Presentation to IoP Manchester 2005. 1

Exotic Hybrid Mesons.

J. D. Kellie

Department of Physics and Astronomy

Glasgow University.

2

Summary of talk.

1. Introduction.

2. Exotic Hybrid Mesons

a) Why they are of interest.

b) How they can be produced.

c) How they can be identified and their properties measured.

d) Current experimental evidence.

3. Conclusions.

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Introduction.

• This talk presents two experiments designed to investigate QUARK CONFINEMENT by attempting to find evidence of gluonic excitations within mesons in both the light (u, d, s) and charmed (c) quark sectors.

• The experiments are GLUEX which forms part of the Jefferson Laboratory upgrade which will increase the electron beam energy from 6 to 12 GeV, and PANDA which is part of the GSI upgrade.

• GLUEX will use tagged linearly polarised photons – a source of vector mesons – incident on a hydrogen target. A hermetic spectrometer, based on a superconducting solenoid, will measure the reaction products. Photons of around 9 GeV are required.

• The upgrade has obtained U.S. Department of Energy support.

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PANDA will be located in the high-energy storage ring HESR at the international FAIR facility at GSI, and will use anti-proton proton annihilation to study gluonic excitations, glueballs and hybrids in the charmonium mass range.

The antiproton beam – momentum extending to 15 GeV/c – is incident on a fixed hydrogen target inside a superconducting solenoid, which, together with a forward large acceptance dipole magnet, will form the basis of the PANDA spectrometer.

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Why Exotic Hybrid Mesons are of interest.

Quark confinement can be explained in terms of the interaction which arises from gluonic exchange between the quarks.

Mesons, with their structure, are ideal for studying the interaction.

states have well defined quantum numbers. If gluonic degrees of freedom are added, the resulting states are hybrid mesons. When hybrid meson states have quantum numbers that do not belong to the basic set of states they are called exotic hybrid mesons.

If states with exotic quantum numbers are discovered, this will clarify the role played by gluons in the confinement of quarks.

GLUEX and PANDA are specifically designed to measure the quantum numbers of excited mesons.

qq

qq

qq

qq

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Lattice QCD

Flux

tube

forms

between

qq

From G. Bali

Confinement arises from flux tubes and their

excitation leads to a newspectrum of mesons

→ Flux Tube Model

Quark Confinement.

neutron (d,u,d)

(d, u ) proton (u,u,d)

π

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Understanding Confinement

The Ideal Experiment The Real Experiment

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LS

S

1

2

S = S + S1 2

J = L + S

C = (-1)L + S

P = (-1)L + 1

Normal Mesons – qq color singlet bound statesSpin/angular momentum configurations & radial excitations generate our known spectrum of light quark mesons.

Starting with u - d - s we expect to find mesons grouped in nonets - each

characterized by a given J, P and C.

JPC = 0– + 0++ 1– – 1+ – 2++ …

Allowed combinations

JPC = 0– – 0+ – 1– + 2+ – …

Not-allowed: exotic

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Hybrid Mesons

1 GeV mass difference (/r)

Hybrid mesons

Normal mesons

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Quantum Numbers of Hybrid MesonsQuarks

Excited Flux Tube Hybrid Meson

S 0

L 0

J PC 0

J PC 1

1

J PC

1

1

, Klike

J PC 0 1 2

0 1 2

S 1

L 0

J PC 1

J PC 1

1

like ,

Exotic

Flux tube excitation (and parallel quark spins) lead to exotic JPC

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Mas

s (G

eV)

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

(L = qq angular momentum)

exoticnonets

0 – +

0 + –

1 + +

1 + –

1– +

1 – –

2 – +

2 + –2 + +

0 – +

2 – +

0 + +

Glueballs

Hybrids

Meson Map – GLUEX mass range

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Meson Map – PANDA mass range.

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12 GeV CEBAF

CHL-2CHL-2

Upgrade magnets Upgrade magnets and power and power suppliessupplies

Enhance equipment in Enhance equipment in existing hallsexisting halls

add Hall D (and beam line)

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GlueX / Hall D Detector

Electron Beam from CEBAF

Lead GlassDetector

Solenoid

Coherent Brem.Photon Beam

Tracking

TargetCerenkovCounter

Time ofFlight

BarrelCalorimeter

Note that tagger is80 m upstream of

detector

Detector ReviewOct 20-22, 2004

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HESR storage ring at FAIR.

PANDA facility.

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PANDA spectrometer.

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Participation of the GLASGOW group at

GLUEX and PANDA.

GLUEX.

•Design of the GLUEX tagging spectrometer.

•Assessment of suitable diamond radiators.

PANDA.

•Solenoid and Forward Spectrometer magnetic field design studies.

•Development of Grid Computing.

•Design of Cerenkov detectors – with Edinburgh.

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PANDA magnetic field calculations – showing the effects of field clamps.

Vertical section:

Field component along the axis.

Vertical section:

Field component transverse to the axis.

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How Exotic Hybrid Mesons are produced at GLUEX

N N

e

X

,,

Basic interaction.

Requirements.

•JLab energy upgraded from 6 to 12 GeV.

•Source of linearly polarised photons, (determine parity).

•New detector to measure reaction products.

The incident photons – vector mesons –are excited into states X which decay into many different channels. A hermetic spectrometer detects the reaction products.

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Production of Linearly Polarised Photons.

• When the electron beam of energy interacts with a carefully aligned thin diamond wafer, linearly polarised coherent bremsstrahlung, as well as incoherent bremsstrahlung, is produced.

• If the energies E of the residual electrons are measured in a tagging spectrometer, the energy of the bremsstrahlung photon is

• The ratio of coherent to incoherent bremsstrahlung is enhanced if the photons pass through a narrow collimator.

0E

E.E0

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Coherent Bremsstrahlung

flu

x

E (GeV)

12 GeV electrons

This technique provides requisite energy, flux and

polarization

collimated

Incoherent &coherent spectrum

tagged

with 0.1% resolution

40%polarization

in peak

electrons in

photons out

spectrometer

diamondcrystal

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The GLUEX tagger.

Two identical magnets, each ~50 tonnes.

Vacuum chamber, designed to withstand vacuum force of ~ 70 tonnes.

Focal Plane

The focal plane consists of a broad-band low resolution hodoscope covering used for diamond alignment and monitoring, and a high resolution microscope positioned for

and operating at per channel.

0γ00.95EE0.25E

9GeVE8.5GeV γ .6 Hz105~

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Rocking curve measurements.

X-ray topography

Diamond Assessment using X-rays.

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dbb

dbg

dbr

Diamond X-ray Analysis - very good diamond.Topographs.

Rocking Curves for bottom slice.

Topographs of 3 slices (100 in thickness) cut from a single synthetic diamond with the top slice nearest the seed. Each shows the same pattern, but the relative area of the central region increases with distance from the seed.

μm

FWHM~10 microradians

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How exotic hybrid mesons can be identified and their properties measured.

Use a large acceptance detectorhermetic coverage for charged and neutral particles.typical hadronic final states: f1KKKK

b1high data acquisition rate.Perform partial-wave analysis

identify quantum numbers as a function of mass.

check consistency of results in different decay modes.

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Rates High statistics means high rates

Initially 107 tagged /s Design detector for 108

JLab CLAS runs at 107 already.

At 107, the total hadronic rate is » 37kHz the tagged hadronic rate is » 1.4kHzAt 108, the total hadronic rate is » 370kHz the tagged hadronic rate is » 14kHz

Running at 107 for 1 year will exceed current photoproductiondata by several orders of magnitude and will exceed current data.

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Topologies

p

X

n,p

t-channel meson photoproduction

photons pions protons

~1

Ge

V/c

10-60o

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Background Topologies

p

X

n,p

photons pions protons

N* production is a significant backgroundto the simple t-channel production.

There is interesting physics in this channel,it is just more complicated to analyze.

backwards slow pionsforward

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The GlueX DetectorTracking Calorimetry Particle ID

Magnetic Field

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CalorimetryForward Calorimeter LGD Existing lead glass detector ~2500 blocks E/E · 0.036+0.073/E1/2

» 100 MeV · E · 8 GeV Barrel Calorimeter BCAL Lead-scifiber sandwich 4m long cylinder E/E · 0.020+0.05/E1/2

~20MeV · E · » 3 GeV 200ps timing resolution z-position of shower time-of-flightUpstream Photon Veto UPV Veto photons ~20MeV · E · 300 MeV

Expected o and resolutions

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TrackingForward Region FDC 4 packages of planar drift chambers anode + cathode readout six planes per package xy=150m active close to the beam line.

Central Region CDC cylindrical straw-tube chamber 23 layers from 14cm to 58cm 6o stereo layers r=150m z» 2mm minimize downstream endplate dE/dx for p<450 MeV/c Necessary for protons

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Particle Identification

Time-of-flight Systems Forward tof ~80ps BCAL ~200ps Start counter

Cherenkov Detector DIRC K p separation

dE/dx Information The CDC will do dE/dx p<450 MeV/c

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Requirements for a good partial wave analysis.

• Hermetic Detector for charged particles • and photons.

• Uniform, understood acceptance.

• Excellent resolution to reduce backgrounds.

• Linear polarized photons.

• High statistics data sets.

• Sensitive to many final states.

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Current Experimental Evidence for Exotic Hybrid Mesons.

BNL E852 Experiment with on a hydrogen target.

a)

- PWA mass peak unstable.

b)

cGeV /18π

1

PC:

1Jπ(1600)pπpπ

1

πρ0

πππ

(2001)pπpπ1

πf1

ππηπ

πb1

00πππππ

Statistics for b) are low, but more in line with LQCD in terms of mass and decay modes. GlueX will have much higher statistics and be able to find nonets of states.

)2001(1

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CONCLUSIONS.

•The discovery of exotic hybrid mesons will provide strong evidence that quarks interact by the exchange of gluons, and hence greatly increase our understanding of quark confinement.

•The GlueX and PANDA experiments have the required versatility, acceptance, resolution and particle ID to determine the quantum numbers of mesonic states.

•In addition to being able to identify exotic hybrid mesons, GlueX and PANDA will measure the spectroscopy of meson states in both the light and charmed quark sectors.

•GlueX and PANDA have the potential to search for glueballs, quark molecular states etc.

•GlueX and PANDA are complementary experiments with common physics goals, but using entirely different types of facilities.