Limits on new strongly interacting matter from measurements of Transverse Energy-Energy Correlations at $\sqrt{s} = 13$ TeV at the LHC

TL;DR

The paper addresses constraining models with new colour-charged fermions by exploiting the sensitivity of Transverse Energy-Energy Correlations (TEEC) to the strong coupling constant $\alpha_s(Q^2)$. It combines ATLAS 13 TeV data with approximate NNLO TEEC predictions across a wide set of BSM scenarios parameterised by $n_{\text{eff}}$ and $m_X$, using SM $K$-factors to proxy NNLO for BSM. A CLs-based likelihood test yields 95% confidence level exclusions, with masses up to about $4$ TeV excluded for several $n_{\text{eff}}$, and stronger limits at higher $H_{T2}$; results are robust to PDF choices and improve upon 8 TeV constraints. The study demonstrates TEEC as a precise probe of strong-sector extensions and sets a baseline for future high-energy collider tests of coloured states without dependence on their decay properties.

Abstract

This work establishes 95% confidence level limits to models incorporating additional fermions sensitive to the strong interaction. Precision measurements of Transverse Energy-Energy Correlations at the ATLAS experiment are used, exploiting their dependence on the strong coupling constant to analyse the effects of introducing new fermions with colour charge on the Renormalisation Group Equation. The comparison between theoretical predictions, corrected up to next-to-next-to-leading order, and the data collected by ATLAS at $\sqrt{s} = 13$ TeV allows to constrain physics models proposing the existence of new fermions with masses up to 4 TeV, independently of assumptions on the fermion decay.

Figures (6)

• Figure 1: Variation of $\alpha_s$ with the renormalisation scale, calculated at NLO for different BSM models, introducing a new colour-charged fermion into the SM. The resulting curves are shown for a fermion with mass $m_X = 600$ GeV, transforming as a colour triplet, octet, sextet and decuplet, respectively.
• Figure 2: Differential cross section of the process $pp \rightarrow \hbox{jj}$ as a function of $H_{T2}$, calculated at NLO for BSM models introducing a new fermion with mass $m_X = 600$ GeV and $n_{\text{eff}} = 1, 3, 5$ and 15, respectively. On the lower panel, the ratio of the theoretical predictions for BSM models to the expected values for the SM is shown.
• Figure 3: Theoretical predictions of transverse energy-energy correlations for two $H_{T2}$ energy ranges. In each individual figure, the upper graph shows the approximate NNLO theoretical prediction of the SM and the data collected at 13 TeV. The lower graph represents the ratio of the prediction of four BSM models including a new fermion with colour charge, mass $m_X =$ 600 GeV, and $n_{\text{eff}} =$ 1, 3, 5, 15 to the prediction of the SM.
• Figure 4: Distributions of the likelihood ratio $t=-2\log(Q)$, evaluated for the TEEC distribution with 1.8 TeV $< H_{T2} <$ 2 TeV, for the SM (blue) and BSM (red) hypotheses for a particular model with $n_{\text{eff}} = 2$ and $m_X = 600$ GeV. The observed value, $t_{\text{obs}}$, is also shown as a vertical dashed line.
• Figure 5: CL$_\text{s}$ for BSM models with $1 < n_\text{eff} < 20$ and 200 GeV $< m_X <$ 4 TeV in 200 GeV intervals calculated with TEEC. The ($m_X$, $n_{\text{eff}}$) plane is shown for four energy ranges, 2.3 TeV $<$$H_{T2}$$<$ 2.6 TeV (top left), 2.6 TeV $< H_{T2} <$ 3.0 TeV (top right), 3.0 TeV $< H_{T2} <$ 3.5 TeV (bottom left) and $H_{T2} >$ 3.5 TeV (bottom right).