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

Search for Light Dark Matter with NA64 at CERN

Thermal dark matter models with particle χ masses below the electroweak scale can provide an explanation for the observed relic dark matter density. This would imply the existence of a new feeble interaction between the dark and ordinary matter. We report on a new search for the sub-GeV χ production through the interaction mediated by a new vector boson, called the dark photon A^{'}, in collisions of 100 GeV electrons with the active target of the NA64 experiment at the CERN SPS. With 9.37×10^{11} electrons on target collected during 2016-2022 runs NA64 probes for the first time the well-motivated region of parameter space of benchmark thermal scalar and fermionic dark matter models. No evidence for dark matter production has been found. This allows us to set the most sensitive limits on the A^{'} couplings to photons for masses m_{A^{'}}≲0.35  GeV, and to exclude scalar and Majorana dark matter with the χ-A^{'} coupling α_{D}≤0.1 for masses 0.001≲m_{χ}≲0.1  GeV and 3m_{χ}≤m_{A^{'}}.

Thermal Dark Matter from Freeze-Out of Inverse Decays

We propose a new thermal dark matter candidate whose abundance is determined by the freeze-out of inverse decays. The relic abundance depends parametrically only on a decay width, while matching the observed value requires that the coupling determining the width-and the width itself-should be exponentially small. The dark matter is therefore very weakly coupled to the standard model, evading conventional searches. This inverse decay dark matter can be discovered by searching for the long-lived particle that decays into the dark matter at future planned experiments.

Resonant Self-Interacting Dark Matter from Dark QCD

We present new models utilizing QCD-like dark sectors to resolve small-scale structure problems. These models of resonant self-interacting dark matter in a dark sector with QCD are based on analogies to the meson spectra in standard model QCD. We introduce a simple model that realizes resonant self-interaction (analogous to the ϕ-K-K system) and thermal freeze-out, in which dark mesons are made of two light quarks. We also consider asymmetric dark matter composed of heavy and light dark quarks to realize a resonant self-interaction (analogous to the ϒ(4S)-B-B system) and discuss the experimental probes of both setups. Finally, we comment on the possible resonant self-interactions already built into SIMP and ELDER mechanisms while using lattice results to determine feasibility.

X17 Discovery Potential in the γN→e^{+}e^{-}N Process at Electron Scattering Facilities

We propose a direct search for the X17 particle, which was conjectured to explain the ATOMKI ^{8}Be and ^{4}He anomalies, through the dilepton photoproduction process on a nucleon in the photon energy range below or around the pion production threshold. For the scenarios of either pseudoscalar, vector, or axial-vector quantum numbers of the conjectured X17, we use existing constraints to estimate the X17 signal process. For dilepton invariant mass resolutions which have been achieved in previous experiments, a signal-to-background ratio of up to an order of magnitude is found for a neutron target, and, in particular, for the pseudoscalar and vector X17 scenarios.

Search for Darkonium in e^{+}e^{-} Collisions

Collider searches for dark sectors, new particles interacting only feebly with ordinary matter, have largely focused on identifying signatures of new mediators, leaving much of dark sector structures unexplored. In particular, the existence of dark matter bound states (darkonia) remains to be investigated. This possibility could arise in a simple model in which a dark photon (A^{'}) is light enough to generate an attractive force between dark fermions. We report herein a search for a J^{PC}=1^{--} darkonium state, the ϒ_{D}, produced in the reaction e^{+}e^{-}→γϒ_{D}, ϒ_{D}→A^{'}A^{'}A^{'}, where the dark photons subsequently decay into pairs of leptons or pions, using 514  fb^{-1} of data collected with the BABAR detector. No significant signal is observed, and we set bounds on the γ-A^{'} kinetic mixing as a function of the dark sector coupling constant for 0.001<m_{A^{'}}<3.16  GeV and 0.05<m_{ϒ_{D}}<9.5  GeV.

Constraints on New Physics in Electron g-2 from a Search for Invisible Decays of a Scalar, Pseudoscalar, Vector, and Axial Vector

We performed a search for a new generic X boson, which could be a scalar (S), pseudoscalar (P), vector (V), or an axial vector (A) particle produced in the 100 GeV electron scattering off nuclei, e^{-}Z→e^{-}ZX, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such a process was found in the full NA64 dataset of 2.84×10^{11} electrons on target. We place new bounds on the S, P, V, A coupling strengths to electrons, and set constraints on their contributions to the electron anomalous magnetic moment a_{e}, |Δa_{X}|≲10^{-15}-10^{-13} for the X mass region 1  MeV≲m_{X}≲1  GeV. These results are an order of magnitude more sensitive compared to the current accuracy on a_{e} from the electron g-2 experiments and recent high-precision determination of the fine structure constant.

Hunting down the X17 boson at the CERN SPS

Recently, the ATOMKI experiment has reported new evidence for the excess ofe + e - events with a mass ∼ 17 MeV in the nuclear transitions of4 He, that they previously observed in measurements with8 Be. These observations could be explained by the existence of a new vector X 17 boson. So far, the search for the decay X 17 → e + e - with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining X 17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X 17 decay with the proposed method. To reach this goal an optimization of the X 17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible.

Evidence for Nonlinear Isotope Shift in Yb^{+} Search for New Boson

We measure isotope shifts for five Yb^{+} isotopes with zero nuclear spin on two narrow optical quadrupole transitions ^{2}S_{1/2}→^{2}D_{3/2}, ^{2}S_{1/2}→^{2}D_{5/2} with an accuracy of ∼300  Hz. The corresponding King plot shows a 3×10^{-7} deviation from linearity at the 3σ uncertainty level. Such a nonlinearity can indicate physics beyond the Standard Model (SM) in the form of a new bosonic force carrier, or arise from higher-order nuclear effects within the SM. We identify the quadratic field shift as a possible nuclear contributor to the nonlinearity at the observed scale, and show how the nonlinearity pattern can be used in future, more accurate measurements to separate a new-boson signal from nuclear effects.

Improved Isotope-Shift-Based Bounds on Bosons beyond the Standard Model through Measurements of the ^{2}D_{3/2}-^{2}D_{5/2} Interval in Ca^{+}

We perform high-resolution spectroscopy of the 3d ^{2}D_{3/2}-3d ^{2}D_{5/2} interval in all stable even isotopes of ^{A}Ca^{+} (A=40, 42, 44, 46, and 48) with an accuracy of ∼20  Hz using direct frequency-comb Raman spectroscopy. Combining these data with isotope shift measurements of the 4s ^{2}S_{1/2}↔3d ^{2}D_{5/2} transition, we carry out a King plot analysis with unprecedented sensitivity to coupling between electrons and neutrons by bosons beyond the standard model. Furthermore, we estimate the sensitivity to such bosons from equivalent spectroscopy in Ba^{+} and Yb^{+}. Finally, the data yield isotope shifts of the 4s ^{2}S_{1/2}↔3d ^{2}D_{3/2} transition at 10 parts per billion through combination with recent data of Knollmann, Patel, and Doret [Phys. Rev. A 100, 022514 (2019)PLRAAN2469-992610.1103/PhysRevA.100.022514].

New Opportunities at the Next-Generation Neutrino Experiments (Part 1: BSM Neutrino Physics and Dark Matter

With the advent of a new generation of neutrino experiments which leverage high-intensity neutrino beams for precision neutrino oscillation parameter and for CP violation phase measurements, it is timely to explore physics topics beyond the standard neutrino-related physics. Given that beyond the standard model (BSM) physics phenomena have been mostly sought at high-energy regimes, such as the LHC at CERN, the exploration of BSM physics in neutrino experiments will enable complementary measurements at the energy regimes that balance that of the LHC. This is in concert with new ideas for high-intensity beams for fixed target and beam-dump experiments world-wide. The combination of the high intensity beam facilities and large mass detectors with highly precise track and energy measurements, excellent timing resolution, and low energy thresholds will help make BSM physics reachable even in low energy regimes in accelerator-based experiments and searches for BSM phenomena from cosmogenic origin. Therefore, it is conceivable that BSM topics could be the dominant physics topics in the foreseeable future. In this spirit, this paper provides a review of the current theory landscape theory in neutrino experiments in two selected areas of the BSM topics - dark matter and neutrino related BSM - and summarizes the current results from existing neutrino experiments for benchmark. This paper then provides a review of upcoming neutrino experiments and their capabilities to set the foundation for potential reach in BSM physics in the two themes. One of the most important outcomes of this paper is to ensure theoretical and simulation tools exist to perform studies of these new areas of physics from the first day of the experiments, such as DUNE and Hyper-K. Tasks to accomplish this goal, and the time line for them to be completed and tested to become reliable tools in a timely fashion are also discussed.

Confirmation of the existence of the X17 particle

In a 2016 paper, an anomaly in the internal pair creation on the M1 transition depopulating the 18.15 MeV isoscalar 1+ state on 8Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass mXc2 = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson (mXc2) and its branching ratio relative to the γ-decay of the 8Be excited state (BX), to mXc2 = 17.01(16) MeV and BX = 6(1) 10−6, respectively. Using the latter setup, the e+e− pairs depopulating the 21 MeV Jπ = 0− 0+ transition in 4He were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass mXc2 = 16.98 ± 0.16(stat) ± 0.20(syst) MeV.

Search for A^{'}→μ^{+}μ^{-} Decays

Searches are performed for both promptlike and long-lived dark photons, A^{'}, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A^{'}→μ^{+}μ^{-} decays using a data sample corresponding to an integrated luminosity of 5.5  fb^{-1} collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the γ-A^{'} kinetic mixing strength. The promptlike A^{'} search explores the mass region from near the dimuon threshold up to 70 GeV and places the most stringent constraints to date on dark photons with 214<m(A^{'})≲740  MeV and 10.6<m(A^{'})≲30  GeV. The search for long-lived A^{'}→μ^{+}μ^{-} decays places world-leading constraints on low-mass dark photons with lifetimes O(1)  ps.

First Direct Search for Inelastic Boosted Dark Matter with COSINE-100

A search for inelastic boosted dark matter (IBDM) using the COSINE-100 detector with 59.5 days of data is presented. This relativistic dark matter is theorized to interact with the target material through inelastic scattering with electrons, creating a heavier state that subsequently produces standard model particles, such as an electron-positron pair. In this study, we search for this electron-positron pair in coincidence with the initially scattered electron as a signature for an IBDM interaction. No excess over the predicted background event rate is observed. Therefore, we present limits on IBDM interactions under various hypotheses, one of which allows us to explore an area of the dark photon parameter space that has not yet been covered by other experiments. This is the first experimental search for IBDM using a terrestrial detector.

Recent results of the NA64 experiment at the CERN SPS

We report on the results of the search 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 → e-ZA’ of 100 GeV electrons dumped against an active target which is followed by the prompt invisible decay A’ $ \to \chi \bar{\chi } $. 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 × 1010 electrons on target no evidence of such a process has been found. The constraints on the A’ mixing strength with photons, 10−5 ≤ ɛ ≤ 10−2, for the A’ mass range mA’ ≤ 1GeV are derived. Significantly more data were taken in 2017 and 2018. The analysis of these data is not yet finished, although the sensitivity is already estimated as about 3 times better than in the previously published result on the 2016 data.

For models considering scalar and fermionic thermal Dark Matter interacting with the visible sector through the vector portal the experiment starts to be sensitive to the regions of the dark-matter parameter $ y = \in^{2} \alpha_{D} (\frac{{m_{\chi } }}{{m_{{A^{\prime}}} }})^{4} $ predicted from the cosmological observations of Dark Matter.

We also report the improved 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 8Be* nucleus decays. Due to its coupling to electrons, the X could be produced in the bremsstrahlung reaction e-Z → e-ZX by a beam of electrons incident on an active target in the NA64 experiment at the CERN SPS and observed through the subsequent decay into a e+e- pair. With 8.4 × 1010 electrons on target no evidence for such decays was found, allowing to set limits on the X - e- coupling in the range 1 × 10−4 ≤ ɛe ≤ 6 × 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 ≤ 23MeV.

Future plans of the NA64 experiment are presented.