A new precise measurement of the X particle August 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.

1. Anonymous - August 15, 2008

If there’s a b-quark (rather than c-quark) analogue to the X(3872), any chance that CDF could find it?? (If so, perhaps that might add some clues regarding its nature…) All you need to do is to look around 9.73 GeV for something decaying to upsilon(1S) + pi+ + pi-… Not easy at CDF, but I think it could be done, no?

2. dorigo - August 15, 2008

Anon, good question. I am sure CDF can and should find such a state if it exists. We have about 100,000 Y(1S) mesons reconstructed in the dimuon final state, and the mass resolution at 9 GeV is not too much worse. However, it is not at 9.73 (=9.45+0.14+0.14) where
we should look, but rather at the B+B* mass, which is above 10.5 GeV (too lazy to look up the exact masses, look in the pdg). And it is good that it is so: a 9.73 GeV particle decaying as you mention would yield zero momentum pions, which would be utterly unobservable.

However, I am not sure that if a D and D* meson decide to bind into a molecule, B and B* mesons should do the same thing by force of logic. Gluon fragmentation into a charm-anticharm pair is a very different process from fragmentation into a bottom-antibottom state. That is evident, for instance, if one looks at the P_t distribution of the J/Psi and Y mesons: the former are boosted, the latter are produced at rest.

Cheers,
T.

3. Anonymous - August 15, 2008

Yes, right, thanks — around 10600 GeV (I was lazily and naively replacing the 1.27 GeV masses of the charm quarks themselves with 4.20 GeV bottom quarks without considering differences in potential etc.) Indeed it isn’t an entirely sure thing, but it is definitely worth a look. Given that there is an X(4260) as well as the 3872, I would be very surprised if there is neither anything analogous nor absolutely anything beyond what’s already been seen in terms of b bbar mesons.

4. Neil B. - August 15, 2008

Mind if I bring up the other “X” particle? That was IIRC some oddball supersymmetric entity, talked about in the late 70s-80s, in books like The Key to the Universe: A Report on the New Physics by Nigel Calder (1978). Since this new particle is called “X” does that mean the other postulated particle is unlikely? Do they care enough about keeping names separate?

Also, is there any more insight into another weird particle, the Ds(2317), maybe a tetraquark or other oddball complex? How many “molecular states” and maybe superpositions and other entities that can’t just be simply put forth as either just fundamental as is, or “this combination of quarks” are there? How many are possible? Could we put things together in odd ways that would produce bizarre results? I remember reading in old World Book Science Year about IIRC strange “spin-2 matter”, issues about Pomeranchukons that would determine how easily one could go close to light speed without having big problems from high-energy protons, and other curious angles (like the X particle) that seem to have faded.

5. dorigo - August 15, 2008

Anon, I might concur, but a particle decaying into Y(1S) plus two pions at 10.5 GeV does exist… It is the Y(4S), mass 10.58 GeV. Also the 3S does that decay (and much more frequently, since the BB channel is not open for it), at 10.35 GeV.

Meson spectroscopy is complicated. And I do not even know much about it…

Cheers,
T.

6. dorigo - August 15, 2008

Hi Neil,

I had not heard of that other X particle, and I suspect the discoverers of the X(3872) did not care much about the name being already taken by some mysterious entity never confirmed.

For the other questions: I am not the right person to ask. I know only basic things about low-energy QCD spectroscopy, unfortunately (unfortunately, since it is a fascinating subject). Maybe Alexei Petrov is a better bet.

Cheers,
T.

7. Marco Frasca - August 15, 2008

Hi Tommaso,

It is not too much clear to me why you think this particle should be thought as a diquark molecular state. As far as I can tell, no evidence whatsoever is known about such states and I think that until we get an unequivocal signature of them, one should avoid any claim about.

Ciao,

Marco

8. dorigo - August 15, 2008

Hello Marco,

well, I do not think it is a molecule of two D mesons. I agree with you that some kind of a ccbar bound state in the right spin-parity configuration ($2^{-+}$) still fits the bill, and that the claim it to be a molecular state is not one to take lightly: it requires strong evidence, much stronger than whatever is still present here. However, it is really remarkable that the mass is so close to that of a (free) D+D* system. Let us wait and see!

Cheers,
T.

9. menchevik - August 15, 2008

I may be wrong, but I think that if nucleons form bound states (a.k.a. “nuclei”) it should not be unconceivable that mesons may form quasi-bound states. I think that is what is meant by “molecular” states of D-D^*, just like nuclei are “molecular” states of nucleons.

But it might also happen that such structure is not enough to describe the X, and that an additional, exotic four-quark component is also present, maybe describable as a diquark-diquark quasi-bound state. I think some people like that idea a lot. (But then, tetraquarks are so discredited that I personally won’t believe it until I actually see it…)

10. dorigo - August 15, 2008

Hi Menchevik,

well, yes. The reasoning is logical, but of course one needs to take in account the binding strength, the radius of the mesons, their angular momenta, and the matrix elements of NRQCD, plus the relative velocity distirbution of the pair of D mesons as they are created in a gluon fragmentation. Highly non-trivial computations. Methinks we basically do not yet know enough to get meaningful cross-section estimates for a D-D* molecule. Just a thought: J/Psi production at hadron colliders is itself just barely understood… Of course, this is just the simple-minded thought of an ignorant.

Cheers,
T.

11. menchevik - August 16, 2008

well, two of them… count me in.

12. Neil B. - August 18, 2008

You folks may be interested in this other reference to a “particle X” which is apparently, again not the X(3872) mentioned above but not the older supersymmetry (IIRC) “X particle” either. This report is strange and very interesting (to me at least), it seems to have “significance potential”:

http://www.physorg.com/news92488817.html

“A particle that may mediate the rare decay of a Sigma-plus hyperon appears to have close affiliations with a light Higgs boson found in one supersymmetric model—an interpretation suggesting unambiguous evidence for physics beyond the standard model (SM), scientists say.”

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