2γ Decay Research Articles

Measurement of the Isolated Nuclear Two-Photon Decay in ^{72}Ge.

The nuclear two-photon or double-gamma (2γ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the 2γ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to ∼100 keV and half-lives as short as ∼10 ms. The half-life for the 2γ decay of the first-excited 0^{+} state in bare ^{72}Ge ions was determined to be 23.9(6)ms, which strongly deviates from expectations.

First-principle calculation of the [formula omitted] decay width from lattice QCD

We perform a lattice QCD calculation of the ηc→2γ decay width using a model-independent method that requires no momentum extrapolation of the off-shell form factors. This method also provides a straightforward and simple way to examine the finite-volume effects. The calculation is accomplished using Nf=2 twisted mass fermion ensembles. The statistically significant excited-state effects are observed and eliminated using a multi-state fit. The impact of fine-tuning the charm quark mass is also examined and confirmed to be well-controlled. Finally, using three lattice spacings for the continuum extrapolation, we obtain the decay width Γηcγγ=6.67(16)stat(6)syst keV, which differs significantly from the Particle Data Group’s reported value of Γηcγγ=5.4(4) keV (2.9σ tension). We provide insight into the comparison between our findings, previous theoretical predictions, and experimental measurements.

Lattice calculation of χ c0 → 2γ decay width * *Support by NSFC of China (12070131001) and also the support by CAS Interdisciplinary Innovation Team and NSFC of China (11935017), NSFC of China (12047505) and State Key Laboratory of Nuclear Physics and Technology, Peking University

We perform a lattice QCD calculation of the decay width using a model-independent method that does not require the momentum extrapolation of corresponding off-shell form factors. The simulation is performed on ensembles of twisted mass lattice QCD gauge configurations with three different lattice spacings. After the continuum extrapolation, the decay width is obtained to be . Regardless of this large statistical error, our result is compatible with the experimental results within 1.3σ. Potential improvements of the lattice calculation in the future are also discussed.

Performance of an emulsion telescope for gamma-ray observations in the GRAINE2018 balloon-borne experiment

Abstract The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) project is aimed at the precise observation of astronomical gamma-ray sources in the energy range of 10 MeV–100 GeV using a balloon-borne telescope utilizing a nuclear emulsion, which can help realize precise imaging with high angular resolution (1.0○ at 100 MeV), polarization sensitivity, and large aperture area (10 m2). In 2018, the third balloon experiment was carried out as a demonstration of the detection of the brightest known astronomical gamma-ray source, the Vela pulsar, with an aperture area of 0.38 m2. In these data, some gamma rays were produced by the π0 → 2γ decay, which was caused by the hadronic interactions of cosmic rays in the detector. These could be used to calibrate the reconstructed angle, energy, and so on. In this study, we establish a method of searching for hadronic interactions and concomitant gamma rays with high statistics and purity. Our analysis indicates that the performance of our detector for gamma rays is as expected in wide incidence angle and energy ranges. We plan to commence scientific observations using the proposed system with the verified high angular resolution and largest aperture area in 2022 or later.

No Anomaly and New Renormalization Scheme

We review the physics of chiral anomaly and show that the anomaly equation of δμJμ5 =e216π2eμνρδ FμνFρδis not connected to any physical observables. This is based on the fact that the reaction process of π0→2γ has no diver- gence at all, and the triangle diagrams with the vertex of γμγ5 describing the Z0→2γ decay do not have any di- vergences either. The recent calculated branching ratio of the Z0→2γ decay rate is found to be ГZ0→2γ/Г□2.4×10-8. Further, we discuss the anomaly equation in the Schwinger model which is known as δμJμ5=e2πeμνFμν , and prove that this anomaly equation disagrees with the exact value of the chiral charge δ5=±1 in the Schwinger vacuum. Therefore, the chiral anomaly is a spurious effect induced by the regularization. In connection with the anomaly prob- lem, we clarify the physical meaning why the self-energy of photon should not be included in the renormalization scheme. Also, we present the renormalization scheme in weak interactions without Higgs particles, and this is achieved with a new propagator of massive vector bosons, which does not give rise to any logarithmic divergences in the vertex corrections. Therefore, there is no necessity of the renormalization procedure of the vertex corrections arising from the weak vector boson propagation.

Effect of Excited and Fourth Generation Leptons in Lepton Flavor Violating μ− → e−2γ Decay

We study the lepton flavor violating μ− → e−2γ decay induced by the mediation of heavy leptons, such as the excited and fourth generation leptons. Effects of these leptons in μ− → e−2γ decay are examined. It is found that the branching ratio of this decay is very sensitive to excited and fourth generation leptons. Furthermore, we obtain the bounds on model parameters by using the current experimental limit of this decay.

CONSISTENTLY COMPUTING THE K → π LONG DISTANCE WEAK TRANSITION

First we extract the long-distance (LD) weak matrix element <π|HW|K> from K → 2π, K → 3π, KL→ 2γ, K+→ π+e+e-, K+→ π+μ+μ-and KL→ μ+μ-data and give compatible theoretical estimates. We also link this LD scale to the single-quark-line (SQL) transition scale βWand then test the latter SQL scale against the decuplet weak decay amplitude ratio <π-Ξ0| HW|Ω->/<Ξ-|HW|Ω->. Finally, we study LD KS→ 2γ decay. All of these experimental and theoretical values for <π|HW|K> are in good agreement. We deduce an average value from 11 experimental determinations of <π|HW|K> = (3.59 ± 0.05) × 10-8GeV2, while the theoretical SQL values average to (3.577 ± 0.004) × 10-8GeV2.

Pion electroproduction in parity violating elastic ep scattering experiment

This study has been developed for electron-proton scattering experiments when only the scattered electrons are detected. Pion electroproduction on the proton including the cascade π0→ 2γ decay and the QED radiative corrections to elastic ep scattering are investigated. Our results are shown in the kinematical configuration of the parity violating electron scattering experiment planned at the Mainz Microtron (MAMI).

Construction of a time-of-flight measurement system to study the low energy positronium production

Abstract To investigates the low energy positronium (Ps) production in terms of target temperatures and materials, surface condition and so on, we adopt the time-of-flight (TOF) method and perform extensive Monte Carlo simulation to design an apparatus. Under reasonable assumptions in the simulation with respect to an initial energy distribution of Ps, we examine how cleanly ortho-positronium (oPs) is observed without being subject to large backgrounds of 2γ decay from e+e− annihilation. The simulation finds that the contribution of 2γ events is suppressed by 10−6 smaller than the generated Ps by using the 2.5-cm thick W and 4.6-cm Pb collimators with slit gap of 1 mm. We inject slow positron beams on the W target installed in the TOF system and move the position of the target. Then we expect the position resolution of 1.5 mm resulting the accuracy of determining the oPs velocity 20%, corresponding to 1.1×106 cm/s for thermal Ps with 30 meV.

Light gluino mass and condensate from properties of η and η′

We investigate whether known properties of the η′ meson are consistent with its being the Goldstone boson of the spontaneously broken anomaly-free R symmetry required in the light gluino scenario. We fit the masses and 2γ decays of the η and η′ mesons, and also their production in radiative J ψ decays. We find that the η - η′ system is well-described in the light gluino scenario, if m λ ⋍ (84–144) MeV and 〈 λ λ〉 ⋍ −(0.15–0.36) GeV 3. These values are in the range expected when the gluino gets its mass entirely from radiative corrections.

Effects of the UA(1) anomaly on η → 2γ decay

We study the η → 2γ decay using an extended three-flavor Nambu-Jona-Lasinio model that includes the 't Hooft instanton induced interaction. The η meson mass and the η → 2γ decay width are reproduced simultaneously with a rather strong instanton induced interaction. The calculated η decay constant is f η = 2.23 f π and it suggests that the η meson is no longer a Goldstone boson.

Photon scattering on quasi-free neutrons in the reaction ?d???np and neutron polarizabilities

The possibility to obtain information on γn scattering at intermediate energies from the reaction γd→γ′np is analyzed. To this aim, the differential cross sectiond 3 σ/dΩ γ ,dΩ n dE n and the scattering asymmetry with linearly polarized photons are calculated at photon energies 100 to 400 MeV in the diagrammatic approach. The pole diagrams of the impulse approximation are evaluated with realistic γn and γp scattering amplitudes. One-loop diagrams withnp rescattering in the final state and with meson-exchange and isobar currents are taken into account as well. The main contribution to the differential cross sectiond 3 σ/dΩ γ ,dΩ n dE n in the kinematics of quasi-free γn scattering arises from the neutron pole diagram. The correction due to other diagrams is typically −30% to −10% and decreases with increasing photon energy and momentum transfer. The sensitivity of the cross sections to the magnitude of the neutron electric polarizability and to the sign of the π0→2γ decay constant is demonstrated.

Nonrenormalization theorem for the Abelian anomaly within dimensional regularization

An approach to the calculation of the Abelian axial anomaly within the dimensional regularization is proposed. This approach allows one to make all computations consistently in the framework of a perturbation theory to any number of loops by using the ordinary dimensional regularization. Handling of the γ5 matrix in the anomaly calculation requires only the possibility of cycle permutation under the trace sign. Our procedure leads to a normalization of the axial current that differs by a finite amount from that of the corresponding vector current. The question of normalization of the axial current in relation to the π0→2γ decay amplitude is briefly considered.

Studies of the reactionπ − p→η ′ π 0 n at 38 and 100 GeV/c

The production ofη′π0 system inπ− p charge-exchange interactions atπ− momenta 38 GeV/c and 100 GeV/c has been studied. The 2γ and ηπ0π0 decay modes ofη′ leading to 4γ and 8γ final states respectively have been detected. No statistically significant peak is observed in theη′π0 mass spectrum in the range of 1.2 GeV÷2.1 GeV. An upper limit of 6·10−3 has been established for thea 2 0 (1320)→η′π0 branching ratio.

Liquid xenon ionization and scintillation studies for a totally active-vector electromagnetic calorimeter

In these experiments MeV's to GeV's energies were deposited in a liquid xenon test cell by an electron accelerator with kinetic energy ≤100 keV, intensity ≤10 6 e and pulse width ≤30 ns FWHM. Purification of the liquid xenon is effected by continuous circulation of boil-off gas through Oxisorb. This technique will allow operation of a liquid xenon calorimeter in a high radiation ..environment because the liquid can be continuously purified and replenished. An ionization signal was detected by collecting drift electrons onto an anode mesh, a scintillation signal by collecting photoelectons in a silicon photodiode immersed in the liquid. The energy to create an ionization electron was measured to be W = 9.76±0.70 eV. The corresponding energy to produce a 175 nm scintillation photon was found to be W s= 14.2 eV. The scintillation signal is observed to be fast ( σ t≤14 ns). Anti-correlation of the ionization and scintillation signals was also seen. Intrinsic energy resolution σ E/ E ≈ 0.07%/√( E/GeV) was determined from ionization and σ E/ E ≈ 0.2%/√( E/GeV) from scintillation. Doping of xenon with up to 2% methane did not adversely affect the ionization/scintillation yields or resolutions but increased the drift velocity hence, the current signal by about 75%. The intrinsic energy resolution of a totally active xenon scintillation calorimeter will be limited by uniformity of light collection and the dead material of the calorimeter to ( σ E/ E ≤ 0.5%). Detection of scintillation with a silicon (or CsI gas) photodiode permits a fast energy measurement which can be used in a first level trigger. The ionization signal will be sampled to determine the vector direction of the photon (or electron) which initiates the shower. Directional resolution σ θ = 5 mrad/√( E/GeV) achievable with this method will allow association of a detected photon to its true vertex point. At LHC ( ≈ 20 interactions per beam crossing) this calorimeter can give a Higgs mass resolution σ M/ M H ≈ 0.6% via the 2γ decay mode compared to 3% for calorimeters with equal energy resolution ( σ E/ E = 0.5%) but without this vertex capability. Sampling of ionization in the early part of the shower will allow discrimination against π 0 production and detect individual photons from π 0 decays thus enhancing discrimination against these important sources of hadronic background. The essential problem in measuring the shower profiles is the large amount of readout electronics needed. Cheap, current sensitive, digital VLSI electronics has already been developed and produced for the FAST RICH detector project. Studies to adapt this electronics for use in liquid xenon are underway.

Electromagnetic form factors in the Nambu-Jona-Lasinio model

The original Nambu-Jona-Lasinio model is treated to leading order in 1/N. The electromagnetic form factors of the constituent quarks and the pion, and the anomalous π0 → 2γ decay amplitude are obtained. Their dependence on the cutoff of the model is made explicit. Two distinct limits (depending on the ratio of the cutoff to the constituent quark mass) in which the cutoff can be taken to infinity are also discussed. We find that to leading order of 1/N the model in one limit corresponds to the linear and in the other to the nonlinear sigma model.

Observation of the decay K s → 2γ and measurement of the decay rates K L → 2γ and K S → 2γ

The decay rates of K L → 2γ and K S → 2γ have been measured at the CERN SPS. The results are Γ(K L→2 γ)/ χ(K L→2 π 0)=0.632±0.004±0.008 and Γ(K S→2 γ)/ Γ(K L→2 γ)=2.3 ±1.0±0.4. This is the first observation of K S→2γ decays.

Nuclear two-photon decay in 0+→0+ transitions

The two-photon decay of the first excited 0 + state of 16 O has been measured using the Heidelberg-Darmstadt crystal ball. A branching ratio of Γ γγ Γ tot = (6.6±0.5) · 10 −4 was obtained. As in the cases of 40 Ca and 90 Zr previously reported by us, the 2γ decay of 16 O proceeds via double E1 and M1 transitions of similar strength; the evidence is the observed interference term in the 2γ angular correlation. The ratio of the matrix elements α E 1 χ for 16 O was restricted to the two inverse values (−6.2±1.5) or (−0.16±0.04). An interpretation of 2γ matrix elements observed for 16 O, 40 Ca and 90 Zr in terms of the electric polarizabilities and magnetic susceptibility is given leading to a qualitative understanding of this decay mode.

The radial excitations of η−η′ mesons

The possibility of identification of a number of the recently discoveredJPC=0−+ isosinglet resonances (the ζ(1275), ι(1460) and others) as radial excitations of the η(549)- and η′(958)-mesons is investigated within the earlier developed phenomenological approach to analyse radial excitations of light mesons. The widths of the electromagnetic and strong decays of these mesons and the branching ratios of their production in the radiativeJ/ψ decay, as well as in the π-p charge exchange reaction, are estimated. It is pointed out that the abundant production of the ι-meson in theJ/ψ-decays and the small width of the ι→2γ decay put forward repeatedly as an evidence for the glueball nature of this meson, can qualitatively be obtained in one of the considered schemes of the radial excitations of the η−η′-system.

Four-quark states in heavy quark systems

Heavy four-quark states R4q = qqq are considered in the framework of potential models. Special attention is paid to the possibility of observing such states in e+e− annihilation. It is shown that in the cccc system there must exist a resonance with JPC = 1−−, spin S = 2, with mass M ≈ 6.75 GeV, Γe+e−∼0.02 eV and a total width of Γtot∼10 MeV. For analogous states in the cscs andssss systems we have obtained Mcscs ≈4.8 GeV, Γe+e−∼0.4 eV and Mssss≈2.7 GeV, Γe+e−∼3 eV. The decay modes of such states and possibilities of their detection are discussed. The masses of C-even 4-quark states are also calculated and their 2γ decay widths and the probabilities for charmonium and upsilonium radiative decays to these states are estimated.