The Hunt for the Hybrid Meson

special relativity changes things

 How might we study these effects?

  consider Z > 1 for Z = 140, a = 1.02

E 0  (

Z

a ) 2

m e c

2 2

11



Confinement in atomic physics

E 0  (

Z

a ) 2

m e c

2 Warning!

  2 relativistic corrections to the Hamiltonian shift the g.s. energy E 1 from this simple extrapolation of E 0 the Dirac equation must be solved  Qualitative results   something new happens when E 1 > 2mc 2 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 < 2mc 2 

The Z=180 nucleus is confined

to the neighborhood of its electrons – i.e. physical states must have Q < 180 !

12

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 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 > 160  Is there some other system for which a real spectroscopy is possible?

~ 1 for which

13

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. > 2mc 2       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

14

Confinement in nuclear physics

 other differences from forces in atomic physics

QED

1 kind of charge (q) force mediated by

photons

photons are

neutral

a is nearly constant

QCD

3 kinds of charge ( r , g , b ) force mediated by

gluons

gluons are

charged

(eg. r g , bb, g b ) a s strongly depends on distance  The underlying theories are formally almost identical!

15

LQCD: the static quark potential

  V(r<

asymptotic freedom

V(r>>r 0 ) ~ r   like electrodynamics in 1d

confinement 16

Lattice field theory: a new frontier

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 quarks

17

LQCD: how well does it do?

 best test is with

heavy quarkonium

(quenched approx.)   a s ~ 0.2

reveals static V qq (r)  contains effects of strong coupling at large distances  

shows confinement!

good agreement with experimental spectrum

18

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

19

Meson Spectroscopy

   production and detection analysis of the final state quantum numbers and

exotic mesons 20

Production

 e + e annihilation  pp annihilation  p p collisions  g p collisions + + + + + + -

21

Detection

Forward Calorimeter Barrel Calorimeter Solenoid Tracking Cerenkov Counter Time of Flight Target 22

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 q lab q cm

23

What do we see?

 Consider a final state that contains a p + p pair  what might decay to p + p ?

 consult

selection rules

 parent mesons are identified by

resonances

in p + p mass spectrum M( p  p   2   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

24

Some assembly required…

Data from E852, BNL: p  p  p  p  p  p at 18 GeV/c M( p  p   2 

suggests

p  p   0 p  p  p  p  p  p M( p  p  p   2 

to partial wave analysis 25

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

26

Ordinary mesons

s d u J=L+S P=(-1)

L+1

C=(-1)

L+S

G=C (-1)

I

u s d

(2S+1)

L

J 1

S

0 3

S

1

= 0

-+

= 1

--

quark-antiquark pairs orbital L=2 

3

, w

3

, f

3

,K

3



2

, w

2

, f

2

,K

2



1

, w

1

, f

1

,K

1

p

2

, h

2

, h ’

2

,K

2

3

--

2

--

1

--

2

-+

L=1 L=0 a 2 ,f 2 ,f’ 2 ,K 2 a 1 ,f 1 ,f’ 1 ,K 1 a 0 ,f 0 ,f’ 0 ,K 0 b 1 ,h 1 ,h’ 1 ,K 1 ,w,f ,K* p,h,h ’,K 2

++

1

++

0

++

1

+-

1

--

0

-+

radial

27

Quantum numbers of hybrids

  start with CQM rules: add angular momentum of the string J PC = 1 -+ or 1 + J=L+S P=(-1)

L+1

C=(-1)

L+S

G=C (-1)

I

CP={(-1)

L+S

}{(-1)

L+1

} ={(-1)

S+1

} Flux-tube Model m=0 CP=(-1)

S+1

m=1 CP=(-1)

S

S=1,L=0,m=1 J=1 CP= J

PC

=0

-+

, 0

+-

1

-+

,1

+-

2

-+

, 2

+ (2S+1)

L

J 1

S

0 3

S

1

= 0

-+

= 1

--

S=0,L=0,m=1 J=1 CP=+ J

PC

=1

++

,1

- 28

qq Mesons 2.5

2.0

1.5

1.0

L = 0 1 2 3 4 2 – + 0 – + 2 + + 2 + – 2 – + 1 – – 1 – + 1 + – 1 + + 0 + – 0 – + Each box corresponds to 4 nonets (2 for L=0) Radial excitations exotic nonets 0 + + 0 ++ 1.6 GeV 29

Searches for Exotic Mesons

   proton-antiproton annihilation pion-excitation experiments

photo-excitation experiments 30

Searches: proton-antiproton annihilation

Crystal Barrel CERN/LEAR + -

31

CBAR Exotic

antiproton-neutron annihilation

PWA of np  hp 0 p 

Same strength as the a 2 .

p 1 (1400) Mass = 1400 ± Width= 310 ± Without p 1 20 ± 20 MeV/c 2 50 +50 -30 MeV/c c 2 /ndf = 3, with = 1.29

2 Produced from states with

one unit

of angular momentum .

32

Significance of exotic signal.

33

Hybrid mass predictions

Flux-tube model: 8 degenerate nonets

1

++

,1

--

0

-+

, 0

+ ,

1

-+

,1

+-

,2

-+

, 2

+ ~1.9 GeV/c 2

S=0 S=1 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

34

Searches: pion excitation experiments

E852 BNL/MPS + +

35

Partial Wave Analysis

PWA of p  p  p  p  p + a 1 p 2 a 2

Benchmark resonances 36

PWA: exotic signal

p  p > hp  p p 1 (1400) Mass = 1370 + 16 +50 -30 MeV/c 2 Width= 385 +- 40 +65 -105 MeV/c 2 (18 GeV) The a 2 (1320) is the dominant signal. There is a small (few %) exotic wave.

a 2

p 1 Interference effects show a resonant structure in 1  .

(Assumption of flat background phase as shown as 3.)

37

A second exotic signal!

1 

Exotic Signal

p

1 (1600) Leakage From Non-exotic Wave due to imperfectly understood acceptance

M( p  p  p   2  3 p m=1593+-8 +28 -47 ph ’ m=1597+-10 +45 -10 G =168+-20 +150 -12 G =340+-40+-50

38

+

Searches: photo-excitation experiments

glueballs + hybrid mesons

39

Photoproduction of hybrids

q

p

or beam



q 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) q

g

beam q Quark spins aligned Almost no data in hand in the mass region where we expect to find exotic hybrids when flux tube is excited 40

Production cross sections

Model predictions for regular vs exotic with photon and pion probes meson prodution

Szczepaniak & Swat

41

Complementary probes

p  p  p 

Compare statistics and shapes

p  p  p g p  p  p  p  n

ca. 1998 @ 18 GeV ca. 1993 @ 19 GeV 28 BNL 4 1.0

M(3 p  2 

1.5

2.0

2.5

42

www.gluex.org

GlueX experiment

 12 GeV gamma beam

Lead Glass Detector Barrel Calorimeter

   MeV energy resolution high intensity (10 8 g /s) plane polarization

Coherent Bremsstrahlung Photon Beam Solenoid Note that tagger is 80 m upstream of detector

Electron Beam from CEBAF

Target Tracking Time of Flight Cerenkov Counter Event rate to processor farm: 10 kHz to and later data rates of 180 kHz 50 corresponding and 900 Mbytes/sec respectively 43

Hall D will be located here

Jefferson Lab site

44

Upgrade plan

Add Cryomodules Add Arc Add Cryomodules

45

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.

46