Measurement of the Inelastic Proton-Proton Cross Section at $\sqrt{s} = 13$ TeV with the ATLAS Detector at the LHC (1606.02625v4)

Abstract: This Letter presents a measurement of the inelastic proton-proton cross section using 60 $\mu$b$^{-1}$ of $pp$ collisions at a center-of-mass energy $\sqrt{s}$ of $13$ TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region ($2.07<|\eta|<3.86$) of the detector. A cross section of $68.1\pm 1.4$ mb is measured in the fiducial region $\xi=M_X^2/s>10^{-6}$, where $M_X$ is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this $\xi$ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with $M_X>13$ GeV. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross-section of $78.1 \pm 2.9$ mb is measured, consistent with the inelastic cross section increasing with center-of-mass energy.

• The paper presents a robust measurement of the inelastic pp cross section at 13 TeV with a fiducial value of 68.1 ± 1.4 mb for ξ > 10⁻⁶.
• It utilizes advanced detector techniques and multiple Monte Carlo generators, including Pythia8 and EPOS, to cross-verify experimental results.
• The study confirms that the cross-section rise aligns with non-perturbative QCD expectations, offering crucial insights for refining hadronic interaction models.

Measurement of the Inelastic Proton-Proton Cross Section at $\sqrt{s} = 13 \TeV$ with the ATLAS Detector at the LHC

The paper presents a detailed measurement of the inelastic proton-proton ($pp$) cross section at a center-of-mass energy $\sqrt{s}$ of 13 TeV using the ATLAS detector at the Large Hadron Collider (LHC). This research harnesses 60 $\mu$b$^{-1}$ of integrated luminosity from $pp$ collisions produced in June 2015. The experimental setup focused primarily on inelastic interactions, excluding elastic scattering and specific diffractive processes, by implementing rings of plastic scintillators in the forward region of the ATLAS detector. In this setup, a fiducial cross section of $68.1 \pm 1.4$ mb was recorded for $\xi = M_X^2/s > 10^{-6}$, where $M_X$ represents the larger invariant mass of the two hadronic systems separated by the maximum rapidity gap within the event.

The rise in the total $pp$ cross section with $\sqrt{s}$ serves as a probe into the non-perturbative field of quantum chromodynamics (QCD), supported by Heisenberg and observed in early experiments such as the CERN Intersecting Storage Rings. The current understanding, underpinned by foundational arguments such as unitarity and factorization, contends that the cross-section growth should not surpass a logarithmic squared relation to energy, hence laying theoretical expectations against which experimental results are compared.

The analysis detailed in the paper further extrapolates the fiducial cross-section measurement to the full inelastic cross-section, yielding a value of $78.1 \pm 2.9$ mb. This estimate aligns with theoretical predictions and confirms the expected increase of the inelastic cross-section with rising center-of-mass energy, consistent with lower energy findings and predictions from earlier models. The measured values affirmed compatibility with several theoretical frameworks, indicating the robustness of the employed methodologies and the experiment's accuracy in capturing the dynamics of $pp$ interactions at high energies.

Different event generators were employed to analyze and corroborate these measurements, including Pythia8, Epos LHC, and QGSJet-II. The DL and MBR models of Pythia8 were notably congruent with the results. Contrarily, predictions from the Epos LHC and QGSJet-II models slightly exceeded observed measurements, emphasizing areas for refinement in the modeling of high-energy hadronic interactions. This research highlights the efficacy of using multiple predictive models to cross-verify measurement accuracy and further elaborates on the subtleties in diffractive processes by adjusting model parameters to match the measured ratio of single-sided to inclusive events, thus guiding future event generator tuning for high-energy physics simulations.

The implementation of advanced Monte Carlo simulations, benchmarked against experimental results, marks a significant step in understanding non-diffractive and diffractive processes in high-energy $pp$ collisions. Such insights form a crucial addition to the corpus of knowledge in QCD, especially towards unveiling the intricate behavior of hadronic cross sections under varying high-energy conditions.

The implications of this paper are profound, suggesting that as experimental accuracy and detector technologies further develop, insights into QCD's non-perturbative aspects will be increasingly accessible. Future research could focus on refining diffractive models and expanding our understanding of the linkage between elastic and inelastic scattering processes, as well as exploring the ramifications of these findings in cosmic ray physics and beyond the standard model phenomena.

In summary, the paper delivers a precise measurement of the inelastic $pp$ cross section at 13 TeV, corroborating theoretical predictions and providing valuable data against which models of hadronic interactions can be calibrated, thus offering insightful implications for both experimental and theoretical advancement in high-energy particle physics.