6. SYMMETRY TESTS
Objects in our everyday macroscopic world are often said to possess symmetry. We say this when there are operations on an object that could alter its appearance, but in fact do not; an example of this is the rotation of a sphere through an angle about its center. Symmetry concepts can be applied not only to the physical processes that govern our local, large scale world, including such things as plant growth, the composition of rocks, and the flight of space ships, but also to the microscopic world.
To fully understand nature’s principles, it is believed that studying the relationship between symmetry and the physical laws is best done at the most fundamental level, the level of quarks and leptons. Over the last few decades, the field of elementary particle physics has made tremendous progress in understanding the nature of fundamental interactions and fundamental structure. New discoveries have propelled us toward a substantial strengthening of the Standard Model of particle physics, while advanced technologies have enhanced our ability to investigate the compelling questions of the origins of mass, matter and antimatter in the universe, and the basic nature of symmetry. Symmetry arguments have a long and extensive history, including the history of many of the basic principles of physical laws. And, with the introduction of successful quantum field theories and more mathematically rigorous techniques over the decades, symmetry principles have been used as tools to help define and extend natural laws. Also, experimental data has been studied for the accuracy of compliance with symmetry principles; and when symmetry is violated, to what degree and by what mechanisms.
The experiments that operated during the Tevatron fixed target era extended the investigation of the relatively old symmetry in nature known as CP, including the search and study of extremely rare decay processes. The C in this mathematical formulation represents the independent symmetry that describes the interchange of matter and antimatter. The P in this combined operation represents the independent symmetry of parity, an operation that changes left to right (like looking in a mirror). The combined operation, CP, was thought to be invariant in particle interactions, i.e., the interactions would be symmetrical in CP terms. However, in some very rare cases, this symmetry is violated. The masses and interactions of particles are nearly identical to those of their corresponding antiparticles, but there is a small difference in this CP symmetry of nature, only observed so far in the decays of K mesons. Since matter and antimatter mutually annihilate, and (at creation) our universe is postulated to include equal amounts of matter and antimatter, this very small CP difference may contribute a crucial bit of information that helps explain the abundance of matter over antimatter in the universe today. Although an ongoing program, differences in the accuracy of the experimental measurements of these processes world wide has provided another motivation for new studies. A number of experiments presented here had a goal, not of just observing this well established symmetry violation, but to measure the parameters of these very rare processes to high precision. The results could improve our understanding of these phenomena in the context of the Standard Model, which so successfully describes many aspects of elementary particle interactions.
These experiments also provided some of the most versatile detector instruments built during this era, making it possible also to study hyperon physics, to make particle lifetime measurements, and to search for supersymmetric particles. Forethought in the design of the beams, particularly in the kaon experiments, also provided for the creation of exceptional, secondary, neutral hyperon beams that allowed dedicated studies of hyperons produced at high energies. The scope of these studies also includes the search for violation of well established symmetries such as CPT, where one includes the operation of time reversal invariance T and searches for rare, short lived new particles. The results gained from the successful study of CP violation in the decay of the neutral kaon, does not answer one intriguing question; why is this symmetry violation seen only in the kaon system? Many non-kaon experiments around the world are currently underway to search for this symmetry violation in other systems. Experiments are underway on CP violation in hyperon decay at Fermilab and on B meson decay here and at other laboratories – with high priority.
The Fermilab fixed target symmetry experiments produced a number of high precision, rare decay measurements, including first measurements of some decay modes. One such measurement observed the first CP violating effect in an angular distribution variable, and included demonstrated CP- and T-odd effects in the asymmetry. From the successful early measurements of CP violating symmetries to unprecedented precision, to the latest fixed target experiments and the results of further analysis to follow, these experiments are well on their way to completing their goals.
6.2 E621 -
MEASUERMENT OF CP VIOLATION PARAMETER h + - 0
Michigan, Minnesota, Rutgers
This experiment measured the CP violation parameter h+ - 0 by measuring the interference between Ks0 and KL0 decays to p+ p- p0 near a kaon production target. The experiment was performed in the “neutral hyperon beam” previously used for E619. The measured interference is dependent on both the proper lifetime of the kaon and the exact acceptance versus decay distance of the spectrometer. By using two different target distances from the spectrometer, the acceptance systematics were minimized and the lifetime of the Ks was measured to 0.5% accuracy. The lifetime measurement allowed a stringent test of any possible momentum dependence of the kaon lifetime as had been predicted for a “fifth force”.
E621 Degree Recipients
Peter Border Ph.D. University of Michigan
Nancy Lee Grossman Ph.D. University of Minnesota
Yu Zou Ph.D. Rutgers University
E621 Degree Recipients
New
Measurement of the amplitude of the CP conserving decay
., Y. Zou, et al., Phys. Lett. B369, 362 (1996).
Test of CPT Symmetry Conservation in K0 Decays., G.B. Thomson, et al., Phys. Rev. D51, 1412 (1995).
Search for CP Violation in the Decay 
., Y. Zou et al., Phys. Lett. B329, 519 (1994).
Measurement
of the amplitude of the CP conserving decay 
., G.B. Thomson, et
al., Phys. Lett. B337, 411
(1994).
Measurement
of the Lifetime of 
Mesons in the
Momentum Range 100 to 350 GeV/c., S. Tiege, et al., Phys. Rev. Lett., 63,
2717 (1989).
Measurement
of the Lifetime of 
Mesons in the
Momentum Range 100 to 350 GeV/c., N.K. Grossman, et al., Phys. Rev. Lett., 59,
18 (1987).
New
Measurement of the amplitude of the CP conserving decay 
., Y. Zou, et al., Phys. Lett. B369, 362 (1996).
Search for
CP Violation in the Decay 
., Y. Zou, et al., Phys. Lett. B329, 519 (1994).
Measurement
of the amplitude of the CP conserving decay 
,
., G.B. Thomson, et
al., Phys. Lett. B337, 411 (1994).
Measurement
of the Lifetime of 
Mesons in the
Momentum Range 100 to 350 GeV/c., S. Tiege, et al., Phys. Rev. Lett., 63, 2717 (1989).
Measurement
of the Lifetime of 
Mesons in the
Momentum Range 100 to 350 GeV/c., N.K. Grossman, et al., Phys. Rev. Lett., 59, 18 (1987).
6.3
E731 - A Precision Measurement of the CP
Violation Parameter (e'/e) in the Neutral Kaon System
Chicago, Elmhurst, Fermilab, Princeton,
Saclay (France)
The E731 proposal was to build a new beam and detector for the sole purpose of measuring e'/e. Indeed no other physics was mentioned in the proposal, which had only seven authors. The 1983 proposal goal was a measurement to a precision of 0.001; the final result was (7.4 ± 5.9) x 10-4, not far enough from zero to claim an effect but enough, together with the CERN result at the time, to launch a new generation of measurements: see the write-up on E832 for the latest results and a fuller description of the motivation behind this measurement. This E731 result remained the highest precision result until quite recently.
The experiment took advantage of the higher energy kaons that could be made with the Tevatron, and of the much superior duty cycle of the machine. In addition, the detector designed solely for the purpose of clean pp reconstruction could make unexpectedly significant measurements in other channels, particularly multi-body modes including gammas and/or electrons. The facility of measuring other channels was due to the high precision and excellent background rejection with which the high-energy secondaries could be reconstructed. It was also in E731 that the advantages for high sensitivity rare kaon decay physics at a high energy machine became apparent. These advantages lead to the proposal and execution of E799I and E799II. The KAMI initiative with the Main Injector, now under development, is also a direct result of this experience.
Many decay channels were studied in E731. The highlights included: the first limit on
the p0e+e‾
decay mode rate, the first limit on the p0 n anti-n decay mode rate, a high statistics study of
the Ke4
mode, and unique observations on the ppg
final state, including the observation of the interference between the KL and KS decays. The first two of these decay channels are
likely to have large CP violating components.
E731 Degree Recipients
P. Cabeza-Orecel Ph.D. University of Paris - Sud
R. Daudin Ph.D. University of Paris - Sud
Lawrence K. Gibbons Ph.D. University of Chicago
G.L. Grazer Ph.D. Princeton University
P. Jarry Ph.D. University of Paris - Sud
Magnus R. Karlsson Ph.D. Princeton University
G. Makoff Ph.D. University of Chicago
V. Papadimitriou Ph.D. University of Chicago
J.R. Patterson Ph.D. University of Chicago
Michael B. Woods Ph.D. University of Chicago
E731 Publications
First Result on a New Measurement of e'/e in the Neutral-Kaon-System., M. Woods, et al., Phys. Rev. Lett. 60, 1695 (1988).
New Limits on KL ® p 0 e+e-.,
L.K. Gibbons et al., Phys. Rev. Lett. 61, 2661 (1988).
Search for KL ® p 0gg, V. Papadimitriou, et al., Phys. Rev. Lett. 63, 28 (1989).
Determination of Re(e'/e ) by the Simultaneous Detection of the Four KL,S to pp Decay Modes., J.R. Patterson, et al., Phys. Rev. Lett. 64, 1491 (1990).
New limit on KL ® p 0e+e- , A. Barker , et al., PR D41, 3546 (1990).
Test of CPT Symmetry through a Deterermination of the Difference in the Phases of h00 and h+‾ in KL ® 2p Decays., M. Karlsson , et al., Phys. Rev. Lett. 64, 2976 (1990).
Measurement of the Branching Ratio of the Decay KL → p 0gg., V . Papadimitriou, et al., Phys. Rev. D44, R573 (1991).
Search for the Decay KL , ® p 0nn., G.E. Graham, et al., Phys. Lett. B295, 169 (1992).
Measurement of the Quadratic Slope Parameter in the KL ® 3p 0 Decay Dalitz Plot., S. Somalwar, et al., Phys. Rev. Lett. 68, 2580 (1992).
New Measurements of the Neutral Kaon Parameters dm , tS , f 0 0 - , f + -0 and f + -., L.K. Gibbons, et al., Phys. Rev. Lett. 70, 1199 (1993).
Measurement of the CP-Violation Parameter Re(e'/e)., L. K. Gibbons , et al., Phys. Rev. Lett. 70, 1203 (1993).
Study of the Decay KL ,→ p ± e± n n.,
G. Makoff , et al., Phys. Rev. Lett. 70, 1591 (1993); Erratum-ibid. 75, 2069 (1995).
Simultaneous Measurement of KS
and KL Decays into p 0gg.,
E.J. Ramberg., et al., Phys. Rev.
Lett. 70, 2525 (1993).
Measurement of the CP-Violation Parameter
h+ -g in
Neutral Kaon Decays., E.J. Ramberg ,
et al., Phys. Rev. Lett. 70, 2529 (1993).
CP and CPT Symmetry Tests from the
Two-Pion Decays of the Neutral Kaon with the Fermilab-E731 Detector., L.K.
Gibbons, et al., Phys. Rev. D55, 6625 (1997).
6.4 E773 - Measurement of the Phase Difference
Between h00 and h+
- to a Precision of 1o
Chicago, Elmhurst, Fermilab, Illinois,
Rutgers
Quantum field theories of the fundamental interactions require that particles and antiparticles have identical masses and lifetimes. It is, however, conceivable that violations of this exact matter-antimatter symmetry could occur at the miniscule distance scales associated with the violent disruptions of space and time caused by quantum gravitational effects. The very small mass difference between long- and short-lived K mesons (KL and KS) provides a possible experimental tool for detecting these effects. Experiment 773 was designed to study the interference between decaying KL and KS mesons in a neutral particle beam at Fermilab in order to search for signs of this CPT violation.
The experiment determined the relative phases between quantum mechanical amplitudes characterizing decays of K mesons into pairs of charged and neutral pions. The ratio of the amplitudes for KL ® p+ p - and KS ® p+ p - decays (h+-) is expected to be nearly identical to the corresponding ratio for KL ® p0 p0 and KS ® p0 p0 (h00). In particular, a difference in the phases of the amplitudes h+- and h00 that was larger than a fraction of a degree would be an indication that the masses of K mesons and anti-K mesons differ. E773 found that this difference in phases was (0.63±1.03) degrees, consistent with CPT conservation. When combined with results from E731, this corresponds to a mass difference between K0 and the K0 antiparticle that is less than roughly 1 part in 1018 of the K0 mass. This precision would correspond to weighing Lake Michigan with an experimental error equal to one teaspoon of water. The E773 result was one of the measurements listed in the American Physical Society's Physics News in 1995.
E773 Degree Recipients
Roy A. Briere Ph.D. University of Chicago
John N. Matthews Ph.D. Rutgers University
Bernhard Schwingenheuer Ph.D. University of Chicago
E773 Publications
CPT tests in the neutral kaon system., B. Schwingenheuer, et al., Phys. Rev. Lett. 74, 4376 (1995).
New measurement of the CP violation parameter h +-g., J.N. Matthews, et al., Phys. Rev. Lett. 75, 2803 (1995).
6.5 E774 -
Electron Beam Dump Particle Search
Fermilab, Illinois, Northeastern
In the early 1980’s, experiments at the GSI heavy ion accelerator in Darmstadt observed a striking anomaly in the production of electrons and positrons in elastic collisions of heavy ions. The anomalous signal consisted of coincident electron-positron pairs having approximately equal energies in the laboratory frame, and was interpreted as evidence for the two-body decay of a neutral particle of mass 1.8 MeV/c2. It was presumed that this new particle was created at rest in the strong electromagnetic fields of the colliding ions. Experimental limits at that time could not exclude such a particle, especially if its coupling was primarily to the electron. Theoretical speculation was that this could be a variant of the axion.
E-774 was one of many electron beam dump experiments mounted around the world to search for this object. The technique was simple. A beam of high energy electrons was directed into a dense beam absorber. A light neutral particle coupled to the electron would be produced in the beam dump by a bremsstrahlung-like process and could then be observed by its decay in flight if its flight path were longer than the beam dump. E-774 was unique in that it used the Wide Band electron beam at Fermilab which at nearly 400 GeV was the highest energy electron beam in the world. This gave the best sensitivity to short lifetimes.
The beam dump for E-774 was a dense tungsten calorimeter, 60 radiation lengths deep and instrumented with scintillating fiber ribbons. The calorimeter was thick enough to completely absorb the high-energy electron showers so that a veto counter at its downstream face could operate quietly and define the beginning of the decay volume. The entire assembly of beam dump calorimeter plus veto counters was about 25 cm long, allowing sensitivity to lifetimes below 10-15 sec. Following the decay volume, an array of scintillation counters and a downstream electromagnetic calorimeter were used to trigger on decays in flight. Track momenta were analyzed using a magnetic spectrometer. Incoming primary beam particles were tagged as electrons using a synchrotron radiation detector. This effectively removed the 10-20% contamination of pions and kaons in the primary beam.
Had the 1.8 MeV/c2 particle been real, E-774 would have seen a considerable flux of events with a clear and striking signature. A beam electron would hit the beam dump calorimeter but deposit only a fraction of its energy and leave no signal in the veto counter at the beginning of the decay space. The counters at the downstream end of the decay space would register two charged particles. The beam energy missing from the beam dump calorimeter would appear in the downstream trigger calorimeter, and this energy would match the momenta of the electron-positron pair reconstructed in the momentum spectrometer. E-774 took data during the 1987-88 and 1990 fixed target runs. While a considerable flux of electron-positron pairs was seen from neutral kaon decays, all of these events were excluded by kinematics and/or by the tagging of the primary beam. No events were found which were consistent with the two-body decay hypothesis, and the experiment was able to exclude the last remaining lifetime region for the 1.8 MeV/c2 object.
E774 Publications
Search for
Short-lived Particles Produced in an Electron Beam Dump., A. Bross, et al., Phys. Rev. Lett. 67, 2942 (1991).
6.6
E799
Rare Decays of Kand Hyperons
Arizona, UCLA, UC/San Diego, Campinas
(Brazil), Chicago, Colorado, Elmhurst,
Fermilab, Osaka (Japan), Rice, Rutgers,
Sao Paulo (Brazil), Virginia, Wisconsin
The high intensity and high energy protons provided by the Tevatron, allowed a major extension of the experimental sensitivity to rare, neutral kaon decays. E799 took advantage of this new opportunity. The sensitivity of the experiment crucially depended on both the number of observed kaon decays (flux and acceptance) and resolution of the detector. The combination of Tevatron and the E799-Phase I detector provided these ingredients.
E799-I published results on a search for the breakdown of the fundamental symmetry of charge-conjugation-parity (CP), which could manifest itself in rare kaon decay processes that include a pion and a pair of leptons. Additional results included tests of the principles of unitarity, lepton-number conservation, hyperon polarization, and kaon form factors. All of these results were the best at the time, and held the record for many years. This experiment also set the stage for the rare kaon decays in the years beyond 2000, especially on the most important "Golden Mode" KLong decay to a neutral pion with two neutrinos. This very rare kaon decay mode provides the cleanest measurement of the CP violating parameter h of the Standard Model. E799-Phase I was a pioneering experiment that proved the viability of research on many key physics questions that are best answered by rare kaon decays.
Besides the importance of the physics of this whole series of experiments, perhaps an unconventional 'parameterization' of these small scale, sharply-focused experiments is the educational benefit that can be measured by what students learn and do during their thesis research. Students had to do everything; including design the detector, do R&D on various detector elements, construct the detector components, make them all work, do the data-taking, understand in detail the detector performance, and finally do the physics analysis. Six out of the seven E799 – Phase-I PhD students who graduated from this experiment are still practicing the art of experimental physics.
E799 I Degree Recipients
Ping Gu Ph.D. Rutgers University
Deborah Appel Harris Ph.D. University of Chicago
Fumihiko Kato M.S. Osaka University
Kevin Scott McFarland Ph.D. University of Chicago
Tsuyoshi Nakaya Ph.D. Osaka University
Douglas Alan Roberts Ph.D. University of California at Los
Angeles
Matthew Brandon Spencer Ph.D. University of California at Los
Angeles
Matthew John Weaver Ph.D. University of California at Los
Angeles
E799 I Publications
Measurement of the branching ratio of p o to e+ e- from p o produced by KL → p o p o p o decays in flight., K.S. McFarland, et al., Phys. Rev. Lett. 71, 31 (1993).
Limit on the branching ratio of KL → p om+ m-., D.A. Harris, et al., Phys. Rev. Lett. 71, 3914 (1993).
Limit on the branching ratio of KL → p oe+ e-., D.A. Harris, et al., Phys. Rev. Lett. 71, 3918 (1993).
A limit on the lepton-family number violating process po →m and e., P. Krolak, et al., Phys. Lett. B320, 407 (1994).
Measurement of the branching ratio and a study of CP for the leptonic decay KL → e+ e+ e- e-., P. Gu, et al., Phys. Rev. Lett 72, 3000 (1994).
Limit on the branching ratio of KL → p on n., M. Weaver, et al., Phys. Rev. Lett.72, 3758 (1994).
Search for the decay KL → p o p o g., D. Roberts, et al., Phys. Rev. D50, 1874 (1994).
Measurement of the branching ratio of KL → e eg g., T. Nakaya, et al., Phys. Rev. Lett 73, 2169 (1994).
Polarization of L and anti-L produced by 800 Gev protons., E.J. Ramberg, et al., Phys. Lett. B338, 403 (1994).
Measurement of the branching ratio and form factor of KL →m m g., M.B. Spencer, et al., Phys. Rev. Lett 74, 3323 (1995).
First evidence for the decay KL → m m e e., P. Gu, et al., Phys. Rev. Lett 76, 4312 (1996).
Search for
the lepton-family number violating decays KL →
p o m e., K. Arisaka, et al., Phys. Lett. B432, 230 (1998).
Following the first run of E799, a new spectrometer and beam line were developed for the combined E799 Phase II and the e'/e experiment E832. This combination is referred to as KTeV. Good pion/electron discrimination was achieved, over 10,000 to 1, by the combination of a precision CsI calorimeter and a transition radiation detector. These capabilities greatly improved the sensitivity of the experiment.
KTeV-E799 addressed the principles of unitarity and lepton-number conservation, the polarization of hyperons, and kaon form factors. A particular kaon decay, KL to a pair of charged pions and a electron-positron pair, was observed to manifest CP and T violations in a decay angle asymmetry. This is the first observation of a CP-odd and T-odd variable in physics, and provides guidance towards understanding symmetry breaking. The experimenters are still in the early phase of analyzing data collected during 1996/97 and 1999, and expect to publish (essentially rewrite the chapters on neutral kaon decays with) many results in future years.
E799 II Degree Recipients
A. Alavi-Harati Ph.D. University of Wisconsin
G. Graham Ph.D. University of Chicago
K. Hanagaki Ph.D. Osaka University
Kazunori Hanagaki M.S. Osaka University
Yuji Matsumiya M.S. Osaka University
P. Mikelsons Ph.D. University of Colorado
B. Quinn Ph.D. University of Chicago
M. Sadamoto Ph.D. Osaka University
K. Senyo Ph.D. Osaka University
Masanori Sogo M.S. Osaka University
Toshihiro Tsuji M.S. Osaka University
Motoharu Yagi M.S. Osaka University
Eric Zimmerman Ph.D. University of Chicago
Measurement of the branching fraction of the decay KL ® p + p - e+ e -., A. Alavi-Harati, et al., Phys. Rev. Lett. 80, 4123 (1998).
Search for the decay KL ® p0 n n., A. Alavi-Harati, et al., Phys.Lett. B447, 240 (1999).
Observation of X0 ® S+ e - n., A. Affolder, et al., Phys. Rev. Lett. 82, 3751 (1999).
Measurement of the decay KL ® p0 g g., A. Alavi-Harati, et al., Phys.Rev.Lett. 83, 917 (1999).
Measurement of the branching ratio of KL ® p0 e+ e - using KL ® 3p0 decays in flight., A. Alavi-Harati, et al., Phys. Rev.Lett. 83, 922 (1999).
Observation of CP Violation in KL ® p + p - e+ e -., A. Alavi-Harati, et al., Phys. Rev. Lett. 84, 408 (2000).
Search for the Weak Decay of a Lightly Bound H0 Dibaryon., A. Alavi-Harati, et al., Phys. Rev. Lett. 84, 2593 (2000).
Search for the decay KL ® p0 n n using p0 ® e+ e - g., A. Alavi-Harati, et al., Phys.Rev. D61, 2006 (2000).
Search for the Decay KL ® p0 m+ m -., A. Alavi-Harati, et al., to be published in Phys. Rev. Lett.
Observation of the Decay KL ® m+ m - g g., A. Alavi-Harati, et al., Submitted to Phys.Rev. D.
6.7 E832/KTeV A Search for
Direct CP Violation in KL0 → 2p
Arizona, UCLA, UC/San Diego, Campinas
(Brazil), Chicago, Colorado, Elmhurst,
Fermilab, Osaka (Japan), Rice, Rutgers,
Sao Paulo (Brazil), Virginia, Wisconsin
One of the long-lasting puzzles in particle physics is the origin of the CP violation, a difference in the behavior of matter and antimatter, where C stands for the charge conjugation (exchange particle and antiparticle) and P stands for the parity (space inversion like mirror image). The CP violation in neutral kaon decays is a tiny effect (about 1 part in 500). Can it be related to the striking asymmetry between matter and antimatter of our universe which appears to be composed entirely of matter, with no astronomical object made of antimatter ever detected? In fact, the only antimatter we find anywhere is the minute quantities produced in high energy particle interactions like the interactions which occur when cosmic rays strike the upper atmosphere and like those studied here at Fermilab.
E832 was one of the two experiments in KTeV (Kaons at the Tevatron), which sought to determine whether or not the effect of CP violation can be fully understood in the context of the present picture of matter (the "Standard Model"). The best way to do this is by making high-precision measurements on decays that are known to manifest CP violation. This requires large numbers of clean long-lived neutral kaons (KLong), each of which is a mixture of matter particle kaon (Ko) and antimatter particle kaon (anti-Ko). These were produced by the proton beam from Fermilab's Tevatron accelerator. The observed decays came from a long, downstream, vacuum region.
Then fast, precise position and energy measurements were necessary for both electrically charged and neutral particles arising from the decays. The KTeV experiment apparatus thus consisted of a tandem series of state-of-the-art detectors, such as CsI calorimeter, multiwire drift chambers, analyzing magnet, anti-coincidence photon counters, and regenerator (producing side-by-side short-lived kaons). For example, better than 0.7% in precision for the photon energy measurement by the CsI crystal calorimeter has been achieved. Data from the detectors were organized, monitored, displayed, culled, and recorded on magnetic tape by programs running on a powerful 34-processor on-line computer system.
The experiment was constructed beginning in 1992 and took its first data beginning in 1996. E832 took data in 1996,1997 and 1999.
The main purpose of E832 was to measure the quantity e'/e (epsilon prime divided by epsilon) which, if non-zero, would signal a new form of CP violation. CP violation was first discovered in the famous experiment of 1964 for which Jim Cronin and Val Fitch received the Nobel Prize. The magnitude of this effect is parameterized by the parameter epsilon, which is about 0.0023. The Cronin-Fitch, or epsilon effect can be described as an asymmetry in the mixing of the neutral kaon with its anti-particle.
Ever since the discovery of the Cronin-Fitch effect, scientists have attempted to observe an effect in the DECAY, rather than the mixing, of the neutral kaon. Such an effect is called "direct" CP violation. The original effect was established by observing the decay of the long-lived neutral kaon to charged pions. To see the new effect, physicists had to study the decay to neutral as well as charged pions, a much more difficult prospect since the neutral pions decay to high energy photons which are difficult to precisely measure. It is necessary to study the decays of the two mixtures of the neutral kaons, called K-long (long-lived) and K-short (short lived), to both the charged and neutral pion final states, which is then parameterized by e'/e.
The first KTeV result on e'/e was announced in early 1999, based on about 23% of the data already collected from 1996-1997. The analysis was done "blind" up until a week prior to the announcement. The data was sufficiently well understood and the systematic uncertainty was sufficiently small to "open the box" Concluding the preliminary analysis. The result on the ratio e'/e is 0.00280 with an error of 0.00041. Thus, the e' effect is about 350 times smaller than the e effect. The result serves to establish, with nearly seven standard deviations, this new CP violating effect, direct CP violation. It definitively rules out the Superweak Model as the sole source of CP violation. While the Standard Model predicts a non-zero effect, the size of the KTeV result is larger than most theorists expected.
This result is consistent with the previous finding from a CERN experiment, although their precision (about 3.5 standard deviations) did not lead to definitely reporting a non-zero result. The previous FNAL experiment (see the writeup of E731, this volume) saw a small effect but it was only 1.2 standard deviations from zero.
For the future, it will be interesting to see what results the additional KTeV data sets provide. Some of KTeV's sources of systematic uncertainty have been largely eliminated for its 1999 run and KTeV has doubled the data sample. It will also be interesting to compare new results with CERN and Frascati experiments to further establish the precise measurement of the new CP violating effect. The large amount of data (about 50 Terabytes) collected in E832 was also used to study the decay dynamics of kaons, such as KL ® p+ p- g and p0 g g, the CP asymmetry in semileptonic KL decays, as well as searches for supersymmetric particles beyond the Standard Model.
E832 Degree Recipients
Peter S. Shawhan Ph.D. University of Chicago
E832 Publications
Light Gluino Search for Decays Containing p + p - or p 0 p 0 from a Neutral Hadron Beam at Fermilab., A. Alavi-Harati, et al., Phys. Rev. Lett. 83, 2128-2132 (1999).
Measurement of the Decay KL → p 0 g g., A. Alavi-Harati, et al., Phys. Rev. Lett. 83, 917 (1999).
Observation of Direct CP Violation in KLS → p p Decays., A. Alavi-Harati, et al., Phys. Rev. Lett. 83, 22 (1999).
Search for Light Gluino via the Spontaneous Appearance of p + p - Pairs with an 800 GeV/c Proton Beam at Fermilab., J. Adams, et al., Phys. Rev. Lett. 79, 4083 (1997).
6.8 E871 -
Search for CP Violation in the Decays of X- / + and L / Hyperons
Academia
Sinica (Taiwan), UC/Berkeley, Fermilab, Guanajuato (Mexico), IIT,
Lausanne
(Switzerland), LBNL, Michigan, South Alabama, Virginia
It has been known for over fifty years that antimatter exists. But until 1964, it was thought that, outside of the difference in charge, antimatter should behave identically to matter. That all changed in 1964 when Cronin and Fitch discovered a slight asymmetry between matter and antimatter, or CP violation, in kaon decays. This tiny asymmetry may have cosmological implications. The matter we see now in the universe; stars, planets, dust, people, may all be a consequence of CP violation. This is because in the Big Bang both matter and antimatter should have been created in equal amounts and hence should have mutually annihilated leaving a very boring universe composed of mainly photons. That this didn't happen is thought to be due to CP violation.
Since Cronin and Fitch's discovery, there has been an ever increasing effort to understand CP violation, both experimentally and theoretically. Despite a number of elegant experiments over the past 35 years, it still remains a mystery; and despite some tantalizing evidence from CDF, it still remains a property unique to the decay of the kaons. It should be a universal property of the weak interaction and hence present at some level in (almost) all weak decays. The HyperCP experimenteres are attempting to see CP violation in the decays of two hyperons: the L and the X. These are particles much like the proton and neutron except that they contain one or two strange quarks.
Any CP-violation, unlike parity violation, is expected to be small. Hence huge numbers of events are needed. To facilitate this, the HyperCP collaboration built an extremely high-rate spectrometer which recorded more events on tape than any other particle physics experiment: about 75 billion and 160 billion respectively in the 1997 and 1999 fixed-target runs, a total of 100 terabytes of data. In building the spectrometer, the collaboration took advantage of Fermilab's longstanding experience and expertise in hyperon physics and Fermilab's pioneering efforts in very high-rate data acquisition systems. Analysis of the data is still underway and will continue on the Fermilab computing farms for another year. Crunching through 235 billion events takes a while! The collaboration expects to publish some rare and forbidden hyperon decay search results this year, a bit later for the CP violation results.
E871 Degree Recipients
Gennifer Gerbi Ph.D. University of Virginia
Durga Ramajaran Ph.D. University of Virginia
E871 Conference Proceedings
Status of the HyperCP Experiment at Fermilab., Proceedings of the 3rd International Conference on Hyperons, Charm and Beauty Hadrons, C. Dukes, et al., Nuc. Phys. 75B ,281 (1999).
Search for Direct CP Violation in L and X Hyperon Decays., Proceedings of the 27th International Symposium on Multi-Particle Dynamics, C. White, et al., Nucl. Phys. Proc. Suppl. 71, 451 (1999).
CP Violation in Strange Baryon Decays: A Report from Fermilab Experiment 871., C. James, et al., Proceedings of the 4th Workshop on Heavy Quarks at Fixed Target (HQ98), Fermilab, October 10, 1998, p.107 (1999).
Search for Direct CP Violation in Decays of Hyperons., Y.C. Chen, et al., Proceedings of the International Conference on Hadron Structure (HS 98), Stara Lesna, Slovakia, September 7, 1998, p.447 (1998).
