronic component of CRs is helium, followed by carbon and oxygen wich make up the bulk of the primary component. The secondary component is made up by boron, lithium and beryllium nuclei. Nitrogen defies a clear classification as primary or secondary, as it is supposed to be an admixture of both a primary component and a secondary component.
Light nuclei in CRs with charge Z ≤ 8 have been recently measured by the AMS-02 collaboration in Refs. [1][2][3] and the results will be summarized in section 3.

The AMS-0experiment
AMS-02 is a general-purpose high-energy physics particle detector operating onboard the International Space Station (ISS) since May 2011. The instrument will be active for the entire ISS lifetime which is currently forseen to last until 2024. With such a long exposure time and its large geometrical acceptance (∼ 0.5 m 2 sr), AMS-02 is capable to provide high-quality data on CR fluxes at the TeV energy scale with unprecedent precision and sensitivity.
The AMS-02 detector is described in details in ref.
[4]. It is composed by several sub-detector systems that allow precise particle identification as well as redundant measurements of the main characteristics of CR particles, such as arrival direction, charge and magnetic rigidity (R = pc/Ze). The particle trajectory in the AMS-02 magnetic field is reconstructed from the position measurements along the 9 silicon layers of the tracking system, which provides a spatial resolution of 10µm in the y (bending) view and 30µm in the x (non-bending) view. From the reconstructed trajectory the particle direction and rigidity can be measured. The velocity β = v/c can be determined from the transit time between the upper and lower TOF scintillator planes (with a resolution, for Z = 1, ∆β/β 2 ∼ 2%), or more precisely using the RICH sub-detector (with a resolution, for Z = 1, ∆β/β ∼ 10 −3 ). The central part of AMS-02 is surrounded by an anti-coincidence system (ACC). The detector is completed with a Transition Radiation Detector (TRD), which is located between the first layer of the silicon tracker and the upper TOF, and a 18-layer electromagnetic calorimeter (ECAL), which is placed at the bottom.

Data analysis
The key elements used in this measurement are the permanent magnet, the silicon tracker, and the four planes of time of flight (TOF) scintillation counters. The dataset used corresponds to the first 60 months of AMS data taking, from May 19 th 2011 to May 26 th 2016, during which AMS recorded 8.5 × 10 10 cosmic ray triggers. Events of a given species were required to be downward going and to have a reconstructed track in the inner tracker which passes through the first tracker layer (L1). In the highest rigidity region, R ≤ 1.13 TV, the track is also required to pass through the last tracker layer (L9). Track fitting quality criteria such as a χ 2 /d.o.f. < 10 in the bending coordinate are applied, similar to [5][6][7].
Charge measurements on L1, the inner tracker, the upper TOF, the lower TOF, and, for R > 1.13 TV, L9 are required to be compatible with charge Z = 2 to 8 for helium to oxygen events N. of events Residual background at 2 GV / 3.3 TV Helium 90 × 10 6 < 0.1% Lithium 1.9 × 10 6 5% / 2% Beryllium 0.9 × 10 6 8% / 13% Boron 2.6 × 10 6 5% / 8% Carbon 8.6 × 10 6 < 0.5% Nitrogen 2.2 × 10 6 < 3% / 6% Oxygen 7.0 × 10 6 < 0.5% The background from interactions on materials above the tracker L1 (a thin support structures made by carbon fiber and aluminum honeycomb) has been estimated from simulation using MC samples generated according to the AMS flux measurements as detailed in [7]. The estimated residual background levels at 2 GV and 3.3 TV are shown in table 1. The bin-to-bin migration of events was corrected using the unfolding procedure described in Ref. [6]. A detailed description of the systematic uncertainties can be found in Refs. [1-3].

Conclusions
The fluxes obtained are shown in Fig. 1 as a function of rigidity with the total errors, the sum in quadrature of statistical and systematic errors. As seen, cosmic ray species can be divided into two distinct families, primary CRs exhibiting a R −2.7 power-law behaviour below 300 GV, and secondary CRs with a R −3.1 power-law behaviour below 300 GV. Besides a different normalisation, all primaries fluxes seem to have the same rigidity shape in common, and all the secondaries flux seem to share the same rigidity dependence although different from the primary shape. Remarkably all the species show a deviation from the single power-law shape at ∼ 200 GV. This can be also seen examining the evolution of the spectral index as a function of rigidity, shown in Fig. 2. The nitrogen flux is the only one not following either of the two behaviors but, as shown in Ref. [3], it can be well described by a mixture of a primary component and a secondary component both following the spectral shape of the primary and secondary elements respectively. corrections, ðN i − ℵ i Þ=ℵ i , where ℵ i is the number of observed events in bin i, are þ 15% at 3 GV, þ 7% at 5 GV, −5% at 200 GV, and −6% at 3.3 TV. Extensive studies were made of the systematic errors. These errors include the uncertainties in the background estimations discussed above, the trigger efficiency, the geomagnetic cutoff factor, the acceptance calculation, the rigidity resolution function, and the absolute rigidity scale. The systematic error on the flux associated with the trigger efficiency measurement is < 0.7% over the entire rigidity range. The geomagnetic cutoff factor was varied from 1.0 to 1.4, resulting in a negligible systematic uncertainty (< 0.1%) in the rigidity range below 30 GV.

References
The effective acceptances A i were calculated using MC simulation and corrected for small differences between the data and simulated events related to (a) event reconstruction and selection, namely, in the efficiency of velocity determination, track finding, charge determination, and tracker quality cuts and (b) the details of inelastic interactions of for N þ C and N þ Ref.
[8] and discuss of the Supplementa be < 3% up to 100 The rigidity resol a pronounced Gaus non-Gaussian tails [18]. The resolutio procedures describe Fig. 6 of the Supple the measured track 5.5 μm is in good yields the MDR o provides the uncer the non-Gaussian ta to the rigidity resol ing the unfolding pr Gaussian core of t independently varyi tails by 10%. The r less than 1% below There are two con on the rigidity scale first is due to resid contribution arises ment and magnetic on the flux due to th and 5% in the last Much effort has tematic errors [18,2 Supplemental Mate surements of the nit performed using ev other using events agreement between atic errors on unf resolution functions tance, due to the d amount of material Most importantly performed on the sam The results of those