stayParticle physicsOfStandard modelThe theory of weak interaction points out thatW and Z bosonsThe weak force is caused by the emission (or absorption) of bosons, so it is aNon-contact force。The most famous of these launches isBeta decay, it isradioactivityA performance of.Heavy particles are unstable in nature. Since Z and W bosons are much heavier than protons or neutrons, the interaction distance of weak interaction is very short.This interaction is called "weak" because the probability of beta decay is much lower than that of strong interaction, which means that its general intensity ratioelectromagnetismandStrong nuclear forceSeveral weakOrder of magnitude。Most particles will decay through weak interaction after a period of time.The weak interaction has a unique characteristic that isquarkTaste changes - other interactions cannot do this.In addition, it will destroyParity symmetryandCP symmetry。The flavor change of quarks enables quarks to“taste”Interchangeable.
The earliest description of weak force is in the 1930s, which is fourFermionContact interactingFermi theory: Contact refers to no operating distance (i.e. completely physical contact).But now it is better to describe it in terms of a field with an effective distance, although that distance is very short.In 1968, electromagnetic andWeak interactionThey are two aspects of the same force, now calledWeak current interaction。
Weak interaction in theBeta decayIs the most obvious in hydrogen productionDeuteriumandheliumThe process (energy source of star thermonuclear reaction) is also very obvious.Radiocarbon datingThis is the decayCarbon-14Decay through weak interactionNitrogen - 14。It can also createRadiant cold light, common inSuperheavy hydrogenLighting;Has also createdBeta voltThis application field (Beta rayThe electrons of are used as current).
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The weak interaction has several characteristics as follows:
becauseBosonThe probability of weak decay is relatively low compared with strong or electromagnetic decay, so it occurs slowly.For example, when a neutral pion decays through electromagnetism, its lifetime is about 10-16Second;When a charged pion decays through weak nuclear force, its lifetime is about 10-8Seconds, 100 million times the former.In contrast, the lifetime of a free neutron (decayed by weak interaction) is about 15 minutes.
For a long time, people thought thatLaws of natureIn MirrorreflexAfter that, it will remain unchanged, and mirror reflection will equal allSpace axisreversal.That is to say, when you look at the experiment in a mirror, the experimental results will be the same as when you turn the experimental equipment to the mirror direction.This so-called law is calledParityConservation, classical gravityelectromagnetismandStrong interactionAll abide by this law;It is supposed to be a universal law.However, in the mid-1950s,Yang ZhenningAndLi ZhengdaoproposeWeak interactionIt may break this law.Wu JianxiongHe and his colleagues discovered the weak interactionParity nonconservation, which brought Yang Zhenning and Li ZhengdaoThe nobel prize in physics。
Although previously usedFermi theoryCan describe the weak interaction, but when it is found that parity nonconservation andRenormalizationAfter the theory, weak interaction needs a new description.In 1957Robert Marshak AndGeorge Sudarshan , and laterRichard FeynmanAndMurray Gelman, proposed weak interactionV−A(vector V minusAxis vectorA or left-handed)Lagrangian quantity。In this theory,Weak interactionOnly acts on left-handed particles (or right-handed antiparticles).Since the mirror reflection of left-handed particles is right-handed particles, this explains the largest destruction of parity.Interestingly, becauseV−AThe Z-boson has not been found at the time of development, so the theory does not include the right-handed field entering the neutral current interaction.
However, the theory allows composite symmetryCPConservation.CPConsists of two parts, parityP(left and right interchanged) and charge conjugationC(Replace particles with antiparticles).A discovery in 1964 completely surprised physicists,James Cronin AndVal Fitch withK mesonDecay, under the action of weak phaseCP symmetry breakingThey provided clear evidence, so they obtainedThe nobel prize in physics。Kobayashi ChengAndToshihide Maskawa In 1972, it was pointed out that the CP destruction of weak interaction requires more than two generations of particles, so this discovery actually predicted the existence of the third generation of particles, and this prediction brought them half the Nobel Prize in physics in 2008.Unlike parity nonconservation, CP violation has a low probability of occurrence, but it is still a key to solving the imbalance between matter and antimatter in the universe;It becameAndrei Sakharov OfBaryon production processOne of the three conditions.
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Weak interactionThere are two kinds.The first is called“Load flow interaction”Because the particles responsible for transmitting it are charged（W+or W −）β decay is caused by it.The second is called“Neutral current interaction”, because the particles responsible for transmitting it,Z boson , is neutral (not charged).
Load flow interaction
In one of the load current interactions, the chargedLepton(e.g. electron or muon, with charge of − 1) can absorbW+
Boson(the charge is+1), and then converted into the correspondingneutrino(charge is 0), while the type (generation) of neutrinos (electrons, μ and τ) is consistent with that of leptons before interaction, such as:
Similarly, the lower typequark(Charge is − ⁄three）It can be converted into an upper quark (with a charge of+⁄) by emitting a W − boson or absorbing a W+bosonthree）。More accurately, the lower quark becomes the upper quarkQuantum superposition stateThat is to say, it has the possibility of transforming into any of the three upper quarks. The possibility is determined byCKM matrixDescribed in.Conversely, an upper quark can emit a W+boson, or absorb a W −
Bosons, and then converted into the following type quarks:
Because the W boson is very unstable, it has a very short life and decays quickly.For example:
W bosons can decay into other products, with different possibilities.
In neutron so-calledBeta decayIn the middle (see the above figure), the lower quark in the neutron emits a virtual W − boson, which is converted into an upper quark, and the neutron is thus converted into a proton.Due to the energy in the process (i.e. the mass difference between the lower quark and the upper quark), W − can only be converted into an electron and an anti electric neutrino.At the quark level, the process can be described by the following formula:
Neutral current interaction
In neutral current interaction, a quark or lepton (such as an electron or muon) emits or absorbs a neutralZ boson 。For example:
Like the W boson, the Z boson also decays rapidly, such as:
According to the weak electricity theory, when the energy is very high, there are four kinds of masslessGauge bosonFields, which are similar to photons, have a complex vectorHiggs fieldDouble state.However, when the energy is low, gauge symmetry will appearSpontaneous breakage, become electromagnetically interactingU(1)Symmetry (one of the Higgs fields hasVacuum expectation）。Although thisSymmetry breakingThree massless bosons will be generated, but they will fuse with three photon like fields, soHiggs mechanismWill bring quality to them.These three fields become the weakly interacting W+, W − and Z bosons, while the fourth gauge field remains massless, that is, the photon of electromagnetic interaction.
Although this theory makes several predictions, including the prediction of their masses before the discovery of Z and W bosonsHiggs bosonIt has not yet been discovered.European Organization for Nuclear ResearchSubordinateLarge Hadron ColliderOne of its main tasks is to produce the Higgs boson.On March 14, 2013, the European Organization for Nuclear Research issued a press release, officially announcing the detection of new particles, namelyHiggs boson。
