a coherent gamma source the gluex experiment the hall d photon beam the requirements for diamonds...

A coherent gamma sourcethe GlueX experimentthe Hall D photon beamthe requirements for diamonds

Richard Jones, University of Connecticut

presented by

GlueX Tagged Photon Beam Working GroupUniversity of Glasgow

University of ConnecticutCatholic University of America

2Richard Jones, CHESS seminar, Aug. 15, 2006

What is GlueX?

ForwardCalorimeter

CerenkovCounter

Time ofFlight

Solenoid

BarrelCalorimeter

Tracking

Target

A new meson spectroscopy experiment at Jefferson Lab which requires:

the 12 GeV upgradethe 12 GeV upgrade

a new experimental halla new experimental hall

a polarized photon beama polarized photon beam

a multi-particle spectrometera multi-particle spectrometer

a new collaboration, presentlya new collaboration, presently~80 physicists~80 physicists~30 institutions~30 institutions


Motivation: gluonic excitations

Consider QCD with Consider QCD with only heavy quarks:only heavy quarks:

the light mesons are glueballs

qq mesons have the conventional positronium low-energy spectrum

spectrum is distorted at higher excitations by a linear potential

for r >> 0.5 fm a tube of gluonic flux forms between q and q

0.4 0.8 1.2 1.6

1.0

2.0

0.0

r (fm)

V0(QQ)

(GeV)glueball decay threshold


Motivation: gluonic excitations

Consider QCD with Consider QCD with only heavy quarks:only heavy quarks:

gluonic excitations give rise to new potential surfaces

for r >> r0 gluonic excitations behave like flux tube oscillations

inspires the flux tube modelflux tube model


Motivation: normal vs hybrid mesons

m=0 CP=(-1)S+1

m=1 CP=(-1)S

Flux-tube Model

ground-state flux-tube m=0

excited flux-tube m=1

CP = (-1)L+S (-1)L+1

= (-1)S+1

S=0, L=0

J=1 CP=+

JPC=1++,1--

(not exotic)

S=1, L=0

J=1 CP=-

JPC = 0-+,00+-+-

11-+-+,1+-

2-+,22+-+-

exoticexotic

normal mesons

11-+-+ or 11+-+-


Motivation: hybrid massesFlux-tube model: 8 degenerate nonetsFlux-tube model: 8 degenerate nonets 1++,1-- 0-+,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2

Lattice calculations --- Lattice calculations --- 11-+-+ nonet is the lightest nonet is the lightest UKQCD (97) 1.87 0.20MILC (97) 1.97 0.30MILC (99) 2.11 0.10Lacock (99) 1.90 0.20Mei(03) 2.01 0.10Bernard (04) 1.79 0.14

~2.0 GeV/c2

In the charmonium sector:In the charmonium sector:1-+ 4.39 0.080+- 4.61 0.11 Splitting = 0.20

1-+

0+-

2+-

Splitting 0.20

S=0 S=1


Lowest mass expected to be 1(1−+) at 1.9±0.2 GeV

Lattice 1-+ 1.9 GeV2+- 2.1 GeV0+- 2.3 GeV


Experiment: hybrid searchesMost of what is presently known about the hybrid spectrum has come from one experiment: BNL E852BNL E852

- p X n at 18 GeV

1(1400) – seen in

1(1600) – seen in , f1, b1, ’

1(2000) – seen in f1b

General observations regarding these analyses exotic intensities are typically 1/10 dominant ones requires large samples (~106 in exclusive channels) requires good acceptance (uniform and well-understood) requires access to high-multiplicity final states


Experiment: hybrid photoproduction

p p

BNL @ 18 GeV

Unexplored territory with unique advantages for hybrid search

ca. 1998@ 19 GeV

28

4

Eve

nts

/50

MeV

/c2

SLAC

p n

SLAC

1.0 2.52.01.5

ca. 1993


Almost no data is available in the mass region

where we expect to find exotic hybridswhen flux tube is excited

Experiment: hybrid photoproduction

A pion or kaon beam, when scattering occurs,

can have its flux tube excitedor

beam

Quark spins anti-aligned

Data from these reactions show evidence for gluonic excitations(small part of cross section)

q

q

bef

ore

q

qaf

ter

q

q

afte

r

q

q

bef

ore

beam

Quark spins aligned

__

__


Experiment: photoproduction phenomemology

N N

Xfinalstateforwardsystem

general framework: VMD in initial stateVMD in initial state t-channel exchanget-channel exchange


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

12 GeV electrons are required In order to produce a 9 GeV photon beam with a significant degree of linear polarization

9 GeV gamma beam MeV energy resolution high intensity (108 /s) linear polarization

www.gluex.org


GlueX Experiment: beam polarization

L 0, 1, or 2

PV P PX 1 L

Suppose we want to distinguish theexchange: O+ from 0- ( AN from AU )

V = vectorphoton

m = 1

m = -1

R

L

AN AU

AN AU

For circular polarization: With linear polarization we

can isolate AN from AU

Circular polarization gives access to their interference

R J=0– or 0+

X

photon

for R with J = 0

Gottfried-Jackson frame

exchange particle


GlueX Experiment: photon beam

flu

x

photon energy (GeV)

12 GeV electronsThe coherent bremsstrahlung technique provides requisite energy, flux and polarization

collimated

Incoherent &coherent spectrum

taggedwith 0.1% resolution

40%polarization

in peak

electrons in

photons out

spectrometer

diamondcrystal


Requirements for photon beam

Energy 9 GeV

Linear polarization

High rates (consistent with tagging) Initial running at 107 /s in the coherent peak Design system with a clear path to 108 /s

Energy resolution E/E ~ 0.1% for use in event reconstruction


6 GeV CEBAF

CHL-2CHL-2

Upgrade magnets Upgrade magnets and power and power suppliessupplies

12

Enhance equipment in Enhance equipment in existing hallsexisting halls

add Hall D (and beam line)


1. energy 2. polarization 3. rate 4. resolution () () ()

5. background

The generic photon source

Techniques:A. Compton backscatterB. BremsstrahlungC. Coherent bremsstrahlung

A B C

() with tagging


Incoherent vs coherent bremsstrahlung

Consider the electromagnetic form-factor of the target in q-space

q

no enhancement

q

strong enhancement


Kinematics of Coherent Bremsstrahlung

effects of collimation at 80 m distance from radiatorincoherent (black) and coherent (red) kinematics

effects of collimation: to enhance high-energy flux and increase polarization


No other solution was found that could meet all of these requirements at an existing or planned nuclear physics facility.

Coherent Bremsstrahlung with Collimation

A laser backscatter facility would need to wait for new construction of a new multi-G$ 20GeV+ storage ring (XFEL?).

Even with a future for high-energy beams at SLAC, the low duty factor <10-4 essentially eliminates photon tagging there.

The continuous beams from CEBAF are essential for tagging and well-suited to detecting multi-particle final states.

By upgrading CEBAF to 12 GeV, a 9 GeV polarized photon beam can be produced with high polarization and intensity.

UniqueUnique::


circular polarization transfer from electron beam reaches 100% at end-point

linear polarization determined by crystal orientation vanishes at end-point not affected by electron

polarization

Polarization from Coherent Bremsstrahlung

Linear polarization arises from the two-body nature of the CB kinematics

Linear polarization has unique advantages for GlueX physics: a requirement

Changes the azimuthal coordinate from a uniform random variable to carrying physically rich information.


Photon Beam Intensity Spectrum

4

nominaltagginginterval

Rates based on:• 12 GeV endpoint• 20m diamond crystal• 100nA electron beam

Leads to 107 /s on target

(after the collimator)

Design goal is to build an experiment with ultimate rate capability as high as 108 /s on target.


II. Optimization

photon energy vs. polarization crystal radiation damage vs. multiple scattering collimation enhancement vs. tagging efficiency

Understanding competing factors is necessary to optimize the design


Optimization: chosing a photon energy

A minimum useful energy for GlueX is 8 GeV;8 GeV; 9-10 GeV9-10 GeV is better for several reasons,

for a fixed endpoint of 12 GeV, the peak polarizationpeak polarization and the coherent gain factorcoherent gain factor are both steep functions of peak energysteep functions of peak energy.

CB polarization is a key factor in the choice of a energy range of 8.4-9.0 GeV for GlueX

but


Optimization: choice of diamond thickness

Design calls for a diamond thickness of 2020mm which is approximately 1010-4-4 rad.len rad.len.

Requires thinningthinning: special fabrication steps and $$.

Impact from multiple-scattering is significant.

Loss of rate is recovered by increasing beam current,

up to a point…up to a point…

The choice of 20m is a trade-off between MS and radiation damage.

-3

-4


Electron Beam Emittance

requirement : << 1010-8-8 m m••rr emittances are r.m.s. values

derivation : virtual spot size: 500 m radiator-collimator: 76 m crystal dimensions: 5 mm

In reality, one dimension (y) is much better than the other (x 2.5)

This is a key issue for achieving the requirements for the GlueX Photon Beam

Optics study: goal is achievable, but close to the limits according to 12 GeV machine models Optics study: goal is achievable, but close to the limits according to 12 GeV machine models


V. Diamond crystal requirements

orientation requirements limitations from mosaic spread radiation damage assessment


Diamond crystal: goniometer mount

temperature profile of crystalat full operating intensity

oC


Diamond Orientation

orientation angle is relatively large at 9 GeV: 3 mr3 mr

initial setup takes place at near-normal incidence

goniometer precision requirements for stable operation at 9 GeV are not severe.

alignmentzone

operatingzone

fixed hodoscope

microscope(m

r)


Driven by beam emittanceand spot size at collimator

X: r.m.s. = 1.7mmY: r.m.s. = 0.7mm

Minimum acceptable size:

5mm x 3mm

How large a diamond is needed?


Mosaic spread goal:

Adds in quadraturewith beam divergence

50µr r.m.s.

25µr r.m.s.

How perfect a crystal is needed?


A HTHP diamond ingot

seed

slice 1

slice 2

slice 3

We brought samples from 3 ingots to Daresbury January 2002

Stone 1407 Stone 1485A Stone 1532

SRS measurements (January, 2002)


Stone 1407 slice 1

2mm

4mm x 4mm X-ray beam rocking curve


Stone 1407 slice 1

2mm

0.5mm x 0.5mm X-ray beam rocking curve


Stone 1482A slice 1

2mm



Stone 1482A slice 2

4mm



Stone 1482A slice 2 (rotated)

4mm



Stone 1482A slice 3

4mm



13

Stones not looked at

Stone 1407 slice 2

Stone 1407 slice 3


Diamond Crystal Quality

rocking curve from X-ray scattering

natural fwhm

reliable source of high-quality synthetics from industry (Univ. of Glasgow contact)(Univ. of Glasgow contact)

established procedure in place for selection and assessment using X-rays

R&D is ongoing towards reliable operation of one 20m crystal (Hall B)(Hall B)


conservative estimate (SLAC) for useful lifetime (before significant degradation):

during initial running at 107 /s this gives 600 hrs of running before a spot move

a “good” crystal accommodates 5 spot moves

R&D is planned that will improve the precision of this estimate.

Diamond Crystal Lifetime

0.25 C / mm0.25 C / mm22


14

Conclusions

Doing X-ray topographs is not sufficient.

Topographs are relatively fast and easy to set up.

Rocking curves tell us what we need to know.

It is hard to tell from looking at the topograph what the rocking

curve will look like.

Large and high quality crystals are available.

Based on an example of 1.

New X-ray tests are needed after thinning, rad. damage.


GlueX ReviewsDecember 1999: PAC Requested Review of the GlueX ProjectD. Cassel (chair), J. Domingo, W. Dunwoodie, D. Hitlin, G. Young.

April 2001: NSAC Long Range Plan Committee.

July 2003: Electronics Review of the GlueX ProjectJ. Domingo, A. Lankford (chair), G. Young

October 2004: Detector ReviewM. Albrow, J. Alexander (Chair), W. Dunwoodie, B. Mecking.

December 2004: Solenoid AssessmentJ. Alcorn, B. Kephart (Chair), C. Rode.

January 2006: Photon Beam and Tagger ReviewJ. Ahrens (chair), B. Mecking, A. Nathan

a coherent gamma source the gluex experiment the hall d photon beam the requirements for diamonds...

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