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A search for new resonances decaying into a pair of jets is reported using the dataset of proton-proton collisions recorded at √ s = 13 TeV with the ATLAS detector at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of 139 fb −1. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the Standard Model background. In addition to an inclusive dijet search, events with jets identified as containing b-hadrons are examined specifically. No significant excess of events above the smoothly falling background spectra is observed. The results are used to set cross-section upper limits at 95% confidence level on a range of new physics scenarios. Model-independent limits on Gaussian-shaped signals are also reported. The analysis looking at jets containing b-hadrons benefits from improvements in the jet flavour identification at high transverse momentum, which increases its sensitivity relative to the previous analysis beyond that expected from the higher integrated luminosity.

A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced in pp collisions at $$ \sqrt{s} $$ s = 13 TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb−1, corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation of c- or b-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.

The improved results on a direct search for a new X(16.7 MeV) boson that could explain the anomalous excess of e + e − pairs observed in the decays of the excited 8 Be * nucleus ("Berillium anomaly") are reported. The X boson could be produced in the bremsstrahlung reaction e − Z → e − ZX by a high energy beam of electrons incident on the 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 × 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 × 10 −4 e 6.8 × 10 −4 , excluding part of the parameter space favored by the Berillium anomaly. The non-observation of the decay A → e + e − allows also to set the new bounds on the mixing strength of photons with dark photons (A ) with a mass 24 MeV.Recently, the search for new light bosons weakly coupled to SM particles was additionally inspired by the observation in the ATOMKI experiment by Krasznahorkay et al. [1,2] of a ∼7σ excess of events in the invariant mass distribution of e + e − pairs produced in the nuclear transitions of the excited 8 Be * to its ground state via internal pair creation. It was shown that this anomaly can be interpreted as the emission of a protophobic gauge boson X with a mass of 16.7 MeV decaying into e + e − pair [3,4]. This explanation of the anomaly was found to be consistent with the existing constraints assuming that the X has non-universal coupling to quarks, coupling to electrons in the range 2 × 10 −4 e 1.4 × 10 −3 and lifetime 10 −14 τ X 10 −12 s. It is interesting that a new boson with such relatively large couplings to charged leptons could also resolve the so-called (g µ − 2 ) anomaly, a discrepancy between measured and predicted values of the muon anomalous magnetic moment. This has motivated worldwide efforts towards the experimental searches, see, e.g., Refs. [5,6], and studies of the phenomenological aspects of light vector bosons weakly coupled to quarks and leptons, see, e.g., and also earlier works of Refs. [13][14][15][16]. The latest experimental results from the ATOMKI group show a similar excess of events at approximately the same invariant mass in the nuclear transitions of another nucleus, 4 He [17]. This further increases the importance of independent searches for a new particle X.Another strong motivation to search for new light bosons decaying into e + e − pair comes from the dark matter puzzle. An interesting possibility is that in addition to gravity a new force between the dark sector and visible matter, carried by a new vector boson A , called dark photon, might exist [18,19]. Such A could have a mass

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