The Hunt for the Hybrid Meson

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The Hunt for the Hybrid Meson
Physics and Astronomy Colloquium Series Dartmouth
College, Feb. 6, 2004

• Exploring the dynamics of quark confinement

Richard Jones University of Connecticut
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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

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Introduction the strong nuclear force
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

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Theoretical foundations

• Hideki Yukawa proposes theory of the nuclear
  force (1935)
• mediated by spinless exchange particle called the
  p meson
• mass of p meson about 250 times that of the
  electron
• p meson later discovered
• (Lattes, Muirhead, Occhialini, Powell, 1947)

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Experimental advances

• 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

many more
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Quark model

• pattern suggests substructure
• Murray Gell-Mann ? quarks
• George Zweig ? aces
• quarks
• fractional electric charge!
• spin 1/2
• come in flavors (up, down, )
• baryons three quarks
• mesons quark-antiquark pair

Gell-Mann
Zweig
2/3e
-1/3e
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More experimental advances

• experiments at Stanford Linear Accelerator Center
  (Friedman, Kendall and Taylor, 1968)
• rendition of Rutherford experiment
• scattered electrons off protons
• looked at large momentum transfers
• found point-like charges inside proton
• new charges initially called partons, but
• fractional charges confirmed
• scattering consistent with massless quarks

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and more quarks

• discovery of J/Y meson in November 1974 (BNL,
  SLAC)
• interpreted as bound state of new flavor of quark
  called charm
• predicted as weak partner of strange quarks
• discovery of U 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

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and yet,

• 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?

confinement
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Confinement in atomic physics
V

• consider the hydrogen atom
• where
• a1/137, weak coupling Þ no confinement
• atom can be ionized with energy E0
• isolated protons exist as physical states

r
n2
n1
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Confinement in atomic physics

• Note the energy scale
• What happens if a 1 or greater?
• ltTgt grows to the same size as mass-energy mc2
• ltUgt is of same order as mc2
• special relativity changes things
• How might we study these effects?
• consider Z gt 1
• for Z 140, a 1.02

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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 gt 2mc2
• the bare nucleus spontaneously grows an electron
  in its g.s.
• a positron (anti-electron) simultaneously flies
  off
• process continues until ionization energy of atom
  lt 2mc2
• The Z180 nucleus is confined to the neighborhood
  of its electrons i.e. physical states must have
  Q lt 180 !

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Confinement in atomic physics

• Can this effect be observed in experiment?
• nuclei with Z gt100 are increasingly unstable and
  radioactive
• compound nuclei can be created in AA collisions
  with a lifetime of order 10-21 s
• lifetime is too short to do atomic 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 gt 160
• Is there some other system for which a 1 for
  which real spectroscopy is possible?

quarks!
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Confinement in nuclear physics

• this atomic physics analogy is imperfect
• only one of the two charges is large
• for true a 1 BOTH charges must grow
• new things happen
• when B.E. gt 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 QCD 1 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) a is nearly
constant as strongly depends on distance
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LQCD the static quark potential

• V(rltltr0) 1/r
• 1-gluon exchange
• asymptotic freedom
• V(rgtgtr0) r
• like electrodynamics in 1d
• confinement

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Lattice field theory a new frontier
quarks
gluons

• 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

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LQCD how well does it do?

• best test is with heavy quarkonium (quenched
  approx.)
• as 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 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 m0
excited flux-tube m1
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Meson Spectroscopy

• production and detection
• analysis of the final state
• quantum numbers and exotic mesons

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Production
-

• ee- annihilation
• pp annihilation
• pp collisions
• gp collisions

-

-




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Detection
Forward Calorimeter
Barrel Calorimeter
Solenoid
Time of Flight
Tracking
Cerenkov Counter
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 p, k, h
  (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

qlab
qcm
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What do we see?

• Consider a final state that
• contains a pp- pair
• what might decay to pp- ?
• consult selection rules
• parent mesons are identified by
• resonances in pp- mass spectrum
• empirical rule isobar model of strong
  interactions
• Two-body decay modes are dominant
• Multiparticle final states should be described by
  a cascading sequence of two-body decays from
  heavier resonances

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Some assembly required
at 18 GeV/c
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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Ordinary mesons
quark-antiquark pairs
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Quantum numbers of hybrids

• start with CQM rules
• add angular momentum
• of the string

JPC 1- or 1-
CP(-1)LS(-1)L1 (-1)S1
S0,L0,m1
J1 CP
JPC0-,0- 1-,1- 2-,2-
JPC1,1--
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Each box corresponds to 4 nonets (2 for L0)
Radial excitations
0 1.6 GeV
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Searches for Exotic Mesons

• proton-antiproton annihilation
• pion-excitation experiments
• photo-excitation experiments

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Searches proton-antiproton annihilation
Crystal Barrel CERN/LEAR
-

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CBAR Exotic
antiproton-neutron annihilation
PWA of np hp0p-
Same strength as the a2.
Mass 1400 20 20 MeV/c2 Width 310 50
50-30 MeV/c2
p1(1400)
Produced from states with one unit of angular
momentum.
Without p1 c2/ndf 3, with 1.29
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Significance of exotic signal.
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Hybrid mass predictions
Flux-tube model 8 degenerate nonets
1,1-- 0-,0-,1-,1-,2-,2- 1.9 GeV/c2
S0
S1
MILC, hep-lat/0301024
Lattice calculations UKQCD (97) 1.87
?0.20 MILC (97) 1.97 ?0.30 MILC (99)
2.11 ?0.10 Lacock(99) 1.90 ?0.20 Mei(02)
2.01 ?0.10
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Searches pion excitation experiments
E852 BNL/MPS
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Partial Wave Analysis
PWA of p- p p-p-p
Benchmark resonances
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PWA exotic signal
p-p -gt hp- p
(18 GeV)
Mass 1370 -1650-30 MeV/c2 Width 385
- 4065-105 MeV/c2
p1(1400)
The a2(1320) is the dominant signal. There is a
small (few ) exotic wave.
p1
a2
Interference effects show a resonant structure in
1- . (Assumption of flat background phase as
shown as 3.)
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A second exotic signal!
p1(1600)
Exotic Signal
3p m1593-828-47 G168-20150-12 ph
m1597-1045-10 G340-40-50
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Searches photo-excitation experiments
-


glueballs
hybrid mesons
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Photoproduction of hybrids
Quark spins anti-aligned
A pion or kaon beam, when scattering occurs, can
have its flux tube excited
Much data in hand with some evidence for gluonic
excitations (tiny part of cross section)
_
_
_
_
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Production cross sections
Model predictions for regular vs exotic meson
prodution with photon and pion probes
Szczepaniak Swat
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Complementary probes
Compare statistics and shapes
_at_ 18 GeV
ca. 1998
BNL
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GlueX experiment
www.gluex.org
Lead Glass Detector
Barrel Calorimeter

• 12 GeV gamma beam
• MeV energy resolution
• high intensity (108 g/s)
• plane polarization

Coherent Bremsstrahlung Photon Beam
Solenoid
Time of Flight
Note that tagger is 80 m upstream of detector
Tracking
Cerenkov Counter
Target
Electron Beam from CEBAF
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Jefferson Lab site
Hall D will belocated here
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Upgrade plan
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Summary and Outlook

• Regularities in the spectrum of light hadrons was
  a key to the discovery of the building blocks of
  the nucleus and of the theory of strong
  interactions.
• Precise predictions of the properties of light
  hadrons are still very difficult within QCD, but
• lattice QCD can overcome these difficulties,
  provided the systematic errors are controlled,
  and
• rapid advances in computing power are leading to
  unprecedented accuracy in predicting observables.
• Recent experimental results have fueled renewed
  interest in hadron spectroscopy to test the
  theory.