On 23 July 2015, ATLAS presented its first measurements of soft strong interaction processes using charged particles produced in proton–proton collisions at 13 TeV centre-of-mass energy delivered by the Large Hadron Collider at CERN. These measurements were performed with a dataset collected beginning of June under special low-luminosity conditions during which the frequency of multiple proton–proton scattering occurring in the same recorded collision event was strongly reduced.

Measurements like this are important for understanding the collision energy dependence of such processes, as well as ensuring a successful description of the data by the Monte Carlo (MC) event generators. The accuracy of these simulations is critical for their subsequent use in ATLAS searches and measurements.

Performing these measurements at an unprecedented collision energy with upgraded detector components in such a short scale was a challenge. During the Long Shutdown, many improvements were made to the detector, most relevant among them for these results was the addition of the innermost pixel layer, the insertable B-layer (IBL). Adding the IBL dramatically improved the accuracy of the track reconstruction and the identification of jets originating from bottom quarks, which is important for many searches. The commissioning of the IBL, alignment of different detector components and assessment of passive detector material is still ongoing. Figure 1 shows the number of hits in pixel layer per track. Generally, good agreement is observed, indicating a very good understanding of the ATLAS four-layer pixel system. The minor disagreements stem from the mismatch in simulation and data of the number of modules not working properly during that period of data-taking.

The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity (which essentially represents the angular position from beam axis) and the dependence of the mean transverse momentum on the charged-particle multiplicity were presented, based on about 9 million events. The events contained at least one charged particle with transverse momentum greater than 500 MeV in the central part of the detector. The data were corrected with minimal model dependence to obtain inclusive distributions. Overall the Monte Carlo models, which were tuned to such similar measurements performed at lower centre-of-mass energies, seem to describe the data reasonably well. Figure 2 shows the mean number of charged particles in the central region compared to previous measurements at different collision energies, together with the MC predictions. The mean number of charged particles increases by a factor of 2.2 when collision energy increases from 0.9 TeV to 13 TeV.

In the events where the leading track had a transverse momentum of at least 1 GeV, the accompanying activity was studied at the detector level. The azimuthal region perpendicular to the direction of the leading track is most sensitive to this accompanying activity, termed the underlying event (UE). The average number of tracks in each event and their transverse momentum sum are seen to show a gradual rise towards a “plateau” with rising leading track transverse momentum, a trend seen in previous measurements. Figure 3 shows the latter in the transverse region. Compared to 7 TeV results a 20% increase to the UE activity is observed and is predicted well by most of the models.

These early measurements show a good understanding of the performance the upgraded ATLAS detector as well as the ability of the Monte Carlo event generators to describe the data at new collision energy.