## Why, oh Y(4140) are you so tantalizing?

Note: I’m posting this from the road, so it will be somewhat lacking in pictures and details. Stay tuned for an update that fills all that in!

I’ve spent a lot of time poring over the results coming out of the Hadron Collider Physics conference this week, and I’ve noticed a trend. Higgs candidate cross sections are consistent with the standard model. B meson branching ratios are consistent with the standard model. Multijet event counts are consistent with the standard model. Maybe you can see where this is going. Vector boson production rates are consistent with the standard model. CMS rediscovered an unknown particle. Meson masses are consistent with—

Wait, what?

Yeah, that happened. CMS announced the second observation ever [PDF] of a mysterious new particle which defies classification.

This mystery object, called the Y(4140) or sometimes X(4140), was first seen in 2009 by CDF, one of the experiments at the Tevatron. It wasn’t just a slight fluctuation, either; the CDF data excluded the background-only hypothesis at more than a $$5\sigma$$ level, which is the threshold physics uses to define a proper discovery. Certainly much ado has been made about less strong statistical evidence.

Unlike the much-lauded Higgs boson, however, the Y(4140) was only ever seen once, by one experiment. No matter how many sigmas you’ve got, it counts for nothing if the result can’t be independently verified. Belle tried to do just that in 2010, and LHCb tried again in 2011, and neither of them found the $$5\sigma$$ detection that CDF had reported, or even as much as a little bump in the right place. When the verifications came up empty, the physics community decided that this particle probably didn’t exist — that the bump was just a statistical fluke. But now that CMS has been able to detect what is presumably the same peak, it’s not so clear anymore. Maybe there is something there after all.

Now, the fact that CMS has seen a bump would not be that big a deal by itself. Previously unseen particles are discovered all the time, because there are a ton of ways you can combine two or three out of the six different quark flavors to make a hadron. And each combination of flavors has an infinite set of energy levels, each of which can be seen as a new particle. So as you keep going to higher and higher energies in your particle accelerator, you’ll keep discovering more and more of these hadronic resonances, as we call them.

The properties of these resonances (particles), are determined by the strong interaction. We understand the strong interaction well enough to predict the masses and most other properties of hadronic resonances pretty precisely. So usually, when a new particle shows up, it doesn’t come as a surprise. It’s just going to match an entry in a table that some theorist has published years prior. But that’s not the case with the Y(4140). The standard model does not predict any hadronic resonance with a mass near $$\SI{4140}{GeV}$$. If it turns out to be real, then this could very well be the first harbinger of beyond-the-standard-model physics that the entire community has been hoping to see in the LHC data all along!

But it’s not time to get excited just yet. Remember that four experiments looked for this particle, and two of them found it while the other two found actual nothing. So it’s not even clear that the particle exists. The first and most important step in characterizing the Y(4140) will be answering that question by figuring out which experiments are right, and why the others didn’t reproduce the outcome.

If the particle turns out to be real, the next thing to look into will be establishing whether it’s fundamental, and if not, its composition. This always starts by looking at what the particle can decay into. In this case, we’re lucky, becaus all the decay products show up directly in the detector, so it’s possible to work backwards reconstruct the properties of the intermediate particles that were involved. The CDF and CMS teams have done this and come up with $$\mathrm{Y}(4140)\to J/\psi\phi\to \ulp\ualp\mathrm{K}^-\mathrm{K}^-$$.

The $$J\psi$$ has a quark composition of $$\chq\chaq$$, and the $$\phi$$ of $$\srq\sraq$$, so there is some speculation that Y(4140) is just a bound state of those four quarks, a tetraquark. Right now, most particle physicists don’t think tetraquarks exist, so discovering one would be a major development indeed! But in a way, this would be a bit of a disappointment, because it wouldn’t be a signal of something totally new. Tetraquarks, exotic as they are, are still within the scope of QCD.

If it’s not a tetraquark, then the Y(4140) is probably going to be something totally new, either a brand new fundamental particle (like a superpartner, although that particular identification seems unlikely) or some composite thing that includes a brand new fundamental particle as one of its constituents. This would be the really exciting option! In the latter case, one might wonder, if the Y(4140) is composite, why haven’t we seen any other evidence of the fundamental particle that would be one of its constituents? There’s a precedent for that, though; this new fundamental particle might interact in a way that never allows it to be isolated, just like a quark. The Y(4140) would play a role analogous to the neutral $$\Lambda$$ hyperon, which is the lightest free particle that contains a strange quark. If this turns out to be the case, we might be looking at adding another symmetry group to the $$U(1)_Y\times SU(2)_L\times SU(3)_\text{color}$$ structure of the standard model. If you don’t know about symmetry groups, just mentally replace “$$U(1)_Y\times SU(2)_L\times SU(3)_\text{color}$$” with “cooooool” (because if you do know what symmetry groups are, I don’t need to tell you, you’ll be doing that anyway :-P)

This is definitely something to watch out for over the next few years.