The CPLEAR experiment used the antiproton beam of the LEAR facility - Low-Energy Antiproton Ring which operated at CERN from 1982 to 1996 - to produce neutral kaon s through proton-antiproton annihilation in order to study CP , T and CPT violation in the neutral kaon system. [1]
According to the theory of the Big Bang, matter and antimatter would have existed in the same amount at the beginning of the Universe. If this was true, particle s and antiparticle s would have annihilated each other, creating photon s, and thus the Universe would have been only compounded by light (one particle of matter for 1018 photons). However, only matter has remained and at a rate of one billion times more particles than expected. What happened then, for the antimatter to disappear in favor of matter? A possible answer to this question is baryogenesis, the hypothetical physical process that took place during the early universe that produced baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe. However, baryogenesis is only possible under the following conditions proposed by Andrei Sakharov in 1967:
The first experimental test of CP violation came in 1964 with the Fitch-Cronin experiment. The experiment involved particles called neutral K-mesons, which fortuitously have the properties needed to test CP. First, as mesons, they're a combination of a quark and an anti-quark, in this case, down and antistrange, or anti-down and strange. Second, the two different particles have different CP values and different decay modes: K1 has CP = +1 and decays into two pions; K2 has CP = -1 and decays into three. Because decays with larger changes in mass occur more readily, the K1 decay happens 100 times faster than the K2 decay. This means that a sufficiently long beam of neutral Kaons will become arbitrarily pure K2 after a sufficient amount of time. The Fitch-Cronin experiment exploits this. If all the K1s are allowed to decay out of a beam of mixed Kaons, only K2 decays should be observed. If any K1 decays are found, it means that a K2 flipped to a K1, and the CP for the particles flipped from -1 to +1, and CP wasn't conserved. The experiment resulted in an excess of 45±9 events around cos(θ) = 1 in the correct mass range for 2-pion decays. This means that for every decay of K2 into three pions, there are (2.0±0.4)×10-3 decays into two pions. Because of this, neutral K mesons violate CP. [2] The study of the ratio of neutral kaon and neutral anti-kaons production is thus an efficient tool to understand what happened in the early Universe that promoted the production of matter. [3]
CPLEAR is a collaboration of about 100 scientists, coming from 17 institutions from 9 different countries. Accepted in 1985, the experiment took data from 1990 until 1996. [1] Its main aim was to study CP, T and 'CPT symmetries in the neutral kaon system.
In addition, CPLEAR performed measurements about quantum coherence of wave function s, Bose-Einstein correlations in multi-pion states, regeneration of the short-lived kaon component in the matter, the Einstein-Rosen-Podolsky paradox using entangled neutral-kaon pair states and the equivalence principle of general relativity. [4]
The CPLEAR detector was able to determine the locations, the momenta and the charges of the tracks at the production of the neutral kaon and at its decay, thus visualizing the complete event.
Strangeness is not conserved under weak interactions, meaning that under weak interactions a
K0
can transform into a
K0
and vice versa. To study the asymmetries between
K0
and
K0
decay rates in the various final states f (f = π+π−, π0π0, π+π−π0, π0π0π+, πlν), the CPLEAR collaboration used the fact that the strangeness of kaons is tagged by the charge of the accompanying kaon. Time-reversal invariance would imply that all details of one of the transformations could be deducible from the other one, i.e. the probability for a kaon to oscillate into an anti-kaon would be equal to the one for the reverse process. The measurement of these probabilities required the knowledge of the strangeness of a kaon at two different times of its life. Since the strangeness of the kaon is given by the charge of the accompanying kaon, and thus be known for each event, it was observed that this symmetry was not respected, thereby proving the T violation in neutral kaon systems under weak interaction. [3]
The neutral kaons are initially produced in the annihilation channels
which happen when the 106 anti-protons per second beam coming from the LEAR facility is stopped by a highly-pressurized hydrogen gas target. The low momentum of the antiprotons and the high pressure allowed to keep the size of the stopping region small in the detector. [5] Since the proton-antiproton reaction happens at rest, the particles are produced isotropically, and as a consequence, the detector has to have a near-4π symmetry. The whole detector was embedded in a 3.6 m long and 2 m diameter warm solenoidal magnet providing a 0.44 T uniform magnetic field. [3]
The antiprotons were stopped using a pressurized hydrogen gas target. A hydrogen gas target was used instead of liquid hydrogen to minimize the amount of matter in the decay volume. The target initially had a radius of 7 cm and subjected to a pressure of 16 bar. Changed in 1994, its radius became equal to 1.1 cm, under a 27 bar pressure. [3]
The detector had to fulfill the specific requirements of the experiment and thus had to be able to:
Cylindrical tracking detectors together with a solenoid field were used to determine the charge signs, momenta and positions of the charged particles. They were followed by the particle identification detector (PID) whose role was to identify the charged kaon. It was compounded by a Cherenkov detector, which carried out the kaon-pion separation; and scintillator s, measuring the energy loss and the time of flight of the charged particles. It was also used for the electron-pion separation. The detection of photons produced in π0 decays was performed by ECAL, an outermost lead/gas sampling calorimeter, complementary to the PID by separating pions and electrons at higher momenta. Finally, hardwired processors (HWK) were used to analyze and select the events in a few microseconds, deleting the unwanted ones, by providing a full event reconstruction with sufficient precision. [3]
In modern physics, antimatter is defined as matter composed of the antiparticles of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charge, parity, and time, known as CPT reversal. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay, but only a tiny fraction of these have successfully been bound together in experiments to form antiatoms. Minuscule numbers of antiparticles can be generated at particle accelerators; however, total artificial production has been only a few nanograms. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Nonetheless, antimatter is an essential component of widely-available applications related to beta decay, such as positron emission tomography, radiation therapy, and industrial imaging.
In particle physics, a meson is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre (10−15 m), which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few tenths of a nanosecond. Heavier mesons decay to lighter mesons and ultimately to stable electrons, neutrinos and photons.
In particle physics, a pion is any of three subatomic particles:
π0
,
π+
, and
π−
. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions
π+
and
π−
decaying after a mean lifetime of 26.033 nanoseconds, and the neutral pion
π0
decaying after a much shorter lifetime of 85 attoseconds. Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.
In particle physics, a kaon, also called a K meson and denoted
K
, is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model they are understood to be bound states of a strange quark and an up or down antiquark.
In physical cosmology, baryogenesis is the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe.
The BaBar experiment, or simply BaBar, is an international collaboration of more than 500 physicists and engineers studying the subatomic world at energies of approximately ten times the rest mass of a proton (~10 GeV). Its design was motivated by the investigation of charge-parity violation. BaBar is located at the SLAC National Accelerator Laboratory, which is operated by Stanford University for the Department of Energy in California.
Jack Steinberger was a German-born American physicist noted for his work with neutrinos, the subatomic particles considered to be elementary constituents of matter. He was a recipient of the 1988 Nobel Prize in Physics, along with Leon M. Lederman and Melvin Schwartz, for the discovery of the muon neutrino. Through his career as an experimental particle physicist, he held positions at the University of California, Berkeley, Columbia University (1950–68), and the CERN (1968–86). He was also a recipient of the United States National Medal of Science in 1988, and the Matteucci Medal from the Italian Academy of Sciences in 1990.
In particle physics the semileptonic decay of a hadron is a decay caused by the weak force in which one lepton is produced in addition to one or more hadrons. An example for this can be
In physical cosmology, the baryon asymmetry problem, also known as the matter asymmetry problem or the matter–antimatter asymmetry problem, is the observed imbalance in baryonic matter and antibaryonic matter in the observable universe. Neither the standard model of particle physics nor the theory of general relativity provides a known explanation for why this should be so, and it is a natural assumption that the universe is neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter. Since this does not seem to have been the case, it is likely some physical laws must have acted differently or did not exist for matter and antimatter. Several competing hypotheses exist to explain the imbalance of matter and antimatter that resulted in baryogenesis. However, there is as of yet no consensus theory to explain the phenomenon, which has been described as "one of the great mysteries in physics".
The Belle experiment was a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan. The experiment ran from 1999 to 2010.
Neutral B meson oscillations are one of the manifestations of the neutral particle oscillation, a fundamental prediction of the Standard Model of particle physics. It is the phenomenon of B mesons changing between their matter and antimatter forms before their decay. The
B
s meson can exist as either a bound state of a strange antiquark and a bottom quark, or a strange quark and bottom antiquark. The oscillations in the neutral B sector are analogous to the phenomena that produce long and short-lived neutral kaons.
In particle physics, B mesons are mesons composed of a bottom antiquark and either an up, down, strange or charm quark. The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's short lifetime. The combination of a bottom antiquark and a bottom quark is not a B meson, but rather bottomonium, which is something else entirely.
The D mesons are the lightest particle containing charm quarks. They are often studied to gain knowledge on the weak interaction. The strange D mesons (Ds) were called "F mesons" prior to 1986.
In particle physics, CP violation is a violation of CP-symmetry : the combination of C-symmetry and P-symmetry. CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C-symmetry) while its spatial coordinates are inverted. The discovery of CP violation in 1964 in the decays of neutral kaons resulted in the Nobel Prize in Physics in 1980 for its discoverers James Cronin and Val Fitch.
OKA is a particle physics detector experiment at the U-70 accelerator in the Institute for High Energy Physics located in Protvino near Moscow (Russia). OKA is specialized experiment with separated charge kaons beam.
NA31 is a CERN experiment which was proposed in 1982 as a "Measurement of |η00 /η+-|2 by the CERN-Edinburgh-Mainz-Pisa-Siegen collaboration. It took data between 1986 and 1989, using a proton beam from the SPS through the K4 neutral beam-line. Its aim was to experimentally prove direct CP-violation.
Maria Fidecaro (1930-2023) was an Italian experimental physicist with a focus on particle physics. She has spent most of her career at CERN, where she after retirement had the status of honorary member of the personnel.
The Enhanced NeUtrino BEams from kaon Tagging or ENUBET is an ERC funded project that aims at producing an artificial neutrino beam in which the flavor, flux and energy of the produced neutrinos are known with unprecedented precision.
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General Introduction to the experiment
