The CPLEAR experiment used the
antiproton
The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.
The exis ...
beam
Beam may refer to:
Streams of particles or energy
*Light beam, or beam of light, a directional projection of light energy
**Laser beam
*Particle beam, a stream of charged or neutral particles
**Charged particle beam, a spatially localized grou ...
of the LEAR facility - Low-
Energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
Antiproton Ring which operated at
CERN
The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
from 1982 to 1996 - to produce neutral
kaon
KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe.
Its first incarnation was developed in 2002 and supported an enhanced version of ...
''s'' through
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
-
antiproton
The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.
The exis ...
annihilation
In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy ...
in order to study ''
CP'', ''
T'' and ''
CPT'' violation in the neutral kaon system.
[
]
Background
According to the theory of the
Big Bang,
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
and
antimatter
In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...
would have existed in the same amount at the beginning of the
Universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...
. If this was true,
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from ...
''s'' and
antiparticle
In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antie ...
''s'' would have annihilated each other, creating
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
''s'', and thus the Universe would have been only compounded by
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
(one particle of matter for 10
18 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
In physical cosmology, baryogenesis (also known as baryosynthesis) 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 (a ...
, the hypothetical physical process that took place during the early universe that produced baryonic asymmetry, i.e. the imbalance of
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
(baryons) and
antimatter
In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...
(antibaryons) in the observed universe. However, baryogenesis is only possible under the following
conditions proposed by
Andrei Sakharov
Andrei Dmitrievich Sakharov ( rus, Андрей Дмитриевич Сахаров, p=ɐnˈdrʲej ˈdmʲitrʲɪjevʲɪtɕ ˈsaxərəf; 21 May 192114 December 1989) was a Soviet nuclear physicist, dissident, nobel laureate and activist for n ...
in 1967:
*
Baryon number
In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as
::B = \frac\left(n_\text - n_\bar\right),
where ''n''q is the number of quarks, and ''n'' is the number of antiquarks. Bary ...
violation.
*
C-symmetry
In physics, charge conjugation is a transformation that switches all particles with their corresponding antiparticles, thus changing the sign of all charges: not only electric charge but also the charges relevant to other forces. The term C-symm ...
and
CP-symmetry
In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics should be the ...
violation.
* Interactions out of
thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
.
The first experimental test of
CP violation
In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics should be the ...
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
A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...
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
Decay may refer to:
Science and technology
* Bit decay, in computing
* Software decay, in computing
* Distance decay, in geography
* Decay time (fall time), in electronics
Biology
* Decomposition of organic matter
* Tooth decay (dental caries ...
modes: K
1 has CP = +1 and decays into two
pion
In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...
s; K
2 has CP = -1 and decays into three. Because decays with larger changes in mass occur more readily, the K
1 decay happens 100 times faster than the K
2 decay. This means that a sufficiently long beam of neutral Kaons will become arbitrarily pure K
2 after a sufficient amount of time. The Fitch-Cronin experiment exploits this. If all the K
1s are allowed to decay out of a beam of mixed Kaons, only K
2 decays should be observed. If any K
1 decays are found, it means that a K
2 flipped to a K
1, 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 K
2 into three pions, there are (2.0±0.4)×10-3 decays into two pions. Because of this, neutral K mesons violate CP.
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.
The experiment
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.
[ 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
A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements ...
''s'', Bose-Einstein correlations in multi-pion
In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...
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
In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (suc ...
of general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
.[
]
Facility description
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
In particle physics, strangeness ("''S''") is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions which occur in a short period of time. The strangeness of a parti ...
is not conserved under weak interactions, meaning that under weak interactions a can transform into a and vice versa. To study the asymmetries
Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
between and decay rates in the various final states f (f = π+π−, π0π0, π+π−π0, π0π0π+, π''l''ν), 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
Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...
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
In particle physics, strangeness ("''S''") is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions which occur in a short period of time. The strangeness of a parti ...
of a kaon at two different times of its life. Since the strangeness of the kaon is given by the charge
Charge or charged may refer to:
Arts, entertainment, and media Films
* '' Charge, Zero Emissions/Maximum Speed'', a 2011 documentary
Music
* ''Charge'' (David Ford album)
* ''Charge'' (Machel Montano album)
* ''Charge!!'', an album by The Aqu ...
of the accompanying kaon, and thus be known for each event
Event may refer to:
Gatherings of people
* Ceremony, an event of ritual significance, performed on a special occasion
* Convention (meeting), a gathering of individuals engaged in some common interest
* Event management, the organization of e ...
, it was observed that this symmetry was not respected, thereby proving the ''T'' violation in neutral kaon systems under weak interaction.
The neutral kaons are initially produced in the annihilation channels
* p → π+
* p → π− [
]
which happen when the 106 anti-protons per second beam coming from the LEAR facility is stopped by a highly-pressurized hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
target. The low momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
of the antiprotons and the high pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
allowed to keep the size of the stopping region small in the detector
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
.[
{{cite journal
, last1=Angelopoulos, first1=A.
, last2=Apostolakis, first2=A.
, last3=Aslanides, first3=E.
, title=Physics at CPLEAR
, journal=Physics Reports
, volume=374
, issue=3
, year=2003
, pages=165–270
, issn=0370-1573
, doi=10.1016/S0370-1573(02)00367-8, bibcode=2003PhR...374..165A
] 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
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...
providing a 0.44 T uniform magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
.
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.
Layout of the detector
The detector had to fulfill the specific requirements of the experiment and thus had to be able to:
* do an efficient kaon identification
* select the annihilation channels mentioned in Facility description among the very large number of multi-pions annihilation channels
* distinguish between the different neutral-kaon decay channels
* measure the decay proper time
* acquire a large number of statistics, and for this, it had to have both a high rate capability and a large geometrical coverage
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
Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...
of the charged particles. It was also used for the electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
-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.
References
External Links
CPLEAR
experiment record on INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1970 ...
CPLEAR
website
CERN experiments
Particle experiments