The development of particle accelerators was previously discussed (see Part 1 and 2 on Vacuum Technology at CERN). The use of the large hadron collider (LHC) located at CERN with its enormous, multiple vacuum systems is intended to investigate the fundamental particles of the universe and one of the most exciting in recent years being the search for the Higgs boson. Here we also take a closer look into the technologies used in the LHC, discussing some of the other discoveries it has led to, as well as reviewing past contributors to the field of vacuum technology whose innovations made the LHC possible.
The Standard Model of Particle Physics
Developed in its current form in the 1970’s, the so-called “Standard Model” of particle physics is a theory of fundamental particles and how they interact. It integrated all that was known about subatomic particles at the time and predicted the existence of additional particles as well. The Standard Model describes three of the four known fundamental forces in the universe – electromagnetic forces, weak nuclear, and strong nuclear interactions.
An explanation for the fourth, namely gravity, has still not been established. Used to categorize all known fundamental particles, the Standard Model effectively explains nearly all experimental results and accurately predicts a wide variety of phenomena. Over several decades of experimentation, the Standard Model has become recognized as a well-tested physics theory. Through the early 2000’s, there were two key questions still not fully explained by the Standard Model, that is (1) what gives particles their mass, and (2) what mechanism is responsible for the force of gravity.
In 1964 British physicist Peter Higgs postulated that a certain type of a subatomic particle existed that gives all other particles their mass, he called this particle a boson. Higgs’ calculations showed this particle should have a mass of 125 GeV (gigaelectronvolts). At the LHC on July 4th, 2012, the high energy collision of two protons resulted in the discovery of a new particle with a mass between 125 and 127 GeV, and physicists suspected that it was the boson particle that Higgs predicted. Later calculations and experiments showed that the particle behaves, interacts, and decays as the Standard Model predicted it would and later it became known as the Higgs boson.
Two fundamental attributes were also discovered for this particle: even parity and zero spin. Upon further experimentation using the LHC at CERN, it was announced in March 2013 that the particle that Higgs predicted was indeed the Higgs boson. As a result of these findings, Higgs and co-discoverer François Englert winning the Nobel prize in Physics in 2013.
How does the Higgs boson give particles their mass?
To answer the question about how the Higgs boson imparts mass on matter, it is necessary to understand the existence of a Higgs field and that the Higgs boson itself is just a “ripple” in this field. Similar to an electromagnetic field, the Higgs field permeates three-dimensionally and interacts with particles to give them their mass. The Higgs field is a quantum phenomenon and it is difficult to intuitively comprehend but can be described as “sticky” to particles passing through it (similar to tar or molasses), with its level of “stickiness” determining the particle’s mass. A lower-mass particle is slowed down or deflected more while traveling through this sticky field than a higher-mass particle.
Another popular explanation analogizes the Higgs field to a crowd at a party. When a celebrity enters the room, his fans surround him, asking for autographs and taking pictures, which slows him down. In comparison, a waiter passes through unimpeded by the crowd. In this analogy, popularity is synonymous with mass and the more popular a person is, the more mass he has. The crowd is analogous to the Higgs field and interacts more with higher mass (more popular) people, affecting their motion to a greater degree.
Particle Detectors at the Large Hadron Collider
The accelerators used at the LHC to accelerate particles to near light speed are only half the story. After the counter-rotating particle beams in the LHC are made to collide at specific locations on the LHC ring, the results are detected using a sophisticated set of sensors and sub-detectors. The four primary detectors (ATLAS, CMS, LHCb, and ALICE), are situated in enormous underground chambers at these collision locations on the LHS ring (Fig. 3).
The largest of these, ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) (Fig. 4), are general purpose detectors used to examine the most basic collision phenomenon. When particle beams from the LHC collide at the center of the ATLAS and CMS detectors, debris are released in all directions. ATLAS incorporates six sub-detectors arranged in strata around the collision point. These sub-detectors record the paths, momentum, and energy of the particles, allowing each particle to be properly classified. A system of precise electromagnets bends the paths of charged particles so that their momenta can be measured. The ATLAS detector is the largest particle detector in the world. It weighs in at 6,400 kg (14,000 lbs.) and is 46 m long x 25 m wide x 25 m high (150 ft. long x 82 ft. wide x 82 ft. high). The CMS detector has the same technical objectives as ATLAS, but uses different technical solutions and a different magnet-system design.
The ALICE (A Large Ion Collider Experiment) detector and the LHCb (Large Hadron Collider beauty) detector are more specialized and designed to examine such phenomena as the matter-antimatter asymmetry of the universe and the physics of strongly interacting matter at extreme energy densities such as existed immediately after the big bang.
Vacuum Technology’s Contribution to the LHC
The breathtaking discoveries made by the LHC would not be possible without the advanced vacuum technologies established by the pioneers of the past.
In 1654, German scientist Otto von Guericke was working on the poorly understood problem of what was left in a container after the air was removed from it. Von Guericke disagreed with Aristotle’s view, still prevalent at the time, that “nature abhors a vacuum.” Aristotle implied that there could actually be no such thing as a vacuum, or a total void. Von Guericke was fascinated by the question “could empty space exist?” and believed a void could be created. Using a cylinder with a hand level and two-way flaps designed to pull air out of whatever vessel it was connected to, von Guericke invented the first vacuum pump. He had the brilliant insight that materials were not pulled by a vacuum, but were pushed by the pressure of the surrounding fluids. He then set about showing, through a series of public experiments, the properties of an evacuated chamber. His most famous demonstration, performed in 1657, used the so-called Magdeburg hemispheres (Fig.5). They were two 508 mm (20 inch) diameter machined copper hemispheres sealed together with grease, from which he pumped out the air, creating a suction that locked them together. The air pressure outside held the halves together so tightly that sixteen horses, eight harnessed to each side of the globe, were not able to separate them (Fig. 6). Atmospheric pressure was holding them together with a force of approximately 2,090 kg (4,600 lbs.).
Another advanced vacuum technology critical to the LHC is the cryogenic pump, a pioneering accomplishment that relies on the groundbreaking achievements of scientists and inventors of the past.
Since cryopump technology depends on advances in the field of cryogenics, the invention and development of cryopumps occurred at the same time as advances were made in the field of low temperature gas liquefaction. Further progress in the field of cryogenics required the liquefaction of hydrogen, a feat that would require a temperature of 20.4 Kelvin, or just 20.4°C above absolute zero (the temperature at which no further thermal energy exists in a material). Nitrogen had previously been liquefied at a temperature of 77.3 Kelvin, but the insulation technology of the day did not allow the ultra-low temperature required for liquid hydrogen. The vessel containing the specimen simply absorbed too much heat from the environment.
Starting in 1878, over 200 hundred years after Otto von Guericke’s invention of the vacuum pump, Scottish chemist and physicist Sir James Dewar (Fig. 7) was studying the properties of gaseous elements separated from atmospheric air with the aid of very low temperatures. Dewar had an intractable curiosity, and was active in a wide range of scientific pursuits, including organic chemistry, hydrogen and its physical constants, the response of the retina to light, the invention of the explosive substance cordite, the temperature of the Sun, electrophotometry, and the chemistry and temperature of the electric arc.
In 1898, while studying a sample of the element palladium, Dewar had an insight. In order to allow further cooling of the sample, he formed a brass chamber that he enclosed in another chamber, then evacuated the air between the two. With the air evacuated, Dewar had drastically reduced the heat transfer by both convection and conduction through the wall of the cooling chamber. The superior insulating properties of this arrangement allowed the palladium to maintain a cooler desired temperature. Dewar had invented the vacuum flask, named the Dewar flask in his honor. He never patented his invention, and it was later patented by others, and the rights sold to the Thermos company, who brought it into mainstream use. Dewar lost a court battle related to the patent.
Soon after Dewar’s invention, it was discovered that vacuum can be produced by charcoal cooled to cryogenic temperatures, and the sorbent vacuum pump was born. In the 1910’s Wolfgang Gaede and Irving Langmuir first demonstrated the use of liquid-nitrogen-cooled traps to prevent oil backstreaming from diffusion pumps, yet another critical contribution of cryogenics to vacuum technology. Gaede is noted for his invention of the diffusion pump in 1915 using mercury vapor. Langmuir is credited for making improvements to its design. Cryopumping took off in the 1950’s in response to the needs of the first space exploration projects, which required operation with liquid hydrogen. Today it enjoys widespread
use and has become a common and valuable technology.
Summary
This series of articles was intended to provide a perspective on more recent developments in particle physics. Our intent was to inform and demonstrate that fundamental science is only possible through a culmination of many connected inventions, such as the development of vacuum pumping technology, critical to the successes at CERN. We now return to more heat treat focused articles.