CMS in 2011: A mountain of particle collision data

This is the proton collision data delivered by the LHC to CMS. The red triangles indicate how much data was delivered up to a given day in 2011.

Datasets are the currency of physics. As data accumulate, measurement uncertainty ranges narrow, which increases the potential of discoveries and makes non-observations more stringent, with more far-reaching consequences. In collider physics, the amount of data is measured by the total number of collisions observed and the rate of those collisions, or the luminosity.

In 2011, the LHC produced more collisions than scientists dared to expect, breaking the world record luminosity in April and then continuing to grow seven-fold. By the end of the proton collision run in November, 240 million protons were colliding each second. Top quarks, once considered rare, were produced at a rate of one pair every two seconds. This provided CMS scientists with enough data to measure known processes with unprecedented precision, improving our understanding of the way protons collide and sharpening theoretical predictions of new phenomena.

One of the most pervasive ideas in theoretical physics is supersymmetry. This is the idea that matter and forces were unified in the early universe. Although this symmetry principle may appear in many guises, CMS scientists cast a wide net of search strategies to look for signs of supersymmetric particles. None were observed, and this means that many models that had been plausible in 2010 are now ruled out. Supersymmetry is still possible, but it has less wiggle room than it did before.

Many other models of new phenomena were studied as well: extra dimensions, substructure within familiar quarks and leptons, excited variants, new generations, conglomerates of these particles, new forces, new long-lived or stable particles, and even microscopic black holes. In each case, the fact that these phenomena did not appear rolls back the scope of speculation with hard facts. Curled-up extra dimensions may exist, but if so, they must be curled 2.5 times smaller than previously supposed.

The last four weeks of the year were dedicated to collisions of lead ions and physics of an entirely different character. When lead ions collide, so many quarks and gluons are produced that the fireball forms a fluid like the one that existed in the early universe. The December 2010 data revealed that this fluid is lumpier and more asymmetric than expected, that it absorbs even very energetic quarks and gluons, and that it melts mesons. It is too soon to say what will be found in this year’s lead ion dataset, which is 16 times larger.

The most dramatic revelation of 2011 concerns the Higgs boson, the as-yet undiscovered cornerstone of the Standard Model of particle physics. Hundreds of CMS scientists have pooled their data to show that the Standard Model Higgs, if it exists, can only have a mass in the narrow range between 115 and 127 GeV. This could mean that the Higgs is hiding within this range, or that the real Higgs boson is not a Standard Model Higgs but has exotic properties, or that it simply does not exist. LHC scientists anticipate a large enough dataset in 2012 to make a definitive statement. Regardless of the outcome, the stage is set for an exciting year.

We would like to thank the LHC accelerator staff for a highly successful physics run, and wish everyone a restful break and a happy new year!

Jim Pivarski