# What is Higgs Boson

What is the Higgs boson? Why is the world of science so excited by its discovery? What are its properties? What impact has this discovery had on theoretical physics?

On 4th July, 2012, scientists at the European Organization for Nuclear Research (CERN) had an important announcement to make — they’d discovered a particle with properties similar to those predicted for the elusive Higgs boson. On 8th October, 2013, Peter Higgs and Francois Englert were awarded the Nobel Prize in Physics for their work on this particle (see Fig. 1). These two events commemorated the culmination of a long quest involving generations of experiments and the construction of the world’s largest and most powerful particle accelerator — the Large Hadron Collider (LHC).

Fig. 1. The Nobel Prize in Physics (2013) was awarded
to two scientists for their work on the Higgs boson:
(a) Peter Higgs. (b) Francois Englert.
Credits: Bengt Nyman, Wikimedia Commons.

Imagining the Higgs boson

The search for the Higgs boson was part of a quest to find answers to a foundational question in physics — what is matter? Science textbooks define it, quite simply, as being any substance with mass and volume. For e.g., you know an object has mass by the resistance you feel when you try to apply force on it. But what is matter made up of, and where does it come from? These are some questions that scientists have been grappling with for centuries.

Fig. 2. The Standard Model offers a framework to order and classify elementary particles.
Credits: MissMJ, PBS NOVA, Fermilab, Office of Science, United States Department of Energy, Particle Data Group, Wikimedia Commons. URL: https://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particle.... License: CC-BY.

By the 1970’s, physicists had begun to assemble mathematical equations in an elegant theoretical model, the so-called Standard Model (see Fig. 2), to describe the fundamental units of matter (see Box 1) and three of the four fundamental forces (see Box 2) that influence their interactions.1 Gravity is not yet a part of the Standard Model. In fact, the unification of gravitation with the other three fundamental forces remains an outstanding and open problem in Physics (see Box 3).

Today, the Standard Model has been found to be largely self-consistent, and many of its experimental predictions have been verified. However, during the early stages of its development, physicists discovered a problem — when applied to nuclear interactions, the equations of the model were found to be inconsistent if its fundamental particles had intrinsic mass (see Box 4). This ‘contradiction’ could be resolved in one of two ways. If it were true that all elementary particles (including photons) possessed intrinsic mass, the Standard model would no longer be valid (in mathematical terms, some of its central predictions would show infinite divergence). On the other hand, if the Standard Model were valid, it would mean that all elementary particles, including the bosons that mediate weak interactions, were inherently massless. The Model would then have to provide for some mechanism to account for their ‘observed’ masses (see Box 5).

By the early 1960’s, physicists like Yoichiro Nambu and Philip Anderson had suggested that it may be possible for ‘some’ elementary particles to ‘acquire’ mass under certain conditions. The fledgling Standard Model was, however, saved by the work of three independent groups of researchers — Robert Brout and Francois Englert; Peter Higgs; and, Gerald Guralnik, Carl Hagen, and Tom Kibble. In papers published (almost simultaneously) in 1964, these groups postulated that fundamental particles were created massless. These particles could, however, acquire their ‘observed’ masses if they happened to interact with a hypothetical,  ubiquitous ‘field’, called the Higgs field, that was believed to permeate the universe (see Box 6). These researchers also suggested a mechanism, called Higgs mechanism after Peter Higgs, that could give fundamental particles their observed masses (see Box 7).

According to the concept of waveparticle duality (see Box 8), all fields must have a fundamental particle associated with them. Thus, the scheme offered by the Standard Model necessitates the existence of a special boson — the Higgs boson (see Box 9). In other words, the Higgs boson can be described as the manifestation of the quantum excitation of the Higgs field. The existence of this field can, therefore, be proved only by detecting the Higgs boson.

Finding the Higgs boson

In 2008, the Large Hadron Collider (LHC) was constructed at CERN France. One of its important goals was to find out if the Higgs boson existed, and to detect it if it does. In the LHC, two beams of hadrons (like protons) are forced to collide with each other while traveling at speeds close to that of light, releasing enormous amounts of energy. This energy is quickly used up to form an array of fundamental particles, the exact nature of which may vary with each collision (see Fig. 3).

Fig. 3. The Large Hadron Collider at CERN.
Credits: CERN. URL: https://www.flickr.com/photos/11304375@N07/2046228644. License: CC-BY.

Since the Higgs boson is fairly heavy (~130 times heavier than a proton), only the latest generation of colliders, such as the LHC, would be energetic enough to produce it. Scientists hoped that the higher the speed of the colliding protons, the greater would be the amount of energy released on collision and, therefore, the higher the probability of it throwing out a Higgs boson. They were aware, however, that even if a Higgs boson were produced, detecting it would be very difficult. Firstly, the probability of a Higgs boson being produced by the collision of two hadrons is extremely small — roughly 1 out of every 1012 (trillion) events. This means that a very large number of collisions would be needed before the Higgs boson could be expected to be produced with any reliability. Secondly, given its high energy, a Higgs boson was predicted to be extremely unstable in nature. If produced, it would decay almost immediately into other types of elementary particles, including photons (electromagnetic force), W bosons (weak force), and leptons (strong force). This means that we’d only be able to deduce the presence of a Higgs boson indirectly through measurements of its decay products. Thirdly, even detecting the presence of the Higgs boson from its decay products would be a challenge unless it showed a distinctive decay pattern. If the decay products of the Higgs boson were similar to those arising from the decay of other known unstable particles, it would be very hard to ascertain the actual source of these products (see Box 10).

In 2012, ATLAS and CMS — the two teams looking for the Higgs boson at CERN's LHC, announced the discovery of a particle ‘compatible’ with it, with an error margin of less than one in a million (see Fig. 3).4, 5 For the next several months, scientists continued to examine this particle and its attributes. Their measurements have shown that the particle behaves, interacts, and decays in many of the ways that the Standard Model predicts for Higgs particles.

Fig. 4. More than 200 Fermilab researchers and staffers (of the 1,700 scientists, engineers,
technicians and graduate students from the United States that helped design, build and
operate the LHC accelerator and particle detectors, and analyze the data from the
collisions) assembled in an auditorium at 2 a.m. EDT on 4th July, 2012 to await the
announcement that a Higgs boson-like particle had been detected.
Credits: US Department of Energy. URL: http://www.publicdomainfiles.com/show_file.
To conclude

With the detection of the Higgs boson, we've found all the fundamental particles predicted by the Standard Model. While this strengthens the case for this model as the theoretical takeawaysedifice of particle physics, it has had remarkably little impact on the discipline per se. This is because the Higgs field has been part of the Standard Model for many years before its detection.

The current focus of particle physics is on determining the existence of elementary particles that are not in the Standard Model, and measuring effects of known fundamental particles that this model gets wrong. For e.g., meticulous experiments and analyses of measurements are being carried out at the LHC and elsewhere to determine if different types of Higgs bosons exist.6 If they do, their discovery may lead us into realms of physics that go beyond our current understanding of the Standard Model.

While the quest for the Higgs boson has furthered technological progress of widespread importance (see Box 11), its discovery does not seem to have had any direct technological benefits. Given that all fundamental discoveries tend to yield practical applications on exploration, this may just be a matter of time. No wonder, then, that the whole world is excited — we now believe we know how elementary particles and everything they build (including ourselves) possess the property that we call mass!

References:

1. Kane G (1996). ‘The Particle Garden’, Perseus Publishing.
2. Kane G (2006). ‘The Mysteries of Mass’. Scientific American, January: 33-38.
3. Lederman L M & Teresi D (1993). ‘The God Particle: If the Universe is the Answer, What is the Question’. Houghton Mifflin Company.
4. CMS Collaboration (2012). 'A new boson with a mass of 125 GeV observed with the CMS experiment at the Large Hadron Collider'. Science, 338: 1569-1575.
5. CMS Collaboration et. al. (2012). ‘Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC’. Phys. Lett. B, 716: 30-61.
6. Carroll S (2015). ‘The Higgs boson and beyond’. Course Guidebook — The Great Courses, Virginia.

Subhash Kumar is an Academic at the Department of Physics, Acharya Narendra Dev College (University of Delhi), New Delhi. His research interests lie in Astrophysical Plasmas and Astronomy. He may be contacted at subhashkumar@andc.du.ac.in.

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