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QCD Exotics – Past, Present and Future
Exotics as a Probe of Confinement
Curtis A. Meyer
Carnegie Mellon University
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The beginning
of time.
Outline
The strong force
and QCD
Color confinement
Spectroscopy
Lattice QCD
Finding Gluonic
Hadrons
Confinement
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The First Seconds of The Universe
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Quark Gluon Plasma
For a period from about 10-12 s to 10-6 s the universe
contained a plasma of quarks, anti quarks and gluons.
Relativistic Heavy Ion Collisions are trying to
produce this state of matter in collisions
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Confinement
From about 10-6 s on, the quark and anti quarks became
confined inside of Hadronic matter. At the age of 1s,
only protons and neutrons remained.
Flux
tube
forms
between
qq
Mesons
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The gluons produce
the 16ton force that
holds the hadrons
together.
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Baryons
The Formation of Nuclei
By the old age of three minutes, the formation of low
mass nuclei was essentially complete.
Primordial hydrogen, deuterium, helium and a
few other light nuclei now exist.
It will be nearly a half a million years before
neutral atoms will dominate matter.
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Quantum Chromo Dynamics
The rules that govern how the quarks
froze out into hadrons are given by QCD.
Just like atoms are
electrically neutral,
hadrons have to be
neutral.
Color Charge
Three charges called RED, BLUE and
GREEN, and three anti colors. The
objects that form have to be color
neutral:
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Gluons Carry the Force
The exchange of gluons
is continually changing the
Individual colors of the
quarks, but the overall
Color remains neutral
G
Meson
G R
G R
Time
R
B
B
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R G
G
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R
Meson
Gluons Carry the Force
The exchange of gluons
is continually changing the
Individual colors of the
quarks, but the overall
Color remains neutral
G
Meson
G R
G R
Time
R
B
B
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R G
G
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R
Meson
Gluons Carry the Force
The exchange of gluons
is continually changing the
Individual colors of the
quarks, but the overall
Color remains neutral
G
Meson
R
G R
G R
Meson
Time
R
B
B
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R G
G
Gluons produce the forces that
confine the quarks, but the gluons
do not appear to be needed to
understand normal hadrons
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Gluon Interactions
R
G
R
B
G
R
3 Colors
3 Anti Colors
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R G
B
G
B
R
G
R
1 color neutral
8 colored objects
8 Gluons
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R
R G
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self-interaction of gluons
leads to both interesting
behavior of QCD, and its
extreme complications.
Flux Tubes
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Flux Tubes
Flux
tube
forms
between
qq
Color Field: Because of self
interaction, confining flux tubes
form between static color charges
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Confinement arises from
flux tubes and their
excitation leads to a new
spectrum of mesons
Quark Confinement
• quarks can never be isolated
• linearly rising potential
– separation of quark from antiquark takes an
infinite amount of energy
– gluon flux breaks, new quark-antiquark pair
produced
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Strong QCD
See
qq
and
qqq systems.
Color singlet objects observed in nature:
Nominally, glue is
not needed to
describe hadrons.
white
white
u u
d d
s s
Focus on “light-quark mesons”
quark-antiquark states
Allowed systems:
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glueballs
hybrids
gg, ggg , qq g , qq qq
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Spectroscopy
A probe of QED
Positronium
e+
Spin: S=S1+S2=(0,1)
e-
Orbital Angular Momentum: L=0,1,2,…
Total Spin: J=L+S
L=0, S=0 : J=0 L=0, S=1 : J=1
L=1 , S=0 : J=1 L=1, S=1 : J=0,1,2
…
…
Reflection in a mirror:
Parity: P=-(-1)(L)
Notation: J(PC)
(2S+1)L
J
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Particle<->Antiparticle:
Charge Conjugation: C=(-1)(L+S)
0-+, 1--, 1+-, 0++, 1++, 2++
1S , 3S , 1P , 3P , 3P , 3P ,…
0
1
1
0
1
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Spectroscopy and QCD
Quarkonium
Mesons
Consider the three lightest quarks
4++
++
L=3 3 ++
2
3+-
u, d , s
u, d , s
3-2-L=2 -1
2-+
L=1
2++
1++
0++
1+-
L=0
1-0-+
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q
q
9 Combinations
us
ds
1
uu  dd 
2
du
sd
S=1
S=0
1
uu  dd  ss 
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ud
su
1
uu  dd  2ss 
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Spectroscopy an QCD
Quarkonium
Mesons
q
4++
++
L=3 3 ++
2
3+-
r,K*,w,f
3-2-L=2 -1
2-+
L=1
L=0
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p,K,h,h’
0-+
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Mesons come in
Nonets of the same
JPC Quantum Numbers
a,K,f,f’
2++
1++
0++
1+1--
q
b,K,h,h’
S=1
S=0
r,K*,w,f
p,K,h,h’
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SU(3) is broken
Last two members mix
Quantum Mechanical Mixing
States with the same quantum numbers mix:
1 
1
uu  dd  ss 
3
1
uu  dd  2ss 
8 
6
 cos  sin   1


SU(3)
f'
  sin  cos   8
f
physical
states
Ideal Mixing:
2
cos 
3
1
sin  
3
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1
f
uu  dd 
 2
f'
ss
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Spectroscopy an QCD
Mesons
Nothing to do
with Glue!
4++
++
L=3 3 ++
2
3+-
L=1
L=0
1-0-+
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q
q
Allowed JPC Quantum numbers:
3-2-L=2 -1
2-+
2++
1++
0++
1+-
Quarkonium
S=1
S=0
0-- 0++ 0-+ 0+1–- 1++ 1-+ 1+2-- 2++ 2-+ 2+3-- 3++ 3-+ 3+4-- 4++ 4-+ 4+5-- 5++ 5-+ 5+Exotic Quantum Numbers
non quark-antiquark description
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Glueball Predictions
Particles that only contain gluons
qq Mesons
2.5
1.5
1.0
Glueballs
2.0
2 –+
0 –+
2 ++
0 ++
L= 01 2 3 4
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All of these are normal
quark-antiquark quantum
numbers.
Lattice QCD Predictions
Gluons can bind to form glueballs
EM analogue: massive globs
of pure light.
Lattice QCD predicts masses
The lightest glueballs have
“normal” quantum numbers.
Glueballs will Q.M. mix
The observed states will
be mixed with normal
mesons.
Strong experimental evidence
For the lightest state.
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Identification of Glueballs
Lightest Glueball predicted near two states of same Q.N..
“Over population” Predict 2, see 3 states
Glueballs should decay in a flavor-blind fashion.
pp : KK : hh : h ' h ' : hh '  3 : 4 : 1 : 1 : 0
Production Mechanisms:
Certain are expected to by Glue-rich, others are
Glue-poor. Where do you see them?
Proton-antiproton
Central Production
J/y decays
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Crystal Barrel Results: antiproton-proton annihilation at rest
Crystal Barrel Results
Discovery of the f0(1500)
f0(1500) a pp, hh, hh’, KK, 4p
Solidified the f0(1370)
f0(1370) a 4p
Establishes the scalar nonet
Discovery of the a0(1450)
700,000 p0p0p0 Events
f2(1565)+s
250,000 hhp0 Events
f0(1500)
f2(1270)
f0(980)
f0(1500)
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Wa102 Results
CERN experiment colliding p
on a hydrogen target.
Central Production Experiment
f 0 (1370)  pp
 2.17  0.90
f 0 (1370)  KK
f 0 (1370)  hh
 0.35  0.21
f 0 (1370)  KK
f 0 (1500)  pp
 5.5  0.84
f 0 (1500)  hh
f 0 (1500)  KK
 0.32  0.07
f 0 (1500)  pp
f 0 (1500)  hh '
 0.52  0.16
f 0 (1500)  hh
f 0 (1710)  pp
 0.20  0.03
f 0 (1710)  KK
Recent comprehensive data set
and a coupled channel analysis.
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f 0 (1710)  hh
 0.48  0.14
f 0 (1710)  KK
f 0 (1710)  hh '
 0.05(90%cl )
f 0 (1710)  hh
A Model for Mixing
meson
meson
meson
meson
Glueball
meson
meson
Glueball
meson
G  qq flavor blind?
uu , dd , ss
1
r2
r3
r
Solve for mixing scheme
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F.Close: hep-ph/0103173
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Experimental Evidence
Glueballs
Scalar (0++) Glueball and two
nearby mesons are mixed.
f0(1710)
f0(1500)
a0(1450)
K*0(1430)
f0(1370)
Glueball
spread
over 3
mesons
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a0(980)
f0(980)
Are there other glueballs?
Higher Mass Glueballs?
Part of the BES-III program will be to search for
glueballs in radiative J/y decays.
Lattice predicts that the 2++ and the 0-+ are the
next two, with masses just above 2GeV/c2.
Radial Excitations of the 2++ ground state
L=3 2++ States + Radial excitations
f2(1950), f2(2010), f2(2300), f2(2340)…
2’nd Radial Excitations of the h and h’,
perhaps a bit cleaner environment! (I would
Not count on it though….)
I expect this to be very challenging.
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Hybrid Meson Predictions
Flux-tube model: 8 degenerate nonets
1++,1-- 0-+,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2
S=0
qqg
S=1
qq Mesons
2.0
1.5
2 +–
2 –+
1 ––
1– +
1 +–
1 ++
0 +–
0 –+
1.0
L= 01 2 3 4
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Hybrids
2.5
exotic
nonets
Start with S=0
1++ & 1-Start with S=1
0-+ & 0+1-+ & 1+2-+ & 2+-
QCD Potential
ground-state
excited flux-tube
flux-tube
m=1
m=0
linear potential
Gluonic Excitations provide an
experimental measurement of
the excited QCD potential.
Observations of the nonets on the excited potentials are
the best experimental signal of gluonic excitations.
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Hybrid Predictions
Flux-tube model: 8 degenerate nonets
1++,1-- 0-+,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2
S=0
S=1
Lattice calculations --- 1-+ nonet is the lightest
UKQCD (97) 1.87 0.20
MILC (97)
1.97 0.30
1-+
1.9+/- 0.2
MILC (99)
2.11 0.10
2+- 2.0+/- 0.11
Lacock(99)
1.90 0.20
0+2.3+/- 0.6
Mei(02)
2.01 0.10
Bernard(04) 1.792§0.139
In the charmonium sector:
1-+
4.39 0.08
Splitting = 0.20
+0
4.61 0.11
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Fit n-D angular distributions
Fit Models of production and
decay of resonances.
Lglue
Meson
Analysis Method
Partial Wave Analysis
Decay Predictions
Meson
Looking for Hybrids
Angular momentum
in the gluon flux stays confined.
This leads to complicated
multi-particle final states.
Detector needs to be
very good.
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Experimental Evidence Hybrids
Exotic Mesons
1-+ mass 1.4
E852 BNL ’97
CBAR CERN ’97
Too light
Not Consistent
Possible rescattering (?)
Decays are wrong (?)
Not a Hybrid
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New York Times,
Sept. 2, 1997
E852 Results


p p p

p p
M(p pp  ) GeV / c2 

p pp p p p


 
At 18 GeV/c

M(p p ) GeV / c2 
to partial wave analysis
suggests

0 
p pr p p
 p p  p  p
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An Exotic Signal
Correlation of
Phase
&
Intensity
1
Exotic
Signal
p1(1600)
Leakage
From
Non-exotic Wave
due to imperfectly
understood acceptance
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M(p pp  ) GeV / c2 
3p m=1593+-8+28-47 G=168+-20+150-12
ph’ m=1597+-10+45-10 G=340+-40+-50
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In Other Channels
1-+ in h’p
p-p  h’p-p at 18 GeV/c
The p1(1600) is the
Dominant signal in h’p.
Mass = 1.5970.010 GeV
Width = 0.3400.040 GeV
p1(1600)  h’p
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E852 Results
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In Other Channels
1-+ in f1p and b1p
p-p  hp+p-p-p
p-p  wp0p-p
E852 Results
p1(1600)  b1p
p1(1600)  f1p
Mass=1.7090.024 GeV
Width=0.4030.08 GeV
In both b1p and f1p, observe
Excess intensity at about
2GeV/c2.
Mass ~ 2.00 GeV,
Mass = 1.6870.011 GeV
Width ~ 0.2 to 0.3 GeV
Width = 0.2060.03 GeV
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New Analysis
Add
Add
Add
Add
Add
p2(1670)->rp (L=3)
p2(1670)->r3p
p2(1670)-> (pp)Sp
a3 decays
a4(2040)
No Evidence for the p1(1670)
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Dzierba et. al. PRD 73 (2006)
10 times statistics in
each of two channels.
Get a better description
of the data via moments
comparison
Exotic Signals
p1(1400) Width ~ 0.3 GeV, Decays: only hp
NOT A
weak signal in pp production (scattering??)
HYBRID
strong signal in antiproton-deuterium.
p1(1600) Width ~ 0.16 GeV, Decays rp,h’p,(b1p) Does
this
Only seen in pp production, (E852 + VES)
exist?
p1(2000) Weak evidence in preferred hybrid The right
modes f1p and b1p
place. Needs
confirmation.
p1 IG(JPC)=1-(1-+)
K1 IG(JPC)=
h1 IG(JPC)=0+(1-+)
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½ (1-)
h’1 IG(JPC)=0+(1-+)
Experimental Evidence
Hybrid Nonets
1-+
Establish other Nonets:
0+-
1-+
New York Times,
Sept. 2, 1997
2+-
Levels
Built on normal mesons
us
ds
1
uu  dd 
2
du
sd
1
uu  dd  ss 
3
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ud
su
1
uu  dd
6
Identify other
states in nonet
 2 ss 
to establish hybrid
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Jefferson Lab Upgrade
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Jefferson Lab Upgrade
Upgrade magnets
and power
supplies
CHL-2
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The GlueX Experiment
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How to Produce Hybrids
Beams of photons may be a more natural
way to create hybrid mesons.
Simple QN counting leads to the exotic mesons
There is almost no data for photon beams at
these energies. GlueX will increase samples by
3-4 orders of magnitude.
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This technique
provides requisite
energy, flux and
polarization
flux
Coherent
Bremsstrahlung
12 GeV electrons
Incoherent &
coherent spectrum
40%
polarization
in peak
Linearly polarized
photons out
collimated
electrons in
spectrometer
diamond
crystal
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tagged
with 0.1% resolution
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photon energy (GeV)
Exotics
p1
In Photoproduction
IG(JPC)=1-(1-+)
1-+ nonet
K1 IG(JPC)=
h1 IG(JPC)=0+(1-+)
Need to establish nonet nature
of exotics: p h h0
Need to establish more than one
nonet: 0+- 1-+ 2+-
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½ (1-)
h’1 IG(JPC)=0+(1-+)
g
N
X
e
N
g  r,w,f
Gluonic Hadrons and Confinement
What are the light quark
Potentials doing?
DE
Potentials corresponding
To excited states of glue.
Non-gluonic mesons –
ground state glue.
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Lattice QCD potentials
Conclusions
The quest to understand confinement
and the strong force is about to make
great leaps forward.
Advances in theory and computing
will soon allow us to solve QCD
and understand the role of glue.
The definitive experiments to
confirm or refute our expectations
are being built.
The synchronized advances in both areas will allow
us to finally understand QCD and confinement.
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