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The higgs particle - a useful analogy for physics classrooms
The higgs particle - a useful analogy for physics classrooms
The higgs particle - a useful analogy for physics classrooms
The higgs particle - a useful analogy for physics classrooms
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The higgs particle - a useful analogy for physics classrooms


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  • 1. This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: This content was downloaded on 17/04/2014 at 06:28 Please note that terms and conditions apply. The Higgs particle: a useful analogy for physics classrooms View the table of contents for this issue, or go to the journal homepage for more 2010 Phys. Educ. 45 73 ( Home Search Collections Journals About Contact us My IOPscience
  • 2. FE A T U R E S The Higgs particle: a useful analogy for physics classrooms Xabier Cid1 and Ramon Cid2 1 Particle Physics Departament USC, 15706 Santiago de Compostela, A Coru˜na, Spain 2 IES de SAR, 15702 Santiago, A Coru˜na, Spain E-mail: and Abstract In November 2009, the largest experiment in history was restarted. Its prime target is the Higgs particle—the last remaining undiscovered piece of our current theory of matter. We present a very simple way to introduce this topic to senior secondary school students, using a comparison with the refractive index of light. The standard model The standard model of particle physics is the best theory that physicists currently have to describe the building blocks of the Universe. It is one of the biggest scientific achievements in twentieth- century science. The standard model describes the Universe using six quarks and six leptons. There are four known interactions, each mediated by a fundamental particle, known as a carrier particle. In the 1970s physicists realized that there are very close ties between two of the four fundamen- tal interactions—namely, the weak interaction and the electromagnetic interaction. The two interactions can be described within the same theory, which forms the basis of the standard model. This ‘unification’ implies that the interaction-carrying particles have no mass. We know from experiments that this is not true. To solve this problem several physicists proposed the existence of a new field with its corresponding quantum particle, the Higgs field and the Higgs particle. The Higgs particle has been nicknamed (by Nobel Prize-winning physicist Leon Lederman) the ‘God particle’ because of its importance to the standard model. Detecting this particle is one of the Large Hadron Collider’s (LHC’s) main purposes. The LHC is the world’s largest and highest-energy particle accelerator, intending to collide opposing particle beams of either protons at an energy of 7 TeV per particle or lead nuclei at an energy of 574 TeV per nucleus. It lies in a tunnel 27 km in circumference, as much as 100 m beneath the Franco–Swiss border near Geneva, Switzerland. The mass of the particles Why do particles have mass? Why are the masses what they are? Why are the ratios of masses what they are? In the early 1970s, Peter Higgs, Franc¸ois Englert, Robert Brout, Gerald Guralnik, Dick Hagen and Tom Kibble independently proposed that the Universe is full of a field later called the Higgs field. Disturbances in this field, as particles move through it, cause objects to have mass. It is important to say that the original basis of Higgs’ work came from the Japanese-born theorist and 2008 Nobel Prize winner Yoichiro Nambu. Different ways of explaining this mechanism to a general audience have been proposed. For example, the discrete units which stir up the field, Higgs particles, act like a kind of cosmic molasses 0031-9120/10/010073+03$30.00 © 2010 IOP Publishing Ltd PH Y S I C S ED U C AT I O N 45 (1) 73
  • 3. X Cid and R Cid which fills all of space. As objects move through space they have to ‘wade’ through these Higgs particles that ‘cling’ to them, causing a drag that shows up as mass. Another analogy often cited describes it well: imagine you are at a party. The crowd is rather thick, and evenly distributed around the room, chatting. When a big star arrives, the people near the door gather around him. By gathering a fawning cluster of people around him, he has gained momentum, an indication of mass. He is harder to slow down than he would be without the crowd. Once he has stopped, it is harder to get him going again. Further analogies have been presented, but we prefer one that is closer to physics. Comparing refractive index and mass When light, composed of photons, passes through a transparent material such as water or glass, its velocity changes according to the refractive index of the material. If a beam of light enters the material at an angle, it is bent or refracted as a result of this decrease in velocity. The reason why photons are slower when passing through a transparent material is the effect of electrical fields surrounding the electrons and nuclei of atoms in the material. The photons are slowed down by interacting with these electrical fields. The effect is greater in materials such as water and glass than it is in air, a result of their greater relative densities. The fields almost act like ‘friction’ on the photons, decreasing their transmission velocity. It is like trying to walk through a muddy field. A measure of how much photon speeds are reduced is given by the refractive index of the material. The refractive index (i) of a material equals the speed of light in a vacuum (c) divided by the speed of light in the material (v): i = c/v. A very important detail is that the speed of light in a transparent material depends on the wavelength (i.e. momentum) of the photons. For instance, consider visible light in water, the refractive indices for the different colours are: Blue (486.1 nm) Yellow (589.3 nm) Red (656.3 nm) 1.337 1.333 1.331 ‘Yellow’ photons travel through water faster than blue, and red photons are even faster. You could say that blue photons have greater difficulty moving in water than yellow and red photons. You could say that blue photons behave as if they have more ‘inertia’, i.e. more ‘mass’. Refractive index gives a measure of the interaction between photons and a material medium through which they travel; it could also be considered an ‘index of mass’, since the bigger its value the lower the photon speed. In a vacuum, all photons travel with identical speed but, if the Universe were filled with water, photons corresponding to different wavelengths would travel with different speeds. As has been said before, they would appear to have ‘different masses’. So you would pass from a symmetrical situation to a nonsymmetrical one. This is what in particle physics is called symmetry breaking. Now we are ready to establish our compari- son. The standard model suggests that all particles were massless just after the big bang but, as the Universe cooled and the temperature fell below a critical value, an invisible field called the ‘Higgs field’ appeared, filling all space. You could also say that the Higgs field was created at the beginning of the Universe, but it only showed its influence once the Universe cooled down enough. The massless symmetry of particles was broken. Unlike magnetic or gravitational fields, which vary from place to place, the Higgs field is exactly the same everywhere. What varies is how the different fundamental particles interact with the field and are given mass. Of course, other kinds of interaction, such as the electromagnetic, weak or strong interaction may also contribute significantly to the resulting mass. Moreover, the degree of resistance of the Higgs field is different depending on the fundamental particle, and this generates, for example, the difference in mass between an electron and a quark. Now, suppose a quark or electron is moving (making up composite particles such as protons, neutrons or various atoms) in a uniform Higgs field. If these atoms (or molecules) change their velocities, that is, if they accelerate, and if the Higgs field exerts a certain amount of resistance or drag, then this is the origin of inertial mass. 74 PH Y S I C S ED U C A T I O N January 2010
  • 4. The Higgs particle: a useful analogy for physics classrooms The situation is similar to refraction, dis- cussed above. The Higgs field acts as a ‘transpar- ent material’ with a specific ‘refractive index’ for each kind of fundamental particle. In this way, the Higgs field gives each different particle its charac- ter, what physics calls ‘mass’. For instance, when a proton interacts with an electron, it undergoes an effect almost 2000 times smaller than the electron, because the ‘index’ in the Higgs field for protons is almost 2000 times greater. The analogy only works if you consider light as particles, and not as waves. The refraction of light happens only in a material medium, which the Higgs field is certainly not. Provided these limitations are made explicit and clear, we think the refraction analogy can help secondary students to understand the Higgs field. From a quantum point of view, however, you can only stir up the Higgs field in discrete units. The smallest possible disturbance is due to a Higgs particle, or more precisely, a Higgs boson. ATLAS and CMS are general-purpose LHC detectors designed to see a wide range of particles and phenomena produced in LHC collisions. The Higgs boson, possibly hundreds of times heavier than a proton, could be created in the proton collisions in the centre of their detectors. More than 4000 physicists from about 40 countries will be using the data collected from both complex detectors to search for Higgs particles. It is widely believed that they exist or at least some sort of Higgs-like particle which plays that role. But there is no real guarantee that the LHC will find it. It should find it, at least in the simplest models, but the simplest models are not always right. Anyway, following Professor Hawking, even a failure would be exciting, because that would pose new questions about the laws of nature. So, if it turns out that we cannot find the Higgs particle, this will leave the field wide open for physicists to develop a completely new theory to explain the origin of particles’ mass. Acknowledgments The authors would like to thank Abraham Gallas Torreira and Diego Mart´ınez Santos for their helpful comments and corrections to this article. Received 28 August 2009, in final form 15 October 2009 doi:10.1088/0031-9120/45/1/008 Further reading Taking a closer look at LHC High school teachers at CERN teachers/ CERN LHC Detector CMS Detector ATLAS Xabier Cid Vidal graduated in physics in 2007. He is currently doing his PhD on experimental particle physics, taking part in the LHCb collaboration at CERN with the University of Santiago’s group. Ramon Cid graduated in physics and chemistry and has taught physics at secondary school since 1980. He participated in the HST programme at CERN in 2003. He has coordinated various European teaching projects and several annual ENCIGA (Association of Science Teachers of Galicia) science teachers’ congresses. January 2010 PH Y S I C S ED U C A T I O N 75