Portal Effective Theories. A framework for the model independent description of light hidden sector interactions

We present a framework for the construction of portal effective theory (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PETs encompass all portal operators up to dimension five, while the strong scale PETs additionally contain all portal operators of dimension six and seven that contribute at leading order to quark-flavour violating transitions. Using the strong scale PETs, we define a set of portal currents that couple hidden sectors to QCD, and construct portal chiral perturbation theory (χPTs) that relate these currents to the light pseudoscalar mesons. We estimate the coefficients of the portal χPT Lagrangian that are not fixed by SM observations using non-perturbative matching techniques and give a complete list of the resulting one- and two-meson portal interactions. From those, we compute transition amplitudes for three golden channels that are used in hidden sector searches at fixed target experiments: i) charged kaon decay into a charged pion and a spin zero messenger, ii) charged kaon decay into a charged lepton and a spin one half messenger, and iii) neutral pion decay into a photon and a spin one messenger. Finally, we compare these amplitudes to specific expressions for models featuring light scalar particles, axion-like particles, heavy neutral leptons, and dark photons.

Meson bunches formed by pulsed laser-induced processes in ultra-dense hydrogen H(0)

Ultra-dense hydrogen H(0) (reviewed in Holmlid and Zeiner-Gundersen, Physica Scripta 2019 ) consists of small strongly bound molecules with interatomic distance of 0.56 pm in spin state s = 1. It is a useful nuclear fuel for energy generation, giving heat above break-even (Holmlid, AIP Advances 2015) in laser-induced processes (Holmlid, Int. J. Hydr. Energy 2021). Nuclear processes in H(0) emit particles in typical meson decay chains with kinetic energy up to 100 MeV. These mesons decay and generate fast muons at up to 500 MeV energy at current densities of several mA cm-2 at 1–2 m distances, which corresponds to 1013 -1014 muons formed per laser pulse. It is shown that the mesons decay in chain processes with well-defined meson time constants in the range 10–60 ns. The time varying signals from H(0) agree well with mesons M in decay chains as A ◊ M ◊ N where N is a signal muon. M may be a charged kaon K± (decay time constant at rest 12.4 ns) or a charged pion π± (decay time constant at rest 26 ns) or a long-lived neutral kaon \({\text{K}}_{L}^{0}\) (decay time constant at rest 51 ns). Ultra-dense protium p(0) gives the same time constants as D(0) but slightly different decay-chains. The meson bunches observed are similar to the meson bunches from nucleon + antinucleon annihilation. The energy gain in the nuclear process is at least 8000, strongly indicating baryon annihilation for which process further evidence is given in other recent publications.

Latest results on rare kaon decays from the NA48/2 experiment @CERN

The NA48/2 experiment at CERN reports the first observation of the [Formula: see text] decay from an exposure of [Formula: see text] charged kaon decays recorded in 2003–2004. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region. The study of the kinematic space shows evidence for a structure-dependent contribution in agreement with predictions based on chiral perturbation theory. Several [Formula: see text]- and [Formula: see text]-violating asymmetries are also evaluated. The most precise measurement of the charged kaon semi-leptonic form factors obtained by NA48/2 with 4.4 million [Formula: see text] and 2.3 million [Formula: see text] events collected in 2004 will also be presented.

Kaon decays shedding light on massless dark photons

We explore kaon decays with missing energy carried away by a massless dark photon, $${{\overline{\gamma }}}$$ γ ¯ , assumed to have flavor-changing dipole-type couplings to the d and s quarks. We consider in particular the neutral-kaon modes $$K_L\rightarrow \gamma {{\overline{\gamma }}}$$ K L → γ γ ¯ and $$K_L\rightarrow \pi ^0\gamma {{\overline{\gamma }}}$$ K L → π 0 γ γ ¯ and their $$K_S$$ K S counterparts, as well as the charged-kaon channel $$K^+\rightarrow \pi ^+\gamma {{\overline{\gamma }}}$$ K + → π + γ γ ¯ , each of which also has an ordinary photon, $$\gamma $$ γ , in the final state. In addition, we look at $$K_{L,S}\rightarrow \pi ^+\pi ^-{{\overline{\gamma }}}$$ K L , S → π + π - γ ¯ and $$K^+\rightarrow \pi ^+\pi ^0{{\overline{\gamma }}}$$ K + → π + π 0 γ ¯ . Interestingly, the same $$ds{{\overline{\gamma }}}$$ d s γ ¯ interactions give rise to the flavor-changing two-body decays of hyperons with missing energy and are subject to model-independent constraints that can be inferred from the existing hyperon data. Taking this into account, we obtain branching fractions $${{{\mathcal {B}}}}(K_L\rightarrow \gamma {{\overline{\gamma }}})$$ B ( K L → γ γ ¯ ) and $$\mathcal{B}(K_L\rightarrow \pi ^0\gamma {{\overline{\gamma }}})$$ B ( K L → π 0 γ γ ¯ ) which can be as high as $$10^{-3}$$ 10 - 3 and $$10^{-6}$$ 10 - 6 , respectively, one or both of which may be within the sensitivity reach of the KOTO experiment. Furthermore, we find that $${{{\mathcal {B}}}}(K^+\rightarrow \pi ^+\gamma {{\overline{\gamma }}})$$ B ( K + → π + γ γ ¯ ) and $$\mathcal{B}(K^+\rightarrow \pi ^+\pi ^0{{\overline{\gamma }}})$$ B ( K + → π + π 0 γ ¯ ) are allowed to be maximally of order $$10^{-6}$$ 10 - 6 as well, which may be probed by NA62. Complementarily, the hyperon modes can have rates which are potentially accessible by BESIII. Thus, these ongoing experiments could soon be able to offer significant tests on the existence of the massless dark photon.

Atmospheric charged $$K/\pi $$ ratio and measurement of muon annual modulation with a liquid scintillation detector at Soudan

We report a measurement of muon annual modulation in a 12-l liquid scintillation detector with a live-time of more than 4 years at the Soudan Underground Laboratory. Muon minimum ionization in the detector is identified by its observed pulse shape and large energy deposition. The measured muon rate in the detector is $$28.69\pm 2.09$$28.69±2.09 muons per day with a modulation amplitude of ($$2.64\pm 0.07$$2.64±0.07)% and a phase at Jul $$22 \pm 36.2$$22±36.2 days. This annual modulation is correlated with the variation of the effective atmospheric temperature in the stratosphere. The correlation coefficient, $$\alpha _{T}$$αT, is determined to be $$0.898 \pm 0.025$$0.898±0.025. This can be interpreted as a measurement of the atmospheric charged kaon to pion ($$K/\pi $$K/π) ratio of $$0.094^{+0.044}_{-0.061}$$0.094-0.061+0.044 for $$E_{p} > 7$$Ep>7 TeV, consistent with the measurement from the MINOS far detector. To further constrain the value of $$K/\pi $$K/π ratio, a Geant4 simulation of the primary cosmic-ray protons with energy up to 100 TeV is implemented to study the correlation of $$K/\pi $$K/π ratio and the muon annual modulation for muon energy greater than 0.5 TeV. We find out that a charged $$K/\pi $$K/π ratio of 0.1598, greater than the upper bound (0.138) from this work at the production point 30 km above the Earth surface in the stratosphere cannot induce muon annual modulation at the depth of Soudan.