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Register a new userSearch for 4th family quarks with the ATLAS detector
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The pair production of heavy fourth-generation quarks, which are predicted under the hypothesis of flavor democracy, is studied using tree-level Monte Carlo generators and fast detector simulation. Two heavy-quark mass values, 500 and 750$gev$, are considered with the assumption that the fourth family mixes primarily with the two light families. It is shown that a clear signature will be observed in the data collected by the ATLAS detector, after the first year of low-luminosity running at the Large Hadron Collider.
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Existence of metastable quarks of new generation can be embedded into phenomenology of heterotic string together with new long range interaction, which only this new generation possesses. We discuss primordial quark production in the early Universe, their successive cosmological evolution and astrophysical effects, as well as possible production in present or future accelerators. In case of a charge symmetry of 4th generation quarks in Universe, they can be stored in neutral mesons, doubly positively charged baryons, while all the doubly negatively charged baryons are combined with He-4 into neutral nucleus-size atom-like states. The existence of all these anomalous stable particles may escape present experimental limits, being close to present and future experimental test. Due to the nuclear binding with He-4 primordial lightest baryons of the 4th generation with charge +1 can also escape the experimental upper limits on anomalous isotopes of hydrogen, being compatible with upper limits on anomalous lithium. While 4th quark hadrons are rare, their presence may be nearly detectable in cosmic rays, muon and neutrino fluxes and cosmic electromagnetic spectra. In case of charge asymmetry, a nontrivial solution for the problem of dark matter (DM) can be provided by excessive (meta)stable anti-up quarks of 4th generation, bound with He-4 in specific nuclear-interacting form of dark matter. Such candidate to DM is surprisingly close to Warm Dark Matter by its role in large scale structure formation. It catalyzes primordial heavy element production in Big Bang Nucleosynthesis and new types of nuclear transformations around us.
This paper has been withdrawn by the authors due to the violation of ATLAS experiment publication policy.
Leptoquarks (LQ) are predicted by many new physics theories to describe the similarities between the lepton and quark sectors of the Standard Model and offer an attractive potential explanation for the lepton flavour anomalies observed at LHCb and flavour factories. The ATLAS experiment has a broad program of direct searches for Leptoquarks, coupling to the first-, second- or third-generation particles. The most recent 13 TeV results on the searches for Leptoquarks and contact interactions with the ATLAS detector are reviewed, covering flavour-diagonal and cross-generational final states.
The large ttbar production cross-section at the LHC suggests the use of top quark decays to calibrate several critical parts of the detectors, such as the trigger system, the jet energy scale and b-tagging.
A direct search for charged lepton-flavour violation in top-quark decays is presented. The data analysed correspond to $79.8 text{fb}^{-1}$ of proton--proton collisions at a centre-of-mass energy of $sqrt{s}=13 text{TeV}$ recorded by the ATLAS experiment at the LHC. The process studied is the production of top-quark pairs, where one top quark decays into a pair of opposite-sign different-flavour charged leptons and an up-type quark, while the other decays semileptonically according to the Standard Model. The signature of the signal is thus characterised by the presence of three charged leptons, a light jet and a $b$-jet. A multivariate discriminant is deployed and its distribution used as input to extract the signal strength. In the absence of a signal, an upper limit on the branching ratio of ${cal B}(t to ell ell q) < 1.86 times 10^{-5}$ is set at the 95% confidence level.
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