The Sciences

LHC - First Inverse Picobarn!

Cosmic VarianceBy John ConwayAug 10, 2010 3:58 PM


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Back in May, I gave a talk at the annual Phenomenology Symposium at UW Madison, showing some of the first physics results from the CMS experiment. At that point we had a data sample of proton-proton collisions corresponding to 1 inverse nanobarn.

This past weekend the LHC crossed a major threshold: 1 inverse picobarn delivered to the experiments - a factor of a thousand more collisions. By late next year we are all hoping to have recorded another factor of a thousand, for a total of 1 inverse femtobarn.

In an earlier post I explained these funny units, inverse whatever-barns. The point here, though, is that as we record exponentially greater numbers of collision events, with the proton beam energy 3.5 times greater than that at the Tevatron at Fermilab we will begin to really probe an unexplored mass scale in the search for new particles. What lies there is completely unknown.

So far the LHC experiments CMS and ATLAS have presented results on about a quarter of the data sample recorded so far, at the biennial International Conference on High Energy Physics, held this year in Paris. To sum it up in a sentence, both experiments have rediscovered our familiar standard model friends, among which the W and Z bosons and the top quark are the most massive.

The W and Z are both produced in proton-proton collisions by the collision of quarks with antiquarks. You should visualize the incoming beam protons as being composed not just of two "up" quarks (charge +2/3) and one "down quark (charge -1/3), but as sort of seething, roiling mass of quarks, antiquarks, and gluons. When the protons collide any two of these constituents, if they have enough energy, can annihilate to form a a W or Z boson.

The W and Z are the "carriers" of the weak force in the standard model. For commonplace processes like nuclear decay (like cesium-137, for example) it's the weak force which allows it to happen. We describe the process as involving a "virtual" W boson which exists fleetingly, by grace of the uncertainty principle, with a mass thousands of times less than its true mass of 80 GeV. It's this virtuality that makes the weak force weak, in fact, for nuclear processes.

But at the energies of the incoming proton constituents, there is plenty to make real W bosons, and also Z bosons. (We don't ordinarily see the effects of Z bosons in nuclear processes, because Z's can only couple a particle to its own antiparticle...) Now, if you have a real W or Z boson sitting there, it will decay in about 10 23 seconds to a quark and an antiquark or to leptons. In the case of the W, which has electric charge +1 or -1, it decays to a charged lepton and its associated neutrino about 33% of the time, and the rest of the time to quark-antiquark pairs. A Z boson will decay to a charged lepton (e, mu, or tau) and antilepton about 10% of the time, or to a pair of neutrinos about 20% of the time, and the rest of the time to a quark-antiquark pair.

The LHC experiments cannot really see the quark-antiquark decays of the W and Z - there is just too much background from quark-quark, quark-gluon, and gluon-gluon scattering giving two outgoing quarks or gluons. When a quark or gluon emerges sideways from one of these collisions it sort of shatters into a collimated spray of high energy particles that we call a jet. This all is governed by the strong force, which, being stronger than the weak force, has much higher rate than the W- and Z-producing processes.

But, ah, the leptonic decays of the W and Z! The sweetest is the lepton-antilepton decay of the Z. About 7% of the time a Z will decay to an electron-positron pair or a muon-antimuon pair. These particles come screaming out of the collision region into the detector carrying about half the Z's total mass-energy of 91.2 GeV. This makes them readily identifiable and reconstructible. High energy electrons and muons leave a very straight track in our charged particle tracking system. Electrons then lose all their energy in the dense calorimeter surrounding the tracker. Muons, being 200 times more massive, tend to sail on through the calorimeter and magnet coil out to the muon tracking system that forms the true bulk of the CMS experiment. Here is a cool display of one of the first such events recorded in CMS:

In fact, the astute reader who knows the size of an atomic nucleus will conclude that the muons in the picture above must have traveled straight through quite a number of nuclei to reach the outer parts of the detector! This is because muons interact only via the weak and electromagnetic forces with nuclear matter, and, those forces are quite weak compared with the strong force.

With two muons, one can calculate the mass of the parent particle from which they came, using relativistic formulae. And by ICHEP the CMS experiment had recorded enough muon pair events to make the following beautiful graph showing the spectrum of masses from which opposite sign muon pairs arose. In the plot, at the far right end, the peak from the Z boson at 91.2 GeV is clear as a bell:

At lower masses one can see the peaks from the upsilon (Y), which is a bound state of a bottom and an antibottom quark, the J/psi which is a bound state of charm-anticharm, and lighter resonances. The broad smear of "continuum" muon pair production comes from virtual photons - the electromagnetic interaction.

These data, and similar data from eletron-positron pairs, is extremely important for calibrating the experiment. By measuring the position of the Z peak we can see whether we have properly calibrated our charge particle momentum scale, and then use that to calibrate the calorimeters via the Z to ee signal. The Z is our standard candle here, but as the saying goes, in high energy physics yesterday's sensation is today's calibration (and tomorrow's background).

All these results and more are there for the world to see at the ICHEP web site. There are plenty more results, including the first glimpse of top-antitop events, and the results of some searches for new phenomena.

Nothing startling has come out yet, and we are eagerly awaiting the exponentially growing samples to analyze, with which we will push past the Tevatron sensitivity in a number of areas. But don't count the Tevatron out just yet! The CDF and Dzero experiments have recorded thousands of times more collisions and results are still pouring out. And, oops, it's time for me to go to that CDF analysis meeting now...

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