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Backward π−p elastic scattering at 3 and 4 GeVc
Volume 42B, number 2
PHYSICS LETTERS
27 November 1972
B A C K W A R D r r - p E L A S T I C S C A T T E R I N G A T 3 A N D 4 GeV/c A. BRABSON .1, G. CALVELLI, S. CITTOLIN, P. De GUIO, F. GASPARINI S. LIMENTANI, P. MITTNER .2, M. POSOCCO, L. VENTURA and C. VOCI Istituto Nazionale di Fisica Nuclear, Sezione di Padova, Istituto di Fisica dell'UniversitY, Padova, Italy
and M. CROZON, A. DIACZEK, R. SIDWELL *a a n d J. TOCQUEVILLE Laboratoire de Physique Atomique et Nucl~aire, College de France, Paris, France Received 29 October 1972 We have measured do/du for n-p elastic scattering at 3 and 4 GeV/c in the ranges -0.119 < u ~; 0.113 and -0.233 < u < 0.088, respectively. A fit of the form do/du = A exp(Bu + Cu2) givesB = 4.34 ± 0.42 and C= 7.0 ± 3.5 at 4 GeV/c with x 2 = 5.7 for 9 degrees of freedom; the simpler form do/du =A exp(Bu) givesB = 3.7 ± 0.3 with x2 = 9.6. At 3 GeV/c we confirm with high statistics the structures already observed. We present here the results of a measurement of 7r-p backward elastic scattering at 3 and 4 GeV/c. The experiment was performed at the CERN PS between January and August 1971. The beam was enriched in antiprotons with an electrostatic separator, since at the same time with the same apparatus we have completed an experiment on p p ~ 7r~r and ~p ~ Fd(. Typical beam fluxes were (6 000 ~ + 24 000 rr-)[ burst at 3 GeV/c with a m o m e n t u m bite of 1.7% and (4 000 ~ + 46 000 7r-)/burst with a m o m e n t u m bite
of 1% at 4 GeV/c. The incident particle was identified by two threshold Cerenkov gas counters; K - contamination in the beam was less than 0.6% and has been taken into account in the results as well as the lepton contamination measured from the counters pressure CUrVeS.
The apparatus is shown in fig. 1. The trigger logic required a forward-backward coincidence F1F2(B1B2 + B3B4). The trigger rate of unwanted events was reduced by requiring the anticoincidence • a Partially supported by lstituto Nazionale di Fisica Nucleare,
• 1 Now at Indiana University. • 2 CERN staff member until the end Of 1971.
Sezione di Padova and by Indiana University through AEC contract AT (11-1)-2009.
tt
I
A
112
Fig. 1. Apparatus lay out .S are b e a m coincidences counters. F and B are coincidence and A are anticoincidence counters. Q is a veto counter. Not shown are two more S counters and the two b e a m ~ e r e n k o v gas counters, that were farther to the left.
283
Volume 42B, n u m b e r 2
PHYSICS LETTERS
Table 1 Differential cross sections for ~r-p backward scattering 3 GeV/c
u
50 ¸
4 GeV/c
do/du [ub/(GeV/c) 2]
u
(GeV/c) 2
(GeV/c) 2
da/du [t.~b/(GeV/c)2]
0.104 0.085 0.065 0.046 0.026 0.007 -0.013 -0.032 -0.051 -0.071 -0.090 -0.109
43.3 42.0 30.2 22.2 19.4 12.1 14.3 11.8 7.2 10.9 10.1 13.0
0.075 0.048 0.022 -0.006 -0.032 -0.059 -0.086 -0.112 -0.139 -0.166 -0.193 -0.220
26.5 21.9 19.6 17.7 15.8 13.3 12.9 13.9 10.9 9.7 11.1 8.0
± ± ± ± ± ± ± ± ± ± ± ±
3.0 2.6 2.1 1.7 1.6 1.4 1.6 1.6 1.4 2.0 2.2 3.3
± ± ± + ± + ± ± ± ± ± ±
1.5 1.2 1.0 1.0 1.0 1.0 1.1 1.3 1.3 1.4 1.7 1.7
4C
~.~ 3C 4~
'i -1.
The errors q u o t e d are purely statistical. The possible systematic error is estimated to be at m o s t 12%.
of a set of lead-plastic scintillation counters (A) surrounding the target. One more anticounter (Q, 10X 12 cm 2) was put in the beam trajectory at the far end of the apparatus, 5.8 m from the target, in order to reduce accidentals. The typical electronics trigger rate, with a target length of 40 cm, was of about two events/burst, one with lr- and one with ~ signature. The rr- and ~ trigger rates were kept at about the same value by a gate system that enabled the ,r- to trigger for approximately 100 ms out of the 400 ms ~pill-out time. Incident beam direction, angles of outgoing par. titles and momentum of the forward going one were measured by five telescopes of wire spark chambers with magnetostrictive readout. Each telescope consisted of four gaps, that is 4x and 4y planes with wires at 90°; only the beam telescope had three gaps. The magnetic deflection was accomplished by a CERN C-magnet with 1.0 X 1.0 X 0.5 m 3 gap and a bending power of 1.4 Tin. A 2116 B, 8K Hewlett-Packard computer was connected on line to the apparatus and recorded all relevant information such as spark coordinates, fluxes, beam particle signature and the coincidence trigger pattern. Data were stored onto magnetic tape while continuous monitoring was available on teletype, CRT and lights display. 2841
27 November 1972
-.8
-.6.
-.4
"~2
0
.2
.4
.6
.8
Zip (GeV/c)
lC~
e,
80
O
41
o,ln -.6oo
n n nnnnn -.3eo
-.12o
qn ~
AO
.12o (tad)
.~o
Fig. 2. Distributions of a p and 4 0 , where Ap is the difference b e t w e e n t h e forward m o m e n t u m computed from the forward angle a n d the measured m o m e n t u m ; A® is t h e analogous difference for the backward angle. Only a coplanarity cut was made on the reconstructed events before these distributions; i.e. neither of them require a proper correlation for the other variable.
Volume 42B, number 2
PHYSICS LETTERS 50-* --
¢
+
+
10
+ '"
v
J
I
~5
I
I
1
9~
2 3
q,
~5
27 November 1972
t
"1
r
I
r
I
rof. S I
0 3.3
re[ 4
I i at~ t~ d.2 ~.~ ~o
'
~1 Ii/ e
A 2.~S Ge~c
I0 ~ ~:"k,z
i
(;eric
I
~ ,*t.4 ~perDw.e~ ~t. 8 c,e~ ~t. o c,o~ rot.7
10-
'-o~
'-o.le '
t
ttt t
40~¢¢ '-o.b4'-o~
. ~'e..,~,l,d ' - z
Fig. 3. do/du versus u for 7r-p ~ ~r-p.
t
1-
Reconstruction and analysis of the events were performed off-line on a CDC 6600 computer. At least three sparks per telescope were required for each track as well as a reasonable reconstruction of the interaction vertex and of the intersection of the forward going track direction before and after the magnet. The distributions of the parameters that measured the goodness of these reconstructions do not show any kind of tail. Kinematical identification of the events was done by requiring coplanarity and appropriate correlations between forward angle and backward angle and between forward angle and measured momentum. Typical examples of these correlations are shown in fig. 2. Background subtraction is always small (less than 4%) and unambiguous. Particular care has been taken to check the results against angular biases that might arise from different efficiencies of the two backward telescopes (they covered partially different angles) and from non-uniform efficiency of the forward chambers. No signifi~;ant effect o f this type has been observed. Data have been corrected for geometrical acceptance, beam attenuation in the target, efficiency of
05-
---7-"
)
0.1
0
'
I
- 0.1
-02
-0.3
Fig. 4. do/du versusu for ~r-p--. 7r-p. the apparatus and interactions and decays of the secondary particles. The efficiency of the apparatus had been estimated in two completely independent ways: a) by comparing the number of events expected to pass the geometrical reconstruction with those for which reconstruction was in fact completed; b) by computing the efficiency of each telescope starting from the spark topologies as seen in the two projections of the elescope. The relative difference of the two results that amounts to approximately 12% may give an estimate of the possible systematic error of the quoted cross-sections which are computed with the first method. The angular distributions are shown in fig. 3 and 285
Volume 42B, number 2
PHYSICS LETTERS
their values are reported in table 1; only events are included for which the geometrical acceptance was at least 0.1. In fig. 3 the previous measurements at the same energies are also given [1, 2, 3]. In fig. 4 we compare our results with other published data [2, 4 - 7 ] . At 3 GeV/c do/du is characterized by a sharp backward peak and by a break in the slope at u ~ - 0 . 0 5 (GeV/c) already observed with poorer statistics by Critenden et al. [2]. The slope at u = Ureax is found to be 12.8 +- l(GeV/c) - 2 . At 4 GeV/c no measurement of do/du existed. A fit to our data of the form do/du =A exp(Bu) leads t o B = 3.7 -+ 0.3 with 7(2 = 9.6 for 10 degrees of freedom. As there is an indication that the experimental distribution is not simply exponential we have also tried do/du = A exp(Bu + Cu2). This gives a value o f B = 4.34 + 0.42 close to that found at higher energies and C = 7.0 +--3.5, with ×2 = 5.7.
286
27 November 1972
We would like to acknowledge the financial and technical support of CERN. In particular we appreciate the contributions of V. Chabaud, L. Mazzone, G. Muratori and B. Taylor. The C.d.F. would like to acknowledge the support in part of the Commisariat ~ l'Energie Atomique.
References [1] S.W. Kormanyos et al., Phys. Rev. Lett. 16 (1966) 709. [2] R.R. Crittenden et al., Phys. Rev. D1 (1970) 3050. [3] C.C. Coffin et al., Phys. Rev. 159 (1967) 1169. [4] W.F. Baker et al., Nucl. Phys. B9 (1968) 249. [5} J. Banaigs et al., Nucl. Phys. B8 (1968) 31. [6] W.F. Baker et al,, Phys. Lett. 28B (1968) 291. [7] J. Orear et al., Phys. Rev. 152 (1966) 1162.
PHYSICS LETTERS
27 November 1972
B A C K W A R D r r - p E L A S T I C S C A T T E R I N G A T 3 A N D 4 GeV/c A. BRABSON .1, G. CALVELLI, S. CITTOLIN, P. De GUIO, F. GASPARINI S. LIMENTANI, P. MITTNER .2, M. POSOCCO, L. VENTURA and C. VOCI Istituto Nazionale di Fisica Nuclear, Sezione di Padova, Istituto di Fisica dell'UniversitY, Padova, Italy
and M. CROZON, A. DIACZEK, R. SIDWELL *a a n d J. TOCQUEVILLE Laboratoire de Physique Atomique et Nucl~aire, College de France, Paris, France Received 29 October 1972 We have measured do/du for n-p elastic scattering at 3 and 4 GeV/c in the ranges -0.119 < u ~; 0.113 and -0.233 < u < 0.088, respectively. A fit of the form do/du = A exp(Bu + Cu2) givesB = 4.34 ± 0.42 and C= 7.0 ± 3.5 at 4 GeV/c with x 2 = 5.7 for 9 degrees of freedom; the simpler form do/du =A exp(Bu) givesB = 3.7 ± 0.3 with x2 = 9.6. At 3 GeV/c we confirm with high statistics the structures already observed. We present here the results of a measurement of 7r-p backward elastic scattering at 3 and 4 GeV/c. The experiment was performed at the CERN PS between January and August 1971. The beam was enriched in antiprotons with an electrostatic separator, since at the same time with the same apparatus we have completed an experiment on p p ~ 7r~r and ~p ~ Fd(. Typical beam fluxes were (6 000 ~ + 24 000 rr-)[ burst at 3 GeV/c with a m o m e n t u m bite of 1.7% and (4 000 ~ + 46 000 7r-)/burst with a m o m e n t u m bite
of 1% at 4 GeV/c. The incident particle was identified by two threshold Cerenkov gas counters; K - contamination in the beam was less than 0.6% and has been taken into account in the results as well as the lepton contamination measured from the counters pressure CUrVeS.
The apparatus is shown in fig. 1. The trigger logic required a forward-backward coincidence F1F2(B1B2 + B3B4). The trigger rate of unwanted events was reduced by requiring the anticoincidence • a Partially supported by lstituto Nazionale di Fisica Nucleare,
• 1 Now at Indiana University. • 2 CERN staff member until the end Of 1971.
Sezione di Padova and by Indiana University through AEC contract AT (11-1)-2009.
tt
I
A
112
Fig. 1. Apparatus lay out .S are b e a m coincidences counters. F and B are coincidence and A are anticoincidence counters. Q is a veto counter. Not shown are two more S counters and the two b e a m ~ e r e n k o v gas counters, that were farther to the left.
283
Volume 42B, n u m b e r 2
PHYSICS LETTERS
Table 1 Differential cross sections for ~r-p backward scattering 3 GeV/c
u
50 ¸
4 GeV/c
do/du [ub/(GeV/c) 2]
u
(GeV/c) 2
(GeV/c) 2
da/du [t.~b/(GeV/c)2]
0.104 0.085 0.065 0.046 0.026 0.007 -0.013 -0.032 -0.051 -0.071 -0.090 -0.109
43.3 42.0 30.2 22.2 19.4 12.1 14.3 11.8 7.2 10.9 10.1 13.0
0.075 0.048 0.022 -0.006 -0.032 -0.059 -0.086 -0.112 -0.139 -0.166 -0.193 -0.220
26.5 21.9 19.6 17.7 15.8 13.3 12.9 13.9 10.9 9.7 11.1 8.0
± ± ± ± ± ± ± ± ± ± ± ±
3.0 2.6 2.1 1.7 1.6 1.4 1.6 1.6 1.4 2.0 2.2 3.3
± ± ± + ± + ± ± ± ± ± ±
1.5 1.2 1.0 1.0 1.0 1.0 1.1 1.3 1.3 1.4 1.7 1.7
4C
~.~ 3C 4~
'i -1.
The errors q u o t e d are purely statistical. The possible systematic error is estimated to be at m o s t 12%.
of a set of lead-plastic scintillation counters (A) surrounding the target. One more anticounter (Q, 10X 12 cm 2) was put in the beam trajectory at the far end of the apparatus, 5.8 m from the target, in order to reduce accidentals. The typical electronics trigger rate, with a target length of 40 cm, was of about two events/burst, one with lr- and one with ~ signature. The rr- and ~ trigger rates were kept at about the same value by a gate system that enabled the ,r- to trigger for approximately 100 ms out of the 400 ms ~pill-out time. Incident beam direction, angles of outgoing par. titles and momentum of the forward going one were measured by five telescopes of wire spark chambers with magnetostrictive readout. Each telescope consisted of four gaps, that is 4x and 4y planes with wires at 90°; only the beam telescope had three gaps. The magnetic deflection was accomplished by a CERN C-magnet with 1.0 X 1.0 X 0.5 m 3 gap and a bending power of 1.4 Tin. A 2116 B, 8K Hewlett-Packard computer was connected on line to the apparatus and recorded all relevant information such as spark coordinates, fluxes, beam particle signature and the coincidence trigger pattern. Data were stored onto magnetic tape while continuous monitoring was available on teletype, CRT and lights display. 2841
27 November 1972
-.8
-.6.
-.4
"~2
0
.2
.4
.6
.8
Zip (GeV/c)
lC~
e,
80
O
41
o,ln -.6oo
n n nnnnn -.3eo
-.12o
qn ~
AO
.12o (tad)
.~o
Fig. 2. Distributions of a p and 4 0 , where Ap is the difference b e t w e e n t h e forward m o m e n t u m computed from the forward angle a n d the measured m o m e n t u m ; A® is t h e analogous difference for the backward angle. Only a coplanarity cut was made on the reconstructed events before these distributions; i.e. neither of them require a proper correlation for the other variable.
Volume 42B, number 2
PHYSICS LETTERS 50-* --
¢
+
+
10
+ '"
v
J
I
~5
I
I
1
9~
2 3
q,
~5
27 November 1972
t
"1
r
I
r
I
rof. S I
0 3.3
re[ 4
I i at~ t~ d.2 ~.~ ~o
'
~1 Ii/ e
A 2.~S Ge~c
I0 ~ ~:"k,z
i
(;eric
I
~ ,*t.4 ~perDw.e~ ~t. 8 c,e~ ~t. o c,o~ rot.7
10-
'-o~
'-o.le '
t
ttt t
40~¢¢ '-o.b4'-o~
. ~'e..,~,l,d ' - z
Fig. 3. do/du versus u for 7r-p ~ ~r-p.
t
1-
Reconstruction and analysis of the events were performed off-line on a CDC 6600 computer. At least three sparks per telescope were required for each track as well as a reasonable reconstruction of the interaction vertex and of the intersection of the forward going track direction before and after the magnet. The distributions of the parameters that measured the goodness of these reconstructions do not show any kind of tail. Kinematical identification of the events was done by requiring coplanarity and appropriate correlations between forward angle and backward angle and between forward angle and measured momentum. Typical examples of these correlations are shown in fig. 2. Background subtraction is always small (less than 4%) and unambiguous. Particular care has been taken to check the results against angular biases that might arise from different efficiencies of the two backward telescopes (they covered partially different angles) and from non-uniform efficiency of the forward chambers. No signifi~;ant effect o f this type has been observed. Data have been corrected for geometrical acceptance, beam attenuation in the target, efficiency of
05-
---7-"
)
0.1
0
'
I
- 0.1
-02
-0.3
Fig. 4. do/du versusu for ~r-p--. 7r-p. the apparatus and interactions and decays of the secondary particles. The efficiency of the apparatus had been estimated in two completely independent ways: a) by comparing the number of events expected to pass the geometrical reconstruction with those for which reconstruction was in fact completed; b) by computing the efficiency of each telescope starting from the spark topologies as seen in the two projections of the elescope. The relative difference of the two results that amounts to approximately 12% may give an estimate of the possible systematic error of the quoted cross-sections which are computed with the first method. The angular distributions are shown in fig. 3 and 285
Volume 42B, number 2
PHYSICS LETTERS
their values are reported in table 1; only events are included for which the geometrical acceptance was at least 0.1. In fig. 3 the previous measurements at the same energies are also given [1, 2, 3]. In fig. 4 we compare our results with other published data [2, 4 - 7 ] . At 3 GeV/c do/du is characterized by a sharp backward peak and by a break in the slope at u ~ - 0 . 0 5 (GeV/c) already observed with poorer statistics by Critenden et al. [2]. The slope at u = Ureax is found to be 12.8 +- l(GeV/c) - 2 . At 4 GeV/c no measurement of do/du existed. A fit to our data of the form do/du =A exp(Bu) leads t o B = 3.7 -+ 0.3 with 7(2 = 9.6 for 10 degrees of freedom. As there is an indication that the experimental distribution is not simply exponential we have also tried do/du = A exp(Bu + Cu2). This gives a value o f B = 4.34 + 0.42 close to that found at higher energies and C = 7.0 +--3.5, with ×2 = 5.7.
286
27 November 1972
We would like to acknowledge the financial and technical support of CERN. In particular we appreciate the contributions of V. Chabaud, L. Mazzone, G. Muratori and B. Taylor. The C.d.F. would like to acknowledge the support in part of the Commisariat ~ l'Energie Atomique.
References [1] S.W. Kormanyos et al., Phys. Rev. Lett. 16 (1966) 709. [2] R.R. Crittenden et al., Phys. Rev. D1 (1970) 3050. [3] C.C. Coffin et al., Phys. Rev. 159 (1967) 1169. [4] W.F. Baker et al., Nucl. Phys. B9 (1968) 249. [5} J. Banaigs et al., Nucl. Phys. B8 (1968) 31. [6] W.F. Baker et al,, Phys. Lett. 28B (1968) 291. [7] J. Orear et al., Phys. Rev. 152 (1966) 1162.