Study of two-photon production process in proton-proton collisions at 216 MeV

The energy spectrum for high energy $\gamma$-rays ($E_\gamma \geq 10$ MeV) from the process $pp \to \gamma \gamma X$ emitted at $90^0$ in the laboratory frame has been measured at 216 MeV. The resulting photon energy spectrum extracted from $\gamma-\gamma$ coincidence events consists of a narrow peak (5.3$\sigma$) at a photon energy of about 24 MeV and a relatively broad peak (3.5$\sigma$) in the energy range of (50 - 70) MeV. This behavior of the photon energy spectrum is interpreted as a signature of the exotic dibaryon resonance $d^\star_1$ with a mass of about 1956 MeV which is assumed to be formed in the radiative process $pp \to \gamma d^\star_1$ followed by its electromagnetic decay via the $d^\star_1 \to pp \gamma$ mode. The experimental spectrum is compared with those obtained by means of Monte Carlo simulations.Comment: 4 pages, LaTex, 1 eps-figure; Talk given at the HADRON 2001, Protvino, Russia, August 27 - September 1,2001, to be published in the proceedin

On a Missed Mechanism of Dielectron Production in Nucleon-Nucleon Collisions

We examine a new mechanism of $e^+e^-$ pair production in $NN$ collisions associated with the $NN$-decoupled dibaryon $d^\star_1$(1956) formation in the process $NN \to \gamma^\star d^\star_1$, where $\gamma^\star$ is the virtual photon which converts into a $e^+e^-$ pair. It is shown that a substantial excess of dielectron yields from $Ca+Ca$ and $C+C$ collisions at 1 GeV/A in the dielectron mass spectra in the region from 0.2 to 0.5 GeV/$c^2$ measured by the \textit{DLS Collaboration} as compared with calculated ones can be attributed to the contribution of this mechanism. A simple means for verification of the existence of such a mechanism is proposed.Comment: 4 pages, 3 figures, Talk presented at MENU 2010, May 31-June 4, 2010. College of William and Mary, Williamsburg, Virginia, USA Subjects: Nuclear Theory (nucl-th); High Energy Physics - Phenomenology (hep-ph); Nuclear Experiment (nucl-ex

On possible existence of the dibaryon resonance d1∗d^*_1 and its role in the npγnp\gamma and pdγpd\gamma processes below the pion threshold

We give reasons for the existence of the NN decoupled dibaryon resonance $d^*_1$(1956). Strong evidence for its presence has first been found in the energy spectrum of coincident photons emitted at $\pm 90^0$ from the $pp \to \gamma\gamma X$ process at 216 MeV measured by the DIB2$\gamma$ collaboration at JINR. As further experimental indications of the $d^*_1$(1956) existence we present those found in the available photon energy spectra of $np\gamma$, $pd\gamma$, and $pA\gamma$ reactions below the pion threshold. It is noted that serious discrepancies between the $np\gamma$ and $pd\gamma$ experimental cross sections and theoretical calculations can reasonably be explained by the fact that latter did not take into account the $d^*_1$ effect.Comment: 4 pages, LaTex, 4 eps-figures, Talk presented at the XVI International Conference on Particle and Nuclei (PANIC02), Osaka, Japan, Sep. 30 - Oct. 4, 200

Understanding the Sources of He II Reionization

The lifetime of quasars is a fundamental parameter for understanding the physical processes governing the growth of supermassive black holes, their co-evolution with galaxies, and the reionization history of the intergalactic helium. However, despite cosmological importance, current estimates of quasar lifetime are uncertain by at least two orders of magnitude, preventing significant progress in our understanding of the above questions. This thesis presents theoretical progress toward constraining quasar lifetimes and properties of helium reionization. A combination of cosmological hydrodynamical simulations and 1D radiative transfer algorithm is used to investigate the structure and evolution of the He II Lya proximity zones around quasars at redshift z = 3-4 and draw the following conclusions. In the first part of this thesis I show that the time evolution of the proximity zones can be described by a simple analytical model for the approach of the fraction of singly ionized helium (He II fraction) to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar luminosity, and the the average He II fraction (or equivalently the metagalactic He II ionizing background). Due to density fluctuations I advocate stacking existing He II quasar spectra at z ∼ 3, and show that the shape of this average proximity zone pro- file is sensitive to lifetimes as long as ~ 30 Myr. I find that at higher redshift z ~ 4 where the He II fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently. In the second part of this thesis I describe the method to constrain the quasar lifetime and the He II fraction (redshifts of the He II reionization) at z ~ 4 using the thermal state of the intergalactic medium around quasars. I show how the temperature of intergalactic gas increases due to quasar activity, producing thermal proximity effect in the Lyα absorption spectra. I investigate how the amplitude and the extent of thermal proximity effect depend on the amount of singly ionized helium in the IGM prior to quasar activity and on quasar lifetime by measuring the power spectrum of the H I Lya absorption spectra. I propose to use Markov Chain Monte Carlo algorithm to estimate the accuracy of the power spectrum measurement, and illustrate how with the mock sample of 50 high-resolution quasar spectra the He II fraction and quasar lifetime can be estimated with ~ 5 - 10% and 2 - 5% precision, respectively

The cosmic baryon partition between the IGM and CGM in the SIMBA simulations

We use the SIMBA suite of cosmological hydrodynamical simulations to investigate the importance of various stellar and active galactic nuclei (AGN) feedback mechanisms in partitioning the cosmic baryons between the intergalactic (IGM) and circumgalactic (CGM) media in the z ≤ 1 Universe. We identify the AGN jets as the most prominent mechanism for the redistribution of baryons between the IGM and CGM. In contrast to the full feedback models, deactivating AGN jets results in ≈20 per cent drop in fraction of baryons residing in the IGM and a consequent increase of CGM baryon fraction by ≈50 per cent.We find that stellar feedback modifies the partition of baryons on a 10 per cent level. We further examine the physical propertiesof simulated haloes in different mass bins, and their response to various feedback models. On average, a sixfold decrease inthe CGM mass fraction due to the inclusion of feedback from AGN jets is detected in 1012 M ≤ M200 ≤ 1014 M haloes. Examination of the average radial gas density profiles of M200 > 1012 M haloes reveals up to an order of magnitude decrease in gas densities due to the AGN jet feedback. We compare gas density profiles from SIMBA simulations to the predictions of the modified Navarro–Frenk–White model, and show that the latter provides a reasonable approximation within the virial radii of the full range of halo masses, but only when rescaled by the appropriate mass-dependent CGM fraction of the halo. The relative partitioning of cosmic baryons and, subsequently, the feedback models can be constrained observationally with fast radio bursts in upcoming surveys