By Achintya Rao

It has been a little over seven (and a half) years since the LHC started delivering collisions to CMS for physics analysis, and just a few days ago we published our 700th research paper. To celebrate this achievement, we thought we would give you a glimpse of seven of several milestone papers CMS has published over the years. Of course, choosing any such list is inherently difficult and we have to leave out many other deserving papers in the process. But we hope the ones shown below give you a flavour of the breadth of the CMS research programme and the many contributions to this endeavour from our collaborators all over the globe. Here’s to (at least) 700 more papers that push the boundaries of our knowledge of the universe.

Three of many memorable “first”s for CMS

1. The first paper about the detector (2008)

“The CMS experiment at the CERN LHC”: https://cds.cern.ch/record/1129810

Technically, this paper predates the start of collision-data collection, but it is arguably one of the most important ones published by CMS, as it sets the stage for all the research papers to follow. Unfortunately, the abstract notes that the detector weighs 12,500 tonnes when in reality this is the weight of the magnet system only (solenoid + return yoke), while the whole detector registers an enormous 14,000 tonnes on the weighing scale (well, a hypothetical weighing scale in this case). It is also noteworthy that the author list of over 20 pages is at the start of the paper; later CMS publications moved this list to the end of the paper so you don’t have to scroll down for a long while before getting to the main content of the paper (and to save trees by stopping your printer before it gets to the author list).

Abstract: The Compact Muon Solenoid (CMS) detector is described. The detector operates at the Large Hadron Collider (LHC) at CERN. It was conceived to study proton-proton (and lead-lead) collisions at a centre-of-mass energy of 14 TeV (5.5 TeV nucleon-nucleon) and at luminosities up to 1034 cm−2−1 (1027 cm−2s−1). At the core of the CMS detector sits a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungstate scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4π solid angle. Forward sampling calorimeters extend the pseudorapidity coverage to high values (|η| ≤ 5) assuring very good hermeticity. The overall dimensions of the CMS detector are a length of 21.6 m, a diameter of 14.6 m and a total weight of 12500 t.

2. The first analysis with LHC data (2009)

“Transverse momentum and pseudorapidity distributions of charged hadrons in pp collisions at √s = 0.9 and 2.36 TeV”: https://cds.cern.ch/record/1237408

This paper is also an exception in the sense that it too was published before the formal start of operations for the LHC. You may recall that the LHC, originally scheduled to begin operating in 2008, suffered from a major setback in the form of a helium leak a few days after the first protons circulated in the accelerator. Following a year-long consolidation effort, the LHC was ready to re-circulate proton beams in late 2009, allowing CMS to record some data from the proton-proton collisions during a brief commissioning run. CMS used these data to publish the collaboration’s first paper using collisions delivered by the LHC, which, at the time, represented the highest energy achieved by a hadron collider.

Abstract: Measurements of inclusive charged-hadron transverse-momentum and pseudorapidity distributions are presented for proton-proton collisions at √s = 0.9 and 2.36 TeV.The data were collected with the CMS detector during the LHC commissioning in December 2009. For non-single-diffractive interactions, the average charged-hadron transverse momentum is measured to be 0.46 ± 0.01 (stat.) ± 0.01 (syst.) GeV/c at 0.9 TeV and 0.50 ± 0.01 (stat.) ± 0.01 (syst.) GeV/c at 2.36 TeV, for pseudorapidities between −2.4 and +2.4. At these energies, the measured pseudorapidity densities in the central region, dNch/dη||η|<0.5, are 3.48 ± 0.02 (stat.) ± 0.13 (syst.) and 4.47 ± 0.04 (stat.) ± 0.16 (syst.), respectively. The results at 0.9 TeV are in agreement with previous measurements and confirm the expectation of near equal hadron production in pp [proton-antiproton] and pp [proton-proton] collisions. The results at 2.36 TeV represent the highest-energy measurements at a particle collider to date.

3. The first joint CMS-ATLAS publication (2015)

“Combined Measurement of the Higgs Boson Mass in pp Collisions at √s = 7 and 8 TeV with the ATLAS and CMS Experiments”: https://cds.cern.ch/record/2004386

OK, we’ve jumped the gun here, because we haven’t yet spoken about the Higgs discovery itself yet. But while we’re talking about firsts, we cannot ignore a monumental milestone in inter-collaboration collaboration: the first paper published by cross-ring friendly rivals, CMS and ATLAS. Both collaborations had, in July 2012, jointly announced the discovery of the Higgs boson by our respective detectors, and by putting our data together subsequently we were able to present a combined measurement of this new particle’s mass to a very high precision using two of the Higgs’s “decay channels”. And to remind us of the incredible effort that went into this publication, the CERN Document Server helpfully gives us the option to “Show all 5154 authors”!

Abstract: A measurement of the Higgs boson mass is presented based on the combined data samples of the ATLAS and CMS experiments at the CERN LHC in the H→γγ and H→ZZ→4ℓ decay channels. The results are obtained from a simultaneous fit to the reconstructed invariant mass peaks in the two channels and for the two experiments. The measured masses from the individual channels and the two experiments are found to be consistent among themselves. The combined measured mass of the Higgs boson is mH = 125.09 ± 0.21 (stat.) ± 0.11 (syst.) GeV.

Precision measurements

4. The best measurement to date of the top quark’s mass (2015)

“Measurement of the top quark mass using proton-proton data at √s = 7 and 8 TeV”: https://cds.cern.ch/record/2052828

The top quark, first observed at the Tevatron in 1995, remains the heaviest particle ever discovered. Measuring its properties and understanding its behaviour with utmost precision is key to testing the limits of the Standard Model of particle physics. The most important property, of course, is the mass of the particle, and CMS was able to use data recorded during the first run of the LHC (with a centre-of-mass collision energy of 7 and 8 TeV) to determine the top quark’s mass to the best precision so far.

Abstract: A new set of measurements of the top quark mass are presented, based on the proton-proton data recorded by the CMS experiment at the LHC at √s = 8 TeV corresponding to a luminosity of 19.7 fb−1. The top quark mass is measured using the lepton+jets, all-jets and dilepton decay channels, giving values of 172.35 ± 0.16 (stat) ± 0.48 (syst) GeV, 172.32 ± 0.25 (stat) ± 0.59 (syst) GeV, and 172.82 ± 0.19 (stat) ± 1.22 (syst) GeV, respectively. When combined with the published CMS results at √s = 7 TeV, they provide a top quark mass measurement of 172.44 ± 0.13 (stat) ± 0.47 (syst) GeV. The top quark mass is also studied as a function of the event kinematical properties in the lepton+jets decay channel. No indications of a kinematic bias are observed and the collision data are consistent with a range of predictions from current theoretical models of tt [top-antitop] production.

5. CMS-LHCb joint measurement of the decay rates of a strange meson (2014)

“Observation of the rare B0s→μ+μ− decay from the combined analysis of CMS and LHCb data”: https://cds.cern.ch/record/1970675

Another way to probe the Standard Model is by testing its predictions of rare particle transformations. For example, theory tells us that for every billion or so Bs (or B0s) mesons produced, around three will transform into a muon-antimuon pair, that is, a “decay rate” of ~3 x 10−9. Any deviation from this prediction would open the door to new beyond-the-Standard-Model physics. CMS and LHCb independently presented their measurement of this decay rate in 2013, later combining our forces to publish a joint measurement. Spoiler alert: The Standard Model continues to hold fort, much to our disappointment.

Abstract: A joint measurement is presented of the branching fractions B0s→μ+μ− and B0→μ+μ− in proton-proton collisions at the LHC by the CMS and LHCb experiments. The data samples were collected in 2011 at a centre-of-mass energy of 7 TeV, and in 2012 at 8 TeV. The combined analysis produces the first observation of the B0s→μ+μ− decay, with a statistical significance exceeding six standard deviations, and the best measurement of its branching fraction so far, and three standard deviation evidence for the B0→μ+μ− decay. The measurements are statistically compatible with SM predictions and impose stringent constraints on several theories beyond the SM.

Surprises and discoveries

6. The so-called “ridge effect” seen in proton-proton collisions (2010)

“Observation of Long-Range, Near-Side Angular Correlations in Proton-Proton Collisions at the LHC”: https://cds.cern.ch/record/1293115

Few would have expected as big a surprise as observing the “ridge effect” in proton-proton collisions for the first time, only months after the LHC began operating at a centre-of-mass energy of 7 TeV. The “ridge” in question corresponds to an apparent correlation between particles travelling in opposite directions after being produced when two protons collide. The phenomenon had previously been observed in collisions of heavy nuclei but had not been seen when individual particles were collided, prior to the LHC. CMS has subsequently seen the ridge make an appearance in collisions of lead nuclei with lead nuclei and in collisions of lead nuclei with protons.

Abstract: Results on two-particle angular correlations for charged particles emitted in proton-proton collisions at center-of-mass [sic] energies of 0.9, 2.36, and 7 TeV are presented, using data collected with the CMS detector over a broad range of pseudorapidity (eta) and azimuthal angle (ϕ). Short-range correlations in Δη, which are studied in minimum bias events, are characterized using a simple “independent cluster” parametrization in order to quantify their strength (cluster size) and their extent in η (cluster decay width). Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity and particle transverse momentum using a 980 inverse nb data set at 7 TeV. In high multiplicity events, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate transverse momentum of 1-3 GeV/c, 2.0 < |Δη| < 4.8 and Δϕ ≈ 0. This is the first observation of such a long-range, near-side feature in two-particle correlation functions in pp [proton-proton] or pp [proton-antiproton] collisions.

7. The discovery of the Higgs boson (2012)

“Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC”: https://cds.cern.ch/record/1471016

The Higgs boson is no doubt the crown jewel of the LHC, having first been seen by CMS (and ATLAS) in data recorded during the LHC’s first run. Predicted by theorists in the 1960s, it took nearly five decades of experimental work to observe this particle. In 2012, CMS and ATLAS were cautious about what we had seen in the data: all we could say was that we had observed a new particle that behaved like the predicted Higgs boson. Later measurements confirmed that it was in fact a Higgs boson, but the jury is still out on whether there are more Higgs bosons for us to find.

Abstract: Results are presented from searches for the standard model Higgs boson in proton-proton collisions at √s = 7 and 8 TeV in the CMS experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at 7 TeV and 5.3 inverse femtobarns at 8 TeV. The search is performed in five decay modes: γγ, ZZ, WW, τ+τ−, and bb [bottom-antibottom]. An excess of events is observed above the expected background, a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, γγ and ZZ; a fit to these signals gives a mass of 125.3 ± 0.4 (stat.) ± 0.5 (syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.

All CMS publications are published as open access under a Creative Commons licence.

And if you want to explore all papers CMS has published since 2010, use this interactive timeline (also available below): https://cern.ch/cms-results/public-results/publications-vs-time/

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