Particle Physics Group,Department of Physics, Graduate School of Science, Kobe University
The ATLAS experiment is a particle physics experiment using proton collisions at the world’s highest energy accelerator, the LHC (Large Hadron Collider). The LHC is a 27 km circumference accelerator located at CERN (European Organization for Nuclear Research) in Geneva, Switzerland. By colliding protons at a center-of-mass energy of 14 TeV (tera-electronvolts, where tera = 10¹²), the experiment produces a large number of particles at an electroweak scale (around 100 GeV) for precision measurements, while also searching for new particles and interactions in the TeV region, i.e. physics at the Terascale.
The LHC is currently undertaking Run 3 (2022–2026). It has already collected more collision data than in the second phase (up to 2018), and is expected to gather more than three times that amount by 2026, enhancing sensitivity to new physics. Afterward, a major upgrade of both the detector and accelerator will take place over approximately three years of a shut-down period, followed by the High-Luminosity LHC (HL-LHC) from 2030. The HL-LHC will achieve about three times the current collision rate, aiming to collect nearly ten times the data of the Run 3.
Kobe University has made significant contributions to the construction of the endcap muon trigger detector (Thin Gap Chamber = TGC), and is actively involved in the development and operation of the trigger system that is necessary for taking efficiently essential physics data. Our activity has been contributing greatly for the smooth data acquisition of the ATLAS experiment. Currently, the final stage of trigger upgrades for the HL-LHC is underway. We are also actively involved in data analysis to find new physics, including supersymmetry.
The Standard Model of particle physics, which describes the currently known elementary particles and their interactions, has been tested through various experiments at ver high precision. However, it does not answer fundamental questions such as: why there are exactly three types of quarks and leptons (e.g., electrons and neutrinos); why their masses differ by more than 11 orders of magnitude etc.
Therefore, the Standard Model is not considered the ultimate theory. It is believed that at the Terascale—i.e., the TeV energy region—elementary particles should be described by a new theory that encompasses the Standard Model. At the LHC, such physics is explored through high-energy collisions, aiming to directly discover new phenomena and investigate their properties.
Two major areas of expected physics are as follows:
The Higgs boson was discovered in July 2012 (announced by CERN), and the Nobel Prize in Physics was awarded in 2013 (reference). Since then, the LHC has been conducting detailed investigations into the properties of the Higgs boson, including whether it is truly the sole origin of mass.
In parallel, searches for new physics such as supersymmetry continue. By improving the precision of these measurements, researchers aim to detect deviations from the Standard Model, which could guide the future direction of particle physics.
These achievements have been recognized by science communities: the ATLAS experiment, along with other LHC experiments, received the Breakthrough Prize in Fundamental Physics.
The ATLAS experiment is thus striving to uncover hints of physics beyond the collision energy scale through a wide range of approaches. Would you like to join us in this exciting research?
Kobe University is the third-largest group in the ATLAS Japan collaboration, following the University of Tokyo and KEK (High Energy Accelerator Research Organization). The group has played a major role in detector development, construction, installation and operation, as well as data analysis.
Current activities are centered around three main pillars:
Current quantum field theories that describe elementary particles and their interactions cannot give mass to the particles of the Standard Model while preserving gauge invariance—the principle that the theory remains unchanged under local phase transformations of the fields.
If particles had no mass, they would move at the speed of light, which contradicts our observations of the real world. To resolve this, physicists propose the existence of the Higgs field, which slows particles down—effectively giving them inertial mass.
Although the Higgs field itself cannot be directly observed, fluctuations in the field caused by interactions with massive particles can be detected as Higgs bosons.
At the LHC, high-energy collisions produce massive particles that excite the Higgs field, resulting in the creation of Higgs bosons, which are then studied to understand the origin of mass.
Since particles can be distinguished by their mass, the Higgs field is thought to hold the key to solving mysteries in the Standard Model, such as the number of generations and the large differences in particle masses. The Higgs field in the Standard Model corresponds to the single type that gives rise to the discovered Higgs boson. Based on this, the Standard Model predicts a match between the masses of fermions (horizontal axis in the upper right figure) and their coupling constants with the Higgs boson (vertical axis). Measurements to date have shown that this relationship holds with extremely high precision. By conducting such measurements with even greater accuracy, researchers aim to uncover clues about whether there is truly only one type of Higgs field, or whether there might be unknown particles that couple to the Higgs field.
Supersymmetry Supersymmetry is a theoretical framework that proposes the existence of partner particles for all known Standard Model particles (shown as “particles” in the right figure), which interact in the same way. These are referred to as supersymmetric “shadow” particles (shown in the right figure). While they are not identical to their Standard Model counterparts, there are two key differences.
The first is spin: fermions in the Standard Model (with spin 1/2) are paired with bosons of spin 0, and bosons are paired with fermions. The second is mass: supersymmetric particles are believed to have masses on the TeV scale, much heavier than Standard Model particles, which is why they have not yet been discovered.
The operator that transforms a particle into its supersymmetric partner is known to cause distortions in spacetime, as described in general relativity. This suggests that supersymmetry is essential for unifying quantum field theory with gravity. Furthermore, if the lightest neutral supersymmetric particle is stable, it becomes a strong candidate for dark matter—a form of matter that is massive and interacts very weakly, whose existence is strongly supported by cosmological observations.
Supersymmetry also offers various theoretical advantages and is considered one of the most promising extensions beyond the Standard Model.
Despite the high energies achieved at the LHC, no supersymmetric particles have yet been observed. One possible reason is that the mass differences between these particles are small. If so, the energy of the Standard Model particles emitted during the decay of a supersymmetric particle into the lightest neutral one would be very low, making them difficult to distinguish from the many other particles produced in collisions. Alternatively, if the particles are heavy, they may move slowly and be detected with a delay after the collision.
At Kobe University, researchers are analyzing data to search for such particles and developing triggers to capture these slow-moving particles.
