ATLAS highlights from LHCP 2023

In 2013, the LHCP conference was established by combining two international conferences: “Physics at Large Hadron Collider Conference” and “Hadron Collider Physics Symposium”, providing an important platform for scientists to discuss their latest results and exchange ideas on collider physics.

The 11th edition of the LHCP conference was held from 22 to 26 May 2023. Over 300 physicists travelled to Belgrade (Serbia) for LHCP’s long-awaited return to in-person gatherings since 2019. The latest results from the LHC experiments were presented and discussed, alongside updates on detector operation, performance and upgrades for the High-Luminosity LHC. In addition, the progress of future large linear and circular colliders drew attention and initiated an extensive discussion.

Scientists from the ATLAS Collaboration presented several new results at the LHCP conference, studying data collected during each of the LHC’s data-taking periods: Run 1 (2010-2013), Run 2 (2015-2018) and Run 3 (2022-ongoing). These newly released results cover a broad range of physics topics, including precision measurements of the W^± and Z bosons, evidence of the Higgs-boson decay into a Z boson and a photon, and searches for a new scalar particle. Selected highlights are presented and discussed below.

Evidence of the Higgs boson decaying into different bosons

Physics,ATLAS — Figure 1: The Zγ invariant mass distribution of events from all ATLAS and CMS analysis categories. The data (dots with error bars) in each category. The fitted signal-background (background) terms are represented by a red solid (blue dashed) line. In the lower panel, the same data and the two models are compared after subtraction of the estimated background. (Image: ATLAS Collaboration/CERN)

Since discovering the Higgs boson in 2012, ATLAS and CMS physicists have been precisely measuring the boson’s properties in order to probe the Standard Model and search for rare production and decay modes. Among these rare decays, the Higgs boson decay into a Z boson and a photon (H→Zγ) garnered special attention during Run 2 of the LHC, due to the large-scale dataset collected.

In the Standard Model, the Higgs boson does not directly decay into a Zγ pair, but via a loop-induced process similar to the Higgs boson decay to two photons. Thus the Standard Model decay probability (branching fraction) of H→Zγ is predicted to be as small as 1.5×10^-3, making it very challenging to observe from LHC data. Contributions from new particles may vary this decay rate and any confirmed deviation from the Standard-Model prediction would be a very useful probe for new physics phenomena in the Higgs sector.

Individual analyses searching for the H→Zγ process were carried out by the ATLAS and CMS Collaborations with the full LHC Run-2 dataset. To maximise the statistical power, these two independent analyses were combined. Both analyses explore the final state with the Z boson decaying into a pair of electrons or muons. The signal presents itself as a narrow peak in the Z𝛄 invariant mass spectrum. To enhance the significance, both analyses introduced advanced machine learning techniques to separate signal and background. Figure 1 presents the observed distribution of the Zγ invariant mass in the combined data sample.

Such collaborative efforts significantly improve the statistical precision of the results. The combined analysis shows first evidence of the H→Zγ decay with an observed significance of 3.4 standard deviations, which means the probability of such a signal being caused by a statistical fluctuation is smaller than 0.04%. The measured branching fraction for H→Zγ is (3.4±1.1)×10^-3, which is 2.2 ± 0.7 times the Standard Model prediction. Figure 2 presents the signal strength.

Though this result still has a large measured uncertainty, this finding establishes an important path for determining the Higgs boson properties and probing the new physics beyond the Standard Model. By including Run 3 data – which is expected to triple the size of ATLAS’ data sample – researchers will be able to perform more precise measurements on this rare decay mode.

New precision measurements of the W^± and Z boson properties

As the carriers of the weak force, precise determinations of the properties of the W^± and Z bosons are crucial for understanding the electroweak sector of the Standard Model.

At the LHCP conference, ATLAS researchers presented new measurements of the W^± and Z boson transverse momentum (p_T) distribution at two different centre-of-mass energies (5.02 TeV and 13 TeV). Distributions at 5.02 TeV were measured for the first time. These extraordinarily precise measurements provide essential input for measurements of other properties, including key studies of the W± boson mass. These measurements also reduce the uncertainty on underlying assumptions about the W^± boson production mechanism. Furthermore, they are critical for scientists to better understand Quantum Chromodynamics across a wide range of transverse momentum.

The measurements were performed with special data samples collected with far fewer proton-proton collisions occurring simultaneously (called low pileup conditions). This provided much cleaner experimental conditions for high-precision measurements.

Unlike the Z boson, whose transverse momentum can be directly reconstructed from charged leptons in its decays, the transverse momentum of the W^± boson is measured by the detectable particles recoiling against it, most of which are hadrons (hadronic recoil). These particles were reconstructed with a new technique (the Particle Flow Algorithm) and their properties were calibrated to sub-percent-level precision. Measurements of the Z boson using both direct reconstruction and hadronic recoil methods were used to validate the technique for W^± boson measurement. Figure 3 presents the measured W^- and Z boson transverse momentum distributions.

LHCP Fig3 — Figure 3: Measured W- (left) and Z (right) boson transverse momentum distributions at 13 TeV, compared to several predictions. (Image: ATLAS Collaboration/CERN)

Precise vector boson scattering measurements and search for anomaly

Studies of vector boson scattering, VV→VV (V=W^±/Z/γ), are crucial for understanding the nature of the electroweak symmetry-breaking mechanism in the Standard Model. They are also crucial for searches for new physics processes, which may be found to impact the scattering at high precision, and studies of the Higgs sector, as the Higgs mechanism impacts the scattering rate.

At the LHCP conference, scientists from the ATLAS Collaboration presented several precision measurements of the vector boson scattering process with the full Run-2 dataset (collected in 2015-2018). These results covered the final states of W±W±, ZZ, and Zγ, and included measurements of both the fiducial and differential cross-sections. In the LHC, the vector boson scattering process occurs in association with two forward quarks which become two forward jets after hadronization. Thus, the measurements were done in a signature of two vector bosons and two forward jets (VV+2jets).

ATLAS Collaboration also presented two fascinating new physics searches with W^±W^±+jets events, looking for doubly charged Higgs boson and Majorana neutrinos. If it exists, the doubly-charged Higgs boson might decay into two same-charged W^±bosons and might be seen in the study of W^±W^±+jets events at the LHC. The Majorana neutrino is a fermion that is its own antiparticle which, if it exists, could help answer open questions about the origin of neutrino mass. By analysing W^±W^±+jets, researchers explored the phase space of having a heavy Majorana neutrino with a mass from a few GeV to a few TeV. This search provides complementary results to neutrinoless double-beta-decay experiments. Figure 4 shows the search results for Majorana neutrino with same-charged W^±W^± final state events.

Search for strongly-coupled dark matter particles

Plots or Distributions,Physics,ATLAS — Figure 5: The expected and observed exclusion contours for semi-visible jets signal as a function of mediator mass on the x-axis, and the invisible fraction on the y-axis. Mediator masses on the left hand side of the red solid line are excluded for the given invisible fraction. (Image: ATLAS Collaboration/CERN)

The nature of dark matter is a long-standing mystery in particle physics. Various astrophysical observations require the presence of dark matter and dark energy to be explained by current theories of gravity. However, no dark matter particle has been found in experiments so far. While dark matter particles are often believed to interact weakly with ordinary matter, other theories take a different approach. There may be dark matter particles with strong interactions, leading to the presence of dark quarks and gluons acting as the “dark” partners of the Standard Model particles. With this feature, a "semi-visible jet" of particles would arise from decays of these dark quarks if they can both interact with the Standard-Model quarks and undetected dark hadrons.

ATLAS researchers presented a search for the non-resonant production of semi-visible jets with Run 2 data. These jets arise from the decay of dark quarks which are produced in pairs with a new mediating particle. Researchers explored two sensitive observables to extract the signal contribution: the transverse momentum balance between jets nearest and farthest in azimuth angle from the missing transverse momentum, and the azimuthal separation between these two jets. They also investigated scenarios with different invisible fractions and mediating particle masses. No evidence of semi-visible jets was found and, for the first time, limits were set on the invisible fraction and mediating particle mass plane (as shown in Figure 5). The mass of the mediating particle was excluded up to 2.7 TeV.

Higgs boson measurement at 13.6 TeV

Last year, the LHC started its third data-taking campaign with a centre-of-mass energy of 13.6 TeV. At the LHCP conference, the ATLAS Collaboration presented new measurements of the production cross-section of the Higgs boson at 13.6 TeV using data collected in 2022. The results explored the Higgs boson's "discovery channels": H→γγ, where the Higgs boson decays into two photons, and H→ZZ*→4l, where the Higgs boson decays to two Z bosons, which subsequently decay into 4 charged leptons (electrons and muons).

In the first step, the cross-section was measured within the coverage of the ATLAS detector (the fiducial cross-section). The measured fiducial cross-section in H→γγ channel is σ_γγ=76±14 fb, and in H→ZZ*→4l channel is σ_4l=2.80±0.74 fb. These results are very compatible with the Standard Model predictions of 67.6±3.7 fb and 3.67±0.19 fb, respectively.

Assuming Standard Model decay rates, these fiducial cross-sections were extrapolated to obtain the total cross-sections of the Higgs boson production measured in each channel. The results are 67±12 pb in H→γγ channel and 46±12 pb in H→ZZ*→4l channel. By combining measurements in both channels, ATLAS researchers also reported a world-best Higgs boson production cross-section measurement of 58.2 ± 8.7 pb, which is in great agreement with the Standard Model prediction of 59.9±2.6 pb. Figure 6 shows the latest cross-section measurements at 13.6 TeV from both channels, overlaid with previous measurements.