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GENERALARTICLES DISCOVERING THE HIGGS Inside the Discovery of the Higgs 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 “ 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 “ 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 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 physics cast of the . 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 , had developed a model to explain ory. However, at the subatomic level, , and we understood strong, weak, and electromagnetic the is not perfect, in high school. Instead the Standard nuclear interactions of fundamental regarding gravitational as neg- Model is based on fundamental par- in our - 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 (2). protons, and neutrons we know – Standard Model of physics. It was the But the Standard Model is what the namely , , , W theory of almost everything (1). physics world, and indeed many other and Z particles, and . 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 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 world around us on an almost infini- neutrons of ; 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 , a theory which works for everything, atoms that used to be the smallest par- particles, , 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 /Antiquark Pairs W Particles scientists accelerated protons to near the speed of light and induced Two Gluons Tau /Antilepton Pairs collisions within huge detectors. /Antiquark Pairs Z Particles These detectors were designed of Two Photons 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 the mass observed. Graphic created by Shree Bose. predicate that heavier particles will, if possible, decay into lighter ones. , 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, . 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 for known as 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 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 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 more than a , 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 , a combination of the electro- ics, and Their Unification. Int. J. Mod. from a standard model Higgs, not magnetic force between charged par- Phys. A, 15, 840 (2000) consistent with those proportions ticles and the weak force dealing with 3. D. Griffiths, Introduction to Elementary that had been initially proposed. In . The Higgs mecha- Particles (Physics Textbook). (John Wiley the experimental results from CMS nism can actually explain how W and and Sons, 2008) and ATLAS, the probabilities for the Z bosons responsible for the weak 4. J. Letessier, J. Rafelski, Hadrons and and tau lepton decay force obtain mass and interact with Quark- Plasma (Volume 18 of modes were smaller in of the massless photon responsible for Cambridge Monographs on Partcle Phys- the new particle and the probability of the electromagnetic force. With more ics, , and Cosmology) . decay to two photons was almost 50% analysis, this interaction may explain (Cambridge University, 2002) 5. L. Lederman, D. Teresi, The God Particle: larger than the predicted (8,9). the unification of these two fundamen- If the Universe is the Answer, What is Only one or at most a few Higgs bo- tal forces as the electroweak force. In the Question? (Houghton Mifflin Har- sons were produced at the Large Had- addition, since the mass of the boson court, 2006) ron Collider with every proton-proton was found to be low, the model still 6. E. Braaten, J.P, Leveille, Higgs-boson collision. Each of the particles then allows for possibility of supersymme- decay and the running mass. Physical decayed within fractions of a second try, a theory which suggests that each Review D, 22, 3 (August 1, 1980) into the unique combinations of prod- particle has a “” particle 7. K. Gospondinova, The Elusive Higgs Bo- ucts detected by the massive under- with higher mass and slightly different son Particle Found. (The Student, October ground structures and then analyzed characteristics (11,12). 16, 2012) by a network of computers all over the For the physics world, as Nima Ar- 8. CMS collaboration, of a new boson at a mass of 125 GeV with the world. To confirm the existence of the kani-Hamed, a physicist at the Insti- CMS Experiment at the LHC. Physics Let- small bump seen in the data, trillions tute for Advanced Study in Princeton ters B, 716, 1: 30-61 (2012) of collisions were necessary to get a put it, “now some fun begins” (13). 9. ATLAS collaboration, Observation of a statistical significance of 5 standard So far, the masses and much of the New Particle in the Search for the Stan- deviations. Together, using the results behavior predicted by the Standard dard Model Higgs Boson with the ATLAS of ATLAS and CMS to confirm one Model has held true for this new Detector at the LHC. Physics Letters B, another, CERN physicists were able subatomic particle, bringing together 716 (1): 1-29 (2012) to reach this level of significance – to one of the major missing pieces of the 10. M. Strassler, Decays of the Standard prove that this new particle wasn’t a Standard Model and validating much Model Higgs Boson. (Of Particular glitch from background interference of what the physics world had accept- Significance: Conversations about Science with Theoretical Physicist Matt Strassler, but rather a new particle decaying at ed to be true. This is a huge step to be 2011) that precise mass. This new particle is made for human knowledge – a huge 11. C. Moskowitz, The Elusive Particle: 5 Im- what is believed to the Higgs Boson, step which may be used in ways in plications of Finding Higgs (LifeScience, despite the fact that other properties the future to do things we could have July 4, 2012) of the particle remain to be analyzed. never even imagined. 12. A. Mann, How the Discovery of the Higgs So what does this momentous discov- July 4, 2012. What could very Boson Could Break Physics (Wired Sci- ery of the physics world imply? In ad- possibly be the last subatomic particle ence, July 2, 2012) dition to explaining much of the origin left to be found had been discovered 13. D. Overbye, Physicists Find Elusive Par- of the mass that our universe, the (14). History was being made in front ticle Seen as Key to Universe. (The New existence of the Higgs Boson validates of my eyes. And as the Director Gen- York Times, July 5, 2012) 14. CERN Press Release, paper presented a part of the Standard Model which eral said those three words we had all at CERN seminar (“CERN experiments had previously been unproven. All of been waiting to hear, the auditorium, observe particle consistent with long- the particles of the Standard Model and probably so many more all over sought Higgs boson”), Geneva, Switzer- except for the Higgs Boson had been the world burst into applause. land, 4 July 2012. found before July 4, 2012. Without “We got it.”

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