When we arrived at the LHCb, we donned hard hats and were scanned and admitted to the experimental area deep underground.
We descended more than 100 meters into the large underground cavern that houses the LHCb and the defunct DELPHI experiment. We first went to DELPHI, a now defunct detector that has been dissected and put on display that formerly occupied the location of the LHCb detector.
At the experiment, we were actually able to touch the copper wire matrix that made up the tracker. The wires alternated between horizontal and vertical alignment, which allows for the detector software to determine the trajectory of a particle. Our guide told us about the improvements made by CMS over DELPHI's design, such as the replacement of DELPHI's copper wire tracker with silicon charge-coupled devices that provide higher resolution imaging of the particles as they curve in the magnetic field.
Depending on the scale of the experiment, particles of different charge can be separated using magnetic fields. By equating the Lorentz Force and the centripetal force, the momentum can be determined, and by analyzing the curvature of the path, the charge of the particle can be resolved.
We then moved across a thick cement radiation screen from the DELPHI exhibit to the actual site of LHCb (Large Hadron Collider-beauty experiment), which is designed to study the differences between matter and anti-matter through precise measurement of different parameters of b-meson physics.
More specifically, its goal is to shed light on why ordinary matter survived to make up our current Universe while seemingly all of the primordial anti-matter was annihilated and not symmetrically replenished. If matter and anti-matter were perfectly symmetric, we would expect to see stars formed from anti-matter among the ordinary stars of our Universe. To date, we have never found a larger anti-nucleus than antihelium, let alone anti-molecules or anti-matter stars.
686 scientists from 48 different institutes and 15 different countries around the world, along with hundreds of technicians and engineers, collaborate on the LHCb. We stood in a fenced off observational area and viewed the different parts of the LHCb, including the calorimeter system, which is designed to measure the energy of the produced particles, the tracker system, the muon tracker and the vertex locator. Unlike CMS, which must provide full azimuthal coverage to detect signatures of all kinds, the LHCb only needs to be able to measure interactions involving b-mesons. These mesons are massive enough that they stay at a very low angle relative to the plane of the LHC tunnel. This allows for the LHCb to have a pyrimidal design instead of the cylindrical design of CMS and ATLAS.
Our next location was the control facility for AMS (the Alpha Magnetic Spectrometer), a NASA-owned detector on the International Space Station. It is designed to detect anti-matter in cosmic rays and search for dark matter. We saw a model of the experiment and through it learned about the different parts of the module, including the vacuum case, tracker, calorimeter and magnet. It bore many similarities to the exhibits we viewed earlier, but unlike the LHCb and DELPHI it is entirely observational. We were guided by Dr. Vincent Smith, a member of the CMS project recently retired from Bristol University, who explained the role of cosmic rays in the ongoing search for new types of particle, such as the strangelet, a theoretically stable mix of top quarks, bottom quarks, and strange quarks that remains undiscovered as of yet.
For lunch, we ate at the CERN cafeteria, which provided awesome food, sampling from a variety of ethnicities. Overall, the meal was delicious and cheap, and we finished our lunch break with a new card game, Musta Maija, we learned from the Finns.
Our lunch ended at 1:00 p.m., just in time to make it to our belated introduction to CERN. Unlike the typical tour program for high schoolers visiting CERN, which include an introductory presentation, one site tour, and a trip to the onsite museum, we had the opprotunity to visit four different sites before we even received an introduction! We were once again graced by lecturer Dr. Vincent Smith, who gave us a very thorough overview of the origins of the organization and the different experiments going on at CERN. CERN, established in 1954, was originally called "Conseil Europeen pour la Recherche Nucleaire." When the name was changed to "Organisation Europeene pour la Recherche Nucleaire," physicists decided against changing the name to the unpronounceable "OERN." However, the original name no longer holds, because CERN is no longer a purely European organization (Israel is a member and Brazil is in the process of joining), and the focus is no longer solely on nuclear research.
After Dr. Smith's presentation, we travelled to SMS-18, where we learned about the testing and role of superconductor-containing magnets in the various experiments at CERN. The magnets must be kept below 2 Kelvin to retain their superconductive nature. If the magnet heats up too much, a quench may occur in which the coil's resistivity increases rapidly. If the quench is not controlled, the increased resistivity of the coil will cause immense amounts of heat to be emitted by the no longer superconductive magnet exacerbating the problem. Quenches are a common occurrence in the operation of the LHC, but an uncontrolled quench may cause the temperature of the magnets to rise rapidly, leading to thermal expansion and damage to the device. In 2008, an catastrophic quenching event cascaded out of control, necessitating the replacement of about fifty magnets.
Protons at the LHC are accelerated to full speed in multiple stages. First, the hydrogen atoms have their electrons striped with a powerful electric field. Then, the protons travel through a linear accelerator that uses progressively larger electromagnetic cavities to accelerate bunches of protons. Eventually, the protons enter the main LHC ring, which continues to accelerate the particles with x-ray frequency cavities. Since the protons are already at near the speed of light, the cavities are all the same size.
In addition, our guides described the theory behind superconductivity. Whereas electrons are traditionally confined to one set of quantum numbers and one location (due to the Pauli Exclusion principle), the formation of Cooper Pairs, caused by electron-phonon interactions, allow sets of pairs to "stack"Â due to their bosonic nature. From this, persistent current can be created, and large amounts of current can be sent through superconductors with no resistance.
After learning about how data is collected, we traveled to the CERN Control Centre (CCC) to learn about the inner mechanism of the collision process and how the different experiments interacted. In a presentation on another transforming window touchscreen, we learned how magnets can be employed with progressively larger radio frequency cavities to allow an amalgam of protons and particles to spread out and accelerate towards a collision detector. We were able to view parts from the different colliders and the room of people in charge of analyzing experimental data. Interestingly, they kept all celebratory champagne bottles from past successful experiments.
At the end of our time at CERN, we returned to the Hostel to work on homework, and we left an hour later for a group dinner with all the Finns at La Veranda. The group dinner included lots of good food, shared experiences, and ultimately an exchange of good-bye gifts, as three of the Finnish students are returning tomorrow to take a national Finnish Physics exam. We are now preparing for tomorrow, our final day at CERN.