arXiv subjects

N. V. Krasnikov

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

At least 55 records · Page 3Linked to original sources

arXiv2020

Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into $e^+e^-$ pairs

The improved results on a direct search for a new X(16.7 MeV) boson which could explain the anomalous excess of $e^+e^-$ pairs observed in the excited 8Be nucleus decays ("Berillium anomaly") are reported. Due to its coupling to electrons, the X boson could be produced in the bremsstrahlung reaction e-Z -> e-ZX by a high-energy beam of electrons incident on active target in the NA64 experiment at the CERN SPS and observed through its subsequent decay into $e^+e^-$ pair. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4\times 10^{10} electrons on target collected in 2017 and 2018. This allows to set the new limits on the $X$--$e^-$ coupling in the range 1.2 \times 10^{-4} < ε_e < 6.8 \times 10^{-4}, excluding part of the parameter space favored by the Berillium anomaly. We also set new bounds on the mixing strength of photons with dark photons (A') from non-observation of the decay $A' \to e^+e^-$ of the bremsstrahlung A' with a mass below 24 MeV.

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arXiv2019

Implications of last NA64 results and the electron $g_{e}-2$ anomaly for the X(16.7) boson survival

We point out that last NA64 bound on coupling constant of hypothetical $X(16.7 MeV)$ vector boson with electrons plus the recent value of the anomalous electron magnetic moment exclude at 90 \% C.L. purely vector or axial vector couplings of $X(16.7)$ boson with electrons. Models with nonzero $V \pm A$ coupling constant with electron survive and they can explain both the electron and muon $g - 2$ anomalies.

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arXiv2019

Dark matter search in missing energy events with NA64

A search for sub-GeV dark matter production mediated by a new vector boson $A'$, called dark photon, is performed by the NA64 experiment in missing energy events from 100 GeV electron interactions in an active beam dump at the CERN SPS. From the analysis of the data collected in the years 2016, 2017, and 2018 with $2.84\times10^{11}$ electrons on target no evidence of such a process has been found. The most stringent constraints on the $A'$ mixing strength with photons and the parameter space for the scalar and fermionic dark matter in the mass range $\lesssim 0.2$ GeV are derived, thus demonstrating the power of the active beam dump approach for the dark matter search.

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arXiv2019

Combined search for light dark matter with electron and muon beams at NA64

We discuss prospects of searching for a dark photon ($A'$) which serves as mediator between Standard model (SM) particles and light dark matter (LDM) by using the combined results from the NA64 experiment at the CERN SPS running in high-energy electron (NA64e) and muon (NA64$μ$) modes. We discuss the most natural values and upper bounds on the $A'$ coupling constant to LDM and show they are lying in the range accessible at NA64. While for the projected $ 5\times10^{12}$ electrons on target (EOT) NA64e is able to probe the scalar and Majorana LDM scenarios, the combined NA64e and NA64$μ$ results with $\simeq 10^{13}$ EOT and a few $10^{13}$ MOT, respectively, will allow covering significant region in the parameter space of the most interesting LDM models. This makes NA64e and NA64$μ$ extremely complementary to each other and increases significantly the discovery potential of sub-GeV DM.

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arXiv2018

Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN

We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e+e- pairs observed in the excited Be-8 nucleus decays. Due to its coupling to electrons, the X could be produced in the bremsstrahlung reaction e- Z -> e- Z X by a 100 GeV e- beam incident on an active target in the NA64 experiment at the CERN SPS and observed through the subsequent decay into an e+e- pair. With 5.4\times 10^{10} electrons on target, no evidence for such decays was found, allowing to set first limits on the X-e^- coupling in the range 1.3\times 10^{-4} < ε_e < 4.2\times 10^{-4} excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A') from non-observation of the decay A'->e+e- of the bremsstrahlung A' with a mass <~ 23 MeV.

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arXiv2018

Search for vector mediator of Dark Matter production in invisible decay mode

A search is performed for a new sub-GeV vector boson ($A'$) mediated production of Dark Matter ($χ$) in the fixed-target experiment, NA64, at the CERN SPS. The $A'$, called dark photon, could be generated in the reaction $ e^- Z \to e^- Z A'$ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay $A' \to χ\overlineχ$. The experimental signature of this process would be an event with an isolated electron and large missing energy in the detector. From the analysis of the data sample collected in 2016 corresponding to $4.3\times10^{10}$ electrons on target no evidence of such a process has been found. New stringent constraints on the $A'$ mixing strength with photons, $10^{-5}\lesssim ε\lesssim 10^{-2}$, for the $A'$ mass range $m_{A'} \lesssim 1$ GeV are derived. For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the 90% C.L. limits $10^{-11}\lesssim y \lesssim 10^{-6}$ on the dark-matter parameter $y = ε^2 α_D (\frac{m_χ}{m_{A'}})^4 $ are obtained for the dark coupling constant $α_D = 0.5$ and dark-matter masses $0.001 \lesssim m_χ\lesssim 0.5 $ GeV. The lower limits $α_D \gtrsim 10^{-3} $ for pseudo-Dirac Dark Matter in the mass region $m_χ\lesssim 0.05 $ GeV are more stringent than the corresponding bounds from beam dump experiments. The results are obtained by using tree level, exact calculations of the $A'$ production cross-sections, which turn out to be significantly smaller compared to the one obtained in the Weizsäcker-Williams approximation for the mass region $m_{A'} \gtrsim 0.1$ GeV.

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arXiv2018

The exact tree-level calculation of the dark photon production in high-energy electron scattering at the CERN SPS

Dark photon ($A'$) that couples to the standard model fermions via the kinetic mixing with photons and serves as a mediator of dark matter production could be observed in the high-energy electron scattering $e^- + Z ~\rightarrow e^- + Z + A'$ off nuclei followed by the $A' \to invisible $ decay. We have performed the exact, tree-level calculations of the $A'$ production cross sections and implemented them in the program for the full simulation of such events in the experiment NA64 at the CERN SPS. Using simulations results, we study the missing energy signature for the bremsstrahlung $A' \rightarrow $ invisible decay that permits the determination of the $γ-A'$ mixing strength in a wide, from sub-MeV to sub-GeV, $A'$ mass range. We refine and expand our earlier studies of this signature for discovering $A'$ by including corrections to the previously used calculations based on the improved Weizsaker-Williams approximation, which turn out to be significant. We compare our cross sections values with the results from other calculations and find a good agreement between them. The possibility of future measurements with high-energy electron beams and the sensitivity to $A'$ are briefly discussed.

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arXiv2018

Probing the muon g_μ-2 anomaly, L_μ - L_τ gauge boson and Dark Matter in dark photon experiments

In the L_μ - L_τ model the 3.6$σ$ discrepancy between the predicted and measured values of the anomalous magnetic moment of positive muons can be explained by the existence of a new dark boson Z' with a mass in the sub-GeV range, which is coupled at tree level predominantly to the second and third lepton generations. However, at the one-loop level, the Z' coupling to electrons or quarks can be induced via the γ-Z' kinetic mixing, which is generated through the loop involving the muon and tau lepton. This loophole has important experimental consequences since it opens up new possibilities, in particular for the complementary searches of the Z' in the ongoing NA64 and incoming dark photon experiments with high-energy electrons. An extension of the model able to explain relic Dark Matter density is also discussed.

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arXiv2017

Light scalars, ($g_μ-2$) muon anomaly and dark matter in a model with a Higgs democracy

We consider isosinglet scalar extension of a model with a Higgs democracy - multihiggs extension of the SM where each quark and lepton has its own Higgs isodoublet. The addition of light isosinglet scalar allows to solve both muon $g_μ-2$ anomaly and dark matter problem. Also we point out that an extension of the model with $L_μ - L_τ$ vector interaction allows not only explain muon $g_μ - 2$ anomaly but also dark matter density.

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arXiv2017

The muon (g - 2) anomaly and a new light vector boson

This mini review aims to outline the main experimental and theoretical results related with the search for a new light vector boson proposed as a solution of the muon $g-2$ anomaly. Additionally, I consider a model with infinite number of light vector bosons which can explain the anomaly, while still remaining consistent with current experimental bounds.

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arXiv2016

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 \to e^- Z A' via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The A's would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with 2.75\cdot 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.

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arXiv2016

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 $10^{-6}\lesssim ε\lesssim 10^{-2}$ 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 $ε$ by using the missing energy spectrum shape is discussed. We consider as an example the $A'$ with the mass 16.7 MeV and mixing $ε\lesssim 10^{-3}$, 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 $ε$ and $M_{A'}$ parameters.

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arXiv2016

The SM extensions with additional light scalar singlet, nonrenormalizable Yukawa interactions and $(g-2)_μ$

We consider the SM extension with additional light real singlet scalar, right-handed neutrino and nonrenormalizable Yukawa interaction for the first two generations. We show that the proposed model can explain the observed $(g - 2)$ muon anomaly. Phenomenological consequences as flavour violating decays $τ\rightarrow μμμ, μμe, μe e $ are briefly discussed. We also propose the $U_R(1)$ gauge generalization of the SM with complex scalar singlet and nonzero right-handed charges for the first two generations.

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arXiv2016

Invisible $K_L$ decays in the SM extensions

In the Standard Model (SM) the branching ratio for the decay $K_L \to ν\barν$ into two neutrinos is helicity suppressed and predicted to be very small $ \leq O(10^{-17})$. We consider two natural extensions of the SM, such as two-Higgs-doublet model (2HDM) and the $ν$MSM with additional singlet scalar, those main features is that they can lead to an enhanced $Br(K_L \to invisible)$. In the 2HDM the smallness of the neutrino mass is explained due to the smallness of the second Higgs doublet vacuum expectation value. Moreover, the $ν$MSM extension with additional singlet field can explain the $(g - 2)$ muon anomaly. The considered models demonstrate that the $K_L \to invisible $ decay is a clean probe of new physics scales well above 100 TeV, that is complementary to rare $K \to π+invisible$ decay, and provide a strong motivation for its sensitive search in a near future low-energy experiment.

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arXiv2015

Invisible $K_L$ decays as a probe of new physics

The decay $K_L \to invisible$ has never been experimentally tested. In the Standard Model (SM) its branching ratio for the decay into two neutrinos is helicity suppressed and predicted to be $Br(K_L \to ν\barν) \lesssim 10^{-10}$. We consider several natural extensions of the SM, such as two-Higgs-doublet (2HDM), 2HDM and light scalar, and mirror dark matter models, those main feature is that they allow to avoid the helicity suppression factor and lead to an enhanced $Br(K_L \to invisible)$. For the decay $K_L \to ν\barν$ the smallness of the neutrino mass in the considered 2HDM model is explained by the smallness of the second Higgs doublet vacuum expectation value. The small nonzero value of the second Higgs isodoublet can arise as a consequence of nonzero quark condensate. We show that taking into account the most stringent constraints from the $K \to π+invisible$ decay, this process could be in the region of $Br(K_L \to invisible) \simeq 10^{-8}-10^{-6}$, which is experimentally accessible. In some scenarios the $K_L \to invisible$ decay could still be allowed while the $K \to π+invisible$ decay is forbidden. The results obtained show that the $K_L \to invisible$ decay is a clean probe of new physics scales well above 100 TeV, that is complementary to rare $K \to π+invisible$ decay, and provide a strong motivation for its sensitive search in a near future experiment.

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arXiv2015

Muon g-2 and searches for a new leptophobic sub-GeV dark boson in a missing-energy experiment at CERN

The 3.6 σdiscrepancy between the predicted and measured values of the anomalous magnetic moment of positive muons can be explained by the existence of a new dark boson Z_μwith a mass in the sub-GeV range, which is coupled predominantly to the second and third lepton generations through the L_μ- L_τcurrent . After a discussion of the present phenomenological bounds on the Z_μcoupling, we show that if the Z_μexists, it could be observed in the reaction μ+Z \to μ+Z+Z_μof a muon scattering off nuclei by looking for an excess of events with large missing muon beam energy in a detector due to the prompt bremsstrahlung Z_μdecay Z_μ\to ννinto a couple of neutrinos. We describe the experimental technique and the preliminary study of the feasibility for the proposed search. We show that this specific signal allows for a the search for the Z_μwith a sensitivity in the coupling constant α_μ> 10^{-11}, which is 3 orders of magnitude higher than the value required to explain the discrepancy. We point out that the availability of high-energy and -intensity muon beams at CERN SPS provides a unique opportunity to either discover or rule out the Z_μin the proposed search in the near future. The experiment is based on the missing-energy approach developed for the searches for invisible decays of dark photons and (pseudo)scalar mesons at CERN and is complementary to these experiments.

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arXiv2013

Proposal for an Experiment to Search for Light Dark Matter at the SPS

Several models of dark matter suggest the existence of dark sectors consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields. These sectors of particles do not interact with the ordinary matter directly but could couple to it via gravity. In addition to gravity, there might be another very weak interaction between the ordinary and dark matter mediated by U'(1) gauge bosons A' (dark photons) mixing with our photons. In a class of models the corresponding dark gauge bosons could be light and have the $γ$-A' coupling strength laying in the experimentally accessible and theoretically interesting region. If such A' mediators exist, their di-electron decays A' -> e+e- could be searched for in a light-shining-through-a-wall experiment looking for an excess of events with the two-shower signature generated by a single high energy electron in the detector. A proposal to perform such an experiment aiming to probe the still unexplored area of the mixing strength 10^-5 < $ε$ < 10^-3 and masses M_A' < 100 MeV by using 10-300 GeV electron beams from the CERN SPS is presented. The experiment can provide complementary coverage of the parameter space, which is intended to be probed by other searches. It has also a capability for a sensitive search for A's decaying invisibly to dark-sector particles, such as dark matter, which could cover a significant part of the still allowed parameter space. The full running time of the proposed measurements is requested to be up to several months, and it could be taken at different SPS secondary beams.

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arXiv2013

Invisible decay of muonium: Tests of the standard model and searches for new physics

In the Standard Model there are several canonical examples of pure leptonic processes involving the muon, the electron and the corresponding neutrinos which are connected by the crossing symmetry: i) the decay of muon, ii) the inverse muon decay, and iii) the annihilation of a muon and an electron into two neutrinos. Although the first two reactions have been observed and measured since long ago, the third process, resulting in the invisible final state, has never been experimentally tested. It may go either directly, or, at low energies, via the annihilation of a muon and an electron from an atomic bound state, called muonium (M=μ^+e^-). The M\to ν_μν_e decay is expected to be a very rare process, with the branching fraction predicted to be Br(M\to ν_μν_e) = 6.6 10^{-12} with respect to the ordinary muon decay rate. Using the reported experimental results on precision measurements of the positive muon lifetime by the MuLan Collaboration, we set the first limit Br(M \to invisible) < 5.7 10^{-6}, while still leaving a big gap of about six orders of magnitude between this bound and the predictions. To improve substantially the limit, we proposed to perform an experiment dedicated to the sensitive search for the M\to invisible decay. A feasibility study of the experimental setup shows that the sensitivity of the search for this decay mode in branching fraction Br(M\to invisible) at the level of 10^{-12} could be achieved. If the proposed search results in a substantially higher branching fraction than predicted, say Br(M \to invisible) < 10^{-10}, this would unambiguously indicate the presence of new physics. We point out that such a possibility may occur due the muonium-mirror muonium conversion in the mirror matter model. A result in agreement with the Standard Model prediction would be a clean check of the pure leptonic bound state annihilation.

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