{"id":325046,"date":"2026-02-07T11:13:48","date_gmt":"2026-02-07T11:13:48","guid":{"rendered":"https:\/\/www.europesays.com\/ie\/325046\/"},"modified":"2026-02-07T11:13:48","modified_gmt":"2026-02-07T11:13:48","slug":"atlas-maps-the-top-quark-higgs-boson-interaction-with-multileptons","status":"publish","type":"post","link":"https:\/\/www.europesays.com\/ie\/325046\/","title":{"rendered":"ATLAS maps the top quark\u2013Higgs boson interaction with multileptons"},"content":{"rendered":"

One of the central questions in particle physics is how fundamental particles acquire mass. The top quark<\/a>, as the heaviest known elementary particle, interacts most strongly with the Higgs boson<\/a>. As such, it may play a crucial role in understanding how the Higgs field gives rise to particle masses.<\/p>\n

The ATLAS Collaboration is studying the production of the Higgs boson together with a top-quark pair (\u201cttH production\u201d). First observed by the ATLAS and CMS Collaborations in 2018<\/a>, this process accounts for just 1% of all Higgs-boson production and yet provides a unique opportunity for researchers to directly measure the top\u2013Higgs interaction.<\/p>\n

Using the complete LHC Run-2 dataset (collected in 2015\u20132018), the ATLAS Collaboration has carried out a new measurement of ttH production<\/a>. The analysis focuses on \u201cmultilepton\u201d events, where particle collisions leave several leptons (electrons, muons or taus) in the final state.<\/p>\n

Multilepton needles in the haystack<\/strong><\/p>\n

\"Physics,ATLAS\"<\/a>Figure 1: The six categories used in the analysis as a function of the number of light leptons (electrons or muons) and hadronically-decaying tau leptons. 2lSS refers to two leptons of the same sign. (Image: ATLAS Collaboration\/CERN)<\/p>\n

To maximise sensitivity, ATLAS physicists divided their data into six exclusive categories based on the number and charge of the leptons (Figure 1). Categories containing one or two hadronically-decaying<\/a> tau leptons would be particularly sensitive to Higgs boson decays into taus, while the other categories would focus on Higgs-boson decays into W bosons. All six categories were analysed simultaneously, allowing the small ttH signal to be separated from much more common background processes.<\/p>\n

The new ATLAS result improves on the previous Run-2 analysis<\/a> by combining a larger dataset with enhanced particle identification and reconstruction techniques. Researchers also drew upon more accurate simulations, dedicated studies of the dominant background processes, data-driven methods to constrain background rates, and a refined treatment of experimental uncertainties to bolster their result.<\/p>\n

Testing predictions<\/strong><\/p>\n

The ATLAS team assessed how well their measurement matched the Standard Model using the so-called \u201csignal strength\u201d, defined as the observed ttH production rate divided by the predicted rate. The ttH production signal strength<\/a> (\u03bcttH) was measured to be 0.63 +0.20\u22120.19 While slightly lower than the Standard Model prediction (1.0), the value is compatible within experimental uncertainties. The analysis provides evidence of ttH production in multilepton final states with a statistical significance of 3.3 standard deviations<\/a> (\u03c3), compared with an expected significance of 5.3\u03c3.<\/p>\n

A complementary measurement of Higgs-boson production in association with a single top quark (tH) was also performed. The measured signal strength, \u03bctH = 7.2+4.6\u22124.0, is slightly above the Standard Model expectation. A similarly high value was reported in previous ATLAS<\/a> and CMS studies<\/a> of this process.<\/p>\n

\"Physics,ATLAS\"<\/a>Figure 2: The observed best-fit values of the ttH cross-section relative to the Standard-Model expectation and their uncertainties in the simplified template cross-section measurement. The inclusive theory systematics in the Standard-Model prediction are shown as a grey dashed band, but are not included in the uncertainties of the measurement. (Image: ATLAS Collaboration\/CERN)<\/p>\n

At the ATLAS experiment, scientists are exploring the relationship between the heavyweights of the subatomic world: the top quark and the Higgs boson.<\/p>\n

The Higgs boson in motion<\/strong><\/p>\n

\"Physics,ATLAS\"<\/a>Figure 3: The observed exclusion contours of the CP-even and CP-odd components of the top-Higgs interaction. Regions contained in the dashed, and in the solid lines are compatible with the best-fit result at 68% and 95% confidence level. \u03bat’ quantifies a deviation of the interaction amplitude from its Standard Model value and \u03b1 is the mixing angle between CP-odd and CP-even states. The orange star represents the Standard Model (\u03b1 = 0\u00b0, \u03bat’=1) and the yellow star represents a pure CP-odd interaction with no modification to the amplitude of the coupling (\u03b1 = 90\u00b0, \u03bat’=1). (Image: ATLAS Collaboration\/CERN)<\/p>\n

Researchers also studied the transverse momentum of the Higgs boson, which can be used to probe different Higgs production mechanisms and possible deviations from the Standard Model interactions. Since the observed leptons can come from either Higgs-boson or top-quark decays and several neutrinos escape detection, the kinematic system cannot be fully reconstructed. Instead, the Higgs boson\u2019s transverse momentum must be inferred statistically using a Graph Neural Network<\/a> trained on the full event topology of simulated ttH events. Applying a simplified template cross-section (STXS<\/a>) framework, the ttH production rate was measured in three ranges of transverse momentum (Figure 2).<\/p>\n

Testing the “symmetry” of the Higgs boson<\/strong><\/p>\n

Having studied the ttH production rate, physicists turned their attention to more detailed properties of the top-Higgs interaction. The Standard Model predicts that the Higgs boson\u2019s interactions with other particles should have even charge-parity (CP) symmetry<\/a>. Any evidence of CP-violating interactions (CP-odd) would indicate the presence of as-yet undiscovered phenomena.<\/p>\n

In their new analysis, ATLAS researchers tested whether the top\u2013Higgs interaction could contain a mixture of different CP components. The degree of CP mixing is parameterised by the mixing angle \u03b1, where \u03b1 = 0\u00b0 corresponds to a purely CP-even interaction and \u03b1 = 90\u00b0 to a purely CP-odd one. Values of \u03b1 > 62\u00b0 were excluded at a 68% confidence level<\/a>, supporting the Standard Model description of the Higgs boson and remaining consistent with the best ATLAS constraint to date<\/a>. Constraints were set on possible CP-odd components of the top-Higgs interaction (Figure 3).<\/p>\n

As larger datasets from LHC Run 3 and the High-Luminosity LHC are analysed, ATLAS researchers will further refine ttH measurements and sharpen their knowledge of the top\u2013Higgs interaction.<\/p>\n

Image banner: Event display for a ttH candidate event, with an electron (green), muon (red) and two hadronically-decaying tau leptons (pink cone) in the final state. (Image: ATLAS Collaboration\/CERN)<\/p>\n

Learn more<\/strong><\/p>\n