NA31 is a

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 Meyrin, western suburb of Gene ...

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.

CP violation

While charge symmetry and parity symmetry are both violated for any transformation under the

weak interaction In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...

, the

CP violation In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics s ...

is known only to appear in particular phenomena -

kaon In particle physics, a kaon, also called a K meson and denoted , 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 (or antiquark ...

and

B-meson 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 ...

decays - under the weak interaction. CP-violation was first theoretically developed for the

Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...

Kobayashi Kobayashi (Japanese language, Japanese: , 'small woods') is the 8th most common Japanese surname. A less common variant is . Notable people with the surname include: Arts Film, television, theater and music *, Japanese actress and voice a ...

and Maskawa in 1973 when they introduced a third generation of

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 nucleus, atomic nuclei ...

(bottom and top) and thus extended the Cabibbo matrix to the 3x3 CKM matrix, parameterizing the couplings between quark-mass eigenstates and the charge weak gauge

bosons In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0, 1, 2, ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have half odd-integer ...

. CP violation then appears through the presence of complex parameters in this matrix. Determined from the relative decay rates of short- and long-lived neutral kaons into two neutral and charged

pion In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (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 ...

s, respectively, the so-called ε'/ε ratio which expresses the relative strength of direct CP-violation was known to be small but expected to be different from zero in the Standard Model. The measurement of this small deviation from zero was the aim of NA31 in order to prove the existence of direct CP-violation in kaon decays under weak interaction.

Evolution of the experiment

NA31 found the first evidence for direct CP violation in 1988 with a ratio deviating about three standards from zero. However, shortly after, another experiment – E731 at

Fermilab Fermi National Accelerator Laboratory (Fermilab), located in Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy particle phys ...

– reported a measurement consistent with zero. A better precision was needed by both NA31 and Fermilab to find consistent results and thus to allow a final conclusion. A new generation of

detectors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

were thus built, both at CERN (for what became the

NA48 experiment The NA48 experiment was a series of particle physics experiments in the field of kaon physics being carried out at the North Area of the Super Proton Synchrotron at CERN. The collaboration involved over 100 physicists mostly from Western Europe ...

) and at Fermilab (KTeV). Finally, in 1999, the two new experiments confirmed both direct CP violation in the decay of neutral kaons (CERN Courier September 1999 p32), a discovery which was later recognized by honours, as one of the most important discoveries made at CERN. In particular the 2005 European Physics Society

High Energy and Particle Physics Prize The High Energy and Particle Physics Prize, established in 1989, is awarded every two years by the European Physical Society (EPS) for an outstanding contribution to high energy and particle physics. Recipients Source: * 1989 Georges Charpak * ...

was awarded jointly to the NA31 Collaboration and its spokesman Heinrich Wahl.

The detector

The detector was compounded by wire chambers combined with

calorimetry In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in '' state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...

in order to determine K⁰ parameters (e.g.

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

, decay vertex). A great precision on these parameters is required to define well the

phase space The phase space of a physical system is the set of all possible physical states of the system when described by a given parameterization. Each possible state corresponds uniquely to a point in the phase space. For mechanical systems, the p ...

for all the decay modes which are to be compared. It consists of : * an evacuated decay region. * proportional wires chambers to measure the charged pion direction. *

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

calorimeter A calorimeter is a device used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimeters ...

with a good energy and

position Position often refers to: * Position (geometry), the spatial location (rather than orientation) of an entity * Position, a job or occupation Position may also refer to: Games and recreation * Position (poker), location relative to the dealer * ...

resolution to measure the

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 particles that can ...

s from the pion decay. * a

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

calorimeter to measure the energy of the charged pions. * a plane of

scintillation Scintillation can refer to: *Scintillation (astronomy), atmospheric effects which influence astronomical observations *Interplanetary scintillation, fluctuations of radio waves caused by the solar wind *Scintillation (physics), a flash of light pro ...

counters serving as

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...

identifiers.

References

External links

NA31 CERN 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 1 ...

{{SPS experiments CERN experiments Particle experiments 1980s in science

CP violation

Evolution of the experiment

The detector

See also

References

External links