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Categories : bensinger lab, blocker lab, cern, Higgs boson, high-energy physics, lawrence group, sciolla lab, wardle group
Three hundred feet beneath Geneva, Switzerland, lies the world’s biggest machine, so vast it spills over the neighboring French border — not just in one place, but in four separate locations.
The Large Hadron Collider (LHC) is the biggest and baddest physics experiment ever created. Inside a subterranean tunnel, a 17-mile ring of high-powered magnets flings subatomic particles along at nearly the speed of light, slamming them together in spectacular collisions that can reveal their basic building blocks.
To the untrained eye, it looks like the Death Star crammed into a mine shaft.
To experimental physicists like Brandeis’ Jim Bensinger, Craig Blocker and Gabriella Sciolla, though, the collider embodies the greatest aspirations of particle physics: the quest to understand, as closely as possible, the forces that are locked inside the most infinitesimal specks of matter, and the laws that govern that matter.
Read the entire Brandeis Magazine article.
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Categories : cern, graduate students, Higgs boson, high-energy physics
by Serdar Gozpinar
The Higgs Boson is an elementary particle predicted by the Standard Model of particle physics. It is associated with the Higgs mechanism that was developed with the contributions of physicists Brout, Englert, Guralnik, Hagen, Kibble and Higgs in 1964. This mechanism proposed an elegant solution to one of the standing problems in particle physics – how particles acquire mass. The elementary particles of nature have masses varying by many orders of magnitude. For instance, the top quark, the heaviest known particle yet, has a mass that is roughly 340000 times that of the electron. Neutrinos on the other hand, have masses that are constrained by experiment to only a small fraction of the electron mass. The Higgs mechanism suggests that all particles acquire their mass by the strength of their couplings to a field that permeates the whole universe – the Higgs field. The top quark does not get its higher mass because it is bigger in size; in fact it is probably no larger than an electron. It simply couples more strongly to the Higgs field to get its enormous mass at around 173 GeV. Photons do not couple to this field at all and hence are massless. The much sought after Higgs boson is the quanta of this hypothetical field, very similar to the photon being the quanta of the electromagnetic field. One of the main goals of the LHC physics program is to find evidence of this quanta, the Higgs boson.
Higgs bosons, if they exist, can be created in proton – proton collisions at the LHC but would decay instantaneously. Any search for them has to be carried out by analyzing data for signatures of Higgs decay. This is not a trivial task however, because such decays are impossible to distinguish from some other well known Standard Model processes. Collision data from LHC would have to be analyzed, and deviations from known background processes would point towards some form of new physics and possibly the existence of the Higgs boson. One would then have to compare the rates in different Higgs decay channels to see if the observed rates match those from predictions for a Higgs decay.
In December 2011, ATLAS and CMS, the two main experiments at LHC, announced their 2011 Higgs search results which led to much hype in the physics world and the media – both experiments were indeed seeing some excess in several Higgs decay channels around 125 GeV. The mass of a proton is about 0.94 GeV, so the finding suggests a Higgs boson mass that is about 133 times heavier than the proton (notice the bump in the above plot from ATLAS at around 125 GeV).
However, it is still very early to come to a conclusive statement about a discovery. Standard Model backgrounds are subject to statistical fluctuations which show themselves at various levels in experiments. A discovery would only be claimed if the deviations from expected backgrounds are over 5 standard deviations. The results announced by CMS and ATLAS in December 2011 were 1.9 and 2.6 standard deviations above expected backgrounds respectively. Though this is interesting, it is far from sufficient to provide a conclusive answer about the existence of the Higgs boson. The search will have to continue in 2012 as more data from LHC becomes available, and physicists will finally be able to give a conclusive answer to either confirm or exclude the Standard Model Higgs boson, ending a puzzle that has been around for several decades.
Serdar Gozpinar is a 5th year graduate student in the high-energy physics group at Brandeis working on the ATLAS experiment at CERN.