Photon Decay Research Articles

Coincident excitation and radiative decay in electron-nucleus collisions

The distorted-wave Born approximation formalism for the description of the $(e,{e}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction, in which emitted photons and scattered electrons are simultaneously detected, is outlined. Both the Coulomb and the magnetic scattering are fully taken into account. The influence of electron bremsstrahlung is estimated within the plane-wave Born approximation. Recoil effects are also discussed. The formalism is applied for the low-energy $(e,{e}^{\ensuremath{'}}\ensuremath{\gamma})^{92}\mathrm{Zr}$ reaction with excitation of the first collective (${2}_{1}^{+}$) and mixed-symmetry (${2}_{2}^{+}$) states. The corresponding transition charge and current densities are taken from a random-phase approximation (RPA) calculation within the quasiparticle phonon model. It is shown, by this example, in which way the magnetic subshell population of the excited state influences the angular distribution of the decay photon. For these quadrupole excitations the influence of magnetic scattering is only prominent at the backmost scattering angles, where a clear distinction of the photon pattern pertaining to the two states is predicted.

Faking ordinary photons by displaced dark photon decays

A light meta-stable dark photon decaying into collimated electron/positron pair can fake a photon, either converted or unconverted, at the LHC. The detailed object identification relies on the specifics of the detector and strategies for the reconstruction. We study the fake rate based on the ATLAS(CMS) detector geometry and show that it can be O(1) with a generic choice of parameters. Especially, the probability of being registered as a photon is angular dependent. Such detector effects can induce bias to measurements on certain properties of new physics. In this paper, we consider the scenario where dark photons in final states are from a heavy resonance decay. Consequently, the detector effects can dramatically affect the results when determine the spin of a resonance. Further, if the decay products from the heavy resonance are one photon and one dark photon, which has a large probability to fake a diphoton event, the resonance is allowed to be a vector. Due to the difference in detector, the cross sections measured in ATLAS and CMS do not necessarily match. Furthermore, if the diphoton signal is given by the dark photons, the SM $Z\gamma$ and $ZZ$ final states do not necessarily come with the $\gamma\gamma$ channel, which is a unique signature in our scenario. The issue studied here is relevant also for any future new physics searches with photon(s) in the final state. We discuss possible ways of distinguishing dark photon decay and real photon in the future.

Lifetime Measurement of the ^{229}Th Nuclear Isomer.

The first excited isomeric state of ^{229}Th possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today's laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground state (internal conversion, γ decay, and bound internal conversion), whose strengths depend on the charge state of ^{229m}Th. We report on the measurement of the internal-conversion decay half-life of neutral ^{229m}Th. A half-life of 7±1 μs has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of ≈10^{4} s of the photonic decay channel, gives further support for an internal conversion coefficient of ≈10^{9}, thus constraining the strength of a radiative branch in the presence of internal conversion.

Impact of residual contamination on inclusive and direct photon flow

Direct photon flow is measured by subtracting the contribution of decay photon flow from the measured inclusive photon flow via the double ratio , which defines the excess of direct over decay photons. The inclusive photon sample is affected by a modest contamination arising from different background sources, which can not always be fully corrected for in measurements. However, due to the sensitivity of the direct photon measurement even a residual contamination may significantly bias the extracted direct photon flow. In particular, for measurements using photon conversions, which are very powerful at low transverse momentum, these effects can be substantial. Assuming three different types of correlated background contributions we demonstrate using the Therminator2 event generator that the impact of the contamination on the magnitude of direct photon flow can be on the level of 50%, even if the purity of the inclusive photon sample is about 97%. Future measurements should attempt to account for the contamination by measuring the background contributions and subtracting them from the inclusive photon flow.

Search for Invisible Decays of Sub-GeV Dark Photons in Missing-Energy Events at the CERN SPS.

We report on a direct search for sub-GeV dark photons (A^{'}), which might be produced in the reaction e^{-}Z→e^{-}ZA^{'} via kinetic mixing with photons by 100GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75×10^{9} electrons on target. We set new limits on the γ-A^{'} mixing strength and exclude the invisible A^{'} with a mass ≲100 MeV as an explanation of the muon g_{μ}-2 anomaly.

Shutdown Dose Rate Analysis for the long-pulse D-D Operation Phase in KSTAR

KSTAR is a medium size fully superconducting tokamak. The deuterium-deuterium (D-D) reaction in the KSTAR tokamak generates neutrons with a peak yield of 3.5x1016 per second through a pulse operation of 100 seconds. The effect of neutron generation from full D-D high power KSTAR operation mode to the machine, such as activation, shutdown dose rate, and nuclear heating, are estimated for an assurance of safety during operation, maintenance, and machine upgrade. The nuclear heating of the in-vessel components, and neutron activation of the surrounding materials have been investigated. The dose rates during operation and after shutdown of KSTAR have been calculated by a 3D CAD model of KSTAR with the Monte Carlo code MCNP5 (neutron flux and decay photon), the inventory code FISPACT (activation and decay photon) and the FENDL 2.1 nuclear data library.

Scintillation and Spectroscopic Characteristics of 90%Lu LGSO With Variable Decay Times

High lutetium content LGSO [Lu1.8Gd0.2SiO5:Ce] scintillators with decay time constants in the range 30-45 ns were introduced some ten years ago. At first an experimental development, mass production was gradually established and mature 90%Lu LGSO scintillators with stable characteristics have become available. The aim of this work was to assess the scintillation characteristics of recent 90%Lu LGSO production with different decay times using avalanche photodiode (APD) readout. Samples of Fast (~ 30 ns), Standard (~ 40 ns) and Slow (~ 47 ns) 90%Lu LGSO, extracted from top, middle and bottom positions into ingots, have been investigated. The decay time and central emission wavelength were found to be stable for samples from the same ingot, irrespective of the position where the sample was obtained. The photoelectron yields of Slow, Standard and Fast LGSO scintillators generally showed good uniformity. Their average photoelectron yields at 511 keV were respectively 34 000 ± 1 000 phe/MeV, 31 200 ± 500 phe/MeV and 28 000 ± 2 000 phe/MeV without any definite trend or bias with respect to position in ingots. The combination of photon yield and decay time results in high initial emission rate and a subnanosecond time resolution performance of 914 ± 18 ps for the three categories of decay times. The average energy resolution at 511 keV of Slow LGSO was 10 ± 2% whereas Standard and Fast LGSO samples had identical energy resolution of 11 ± 3%. The light yield non-proportionality as a function of irradiation energy was also investigated. At 60 keV, the departure from linearity reached 19 ± 2%.

The CBM ECAL

We present the design and performance of the Electromagnetic Calorimeter (ECAL) of the Compressed Baryonic Matter (CBM) Experiment at FAIR. The main purpose of the ECAL detector is the identification and the measurement of energy and position of photons and electrons, which are produced in high-energy heavy-ion collisions in the beam energy range from 4 to 40 AGeV. ECAL will measure spectra of photons and neutral mesons decaying in their photonic decay channels. Precise measurement of masses and widths of short-living mesons (η, η′, ϕ χc etc.) will shed light on the chiral symmetry restoration which is expected to occur in dense nuclear matter. Measurements of the π0 and meson spectrum are important to study dependence of the particle yield on thermodynamical parameters of nuclear mater.

Rigorous-two-Steps scheme of TRIPOLI-4® Monte Carlo code validation for shutdown dose rate calculation

After fission or fusion reactor shutdown the activated structure emits decay photons. For maintenance operations the radiation dose map must be established in the reactor building. Several calculation schemes have been developed to calculate the shutdown dose rate. These schemes are widely developed in fusion application and more precisely for the ITER tokamak. This paper presents the rigorous-two-steps scheme implemented at CEA. It is based on the TRIPOLI-4® Monte Carlo code and the inventory code MENDEL. The ITER shutdown dose rate benchmark has been carried out, results are in a good agreement with the other participant.

Single-sideband photonic microwave generation with an optically injected quantum-dot semiconductor laser.

We studied single-sideband (SSB) photonic microwave generation with a high sideband rejection ratio (SRR) based on the period-one dynamical states of an optically injected quantum-dot (QD) semiconductor laser and demonstrated that the SSB signals have SRRs of approximately 15 dB higher than those generated with a conventional quantum-well semiconductor laser under conditions of optimal microwave power. The enhancement of SRR in the QD laser, which is important in mitigating the power penalty effect in applications such as radio-over-fiber optical communications, could be primarily attributed to a lower carrier decay rate in the dots, smaller linewidth enhancement factor, and reduced photon decay rate.

Missing energy signature from invisible decays of dark photons at the CERN SPS

The dark photon ($A'$) production through the mixing with the bremsstrahlung photon from the electron scattering off nuclei can be accompanied by the dominant invisible $A'$ decay into dark-sector particles. In this work we discuss the missing energy signature of this process in the experiment NA64 aiming at the search for $A'\to invisible$ decays with a high-energy electron beam at the CERN SPS. We show the distinctive distributions of variables that can be used to distinguish the $A'\to invisible$ signal from background. The results of the detailed simulation of the detector response for the events with and without $A'$ emission are presented. The efficiency of the signal event selection is estimated. It is used to evaluate the sensitivity of the experiment and show that it allows to probe the still unexplored area of the mixing strength $10^{-6}\lesssim \epsilon \lesssim 10^{-2}$ and masses up to $M_{A'} \lesssim 1$ GeV. The results obtained are compared with the results from other calculations. In the case of the signal observation, a possibility of extraction of the parameters $M_{A'}$ and $\epsilon$ by using the missing energy spectrum shape is discussed. We consider as an example the $A'$ with the mass 16.7 MeV and mixing $\epsilon \lesssim 10^{-3}$, which can explain an excess of events recently observed in nuclear transitions of an excited state of $^8$Be. We show that if such $A'$ exists its invisible decay can be observed in NA64 within a month of running, while data accumulated during a few months would allow also to determine the $\epsilon$ and $M_{A'}$ parameters.

Application of Origen2.1 in the decay photon spectrum calculation of spallation products**Supported by Strategic Priority Research Program of Chinese Academy of Sciences (XDA03030102)

Origen2.1 is a widely used computer code for calculating the burnup, decay, and processing of radioactive materials. However, the nuclide library of Origen2.1 is used for existing reactors like pressurized water reactors. To calculate the photon spectrum released by the decay of spallation products, we have made specific libraries for the ADS tungsten spallation target, based on the results given by the FLUKA Monte Carlo code. All the data used to make the Origen2.1 libraries are obtained from Nuclear structure & decay Data (NuDat2.6). The accumulated activity of spallation products and the contribution of nuclides to photon emission are given in this paper.

Vacuum Cherenkov radiation and photon decay rates from generic Lorentz Invariance Violation

Among the most studied approaches to introduce the breaking of Lorentz symmetry, the generic approach is one of the most frequently used for phenomenology, it converges on the modification of the free particle dispersion relation. Using this approach in the photon sector, we have calculated the squared probability amplitude for vacuum Cherenkov radiation and photon decay by correcting the QED coupling at tree level and first order in LIV parameters. For the lower order energy correction we calculate the emission and decay rate for each process.

The spectrum of darkonium in the Sun

Dark matter that gets captured in the Sun may form positronium-like bound states if it self-interacts via light dark photons. In this case, dark matter can either annihilate to dark photons or recombine in bound states which subsequently also decay to dark photons. The fraction of the dark photons that leave the Sun without decaying to Standard Model particles have a characteristic energy spectrum which is a mixture of the direct annihilation process, the decays of ortho- and para- bound states and the recombination process. The ultimate decay of these dark photons to positron-electron pairs (via kinetic mixing) outside the Sun creates a distinct signal that can either identify or set strict constraints on dark photon models.

Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections.

The double polarization observable E and the helicity dependent cross sections σ_{1/2} and σ_{3/2} were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π^{0}. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ_{1/2} and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in η production are only N 1/2^{-} (S_{11}) and N 1/2^{+} (P_{11}). Furthermore, the extracted Legendre coefficients of the angular distributions for σ_{1/2} are in good agreement with recent reaction model predictions assuming a narrow resonance in the P_{11} wave as the origin of this structure.

Few remarks on the Higgs boson decays in gauge-Higgs unification

In the scenario of gauge-Higgs unification, the origin of the Higgs boson is higher dimensional gauge boson. Reflecting its origin, very characteristic predictions are made of the Higgs boson interactions in this scenario. Especially, a remarkable claim has been made: the contribution of non-zero Kaluza-Klein modes to the Higgs decay $H \to Z \gamma$ exactly vanishes in the minimal SU(3) electro-weak unified model, at least at the one-loop level. In this brief report, in order to see whether this prediction is a general feature of the scenario or the consequence of the specific choice of the model, matter content or the order of perturbative expansion, we perform an operator analysis. We demonstrate that there does not exist any relevant operator, respecting the gauge symmetry SU(3) in the bulk. We also comment on the possibly important contribution to the photonic decay $H \to \gamma \gamma$ due to the non-zero Kaluza-Klein modes of light quarks.

On the electrodynamics of moving particles in a quasi flat spacetime with Lorentz violation and its cosmological implications

This research aims to develop a new approach towards a consistent coupling of electromagnetic and gravitational fields, by using an electron that couples with a weak gravitational potential by means of its electromagnetic field. To accomplish this, we must first build a new model which provides the electromagnetic nature of both the mass and the energy of the electron, and which is implemented with the idea of [Formula: see text]-photon decay into an electron–positron pair. After this, we place the electron (or positron) in the presence of a weak gravitational potential given in the intergalactic medium, so that its electromagnetic field undergoes a very small perturbation, thus leading to a slight increase in the field’s electromagnetic energy density. This perturbation takes place by means of a tiny coupling constant [Formula: see text] because gravity is a very weak interaction compared with the electromagnetic one. Thus, we realize that [Formula: see text] is a new dimensionless universal constant, which reminds us of the fine structure constant [Formula: see text]; however, [Formula: see text] is much smaller than [Formula: see text] because [Formula: see text] takes into account gravity, i.e. [Formula: see text]. We find [Formula: see text], where [Formula: see text] is the speed of light and [Formula: see text][Formula: see text]m/s) is a universal minimum speed that represents the lowest limit of speed for any particle. Such a minimum speed, unattainable by particles, represents a preferred reference frame associated with a background field that breaks the Lorentz symmetry. The metric of the flat spacetime shall include the presence of a uniform vacuum energy density, which leads to a negative pressure at cosmological scales (cosmological anti-gravity). The tiny values of the cosmological constant and the vacuum energy density will be successfully obtained in agreement with the observational data.

Generation of an optical Schrödinger-cat-like state in a nonideal cavity by injecting opposite-phase atomic dipoles

We propose a method to generate an optical Schr\"odinger-cat-like state in a cavity in a substantial decoherence regime. Even when the cavity decay rate is considerably large, a cat-like state can be generated in a laser-like setting if the gain for the field is larger than the loss. Under the condition that opposite-phase atomic dipoles repeatedly traverse the cavity, the cavity field converges to a squeezed vacuum state in a steady state. A Schr\"odinger-cat-like state is then generated when a single photon decay occurs. The phase-space distribution of the cat state can be revealed in homodyne detection by using the decaying photon as a herald event. Quantum trajectory simulation was used to identify the conditions for the Schr\"odinger-cat-like state formation as well as to analyze the properties of those states. Based on these simulations, possible experiments are proposed within the reach of the current technology.

Threshold enhancement of diphoton resonances

The data collected by the LHC collaborations at an energy of 13 TeV indicates the presence of an excess in the diphoton spectrum that would correspond to a resonance of a 750 GeV mass. The apparently large production cross section is nevertheless very difficult to explain in minimal models. We consider the possibility that the resonance is a pseudoscalar boson $A$ with a two--photon decay mediated by a charged and uncolored fermion having a mass at the $\frac12 M_A$ threshold and a very small decay width, $\ll 1$ MeV; one can then generate a large enhancement of the $A\gamma\gamma$ amplitude which explains the excess without invoking a large multiplicity of particles propagating in the loop, large electric charges and/or very strong Yukawa couplings. The implications of such a threshold enhancement are discussed in two explicit scenarios: i) the Minimal Supersymmetric Standard Model in which the $A$ state is produced via the top quark mediated gluon fusion process and decays into photons predominantly through loops of charginos with masses close to $\frac12 M_A$ and ii) a two Higgs doublet model in which $A$ is again produced by gluon fusion but decays into photons through loops of vector--like charged heavy leptons. We also comment on a minimal scenario in which the $A$ state couples only to photons through a heavy lepton loop and is both produced and decays through this coupling. In all these scenarios, while the mass of the charged fermion has to be adjusted to be extremely close to half of the $A$ resonance mass, the small total widths are naturally obtained if only suppressed three-body decay channels occur. Finally, the implications of some of these scenarios for dark matter are discussed.

Gluon versus photon production of a 750 GeV diphoton resonance

The production mechanism of a 750 GeV diphoton resonance, either via gluon or photon fusion, can be probed by studying kinematic observables in the diphoton events. We perform a detector study of the two production modes of a hypothetical scalar or tensor diphoton resonance in order to characterize the features of the two scenarios. The nature of the resonance production can be determined from the jet multiplicity, the jet and diphoton rapidities, the rate of central pseudorapidity gaps, or the possible detection of forward protons from elastic photoproduction for events in the signal region. Kinematic distributions for both signals and expected irreducible diphoton background events are provided for comparison along with a study of observables useful for distinguishing the two scenarios at an integrated luminosity of 20 fb$^{-1}$. We find that decay photons from a 750 GeV scalar resonance have a preference for acceptance in the central detector barrel, while background events are more likely to give accepted photons in the detector end caps. This disfavors the interpretation of the large number of excess events found by the the Run-2 CMS diphoton search with one photon detected in the end cap as a wide spin-0 resonance signal. However, one expects more end cap photons in the case of spin-2 resonance.