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## A new precise measurement of the X particleAugust 14, 2008

Posted by dorigo in news, physics, science.
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Remember the X(3872) ? It is a new neutral particle decaying into a J/Psi meson and a pair of pions, which was discovered by BaBar in 2003. Its mass was immediately recognized to be really close to the sum of a D and a D* meson, making the interpretation that it is actually a molecular state of those two hadrons a viable explanation for its nature. CDF produced a detailed study of its nature in 2006 (see link above), and has continued to pay close attention to it as more data were being collected.

Thanks to the careful, dedicated work of Michael Feindt and collaborators (J. Heuser, M. Kreps, and Th. Kuhr), CDF now has the most precise measurement of the mass of this particle in the world. Quite a remarkable result, if you ask me: one would think that heavy flavor physics was a business for the B factories, but it turns out that good old CDF is competitive wherever it looks.

So take a look at the mass spectrum of fully reconstructed X particles, from their decay to a J/Psi meson (which is identified through its disintegration into a pair of muon tracks) plus two charged pions. Ain’t it pretty ?

The mass distribution is extracted from 2.4 inverse femtobarns of data collected by a J/Psi trigger. What I find impressive is the fact that the mass turns out to be inches below the sum of masses of a D and a D* meson, thus making the hypothesis that X is a bound state of these objects a real possibility. Instead, the analysis excludes that there are two neutral states of different mass contributing to the peak, as would be the case if the particle was a combination of multiple quarks, a hypothesis put forth by Luciano Maiani et al. in Hep-Ph/0412098. The mass difference between those states would have to be smaller than 3.6 MeV for that to be still allowed by CDF data.

Now, the mass turns out to be $m = 3871.61 \pm 0.16 \pm 0.19 MeV$, a measurement to 60 parts per million which is the most precise in the world. It also dominates the world average, as you can see in the bar plot below.

In the plot above you see the new CDF measurement (fourth-to-last point), the old and new world averages (third- and second-to-last), and for comparison the sum of D° and D* masses.

The mystery remains thick with the origin of this particle. I look forward to a spin-parity analysis with the larger dataset that has allowed the mass measurement: that should probably allow a more conclusive understanding. Is it just another charm-anticharm bound state ? Or is it a more exotic combination of two color-neutral hadrons containing charm ? Who said the high-energy frontier of particle physics is the most exciting one these days ?

More detail and plots on this very interesting new measurement are available from the public web page of the analysis.