About Us

Theoretical High Energy Physics (THEP) Group

In Theoretical High Energy Physics Group, we are working on high-energy phenomena in particle physics and cosmology and their implications for physics beyond the Standard Model (SM) and the Standard Big Bang Cosmology (SBBC). Our researches are mainly motivated by theoretical problems such as initial conditions for SBBC (such as homogeneity, isotropy), the hierarchy problem associated with Higgs mass in the SM, and the cosmological constant problem.

Covered are a variety of topics, ranging from cosmic inflation, dark matter at galaxies and cosmological scales, and self-tuning solutions to cosmological constant problem, to Higgs properties and new particles in supersymmetric models and BSM at colliders. We are also building new particle physics models to explain the observed data and are testing them via interplay between different experiments. Our researches are initiated in the theoretical frameworks based on quantum field theory and gauge (or global) symmetries, but their further developments rely strongly on the numerical analysis and simulations with various computational tools.

Research areas: Cosmology and Dark Matter

- Cosmic inflation and early universe

"It is said that there is no such thing as a free lunch.

But the universe is the ultimate free lunch"

- Alan Guth 1981

Cosmic inflation was introduced by Alan Guth (MIT) in 1981 to solve the horizon and flatness problems in the Standard Big Bang Cosmology. It is based on an exponential expansion of the Universe shortly after the Big Bang. Inflation can explain the homogeneity and isotropy of Cosmic Microwave Background (CMB) as well as anisotropies of CMB due to quantum fluctuations generated during inflation. Inflation models and comparisons to CMB measurements such as Planck are an active area in cosmology and can provide us a deep understanding of the early Universe through primordial gravitational waves.

A scalar field, the so called inflaton, plays a role of carrying a huge potential energy for inflation and ending the inflation after it relaxes into the minimum of the potential with a zero (very tiny) vacuum energy. Higgs boson is a fundamental scalar introduced in the SM for electroweak symmetry breaking and generation of masses. As the Higgs boson has been finally discovered at the LHC in 2012, it has become an important question whether the Higgs boson can be the inflaton.

- Dark matter and cosmic data

Dark matter is the ubiquitous but unknown component of matter in the Universe, supported by rotation curves of galaxies, gravitational lensing, collision of galaxy clusters. Moreover, CMB, BAO (Baryon Accoustic Oscillation) and SN (Supernovae) data are all consistently pointing to the fact that the energy density of the Universe is dominated by dark energy (68%) and dark matter (27%), while baryonic matter (based on protons and neutrons) is just 5%. However, the properties of dark matter such as mass and non-gravitational interactions in particle physics are not known.

Weak interactions, similarly to those of neutrinos, may be responsible for communicating between dark matter and the SM, named the WIMP (Weakly Interacting Massive Particle) paradigm. WIMP dark matter can annihilate into a pair of the SM particles such as quarks and leptons, so the indirect evidences for WIMP have been searched for in cosmic ray experiments on balloon and satellites. Dark matter, pervading near the Earth, can be also probed by the recoil energy of nucleons in elastic scattering between dark matter and heavy nucleus underground, which is named the direct detection.

SIMP (Strongly Interacting Massive Particle) paradigm has strongly motivated a new species of dark matter with mass below GeV scale, solving small problems in galaxies and galaxy clusters due to strong self-interactions of dark matter. Cosmic, astrophysical as well as collider data can be used to test the SIMP scenario in next years.

- Dark matter and collider physics

Dark matter can be produced at particle colliders such as the LHC (Large Hadron Collider).

Dark matter is not directly observed, but instead it leaves missing energy and momentum against visible particles. Direct production of dark matter is complementary to indirect searches for dark matter in cosmic data.

- Cosmological constant problem and dark energy

Cosmological constant is the vacuum energy in the Universe's ground state, that contains contributions from various unrelated dynamics, ranging from Electroweak phase transition (by which electroweak symmetry of the SM is broken into electromagnetism), QCD (Quantum Chromodynamics) phase transition (quarks to hadrons), etc, in the early Universe, to even the quantum fluctuations from electrons. It is known that dark energy dominates the current Universe and it is consistent with a cosmological constant. The problem of cosmological constant is why it is so small as compared to typical mass scales appearing in particle physics such as electroweak scale or QCD scale.

Research areas: Beyond the Standard Model and Unified Theories

- Higgs data and new physics

Higgs boson (spin-0 or scalar) has been discovered at the LHC on July 4th in 2012. From the decay products of the Higgs boson into the SM particles such as photons and leptons, etc, the Higgs mass and couplings to the SM particles are measured to be consistent with the SM predictions. As of summer 2015, the measured Higgs mass is (1GeV is about proton mass) mH=125.09±0.21(stat)±0.11(syst)

and the Higgs couplings are shown to be consistent with the SM values within 10% at 68% Confidence Level. Up to now, Higgs boson appears a fundamental scalar that has been discovered in nature for the first time. If true, the couplings of the SM particles to Higgs boson would lead to huge quantum corrections to the Higgs mass, so there must have been an extreme fine-tuning to get the measured Higgs mass. This problem is called the hierarchy problem, which is restated as the problem of the large difference between gravity and weak interactions. In our group, already proposed solutions to the hierarchy problem such as supersymmetry and warped extra dimensions, as well as new proposals, are investigated.

- Collider physics and new particles at the LHC and future colliders

LHC has been upgraded to 13TeV energy (1TeV = 1000 GeV) in last June in 2015 and it will make it possible to discover new heavy resonances (particles) in a near future. LHC will keep running until 2035 with maintenance short breaks. Future colliders with higher energies such as ILC (International Linear Collider, with electron and positron of 1TeV energies) and FCC (Future Circular Colliders, about 100 TeV) are scheduled or under discussion in the international particle physics community. Therefore, it is timely and important to prepare for the analysis of new data from the LHC and future colliders.

- Supersymmetric models & supergravity

“We are, I think, in the right Road of Improvement, for we are making Experiments.”

– Benjamin Franklin

Supersymmetry(SUSY) is a symmetry exchanging between fermions and bosons, that are thought to be separate in the Standard Model. It is the minimal extension of spacetime symmetries in four dimensions, including translations and rotations of spacetime. Each particle in the SM has its superpartner with the same mass and charge but differing by 1/2 in spin. For instance, electron has its superpartner, selectron (scalar electron), while quark has its superpartner, squark (scalar quark), and gauge boson has its superpartner, gaugino, Higgs boson has its superpartner, Higgsino. SUSY is broken in nature, meaning that superpartners are heavier than SM partners, but the interactions of superpartners inherited by SUSY remain. In particular, the lightest superpartner is stable and, if charge neutral, it could be a dark matter candidate and be searched for from missing momentum at colliders. SUSY has been regarded as one of the elegant solutions to the hierarchy problem so searches for superpartners have been one of the main physics goals at collider experiments, such as LEP, Tevatron and LHC. Supergravity, on the other hand, is the gravity version of supersymmetry, where SUSY is extended to graviton (the quantum of gravitational interaction). As a consequence, the superparner of graviton is so called gravitino. Although gravitino interacts with the SM particles very weakly like graviton, it has important effects on cosmology in the early Universe. For instance, the decays of gravitino can produce the dark matter abundance before BBN(Big Bang Nuclearsynthesis). If gravitino is the lightest superparticle, it can be a dark matter candidate on its own.

- Brane models & string theory

String theory has been known as Theory of Everything, encompassing the quantum theories of gauge and gravitational interactions in nature. It replaces point-like elementary particles by the excitations of a one-dimensional object, the string, and it requires supersymmetry for the consistency, being superstring.

String theory, depending on whether it is weakly or strongly coupled, is defined only in 10 or 11 spacetime dimensions. Therefore, extra space dimensions, 5 or 6, must be hidden, through the process of compactifications, meaning that they are too small to be observed in experiments. The standard picture of compactification is that there must be no extra charged particles up to the unification scale (about 10^16 GeV/c^2) where three gauge couplings of the SM are unified into a single value. In this case, the string scale, which is defined by the tension of the string, is also close to the unification scale. Then, there would be no observable effects in terrestrial experiments on Earth, because extra particles or string states are too heavy. However, due to the 2nd string revolution with D-branes in 90's, it is known that the string scale can be lowered, due to the fact that gauge and matter fields in the SM can be confined to D-branes (or branes in wide term) while graviton (the quantum of gravitational force) propagate into all the space dimensions. Furthermore, extra space dimensions can be warped such that the strength of gravitational interactions becomes different depending on the location in extra dimensions. This is so called warped extra dimensions, which have been proposed by Lisa Randall and Raman Sundrum in 1999. In brane models, it is possible to have a lower string scale, which might be close enough to the energies at LHC. In our group, we investigate Kaluza-Klein gravitons (graviton carrying extra momentum) and radion (the fluctuation of the volume of extra space dimensions) in the models with warped extra dimensions, focusing on the connection to collider and dark matter (in)direct detection experiments.

Computational tools

- Mathematica, Micromegas, LanHEP, FeynRules, Calchep, Madgraph, Pythia, Delphes, Root, etc.