RIKEN Nishina Center

Normal particles carry a microscopic arrow of causality.Lee-Wick ghosts carry the reversed arrow, mediating characteristic collider signals in flat space:opposite-sign scattering amplitudes that violate positivity bounds;acausality on time scales set by their negative decay rate.During inflation, the corresponding cosmo-collider ghost signals are:opposite-sign non-Gaussianities;Boltzmann-unsuppressed local oscillatory signals without their non-local counterparts;IR-enhanced bi-spectrum and power spectrum, depending on the dimension of the interaction operator, whichdecreases if the ghost decay rate is comparable to the Hubble rate.

Abstract We present high-resolution X-ray spectroscopy of the merging cluster Abell 754 using XRISM/Resolve. In GO1 phase, XRISM/Resolve observed Abell 754 in two deep pointings, targeting the eastern primary core (114 ks) and the middle of the X-ray filamentary structure (190 ks). Spectral fits to full field-of-view data reveal a line-of-sight velocity difference of $656 \pm 35$ km s$^{-1}$ between the two pointing, corresponding to a bulk Mach number of 0.45$\pm$0.03. Velocity dispersions are measured to be $220^{+26}_{-29}$ km s$^{-1}$ and $279^{+24}_{-23}$ km s$^{-1}$ in the eastern and middle pointing, respectively. Within the eastern core, the velocity dispersion shows spatial variation, reaching $497^{+144}_{-117}$ km s$^{-1}$ in the southern core with high temperature—among the largest values yet reported in galaxy clusters to date. Narrow-band analysis of the Fe–K complex in this region reveals systematically higher temperatures derived from He-like and H-like Fe line ratio compared to those obtained via broadband fits, indicating multi-phase structures. Two-temperature modeling further separates a cooler core phase from a hotter, shock or turbulence-heated phase whose velocity is blueshifted, similar to that of the middle pointing. These results point to a mixing interface where post-shock gas from the south overlaps, in projection, with cooler core gas, inflating the observed line widths in this region. Weak-lensing analysis with Subaru/HSC and Suprime-cam confirms that the eastern component is about twice as massive as the western one, consistent with disruption and gas stripping of the latter. The curved morphology of the eastern X-ray core, together with the measured kinematics, is naturally explained by an off-axis, post–core-passage merger that imparts angular momentum and drives large-scale rotational and fallback flows.

The FAMU experiment at RIKEN-RAL aims to measure with high precision the ground state hyperfine splitting (1S HFS) of muonic hydrogen and thus determine the proton Zemach radius. A novel method based on the detection of X-rays emitted by μO after the muon transfer from μH to oxygen is used. A high performance X-ray detectors' system and an innovative mid-infrared (MIR) laser system were therefore developed. The X-ray detector's system is mainly based on LaBr3:Ce crystals read by SiPM arrays. From the MIR laser frequency value, known with precision < 10-5, the energy of the HFS transition may be accurately measured, as ΔE HFS = hν HFS.

Abstract We report 14 proton-rich new isotopes with atomic numbers (Z) of 63–70 produced via projectile fragmentation of a 345-MeV/nucleon $^{238}$U beam at the RI Beam Factory, RIKEN, based on the new-isotope evaluation shown in the National Nuclear Data Center as of October 2025: $^{132,133}_{\qquad \! 63}$Eu, $^{133,134,136}_{\qquad \quad \, \, 64}$Gd, $^{136,137,138}_{\qquad \quad \, \, 65}$Tb, $^{138}_{\ 66}$Dy, $^{143}_{\ 67}$Ho, $^{143}_{\ 68}$Er, $^{144}_{\ 69}$Tm, and $^{147,148}_{\qquad \! 70}$Yb. These highly proton-rich radioactive isotopes were separated and identified using the large-acceptance two-stage separator, BigRIPS. A 10-µm Ta charge stripper was used in the first stage of the separator to efficiently remove less-exotic lower-Z contaminants than the isotopes of interest: Hydrogen-like and fully stripped ions were transported before and after the charge stripper, respectively. Production cross sections of the new isotopes and their neighboring nuclei were determined and compared with semi-empirical formulas, among which EPAX2.15 showed the best agreement with the experimental data.

Abstract We report a long-term, high-cadence timing and spectral observation of the X-ray pulsar SMC X-1 using NinjaSat—a 6U CubeSat in low-Earth orbit—covering nearly a full superorbital cycle. SMC X-1 is a high-mass X-ray binary exhibiting a 0.7 s X-ray pulsar and a nonstationary superorbital modulation with periods ranging from approximately 40 to 65 days. Its peak luminosity of 1.3 × 10 39 erg s −1 makes it a local analog of ultraluminous X-ray pulsars powered by supercritical accretion. We find that the spinup rate during the high state remains consistent with the long-term average, with no significant correlation between the spinup rate and flux. This result indicates that the modulation is primarily geometric rather than accretion-driven. The hardness ratio and spectral shape are stable throughout the entire superorbital cycle, supporting obscuration by optically thick material or energy-independent scattering. In addition, the 2–20 keV pulse profile varies with superorbital phase, which may be explained either by a variable covering fraction, due to geometric obscuration, or by free precession of the neutron star. This represents the first complete measurement of the spinup rate and spectral evolution across a single superorbital cycle in SMC X-1, highlighting the scientific capability of CubeSat-based observatories.

We report atomic mass measurements of the unstable nuclides 73−75 Ni, 73−78 Cu, and 74−78 Zn, which have been accomplished using multi-reflection time-of-flight mass spectrometry combined with new technical developments to resolve challenges for exotic-isotope identification and selection. The isotopes were produced in-flight at the RIKEN’s Radioactive Ion Beam Facility and delivered to the combined gas cell and multi-reflection system installed downstream of the ZeroDegree spectrometer. The incoming high-energy beam was energy-degraded and subsequently stopped in a helium gas cell. The energy degrader thickness was optimized using a new method that employs signals from plastic scintillators located upstream and downstream of the helium-filled gas cell. Extracted isotopes of interest were mass-selected by the in-MRTOF deflector method, for which we discuss simultaneous selection of multiple isobar chains. The ions of interest were identified unambiguously using β-decay-correlated mass measurements for the first time, which is demonstrated for 78 Zn. The new mass values are compared with literature values and recent measurements performed at JYFLTRAP and ISOLTRAP, where a generally good agreement is observed.

Morton and Vivaldi defined the polynomials whose roots are parabolic parameters for a one-parameter family of polynomial maps (called delta factors here). They conjectured that delta factors are irreducible for the family z↦z2+c. One can easily show the irreducibility for periods 1 and 2 by reducing it to the irreducibility of cyclotomic polynomials. However, for periods 3 and more, this becomes a challenging problem. We prove the irreducibility of delta factors for period 3 and demonstrate the existence of infinitely many irreducible delta factors for periods greater than 3.

Abstract We report proton-rich new isotopes obtained in the region of atomic number $Z$ = 60 produced by the projectile fragmentation of a 345-MeV/nucleon $^{238}$U beam at the RI Beam Factory, RIKEN. Based on the evaluation shown in the National Nuclear Data Center as of June 2025, the following 13 new isotopes have been discovered: $^{118,119}_{\qquad \! 57}$La, $^{119,120}_{\qquad \! 58}$Ce, $^{122,123}_{\qquad \! 59}$Pr, $^{123,124,126}_{\qquad \quad \, \, 60}$Nd, $^{125,126,127}_{\qquad \quad \, \, 61}$Pm, and $^{128}_{\ 62}$Sm. These highly proton-rich radioactive isotopes were separated and identified using BigRIPS, the large-acceptance two-stage separator at RIKEN. Hydrogen-like ions of the isotopes of interest were transported between the production target and the first degrader, following which the ions were fully stripped to increase the purity of the new isotopes in the secondary beams. The cross sections of the highly proton-rich isotopes were deduced and compared with the predictions using semi-empirical formulas.