Neutron Stars Collide

Shortly after 8:41 a.m. EDT on Aug. 17, 2017, Fermi's Gamma-ray Burst Monitor detected a pulse of high-energy light from a powerful explosion, which was immediately reported to astronomers around the globe as a short gamma-ray burst. Less that two seconds earlier, scientists at the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational-waves dubbed GW170817 from a pair of smashing super-dense stars, encouraging astronomers to look for the aftermath of the explosion. Shortly thereafter, the burst was detected as part of a follow-up analysis by ESA's (European Space Agency's) INTEGRAL satellite. NASA's Swift, Hubble, Chandra, and Spitzer missions, along with dozens of ground-based observatories, later captured the fading glow of the blast's expanding debris.

This is the first time scientists have conclusively detected light associated with a gravitational-wave event, thanks to two merging neutron stars in the galaxy NGC 4993, located about 130 million light-years from Earth. The figure shows an artist's impression of the aftermath of this merger.

Neutron stars are the crushed, leftover cores of massive stars that exploded as supernovae long ago. The merging stars likely had masses between 10 and 60 percent greater than that of our Sun, but they were no wider than Washington, D.C. The pair whirled around each other hundreds of times a second, producing gravitational-waves at the same frequency. As they drew closer and orbited faster, the stars eventually broke apart and merged, producing both a gamma-ray burst and a rarely seen flare-up called a "kilonova."

GW170817 showed astronomers that heavy elements found on Earth like gold, platinum, and uranium were likely produced kilonovae long ago.

» https://svs.gsfc.nasa.gov/12740

Gamma-ray Bubbles

Using data from Fermi's Large Area Telescope, scientists have discovered a gigantic, mysterious structure in our galaxy. This never-before-seen feature looks like a pair of bubbles extending above and below our galaxy's center, as shown in false color in this figure.

Each lobe is 25,000 light-years tall, and the whole structure may be only a few million years old. Within the bubbles, extremely energetic electrons are interacting with lower-energy light to create gamma-rays, but right now, no one knows the source of these electrons.

Are the bubbles remnants of a massive burst of star formation? Are they leftovers from an eruption by the supermassive black hole at our galaxy's center? Or did these forces work in tandem to produce them? Scientists aren't sure yet, but the more they learn about this amazing structure, the better we'll understand the Milky Way.

https://svs.gsfc.nasa.gov/10688

Neutrino asociado con blazar distante

Con equipos congelados en el hielo profundo bajo el polo Sur de la Tierra, la humanidad parece haber descubierto un neutrino procedente del Universo lejano. Si se confirma, esto supondría la primera detección clara de neutrinos cosmológicamente distantes así como la primera asociación observada entre energéticos neutrinos y rayos cósmicos creados por potentes chorros provenientes de cuásares (blázares). Después de que el detector del IceCube Antártico midiera un neutrino energético en septiembre de 2017, muchos observatorios importantes comenzaron a trabajar con el fin de identificar una contraparte en la luz. Y lo hicieron. Una contraparte en erupción fue identificada por observatorios de alta energía, como AGILE, Fermi, HAWC, HESS, INTEGRAL, Ñusta, Swift y VERITAS, que encontraron que el rayo gamma Blazer TXS 0506 + 056 se encontraba en la dirección correcta y con los rayos gamma procedentes de una erupción que llegan casi al mismo tiempo que el neutrino. Aunque esta y otras coincidencias en el momento y en la posición sean estadísticamente fuertes, los astrónomos esperan encontrar otras asociaciones neutrino-blazar similares para estar completamente seguros.
La imagen ilustra un chorro de partículas que emana de un agujero negro en el centro de un blazar.

Fuente: http://observatorio.info/2018/07/neutrino-asociado-con-blazar-distante/

The Higgs boson reveals its affinity for the top quark

04 Jun 2018

New results from the ATLAS and CMS experiments at the LHC reveal how strongly the Higgs boson interacts with the heaviest known elementary particle, the top quark, corroborating our understanding of the Higgs and setting constraints on new physics.

Geneva, 4 June 2018. The Higgs boson interacts only with massive particles, yet it was discovered in its decay to two massless photons. Quantum mechanics allows the Higgs to fluctuate for a very short time into a top quark and a top anti-quark, which promptly annihilate each other into a photon pair. The probability of this process occurring varies with the strength of the interaction (known as coupling) between the Higgs boson and top quarks. Its measurement allows us to indirectly infer the value of the Higgs-top coupling. However, undiscovered heavy new-physics particles could likewise participate in this type of decay and alter the result. This is why the Higgs boson is seen as a portal to new physics.

A more direct manifestation of the Higgs-top coupling is the emission of a Higgs boson by a top-antitop quark pair. Results presented today, at the LHCP conference in Bologna, describe the observation of this so-called "ttH production" process. Results from the CMS collaboration, with a significance exceeding five standard deviations (considered the gold standard) for the first time, have just been published in the journal Physical Review Letters; including more data from the ongoing LHC-run, the ATLAS collaboration just submitted new results for publication, with a larger significance. Together, these results are a great step forward in our knowledge of the properties of the Higgs boson. The findings of the two experiments are consistent with one another and with the Standard Model, and give us new clues for where to look for new physics.

“These measurements by the CMS and ATLAS Collaborations give a strong indication that the Higgs boson has a key role in the large value of the top quark mass. While this is certainly a key feature of the Standard Model, this is the first time it has been verified experimentally with overwhelming significance,” said Karl Jakobs, Spokesperson of the ATLAS collaboration.

“The CMS analysis teams, and their counterparts in ATLAS, employed new approaches and advanced analysis techniques to reach this milestone. When ATLAS and CMS finish data taking in November of 2018, we will have enough events to challenge even more strongly the Standard Model prediction for ttH, to see if there is an indication of something new,” declared Joel Butler, Spokesperson of the CMS collaboration.

Measuring this process is challenging, as it is rare: only 1% of Higgs bosons are produced in association with two top quarks and, in addition, the Higgs and the top quarks decay into other particles in many complex ways, or modes. Using data from proton–proton collisions collected at energies of 7, 8, and 13 TeV, the ATLAS and CMS teams performed several independent searches for ttH production, each targeting different Higgs-decay modes (to W bosons, Z bosons, photons, τ leptons, and bottom-quark jets). To maximise the sensitivity to the experimentally challenging ttH signal, each experiment then combined the results from all of its searches.

It is gratifying that this result has come so early in the life of the LHC programme. This is due to the superb performance of the LHC machine, and of the ATLAS and CMS detectors, the use of advanced analysis techniques and the inclusion of all possible final states in the analysis. However, the precision of the measurements still leaves room for contributions from new physics. In the coming years, the two experiments will take much more data and improve the precision to see if the Higgs reveals the presence of physics beyond the Standard Model.

“The superb performance of the LHC and the improved experimental tools in mastering this complex analysis led to this beautiful result,” added CERN1 Director for Research and Computing Eckhard Elsen. “It also shows that we are on the right track with our plans for the High-Luminosity LHC and the physics results it promises.”

For more information:
On the CMS portal
On the ATLAS portal
Video about the results

An event candidate for the production of a top quark and top anti-quark pair in conjunction with a Higgs Boson in CMS. The Higgs decays into a  tau+ lepton, which in turn decays into hadrons and a tau- , which decays into an electron. The decay product symbols are in blue. The top quark decays into three jets (sprays of lighter particles) whose names are given in purple. One of these is initiated by a b-quark. The top anti-quark decays into a muon and b-jet, whose names appear in red.
 

Visualization of a data event from the tt ̄H(γγ) Had BDT bin with the largest signal over background ratio. The event contains two photon candidates, with a diphoton mass of 125.4 GeV. In addition, six jets are reconstructed using the anti-kt algorithm and R = 0.4, including one jet that is b-tagged using a 77% efficiency working point. The photons correspond to the green towers in the electromagnetic calorimeter, while the jets (b-jets) are shown as yellow (blue) cones.

Footnote(s)

1. CERN, the European Organization for Nuclear Research, is one of the world's leading laboratories for particle physics. The Organization is located on the French-Swiss border, with its headquarters in Geneva. Its Member States are: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Spain, Sweden, Switzerland and United Kingdom. Cyprus, Serbia and Slovenia are Associate Member States in the pre-stage to Membership. India, Lithuania, Pakistan, Turkey and Ukraine are Associate Member States. The European Union, Japan, JINR, the Russian Federation, UNESCO and the United States of America currently have Observer status.