Photon Decay Research Articles

Shielding proposal to mitigate the PFC #2 heating due to 16N decay photon radiation from the activated cooling water

In this work, the total heating of the ITER superconducting Poloidal Field Coil #2 (PFC #2) has been calculated due to 16N decay photon radiation emitted from the activated water in the Blanket Manifolds, the Chimney pipes and the Tokamak Cooling Water System (TCWS) manifolds. In order to reduce the total nuclear heating of the PFC #2, shielding measures have been proposed; a set of conceptual stainless-steel shields has been designed and inserted into the computational model of the ITER machine covering the Upper Port Stub (UPS) Chimney and the UPS Chimneybox. The results have shown a 42% reduction of the PFC #2 heating, i.e. from 275 W down to 160 W, when 10 cm thick plates have been placed on the mentioned UP structures. At the same time, the maximum specific heating of the PFC #2 has been decreased by 63%, from 40 W/m3 down to 15 W/m3. A recommendation has been given for further reduction of the PFC #2 heating by improving the on-going design of the TCWS duct.

Single production of composite electrons at future SPPC-based lepton–hadron colliders

We consider the production of excited electrons with spin-1/2 at future SPPC-based electron–proton colliders with center-of-mass energies of 8.4, 11.6, 26.6, and 36.8 TeV. These exotic particles are predicted in the composite models. We calculate the production cross sections and concentrate on the photon decay channel of the excited electrons with the process of ep → e∗X → eγX. The pseudo rapidity and transverse momentum distributions of the electrons and photons in the final state have been plotted to determine the kinematical cutoffs best suited for discovery of the excited electrons. By applying these cutoffs we compute 2σ, 3σ, and 5σ contour plots of the statistical significance of the expected signal in the parameter space (L, m∗), where L denotes the integrated luminosity of the collider and m∗ is the mass of the composite electrons.

Rate of dark photon emission from electron positron annihilation in massive stars

We calculate the rate of production of dark photons from electron-positron pair annihilation in hot and dense matter characteristic of supernova progenitors. Given the non-linear dependence of the emission rate on the dark photon mass and current astrophysical constraints on the dark photon parameter space, we focus on the mass range of 1--10 MeV. For the conditions under consideration both mixing with the in-medium photon and plasma effects on the electron dispersion relation are non-negligible and are explored in detail. We perform our calculations to the leading order in the fine-structure constant. Transverse and longitudinal photon modes are treated separately given their different dispersion relations. We consider the implications for the evolution of massive stars when dark photons decay either into particles of the standard model or of the dark sector.

Making dark matter out of light: Freeze-in from plasma effects

Dark matter (DM) could couple to particles in the Standard Model (SM) through a light vector mediator. In the limit of small coupling, this portal could be responsible for producing the observed DM abundance through a mechanism known as freeze-in. Furthermore, the requisite DM-SM couplings provide a concrete benchmark for direct and indirect searches for DM. In this paper, we present updated calculations of the relic abundance for DM produced by freeze-in through a light vector mediator. We identify an additional production channel: the decay of photons that acquire an in-medium plasma mass. These plasmon decays are a dominant channel for DM production for sub-MeV DM masses, and including this channel leads to a significant reduction in the predicted signal strength for DM searches. Accounting for production from both plasmon decays and annihilations of SM fermions, the DM acquires a highly non-thermal phase space distribution which impacts the cosmology at later times; these cosmological effects will be explored in a companion paper.

Revisiting the dark photon explanation of the muon anomalous magnetic moment

A massive $U(1)^{\prime}$ gauge boson known as a "dark photon" or $A^{\prime}$, has long been proposed as a potential explanation for the discrepancy observed between the experimental measurement and theoretical determination of the anomalous magnetic moment of the muon, ($g_{\mu} - 2$) anomaly. Recently, experimental results have excluded this possibility for a dark photon exhibiting exclusively visible or invisible decays. In this work, we revisit this idea and consider a model where $A^{\prime}$ couples inelastically to dark matter and an excited dark sector state, leading to a more exotic decay topology we refer to as a semi-visible decay. We show that for large mass splittings between the dark sector states this decay mode is enhanced, weakening the previous invisibly decaying dark photon bounds. As a consequence, $A^{\prime}$ resolves the $g_{\mu} - 2$ anomaly in a region of parameter space the thermal dark matter component of the Universe is readily explained. Interestingly, it is possible that the semi-visible events we discuss may have been vetoed by experiments searching for invisible dark photon decays. A re-analysis of the data and future searches may be crucial in uncovering this exotic decay mode or closing the window on the dark photon explanation of the $g_{\mu} - 2$ anomaly.

Light-by-light scattering in ultraperipheral heavy-ion collisions at low diphoton masses

We present a study of photon-photon scattering in the mass range $W_{\gamma\gamma} < 5$ GeV. We extend earlier calculations of this cross section for $W_{\gamma\gamma}>$ 5 GeV into the low mass range where photoproduction of the pseudoscalar resonances $\eta$, $\eta^{'}(958)$ contributes to two-photon final states. We present the elementary photon-photon cross section as a function of diphoton mass $M_{\gamma\gamma}$ arising from lepton and quark loop diagrams, and the visible cross section obtained with the gamma-gamma decay branching fractions of the resonances $\eta$, $\eta^{'}(958)$, $\eta_c(1S)$, $\eta_c(2S)$, $\chi_{c0}(1P)$. We derive the corresponding cross sections in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV by folding the elementary cross section with the heavy-ion photon fluxes. We consider the dominating background of the two-photon final state which arises from gamma decays of photoproduced $\pi^{0}$-pairs. Such $\pi^{0}$-pairs contribute to the background when only two of the four decay photons are within the experimental acceptance, while the other two photons escape undetected. We reduce this background by applying cuts on asymmetries of transverse momenta of the two photons and indicate how the background can be further suppressed using a multivariate sideband analysis. We present the cross section for the signal and the background at midrapidity $|\eta| <$ 0.9, and in the forward rapidity range 2.0 $< \eta <$ 4.5.

Lasing thresholds and photon statistics in high-βburied multiple quantum well photonic crystal nanocavity lasers

We investigate the lasing thresholds and photon statistics of buried multiple quantum well (MQW) photonic crystal (PhC) lasers with a high spontaneous emission coupling coefficient $\ensuremath{\beta}$ and a large carrier transparency number ${N}_{0}$. Using measured input-output curves and delay-dependent second-order photon correlations ${g}^{(2)}(\ensuremath{\tau})$, we estimate that our population inversion threshold is near the kink threshold of the input-output curve, which indicates that the buried MQW PhC lasers operate close to the ``lasers without inversion'' (intensity jump without inversion) regime proposed by Yamamoto and Bj\ork. With respect to static photon statistics ${g}^{(2)}(0)$, the super-Poissonian photon statistics ${g}^{(2)}(0)g1$ are observed even one order of magnitude above the kink threshold. The super-Poissonian photon statistics are specific to the class-B lasers where the photon decay rate ${\ensuremath{\gamma}}_{c}$ is much larger than that of carriers ${\ensuremath{\gamma}}_{\ensuremath{\parallel}}$ and the relaxation oscillation occurs. We also propose that the pump power needed to maximize the damping time of the relaxation oscillation could be the noise threshold at which the photon statistics change from thermal to Poissonian.

(Invited) (sSNOM) Imaging of 2D Encapsulated Ga Layered Metal

Plasmonic properties of newly discovered class of 2‐dimensional materials (2DM): one‐to‐few monolayer thick metals synthesized using confinement heteroepitaxy (CHet)1 will be discussed. Optical properties of 2DM are modulated at the ultimate scale of (a few) nanometers, thus the characterization also requires a similar spatial resolution. Scattering‐type scanning near‐field optical microscope (sSNOM) allowed us to reveal surface impedance of the 2D‐Ga layers coated with bilayer graphene. 2D‐Ga has extremely large optical contrast due to high density of states, resulting in ultimate light‐matter interactions within an atomic layer. 2D‐Ga was shown to not cover the surface uniformly, leaving the valleys of lower contrast. sSNOM data has been analyzed with two novel algorithms: impedance clustering (revealing metal encapsulation) and photonic decay curves (via 3D sSNOM mapping). Photonic decay is directly related to surface plasmonic density of states, the quantity of interest for nanophotonics applications of 2D‐metals. Author acknowledges Mr. M. Blades for help with data processing, Dr. T. Gokus (Neaspec) – for sSNOM instrumentation, and Prof. J. Robinson (PSU) – for CHet material synthesis. This research was partially supported by the NSF ECCS‐1509786, and Penn State Quantum Characterization Infrastructure initiative. [1] N.Briggs, B.Bersch, A.De La Fuente Duran, Y.Wang, A.Li, C.Navarro, J.Robinson, EP03.15.03 : Two‐Dimensional Monoelemental Materials via Confinement Heteroepitaxy, MRS Fall Meeting, Boston MA, 11/25‐30/2018. Figure 1

Light in the beam dump. Axion-Like Particle production from decay photons in proton beam-dumps

The exploration of long-lived particles in the MeV-GeV region is a formidable task but it may provide us a unique access to dark sectors. Fixed-target facilities with sufficiently energetic and intense proton beams are an ideal tool for this challenge. In this work we show that the production rate of Axion-Like-Particles (ALPs) coupled pre-dominantly to photons receives a significant contribution from daughter-photons of secondary π0 and η mesons created in the proton shower. We carefully compare the PYTHIA simulated spectra of such secondaries to experimental literature, compute the ALP flux from the Primakoff conversion of these photons, and finally revisit existing limits on ALPs and update the prospects for a set of existing and future searches. Our results show that taking this production mechanism into account significantly enhances the sensitivity compared to previous studies based on coherent ALP production in primary proton-nucleus interactions.

Electron-impact excitations of highly charged tungsten ions and polarization study of their successive photon decay

Fully relativistic distorted wave theory has been utilized to study the electron-impact excitation of highly charged tungsten ions viz. Y-like (W35+), Sr-like (W36+), Rb-like (W37+) and Br-like (W39+). Electric dipole transitions pertaining to n = 4 → n′ = 4 states have been considered in these four ions. The electron excitation cross-sections are calculated for these ions in the wide range of electron-impact energies from excitation threshold to 20 keV. The required relativistic bound states wave functions for the initial and excited states of the ions are obtained using GRASP2k code. The accuracy of the computed wave functions is ascertained by comparing the calculated energies and oscillator strengths for the considered transitions with the available theoretical and experimental results. For the direct application of the cross-sections to the plasma modelling our calculated cross-sections are fitted to suitable analytical expressions. Further, the degree of linear polarization of the emitted photons due to the decay of the electron from the excited tungsten ions to their respective ground states is also calculated using density matrix theory. There are no previous calculations available for the cross-sections and polarizations to compare with our results of these ions.

FoCal: A highly granular digital calorimeter

This contribution discusses the recent progress in the development of a highly granular digital electromagnetic calorimeter proposed as an upgrade to the ALICE detector. This forward electromagnetic calorimeter (FoCal) must be able to discriminate decay photons from direct photons at very high energy, which requires extremely high granularity and a small Molière radius. A dedicated R&D program is ongoing to develop the technology needed for such a high-granularity device. Within this program we have constructed a prototype of a digital electromagnetic calorimeter based on CMOS monolithic active pixel sensors (MAPS). This prototype has demonstrated the unique capabilities of such a highly granular digital calorimeter, providing unprecedented shower profile measurements and good linearity and energy resolution. The prototype was based on the MIMOSA chip, which is however not fast enough for application in a full detector at the LHC. As a next step, the ALPIDE chip developed for the ALICE Inner Tracker Upgrade is being investigated for performance with high occupancy. This contribution presents results from the current prototype, the performance of the ALPIDE and plans for the next prototype.

The rare and forbidden: Testing physics beyond the standard model with Mu3e

The upcoming Mu3e experiment aims to search for the lepton flavour violating decay \boldsymbol{\muposeeemath} with an unprecedented final sensitivity of one signal decay in \boldsymbol{\num{e16}} observed muon decays by making use of an innovative experimental design based on novel ultra-thin silicon pixel sensors. In a first phase, the experiment is operated at an existing muon beam line with rates of up to \boldsymbol{\num{e8}} muons per second. Detailed simulation studies confirm the feasibility of background-free operation and project single event sensitivities in the order of \boldsymbol{\num{e-15}} for signal decays modelled in an effective field theory approach. The precise tracking of the decay electrons and large geometric and momentum acceptance of Mu3e enable searches for physics beyond the Standard Model in further signatures. Examples of which are searches for lepton flavour violating two-body decays of the muon into an electron and an undetected boson as well as for electron-positron resonances in \boldsymbol{\muposeeenunumath} which could result for instance from a dark photon decay. The Mu3e experiment is expected to be competitive in all of these channels already in phase I.

Direct photon production at low transverse momentum in proton-proton collisions at s=2.76 and 8 TeV

Measurements of inclusive and direct photon production at mid-rapidity in pp collisions at $\sqrt{s}=2.76$ and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of $0.4<p_{T}<10$ GeV/$c$ and $0.3<p_{T}<16$ GeV/$c$, respectively. Photons are detected with the electromagnetic calorimeter~(EMCal) and via reconstruction of e$^+$e$^-$ pairs from conversions in the ALICE detector material using the central tracking system. For the final measurement of the inclusive photon spectra the results are combined in the overlapping $p_{T}$ interval of both methods. Direct photon spectra, or their upper limits at 90% C.L. are extracted using the direct photon excess ratio $R_{\gamma}$, which quantifies the ratio of inclusive photons over decay photons generated with a decay-photon simulation. An additional hybrid method, combining photons reconstructed from conversions with those identified in the EMCal, is used for the combination of the direct photon excess ratio $R_{\gamma}$, as well as the extraction of direct photon spectra or their upper limits. While no significant signal of direct photons is seen over the full $p_{T}$ range, $R_{\gamma}$ for $p_{T}>7$ GeV/$c$ is at least one $\sigma$ above unity and consistent with expectations from next-to-leading order pQCD calculations.

Measuring the boiling point of the vacuum of quantum electrodynamics

It is a long-standing non-trivial prediction of quantum electrodynamics that its vacuum is unstable in the background of a static, spatially uniform electric field and, in principle, sparks with spontaneous emission of electron-positron pairs. However, an experimental verification of this prediction seems out of reach because a sizeable rate for spontaneous pair production requires an extraordinarily strong electric field strength $|\mathbf E|$ of order the Schwinger critical field, ${\rm E}_c=m_e^2/e\simeq 1.3\times 10^{18}\ {\rm V/m}$, where $m_e$ is the electron mass and $e$ is its charge. Here, we show that the measurement of the rate of pair production due to the decays of high-energy bremsstrahlung photons in a high-intensity laser field allows for the experimental determination of the Schwinger critical field and thus the boiling point of the vacuum of quantum electrodynamics.

Direct photon elliptic flow in Pb–Pb collisions at [formula omitted

The elliptic flow of inclusive and direct photons was measured by ALICE for central and semi-central Pb–Pb collisions at sNN=2.76TeV. The photons were reconstructed using the electromagnetic calorimeter PHOS and the central tracking system. The inclusive photon flow reconstructed with both methods are combined and used to extract the direct photon flow, using a decay photon simulation and the direct photon excess Rγ, in the transverse momentum range 0.9 <pT < 6.2 GeV/c. We find that the theoretical predictions generally under-predict the results.

Inelastic dark matter at the LHC lifetime frontier: ATLAS, CMS, LHCb, CODEX-b, FASER, and MATHUSLA

Visible signals from the decays of light long-lived hidden sector particles have been extensively searched for at beam dump, fixed-target, and collider experiments. If such hidden sectors couple to the Standard Model through mediators heavier than $\sim 10$ GeV, their production at low-energy accelerators is kinematically suppressed, leaving open significant pockets of viable parameter space. We investigate this scenario in models of inelastic dark matter, which give rise to visible signals at various existing and proposed LHC experiments, such as ATLAS, CMS, LHCb, CODEX-b, FASER, and MATHUSLA. These experiments can leverage the large center of mass energy of the LHC to produce GeV-scale dark matter from the decays of dark photons in the cosmologically motivated mass range of $\sim 1-100$ GeV. We also provide a detailed calculation of the radiative dark matter-nucleon/electron elastic scattering cross section, which is relevant for estimating rates at direct detection experiments.

Revealing microscopic origins of shape coexistence in the Ni isotopic chain

In a two–neutron transfer experiment, performed in Bucharest in July 2016 at sub–Coulomb barrier energy,a photon decay hindered – solely – by a nuclear shape change was identified in the 66Ni nucleus. Such a rare process, at spin zero, was clearly observed before only in actinide nuclei in the 1970’s,where fission isomers were found. The experimental findings on 66Ni have been well reproduced by the Monte Carlo Shell Model Calculations of the Tokyo group, which predict a multifaceted scenario of coexistence of spherical, oblate and prolate shapes in neutron–rich Ni isotopes. The results on 66Ni encouraged a comprehensive gamma–spectroscopy investigation of neutron–rich Ni isotopes, in particular 62Ni and 64–Ni, at IFIN–HH (Bucharest), IPN Orsay and ILL (Grenoble), employing different reaction mechanisms to pin down the wave function composition of selected excited states. The aim is to shed light on the microscopic origin of deformation in neutron–rich Ni nuclei, possibly locating other examples of “shape–isomer–like” structures inthis region.

Photon--Photon Scattering in the Resonance Region at Midrapidity at the LHC

A study is presented to extend the measurements of photon-photon scattering in ultra-peripheral Pb-Pb collisions at the LHC into the mass region of the pseudoscalar resonances $\eta$ and $\eta'$. The elementary photon-photon scattering cross section is presented. The cross section for photon-photon scattering in Pb-Pb is derived by convoluting the elementary photon-photon cross section with the Pb-Pb photon luminosity. The main background to two-photon final states, arising from double $\pi^{0}$ production with two of the four decay photons escaping detection, is examined, and possible kinematical conditions are discussed to optimize the signal-to-background ratio for such measurements at mid-rapidity.

Research on cluster reconstruction algorithm for the STAR forward electromagnetic calorimeter

The Forward Meson Spectrometer, FMS, is a forward electromagnetic calorimeter at the STAR detector at RHIC, covering pseudorapidity from 2.6 to 4.0. It is an electromagnetic calorimeter comprised of two different types of 1264 lead-glass cells where the two types vary in both composition and size. The FMS was built primarily to unravel the novel spin effects seen in transversely polarized proton collisions. These effects relate to the spin-orbit correlation of the partons in the proton to the spin of the proton, which is a consequence of the confined motion of partons in nucleons. The reconstruction of neutral pions in the FMS is essential to study these effects. The gain calibration of the FMS is solely based on the reconstruction of the invariant mass of neutral pions at a fixed energy – because there is no tracker or hadronic calorimeter around – which poses the complication that the calibration gets intertwined with the reconstruction algorithm. Also, in order to cover the kinematic region of interest, the FMS needs to measure pion energies as high as 80 GeV at pp collisions with center-of-mass energies of 200 and 500 GeV. It was found that there was a strong correlation between the reconstructed neutral pion mass and its energy due to a combination of biases in the photon finding, namely the energy and the opening angle of the decay photons. The impact of the non-linear response of the lead-glass on the energy-scale was verified in detailed simulations of the light attenuation in the lead-glass and it was found that a correction function could be established. The fitting of the electromagnetic shower shape was modified to include non-zero incident angles and non-zero vertex positions. The reconstruction algorithm was optimized for clusters with two showers, especially when the separation of the two photons nears the physical limit in terms of the cell sizes. This article will present details of all these improvements and their impact on the reconstruction of neutral pions with the FMS.

Fast and robust generation of singlet state via shortcuts to adiabatic passage

In this paper, we propose a protocol to fast and robustly generate two-atom singlet state by designing the evolution operator with the help of quantum Zeno dynamics. The population of the intermediate state can be controlled by system parameters. The pulses in the protocol can be fitted as Gaussian functions, which are beneficial to the experimental feasibility. Besides, the performance of various decoherence factors, such as spontaneous emission, cavity decay and fiber photon leakage, is discussed by numerical simulations. The results show that the protocol is fast and robust against decoherence and operational imperfection. Finally, the protocol is generalized to realize three-atom singlet state by the same principle.