The X(3872) particle -- What is it?

April 15, 2004

Since times immemorial, human beings have sought the fundamental building blocks of matter. At one time, it was thought that atoms were the smallest units in nature. We now know different, and believe that we truly understand the building blocks and how they interact with each other. These building blocks can combine to form observable particles such as the neutron and proton. Recently, the "BELLE" experiment in Japan discovered a new particle they named the X(3872). This particle may represent a new kind of combination of building blocks. As it often happens with new discoveries, only a small number of events were found by BELLE, making it difficult to understand the true nature of X(3872). Fortunately, detectors at Fermilab are also able to study this particle and help determine its nature.

A theory that describes all known matter and forces (except gravity) is called the Standard Model. It proposes that matter is composed of leptons and quarks. For example, the nucleus of an atom consists of protons and neutrons. The protons and neutrons are composed of more fundamental objects called quarks. The electrons that orbit about the nucleus are examples of the fundamental leptons.

There appear to be only six different types of quarks called up, down, charm, strange, top and bottom. Each of these quarks has a partner antiquark. These quarks and antiquarks combine in different ways to make the particles we can detect. For example, a charm quark and an anticharm quark can make a particle called a J/psi. A meson is a generic term for any particle made from a quark-antiquark pair. Particles can also be made up of three quarks, e.g., a proton is made up of two up quarks and one down quark - the generic term for particles containing three quarks is "baryon".

Because many of the particles we study disintegrate (or decay) very quickly (a particle that lives as long as a trillionth of a second is considered to be long-lived!), we look for new particles by studying remnants of their disintegration. For instance, when the atom was split apart, it was discovered that it was made up of a nucleus (containing protons and neutrons) and electrons. When a particle decays, we can determine the mass of the original particle by measuring the energy and momentum of its decay products. The rest mass (m) of a particle is related to its energy (E) and its momentum (p) by (mc2)2=E2-p2c2, where c is the speed of light. By plotting this quantity for many different particle combinations, we can look for 'mass peaks' in the data.

Below is a plot showing the mass spectrum collected from individual proton-antiproton collisions. In many of these collisions, pairs of muons are created. A muon is another example of a lepton, one can think of it as a "heavy" electron. For each muon pair, the mass of the parent particle can be calculated. Significant peaks are clearly seen in the data. These peaks indicate the presence of known particles that decay to two muons.

The particle of lowest mass in the plot is the omega meson, which consists of a combination of up, antiup, down and antidown quarks. The phi (consisting of strange-antistrange quarks), J/psi and psi'(charm-anticharm quark pair), and the Upsilon(1S,2S,3S) (bottom-antibottom pair) can also be observed.

Knowing the masses of the quarks and how they interact, we can formulate a model to predict the masses of the observed particles. The X(3872) particle does not fit into such conventional models, and we therefore would like to learn more about its properties.

Physicists working at the Tevatron collider at Fermilab (near Chicago), have observed the X(3872) particle using the D detector. The decay of the X(3872) is more complicated than simply into two muons, but the idea is the same as in the above plot. In this case we look for the decay of the X(3872) into a J/psi and two pions (a pion consists of an up and anti-down quark). The J/psi then decays into two muons, and so we search for the X(3872) by combining two muons and two pions together. By measuring the momentum and energy of these four particles, we can determine the rest mass of the X(3872). To make the peaks as clear as possible, we calculate the mass of the two muons and two pions M(muon muon pion pion) and then subtract off the mass of only the two muons M(mu mu).

In the mass spectrum on the left we see two clear peaks. The lower one corresponds to the particle psi(2S) or psi', and the peak on the right is the X(3872) particle.

We are in the process of measuring as many properties of this particle as possible, to determine if this new X(3872) particle is another charm-anticharm state similar to the J/psi and psi' or something more exotic.

Some theories have predicted that the X(3872) is a new type of particle called a meson-molecule. The mass of the X(3872) has almost the same mass as adding up two other mesons called the D and D*, and it is thought that X(3872) may be a molecule comprised of a D and an anti-D*. If this were the case, it would be the first evidence of this type of matter.

Our first measurements of the properties of the X(3872) show no significant differences between the X(3872) particle and the psi(2S). We are continuing our study and soon hope to shed light on its true nature.

For further information contact Dr. Brad Abbott, University of Oklahoma, email:


 The X(3872), a quark molecule?
A new subatomic particle called the X(3872) is under study by the D experiment at Fermi National Accelerator Laboratory, Chicago Illinois. The excitement surrounding this study can be attributed to the difficulty placing the X(3872) into conventional models of subatomic particles.

One leading possibility suggests that the X(3872) may be a composite of already well known fundamental particles. In the composite model two particles (each itself an assembly of quarks) are thought to bind together much like two atoms might combine to make a molecule. Although composite states have been predicted, no experimental verification exits. By measuring properties of the X(3872), physicists hope to unravel its mysteries.

In a paper recently accepted by Physical Review Letters, the D experiment has compared a number of properties of the X(3872) to a well-known particle containing a charm quark and charm anti-quark. Within the uncertainties of the measurements, all of the properties of the X(3872) measured by the D experiment behave similarly to this standard form of matter. The debate continues on where the X(3872) fits in particles models and whether it is actually a new type of particle. The D experiment is working toward a definitive answer to this question.