The Hunt for the Hybrid Meson
Experimental Nuclear Physics Research at Jefferson Lab
Richard JonesUniversity of Connecticut
Frontiers in Physics Colloquium Series
2
Outline Introduction
the strong interaction confinement in QCD quark potentials and the quarkonium spectrum
Meson Spectroscopy production and detection analysis of the final state quantum numbers and exotic mesons
Experimental Searches for Exotics proton-antiproton annihilation pion-excitation experiments photo-excitation experiments
3
Introduction
electricity + magnetism
electroweak
Four fundamental forces:
Which ones are relevant to nuclear physics?
4
Historical Origins
Dimitri Mendeleev’s periodic table of the elements 1869
pattern substructure
5
Discovery of the atomic nucleus Ernest Rutherford, Geiger, and Marsden (1909)
6
Modern theory of the atom regularities in Mendeleev’s periodic table Rutherford’s particle scattering experiments discovery of electron (J.J. Thomson, 1897) lines in atomic spectra (Balmer, Lyman, Rydberg)
7
What holds the nucleus together? protons: positive electric charge neutrons: no charge like charges repel
new force must be present strong to overcome electrostatic repulsion short-ranged to prevent collapse
8
Yukawa’s strong force Hideki Yukawa proposes theory of the nuclear force (1935)
mediated by spinless exchange particle called the meson mass of meson about 250 times that of the electron
meson later discovered(Lattes, Muirhead, Occhialini, Powell, 1947)
9
Particle zoo experiments soon revealed many more new particles
involved in strong interactions protons and neutrons lightest particles in a large spectrum of
strongly-interacting fermions called baryons pions lightest member of equally numerous sequence of
strongly-interacting bosons called mesons
manymore…
10
Quark hypothesis pattern suggests substructure
Murray Gell-Mann quarks George Zweig aces
quarks: fractional electric charge! spin 1/2 come in flavors (up, down, strange, …)
baryons = three quarks mesons = quark-antiquark pair
Gell-Mann Zweig
-1/3e
+2/3e
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Gell-Mann’s paper
quark hypothesis brought order to the particle zoo, but
fractional electric charge never observed
quarks were initially considered as purely mathematical objects
but on last page of paper:
“It is fun to speculate about the way quarks would behave if they were physical particles of finite mass (instead of purely mathematical entities…).”
12
Discovery of the quark experiments at Stanford Linear Accelerator Center
(Friedman, Kendall and Taylor, 1968) modern rendition of Rutherford experiments scattered electrons off protons found point-like charges inside proton
new charges initially called partons, but scattering consistent with quark hypothesis! fractional charges confirmed
quarks not observed directly, only by “kick” they give the scattered electron
interactions between quarks weaken at short distances asymptotic freedom
13
… and more quarks discovery of J/mesonin November 1974 (BNL, SLAC)
interpreted as bound state of new flavor of quark called charm predicted as weak partner of strange quarks
discovery of meson in August, 1977 (Fermilab) interpreted as bound state of new flavor called bottom new partner predicted at higher mass, to be called top
ultra-heavy quark finally observed in 1995 (Fermilab) weak interaction comparable with strong at 180 GeV/c2 !
no more quarks expected below mass scale ~1 TeV/c2
14
no single isolated quark was ever seen in a detector heavy quarks decay to light quarks via weak interactions light quarks “dress” themselves in anti-quarks to form mesons mesons are seen in detectors
What kind of theory might explain this?
… and yet,
confinement
15
Confinement in atomic physics
consider the hydrogen atom
where
=1/137, weak coupling no confinement atom can be ionized with energy E0
isolated protons exist as physical states
20
n n
E E
V
r
n=1
n=2
2E
22
0
cme
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Confinement in atomic physics
Note the energy scale:
What happens if ~ 1 or greater? <T> grows to the same size as mass-energy mc2
<U> is of same order as mc2
special relativity changes things
How might we study these effects? consider Z > 1 for Z = 140, = 1.02
2 EUT
22
0
cm- e
2)(E
22
0
cmZ e
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Confinement in atomic physics
Warning! relativistic corrections to the Hamiltonian shift the g.s. energy
E1 from this simple extrapolation of E0
the Dirac equation must be solved
Qualitative results something new happens when E1 > mc2
the bare nucleus spontaneously creates an electron in its g.s.
a positron (anti-electron) simultaneously flies off process continues until ionization energy of atom < mc2
The Z=150 nucleus is confined to the neighborhood of its electrons – i.e. physical states must have Q < 150 !
2)(E
22
0
cmZ e
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Confinement in atomic physics Can this effect be observed in experiment?
nuclei with Z >100 are increasingly unstable and radioactive compound nuclei can be created in A+A collisions with a
lifetime of order 10-21 s lifetime is too short to do spectroscopy
Experiment with heavy ion collider was performed at G.S.I. in Darmstadt, Germany
positron emission rate was monitored vs. Z of beams some excess yield was seen for Z > 160
Is there some other system for which ~ 1 for which real spectroscopy is possible?
19
Confinement in nuclear physics this atomic physics analogy is imperfect
only one of the two charges is large for true ~ 1 BOTH charges must grow new things happen
when B.E. > 2mc2
new matter-antimatter pairs spontaneously created vacuum is unstable! a new phase is formed to replace the ordinary vacuum “empty space” becomes full of particles the Dirac equation is of little use field theory is the only approach
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Confinement in nuclear physics other differences from forces in atomic physics
The underlying theories are formally almost identical!
QED QCD1 kind of charge (q) 3 kinds of charge (r,g,b)
force mediated by photons force mediated by gluons
photons are neutral gluons are charged (eg. rg, bb, gb)
is nearly constant s strongly depends on distance
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Meson Spectroscopy
production and detection analysis of the final state quantum numbers and exotic mesons
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Production
e+e- annihilation
pp annihilation
p collisions
p collisions
+ -
+-
- +
+
+
+
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DetectionForward
Calorimeter
CerenkovCounter
Time ofFlight
Solenoid
BarrelCalorimeter
Tracking
Target
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Analysis reactions tend to produce all sorts of mesons
many flavors (mixtures of up, down, strange …) many spins and parities
only the lightest are “stable”: , k, pseudoscalar nonet) all other mesons decay to pseudoscalars and photons must be reconstructed by their kinematics
energies of decay products angles of decay products respect special relativity, i.e. use rest frame of decaying particle
lab cm
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Consider a final state that
contains a +- pair what might decay to +- ? consult selection rules
parent mesons are identified by
resonances in +- mass spectrum
empirical rule: isobar model of strong interactions Nature prefers to invest in mass Multiparticle final states should be described by a cascading
sequence of two-body decays from heavier resonances
M( ) GeV / c2
What do we see?
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p p at 18 GeV/c
suggests p 0 p
p
to partial wave analysis
M( ) GeV / c2 M( ) GeV / c2
Some assembly required…Data from E852, BNL:
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Classification Ordinary mesons (qq)
defined by the Constituent Quark Model decay model built on CQM generally successful spectrum is well understood (experiment, CQM, QCD)
Exotic mesons new states predicted on the basis of confinement in QCD of special interest are gluonic excitations
Glueballs Hybrids
spectrum not well understood little is known about decays
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quark-antiquark pairs
ud
s u d
s
J=L+S
P=(-1) L+1
C=(-1) L+S
G=C (-1) I
(2S+1) L J
1S0 = 0 -+
3S1 = 1--,K*’,KL=0
1--
0-+
a2,f2,f’2,K2
a1,f1,f’1,K1
a0,f0,f’0,K0
b1,h1,h’1,K1
L=1
2++
1++
0++
1+-
3,3,3,K3
2,2,2,K2
1,1,1,K1
2,2,’2,K2
L=2
3--
2--
1--
2-+
radial
orbital
Ordinary Mesons
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1.0
1.5
2.0
2.5
qq Mesons
L = 0 1 2 3 4
Each box correspondsto 4 nonets (2 for L=0)
Radial excitations
exoticnonets
0 – +
0 + –
1 + +
1 + –
1– +
1 – –
2 – +
2 + –2 + +
0 – +
2 – +
0 + +
Glueballs
Hybrids
0++ 1.6 GeV
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Searches for Exotic Mesons
proton-antiproton annihilation pion-excitation experiments photo-excitation experiments
But first,
How do we know what to look for?
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Birth of Lattice QCD
Ken Wilson
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Lattice field theory: a new frontier hypercubic space-time lattice quarks reside on sites,
gluons reside on links between sites
lattice excludes short wavelengths from theory (regulator)
regulator removed using standard renormalization
systematic errors discretization finite volume
quarksgluons
33
LQCD: how well does it do? best test is with heavy quarkonium (quenched approx.)
s ~ 0.2
reveals static Vqq(r)
contains effects of
strong coupling at
large distances
shows confinement! good agreement with experimental spectrum
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LQCD: the static quark potential V(r<<r0) ~ 1/r
1-gluon exchange asymptotic freedom
V(r>>r0) ~ r like electrodynamics in 1d confinement
35
LQCD: what is a hybrid meson? Intuitive picture within Born-Oppenheimer approximation
quarks are massive –
slow degrees of freedom gluons are massless –
generate effective potential
Glue can be excited
ground-state flux-tube m=0
excited flux-tube
m=1
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m=0 CP=(-1) S+1
m=1 CP=(-1) S
Flux-tube Model
CP={(-1)L+S}{(-1)L+1} ={(-1)S+1} S=0,L=0,m=1
J=1 CP=+
JPC=1++,1--
S=1,L=0,m=1
J=1 CP=-JPC=0-+,0+-
1-+,1+-
2-+,2+-
JPC = 1-+ or 1+-
Quantum numbers of hybridsJ=L+S
P=(-1) L+1
C=(-1) L+S
G=C (-1) I
(2S+1) L J
1S0 = 0 -+
3S1 = 1--
start with CQM rules: add angular momentum
of the string
37
linear potential
ground-state flux-tube m=0
excited flux-tube m=1
Gluonic Excitations provide anexperimental measurement of the excited QCD potential.
Observations of exotic quantum number nonets are thebest experimental signal of gluonic excitations.
QCD Potential
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Searches: proton-antiproton annihilation
+-
Crystal BarrelCERN/LEAR
39
1(1400)
antiproton-neutron annihilation
Mass = 1400 ± 20 ± 20 MeV/c2
Width= 310 ± 50 +50-30 MeV/c2
Same strength as the a2.
Produced from states withone unit of angular momentum.
Without 1 2/ndf = 3, with = 1.29
PWA of np
CBAR Exotic
40
Significance of signal.
41
Hybrid Mass PredictionsFlux-tube model: 8 degenerate nonets 1++,1-- 0-+,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2
Lattice calculationsUKQCD (97) 1.87 0.20MILC (97) 1.97 0.30MILC (99) 2.11 0.10Lacock(99) 1.90 0.20Mei(02) 2.01 0.10
S=0 S=1 MILC, hep-lat/0301024
42
Searches: pion excitation experiments
- +
+
E852BNL/MPS
43
Partial Wave Analysis
a1
a2
Benchmarkresonances
2
PWA of p +
44
An Exotic Signal
LeakageFrom
Non-exotic Wavedue to imperfectly
understood acceptance
ExoticSignal
1
M( ) GeV / c2
1(1600)
3 m=1593+-8+28-47 =168+-20+150
-12
’ m=1597+-10+45-10 =340+-40+-50
45
Searches: photo-excitation experiments
glueballs hybrid mesons
+ - +
46
Photoproduction of hybrids
A pion or kaon beam, when scattering occurs,
can have its flux tube excitedor
beam
Quark spins anti-aligned
Much data in hand with some evidence for gluonic excitations
(tiny part of cross section)
q
q
befo
req
q
aft
er
q
q
aft
er
q
q
befo
re
beamAlmost no data in hand
in the mass regionwhere we expect to find exotic hybrids
when flux tube is excited
Quark spins aligned
__
__
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p p
BNL
@ 18 GeV
Compare statistics and shapes
ca. 1998
28
4
Eve
nts
/50
MeV
/c2
SLAC
p n
@ 19 GeV
SLAC
1.0 2.52.01.5
ca. 1993
M(3) GeV / c2
Complementary probes
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Production cross sectionsModel predictions for regular vs exotic meson prodution with photon and pion probes
Szczepaniak & Swat
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GlueX ExperimentLead GlassDetector
Solenoid
Electron Beam from CEBAF
Coherent BremsstrahlungPhoton Beam
Tracking
Target
CerenkovCounter
Time ofFlight
BarrelCalorimeter
Note that tagger is80 m upstream of
detector
Event rate to processor farm:10 kHz and later 180 kHz correspondingto data rates of 50 and 900 Mbytes/sec
respectively
12 GeV gamma beam MeV energy resolution high intensity (108 /s) plane polarization
www.gluex.org
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Jefferson Lab SiteHall D will belocated here
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Add Arc
Add Cryomodules
Add Cryomodules
The Upgrade Plan
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Summary and Outlook Regularities in the spectrum of light hadrons was
the key to unlocking the nature of the strong interactions.
Precise predictions of the properties of light hadrons are very difficult within QCD, but
Lattice QCD can overcome these difficulties, with some care as to systematic errors, and
Rapid advances in computing power are leading to unprecedented accuracy in predicting observables.
New experimental results have fueled a revival of interest in meson spectroscopy to test the theory.
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pp (18 GeV)
The a2(1320) is the dominantsignal. There is a small (few %)exotic wave.
Interference effects showa resonant structure in .(Assumption of flat backgroundphase as shown as 3.)
Mass = 1370 +-16+50
-30 MeV/c2
Width= 385 +- 40+65-105 MeV/c2
a2
E852 Results