Particle discovery from 1897 to 2000 can be represented by three pulses, well reproduced by logistic curves. A long shallow wave stretches the whole 20th century, while the other two are much steeper, with midpoints in 1965 and 1983. The second wave is over by 1970, and the third longer one is exhausted by 1998. (The discovery of chemical elements from antiquity to the present day also occurred in waves, related to the technology available for the discovery [1,2]). The construction of particle accelerators, starting in the late '40s, is almost complete by the year 2000, and shows two waves, with midpoints in 1958 and 1975 [2].

Waves 2 and 3 of particle discovery follow accelerator waves 1 and 2 by 7 and 8 years respectively, as new states became accessible with the availability of higher energies and the expression of heavier flavors. After a gap of a few years when no new hadron states were announced, a fresh wave of particle discoveries has started in 2003 with the detection of the Ds*(2317) by BaBar. The 2004 PDG RPP state count increased by 10 with respect to 2002, 9 states were added in the 2006 release, and 15 further candidates were announced in 2006 by various experiments.

Several of the new states do not seem to fit the current classification scheme, and various mechanisms have been proposed to explain their existence: hybrids, molecules, and so on. Most likely a considerable number of yet-undetected and unexpected hadron states are just waiting to be discovered. Actually, dozens of additional new states have been reported, but did not make it to the PDG RPP yet (probably because they were seen only by one group):

• the HyperCP boson at 214.13 MeV/c2,
• 10 distinct low-mass f0 candidates from JINR,
• 10 S=1 baryonic resonances also seen at JINR,
• 17 narrow baryons detected at SPES3 and SPES4, at the SATURNE machine,
• 7 equally spaced Theta+ exotic baryons from JINR, of which only the (1540) is listed by the PDG,
• and more.

The fourth wave of particle discovery is by now in full swing, and a revival of hadron spectroscopy might well be around the corner. The new wave is not associated with new flavors nor really with a new generation of accelerators, but then, WHAT DOES IT CORRESPOND TO? In view of the current plans of BaBar, Belle and FNAL, and considering that the European strategy for particle physics does not emphasize hadron spectroscopy, WHERE WILL THESE NEW PARTICLES BE DISCOVERED?

References:

1. C. Marchetti, Society as a Learning System - Discovery, Invention, and Innovation Cycles Revisited, Technological Forecasting and Social Change, 18, 267-282 (1980).

2. T. Modis, Predictions - Society's Telltale Signature Reveals the Past and Forecasts the Future, Simon and Schuster, New York, 63Ð66 and 244 (1992).