CMS

CMS is a particle physics experiment in construction at the CERN LHC – the Large Hadron Collider, an accelerator for protons that is designed to provide the highest energy proton-proton collisions ever achieved, seven times more than the present record set by Fermilab’s Tevatron collider.

The acronym CMS stands for “Compact Muon Solenoid” (not “Central” as I had mistyped this morning): the focus of the name is on the detection of muons, which are particles very infrequently produced in proton collisions. Their detection provides a clean handle to identify the production of new particles that physicists are trying to discover: the main example is the long-sought Higgs boson, which might be detected when it decays to pairs of Z bosons, each in turn producing a high-energy muon.

Muons are penetrating particles: they can traverse several meters of dense matter without interacting with it other than leaving a ionization trail. All other produced particles in the collision – hundreds of them – can be screened with a meter of steel, so one could just place a ionization detector outside a thick slab of steel to see just muons coming out.

However, physicists are greedy, and they want to know everything about the collisions, not just measuring a muon here and another there. So CMS is actually an enormously complex device, endowed with millions of electronic channels reading tens of different detecting devices. More than 2000 physicists have been working for more than 10 years to put together this incredibly complex device. Another such detector is being built just a few kilometers away: it is the ATLAS detector, a competitor of CMS. Both will benefit from the protons accelerated by the LHC.

CMS is built like an onion: around the interaction point, where protons are smashed together by the intersection of two opposing beams, there are several layers of microstrips detectors – where all electrically charged particles (not just muons) can be tracked thanks to the ionization they leave in 300 microns-thick silicon. As particles escape these inner layers, they enter crystals of lead tungstate – a material that produces light when traversed. Outside of these crystals, thick layers of heavy absorber sandwiching slabs of plastic scintillators provide a further measurement of both charged and neutral particles. Finally, four layers of muon detectors encircle the whole detector.

All in all, CMS weighs more than a battleship. It is an impressive, huge construction, which will be installed deep underground, where the 27-km long tunnel of the collider rests, screening with tens of meters of rock the happy citizens of Geneva from the harmful radiation.

The experiment is expected to start data taking in 2008. If the Higgs boson exists, it will be discovered in not more than one or two years. But things will not end there: many theoretical physicists are convinced that a whole host of yet undiscovered new particles are within CMS and ATLAS reach. These “supersymmetric” particles (there should exist one supersymmetric particle for all ordinary particles already known), if discovered, will change wholly the way we describe the world at the subatomic scales.

What would be the benefit of discovering these new objects to our lives ? We do not know. The idea of finding out more about the physical world is just to deepen our knowledge. Roentgen did not know whether he was doing anything good when he played with vacuum tubes, but as soon as he saw his newly-discovered x rays were able to provide an image of the structure of thick objects, an application was instantly found: a arm bone was fixed after x-raying it just months after the discovery. With supersymmetric particles it might be harder to find an easy application, but science cannot stop…