CMS-PAS-SMP-26-008

CMS-PAS-SMP-26-008
Observation of lepton-lepton scattering in proton-proton collisions at $ \sqrt{s} = $ 13 and 13.6 TeV
CMS Collaboration
2026-07-25

Abstract: The scattering through a photon exchange of same-sign charge electrons and muons originating from protons is observed for the first time using a data sample of proton-proton collisions at $ \sqrt{s}= $ 13 and 13.6 TeV collected with the CMS detector between 2016 and 2026, corresponding to an integrated luminosity of 446 fb$ ^{-1} $. The $ e^\pm\mu^\pm $ and $ \mu^\pm\mu^\pm $ scatterings are searched for in the $ e^\pm\mu^\pm $, $ \mu^\pm\mu^\pm $, and $ \mu^\pm\mu^\pm\mu^\mp $ final states. The signal is separated from backgrounds on the basis of the track multiplicity at the multilepton vertex and of the ratio between the vectorial and scalar transverse momentum sums of the leptons. The lepton-lepton scattering is observed with a significance of 5.4 standard deviations with respect to the background-only hypothesis. A strong evidence for the existence of muon-muon scattering is obtained with a significance of 4.0 standard deviations. The fiducial cross sections for $ e^\pm\mu^\pm $ and $ \mu^\pm\mu^\pm $ are measured at $ \sqrt{s}= $ 13 and 13.6 TeV and agree with the theoretical predictions.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Tree-level Feynman diagram for the $ \ell\ell $ scattering at the LHC. The $ \mathrm{e}^\pm\mu^\pm $ and $ \mu^\pm\mu^\pm $ scatterings targeted in this analysis are shown. The protons may or may not fragment.
png pdf	Figure 2: Observed and expected distributions of the $ p_{\mathrm{T}} $ balance for events passing the SR selection in the $ \mu^\pm\mu^\pm\mu^\mp $ final state and in the "2 tracks" and "3 tracks $ \|q_\text{tot}\|= $ 1" categories of the $ \mathrm{e}^\pm\mu^\pm $ and $ \mu^\pm\mu^\pm $ final states. The predicted distributions are adjusted to the result of the global fit, and the signal distribution is normalized to its best fit signal strength. The lower panel shows the difference between the observed events and the backgrounds, as well as the signal contribution.
png pdf	Figure 3: Predicted and measured fiducial cross sections for the $ \mathrm{e}^\pm\mu^\pm $ and $ \mu^\pm\mu^\pm $ scatterings at $ \sqrt{s}= $ 13 and 13.6 TeV. The theoretical predictions are taken from the SFGEN generator [34,35] and shown with their theoretical uncertainties, where the leading source is the survival factor. The measured uncertainty includes all systematic and statistical uncertainties, and is largely statistically driven.

Tables

png pdf

Table 1:
Summary of the selection criteria in the SRs. The symbols $ \ell_1 $ and $ \ell_2 $ denote the leading and subleading leptons, respectively. The track multiplicity $ N_\text{tracks} $ includes tracks matched or not to the selected leptons. The expression "OSSF from $ \mathrm{Z} \ell\ell $" denotes an OS same-flavor lepton pair with invariant mass compatible with $ m_{\mathrm{Z}} $, and the symbol $ N $ denotes a multiplicity.

Summary

In summary, the scattering of leptons originating from the protons has been observed for the first time in proton-proton collisions with a significance of 5.4 standard deviations with respect to the background-only hypothesis. A strong evidence for the existence of muon-muon scattering, which had never been investigated in any previous experiment, is obtained with a significance of 4.0 standard deviations. These results rely on 446 fb$ ^{-1} $ of data collected with the CMS detector in 2016--2026 at $\sqrt{s}=13$ and 13.6 TeV. The fiducial $ e^\pm\mu^\pm $ and $ \mu^\pm\mu^\pm $ scattering cross sections have been measured at $\sqrt{s}=13$ and 13.6 TeV and show good agreement with the predictions of the standard model. This result supports the concept of lepton content of the proton and lays the foundations for future beyond-the-standard-model searches for lepton-induced processes at LHC.

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