GENERALARTICLES DISCOVERING THE HIGGS Inside the Discovery of the Higgs Boson and Understanding What it Means for Us BY SHREE BOSE uly 4, 2012. Geneva, Switzerland. the border of France and Switzerland, I was near the back of the room, I can’t believe I’m alive to a testament to the cooperation of Jnext to a girl who had her phone see this. the countries who had agreed to be a out frantically texting. I remember part of the Conseil Européen pour la looking over at her phone to see a “ Peter Higgs Recherche Nucléaire. There was Peter long string of French interjected with Physicist, Univeristy of Edinburgh Higgs, the renowned British theoreti- the words “Higgs,” and I knew that Proponent of the Higgs Boson“ cal physicist who had traveled in from the University of Edinburgh to be here there was nothing else on everyone’s for this seminar. When I had met him mind. Everyone around me whispered the day before and asked about what excitedly, and as I looked around the he believed would be discussed at the room, a small auditorium in the heart seminar, he had responded with a of the CERN, the European Organiza- simple “I can’t believe I’m alive to see tion for Nuclear Research, I saw a col- this.” There were the young men and lection of people who had worked women, some of whom had just gradu- for years to hear the results of ated with their degrees in physics and what this seminar would re- were living out their dream careers, all veal. There were old, wizened seemingly trembling with excitement. men and women who had And there were the millions of people been there for the very begin- all over the world tuning into the web- nings of the particle physics cast of the event. Something big was in research, for when the giant the air. detectors had been conceived The Director General of CERN, Dr. Rolf Heuer took the stage, and within Meeting with Peter Higgs the day before and built into the massive 27 moments the room was completely the seminar. Photo courtesy of Shree kilometer underground ring on silent. He went through the basics, how Bose. 19 | HARVARD SCIENTIFIC REVIEW theoretical physicists over the years strived for - the Grand Unified The- and even smaller than the electrons, had developed a model to explain ory. However, at the subatomic level, protons, and neutrons we understood strong, weak, and electromagnetic the Standard Model is not perfect, in high school. Instead the Standard nuclear interactions of fundamental regarding gravitational forces as neg- Model is based on fundamental par- particles in our universe - a model ligible and, until July 2012, not being ticles which make up the electrons, which had come to be known as the able to explain the role of mass (2). protons, and neutrons we know – Standard Model of physics. It was the But the Standard Model is what the namely quarks, leptons, gluons, W theory of almost everything (1). physics world, and indeed many other and Z particles, and photons. Quarks, The world of modern particle phys- fields and applications beyond, have one of the fundamental particles, ics currently has consolidated a vast built on, and what has been revali- come in six different “flavors” – up, amount of knowledge and experimen- dated by experimental results time and down, charm, strange, bottom, and tal evidence into a set of laws and the- time again. The Standard Model has top. Combinations of quarks with the ories which explain concepts through- been our way of understanding the correct charges create the protons and out all of physics, from the quantum world around us on an almost infini- neutrons of atoms; however, electrons field theory to symmetry breaking, tesimally small scale. are elementary particles themselves. and even extra dimensions. However, The Standard Model is based on In fact, electrons are a class of the six the ultimate aim is, of course, to have elementary particles – smaller than the types of leptons, along with muons, a theory which works for everything, atoms that used to be the smallest par- tau particles, electron neutrinos, muon what physicists all over the world have ticles we understood in middle school neutrinos, tau neutrons (3). During The Higgs Boson Party Analogy Figure 1: The Higgs field is comparable to a crowd of reporters; when someone who is not as notable walks by, they can pass through the field with little interaction with it (see left). However if a celebrity like Justin Bieber walks, the reporters would congregate around him, and he would have more interaction with the field. If each of the reporters was a Higgs boson, then the celebrity would have more interaction with the Higgs Field and therefore a much higher mass. Graphic created by Shree Bose. HARVARD SCIENTIFIC REVIEW | 20 GENERALARTICLES (each of the reporters). The more the particles interact with the Higgs field, the higher their mass is (6). This idea of Higgs field interac- tion endowing particles with mass was one of the keystones of the Standard Model, without which the entire model would fall apart, and particles would fly around at the speed of light, not slowed enough by their size to bind to anything else and hold our universe together. At CERN, in a 27 kilometer under- ground circular tunnel across the border of France and Switzerland, Bottom Quark/Antiquark Pairs W Particles scientists accelerated protons to near the speed of light and induced Two Gluons Tau Lepton/Antilepton Pairs collisions within huge detectors. Charm Quark/Antiquark Pairs Z Particles These detectors were designed of Two Photons Photon and Z Particle layers of impact sensitive material to sense subatomic particles produced in the collisions. However, the Figure 2: Decay modes that were predicted by the Standard Model Higgs at principles of quantum mechanics the mass observed. Graphic created by Shree Bose. predicate that heavier particles will, if possible, decay into lighter ones. particle decay, muons, tau particles, after even attracting the attention of In accordance with these principles, and electrons all decay into their the mainstream media as the sub- physicists at CERN structured their complementary neutrinos. ject of physicist, Leon Lederman’s, expectations around detecting the Together leptons and quarks make book, The God Particle, evidence standard model predictions of the up a class of particles known as for this elusive particle of mass had products of the Higgs Boson decay, fermions. However, fermions them- not been found (5). Yet. rather the particle itself. selves cannot be responsible for At the physics powerhouse of The Higgs Boson is expected to their interactions with one another. the world at CERN, theoretical decay into various fermions based Instead, another class of particles, and experimental physicists came on its mass (7). At higher masses for known as bosons is responsible together to find conclusive evidence the Higgs Boson, the decay pat- for the forces associated with each for the existence of the Higgs field. terns would be expected to favor interaction. Bosons include W and To understand this field, imagine a heavier particles like top-antitop Z particles, photons, and gluons, mob of reporters. As some relatively quark pairs in a higher proportion and, when paired with fermions, can unknown person walks through the than quark-antiquark pairs. At two create completely distinct subatomic mob, they could pass through with of the major detectors at CERN, particles. For example quarks held relatively little interaction with the ATLAS and CMS (compact muon together by gluons create hadrons reporters, and would not be slowed solenoid), scientists used different (4). down. Now if we were to imagine channels to detect either photons or Now, based the calculations of the Justin Bieber walking into this crowd leptons released in the decay of the interactions of these different par- of reporters, he would probably Higgs. In both channels, they found ticles and their arrangements in our be mobbed and would have much a bump in the energy at around 125 universe, six physicists including Pe- more interaction and would not get gigaelectronvolts (GeV), almost 125 ter Higgs had formulated a hypoth- through as fast. In this case, Biebs times more than a proton, a mass esis for the origin of mass in 1964. would have a much higher mass in inconsistent with any other known Together, they proposed a so-called this field than our initial unknown particle, leading them to speculate “Higgs field” made up of a new person, and collectively the mob that this bump in the data was actu- class of leptons, known as the Higgs of reporters would be a Higgs field ally indicative of the mass of the Boson. But for almost 50 years and made up of individual Higgs bosons 19 | HARVARD SCIENTIFIC REVIEW Higgs (8,9). evidence for the Higgs Boson, much - Shree Bose is a freshman in Struus from Using the calculations of the Standard of the model would have been invali- Fort Worth, Texas. Model using this mass, physicists were dated, and the physics world would Sources able to predict certain proportions have been back to the drawing board 1. R. Oerter, The Theory of Almost Every- of the decay products expected from in developing their hypotheses. thing: The Standard Model, the Unsung a Higgs Boson (10). However, the In addition, this discovery provides Triumph of Modern Physics. (Penguin, particle they found had much different some insight into the electroweak USA, 2006). 2. J.M. Maldacena, Gravity, Particle Phys- decay patterns than what was expected force, a combination of the electro- ics, and Their Unification.