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Spin effects in low energy proton-antiproton forward elastic scattering
Volume 124B, number 6
PHYSICS LETTERS
12 May 1983
SPIN EFFECTS IN LOW ENERGY PROTON-ANTIPROTON FORWARD ELASTIC SCATTERING M. LACOMBE, B. LOISEAU, B. MOUSSALLAM and R. VINH MAU Division de Physique Th~orique 1, Institut de Physique Nucl~aire, 91406 Orsay, France and LPTPE, Universit~ Pferre et Marie Curie, 75230 Paris Cedex 05, France Received 6 July 1982 Revised manuscript received 11 February 1983
Predictions of the ~p forward elastic cross section and of the real to imaginary ratio ,o given by the NI~ interaction we proposed recently are reported. Our cross sections compare very well with recent measurements at 400 < PL < 730 MeV/c and 700 MeV/c. It is also shown that spin effects cannot be neglected in the determination ofp as usually assumed.
In a recent letter [1], new measurements of the ~p forward elastic cross sections at low energies (400 < PL < 730 MeV/c) were reported. These data complete earlier bubble chamber results [2] obtained at 700 MeV/c. In both letters, the data were used in conjunction with p~ total cross sections measurements to deduce the real to imaginary ratio Re [ ~ l ( t = 0) + qb3(t = 0)]
tion that only the spin independent amplitude is dominant in the forward direction. If spin dependence is not neglected, and if one assumes the same slope for all amplitudes, eq. (1) should be replaced by
do/dt = 47r(cthc/~t)2F(t) 2 + (l/rr)(Otot/4~c)2(l + p2)(1 + r/2) exp ( - b t ) + (ctOtot/~t) F ( t ) e x p ( - ~ l b t ) ( p c o s 6
sin6),
(2)
P = I m [qbl(t = 0) + qb3(t = 0)] ' where o f the forward spin independent nuclear amplitude via the following approximation:
r/2 = 2lqb2(t = 0)12+ {qb3(t = 0) - ~ 1 ( t = 0)12 Iq~l(t = 0) + ~ 3 ( t = 0){ 2
do/d t = 4 n ( a h c / ~ t ) 2 F ( t ) 2
+ (l/rr)(Otot/4hc)2(l + ,o2)exp ( - b t ) + (ctOtot//3t)F(t)exp(--~bt)(Ocos5
- sin6),
(1)
where - t is the four-momentum transfer, a the fine structure constant, 13 the velocity o f the incident ~ in the laboratory, b the slope parameter o f the ~p diffraction peak, and the qbi's are the usual helicity amplitudes [3]. F ( t ) = (1 + t/0.71) ~ is the Coulomb form factor o f the proton and 8 = - [In 9.5 t + 0.5772]a/3 -1 is the phase o f Coulomb amplitude as usedin refs. [1J and [2]. In these two last expressions t is to be expressed in (GeV) 2. Formula (1) is, however, written with the assump1 Laboratoire associ4 au CNRS. 0 0 3 1 - 9 1 6 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 03.00 © 1983 North-Holland
Formula (1) was used in refs. [ll and [2] to deduce p from the data obtained on do/dt and Otot at different energies. Comparing the results thus obtained with those given by a dispersion relation analysis, thcy concluded that it was necessary to include an effective pole term in the unphysical region near the ~p threshold (at m = 1884 -+ 9 MeV with a residue = 0.97 -+
0.08). The purpose o f this note is twofold. Firstly, we wish to report predictions of the ~p forward elastic cross section and of the ratio p given by the Nlq interaction we proposed recently [4]. Secondly, we want to show that the widely used formula ( I ) with the neglect of spin dependence can lead to incorrect results for p, and consequently, the comparison with theoretical models as done in refs. [ 1] and {2] can be 443
Volume 124B, number 6
PHYSICS LETTERS
12 May 1983
d5
(r'n6/sr-)
Tlab:192.4 MeV
TIQb= 96.3 MeV 10C
~b =436 M~V/~
75
~,:,b=631 M~V.A
100
i
75
50
50
\j ......
~
25
T A-~"'~--- ..............
t.~
..............
25
,ecre(de,,9)
o
lb
20
30
Fig. 1. Differential cross section fo p~ forward elastic scatterbag at TLab = 96.3 MeV. The data points (~) are from ref. [ 1]. The solid curve is the prediction of our Nlq model and the dashed curve the contribution of our nuclear amplitude.
misleading. This remark applies n o t only to the analysis o f earlier works but should be borne in mind also in new projects currently under investigation, for example at CERN [5]. In ref. [4], the existing data for observables on ~p scattering were used to constrain the phenomenological parameters of the NN optical model. However, in this fit, the forward elastic cross sections of refs. [ 1 l and [2] were not included. The results presented here are therefore predictions of the model. The agreement of our results with the data of refs. [1] and [2] is very good, the x2/data being of 1.06 and 1.18 respectively. Some samples of these results are shown in figs. 1 - 4 . Along with the excellent fit to the elastic cross sections at m e d i u m and backward angles, as well as to the total cross sections [4], this result confirms the ability of our model to describe accurately both ~p elastic scattering and annihilation. The values of p, r~ and ato t calculated with our model [4J, are shown in table 1. It should be noted that for Tlab > 100 MeV our values for ,o are s m a l l much smaller than those extracted from the data of refs. [ 1] and [2], using formula (1). We also found 444
o
lb
3b
Fig. 2. As in fig. 1 but for TLa b = 192.4 MeV. that the values for ~ are significant in comparison with those of,o and therefore c a n n o t be neglected. These results, given the good agreement of our model with experiment (in particular for the forward differ-
do- (rnb/sr)
100
75
i
Tleb=138 4 Me',/
i',.
~b =528 MeV/c
i
i* i'
50
25 ..........
.....................................
.¢:m@,~9) 0
10
20
30
Fig. 3. As in fig. 1 but for TLa b = 138.4 MeV.
Volume 124B, number 6
PHYSICS LETTERS
Table 1 The values ofo, r/and Oto t from our model and of the slope parameterb to be used in formula (2).
E~ TIm5 = 233 0 MeV
~ b = 697 MeV/c
100
75
r
50
",,
'
L
2b
30
12 May 1983
"
Fig. 4. As in fig. 1 but lor TLab = 233 MeV. Also shown are the data points (~)of ref. [2] at TLab = 230.5 MeV. ential croSS sections), suggest that the values o f p found in earlier analyses where r/has been neglected might be erroneous. Actually, formula (2) by itself is a relationship between d a / d t , Otot, p and r / i n the approximation that in the forward region the t dependence is essentially given by the slope parameter b. Using do/dt, p, rl and Otot given by our model we have checked that this parametrization of do/dt is indeed quite accurate in the range 0 ° < 0CM < 40 °. The resulting values for b are shown in table 1. These values o f b are slightly higher than those of ref. [2] but agree with more recent values [61. In view of this, we would like to urge experimentalists to use formula (2) rather than formula (1) in the analysis of their forward scattering data. In this case however accurate d e t e r m i n a t i o n o f both p and r / f r o m the data only is difficult as noticed by Jenni et al. [7] in a different energy d o m a i n (1.2 GeV/c
Tlab (MeV)
o
77
b (GeV)-2
ato t (mb)
50.0 96.3 138.4 192.4 233.0 250.0 300.0 350.0
-0.181 .0.0972 -0.0464 . 0.0073 +0.0098 + 0.0143 + 0.0201 +0.0193
0.325 0.309 0.299 0.284 0.272 0.268 0.257 0.247
34.4 25.7 22.4 19.6 18.4 17.9 17.0 16.2
231.4 187.0 164.9 146.7 137.4 134.3 126.6 120.4
GeV/c). Some additional constraints may have then to be imposed in a search for the best fit. Otherwise, the solution might not be unique. In conclusion, we suggest that, in future analyses for determining the ratio,o o f the real to imaginary anlplitudes, (i) care should be taken on spin effects, (ii) one should use experimental data of the forward differential and the total cross sections in combination with theoretical values o f t / a n d b such as those given in table 1. We would like to thank Dr. T. Kamae and Dr. K. Nakamura for sending us the numerical values of their data.
References [ 1 ] H. lwasaki et al., Phys. Lett. 103 B ( 1981) 247. [2] H. Kaseno et al., Phys. Lett. 61B (1976) 203; 68B (1977) 487 (E). [3] N. Hoshizaki, Suppl. Prog. Theor. Phys. 42 (1968) 107. [4] J. Cft~, M. Lacombe, B. Loiseau, B. Moussallam and R. Vinh Mau, Phys. Rev. Lett. 48 (1982) 1319. [5] l h . Walcher, private communication. [6] S. Sakamoto, T. ttashimoto, F. Sai and S.S. Yamamoto, Nucl. Phys. B195 (1981) 1. [71 P. Jenni et al., Nucl. Phys. B94 (1975) 1.
445
PHYSICS LETTERS
12 May 1983
SPIN EFFECTS IN LOW ENERGY PROTON-ANTIPROTON FORWARD ELASTIC SCATTERING M. LACOMBE, B. LOISEAU, B. MOUSSALLAM and R. VINH MAU Division de Physique Th~orique 1, Institut de Physique Nucl~aire, 91406 Orsay, France and LPTPE, Universit~ Pferre et Marie Curie, 75230 Paris Cedex 05, France Received 6 July 1982 Revised manuscript received 11 February 1983
Predictions of the ~p forward elastic cross section and of the real to imaginary ratio ,o given by the NI~ interaction we proposed recently are reported. Our cross sections compare very well with recent measurements at 400 < PL < 730 MeV/c and 700 MeV/c. It is also shown that spin effects cannot be neglected in the determination ofp as usually assumed.
In a recent letter [1], new measurements of the ~p forward elastic cross sections at low energies (400 < PL < 730 MeV/c) were reported. These data complete earlier bubble chamber results [2] obtained at 700 MeV/c. In both letters, the data were used in conjunction with p~ total cross sections measurements to deduce the real to imaginary ratio Re [ ~ l ( t = 0) + qb3(t = 0)]
tion that only the spin independent amplitude is dominant in the forward direction. If spin dependence is not neglected, and if one assumes the same slope for all amplitudes, eq. (1) should be replaced by
do/dt = 47r(cthc/~t)2F(t) 2 + (l/rr)(Otot/4~c)2(l + p2)(1 + r/2) exp ( - b t ) + (ctOtot/~t) F ( t ) e x p ( - ~ l b t ) ( p c o s 6
sin6),
(2)
P = I m [qbl(t = 0) + qb3(t = 0)] ' where o f the forward spin independent nuclear amplitude via the following approximation:
r/2 = 2lqb2(t = 0)12+ {qb3(t = 0) - ~ 1 ( t = 0)12 Iq~l(t = 0) + ~ 3 ( t = 0){ 2
do/d t = 4 n ( a h c / ~ t ) 2 F ( t ) 2
+ (l/rr)(Otot/4hc)2(l + ,o2)exp ( - b t ) + (ctOtot//3t)F(t)exp(--~bt)(Ocos5
- sin6),
(1)
where - t is the four-momentum transfer, a the fine structure constant, 13 the velocity o f the incident ~ in the laboratory, b the slope parameter o f the ~p diffraction peak, and the qbi's are the usual helicity amplitudes [3]. F ( t ) = (1 + t/0.71) ~ is the Coulomb form factor o f the proton and 8 = - [In 9.5 t + 0.5772]a/3 -1 is the phase o f Coulomb amplitude as usedin refs. [1J and [2]. In these two last expressions t is to be expressed in (GeV) 2. Formula (1) is, however, written with the assump1 Laboratoire associ4 au CNRS. 0 0 3 1 - 9 1 6 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 03.00 © 1983 North-Holland
Formula (1) was used in refs. [ll and [2] to deduce p from the data obtained on do/dt and Otot at different energies. Comparing the results thus obtained with those given by a dispersion relation analysis, thcy concluded that it was necessary to include an effective pole term in the unphysical region near the ~p threshold (at m = 1884 -+ 9 MeV with a residue = 0.97 -+
0.08). The purpose o f this note is twofold. Firstly, we wish to report predictions of the ~p forward elastic cross section and of the ratio p given by the Nlq interaction we proposed recently [4]. Secondly, we want to show that the widely used formula ( I ) with the neglect of spin dependence can lead to incorrect results for p, and consequently, the comparison with theoretical models as done in refs. [ 1] and {2] can be 443
Volume 124B, number 6
PHYSICS LETTERS
12 May 1983
d5
(r'n6/sr-)
Tlab:192.4 MeV
TIQb= 96.3 MeV 10C
~b =436 M~V/~
75
~,:,b=631 M~V.A
100
i
75
50
50
\j ......
~
25
T A-~"'~--- ..............
t.~
..............
25
,ecre(de,,9)
o
lb
20
30
Fig. 1. Differential cross section fo p~ forward elastic scatterbag at TLab = 96.3 MeV. The data points (~) are from ref. [ 1]. The solid curve is the prediction of our Nlq model and the dashed curve the contribution of our nuclear amplitude.
misleading. This remark applies n o t only to the analysis o f earlier works but should be borne in mind also in new projects currently under investigation, for example at CERN [5]. In ref. [4], the existing data for observables on ~p scattering were used to constrain the phenomenological parameters of the NN optical model. However, in this fit, the forward elastic cross sections of refs. [ 1 l and [2] were not included. The results presented here are therefore predictions of the model. The agreement of our results with the data of refs. [1] and [2] is very good, the x2/data being of 1.06 and 1.18 respectively. Some samples of these results are shown in figs. 1 - 4 . Along with the excellent fit to the elastic cross sections at m e d i u m and backward angles, as well as to the total cross sections [4], this result confirms the ability of our model to describe accurately both ~p elastic scattering and annihilation. The values of p, r~ and ato t calculated with our model [4J, are shown in table 1. It should be noted that for Tlab > 100 MeV our values for ,o are s m a l l much smaller than those extracted from the data of refs. [ 1] and [2], using formula (1). We also found 444
o
lb
3b
Fig. 2. As in fig. 1 but for TLa b = 192.4 MeV. that the values for ~ are significant in comparison with those of,o and therefore c a n n o t be neglected. These results, given the good agreement of our model with experiment (in particular for the forward differ-
do- (rnb/sr)
100
75
i
Tleb=138 4 Me',/
i',.
~b =528 MeV/c
i
i* i'
50
25 ..........
.....................................
.¢:m@,~9) 0
10
20
30
Fig. 3. As in fig. 1 but for TLa b = 138.4 MeV.
Volume 124B, number 6
PHYSICS LETTERS
Table 1 The values ofo, r/and Oto t from our model and of the slope parameterb to be used in formula (2).
E~ TIm5 = 233 0 MeV
~ b = 697 MeV/c
100
75
r
50
",,
'
L
2b
30
12 May 1983
"
Fig. 4. As in fig. 1 but lor TLab = 233 MeV. Also shown are the data points (~)of ref. [2] at TLab = 230.5 MeV. ential croSS sections), suggest that the values o f p found in earlier analyses where r/has been neglected might be erroneous. Actually, formula (2) by itself is a relationship between d a / d t , Otot, p and r / i n the approximation that in the forward region the t dependence is essentially given by the slope parameter b. Using do/dt, p, rl and Otot given by our model we have checked that this parametrization of do/dt is indeed quite accurate in the range 0 ° < 0CM < 40 °. The resulting values for b are shown in table 1. These values o f b are slightly higher than those of ref. [2] but agree with more recent values [61. In view of this, we would like to urge experimentalists to use formula (2) rather than formula (1) in the analysis of their forward scattering data. In this case however accurate d e t e r m i n a t i o n o f both p and r / f r o m the data only is difficult as noticed by Jenni et al. [7] in a different energy d o m a i n (1.2 GeV/c
Tlab (MeV)
o
77
b (GeV)-2
ato t (mb)
50.0 96.3 138.4 192.4 233.0 250.0 300.0 350.0
-0.181 .0.0972 -0.0464 . 0.0073 +0.0098 + 0.0143 + 0.0201 +0.0193
0.325 0.309 0.299 0.284 0.272 0.268 0.257 0.247
34.4 25.7 22.4 19.6 18.4 17.9 17.0 16.2
231.4 187.0 164.9 146.7 137.4 134.3 126.6 120.4
GeV/c). Some additional constraints may have then to be imposed in a search for the best fit. Otherwise, the solution might not be unique. In conclusion, we suggest that, in future analyses for determining the ratio,o o f the real to imaginary anlplitudes, (i) care should be taken on spin effects, (ii) one should use experimental data of the forward differential and the total cross sections in combination with theoretical values o f t / a n d b such as those given in table 1. We would like to thank Dr. T. Kamae and Dr. K. Nakamura for sending us the numerical values of their data.
References [ 1 ] H. lwasaki et al., Phys. Lett. 103 B ( 1981) 247. [2] H. Kaseno et al., Phys. Lett. 61B (1976) 203; 68B (1977) 487 (E). [3] N. Hoshizaki, Suppl. Prog. Theor. Phys. 42 (1968) 107. [4] J. Cft~, M. Lacombe, B. Loiseau, B. Moussallam and R. Vinh Mau, Phys. Rev. Lett. 48 (1982) 1319. [5] l h . Walcher, private communication. [6] S. Sakamoto, T. ttashimoto, F. Sai and S.S. Yamamoto, Nucl. Phys. B195 (1981) 1. [71 P. Jenni et al., Nucl. Phys. B94 (1975) 1.
445