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New Names for Old Mesons

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The CERNINaples!PalaiseaulStrasbourg! ETH Zurich experiment at CERN studies muon pairs produced by high intensity pion beams. copper recycled from the main ring magnets of the Argonne Zero Gradient Synchrotron. During the first running period of the Teva­ tron, data was taken using an 80 GeV pion beam derived from 400 GeV protons, and a 255 GeV beam from 800 GeV protons. Three effects are seen with these 'quarklike' pions, in the an­ gular distribution of the muon pairs, in their average transverse momentum, and in the pion struc­ ture function itself. These effects tie in with QCD calculations (by E. Berger and S. Brodksy) which take into account the fact that the anni­ hilating quark and antiquark are contained in a nucleon and a pion. This is encouraging, as not only is the internal structure of the pion unfolding, but the first steps are being taken to understand it. sea quark dressing of the pions However the measured quark New names for old means that the predicted and ob­ structure of the pion shows that a mesons served muon pair spectra become quark in a pion can carry more of compatible, dispensing with the the total momentum than can a need for a K factor. quark in a proton. The Particle Data Group collates However for heavier muon pairs, The aim of the new Chicago/Fer- information on particle properties above the upsilon region, a K vari­ milab/lowa/Princeton study at the from experiments carried out at able, depending on kinematics, Fermilab Tevatron is to look spe­ Laboratories all over the world rather than just a numerical con­ cifically at the forward production and brings out regular editions of stant, is required to bring the theo­ of muon pairs. According to the its 'Review of Particle Properties' - retical and observed distributions measured quark structure of the the particle physicists' handbook. into line. The quark structure of pion, under these conditions the Twenty years ago, the Group in­ the pion is also seen to evolve, the pion antiquark takes up most of troduced a new convention for changes being larger than any­ the total pion momentum. Thus a naming baryons (half-integer spin thing which can be attributed to laboratory particle, at least kine- particles), which has gone on to uncertainties in the input quan­ matically, becomes a close ap­ become standard. This contrasts tities. Thus the results from run­ proximation to a quark (of course with the mesons (integer spin par­ ning with 140 and 300 GeV pion the quantum numbers of a pion ticles), which use an alphabet beams are eagerly awaited. and a quark are different). soup of largely uninformative Many of the problems of quark In this experiment, a 'selection' names, some particles with related calculations stem from the intrin­ magnet upstream of the large quantum numbers having very dif­ sic difficulty that the quarks are aperture magnetic spectrometer ferent names, while other dispar­ not free, but are bound tightly in­ focuses heavy muon pairs. This ate particles have inherited side nucleons or other particles. magnet was built from steel and confusingly similar labels. 390 CERN Courier, November 1985 People and things It is easy to invent logical and lift, S*(975) becoming f;(975). On people consistent naming schemes, how­ For mesons containing strange ever it is unpleasant and confusing and other heavy quarks, the heav­ Ian Butterworth, presently Re­ to have to learn lots of new ier of the two quarks provides the search Director at CERN, has been names. Thus having considered label - K for a strange quark, D for appointed Principal of Queen Mary schemes of various degrees of a charmed quark, B for beauty and College, London, from 1 August radicalism (and having exposed T for top. A letter subscript is 1986. Dart of the physics community to added for the lighter quark, unless Gustav-Ado/f Voss, leading figure :hem) the Particle Data Group now it is 'up' or 'down'. Thus the F on the world particle accelerator pnds up with a set of new names. (charm/strange) meson becomes scene and Vice Chairman of the The exception is the celebrated Ds. Another subscript gives the DESY Directorate since 1973, has J/psi, which has such a monstrous spin (again omitted for pseudosca­ been awarded the German Federal name that nearly everybody wants lar and vector mesons). Finally a Service Cross (Bundesverdienst- to keep it! superscript asterisk is added for kreuz) 1st Class. The award comes Under the new meson scheme, states with 'normal' spin-parity as­ in recognition of his work in devel­ the quantum numbers (spin, par­ signments from a quark model oping and constructing storage ity, isospin, etc.) and the quark picture (zero plus, one minus, two rings, in particular the content define the names. The plus, etc.). Thus the names K, K*, PETRA electron-positron collider at spin is indicated by subscript, ex­ D, D* and B do not change, but K* DESY, and for his contributions to cept for the spin zero, negative (1430) becomes K*2 (1430), L accelerator physics and technol­ parity (pseudoscalar) and spin (1770) becomes K2 (1770), etc. ogy. At present Gus Voss is re­ one, negative parity (vector) parti­ The new scheme admittedly can sponsible for the civil engineering cles, where the subscript is omit­ lead to cumbersome notations, but and for the 30 GeV electron stor­ ted. not for states that are likely to be age ring of the new HERA proton- common. To facilitate the transi­ For mesons which are not electron collider now under con­ tion, the Particle Data Group will bound states of. quarks and anti- struction at DESY. He is also a use both the old and the new me­ quarks (such as 'glueballs'), the member of the Working Group on son labelling schemes for a few quantum numbers (when non-ex­ the scientific and technological otic) determine their names, just editions of the Review of Particle as for quark-antiquark bound Properties. states. This seems appropriate since such states will be difficult to From Matts Roos distinguish from quark-antiquark ones and will likely mix with them. A difficulty arises with related isospin singlet particles (eta and eta prime, omega and phi, f and f prime). For the lightest such states, the existing conventions will be followed. Primes are being reserved for use in these cases, so the old habit of using primes to denote radial excitations has been dropped. Many familiar names stay - pi, eta, rho, omega, eta prime, phi, J/ psi, chi, upsilon, etc. Some names undergo minor changes: A2(1320) for example, becoming a (1320), CERN Research Director Ian Butterworth, 2 soon to become Principal of London's while others get a complete face­ Queen Mary College. CERN Courier, November 1985 391 .
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