Research Outputs (School of Physics)
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Item First limits on the very-high energy gamma-ray afterglow emission of a fast radio burst HESS observations of FRB 150418(EDP Sciences, 2017-01-01) Colafrancesco, S.; Jingo, M.; Shafi, N.; Abdalla, H.; Abramowski, A.; Aharonian, F.; Ait Benkhali, F.; Akhperjanian, A. G.; Andersson, T.; Angüner, E. O.; Arakawa, M.; Arrieta, M.; Aubert, P.; Backes, M.; Balzer, A.; Barnard, M.; Chen, A.; Jingo, M.; Komin, N.; Shafi, N.Aims. Following the detection of the fast radio burst FRB150418 by the SUPERB project at the Parkes radio telescope, we aim to search for very-high energy gamma-ray afterglow emission. Methods. Follow-up observations in the very-high energy gamma-ray domain were obtained with the H.E.S.S. imaging atmospheric Cherenkov telescope system within 14.5 h of the radio burst. Results. The obtained 1.4 h of gamma-ray observations are presented and discussed. At the 99% C.L. we obtained an integral upper limit on the gamma-ray flux of Φγ(E > 350 GeV) < 1.33 × 10-8 m-2 s-1. Differential flux upper limits as function of the photon energy were derived and used to constrain the intrinsic high-energy afterglow emission of FRB 150418. Conclusions. No hints for high-energy afterglow emission of FRB 150418 were found. Taking absorption on the extragalactic background light into account and assuming a distance of z = 0.492 based on radio and optical counterpart studies and consistent with the FRB dispersion, we constrain the gamma-ray luminosity at 1 TeV to L < 5.1 × 1047 erg/s at 99% C.L.Item Characterizing the gamma-ray long-term variability of PKS2155 304 with HESS and Fermi-LAT(EDP Sciences, 2017-02) Chen, A.; Colafrancesco, S.; Jingo, M.; Collaboration, H.E.S.S.; Abdalla, H.; Abramowski, A.; Aharonian, F.; Ait Benkhali, F.; Akhperjanian, A.G.; Andersson, T.; Angüner, E.O.; Arrieta, M.; Aubert, P.; Backes, M.; Balzer, A.; Barnard, M.; Becherini, Y.; Becker Tjus, J.; Berge, D.; Bernhard, S.; Komin, Nu.Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in the high (HE, 100 MeV < E < 300 GeV) and very high energy (VHE, E > 200 GeV) γ-ray domain. Over the course of ∼9 yr of H.E.S.S. observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index βVHE = 1.10-0.13 +0.10) on timescales larger than one day. An analysis of ∼5.5 yr of HE Fermi-LAT data gives consistent results (βHE = 1.20-0.23 +0.21), on timescales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior (β ∼ 2) seen on shorter timescales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.Item Inelastic magnon scattering(Elsevier, 2017-05) De Mello Koch, R.; Van Zyl, H.J.R.We study the worldsheet S-matrix of a string attached to a D-brane in AdS5×S5. The D-brane is either a giant graviton or a dual giant graviton. In the gauge theory, the operators we consider belong to the su(2|3)sector of the theory. Magnon excitations of open strings can exhibit both elastic (when magnons in the bulk of the string scatter) and inelastic (when magnons at the endpoint of an open string participate) scattering. Both of these S-matrices are determined (up to an overall phase) by the su(2|2)2global symmetry of the theory. In this note we study the S-matrix for inelastic scattering. We show that it exhibits poles correspondingto boundstates of bulk and boundary magnons. A crossing equation is derived for the overall phase. It reproduces the crossing equation for maximal giant gravitons, in the appropriate limit. Finally, scattering in the su(2)sector is computed to two loops. This two loop result, which determines the overall phase to two loops, will be useful when a unique solution to the crossing equation is to be selected.Item Probing anomalous couplings using di-Higgs production in electron–proton collisions(Elsevier, 2017-01) Kumar, M.; Ruan, X.; Islam, R.; Klein, M.; Klein, U.; Mellado, B.; Cornell, A.S.A proposed high energy Future Circular Hadron-Electron Collider would provide sufficient energy in a clean environment to probe di-Higgs production. Using this channel we show that the azimuthal angle correlation between the missing transverse energy and the forward jet is a very good probe for the non-standard hhh and hhWW couplings. We give the exclusion limits on these couplings as a function of integrated luminosity at a 95% C.L. using the fiducial cross sections. With appropriate error fitting methodology we find that the Higgs boson self coupling could be measured to be g(hhh)((1)) = 1.00(-0.17(0.12))(+0.24(0.14)) of its expected Standard Model value at root s = 3.5(5.0) TeV for an ultimate 10 ab(-1) of integrated luminosity.Item Effects of spatial fluctuations in the extragalactic background light on hard gamma-ray spectra(Oxford University Press (OUP), 2017-06) Kudoda, A.M.; Faltenbacher, A.This study investigates the impact of the fluctuations in the extragalactic background light (EBL) on the attenuation of the hard gamma-ray spectra of distant blazars. EBL fluctuations occur on the scales up to 100 Mpc and are caused by the clustering of galaxies. The EBL photons interact with high-energy gamma-rays via the electron-positron pair production mechanism: gamma + gamma '-> e(+) + e(-). The attenuation of gamma-rays depends on their energy and the density of the intervening EBL photon field. Using a simple model for the evolution of the mean EBL photon density, we implement an analytical description of the EBL fluctuations. We find that the amplitudes of the EBL energy density can vary by +/- 1 per cent as a function of environment. The EBL fluctuations lead to mild alterations of the optical depth or equivalently the transmissivity for gamma-rays from distant blazars. Our model predicts maximum changes of +/- 10 per cent in the gamma-ray transmissivity. However, this translates into marginal differences in the power-law slopes of currently observed gamma-ray spectra. The slopes of deabsorbed gamma-ray spectra differ by not more than +/- 1 per cent if EBL fluctuations are included.Item Effect of the non-thermal Sunyaev–Zel’dovich effect on the temperature determination of galaxy clusters(Oxford University Press (OUP), 2017-08) Marchegiani, P.; Colafrancesco, S.A recent stacking analysis of Planck HFI data of galaxy clusters led to the derivation of the cluster temperatures using the relativistic corrections to the Sunyaev-Zel'dovich effect (SZE). However, the temperatures of high-temperature clusters, as derived from this analysis, were basically higher than the temperatures derived from X-ray measurements, at a moderate statistical significance of 1.5 sigma. This discrepancy has been attributed by Hurier to calibration issues. In this paper, we discuss an alternative explanation for this discrepancy in terms of a non-thermal SZE astrophysical component. We find that this explanation can work if non-thermal electrons in galaxy clusters have a low minimum momentum (p(1) similar to 0.5-1), and if their pressure is of the order of 20-30 per cent of the thermal gas pressure. Both these conditions are hard to obtain if the non-thermal electrons are mixed with the hot gas in the intracluster medium, but can be possibly obtained if the non-thermal electrons are mainly confined in bubbles with a high amount of non-thermal plasma and a low amount of thermal plasma, or are in giant radio lobes/relics in the outskirts of the clusters. To derive more precise results on the properties of the non-thermal electrons in clusters, and in view of more solid detections of a discrepancy between X-ray- and SZE-derived cluster temperatures that cannot be explained in other ways, it would be necessary to reproduce the full analysis done by Hurier by systematically adding the non-thermal component of the SZE.Item Nonlinear interglitch dynamics, the braking index of the Vela pulsar and the time to the next glitch(Oxford University Press (OUP), 2017-08) Akbal, O.; Buchner, S.; Pines, D.; Alpar, M.A.The interglitch timing of the Vela pulsar is characterized by a constant second derivative of the rotation rate. This takes over after the post-glitch exponential relaxation and is completed at about the time of the next glitch. The vortex creep model explains the second derivatives in terms of nonlinear response to the glitch. We present interglitch timing fits to the present sample covering 16 large glitches, taking into account the possibility that in some glitches part of the step in the spin-down ratemay involve a 'persistent shift', as observed in the Crab pulsar. Modifying the expression for the time between glitches with this hypothesis leads to better agreement with the observed interglitch time intervals. We extrapolate the interglitch model fits to obtain spin-down rates just prior to each glitch and use these to calculate the braking index n = 2.81 +/- 0.12. The next glitch should occur around 2017 December 22, +/- 197 d if no persistent shift is involved, but could occur as early as 2016 July 27, +/- 152 d if the 2013 glitch gave rise to a typical Vela persistent shift. Note added: Literally while we were submitting the first version of this paper on 2016 December 12, we saw ATel # 9847 announcing a Vela pulsar glitch which has arrived 138 d after our prediction with a persistent shift, within the 1 sigma uncertainty of 152 d.Item Confinement and diffusion time-scales of CR hadrons in AGN-inflated.(Oxford University Press (OUP), 2017-09) Prokhorov, D.A.; Churazov, E.M.While rich clusters are powerful sources of X-rays, γ -ray emission from these large cosmic structures has not been detected yet. X-ray radiative energy losses in the central regions of relaxed galaxy clusters are so strong that one needs to consider special sources of energy, likely active galactic nucleus (AGN) feedback, to suppress catastrophic cooling of the gas. We consider a model of AGN feedback that postulates that the AGN supplies the energy to the gas by inflating bubbles of relativistic plasma, whose energy content is dominated by cosmic-ray (CR) hadrons. If most of these hadrons can quickly escape the bubbles, then collisions of CRs with thermal protons in the intracluster medium (ICM) should lead to strong γ -ray emission, unless fast diffusion of CRs removes them from the cluster. Therefore, the lack of detections with modern γ -ray telescopes sets limits on the confinement time of CR hadrons in bubbles and CR diffusive propagation in the ICM.Item Systematics of quark/gluon tagging(Springer Verlag, 2017-07) Gras, P.; Höche, S.; Kar, D.; Larkoski, A.; Lönnblad, L.; Plätzer, S.; Siódmok, A.; Skands, P.; Soyez, G.; Thaler, J.By measuring the substructure of a jet, one can assign it a “quark” or “gluon” tag. In the eikonal (double-logarithmic) limit, quark/gluon discrimination is determined solely by the color factor of the initiating parton (CF versus CA). In this paper, we confront the challenges faced when going beyond this leading-order understanding, using both parton-shower generators and first-principles calculations to assess the impact of higher-order perturbative and nonperturbative physics. Working in the idealized context of electron-positron collisions, where one can define a proxy for quark and gluon jets based on the Lorentz structure of the production vertex, we find a fascinating interplay between perturbative shower effects and nonperturbative hadronization effects. Turning to proton-proton collisions, we highlight a core set of measurements that would constrain current uncertainties in quark/gluon tagging and improve the overall modeling of jets at the Large Hadron Collider.Item Mirror quintic vacua: hierarchies and inflation.(Springer Verlag, 2016-10) Bizet, N.C; Loaiza-Brito, O.; Zavala, IWe study the moduli space of type IIB string theory flux compactifications on the mirror of the CY quintic 3-fold in ℙ 4. We focus on the dynamics of the four dimensional moduli space, defined by the axio-dilaton τ and the complex structure modulus z. The z-plane has critical points, the conifold, the orbifold and the large complex structure with non trivial monodromies. We find the solutions to the Picard-Fuchs equations obeyed by the periods of the CY in the full z-plane as a series expansion in z around the critical points to arbitrary order. This allows us to discard fake vacua, which appear as a result of keeping only the leading order term in the series expansions. Due to monodromies vacua are located at a given sheet in the z-plane. A dS vacuum appears for a set of fluxes. We revisit vacua with hierarchies among the 4D and 6D physical scales close to the conifold point and compare them with those found at leading order in [1, 2]. We explore slow-roll inflationary directions of the scalar potential by looking at regions where the multi-field slow-roll parameters ϵ and η are smaller than one. The value of ϵ depends strongly on the approximation of the periods and to achieve a stable value, several orders in the expansion are needed. We do not find realizations of single field axion monodromy inflation. Instead, we find that inflationary regions appear along linear combinations of the four real field directions and for certain configurations of fluxes.
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