2015 has been extraordinary: after 2 years of shutdown (LS-1) the LHC and its experiments have come back into operation for a new exciting running period (Run-2) to push the research about the infinitely small structure of matter, related to the origin of the universe, into a new territory and shed further light on the fundamental laws and interactions of nature.

ATLAS, one of the major experiments at the LHC, has undergone an intense programme of upgrade and maintenance preparing for the new running period with protons colliding at the unprecedented energy of 6.5 TeV per beam.

Closest to the interaction point, placed around a new lighter and smaller beampipe, the new Insertable B-Layer (IBL) has been installed adding a fourth silicon layer to the existing pixel detector. All of the remainder sub-detectors and the infrastructure they require have been refurbished to allow more precise measurements and reliable operation.

New muon chambers have been mounted and instrumented to improve the detector coverage. In addition, new luminosity detectors and beam instrumentation monitors have been installed.

Side A of the ATLAS Cavern.

The trigger and data acquisition architecture has been upgraded to provide a higher trigger accuracy and data bandwidth in view of the expected higher luminosity pushing up the trigger rate from 75 kHz of Run-1 to 100 kHz for Run-2 at Level 1, and 1 kHz at high level trigger.

Starting already from 2014 several periodic test runs, including more and more sub-detectors, were organized to get all of ATLAS fully prepared for the operation with beams. ATLAS was indeed ready when, on Easter Sunday, 5 April, the first beams circulated in the LHC and beam-splash events were seen with great excitement in the control room.

The first Run-2 collisions at injection energy were recorded soon after on 5 May followed by collisions at 13 TeV two weeks later. All these events have been used to validate and adjust the detector settings, the data acquisition and the trigger. All detectors including the new IBL, were successfully switched on for these first data. Stable beams were delivered starting on 3 June 2015: one week with nominal optics, one week devoted to collecting data at low pileup (number of collisions per bunch) with events triggered by ATLAS and by the LHCf experiment to simulate the collisions of very high energy cosmic rays.

A 13 TeV collision recorded by ATLAS. The yellow and green bars indicate the presence of particle jets, which leave behind lots of energy in the calorimeters. (Image: ATLAS)

The following weeks have seen the beam intensity rising, with the LHC running first with 50ns bunch spacing (as in Run-1) and later with 25ns trains, from few tens of colliding bunches at the beginning of June up to instantaneous luminosities of 5x103³³cm^-2s^-1 and more than 2200 bunches at the end of October. ATLAS, has followed day and night the LHC operation taking almost continuously data with deadtime and efficiencies comparable with those of the most stable Run1 operating conditions.

The nominal high energy proton collisions at 13 TeV were interleaved by periods of machine development and several dedicated special runs. Each step required careful expert action, tuning of the detectors, the trigger and the data acquisition system:

- Van der Meer scans where the beams are moved apart in steps to allow for precise luminosity calibration.

- Data aiming at the study of elastic and diffactive interactions using the ALFA Roman Pots; these are detectors located at about 240 meters from the interaction point along the beam line and move up to few millimiters from the beam during operation.

- Special alignment runs (done with the ATLAS toroids off and when the LHC was operating with a limited number of bunches) to collect straight muon tracks for precise detector alignment.

On 3 November, the protons-protons 13 TeV programme stopped, with the LHC delivering luminosity at a slope at times steeper than the one of Run-1 and ATLAS recording about 4 fb^-1 of collisions with a data taking efficiency above 92%.

The following four weeks have seen the LHC switching to Heavy Ions with collisions of lead nuclei at a center of mass energy of 5 TeV per nucleon pair, summing up to energies of more that 1000 TeV per lead nucleus and an instantaneous luminosity of 3.5x10²⁷ cm^-2s^-1 that is well above the initial design. ATLAS, with the trigger and detectors tuned (including a dedicated Zero Degree Calorimeters installed before the start of the ion run at 140 m from the interaction point along the beam line), has continued taking data, recording 686 out of 703 μb^-1 delivered by the accelerator.

LHC and the experiments are preparing for the coming years as the 2015 data is being intensively analyzed. First results, using almost the full 2015 recorded sample have been recently presented extending the range of the investigations and searches from Run-1 and bringing intriguing fluctuations which require more data to be confirmed.

One of the first heavy-ion collisions with stable beams recorded by ATLAS in November 2015. 

With such an incredible year just over, ATLAS is looking forward with even greater expectations to 2016 and new results from Run-2.

Read more about the latest 2015 results presented by the ATLAS and CMS experments: here