ATLAS highlights from LHCP 2017

23 May 2017 | By

The fifth annual Large Hadron Collider Physics (LHCP2017) conference was held last week at Shanghai Jiao Tong University in a leafy suburb in the former French concession in Shanghai, China. This year there were more participants than ever before: 470 people from universities across the globe. ATLAS presented an interesting set of new results exploiting the high statistics of the combined 2015 and 2016 dataset. Selected highlights are discussed below, and a more complete list of recent results using the full 13 TeV dataset can be found here.

Figure 1
​Figure 1: Excluded regions of neutralino and stop masses from recent ATLAS stop searches. (Image: ATLAS Collaboration/CERN)

Probing the Standard Model

Although no longer a brand-new result, the ATLAS measurement of the W mass is still stimulating significant interest and excitement. It has been measured to an outstanding precision of 19 MeV and is consistent with the predictions from the Standard Model.

The larger Run 2 dataset allows the coupling of the top quark to a range of other particles to be explored. ATLAS presented a new result, based on a careful analysis of the 8 TeV data, measuring the cross-section of the production of top quarks in association with photons. It is a test case for our understanding of the production of top-quark pairs together with bosons.

The search for supersymmetry continues

Supersymmetry is a popular, albeit so far unconfirmed, theory that provides a solution for some critical issues in the Standard Model. ATLAS has produced a wide range of new supersymmetry searches for LHCP exploiting the high statistics of the combined 2015 and 2016 datasets. These include several searches [1, 2] for the super-partner of the top (the ‘stop’), which is particularly important in the context of “natural” supersymmetry, proposed to solve a dramatic fine-tuning problem in the Standard Model. Figure 1 shows that these searches now exclude stop masses almost up to 1 TeV in the most favourable scenarios (in other scenarios the exclusion can be much weaker). Other results presented include a new search for electroweak production of supersymmetric particles using events with leptons and new searches for the strong production of super-particles [1,2, 3] . Among the latter searches are analyses exploiting jet multiplicities of up to 12 jets [1, 2]. No significant excess was observed in any of these searches.

Figure 2
Figure 2. Left: differential cross-section of the Higgs transverse momentum compared to various theoretical predictions. Right: measured and predicted differential Higgs cross-sections for events with different numbers of jets. (Image: ATLAS Collaboration/CERN)

Studying the Higgs Boson

A highlight of the Higgs talks at LHCP were the new measurements from ATLAS of the inclusive and differential cross-sections using the clean H→ZZ*→4ℓ decay signature. The results are unfolded to allow easy comparison to different theoretical predictions. Measurements of the Higgs boson’s transverse momentum probe different Higgs production mechanisms and are sensitive to possible deviations from the Standard Model. Figure 2 (left) shows the measured differential cross-section of the four-lepton transverse momentum compared to various predictions. Figure 2 (right) shows the measured and predicted differential cross-sections as a function of the jet multiplicity, which will help to improve the theoretical modelling of Higgs boson production via gluon-gluon fusion.

Figure 3
Figure 3: Measurements and predictions of the ZZ cross-section as a function of collision energy. (Image: ATLAS Collaboration/CERN)

The main background to the Higgs search to four leptons is the background from direct ZZ production in the Standard Model. A detailed ATLAS study of this process at a centre-of-mass energy of 13 TeV provides crucial input to the understanding of that process. Figure 3 shows how the cross-section for ZZ production increases as a function of the energy of the colliding protons. Good agreement between all measured and predicted results is observed.

ATLAS also presented results from a recently submitted paper that places a stringent limit of less than three times the Standard Model prediction on the coupling of the Higgs boson to muons, emphasising the highly non-universal coupling of the Higgs boson to fermions of different generations. The Higgs decay to a muon pair represents a clean signature, but the small branching ratio means that significantly more data is required to reach Standard Model sensitivity.

The success of the LHCP conference demonstrated that the LHC experiments are just beginning to harvest the fascinating physics opportunities provided with the large Run 2 dataset.

Preparing for high performance

A thorough understanding of the detector is required to obtain precise physics measurements using the full statistics of the large Run 2 dataset. ATLAS presented new results from a detailed study of the material of the upgraded ATLAS Inner Detector that is responsible for accurately measuring charged particles. These results combine studies of photon conversions, hadronic interactions, and the stopping of tracks to identify and improve limitations in the detector modelling, particularly in the forward region. Figure 4 (left) shows the reconstructed location of vertices from interactions of charged hadrons originating from proton collisions with the detector material, where the density of the vertices is proportional to the amount of material. The structure of the individual modules and their cooling pipes is clearly visible. Figure 4 (right) compares the rate of hadronic interactions and conversions between data and simulation (shown is their ratio) for the different layers and support components in the Inner Detector. There is good agreement both between the two methods and between the data and the simulation, which demonstrates that ATLAS has an excellent level of understanding of the detector geometry. Such an understanding is critical to accurately estimate the backgrounds in specific SUSY searches, such as a recent study in which ATLAS looked for late decays of a putative SUSY particle yielding a signature similar to that of hadronic interactions.

Figure 4
Figure 4. Left: radiography of the innermost layer of the pixel detector (denoted IBL and newly installed for Run 2) of the ATLAS pixel detector via the location of hadronic interactions; the individual modules and cooling pipes are clearly visible. Right: comparison of the rate of photon conversions and hadronic interactions in the Inner Detector. (Image: ATLAS Collaboration/CERN)

ATLAS presented other new techniques, which have further improved its performance and, thus, its physics capabilities. These include a new tagging algorithm which discriminates between jets originating from quarks and gluons based on the number of tracks and a new technique for particle-flow reconstruction, which combines information from the calorimeter and tracker to mitigate the impact of pile-up.

Searches for heavy vector bosons

There were a number of talks related to lepton-flavour violation at the conference. ATLAS presented the direct search for heavy vector bosons decaying to pairs of leptons (see Fig. 5), which could give rise to lepton-flavour violating effects. As no excess was observed, the larger dataset means that any such boson with the properties studied must have a mass greater than 4-5 TeV.

Figure 5
Figure 5: The dimuon (left) and dielectron (right) invariant mass distributions from a search for new heavy vector bosons. (Image: ATLAS Collaboration/CERN)

The success of the LHCP conference demonstrated that the LHC experiments are just beginning to harvest the fascinating physics opportunities provided with the large Run 2 dataset. With this year's data-taking just beginning, there will be many more interesting results still to come, just like the ever-changing skyline of Shanghai.