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Particle Zoo and Quark Model

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Particle Zoo and Quark Model PHY771, 1/26/2016 Tomasz Skwarnicki 1 Historical introduction to Elementary Particles: Particle Zoo and Quark Model Tomasz Skwarnicki Syracuse University • Griffiths, 2 nd ed., 1.6-1.8, 4.3, 5.5, 5.6.1 • Recommend reading about quark model in other books as well (e.g. D.H.Perkins Introduction to High Energy Physics, 4 th ed., 4.3-4.11) PHY771, 1/26/2016 Tomasz Skwarnicki 2 Pion discovery 1946 • C.F.Powell distinguished muons from charged Particle Mass pions in 1946 in cosmic 2 Cecil Frank Powell MeV/c 1903-1969 rays e 0.5 England – Pion is the particle Yukawa µ 105.7 − had predicted as carrier of e − strong interactions in nuclei π 139.6 – Yukawa received Nobel p 938.3 prize in 1947, Powell in n 939.6 1950 µ− • Except for “spurious” muon, it seemed in 1947 that our understanding of elementary ingredients of matter was complete π− PHY771, 1/26/2016 Tomasz Skwarnicki 3 Discovery of strange particles - 1947 • Studying cosmic rays with a cloud chamber at Manchester University in late 1947, G.D. Rochester and C.C. Butler observed a K0 → π+π− neutral particle decaying to π+π− with mass about half of nucleon mass – meson. Nowadays we call it a neutral kaon (K 0). • Charged kaon was first observed in cosmic rays in 1949 via K + → π+π−π+ • In 1950 another “V 0 particle” was observed in cosmic rays: Λ0 → pπ− (V.D. Hopper, S. Biwas, Univ. of Melbourne; also ΛΛΛ0 → pppπππ−−− Anderson group in Caltech) K+ • They were completely unexpected and had very strange lifetimes (see next) π+ π− π+ K+ → π+π−π+ PHY771, 1/26/2016 Tomasz Skwarnicki 4 First accelerators • In 1952 first modern particle accelerator came to operation at Brookhaven National Lab: Bubble chamber photograph Cosmotron (3.3 GeV p beam) p π− Λ0 lives long Lots of particles Incoming p produced; most decay to beam pions right away Kaons also live “strangely” long PHY771, 1/26/2016 Tomasz Skwarnicki 5 Particle Zoo - Meson • Lots of new particles were discovered at accelerators. • Lightest mesons: Spin Mass Main decay mode Lifetime cτ Decay type (width= h /τ) τ ± ± -8 π 0 140 MeV µ νµ 2.6x10 s 7.8 m weak π0 0 135 MeV γγ 8.5x10 -17 s 2.5 10 -8 m electromagnetic η 0 548 MeV γγ 3.1x10 -18 s 9.5 10 -10 m electromagnetic ρ0,ρ ± 1 775 MeV π+π−, π ±π0 2.8x10 -23 s 8.3 10 -15 m strong (149 MeV) ω 1 783 MeV π+π−π0 4.9x10 -22 s 1.5 10 -13 m strong ( 8.2 MeV) η’ 0 958 MeV π+π−η 2.1x10 -20 s 6.3 10 -12 m strong ( 0.2 MeV) 0 + − 0 0 -11 Κ s 0 498 MeV π π , π π 8.9x10 s 0.027 m weak ± ± -8 Κ 0 494 MeV µ νµ 1.2x10 s 3.7 m weak Κ∗0 ,Κ ∗± 1 892 MeV Κ+π−, Κ ±π0 8.3x10 -23 s 2.5 10 -14 m strong (~50 MeV) φ 1 1020 MeV Κ+Κ− 9.6x10 -22 s 2.9 10 -13 m strong (4.3 MeV) 0 + − Why K s does not decay strongly to π π ? [next slide] PHY771, 1/26/2016 Tomasz Skwarnicki 6 Conservation of strangeness - Pais • Abraham Pais suggested: – new quantity “strangeness”: • S( strange particle e.g. K +) = +1 • S( strange antiparticle e.g. K -) = -1 • S( non-strange particle ) = 0 – total strangeness ( ΣiSi) is conserved in strong and electromagnetic interactions – weak interactions can violate strangeness • Lightest strange particles must decay weakly, and therefore live long With particles being produced via 0 + − Κ s → π π proton beam incident at nuclear target (strong interactions!), strange particles ΣiSi 1 0+0=0 weak can only be produced in strange – ∗0 + − Κ → Κ π antistrange pairs ΣiSi 1 1 +0= 1 strong → ... φ → Κ+Κ− pA strong ΣiSi 0+0=0 0 ΣiSi 0 1 +( -1)=0 strong PHY771, 1/26/2016 Tomasz Skwarnicki 7 Particle Zoo - Baryons • Lightest baryons: Spin Strang Mass Main Lifetime cτ Decay type ness (width= h /τ) decay τ mode 1 p /2 0 938 MeV 1 - +2 +8 n /2 0 940 MeV pe νe 8.8x10 s 2.6 10 m weak - 0 + ++ 3 -23 -16 ∆ ,∆ ,∆ ,∆ /2 0 ~1232 MeV pπ, nπ 3.5x10 s 1.1 10 m strong (~117 MeV) 1 − 0 -10 Λ /2 -1 1116 MeV pπ , nπ 2.6x10 s 0.079 m weak + 1 0 + -10 Σ /2 -1 1189 MeV pπ , nπ 0.8x10 s 0.024 m weak 0 1 -20 -11 Σ /2 -1 1193 MeV Λγ 7.4x10 s 2.2 10 m electromagnetic - 1 − -10 Σ /2 -1 1197 MeV nπ 1.5x10 s 0.044 m weak ∗- ∗0 ∗+ 3 -22 -14 Σ ,Σ ,Σ /2 -1 1383 MeV Λπ 1.2x10 s 3.5 10 m strong (~36 MeV) 0 1 0 -10 Ξ /2 -2 1315 MeV Λπ 2.9x10 s 0.087 m weak − 1 − -10 Ξ /2 -2 1322 MeV Λπ 1.6x10 s 0.049 m weak ∗- ∗0 3 -22 -13 Ξ ,Ξ /2 -2 ~1532 MeV Ξπ 4.5x10 s 1.3 10 m strong (~9 MeV) 1) Why proton doesn’t decay (e.g. e +γ)? Pions do. [see next slide] 2) Λ,Σ are strange as seen from lifetimes of their lightest versions 3) Ξ are doubly strange, since their decay to singly strange baryons is weak (i.e. slow) PHY771, 1/26/2016 Tomasz Skwarnicki 8 Conservation of baryon number - Stueckelberg • Ernst Stueckelberg introduced baryon number (~1938) as a conserved quantity to explain why proton does not decay – baryon number • A( baryon e.g. p ) = +1 • A( antibaryon e.g. p ) = -1 • A( not a baryon ) = 0 – total baryon number ( ΣiAi) is conserved in all interactions – For nuclei baryon number is equal to their mass number (justifies use of “A” for the baryon number) • Lightest baryon (p) cannot decay • All baryons discovered in cosmic rays and at accelerators obey this rule, thus subsequent decays of baryons all eventually lead to a proton (unless baryon interacts with a nucleus in a material of apparatus before it decays) e.g. Ξ∗- → Ξ0π− , Ξ 0→ Λπ 0 , Λ → pπ− • Please note, there is no conservation of meson number! Notice, that decay of a meson to baryon-antibaryon pair is possible and does happen e.g. 0 B → pΛπ− ΣiAi 0 1+(-1) +0=0 PHY771, 1/26/2016 Tomasz Skwarnicki 9 Terminology related to strange baryons • Λ : “ Lambda ” • Σ : “ Sigma ” • Ξ : “ Xi ” or “ cascade ” • Any strange baryon ( Λ,Σ,Ξ ) : “ hyperon ” PHY771, 1/26/2016 Tomasz Skwarnicki 10 Particle Zoo • In addition to the lightest mesons and baryons listed on the previous slides, many more, usually short lived, particles were discovered in 1950ies contemporary catalog at http://pdg.lbl.gov Way out of crisis - build “periodic table of elements” PHY771, 1/26/2016 Tomasz Skwarnicki 11 Isospin symmetry - 1932 • Neutron was discovered in 1932: – p and n had almost identical masses – p and n interact in nucleus the same way • Heisenberg proposed to think about p,n as the same particle (nucleon), different via projections of spin-like quantum number “isospin” onto quantization axis: Werner Heisenberg Nucleon I= ½ , I = +½ (p), -½ (n) 1901-1976 z Germany Just like electron ( J= ½ ) with spin projection ( J ) +½ is not z Nobel Prize 1932 considered to be a different particle than electron with spin For development of QM projection -½ (two different quantum states of the same particle) Interaction Lagrangian Conserved Intrinsic particle Possible values Describe particle symmetrical quantity quantum number properties under under rotation in 3 any rotation in Angular Spin ( J) 0, ½, 1, /2, 2,… real space real space momentum 3 strong rotation in Isospin Isospin ( I) 0, ½, 1, /2, 2,… Isospin space isospin space The difference in n,p masses was blamed on electric charge of p Nuclei have well defined isospin (often called isobaric spin ), which is a sum of nucleon isospins. Nuclear reactions conserve isospin – very useful concepts in nuclear physics. PHY771, 1/26/2016 Tomasz Skwarnicki 12 Particle Zoo and isospin symmetry - baryons • Newly discovered baryons clearly formed isospin multiplets: particle Isospin Members of isospin multiplet ( 2I+1 ): (I) Iz , symbol, (mass in MeV) nucleon 1/2 -1/2 n (940) +1/2 p (938) Delta baryon 3/2 -3/2 ∆− -1/2 ∆0 +1/2 ∆+ +3/2 ∆++ Lambda 0 0 Λ (1116) Sigma 1 -1 Σ− (1197) 0 Σ0 (1192) 1 Σ+ (1189) Cascade 1/2 -1/2 Ξ− (1322) +1/2 Ξ0 (1315) The mass differences within multiplets were “blamed” on electromagnetic interactions - though mechanism for that was never really explained and would have been inconsistent Empirical formula for electric charge (Gell-Mann Nishijima): Q = Ιz+ ½ (A+S) A=1 for all above Antibaryons exist for each above e.g.: Σ+ ≠Σ − , Σ≠Σ0 0 PHY771, 1/26/2016 Tomasz Skwarnicki 13 Particle Zoo and isospin symmetry - meson • Mesons formed isospin multiplets as well: particle Isospin Members of isospin multiplet ( 2I+1 ): (I) Iz , symbol, (mass in MeV) pion 1 -1 π− (139) 0 π0 (135) 1 π+ (139) rho meson 1 -1 ρ− (766) 0 ρ0 (769) 1 ρ+ (766) eta meson 0 0 η (548) omega 0 0 ω (782) eta’ meson 0 0 η’ (958) kaon 1/2 -1/2 Κ0 (498) +1/2 K ± (494) Empirical formula for electric charge (Gell-Mann Nishijima): Q = Ιz+ ½ (A+S) A=0 for all above Antiparticles already included above e.g.: π+= π − , π0 = π 0 PHY771, 1/26/2016 Tomasz Skwarnicki 14 “Eightfold Way” symmetry - 1961 • Gell-Mann proposed in 1961 a higher level of symmetry (incorporating Isospin symmetry): – Lightest mesons and baryons formed “ octets ” in space of Y=A+S (“hypercharge”) vs Ιz Y Spin = 0 Y Spin = ½ Q Q +1 +1 0 0 Ιz Ιz -1 -1 -1 0 1 -1 0 1 The differences in masses are now much larger than in isospin multiplets, but there were some regularties within [do related problems 1.4 and 1.5] Nevertheless, this symmetry seemed to “explain” why there were no other (pseudo)scalar mesons or spin ½ baryons in these mass ranges.
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