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25 Years of CP Mystery

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25 Years of CP Mystery layers of thin fibres in front of the (TPCs) and their derivatives in to­ Physicists grappling with the chamber. Like the Novosibirsk clini­ day's collider experiments, includ­ problems of the next generation of cal chamber, it is filled with a xe­ ing those for the experiments being proton colliders were reassured by non mixture to convert X-rays. installed at CERN's new LEP collid­ Gail Hanson of Stanford who has Just as Cherenkov light can be er. Despite detector diameters and been looking into new kinds of used to identify particle masses up lengths of several metres, LEP ex­ central tracking devices. While the to a few tens of GeV, so transition periments are citing track preci­ simulated raw data is frightening, radiation is used to identify highly sions of 100 microns. track-finding algorithms quickly energetic particles and to separate The smaller pictorial drift cham­ clean up the ptcture, and tracks pions from electrons up to bers, called 'vertex chambers' be­ soon begin to show up. With phy­ hundreds of GeV. The NA31 neu­ cause they help extrapolate tracks sicists pushing for bigger hadron tral kaon study at CERN (July/Au­ back to the interaction vertex in­ colliders (such as LHC for the LEP gust 1988, page 7) has also just side the collider beampipe, are now tunnel at CERN and the proposed been fitted with a transition radi­ achieving 20 micron resolution. To US Superconducting Supercollider, ation detector (reported by Harry do this they use a 'slow' gas such SSC), this work suggests that the Nelson of CERN) to help reject as dimethylether (DME). Unfortu­ detectors needed to extract the background from the different de­ nately DME attacks many other­ physics from these machines are cays of neutral kaons. wise useful materials, but M. Jilaby no longer a dream. Impressive were the results from of Florida took a close look at DME the giant Time Projection Chambers compatibility. From David Miller 25 years of CP mystery A quarter of a century ago, great AGS milestone has to be handled cists searching for a good weak physics was happening at Brook- more circumspectly. Its message force invariant turned to 'CP' - a haven's Alternating Gradient for the weak nuclear force (beta combined particle/antiparticle and Synchrotron (AGS). February 1964 decay) is no less important, but 25 left/right reversal. saw the discovery of the omega- years down the line, the real phy­ minus particle, confirming the un­ sics behind this subtle effect is still derlying symmetry of the strongly largely uncharted. interacting particles and opening In 1956, Tsung Dao Lee and The strange world of the neutral the door to today's quark picture Chen Ning Yang had suggested kaon of particle constituents. Just a few that the weak nuclear force might months later came another major be very sensitive about the direc­ When physicists say the neutral AGS discovery, eventually earning tion in which things happen. This kaon is a strange particle, as usual the Nobel Prize for James Cronin was soon confirmed by a series of they are being very precise. It car­ and Val Fitch in 1980. The two precision measurements of beta ries an extra quantum number - discoveries make an interesting decay, showing that if the parent strangeness - not found with ordi­ physics counterpoint. nuclei are lined up, the electrons nary nucleons but nevertheless ri­ The omega-minus and its impli­ prefer to come off on one side gidly conserved when the strong cations for the strong nuclear force rather than the other. nuclear force is at work. However can be packaged relatively easily This intrinsic 'handedness' also under the weak nuclear force the for mass consumption (March is­ switches from particle to its anti- neutral kaon behaves strangely by sue, page 1), but the second 1964 particle counterpart, so that physi­ anybody's standards. 4 CERN Courier, April 1989 The electrically neutral kaon and pion pair detector from the old 'forbidden' way. its antiparticle are distinguished Brookhaven Cosmotron and put it This mixing of the neutral kaon only by their strangeness labels - in the neutral beam from the new and its antiparticle by the weak nu­ good for strong interactions, but AGS. In addition, they would run clear force is unique among the which get mixed up by the weak parasitically with an upcoming commonly known particles and force. In the CP framework, the study using muon beams. sets the stage for some enigmatic neutral kaon has two variants, use­ 'If you can show that you don't physics. (In 1987 experiments at ful because in the early 1960s phy­ cost anybody anything, they'll let CERN and at the German DESY sicists knew that the neutral kaon you in,' said Val Fitch. And they Laboratory sa#w the first signs of comes in two kinds - short-lived went in fast. Just two months after analogous behaviour, but not CP ones decaying into a pion pair, and the proposal was written they were violation, with the neutral B me­ others living about a hundred times taking data. The original aim of the sons carrying the beauty quantum longer before decaying into three experiment was to look at the deli­ number.) pions. In a neutral kaon beam, the cate interplay of strong and weak CP violation is surely telling us short-lived kind decay away in neutral kaon interactions ('regener­ something. It can be accommo­ about 10~10 seconds, so that even ation'), but by October, they had dated, but not explained, in a after a few metres only the long- seen something else and, thinking scheme linking the weak decays of lived variety remains. The CP idea they had fallen victim to unrepre­ particles using a total of six kinds seemed to point in the right direc­ sentative statistics, waited for it to of constituent quarks - possibly tion. 'go away'. more, but certainly no less. To un­ To dig deeper into neutral kaon In a sample of 22,700 long-lived cover the mechanism driving the physics, Cronin and Fitch, working kaon decays, and after painstaking­ effect is one of the major chal­ with J.H. Christenson and Rene ly eliminating all spurious back­ lenges facing today's particle phy­ Turlay, wrote in 1963 an exper­ ground effects, they were left with sicists. imental proposal which ran to just 45 examples where only two pions The high kaon fluxes promised a page and a half, in marked con­ emerged (as in the short-lived de­ by proposed new meson 'factories' trast to the detailed descriptions cays). After trying for about six would provide a useful new probe tabled by today's big collabora­ months to find a conventional ex­ of CP violation (see page 16). tions. 'Fortuitously the equipment planation, they finally published of this experiment already exists,' their result - about one neutral they wrote. The idea was to take a kaon in five hundred decayed in a The modest setup of the Brookhaven experi­ ment which 25 years ago discovered the puzzle of CP violation. CERN Courier, April 1989 5 .
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