Time filter
Source Type

Boito D.,University of Sao Paulo | Jamin M.,IFAE | Miravitllas R.,IFAE
Physical Review Letters | Year: 2016

The quantum chromodynamics (QCD) coupling αs is not a physical observable of the theory, since it depends on conventions related to the renormalization procedure. We introduce a definition of the QCD coupling, denoted by α, whose running is explicitly renormalization scheme invariant. The scheme dependence of the new coupling α is parametrized by a single parameter C, related to transformations of the QCD scale Λ. It is demonstrated that appropriate choices of C can lead to substantial improvements in the perturbative prediction of physical observables. As phenomenological applications, we study e+e- scattering and decays of the τ lepton into hadrons, both being governed by the QCD Adler function. © 2016 American Physical Society.

Jamin M.,IFAE | Jamin M.,Catalan Institution for Research and Advanced Studies | Miravitllas R.,IFAE
Journal of High Energy Physics | Year: 2016

In this work, the perturbative QCD series of the scalar correlation function Ψ(s) is investigated. Besides ImΨ(s), which is relevant for Higgs decay into quarks, two other physical correlators, Ψ′′ (s) and DL(s), have been employed in QCD applications like quark mass determinations or hadronic τ decays. DL(s) suffers from large higher-order corrections and, by resorting to the large-β0 approximation, it is shown that this is related to a spurious renormalon ambiguity at u = 1. Hence, this correlator should be avoided in phenomenological analyses. Moreover, it turns out advantageous to express the quark mass factor, introduced to make the scalar current renormalisation group invariant, in terms of the renormalisation invariant quark mass m^ q.To further study the behaviour of the perturbative expansion, we introduce a QCD coupling α^ s, whose running is explicitly renormalisation scheme independent. The scheme dependence of α^ s is parametrised by a single parameter C, being related to transformations of the QCD scale parameter Λ. It is demonstrated that appropriate choices of C lead to a substantial improvement in the behaviour of the perturbative series for Ψ′′ (s) and ImΨ(s). © 2016, The Author(s).

Ortolan L.,IFAE
20th International Workshop on Deep-Inelastic Scattering and Related Subjects, DIS 2012 | Year: 2012

The CDF collaboration has an interesting and comprehensive program on the study of jet production in association with a Z/γ*boson. These measurements are important because Z/γ*+jets events are a background in many searches of new physics and in particular Z/γ*+b-jet represents the main and irreducible background for Higgs boson produced in association with a Z/γ* where the Higgs decays in two b quarks. In this contribution new measurements of Z/γ*+jets differential cross section and the Z/γ*+b-jet cross section ratio with respect to the Z inclusive cross section are presented. The results are performed with the complete dataset collected at CDF and are compared with the next-to-leading order predictions.

Bednarek W.,University of Lodz | Sitarek J.,IFAE
Monthly Notices of the Royal Astronomical Society | Year: 2013

We consider nebulae which are created around binary systems containing rotation-powered pulsars and companion starswith strong stellar winds. It is proposed that the stellar and pulsar winds have to mix at some distance from the binarysystem, defined by the orbital period of the companion stars and the velocity of the stellar wind. The mixed pulsar-stellar wind expands with a specific velocity determined by the pulsar power and the mass loss rate of the companion star. Relativistic particles, either from the inner pulsar magnetosphere and/or accelerated at the shocks between stellar and pulsar winds, are expected to be captured and isotropized in the reference frame of the mixed wind. Therefore, they can efficiently Comptonize stellar radiation producing GeV-TeV γ -rays in the inverse Compton process. We calculate the γ -ray spectra expected in such scenario for the two example binary systems: J1816+4510 which is the Redback-type millisecondbinary and LS 5039 which is supposed to contain energetic pulsar. It is concluded that the steady TeV γ -ray emission from J1816+4510 should be on the 100 h sensitivity limit of the planned Cherenkov Telescope Array, provided that ε ~ 10 per cent of the rotational energy lost by the pulsar is transferred to TeV electrons. On the other hand, the comparison of the predicted steady TeV γ -ray emission, expected from γ -ray binary LS 5039, with the observations of the TeV emission in a low state, reported by the High Energy Stereoscopic System Collaboration, allows us to put stringent upper limiton the product of the part of the hemisphere in which the mixed pulsar-stellar wind is confined, δmix, and the energy conversion efficiency, ε, from the supposed pulsar to the TeV electrons injected in this system, δmixε < 1 per cent. This lower limit can be understood provided that either the acceleration efficiency of electrons is rather low (ε ~ 1 percent), or the parameters of the stellar wind from the companion star are less extreme than expected, or the injection/acceleration process of electrons occurs highly anisotropically with the predominance towards the companion star. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Reichardt I.,IFAE | De Ona-Wilhelmi E.,Max Planck Institute for Nuclear Physics | De Ona-Wilhelmi E.,Now At Institute Of Ciencies Of Lespai Ieec Csic | Rico J.,IFAE | And 2 more authors.
Astronomy and Astrophysics | Year: 2012

We analyze 3.5 years of public Fermi/LAT data around the position of the supernova remnant HB 21, where four point-like sources from the 2nd Fermi/LAT catalog are located. We determine that the gamma-ray source is produced by a single extended source. We model the observed morphology as a uniform circle. The spectral energy distribution is best described by a curved power law, with a maximum at 413 ± 11 MeV. We divide the circle into three regions defined by previously identified shocked molecular clouds, and find that one of these regions has a softer spectrum. The >3 GeV gamma-ray emission of the soft spectrum region is bow-shaped and coincident with the supernova remnant shell seen at radio wavelengths. We suggest that the gamma-ray emission from HB 21 can be understood as a combination of emission from shocked/illuminated molecular clouds, one of them coincident with the supernova remnant shell itself. ©2012 ESO.

Szanecki M.,University of Lodz | Bernlohr K.,Max Planck Institute for Nuclear Physics | Sobczynska D.,University of Lodz | Niedzwiecki A.,University of Lodz | And 2 more authors.
Astroparticle Physics | Year: 2013

We investigate the influence of the geomagnetic field (GF) on the Imaging Air Cherenkov Telescope technique for two northern (Tenerife and San Pedro Martir) and three southern (Salta, Leoncito and Namibia (the H.E.S.S.-site)) site candidates for Cherenkov Telescope Array (CTA) observatories. We use the CORSIKA and sim-telarray programs for Monte Carlo simulations of gamma ray showers, hadronic background and the telescope response. We focus here on gamma ray measurements in the low energy, sub-100 GeV, range. Therefore, we only consider the performance of arrays of several large telescopes. Neglecting the GF effect, we find (in agreement with previous studies) that such arrays have lower energy thresholds, and larger collection areas below 30 GeV, when located at higher altitudes. We point out, however, that in the considered ranges of altitudes and magnetic field intensities, 1800-3600 m a.s.l. and 0-40 μT, respectively, the GF effect has a similar magnitude to this altitude effect. We provide the trigger-level performance parameters of the observatory affected by the GF effect, in particular the collection areas, detection rates and the energy thresholds for all five locations, which information may be useful in the selection of sites for CTA. We also find simple scaling of these parameters with the magnetic field strength, which can be used to assess the magnitude of the GF effect for other sites; in this work we use them to estimate the performance parameters for five sites: South Africa-Beaufort West, USA-Yavapai Ranch, Namibia-Calapanzi, Chile-La Silla and India-Hanle. We roughly investigate the impact of the geophysical conditions on gamma/hadron separation procedures involving image shape and direction cuts. We note that the change of altitude has an opposite effect at the trigger and analysis levels, i.e. gains in triggering efficiency at higher altitudes are partially balanced by losses in the separation efficiency. In turn, a stronger GF spoils both the shape and the direction discrimination of gamma rays, thus its effects at the trigger and analysis levels add up resulting in a significant reduction of the observatory performance. Overall, our results indicate that the local GF strength at a site can be equally important as its altitude for the low-energy performance of CTA. © 2013 Elsevier B.V. All rights reserved.

Sitarek J.,IFAE | Bednarek W.,University of Lodz
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

The Fermi-LAT telescope has unexpectedly discovered GeV γ-ray emission from the symbiotic Nova V407 Cygni. We investigate the radiation processes due to electrons and hadrons accelerated during the explosion of this nova. We consider a scenario in which GeV γ-ray emission observed by Fermi is produced by the electrons with energies of a few tens of GeV in the inverse Compton scattering of stellar radiation. On the other hand, the hadrons are expected to reach larger energies, due to the lack of radiation losses during acceleration process, producing TeV γ rays and neutrinos. We predict the fluxes of very high energy γ rays and neutrinos from novas of the V407 type for two models of hadron acceleration and discuss their possible detectability by the present and future telescopes (e.g., IceCube, CTA). © 2012 American Physical Society.

Klepser S.,IFAE
Astroparticle Physics | Year: 2012

Astrophysical sources of TeV gamma rays are usually established by Cherenkov telescope observations. These counting type instruments have a field of view of few degrees in diameter and record large numbers of particle air showers via their Cherenkov radiation in the atmosphere. The showers are either induced by gamma rays or diffuse cosmic ray background. The commonly used test statistic to evaluate a possible gamma-ray excess is Li and Ma [6], Eq. (17), which can be applied to independent on- and off-source observations, or scenarios that can be approximated as such. This formula however is unsuitable if the data are taken in so-called "wobble" mode (pointing to several offset positions around the source), if at the same time the acceptance shape is irregular or even depends on operating parameters such as the pointing direction or telescope multiplicity. To provide a robust test statistic in such cases, this paper explores a possible generalization of the likelihood ratio concept on which the formula of Li and Ma is based. In doing so, the multi-pointing nature of the data and the typically known instrument point spread function are fully exploited to derive a new, semi-numerical test statistic. Due to its flexibility and robustness against systematic uncertainties, it is not only useful for detection purposes, but also for skymapping and source shape fitting. Simplified Monte Carlo simulations are presented to verify the results, and several applications and further generalizations of the concept are discussed.© 2012 Elsevier B.V. All rights reserved.

Michela I.,IFAE
Proceedings of the 15th Lomonosov Conference on Elementary Particle Physics: Particle Physics at the Tercentenary of Mikhail Lomonosov | Year: 2013

The T2K experiment studies the phenomenon of neutrinos oscillations as they travel from the Japan Proton Accelerator Research Complex (J-PARC) in Tokai (Japan) to the Super-K detector in the Kamioka Mine, 295 Km away. The aim of the experiment is to measure the mixing angle θ13 using the νe appearance and to improve the atmospheric parameters from the νμ disappearance measurements. The neutrino beam created at J-PARC is monitored by the near detectors site located at 280m downstream of neutrino target. It is split in two parts: the on-axis INGRID detector and the main off-axis detectors ND280. In this article we will briefly describe the setup of near detectors and the analyses finalized in 2010 used as input to the T2K oscillation studies. © 2013 by World Scientific Publishing Co. Pte. Ltd.

News Article | November 8, 2016
Site: www.eurekalert.org

In a study published last Friday in the journal Astronomy & Astrophysics, scientists of the international collaboration of the MAGIC telescopes (Major Atmospheric Gamma Imaging Cherenkov Telescope), located at the Roque de los Muchachos Observatory, in Garafía (La Palma), and among which are researchers of the Instituto de Astrofísica de Canarias (IAC), have announced the discovery of gamma-ray emission more distant than any previous detection. The discovery was made possible by the gravitational lensing caused by a massive galaxy between the quasar and Earth, that "repeated" the light produced by the source. According to Einstein's General Relativity, light is deflected passing close to a large mass. To a distant observer the mass focuses light like a giant lens. The result is a much brighter, although distorted, image of the source and a chance to see distant objects that might otherwise be far too faint to detect. And just like a lens, light can pass through the lens with slightly different path lengths. On cosmic scales, this means photons -- parcels of light -- traveling along different lines of sight arrive at slightly different times. If, in addition, the source is variable, this is "imprinted" on the light with a time delay relative to a fixed first arrival. And this should not depend on the energy of the photons, according to the theory. That makes such observations especially important. QSO B0218+357 harbors a supermassive black hole in a galaxy located halfway across the Universe from Earth. Over 7 billion years ago a gigantic explosion occurred in this object, which led to the emission of an intense flare of gamma rays, which is the highest-energy form of light. In its long journey toward Earth, these photons passed in the vicinity of a foreground -- still distant -- galaxy, B0218+357G, over one billion years later. In passing and being deflected, those photons traveling along the shorter path finally arrived at Earth on July 14th, 2014 and were observed by the Large Area Telescope on board the orbiting Fermi satellite, which scans the entire sky every 3 hours. The detection of this gamma-ray outburst alerted the astronomical community, and many telescopes worldwide were immediately pointed at QSO B0218+357 to learn more from this distant cosmic explosion. Researchers operating the MAGIC telescopes, located on La Palma in the Canary Islands, became excited about the possible observation of this object in very-high-energy gamma rays. These could provide the most extreme perspective of this outburst, but, unfortunately, at that time there was full moon in La Palma, which prevented the operation of the MAGIC telescopes. The MAGIC telescopes measure very-high-energy gamma rays, which are a thousand times more energetic than those measured by Fermi, and a hundred billion times more energetic than any light we see from our Sun. From the earlier measurements of this object in 2012 by Fermi and by radio telescopes the scientists knew that photons arriving along the longer path should arrive about 11 days later. "In other words, Nature could award us with a replay, a second chance to look at the same interesting phenomenon." says the MAGIC Collaboration member Julian Sitarek (University of ?ódz, Poland and IFAE former member of the Institut de Fisica d'Altes Energies in Barcelona, Spain, when he started this project) who led this study, and continues: "When the time came, the MAGIC telescopes were pointed at QSO B0218+357, and, in accordance with the prediction, a flare of very-high-energy gamma rays was observed, making QSO B0218+357 the most distant object detected in the very-high-energy gamma-ray domain to date." These very-high-energy gamma-rays from any distant source have a high chance to interact with the numerous low-energy photons emitted by galaxies and stars, being lost in the process. With this observation, MAGIC has doubled the previously known visibility range of the Universe in very-high-energy gamma rays. Observation of the delayed signal from QSO B0218+357 by MAGIC showed for the first time that these very energetic photons are also deflected in agreement with General Relativity, a result that is both striking and potentially profound. The signal arriving at the predicted time may rule out some theories of the structure of the vacuum. That awaits further analysis. For the moment, this observation demonstrates a new capability of the very-high-energy gamma-ray observatories and highlights what awaits the next generation of such telescopes, the Cherenkov Telescope Array (CTA) project. MAGIC is a ground-based gamma-ray instrument located on the Canary island of La Palma, Spain. The system of two 17m diameter Cherenkov telescopes is currently one of the three major imaging atmospheric Cherenkov instruments in the world. It is designed to detect gamma rays tens of billions to tens of trillions times more energetic than visible light. MAGIC has been built with the joint efforts of a largely European collaboration that includes about 160 researchers from Germany, Spain, Italy, Switzerland, Poland, Finland, Bulgaria, Croatia, India and Japan. The Observatories of the Instituto de Astrofísica de Canarias (IAC) are part of the Unique Scientific And Technical Infraestructures (ICTS) network.

Loading IFAE collaborators
Loading IFAE collaborators