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Study of double pomeron exchange in pp collisions at √s=31GeV
Volume 65B, number 4
PHYSICS LETTERS
6 December 1976
S T U D Y O F D O U B L E P O M E R O N E X C H A N G E I N p p C O L L I S I O N S A T x / s = 31 G e V M. DELLA NEGRA, D. DRIJARD, H.G. FISCHER, G. FONTAINE, H. FREHSE, P. FRENKIEL, C. CHESQUIERE, R. GOKIELI, P. HANKE, W. HOFMANN, W. ISENBECK, E.E. KLUGE, V. KORBEL, D. LINGLIN, A. MINTEN, A. NORTON, A. PUTZER, R. SOSNOWSKI, S. STEIN and D. WEGENER CERN, Geneva, Switzerland CERN-Coll~ge de Franee-Heidelberg-Karlsruhe Collaboration
Received 14 June 1976 The reaction pp ~ ppn+~- at x/s = 31 GeV was studied at the Split Field Magnet Facility of the CERN-ISR. Selecting events with two leading protons ofx > 0.9 and a rapidity gap ~,y > 2 between the protons and both pions a sample of 720 events is obtained representing a cross section of 25 + 10 #b. The mass distribution of the dipion system, x and PT distributions for the protons are presented and analyzed for the presence of double pomeron exchange.
Diffraction scattering in high energy particle collisions is described either in terms of the optical model or by pomeron exchange within the framework of the particle exchange picture. In the latter case the occurrence o f single diffractive excitation implies the possibility of double pomeron exchange the presence of which remains to be verified [1 ]. In this experiment the process pp ~ p p n + n - was analyzed for the presence of double pomeron exchange mechanism. Although this reaction is dominated by diffraction the double pomeron contribution becomes separable at very high energies. To enrich the sample with candidates o f the double pomeron exchange type, events are selected by requiring x > 0.9, where x is the reduced longitudinal momentum. Furthermore, large rapidity gaps Ay between the pions and the protons ensure that the background from peripherally produced pions is suppressed. The requirement Ay = lYp -Y,r[ > 2 makes it necessary to study double pomeron exchange at the highest energies available where the total rapidity interval is sufficiently large. Several experiments [2] have been performed during the last year to establish the presence of double pomeron exchange. Due to the lack of statistical significance or due to the incomplete measurement of kinematical quantities involved these studies have so far remained inconclusive [3]. 394
The present experiment was performed at the CERN-ISR at x/S~= 31 GeV using the Split Field Magnet Facility [4] (SFM) to detect all charged particles occurring in an event and measure their momenta. The acceptance of the SFM detector covers nearly the full solid angle. Due to the presence of the beam tubes leading protons with transverse momentum o f P T2 < 0.03 (GeV/c) 2 remain undetected. To select events of the reaction pp -+ ppX with two leading protons and centrally produced charged particles the Multi-Wire Proportional Chamber (MWPC) system o f the SFM was used in a self-triggering mode. The presence of leading protons was signalled by a six-fold coincidence of wire planes in both compensator telescopes [4] of the SFM, consisting of 3 MWPC each. To ensure particle production in the central region of rapidity and to suppress elastic scattering, coincident signals were required in addition from at least 3 pairs of wire planes of the SFM central detector [5]. At x / s = 31 GeV a sample of 350000 events was recorded. The trajectories of the charged secondaries were reconstructed thus determining their charge and momentum [6]. In about 25 000 of the reconstructed events, the reduced longitudinal momentum of two positive particles was bigger than 0.7. Of those, events with an additional particle pair of opposite charge were subjected to a four constraint fit procedure [7] to obtain events of the type pp -+ p p , + n - . By requir-
Volume 65B, number 4
PHYSICS LETTERS
6 December 1976
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F i g . 1. S c a t t e r p l o t o f t h e r e d u c e d
longitudinal
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x of
ing a chi-square probability greater than 1% the number o f events remaining was 2000. By applying the cuts mentioned (x > 0.9, Ay > 2) the final sample consisted of 720 events. To establish the occurrence of double pomeron exchange a standard procedure is to study the x distributions o f the scattered protons. In fig. 1 the reduced longitudinal m o m e n t u m x 1 o f one proton is plotted versus x 2 o f the other proton. For diffractive events x 1 or x 2 are near one. The component from single diffraction is predicted to decrease fast i f x 1 and x 2 approach x -~ 1. In contrast a rather flat population is observed for this region as expected if double pomeron exchange is present, whereas in the case o f single diffraction this area would be depopulated. Restricting the analysis to the sample o f 720 events defined above one observes the absence of correlations between the azimuthal angles ¢ o f the two protons. The transverse momenta PT o f the two protons are also uncorrelated. Parametrizing the PT distribution as
d2N/(dp21 • dp22)
= A exp
2
PTI + P T 2
X2
the two protons.
B(p21
+
p22)
~
I
0,4
L
0,6
0,8
[ GeV/c ]z
F i g . 2. D i s t r i b u t i o n o f t h e s u m o f s q u a r e s o f t h e p r o t o n transverse momenta.
B
( - 5 . 5 -+0.9) (GeV/c) - 2
=
was obtained. In fig. 2 the distribution is displayed. A slope parameter B of this magnitude and the absence o f correlations in ¢ and PT for the two protons are expected if compared to elastic scattering analyzed in terms of factorization. Using this value o r b for extrapolation and correcting for other acceptance losses a cross section of 25 80
I
I
I
I
I
pp--pp 6O
0
,,¢
l
I
rr+ -rr-
= 31GeV
40
w
2C
500 I000 M ('rr+'rr - ) [MeV/c z ]
2000
a value F i g . 3. I n v a r i a n t m a s s d i s t r i b u t i o n
m 0r÷~r-).
395
Volume 65B, number 4
PHYSICS LETTERS
-+1012b is obtained for the reaction pp -+ pwr+n with the cuts given above. The presence of double pomeron exchange can also be studied by analyzing the dipion system. In fig. 3 the mass distribution m (n+n - ) is presented. Assuming the dipion system to originate from the scattering of two pomerons, one expects the presence o f a low mass s-wave n+n - - s t a t e , the absence of the p meson and eventually, i.e. depending on the coupling strength the occurrence of the f meson. The mass distribution displayed in fig. 3 is peaked around 500 MeV/c and no signal from the p meson is observed. The absence of a pronounced peak in the region of the f mass is possibly due to our trigger condition. Since we require at least one o f the pions to enter the SFM central detector, evens with a highly asymmetric f decay remain undetected. The angular m o m e n t u m composition o f the dipion system can be studied by analyzing the 1r+ angular distribution in the n+n - CMS system. The observed isotropic distribution does agree with the assumption that the dipion system is dominantly an s-wave state. Although the notion o f double pomeron exchange cannot be verified directly in experiments, a number o f predictions from phenomenological models is accessible to experimental tests. In this experiment the centre o f mass energy is large enough to obtain a statistically significant sample of events even if stringent cuts are applied to suppress the background from single diffraction. The remaining sample exhibits all features expected for events from pomeron-pomeron scattering: Both protons have nearly the full longitudinal momentum, x ~ 1, they are neither correlated in their transverse momenta PT nor in their azimuthal angle q5 and the exponential slope B of the PT distribution is close to that expected from elastic scattering if factorization is invoked. The invariant mass of the
396
6 December 1976
dipion system is concentrated at low masses (below 1 GeV/c 2) and the angular distribution of the pions in the dipion rest system is isotropic in agreement with dominant s-wave. Although under conspiratorial circumstances an interpretation of our data as the tail o f single diffraction cannot be ruled out completely, it has to be tested that all features observed in our experiment are in perfect agreement with the expectation for double Pomeron exchange. This experiment was greatly helped in different phases b y contributions from Drs. D.R.O. Morrison, H. Schneider, W.J. Schwille and R. Stroynowski. We are grateful to the SFM D e t e c t o r Group and the ISR Division for their support. We are indebted for useful discussions to Drs, M. Jacob and A. Kaidalov. The Heidelberg and Karlsruhe groups were supported by a grant from the Bundesministerium fiir Forschung und Technology, Federal Republic Germany.
References [ 1] D.M. Chew and G.F. Chew, Prediction of double pomeron cross sections from single-diffraction measurements, LBL3334. D.M. Chew, Nucl. Phys. B82 (1974) 422. [2] M. Derrick et al., Phys. Rev. D9 (1974) 1215; L. Baksay et al., Evidence for double pomeron exchange at the CERN-ISR, submitted to Phys. Letters B; D.M. Chew et al., LBL-2106 (1973). [3] H.I. Miettinen, Diffractive processes and the triple pomeron, plenary session talk at the EPS Intern. Conf. on High energy physics, Palermo Th 2072, CERN. [4] The split field magnet facility, ed. A. Minten, CERN/ SFMD-Note 72-4 (1972). [5] R. Bouclier et al., Nucl. Instr. Methods 125 (1975) 19. [6] H. Grote et al., Users guide to the SFM off-line program chain, OM Dev. Note AP/18 -CERN (1975).
PHYSICS LETTERS
6 December 1976
S T U D Y O F D O U B L E P O M E R O N E X C H A N G E I N p p C O L L I S I O N S A T x / s = 31 G e V M. DELLA NEGRA, D. DRIJARD, H.G. FISCHER, G. FONTAINE, H. FREHSE, P. FRENKIEL, C. CHESQUIERE, R. GOKIELI, P. HANKE, W. HOFMANN, W. ISENBECK, E.E. KLUGE, V. KORBEL, D. LINGLIN, A. MINTEN, A. NORTON, A. PUTZER, R. SOSNOWSKI, S. STEIN and D. WEGENER CERN, Geneva, Switzerland CERN-Coll~ge de Franee-Heidelberg-Karlsruhe Collaboration
Received 14 June 1976 The reaction pp ~ ppn+~- at x/s = 31 GeV was studied at the Split Field Magnet Facility of the CERN-ISR. Selecting events with two leading protons ofx > 0.9 and a rapidity gap ~,y > 2 between the protons and both pions a sample of 720 events is obtained representing a cross section of 25 + 10 #b. The mass distribution of the dipion system, x and PT distributions for the protons are presented and analyzed for the presence of double pomeron exchange.
Diffraction scattering in high energy particle collisions is described either in terms of the optical model or by pomeron exchange within the framework of the particle exchange picture. In the latter case the occurrence o f single diffractive excitation implies the possibility of double pomeron exchange the presence of which remains to be verified [1 ]. In this experiment the process pp ~ p p n + n - was analyzed for the presence of double pomeron exchange mechanism. Although this reaction is dominated by diffraction the double pomeron contribution becomes separable at very high energies. To enrich the sample with candidates o f the double pomeron exchange type, events are selected by requiring x > 0.9, where x is the reduced longitudinal momentum. Furthermore, large rapidity gaps Ay between the pions and the protons ensure that the background from peripherally produced pions is suppressed. The requirement Ay = lYp -Y,r[ > 2 makes it necessary to study double pomeron exchange at the highest energies available where the total rapidity interval is sufficiently large. Several experiments [2] have been performed during the last year to establish the presence of double pomeron exchange. Due to the lack of statistical significance or due to the incomplete measurement of kinematical quantities involved these studies have so far remained inconclusive [3]. 394
The present experiment was performed at the CERN-ISR at x/S~= 31 GeV using the Split Field Magnet Facility [4] (SFM) to detect all charged particles occurring in an event and measure their momenta. The acceptance of the SFM detector covers nearly the full solid angle. Due to the presence of the beam tubes leading protons with transverse momentum o f P T2 < 0.03 (GeV/c) 2 remain undetected. To select events of the reaction pp -+ ppX with two leading protons and centrally produced charged particles the Multi-Wire Proportional Chamber (MWPC) system o f the SFM was used in a self-triggering mode. The presence of leading protons was signalled by a six-fold coincidence of wire planes in both compensator telescopes [4] of the SFM, consisting of 3 MWPC each. To ensure particle production in the central region of rapidity and to suppress elastic scattering, coincident signals were required in addition from at least 3 pairs of wire planes of the SFM central detector [5]. At x / s = 31 GeV a sample of 350000 events was recorded. The trajectories of the charged secondaries were reconstructed thus determining their charge and momentum [6]. In about 25 000 of the reconstructed events, the reduced longitudinal momentum of two positive particles was bigger than 0.7. Of those, events with an additional particle pair of opposite charge were subjected to a four constraint fit procedure [7] to obtain events of the type pp -+ p p , + n - . By requir-
Volume 65B, number 4
PHYSICS LETTERS
6 December 1976
. ~. ?."~;... .~.; ~,,.",, .:.{:;v.:.-. 4 ~'-:'f-,'. ",.:::tr:.;.e'.;~t ~Z,t; ,,;..,~ :../ .. ........... :., .,-%.r:tr~:: ..'.~;,,~:~ .:~,,~<, .
.
. . .
,
.
•
•
"
"
•
•
'
"..
•
96
.
I
I
I
I
I
I
I
.
.
"
"
,
-,"
g.'.:
....
.
.
"
.-
,
.
~:.t . , : " , . ~ t , l ~ ' : ~ ; "~ . . : ...-.,,:.:,
.~...;~
: :.
.
.
.
.
"
.
:...
'"
.
pp_
...'..:,...:~..;/;
, .~,:::~:',
•
.
I000 -
-T .;~.-, :'~'J" ".~"
. ," ".'.L. ;:~-~: .
92
,
.
p prr+rr -
:..
8B
;:.~
•
"
~.
Loo
.:..i.!. .;:::
.::[':!
.8o
--N
10 -
2" No_~.76
'"
.i: ?,
.'72 I .72
I .76
I .80
I .Sq
8B
I
I
92
.96
I
I
I
I
0,2
0
I.
2
Xl
V5
F i g . 1. S c a t t e r p l o t o f t h e r e d u c e d
longitudinal
momenta
x of
ing a chi-square probability greater than 1% the number o f events remaining was 2000. By applying the cuts mentioned (x > 0.9, Ay > 2) the final sample consisted of 720 events. To establish the occurrence of double pomeron exchange a standard procedure is to study the x distributions o f the scattered protons. In fig. 1 the reduced longitudinal m o m e n t u m x 1 o f one proton is plotted versus x 2 o f the other proton. For diffractive events x 1 or x 2 are near one. The component from single diffraction is predicted to decrease fast i f x 1 and x 2 approach x -~ 1. In contrast a rather flat population is observed for this region as expected if double pomeron exchange is present, whereas in the case o f single diffraction this area would be depopulated. Restricting the analysis to the sample o f 720 events defined above one observes the absence of correlations between the azimuthal angles ¢ o f the two protons. The transverse momenta PT o f the two protons are also uncorrelated. Parametrizing the PT distribution as
d2N/(dp21 • dp22)
= A exp
2
PTI + P T 2
X2
the two protons.
B(p21
+
p22)
~
I
0,4
L
0,6
0,8
[ GeV/c ]z
F i g . 2. D i s t r i b u t i o n o f t h e s u m o f s q u a r e s o f t h e p r o t o n transverse momenta.
B
( - 5 . 5 -+0.9) (GeV/c) - 2
=
was obtained. In fig. 2 the distribution is displayed. A slope parameter B of this magnitude and the absence o f correlations in ¢ and PT for the two protons are expected if compared to elastic scattering analyzed in terms of factorization. Using this value o r b for extrapolation and correcting for other acceptance losses a cross section of 25 80
I
I
I
I
I
pp--pp 6O
0
,,¢
l
I
rr+ -rr-
= 31GeV
40
w
2C
500 I000 M ('rr+'rr - ) [MeV/c z ]
2000
a value F i g . 3. I n v a r i a n t m a s s d i s t r i b u t i o n
m 0r÷~r-).
395
Volume 65B, number 4
PHYSICS LETTERS
-+1012b is obtained for the reaction pp -+ pwr+n with the cuts given above. The presence of double pomeron exchange can also be studied by analyzing the dipion system. In fig. 3 the mass distribution m (n+n - ) is presented. Assuming the dipion system to originate from the scattering of two pomerons, one expects the presence o f a low mass s-wave n+n - - s t a t e , the absence of the p meson and eventually, i.e. depending on the coupling strength the occurrence of the f meson. The mass distribution displayed in fig. 3 is peaked around 500 MeV/c and no signal from the p meson is observed. The absence of a pronounced peak in the region of the f mass is possibly due to our trigger condition. Since we require at least one o f the pions to enter the SFM central detector, evens with a highly asymmetric f decay remain undetected. The angular m o m e n t u m composition o f the dipion system can be studied by analyzing the 1r+ angular distribution in the n+n - CMS system. The observed isotropic distribution does agree with the assumption that the dipion system is dominantly an s-wave state. Although the notion o f double pomeron exchange cannot be verified directly in experiments, a number o f predictions from phenomenological models is accessible to experimental tests. In this experiment the centre o f mass energy is large enough to obtain a statistically significant sample of events even if stringent cuts are applied to suppress the background from single diffraction. The remaining sample exhibits all features expected for events from pomeron-pomeron scattering: Both protons have nearly the full longitudinal momentum, x ~ 1, they are neither correlated in their transverse momenta PT nor in their azimuthal angle q5 and the exponential slope B of the PT distribution is close to that expected from elastic scattering if factorization is invoked. The invariant mass of the
396
6 December 1976
dipion system is concentrated at low masses (below 1 GeV/c 2) and the angular distribution of the pions in the dipion rest system is isotropic in agreement with dominant s-wave. Although under conspiratorial circumstances an interpretation of our data as the tail o f single diffraction cannot be ruled out completely, it has to be tested that all features observed in our experiment are in perfect agreement with the expectation for double Pomeron exchange. This experiment was greatly helped in different phases b y contributions from Drs. D.R.O. Morrison, H. Schneider, W.J. Schwille and R. Stroynowski. We are grateful to the SFM D e t e c t o r Group and the ISR Division for their support. We are indebted for useful discussions to Drs, M. Jacob and A. Kaidalov. The Heidelberg and Karlsruhe groups were supported by a grant from the Bundesministerium fiir Forschung und Technology, Federal Republic Germany.
References [ 1] D.M. Chew and G.F. Chew, Prediction of double pomeron cross sections from single-diffraction measurements, LBL3334. D.M. Chew, Nucl. Phys. B82 (1974) 422. [2] M. Derrick et al., Phys. Rev. D9 (1974) 1215; L. Baksay et al., Evidence for double pomeron exchange at the CERN-ISR, submitted to Phys. Letters B; D.M. Chew et al., LBL-2106 (1973). [3] H.I. Miettinen, Diffractive processes and the triple pomeron, plenary session talk at the EPS Intern. Conf. on High energy physics, Palermo Th 2072, CERN. [4] The split field magnet facility, ed. A. Minten, CERN/ SFMD-Note 72-4 (1972). [5] R. Bouclier et al., Nucl. Instr. Methods 125 (1975) 19. [6] H. Grote et al., Users guide to the SFM off-line program chain, OM Dev. Note AP/18 -CERN (1975).