The Hunt for the Hybrid Meson

Exploring the dynamics of quark confinement

Richard JonesUniversity of Connecticut

Physics and Astronomy Colloquium SeriesDartmouth College, Feb. 6, 2004

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: the strong nuclear force

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

What holds the nucleus together?

4

Theoretical foundations

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)

5

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

manymore…

6

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

-1/3e

+2/3e

7

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

8

… 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

9

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

10

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

11

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

cm α EUT

2e2

02

1

2)(E

22

0

cmZ e

12

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 > 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 < 2mc2

The Z=180 nucleus is confined to the neighborhood of its electrons – i.e. physical states must have Q < 180 !

2)(E

22

0

cmZ e

13

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 ~ 1 for which real spectroscopy is possible?

14

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

15

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

16

LQCD: the static quark potential

V(r<<r0) ~ 1/r 1-gluon exchange asymptotic freedom

V(r>>r0) ~ r like electrodynamics in 1d confinement

17

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

18

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

19

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

20

Meson Spectroscopy

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

21

Production

e+e- annihilation

pp annihilation

p collisions

p collisions

+ -

+-

- +

+

+

+

22

DetectionForward

Calorimeter

CerenkovCounter

Time ofFlight

Solenoid

BarrelCalorimeter

Tracking

Target

23

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

24

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 Two-body decay modes are dominant Multiparticle final states should be described by a cascading

sequence of two-body decays from heavier resonances

M( ) GeV / c2

What do we see?

25

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:

26

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

27

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

28

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 hybrids

J=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

29

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

30

Searches for Exotic Mesons

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

31

Searches: proton-antiproton annihilation

+-

Crystal BarrelCERN/LEAR

32

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

33

Significance of exotic signal.

34

Hybrid mass predictions

Flux-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

35

Searches: pion excitation experiments

- +

+

E852BNL/MPS

36

Partial Wave Analysis

a1

a2

Benchmarkresonances

2

PWA of p +

37

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

PWA: exotic signal

38

A second 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

39

Searches: photo-excitation experiments

glueballs hybrid mesons

+ - +

40

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

__

__

41

Production cross sectionsModel predictions for regular vs exotic meson prodution with photon and pion probes

Szczepaniak & Swat

42

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

43

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

44

Jefferson Lab siteHall D will belocated here

45

Add Arc

Add Cryomodules

Add Cryomodules

Upgrade plan

46

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.