Los ntawm nruab nrab ntawm lub xyoo pua 20th, lub tswv yim ntawm "particle zoo" tau tshwm sim hauv physics, txhais tau hais tias muaj ntau yam ntawm cov teeb meem, uas cov kws tshawb fawb tau ntsib tom qab tsim cov accelerators muaj zog txaus. Ib qho ntawm ntau cov neeg nyob hauv "zoo" yog cov khoom hu ua mesons. Cov tsev neeg no, nrog rau baryons, suav nrog hauv pawg loj ntawm hadrons. Lawv txoj kev tshawb fawb tau ua kom nws nkag mus rau qhov tob ntawm cov qauv ntawm cov teeb meem thiab pab txhawb kev txiav txim siab txog kev paub txog nws mus rau hauv txoj kev xav niaj hnub ntawm cov ntsiab lus thiab kev sib cuam tshuam - tus qauv qauv.
Discovery keeb kwm
Nyob rau xyoo 1930, tom qab cov ntsiab lus ntawm atomic nucleus tau qhia meej, cov lus nug tshwm sim txog qhov xwm txheej ntawm cov rog uas ua kom nws muaj sia nyob. Nws tau pom tseeb tias kev sib cuam tshuam uas khi cov nucleons yuav tsum muaj zog heev thiab ua tiav los ntawm kev sib pauv ntawm qee qhov hais. Kev suav ua nyob rau xyoo 1934 los ntawm Japanese theorist H. Yukawa qhia tias cov khoom no yog 200-300 npaug loj dua li cov hluav taws xob hauv huab hwm coj thiab,Feem ntau, ob peb zaug qis dua rau cov proton. Tom qab ntawd lawv tau txais lub npe mesons, uas nyob rau hauv Greek txhais tau tias "middle". Txawm li cas los xij, lawv thawj qhov kev tshawb pom ncaj qha tau dhau los ua "misfire" vim qhov sib thooj ntawm cov khoom sib txawv heev.
Hauv xyoo 1936, cov khoom (lawv hu ua mu-mesons) nrog cov huab hwm coj sib luag rau Yukawa qhov kev suav tau pom hauv cosmic rays. Nws zoo nkaus li tias nrhiav quantum ntawm nuclear rog tau pom. Tab sis tom qab ntawd nws tau muab tawm tias mu-mesons yog cov khoom uas tsis muaj feem cuam tshuam nrog kev sib pauv ntawm nucleons. Lawv, ua ke nrog cov electron thiab neutrino, koom nrog lwm chav kawm ntawm cov khoom hauv microcosm - lepton. Cov khoom tau hloov npe hu ua muons thiab tshawb nrhiav txuas ntxiv.
Yukawa quanta tau pom tsuas yog xyoo 1947 thiab raug hu ua "pi-mesons", lossis pions. Nws tau muab tawm tias qhov hluav taws xob uas muaj hluav taws xob lossis nruab nrab pi-meson yog qhov tseeb ntawm cov khoom uas nws qhov kev sib pauv tso cai rau nucleons los koom ua ke hauv lub nucleus.
Meson structure
Nws tau paub meej yuav luag tam sim ntawd: peonies tuaj rau "particle zoo" tsis yog ib leeg, tab sis nrog ntau tus txheeb ze. Txawm li cas los xij, nws yog vim muaj pes tsawg leeg thiab ntau yam ntawm cov khoom no uas nws muaj peev xwm tsim tau tias lawv yog kev sib txuas ntawm cov khoom siv me me. Quarks tau dhau los ua cov txheej txheem zoo li no.
Meson yog lub xeev khi ntawm quark thiab antiquark (kev sib txuas yog ua los ntawm quanta ntawm kev sib cuam tshuam zoo - gluons). Tus nqi "muaj zog" ntawm quark yog tus lej quantum, conventionally hu ua "xim". Txawm li cas los xij, tag nrho cov muajthiab mesons ntawm lawv, tsis muaj xim. Nws txhais li cas? Ib lub meson tuaj yeem tsim los ntawm quark thiab antiquark ntawm ntau hom (los yog, raws li lawv hais, flavors, "flavors"), tab sis nws yeej ib txwm sib xyaw xim thiab anticolor. Piv txwv li, π+-meson yog tsim los ntawm ib khub ntawm u-quark - anti-d-quark (ud̄), thiab kev sib xyaw ntawm lawv cov xim xim tuaj yeem yog "xiav - tiv thaiv- xiav", "liab - anti-liab" los yog ntsuab-tiv thaiv ntsuab. Kev pauv ntawm gluons hloov xim ntawm quarks, thaum lub meson tseem tsis muaj xim.
Quarks ntawm cov neeg laus, xws li s, c thiab b, muab cov khoom sib xws rau mesons lawv tsim - txawv, ntxim nyiam thiab ntxim nyiam, qhia los ntawm lawv tus kheej quantum tooj. Tus nqi hluav taws xob ntau ntawm meson yog tsim los ntawm cov nqi qis ntawm cov khoom thiab cov tshuaj tiv thaiv uas tsim nws. Ntxiv rau cov khub no, hu ua valence quarks, lub meson suav nrog ntau (" hiav txwv") cov khub virtual thiab gluons.
Mesons thiab fundamental rog
Mesons, lossis theej, cov quarks uas ua rau lawv, koom nrog txhua hom kev sib cuam tshuam tau piav qhia los ntawm Tus Qauv Qauv. Kev siv zog ntawm kev sib cuam tshuam ncaj qha rau qhov sib luag ntawm cov tshuaj tiv thaiv los ntawm nws, uas yog, kev txuag ntawm qee qhov ntau.
Cov txheej txheem tsis muaj zog yog qhov mob siab tshaj plaws, lawv txuag lub zog, hluav taws xob, lub zog, lub zog ntawm lub zog (spin) - hauv lwm lo lus, tsuas yog universal symmetries ua. Hauv kev sib cuam tshuam electromagnetic, qhov sib luag thiab tsw quantum tooj ntawm mesons kuj tau khaws cia. Cov no yog cov txheej txheem uas ua lub luag haujlwm tseem ceeb hauv cov tshuaj tiv thaivlwj.
Kev sib cuam tshuam muaj zog yog qhov sib luag tshaj plaws, khaws cia lwm qhov ntau, tshwj xeeb, isospin. Nws yog lub luag haujlwm rau khaws cov nucleon hauv lub nucleus los ntawm kev sib pauv ion. Los ntawm emitting thiab nqus cov nqi pi-meson, cov proton thiab neutron tau txais kev sib hloov pauv, thiab thaum lub sij hawm sib pauv ntawm ib qho nruab nrab, txhua tus nucleons tseem nyob ntawm nws tus kheej. Yuav ua li cas qhov no tuaj yeem sawv cev ntawm qib ntawm quarks yog qhia hauv daim duab hauv qab no.
Kev sib cuam tshuam zoo kuj tseem tswj hwm kev tawg ntawm mesons los ntawm nucleons, lawv cov khoom tsim hauv hadron kev sib tsoo thiab lwm yam txheej txheem.
Dab tsi yog quarkonium
Kev sib xyaw ntawm quark thiab antiquark ntawm tib lub tsw yog hu ua quarkonia. Lo lus no feem ntau yog siv rau mesons uas muaj c- thiab b-quarks loj heev. Ib qho hnyav heev t-quark tsis muaj sijhawm los nkag mus rau hauv lub xeev khi txhua, tam sim ntawd lwj mus rau hauv cov sib dua. Kev sib xyaw ua ke cc̄ yog hu ua charmonium, los yog ib qho particle nrog zais ntxim nyiam (J / ψ-meson); kev sib xyaw ua ke bb̄ yog bottomonium, uas muaj qhov ntxim nyiam zais (Υ-meson). Ob leeg yog tus yam ntxwv los ntawm muaj ntau resonant - zoo siab - xeev.
Cov khoom tsim los ntawm cov khoom siv lub teeb - uū, dd̄ lossis ss̄ - yog ib qho superposition (superposition) ntawm flavors, txij li cov pawg ntawm cov quarks nyob ze ntawm tus nqi. Yog li, qhov nruab nrab π0-meson yog ib qho superposition ntawm lub xeev uū thiab dd̄, uas muaj tib pawg ntawm quantum tooj.
Meson instability
Kev sib xyaw ua ke ntawm particle thiab antiparticle ua rautias lub neej ntawm ib qho meson xaus rau hauv lawv txoj kev puas tsuaj. Lub neej nyob ntawm qhov kev sib cuam tshuam tswj kev lwj.
- Mesons that decay through the channel of "strong" annihilation, say, into gluons with the following mesons of new mesons, tsis txhob nyob ntev - 10-20 - 10 - 21 p. Ib qho piv txwv ntawm cov khoom no yog quarkonia.
- Electromagnetic annihilation kuj tseem khaus: lub neej ntawm π0-meson, uas nws quark-antiquark khub annihilates rau hauv ob photons nrog qhov tshwm sim ntawm yuav luag 99%, yog hais txog 8 ∙ 10-17 s.
- Tsis muaj kev puas tsuaj (lwj mus rau hauv lepton) pib nrog kev siv zog tsawg dua. Yog li, tus nqi pion (π+ - ud̄ - lossis π- - dū) nyob ntev heev - qhov nruab nrab 2.6 ∙ 10-8 s thiab feem ntau lwj mus rau hauv muon thiab neutrino (los yog cov khoom sib txuas).
Feem ntau mesons yog qhov hu ua hadron resonances, luv luv (10-22 - 10-24 c) phenomena uas tshwm sim hauv qee lub zog siab, zoo ib yam li lub xeev zoo siab ntawm lub atom. Lawv tsis tau sau npe rau ntawm cov khoom ntes, tab sis suav nrog lub zog sib npaug ntawm cov tshuaj tiv thaiv.
Spin, orbital momentum and parity
Tsis zoo li baryons, mesons yog cov khoom me me uas muaj tus lej ntawm tus lej spin (0 lossis 1), uas yog, lawv yog bosons. Quarks yog fermions thiab muaj ib nrab-tus lej spin ½. Yog hais tias lub sij hawm ntawm lub zog ntawm quark thiab antiquark yog tib yam, ces lawvcov sum - meson spin - yog sib npaug rau 1, yog antiparallel, nws yuav muab sib npaug rau xoom.
Vim kev sib koom ua ke ntawm ib khub ntawm cov khoom, meson kuj muaj tus lej orbital quantum, uas ua rau nws loj. Lub orbital momentum thiab spin txiav txim siab tag nrho cov angular momentum ntawm lub particle, txuam nrog lub tswvyim ntawm spatial, los yog P-parity (ib yam symmetry ntawm yoj ua haujlwm nrog rau daim iav inversion). Raws li kev sib xyaw ua ke ntawm spin S thiab sab hauv (muaj feem xyuam rau cov khoom siv ntawm tus kheej) P-parity, cov nram qab no hom meson yog qhov txawv:
- pseudoscalar - lub lightest (S=0, P=-1);
- vector (S=1, P=-1);
- scalar (S=0, P=1);
- pseudo-vector (S=1, P=1).
Peb hom kawg yog meson loj heev, uas yog lub xeev muaj zog.
Isotopic thiab unitary symmetries
Rau kev faib tawm ntawm mesons nws yooj yim siv tus lej tshwj xeeb quantum - isotopic spin. Hauv cov txheej txheem muaj zog, cov khoom siv nrog tib tus nqi isospin koom nrog symmetrically, tsis hais lawv cov nqi hluav taws xob, thiab tuaj yeem sawv cev raws li cov nqi sib txawv (isospin projections) ntawm ib yam khoom. Ib txheej ntawm cov khoom no, uas nyob ze heev hauv pawg, hu ua isomultiplet. Piv txwv li, pion isotriplet suav nrog peb lub xeev: π+, π0 thiab π--meson.
Tus nqi ntawm isospin yog xam los ntawm tus qauv I=(N–1)/2, qhov twg N yog tus lej ntawm cov khoom hauv multiplet. Yog li, lub isospin ntawm pion yog sib npaug rau 1, thiab nws qhov projections Izhauv tus nqi tshwj xeebqhov chaw yog feem ntau +1, 0 thiab -1. Plaub lub meson txawv txawv - kaons - tsim ob isodoublets: K+ thiab K0 nrog isospin +½ thiab txawv +1 thiab ob npaug ntawm cov tshuaj tiv thaiv K- thiab K̄0, uas cov nqi no tsis zoo.
Tus nqi hluav taws xob ntawm hadrons (xws li mesons) Q yog cuam tshuam nrog isospin projection Iz thiab hu ua hypercharge Y (cov lej ntawm baryon thiab tag nrho cov tsw tus lej). Qhov kev sib raug zoo no tau qhia los ntawm Nishijima–Gell-Mann mis: Q=Iz + Y/2. Nws yog tseeb hais tias tag nrho cov tswv cuab ntawm ib tug multiplet muaj tib lub hypercharge. Tus lej baryon ntawm meson yog xoom.
Tom qab ntawd, cov mesons tau koom nrog kev sib tw ntxiv thiab kev sib luag rau hauv supermultiplets. Yim pseudoscalar mesons tsim ib qho octet, vector hais ua ib qho nonet (cuaj), thiab lwm yam. Qhov no yog ib qho kev tshwm sim ntawm qib siab symmetry hu ua unitary.
Mesons thiab kev tshawb nrhiav rau New Physics
Tam sim no, physicists tab tom tshawb nrhiav qhov tshwm sim, cov lus piav qhia uas yuav ua rau nthuav dav ntawm Tus Qauv Qauv thiab mus dhau nws nrog kev tsim kho qhov tob thiab ntau dua kev xav ntawm microworld - New Physics. Nws yog assumed tias Standard Model yuav nkag mus rau nws raws li ib tug txwv, tsawg-zog rooj plaub. Hauv kev tshawb nrhiav no, kev kawm ntawm mesons ua lub luag haujlwm tseem ceeb.
Ntawm qhov kev txaus siab tshwj xeeb yog cov kab txawv mesons - cov khoom nrog cov qauv uas tsis haum rau hauv lub moj khaum ntawm cov qauv niaj hnub. Yog li, ntawm Loj HadronCollider hauv 2014 tau lees paub Z (4430) tetraquark, lub xeev khi ntawm ob lub ud̄cc̄ quark-antiquark khub, ib qho khoom tawg nruab nrab ntawm qhov zoo nkauj B meson. Cov kev puas tsuaj no kuj tseem nthuav txog qhov muaj peev xwm nrhiav tau ntawm chav kawm tshiab ntawm cov lus hais - leptoquarks.
Cov qauv kuj kwv yees lwm lub xeev kab txawv uas yuav tsum tau muab cais ua mesons, txij li lawv koom nrog cov txheej txheem muaj zog, tab sis muaj pes tsawg baryon tus lej, xws li glueballs, tsim los ntawm gluons nkaus xwb tsis muaj quarks. Tag nrho cov khoom zoo li no tuaj yeem ua kom peb cov kev paub txog qhov tseem ceeb ntawm kev sib cuam tshuam thiab pab txhawb rau kev txhim kho ntxiv ntawm physics ntawm microworld.
