G. Rubtsov

Narrowing the window for millicharged particles by CMB anisotropy

We calculate the cosmic microwave background (CMB) anisotropy spectrum in models with millicharged particles of electric charge q∼10−6−10−1 in units of electron charge. We find that a large region of the parameter space for the millicharged particles exists where their effect on the CMB spectrum is similar to the effect of baryons. Using WMAP data on the CMB anisotropy and assuming the Big Bang nucleosynthesis value for the baryon abundance, we find that only a small fraction of cold dark matter, Ωmcp<0.007 (at 95% CL), may consist of millicharged particles with the parameters (charge and mass) from this region. This bound significantly narrows the allowed range of the parameters of millicharged particles. In models without paraphotons, millicharged particles are now excluded as a dark matter candidate. We also speculate that recent observation of 511-keV γ rays from the Galactic bulge may be an indication that a (small) fraction of cold dark matter is comprised of millicharged particles.

Muon content of ultrahigh-energy air showers: Yakutsk data versus simulations

A sample of 33 extensive air showers (EASs) with estimated primary energies above 2 × 1019 eV and high-quality muon data recorded by the Yakutsk EAS array is analyzed. The observed muon density is compared event-by-event to that expected from CORSIKA simulations for primary protons and iron using SIBYLL and EPOS hadronic interaction models. The study suggests the presence of two distinct hadronic components, “light” and “heavy.” Simulations with EPOS are in good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SIBYLL, simulated muon densities for iron primaries are a factor of ∼ 1.5 less than those observed for the heavy component for the same electromagnetic signal. Assuming a two-component proton-iron composition and the EPOS model, the fraction of protons with energies E > 1019 eV is 0.52−0.20+0.19 at the 95% C.L.

Constraints on millicharged particles from Planck data

We revisit cosmic microwave background (CMB) constraints on the abundance of millicharged particles based on the Planck data. The stringent limit Omega_{mcp}h^2 < 0.001 (95% CL) may be set using the CMB data alone if millicharged particles participate in the acoustic oscillations of baryon-photon plasma at the recombination epoch. The latter condition is valid for a wide region of charges and masses of the particles. Adding the millicharged component to LCDM shifts prefered scalar spectral index of primordial perturbations to somewhat larger values as compared to minimal model, even approaching Harrison-Zeldovich spectrum under some assumptions.

Constraining anisotropic models of the early universe with WMAP9 data

We constrain several models of the early Universe that predict a statistical anisotropy of the cosmic microwave background (CMB) sky. We make use of WMAP9 maps deconvolved with beam asymmetries. As compared to previous releases of WMAP data, they do not exhibit the anomalously large quadrupole of statistical anisotropy. This allows us to strengthen the limits on the parameters of models established earlier in the literature. In particular, the amplitude of the special quadrupole is constrained as

Radio measurements of the energy and the depth of the shower maximum of cosmic-ray air showers by Tunka-Rex

We reconstructed the energy and the position of the shower maximum of air showers with energies E & 100PeV applying a method using radio measurements performed with Tunka-Rex. An event-to-event comparison to air-Cherenkov measurements of the same air showers with the Tunka-133 photomultiplier array confirms that the radio reconstruction works reliably. The Tunka-Rex reconstruction methods and absolute scales have been tuned on CoREAS simulations and yield energy and Xmax values consistent with the Tunka-133 measurements. The results of two independent measurement seasons agree within statistical uncertainties, which gives additional confidence in the radio reconstruction. The energy precision of Tunka-Rex is comparable to the Tunka-133 precision of 15 %, and exhibits a 20% uncertainty on the absolute scale dominated by the amplitude calibration of the antennas. For Xmax, this is the first direct experimental correlation of radio measurements with a different, established method. At the moment, the Xmax resolution of Tunka-Rex is approximately 40 g/cm2. This resolution can probably be improved by deploying additional antennas and by further development of the reconstruction methods, since the present analysis does not yet reveal any principle limitations.

Structure Dependent Energy Transport: Relaxation-Assisted 2DIR Measurements and Theoretical Studies

Vibrational energy relaxation and transport in a molecule that is far from thermal equilibrium can affect its chemical reactivity. Understanding the energy transport dynamics in such molecules is also important for measuring molecular structural constraints via relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy. In this paper we investigated vibrational relaxation and energy transport in the ortho, meta, and para isomers of acetylbenzonitrile (AcPhCN) originated from excitation of the CN stretching mode. The amplitude of the cross-peak among the CN and CO stretching modes served as an indicator for the energy transport from the CN group toward the CO group. A surprisingly large difference is observed in both the lifetimes of the CN mode and in the energy transport rates for the three isomers. The anharmonic DFT calculations and energy transport modeling performed to understand the origin of the differences and to identify the main cross-peak contributors in these isomers described well the majority of the experimental results including mode excited-state lifetimes and the energy transport dynamics. The strong dependence of the energy transport on molecular structure found in this work could be useful for recognizing different isomers of various compounds via RA 2DIR spectroscopy.

Upper limit on the ultrahigh-energy photon flux from AGASA and Yakutsk data

We present the interpretation of the muon and scintillation signals of ultrahigh-energy air showers observed by AGASA and Yakutsk extensive air shower array experiments. We consider case-by-case ten highest-energy events with known muon content and conclude that at the 95% confidence level none of them was induced by a primary photon. Taking into account statistical fluctuations and differences in the energy estimation of proton and photon primaries, we derive an upper limit of 36% at a 95% confidence level on the fraction of primary photons in the cosmic-ray flux above 10{sup 20} eV. This result disfavors the Z-burst and superheavy dark-matter solutions to the Greisen-Zatsepin-Kuzmin-cutoff problem.

Breaks in gamma-ray spectra of distant blazars and transparency of the Universe

Energetic gamma rays scatter on soft background radiation when propagating through the Universe, producing electron-positron pairs (A.I. Nikishov, Sov. Phys. JETP 14, 393 (1962)). Gamma rays with energies between 100 GeV and a few TeV interact mostly with infrared background photons whose amount is poorly known experimentally but safely constrained from below by account of the contribution of observed light from known galaxies (R.C. Keenan, A.J. Barger, L.L. Cowie, and W.-H. Wang, Astrophys. J. 723, 40 (2010); arXiv: 1102.2428). The expected opacity of the intergalactic space limits the mean free path of TeV gamma rays to dozens of Megaparsecs. However, TeV photons from numerous more distant sources have been detected (S.P. Wakely and D. Horan, http://tevcat.uchicago.edu/). This might be interpreted, in each particular case, in terms of hardening of the emitted spectrum caused by presently unknown mechanisms at work in the sources (S. Archambault et al. (VERITAS and Fermi LAT Collaborations), Astrophys. J. 785, L16 (2014); arXiv: 1403.4308). Here we show that this interpretation is not supported by the analysis of the ensemble of all observed sources. In the frameworks of an infrared-background model with the lowest opacity (R.C. Gilmore, R.S. Somerville, J.R. Primack, and A. Dominguez, Mon. Not. Roy. Astron. Soc. 422, 3189 (2012); arXiv: 1104.0671), we reconstruct the emitted spectra of distant blazars and find that upward spectral breaks appear precisely at those energies where absorption effects are essential. Since these energies are very different for similar sources located at various distances, we conclude that the breaks are artefacts of the incorrect account of absorption and, therefore, the opacity of the Universe for gamma rays is overestimated even in the most conservative model. This implies that some novel physical or astrophysical phenomena should affect long-distance propagation of gamma rays. A scenario in which a part of energetic photons is converted to an inert new particle in the vicinity of the source and reconverts back close to the observer (M. Simet, D. Hooper, and P. Serpico, Phys. Rev. D 77, 063001 (2008); arXiv: 0712.2825; M. Fairbairn, T. Rashba, and S. Troitsky, Phys. Rev. D 84, 125019 (2011); arXiv:0901.4085) does not contradict our results. This new axion-like particle appears in several extensions of the Standard Model of particle physics (J. Jaeckel and A. Ringwald, Ann. Rev. Nucl. Part. Sci. 60, 405 (2010); arXiv: 1002.0329) and may constitute the dark matter (P. Arias et al., JCAP 1206, 013 (2012); arXiv: 1201.5902).

Constraints on the Fraction of Primary Gamma Rays at Ultra-High Energies from the Muon Data of the Yakutsk EAS Array ¶

By using data on the total signal and on the muon component of air showers detected at the Yakutsk array, the possible upper limits on the fraction of primary gamma rays at ultra-high energies is analyzed in the framework of the recently suggested event-by-event approach. Upper limits on the photon fraction in the integral flux of primary cosmic rays are derived. At 95% C.L., these limits are 22% for primary energies E0 > 4 × 1019 eV and 12% for E0 > 2 × 1019 eV. Despite the presence of muonless events, the data are consistent with the complete absence of photons at least at 95% C.L. The sensitivity of the results to systematic uncertainties, in particular to those of the energy determination for nonphoton primaries, is discussed.

Fully automated dual-frequency three-pulse-echo 2DIR spectrometer accessing spectral range from 800 to 4000 wavenumbers

A novel dual-frequency two-dimensional infrared instrument is designed and built that permits three-pulse heterodyned echo measurements of any cross-peak within a spectral range from 800 to 4000 cm(-1) to be performed in a fully automated fashion. The superior sensitivity of the instrument is achieved by a combination of spectral interferometry, phase cycling, and closed-loop phase stabilization accurate to ~70 as. The anharmonicity of smaller than 10(-4) cm(-1) was recorded for strong carbonyl stretching modes using 800 laser shot accumulations. The novel design of the phase stabilization scheme permits tuning polarizations of the mid-infrared (m-IR) pulses, thus supporting measurements of the angles between vibrational transition dipoles. The automatic frequency tuning is achieved by implementing beam direction stabilization schemes for each m-IR beam, providing better than 50 μrad beam stability, and novel scheme for setting the phase-matching geometry for the m-IR beams at the sample. The errors in the cross-peak amplitudes associated with imperfect phase matching conditions and alignment are found to be at the level of 20%. The instrument can be used by non-specialists in ultrafast spectroscopy.