The High‑Luminosity Large Hadron Collider (HL‑LHC) will produce many more simultaneous proton collisions than today. That makes it harder for experiments to trace charged particles back to where they started. The CMS collaboration has added a new step to a fast, parallel tracking algorithm called Line Segment Tracking (LST). This new step, called a quadruplet or T4, increases the radius at which CMS can reliably reconstruct tracks that start away from the collision point — from about 40 cm to about 60 cm in the tests reported here. By comparison, the legacy tracking approach without LST reached only around 8–10 cm for such displaced tracks.

LST builds track candidates using hits in the outer tracker only. It first groups two close hits in the same module into a “minidoublet,” then links nearby minidoublets into a short straight piece called a line segment. Line segments are linked into triplets (T3) and then into longer objects like quintuplets (T5). The new quadruplet (T4) is defined as a linked pair of triplets that share a line segment. T4s are built only from triplets that were not already used to make T5s or pixel‑linked objects, and only in specific detector regions. The T4s are added to the final list of track candidates that the rest of the reconstruction uses.

Because displaced tracks are rarer than prompt tracks that come from the collision point, the team used a small deep neural network (DNN) to separate real displaced T4s from prompt or fake ones. The DNN labels each T4 candidate as real prompt, real displaced, or fake. “Displaced” here means a simulated track that begins more than 10 cm away from the primary interaction point in the plane perpendicular to the beam. The DNN uses simple geometric features of the hits and the scores of the triplets that make up the T4. The selection thresholds were tuned to keep between 95% and 99% of real displaced T4s while rejecting many prompt and fake candidates.