This article is a not-too-technical exposition to the Higgs boson. This installment gives a summary of the Higgs mechanism and an introduction to CERN, the LHC and the experiments.
Source/Credit: The Daily Start
By Dr. Lashkar Kashif | January 8, 2013
The Higgs boson, dubbed the 'God Particle', has been much in the news during 2012. The discovery, announced on July 4, became Science magazine's Discovery of the Year, and the boson was briefly considered for the honor of Time's Person Of The Year.
Being a member of the ATLAS experiment at CERN and directly involved in a Higgs discovery analysis, I was fortunate in getting a first-hand experience of the tremendous excitement and elation that have pervaded the atmosphere here over the past months.
This article is a not-too-technical exposition to the Higgs boson. This installment gives a summary of the Higgs mechanism and an introduction to CERN, the LHC and the experiments.
The Higgs mechanism
The Higgs mechanism is an integral part of the Standard Model of Particle Physics, which attempts to describe all elementary particles and their interactions. Elementary particles are entities that cannot be broken down further; to the best of our current knowledge, these are six quarks, six leptons, four force carriers or 'gauge bosons', and the newly-discovered Higgs boson. The Standard Model was devised in the 1960s, principally owing to the work of Sheldon Glashow, Steve Weinberg and the late Abdus Salam from Pakistan. As an aside, the word 'boson' was coined after our own Satyen Bose, a Bengali and a former professor and Chair in the Physics Department at Dhaka University. Prof. Abdus Salam was a Foreign Fellow of the Bangladesh Academy of Sciences, and remains the only Muslim to have received a physics Nobel Prize.
A central question that the Standard Model must answer is: how do particles acquire mass? In particular, the Model contains the massless photon, and two other gauge bosons named the W and the Z, which are massive. Since all three are carriers of the electroweak interaction, how would the W and the Z acquire mass if the photon is to remain massless? The Higgs mechanism, first proposed in 1964 by Peter Higgs and others, was adapted by Weinberg and Salam to answer this question. (The story is a bit more complicated, but this captures the essence.) The mechanism invokes a Higgs field, whose presence breaks the electroweak symmetry in such a way as to yield one massless boson and two massive bosons, one of which has so-called longitudinal and transverse polarization modes. The process is known as spontaneous symmetry breaking, occurs in many contexts of physics, and is one of the most elegant theoretical mechanisms ever developed.
We physicists adore economy. We want to economize on the content of any theory: the fewer parameters, particles and interactions needed in a theory, the better. Therefore, once the Higgs mechanism was invoked to explain the masses of the gauge bosons, the natural question was whether it can explain the masses of all elementary particles. It turns out that, within the Standard Model, it can do so. Strictly speaking, the Higgs mechanism is not necessary to generate masses for the quarks and leptons, but it is economic if a single mechanism can explain their masses in addition to those of the massive bosons, and so it came about that this mechanism was hypothesized to give masses to all elementary particles.
So much for the Higgs mechanism; what is the Higgs boson? In particle physics, every field is associated with a field particle, and the Higgs boson is the particle associated with the Higgs field. More formally, the spontaneous breaking of electroweak symmetry leaves an elementary scalar particle in addition to the gauge bosons, this scalar being the Higgs boson. By the mid-1990s, all particles of the Standard Model had been discovered, except for the Higgs. But this was a crucial missing part, since unless the existence of the Higgs boson was confirmed, we would not know the mechanism of electroweak symmetry breaking, upon which the Standard Model is largely based. The Large Electron Positron (LEP) collider in Europe and the Tevatron collider in the US were unable to find the Higgs after two decades of search. Then the Large Hadron Collider turned on in 2010.
CERN and the Large Hadron Collider
The European Council for Nuclear Research (CERN) is located partly in Geneva, Switzerland and partly in nearby France. It has twenty member states, all European countries, plus a number of so-called observer states. Together, the member and observer states provided the funding and technical expertise required to build the Large Hadron Collider (LHC), easily the most complicated single machine ever built. The LHC is a circular collider, 27 kilometers in circumference, situated 80-175 meters underground, and is by far the highest-energy particle accelerator on earth. It collides protons with protons at a center-of-mass energy of 8 TeV, which is the energy particles had less than a 100 trillionths of a second after the Big Bang. To rephrase, the LHC is able to recreate the conditions in the universe immediately after the Big Bang.
The LHC, however, is simply a tool to collide protons; we need more than the LHC to do physics. The collisions create heavy particles that have not existed since right after the Big Bang, and these particles decay to lighter particles that we can detect. This detection requires massive particle detectors, also called experiments. There are four particle detectors at the LHC, with cryptic names like ATLAS, CMS, ALICE and LHCb. ATLAS and CMS are so-termed general-purpose detectors, built to enable us to perform a wide range of physics analyses. Being a member of the ATLAS collaboration, I will give a short introduction to the ATLAS detector. ATLAS stands for A Toroidal Lhc ApparatuS, the largest particle detector ever constructed. A cylindrical apparatus built in a number of layers, it stands 7 stories tall, weighs about 7000 tons, and took 22 years to design, prototype, build, assemble and commission. The detector has close to a 100 million data-taking channels, which record particle collisions occurring 400 million times every second, 24 hours a day. At this time, about 3200 physicists from 176 institutes in 38 countries are members of the ATLAS collaboration. The CMS detector and collaboration are of similar size and complexity.
The first physics run of the LHC started in March 2010 and ended in December 2012, with some very impressive results. A major goal of the run was the discovery of the Higgs boson, which was achieved independently by the ATLAS and CMS collaborations. I want to emphasize here that the driving force behind this accomplishment was not senior scientists but young physicists: PhD students and post-doctoral researchers.
The writer did his undergraduate studies at Yale University, received a PhD in Physics from Harvard University, and is now a postdoctoral researcher with CERN/University of Wisconsin-Madison. He is based at CERN in Geneva, Switzerland.
Read original post here: Faith and Science: Chasing the Higgs boson - How it all began
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