arXiv subjects

N. V. Krasnikov

Publications and source records attributed to N. V. Krasnikov.

At least 19 recordsLinked to original sources

arXiv2026

Strictly solvable $d=4$ quantum field theory models with unitarity violation. Possible implications for baryon asymmetry of the Universe and dark matter

In this paper we consider quantum field theory with unitarity violation. We construct several $d = 4$ strictly solvable models with unitarity violation. As a possible application of the models with unitarity violation we construct two models explaining the observed baryon asymmetry of the Universe and the dark matter existence. In considered models dark matter particles have nonzero baryon number and the total baryon number of the Universe being the sum of the baryon number of the matter and dark matter is equal to zero. Dark matter paricles have mass $\sim 6~GeV$. Proposed models predict negligible cross sections of the dark matter with nuclei and electrons. It means that underground experiments will fail to discover dark matter.

hep-ph↗

arXiv2026

Improved limits on a new $Z'$ in $B-L$ scenarios with the NA64 experiment at CERN

Extensions of the Standard Model featuring an additional $U(1)_{B-L}$ gauge symmetry provide a compelling framework linking the origin of neutrino masses to possible dark matter candidates. The associated gauge boson, $Z'$, couples directly to Standard Model fermions and can be produced in fixed-target experiments through electron-nucleus interactions. In this work, we present new constraints on the coupling constant $g_{B-L}$ obtained with the NA64 experiment using the full electron-beam dataset collected between 2016 and 2022, corresponding to $(9.4\pm0.5)\times10^{11}$ electrons on target. The analysis includes the resonant $e^{+}e^{-}$ annihilation production channel, which enhances sensitivity in the mass range $m_{Z'}\in[200,300]$ MeV. The larger dataset provides approximately three times the statistics of previous analyses, thereby improving sensitivity. For the unbroken $U(1)_{B-L}$ case, the new limits exceed those from dedicated neutrino-scattering experiments, providing the most stringent laboratory bounds on $g_{B-L}$ for sub-GeV masses of the new boson. In scenarios where the $Z'$ couples to dark matter, the decay width is dominated by invisible channels, and the corresponding exclusion limits can be directly derived from the NA64 invisible-mode analysis.

hep-ex↗

arXiv2026

Missing energy signatures of inelastic magnetic dipole DM at NA64e

Some extensions of the Standard Model consider inelastic dark matter (iDM) as an attractive candidate for sub-GeV DM of thermal origin that could be detected at modern accelerators. In the present paper, we calculate the production rate of iDM pairs $χ_{1} \barχ_0$ interacting with the ordinary photon via dipole magnetic moment in the reaction of high-energy electron scattering on nuclei, $e^- N \to e^- N χ_{1} \barχ_0$, in the NA64e experiment at the CERN SPS. We derive the projected sensitivity of NA64e to such particles assuming of $\simeq10^{13}$ 100 GeV electrons on target. We also show, that incorporating heavy vector meson decays, $γ^* N \to N V (\to χ_1 \barχ_0)$, alongside bremsstrahlung-like emission of inelastic dark matter pairs, $e^- N \to e^- N γ^* (\to χ_1 \barχ_0)$, will allow NA64e to probe previously unexplored regions of the iDM parameter space, in particular for modest mass splittings, $Δ\simeq 5 \times 10^{-2}$, and relatively light masses, $m_{χ_0} \lesssim 100~\mbox{MeV}$.

hep-ph↗

arXiv2026

On the ultraviolet behavior of the invariant charge in quantum electrodynamics

In this paper we study the ultraviolet behavior of the invariant charge in QED. We show that for complex momenta the invariant charge does not have Landau pole singularity. We can define new invariant charge as real part of standard invariant charge. New invariant charge is limited from above and does not have Landau pole singularity. Also we use the $1/N$ perturbation theory for the investigation of the ultraviolet behavior of the invariant charge. To this aim we consider QED with imaginary charge which is asymptotically free but nonphysical model. In QED with nonphysical imaginary charge we can reliably calculate the ultraviolet asymptotics for the $(1/N)^k$ correction to the invariant charge, namely: $α_k(\frac{p^2}{μ^2}, α) \sim (\ln(\frac{p^2}{μ^2}))^{-k-1}$ at $k > 1$ and $α_1(\frac{p^2}{μ^2}, α) \sim (\frac{\ln(\ln(\frac{p^2}{μ^2})}{\ln^2(\frac{p^2}{μ^2})})$ at $k =1$. The $1/N$ perturbation theory coincides for QED with imaginary charge and standard QED with real charge. It means in particular that ultraviolet behavior of the $(1/N)^k$ correction $α_k(\frac{p^2}{μ^2}, α)$ in real QED coincides with the corresponding asymptotics for QED with imaginary charge. We propose also to use the modified $1/N$ expansion which is ultraviolet finite. The comparison of the standard QED and nonphysical QED with imaginary charge gives hint that in other non asymptotically free models like supersymmetric QED, scalar QED or Wess-Zumino model ultraviolet asymptotics of the invariant charge coincides with leading log approximtion.

hep-th↗

arXiv2026

Constraints on dark axion portal: missing energy and fermion EDMs

We study a model in which a new interactions between the Standard Model (SM) photon and both the dark photon ($γ_D)$ and an ALP ($a$) are described by the dark axion portal operator. The implications of this dark axion portal scenario for electron fixed-target experiments are presented. In particular, we investigate the missing energy signatures associated with production of dark photons and their subsequent invisible decays into stable dark sector fermions, $γ_D \to χ\barχ$. We discuss the discovery potential for such a scenario and derive projected sensitivity curves for the NA64$e$ and LDMX experiments. Furthermore, novel constraints of the NA64$e$ for $9.37\times 10^{11}$ electrons on target are derived by considering two production mechanisms for invisible states: (i)~the bremsstrahlung-like emission of an $aγ_D$ pair, $e N \to e N γ^* (\to a γ_D)$, and (ii)~the exclusive vector meson photoproduction, $γ^* N \to N V$, followed by the invisible decays of vector mesons, $V \to a γ_D$. Additionally, the constraints on the parameter space of $CP$-violating, fermion-specific ALP and dark photon couplings are established. These constraints are derived from current experimental bounds on the electric dipole moments (EDMs) of SM fermions, incorporating loop-induced contributions to the EDMs of the electron, muon, and neutron.

hep-ph↗

arXiv2026

Probing millicharged particles with NA64$μ$ and LDMX

Millicharged particles emerge as compelling candidates in numerous theoretically well-motivated extensions of the Standard Model. These hypothetical particles, characterized by an electric charge that is a small fraction of the elementary charge, have attracted significant attention in contemporary experimental physics. Their potential existence motivates dedicated search strategies across multiple experimental platforms, leveraging their distinctive electromagnetic interactions while evading conventional detection methods. In the present paper we estimated the projected sensitivity of fixed-target experiments, specifically NA64$μ$ and LDMX, to the parameter space of millicharged particles. For the NA64$μ$ experiment, with an anticipated muon flux of $\mbox{MOT}\lesssim 10^{14}$, our analysis reveals a detectable mass window of $10~\mbox{MeV} \lesssim m_χ\lesssim 150~\mbox{MeV}$ and charge parameter range $10^{-4} \lesssim ε\lesssim 7\times 10^{-4}$. This sensitivity arises from the bremsstrahlung-like missing energy signature $μN \to μN γ^{*}( \to χ\barχ)$. Furthermore, we evaluate the discovery potential of the LDMX facility, considering its projected electron beam statistics, $\mbox{EOT}\lesssim 2\times 10^{16}$, and energy, $E_{\rm e}\simeq 8~\mbox{GeV}$. Our results demonstrate that LDMX can probe heavier MCPs in the mass range $250~\mbox{MeV} \lesssim m_χ\lesssim 400 ~\mbox{MeV}$, with sensitivities reaching $10^{-3} \lesssim ε\lesssim 1.5 \times 10^{-3}$. This parametric window can be accessible through the distinctive invisible decay channel $ρ\to χ\barχ$, where $ρ$-meson photo-production $γN \to N ρ$ plays a pivotal role.

hep-ph↗

arXiv2025

On the internal inconsistency of the Wess-Zumino model

We prove the internal inconsistency of the supersymmetric Wess-Zumino model. Our proof is based on three assumptions. The first assumption is that in the full theory the structure of counter temcs coincides with the structure of the counter terms in the perturbation theory. The second assumption is the positivity of norm states - no ghosts in the spectrum of the model. The third assumption is that the canonical commutation relations among generalized coordinates and momenta are valid in renormalized theory. The obtained results mean that there are negative norm states in the spectrum of the WZ model.

hep-th↗

arXiv2025

Search for mirror hidden sector with neutral kaons at NA64

Mirror matter from the dark hidden sector with the same particle content and gauge interactions as in the standard model is still an interesting candidate for dark matter. Several experiments on search for positronium and neutron oscillations into their mirror partner have been conducted recently. In this work we consider the transitions $K^0-K^0_m$ of a neutral kaon into a hidden mirror kaon. It is shown that their probability can be probed with the sensitivity of $P(K^0-K^0_m )\lesssim 10^{-7}$ from the search for missing-energy events in the NA64 experiment at the CERN SPS with $\sim 10^{11}$ $K^+$ on target.This would result in a limit on the $K^0-K^0_m$ mixing parameter much stronger than the similar bound estimated from the cosmological considerations.

hep-ph↗

arXiv2025

Proof of principle for a light dark matter search with low-energy positron beams at NA64

Thermal light dark matter (LDM) with particle masses in the 1 MeV - 1 GeV range could successfully explain the observed dark matter abundance as a relic from the primordial Universe. In this picture, a new feeble interaction acts as a "portal" between the Standard Model and LDM particles, allowing for the exploration of this paradigm at accelerator experiments. In the last years, the "missing energy" experiment NA64e at CERN SPS (Super Proton Synchrotron) has set world-leading constraints in the vector-mediated LDM parameter space, by exploiting a 100 GeV electron beam impinging on an electromagnetic calorimeter, acting as an active target. In this paper, we report a detailed description of the analysis of a preliminary measurement with a 70 GeV positron beam at NA64e, performed during summer 2023 with an accumulated statistic of 1.6 x 10^10 positrons on target. This data set was analyzed with the primary aim of evaluating the performance of the NA64e detector with a lower energy positron beam, towards the realization of the post-LS3 program. The analysis results, other than additionally probing unexplored regions in the LDM parameter space, provide valuable information towards the future NA64e positron campaign.

hep-ex↗

arXiv2025

Searching for Light Dark Matter and Dark Sectors with the NA64 experiment at the CERN SPS

Since its approval in 2016, NA64 has pioneered light dark matter (LDM) searches with electron, positron, muon, and hadron beams. The experiment has successfully met its primary objectives, as outlined in the EPPS input (2018), and even exceeded them, producing results that demonstrate its ability to operate in a near-background-free environment. The Physics Beyond Collider (PBC) initiative at CERN recognizes NA64's contributions as complementary and worthy of continued exploration. Its key advantage over beam-dump approaches is that the signal rate scales as the square of the coupling rather than the fourth power, reducing the required number of beam particles for the same sensitivity. To fully exploit the NA64 physics potential, an upgrade during LS3 will enable the experiment to run in background-free mode at higher SPS beam rates. Planned upgrades include: (a) improved detector hermeticity with a new veto hadron calorimeter, (b) enhanced particle identification with a synchrotron radiation detector, and (c) increased beam rates via upgraded electronics. With the recently strengthened NA64 collaboration, stable operations and timely data analysis are planned for LHC Run 4. The expected beam exposures are approximately 1e13 electrons, 1e11 positrons (at 40 and 60 GeV), and 2e13 muons on target. This will allow NA64 to explore new LDM parameter space, with the potential for discovery or conclusive exclusion of many well-motivated models.

hep-ex↗

arXiv2025

Bounds on the Gell-Mann--Low functions in quantum electrodynamics and in the Wess-Zumino model

We derive bounds $ |\frac{dψ(α)}{dα}| \leq 1 $, $ \frac{d(\frac{dψ(α)}{dα}ψ(α))}{dα} \leq 1 $ on the GL (Gell-Mann--Low) function $ψ(α)$ from the Kallen-Lehmann dispersion representation in quantum electrodynamics. We also derive analogous bounds for the GL function in the Wess-Zumino model. The implications of the obtained inequalities are discussed. In particular, we obtain bounds on coupling constants in dark photon model and in dark matter model with vector $(B-L)$ messenger.

hep-ph↗

arXiv2025

High efficiency veto hadron calorimeter in the NA64 experiment at CERN

NA64 is a fixed-target experiment at the CERN SPS designed to search for Light particle Dark Matter (LDM) candidates with masses in the sub-GeV range. During the 2016-2022 runs, the experiment obtained the world-leading constraints, leaving however part of the well-motivated region of parameter space suggested by benchmark LDM models still unexplored. To further improve sensitivity, as part of the upgrades to the setup of NA64 at the CERN SPS H4 beamline, a prototype veto hadron calorimeter (VHCAL) was installed in the downstream region of the experiment during the 2023 run. The VHCAL, made of Cu-Sc layers, was expected to be an efficient veto against upstream electroproduction of large-angle hadrons or photon-nuclear interactions, reducing the background from secondary particles escaping the detector acceptance. With the collected statistics of $4.4\times10^{11}$ electrons on target (EOT), we demonstrate the effectiveness of this approach by rejecting this background by more than an order of magnitude. This result provides an essential input for designing a full-scale optimized VHCAL to continue running background-free during LHC Run 4, when we expect to collect $10^{13}$ EOT. Furthermore, this technique combined with improvements in the analysis enables us to decrease our missing energy threshold from 50 GeV to 40 GeV thereby enhancing the signal sensitivity of NA64.

hep-ex↗

arXiv2025

Search for $K_{S,L}$ oscillations and invisible decays into the dark sector at NA64

The decays $K_{S,L}$ have never been experimentally tested. In the Standard Model their branching ratios for the decay into two neutrinos are predicted to be extremely small, $Br(K_{S,L} \to ν\barν) \lesssim 10^{-16}$. We consider several natural extensions of the SM, such as two-Higgs-doublet (2HDM), 2HDM and light scalar, and dark mirror sector models, that allow to enhance the $Br(K_{S,L})$ up to a measurable level. We briefly discuss the possible search for $K_{S,L}$ decays and $K_{S,L}$ oscillations into the dark sector at the NA64 experiment at CERN with the sensitivity to $Br(K_{S,L}) \lesssim 10^{-7}-10^{-5}$.

hep-ph↗

arXiv2024

Shedding light on Dark Sectors with high-energy muons at the NA64 experiment at the CERN SPS

A search for Dark Sectors is performed using the unique M2 beam line at the CERN Super Proton Synchrotron. New particles ($X$) could be produced in the bremsstrahlung-like reaction of high energy 160 GeV muons impinging on an active target, $μN\rightarrowμNX$, followed by their decays, $X\rightarrow\text{invisible}$. The experimental signature would be a scattered single muon from the target, with about less than half of its initial energy and no activity in the sub-detectors located downstream the interaction point. The full sample of the 2022 run is analyzed through the missing energy/momentum channel, with a total statistics of $(1.98\pm0.02)\times10^{10}$ muons on target. We demonstrate that various muon-philic scenarios involving different types of mediators, such as scalar or vector particles, can be probed simultaneously with such a technique. For the vector-case, besides a $L_μ-L_τ$ $Z'$ vector boson, we also consider an invisibly decaying dark photon ($A'\rightarrow\text{invisible}$). This search is complementary to NA64 running with electrons and positrons, thus, opening the possibility to expand the exploration of the thermal light dark matter parameter space by combining the results obtained with the three beams.

hep-ex↗

arXiv2024

First constraints on the $L_μ-L_τ$ explanation of the muon $g-2$ anomaly from NA64-$e$ at CERN

The inclusion of an additional $U(1)$ gauge $L_μ-L_τ$ symmetry would release the tension between the measured and the predicted value of the anomalous muon magnetic moment: this paradigm assumes the existence of a new, light $Z^\prime$ vector boson, with dominant coupling to $μ$ and $τ$ leptons and interacting with electrons via a loop mechanism. The $L_μ-L_τ$ model can also explain the Dark Matter relic abundance, by assuming that the $Z'$ boson acts as a "portal" to a new Dark Sector of particles in Nature, not charged under known interactions. In this work we present the results of the $Z'$ search performed by the NA64-$e$ experiment at CERN SPS, that collected $\sim 9\times10^{11}$ 100 GeV electrons impinging on an active thick target. Despite the suppressed $Z'$ production yield with an electron beam, NA64-$e$ provides the first accelerator-based results excluding the $g-2$ preferred band of the $Z'$ parameter space in the 1 keV $ < m_{Z'} \lesssim 2$ MeV range, in complementarity with the limits recently obtained by the NA64-$μ$ experiment with a muon beam.

hep-ex↗

arXiv2024

50 GeV $π^-$ in, nothing out: a sensitive probe of invisible $η$ and $η'$ decays with NA64h

We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar $η$ and $η'$ mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV $π^-$ on nuclei of an active target as the source of neutral mesons. The $η, η' \to invisible$ events would exhibit themselves via a striking signature - the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with $2.9\times10^{9}$ pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio ${\rm Br}(η' \to invisible) < 2.1 \times 10^{-4}$ improving the current bound by a factor of $\simeq3$. We also set a limit on ${\rm Br}(η\to invisible) < 1.1 \times 10^{-4}$ comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.

hep-ex↗

arXiv2024

Dark matter models with suppressed dark matter nuclei elastic cross section

We propose two generalizations of the dark photon model which predict the suppressed elastic dark matter nuclei cross section in comparison with the corresponding prediction of the dark photon model. In the first model the main difference from dark photon model is that the mixing parameter $ε$ is nonlocal formfactor $ε(q^2)= \frac{q^2}{Λ^2}V(\frac{q^2}{Λ^2}) $ depending on the square of the momentum transfer $q^2$. Here $V(\frac{q^2}{Λ^2})$ is an entire function of the growth $ρ\geq \frac{1}{2}$ and $Λ$ is nonlocal scale. In this model our world and dark world are described by renormalizable field theories while the communication between them is performed by nonlocal interaction. The second model is renormalizable model where besides dark photon field $A'$ additional vector boson $Z'$ interacts with $B - L$ current. The communication between our world and dark world is performed due to nonzero kinetic mixing between $Z'$ and $A'$ fields. The predictions of the models for the search for dark matter at the accelerators don't contain additional suppression factors.

hep-ph↗

arXiv2024

Development of the fully Geant4 compatible package for the simulation of Dark Matter in fixed target experiments

The search for new comparably light (well below the electroweak scale) feebly interacting particles is an exciting possibility to explain some mysterious phenomena in physics, among them the origin of Dark Matter. The sensitivity study through detailed simulation of projected experiments is a key point in estimating their potential for discovery. Several years ago we created the DMG4 package for the simulation of DM (Dark Matter) particles in fixed target experiments. The natural approach is to integrate this simulation into the same program that performs the full simulation of particles in the experiment setup. The Geant4 toolkit framework was chosen as the most popular and versatile solution nowadays. The simulation of DM particles production by this package accommodates several possible scenarios, employing electron, muon or photon beams and involving various mediators, such as vector, axial vector, scalar, pseudoscalar, or spin 2 particles. The bremsstrahlung, annihilation or Primakoff processes can be simulated. The package DMG4 contains a subpackage DarkMatter with cross section methods weakly connected to Geant4. It can be used in different frameworks. In this paper, we present the latest developments of the package, such as extending the list of possible mediator particle types, refining formulas for the simulation and extending the mediator mass range. The user interface is also made more flexible and convenient. In this work, we also demonstrate the usage of the package, the improvements in the simulation accuracy and some cross check validations.

hep-ph↗