Magnet Seniors go to Switzerland
The Trans-Atlantic Science Schools (TASS) program is a joint endeavor for research and collaborative learning formed between the school systems of Maryland and Finland that fosters creativity, cooperation, and collegiality while focusing on advanced science topics in physics. Six Blair Magnet seniors traveled to the CERN laboratory in Switzerland as part of TASS 2015: Matthew Das Sarma, Jared Marx-Kuo, Arjuna Subramanian, Sarah Wagner, Catherine Xue, and Dennis Zhao. The students were accompanied by Blair High School Physics teacher James Schafer and Wootton High School Physics teacher Michael Thompson. While in Switzerland, the students kept a running blog, and after returning home to Maryland, the students wrote this summary for the Magnet Foundation newsletter:
At CERN, the group visited eight different sites where novel particle physics research takes place, a museum cataloguing the accomplishments and history of the organization, and many displays of equipment formerly used in groundbreaking experiments. The eight active research sites were the Low Energy Antiproton Ring (LEAR), the CERN Computing Center, the Large Hadron Collider “Beauty” Collaboration (LHCb), the Alpha Magnetic Spectrometer (AMS), the CERN Control Center, CERN’s superconducting magnet testing facility (SMS-18), the ATLAS Experiment, and the Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS). What follows are the highlights of the trip with many details taken from the blog that was maintained as part of the experience.
Inside the Universe of Particles museum, the crystal tiles used in the forward calorimetry of LHC detectors to measure the momentum of scattered particles of very high pseudorapidity and cross-sections of the LHC magnets were on display. Other objects of interest included copies of the three papers from 1964 that predicted the existence of the Higgs Boson. The museum also featured Tim Berners-Lee's original proposal for the World Wide Web, which was originally intended as a method of collaboration between particle physicist researchers.
Independent of the active research sites, equipment from the early days of CERN in the 1940s and 1950s was on display in a collection that had the feel of a sculpture garden, featuring such items as an 8 foot logical flip-flop, which was originally used as an important component of the injectors for particle accelerators, and bubble chambers that were once pumped full of liquid freons to discover neutrinos and photograph particle jets. The DELPHI detector, decommissioned in 2000 in order to convert the Large Electron-Positron collider (LEP) into the modern Large Hadron Collider (LHC), was visible in a dissected form in the LHCb cavern, where it was actually possible to touch the copper wire matrix that made up its particle tracker. CERN scientist guides also explained how the Compact Muon Solenoid Experiment (CMS) - notable for being one of the two LHC detectors to discover a Higgs boson - improved on DELPHI's design with advancements such as silicon charge-coupled devices that provide higher resolution imaging of the particles as they curve in the magnetic field.
At the CERN data center, the guide outlined the flow of data from analogue input devices like the CMS lead tungstate calorimetry cells to the magnetic tape memory that will store the data forever. Originally, CERN computers were colossal mainframes with only a fraction of the power of some modern day kitchen appliances. Later, Cray Industries supercomputers became the backbone of CERN’s computing platform to such an extent that Cray measured their later computing devices in terms of CERN's processing power. Today, the supercomputing paradigm has yielded to massive parallel processor farms built up of everyday desktop PC's. This new paradigm allows for hundreds or even thousands of particle collision events to be reconstructed simultaneously on separate computers.
All of the different accelerator rings at CERN feed in to each other, using magnetic fields and synchronized electromagnetic radiation cavities of variable lengths to accelerate particles to very near the speed of light. Protons 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. Students visited one of the smaller accelerator rings, the Low Energy Ion Ring (LEIR), which receives lead ions from Linac 3 (a linear accelerator) and strips them down to lead nuclei that can be injected into the LHC. In the past, the LEIR infrastructure served as LEAR, the Low Energy Antiproton Ring, which circulated antiprotons and positrons in close enough proximity to synthesize anti-hydrogen.
A highlight of the site visits was the chance for the students to don hard hats and head 30 stories underground into the LHCb experimental area. LHCb (the Large Hadron Collider-beauty experiment), one of the LHC’s four major experiments, studies the differences between matter and anti-matter through precise measurement of different parameters of b-meson physics.
Specifically, the goal of LHCb 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 anti-helium, let alone anti-molecules or anti-matter stars.
The control facility for AMS (the Alpha Magnetic Spectrometer), a NASA-owned detector on the International Space Station, is located on the CERN campus because of its pertinence to particle physics. It is designed to detect anti-matter in cosmic rays and search for dark matter. The AMS experiment bears many similarities to the exhibits we viewed earlier, but unlike the LHCb and DELPHI it is entirely observational. Dr. Vincent Smith, a member of the CMS project recently retired from Bristol University, 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.
After two days at CERN, the group received a belated introduction. 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, TASS students enjoyed the opportunity to visit four different sites before even receiving an introduction! The group was once again graced by lecturer Dr. Vincent Smith, who gave 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.
At SMS-18, students learned about the theory of superconductivity and its role in the construction of LHC tunnel magnets, which 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.
ATLAS, another of the LHC’s four primary experiments, is the only LHC detector experiment located in Switzerland, directly across the street from the main CERN campus. Unlike the LHCb, which is designed exclusively for b-meson physics, ATLAS is a general purpose detector that can be used for a very wide variety of particle physics experiments. The ATLAS chamber had already been sealed off from the public by the time of the trip to prepare for the April/May return to normal operations, allowing engineers to work more closely with the detector.
Guides emphasized that ATLAS must be able to extract meaningful physics from the 40 million beam crossings that occur every second inside the detector chamber. At 25 megabytes of data per crossing, ATLAS has the capacity to generate approximately one petabyte of data per second, which would be intractable for any computing network within the budget of CERN. A vast majority of the calorimetry and spectroscopy data generated by ATLAS is extraneous, describing physical phenomena that we already understand fully. To use the Higgs boson as an example, two of the most common decay channels (two muons and four electrons) occur once in less than one billion proton-proton collisions. The low-level trigger must be able to differentiate between definitely not Higgs-containing and possibly Higgs-containing events efficiently, effectively, and, most importantly, without error. Within 25 nanoseconds, the trigger must make a decision on whether the event should be accepted or rejected.
Students and guides also discussed the different layers of ATLAS, which are, from interior to exterior: the pixel detector, semiconductor tracker, transition radiation tracker, internal solenoid, electromagnetic calorimeter, hadronic calorimeter, and finally muon spectrometer. The pixel detector and trackers lie inside of a powerful solenoid magnet, allowing them to capture the curvature of particles' trajectories. The ECAL and HCAL are designed to interact strongly with certain particles (i.e. electrons and hadrons, respectively), allowing precise measurement of particle energies. Additionally, ATLAS measures missing transverse energy, which may be due to the expulsion of high energy neutrinos, but also could be an indication of weakly interacting massive particles.
COMPASS (the Common Muon and Proton Apparatus for Structure and Spectroscopy), near the center of the LHC ring in France, collides protons from the SPS (Super Proton Synchrotron) with spin-aligned 6mK superfluid protons within a powerful magnetic field, with the goal of accurately measuring the spin of the proton. Unlike the LHC experiments that collide protons head-on, COMPASS exemplifies a fixed target experiment in which high speed protons are fired at stationary hydrogen atoms.
In addition to the days spent at CERN, time was allotted to traverse the city of Geneva and the surrounding countryside in the canton of Vaud (along the north shore of Lake Geneva). These excursions were spent admiring the region’s picturesque landscapes and castles while discussing contrasts in cultures, lifestyles, and schooling with the Finnish students (endless topics that were also expounded on over just about every meal as well). A full version of the blog that was maintained on a daily basis during the trip, as well as many pictures of the various items mentioned in this summary, can be accessed at http://tass.mbhs.edu/blog.php.
Looking forward to next year, the TASS program hopes to invite the Finnish students and teachers to the United States, as the current plan revolves around a trip to Fermilab. As always, the group is thankful for the support and encouragement from the Magnet program and the Magnet Foundation. Any support, suggestions, or information that alumni, family, or friends may be able to provide for future endeavors will be greatly appreciated.