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In a paper published in the journal Nature Physics on Monday, the Compact Muon Solenoid (CMS) experiment at the European Organization for Nuclear Research (CERN) reports new results on an important property of the Higgs particle, the discovery of which was announced by the ATLAS (A Toroidal LHC Apparatus) and CMS experiments on July 4, 2012. The new CMS result follows preliminary results from both experiments, each of which both reported strong evidence for the Higgs boson’s decay into fermions late in 2013. The discovery of the Higgs boson validated a half-century-old theory about how subatomic particles acquire mass. It is associated with a mechanism first put forward in 1964 by Robert Brout, François Englert and Peter Higgs to account for the different ranges of two fundamental forces of nature. “The main reason for the Higgs boson idea initially was to explain how the very heavy (W and Z) bosons got heavy,” said Joseph Incandela, UC Santa Barbara professor of physics and spokesman for the CMS experiment at the time of the Higgs boson discovery. The Higgs boson is unique in that it presents zero angular momentum, or “spin,” while other bosons exhibit integer — whole number — values of spin, and fermions have half-integer values of spin. Later, explained Incandela, the idea was adapted for use to give mass to fermions. But, in order to test that idea fully, it was necessary to measure the direct decay of the Higgs boson into all kinds of particles. The Higgs boson discovery was based on measurements of the decay of the Higgs to other bosons, the carriers of nature’s forces. The recent results reported by ATLAS and CMS discuss the decay of Higgs bosons directly to fermions, the particles that make up matter. Now referred to as BEH (Brout-Englert-Higgs), this mechanism is postulated to give rise to the masses of all the fundamental particles. “When Fabiola Gianotti — the former spokeswoman for the ATLAS experiment — and I announced the discovery for our respective experiments, the evidence for a signal was only clear in the decays to bosons,” Incandela said. “It is harder to see the decays to fermions so this was not so surprising, but it was a major open question that was on my mind that day, and one that we knew we’d have the opportunity to address with more data.” The measurements from both experiments have given substantial evidence that the Higgs boson decays directly to fermions at a rate consistent with that predicted by the Standard Model of particle physics, the theory that accounts for the fundamental particles of visible matter and the interactions that work between them, giving structure to matter. “With our on-going analyses, we are really starting to understand the BEH mechanism in depth,” CMS spokesperson Tiziano Camporesi said. “So far, it is behaving exactly as predicted by theory.” It could have been that another particle would be required for the fermions, said Incandela, but the result not only confirms the Higgs’ link to both fermions and bosons, but also the powerful simplicity of nature. “This result verifies to us in a direct and clear way that nature has to a large extent chosen the most simple and efficient possible way to generate mass,” he said. There’s more to come, according to Incandela, including further research to address theories such as supersymmetry and predictions that many Higgs bosons exist that behave like the ones being studied. “These results show the power of the detectors in allowing us to do precision Higgs physics,” ATLAS spokesman Dave Charlton said. “We’re coming close to achieving all we can on the Higgs analysis with Run 1 data, and are all looking forward to new data when the LHC restarts in 2015.” The Large Hadron Collider, CERN’s flagship research facility, has been off-line for maintenance and upgrading in last 18 months. Preparations are under way for it to restart early in 2015. More information will be available as results are announced at the International Conference on High Energy Physics (ICHEP), which begins at Valencia, Spain, on July 2.
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ZAGREB - Naše aktualno razumijevanje fizike čestica počiva na Standardnom Modelu, razvijenom u sedamdesetim godinama prošlog stoljeća, koji je uspješno opisao sve elementarne čestice poznate u to vrijeme, i odnose među njima. Standardni model također je predviđao postojanje i čestica koje do tada još nisu bile opažene, poput b kvarka, t kvarka i tau neutrina. Rolf Heuer, generalni direktor CERN-a: Mislim da ga imamo. Imamo otkriće! Standardni model također je predviđao i postojanje Higgsovog bozona, čestice čija je teoretska razina energije bila iznad razine koju su ljudi mogli postići u akceleratorima čestica prije pokretanja LHC-a. Ukoliko Higgsov bozon ne bi bio pronađen, to bi značilo da postoje funtamentalne pogreške u Standardnom Modelu, te da je potrebna nova teorija fizike čestica koja bi objasnila zašto čestice imaju masu. Otkriće Higgsovog bozona, s druge strane, konačna je potvrda znanstvenicima da Standardni Model točno opisuje svijet koji nas okružuje. Standardni Model Prema Standardnom Modelu, sve čestice su ili fermioni, od kojih se sastoji sva materija, ili bozoni, koji služe kao nositelji sile. Fermioni su kvarkovi, koji sačinjavaju 'klasične' subatomske čestice poput protona i neutrona, te leptoni, poput elektrona ili neutrina. Standardni Model matematički je predvidio postojanje šest kvarkova - u, c, t, d, s i b, čije je postojanje i eksperimentalno potvrđeno, te šest leptona - elektron, muon, tau, elektron neutrino, muon neutrino i tau neutrino. Eksperimentalno je potvrđeno i postojanje četiri bozona - fotona, koji prenosi informacije o elektromagnetskoj sili, gluona, koji djeluje na kvarkove i prenosi informacije o jakoj nuklearnoj sili, te W i Z bozona, koji prenose informacije o slaboj nuklearnoj sili. Higgsov bozon, prema Standardnom Modelu, djeluje na druge čestice te im 'daje' masu - tj. interakcija između Higgsovog bozona i drugih čestica određuje kolika je njihova masa. Jedina preostala neotkrivena čestica Standardnog Modela je graviton, kojeg vjerojatno nikada neće biti moguće izravno eksperimentalno potvrditi, zbog njegove iznimno slabe interakcije sa svim drugim poznatim česticama. No, otkriće Higgsovog bozona nedvosmisleno je potvrdilo ispravnost teorije Standardnog Modela, te možemo biti gotovo sigurni kako i gravitoni postoje.