CMS+Startup+Research

Big Questions of Particle Physics
One topic that fascinates CMS scientists is the question of how and why the Universe came to be exactly how it is. The dominating theory on what took place just after the big bang involves a "quark-gluon plasma" that can only exist at temperatures above roughly 2000 billion degrees, 100,000 times hotter than the core of the sun. The theory suggests that all the particles of the universe were then formed as this plasma cooled and the quarks and gluons condensed into composite particles such as protons. So, CMS scientists set up experiments at the Relativistic Heavy Ion Collider which involved colliding heavy nuclei with each other in order to create an extremely small-scale version of the quark-gluon plasma which was likely the source of the Universe. A larger scale experiment using the Large Hadron Collider is the next step in discerning how it is that quark-gluon plasma behaves and consequently discovering what it is that made protons and neutrons form and what it is that keeps them together.

Another vexing topic for even some of the brightest thinkers is the question of how many dimensions we truly live in. String theory is a modern theory which asserts that in fact, six dimensions aside from the four commonly accepted dimensions actually exist. The theory is based on the idea that rather than points or dots, the smallest particles are actually bundles of a string-like substance, and different particles are merely different vibrations or oscillations of this string. This idea supports the idea that as we dig deeper into smaller and smaller scales and particles, we can uncover new dimensions that merely are not perceived by humans. One extra fascinating element this school of thought provides is an explanation for gravity. Maybe it is so weak because its force is being spread across many more dimensions than we can perceive it affecting. This concerns the LHC because some theories about the smaller dimensions actually provide that they might be big enough to be observed in experiments in the LHC.

One last captivating topic for CMS scientists is the concept of antimatter. Scientists have concluded that for each particle of matter in the Universe, there is a particle with the same mass, but opposite charge. If these opposite particles meet, their mass is turned into energy and they both vanish in a flash. In fact, the electron's antimatter counterpart, the positron, has been discovered in cosmic rays. The LHC is the pioneer in the creation and study of antimatter, having routinely created antimatter particles and in fact having artificially created antimatter particles. Every time antimatter has been created, though, a corresponding particle of matter has also been created. Scientists have found no way to avoid this. Thus, they are baffled by the question of why we still exist and why antimatter has seemed to have largely disappeared in the Universe but matter has not. Theories for this question cover a wide range of approaches, but it is the discoveries made at CERN's LHC that find the new evidence to shape more and more accurate theories about antimatter.

The Compact Muon Solenoid Detector


The CMS Detector is measuring infinitesimally small particles, yet it is 21.5 meters in length and 15 meters in diameter. This is the case because although the particles it is measuring are almost unfathomably small, the forces these particles carry are equally as astonishingly great. The center-of-mass energy of the LHC is 14 TeV, or more specifically 14 trillion times the charge energy of a single electron. So, in order to measure these extremely large outputs of energy, the components of the CMS need to be quite large. The CMS Detector is broken down into the following sub-detectors:
 * First is the Silicon Tracker. It contains 205 square meters of silicon sensors, which serve to track the exact curvature of particles through the detector. This data of the curvature can then consequently be used to calculate the momentum and charge of each particle.
 * Next is the Electromagnetic Calorimeter. A network of crystals of the extremely dense lead tungstate serves as a stopping mechanism for photons and electrons. They are backed by silicon detectors which provide the data readout of the amount of energy involved in the stopping process.
 * The third component down the line of the CMS Detector is the Hadronic Calorimeter. This is designed to do two tasks. One task is to measure the energy of each of the hadrons produced in each event. It does this using a system of dense metals, scintillators which release absorbed energy in the form of light, and wavelength-shifting fibres. The other task performed by the Hadronic Calorimeter is the detection of events with missing energy.
 * The penultimate component of the CMS Detector is the Superconducting Solenoid, also known as the magnet. The Superconducting Solenoid is a 13 meter long, 6 meter in diameter helix coil magnet. The magnet is equipped to handle the energy of half a ton of TNT. The function of this magnet is to determine the ratio of charge to mass of particles based on the curve they follow through the magnet.
 * The final component of the CMS Detector is the Ion Return Yoke Interspersed with Muon Chambers. This largest component of the detector consists of barrels called Drift Tubes capped by Cathode Strip Chambers and lined with Resistive Plate Chambers. Together these components can precisely measure the paths of muons and calculate their momenta.

Particles in the CMS Detector


The LHC deals mainly with collisions of one proton with another proton. If two protons are to collide, then one specific part of one proton will collide with a specific part of the other proton, and the improbability of situations in which multiple parts of each proton collide at once makes those situations negligible. So, we are concerned with the probabilities of certain collisions. The collision of an up quark with an up quark is four times as likely as the collision of a down quark with a down quark, because there are four more combinations of up quarks between the two protons than down quarks, as illustrated in the picture above. Similarly, it can be deduced that there are nine possible collisions of gluons with gluons and thus a gluon-gluon collision is nine times as more likely that a down quark-down quark collision. This phenomenon explains why there are different volumes of data produced for the different kinds of collisions.

The CMS Detector is able to take data about particles that are not actually directly observed because they are there and gone in a flash and are astonishingly small. The way it is able to accurately record data about these particles is through two main activities: absorbing the energy of these particles and tracking the path of the particles. The massive number of sensors and stopping mechanisms in the Electromagnetic Calorimeter and the Hadron Calorimeter allows for extremely accurate data to be taken about the energy of these mysterious, unobservable particles. The huge array of detectors and the precisely constructed and positioned magnets, structures, and chambers in the Silicon Tracker, Superconducting Solenoid, and the Ion Return Yoke Interspersed with Muon Chambers allows for extremely accurate data to be taken about the flight path of these particles through the CMS Detector. In turn, features of these particles such as the mass can be determined by synthesizing data about the energy of the particles with data of the flight paths of the particles through the detector.