KEK Experiment Research Articles

Searches for New Physics with the TREK Detector

The KEK Experiment E246 has provided the best upper limit of the transverse muon polarization in Kμ3 decays to date. The E246 detector is now being upgraded by the TREK collaboration and upcoming experiments with this detector at the J-PARC facility will allow new high-precision measurements in search for physics beyond the Standard Model. The first round of planned experiments include the search for lepton universality violation in a measurement of the ratio of the Ke2 and Kμ2 decay widths, as well as the search for heavy sterile neutrinos and possible massive gauge boson A' in the 10 to 100 MeV mass range. The search for T violation in kaon decays requires a higher intensity hadron beam and will be carried out later.

Gamma-ray spectroscopy of hypernuclei — present and future

Recent results and future directions of the γ-ray spectroscopy of Λ hypernuclei are discussed. A further analysis of the last KEK experiment (E566) has revealed a new γ transition in 12ΛC at 6048keV (preliminary), which is ascribed to the M1(13−→11−) transition. At J-PARC, studies will be extended from p-shell to s-shell and sd-shell hypernuclei. In the first part of the J-PARC E13 experiment, we will measure the 4ΛHe(1+→0+) transition to study charge-symmetry breaking in the ΛN interaction and investigate the 19ΛF hypernucleus to study ΛN spin–spin interaction further using one of the sd-shell hypernuclei. In the second part of E13, we will precisely measure the spin-flip B(M1) value for the 7ΛLi(3/2+→1/2+) transition. Beyond E13, we plan to study impurity effects by observing possible changes in nuclear deformation caused by the presence of a Λ in sd-shell hypernuclei, as well as to investigate various heavier Λ hypernuclei in order to precisely determine the energies of Λ single-particle orbits.

Many-body cascade calculation of final state interactions in C nonmesonic weak decay

We study the nonmesonic weak decay (NMWD) ΛN → nN of hypernucleus induced by one nucleon N = n, p. The whole process is described by two coupled models: one for the primary NMWD of Λ hyperon in the nuclear environment, and the other taking into account the final state interactions (FSI) of the two outgoing nucleons within the residual nucleus. The NMWD dynamics is represented by the one-meson-exchange potential with usual parametrization, while the independent-particle shell model is used as the nuclear structure framework. The FSI are accounted for by a time-dependent multicollisional Monte Carlo cascade scheme, implemented within the CRISP code (Collaboration Rio-São Paulo), which describes in a phenomenological way both the nucleon–nucleon scattering inside the nucleus and the escaping of nucleons from the nuclear surface. An evaporation phase is included to cool down the cascade residual nucleus as well. We analyze the single-nucleon kinetic energy spectra, and the two-nucleon coincidence spectra as a function of both: (i) the sum of the kinetic energies and (ii) the opening angle. The theoretical results are compared with recent data from KEK and FINUDA experiments, obtaining fairly good agreement for inclusive and exclusive kinetic energy spectra. Further theoretical improvements are required to explain angular distributions.

Nonmesonic Hyperon Weak Decay Spectra in 12ΛC

We study the nonmesonic weak decay (NMWD) ΛN → nN of the 12ΛC hypernucleus induced by the nucleon N = n,p with transition rate ΓN. The nuclear process is described by the interplay of two models; one describing the NMWD of hyperon Λ in the nuclear environment, and the other taking into account the Final State Interaction (FSI) of the two outgoing nucleons with the residual nucleus. The first one is done in the framework of the Independent-Particle Shell-Model (IPSM), with the decay dynamics represented by the exchange of π+η+K+ρ+ω+ K*mesons with usual parametrization. For the second one is used a time dependent multicollisional intranuclear cascade schema (implemented in the CRISP code - Collaboration Rio-São Paulo). The results obtained for inclusive and exclusive kinetic energy spectra, and the angular correlation are compared with recent data from KEK and FINUDA experiments. The calculated ratio (Γn/Γp)FSI, between the numbers of emitted back-to-back nn and np pairs, is in good agreement with the experimental data.

Kaonic helium atoms

The KEK experiment E570 has measured the Balmer-series x-rays of kaonic 4He atoms using large-area silicon drift x-ray detectors (SDDs) in order to study the low-energy K ¯ -nucleus strong interaction. The energy of the 3 d → 2 p transition was determined to be 6467 ± 3 (stat) ± 2 (syst) eV . The resulting strong-interaction energy-level shift is in agreement with theoretical calculations, thus eliminating a long-standing discrepancy between theory and experiment. Using a similar setup, the E17 experiment at J-PARC, the first experiment to be carried out in the hadron hall of J-PARC, will measure the 3 d → 2 p x-rays of kaonic 3He.

KAONIC HELIUM ATOMS

The KEK experiment E570 has measured the Balmer-series x-rays of kaonic 4 He atoms using large-area silicon drift x-ray detectors (SDDs) in order to study the low-energy [Formula: see text]-nucleus strong interaction. The energy of the 3d → 2p transition was determined to be 6467 ± 3( stat ) ± 2( syst ) eV . The resulting strong-interaction energy-level shift is in agreement with theoretical calculations, thus eliminating a long-standing discrepancy between theory and experiment. Using a similar setup, the E17 experiment at J-PARC, the first experiment to be carried out in the hadron hall of J-PARC, will measure the 3d → 2p x-rays of kaonic 3 He .

Extended-soft-core baryon-baryon model. II. Hyperon-nucleon interaction

The YN results are presented from the extended soft-core (ESC) interactions. They consist of local and nonlocal potentials because of (i) one-boson exchanges (OBE), which are the members of nonets of pseudoscalar, vector, scalar, and axial mesons; (ii) diffractive exchanges; (iii) two-pseudoscalar exchange; and (iv) meson-pair exchange (MPE). Both the OBE and pair vertices are regulated by Gaussian form factors producing potentials with a soft behavior near the origin. The assignment of the cutoff masses for the baryon-baryon-meson (BBM) vertices is dependent on the SU(3) classification of the exchanged mesons for OBE and a similar scheme for MPE. The particular version of the ESC model, called ESC04 [T. A. Rijken, Phys. Rev. C 73, 044007 (2006)], describes nucleon-nucleon (NN) and hyperon-nucleon (YN) interactions in a unified way using broken SU(3) symmetry. Novel ingredients are the inclusion of (i) the axial-vector meson potentials and (ii) a zero in the scalar- and axial-vector meson form factors. These innovations made it possible for the first time to keep the parameters of the model close to the predictions of the ${}^{3}{P}_{0}$ quark-antiquark creation model. This is also the case for the $F/(F+D)$ ratios. Furthermore, the introduction of the zero helped to avoid the occurrence of unwanted bound states. Broken SU(3) symmetry serves to connect the NN and the YN channels, which leaves after fitting NN only a few free parameters for the determination of the YN interactions. In particular, the meson-baryon coupling constants are calculated via SU(3) using the coupling constants of the NN analysis as input. Here, as a novel feature, medium-strong flavor-symmetry breaking (FSB) of the coupling constants was allowed, using the ${}^{3}{P}_{0}$ model with a Gell-Mann-Okubo hypercharge breaking for the BBM coupling. Very good fits for ESC model with and without FSB were obtained. The charge-symmetry breaking in the $\ensuremath{\Lambda}p$ and $\ensuremath{\Lambda}n$ channels, which is an SU(2) isospin breaking, is included in the OBE, TME, and MPE potentials. Simultaneous fits to the NN- and the YN-scattering data are described, using different options for the ESC model. For the selected 4233 NN data with energies $0\ensuremath{\le}{T}_{\mathrm{lab}}\ensuremath{\le}350$ MeV, a ${\ensuremath{\chi}}^{2}/{N}_{\mathrm{data}}=1.22$ was typically reached. For the usual set of 35 YN data and 3 ${\ensuremath{\Sigma}}^{+}p$ cross sections from a recent KEK experiment E289 ${\ensuremath{\chi}}^{2}/{\mathit{YN}}_{\mathrm{data}}\ensuremath{\approx}0.63$ was obtained. In particular, we were able to fit the precise experimental datum ${r}_{R}=0.468\ifmmode\pm\else\textpm\fi{}0.010$ for the inelastic capture ratio at rest rather well. The four versions (a,b,c, and d) of ESC04 presented in this article, give different results for hypernuclei. The reported G-matrix calculations are performed for YN ($\ensuremath{\Lambda}N,\ensuremath{\Sigma}N,\ensuremath{\Xi}N$) pairs in nuclear matter. The obtained well depths (${U}_{\ensuremath{\Lambda}},{U}_{\ensuremath{\Sigma}},{U}_{\ensuremath{\Xi}}$) reveal distinct features of ESC04a--d. The $\ensuremath{\Lambda}\ensuremath{\Lambda}$ interactions are demonstrated to be consistent with the observed data of ${}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}^{6}$He. The possible three-body effects are investigated by considering phenomenologically the changes of the vector-meson masses.

Experimental Progress in Hypernuclear Physics

Recent experimental progress of hypernuclear physics is reviewed. Detailed level structure of various p-shell A hypernuclei has been investigated by 7-ray spectroscopy with Hyperball and an NaI counter array, and the strengths of all the spin-dependent components of the AN interaction have been determined. The (e,e' K + ) reaction has been successfully introduced to hypernuclear spectroscopy at Jefferson Laboratory with a resolution better than 1 MeV (FWHM). The first experiment with the (π - ,K + ) reaction was carried out at KEK to produce neutron-rich hypernuclei. Double hypernuclear events interpreted as 4 Λ Λ H were observed by detecting two π - 's from sequential weak decay. In an emulsion-counter hybrid experiment, a double A hypernucleus 6 Λ Λ He was observed and the strength of the AA interaction was unambiguously derived. A series of KEK experiments studying weak decays of 5 Λ He and 1 2 Λ C have solved the so-called Γ n /Γ p ratio puzzle.

Spectrometer for measurements of φ mesons in nuclear matter produced through 12-GeV p+A reactions

A spectrometer was constructed to measure the invariant mass spectra of ρ, ω and φ mesons through their e +e − or K +K − decays. The KEK experiment E325 was successfully carried out using the new spectrometer, and observed the signature of an in-medium mass modification of the ρ and/or ω mesons for the first time. The details of the spectrometer are described.

High resolution spectroscopy of Λ hypernuclei: present status and perspectives

Recent results and future directions of the γ-ray spectroscopy of Λ hypernuclei are discussed. A further analysis of the last KEK experiment (E566) has revealed a new γ transition in 12ΛC at 6048keV (preliminary), which is ascribed to the M1(13−→11−) transition. At J-PARC, studies will be extended from p-shell to s-shell and sd-shell hypernuclei. In the first part of the J-PARC E13 experiment, we will measure the 4ΛHe(1+→0+) transition to study charge-symmetry breaking in the ΛN interaction and investigate the 19ΛF hypernucleus to study ΛN spin–spin interaction further using one of the sd-shell hypernuclei. In the second part of E13, we will precisely measure the spin-flip B(M1) value for the 7ΛLi(3/2+→1/2+) transition. Beyond E13, we plan to study impurity effects by observing possible changes in nuclear deformation caused by the presence of a Λ in sd-shell hypernuclei, as well as to investigate various heavier Λ hypernuclei in order to precisely determine the energies of Λ single-particle orbits.

Formula omitted] interaction study by kaonic atom at KEK and DAΦNE

The long-standing problem, known to be “Kaonic Hydrogen Puzzle”, has been solved by recent KEK experiment, which reports a repulsive shift of the Kaonic Hydrogen 1 s atomic state. Physics interest is now in the precise determination of the interaction including isospin dependence. We report a brief summary of the kaonic atom experiment at KEK (KpX) and present status of the precise experiment at DAΦNE (DEAR).

Comments to the partial wave analysis of the ηπ system produced in peripheral πp-interactions

Results of the partial wave analysis of the ηπ system obtained in the GAMS, E852, KEK and VES experiments are considered and compared. The status of a 0(1300) and π 1(1370) ϱ(1405) states is discussed in detail.

Hypernuclear γ spectroscopy with a Ge ball

We are constructing a large-acceptance germanium detector system (Ge ball) dedicated to hypernuclear γ-ray spectroscopy. Through studies of detailed hypernuclear structure we can obtain information on the spin-dependent ΛN interactions and investigate modifications of nuclear structure caused by a Λ as an impurity. Experimental plans are described in detail, in particular, for a KEK experiment on Λ 7 and for a BNL experiment on Λ 9Be, Λ 15 N, and Λ 16 O.

T-violating muon polarization inK+→μ+νγ

We study the T violating transverse muon polarization in the decay of $K^+\to \mu^+\nu\gamma$ due to CP violation in theories beyond the standard model. We find that the polarization asymmetry could be large in some CP violation models and it may be detectable at the ongoing KEK experiment of E246 as well as the proposed BNL experiment.

Supersymmetric time reversal violation in semileptonic decays of charged mesons

We provide a general analysis of time reversal violation arising from misalignment between quark and squark mass eigenstates. In particular, we focus on the possibility of large enhancement effects due to the top quark mass. For semileptonic decays of the charged mesons, K + → π 0 μ + ν μ , D + → K 0μ +ν μ , and B + → D 0τ +ν τ , the transverse polarization of the lepton P l ⊥ is a T-odd observable that is of great experimental interest. It is noted that under favorable choice of parameters, P μ ⊥ in K μ3 + decay can be detectable at the ongoing KEK experiment and it holds a promising prospect for discovery at the proposed BNL experiment. Furthermore, P τ ⊥ in B ± decay could well be within the reach of B factories, but P μ ⊥ in D ± decay is not large enough for detection at the proposed τ-charm factory.

Time Reversal Violation in K+ -> + Decay and Three Higgs Model

Transverse muon polarization in the $K^+ \rightarrow \mu^+ \nu \gamma$ decay is calculated in the model with scalar and pseudo scalar four-Fermi interactions. Combined with a similar calculation in the $K^+ \rightarrow \mu^+ \nu \pi^0$ decay, a possible constraint on parameters in the three-Higgs model is obtained assuming sensitivity of the up-coming KEK experiment. It is pointed out that the predictions for the two polarizations are strongly correlated in the three Higgs model.

The (π +, K +) Λ-hypernucleus production reaction in the response-function formalism

We present a detailed study of the excitation spectra of the Λ-hypernuclei obtained through the (π +, K +) reaction, and we compare our results for the cross sections with the most recent data from BNL and KEK experiments. The cross sections are obtained in the distorted-wave-impulse approximation (DWIA) with a proper optical potential for pions and kaons, obtained in the eikonal approximation. For the first time, a completely self-consistent treatment of the Λ-hypernuclei structure has been done, by means of the response-function method in the Hartree-Fock (HF) approximation with a Skyrme-type interaction. Moreover, the continuum is treated exactly with the Green-function method. Our results, from Λ 12C to Λ 89Y, agree quite well with the experimental data. The shell model seems to give a very good description of these systems, where the Λ-particle occupies single-particle levels with no sign of collective effects and is uninhibited by the Pauli principle. Possible improvements of our model are suggested.

TKO 32-channel pipeline TDC module using a 1 GHz GaAs shift register

A high-density, high-speed pipeline TDC (time-to-digital converter) that operates at a 11-GHz clock frequency has been developed. The depth of each internal shift register is 256. Special GaAs shift register and GaAs clock controller/driver ICs were developed for this module. Operating at 1-GHz clock speed, it can provide a time resolution of 0.4 ns (RMS) and a time range of 256 ns. This module is also a reasonably low-power unit for its class due to an inherent property of GaAs ICs. This pipeline TDC module is at the mass-production stage and will be used for a KEK experiment starting at the end of 1990. The pipeline nature of the TDC provides multihit capability, which is essential for experiments with a high hit rate on each detector channel. >

Strangeness production in Madron-nucleus reactions

Strange-particle production in hadron-nucleus reactions is discussed. Emphases are: (1) Although enhancements of strangeness production may be a good signature of exotic phenomena associated with highly-excited nuclear matter, one must carefully separate effects of secondary processes. An enhancement of Λ production in p̄-nucleus reactions observed in a KEK bubble-chamber experiment was explained by secondary processes. (2) Hyperon polarization provides a good tool to probe quark structures of hadrons and nuclei. A KEK experiment on hyperon-beam polarization indicated that the quark-diquark recombination model seemed to work not only at high energies but also at low energies. It is interesting to study possibilities and/or limitations of applying such parton-recombination pictures to low-energy phenomena in the target-rapidity region. (3) Hyperon polarization also provides a variety of applications to spin physics of hypernuclei.

Spectroscopy of hypernuclei with stopped kaons — recent result of KEK experiment

Pion momentum spectra from K − absorption at rest on various nuclear targets were measured by means of a magnetic spectrometer with wide momentum range (100 ∼ 300 MeV/c). The ground state of Σ − hypernucleus, if well bound with a narrow width, is expected to be populated in 12C(stopped K −, π +) inclusive (untagged) spectrum, but such a peak was not observed. The spectrum is compared with the DWIA calculations by Morimatsu and Yazaki, indicating that the depth of the nuclear potential of Σ − is shallower than 12 MeV, and that its imaginary part is larger than 5 MeV if the potential depth is about 10 MeV. In the 12C(stopped K −, π −) spectrum, the ground state (( p3 2 ) n −1 ( s1 2 )Λ) and the excited states (( p3 2 ) n −1( p)Λ) of Λ 12C were observed, and their formation probabilities were roughly in agreement with the results of DWIA calculations. In the π − spectra on 12C, 9Be, and 7Li targets a distinct peak was observed at 132.1±0.7 MeV/c. It is ascribed to π − from the mesic decay of Λ 4H; Λ 4 H → 4He π −. The formation probability of Λ 4H on C, Be, or Li target is much larger than those of the discrete states of Λ 12C via the direct (stopped K −, π −) reaction.