Kensuke Homma is a particle physicist covering a wide range of collision systems where he studied the internal structure of proton, Higgs hunting, and the QCD phase structure. These studies are relevant to the dominant part of “visible” Universe. His current interest is further extended to “invisible” part of the Universe, that is, the vacuum structure at the present Universe where something dark such as dark matter and dark energy occupy more than 90% of the energy density of the Universe. These dark components are not necessarily probed by conventional charged particle colliders. His focus under IZEST is to challenge to probe such dark fields via quantum optical observables in vacuum by combining high-intensity and high repetition rate laser fields. He is also conducting experiments planned for Extreme Light Infrastructure in the Romania site where searches via multi-energy-scale photon interactions will be performed.
Figure 1 Possible photon-photon interactions in various energy scales.
Interactions between photons contain rich information on particle physics and the background vacuum state. As illustrated in Fig.1, depending on center of mass energies in photon-photon collisions, we can discuss different kinds of dynamics. At the 100 GeV scale, the Higgs boson, relevant to electroweak dynamics, has been recently discovered dominantly via the decay channel into two photons. At the 100 MeV, we know, for instance, 0 , relevant to QCD, decays into two photons. These facts imply that we can inversely produce resonance states by colliding photons at different energy scales more than three orders of magnitude separated. Apart from any theoretical models, these experimental facts are enough for us to expect something similar in very different energy scales too which have not been explored intensively yet. Especially below MeV scales, systematic experimental investigations on photon-photon interactions have not been performed to date. Some of them may be caused by interactions via weakly coupling resonance states which can be candidates to explain something dark in the Universe [1,2]. High-intensity and high repetition rate lasers can make such searches viable depending on how we combine those technologies.
At sub-eV domains, we have conducted searches for sub-eV dark matter candidates based on stimulated laser colliders, in other words, searches for four-wave mixing photons from the vacuum as illustrated in Fig.2 [3,4]. This search strategy consists of two parts . The one is the resonance creation via photon-photon collisions in a laser field, which is the exactly same approach as that in conventional particle colliders. However, the significant difference is its collision geometry. We consider a quasi-parallel colliding system (QPS). This colliding system allows us to reach extremely low center-of-mass energy such as sub-eV domains via the small incident angle even if we use a photon energy of 1eV. The second part is to stimulate decay of a produced resonance state by the other background laser field. This feature is never utilized in high-energy particle colliders, because controlled coherent fields are not available at higher energy scales so far. The overall rate of the creation and stimulated decay has cubic dependence on the number of photons per laser pulse. This cubic nature can enhance the interaction rate especially in the case of strong laser fields. Compared to the number of charged particles, typically 1011 particles per bunch in conventional accelerators, MJ laser, for instance, can provide 10 times of Avogadro’s number of photons per pulse. The cubic of this number provides a huge enhancement factor on the interaction rates, in other words, high sensitivity to weakly coupling interactions which have never been probed by conventional charged particle accelerators to date .
Figure 2 Four-wave mixing process in the vacuum via a resonance particle exchange .
We have proposed the extension of the same experimental method in the technical design report (RA5 experiment) submitted to ELI-NP  where up to 10 PW lasers are available and the proposal has been officially approved. We are now in the preparatory phase.
In the same report, we also have proposed an all-optical gamma-gamma collider exploiting the laser plasma accelerator (LPA) technology as illustrated in Fig. 3. In this proposal we aim at the first direct observation of helicity dependent real photon – real photon scatterings at the MeV range  where the QED-based cross section is maximally enhanced.
Figure 3: An all-optical table-top (3.4 m x 1.3 m) gamma-gama collider : a) top-view including two LPAs and the detector system to capture the gamma+gamma→gamma+gamma scattering, b) collision geometry around the interaction point, IP, where gamma-rays are produced at each Compton scattering point (CP) in head-on collisions and D is the distance between IP and CP. c) a QED-based gamma+gamma → gamma + gamma event simulated by GEANT4. d) a background e- + e- → e- + e- event simulated by GEANT4. The blue and black trajectories in the event displays denote photons and electrons, respectively.
As a further upgraded subject in the report, we plan the measurement of the vacuum birefringent effect with focused 10 PW laser fields available at ELI-NP by utilizing GeV gamma-rays as the probe radiation . For this proposal we will develop a gamma-ray polari-calorimeter . These experimental proposals are very challenging from the technological point of view, because they both need high energy electrons from LPA. However, in principle, all are possible if the combinations of 0.1-10 PW laser systems are properly designed.
Figure 4: Conceptual design for the vacuum birefringence measurement by combining 10 PW laser pulses and 1 GeV gamma-ray probes .
A high-intensity and high-repetition rate laser system such as ICAN laser  will be commonly beneficial for the extension of these proposals relevant to intriguing questions on particle physics and cosmology.
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 Y. Fujii and K. Maeda, The Scalar-Tensor Theory of Gravitation Cambridge Univ. Press (2003).
 Kensuke Homma, Takashi Hasebe, Kazuki Kume, Prog. Theor. Exp. Phys. 2014, 8, 083C01 (2014).
 T. Hasebe, K. Homma, Y. Nakamiya, K. Matsuura, K. Otani, M. Hashida, S. Inoue, S. Sakabe, Prog. Theor. Exp. Phys. 2015, 7, 073C01 (2015).
 Y. Fujii and K. Homma, Prog.Theor. Phys. 126 (2011) 531-553; K. Homma, D. Habs, T. Tajima, Appl. Phys. B 106:229-240 (2012); K. Homma, Prog. Theor. Exp. Phys. 2012 04D004.
 T. Tajima and K. Homma, International Journal of Modern Physics A, Vol. 27, No. 25 (2012) 1230027; K. Homma, Eur. Phys. J. ST 223 (2014) 6, 1131-1137.
 K. Homma et al., “Highlights of RA5: Combined Laser - Gamma Experiments”, to be published in Romanian Reports in Physics (2016).
 K. Homma, K. Matsuura, K. Nakajima, “Testing helicity dependent γγ→γγ scattering in the region of MeV”, to be published in Prog. Theor. Exp. Phys. (2016), http://arxiv.org/abs/1505.03630.
 Y. Nakamiya, K. Homma, T. Moritaka, K. Seto, “Probing vacuum birefringence under a high-intensity laser field with gamma-ray polarimetry at the GeV scale”, http://arxiv.org/abs/1512.00636.
 K. Homma and Y. Nakamiya, “Gamma Polari-Calorimetry with SOI pixels for proposals at Extreme Light Infrastructure (ELI-NP)”, Proceedings of International Workshop on SOI Pixel Detector (SOIPIX2015), Tohoku University, Sendai, Japan, 3-6, June, 2015. C15-06-03, http://arxiv.org/abs/1507.06179.
 G. Mourou, B.