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The angular distribution of charged particles produced in pp collision at the CERN intersecting storage rings
Volume 40B, number 4
THE ANGULAR
PHYSICS LETTERS
DISTRIBUTION
24 July 1972
OF CHARGED PARTICLES PRODUCED
IN pp C O L L I S I O N A T T H E C E R N I N T E R S E C T I N G
STORAGE RINGS
J. BABECKI, Z. CHYLINSKI, Z. CZACHOWSKA, O. CZYZEWSKI*, B. FURMAI~SKA, J. GIERULA, R. HOLYI~SKI, A. JURAK, K. KARNICKA, S. KRZYWDZI/~SKI, G. NOWAK, W. WOLTER Institute o f Nuclear Physics, Cracow, Poland
and A, GURTU** and A.J. HERZ CERN, Geneva, Switzerland
Received 5 June 1972 The angular distribution of secondary relativistic charged particles was measured at the ISR by means of nuclear emulsions. Within the c.m. angular interval from 90° to 22.6° the In tan (0cm/2) distribution is flat.
In this letter we present distributions for centre-ofmass angles between 22.6 ° and 90 ° obtained by means of nuclear emulsions. The details of the exposures, carried out in Intersection 1, are: /
Date
719.71 829.71 9.9.7l
Energy in c.m.s. (GeV)
Currents (A2 )
Duration (min)
44.4 44.4 52.6
116 X 125 216 X 2~5 2.6 X 2.5
580 556 327
Details and dimensions of the lay-out are shown in fig. 1 (see also ref. [ 1] ). The plates were scanned along a line parallel to the emulsion edge facing the diamond. Particles were counted if they had a velocity/3 ~> 0.7 in the laboratory frame. After allowing for the motion of the center-of-mass system and for energy loss in the 2-ram steel walls, the lower limit to the m o m e n t u m in the c.m. system of the pions accepted in scanning becomes approximately 120 MeV/c. The angle subtended by the interaction diamond at each emulsion is quite small, about 10 ° or less in the horizontal plane, so that tracks due to beam-beam in-
* The measurements and the manuscript were completed after Oleg Czy~ewski's untimely death. ** On leave from the Tata Institute of Fundamental Research, Bombay.
'-I
" l" I
ill
J
\
\
x
~ \
',",, \x\s
\,~.\
~
I
I
j
~22-6°
j
00° Fig. 1. General lay-out of the experiment in Intersection 1. The black rectangles show some examples of how the emulsion plates were located in positions ranging from 90° to 22.6° in the c.m. system. Plates lay in the plane of the diamond. + and - indicate the signs of angles as used in the scales in fig. 2. teractions produce a narrow peak in the angular distribution observed in the emulsion, superposed on the general background due to particles from beam-gas and beam-wall interactions. With adequate statistics it is easy to find the background under the peak by interpolation between the pure-background regions on either side. In fig. 2 we show three examples of raw angular distributions; the method of background subtraction is indicated. The gradual change of the shape of the background distribution with decreasing c.m. 507
Volume 40B, number 4
PHYSICS LETTERS
24 July 1972
400 d7.7 ° ~umoer oF"track
300
[)er2° inlervO[
200
100 .
~
~,~--~
ZiO
--
01 II
'/00
80o
70 o
60o
oe
25.4*
50 o
08
~o
I
joo ~2
20o ~*
(a)
~e ~8 l . ta. (o~.#)
500 400
} }{\
~0 200
I00 +Z0* .10"
00
-10"
-200
Fig. 2. Examples of raw angular distributions of minimumionization tracks observed in emulsions located in positions corresponding to c.m. angles indicated at the left side of bach diagram. The procedure of evaluation of the background under the peak by interpolation between pure-background regions is illustrated. angle is an obvious consequence of the angular characteristics of the beam-gas and beam-wall interactions, as well as of the geometry of the lay-out in which the plates were exposed. From these data we calculated the differential cross-section at various angles using the luminosity as determined by the v.d. Meer method [2], the counting rates of the beam-beam monitor, the geometry, and the duration of the exposures. Fig. 3 shows (1/Oinel ) do/d in tan 10 cm as function of In tan (0cm/2). The error bars indicate the standard errors due to statistics and to the uncertainties in geometry. We assumed ainel = 32 mb. As the precision with which the luminosity was determined is not known, there may be an additional systematic 50 8
i
80 o
70 ~
~0 °
50 o
~0 o
jO o
20 o
(b) Fig. 3. Normalized differential cross sections (1/Oinel)do/d In tanl0cm.~ for 44.4 GeV and 5_ for 52.6 GeV c.m. energy, a) without correction; b) corrected data (see text). The uncertainty in the vertical scales is about 17%. error of up to 17% which does not affect the shapes of the distributions. The experimental points in fig. 3a represent the angular distribution of fast charged particles associated with beam-beam interactions and detected in the emulsion. To obtain the distribution of charged hadrons emitted in the original pp collisions, we have applied corrections for the contamination with electrons due to conversion of photons from the decay of neutral pions, and for loss of particles due to decay and nuclear interactions. The correction for electrons was computed using the data ofNeuhofer et al. [3] and
Volume 40B, number 4
PHYSICS LETTERS
assuming that 7r±/Tr° = 2. This correction changes with the c.m. angle from 8% at 90 ° to 18% at 22.6 °. It was checked experimentally by measuring the flux o f highenergy photons coming from the interaction region in the emulsion exposed at the centre-of-mass angle of 22.6 °. The correction due to the decay of pions changes with angle from 3% at 90 ° to 1% at 22.6 ° . The smallness o f this correction is a consequence of the fact that a large fraction of the decay muons enter the emulsions and are accepted in the scanning. The loss due to nuclear interactions in the wall of the pipe is less than 4%. Fig. 3b shows the angular distribution after the corrections have been applied. The distribution is compatible with a flat one and agrees with the other data obtained at the ISR [4, 5]. The results reported here supersede those we reported previously [ 1] which gave an indication o f a d e e p m i n i m u m in do/d log tan (0 cm/2) at 90 °. These results had been obtained with statistics one order of magnitude lower than those we have now, and in experimental conditions which gave rise to very high background at small angles. It is still an open question why the angular distribution observed in inclusive proton-proton reactions
24 July 1972
at the ISR is different from that found in an unbiased sample o f interactions o f ~ 1000 GeV cosmic-ray nucleons with emulsion nuclei [6]. This discrepancy might well be due to a difference between nucleonnucleon and nucleon-nucleus interactions at ultra-high energies. We should like to express our gratitude to all those who contributed their work, support or encouragement to this investigation. Particular thanks are due to the team of scanners in Cracow who gathered the data. Three of us, A.G., J.G. and W.W. are indebted to CERN for financial support.
References [ 1] Bombay-CERN-Cracow Collaboration, Phys. kett. 36B (1971)611. [2] van der Meer, CERN Internal Report, ISR-PO/68-31 (1968). [3] G. Neuhofer et al., Phys. Lett. 37B (1971) 438. [4] G. Barbiellini et al., Phys. Lett. 39B (1972) 294. [5] M. Breidenbach et al., CERN preprint 14751; Phys. Lett. 39B (1972) 654. [6] J. Gierula and W. Wolter, Acta Physica Polonica B2 (1971) 95.
5()t)
THE ANGULAR
PHYSICS LETTERS
DISTRIBUTION
24 July 1972
OF CHARGED PARTICLES PRODUCED
IN pp C O L L I S I O N A T T H E C E R N I N T E R S E C T I N G
STORAGE RINGS
J. BABECKI, Z. CHYLINSKI, Z. CZACHOWSKA, O. CZYZEWSKI*, B. FURMAI~SKA, J. GIERULA, R. HOLYI~SKI, A. JURAK, K. KARNICKA, S. KRZYWDZI/~SKI, G. NOWAK, W. WOLTER Institute o f Nuclear Physics, Cracow, Poland
and A, GURTU** and A.J. HERZ CERN, Geneva, Switzerland
Received 5 June 1972 The angular distribution of secondary relativistic charged particles was measured at the ISR by means of nuclear emulsions. Within the c.m. angular interval from 90° to 22.6° the In tan (0cm/2) distribution is flat.
In this letter we present distributions for centre-ofmass angles between 22.6 ° and 90 ° obtained by means of nuclear emulsions. The details of the exposures, carried out in Intersection 1, are: /
Date
719.71 829.71 9.9.7l
Energy in c.m.s. (GeV)
Currents (A2 )
Duration (min)
44.4 44.4 52.6
116 X 125 216 X 2~5 2.6 X 2.5
580 556 327
Details and dimensions of the lay-out are shown in fig. 1 (see also ref. [ 1] ). The plates were scanned along a line parallel to the emulsion edge facing the diamond. Particles were counted if they had a velocity/3 ~> 0.7 in the laboratory frame. After allowing for the motion of the center-of-mass system and for energy loss in the 2-ram steel walls, the lower limit to the m o m e n t u m in the c.m. system of the pions accepted in scanning becomes approximately 120 MeV/c. The angle subtended by the interaction diamond at each emulsion is quite small, about 10 ° or less in the horizontal plane, so that tracks due to beam-beam in-
* The measurements and the manuscript were completed after Oleg Czy~ewski's untimely death. ** On leave from the Tata Institute of Fundamental Research, Bombay.
'-I
" l" I
ill
J
\
\
x
~ \
',",, \x\s
\,~.\
~
I
I
j
~22-6°
j
00° Fig. 1. General lay-out of the experiment in Intersection 1. The black rectangles show some examples of how the emulsion plates were located in positions ranging from 90° to 22.6° in the c.m. system. Plates lay in the plane of the diamond. + and - indicate the signs of angles as used in the scales in fig. 2. teractions produce a narrow peak in the angular distribution observed in the emulsion, superposed on the general background due to particles from beam-gas and beam-wall interactions. With adequate statistics it is easy to find the background under the peak by interpolation between the pure-background regions on either side. In fig. 2 we show three examples of raw angular distributions; the method of background subtraction is indicated. The gradual change of the shape of the background distribution with decreasing c.m. 507
Volume 40B, number 4
PHYSICS LETTERS
24 July 1972
400 d7.7 ° ~umoer oF"track
300
[)er2° inlervO[
200
100 .
~
~,~--~
ZiO
--
01 II
'/00
80o
70 o
60o
oe
25.4*
50 o
08
~o
I
joo ~2
20o ~*
(a)
~e ~8 l . ta. (o~.#)
500 400
} }{\
~0 200
I00 +Z0* .10"
00
-10"
-200
Fig. 2. Examples of raw angular distributions of minimumionization tracks observed in emulsions located in positions corresponding to c.m. angles indicated at the left side of bach diagram. The procedure of evaluation of the background under the peak by interpolation between pure-background regions is illustrated. angle is an obvious consequence of the angular characteristics of the beam-gas and beam-wall interactions, as well as of the geometry of the lay-out in which the plates were exposed. From these data we calculated the differential cross-section at various angles using the luminosity as determined by the v.d. Meer method [2], the counting rates of the beam-beam monitor, the geometry, and the duration of the exposures. Fig. 3 shows (1/Oinel ) do/d in tan 10 cm as function of In tan (0cm/2). The error bars indicate the standard errors due to statistics and to the uncertainties in geometry. We assumed ainel = 32 mb. As the precision with which the luminosity was determined is not known, there may be an additional systematic 50 8
i
80 o
70 ~
~0 °
50 o
~0 o
jO o
20 o
(b) Fig. 3. Normalized differential cross sections (1/Oinel)do/d In tanl0cm.~ for 44.4 GeV and 5_ for 52.6 GeV c.m. energy, a) without correction; b) corrected data (see text). The uncertainty in the vertical scales is about 17%. error of up to 17% which does not affect the shapes of the distributions. The experimental points in fig. 3a represent the angular distribution of fast charged particles associated with beam-beam interactions and detected in the emulsion. To obtain the distribution of charged hadrons emitted in the original pp collisions, we have applied corrections for the contamination with electrons due to conversion of photons from the decay of neutral pions, and for loss of particles due to decay and nuclear interactions. The correction for electrons was computed using the data ofNeuhofer et al. [3] and
Volume 40B, number 4
PHYSICS LETTERS
assuming that 7r±/Tr° = 2. This correction changes with the c.m. angle from 8% at 90 ° to 18% at 22.6 °. It was checked experimentally by measuring the flux o f highenergy photons coming from the interaction region in the emulsion exposed at the centre-of-mass angle of 22.6 °. The correction due to the decay of pions changes with angle from 3% at 90 ° to 1% at 22.6 ° . The smallness o f this correction is a consequence of the fact that a large fraction of the decay muons enter the emulsions and are accepted in the scanning. The loss due to nuclear interactions in the wall of the pipe is less than 4%. Fig. 3b shows the angular distribution after the corrections have been applied. The distribution is compatible with a flat one and agrees with the other data obtained at the ISR [4, 5]. The results reported here supersede those we reported previously [ 1] which gave an indication o f a d e e p m i n i m u m in do/d log tan (0 cm/2) at 90 °. These results had been obtained with statistics one order of magnitude lower than those we have now, and in experimental conditions which gave rise to very high background at small angles. It is still an open question why the angular distribution observed in inclusive proton-proton reactions
24 July 1972
at the ISR is different from that found in an unbiased sample o f interactions o f ~ 1000 GeV cosmic-ray nucleons with emulsion nuclei [6]. This discrepancy might well be due to a difference between nucleonnucleon and nucleon-nucleus interactions at ultra-high energies. We should like to express our gratitude to all those who contributed their work, support or encouragement to this investigation. Particular thanks are due to the team of scanners in Cracow who gathered the data. Three of us, A.G., J.G. and W.W. are indebted to CERN for financial support.
References [ 1] Bombay-CERN-Cracow Collaboration, Phys. kett. 36B (1971)611. [2] van der Meer, CERN Internal Report, ISR-PO/68-31 (1968). [3] G. Neuhofer et al., Phys. Lett. 37B (1971) 438. [4] G. Barbiellini et al., Phys. Lett. 39B (1972) 294. [5] M. Breidenbach et al., CERN preprint 14751; Phys. Lett. 39B (1972) 654. [6] J. Gierula and W. Wolter, Acta Physica Polonica B2 (1971) 95.
5()t)