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

QCD: The Theory of the Strong The Theory of the Strong InteractionInteraction

QCD = quantum chromodynamics, ca. 1973

Theory of the strong “nuclear” force. It’s due to the exchange of

spin-1 particles “gluonsgluons” gg between spin-1/2 matter particles,

“quarks” q and antiquarks q.

Similar to QED (quantum electrodynamics), spin-1 photons 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,

and [novel] gluons interact with gluonsgluons 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/010032)

LGT simulation showing

the QCD flux tube

Q Q

R = 1.2 [fm]

“funnel-shaped” VQQ(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 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, = uuu, = uds, + = ud, K+ = us, etc.

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 +(135)=ud -(135)=du,…

s strange -1/3 0 (etc) 150 MeV strange hadrons; =uds,K+=us,…c charm +2/3 1500 MeV family (cc); open charm hadrons; Do =cu, D+=cd; Ds

+=cs c+=udc, …

b bottom -1/3 5 GeV family (bb); open b hadronst top +2/3 175 GeV t decays too quickly to hadronize

““Naïve” physically allowed hadrons (Naïve” physically allowed hadrons (color color singletssinglets))

qq

q3 Conventional quark modelmesons and baryons.

q2q2, q4q,…

multiquarks

g2, g3,…

glueballs

maybe 1 e.g.

qqg, q3g,…

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

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

First, some conventional hadrons (qq mesons) to illustrate forces.

Parity Pqq

= (-1) (L+1)

C-parity Cqq

= (-1) (L+S)

qq mesons quantum numbers

1S: 3S1 1 ; 1S

0 0 2S: 23S

1 1 ; 21S

0 0 …

1P: 3P2 2 ; 3P

1 1 ; 3P

0 0 ; 1P

1 1

2P …

1D: 3D3 3 ; 3D

2 2 ; 3D

1 1 ; 1D

2 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).

The resulting qq NL states N2S+1LJ have JPC =

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 ee hadrons.

e.g.s

SLAC, DESY 1970s

Now:

CLEO-c, BES cc

B-factories bb(SLAC, KEK)

W,Z machines (LEP@CERN)

J/ 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 statesexcept 1D

2 22

3.73 GeV

Below 3.73 GeV: Annihilation and EM decays.

, KK* , cc, , ll..):narrow states.

s = 0.5538

b = 0.1422 [GeV2]m

c = 1.4834 [GeV]

= 1.0222 [GeV]

Fitted and predicted cc spectrumCoulomb (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/0210030 (quenched – no decay loops)Broadly consistent with the cc potential model spectrum. No radiative or strong decay predictions yet.

S P

Sector of the 1st

shocking new discovery:cs

PS

LGT 0+: 2.44 - 2.47 GeV.

Where it all started. BABAR: D*sJ

(2317)+ in Ds+

0

D.Aubert et al. (BABAR Collab.), PRL90, 242001 (2003).

M = 2317 MeV (2 Ds channels),

< 9 MeV (expt. resolution)

(Theorists expected L=1 cs states, e.g. JP=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 Ds*+

0

Since confirmed by BABAR and Belle. M = 2457 MeV.

D.Besson et al. (CLEO Collab.), PRD68, 032002 (2003).

M = 2463 MeV,

< 7 MeV (expt. resolution)

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)

Belle Collab. K.Abe et al, hep-ex/0308029;S.-K.Choi et al, hep-ex/0309032, PRL91 (2003) 262001.

J

MeV

= 3D1 cc.

If the X(3872) is 1D cc,an L-multiplet is split much more than expected assuming scalar conft.

MeV

Another recent shockto the system:

(From ee collisions at KEK.)

cc sector

Fitted and predicted cc spectrumCoulomb (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. 2003.

n.b. most recent CDF II: M = 3871.3 pm 0.7 pm 0.4 MeV

CDF II Collab. D.Acosta et al, hep-ex/0312021, 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.DD*MeV

DD*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 PeardonPhys. Rev. D60 (1999) 034509

The spectrum of glueballs below 4 GeVin 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 poorlyunderstood.

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. (LEAR@CERN, ca. 1995)

pp 0 0 0

Evidence for a scalar resonance,

f 0 0

n.b. Some prefer a different scalar,

f

PROBLEM: Neither f0 decays in a naïve glueball flavor-symmetric way to KK.qq G mixing?

Hybrid meson? JPC = 1 exotic. (Can’t be qq.)E852@BNL, ca. 1996

p (’) p

aqq

(Current best ofseveral reactions

and claimed exotics.)

Follow up exptsplanned at a newmeson facilityat CEBAF;“HallD” or GlueX.

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)

An experiment expressly designed to detect “pentaquarks” confirmsthe existence of these exotic physics particles, researchers reportedSunday. […]

Physicists are cautious about leaping onto the pentaquark bandwagonbecause 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.

q2q2 “baryonia”

q6 “dibaryons”

q4q “Z*” for q = s … now “pentaquarks”

MANY theoretical predictions of a very rich spectrum ofmultiquark resonances followed in the 1970s/early 1980s.

(Bag model, potential models, QCD_SRs, color chemistry,…)

M [GeV]

I=2 S-wave

[deg]

No I=2 q2q2 resonance at 1.2 GeV.(Bag model prediction, would give = + 180 [deg] there.)

Expt sees only repulsive 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 channelNo qq states. u2d2?

Why are there no multiquark resonances? “Fall-Apart Decay” (actually not a decay at all: no H

I )

Most multiquark models found thatmost channels showed short distance repulsion:

E(cluster) > M1 + M

2.

Thus no bound states.

Only 1+2 repulsive scattering.

nuclei and hypernuclei

weak int-R attraction allows “molecules”

E(cluster) < M1 + M

2,

bag model:

u2d2s2 H-dibaryon, MH - M

= 80 MeV.

n.b.

hypernuclei exist, so this H was wrong.

Exceptions:

VNN

(R)

2mN

R R

“V

(R)”

2m

Q2q2 (Q=b, c?)

2)

1)

3) Heavy-light

““Naïve” physically allowed hadrons (color Naïve” physically allowed hadrons (color singlets)singlets)

qq

q3 Conventional quark modelmesons and baryons.

q2q2, q4q,…

multiquarks

g2, g3,…

glueballs

maybe 1 e.g.

qqg, q3g,…

hybrids

maybe 1-3 e.g.s

100s of e.g.s

”exotica” :ca. 106 e.g.s of (q3)n, maybe 1-3 others

(q3)n, (qq)(qq), (qq)(q3),…

nuclei / molecules

(q2q2),(q4q),…

multiquark clusters ???

controversiale.g.

_

Basis state mixing may be very important in some sectors.

Post-fiasco physically allowed hadrons (color Post-fiasco physically allowed hadrons (color singlets)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.