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The polarised target asymmetry in photoproduction of eta mesons from protons at 4 GeV
Nuclear Physics BI85 (1981) 269-273 © North-Holland Publishing Company
T H E P O L A R I S E D T A R G E T A S Y M M E T R Y IN PHOTOPRODUCTION O F ETA M E S O N S F R O M PROTONS AT 4 GeV P.J. BUSSEY, C. RAINE and J.G. RUTHERGLEN University of Glasgow, Glasgow G12 8QQ, UK
P.S.L. BOOTH, L.J. CARROLL, G.R. COURT, P.R. DANIEL, A.W. EDWARDS, R. GAMET, C.J. HARDWICK, P.J. HAYMAN, J.R. HOLT, J.N. JACKSON and W.H. RANGE University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK
W. GALBRAITH, F.H. COMBLEY, V.H. RAJARATNAM and C. SUTTON University of Sheffield, Sheffield SI 0 2 TN, UK
Received 17 November 1980
Measurements have been made of the polarjsed target asymmetry parameter T for eta photoproduction from protons at a photon energy of 4 GeV and t-valuesof -0.4, -0.6 and -0.8 (GeV/c) 2. The results are compared with predictions for this quantity.
1. Introduction Measurements of the cross sections for photoproduction of pi-zero and eta mesons at energies above the resonance region have provided a severe test of Regge theories. The different angular variations of the differential cross sections in the two cases, in which the pi-zero cross section has a pronounced dip at a t-value of about - 0 . 5 ( G e V / c ) 2, whereas the eta cross section is quite smooth, are impossible to explain on the basis of simple exchange of ~0 (in the pi-zero case) and p (in the eta case). The addition of exchanges involving other particles, or various absorptive cuts, introduces unnatural parity components which have to be greater for eta production than for pi-zero production in order to fill the dip in the t-variation in the former case. However, measurements of the polarised beam asymmetry parameter (Z) for pi-zero and for eta production [1, 2] have shown that this is close to + 1 in both cases for t-values less than about - 0 . 8 ( G e V / c ) 2, indicating that the contribution from unnatural parity exchange is small. A critical summary of the various classes of Regge theory has been given by Irving and Vanryckeghem [3]. In discussing the features of the different exchange models 269
270
P.J. Bussey et al. / Polarised target asymmetry
they have carried out a simultaneous analysis of pi-zero and eta photoproduction in order to separate out exchanges of definite isospin and parity. The lack of measurements of Z in the reaction 3,n ~ ~r°n precluded a definitive analysis, but within certain bounds values for the natural and unnatural isospin components were obtained. By combining their analysis with the helicity amplitude decomposition of Barker and Storrow [4], they were able to determine helicity flip to non-flip ratios for the vector exchanges ~0 and p. By assuming SU(3) symmetry for pi-zero and eta couplings to these mesons they were then able to predict the quantity ½(P + T) for eta photoproduction, where P is the recoil nucleon polarisation and T the polarised target asymmetry. For the case Y~~ + 1, corresponding to zero component of unnatural parity exchange, the general condition I T - P I ~< 1 - Z [5], forces P = T and the predicted quantity becomes identical to the polarised target asymmetry. We have measured this asymmetry ( T ) for the process 3,p ~ ~/p, at a mean photon energy of 4 GeV, and compared the results with the predictions of Irving and Vanryckeghem [3].
2. The experiment The measurements were carried out in a 4 GeV bremsstrahlung beam from an internal target of the Daresbury electron synchrotron. The butanol polarised target, in which protons were polarised in the vertical direction, was 15 mm in diameter and 20 mm long. The beam was collimated to a circular cross section which closely matched that of the target. The procedure for aligning the target with the beam has been described in a previous paper [6], in which details of the polarised target are to be found. The process 3'P ~ ~P was detected via triple coincidences between the proton and the photons of the 23, decay mode of the eta meson. The proton momentum and angle were determined with a magnetic spectrometer, in which water (~erenkov and glass (~erenkov detectors distinguished protons from pions. The momenta and angles of the photons were determined by means of two arrays of lead-glass (2erenkov detectors, placed one above the other so that their centres subtended the appropriate 3,3, decay angle at the target. To allow the photons to emerge unobstructed, the eta decay angle being larger than that for pi-zero mesons detected in a previous experiment, new superconducting coils were provided for the target with an enlarged vertical aperture of _+ 13 ° [7]. More detailed information concerning the proton spectrometer and the photon detectors may be found in previous publications [8, 9]. The analysis of the data followed closely the procedure used in our measurements of the beam asymmetry parameter for eta photoproduction [2]. The determination of the 6 angles and 3 energies for the proton and two photons enabled a 4C fit to be made to the hypothesis of single eta photoproduction. The resulting X2 distribution showed the expected peak in the range 0 - 1 0 for a 4C fit. Separate runs with a
P.J. Bussey et al. / Polarised target asymmetry
271
TABLE 1 Polarised target asymmetry parameter T for photoproduction of eta mesons from protons at 4 GeV incident energy - t(GeV/c) 2
T
0.39 ~ 0.07 0.58 ~0.11 0.77 ± 0 . 1 3
-0.04-+-0.11 -0.17±0.09 - 0 . 4 3 __+0.14
carbon target enabled the background due to the non-hydrogen component of the butanol to be subtracted. This amounted to about 25%. The results of the experiment are given table 1, where the polarised target asymmetry T is shown for three values of the four-momentum transfer. The asymmetry is defined as usual as ( N 1' - N $ ) / ( f ~, N 1' + f 1' N $), where N is the normalised counting rate, and f the fractional polarisation. The upward arrow indicates the direction k × p, where k is the incident beam momentum and p the momentum of the meson. The quoted errors for T include statistical errors, those due to uncertainties in the background subtraction and those in the measurement of the target polarisation.
-0.2-
i
-0.4-
T ± -0.6-
-0.8-
-1.0-
0.T2
1 0~4 0.6 -t (GeV/c) 2
0T8
Fig. l. The black circles are the measured values of the polarised target asymmetry T for photoproduction of eta mesons from protons at 4 GeV. The open circles are the predictions for ½(T+ P) from ref. [3]. The extensions to the error bars indicate the possible deviations of T from this quantity due to the differences between T and P estimated from measurements of Y..
272
P.J. Busseyet al. / Polarisedtarget asymmetry
The data are plotted in fig. 1, together with predicted values from Irving and Vanryckeghem [3]. The latter were obtained from the expression
½(T+ P ) , = x z ° ( ~ r ° p ) [ l + Y'(~r°P)](1 + 9C++)(9 + C+_) I ( T + P),o. 27o(~p)[1 + Y.(~p)](1 + C++)(1 + C+_)
Here x = cos0 - rsin0, where r is the quark model singlet to octet ratio and 0 the quadratic mixing angle. Values of i2 and - 1 0 ° are assigned to these, in keeping with measurements of the ~ width. The quantities C+ + and C+_ are the ratios of s-channel helicity amplitudes for the exchange of I-spin 1 and 0, with non-spin-flip and spin-flip, respectively. They are assumed to be real constants, which is equivalent to assuming that the ~ and O exchanges satisfy SU(3) with constant F / D mixing parameters. This constraint, together with the information concerning the helicity amplitudes from their own analysis and from that of Barker and Storrow [4], lead to values of C+ + = 0.071 and C+_ = 0.28. These incidently provide predictions for the polarised beam asymmetry ~ for yn ~ ~r°n. We then have
½(T+P)~=0.62
°(~r°P)[ l + £ ( ~ r ° p ) ] l ( r + p ) # o(~p)[l+Z(~p)] 2
'
and the predictions in fig. 1 were calculated with values of ½(T+ P)~0 taken from the analysis of Barker and Storrow [4]. The spreads in the predicted values come from the errors in the experimental measurements which were used in their derivation. It should be noted that E(~r°p) and Z(,/p) are close to + 1, so that ½(T+ P) is not very different from T in both cases. The measured deviations of Z(~qp) from + 1 have been used to calculate the maximum possible deviation of Tn from ½(T+ P)n, and these are indicated by the extensions to the error bars in fig. 1. It can be seen that the measurements of Tn are consistently smaller in absolute value than the predictions, although the sign is in agreement and the trend with variation of t is the same. This trend seems different from the corresponding trend of T~,,, which decreases to a negative maximum at t = - 0 . 6 ( G e V / c f and then increases. However, the results cast some doubt on the value of the constant term in the expression for ½(T+ P)n, which depends on considerations based on SU(3), and it would be useful to have measurements of E(~°n) to provide an independent check of the quantities used in its derivation. Such measurements would help to define the amplitudes involved in pseudoscalar meson photoproduction and perhaps point the way to a solution of the difficulties of the Regge exchange models mentioned in sect. 1.
P.J. Bussey et al. / Polarised target asymmetry
273
It is a pleasure to thank the Director and staff of the Daresbury Laboratory for their help and support. We acknowledge with gratitude the collaboration of our late colleague J.G. Rutherglen. References [1] P.J. Bussey, C. Raine, J.G. Rutherglen, P.S.L. Booth, L.J. Carroll, G.R. Court, P.R. Daniel, A.W. Edwards, R. Gamet, C.J. Hardwick, P.J. Hayman, J.R. Holt, J.N. Jackson, W.H. Range, F.H. Combley, W. Galbraith, V.H. Rajaratnam and IC. Sutton, Nucl. Phys. B154 (1979) 492 [2] P.J. Bussey, C. Raine, J.G. Rutherglen, P.S.L. Booth, L.J. Carroll, P.R. Daniel, A.W. Edwards, C.J. Hardwick, J.R. Holt, J.N. Jackson, J. Norem, W.H. Range, W. Gaibraith, V.H. Rajaratnam, C. Sutton, M.C. Thorne and P. Wailer, Phys. Lett. 61B (1976) 479 [3] A.C. Irving and L.G.F. Vanryckeghem, Nucl. Phys. B93 (1975) 324 [4] I.S. Barker and J. Storrow, Nucl. Phys. B137 (1978) 413 [5] I.S. Barker, A. Donnachie and J. Storrow, Nucl. Phys. B95 (1975) 347 [6] P.S.L. Booth, L.J. Carroll, G.R. Court, P.R. Daniel, R. Gamet, C.J. Hardwick, P.J. Hayman, J.R. Holt, A.P. Hufton, J.N. Jackson, J.H. Norem and W.H. Range, Nucl. Phys. B121 (1977) 45 [7] P.J. Bussey, J.G. Rutherglen, P.S.L. Booth, L.J. Carroll, G.R. Court, A.W. Edwards, R. Gamet, P.J. Hayman, J.R. Holt, J.N. Jackson, W.H. Range, C. Wooff, F.H. Combley, W. Galbraith, A. Phillips and V.H. Rajaratnam, Nucl. Phys. B159 (1979) 383 [8] J.S. Barton, P.S.L. Booth, L.J. Carroll, J.R. Holt, J.N. Jackson, G. Moscati and J.R. Wormald, Nucl. Phys. B84 (1975) 449 [9] P.S.L. Booth, L.J. Carroll, J.R. Holt, J.N. Jackson, W.H. Range, K.A. Sprakes and J.R. Wormald, Nucl. Phys. B84 (1975) 437
T H E P O L A R I S E D T A R G E T A S Y M M E T R Y IN PHOTOPRODUCTION O F ETA M E S O N S F R O M PROTONS AT 4 GeV P.J. BUSSEY, C. RAINE and J.G. RUTHERGLEN University of Glasgow, Glasgow G12 8QQ, UK
P.S.L. BOOTH, L.J. CARROLL, G.R. COURT, P.R. DANIEL, A.W. EDWARDS, R. GAMET, C.J. HARDWICK, P.J. HAYMAN, J.R. HOLT, J.N. JACKSON and W.H. RANGE University of Liverpool, P.O. Box 147, Liverpool L69 3BX, UK
W. GALBRAITH, F.H. COMBLEY, V.H. RAJARATNAM and C. SUTTON University of Sheffield, Sheffield SI 0 2 TN, UK
Received 17 November 1980
Measurements have been made of the polarjsed target asymmetry parameter T for eta photoproduction from protons at a photon energy of 4 GeV and t-valuesof -0.4, -0.6 and -0.8 (GeV/c) 2. The results are compared with predictions for this quantity.
1. Introduction Measurements of the cross sections for photoproduction of pi-zero and eta mesons at energies above the resonance region have provided a severe test of Regge theories. The different angular variations of the differential cross sections in the two cases, in which the pi-zero cross section has a pronounced dip at a t-value of about - 0 . 5 ( G e V / c ) 2, whereas the eta cross section is quite smooth, are impossible to explain on the basis of simple exchange of ~0 (in the pi-zero case) and p (in the eta case). The addition of exchanges involving other particles, or various absorptive cuts, introduces unnatural parity components which have to be greater for eta production than for pi-zero production in order to fill the dip in the t-variation in the former case. However, measurements of the polarised beam asymmetry parameter (Z) for pi-zero and for eta production [1, 2] have shown that this is close to + 1 in both cases for t-values less than about - 0 . 8 ( G e V / c ) 2, indicating that the contribution from unnatural parity exchange is small. A critical summary of the various classes of Regge theory has been given by Irving and Vanryckeghem [3]. In discussing the features of the different exchange models 269
270
P.J. Bussey et al. / Polarised target asymmetry
they have carried out a simultaneous analysis of pi-zero and eta photoproduction in order to separate out exchanges of definite isospin and parity. The lack of measurements of Z in the reaction 3,n ~ ~r°n precluded a definitive analysis, but within certain bounds values for the natural and unnatural isospin components were obtained. By combining their analysis with the helicity amplitude decomposition of Barker and Storrow [4], they were able to determine helicity flip to non-flip ratios for the vector exchanges ~0 and p. By assuming SU(3) symmetry for pi-zero and eta couplings to these mesons they were then able to predict the quantity ½(P + T) for eta photoproduction, where P is the recoil nucleon polarisation and T the polarised target asymmetry. For the case Y~~ + 1, corresponding to zero component of unnatural parity exchange, the general condition I T - P I ~< 1 - Z [5], forces P = T and the predicted quantity becomes identical to the polarised target asymmetry. We have measured this asymmetry ( T ) for the process 3,p ~ ~/p, at a mean photon energy of 4 GeV, and compared the results with the predictions of Irving and Vanryckeghem [3].
2. The experiment The measurements were carried out in a 4 GeV bremsstrahlung beam from an internal target of the Daresbury electron synchrotron. The butanol polarised target, in which protons were polarised in the vertical direction, was 15 mm in diameter and 20 mm long. The beam was collimated to a circular cross section which closely matched that of the target. The procedure for aligning the target with the beam has been described in a previous paper [6], in which details of the polarised target are to be found. The process 3'P ~ ~P was detected via triple coincidences between the proton and the photons of the 23, decay mode of the eta meson. The proton momentum and angle were determined with a magnetic spectrometer, in which water (~erenkov and glass (~erenkov detectors distinguished protons from pions. The momenta and angles of the photons were determined by means of two arrays of lead-glass (2erenkov detectors, placed one above the other so that their centres subtended the appropriate 3,3, decay angle at the target. To allow the photons to emerge unobstructed, the eta decay angle being larger than that for pi-zero mesons detected in a previous experiment, new superconducting coils were provided for the target with an enlarged vertical aperture of _+ 13 ° [7]. More detailed information concerning the proton spectrometer and the photon detectors may be found in previous publications [8, 9]. The analysis of the data followed closely the procedure used in our measurements of the beam asymmetry parameter for eta photoproduction [2]. The determination of the 6 angles and 3 energies for the proton and two photons enabled a 4C fit to be made to the hypothesis of single eta photoproduction. The resulting X2 distribution showed the expected peak in the range 0 - 1 0 for a 4C fit. Separate runs with a
P.J. Bussey et al. / Polarised target asymmetry
271
TABLE 1 Polarised target asymmetry parameter T for photoproduction of eta mesons from protons at 4 GeV incident energy - t(GeV/c) 2
T
0.39 ~ 0.07 0.58 ~0.11 0.77 ± 0 . 1 3
-0.04-+-0.11 -0.17±0.09 - 0 . 4 3 __+0.14
carbon target enabled the background due to the non-hydrogen component of the butanol to be subtracted. This amounted to about 25%. The results of the experiment are given table 1, where the polarised target asymmetry T is shown for three values of the four-momentum transfer. The asymmetry is defined as usual as ( N 1' - N $ ) / ( f ~, N 1' + f 1' N $), where N is the normalised counting rate, and f the fractional polarisation. The upward arrow indicates the direction k × p, where k is the incident beam momentum and p the momentum of the meson. The quoted errors for T include statistical errors, those due to uncertainties in the background subtraction and those in the measurement of the target polarisation.
-0.2-
i
-0.4-
T ± -0.6-
-0.8-
-1.0-
0.T2
1 0~4 0.6 -t (GeV/c) 2
0T8
Fig. l. The black circles are the measured values of the polarised target asymmetry T for photoproduction of eta mesons from protons at 4 GeV. The open circles are the predictions for ½(T+ P) from ref. [3]. The extensions to the error bars indicate the possible deviations of T from this quantity due to the differences between T and P estimated from measurements of Y..
272
P.J. Busseyet al. / Polarisedtarget asymmetry
The data are plotted in fig. 1, together with predicted values from Irving and Vanryckeghem [3]. The latter were obtained from the expression
½(T+ P ) , = x z ° ( ~ r ° p ) [ l + Y'(~r°P)](1 + 9C++)(9 + C+_) I ( T + P),o. 27o(~p)[1 + Y.(~p)](1 + C++)(1 + C+_)
Here x = cos0 - rsin0, where r is the quark model singlet to octet ratio and 0 the quadratic mixing angle. Values of i2 and - 1 0 ° are assigned to these, in keeping with measurements of the ~ width. The quantities C+ + and C+_ are the ratios of s-channel helicity amplitudes for the exchange of I-spin 1 and 0, with non-spin-flip and spin-flip, respectively. They are assumed to be real constants, which is equivalent to assuming that the ~ and O exchanges satisfy SU(3) with constant F / D mixing parameters. This constraint, together with the information concerning the helicity amplitudes from their own analysis and from that of Barker and Storrow [4], lead to values of C+ + = 0.071 and C+_ = 0.28. These incidently provide predictions for the polarised beam asymmetry ~ for yn ~ ~r°n. We then have
½(T+P)~=0.62
°(~r°P)[ l + £ ( ~ r ° p ) ] l ( r + p ) # o(~p)[l+Z(~p)] 2
'
and the predictions in fig. 1 were calculated with values of ½(T+ P)~0 taken from the analysis of Barker and Storrow [4]. The spreads in the predicted values come from the errors in the experimental measurements which were used in their derivation. It should be noted that E(~r°p) and Z(,/p) are close to + 1, so that ½(T+ P) is not very different from T in both cases. The measured deviations of Z(~qp) from + 1 have been used to calculate the maximum possible deviation of Tn from ½(T+ P)n, and these are indicated by the extensions to the error bars in fig. 1. It can be seen that the measurements of Tn are consistently smaller in absolute value than the predictions, although the sign is in agreement and the trend with variation of t is the same. This trend seems different from the corresponding trend of T~,,, which decreases to a negative maximum at t = - 0 . 6 ( G e V / c f and then increases. However, the results cast some doubt on the value of the constant term in the expression for ½(T+ P)n, which depends on considerations based on SU(3), and it would be useful to have measurements of E(~°n) to provide an independent check of the quantities used in its derivation. Such measurements would help to define the amplitudes involved in pseudoscalar meson photoproduction and perhaps point the way to a solution of the difficulties of the Regge exchange models mentioned in sect. 1.
P.J. Bussey et al. / Polarised target asymmetry
273
It is a pleasure to thank the Director and staff of the Daresbury Laboratory for their help and support. We acknowledge with gratitude the collaboration of our late colleague J.G. Rutherglen. References [1] P.J. Bussey, C. Raine, J.G. Rutherglen, P.S.L. Booth, L.J. Carroll, G.R. Court, P.R. Daniel, A.W. Edwards, R. Gamet, C.J. Hardwick, P.J. Hayman, J.R. Holt, J.N. Jackson, W.H. Range, F.H. Combley, W. Galbraith, V.H. Rajaratnam and IC. Sutton, Nucl. Phys. B154 (1979) 492 [2] P.J. Bussey, C. Raine, J.G. Rutherglen, P.S.L. Booth, L.J. Carroll, P.R. Daniel, A.W. Edwards, C.J. Hardwick, J.R. Holt, J.N. Jackson, J. Norem, W.H. Range, W. Gaibraith, V.H. Rajaratnam, C. Sutton, M.C. Thorne and P. Wailer, Phys. Lett. 61B (1976) 479 [3] A.C. Irving and L.G.F. Vanryckeghem, Nucl. Phys. B93 (1975) 324 [4] I.S. Barker and J. Storrow, Nucl. Phys. B137 (1978) 413 [5] I.S. Barker, A. Donnachie and J. Storrow, Nucl. Phys. B95 (1975) 347 [6] P.S.L. Booth, L.J. Carroll, G.R. Court, P.R. Daniel, R. Gamet, C.J. Hardwick, P.J. Hayman, J.R. Holt, A.P. Hufton, J.N. Jackson, J.H. Norem and W.H. Range, Nucl. Phys. B121 (1977) 45 [7] P.J. Bussey, J.G. Rutherglen, P.S.L. Booth, L.J. Carroll, G.R. Court, A.W. Edwards, R. Gamet, P.J. Hayman, J.R. Holt, J.N. Jackson, W.H. Range, C. Wooff, F.H. Combley, W. Galbraith, A. Phillips and V.H. Rajaratnam, Nucl. Phys. B159 (1979) 383 [8] J.S. Barton, P.S.L. Booth, L.J. Carroll, J.R. Holt, J.N. Jackson, G. Moscati and J.R. Wormald, Nucl. Phys. B84 (1975) 449 [9] P.S.L. Booth, L.J. Carroll, J.R. Holt, J.N. Jackson, W.H. Range, K.A. Sprakes and J.R. Wormald, Nucl. Phys. B84 (1975) 437