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Bottom-quark jet tagging identifies jets compatible with containing a hadron carrying a bottom quark. A sufficiently long-lived bottom hadron travels away from the primary interaction before its particle decay, so displaced particle tracks and a secondary decay vertex provide evidence about its identity. A lepton within the jet can supply additional evidence. A top quark instead decays before hadronization.

A tag is a statistical classification, so both the efficiency for bottom jets and mistaken tags from charm quarks or light jets require calibration. That calibration concerns the flavor tagging response; it is not interchangeable with a calorimeter calibration of deposited energy. Control samples in collision data can measure the tagging efficiency for specified momentum and topology selections; those selections determine where the result applies.^[ref]

John G. Wood was an accelerator technician who helped develop the hydrogen bubble chamber in Luis Walter Alvarez's group. Alvarez's retrospective describes Wood constructing an all-glass chamber about $1.5$ inches across and observing particle tracks in liquid hydrogen in early 1954. The visible tracks established more than an increase in bulk boiling under irradiation: a usable particle detector needed enough localized bubbles to resolve individual trajectories.^[ref]

Wood's observations also helped show that a long-lived state of superheating was unnecessary. With rapid expansion, useful particle tracks could form in the chamber's interior before boiling at the walls destroyed its sensitivity. That operational insight made larger bubble chambers with joints and windows practical, extending the instrument beyond small, exceptionally smooth all-glass vessels.^[ref]

A particle track is a reconstructed trajectory inferred from spatially separated detector measurements associated with a particle. Individual hits are observations; fitting a track combines them with a propagation model, measurement errors and possible scattering.

In a uniform magnetic field, a charged particle follows a helical trajectory in the ideal limit. Its transverse momentum is $p_{\perp }=|q|BR$, where $R$ is the projected curvature radius in SI units. Curvature therefore constrains momentum, while its direction constrains the charge sign. Neutral particles generally require indirect reconstruction through secondary charged particles or other interaction signatures rather than an ordinary continuous charged-particle track.

A particle track projected onto the plane perpendicular to a uniform magnetic field $B$ is a circular arc with radius $R=p_{\perp }/(|q|B)$. Here $p_{\perp }$ is the momentum component perpendicular to the magnetic field; it is the collider transverse momentum when the field follows the beam axis. For chord length $L$ and sagitta $s$, circle geometry givesFor a short arc with $s\ll L$, this yieldsThe measurement uncertainty follows by differentiating the inverse relation, treating $B$ and $L$ as known. A fixed spatial measurement uncertainty therefore makes fractional momentum uncertainty grow with momentum, whereas increasing the lever arm improves it quadratically in this idealization. Multiple Coulomb scattering, field uncertainties and alignment add other contributions; a helical particle track also requires its pitch to recover the full momentum.^[ref]