RUN 3

BEAM MONITORING

RUN 3 RUN 3

o. -o.

o. 2.)7 0.69 o. o. o. o. o. o. o. o. o. c. o. o. o. -o.

o. o. o.•1 o. 21 o. o. o. o. o. o. o. o. o. o. o. o. -o.

600 ^_o. o. o. o. o. o. o. ^o. o. o. o. o. o. o. ^1.11 c. ^{.... 61} 1.11 c. o. ^0.69 0.59 o. 3.07 2 •• o ^{o. 0.46} z. 2. 1.02 85 7• ^o. c. o. 50 2. 72 3.3" ^o. o. o. 0.21 1. 5e ^o. o. o. o. 0.5. ^o. o. o. o. 0.06 ^o. o. o. o. c. ^o. c. c. c. c. ^o. c. c. c. c. ^o. o. o. o. o. ^o. o. o. o. o. ^o. o. c. cc. . ^-o. -o. --o. -o. c.

100- g: ^o. _o. ^o. _{o. c.} ^{o. o.} _o. ^o. _{c. c.}^1.2• ₀₁ ^4.CC _1.H ^{1. 72} _{3. 52} ^C_1.21 ^{.26 c .c5} _c.2c ^c. _c. ^c. _c. ^o. _o. ^o. _o. ^c. _c. ^-o. _-c.

o. o. o. o. o. o. o. 0.21 1.15 2.31 1.12 C. l l c.02 o. o. c. -o. s

o. o. o. c. o. o. c. o. 0.25 1.01 1.~. I. IC c. 14 o. o. 0. -o. _::I.

o. o. o. c. o. o. o. o. o. ^{(j . 4 ]} l.H 1.5< C.61 o.ce o.co c. -c. _::I.

800-~: ^o. _o. ^o. _o. ^c. _{c •.} ^c. _o. ^o. _o. ^c. _c. ^o. _o. ^o. _o. ^0.06 _o. ^c.t2 _{c.c. c.}^1.41 _5' ^C.96 _C.96 ^{0.11 o.c2 c .oo -o.}

0 · ' ' o.o• 0 .co -o. .l.

o. o. o. o. o. o. o. c. o. o. o. o. c. o. o. o. o. o. o. o. c. o. c.co c. C.69 c.u o.,. ^0.3] c.o.u ct c .co -o. c.co -o.

o. o. o. o. o. o. c. c. o. o. o. o. c. o.o. o.o• c.co -o.

900-g: -o. -o. -o. -o. -o. -o. -o. -o. -o. -o. ^o. _o. -o. -o. -o. -o. -o. ^o. _o. -c. -o. --o. -o. ^o. _c. c. -o. -o. -o. -o. -o. ^o. _o. -o. --o. -o. -o. ^o. _o. o. -o. -o. -o. -o-o. ^o. _o. . -o--o. -o. -o. ^o. o. _o. . -o. -o. -o. -o. -o. ^o. _o. -o. -o. -o. -o. -o. ^o. _o. -c. -o. -o. -o. -c. ^c. _c. -c. --c. -c. -o. ^c. _c. c. -c. -c. --c. -c. ^c. _c. c. -o. -o. -o-o. -o. ^o. _o. . -o. -o. -o. --o. ^0.01 _o.oo o. -o. -o. -c. -c, -o. ^{c .co} c .co -c. ^-o. o. o. o. o. o.

RUN 1

2.11 o. o. o. o. o. o. o. o. o. o. o. o. o. o. o. -o.

2 •• , 3. l4 o. o. o. o. o. o. o. o. o. o. o. o. o. o. -o.

o. •·2' ^{1.99 o. o. o. o. o. o. o. o. o. o.} o. o. o. -o.

600 ^o. o. o. o. o. ^o. o. o. o. o. ^{•• 2.} o.Jo. o. o. ~ ^3.22 a.c. 3.c. 01 22 ^0.10 2 .za •·2• l .06 o ... ^o. o.o.

··"

l .'l't 6.19 ^o. o. o.oq 1.35 ^{'· 50 o.} o. o. o.c1 ^{1.11 o.} o. o. o. ^{0.10 o.} o. o. o. ^{o. o.} o. o. o. ^{c. o.} o. o. c. ^{c. o.} o. o. c. ^{c. o.} o. o. o. ^{o. o.} o. o. o. ^o. o. o. o. c. ^c. -o. -o. -o. -^-o. o.

700 - ^o. o. _o. ^o. o. _o. ^o. o. _o. c. _o. ^{c. o.} o. _o. o. _o. ^{o. 22} _c. '1.05 · 59 ^{4. 64 6}_1.12 ^.14

•·37

1. 28 _{5. 22} 0.2. ^o.ao _{3. 70} c. ^o.11t _c.ec c. ^o. _c.10 c. ^c. _c. o. ^o. _o. o. ^o. _o. c. ^c. _c. -o. ^-_-o. ^o. _s

o. o. o. c. o. o. o. c.01 2.35 •• 01 2.n c.11 c.1• o. o. c. -c. ::I.

o. o. o. c. o. o. c. o. o.37 2.13 1.1. 2.•0 c • ., o.c1 o. c. -o. ::I.

800 - o . ^o. o. o. o. ^o. o. o. o. o. ^o. o. o. o. o. ^c. c. o. o. o. o. ^o. o. o. o. ^o. o. o. o. o. ^o. o. c. o. o. ^o. o. o. o. o. ^o. o. o. o. o. ^0.9l o. o. o. o. ^2ol9 c.c. o. o. 11 o.u c. ^2.35 'c.11 .29 c.29 ^1.•1 c.10 1.34 1.0. ^O.H 0 .t5 o.B 0.12 o.t4 ^0.01 o.c1 o.C6 0.11 ColC ^o. c c.co -o. c .co -o. c .co -o. .co ^--c. ^o. .l.

900 - o . -o. -o-o. -o. -o. ^o. . -o. -o. -o. -o. -o. ^o. o. -o. -o. -o. -o. -o. ^o. o. --o. -o. -c. -o. ^c. c. o. -o. -o. -o. -o. -o. ^o. o. -o. -o. -o. -o. -o. ^o. o. -o. -o. -o. --o. ^o. o. o. -o. -o. -o. -o. -o. ^o. o. -o. -o. -o. -o. -o. ^o. o. -o. -o. -o. -o. -o. ^o. c. -o. -o. -o. -o. -o. ^o. c. -c. --c. --o. ^c. c. c. o. --c. -c. -c. -c. ^c. c. c. -o. -o. -o. -o. -o. ^0.2C o. -o. -o. -o. --o. ^o.o.o. o. ^oe -o. -o. -o. -c. -o. c.co -o. ^{c.cc -}o. c. o. ^o. o. o.

RUN 2

0.37 o. o. o. o. o. o. o. o. o. o. . o. o. o. o. o. -o.

o.n 0.26 o. o. o. o. o. o. o. o. o. o. . o. o. o. o. -o.

o. 3,15 o.69 o. o. o. o. o. o. o. o. o. o. o. o. o. -o.

600 - ⁰o. o. o. o. • o. o. o. o. o. ^3.97 1.56 o. o. o. o ..50 2.47 o. o. • ., o. 0.21 ..59 ....,1 1.01 o. o. o. 1• 3. 2. 79 79 o. o. o. 0.'7 1.•I o. o. o. o. 0.51 o. o. o. o. 0.03 o. o. o. o. o. o. o. o. o. o. o. o. o. c. c. c. o. o. c. c. o. o. o. o. o. o. o. o. o. o. o. o. o. o. o. -o. --o. -o. -o. o.

700 - ^o. o. o. _o. ^o. o. o. _o. ^o. o. o. _o. ^o. c. o. _o. ^o. o. o. _o. o. o. ^1.56 o. ^3.H 1.19 ^l.86 3.21 2.ZJ ^o.•9 2.c1 3.07 ^0.04 o. 22 1.51 ^o. c.c1 c:. ]~ ^o. c. c .o. ^c. c. c. ^o. o. o. ^o. o. o. ^c. c. c. -c. -c. -c. s

o. o. o.u 2ol7 2.35 1.0 c. 30 ^c.01 o. o. c. -c. ^::I.

o. o. o. o. o. o. o. o. o.o 2 .a 7 .z.31 c. 76 c.11 o .c 1 o. c. -o. ^::I.

800 ^{_ a. o. o. o. a. o. o. o. a. 1.21 2. zc l.6l c. 54 o.c~ 0 .co c.} -c.

o. o. a. c. o. o. o. o. o. c .o• ^c.t~ 1.,. 1.35 ^0.2~ 0.01 c.co -c. .l.

o. o. o. o. o. o. o. o. o. o. 0.11 C.93 C,90 0. ,. o.c. c.co -c.

o. o. o. c. o. o. o. o. o. o. o. c.1• c.11 o. 52 o.et c .co -o.

o. o. o. o. o. o. o. o. o. o. c. c. c.12 0 .41 O.tl. c .cc -c.

900 - o . -o. -o. -o. -o. -o. ^o. -o. --o. -o. -o. ^o. o. o. -o. -o. -o. -o. -o. o. ^o. -o. -o. -o. -c. -o. ^o. c. I -o. -o. -o. -o. ^{-o. o.} o. -o. -o. -o. -o. ^{-o. o.} o. --o-c. -o. ^{-o. o.} o. o. . -o. -^-o. -o. -o. o. ^o. o. I -o-o. -o. -o. ^{-o. o.} o. . -o. -^-o. -c. -o. ^o. o. o. -c. -c. -c. -o. ^{-o. o.}o. ^' -c. -c. -c. -o. ^{-a. c.} c.

I

-c. -c-c. -c. ^{-c. c.} c. . -o. -o. -o. -^-o0.2) o.u o. ^. -o. -o. -o-o. ^{-o. o.u} o.oJ . -o. -c. -c. -c. ^{-o. c.co} c .co -^-c. o. o. o. o. c. ^o.

1.0 1.2 1.4 k(BeV)

Figure 10 shows the fit between various p ⁰ models and the experimental mass distributions, and Table III summarizes the final parameters from each fit. Figure 11 compares the angular distribution from the various p ⁰ models. Figure 12. 1 shows the experimental

angular distributions for various di pion mass intervals. These were obtained by dividing the data into .02 bins in cos 8 and calculating

cm the weighted average of d2

cr/dodm in each cos 8 bin. Figure 12. 2

. cm

shows the angular distributions and total cross sections determined by previous experiments.

The resonant mass m deduced by the various fits is in the

p

range 737 < m < 750 which is consistent with previous experiments.

p

The history of the p ⁰ mass deduced from photoproduction experiments is worth noting hereo The first observation of the p ⁰ resonance (l2

) found m = 725 ± 5 MeV, much lower than the value of 755 MeV from productfon by pions. <25) Thff CEA group(22) finds m = 728 ± 8 MeV,

p

and the DESY collaboration the mass decreasing from 760 MeV at

threshold to about 725 MeV at a photon energy of 5 BeV. Recent experi- ments in production from complex nuclei, <43

, 44) find 765 ± 5 MeV. In most cases, the rho mass found in photoproduction is lower than the currently accepted value of 755 ± 3 MeV from production by pions<49

>.

This raises some interesting questions as to the existence of inter- ference effects between p ⁰ production and the background. Also, if an

e: ⁰ meson with JPG = o++ exists, it will obviously have a strong influence on the interpretation of the results from di.pion production •

. The width of the resonance varied over the range 122 <

r

0 < 150.

The values of

r

from previous experiments vary widely. The DESY

0

group found 112 <

r

< 198 MeV depending on the photon energy

0

considered. The CEA group also found 125 <

r

0 < 225 depending on the photon energy interval. The production of pO's from complex nuclei <43' 44) gives

r

₀ = 124 MeV found in production by pions. <49

)

Figure 10

There are a pair of figures for each p ⁰ model - one comparing the expected and observed TT-TT mass plots for each run and the other comparing the expected and observed distri- bution in photon energy for. each run. For the mass plot, the curve indicating the final fit was 9btained from the summation over photon energy.

\'

L

[b N (k, m, i) p p + b N (k, m, i) ps ps + b*N*(k, m, i)J • k

For the photon energy distributions, the curve indicating the final fit was the summation over dipion mass

\' [b N (k, m, i) +

L

p p b N (k, m, i) ps ps + b*N*(k, m, i)J • m

Also shown are the individual contributions from phase space and N* production. The N* contribution is indicated by the dashed lines and the phase space contribution by the dash- dotted lines (when bps

-f

O).

42

0 8> ,_ "' .-f g Q) ~ "Cl ~

~

E

N z ~ ii ~ 0

0 m 0 0

II II Ul a. 0. ..c ..c

co M ~ M M Q:; 1:-...-4 II II a.

s

0 h

~ 2 8

Figure 10. 1

43

!

r-1 (J)

>

"O

N Q) 0 _, ..0 ~ ..x µ;:i P-4 0 g

"!

10. 2

44

1-1 0 ... C,.)

> ~ ~

Q)

s .s

.,. 1-1

..

0 E r-~ ..c

...

... ~ i:il ~ 0

0 0 O') 0

II II

rl.l a. c.. .Q .Q

:>

Q) ~

0 0 CD ID ID CD z t-..-1 N ::::> a: II II II a. 0

s

r... :<

Figure 10. 3

r<>. z :'.) a::

N z :'.) a::

z :'.) a:: 45

I I I ! I I I I I I ~ I I I \ g

'"'

I I I I ::!; I I I I > I N Q) I -' ..0 I ~ I I I I 9

"'

2 Sl

Figure 10. 4 H 0 ~ u ro ~

s

H 0 ~ ..c: ~ •...C ;::-: ^{i:;il ~} 0

200

eo 1-tj

...

~ "1

(t) IOO

~

.

0 01

~

m = 743

p

r

= 142

0

t ₀ = _• 956 Bev2

RlJ-J I

b _p

=.

⁹⁰

b _ps = _• 00

RUN 2

700

~7r(mev)

Diffraction Model

RUN 3

'800 900

(j) H':-

r<) z ::> oc

N z ::> oc

0 ~ 47

0 . Q

Figure 10. 6 0 "' I I I I I I I I I I I I I I 9 <I!

... :! (l) 'O 0 ~

i:: N~ 0 -' ~ •r-4 B ..., C) .:it:. ro f.-4 <H ~ 9 Cl

<I!

II 11

C/l a. 0. .0 .0

t-CN M CN t-.-I II II a.

s

r-..0

~ 48

(\j z ::> 0::

2 8 Figure 10. 7

/ : i I

~

i I

^{\ J}

. ,

^\\

0 I R

,, : \

1. \I

~~

s

~ ~ E ... (]) "t:S

~

... ro C) "bh 0

-

0 ~ (])

s

0 ~ (]) .c:: ~

1501 RUN I RUN 2

JJ ^{I \}

^{RUN 3}

"tj

...

~ ~

: ₀ 1001

l/LJ \1 / I I \ ri ^\--,

^{-~ co}

•

°'

50

-

^...

---

. 8 LO L2 1.4 .8 LO L2 L4 .8 LO L2 l4

k (bev)

Phenomenological Model

30

20

10

.4

20

a;.(14

b)

10

1.0

Phenomenolo~icol Model OPE

OPE with Form Factor Oiffroctlon Model

.6 .. I

···

1 /

//

I

"'//'

//

.··""·.

/ / I •.••. •·· .

/ ./ ....

... ···;,

...

....

....

....

···;;;;"' ^{, / '} /

....

^{. /}

-- --- - ^- -- ^---

---- --- ---

.8 1.0

cos Bcm

-- --

-- -- -- --

-- --

---~-=-:..:-==-==:.:.:-=-::..:-=----::::

-~ -

---

_.,..--·~-'

...

...

. ..

____

,,,,,,

7 ,,,,

,

,,

/ , / "

, ,, ,/,,,/

'" t/

/ I

,/

12

k(bev)

Figure 11

1.4

20 20

al I mlrlr 620 ~ mlrlr ~ 790

I ^{f 1}

10 10

~

p

f H

f

~

.7 .8 .9 1.0 .7 .8 .9 1.0

"-

en . ...

..c ::t

20 20

I _! ^I

520 s mlrlr s 720 700 s mlrlr s 920

f f

10 10

f _{~ f} ^~

f f _~

7 .8 .9 1.0 .7 .8 .9 1.0

cos Bern Figure 12. 1

20

t> CEA

l3 DESY

t Fretwell and Mullins

~ -This Experiment (Form Factor Fit)

~ :1.. ..0

b\c:!

-0,, 10

40

-

_~ ^-4 30

~ 20

10 0

1.0

.2 .4 .6 .8

cos

e

DIFFERENTIAL CROSS SECTION AT 1.4 BEV

f f ·

2.0 Z.!I

k(bev) TOTAL CROSS SECTION

Figure 12. 2

1.0

~

3.0

Model description 1 m (MeV)

r

(Me~ b I extra parameters _IX ² I Prob.

p 0 p ps

OPE ·

I

738 ± 5 133 ± 7 • 90 ::

~~ o o

• -. 00

+.

⁰⁵

I

none

_I

331

I .

0006

r

150± 17 • 90 ::

~~

^{+ 06}

I I .

345

OPE (with form 750 ± 6 0 -: 00 mx =266 ± 8 MeV

I

264 factor)

Diffraction 743 ± 5 142±10

.9o::~~

^{• 20 ± • 06 t =}.956±.03Bev2 314 • 005

0 Cl

""

-1/2

PhenC:c:fe~logical

I

^{737 ±} 5 1122±111. 77 ±. 05

I .

^{13 ±. 05}

I

^a1 = • 013 MeV 297 • 033 et2 = 4. 1

0.3 = • 95 Ct4 ~ • 126

Figure 12 compares the cross sections observed here with. previous experiments. The differential cross section given by this experiment is somewhat more sharply peaked forward than observed before. However, since this experiment covered a limited range of angles at a given dipion mass and photon energy, statistical variations can strongly influence the fit to the angular dependence of the cross section. Bubble chamber experiments which cover all production angles should be considered a more reliable source of information on the differential cross section.

The total cross section given by the various fits are

strikingly similar and the curve shown in Figure 12.2B represents the OPE form factor model. The p ⁰ cross section from this experiment is slightly · lower than observed in previous experiments, but the difference is well within the error limits. However, the amount of phase space production determined by this experiment is much less than found elsewhere. For a comparison of our phase space

production with others, the significant parameter for comparison is the ratio of the p 0

and phase space contributions to the data.

CEA and DESY both find the ratio

Whereas we find

b /b = ps P

b /b ^{R;j •} 9 ± • 2 • ps P

o 00

~: g~

OPE model

o.

0

~: g~ ·

OPE with form factor

• 00

~: g6

Diffraction

• 14 ±. 06 Phenomenological Model.

Since this experiment observed only a limited range of pro- duction angles, a slight distortion of the observed angular distribution

could change the ratio b /b by 5-15%. Also, since we were only ps P

sensitive to dipions which decayed more or less symmetrically, an anisotropic decay distribution for the p ⁰ would influence the results.

In particular, if the p ⁰ decay distribution is sin ²

e

(see Appendix V), as expected from totally polarized p ⁰¹s, then our total cross section for

p ⁰ production must be divided by two. Since the phase space cross section is uninfluenced by the ^{p 0} decay distribution, the ratio b

/b

ps P would then increase by a factor of two. Also, there is no reason to require that the background production of dipions behave exactly like phase space. If the background produces fewer events in the forward direction (cos

e

cm ~ 1) than phase space, then we would expect to observe a negligible background contribution in the interval of cos

e

cm to which we were sensitive.

The OPE model gives a total cross section which is a factor of two higher than any of the other models, so the valuer = • 12 MeV

pny

deduced from the OPE fit is quite meaningless. Assuming the form factor factor model introduces· a factor

g m 2 2 ( nx ~)

t+ mx

into the differential cross section, we find r g2

= ·

^i. 07 MeV •

pny nx

Also, by looking at Figure 12. 1 we note that the data prefer a differential section which is quite peaked in the forward direction.

This makes the OPE model fit badly, whereas the other models have adjustable parameters to allow for a sharply peaked angular distri-

bution.

In conclusion, the major difficulties encountered in

interpreting this experiment are due to the limited kinematic regions which were observed. In particular, many more experimental

configurations could have covered a larger range of production angles,

p ⁰ decay angles, and photon energy-dipion mass values. Future experiments along these lines would prove very interesting. The Caltech synchrotron is very well suited for an investigation of p ⁰

photoproduction near threshold, and more conclusive information on

· the production angular distribution, ^{p 0} decay distribution, and the

existence of interference effects between various production amplitudes would be invaluable.