CMS Tracking Performance Results from early LHC Operation (1007.1988v2)

Abstract: The first LHC pp collisions at centre-of-mass energies of 0.9 and 2.36 TeV were recorded by the CMS detector in December 2009. The trajectories of charged particles produced in the collisions were reconstructed using the all-silicon Tracker and their momenta were measured in the 3.8 T axial magnetic field. Results from the Tracker commissioning are presented including studies of timing, efficiency, signal-to-noise, resolution, and ionization energy. Reconstructed tracks are used to benchmark the performance in terms of track and vertex resolutions, reconstruction of decays, estimation of ionization energy loss, as well as identification of photon conversions, nuclear interactions, and heavy-flavour decays.

• The paper demonstrates that the CMS Tracker achieves high precision in reconstructing charged particle trajectories using all-silicon detectors.
• It details the effective use of a combinatorial track finder algorithm to enhance track reconstruction accuracy over varied energy ranges.
• The study benchmarks key observables like momentum and impact parameter resolutions, confirming robust performance for early LHC physics analyses.

Overview of CMS Tracking Performance Results from Early LHC Operation

The manuscript provides a comprehensive examination of the CMS detector's tracking performance during the initial phase of LHC operations. This paper is grounded in data from the first proton-proton (pp) collisions registered at center-of-mass energies of 0.9 and 2.36 TeV, recorded by the CMS detector in December 2009. The authors focus on the all-silicon Tracker's capabilities, emphasizing its ability to reconstruct charged particle trajectories within the robust 3.8 T axial magnetic field.

Key Components and Methodology

The paper methodically details the CMS Tracker's architecture, comprising both the silicon pixel detector and silicon strip detector. The pixel detector, characterized by its high granularity and precision, plays a critical role in defining three-dimensional points on track trajectories. Meanwhile, the strip detector extends this functionality by covering a larger volume with a layered configuration to enable precise momentum measurements.

The tracking algorithm employed leverages patterns in detector hits to extrapolate particle tracks. The combinatorial track finder (CTF) serves as the backbone of this analysis, facilitating multiple iterations of track finding and validation to improve reconstruction accuracy.

Performance Metrics and Results

Numerical assessments presented in the manuscript report nominal momentum resolutions of 0.7% and 5.0% at momenta of 1 and 1000 GeV/c, respectively, in the central region. The impact parameter resolution for high-momentum tracks is approximately 10 microns. Such precision is pivotal for the accurate determination of primary interaction vertices as well as for distinguishing b-jets through evidence of displaced vertices associated with specific jets.

In the field of particle identification, dE/dx measurements are highlighted as a robust method for distinguishing charged hadrons below the MIP (Minimum Ionizing Particle) region, validated through decay studies of particles like the K$^0_S$ and $\Lambda^0$.

The paper effectively benchmarks the Tracker's performance using various observables, including primary and secondary vertex resolutions and reconstruction of particle decays. Intriguing results are provided for reconstructed V$^0$ decays, which test the CMS detector's ability to manage displaced vertices and adhere closely to simulation predictions.

Discussion and Implications

The findings affirm the CMS Tracker's robust functionality even at the early stages of LHC operations, accomplishing significant strides in precision tracking and vertexing that meet design expectations. The alignment of the Tracker, refined through cosmic rays prior to collisions, plays an instrumental role in these outcomes, demonstrating the benefit of extensive pre-collision testing and calibration.

The implications of this work extend to foundational physics research and experimental high-energy physics. The Tracker's precision is crucial for accurate measurements of fundamental properties of particles and interactions, thereby aiding in the discovery potentials for new physics beyond the standard model.

Speculations for Future Developments

Looking forward, the advancements in sensor technology and data processing algorithms could further enhance the CMS Tracker's efficiency and resolution, particularly under higher luminosity conditions of future LHC runs. These improvements might enable more refined b-tagging capabilities and foster greater insight into the underlying particle physics phenomena.

The results from this paper set a benchmark for ongoing and future analyses by detailing both the capabilities and limitations of the CMS Tracker, motivating advances in detection algorithms and experimental setups to capitalize on the dataset from increasingly energetic and complex collisions at the LHC.