13.07.2012

Particle Physicists Study Mystery of Hadronization

HERMES experiment sheds light on hadron formation in nuclei

All ordinary matter, including ourselves, is built from only three particles: protons, neutrons, and electrons. In atoms, protons and neutrons assemble into the nucleus, which is surrounded by a cloud of electrons. In contrast to electrons, which are elementary particles and cannot be split into smaller particles, protons and neutrons are composite particles of elementary quarks. Individual quarks, however, are never observed alone and are doomed to be confined to composite particles, so-called hadrons. A recent study by the international HERMES collaboration has shed light on the process of hadron formation from quarks. The work has been published in The European Physical Journal and is featured in the latest DESY Particle Physics Annual Report.

Illustration of hadronization in nuclei (Picture: Joshua Rubin).

Using data from DESY’s particle accelerator HERA, the scientists analysed the production of hadrons when bombarding various noble gas targets with HERA’s powerful electron (or positron) beam. The experimental findings refine existing models of hadron formation, or hadronization. Since it is believed that quarks freely existed shortly after the Big Bang, understanding hadronization will also improve our view of the early stages of the universe.

Interactions between quarks and the forces “gluing” them together inside hadrons are described by a theory called quantum chromodynamics (QCD). However, the formation of hadrons from quarks is not understood in detail. “At present, it is impossible to calculate hadronization from first principle QCD,” says HERMES scientist Gunar Schnell, a particle physicist at DESY and the University of the Basque Country in Bilbao, Spain. “None of the existing models are able to describe all aspects of hadronization.” New results from the HERMES experiment promise to improve these models.

The researchers have now examined the production of six different types of hadrons in the HERMES experiment. Operating from 1995 to 2007, HERMES was one of four experiments at DESY’s largest particle accelerator HERA and produced such a wealth of data that its analysis is still an ongoing process today. At HERMES, scientists inserted targets of the noble gases neon, krypton, and xenon into HERA’s beam of electrons (or their anti-particles positrons). The electrons, traveling at almost the speed of light, penetrate the targets’ nuclei, where they interact with the quarks inside the nuclei’s protons and neutrons. “If I kick out a quark [in this process], a hadron must form,” says Schnell, “The quark itself is not observed.”

The nuclei of the noble gas targets are so large that the hadrons form (partially) inside the nuclei. “Previously, most hadronization studies were based on electron-positron collisions in the vacuum,” Schnell says. “The situation inside the nucleus is quite different. It is comparable to flying through an asteroid belt. On its way out of the nucleus, the forming hadron can interact with other particles.”

The nuclear environment enabled the HERMES scientists to study the length and time dependence of hadronization. An example is the effect of hadron attenuation. As the target nucleus gets larger, the interactions of the forming hadron inside the nucleus become stronger. A heavier nucleus makes it harder for the hadron to emerge, mitigating its production.

For the production of protons, however, the HERMES researchers observed a completely different behavior. When the energy deposited inside the nucleus is large, the number of slow protons emerging from the target increases with the target size instead of being attenuated. “If enough energy is transferred, hadron formation is not the only process. An excess in energy can eject an additional proton that had already been present in the nucleus,” Schnell says. “Whereas previous studies only hinted at this increase in the number of slow protons, the present results clearly confirm an excess of such protons in contrast to fast ones, which are attenuated in a similar way as the other hadrons examined.”

The researchers will use their new results to refine existing models of hadron formation. They particularly hope to gain new insights into the time and length scales of hadronization processes. In addition, the HERMES results will benefit research at the Large Hadron Collider (LHC, Switzerland), the world’s most powerful particle accelerator. “In the ALICE experiment at LHC, hadronization will be observed at about one thousand times larger energies,” Schnell points out. “It will be interesting to see if our low-energy models are valid at much larger energies.”

The implications of the new results may even go beyond earthly experiments. Within the first millionth of a second after the Big Bang, our universe was so hot that free quarks could exist. Cooling and expansion of the early universe led to an assembly of quarks into hadrons. Thus, studies of hadronization may take us way back in time.

More information: DESY Particle Physics 2011 Report