By CMS Collaboration

On Sunday 2nd December CMS recorded the last collisions to bring to a close a very successful LHC second running period (Run 2) that began in 2015. LHC has delivered to CMS 68 fb -1 of proton-proton collisions and almost 1.8 nb-1 of Pb-Pb collisions in 2018, and more than 160 fb-1 overall of proton-proton collisions in Run 2.

CMS collected more than 92% of the Run 2 data successfully yielding a total dataset of 150 fb-1. Looking back, 2018 was our most successful year to date, recording almost 95% of the LHC delivered luminosity.


This last year of Run 2 also had several special runs which took special planning and for which we ran special detector and trigger configurations. These include a Beta*=90m run joint with TOTEM,  a low-Pile Up run, and an intense period at the end of the year where we take Lead-Lead collisions. These runs all had different requirements on our detector and trigger/daq system and so early planning was crucial to ensure all these special runs were taken successfully.


Figure 2: Event display taken on April 17th: start of 2018 data taking. (Image: Thomas Mc Cauley /CMS)


Figure 3: Event display of Lead-Lead collision registered in the CMS detector on 8th November 2018. (Image: Thomas Mc Cauley /CMS)

During Run 2, CMS commissioned several important upgrades to the detector including a new Level 1 trigger, new readout for the hadronic calorimeter and a new pixel detector.
  • To cope with the increasing luminosity delivered by the LHC we installed an upgrade to our Level-1 Trigger system during Run 2. We were able to run this system successfully in parallel with the legacy system during some collision runs in 2015 and then deployed it fully for use from 2016. This system used state-of-the-art FPGAs and custom designed electronics boards which allow us to use much advanced algorithms with which to filter our data.​
  • During Run 2 installed and commissioned part of the Phase 1 upgrade to HCAL. In the Forward Calorimeter we upgrade the electronics to discriminate good physics signal from anomalous ones. In the endcap we replaced the Hybrid Photo-diodes (HPDs) with SiPMs, and also replaced the front-end electronics. We obtain much improved signal-to-noise with this system as well as the ability to mitigate pile-up effects with a finer granularity read-out. The final part of this upgrade to HCAL, upgrading the HCAL Barrel will take place during LS2.​
  • During the Extended-Year-End-Technical-Stop (EYETS) in 2016-2017, we also installed a brand new 4 layer pixel detector. Main pixel characteristics: innermost layer moved closer to the IP, outermost layer moved further from the IP. The new pixel detector is able to cope with higher rate capability, increased tracking efficiency, reduction of fake rate, improvements in impact parameter resolution. ​
  • Another important milestone achieved during Run 2 was the inclusion of a new muon detector slice in collision data-taking. This upgrade uses Gas Electron Multiplier (GEM) technology to detect muons in the endcap region of CMS. By integrating a slice into CMS much was learned about the commissioning and operation of this detector, prior to its installation in LS2.​
  • The Computing system was extensively modified for Run 2 to cope with the expectation of a large increase in the amount of collision events collected. Not only was the recorded collision data larger (36 PB of RAW data collected versus 3 PB in Run 1), but the data produced through Monte Carlo simulations followed the same trend. In addition, CMS analyses became more precise, due to increased statistics, improvements in the underlying theory and the better understanding of the LHC and the CMS detector. As a result CMS has had to increase the use of automatic tools for data movement, job dispatching, and recovery of workflows in order to cope with Run 2 without substantial additions to manpower for software and computing.

Figure 4:  The new Pixel Tracker being installed at the heart of the CMS detector - March 2017. (Photo: Max Brice/CERN)

Run 2 data promises to open new physics chapters


The higher collision energies and the more than five times larger size of the datasets recorded in Run 2 were used to improve results and open new chapters throughout CMS’s physics program. At this time, the analysis of the first years of this data taking period have given rise to about 200 published results, with many more results still in preparation.


Among the highlights are the advances in the investigations of the properties of the Higgs boson (see Fig.5). While the discovery of the Higgs boson was based on its decays to photons, W and Z bosons, the couplings of the Higgs boson to the three heavy third-generation fermions tau, bottom and top have now been observed using Run 2 data. The Higgs boson is also routinely used in searches for other, new particles.


The mass range covered by direct searches for the production of heavy, new particles was substantially extended compared to the results obtained in Run 1. One example are searches for dijet and dilepton resonances that set limits on the masses of new W’ or Z’ gauge bosons in the 2-5 TeV range. Searches for the supersymmetric partners of gluons and quarks probe masses up to 2 TeV. Naturally, a central topic were the searches for dark matter that were performed in a multitude of topologies, including production in association with bosons or pairs of top or bottom quarks. The larger datasets, higher collision energy and higher mass reach have motivated development and application of new analysis techniques in order to reconstruct the decay products of highly boosted bosons or top quarks.


Our diverse search program was supported by an extensive set of precision standard model measurements. This included topics ranging from the production of top quarks, W/Z bosons, dibosons, and, tribosons including photons and the use of Higgs bosons (see Fig. 6). Recent highlights included the first observation of electroweak production of same-charge W boson pairs, first evidence for production of a top quark with a photon and detailed investigations of single top quark and top quark pair production as a function of the event characteristics that can be used to measure parameters of the standard model such as the top quark mass and the strong coupling constant.


The proton-proton program was complemented by a large number of results from p-Pb, Pb-Pb, and Xe-Xe ion collisions, allowing detailed studies of the properties of the hot and dense medium produced in these events. A couple of recent highlights include evidence for light-by-light scattering in Pb-Pb collisions and the first observation of top quark production in p-Pb collisions.


Most of the already published CMS results from Run 2 are based on data recorded in 2015 or 2016, while the total number of proton-proton collisions accumulated in Run 2 is more than three times higher, and the most important heavy-ion run of this period has just ended now. The large new dataset will be used to expand the direct searches for new physics, in particular for rare events with unusual signatures such as long-lived, heavy particles. The area of B physics will also profit from the large dataset when measuring rare B-meson decays and investigating hints for anomalies in the flavour sector. The properties of events with Higgs boson will be studied in more detail, and large progress is expected in the search for decays to second-generation fermions. Measurements of the production of top quarks and gauge bosons with the full dataset and using the most recent calibration and reconstruction procedures will allow for a verification of the predictions of the standard model at an unprecedented level of precision.

Figure 5: Fit results for the signal strength of different Higgs boson production modes w.r.t. standard model predictions.


Figure 6: Summary of measurements of standard model cross sections in Run 1 and Run 2.


Many interesting physics results have already been published using the data collected in Run 2 and many more are still to come. The next two years will be a very busy time for the CMS collaboration. We will be analyzing the data collected over Run 2 data sample, improving the detector and software performance, and updating the infrastructure at the experiment. CMS scientists are looking forward to the next LHC running period that will start after the shutdown.