词条 | Kaon | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
释义 |
| bgcolour = | name = Kaon | image = | caption = | num_types = 4 | composition = {{SubatomicParticle|Kaon+}}: {{SubatomicParticle|Up quark}}{{SubatomicParticle|Strange antiquark}} {{SubatomicParticle|Kaon0}}: {{SubatomicParticle|Down quark}}{{SubatomicParticle|Strange antiquark}} / {{SubatomicParticle|Strange quark}}{{SubatomicParticle|Down antiquark}} {{SubatomicParticle|Kaon-}}: {{SubatomicParticle|Strange quark}}{{SubatomicParticle|Up antiquark}} | statistics = Bosonic | group = Mesons | generation = | interaction = Strong, weak, electromagnetic, gravitational | antiparticle = | theorized = | discovered = 1947 | symbol = {{SubatomicParticle|Kaon+}}, {{SubatomicParticle|Kaon0}}, {{SubatomicParticle|Kaon-}} | mass = {{SubatomicParticle|Kaon+-}}: {{val|493.677|0.013|ul=MeV/c2}} {{SubatomicParticle|Kaon0}}: {{val|497.648|0.022|u=MeV/c2}} | decay_time = {{SubatomicParticle|Kaon+-}}: {{val|1.2384|0.0024|e=-8|ul=s}} {{SubatomicParticle|K-short}}: {{val|8.958|e=-11|u=s}} {{SubatomicParticle|K-long}}: {{val|1.2384|0.0024|e=-8|u=s}} | decay_particle = | electric_charge = {{SubatomicParticle|Kaon+-}}: ±1 e {{SubatomicParticle|Kaon0}}: 0 e | color_charge = | spin = 0 | strangeness = {{SubatomicParticle|Kaon+}}: +1 {{SubatomicParticle|Kaon0}}: ±1 {{SubatomicParticle|Kaon-}}: -1 | num_spin_states = }}In particle physics, a kaon {{IPAc-en|ˈ|k|eɪ|.|ɒ|n}}, also called a K meson and denoted {{SubatomicParticle|Kaon}},[1] 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 an up or down antiquark (or quark). Kaons have proved to be a copious source of information on the nature of fundamental interactions since their discovery in cosmic rays in 1947. They were essential in establishing the foundations of the Standard Model of particle physics, such as the quark model of hadrons and the theory of quark mixing (the latter was acknowledged by a Nobel Prize in Physics in 2008). Kaons have played a distinguished role in our understanding of fundamental conservation laws: CP violation, a phenomenon generating the observed matter–antimatter asymmetry of the universe, was discovered in the kaon system in 1964 (which was acknowledged by a Nobel Prize in 1980). Moreover, direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 experiment at CERN and the KTeV experiment at Fermilab. Basic propertiesThe four kaons are :
As the quark model shows, assignments that the kaons form two doublets of isospin; that is, they belong to the fundamental representation of SU(2) called the 2. One doublet of strangeness +1 contains the {{SubatomicParticle|Kaon+}} and the {{SubatomicParticle|Kaon0}}. The antiparticles form the other doublet (of strangeness −1).
[b] {{note|Kaon}}Weak eigenstate. Makeup is missing small CP–violating term (see notes on neutral kaons below). [c] {{note|Kaonmass}} The mass of the {{SubatomicParticle|K-long0}} and {{SubatomicParticle|K-short0}} are given as that of the {{SubatomicParticle|Kaon0}}. However, it is known that a difference between the masses of the {{SubatomicParticle|K-long0}} and {{SubatomicParticle|K-short0}} on the order of {{val|3.5|e=-12|u=MeV/c2}} exists.[5] Although the {{SubatomicParticle|Kaon0}} and its antiparticle {{SubatomicParticle|AntiKaon0}} are usually produced via the strong force, they decay weakly. Thus, once created the two are better thought of as superpositions of two weak eigenstates which have vastly different lifetimes:
(See discussion of neutral kaon mixing below.) An experimental observation made in 1964 that K-longs rarely decay into two pions was the discovery of CP violation (see below). Main decay modes for {{SubatomicParticle|Kaon+}}:
The intrinsic parity of a pion is P = −1, and parity is a multiplicative quantum number. Therefore, the two final states have different parity (P = +1 and P = −1, respectively). It was thought that the initial states should also have different parities, and hence be two distinct particles. However, with increasingly precise measurements, no difference was found between the masses and lifetimes of each, respectively, indicating that they are the same particle. This was known as the τ–θ puzzle. It was resolved only by the discovery of parity violation in weak interactions. Since the mesons decay through weak interactions, parity is not conserved, and the two decays are actually decays of the same particle,[6] now called the {{SubatomicParticle|Kaon+}}. CP violation in neutral meson oscillationsInitially it was thought that although parity was violated, CP (charge parity) symmetry was conserved. In order to understand the discovery of CP violation, it is necessary to understand the mixing of neutral kaons; this phenomenon does not require CP violation, but it is the context in which CP violation was first observed. Neutral kaon mixingSince neutral kaons carry strangeness, they cannot be their own antiparticles. There must be then two different neutral kaons, differing by two units of strangeness. The question was then how to establish the presence of these two mesons. The solution used a phenomenon called neutral particle oscillations, by which these two kinds of mesons can turn from one into another through the weak interactions, which cause them to decay into pions (see the adjacent figure). These oscillations were first investigated by Murray Gell-Mann and Abraham Pais together. They considered the CP-invariant time evolution of states with opposite strangeness. In matrix notation one can write where ψ is a quantum state of the system specified by the amplitudes of being in each of the two basis states (which are a and b at time t = 0). The diagonal elements (M) of the Hamiltonian are due to strong interaction physics which conserves strangeness. The two diagonal elements must be equal, since the particle and antiparticle have equal masses in the absence of the weak interactions. The off-diagonal elements, which mix opposite strangeness particles, are due to weak interactions; CP symmetry requires them to be real. The consequence of the matrix H being real is that the probabilities of the two states will forever oscillate back and forth. However, if any part of the matrix were imaginary, as is forbidden by CP symmetry, then part of the combination will diminish over time. The diminishing part can be either one component (a) or the other (b), or a mixture of the two. MixingThe eigenstates are obtained by diagonalizing this matrix. This gives new eigenvectors, which we can call K1 which is the sum of the two states of opposite strangeness, and K2, which is the difference. The two are eigenstates of CP with opposite eigenvalues; K1 has CP = +1, and K2 has CP = −1 Since the two-pion final state also has CP = +1, only the K1 can decay this way. The K2 must decay into three pions. Since the mass of K2 is just a little larger than the sum of the masses of three pions, this decay proceeds very slowly, about 600 times slower than the decay of K1 into two pions. These two different modes of decay were observed by Leon Lederman and his coworkers in 1956, establishing the existence of the two weak eigenstates (states with definite lifetimes under decays via the weak force) of the neutral kaons. These two weak eigenstates are called the {{SubatomicParticle|K-long}} (K-long) and {{SubatomicParticle|K-short}} (K-short). CP symmetry, which was assumed at the time, implies that {{SubatomicParticle|K-short}} = K1 and {{SubatomicParticle|K-long}} = K2. Oscillation{{Main article|Neutral particle oscillation}}An initially pure beam of {{SubatomicParticle|Kaon0}} will turn into its antiparticle while propagating, which will turn back into the original particle, and so on. This is called particle oscillation. On observing the weak decay into leptons, it was found that a {{SubatomicParticle|Kaon0}} always decayed into an electron, whereas the antiparticle {{SubatomicParticle|AntiKaon0}} decayed into the positron. The earlier analysis yielded a relation between the rate of electron and positron production from sources of pure {{SubatomicParticle|Kaon0}} and its antiparticle {{SubatomicParticle|AntiKaon0}}. Analysis of the time dependence of this semileptonic decay showed the phenomenon of oscillation, and allowed the extraction of the mass splitting between the {{SubatomicParticle|K-short}} and {{SubatomicParticle|K-long}}. Since this is due to weak interactions it is very small, 10−15 times the mass of each state. RegenerationA beam of neutral kaons decays in flight so that the short-lived {{SubatomicParticle|K-short}} disappears, leaving a beam of pure long-lived {{SubatomicParticle|K-long}}. If this beam is shot into matter, then the {{SubatomicParticle|Kaon0}} and its antiparticle {{SubatomicParticle|AntiKaon0}} interact differently with the nuclei. The {{SubatomicParticle|Kaon0}} undergoes quasi-elastic scattering with nucleons, whereas its antiparticle can create hyperons. Due to the different interactions of the two components, quantum coherence between the two particles is lost. The emerging beam then contains different linear superpositions of the {{SubatomicParticle|Kaon0}} and {{SubatomicParticle|AntiKaon0}}. Such a superposition is a mixture of {{SubatomicParticle|K-long}} and {{SubatomicParticle|K-short}}; the {{SubatomicParticle|K-short}} is regenerated by passing a neutral kaon beam through matter. Regeneration was observed by Oreste Piccioni and his collaborators at Lawrence Berkeley National Laboratory. Soon thereafter, Robert Adair and his coworkers reported excess {{SubatomicParticle|K-short}} regeneration, thus opening a new chapter in this history. CP violationWhile trying to verify Adair's results, J. Christenson, James Cronin, Val Fitch and Rene Turlay of Princeton University found decays of {{SubatomicParticle|K-long}} into two pions (CP = +1) in an experiment performed in 1964 at the Alternating Gradient Synchrotron at the Brookhaven laboratory.[7] As explained in an earlier section, this required the assumed initial and final states to have different values of CP, and hence immediately suggested CP violation. Alternative explanations such as nonlinear quantum mechanics and a new unobserved particle were soon ruled out, leaving CP violation as the only possibility. Cronin and Fitch received the Nobel Prize in Physics for this discovery in 1980. It turns out that although the {{SubatomicParticle|K-long}} and {{SubatomicParticle|K-short}} are weak eigenstates (because they have definite lifetimes for decay by way of the weak force), they are not quite CP eigenstates. Instead, for small ε (and up to normalization), {{SubatomicParticle|K-long}} = K2 + εK1 and similarly for {{SubatomicParticle|K-short}}. Thus occasionally the {{SubatomicParticle|K-long}} decays as a K1 with CP = +1, and likewise the {{SubatomicParticle|K-short}} can decay with CP = −1. This is known as indirect CP violation, CP violation due to mixing of {{SubatomicParticle|Kaon0}} and its antiparticle. There is also a direct CP violation effect, in which the CP violation occurs during the decay itself. Both are present, because both mixing and decay arise from the same interaction with the W boson and thus have CP violation predicted by the CKM matrix. Direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 and KTeV experiments at CERN and Fermilab.{{citation needed|date=September 2016}} See also
Notes and references
1. ^The positively charged kaon used to be called τ+ and θ+, as it was supposed to be two different particles until the 1960s. See the parity violation section. 2. ^J. Beringer et al. (2012): Particle listings – {{SubatomicParticle|Kaon+-}} 3. ^M. Tanabashi et al. (2018): Particle listings – {{SubatomicParticle|Kaon0}} 4. ^J. Beringer et al. (2012): Particle listings – {{SubatomicParticle|K-short0}} 5. ^1 J. Beringer et al. (2012): Particle listings – {{SubatomicParticle|k-long0}} 6. ^{{cite journal |last1=Lee |first1=T. D. |authorlink1=Tsung-Dao Lee |last2=Yang |first2=C. N. |authorlink2=Chen Ning Yang |title=Question of Parity Conservation in Weak Interactions |journal=Physical Review |date=1 October 1956 |volume=104 |number=1 |doi=10.1103/PhysRev.104.254 |page=254 |quote=One way out of the difficulty is to assume that parity is not strictly conserved, so that {{SubatomicParticle|Theta+}} and {{SubatomicParticle|Antitauon}} are two different decay modes of the same particle, which necessarily has a single mass value and a single lifetime.|bibcode = 1956PhRv..104..254L }} 7. ^{{cite journal|url=http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.13.138|title=Evidence for the 2π Decay of the K20 Meson|first1=J. H.|last1=Christenson|first2=J. W.|last2=Cronin|first3=V. L.|last3=Fitch|first4=R.|last4=Turlay|date=27 July 1964|journal=Physical Review Letters|volume=13|issue=4|pages=138–140|doi=10.1103/physrevlett.13.138}}
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