QCD = quantum chromodynamics, ca. 1973 Ted Barnes Physics Div. ORNL+UT (+DOE) QCD: The Theory of the Strong Interaction QCD = quantum chromodynamics, ca. 1973 Theory of the strong interaction, including “nuclear” forces. It’s due to the exchange of spin-1 particles “gluons” g between spin-1/2 matter particles, “quarks” q and antiquarks q. Similar to QED (quantum electrodynamics), spin-1 photons g are exchanged between spin-1/2 electrons e- and positrons e+. The basic rules of interaction “Feynman vertices” in this “non-Abelian quantum field theory” are that quarks and antiquarks can emit/absorb gluons, and [novel] gluons interact with gluons.

basic physics of QCD Small qq separation Large qq separation

Comparing QED and QCD. (lagrangians) “It’s déjà vu all over again.” -Y.Berra

LGT simulation showing the QCD flux tube R = 1.2 [fm] “funnel-shaped” VQQ(R) Coul. (OGE) linear conft. (str. tens. = 16 T) The QCD flux tube (LGT, G.Bali et al; hep-ph/010032)

“Naïve” physically allowed hadrons (color singlets) _ Conventional quark model mesons and baryons. qq q3 100s of e.g.s “exotica” : g2, g3,… glueballs maybe 1 e.g. qqg, q3g,… hybrids maybe 1-3 e.g.s q2q2, q4q,… multiquarks

u Q = +2/3 e (u,d very similar in mass) d Q = -1/3 e Quarks Minimal solution for quarks needed to explain the known light hadrons: (1964, Gell-Mann, Zweig; Ne’eman): All JP = ½ + (fermions) u Q = +2/3 e (u,d very similar in mass) d Q = -1/3 e s Q = -1/3 e (somewhat heavier) Thus p = uud, n = udd, D++ = uuu, L = uds, p+ = ud, K+ = us, etc.

3 x 3 x 3 = 10 + 8 + 8 + 1 qqq baryons The lightest qqq baryon octet. (SU(3) symmetry.) 3 x 3 x 3 = 10 + 8 + 8 + 1

qq meson The lightest qq meson octet. (SU(3) symmetry.) 3 x 3 = 8 + 1

The six types or “flavors” of quarks : Gens. I,II,III. Label Name Q/|e| I Iz ca. “mass” habitat . u up +2/3 ½ +½ 5 MeV p(938)=uud, n(940)=udd,… d down -1/3 ½ -½ 10 MeV p+(135)=ud p-(135)=du,… s strange -1/3 0 (etc) 150 MeV strange hadrons; L=uds,K+=us,… c charm +2/3 1500 MeV y family (cc); open charm hadrons; Do =cu, D+=cd; Ds+=cs Lc+=udc, … b bottom -1/3 5 GeV U family (bb); open b hadrons t top +2/3 175 GeV t decays too quickly to hadronize

First, some conventional hadrons (qq mesons) to illustrate forces. qq mesons states The quark model treats conventional mesons as qq bound states. Since each quark has spin-1/2, the total spin is Sqq tot = ½ x ½ = 1 + 0 Combining this with orbital angular momentum Lqq gives states of total Jqq = Lqq spin singlets Jqq = Lqq +1, Lqq, Lqq -1 spin triplets

qq mesons quantum numbers Parity Pqq = (-1) (L+1) C-parity Cqq = (-1) (L+S) The resulting qq NL states N2S+1LJ have JPC = 1S: 3S1 1- - ; 1S0 0 - + 2S: 23S1 1- - ; 21S0 0 - + … 1P: 3P2 2+ + ; 3P1 1+ + ; 3P0 0+ + ; 1P1 1+ - 2P … 1D: 3D3 3- - ; 3D2 2- - ; 3D1 1- - ; 1D2 2- + 2D … JPC forbidden to qq are called “JPC-exotic quantum numbers” : 0 - - ; 0 + - ; 1 - + ; 2 + - ; 3 - + … Plausible JPC-exotic candidates = hybrids, glueballs (high mass), maybe multiquarks (fall-apart decays).

How to make new hadrons (strongly int. particles): Hit things together. A + B -> final state You may see evidence for a new resonance in the decay products. J/y and other 1-- cc Some reactions are “clean”, like e+e- -> hadrons. e.g.s SLAC, DESY 1970s Now: CLEO-c, BES cc B-factories bb (SLAC, KEK) W,Z machines (LEP@CERN)

Charmonium (cc) A nice example of a QQ spectrum. Expt. states (blue) are shown with the usual L classification. Above 3.73 GeV: Open charm strong decays (DD, DD* …): broader states except 1D2 2- +, 2- - 3.73 GeV Below 3.73 GeV: Annihilation and EM decays. (rp, KK* , gcc, gg, l+l-..): narrow states.

Fitted and predicted cc spectrum Coulomb (OGE) + linear scalar conft. potential model blue = expt, red = theory. L*S OGE – L*S conft, T OGE as = 0.5538 b = 0.1422 [GeV2] mc = 1.4834 [GeV] s = 1.0222 [GeV] S*S OGE

cc from LGT <- 1- + exotic cc-H at 4.4 GeV What about LGT??? An e.g.: X.Liao and T.Manke, hep-lat/0210030 (quenched – no decay loops) Broadly consistent with the cc potential model. cc from LGT <- 1- + exotic cc-H at 4.4 GeV Small L=2 hfs.

Sector of the 1st shocking new discovery: cs S P

LGT 0+: 2.44 - 2.47 GeV. S P

Where it all started. BABAR: D*sJ(2317)+ in Ds+ p0 D.Aubert et al. (BABAR Collab.), PRL90, 242001 (2003). M = 2317 MeV (2 Ds channels), G < 9 MeV (expt. resolution) “Who ordered that !?” - I.I.Rabi (about the m- ) Since confirmed by CLEO, Belle and FOCUS. (Theorists expected L=1 cs states, e.g. JP=0+, but with a LARGE width and at a much higher mass.) …

And another! CLEO: D*sJ(2463)+ in Ds*+ p0 D.Besson et al. (CLEO Collab.), PRD68, 032002 (2003). M = 2463 MeV, G < 7 MeV (expt. resolution) Since confirmed by BABAR and Belle. M = 2457 MeV. A JP=1+partner of the possibly 0+ D*sJ(2317)+ cs ?

(Godfrey and Isgur potential model.) Prev. (narrow) expt. states in gray. DK threshold

Theorists’ responses to the BaBar states Approx. 100 theoretical papers have been published since the discovery. There are two general schools of thought: 1) They are cs quark model mesons, albeit at a much lower mass than expected by the usual NRQPMs. [Fermilab] 2) They are “multiquark” states. (DK molecules) [UT,Oxon,Weiz.] 3) They are somewhere between 1) and 2). [reality]

2. They are multiquark states (DK molecules) [UT,Oxon,Weiz.] T.Barnes, F.E.Close, H.J.Lipkin, hep-ph/0305025, PRD68, 054006 (2003). 3. reality Recall Weinstein and Isgur’s “KKbar molecules”.

X(3872) Another recent shock to the system: cc sector Belle Collab. K.Abe et al, hep-ex/0308029; S.-K.Choi et al, hep-ex/0309032, PRL91 (2003) 262001. B+ / - -> K+ / - p+p- J / Y X(3872) (From e+e- collisions at KEK.) cc sector y(3770) = 3D1 cc. If the X(3872) is 1D cc, an L-multiplet is split much more than expected assuming scalar conft. G < 2.3 MeV M = 3872.0 +- 0.6 +- 0.5 MeV

Fitted and predicted cc spectrum Coulomb (OGE) + linear scalar conft. potential model blue = expt, red = theory. X(3872) not cc ???

X(3872) confirmation (from Fermilab) CDF II Collab. D.Acosta et al, hep-ex/0312021, PRL to appear X(3872) confirmation (from Fermilab) G.Bauer, QWG presentation, 20 Sept. 2003. n.b. most recent CDF II: M = 3871.3 pm 0.7 pm 0.4 MeV X(3872) also confirmed by D0 Collab. at Fermilab. Perhaps also seen by BaBar OK, it’s real…

X(3872) M = 3872.0 +- 0.6 +- 0.5 MeV M( Do + D*o) = 3871.5 +- 0.5 MeV Accidental agreement? If not cc 2- + or 2- - or …, a molecular (DD*) state? M = 3872.0 +- 0.6 +- 0.5 MeV n.b. M( D+ + D*-) = 3879.5 +- 0.7 MeV Charm in nuclear physics???

Glueballs: Theor. masses (LGT) The glueball spectrum from an anisotropic lattice study Colin Morningstar, Mike Peardon Phys. Rev. D60 (1999) 034509 The spectrum of glueballs below 4 GeV in the SU(3) pure-gauge theory is investigated using Monte Carlo simulations of gluons on several anisotropic lattices with spatial grid separations ranging from 0.1 to 0.4 fm.

How to make new hadrons (strongly int. particles) (II): Hit more things together. A + B -> final state You may see evidence for a new resonance in the decay products. Reactions between hadrons (traditional approach) are “rich” but usually poorly understood. All light-q and g mesons, incl. qq, glueballs, hybrids, multiquarks. e.g.s BNL p-p -> mesons + baryon LEAR (CERN) pp annih.

Glueball discovery? Crystal Barrel expt. (LEAR@CERN, ca. 1995) pp -> p0 p0 p0 Evidence for a scalar resonance, f0(1500) -> p0 p0 n.b. Some prefer a different scalar, f0(1710) -> hh, KK. PROBLEM: Neither f0 decays in a naïve glueball flavor-symmetric way to pp, hh, KK. qq <-> G mixing?

p-p -> (p-h’) p Hybrid meson? JPC = 1-+ exotic. (Can’t be qq.) E852@BNL, ca. 1996 p-p -> (p-h’) p (Current best of several reactions and claimed exotics.) p1(1600) Follow up expts planned at a new meson facility at CEBAF; “HallD” or GlueX. a2(1320) qq exotic

(Too?) exciting news: the pentaquark at CLAS (CEBAF). nK+ = (udd)(us) = u2d2s. Can’t be a 3 quark baryon! A “flavor exotic” multiquark (if it exists). ( > 200 papers)

The multiquark fiasco “These are very serious charges you’re making, and all the more painful to us, your elders, because we still have nightmares from five times before.” - village elder, “Young Frankenstein”

Upper Limit on the Q+ Yield Counts/4 MeV Counts/4 MeV -0.8 < cosqCM < -0.6 preliminary Q+(1540) ? no structure is observed at a mass of ~1540 MeV the nK+ mass spectrum is smooth M(nK+)(GeV) Counts/4 MeV 0.6 < cosqCM < 0.8 M(nK+)(GeV) M(nK+)(GeV)

The dangerous 1970s multiquark logic: (which led to the multiquark fiasco) The known hadron resonances, qq and qqq (and qqq) exist because they are color singlets. Therefore all higher Fock space “multiquark” color singlet sectors will also possess hadron resonances. q2q2 “baryonia” q6 “dibaryons” q4q “Z*” for q = s … now “pentaquarks” MANY theoretical predictions of a very rich spectrum of multiquark resonances followed in the 1970s/early 1980s. (Bag model, potential models, QCD_SRs, color chemistry,…)

Mpp [GeV] The simplest e.g. of had-had scat: I=2 pp. Q = +2 channel (A flavor-exotic 27 channel, no s-channel qq resonances, so no qq annihilation. Similar to the NN and BB’ problems.) Q = +2 channel No qq states. u2d2? I=2 pp S-wave d 0I=2 [deg] No I=2 q2q2 resonance at 1.2 GeV. (Bag model prediction, would give Dd = + 180 [deg] there.) Expt sees only repulsive pp scat. Mpp [GeV]

Why are there no multiquark resonances? “Fall-Apart Decay” (actually not a decay at all: no HI ) Most multiquark models found that most channels showed short distance repulsion: E(cluster) > M1 + M2. Thus no bound states. Only 1+2 repulsive scattering. Exceptions: 2) E(cluster) < M1 + M2, bag model: u2d2s2 H-dibaryon, MH - MLL = - 80 MeV. n.b. LLhypernuclei exist, so this H was wrong. 1) nuclei and hypernuclei weak int-R attraction allows “molecules” VNN(R) -2mN “VLL(R)” -2mL 3) Heavy-light R R Q2q2 (Q=b, c?)

“Naïve” physically allowed hadrons (color singlets) Post-fiasco physically allowed hadrons (color singlets) “Naïve” physically allowed hadrons (color singlets) _ Conventional quark model mesons and baryons. qq q3 100s of e.g.s ca. 106 e.g.s of (q3)n, maybe 1-3 others (q3)n, (qq)(qq), (qq)(q3),… nuclei / molecules Basis state mixing may be very important in some sectors. ”exotica” : g2, g3,… glueballs maybe 1 e.g. qqg, q3g,… hybrids maybe 1-3 e.g.s q2q2, q4q,… multiquarks (q2q2),(q4q),… multiquark clusters ??? controversial e.g. the ex-pentaquark Q(1542)

Summary and conclusions: The strong interaction is described by QCD, a gauge theory of quarks and gluons, which possesses the remarkable property of “confinement”. This implies that a rich spectrum of qq mesons, qqq baryons, glueballs and hybrids, and possibly multiquarks, should exist and be observable experimentally. This theory should also predict the nuclear forces that underlie NP. Techniques used by theorists to study these states include models (esp. the NR quark model), and most recently lattice QCD. Recent developments are concerned with the possible existence of the “exotica” - glueballs, hybrids and multiquarks, charmed mesons found at much lower masses than expected, and an assortment of charmed quark hadrons, esp. cc “charmonia”. Derivation of nuclear forces (e.g. NN) from QCD is an interesting, open topic.