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

11

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

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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

16

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

17

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

20

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

21

Meson Spectroscopy

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

22

Production

e+e- annihilation

pp annihilation

p collisions

p collisions

+ -

+-

- +

+

+

+

23

DetectionForward

Calorimeter

CerenkovCounter

Time ofFlight

Solenoid

BarrelCalorimeter

Tracking

Target

24

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

25

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?

26

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

28

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

30

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

32

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

34

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

36

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

38

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

__

__

47

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

48

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

Szczepaniak & Swat

49

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.

53

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