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ATLAS Data Collection

The ATLAS experiment at the Large Hadron Collider (LHC) represents one of the most ambitious scientific experiments of our time, aiming to unravel the mysteries of the fundamental particles and forces that shape our universe. Central to this search is the process of data collection and analysis, capturing the aftermath of proton-proton (pp) or ion-ion collisions at unprecedented energies. This document outlines the operational framework of the ATLAS runs and the process of collision event reconstruction, offering insights into the complexities and achievements of high-energy physics research.

ATLAS Runs and LHC Operations

An ATLAS run, typically lasting about 12 hours, is a concerted data acquisition effort that coincides with the LHC fill cycle. It is a precise operation of the ATLAS detector, capturing collision data for particle physics research. Distinct from multi-year "Runs," these "runs" represent shorter periods within the broader operational cycles of the LHC.

During ATLAS runs, the detector operates in sync with the LHC fill cycle, which involves the stages of beam injection, ramp-up to high energies, beam focusing (squeeze and adjust), and the stable beams phase, where data from collisions are recorded.

The LHC fill cycle phases are described below:

  • Injection: The magnets are charged to appropriate levels, and beams are injected into the LHC rings following a specified filling scheme, detailing proton bunch numbers and their spacings.
  • Ramp: Particle acceleration to the desired collision energy occurs, involving both the radio frequency systems and increasing magnet currents.
  • Squeeze and Adjust: Beam sizes at the interaction points are reduced (squeeze), followed by adjustments for optimal collisions (adjust).
  • Stable Beams: This phase marks when collisions occur, and it is safe to record data.
  • Dump and Ramp Down: Beams are safely extracted and discarded, and magnetic fields are reduced.

Collisions at ATLAS

The LHC features 3564 bunch crossings per revolution, with each crossing containing paired, unpaired, or empty bunches. "Paired" refers to crossings where both beams have bunches, "unpaired" where only one beam has a bunch, and "empty" where no bunches are present. These are organized into bunch groups, defined per fill and used in conjunction with trigger conditions. Proton or ion beams are accelerated and collided within the ATLAS detector, producing new particles detectable as result of the collision. These events occur over a billion times per second.

The reconstruction of these events is influenced by "pile-up," which refers to multiple inelastic pp collisions in a single bunch crossing and neighboring bunch crossings.

Event Reconstruction and Luminosity

Each collision event is a gold mine of data, with multiple interaction vertices produced along the beam line. The primary vertex, defined by the largest sum of the squares of the transverse momenta of associated tracks, is crucial for identifying various particles and searching for new physics phenomena.

Luminosity Blocks and Event Selection

Data acquisition in an ATLAS run is segmented into Luminosity Blocks (LBs), which are fundamental units evaluated for physics analysis. Events are selected based on criteria such as the number of vertices and the presence of high-transverse-momentum particles, ensuring the highest data quality for analysis. The events are then organized into a Good Run List, explained in the next section.

Multi-Year Operational Periods

"Run 1" through "Run 4" refer to the extended operational periods of the LHC and ATLAS, each marked by different energy levels, luminosities, and advancements. These periods are vital for discoveries like the Higgs boson and for delving into the laws of the universe.

  • Run 1 (2009-2013): The inaugural operational phase of the LHC, including the discovery of the Higgs boson. Initial energies were up to 7 TeV per beam, later increased to 8 TeV.
  • Run 2 (2015-2018): Characterized by an increase in collision energy to 13 TeV and improvements in luminosity, enabling deeper studies of the Higgs boson and searches for new particles.
  • Run 3 (2021-present): Marks further increases in LHC's luminosity and advancements in detector and data processing technology, focusing on detailed Higgs boson studies and exploring previous anomalies.
  • Run 4 (mid-2020s): Expected to initiate the High-Luminosity LHC (HL-LHC) project, significantly enhancing luminosity and data volume, preparint the way for more precise measurements and potential new discoveries.