[论文解读] First determination of the $ρ$ parameter at $\sqrt{s} = 13$ TeV -- probing the existence of a colourless three-gluon bound state

TOTEM Collaboration, G. Antchev+94 more|arXiv (Cornell University)|Dec 11, 2018

Particle physics theoretical and experimental studies被引用 46

一句话总结

TOTEM 协作在13 TeV 下以史无前例的精度测量 ρ 参数，测试包括无色的 3-gluon 结合态等模型，并通过库伦归一化确定新的总截面，排除 COMPETE 模型。

ABSTRACT

The TOTEM experiment at the LHC has performed the first measurement at $\sqrt{s} = 13$ TeV of the $ρ$ parameter, the real to imaginary ratio of the nuclear elastic scattering amplitude at $t=0$, obtaining the following results: $ρ= 0.09 \pm 0.01$ and $ρ= 0.10 \pm 0.01$, depending on different physics assumptions and mathematical modelling. The unprecedented precision of the $ρ$ measurement, combined with the TOTEM total cross-section measurements in an energy range larger than 10 TeV (from 2.76 to 13 TeV), has implied the exclusion of all the models classified and published by COMPETE. The $ρ$ results obtained by TOTEM are compatible with the predictions, from alternative theoretical models both in the Regge-like framework and in the QCD framework, of a colourless 3-gluon bound state exchange in the $t$-channel of the proton-proton elastic scattering. On the contrary, if shown that the 3-gluon bound state $t$-channel exchange is not of importance for the description of elastic scattering, the $ρ$ value determined by TOTEM would represent a first evidence of a slowing down of the total cross-section growth at higher energies. The very low-$|t|$ reach allowed also to determine the absolute normalisation using the Coulomb amplitude for the first time at the LHC and obtain a new total proton-proton cross-section measurement $σ_{tot} = 110.3 \pm 3.5$ mb, completely independent from the previous TOTEM determination. Combining the two TOTEM results yields $σ_{tot} = 110.5 \pm 2.4$ mb.

研究动机与目标

• Motivate and determine the real-to-imaginary ratio ρ of the forward hadronic amplitude at √s = 13 TeV to probe potential colourless 3-gluon exchange in the t-channel.
• Measure the total proton-proton cross-section σ_tot at 13 TeV using Coulomb-nuclear interference for absolute normalisation.
• Evaluate how the new ρ value constrains phenomenological models and COMPETE predictions across high-energy elastic scattering.

提出的方法

• Use the TOTEM forward proton spectrometer with Roman Pots in a β* = 2500 m optics to access very low |t| and Coulomb-nuclear interference (CNI).
• Reconstruct proton kinematics from two-arm (2RP) or four-arm (4RP) analyses to extract dσ/dt across |t| down to ~8×10^-4 GeV^2.
• Apply careful alignment, optics calibration, and resolution unfolding to obtain the differential cross-section with corrections for background, acceptance, and inefficiencies.
• Extract ρ from the forward region dσ/dt incorporating Coulomb and nuclear amplitudes and determine σ_tot via Coulomb normalisation.
• Separate data by fill and diagonal configurations, then merge statistically with controlled systematic uncertainties.
• Present comparisons to models in Regge-like and QCD frameworks, and discuss implications for Odderon-like exchanges and cross-section growth.]
• research_questionsatim 1: "What is the ρ parameter at √s = 13 TeV and how does it compare with Pomeron-only predictions and alternative models?"
• research_questionsatim 2: "Can the measured ρ and σ_tot at 13 TeV constrain or exclude COMPETE parametrisations for high-energy elastic scattering?"
• research_questionsatim 3: "Does the data provide evidence for a colourless three-gluon bound state exchange in the t-channel?"
• research_questionsatim 4: "What is the total cross-section σ_tot at 13 TeV when using Coulomb normalisation, and how does it compare to previous TOTEM measurements?"
• research_questionsatim 5: "How do the TOTEM results influence interpretations of cross-section growth and potential saturation at high energies?"

实验结果

研究问题

• RQ1 What is the ρ parameter at √s = 13 TeV and how does it compare with Pomeron-only predictions and alternative models?
• RQ2 Can the measured ρ and σ_tot at 13 TeV constrain or exclude COMPETE parametrisations for high-energy elastic scattering?
• RQ3 Does the data provide evidence for a colourless three-gluon bound state exchange in the t-channel?
• RQ4 What is the total cross-section σ_tot at 13 TeV when using Coulomb normalisation, and how does it compare to previous TOTEM measurements?
• RQ5 How do the TOTEM results influence interpretations of cross-section growth and potential saturation at high energies?

主要发现

• ρ = 0.09 ± 0.01 and ρ = 0.10 ± 0.01, depending on modelling assumptions.
• The unprecedented ρ precision combined with TOTEM σ_tot measurements excludes all COMPETE models.
• TOTEM results are compatible with predictions from both Regge-like and QCD-based models that include a colourless 3-gluon bound state exchange.
• Coulomb normalisation yields σ_tot = 110.3 ± 3.5 mb, and combining with differential cross-section results gives σ_tot = 110.5 ± 2.4 mb.
• The measurement supports the possibility of a 3-gluon bound state exchange in the t-channel, while also allowing interpretations involving cross-section growth saturation if such exchange is not dominant.