Heavy-quark hybrid mesons and the Born-Oppenheimer approximation

Transcription

1 Heavy-quark hybrid mesons and the Born-Oppenheimer approximation Colin Morningstar Carnegie Mellon University Quarkonium Workshop, Fermilab Sept 20, /20/2003 Hybrid mesons (C. Morningstar) 1

2 Outline! Introduction! Heavy-quark mesons: leading Born-Oppenheimer approximation " Stationary states of gluons in presence of static qq-pair " Leading order spectrum (no light quark pairs) " Testing the leading Born-Oppenheimer approximation! Quark spin effects! Incorporation of light quark loops! Conclusion 9/20/2003 Hybrid mesons (C. Morningstar) 2

3 Heavy-quark hybrid mesons! more amenable to theoretical treatment than light-quark hybrids! early work: Hasenfratz, Horgan, Kuti, Richard (1980), Michael, Griffiths, Rakow (1983)! possible treatment like diatomic molecule (Born-Oppenheimer) " slow heavy quarks #$ nuclei " fast gluon field #$ electrons (and light quarks)! gluons provide adiabatic potentials V QQ (r) " gluons fully relativistic, interacting " potentials computed in lattice simulations! nonrelativistic quark motion described in leading order by solving Schrodinger equation for each V QQ (r) 2 p + V ( r) ( r) E ( r) QQ ψ = ψ QQ QQ 2µ +! conventional mesons from Σ g ; hybrids from Π, Σ,... 9/20/2003 Hybrid mesons (C. Morningstar) 3 u u

4 First step in Born-Oppenheimer! first step in Born-Oppenheimer approximation " determine the gluonic terms! calculational approach $ resort to Monte Carlo methods " familiar perturbative Feynman diagram techniques fail " Schwinger-Dyson equations intractable! estimate path integrals: very high dimensionality " Markov chain methods! lattice regularization permits formulation of field theory suitable for computer simulations 9/20/2003 Hybrid mesons (C. Morningstar) 4

5 Generalized Wilson loops! gluonic terms extracted from generalized Wilson loops! large set of gluonic operators $ correlation matrix 9/20/2003 Hybrid mesons (C. Morningstar) 5

6 Excitations of static quark potential! gluon field in presence of static quark-antiquark pair can be excited! classification of states: (notation from molecular physics) " magnitude of glue spin projected onto molecular axis Λ = 0,1,2,... = Σ, Π,,... " charge conjugation + parity about midpoint η = g (even) = u (odd) " chirality (reflections in plane containing axis) Σ +, Σ Π,, doubly degenerate (Λ doubling) several higher levels not shown g, g, Juge, Kuti, Morningstar, PRL 90, (2003) 9/20/2003 Hybrid mesons (C. Morningstar) 6 Π Φ u, Π u, Σ u g, Σ,... + u,

7 Three scales! small quark-antiquark separations r " excitations consistent with states from multipole OPE! crossover region 0.5fm < r < 2fm " dramatic level rearrangement! large separations r > 2fm " excitations consistent with expectations from string models Juge, Kuti, Morningstar, PRL 90, (2003) 9/20/2003 Hybrid mesons (C. Morningstar) 7

8 Possible interpretation! small r " strong E field of qq-pair repels physical vacuum (dual Meissner effect) creating a bubble " separation of degrees of freedom gluonic modes inside bubble (low lying) bubble surface modes (higher lying)! large r " bubble stretches into thin tube of flux " separation of degrees of freedom collective motion of tube (low lying) internal gluonic modes (higher lying) " low-lying modes described by an effective string theory (Nπ/r gaps Goldstone modes) 9/20/2003 Hybrid mesons (C. Morningstar) 8

9 Gluon excitation gaps (N=1,2( N=1,2)! comparison of gaps with N π / R 9/20/2003 Hybrid mesons (C. Morningstar) 9

10 Gluon excitation gaps (N=3( N=3)! comparison of gaps with N π / R 9/20/2003 Hybrid mesons (C. Morningstar) 10

11 Gluon excitation gaps (N=4( N=4)! comparison of gaps with N π / R 9/20/2003 Hybrid mesons (C. Morningstar) 11

12 Leading Born-Oppenheimer 2! replace covariant derivative D r 2 by r $ neglects retardation! neglect quark spin effects! solve radial Schrodinger equation d u( r) Lqq + 2 2µ dr 2µ r! angular momentum r r r r J = L + S S! in LBO, L and S are good quantum numbers! centrifugal term r2 L = L qq PC! J eigenstates $ Wigner rotations LSJM ; Λ η + ε LSJM ; Λη + Vqq ( r) u( r) = Eu( r) r r r r r = s + s L = L + J ± " η is CP, ε = ± 1 for Λ 1, ε = ± 1 for Σ PC L+Λ+ 1! LBO allowed J $ P = ε 1, C = ηε ( L + 1) 2Λ + J g q 9/20/2003 Hybrid mesons (C. Morningstar) 12 r q r J 2 g qq = 0 = 2 ( ) ( ) g + ( Σ g ) ( Π, Σ ) u L+ S+Λ u

13 Leading Born-Oppenheimer spectrum! results obtained (in absence of light quark loops)! good agreement with experiment below BB threshold! plethora of hybrid states predicted (caution! quark loops)! but is a Born-Oppenheimer treatment valid? _ LBO degeneracies: Σ Σ Π + g + g u ( S) : ( P) : ( P) : ,1,1,1 ++, 0 + +, 2, 2 ++,1 ++ +,1 +,1, 2 +, Juge, Kuti, Morningstar, Phys Rev Lett 82, 4400 (1999) 9/20/2003 Hybrid mesons (C. Morningstar) 13

14 Charmonium LBO! same calculation, but for charmonium 9/20/2003 Hybrid mesons (C. Morningstar) 14

15 Testing LBO! test LBO by comparison of spectrum with NRQCD simulations " include retardation effects, but no quark spin, no p r 4, no light quarks " allow possible mixings between adiabatic potentials! dramatic evidence of validity of LBO " level splittings agree to 10% for 2 conventional mesons, 4 hybrids H 1, H H H ' = 1 = 1 = , 0,0, ,1,1 + +, 2, higher order NRQCD lowest hybrid 1.49(2)(5) GeV above 1S 9/20/2003 Hybrid mesons (C. Morningstar) 15

16 Compelling physical picture! Born-Oppenheimer provides simple physical picture for heavy-quark conventional and hybrid meson states " partial explanation of quark model success " insight into light quarks? " allows incorporation of gluon dynamics (beyond quark model)! does this BO picture survive inclusion of " quark spin? " light-quark effects? 9/20/2003 Hybrid mesons (C. Morningstar) 16

17 Quark spin effects! Burch and Toussaint, hep-lat/ " NRQCD simulations, measured mixing via " mixing in bottomonium seems not to spoil BO picture " larger effect in charmonium c σ B / M 1 1H 1H ϒ H J / Ψ η H η b c 9/20/2003 Hybrid mesons (C. Morningstar) 17

18 Light quark spoiler?! spoil B.O.? $ unknown! light quarks change V QQ (r) " small corrections at small r fixes low-lying spectrum " large changes for r>1 fm $ fission into ( Qq)( Qq)! states with diameters over 1 fm " most likely cannot exist as observable resonances! dense spectrum of states from pure glue potentials will not be realized " survival of a few states conceivable given results from Bali et al.! discrepancy with experiment above BB _ " most likely due to light quark effects with light quarks 9/20/2003 Hybrid mesons (C. Morningstar) 18

19 String breaking! string breaking using 2 body operators! two flavors of dynamical staggered quarks near 1.2 fm lattice spacing 0.16 fm pion/rho mass body operators string operator Bernard et al., PRD64, (2001) 9/20/2003 Hybrid mesons (C. Morningstar) 19

20 Bottomonium hybrids! recent calculation of bottomonium hybrids confirms earlier results " quenched, several lattice spacings so a 0 limit taken " improved anisotropic gluon and fermion (clover) actions " good agreement with Born-Oppenheimer (but errors large) Liao, Manke, PRD65, (2002) hybrids 9/20/2003 Hybrid mesons (C. Morningstar) 20

21 Charmonium hybrids! recent determination of some charmonium hybrids " quenched, several lattice spacings for continuum limit " improved, anisotropic gluon and fermion (clover) actions " results suggest significant (but not large) corrections from LBO Liao, Manke, hep-lat/ /20/2003 Hybrid mesons (C. Morningstar) 21

22 Conclusion! hadronic states bound by an excited gluon field " interesting new form of matter " shed new light on confinement in QCD! heavy-quark hybrid mesons " validity of a Born-Oppenheimer treatment " relationship to excitations of the static quark potential " compelling physical picture " quark spin effects do not spoil BO " light quark loops $ survival issue 9/20/2003 Hybrid mesons (C. Morningstar) 22