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Pentaquark Search Shows How Science Moves Forward

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Pentaquark Search Shows How Science Moves Forward Although initial results were encouraging, physicists searching for an exotic five-quark particle now think it probably doesn’t exist. The debate over the pentaquark search shows how science moves forward. The rise and fall of the PE NTAQUAR K by Kandice Carter, Jefferson Lab 16 Three years ago, research teams around the stars are made. Yet he and his peers had no world announced they had found data hinting at means to verify its existence. the existence of an exotic particle containing Luminaries of 16th-19th century physics, five quarks, more than ever observed in any other including Newton, Fresnel, Stokes, and Maxwell, quark-composite particle. More than two dozen debated at length the properties of their physi- experiments have since taken aim at the particle, cal version of the philosophical concept, which dubbed the pentaquark, and its possible partners, they called ether. in the quest to turn a hint into a discovery. It’s a The ether was a way to explain how light scenario that often plays out in science: an early could travel through empty space. In 1881, theory or observation points to a potentially Albert A. Michelson began to explore the ether important discovery, and experimenters race to concept with experimental tools. But his first corroborate or to refute the idea. experiments, which seemed to rule out the “Research is the process of going up alleys to existence of the ether, were later realized to be see if they are blind,” said Marston Bates, a zool- inconclusive. ogist whose research on mosquitoes led to Six years later, Michelson paired up with an understanding of how yellow fever is spread. Edward W. Morley to devise the now-famous Throughout history, taking different pathways experiment that ruled out the ether by examining has often led to new research tools, far-reaching the speed of light in two different directions. insights and even great scientific discoveries. Known as the Michelson-Morley experiment, it But whether any particular alley leads to a produced the most famous null result in history, discovery or a dead end can only be determined signaling the scientific end of an idea that pre- by experiment. dates science itself. Like many scientific debates today, getting to the conclusion about the ether A historical debate was a back and forth process between theory and The æther is a classic example of an alley that experiment, with no lightning-flash answer deliv- led to nowhere. Greek philosophers introduced ered from a single experiment. the idea of the æther thousands of years ago, arguing about the nature of the universe. Ancient Sanksrit tablets refer to the æther as the fifth Experiments that first hinted at signs of a pentaquark produced graphs like the one shown. The peak seemed like it might be element (after earth, air, fire, and water), and the signal for a pentaquark but determining whether it was a Aristotle proposed it as the substance of which random fluctuation or a real signal required more data collection. symmetry | volume 03 | issue 07 | september 06symmetry | volume 03 issue 07 Predicting a five-quark particle Hint at a discovery Experiments in the 1950s and 1960s were crucial In the spring of 2003, experimenters from the to establish quarks as the fundamental building SPring-8 laboratory in Japan announced the first blocks of matter. Clumping in pairs and triplets, potential pentaquark sighting, finding hints of quarks and antiquarks make up particles such as the particle in experiments in which energetic pions and protons. gamma rays hit carbon-12 atoms, creating quark In 1997, theorists from the Petersburg Nuclear clusters. That same year, researchers from Physics Institute in Russia were investigating Jefferson Lab, the Alikhanov Institute for the implications of quantum chromodynamics, the Theoretical and Experimental Physics (ITEP) in theory that describes the behavior of quarks. the Russian Federation, and the Electron QCD, formulated in the 1970s, is a complicated Stretcher and Accelerator (ELSA) in Germany, theory; performing calculations with it often announced that they, too, might have spotted involves approximate models and techniques. tantalizing hints of the Theta-plus in data previ- Maxim Polyakov, Dmitri Diakonov, and Victor ously taken in other experiments. Petrov were using a specific model that had pre- It is in a hint of a new particle that the chal- viously had some impressive predictive suc- lenges of particle physics become very obvious. cesses to better understand some features of The data recorded in a particle physics experi- quark behavior. ment are rather messy as lots of particles As they studied the model, they discovered emerge from a single high-energy particle colli- that it predicted the existence of a new family of sion. Because of their short-lived nature, most particles, the pentaquarks. Unlike protons, which particles leave no obvious signature in the parti- consist of three quarks, or pions, which contain cle detectors, and experimenters need to rely on a quark and an antiquark, each pentaquark has sophisticated particle reconstruction software four quarks and one antiquark. Although pen- and detailed statistical analyses to determine the taquarks had never been observed, the general properties, or even the existence of a particle. rules of quark theory allow this combination of quarks and antiquarks to exist. Raising doubts The calculations performed by the Petersburg In the case of the pentaquark, physicists not trio predicted a pentaquark, named “Theta-plus,” involved in the initial experiments soon ques- that contains three different flavors of quarks: two tioned the observations. Simultaneously, authors up quarks, two down quarks, and a strange anti- of some of the original papers began refining quark. The presence of the strange antiquark the statistical analyses of their results; careful should make the particle uniquely identifiable in re-evaluations of the data and more sophisti- experimental searches, giving it a property called cated statistical methods led to less confidence strangeness. Armed with this specific informa- in the existence of the pentaquark. tion, several experimenters turned their attention Pentaquark enthusiasm, however, had taken to the finding the Theta-plus and its partners. hold, and many more experimental groups were 2003 2004 2005 Key Experiments searching for the pentaquarks either showed evidence for their existence existence (yellow dots) or found no evidence of pentaquarks (blue dots). After no evidence a series of early suggestive results, an increasing number of experiments seemed to rule out the pentaquarks. Source: Chart created by Sandbox Studio from data in R.A. Schumacher, Particles and Nuclei International Conference (PANIC'05), 2005, nucl-ex/0512042 18 � � � Pentaquark ������� � � � � � �� �� � � ���������� �� ��������� � ������� � � � ������ �� � �� � � � ������ In one type of experiment, gamma rays hit carbon-12 nuclei, ��������������� which leads to the creation of various other particles. Theoretically, these could include a pentaquark, which would quickly decay into other particles. Physicists try to reconstruct this process from the final particles seen in the experiment to determine whether the pentaquark was actually created. Illustration: Sandbox Studio conducting their own searches for the exotic parti- light—does not exist, space is not empty either. cle. While some higher-statistics searches for Quantum theory describes the vacuum less like the pentaquark began reporting null results, others an expanse of emptiness between subatomic seemed to find signs that pentaquarks existed. particles and more like a field transmitting force Meanwhile, theorists were busy trying to and energy, a seething breeding ground of sub- understand the potential particle, performing cal- atomic particles. Although this description of the culations for production mechanisms and proba- vacuum doesn’t fit with Newton’s narrower defi- bilities. Some of those calculations confirmed nition, it does mesh eerily well with the ancient the possible existence of pentaquarks, but not concept of the æther as an unseen, all-pervasive always at the same energies where experiments medium filling space. were seeing hints. And the pentaquark is not dead either. Some To resolve the issue, physicists began to set production channels remain open. “The evidence up dedicated experiments that would either is not conclusive. Is there something that is irrefut- make precise measurements of the pentaquark able? No, not yet,” says Andrew Sandorfi, a scien- or rule out certain production mechanisms. In tist at Brookhaven National Lab who’s reviewed April 2005, a group from Italy’s Istituto Nazionale the literature. di Fisica Nucleare in Genova announced results The pentaquark’s legacy is still unknown. “It from a high-precision measurement at Jefferson certainly stimulated a lot of thinking in the area Lab. Examining the same reaction that pro- of quantum chromodynamics,” says pentaquark duced the positive result at ELSA in 2003, the expert George Trilling, a Lawrence Berkeley new experiment, which collected one hundred National Laboratory researcher who provided times more data and hence produced much bet- the pentaquark summary in the Particle Data ter statistics, found no evidence of the pen- Group’s Review of Particle Physics. taquark. Other higher-statistics results have done Whether positive or null, each experimental likewise. “If it’s there, I don’t see how it could have result brings physicists closer to an understand- escaped the higher statistics in this experiment,” ing of the universe. Ruling out the presence of says Ken Hicks, an Ohio University researcher in possible subatomic particles is just as important the field. as finding new ones. Without the searches, sci- entists would never know if these particles were What is the final answer? out there, waiting to be discovered. As it turns out, the tales of both the ether and Even though the pentaquark seems to be the pentaquark are still incomplete; hidden doors illusory, at least in the form physicists have may lurk at the end of what appeared to be blind pursued so far, the alley leading toward it has alleys.
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