Science goals

 

                       I.   The electron spectrum

The primary science goal of CALET is to perform high precision measurements of the electron spectrum from 1 GeV to 20 TeV.  Taking advantage of an excellent energy resolution and a low background contamination, CALET has been scanning the energy region already covered by previous experiments for more than six years. By integrating a sufficient exposure on the ISS, CALET has started the esploration of the energy region above 1 TeV where the presence of nearby sources of acceleration is expected to shape the high end of the electron spectrum and leave faint, but detectable, footprints in the anisotropy. In order to meet this experimental goal, CALET has been designed to achieve a large proton rejection capability (>105) thanks to a full containment of electromagnetic showers in the calorimeter and a fine-grained imaging of the first 3 radiation lengths.

 

 

The TeV region.  CALET has observed a significant flux reduction of electrons above 1 TeV. An exciting possibility is that the observation of the electron spectrum in the TeV region may result in a direct detection of nearby astrophysical sources of high energy electrons. In fact, the most energetic galactic cosmic-ray (GCR) electrons that can be observed on Earth are likely to originate from sources younger than ~105 years and located at a distance less than 1 kpc from the Solar System. This is due to the radiative energy losses that limit the propagation lifetime of high energy electrons and, consequently, the distance they can diffuse away from their source(s).

  

 

Vela-spectrum

 

Expected energy spectra of electrons as calculated by a model of Kobayashi et al., Astrophys. J. 601 , 340-351 (2004) with a particular choice of parameters (shown in the picture). The predicted electron spectra are compared to a compilation of previous electron measurements. Possible contributions from Vela, Monogem and Cygnus Loop are shown as an example. Adding these three sources to the "distant component" gives the topmost curve with the expected CALET measurements (red points). 

  

Since the number of potential sources satisfying the above constraints are very limited, the energy spectrum of electrons might have a characteristic structure (Nishimura et al. 1980), and the arrival directions are expected to show a detectable anisotropy (Ptuskin and Ormes 1995; Nishimura et al.1997). There are at least nine candidate Supernova Remnants (SNR) with ages < 105 years and distances less than 1 kpc from the solar system. Possible contributions to the observed GCR electron spectrum from both distant and nearby sources were calculated. Known candidates that may give a contribution in the TeV region include Vela, Cygnus loop and Monogem, in order of strength. Among these, Vela is quite promising as both the distance, ~ 0.25 kpc, and the age, ~ 104 years, are very suitable for the observation.

 

electron-spectrum

 

Inclusive electron (+ positron) spectrum as expected after 5 years of data taking by CALET (red points) according to a SUSY inspired model, consistent with the present data on the observed positron excess.

  

The TeV region might as well conceal a completely different scenario, as in the example shown in the picture above, where "nearby" acceleration sources are not detected and the spectrum rolls off at a characteristic cutoff energy.  In this example, the shape of the spectrum near the cutoff is predicted by a model of dark matter (with neutralino as LSP) that takes into account the recent data on the positron excess. The measurement of the "end point" of the electron spectrum can be used to constrain the cosmic-ray diffusion coefficient.
 
The sub-TeV region. The electron energy spectrum from 10 GeV to 1 TeV is probably the result of the contribution of several unresolved sources. In this energy region CALET accuracy and exposure will allow to significantly improve the knowledge of the detailed spectral shape and angular distribution of the inclusive electron + positron spectrum. This will provide information on the average features of the source spectrum, the diffusion time, the density of sources and possibly their nature, either as astrophysical objects (e.g. a nearby pulsar) or the result of the annihilation/ decay of dark matter particles. Both possibilities have been proposed to explain recent measurements suggesting a hardening of the inclusive electron+positron spectrum in the range 200 GeV - 1 TeV.  The presence of an additional spectral component is also required to explain the now established rise of positron fraction above ~10 GeV as measured by PAMELA and extended to the hundreds GeV range by AMS-02.

  The flux below 10 GeV is strongly modulated by solar activity. Long-term observations can provide accurate data on the evolution of the electron spectrum as a function of time. This information can be used to validate models of the transport of electrons into and within the heliosphere and improve our understanding of the modulation mechanism. 
 
UPDATE after 7.5 years of CALET electron observations on the ISS

 The above predictions for the electron spectrum refer to our estimates before CALET was launched.  A first important deviation from a scale-invariant power-law spectrum was found for electrons near 1 TeV. A significant flux reduction was observed by CALET[1] and DAMPE[2] as expected from to the large radiative losses of electrons during their travel in space. The astrophysical nature and location of the source(s) are still an open question and on of the main targets of CALET research. 

 

 

Compilation of the electron+positron flux multiplied by E2.7 as a function of kinetic energy/nucleon E (in GeV/n) with CALET experimental points (in red) from [1].

 
II.  Charged Cosmic Rays
  
The main science objectives of the experimental study of very high energy (VHE) charged cosmic rays (CR) include:
 
  • the understanding of the acceleration mechanism of primary cosmic rays;
  • the identification of the acceleration sites (sources);
  • the clarification of the interactions of primary cosmic rays with the inter-galactic medium.
     
Direct measurements of the composition and energy spectra of VHE cosmic rays are carried out by instruments flying on balloons (above the atmosphere at ~40 km altitude) and by space-borne experiments (on satellites and on the ISS). Their observations are known as direct because these payloads are equipped with instruments capable of identifying the incoming cosmic particle, in contrast with ground-based (indirect) measurements, where the identity of the impinging nucleus is inferred only indirectly via its interaction with the atmosphere, with large systematic uncertainties. The direct observations collected so far provide the experimental ground for the contemporary standard model of galactic cosmic rays (GCR).  
However, there are still many open questions, as for example:
 
  • are high energy spectra described by a pure power-law (as predicted by standard SNR diffusive shock acceleration models) ?Significant departures from a power-law have been observed to date by CALET and by other experiments. Curvature of the spectrum may occur as suggested by several theoretical models including  acceleration models that take into account the dynamical interaction between the shock and the accelerated particle, or by other models considering the possible presence of one (or more) local sources.
 
  • is there a mass-dependent spectral cutoff for individual elements below the PeV scale?  (as suggested by models that try to explain the all-particle spectral "knee" observed around 3-4 PeV)
  
  • why large anisotropies are not observed?  (contrary to the expectations based on the extrapolation of the propagation pathlength, as derived from measurements at a few GeV/n) 
 
 Some of these problems are being addressed by a new generation of space missions (including CALET) extending the existing measurements to higher energies. Taking advantage of a long observation time on the JEM-EF, favourable duty cycle and a relatively large geometric factor, CALET is extending the existing spectral measurements and studies of cosmic ray elemental composition by more than an order of magnitude in energy, and improving the quality of the data at lower energies by reducing the systematic errors 
 

pHe-spectra-scaled

 

Expected CALET measurement (red points) of the energy spectra of proton and He on the ISS, compared with a compilation of data from direct measurements.  

 
UPDATE after 7.5 years of CALET proton and helium observations on the ISS
The above predictions for the proton and helium spectra refer to our estimates before CALET was launched. Recent direct measurements of cosmic rays have shown the presence of unexpected spectral structures significantly departing from a simple-power-law dependence.
 An unexpected finding was the observation of a significant reduction of the proton flux[3] around 10 TeV also observed by DAMPE[4] and anticipated by the balloon experiment CREAM, albeit with low statistics. A precise measurement of the flux allowed CALET to fit the spectrum with a Double Broken Power-Law (DBPL). After a spectral hardening starting at a few GeV (also observed by AMS-02[5] and PAMELA[6]) and progressively increasing above 500 GeV, a steep softening was found to take place above ~10 TeV.  In the picture below the proton spectrum (multiplied by E2.7) exhibits a broad “bump”, with a deviation from a power-law by more than 20 standard deviations.  A similar trend is observed for He.

 

 

Compilation of direct measurements of proton flux multiplied by E2.7 as a function of kinetic energy/nucleon E (in GeV/n) with CALET experimental points (in red) from [3].

 
Energy spectra of cosmic nuclei.  Equipped with a charge identifier module, placed at the top of the experimental apparatus and capable to identify the atomic number Z of the incoming cosmic ray, CALET is performing long term observations of cosmic nuclei from proton to nickel and of trans-iron elements up to Z=40. CALET is able to identify CR nuclei with individual element resolution and measure their energies in the range from a few tens GeV to several hundreds TeV. CALET is extending the proton and helium energy spectra to hundreds TeV and is measuring the energy spectra of the most abundant heavy nuclei in the multi TeV/amu region C and O as well as Ne, Mg, Si, Fe and Ni with an energy reach only limited by the collected statistics.
 
nuclei-spectra-scaled
 
Expected CALET measurement (red points) of the energy spectra of C, O, Ne, Mg, Si, Fe nuclei,
 on the ISS, compared with a compilation of data from direct measurements.
 
 

Secondary-to-primary flux ratios.  Direct measurements of the energy dependence of the flux ratio of secondary-to-primary elements (e.g.: boron/carbon, sub-iron/iron) can discriminate among different models of CR propagation in the galaxy.  These observables are less prone to systematic errors than absolute flux measurements. Above 10 GeV/amu, the energy dependence of the propagation pathlength is often parametrized in the form E−δ. An accurate measurement of the spectral index parameter δ is crucial to derive the spectrum at the source by correcting the observed spectral shape for the energy dependence of the propagation term.  These measurements have been pushed to high energies with Long Duration Balloon (LDB) experiments. However, at present, they remain statistics limited to a few hundred GeV/amu and suffer from a systematic uncertainty, due to the production of secondary nuclei in the residual atmospheric grammage at balloon altitude, that may become dominant in the TeV/amu region.

 

B-to-C-5years

 

A partial compilation of the B/C ratio measurements as a function of the energy/nucleon and the expected statistical uncertainty of  CALET measurements (red points) after at least 5 years of observations.

 

With a long exposure and in the absence of atmosphere, CALET can provide new data to improve the accuracy of the present measurements of the B/C ratio above 100 GeV/amu and extend them beyond 1 TeV/amu. A compilation of B/C data from direct measurements is shown in the picture above, where the data points expected from CALET are marked as red filled circles in the energy range per nucleon from 15 GeV to ~ 8 TeV.

 

UPDATE after 7.5 years of CALET observations of cosmic nuclei on the ISS
 
The above predictions for cosmic nuclei spectra refer to our estimates before CALET was launched.
To date, the presence of a spectral hardening has been established for several nuclear species around a few hundred GeV/n and high statistics measurements have shown that the rigidity dependence of primary and secondary cosmic nuclei is different.
Measurements of the spectra of light nuclei B, C and O and of their ratios have been extended, in energy reach and precision, by CALET providing evidence[7] of a progressive spectral hardening for most of the primary elements above a few GeV/n. 
 

 

 

CALET  boron (a), carbon (b) flux (multiplied by E2.7) and (c) ratio of boron to carbon (red points), as a function of kinetic energy per nucleon E (from [7]).

 

Heavy cosmic ions and trans-iron elements

  In addition to Iron flux[8], CALET published the first high resolution measurement of Nickel[9] in 2022. The two high mass nuclei were shown to share very similar spectra but provided no evidence of a hardening so far.

 


 

Compilation of iron and nickel fluxes multiplied by E2.6 as a function of kinetic energy/nucleon E (in GeV/n) with CALET experimental points in red (from [8]).

 

Ultra High cosmic rays (UHCR) are quite rare. Their relative abundances to Fe drop down by orders of magnitude with increasing atomic number. Nevertheless, using a special trigger with large acceptance, CALET could study their abundances up to Z=40 with an excellent match with previous measurements from ACE-CRIS, SuperTIGER, and HEAO-3. 

 

III.  Dark Matter searches and gamma-ray astrophysics

 

Dark Matter (DM) candidates (for a concise review see, for instance Drees and Gerbier, 2004) include WIMPs (Weakly Interacting Particles) from supersymmetric theories, such as the LSP neutralino, that may annihilate and produce gamma rays and positrons as a signature. CALET is performing a sensitive search for signatures of DM candidates in both the electron (+positron) spectrum, as discussed above, and in gamma-ray spectra. 

According to a class of models, the annihilation / decay of dark-matter particles in the galactic halo could produce sharp gamma-ray lines in the sub-TeV to TeV energy region, superimposed to a diffuse photon background spectrum. CALET is capable of investigating such a distinctive signature, thanks to a gamma-ray energy resolution of 3% above 100 GeV, that can be improved to 1% with a reduced (75%) on-axis effective area (fiducial volume acceptance cuts to require a total lateral containment of the shower).

The precise determination of the line shape of any spectral feature is expected to play a crucial role in the discrimination among different models of dark matter candidates, or it might suggest an alternative astrophysical interpretation. 

DM-line-Yoshida-2013

  

A simulation of an hypothetical CALET detection of a possible 1.4 TeV gamma-ray line from dark matter in the region of the galactic centre, including galactic diffuse background [K. Yoshida et al., Proc. of 33rd ICRC 0735,1-4 (2013)].

Another class of DM candidates, as suggested by Cheng, Feng and Matchev (2002), are Kaluza-Klein (KK) particles, resulting from theories involving compactified extra-dimensions. They may annihilate in the galactic halo and produce an excess of positrons observable at Earth. Unlike neutralinos, however, direct annihilation of KK particles to leptons is not suppressed and, consequently, the KK electron “signal” is enhanced relative to that from neutralinos. The example in the picture below shows the predicted positron signal (a corresponding number of electrons are produced along with the positrons during the KK annihilation) for possible KK particle masses (dark shaded regions). The estimated background flux (light shaded region) of secondary particles from interactions of cosmic rays with the interstellar material is shown as well. The sharp cutoff in the positron excess close to an hypothetical KK mass, might produce a detectable “feature” in the inclusive electron/positron energy spectrum.

 

KK-spetrum-2002

 

Predicted positron signal from annihilation of Kaluza-Klein dark matter candidate particles according to the model of Cheng, Feng and Matchev (2002).

 

Dark matter KK particles can also decay into gamma rays. This opens the question on how to decide between a neutralino and a possible KK origin of an hypothetically observed gamma ray line. Bergström et al. (2006) have shown that a difference in the line shape between the two types of dark matter candidates has to be expected.  Thus, CALET would have the best capability to resolve the nature of the dark matter for any high energy gamma ray "line" observed.

 

Gamma-ray sources.    Observation of gamma-ray sources[10] is not a primary objective for CALET. However, its excellent energy resolution and good angular resolution (better than 0.4◦, including pointing uncertainty) allow for accurate measurements of diffuse gamma-ray emission and detection of more than 100 bright sources at high latitude from the Fermi-LAT catalogue. Given an on-axis effective area of ≈ 600 cm2 for energies above 10 GeV (reduced by ∼ 50% at 4 GeV) and field of view of 45◦ from the vertical direction, CALET can detect ~25000 (~7000) photons from the galactic (extra-galactic) background with E > 4 GeV and ~300 photons from the Vela pulsar with E > 5 GeV.
 

Gamma-ray Transients.   CALET is also monitoring X-ray/ gamma-ray transients in the energy region 7 keV – 20 MeV with a dedicated Gamma-ray Burst Monitor (CGBM).It is extending GRB studies performed by other experiments (e.g. Swift and Fermi/LAT) and providing added exposure when the other instruments are not available or pointing to other directions.  Moreover, higher energy photons associated with a burst event can be recorded over the entire CALET energy range down to 1 GeV where the CALET main telescope has still (limited) sensitivity, albeit with low resolution. Upon the detection of a GRB, an alert is transmitted to a network of ground "antennas" (e.g.: LIGO, VIRGO) for the possible simultaneous detection of gravitational waves associated with the event.

    

[1] O. Adriani et al. (CALET Collaboration) Physical Review Letters 120, 261102 (2018)

[2] G. Ambrosi et al., Nature volume 552, pages63–66 (2017)

[3] O. Adriani et al. (CALET Collaboration) Physical Review Letters 129, 101102 (2022)

[4] Q. An et al. (DAMPE Collaboration), Sci. Adv. 5, eaax3793 (2019).

[5] M. Aguilar et al. (AMS-02 Collaboration) Physical Review Letters 114, 171103 (2015)

[6] O. Adriani, et al. (PAMELA Collaboration) Science 332, 69 (2011);

[7] O. Adriani et al. (CALET Collaboration) Physical Review Letters 129, 251103 (2022)  

[8] O. Adriani et al. (CALET Collaboration) Physical Review Letters 126, 241101 (2022)  

[9] O. Adriani et al. (CALET Collaboration) Physical Review Letters 128, 131103 (2022)

[10] N. Cannady et al., The Astrophysical Journal Supplement Series, 238:5 (2018)

 

 

 

 

 

 

 

 

 

 

 

 

 

.