  Recognizing hadronically decaying top-quark jets in a sample of jets, or even
its total fraction in the sample, is an important step in many LHC searches for
Standard Model and Beyond Standard Model physics as well. Although there exists
outstanding top-tagger algorithms, their construction and their expected
performance rely on Montecarlo simulations, which may induce potential biases.
For these reasons we develop two simple unsupervised top-tagger algorithms
based on performing Bayesian inference on a mixture model. In one of them we
use as the observed variable a new geometrically-based observable
$\tilde{A}_{3}$, and in the other we consider the more traditional
$\tau_{3}/\tau_{2}$ $N$-subjettiness ratio, which yields a better performance.
As expected, we find that the unsupervised tagger performance is below existing
supervised taggers, reaching expected Area Under Curve AUC $\sim 0.80-0.81$ and
accuracies of about 69% $-$ 75% in a full range of sample purity. However,
these performances are more robust to possible biases in the Montecarlo that
their supervised counterparts. Our findings are a step towards exploring and
considering simpler and unbiased taggers.
