Our God particle accelerators who art in Heaven...

...Give us our daily cosmic rays

Cosmic rays were discovered about one century ago by Victor Hess. Hess was awarded with the Nobel prize in 1936 for his studies, but he was never able to actually perform a direct detection of the cosmic rays due to the technological limitations of balloon flights and of the detectors at his times... It took several years to understand that the main component of cosmic rays is made of protons with a steeply falling flux as function of energy. The study of cosmic radiation and its interaction with the Earth atmosphere led to the discovery of new particles and set the basis for the experimental particle physics that is carried out today at accelerators. 

The origin, acceleration and propagation mechanisms of charged particles traveling in the Space have been the main topics in the studies of cosmic radiation since its discovery. In the aim of solving these puzzling issues, in the 80s and 90s a massive campaign of experiments was carried out on stratospheric balloon flights and small satellites. With increasing knowledge on the cosmic rays, it began to be clear that it is very difficult to provide a satisfactory and self-consistent global model.  

The cosmic ray all-particle spectrum is shown in figure 1 [1]. Most experiments agree, at least qualitatively, that the spectrum consists of at least three regions. At the lowest energies, from tens of MeV to tens of GeV (yellow band in figure), the particles coming from the interstellar space are deflected and influenced by the magnetic region generated by the Sun, the heliosphere. As a consequence, the observed spectrum is flattening. At higher energies, instead, the direct measurements of cosmic rays represent the interstellar flux and composition. For energies between tens of GeV and about 10¹⁵eV, the spectrum can be fit with a power law with slope ∼2.7. Due to such a steep spectrum, with current technology, a direct measurement (cyan band in figure) is possible only up to about 10¹⁵eV, the so called “knee”. Beyond the knee (purple band), the slope grows to ∼3.1. Only indirect measurements are possible by exploiting the atmosphere as a large calorimeter and by making use of ground based detectors. At the highest energies particles have energies comparable to the Greisen-Zatsepin-Kuzmin limit (GZK cutoff), which occurs at about 5×10¹⁹ eV. 

At the end of the 90s, experimental cosmic ray direct measurements were limited to few hundreds of GeV for protons and helium nuclei (major component of cosmic radiation) and to few tens of GeV for antiparticles. Due to the limited statistics and to quite large systematic uncertainties it was still not possible to answer to many fundamental questions concerning the cosmic rays. As a consequence, the experimental study of cosmic rays took three paths that are still effective. The first research line aims to push the direct measurements at the highest energies, possibly reaching the knee, in order to study sources and acceleration mechanisms. The second research line is dedicated to study the chemical composition of cosmic rays, measuring highly charged nuclei spectra, with the aim of understanding the source material, dust and gas, the nucleosynthesis and the propagation of cosmic rays in the interstellar medium. The third path, finally, is dedicated to the study of the rare antiparticle and anti-matter component, trying to search for signal of the elusive dark-matter, set anti-matter limits and understand the matter-antimatter asymmetry in the Universe... 

Direct detection of cosmic rays: through a new era of precision measurements of particle fluxes, Emiliano Mocchiutti

(Submitted on 4 Jul 2014)

Lead us not to the wrong cosmic ray standard model, 

Depending on the research line, different platforms and detection techniques have been adopted. In the following, I will describe the latest missions conducted on stratospheric balloons, satellites and on the International Space Station (ISS) while discussing the main physics results obtained in the recent years. I will categorize the results by type of particles and their role in the cosmic ray “standard model”. With “standard model”, figure [below],  

The latest generation of cosmic ray particle detectors has brought and is bringing many exciting results. Proton, helium and, possibly, highly charged nuclei spectra seems to harden at similar rigidities. Moreover, there is a strong indication that proton and helium nuclei have indeed a different spectral index. 

The measurement of antiparticles in the cosmic rays has been very popular in the last years, with a possible indication of dark matter detection in the positron fraction has to take into account not only the “missing signal” in the antiproton measurement but also the possibility of nearby astrophysical sources capable of accelerating electrons and positrons. 

All these measurements are challenging the cosmic ray standard model. New results from current and future experiments will probably contribute in developing a more precise description of the sources, acceleration and propagation of cosmic rays. 

A special care, however, must be used when interpreting experimental data: thanks to larger acceptances and acquisition time, it is likely that systematic uncertainties will dominate in a big part of the detected energy window. In such cases, it is always important to carefully describe the sources of these uncertainties and their effects on the measurements. Unlikely statistical errors, systematic uncertainties are estimated and they are strongly related to the experimental apparatus and their effect can bring not only renormalization problems but also distortions in the flux measurements.

Id.

But deliver us from darkness

Recently, several groups including Daylan et al. [7], Calore et al. [8], and the Fermi Collaboration [9] re-analyzed data from the Fermi-LAT [17] and concluded that the 1–3 GeV gamma ray signal is statistically significant and appears to originate from dark matter particles annihilating rather than standard astrophysical sources. The peak in the energy distribution is broadly consistent with gamma rays originating from self-annihilation of dark matter particles [7, 18–23]. The intensity of the signal suggests a dark matter annihilation cross section at thermal freeze out [24–29]. The diffuse nature and morphology of the gamma ray excess is consistent with a Navarro-Frenk-White-like Galactic distribution of dark matter [8]. This gamma ray excess thus drew the attention of a number of particle model builders and phenomenologists [10, 14, 24, 30–32]. The conclusion that we have discovered dark matter particles, however, cannot be drawn yet. First, we have to be able to exclude the possibility of a standard astrophysical explanation. Second, we need to demonstrate that a dark matter particle that explains the gamma ray excess (with a given mass, spin, and interaction strength to the standard sector) is consistent with a large number of other observations. The latter concerns our paper. We aim to determine the microscopic properties of the dark matter particle from the gamma ray excess and check that these properties comply with limits from other experiments. We use dark matter abundance and direct detection data, measurements of the gamma ray flux from the Galactic Center, near Earth positron and anti-proton flux data, Cosmic Microwave Background (CMB) observations, and measurements of galactic radio emission as experimental constraints. 

In this work we perform a comprehensive statistical analysis of the gamma ray excess from the Galactic Center in a simplified dark matter model framework. According to our previous study, Majorana fermion dark matter interacting with standard model fermions via a scalar mediator is the most favoured explanation of the galactic center excess when characterised by Bayesian evidence. We locate the most plausible parameter regions of this theoretical hypothesis using experimental data on the dark matter abundance and direct detection interactions, the gamma ray flux from the Galactic center, near Earth positron and anti-proton fluxes, the Cosmic Microwave Background, and galactic radio emission. We find that the radio data excludes the model if we include synchrotron radiation as the only energy loss channel. Since it was shown that inclusion of other types of energy losses lifts this exclusion we discard the single radio data point from our combined likelihood [34]. The rest of the data prefers a dark matter (mediator) mass in the 10–100 (3–1000) GeV region and weakly correlated couplings to bottom quarks and tau leptons with values of 10−3–1 at the 68% credibility level.

Interpreting the Fermi-LAT gamma ray excess in the simplified framework

Csaba Balázs, Tong Li, Chris Savage, Martin White

(Submitted on 25 May 2015)

Which drives a wedge between us(?)

The origin of the neutrino background radiation (NBR) above 35 TeV discovered with [1] IceCube, of the sub-TeV Galactic cosmic ray (CR) positrons measured recently with [2] PAMELA, Fermi-LAT and AMS and of the sub TeV gamma ray background (GBR) measured with [3] Fermi-LAT are still among the unsolved major cosmic puzzles. High energy particle physics offers three main mechanisms, which can produce simultaneously high energy neutrinos (ν’s), gamma rays (γ’s) and positrons (e+’s): 

• (1) meson production in hadronic collisions of high energy CRs with diffuse matter in the interstellar medium of galaxies [4] , in the intergalactic medium (IGM) of Galaxy clusters [5], or inside the cosmic ray sources [6a,6b,6c], 
• (2) photo production of mesons in CR collisions with radiation in/near gamma ray sources [7], and 
• (3) decay of massive dark matter particle [8] relics from the Big Bang 

 But, so far, no connection has been found [9a,9b] between the NBR, GBR and CR positrons and their origin and observed properties are still unsolved cosmic puzzles [1,2,3].  

In this letter, however, using only priors and no adjustable parameters, we show that if the high energy cosmic ν’s, γ’s, and e+’s are mainly produced in hadronic collisions of the CRs inside their main accelerators [the highly relativistic jets ejected in supernova explosions and by active Galactic nuclei [12][6a,6b,6c], then the NBR discovered ... with IceCube at energies above 35 TeV is that expected from the sub-TeV GBR measured ... with Fermi-LAT and that the Galactic GBR itself is that expected from the flux of sub-TeV CR positrons measured with AMS2. Moreover, the sky distributions of the NBR and GBR are predicted to be similar, while the predicted large spectral index of the NBR and low statistics make the Glashow resonance [10] undetectable in the current Icecube data... 

In the energy range between several GeV and PeV, the total flux of primary nucleons is well described by [15] 

Φ_p(E) ≈ 1.8 (E/GeV)^−β fu (2) 

where β ≈ 2.70 and fu=(GeV cm² s sr)⁻¹  is the flux unit... 

When in-source CR production of mesons dominates the Galactic production of high energy γ-rays and e⁺’s, their fluxes, which are given by Eq. (1), are simply related by

Φγ(E) ≈ (Fγ/Fe⁺)×(h/c×τ_e) Φ_e+ , (11)

where Fγ/Fe+=4.0 for βs = 1.367, h≈3.5 kpc is the typical distance from the center of the Galaxy of the supernova remnants and pulsars, the tracers of its main CR accelerator (supernova explosions and gamma ray bursts) are located [20]...  

The precise measurements with AMS of the spectrum of the sub-TeV CR positrons [2] can be used as an additional test of their proposed origin. In a steady state, the expected Galactic flux of CR e+’s is a sum Φ_e⁺(CR)=Φ_e⁺(source)+Φ_e⁺(ISM) where the in-source flux is given by Eq. (3), and the interstellar medium (ISM) flux is  

Φ_e+(ISM)≈F_e+×σin×nISM ×c×τ_e+×Φ_p/(β−1) (13) 

with F_e⁺≈0.007 for β=2.70, a mean ISM density in the Gallactic CR halo ≈ nISM  ≈ 0.05 cm^-3 and Φ_p given by Eq.(2). In order to test the inside-source production hypothesis, Fig.3 compares the predicted Φ_e+(source)+Φ_e+(ISM) for βs=2.33 and the Φ_e+(CR) measured with AMS2. The normalization of Φ_e+(CR) (source) at 100 GeV was adjusted to reproduce the Galactic contribution to the GBR at 100 GeV, as given by Eq(11). For completeness, in Fig.3  we have included a phenomenological heliosphere modulation of the e⁺ spectrum, which affect the spectrum only below 10 GeV. In order to demonstrate the possible effect of pair production in e+ collisions with ∼eV photons of a dense radiation field inside the source we have also included a cutoff in the ee⁺ spectrum [9a,9b], which affects its behavior only above 800 GeV. Such a cutoff is expected for cosmic accelerators such as the highly relativistic jets of supernova explosions of Type Ic, which produce the long duration highly beamed gamma ray bursts, most of which do not point in our direction [12]. As can be seen from Fig. 3, the agreement between the predicted and observed Φ_e⁺(CR) is quite satisfactory...

A critical test of the inside-source production hypothesis is whether the sky distributions of the NBR and GBR are nearly equal, which, however, requires much larger statistics than currently available from the IceCube [1].

Common Solution Of Three Cosmic Puzzles

Shlomo Dado, Arnon Dar

(Submitted on 19 May 2015 (v1), last revised 28 May 2015 (this version, v3))