## CDF discovers a new hadron! March 13, 2009

Posted by dorigo in news, physics, science.
Tags: , , , ,

This morning CDF released the results of a search for narrow resonances produced in B meson decays, and in turn decaying into a pair of vector mesons: namely, $Y \to J/\psi \phi$. This Y state is a new particle whose exact composition is as of yet unknown, except that CDF has measured its mass (4144 MeV) and established that its decay appears to be mediated by strong interactions, given that the natural width of the state is in the range of a few MeV. I describe succintly the analysis below, but first let me make a few points on the relevance of area of investigation.

Heavy meson spectroscopy appears to be a really entertaining research field these days. While all eyes are pointed at the searches for the Higgs boson and supersymmetric particles, if not at even more exotic high-mass objects, and while careers are made and unmade on those uneventful searches, it is elsewhere that action develops. Just think about it: the $\Xi_b$ baryon, the $\Omega_b$, those mysterious X and Y states which are still unknown in their quark composition. Such discoveries tell the tale of a very prolific research field: one where there is really a lot to understand.

Low-energy QCD  is still poorly known and not easily calculable. In frontier High-Energy Physics we bypassed the problem for the sake of studying high-energy phenomena by tuning our simulations such that their output well resembles the result of low-energy QCD processes in all cases where we need them -such as the details of parton fragmentation, or jet production, or transverse momentum effects in the production of massive bodies. However, we have not learnt much with our parametrizations:  those describe well what we already know, but they do not even come close to guessing whatever we do not know. Our understanding of low-energy QCD is starting to be a limiting factor in cosmological studies, such as in baryogenesis predictions. So by all means, let us pursue low-energy QCD in all the dirty corners of our produced datasets at hadron colliders!

CDF is actively pursuing this task. The outstanding spectroscopic capabilities of the detector, combined with the huge size of the dataset collected since 2002, allow searches for decays in the one-in-a-million range of branching ratios. The new discovery I am discussing today has indeed been made possible by pushing to the limit our search range.

The full decay chain which has been observed is the following: $B^+ \to Y K^+ \to J/\psi \phi K^+ \to \mu^+ \mu^- K^+ K^- K^+$. That $J/\psi$ mesons decay to muon pairs is not a surprise, as is the decay to two charged kaons of the $\phi$ vector meson. Also the original decay of the B hadron into the $J/\psi \phi K$ final state is not new: it had been in fact observed previously. What had not been realized yet, because of the insufficient statistics and mass resolution, is that the $J/\psi$ and $\phi$ mesons produced in that reaction often “resonate” at a very definite mass value, indicating that in those instances the $B \to J/\psi \phi K$ decay actually takes place in two steps as the chain of two two-body decays: $B \to Y K$ and $Y \to J/\psi \phi$.

The new analysis by CDF is a pleasure to examine, because the already excellent momentum resolution of the charged particle tracking system gets boosted when constraints are placed on the combined mass of multi-body systems. Take the B meson, reconstructed with two muons and three charged tracks, each assumed to be a kaon: if you did not know that the muon pair comes from a $J/\psi$ nor that two of the kaons come from a $\phi$, the mass resolution of the system would be in the few tens of MeV range. Instead, by forcing the momenta of the two muons to be consistent with the World average mass of the $J/\psi$, $M_{J/\psi}=3096.916 \pm 0.011 MeV$ , and by imposing that the two kaons make exactly the extremely well-known $\phi$ mass ($M_\phi=1019.455 \pm 0.020 MeV$), much of the uncertainty on the daughter particle momenta disappears, and the B meson becomes an extremely narrow signal: its mass resolution is just 5.9 MeV, a per-mille measurement event-by-event!

The selection of signal events requires several cleanup cuts, including mass window cuts around the J/Psi and phi masses, a decay length of the reconstructed B+ meson longer than 500 microns, and a cut on the log-likelihood ratio fed with dE/dx and time-of-flight information capable of discriminating kaon tracks from other hadrons. After those cuts, the B+ signal really stands above the flat background. There is a total of 78+-10 events in the sample after these cuts, and this is the largest sample of such decays ever isolated. It is shown above (left), together with the corresponding distribution in the $\phi \to KK$ candidate mass (right).

A Dalitz plot of the reconstructed decay candidates is shown in the figure on the right. A Dalitz plot is a scatterplot of the squared invariant mass of a subset of the particles emitted in the decay, versus the squared invariant mass of another subset. If the decay proceeds via the creation of an intermediate state, one may observe a horizontal or vertical cluster of events. Judge by yourself: do the points appear to spread evenly in the allowed phase space of the B+ decays ?

The answer is no: a significant structure is seen corresponding to a definite mass of the $J/\psi \phi$ system. A histogram of the difference between the reconstructed mass of the $J/\psi \phi$ system and the $J/\psi$ mass is shown in the plot below: a near-threshold structure appears at just above 1 GeV energy. An unbinned fit to a relativistic Breit-Wigner signal shape on top of the expected background shape shows a signal at a mass difference of $\Delta M=1046.3 \pm 2.9 MeV$, with a width of 11.7+-5.7 MeV.

The significance of the signal is, after taking account of trial factors, equal to 3.8 standard deviations. For the non-zero width hypothesis, the significance is of 3.4 standard deviations, implying that the newfound structure has strong decay. The mass of the new state is thus of 4143+-2.9 MeV.

The new state is above the threshold for decay to pair of charmed hadrons. The decay of the state appears to occur to a pair of vector mesons, $J/\psi \phi$, in close similarity to a previous state found at 3930 MeV, the Y(3930), which also decays to two vector mesons in $Y \to J/\psi \omega$. Therefore, the new state can be also called a Y(4140).

Although the significance of this new signal has not reached the coveted threshold of 5 standard deviations, there are few doubts about its nature. Being a die-hard sceptic, I did doubt about the reality of the signal shown above for a while when I first saw it, but I must admit that the analysis was really done with a lot of care. Besides, CDF now has tens of thousands of fully reconstructed B meson decays available, with which it is possible to study and understand even the most insignificant nuances to every effect, including reconstruction problems, fit method, track characteristics, kinematical biases, you name it. So I am bound to congratulate with the authors of this nice new analysis, which shows once more how the CDF experiment is producing star new results not just in the high-energy frontier, but as well as in low-energy spectroscopy. Well done, CDF!

1. Eric Weinstein - March 13, 2009

Wow. That seems very exciting and of interest to readers beyond the community of practicing professionals. As you write very clearly, this blog is a favorite of several people such as myself who do not speak this language of experimental accelerator physics as natives. What can be surmised from this about the nature of the hadron at this point beyond the inference that there is a non-trivial SU(3) representation involved? Is there any ability at this early stage to translate this for visitors to this field who are more comfortable with descriptions of group representations and internal quantum numbers?

Should this question be mal-formed, please feel free to answer the nearest good question available!

Thanks for the write-up,

Eric

2. mfrasca - March 13, 2009

Tommaso,

some formulae do not appear to parse correctly.

Cheers,

Marco

dorigo - March 13, 2009

Hi Marco, do you have a Mac ? I see them fine with my windoze Pc.

Thanks anyways… Will try to figure out what’s going on, another person with a Mac does not see the formulas.
T.

3. mfrasca - March 13, 2009

Tommaso,

I am Windows addicted since the pioneering times of PC-AT. It seems that the problem is still there.

Marco

4. Andrea Giammanco - March 13, 2009

I’m using firefox with windows, and I have the same problem.

5. Sumar Ongi - March 13, 2009

So, another meson! This one with strangeness -1, no charm or beauty, JP=0+,1-, 2+, … mass 4 MeV, an decays to J/psi phi…
Well, beats me… I have no idea what this is…

6. Sumar Ongi - March 13, 2009

(I mean, 4GeV)

7. dorigo - March 13, 2009

Dear Eric,

the question is well-put, but there is not a clear answer. What I think is that it is most likely some charm-anticharm excitation, above the DD threshold, and possibly some form of bound state of those particles.

Sumar, no, the newfound object has zero strangeness, since it decays strongly to a S=0 state.

I do not know what is wrong with the formulas, I will ask the experts of wordpress.

Cheers,
T.

8. Markk - March 13, 2009

Could these types of studies be called Strong Chemistry? That is, looking at collections of particles and their excitations with the Strong Force as the dominant thing? Just like what chemistry does with new molecules and their excited states? Looking at excited bound states of particles and their decays. its like, molecules, atomic nuclei, and hadrons themselves all have slightly similar happenings with different forces acting. (Well the last two I guess are both the strong force, but in a different way). You get collections of particles in bound states with excitations internal excitations that have coherent properties from the “outside”.

It looks like we are getting to the point of doing this at the hadron level. Was this the kind of thing that ILS (? name) accelerator would be trying to do? Really tune things to look for resonances or other fine details through a range of energies?

9. Dr BDO Adams - March 13, 2009

There so many mystery mesons (especially) in the JP=0+ states, that it might be time to invent some new quarks. Lets invent an exicted S* quark with opposite parity to normal S quarks. Then Y could be d (bar S*), which letter decays via bar S* -> bar c + c + s. S* would be around 4000 GeV, and there be a full spectrum of S* meson around there. Could that fit?

10. Carl Brannen - March 13, 2009

My Mozilla browser loses most of the LaTeX (I get them back by running the mouse over them), but the windows browser gets them all okay. Interesting.

Meanwhile, Phys Math Central has rejected my paper, for its lack of use of the dyanmics of QCD. I think this is unfair in that QCD dynamics is perturbative field theory and I’m looking at a QM approximation. A similar calculation is the Lamb shift in hydrogen-like atoms, where one uses QM and Schroedinger’s equation for the first order approximation to the bound states, and only then uses QED to make corrections. So I’ll send a note back to that effect.

11. dorigo - March 13, 2009

Hi Carl,

I am sorry to hear your paper has been rejected. I cannot judge on that decision by the journal, but I hope you can find some publisher who is less constrained by mainstream thinking.

Cheers,
T.

12. dorigo - March 13, 2009

Markk, yes, actually a colleague at the conference today was calling it the same way, hadronic chemistry. But no, the ILC (if that is the accelerator you have in mind) would study higher-energy particles, like SUSY ones, not hadrons.

BDO, no, I do not think it would fit, for a number of reasons. An additional quark would contribute to R, change electroweak precision parameters, create havoc in the theory. In any case, the phi is a s-sbar
state, so we need something decaying to ccbar ssbar, strongly. Could be a molecular state of two D_s mesons, or an excited charmonium state of some kind. I have not studied in detail the problem of classifying these states yet…

Cheers,
T.

13. Sumar Ongi - March 14, 2009

Geeeezzzz!! The first decay is obviously weak, and I was silly enough to compute the strangeness from it. Anyway, it’s quite likely the Y is a D\bar{D} molecule of some type.

Regarding the problem with some of the equations, all I can say is that all of the problematic ones contain an URL with “&” instead of “&”. That usually leads to problems with links. Copy and paste one of the links to a problem formula into your browser, change all “&” for “&” and the formula is displayed correctly.

14. Sumar Ongi - March 14, 2009

Well, today is not my day… I meant “&” instead of “& amp;” (without the blank…)

15. Kea - March 14, 2009

Sheeeesh. Requiring QCD when it might not be the right theory to describe particle masses to first order seems a bit self defeating, Carl. Maybe another journal will be more sensible.

16. Tony Smith - March 15, 2009

Carl and Kea, since the rejected paper acknowledges substantial contributions by Kea, perhaps it could be resubmitted as a joint paper with Kea as co-author.
If so, maybe Kea’s Oxford affiliation might get it accepted.

Tony Smith

17. carlbrannen - March 15, 2009

Tony and Kea, while I appreciate your support, this is not the place to discuss this. I believe that my being an amateur did not have any effect on acceptance; professionals also complain when their revolutionary papers get rejected. The big difference between the pros and amateurs is that the pros have to publish and so work a lot harder at it than I have. I should have a dumbed down version that is easier to accept in a few days.

18. Streaming video for Y(4140) discovery « A Quantum Diaries Survivor - March 17, 2009

[…] Streaming video for Y(4140) discovery March 17, 2009 Posted by dorigo in news, physics, science. Tags: B physics, CDF, discoveries, QCD, standard model, Tevatron trackback The CDF collaboration will present at a public venue (Fermilab’s Wilson Hall) its discovery of the new Y(4140) hadron, a mysterious particle created in B meson decays, and observed to decay strongly into a state, a pair of vector mesons. I have described that exciting discovery in a recent post. […]

19. Neil B. - March 18, 2009

I used to figure, it’s obvious that every hadron is a straightforward combination of any 2 (well, for mesons typ. quark+antiquark) or 3 quarks from the set u d s c b t. But now I gather, it’s more complicated because of superpositions. Indeed, the proton was said to have a bit of “strangeness” despite canonical composition uud (well, does it?) So maybe this new particle can’t be adequately written up in the simple way. (Pardon any lack of attention to answers in the post or comments, or literature – I’m a nuclear dilettante despite savvy on neglected subjects like the dynamics of extended bodies subject to forces and accelerations, in relativity theory.)

20. Neil B. - March 18, 2009

Followup: I already can find some superpositions, from Wikipedia on mesons. I copied the (?tXt?) direct for two examples but you can figure what it shows:

Neutral rho meson[22] ρ0(770) Self \mathrm{\tfrac{u\bar{u}-d\bar{d}}{\sqrt 2}}\, 0,775.49 ± 0.34

Omega meson[23] ω(782) Self \mathrm{\tfrac{u\bar{u} + d\bar{d} – 2s\bar{s}}{\sqrt{6}}}\, 0,782.65 ± 0.12

If mesons were limited to all q+qbar combos, there wouldn’t be as many – so that leads to, how many superposition variations are possible? “Where does it all end”?

21. La partícula Y(4140) descubierta en el Fermilab podría ser un error de cálculo « Francis (th)E mule Science’s News - March 19, 2009

[…] El 13 de marzo de 2009, la colaboración CDF del Fermilab anunció el descubrimiento de una nueva partícula algo “rara” (artículo técnico aparecido en el ArXiv). El 17 de marzo se ha realizado el anuncio oficial, fichero de transparencias Powerpoint y  vídeo de la presentación, momento en el que muchos se han hecho eco de este “gran” descubrimiento, por ejemplo, Ciencia Kanija “Extraña bola de partículas sorprende a los físicos del Fermilab,” poco Meneada quizás porque un titular sobre una “extraña bola” no es tan espectacular como debería, una “nueva partícula fuera del Modelo Estándar” hubiera sido más meneado. […]

22. anonymous - March 19, 2009

The previous comment links to an article on a spanish blog. If I understand it correctly it says the following (very briefly):

“CDF found that new hadron. DZero does not see it, although it should be able to see it, i.e. DZero contradicts the CDF findings.”

In the article there are links to presentations of the CDF analysis, however I can’t find any link to a reference for the DZero point of view.

Does anyone know what is going on?

23. dorigo - March 19, 2009

Anon, I think the person who wrote that piece misunderstood totally my blog post about the CDF multi-muon excess not seen by DZERO. In other words, he thought my post was about the Y(4140), and he got confused and carried away.