Monday: Quarks and QCD. Quarks and gluons: QCD, another gauge theory! Basic physics of QCD Quarks and their properties The strong interaction: mesons and baryons

Today: (mainly) mesons + recent discoveries. QCD reminder Conventional qq mesons (cc) Making new hadrons (hit things together) Glueballs and hybrids (gluonic excitations) MOST RECENT ARE: Trouble in charmed mesons Molecules, multiquarks and pentaquarks

The Theory of the Strong Interaction QCD: The Theory of the Strong Interaction QCD = quantum chromodynamics, ca Theory of the strong “nuclear” force. It’s due to the exchange of gluonsg spin-1 particles “gluons” g between spin-1/2 matter particles, “quarks” q and antiquarks q. photons  Similar to QED (quantum electrodynamics), spin-1 photons  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, gluons interact with gluons and [novel] gluons interact with gluons.

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

Small qq separation Large qq separation basic physics of QCD

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

Quarks Minimal solution for quarks needed to explain the known light hadrons: (1964, Gell-Mann, Zweig; Ne’eman): All J P = 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,   = uuu,  = uds,  + = ud, K + = us, etc.

qqq baryons The lightest qqq baryon octet. (SU(3) symmetry.) 3 x 3 x 3 =

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 I z 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  + (135)=ud  - (135)=du,… s strange -1/3 0 (etc) 150 MeV strange hadrons;  =uds,K + =us,… c charm +2/ MeV  family (cc); open charm hadrons; D o =cu, D + =cd; D s + =cs  c + =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

“Naïve” physically allowed hadrons (color singlets) qq q3q3 Conventional quark model mesons and baryons. q 2 q 2, q 4 q,… multiquarks g 2, g 3,… glueballs maybe 1 e.g. qqg, q 3 g,… hybrids maybe 1-3 e.g.s 100s of e.g.s “exotica” : _

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

Parity P qq = (-1) (L+1) C-parity C qq = (-1) (L+S) qq mesons quantum numbers 1S: 3 S 1 1   ; 1 S 0 0   2S: 2 3 S 1 1   ; 2 1 S 0 0   … 1P: 3 P 2 2  ; 3 P 1 1  ; 3 P 0 0  ; 1 P 1 1    2P … 1D: 3 D 3 3  ; 3 D 2 2  ; 3 D 1 1  ; 1 D 2 2    2D … J PC forbidden to qq are called “J PC -exotic quantum numbers” : 0   ; 0  ; 1  ; 2  ; 3  … Plausible J PC -exotic candidates = hybrids, glueballs (high mass), maybe multiquarks (fall-apart decays). The resulting qq NL states N 2S+1 L J have J PC =

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. 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 J/y and other 1 -- cc

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 1D 2 2   2  3.73 GeV Below 3.73 GeV: Annihilation and EM decays. , KK*,  cc, , l  l ..): narrow states.

 s = b = [GeV 2 ] m c = [GeV]  = [GeV] Fitted and predicted cc spectrum Coulomb (OGE) + linear scalar conft. potential model blue = expt, red = theory. S*S OGE L*S OGE – L*S conft, T OGE

cc from LGT   exotic cc-H at 4.4 GeV oops…   cc has been withdrawn. Small L=2 hfs. What about LGT??? An e.g.: X.Liao and T.Manke, hep-lat/ (quenched – no decay loops) Broadly consistent with the cc potential model spectrum. No radiative or strong decay predictions yet.

SP Sector of the 1 st shocking new discovery: cscs

PS LGT 0 + : GeV.

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

And another! CLEO: D * sJ (2463) + in D s * +  0 Since confirmed by BABAR and Belle. M = 2457 MeV. D.Besson et al. (CLEO Collab.), PRD68, (2003). M = 2463 MeV,  < 7 MeV (expt. resolution) A J P =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/ , PRD68, (2003). 3. reality Recall Weinstein and Isgur’s “KKbar molecules”.

X(3872 ) Belle Collab. K.Abe et al, hep-ex/ ; S.-K.Choi et al, hep-ex/ , PRL91 (2003)         J   MeV  = 3 D 1 cc. If the X(3872) is 1D cc, an L-multiplet is split much more than expected assuming scalar conft. MeV Another recent shock to the system: (From e + e - collisions at KEK.) cc sector

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

X(3872 ) n.b.  D   D*   MeV  D   D*   MeV Accidental agreement? If not cc 2  or 2  or …, a molecular (DD*) state? MeV Charm in nuclear physics???

The glueball spectrum from an anisotropic lattice study Colin Morningstar, Mike Peardon Phys. Rev. D60 (1999) 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. Glueballs: Theor. masses (LGT)

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. e.g.s BNL   p -> mesons + baryon LEAR (CERN) pp annih. All light-q and g mesons, incl. qq, glueballs, hybrids, multiquarks.

Glueball discovery? Crystal Barrel expt. ca. 1995) pp -> p 0 p 0 p 0 Evidence for a scalar resonance, f 0 (1500) -> p 0 p 0 n.b. Some prefer a different scalar, f 0 (1710) - > hh, KK. PROBLEM: Neither f 0 decays in a naïve glueball flavor-symmetric way to pp, hh, KK. qq G mixing?

Hybrid meson? J PC = 1 -+ exotic. (Can’t be qq.) ca p - p -> (p - h’) p p 1 (1600) a 2 (1320)q (Current best of several reactions and claimed exotics.) Follow up expts planned at a new meson facility at CEBAF; “HallD” or GlueX. exotic

(Too?) exciting news: the pentaquark at CLAS (CEBAF). nK + = (udd)(us) = u 2 d 2 s. Can’t be a 3 quark baryon! A “flavor exotic” multiquark (if it exists). ( > 200 papers)

An experiment expressly designed to detect “pentaquarks” confirms the existence of these exotic physics particles, researchers reported Sunday. […] Physicists are cautious about leaping onto the pentaquark bandwagon because of past bad experiences […] USA Today 3 May 2004

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”

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. q 2 q 2 “baryonia” q 6 “dibaryons” q 4 q “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,…)

M pp [GeV] I=2 pp S-wave d 0 I=2 [deg] No I=2 q 2 q 2 resonance at 1.2 GeV. (Bag model prediction, would give Dd = [deg] there.) Expt sees only repulsive pp scat. The simplest e.g. of had-had scat: I=2  (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. u 2 d 2 ?

Why are there no multiquark resonances? “Fall-Apart Decay” (actually not a decay at all: no H I ) Most multiquark models found that most channels showed short distance repulsion: E(cluster) > M 1 + M 2. Thus no bound states. Only 1+2 repulsive scattering. nuclei and hypernuclei weak int-R attraction allows “molecules” E(cluster) < M 1 + M 2, bag model: u 2 d 2 s 2 H-dibaryon, M H - M  =  80 MeV. n.b.   hypernuclei exist, so this H was wrong. Exceptions: V NN (R)  2m N RR “V  (R)”  2m  Q 2 q 2 (Q=b, c?) 2) 1) 3) Heavy-light

“Naïve” physically allowed hadrons (color singlets) qq q3q3 Conventional quark model mesons and baryons. q 2 q 2, q 4 q,… multiquarks g 2, g 3,… glueballs maybe 1 e.g. qqg, q 3 g,… hybrids maybe 1-3 e.g.s 100s of e.g.s ”exotica” : ca e.g.s of (q 3 ) n, maybe 1-3 others (q 3 ) n, (qq)(qq), (qq)(q 3 ),… nuclei / molecules (q 2 q 2 ),(q 4 q),… multiquark clusters ??? controversial e.g.  _ Basis state mixing may be very important in some sectors. Post-fiasco physically allowed hadrons (color singlets)

Follow-up expts. at CLAS (CEBAF) in progress. They aren’t talking (in public). Sell now. Does it exist? ca. 10 expt. refs confirm and 10 don’t (incl. HEP).

Summary and conclusions: 1) We now understand EM, weak and strong forces as a single theory, called the standard model (SM). Gravity is not yet included. 2) Both SM components (electroweak and strong int) are very similar renormalizable QFTs of the type known as “non-Abelian gauge theories”. 3) The strong int is described by QCD, a gauge theory of quarks and gluons. Recent developments are concerned with the possible existence of “exotica” - glueballs, hybrids and multiquarks, and charmed mesons much at lower masses than expected. Derivation of nuclear forces (e.g. NN) from QCD is an interesting, open topic.