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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 q q mesons (c c )

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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)


Trouble in charmed mesons

Molecules, multiquarks and pentaquarks


QCD: The Theory of the Strong Interaction

QCD= quantum chromodynamics, ca. 1973

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

spin-1 particles “gluons”gbetween spin-1/2 matter particles,


Similar toQED (quantum electrodynamics), spin-1photons gare exchanged

between spin-1/2electrons e- andpositrons 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.


Comparing QED and QCD. (lagrangians)

“It’s déjà vu all over again.” -Y.Berra


basic physics of QCD

Small qq


Large qq



LGT simulation showing

the QCD flux tube



R = 1.2 [fm]

“funnel-shaped” VQQ(R)



linear conft.

(str. tens. = 16 T)

The QCD flux tube

(LGT, G.Bali et al; hep-ph/010032)



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.


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
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)=udp-(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);

opencharm hadrons;

Do =cu, D+=cd; Ds+=csLc+=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

na ve physically allowed hadrons color singlets

g2, g3,…

qqg, q3g,…

q2q2, q4q,…




maybe 1 e.g.

maybe 1-3 e.g.s

“Naïve” physically allowed hadrons (color singlets)


Conventional quark model

mesons and baryons.



100s of e.g.s

“exotica” :

q q mesons states
qq mesonsstates

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

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 Lqqgives states

of total

Jqq = Lqqspin singlets

Jqq = Lqq+1, Lqq, Lqq-1spin triplets

q q mesons quantum numbers
qq mesonsquantum numbers

ParityPqq = (-1)(L+1) C-parity Cqq = (-1)(L+S)

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

1S: 3S11-- ; 1S00 -+ 2S: 23S11-- ; 21S00 -+ …

1P: 3P22+ + ; 3P11+ + ; 3P00+ + ; 1P11+-2P …

1D: 3D33- - ; 3D22- - ; 3D11- - ; 1D22-+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.


SLAC, DESY 1970s


CLEO-c, BES cc

B-factories bb


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,


as = 0.5538

b = 0.1422 [GeV2]

mc = 1.4834 [GeV]

s = 1.0222 [GeV]


c c from lgt

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.

cc from LGT

<-1- + exotic cc-H at 4.4 GeV


L=2 hfs.


1+ - cc has

been withdrawn.


Sector of the 1st

shocking new discovery:





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
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:

BelleCollab. K.Abe et al, hep-ex/0308029;

S.-K.Choi et al, hep-ex/0309032, PRL91 (2003) 262001.

B+ / - -> K+ / -p+p-J /Y

(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 < 2.3MeV

M = 3872.0 +- 0.6 +- 0.5 MeV


Fitted and predicted cc spectrum

Coulomb (OGE) + linear scalar conft. potential model

blue = expt, red = theory.


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

D0Collab. at Fermilab.

Perhaps also seen by BaBar

OK, it’s real…

x 38721

M = 3872.0 +- 0.6 +- 0.5 MeV

Accidental agreement?

If not cc 2- + or 2- - or …,

a molecular (DD*) state?

M( Do + D*o) = 3871.5 +- 0.5 MeV


n.b. M( D+ + D*-) = 3879.5 +- 0.7MeV

Charm in nuclear physics???



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


All light-q and g mesons,

incl. qq, glueballs, hybrids, multiquarks.


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


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?


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.)


Follow up expts

planned at a new

meson facility


“HallD” or GlueX.





(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” 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.

q2q2 “baryonia”

q6 “dibaryons”

q4q “Z*” for q = s …

now “pentaquarks”

MANY theoretical predictions of avery rich spectrumof

multiquark resonances followed in the 1970s/early 1980s.

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


The simplest e.g. of had-had scat: I=2pp.

(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.


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


E(cluster) > M1 + M2.

Thus no bound states.

Only 1+2 repulsive scattering.



E(cluster) < M1 + M2,

bag model:

u2d2s2 H-dibaryon, MH - MLL = - 80 MeV.

n.b. LLhypernuclei exist, so this H was wrong.


nuclei and hypernuclei

weak int-R attraction allows










Q2q2 (Q=b, c?)

na ve physically allowed hadrons color singlets1


(q3)n, (qq)(qq),(qq)(q3),…

multiquark clusters ???

nuclei / molecules


e.g. Q(1542)?

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

g2, g3,…

qqg, q3g,…

q2q2, q4q,…




maybe 1 e.g.

maybe 1-3 e.g.s

Post-fiasco physically allowed hadrons (color singlets)

“Naïve” physically allowed hadrons (color singlets)


Conventional quark model

mesons and baryons.



100s of e.g.s

Basis state mixing may be

very important in some sectors.

”exotica” :


Does it exist? ca. 10 expt. refs confirm and 10 don’t (incl. HEP).

Follow-up expts. at CLAS (CEBAF)

in progress. They aren’t talking

(in public). Sell now.

summary and conclusions
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.