The year 2015 began for the ATLAS experiment with an intense phase of commissioning using cosmic-ray data and first proton–proton collisions, allowing ATLAS physicists to test the trigger and detector systems as well as to align the tracking devices. Then the collection of physics data in LHC Run 2 started in June, with proton–proton collisions at a centre-of-mass energy of 13 TeV (CERN Courier July/August 2015 p25). Measurements at this new high-energy frontier were among the highlights of the many results presented by the ATLAS collaboration at EPS-HEP 2015.
An important early goal for ATLAS was to record roughly 200 million inelastic proton–proton collisions with a very low level of secondary collisions within the same event (“pile-up”). This data sample allowed ATLAS physicists to perform detailed studies of the tracking system, which features a new detector, the “Insertable B-layer” (IBL). The IBL consists of a layer of millions of tiny silicon pixels mounted in the innermost heart of ATLAS at a distance of 3.3 cm from the proton beam (CERN Courier October 2013 p28). Together with the other tracking layers of the overall detector, the IBL allows ATLAS to measure the origin of charged particles with up to two times better precision than during the previous run. Figure 1 shows the resolution achieved for the longitudinal impact parameter of the beam.
ATLAS exploited the early data sample at 13 TeV for important physics measurements. It allowed the collaboration to characterize inelastic proton–proton collisions in terms of charged-particle production and the structure of the “underlying event” – collision remnants that are not directly related to the colliding partons in the proton. This characterization is important for validating the simulation of the high-luminosity LHC collisions, which contain up to 40 inelastic proton–proton collisions in a given event (one event involves the crossing of two proton bunches with more than 100 billion protons each). Figure 2 shows the evolution of the charged-particle multiplicity with centre-of-mass energy.
ATLAS also measured the angular correlation among pairs of the produced charged particles, confirming the appearance of a so-called “ridge” phenomenon in events with large particle multiplicity at a centre-of-mass energy of 13 TeV. The “ridge” (figure 3) consists of long-range particle–particle correlations not predicted by any of the established theoretical models describing inelastic proton–proton collisions.
After the low-luminosity phase, the LHC operators began to increase the intensity of the beams. By the time of EPS-HEP 2015, ATLAS had recorded a total luminosity of 100 pb–1, of which up to 85 pb–1 could be exploited for physics and performance studies. ATLAS physicists measured the performance of electron, muon and τ-lepton reconstruction, the reconstruction and energy calibration of jets, and the reconstruction of “displaced” decays of long-lived particles, such as weakly decaying hadrons containing a bottom quark. The precision of the position measurements of displaced decay locations (vertices) is significantly improved by the new IBL detector.
ATLAS used these data to classify the production of J/ψ particles at 13 TeV in terms of their immediate (“prompt”) and delayed (“non-prompt”) origin. While non-prompt J/ψ production is believed to be well understood via the decay of b hadrons, prompt production continues to be mysterious in some aspects.
ATLAS also performed a first study of the production of energetic, isolated photons and a first cross-section measurement of inclusive jet production in 13 TeV proton–proton collisions. Both are correctly described by state-of-the-art theory.
The data samples at high collision energy contain copious numbers of Z and W bosons, the mediators of the weak interaction, whose leptonic decays provide a clean signature in the detector that can be exploited for calibration purposes. ATLAS has studied the kinematic properties of these bosons, also in association with jet production. Their abundance in 13 TeV proton–proton collisions is found to be consistent with the expectation from theory. ATLAS has also observed some rare di-boson (ZZ) events, which – with a hundred times more data – should allow the direct detection of Higgs bosons. Figure 4 shows a candidate ZZ event.
In higher-energy proton collisions, the rate of particle production for many heavier particles for a given luminosity increases. The heaviest known particle, the top quark – with a mass approximately 170 times that of a proton – is predominantly produced in pairs at the LHC, and the cross-section for the production of top-quark pairs is expected to increase by a factor of 3.3 at 13 TeV, compared with the 8 TeV collisions of Run 1. ATLAS has performed an early measurement of the top-pair production cross-section in the cleanest channels where one top quark decays to an electron, an electron-neutrino and a jet containing a b-hadron (“b-jet”), while the other top-quark decays to a muon, a muon-neutrino and a b-jet. The small backgrounds from other processes in this channel allow a robust measurement with small systematic uncertainties. The measured cross-section agrees with the predicted increase of a factor of 3.3. The precision of the measurement is limited by the 9% uncertainty in luminosity, which is expected to improve significantly during the year. Figure 5 shows the evolution of the top-pair production cross-section.
Although the available data sample does not yet allow a significant increase in the sensitivity to the most prominent new physics phenomena, ATLAS has exploited the data to perform important early measurements. The excellent detector performance has allowed the confirmation of theoretical expectations with 13 TeV proton–proton collision energies.