- Email: [email protected]
Electroproduction of ϱ0 mesons at 0.3 < Q2 < 1.5 GeV2 and 1.7 < W < 2.8 GeV
Volume
43B, number
3
PHYSICS
LETTERS
ELECTROPRODUCTION
5 February
OF p” MESONS AT
0.3 < Q2 < 1.5 GeV2 AND 1.7 < V. ECKARDT*,
W < 2.8 GeV
H.J. GEBAUER*, P. JOOS, H. MEYER, B. NAROSKA**, W.J. PODOLSKY***, G. WOLF and S. YELLIN Deutsches Elektronen-Synchrotron
1973
D. NOTZ,
DESY, Hamburg, Germany
and G. DREWS, H. NAGEL and E. RABE II. Institut ftir Experimentalphysik der Universitiit Hamburg?, Germany Received Production
and decay
characteristics
30 November
of electroproduced
Rho production by real photons has been found to behave very much like elastic hadron-hadron scattering with an exponential fall off with momentum transfer, and a nearly energy independent cross section. Furthermore the s-channel helicity conserving amplitudes are dominating. It is an interesting question whether a qualitatively different behaviour is found when the mass of the photon is changed, as for example in electroproduction. Up to now only a few attempts [ 1, 21 have been made to study rho electroproduction. In this letter we report first results from a streamer chamber experiment studying p” electroproduction, ep + epp’
(1)
in the final state + ep + eprr 71
(2)
A 7.2 GeV electron beam from the DESY synchrotron strikes a 9 cm long liquid hydrogen target inside a streamer chamber. The streamer chamber (dimensions: 104 X 60 X 46 cm3) is placed in an 18 kG magnet. Scattered electrons are tagged by a hodoscope of scintillation counters and a shower counter. If the * Now at the Max-Planck-Institut
fur Physik und Astrophysik, Miinchen, Germany. ** Now at CERN, Geneva, Switzerland. *** U.S. National Science Foundation postdoctoral fellow, now at University of Washington, Seattle, Wash., USA. $ Supported in part by the Bundesministerium fur Bildung und Wissenschaft.
240
1972
rho mesons
were studied
in the final state epn+n-.
output pulse from the shower counter exceeds a preset threshold, the Marx generator is fired and a photograph of the streamer chamber is taken. With this setup 200 000 pictures were taken. The scanning and measuring procedure is similar to that for bubble chamber photographs. A more detailed description of the experiment can be found elsewhere [3]. The results presented below come from about one quarter of the data with 3 X 1011 electrons passing through the hydrogen target. From 1906 three and four prong events 890 events were found to give a GRIND fit (x2/n,, < 9) for reaction (2) consistent with ionization and with the identification procedure for the scattered electron [2, 31. Of these events 492 lie in the kinematical region 1.7 < W < 2.8 GeV and 0.3 < Q2 < 1.5 GeV2 where W is the total ems energy of the hadron system and -Q2 the mass squared of the virtual photon. For the purpose of cross section measurements corrections for radiative effects were made [3] (+19 f 6% for external and internal bremsstrahlung, -7% for vertex and propagator corrections [4] ). Fig. 1 shows the pn+, pn- and 7r+n- mass distributions. For I .7 < W < 2.0 GeV both Ai’ and p” production show up (note that W = 1.7 GeV is the threshold for p production) whereas for 2.0 < W < 2.8 GeV p” production dominates. The cross section for A”, A0 and p” have been determined by a maximum likelihood fit to the event distribution in the Dalitz plot, m(M&r+, M:+,-). The Dalitz plot density was assumed to be a sum of contributions from A+‘, A”,
Volume
43B, number
3
PHYSICS
5 February
LETTERS
the corresponding (normalized) Breit-Wigner terms weighted by a factor exp (At,,+), exp (A tprr-) and exp (At,+,-), respectively (t = corresponding four momentum transfer, fpn* = (yvP7rr’)2, t,+,- = (-y-(n++r))2) in order to account for the peripheral production mechanisms. For the slope A a value of 4 GeVP2 was used. The p Breit-Wigner was multiplied by a factor (MP/Mn+rr-)4 which in photoproduction has been found to give a good description of the n+n- mass distribution if integration is done over all production angles [S, 61. Since the p7-P and pn- mass spectra and the distribution of the p” polar decay angle in the p” helicity system, OH, are closely related by kinematics, the distribution of cos 8H was included in the fit:
ep -eprc+n-
W(cosCIH) = l{(l-r04) 00 + (3rO4 oop 1) cos2eH}
Fig. 1. Reaction ep * epn+n-:pn+, pY and R+T- mass distributions for 1.7 < W < 2.0 GeV and 2.0 < W < 2.8 GeV. The curves show the results of the maximum likelihood fits. For the solid curves the po density matrix element rt: was treated as a free parameter. For the dashed curves r$z was fixed to a value of l/3.
po production
and a phase space like background
[S] :
dN@,+,A4;+n-) = = IQA++fA++Wpn+’tpn+)+~*Of~O(Jfpn-’ tpn-)
1973
First a fit was made to all events (i.e. 0.3 < Q2 < 1.5 GeV2) and the p” density matrix element ~8: was treated as a free parameter. A good description of the mass distributions is obtained (see fig. 1). In a second step the fits were repeated for the three Q2 intervals (see table 1) separately with rii now being fixed to the value obtained for all Q2. Fig. 2 displays the Q2 dependence of the total p” CroSS SeCtiOn,u(pp")=uT(p~o)+ EuL(ppo) where UT and uL are the Hand [7] cross sections for scattering of virtual transverse and longitudinal photon on protons; E is the polarization parameter which is of the order of 0.9. The curves in fig. 2 show for comparison the Q2 behaviour of the total inelastic cross section utot normalized* to the photoproduction points for yp + pp” 151. Whereas up has approximately the same Q2 dependence as utot in the low W interval, it drops off faster with Q2 than utot in the higher W region (see table 1). About 25% of this decrease are due to the kinematical cutoff in the four momentum transfer, t, between incoming and outgoing proton (see table 1). The curve in fig. 2 labelled VDM was calculated from the expression [8]
n,(Q’) (3)
(4)
= Im~l(m~+Q2)12(l+~RQ2/~~)
X exp (A I tImin) o,,(Q2 = 0)
+ Qpfp(Mn+n-’ tn+n-) W~cose~~+npsfps~~~~+~~+~-
In contradistinction
The a’s are fit parameters asd measure the size of the individual contributions;j’A++, fAo and fp represent
* We are indebted
(5)
to ref. [8] a factor R was intro-
to Dr. V. Korbel for supplying us with his computer program which interpolates the existing data on total inelastic cross sections.
241
Volume
43B, number
PHYSICS
3
5 February
LETTERS
1973
Table 1 Reaction ep * eppo: Cross sections and fraction of production of the total cross section together with the modification for the ltlmm cutoff; fit parameters of do/df to the form doo/df exp (At)(lfl < 1 GeV*), where do/dt is the differential cross section for to the total p cross section. The photoproduction data stem events in the p mass region (0.60 < Ma +71- < 0.85 GeV) normalized from ref. [6] . For the total inelastic cross section, etot, we use the values obtained in this experiment.
W (GeV)
Q* (GeV’)
opolb)
2.0-2.8
0 0.3 -0.5 0.5 -0.8 0.8-1.5
21.0* 5.92 5.82 2.7*
1.0 1.4 1.5 1.5
opletot
cpletot X expAltlmin
doo/dt bb GeV2)
A (GeV-* )
0.16 +0.008 0.065 kO.016 0.076+0.021 0.043 * 0.029
0.186+0.009 0.082+0.20 0.010+0.029 0.060+0.041
148.2 f 6.7 27.7t7.1 25.8k8.7 9.6k5.6
6.25 4.3 4.8 2.9
w-
ep*ePP”
w”
060
I
%X,
100
,
,
,
,
17-=W-=
c
,
,
2.0
,
,
,
,
The
$
SET,Q2~0,
GeV . LO’ -0 --o :
GeV
f
_
’ P
’
exp.
w i 2.40 ’
’
’
i 0.20 kO.8 kO.9 +1.2
’
GeV ’
20cWc2.8GeV
L.
10 5
5
,,d 0
r, g
1 I
1.0
05 Q2
1.5 Ii
(GeV2) 1
Fig. 2. Reaction ep + eppo. Total cross section as a function of Q* for different W intervals (r). The values at Q* = 0 have been measured by the ABBHHM Collaboration (ref. [5] ). The dashed-dotted curves show the Q* dependence of the total inelastic ep cross section [8], normalized to the cross section at Q* = 0. The curve labelled VDM has been calculated according to eq. (5).
duced which was set equal to 0.3 (see below). Eq. (5) is seen to agree with the measured cross sections. In fig. 3 the t dependence is shown for all events in the p” region (0.60
1
08
c i
0
1
1 ,I,
I 1.0
0
:tiplp
,,,,,
10
2
(GeV*)
Fig. 3. Reaction ep --* epn+rr-. Differential cross section, do/dt, (t = square of the four momentum transfer between incoming and outgoing proton) for events in the p region (# for different W, Q* intervals. The cross sections have been normalized to the total p cross section. The open points ($) show the photoproduction data of the SBT collaboration (ref. [6] ) treated in the same manner.
parameters from fits to the form do/dt = do”/dt X exp (At) are given in table 1. The slope decreases with
Volume
43B, number
PHYSICS
3
ep+epP”
0.60~Mm+-
0.3<@<1% GeVz
o(
00
’
’ l60'
I
’
360’ 0. J,
’
’ 180.
’
’ 360’
Fig. 4. Reaction ep + epn+n-: Decay angular distributions in the helicity system for events in the p region. The curves for the cos 8H distributions show the result of the maximum likelihood fits to the Dalitz plot density: for the solid curves rzz was treated as a free parameter. For the dashed curve Y:: was fixed to a value of l/3.
increasing Q2. Whether this is caused by a shrinking of the photon-interaction radius [9] or by a different t behaviour of background in the p” region cannot be decided at this point. p” decay angular distribution: The p” decay angular distribution was analyzed in the helicity system in terms of OH, $$I, the polar and azimuthal angles of the decay rr+ in the p rest frame and @, the angle between the e,e’ scattering plane and the production plane for yvirrualp + pp O in the overall ems. In fig. 4 the distributions Of cos OH and $H = $H - $ are shown. All events in the p mass region were used. As emphasized above the cos OH distribution for p mesons can be affected by reflections from A++ and A0 production which will show up near cos OH = + 1, respectively. The mass fits, where such reflections are taken care of autmoatically, yield for 1.7 < W< 2.0 GeV a large fraction of longitudinal aligned p’s,
LETTERS
5 February
1973
namely rti = 0.89 + 0.05. At higher W values this fraco. - 0.2 1 f 0.08. The $H distribution tion is smaller* , ro4 in the higher W interval shows the characteristic cos2$ behaviour observed also in the production of p’s by linearly polarized photons [6] . An analysis in terms of the p spin density matrix has been made for all events in the p region [3]. It was found that for 2.0 < W < 2.8 GeV all terms receiving only contributions from helicity flip terms are small which is suggestive for s-channel helicity conservation (SCHC). Within the present limits of accuracy no interference between p production by transverse and longitudinal photons has been observed. Assuming SCHC a nonzero value of rf$ (eq. (4)) can only arise from production by longitudinal photons and one finds at ( W) = 2.3 GeV, (Q2j = 0.6 GeV2: R = oL(pp)/aT(pp) = 0.30 2 0.14. This value disagrees with the prediction [9] R = Q2/m% = I at Q2 = m2. We have investigated electroproduction in the &al state ep -+ epn+n- . The ratio of p” to total inelastic production drops with increasing Q2 by roughly a factor of two to four between Q2 = 0 and 1 GeV2 (table 1). The t distribution for events in the p region broadens with increasing Q2 (table 1). The p” distribution was analyzed in the helicity system. For 1.7 < W < 2.0 GeV the p” is predominantly longitudinally aligned. For 2.0 < W < 2.8 GeV more transverse p” mesons are observed. Under the assumption of SCHC, R = oL(pp)/oT(pp) = 0.30 * 0.14 at (Q2) = 0.6 CeV2. We wish to thank Professor E. Lohrmann and Professor M.W. Teucher for their encouragement. We are greatly indebted to the machine group, to the Hallendienst and to our technical and scanning personnel. * Since rtz decreases
rapidly from 1.7 < W < 2.0 GeV to 2.0 < W < 2.8 GeV one may suspect that rzz is even smaller at higher values of W. We have therefore repeated the fit for 2.2 < W < 2.8 GeV and found that ($ remains unchanged within errors namely: ri: = 0.3 1 * 0.12.
References [I] C. Driver et al., Nucl. Phys. B38 (1972) 1. 121 D. Andrews et al., Phys. Rev. Letters 26 (1971) E.D. Bloom et al., Phys. Rev. Letters 28 (1972)
864; 516.
243
Volume
43B, number
[3] V. Eckardt published.
PHYSICS
3
et al., DESY report
72/67
(19671,
to be
[4] L.Y. MO and Y.S. Tsai, Rev. Mod. Phys. 41 (1969) 205. [ 51 Aachen-Berlin-Bonn-Hamburg-Heidelberg-Munchen Collaboration, Phys. Rev. 175 (1968) 1669. [ 61 SLAC-Berkeley-Tufts Collaboration, Phys. Rev. D5 (1972) 545.
244
LETTERS
5 February
1973
[7] L. Hand, Phys. Rev. 129 (1964) 1834. [8] H. Fraas and D. Schildknecht, Nucl. Phys. B14 (1969) 543. [9] H. Cheng and T.T. Wu, Phys. Rev. 183 (1969) 1324; J.D. Bjorken, I. Kogut and D. Soper, Phys. Rev. D3 (1971) 1382; H.T. Nieh, Phys. Letters B38 (1972) 100.
43B, number
3
PHYSICS
LETTERS
ELECTROPRODUCTION
5 February
OF p” MESONS AT
0.3 < Q2 < 1.5 GeV2 AND 1.7 < V. ECKARDT*,
W < 2.8 GeV
H.J. GEBAUER*, P. JOOS, H. MEYER, B. NAROSKA**, W.J. PODOLSKY***, G. WOLF and S. YELLIN Deutsches Elektronen-Synchrotron
1973
D. NOTZ,
DESY, Hamburg, Germany
and G. DREWS, H. NAGEL and E. RABE II. Institut ftir Experimentalphysik der Universitiit Hamburg?, Germany Received Production
and decay
characteristics
30 November
of electroproduced
Rho production by real photons has been found to behave very much like elastic hadron-hadron scattering with an exponential fall off with momentum transfer, and a nearly energy independent cross section. Furthermore the s-channel helicity conserving amplitudes are dominating. It is an interesting question whether a qualitatively different behaviour is found when the mass of the photon is changed, as for example in electroproduction. Up to now only a few attempts [ 1, 21 have been made to study rho electroproduction. In this letter we report first results from a streamer chamber experiment studying p” electroproduction, ep + epp’
(1)
in the final state + ep + eprr 71
(2)
A 7.2 GeV electron beam from the DESY synchrotron strikes a 9 cm long liquid hydrogen target inside a streamer chamber. The streamer chamber (dimensions: 104 X 60 X 46 cm3) is placed in an 18 kG magnet. Scattered electrons are tagged by a hodoscope of scintillation counters and a shower counter. If the * Now at the Max-Planck-Institut
fur Physik und Astrophysik, Miinchen, Germany. ** Now at CERN, Geneva, Switzerland. *** U.S. National Science Foundation postdoctoral fellow, now at University of Washington, Seattle, Wash., USA. $ Supported in part by the Bundesministerium fur Bildung und Wissenschaft.
240
1972
rho mesons
were studied
in the final state epn+n-.
output pulse from the shower counter exceeds a preset threshold, the Marx generator is fired and a photograph of the streamer chamber is taken. With this setup 200 000 pictures were taken. The scanning and measuring procedure is similar to that for bubble chamber photographs. A more detailed description of the experiment can be found elsewhere [3]. The results presented below come from about one quarter of the data with 3 X 1011 electrons passing through the hydrogen target. From 1906 three and four prong events 890 events were found to give a GRIND fit (x2/n,, < 9) for reaction (2) consistent with ionization and with the identification procedure for the scattered electron [2, 31. Of these events 492 lie in the kinematical region 1.7 < W < 2.8 GeV and 0.3 < Q2 < 1.5 GeV2 where W is the total ems energy of the hadron system and -Q2 the mass squared of the virtual photon. For the purpose of cross section measurements corrections for radiative effects were made [3] (+19 f 6% for external and internal bremsstrahlung, -7% for vertex and propagator corrections [4] ). Fig. 1 shows the pn+, pn- and 7r+n- mass distributions. For I .7 < W < 2.0 GeV both Ai’ and p” production show up (note that W = 1.7 GeV is the threshold for p production) whereas for 2.0 < W < 2.8 GeV p” production dominates. The cross section for A”, A0 and p” have been determined by a maximum likelihood fit to the event distribution in the Dalitz plot, m(M&r+, M:+,-). The Dalitz plot density was assumed to be a sum of contributions from A+‘, A”,
Volume
43B, number
3
PHYSICS
5 February
LETTERS
the corresponding (normalized) Breit-Wigner terms weighted by a factor exp (At,,+), exp (A tprr-) and exp (At,+,-), respectively (t = corresponding four momentum transfer, fpn* = (yvP7rr’)2, t,+,- = (-y-(n++r))2) in order to account for the peripheral production mechanisms. For the slope A a value of 4 GeVP2 was used. The p Breit-Wigner was multiplied by a factor (MP/Mn+rr-)4 which in photoproduction has been found to give a good description of the n+n- mass distribution if integration is done over all production angles [S, 61. Since the p7-P and pn- mass spectra and the distribution of the p” polar decay angle in the p” helicity system, OH, are closely related by kinematics, the distribution of cos 8H was included in the fit:
ep -eprc+n-
W(cosCIH) = l{(l-r04) 00 + (3rO4 oop 1) cos2eH}
Fig. 1. Reaction ep * epn+n-:pn+, pY and R+T- mass distributions for 1.7 < W < 2.0 GeV and 2.0 < W < 2.8 GeV. The curves show the results of the maximum likelihood fits. For the solid curves the po density matrix element rt: was treated as a free parameter. For the dashed curves r$z was fixed to a value of l/3.
po production
and a phase space like background
[S] :
dN@,+,A4;+n-) = = IQA++fA++Wpn+’tpn+)+~*Of~O(Jfpn-’ tpn-)
1973
First a fit was made to all events (i.e. 0.3 < Q2 < 1.5 GeV2) and the p” density matrix element ~8: was treated as a free parameter. A good description of the mass distributions is obtained (see fig. 1). In a second step the fits were repeated for the three Q2 intervals (see table 1) separately with rii now being fixed to the value obtained for all Q2. Fig. 2 displays the Q2 dependence of the total p” CroSS SeCtiOn,u(pp")=uT(p~o)+ EuL(ppo) where UT and uL are the Hand [7] cross sections for scattering of virtual transverse and longitudinal photon on protons; E is the polarization parameter which is of the order of 0.9. The curves in fig. 2 show for comparison the Q2 behaviour of the total inelastic cross section utot normalized* to the photoproduction points for yp + pp” 151. Whereas up has approximately the same Q2 dependence as utot in the low W interval, it drops off faster with Q2 than utot in the higher W region (see table 1). About 25% of this decrease are due to the kinematical cutoff in the four momentum transfer, t, between incoming and outgoing proton (see table 1). The curve in fig. 2 labelled VDM was calculated from the expression [8]
n,(Q’) (3)
(4)
= Im~l(m~+Q2)12(l+~RQ2/~~)
X exp (A I tImin) o,,(Q2 = 0)
+ Qpfp(Mn+n-’ tn+n-) W~cose~~+npsfps~~~~+~~+~-
In contradistinction
The a’s are fit parameters asd measure the size of the individual contributions;j’A++, fAo and fp represent
* We are indebted
(5)
to ref. [8] a factor R was intro-
to Dr. V. Korbel for supplying us with his computer program which interpolates the existing data on total inelastic cross sections.
241
Volume
43B, number
PHYSICS
3
5 February
LETTERS
1973
Table 1 Reaction ep * eppo: Cross sections and fraction of production of the total cross section together with the modification for the ltlmm cutoff; fit parameters of do/df to the form doo/df exp (At)(lfl < 1 GeV*), where do/dt is the differential cross section for to the total p cross section. The photoproduction data stem events in the p mass region (0.60 < Ma +71- < 0.85 GeV) normalized from ref. [6] . For the total inelastic cross section, etot, we use the values obtained in this experiment.
W (GeV)
Q* (GeV’)
opolb)
2.0-2.8
0 0.3 -0.5 0.5 -0.8 0.8-1.5
21.0* 5.92 5.82 2.7*
1.0 1.4 1.5 1.5
opletot
cpletot X expAltlmin
doo/dt bb GeV2)
A (GeV-* )
0.16 +0.008 0.065 kO.016 0.076+0.021 0.043 * 0.029
0.186+0.009 0.082+0.20 0.010+0.029 0.060+0.041
148.2 f 6.7 27.7t7.1 25.8k8.7 9.6k5.6
6.25 4.3 4.8 2.9
w-
ep*ePP”
w”
060
I
%X,
100
,
,
,
,
17-=W-=
c
,
,
2.0
,
,
,
,
The
$
SET,Q2~0,
GeV . LO’ -0 --o :
GeV
f
_
’ P
’
exp.
w i 2.40 ’
’
’
i 0.20 kO.8 kO.9 +1.2
’
GeV ’
20cWc2.8GeV
L.
10 5
5
,,d 0
r, g
1 I
1.0
05 Q2
1.5 Ii
(GeV2) 1
Fig. 2. Reaction ep + eppo. Total cross section as a function of Q* for different W intervals (r). The values at Q* = 0 have been measured by the ABBHHM Collaboration (ref. [5] ). The dashed-dotted curves show the Q* dependence of the total inelastic ep cross section [8], normalized to the cross section at Q* = 0. The curve labelled VDM has been calculated according to eq. (5).
duced which was set equal to 0.3 (see below). Eq. (5) is seen to agree with the measured cross sections. In fig. 3 the t dependence is shown for all events in the p” region (0.60
1
08
c i
0
1
1 ,I,
I 1.0
0
:tiplp
,,,,,
10
2
(GeV*)
Fig. 3. Reaction ep --* epn+rr-. Differential cross section, do/dt, (t = square of the four momentum transfer between incoming and outgoing proton) for events in the p region (# for different W, Q* intervals. The cross sections have been normalized to the total p cross section. The open points ($) show the photoproduction data of the SBT collaboration (ref. [6] ) treated in the same manner.
parameters from fits to the form do/dt = do”/dt X exp (At) are given in table 1. The slope decreases with
Volume
43B, number
PHYSICS
3
ep+epP”
0.60~Mm+-
0.3<@<1% GeVz
o(
00
’
’ l60'
I
’
360’ 0. J,
’
’ 180.
’
’ 360’
Fig. 4. Reaction ep + epn+n-: Decay angular distributions in the helicity system for events in the p region. The curves for the cos 8H distributions show the result of the maximum likelihood fits to the Dalitz plot density: for the solid curves rzz was treated as a free parameter. For the dashed curve Y:: was fixed to a value of l/3.
increasing Q2. Whether this is caused by a shrinking of the photon-interaction radius [9] or by a different t behaviour of background in the p” region cannot be decided at this point. p” decay angular distribution: The p” decay angular distribution was analyzed in the helicity system in terms of OH, $$I, the polar and azimuthal angles of the decay rr+ in the p rest frame and @, the angle between the e,e’ scattering plane and the production plane for yvirrualp + pp O in the overall ems. In fig. 4 the distributions Of cos OH and $H = $H - $ are shown. All events in the p mass region were used. As emphasized above the cos OH distribution for p mesons can be affected by reflections from A++ and A0 production which will show up near cos OH = + 1, respectively. The mass fits, where such reflections are taken care of autmoatically, yield for 1.7 < W< 2.0 GeV a large fraction of longitudinal aligned p’s,
LETTERS
5 February
1973
namely rti = 0.89 + 0.05. At higher W values this fraco. - 0.2 1 f 0.08. The $H distribution tion is smaller* , ro4 in the higher W interval shows the characteristic cos2$ behaviour observed also in the production of p’s by linearly polarized photons [6] . An analysis in terms of the p spin density matrix has been made for all events in the p region [3]. It was found that for 2.0 < W < 2.8 GeV all terms receiving only contributions from helicity flip terms are small which is suggestive for s-channel helicity conservation (SCHC). Within the present limits of accuracy no interference between p production by transverse and longitudinal photons has been observed. Assuming SCHC a nonzero value of rf$ (eq. (4)) can only arise from production by longitudinal photons and one finds at ( W) = 2.3 GeV, (Q2j = 0.6 GeV2: R = oL(pp)/aT(pp) = 0.30 2 0.14. This value disagrees with the prediction [9] R = Q2/m% = I at Q2 = m2. We have investigated electroproduction in the &al state ep -+ epn+n- . The ratio of p” to total inelastic production drops with increasing Q2 by roughly a factor of two to four between Q2 = 0 and 1 GeV2 (table 1). The t distribution for events in the p region broadens with increasing Q2 (table 1). The p” distribution was analyzed in the helicity system. For 1.7 < W < 2.0 GeV the p” is predominantly longitudinally aligned. For 2.0 < W < 2.8 GeV more transverse p” mesons are observed. Under the assumption of SCHC, R = oL(pp)/oT(pp) = 0.30 * 0.14 at (Q2) = 0.6 CeV2. We wish to thank Professor E. Lohrmann and Professor M.W. Teucher for their encouragement. We are greatly indebted to the machine group, to the Hallendienst and to our technical and scanning personnel. * Since rtz decreases
rapidly from 1.7 < W < 2.0 GeV to 2.0 < W < 2.8 GeV one may suspect that rzz is even smaller at higher values of W. We have therefore repeated the fit for 2.2 < W < 2.8 GeV and found that ($ remains unchanged within errors namely: ri: = 0.3 1 * 0.12.
References [I] C. Driver et al., Nucl. Phys. B38 (1972) 1. 121 D. Andrews et al., Phys. Rev. Letters 26 (1971) E.D. Bloom et al., Phys. Rev. Letters 28 (1972)
864; 516.
243
Volume
43B, number
[3] V. Eckardt published.
PHYSICS
3
et al., DESY report
72/67
(19671,
to be
[4] L.Y. MO and Y.S. Tsai, Rev. Mod. Phys. 41 (1969) 205. [ 51 Aachen-Berlin-Bonn-Hamburg-Heidelberg-Munchen Collaboration, Phys. Rev. 175 (1968) 1669. [ 61 SLAC-Berkeley-Tufts Collaboration, Phys. Rev. D5 (1972) 545.
244
LETTERS
5 February
1973
[7] L. Hand, Phys. Rev. 129 (1964) 1834. [8] H. Fraas and D. Schildknecht, Nucl. Phys. B14 (1969) 543. [9] H. Cheng and T.T. Wu, Phys. Rev. 183 (1969) 1324; J.D. Bjorken, I. Kogut and D. Soper, Phys. Rev. D3 (1971) 1382; H.T. Nieh, Phys. Letters B38 (1972) 100.