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What is the Y(4140)? The plot thickens April 6, 2009

Posted by dorigo in news, physics, science.
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I read with interest -but it would probably be more honest to say I browsed, since I could understand less than 50%- a preprint released three days ago on “The hidden charm decay of Y(4140) by the rescattering mechanism“, by Xiang Liu, from Peking University (now at Coimbra, PT). The Y particle has been recently discovered by CDF.

The existence of the several new resonances of masses above 3 GeV recently unearthed by B factories and by the CDF experiment poses a challenge to our interpretation of these states as simple quark-antiquark bound states, because of their properties -in particular, their decay pattern and their natural widths.

Already with the first “exotic” meson discovered a few years ago (and recently measured with great precision by CDF), the X(3872), the puzzle was evident: at a mass almost coincident with twice the mass of conventional charmed mesons (states which are labeled “D”, which are composed of two quarks: a charm and a up or down quark, like D^+ = c \bar d or D^\circ = c \bar u), the X was immediately suggested to be a molecular state of two D particles. I wrote an account of the studies of the nature of the X particle a few years ago if you are interested -but mind you, the advancements in this research field are quick, and I believe the material I wrote back then is a bit aged by now.

The paper by Liu tries to determine whether the interpretation of the Y particle as a pure second radial excitation of P-wave charmonium (\chi_{cJ}'', with J=0,1) holds water once the observed branching ratio of the Y into the final state seen by CDF (Y \to J/\psi \phi), and the measured decay width, are compared to a theoretical calculation.

The nice thing about the decay of the Y into the observed final state is that it occurs only through a so-called “rescattering” mechanism, by means of the diagrams shown in the graph below (the ones shown refer to the J=0 hypothesis of the \chi_{cJ}'', but similar diagrams are discussed for the J=1 state in the paper).

As you can see, the Y produces the two final state particles by means of a triangle loop of D mesons. These diagrams usually describe rare processes, and in fact Liu’s calculations end up finding a small branching fraction. I am unable to delve into the details of the computation, so I will just state the result: the typical values of the branching ratio B(Y \to J/\psi \phi) depend on a parameter, \alpha which, if taken in a “reasonable” range of values, provides estimates in the ballpark of a few 10^{-4}. This appears inconsistent with the observation provided by the CDF experiment.

Clearly, work is in progress here, so I would abstain from concluding anything definite on the matter. So, for now, let us call this an indication that the simple interpretation of the Y as a excited charmonium state is problematic.

Three exquisite exclusive charmonium signals December 29, 2008

Posted by dorigo in news, physics, science.
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It is my pleasure today to report on an extremely interesting measurement recently produced by the CDF experiment, one which is dear to me for several reasons.

First of all, the measurement involves the study of charmonium -a fascinating system made by a charm-anticharm quark pair, the two quarks orbiting around each other for a tiny fraction of a second before annihilating. Charmonium -in the form of the J/\psi particle- is the state of matter discovered in November 1974 by Nobel Laureates Samuel Ting (at Brookhaven) and Burton Richter (at SLAC), and which immediately confirmed the reality of quarks -entities whose reality had kept many brilliant minds skeptical during the previous decade. The J/\psi and the other charmonium states have been since then a gold mine for the study of Quantum Chromodynamics, of B hadron phenomenology (through the easy-to-spot decays of the type B \to J/\psi X), and an invaluable calibration tool for particle trackers. Charmonium may one day be even used with profit to measure with high precision the top-quark mass at the LHC!

Second, the study focuses on what is called “exclusive production”, a spectacular signature at hadron colliders: it involves the production of the particle without the disintegration of the projectiles, through a colourless exchange. Basically, the charmonium meson appears out of nothing, and in the company of exactly nothing else. It is as if by a stroke of a magic wand, a spark is exchanged between a proton and an antiproton coming close together, and a single particle is created. Those of you who know what kind of a mess is routinely yielded by proton-antiproton collisions at 2 TeV of energy in the core of CDF and D0 cannot but join my awe for the very concept of clean single-body production.

Third, exclusive production processes are quite rare in hadron collisions, and they are extremely interesting because they may, one day, provide the means by which we can access a direct measurement of the quantum numbers of the Higgs boson at the LHC. Also, their theoretical calculation is not on very solid ground, so a measurement is of great help. This one by CDF is the first ever on exclusive charmonium in hadron-hadron collisions.

Some notes on the production mechanisms

Before I come to describe the measurement details, let me present you with two Feynman diagrams of the simplest reactions responsible for the exclusive creation of a charmonium system. As I said above, the exchange between proton and antiproton is colourless: this, while still being a QCD interaction! But how is it possible for quantum chromodynamics to mediate a colourless reaction ? We know that the carrier of the strong force is the gluon, a bi-coloured object. On the other hand, if a proton emits or absorbs a gluon, it will never keep intact: it will become coloured, and explode like a firework. Unless…

Unless a second gluon immediately comes to the rescue, restoring the neutrality of the proton! This is what is needed for a strong interaction to produce a charmonium state: two gluons are emitted and absorbed by the proton and the antiproton, and the pair can thus leave the interaction point, slightly rattled but unharmed. The first pair of gluons meets halfways, creating a charm-quark loop. The loop then “closes” into a \chi_c meson, a charmonium state with the right quantum numbers. This is the process shown on the right.

Notice that a two-gluon exchange cannot give rise to a J/\psi meson. This is because of the invariance of strong interactions for the operation called “charge conjugation”. A discussion of the invariance properties of particle reactions under the operation of exchanging every particle with its antiparticle would take us too far, and is the proper subject of another post. I will just say here that the reaction is impossible: in quantum mechanics, one says that the “transition amplitude vanishes” for a two-gluon state converting into a vector particle.

Not so for the \chi_c though, since that particle has zero angular momentum: it is not a vector, but a pseudoscalar meson. That is why we have examined \chi_c production first. But exclusive J/\psi production does occur anyway.

How, however, if we are to keep the two incoming baryons intact? Let us think at the problem from the point of view of the J/\psi then. This particle is known to be quite slow to decay into hadrons. In fact, it has a lifetime much longer than similar hadronic states. So much so, in fact, that the very suppressed electromagnetic decay into electron or muon pairs gets a sizable share of the probability. An electromagnetic process competing with a strong process ? This is very weird: we know that the electromagnetic decay must have a rate proportional to the coupling constant \alpha, which is fifty times smaller than the strong coupling constant \alpha_s for such low-energy phenomena. However, how is quantum chromodynamics going to disintegrate a J/\psi ?

Phase one is simple: the charm and the anticharm get together in the same point of space, annihilating. Phase two is the emission of gluons: not a single one though, because the J/\psi is colourless! And not just two, because of the quantum mechanical selection rule concerning C-parity we mentioned above! The minimum number of gluons is three. That means that we have to take the third power of \alpha_s when computing the decay rate through QCD interactions, and so electromagnetic processes become competitive.

In reality, these three-gluon exchanges are even more suppressed than they ought to be. This is encoded in what is called “OZI rule”, by the name of Okubo, Zweig, and Iizuka: at any order of the strong coupling constant \alpha_s, “disconnected” strong interaction processes -ones that have no fermion lines propagating from the initial to the final state, but only gluons- occur with smaller frequency. This is a consequence of the variability of \alpha_s with the energy of the gluon emission: in disconnected diagrams gluons have to carry an energy at least equal to the mass of the particles they are going to materialize in the final state, and the corresponding value of the coupling constant is therefore evaluated at a higher energy, and is thus much smaller than the one involved in the emission of softer gluons in non-disconnected diagrams. Confused ? Not a problem, forget the argument and let us return to J/\psi production.

The above discussion just meant to show that we need three gluons to produce one J/\psi, since we now know that one such particle can decay to no less than three gluons. Now, let us take the protons -the projectiles producing the reaction- in the equation. Who is going to provide two gluons, and who is going to provide only one ? I can picture the proton and antiproton starting a bitter argument on this issue. The one who provides a single gluon is a dead baryon, since it will get its colourlessness unbalanced, and it will disgregate. This is bad, because the production process is not exclusive any longer if, together with a J/\psi meson, we get additional hadrons.

How then can we produce a vector charmonium particle in an exclusive fashion ? Electrodynamics comes to the rescue. One of the two projectiles yields a pair of gluons, and the other emits a photon. Both can remain intact this way, and angular momentum does not object. The process is shown on the right: as you see, the charm-anticharm pair becomes a colourless one after the second gluon is exchanged, and a J/\psi (or a \psi(2S) meson, which has the same properties except angular momentum) arises.

The CDF search

Now that we know how proton-antiproton collisions are expected to yield exclusive charmonium, we may look for it more meaningfully.

The search uses an integrated luminosity of 1.5 inverse femtobarns of proton-antiproton collisions, recorded by the CDF experiment during Run II. The trigger used to collect exclusive production candidates selects data with two muon tracks of opposite charge, with no energy recorded in the BSC1 system, a set of scintillation counters covering the rapidity intervals 5.4 < |\eta| < 5.9. Such a requirement excludes data which may be similar kinematically to real exclusive production, but feature a few hadrons produced almost parallel to the proton-antiproton direction.

Offline, events are excluded if they contain any other detected particle but the two muons. However, the search for \chi_c allows one electromagnetic energy cluster in the calorimeter, signalling the potential photon emitted along with two muons in the \chi_c \to \mu^+ \mu^- \gamma decay.

After those cuts, the only potential background to address is cosmic-ray muons that cross the detector, leaving a long track that may be fit as a pair of opposite-charge muons if it crosses the detector in proximity of the beam. These events are rejected by requiring that the two muons have an azimuthal opening angle smaller than 3.0 radians, and rejecting ones that have timing information incompatible with the instant when the proton and antiproton bunches cross in the detector.

The invariant mass of the 402 events surviving the selection is histogrammed in the graph below, if it is in the 3.0 – 4.0 GeV range (ignore the inset, which shows a fit to the background). A clear peak of J/\psi and \psi(2S) candidates is apparent on a flat background of non-resonant QED dimuon pairs; the set of events in the J/\psi peak which contain one additional photon candidate make the \chi_c signal (65 events).

The calculation of efficiencies is complicated by the fact that the exclusivity requirements remove bunch crossings which produced more than a single proton-antiproton interaction; this is a function of the number of particles in the bunches. After all is accounted properly, CDF can however determine the cross-section for charmonium production in exclusive processes with a good accuracy. The results are as follows:

  • \frac{d \sigma}{dy} (y=0) = 3.92 \pm 0.62 nb for J/\psi
  • \frac {d \sigma}{dy} (y=0) = 0.54 \pm 0.15 nb for \psi(2S)
  • \frac {d \sigma}{dy} (y=0) = 75 \pm 14 nb for \chi_c

The above results are in agreement with theoretical calculations, which have very large uncertainties. This measurement was long overdue: it is a very important input for theoretical calculations of exclusive processes.

The kinematical characteristics of the resulting J/\psi and \psi(2S) decay signals are studied by comparing data with a Monte Carlo simulation of photoproduction, finding good agreement. Below are shown the transverse momentum distributions of the resonances in the data (red points), compared to simulation (black histograms).

In conclusion, I am happy to see these very intriguing signals appear in CDF data, and I congratulate with the main authors of the analysis, Long Zhang, Mike Albrow, and James Pinfold! You can find more information on the analysis in the public web page of the result.

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.