First KK gluon at the LHC: couplings, widths, tt̄ / dijet limits and the m(tt̄) spectrum
Compute the first KK gluon of flat GHU (√2 g_s to every quark) or of a warped extra dimension (zero-mode quarks of bulk mass c), its widths and branching fractions, and compare its leading-order σ × BR with the ATLAS tt̄ and CMS dijet limits as r = prediction / limit. Then compute the m(tt̄) spectrum with the interference between the KK gluon and the QCD gluon, in the bins of the CMS parton-level measurement, and the expected Δχ² against its covariance.
Control names are kept as they appear in the application. Start with a reference, change one input at a time, and retain the assumptions with your result.
What this tool is for
Use this to see which LHC channel constrains which realisation: the flat coloron couples democratically (BR(tt̄) ≈ 1/6) and is probed by dijets and, through its large light-quark production, by tt̄; a warped KK gluon is top-philic (BR(tt̄) > 0.8) and broad, so tt̄ resonance searches are its channel.
First steps
- New here? Press 🎬 Demo in the card heading: a forty-second guided simulation presses the buttons for you and ends with how to read the result.
- Choose a preset: the flat GHU coloron, the published RS point of arXiv:0807.4937, or illustrative warped values.
- Change one input: the mass, kL or one of the c values.
- Read Γ/M, BR(tt̄) and the two r values at the chosen mass, then the two mass scans.
- Read the m(tt̄) paragraph: the sign of the interference below the pole, the largest change of the spectrum, Δχ² and the mass where it falls to 3.84; then the two spectrum figures.
- Open the certificates and the PDF systematic before quoting a crossing mass.
Controls and inputs
| Control | Meaning | Try this |
|---|---|---|
| Realisation | Flat GHU (Part VII coloron) or warped zero-mode quarks. | Switch at fixed mass and compare BR(tt̄). |
| KK-gluon mass [TeV] | The resonance mass at which the r values are read. | Move it across the crossing and watch r pass 1. |
| kL and the c values | Warp factor and bulk masses (left-handed UV-localised for c > 1/2, right-handed for c < −1/2), as in the RS localisation probe. | Raise c of t_R and watch the top coupling and the width grow. |
| α_s(M_Z) | 0.130, the PDF set's value at M_Z, or this laboratory's 0.118; either is run to the mass at one loop with six flavours (the PDF set's own α_s table differs by about 0.6% at 1.8 TeV). | Compare the two; the production scales with the coupling. |
| SM theory uncertainty per m(tt̄) bin | An uncorrelated fraction of each measured bin added to the CMS covariance, standing for the scale and PDF errors of the SM prediction. | Set it to 0 and to 0.2: the Δχ² and the mass where it reaches 3.84 move a lot, which is the honest size of this comparison. |
Reading the results
| Result | Meaning |
|---|---|
| Couplings g₁/g_s | Zero-mode couplings to the first KK gluon, certified against a 40-digit reference. |
| Γ/M, BR(tt̄), BR(dijet) | Leading-order widths to quark pairs; the top threshold depends on chirality. |
| r(tt̄) and r(dijet) | Prediction over the ATLAS observed limit (Γ/M = 30% template) and over the CMS spin-1 limit interpolated to this width. |
| Low-tail and pole shares | Where σ(tt̄) comes from; a large low-tail share means the interference with QCD matters (computed in the spectrum). |
| Interference below the pole | Its sign is −sign(v_u v_t), v = (c_L + c_R)/2: destructive for same-sign couplings (flat GHU), constructive for the warped reference point. |
| Δχ² of the m(tt̄) spectrum | Separation of SM and SM + KK gluon in units of the CMS covariance, if the data equal the SM: a sensitivity, not an exclusion. The shift is applied as R × data because the LO SM shape is not the measured one. |
r ≥ 1 means the LO prediction exceeds the published observed limit of that experiment's benchmark. A crossing is a comparison with that benchmark, not a validated exclusion at another width; the low-tail share tells you when the result is mostly off-shell exchange.
Worked example
The published RS point at the edge of the ATLAS limit
- Load the published RS point preset.
- Read r(tt̄) at 3.75 TeV.
- Move the mass to 3.67 TeV, the point's own first KK gluon.
- Open the certificates.
What to expect: r(tt̄) is close to 1: the 2008 reference point sits at the edge of the 2025 ATLAS comparison, within the ±15% of the control.
Scope and limitations
- Leading order; no K-factor for tt̄. The interference enters the m(tt̄) spectrum only; the resonance-search comparison uses the Breit–Wigner alone, as the experiments' templates do.
- The spectrum's Δχ² assumes the K-factor of the KK gluon and of the interference equals the SM's, and depends strongly on the SM theory uncertainty chosen.
- A limit set with one width template is not guaranteed conservative for another width.
- Above about 6 TeV the q q̄ luminosity differs between PDF sets by more than 20%.
- Four-top production and fermion KK modes are not computed.
If something is unclear
An r value reads 'not evaluated'.
The mass or width lies outside the published grid (ATLAS 0.5–5 TeV; CMS width interpolation needs 10–30% and 2.1–6 TeV).
Sources and attribution
- ATLAS tt̄ resonances, arXiv:2512.17856
- HEPData: ATLAS Fig. 10c (KK gluon limits)
- CMS dijet resonances, arXiv:1911.03947
- HEPData: CMS spin-1 qq limits by width
- Casagrande et al., RS reference point, arXiv:0807.4937
- Atre et al., colour-octet widths, arXiv:1206.1661
- CMS differential tt̄ (TOP-20-001), arXiv:2108.02803
- HEPData: CMS parton-level dσ/dm(tt̄)
- HEPData: its covariance matrix
Current instrument guide. Archived editions retain their own scope.
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