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Exclusive diphoton production observed! November 17, 2006

Posted by dorigo in news, physics, science.

A paper seminar in CDF was given yesterdat by Andrew Hamilton on exclusive production of two central photons in proton-antiproton collisions at the Tevatron collider. As far as I know the paper is not in the arxiv yet, I will post the link here as soon as it’s out. A matter of days.

The process is very rare, and in more than 500 inverse picobarns of collected data only three events have been identified. Three events may look like shaky evidence, but the background is so tiny – 0 events expected, with an uncertainty of 0.2 – that the three diphoton candidates represent a solid evidence for the sought process. The measured cross section equals 140 femtobarns, with a large uncertainty:  the measurement error is +140-40 (stat) +-30 (syst) fb.

But what is the process giving two photons and nothing else in the detector, anyway ? It is a very peculiar one indeed. One in which the two protons produce the two central photons by exchanging a pair of gluons. One gluon creates a virtual loop of quarks, which emits the photons. The other gluon has the hefty responsibility of keeping the two protons colorless, such that they do not shatter into a stream of hadrons, but leave the interaction point unscathed through the beampipe.

If you have ever seen a typical result of a 2-TeV proton-antiproton collision, and the mess these high-energy bodies do when they collide, you cannot but be awed at the incredible cleanliness of these three events, which display  two central photons with transverse energy above 5 GeV, and nothing -nothing- else.

Here is a diagram of the production process. The predictions from QCD give a cross section in the kinematic range Et>5 GeV,  absolute rapidity less than 1.0, of about 40 femtobarns, with a large uncertainty (at least a factor four).

As can be seen on the left, you really need a fancy process to produce two photons while retaining the individuality of the projectiles. Of course, protons emit bremsstrahlung, but they do so only in the presence of material, not in the vacuum of the beam pipe – and the radiation is emitted forwards, not orthogonally to the beam! You really need two gluon exchanges to do the trick. The process is one of the fanciest pieces of QCD magic we have observed so far in CDF.

There is also one other very specific reason for being excited about this observation. Higgs production can occur via the same exclusive process, and a measurement of diphoton production by the exclusive process allows to test the prediction for exclusive Higgs production cross section at the LHC, whose uncertainty is large.  And exclusive production of the Higgs is crucial to demonstrate that the Higgs is a scalar particle.

Another good piece of physics by the CDF experiment. I am proud of it!


1. Fred - November 18, 2006

Props to all of you and ‘down the hatch’. Roughly speaking, how many people were involved, and how much time, money and labor was expended to realize the 500 inverse picobarns of collected data? Or is this just another mild event that has been exaggerated to give your field an excuse to celebrate with wine, colleagues and song. A very clever ruse, I might add, as a few of us have to take your word for it, lest we be bombarded with extreme formulae.

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