T. A. Dzhatdoev

Method for measuring the PCR proton spectrum in the energy range of > 1016 eV

Criteria for selecting proton events among the total sequence of events from primary nuclei of cosmic rays with zenith angles θ < 20° are analyzed in the energy region of E0 ≈ 1016 eV. These criteria are concretized for the case of the SPHERE-2 experiment geometry. The QGSJET-I and QGSJET-II model calculations show that the criteria based on the shape of the transverse distribution of Cherenkov light allow detection of more than 10% of proton events and rejection of 99% nuclear events.

Electromagnetic cascade masquerade: a way to mimic γ-axion-like particle mixing effects in blazar spectra

Context. Most of the studies on extragalactic γ -ray propagation performed up to now only accounted for primary γ -ray absorption and adiabatic losses, known as the “absorption-only model”. However, there is growing evidence that this model is oversimplified and must be modified in some way. In particular, it was found that the intensity extrapolated from the optically-thin energy range of some blazar spectra is insufficient to explain the optically-thick part of these spectra. This effect was interpreted as an indication for γ -axion-like particle (ALP) oscillation. On the other hand, there are many hints that a secondary component from electromagnetic cascades initiated by primary γ -rays or nuclei may be observed in the spectra of some blazars. Aims. We study the impact of electromagnetic cascades from primary γ -rays or protons on the physical interpretation of blazar spectra obtained with imaging Cherenkov telescopes. Methods. We used the publicly-available code ELMAG to compute observable spectra of electromagnetic cascades from primary γ -rays. For the case of primary proton, we developed a simple, fast and reasonably accurate hybrid method to calculate the observable spectrum. We performed the fitting of the observed spectral energy distributions (SEDs) with various physical models: the absorption-only model, the “electromagnetic cascade model” for the case of primary γ -rays, and several versions of the hadronic cascade model for the case of primary protons. We distinguish the following species of hadronic cascade models: 1) the “basic hadronic model”, in which it is assumed that the proton beam travels undisturbed by the extragalactic magnetic field and that all observable γ -rays are produced by primary protons through photohadronic processes with subsequent development of electromagnetic cascades; 2) the “intermediate hadronic model”, which is the same as the basic hadronic model, but the primary beam is terminated at some redshift z c ; and 3) the “modified hadronic model” that includes the contribution from primary, redshifted and partially-absorbed, γ -rays. Results. Electromagnetic cascades show at least two very distinct regimes labelled by the energy of the primary γ -ray ( E 0 ): the one-generation regime for the case of E 0 E 0 > 100 TeV and redshift to the source z s > 0.02. Spectral signatures of the observable spectrum for the case of the basic hadronic model, z s = 0.186 and low energy ( E E > 200 GeV the spectrum is much harder for the case of the basic hadronic model. In the framework of the intermediate hadronic model, the observable spectrum depends only slightly on the primary proton energy, but it strongly depends on z c at E > 500 GeV. As a rule, both electromagnetic and hadronic cascade models provide acceptable fits to the observed SEDs. We show that the best-fit model intensity in the multi-TeV region of the spectrum in the framework of the electromagnetic cascade model is typically greater than the one for the case of the absorption-only model. Finally, for the case of blazar 1ES 0229+200 we provide strong constraints on the intermediate hadronic model, assuming models for the blazar emission and the magnetic field around the source.

Results on the primary CR spectrum and composition reconstructed with the SPHERE-2 detector

First preliminary results of the balloon-borne experiment SPHERE-2 on the all-nuclei primary cosmic rays (PCR) spectrum and primary composition are presented. The primary spectrum in the energy range 1016–5 1017 eV was reconstructed using characteristics of Vavilov-Cherenkov radiation of extensive air showers (EAS), reflected from a snow surface. Several sources of systematic uncertainties of the spectrum were analysed. A method for separation of the primary nuclei groups based on the lateral distribution function (LDF) steepness parameter is presented. Preliminary estimate of the mean light nuclei fraction f30-150 at energies 3 1016–1.5 1017 eV was performed and yielded f30-150 = (21±11) %.

Search for the νμ→ντ Oscillation with the OPERA Hybrid Detector

This work reviews the status of the international OPERA experiment aimed at searching for the νμ → ντ oscillation. Development of the emulsion techniques in the preceding DONUT and CHORUS experiments is described. A brief overview of the NOMAD experiment is given, in which properties of ντ interactions were thoroughly investigated. Characteristics of the CERN-Gran Sasso (CNGS) neutrino beam are given, the structure of the OPERA hybrid detector and the functions of its subsystems are considered, and the automatic emulsion scanning technique is briefly reviewed. Uncertainties in reconstruction of event parameters using the electronic and emulsion detectors are discussed and the procedure adopted in the experiment for identification of ντ interactions is described. The main sources of background in the search for νμ → ντ oscillation are considered. Topologies of the first two ντ candidates observed in the OPERA emulsion detector are presented. Possibilities of increasing the ντ interaction selection efficiency using multidimensional criteria are briefly discussed.

Event-by-event study of CR composition with the SPHERE experiment using the 2013 data

We present an event-by-event study of cosmic ray (CR) composition with the reflected Cherenkov light method. The fraction of CR light component above 5 PeV was reconstructed using the 2013 run data of the SPHERE experiment which observed optical Vavilov-Cherenkov radiation of extensive air showers, reflected from snow surface of Lake Baikal. Additionally, we discuss a possibility to improve the elemental groups separability by means of multidimensional criteria.

Present status of the opera experiment for the direct observation of neutrino oscillations in the νμ → ντ channel

The OPERA experiment (Oscillation Project with an Emulsion-Tracking Apparatus) for the direct observation of neutrino oscillations in the νμ → ντ channel has been underway at the underground laboratory in Gran Sasso since 2007. In the course of data collection, more than 3000 νμ interactions have been registered in emulsion detectors. Twenty charmed neutrino interaction candidates have been found. Multidimensional criteria have been developed in order to select ντ interactions against νμ interactions accompanied by charmed hadron production. The first results from automated PAVIKOM emulsion scanning have been obtained. We discuss the measurements of the muon charge ratio of μ+ to μ−.

On the sensitivity of the spatial-angular distribution of the Cherenkov light in extensive air showers to the mass composition of primary cosmic rays with energies of 1015–1016 eV

This paper analyses the possibility of separating distinct groups of nuclei of primary cosmic rays with energies of 1015–1016 eV from the data on the spatial-angular distribution of Cherenkov light in extensive air showers. The paper shows that using an array of a few (3–4) telescopes with a moderately sized angular cell ∼0.5° placed at a distance ∼100 m from one another, one can achieve almost complete separation of the showers initiated by these nuclei (the Bayesian classification error is a few percentage points for the case of separating primary protons and nitrogen nuclei). The authors propose new parameters of the angular Cherenkov image that can greatly enhance the separability of the shower classes as compared to the approach based on the traditional parameters.

Cascades from primary gamma rays and nuclei as a source of background in searches for oscillations between photons and axion-like particles

Methods of searching for oscillations of photons to axion-like particles (γ → ALP) in spectra of extragalactic sources are considered. It is shown that the secondary component from the cascades initiated by primary gamma rays or nuclei can produce an appreciable background in searches for ALPs in the optically thick region of the spectrum. Experiments with a high energy threshold (~1 TeV and higher) are insufficient for searching for γ → ALP oscillations; low-energy instruments (E < 100 GeV) like CTA should also be used.

Testing models of extragalactic γ-ray propagation using observations of extreme blazars in GeV and TeV energy ranges

We briefly review contemporary extragalactic {gamma}-ray propagation models. It is shown that the Extragalactic Magnetic Field (EGMF) strength and structure are poorly known. Strict lower limits on the EGMF strength in voids are of order 10^{-17}--10^{-20} G, thus allowing a substantial contribution of a secondary component generated by electromagnetic cascades to the observable spectrum. We show that this electromagnetic cascade model is supported by data from imaging Cherenkov telescopes and the Fermi LAT detector.

Cosmic ray study by means of reflected EAS Cherenkov light method with the SPHERE-2 detector

The approach to cosmic ray (CR) study with reflected optical Vavilov-Cherenkov radiation (Cherenkov light) was proposed long ago. At present the SPHERE-2 detector is the only existing apparatus that have detected a significant sample of extensive air showers (EAS) by means of this method. At the same time the recorded data allows detailed reconstruction of EAS lateral distribution function (LDF) used to study primary CR mass composition. n nWe report on the status and results of the SPHERE experiment with the emphasis on the peculiarities of the reflected Cherenkov light technique. Detector response simulation was performed by means of full direct Monte Carlo simulation with account of realistic background and noise patterns recorded during the observational runs. Instrumental acceptance was simulated for various energies, charge numbers and zenith angles of primary nuclei. n nPrimary energy of observed showers was estimated with a typical statistical uncertainty 10-20% depending on the primary nucleus parameters. The typical systematic uncertainty of the estimated energy vs the primary charge number was found to be below 3%. The primary all-nuclei spectrum was reconstructed. The fraction of light nuclei vs energy in the energy range 10-100 PeV was estimated by means of an event-by-event approach using the LDF steepness parameter.