Charmed Pentaquark at HERA?

The H1 experiment reports the observation of a new particle made up of five quarks – the ZEUS experiment, however, cannot confirm the observation.

Figure: Possible creation of charmed pentaquarks with a mass of around 3100 MeV/c2 at H1
The data of the H1 experiment (dots) show a clear peak (“resonance”) at a mass of the particle combinations D*- and proton, and D*+ and antiproton of around 3100 MeV/c2, which gives evidence for the creation of a charmed pentaquark. The quark composition of these particles indicates that this was a pentaquark with two up quarks, two down quarks and a charm antiquark (resp. the corresponding antiparticle, a combination of two up antiquarks, two down antiquarks and a charm quark). The solid line describes the signal including the background, whereas the broken line indicates the background only. The bars show the errors of the measurements. No resonance is to be found in this region in the data of the ZEUS experiment. (1 GeV = 1000 MeV)

One of the experiments studying electron-proton collisions at DESY’s HERA accelerator may have discovered a new particle: As the international team of scientists working at the H1 experiment reports, the H1 data contain clear evidence of a particle made up of five quarks – a so-called “pentaquark”. The signal recorded by H1 points to a pentaquark about three times as heavy as the proton and containing four light quarks and a charm antiquark – such a particle has never been observed before. There is a catch, however: The HERA experiment ZEUS cannot confirm the discovery. In their own data, the ZEUS physicists see no evidence for such a particle. The two HERA experiments have thus given the start-signal for an intense search for the charmed pentaquark. “Should the results of the H1 experiment be confirmed, this would be an important observation,” says Professor Robert Klanner, research director of DESY. “We’re thus eagerly awaiting further results from HERA and other experiments. These will help to either confirm or refute the H1 observation.”

For decades, particles made up of quarks were thought to contain either three quarks (“baryons” such as the proton and the neutron) or a quark and an antiquark (“mesons”). Although theory predicts more exotic combinations of quarks and antiquarks, such particles were not observed – until the year 2003, when several research teams succeeded in detecting a first pentaquark, among them the HERMES and ZEUS experiments at DESY. This raised the question of whether further pentaquarks exist which contain heavier types of quarks, such as the charm quark. Since the electron-proton collisions in the HERA storage ring at DESY produce lots of particles with charm quarks, the HERA experiments H1 and ZEUS quickly took up the search for the charmed pentaquark. The goal is not just to add another member to the “zoo” of particles. Thanks to their properties, the pentaquarks can yield information about important aspects of the strong force – one of the fundamental forces of nature, which binds the quarks together inside protons and neutrons and holds these together inside the atomic nucleus.

What is remarkable is that the evidence for a charmed pentaquark in the H1 data is particularly convincing (see figure): The signal stands out clearly above the remaining “background”. It is extremely improbable that such a pronounced “resonance”, as the characteristic particle signature is called, is generated by background fluctuations. The H1 data point to a particle containing two up quarks, two down quarks and a charm antiquark (and the corresponding antiparticle, a combination of two up antiquarks, two down antiquarks and a charm quark).

It is all the more astonishing that the scientists at the ZEUS experiment could find no such resonance in their data. Both experiments are now carrying out further analyses and very detailed comparisons of their data in order to confirm or refute the observation. With the publication of their results, the H1 physicists are also putting these up for discussion in the worldwide scientific community. The solution of the puzzle, however, will not be too long in coming: Even if no other experiment succeeds in finding the charmed pentaquark in the near future, the data the HERA experiments will gather in the next one or two years will provide clarity in any case. It will then be clear if the observed signal is really due to a new particle or if it is just a statistical “runaway” effect.

The 6.3-kilometer-long “Hadron-Electron Ring Accelerator” facility HERA at DESY is the worldwide first and only storage ring in which two different types of matter particles collide: protons and electrons (or their antiparticles, the positrons). There are four experiments at the HERA storage ring. Whereas H1 and ZEUS study the high-energy collisions of electrons and protons, the experiments HERMES and HERA-B use just one of the storage ring’s particle beams.