Review of “David Krofcheck on the Higgs Boson”
New Zealand at CERN and the Discovery of a new Particle: The Review
In July the physics world was rocked by the discovery of a new particle that looks suspiciously like the long hypothesized Higgs boson. The announcement was met with huge excitement and extensive media coverage. We all knew that something significant had been discovered, but what exactly? To the non-physicist it all sounded pretty perplexing. What exactly is a Higgs boson, Higgs field and why is it important? With these questions in mind we invited Dr. David Krofcheck from the department of physics to discuss the recent discovery. David and his NZ team are part of CMS collaboration at CERN, so he was able to give us a fascinating first-hand account of what goes on at the CERN laboratory.
David began by explaining how, much to the embarrassment of physicists everywhere, there are lots of really basic unanswered questions in modern physics. For instance our best theories for understanding matter and its interactions only describe 4% of the known “stuff” in the universe. There is good reason to believe the rest of the universe is composed of (as yet) unidentified dark matter, and even more nebulous dark energy. We don’t know why there is matter in the universe at all, and more fundamentally we don’t even know what mass is! The Large Hadron Collider (LHC) was built to try and answer some of these questions.
From these motivating questions David went on to summarize what (we think) we know: the standard model of particle physics. The standard model divides the world into two types of particles, fermions and bosons. Fermions are the building blocks for protons, neutrons and all of the other “normal matter” in the universe. Bosons are the particles that mediate the known forces e.g. photons are bosons which carry the electromagnetic force. In a sense the standard model is a bit like the periodic table in chemistry. It is a carefully structured table of the fundamental “ingredients” of matter which you can combine in different ways to build more complicated particles/substances.
Although the standard model has worked exceptionally well in making successful experimental predictions, for decades there has been one major missing piece in the puzzle: the Higgs boson. The Higgs boson is conjectured to explain “what gives particles mass”, but what exactly does this mean? David presented us with an analogy to help us understand the theory. Consider a room full of uniformly-distributed journalists, and suppose that an especially famous person (e.g. Margaret Thatcher) enters the room. As she tries to cross the room, she will of course be swamped by journalists that impede her progress and slow her down. The more famous the person, the more they are slowed down inside the room as more journalists “clump” to them. In this analogy the journalists represent the Higgs field, and famous people crossing the room are particles with mass: the more famous the person, the greater the mass of the particle (and the more they are slowed down by Higgs field/clumping of journalists). The Higgs field permeates all of space such that particles everywhere are slowed down and thus appear to have mass. If there were no Higgs field every particle would move at the speed of light since nothing would impede their progress. The Higgs boson can also be understood via this analogy. Sometimes it doesn’t even take the presence of a famous person for journalists to clump together; they may excitedly clump together at the mere mention of a famous person. This represents the idea that given enough energy, the Higgs field may be excited to a state whereby massive particle spontaneously form from the field itself (no particle is required – the field spontaneously ‘clumps’ into its own particle). Although it is the Higgs field that is of interest from a theoretical perspective, we cannot measure the Higgs field directly. Instead we try to observe the Higgs boson, and in doing so test the existence of the associated Higgs field.
David then went on to explain how exactly the LHC is being used to test for the existence of the Higgs boson. The LHC accelerates protons to extremely high speeds and smashes them together inside huge particle detectors. The NZ team plays a fairly crucial role in this process. They are responsible for the construction and operation of the Beam Radiation Monitor, a device that detects if the beam of protons start to drift off target. If the beam were allowed to drift this could be disastrous as super-high energy particles would collide with fragile multi-million dollar electronic equipment. The Beam Radiation Monitor acts as a warning system: if the particles begin to drift off target the beam is safely dumped into a solid wall.
How exactly then is the LHC being used to test for the existence of the Higgs boson? During a collision of two protons, the massive kinetic energy of the protons is converted into new particles which stream outwards in all directions and are detected by large particle detectors. If the energy of the incoming protons is sufficiently large, a Higgs boson may be among the new particles created by the collision. Theory describes how often we should see certain particles created at different energy levels. If we perform many collisions we can compare the experimental frequency distribution (of particles created in the collisions) to the theoretical frequency distribution. In particular if the Higgs boson exists, we expect a slight blip in the frequency distribution at a particular energy level, and indeed such a blip has just been observed. Interestingly some of the characteristics of the distributions suggest that the observed Higgs may not quite have the same properties as the simplest Higgs model predict, which opens the door to a potential family of Higgs particles and exciting new physics beyond the standard model. We await further results from the LHC with great anticipation.