1. 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.

2. basic physics of QCD
Small qq
separation
Large qq
separation

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

4. 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)

5. “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

6. 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.

7. 3 x 3 x 3 = 10 + 8 + 8 + 1 qqq baryons The lightest qqq baryon octet.
(SU(3) symmetry.)
3 x 3 x 3 =

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

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

10. 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 spin triplets

11. 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: 3S ; 1S S: 23S ; 21S …
1P: 3P ; 3P ; 3P ; 1P P …
1D: 3D ; 3D ; 3D ; 1D D …
JPC forbidden to qq are called “JPC-exotic quantum numbers” :
; ; ; ; …
Plausible JPC-exotic candidates =
hybrids, glueballs (high mass), maybe multiquarks (fall-apart decays).

12. 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

13. 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.

14. 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 =
b = [GeV2]
mc = [GeV]
s = [GeV]
S*S OGE

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

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

17. LGT 0+: GeV. S P

18. Where it all started. BABAR: D*sJ(2317)+ in Ds+ p0
D.Aubert et al. (BABAR Collab.),
PRL90, (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.) …

19. And another! CLEO: D*sJ(2463)+ in Ds*+ p0
D.Besson et al. (CLEO Collab.),
PRD68, (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 ?

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

21. Theorists’ responses to the BaBar states
Approx 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]

22. 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”.

23. X(3872) Another recent shock to the system: cc sector
Belle Collab. K.Abe et al, hep-ex/ ;
S.-K.Choi et al, hep-ex/ , PRL91 (2003)
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 = MeV

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

25. X(3872) confirmation (from Fermilab)
CDF II Collab.
D.Acosta et al, hep-ex/ ,
PRL to appear
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
X(3872) also confirmed by
D0 Collab. at Fermilab.
Perhaps also seen by BaBar
OK, it’s real…

26. 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 = MeV
n.b. M( D+ + D*-) = MeV
Charm in nuclear physics???

27. Glueballs: Theor. masses (LGT)
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.

28. 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.

30. 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?

31. p-p -> (p-h’) p Hybrid meson? JPC = 1-+ exotic. (Can’t be qq.)
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

32. (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)

33. 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”

34. 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)

35. 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”
q “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,…)

36. 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 = [deg] there.)
Expt sees only repulsive pp scat.
Mpp [GeV]

37. 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 = 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?)

38. “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)

39. 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.