Part of Tim’s professional activities includes collaboartions that have brought him to CERN a handful of times over the past two decades. This page is to capture the adventure more than the technical aspects, so this is only a brief technical summary of his most recent activity (2018) taking him to CERN.
Tim stands beside the Large Hadron Collider (LHC) in CERN’s underground accelerator tunnel. The LHC is a 27-kilometer (17-mile) superconducting synchrotron that accelerates two counter-rotating particle beams to nearly the speed of light and brings them into collision at designated interaction points around the ring. For proton operation, the LHC has reached 6.8 TeV per proton, producing 13.6 TeV proton-proton collisions. In heavy-ion operation, fully stripped lead nuclei (208Pb82+) are accelerated to energies exceeding 500 TeV per nucleus, producing lead-lead collisions with a nucleon-pair center-of-mass energy of about 5.36 TeV. A lead nucleus itself is only about 14 femtometers across—roughly 10 trillion times smaller than a human hair—yet when two such nuclei collide, hundreds of their constituent protons and neutrons can interact, producing thousands of particles and briefly creating quark-gluon plasma at temperatures of several trillion degrees. During LHC heavy-ion running, these Pb-Pb interactions occur at rates of many thousands of collisions per second. The enormous blue cryostat beside Tim contains superconducting accelerator magnets operating at approximately 1.9 K (−271 °C), colder than outer space, which generate the intense magnetic fields required to bend these extraordinarily energetic particles around the LHC ring.
Shown above with Tim standing beside it for scale, the Compact Muon Solenoid (CMS) is one of the two largest general-purpose particle detectors at CERN’s Large Hadron Collider (LHC). Located about 100 meters underground near the French village of Cessy, CMS surrounds one of the points where the LHC brings beams of protons—or, during heavy-ion experiments, lead nuclei—into collision at nearly the speed of light. Despite the word “compact” in its name, CMS is enormous: roughly 21 meters long, 15 meters in diameter, and 14,000 tonnes. Its many concentric layers function much like an extraordinarily sophisticated three-dimensional camera, measuring the paths, energies, and identities of particles produced in each collision. At its heart is one of the world’s largest superconducting solenoid magnets, generating a 3.8-tesla magnetic field that bends the trajectories of electrically charged particles so their momentum can be determined. CMS was one of the experiments responsible for the 2012 discovery of the Higgs boson, and today it continues to investigate the fundamental structure of matter, the forces governing elementary particles, and the extraordinary state of matter created when heavy nuclei collide at LHC energies.
CERN’s Large Hadron Collider does more than collide protons. During special heavy-ion runs, it collides lead nuclei to recreate tiny droplets of quark-gluon plasma — the extraordinarily hot state of matter believed to have filled the universe shortly after the Big Bang. The geometry of each collision matters: some nuclei collide head-on, while others strike off-center.
To help determine that geometry, University of Maryland radiochemist Alice Mignerey and collaborators developed the Spectator Reaction Plane Detector (SRPD), an upgrade to the Zero Degree Calorimeters of the LHC’s CMS experiment. In an off-center collision, some neutrons do not participate directly in the collision and instead continue almost straight down the beam line. These “spectator neutrons” preserve information about the orientation of the original nuclear collision.
Located approximately 140 meters on either side of the CMS collision point, the SRPD uses a compact array of quartz Cherenkov radiators to measure the spatial distribution of these spectator neutrons. Particle showers created in the ZDC produce flashes of Cherenkov light in the quartz; wavelength-shifting fibers carry the light to photodetectors. From the pattern of light across the detector, physicists can reconstruct the spectator plane and use it as a reference for studying the behavior of the quark-gluon plasma. Tim participated in the detector program through test-beam measurements at CERN and radiation-damage studies at the University of Maryland. Because the SRPD operates in one of the LHC’s harshest radiation environments, its quartz radiators and optical components gradually lose transparency. Koeth’s work helped characterize how irradiation changes the detector’s optical response, information important both for interpreting detector performance and for developing more radiation-resistant future detectors.
The Spectator Reaction Plane Detector (SRPD) is a specialized detector developed for the CMS heavy-ion physics program at CERN’s Large Hadron Collider. During collisions of lead nuclei, not every proton and neutron participates directly in the collision. Some continue traveling forward as “spectators.” The SRPD is designed to detect signals associated with these spectator particles, allowing physicists to determine the orientation, or reaction plane, of the nuclear collision—an important reference for studying the collective behavior of the extremely hot, dense matter known as the quark-gluon plasma. The detector uses segmented radiation-sensitive elements and fast electronics to measure these signals in the exceptionally intense radiation environment close to the LHC beam. Shown above is one of the SRPD detector assemblies undergoing laboratory bench testing, where its response is being measured with an oscilloscope before placement in the CMS experiment.
Above, Tim holds one of the Spectator Reaction Plane Detectors (SRPDs) shortly before its installation with the CMS Zero Degree Calorimeter (ZDC) behind the LHC’s Target Absorber Neutral (TAN). The SRPD was designed to fit into the exceptionally confined detector space associated with the ZDC, where it would measure the spatial distribution of spectator neutrons emerging from lead-lead collisions. Here, the detector and ZDC components are being prepared above ground before the assembly is transported down into the LHC tunnel for installation behind the TAN. The photograph provides a useful sense of scale for the SRPD itself before it became part of the operating CMS heavy-ion experiment.
The two above photographs show Tim and Michael Murray installing the Spectator Reaction Plane Detector (SRPD) into the Zero Degree Calorimeter (ZDC) system at CERN’s Large Hadron Collider. The work is being performed approximately 140 meters from the CMS collision point, where the ZDC sits in the extremely confined region of the LHC tunnel near the Target Absorber Neutral (TAN). The TAN separates the two outgoing LHC beam pipes while allowing neutral particles produced in collisions to continue toward the ZDC. The SRPD is installed with the ZDC to measure the spatial distribution of spectator neutrons emerging from lead-ion collisions, providing information used to reconstruct the orientation of the collision geometry, or reaction plane. These photographs give a particularly useful sense of the practical engineering behind a particle-physics experiment: a detector developed and tested on the laboratory bench ultimately has to be physically installed, connected, and commissioned within the crowded infrastructure of the operating LHC.
The two above photos are of Tim performing signal and readout tests of the installed Spectator Reaction Plane Detectors (SRPDs) on both the positive and negative sides of the CMS interaction point. Each are located roughly 140 meters on either side of CMS. Following installation, Tim used portable test equipment to verify detector signals, cabling, and readout performance from each end of the experiment. Testing both sides was essential because the two SRPD systems independently observe spectator neutrons traveling in opposite directions following lead-lead collisions. Together, their measurements provide information about the geometry and orientation of the nuclear collision and support CMS studies of the quark-gluon plasma. These photographs show the less-visible side of experimental particle physics: commissioning precision instrumentation deep underground, directly alongside the LHC beamline, before it can begin collecting physics data.
Tim stands in the underground service tunnel connecting the CMS detector hall with the experiment’s electronics and service areas. The tunnel provides a physical link between the massive detector and the infrastructure used to power, control, monitor, and read out its many detector systems—a reminder of the extensive underground complex required to operate CMS beyond the detector itself.
Shiva Nataraja at CERN
Standing near CERN’s residential buildings is a two-meter-tall statue of Shiva Nataraja — Shiva as the “Lord of the Dance.” The statue was presented to CERN by India and unveiled in 2004 to commemorate the scientific collaboration between India and CERN that began in the 1960s.
In this traditional representation, Shiva performs the cosmic dance, surrounded by a ring of flames symbolizing the continual processes of creation, transformation, and destruction. CERN notes an especially appropriate connection to particle physics: the imagery of Shiva’s cosmic dance has been used as a metaphor for the ceaseless interactions and transformations of matter at the subatomic scale. The statue was made in India using a traditional lost-wax casting technique and now stands permanently in the square between CERN Buildings 39 and 40.
Shiva Nataraja at CERN — This two-meter statue of Shiva as the cosmic dancer was a gift from India to CERN in 2004, commemorating decades of Indian participation in CERN research. Shiva’s dance symbolizes the continual creation, transformation, and destruction of the universe—an evocative parallel to CERN’s exploration of the dynamic world of subatomic particles.