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Measurement of total hadronic photoabsorption cross section on carbon from 14 to 34 GeV
Volume 56B, number 2
PHYSICS LETTERS
14 April 1975
MEASUREMENT OF TOTAL HADRONIC PHOTOABSORPTION CROSS SECTION O N C A R B O N F R O M 14 T O 34 G e V G.L. BAYATYAN, N.K. GRIGORYAN, S.G. KNYAZYAN, A.T. MARGARYAN, G.S. VARTANYAN
Yerevan Physws Institute, USSR A.I. ALIKHANYAN
Lebedev PhysicsInstitute, USSR A.M. FROLOV
Institute of High Energy Physics, USSR Recelved 7 February 1975 The data on total cross section of hadronic photoabsorption in carbon nuclei as obtained by conventional tagged photon method are given for five values of energy in 14-34 GeV interval. A slight drop of cross section curve with the increase in energy is observed, indicating the contribution of p'-meson to the total cross section.
Introduction. The measurement of total hadromc photoproduction cross section supplies necessary information for the understanding of photon-nucleon interaction. According to vector-meson-dominance model [1,2], VDM, the effects of shadowing, i.e. the reduction of the number of interacting nucleons, may take place when the interaction proceeds via hadron state. When the photon's travel in an hadron state is much larger than the vector-meson-interaction mean free path in a nucleus, the interacting nucleons lie predominantly on the nuclear surface and the interaction cross section will increase with the number of nucleons in a target as o t ~ A 2 / 3 , and not as o t ~ A , as in the case of all the nucleons being likely to contribute. It is convenient, however, to work with the effective number of nucleons which is the ratio of the total cross section of hadron photoproduction on the nucleus with an atomic number A to that on the hydrogen Aeff = ot(TA )/ot(TN ) . The experimental values of Aeff obtained in refs. [3-5] lie between A eft = A and A eft = A 2/3. The falloff of Aeff/A ratio with the increase of photon energy up to ( 4 - 5 ) GeV is observed [3, 4] followed by a nearly constant course up to 18 GeV [5]. The value of Aeff[A in this region exceeds the vector-dominance model prediction with due regard for pO, 6o and ~0contribution
[5]. This discrepancy suggests an additional short-range interaction possibly due to either a heavy vector meson p' or to a partially bare-photon interaction. These two models predict [6] different behaviour of Aeff/A ratio in the energy region above 20 GeV. The hypothesis of the existence of heavy vector meson p predicts at such energies the diminution of Aeff/A ratio with energy - the effect not expected in bare-photon model. Apparatus and measurement technique. To obtain the monochromatic photon beam the conventional tagging photons method [7J was used. The 40 GeV/c electron beam with the momentum spread A p / p ~--+. 3% was monitored by C 1 and C2 counters and struck a lead radiator R of about 0.08 x 0 thick. The tagging hodoscope consisted of two arrays of counters T 1-6 and T~_ 6 coincident in parrs. The separanon of electrons corresponding to the energy interval of tagged photons E.l = 14-34 GeV by means of C 1 - C 3 counters brought in coincidence with the tagging hodoscope. The lower energy interval proceeded from our experimental program [8], while the upper one was defined by the performance of analysing magnet. Anticounters A 1- A 4 were used to prevent the spurious tags from low-energy positrons produced in the radiator. The coincidences between tagging system and the shower counter SO (see fig. 1) give the number of noninteractmg photons. 197
Volume 56B, number 2
PHYSICS LETTERS
14 Aprd 1975
liii' .Ds, D=
III11
I111
p~ Pg
Fig. 1. Layout of hadronic photoabsorption experiment. The hadronic photoabsorption events were detected in lead-scintallator sandwiches AD 1 and AD 2, 100 cm and 350 cm distant from the target respectively. Each of these hadron detectors AD 1 and AD 2 consisted of two (35 X 35 X 2) cm 3 scintillators H 3 - H 6 and with 10 cm center holes. The scintillators were interleaved with 5 cm lead sheets. The high rate of these counters due to the #-meson background caused the large number of random coincidences between hadronic detectors and the tagging system. The addition of H 1 scintillator coincident with hadronic detectors [9] helped to reduce the effective rate of hadronic detectors by approximately a factor of 100. H 2 counter provided the high multiplicity of hadronic coincidences. Hadronic photoabsorption event was indicated by the coincidence of (HIH2H3H4) or (H1H2HsH6) with tagging signal at the absence of SO pulse. It is noteworthy that the photoabsorption events with only neutral hadrons produced were not detected by this experimental setup. However, these events were shown [10] at 2 - 4 GeV to make up less than 2% of the total hadronic photoproduction cross section and to decrease with energy due to the increase of hadron production multiplicity.
Data on 12C nuclei and corrections. To measure the total hadronic photoabsorption cross section in carbon 5 X 106 photons with energy from 14 to 34 GeV were passed through the 0.14x 0 thick target. The total cross section was calculated from the formula Ctot(7C ) = (1/No)N/M , where N O is the number of target nuclei per unit area; N is the number of hadronically interacting photons; M is the number of incident photons. The corrections applied included: 1) The absorption of photon beam in the target due to the e+e--pair production; 2) The attenuation of incident photon beam due to the inconsistency of its cross section with target area. For the target of 5 cm in diameter and the cross section of ?-beam measured this correction made up 5%; 3) The contribution.of random coincidences proved to be negligible; 4) The empty target background for five energy values as given in table 1; 5) The account of multiple bremsstrahlung processes in the radiator. The Monte Carlo calculations of this correction are given in table 1;
Table 1 II channel ( 1 9 - 2 3 ) C-eV
III channel (23 -27) GeV
IV channel
(14-19)GeV
(27-30) GeV
V channel (30-34) GeV
0.157 + 0.049
0.189 ± 0.06
0.207 ± 0.06
0.276 + 0.084
0.381 + 0.099
Correction for multiple scattefing in radlatoz_(in %)
10.5
12
12.5
13.5
15.5
(Otot(-tC) (in #b)
1082 -+56
1105 ± 50.5
1080 ± 57
980 ± 67.7
930 ± 44
I channel
Empty target background tJV/M) x 10'4)
198
Volume 56B, number 2
PHYSICS LETTERS
~-Ser.pu~hov f500
14 April 1975
Ctot("/C) in the energy region (1-34) GeV and in fig. 3 the energy dependence o f A e f f / A . The ratio Aeff[A was calculated from the formula Aeff Otot(TA) A Zotot(TP) + (A - Z) Otot(Tn) ' where
fOOO
ot(TP) = [(98.7-+3.6) + (65-+ 10)/x/E]lab (Em GeV), ot(7 n) = [ot(TP) - (18.3 + 6.1)/x/E]lab (E in GeV).
Fig. 2. The dependence of at(TC) on photon energy in ( 2 - 3 4 ) GeV interval.
6) The 2% background from electromagnetic processes in the target; 7) The geometric corrections which were due to the loss of a-meson photoproduction events. The Monte Carlo simulation of this loss gave the figures from 0.5% to 1.5% for different channels of tagging; 8) The approximately 8% inefficiency of hadron detectors. Our values of Otot(TC) at different energies from 14 to 34 GeV are given in table 1. In fig. 2 we show the measured total cross section
The ratio A eff[A is seen in the figure to decrease with the increase of energy in agreement with the VDM predictions. It is worth-while to note, that a certain amount of multiparticle events with n°-mesons might have been neglected, if 7-quanta from 7r°-mesons decay had gated anticoincidence signal in the shower counter. We have not considered this effect and, so, the decrease of Ct(TC) may well be partly due to it. In conclusion the authors wish to thank A.Ts. Amatuni, A.A. Logunov, S.G. Matinyan, S.S. Gershtein, S.P. Denisov and V.M. Kut'in for the interest and support, O.M. Vinnitski, K.A. Ispiryan and Yu.M. Sapunov for their assistance at different stages of this experiment.
O,y"
0,7-
A -..DEs ..~ " x -~ezp~hAov -V,S M --- w*~ +eoX
0
S,~o~ f -
--
,.-f
Fig. 3. The dependence of Aeff/A ratio on photon energy in ( 2 - 3 4 ) GeV interval.
199
Volume 56B, number 2
PHYSICS LETTERS
References [1] K. Gottfried and D.R. Yennie, Phys. Rev. 182 (1969) 1595. [2] S. Brodsky et al., Phys. Rev. 182 (1969) 1794. [3] G.R. Brooks et al., Phys. Rev. D8 (1973) 2826. [4] V. Heynen et al., Phys. Lett. 34B (1971) 651.
200
[5] [61 [7] [8] [9] [10]
14 April 1975
D.O. Caldwell et al., Phys. Rev. D7 (1973) 1362. D. Sehildknecht, SLAC-PUB-1230 (197,3). G.L. Bayatyan et al., Scientific report EFI-64 (74). A.I. Alikhanyan et al., preprint EFI-EP-1 (70), Yerevan. G.L. Bayatyan, Scientific report E F I 4 7 (73). E. Gabathuler, DL/P 190 (1974).
PHYSICS LETTERS
14 April 1975
MEASUREMENT OF TOTAL HADRONIC PHOTOABSORPTION CROSS SECTION O N C A R B O N F R O M 14 T O 34 G e V G.L. BAYATYAN, N.K. GRIGORYAN, S.G. KNYAZYAN, A.T. MARGARYAN, G.S. VARTANYAN
Yerevan Physws Institute, USSR A.I. ALIKHANYAN
Lebedev PhysicsInstitute, USSR A.M. FROLOV
Institute of High Energy Physics, USSR Recelved 7 February 1975 The data on total cross section of hadronic photoabsorption in carbon nuclei as obtained by conventional tagged photon method are given for five values of energy in 14-34 GeV interval. A slight drop of cross section curve with the increase in energy is observed, indicating the contribution of p'-meson to the total cross section.
Introduction. The measurement of total hadromc photoproduction cross section supplies necessary information for the understanding of photon-nucleon interaction. According to vector-meson-dominance model [1,2], VDM, the effects of shadowing, i.e. the reduction of the number of interacting nucleons, may take place when the interaction proceeds via hadron state. When the photon's travel in an hadron state is much larger than the vector-meson-interaction mean free path in a nucleus, the interacting nucleons lie predominantly on the nuclear surface and the interaction cross section will increase with the number of nucleons in a target as o t ~ A 2 / 3 , and not as o t ~ A , as in the case of all the nucleons being likely to contribute. It is convenient, however, to work with the effective number of nucleons which is the ratio of the total cross section of hadron photoproduction on the nucleus with an atomic number A to that on the hydrogen Aeff = ot(TA )/ot(TN ) . The experimental values of Aeff obtained in refs. [3-5] lie between A eft = A and A eft = A 2/3. The falloff of Aeff/A ratio with the increase of photon energy up to ( 4 - 5 ) GeV is observed [3, 4] followed by a nearly constant course up to 18 GeV [5]. The value of Aeff[A in this region exceeds the vector-dominance model prediction with due regard for pO, 6o and ~0contribution
[5]. This discrepancy suggests an additional short-range interaction possibly due to either a heavy vector meson p' or to a partially bare-photon interaction. These two models predict [6] different behaviour of Aeff/A ratio in the energy region above 20 GeV. The hypothesis of the existence of heavy vector meson p predicts at such energies the diminution of Aeff/A ratio with energy - the effect not expected in bare-photon model. Apparatus and measurement technique. To obtain the monochromatic photon beam the conventional tagging photons method [7J was used. The 40 GeV/c electron beam with the momentum spread A p / p ~--+. 3% was monitored by C 1 and C2 counters and struck a lead radiator R of about 0.08 x 0 thick. The tagging hodoscope consisted of two arrays of counters T 1-6 and T~_ 6 coincident in parrs. The separanon of electrons corresponding to the energy interval of tagged photons E.l = 14-34 GeV by means of C 1 - C 3 counters brought in coincidence with the tagging hodoscope. The lower energy interval proceeded from our experimental program [8], while the upper one was defined by the performance of analysing magnet. Anticounters A 1- A 4 were used to prevent the spurious tags from low-energy positrons produced in the radiator. The coincidences between tagging system and the shower counter SO (see fig. 1) give the number of noninteractmg photons. 197
Volume 56B, number 2
PHYSICS LETTERS
14 Aprd 1975
liii' .Ds, D=
III11
I111
p~ Pg
Fig. 1. Layout of hadronic photoabsorption experiment. The hadronic photoabsorption events were detected in lead-scintallator sandwiches AD 1 and AD 2, 100 cm and 350 cm distant from the target respectively. Each of these hadron detectors AD 1 and AD 2 consisted of two (35 X 35 X 2) cm 3 scintillators H 3 - H 6 and with 10 cm center holes. The scintillators were interleaved with 5 cm lead sheets. The high rate of these counters due to the #-meson background caused the large number of random coincidences between hadronic detectors and the tagging system. The addition of H 1 scintillator coincident with hadronic detectors [9] helped to reduce the effective rate of hadronic detectors by approximately a factor of 100. H 2 counter provided the high multiplicity of hadronic coincidences. Hadronic photoabsorption event was indicated by the coincidence of (HIH2H3H4) or (H1H2HsH6) with tagging signal at the absence of SO pulse. It is noteworthy that the photoabsorption events with only neutral hadrons produced were not detected by this experimental setup. However, these events were shown [10] at 2 - 4 GeV to make up less than 2% of the total hadronic photoproduction cross section and to decrease with energy due to the increase of hadron production multiplicity.
Data on 12C nuclei and corrections. To measure the total hadronic photoabsorption cross section in carbon 5 X 106 photons with energy from 14 to 34 GeV were passed through the 0.14x 0 thick target. The total cross section was calculated from the formula Ctot(7C ) = (1/No)N/M , where N O is the number of target nuclei per unit area; N is the number of hadronically interacting photons; M is the number of incident photons. The corrections applied included: 1) The absorption of photon beam in the target due to the e+e--pair production; 2) The attenuation of incident photon beam due to the inconsistency of its cross section with target area. For the target of 5 cm in diameter and the cross section of ?-beam measured this correction made up 5%; 3) The contribution.of random coincidences proved to be negligible; 4) The empty target background for five energy values as given in table 1; 5) The account of multiple bremsstrahlung processes in the radiator. The Monte Carlo calculations of this correction are given in table 1;
Table 1 II channel ( 1 9 - 2 3 ) C-eV
III channel (23 -27) GeV
IV channel
(14-19)GeV
(27-30) GeV
V channel (30-34) GeV
0.157 + 0.049
0.189 ± 0.06
0.207 ± 0.06
0.276 + 0.084
0.381 + 0.099
Correction for multiple scattefing in radlatoz_(in %)
10.5
12
12.5
13.5
15.5
(Otot(-tC) (in #b)
1082 -+56
1105 ± 50.5
1080 ± 57
980 ± 67.7
930 ± 44
I channel
Empty target background tJV/M) x 10'4)
198
Volume 56B, number 2
PHYSICS LETTERS
~-Ser.pu~hov f500
14 April 1975
Ctot("/C) in the energy region (1-34) GeV and in fig. 3 the energy dependence o f A e f f / A . The ratio Aeff[A was calculated from the formula Aeff Otot(TA) A Zotot(TP) + (A - Z) Otot(Tn) ' where
fOOO
ot(TP) = [(98.7-+3.6) + (65-+ 10)/x/E]lab (Em GeV), ot(7 n) = [ot(TP) - (18.3 + 6.1)/x/E]lab (E in GeV).
Fig. 2. The dependence of at(TC) on photon energy in ( 2 - 3 4 ) GeV interval.
6) The 2% background from electromagnetic processes in the target; 7) The geometric corrections which were due to the loss of a-meson photoproduction events. The Monte Carlo simulation of this loss gave the figures from 0.5% to 1.5% for different channels of tagging; 8) The approximately 8% inefficiency of hadron detectors. Our values of Otot(TC) at different energies from 14 to 34 GeV are given in table 1. In fig. 2 we show the measured total cross section
The ratio A eff[A is seen in the figure to decrease with the increase of energy in agreement with the VDM predictions. It is worth-while to note, that a certain amount of multiparticle events with n°-mesons might have been neglected, if 7-quanta from 7r°-mesons decay had gated anticoincidence signal in the shower counter. We have not considered this effect and, so, the decrease of Ct(TC) may well be partly due to it. In conclusion the authors wish to thank A.Ts. Amatuni, A.A. Logunov, S.G. Matinyan, S.S. Gershtein, S.P. Denisov and V.M. Kut'in for the interest and support, O.M. Vinnitski, K.A. Ispiryan and Yu.M. Sapunov for their assistance at different stages of this experiment.
O,y"
0,7-
A -..DEs ..~ " x -~ezp~hAov -V,S M --- w*~ +eoX
0
S,~o~ f -
--
,.-f
Fig. 3. The dependence of Aeff/A ratio on photon energy in ( 2 - 3 4 ) GeV interval.
199
Volume 56B, number 2
PHYSICS LETTERS
References [1] K. Gottfried and D.R. Yennie, Phys. Rev. 182 (1969) 1595. [2] S. Brodsky et al., Phys. Rev. 182 (1969) 1794. [3] G.R. Brooks et al., Phys. Rev. D8 (1973) 2826. [4] V. Heynen et al., Phys. Lett. 34B (1971) 651.
200
[5] [61 [7] [8] [9] [10]
14 April 1975
D.O. Caldwell et al., Phys. Rev. D7 (1973) 1362. D. Sehildknecht, SLAC-PUB-1230 (197,3). G.L. Bayatyan et al., Scientific report EFI-64 (74). A.I. Alikhanyan et al., preprint EFI-EP-1 (70), Yerevan. G.L. Bayatyan, Scientific report E F I 4 7 (73). E. Gabathuler, DL/P 190 (1974).