Volume 55 B, number 3 PHYSICS LETTERS 17 February 1975

D O U B L E D I F F R A C T I O N D I S S O C I A T I O N A N D

T E S T O F P O M E R O N F A C T O R I Z A T I O N A T T H E C E R N I S R

R. WEBB, G. TRILLING .1 , V. TELEGD1.2 , P. STROLIN, A. STAUDE, B. SHEN, P. SCHLEIN, J. RANDER, B. NAROSKA, F. MULLER,

T. MEYER, W. MARSH, W. LOCKMAN, J. LAYTER, A. KERNAN, H. FOETH, R. ELLIS, A. BOHM and L. BAKSAY

Ili. Physikalisches Institut der Technischen Hochschule * a, Aachen, Germany Department o f Physics, University o f California .4, Los Angeles, Calif., USA Department o f Physics, University o f California *s , Riverside, Calif., USA

CERN, Geneva, Switzerland

Received 14 January 1975

We report the observation of double diffraction dissociation in the process p + p ~ (pTr+lr ") + X at x/~of 45 GeV and momentum transfer in the - t range 0.15-0.53 GeV 2 at the CERN ISR. The relative rates for elastic, single and double dissociation reactions measured here are found to agree with the prediction of Pomeron faetorization.

The process of single diffraction dissociation in which one of the incident particles is excited through the exchange of a Pomeron has been studied at high energies mainly via the inclusive reaction

P + P ~ P l +X2" (1)

The presence of the low-mass final system X 2 is manifested by a marked peak in the proton momen- tum spectrum near its upper limit [ 1 ].

The substantial rate for reaction (1) raises the question as to whether double diffraction dissociation

p + p-+ X 1 + X 2 (2)

is also a significant contributor to hadronic processes. Indeed, if the Pomeron is factorizable, one can make the quantitative prediction

do X do do X , , do d-/(Pl 2 ) /d - t (P lP2)=d- t (X1 2)/d-t(XlP2) (3)

,1 Gugger, heim Fellow, on sabbatical leave from the Uni- versity of California, Berkeley, USA.

.2 Permanent address: University of Chicago, USA.

.a Supported by the Bundesministerium fiir Forschung und Technologie, Germany.

.4 Supported by the US National Science Foundation (GP-33565).

,s Supported by the US Atomic Energy Commission.

where the terms in parentheses are the various final states.

In spite of the non-negligible rate implied by (3), reaction (2) is rather difficult to isolate, because it in- volves the measurement of the entire dissociated sys- tem X 1 rather than of a single recoil prOton. Evidence for its occurrence at small momentum transfers ((p2) = 0.03 GeV 2) and for x/s-= 53 GeV has been re- ported recently by Baksay et al. [2], who showed that for those events in the reaction

p + p -~ (p~r+rr-)l + X2, (4)

where X 2 involves more than a single proton, the mo- mentum spectrum of (pzr+rr-)l shows a peak at the beam momentum, resembling the spectrum of Pl in reaction (1). To select the double dissociation proc- ess of reaction (4), all events in which the X 2 is just a single proton must be excluded. In the data of Baksay et al. this exclusion was based on detecting signals in scintillation counter telescopes which could only arise if X 2 was not a single proton.

In this paper we report results on the double dis- sociation process from new data taken at the CERN intersecting storage rings (ISR). As detailed in ref. [3], the multiparticle spectrometer used in the work of ref. [2] for the detection of X 1 (arm 1), is supple- mented in the opposite arm (arm 2) by a multiwire

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Volume 55B, number 3 PHYSICS LETTERS 17 February 1975

proportional chamber telescope, which detected de- cay particles from X 2. With this setup we study simul- taneously the inclusive reaction (4) as well as the ex- clusive reaction:

P + P -+ (Prt+lr-)l + P2 (5)

which is reported in ref. [3]. The arm 2 wire chamber telescope identifies P2 by coUinearity in the c.m. sys- tem with (plr+rt-)l, whereas the production of a mul- tiparticle system X 2 is associated with a non-collinear track in arm 2 (the subscripts 1 and 2 refer to the ISR arm in which the outgoing particles are seen).

The data confirm the occurrence of the double dif- fraction dissociation process. Furthermore, by com- paring the ratio of the rates for reactions (4) and (5) with the ratio of rates for reaction (1) and the elastic reaction

P + P ~ Pl + P2 (6)

observed in the same apparatus, we find good agree- ment with the predictions of factorization.

Data for reactions (4) and (5) [(1) and (6)] were recorded requiring one or more particles in the upper and [or] lower spectrometers in arm 1. In addition, at least one particle was required to pass through the arm 2 chambers. Events with particles emitted at large angles (0 > 0.4 radian with respect to either beam) were vetoed ,1. In order to isolate reactions (4) and (5) we first require three tracks with the proper charges ( + + - ) in the spectrometer and a common vertex. The signature for reaction (5) is then coUinear- ity between the vector sum of the momenta of the three particles in arm 1 and the single track detected in arm 2. We accept only those events in which there is one and only one wire struck in each of the four chambers in arm 2, in order to define unambiguously the track. The measured collinearity distribution (figs. 1 abc) has in both vertical and horizontal planes a width of 3 mrad (FWHM) and thus provides a sharp identification of reaction (5). We classify the events as follows:

, l The scintillation counter coincidences (see fig. 1 of ref. [ 3]) used to initiate read-out were UI. D1. S- g for reac- tions (4) and (5); and [ U1 + D1 ]. S. X for reactions (1) and (6). The data reported here come from 2.3 × 10 6 trig- gers of the first type and 5 X l0 s triggers of the second type at colliding beam momenta of 22.6 GeV/c.

Collinear (reaction 5) 160xl < 5 mrad

and 180yl < 5 mrad

Non-collinear (not reaction 5) lSOxl > 5 mrad

and I~Oyl > 5 mrad,

where fox, 5Oy are the horizontal and vertical angular deviations from collinearity. The requirement that in the definition of non-collinearity both I~Oxl and I~Oyl > 5 mrad is intentionally stringent to help en- sure that no significant tail of the numerous events from reaction (5) be included in the sample attributed to reaction (4).

The non-collinearity condition allows a preliminary selection of reaction (4) events. To eliminate from this sample events of reaction (5) in which scattering in the vacuum chamber walls produce non-collinearity, we investigate the closest distance of approach (CDA) of the summed vector momentum of the arm 1 sys- tem to the track detected in arm 2. Fig. 1 d shows the CDA distribution for collinear events. The fact that these events satisfy the collinearity requirement im- plies that no significant scattering of the outgoing par- ticles has occurred, and the width of the distribution ( " 3 mm) reflects entirely the measurement errors. Fig. 1 e is the same distribution for non-collinear events. It shows a signal similar to that seen in fig. 1 d plus a small, wide tail presumably arising from scat- ters in the vacuum chamber walls. The geometry is such that for each event for which scattering in the wall is the source of non-collinearity, the CDA plotted in fig. 1 must be at least 6 mm. Thus the condition CDA< 6 mm removes all background due to scattered particles from reaction (5), and is therefore used as an additional criterion for reaction (4).

Fig. 2a shows the (plr+Ir-)l momentum spectrum for collinear events, consisting of a peak at the value of the beam momentum broadened by the measure- ment resolution (7% FWHM) :~2. Fig. 2b shows the corresponding spectrum for non-coUinear events satis- fying the CDA cut mentioned above. The peak at the upper end of the momentum distribution is produced by reaction (4), X 2 being a low-mass diffractive sys-

,2 The broadening produced by the p~r*n- mass spectrum ac- cepted in the arm 1 spectrometer, namely 1.3 < M(p~rn) < 2.5 GeV, is only 0.2%, and hence totally negligible in comparison with the resolution.

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Volume 55 B, number 3 PHYSICS LETTERS 17 February 1975

20

I0

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c~ ~ 0

-I0

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Fig. 1. a), b), c): Scatter plot of 80y versus fO x and projections for events with three tracks in arm 1. d), e): Closest distance of approach between the resultant of the three arm 1 momenta and the arm 2 track for collinear and non-coUinear events.

tern. The shaded histogram in fig. 2b is obtained by further imposing both of the following requirements:

a) a charged particle must be detected either in the Y2 or in the D2 counters [3] of arm 2;

b) no charged particles detected in the YI counters [3] in arm 1. Requirement (a) directly ensures at least two particles in arm 2. Requirement (b) removes single diffraction events with charged particles in arm 1 outside the spectrometer. The pronounced diffractive peak in the

spectrum in fig. 2b gives conclusive evidence for the occurrence of double diffractive dissociation.

To test factorization, the events with a single track Pl in arm 1 [reactions (1) and (6)] are also separated into elastic (collinear) and inelastic (non-collinear) events by cuts similar to the ones discussed above. Their respective momentum spectra are shown in figs. 2c and 2d with the inelastic events required to satisfy (a) and (b) above. The peak in the momentum spec- trum of the inelastic events arises from the single dis-

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Volume 55B, number 3 PHYSICS LETTERS 17 February 1975

160(

120(

p p • (p 7r+w'-), p=

o) Collinear 6810 events

12K

90C 8 K

60C

~" ~ 4K 300 ,~

to

(5 --'

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> [ ] With *~ 500 counter requirements

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events

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c)

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60

40

20

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Collinear

58942 events

,J, p p b p,

d)

Non- collineor !

2537 events /

L X z

~ ' % . ' \ i - - o I ~ " \ 14 18 22 26 30 0 I0 2 0 30

p (p r r+r r - )~ (GeV/c) p ( p j ) (GeV /c )

Fig. 2. The momentum spectra of the (l~r+~r-), system for (a) coUinear events, and (b) non-eollinear events, and of the single pro- ton Pl for (c) collinear events, and (d) non-collinear events.

sociation process. The width of the elastic distribution in fig. 2c corresponds to the momentum measurement resolution, 22% (FWHM) for a single proton Pl at 22 c, ev/c.

The same geometrical cuts are applied to Pl in each of the two reactions in which it enters in eq.(3); and similarly for P2, X1 and X 2 enter into eq. (3) the effects of acceptance cancel out, and do not need to be known. Relation (3) can therefore be tested direct- ly comparing the experimental rates for reactions (1) and (6) and reactions (4) and (5). The comparison is made for three It[ intervals: a) 0 .15-0 .275 GeV2; b) 0 .275-0 .4 GeV2; and c) 0.4--0.525 GeV 2. The

minimum Itl o f 0.15 GeV 2 comes from the accept- ance of the arm 1 spectrometer for reactions (1) and (6). The data is confined to events with M(prr+Tr - ) below 1.85 GeV ,3 . The numerators in relation (3) are evaluated by integrating the high momentum peaks with a small background correction for the low momentum tail.

The results of these comparisons, given in table 1, are consistent with factorization. We have checked

,3 Strictly speaking the factorization relation (3) only holds for specific states of the XI, X2 systems. As discussed in ref. [3] the bulk of the diffractive mass spectrum is below 1.85 GeV and is dominated by N(1688).

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Volume 55 B, number 3 PHYSICS LETTERS 17 February 1975

Table 1 Test of factorization hypothesis. The quoted errors are statistical only; systematic errors are estimated to be of similar magnitude. The values of B/A should be reduced by about 5% to correct for the finite t-range.

I tl range A = (dN/dt) [Pi X2] B = (dN/dt) [(pTr+~r-)l X2] B/A (GeV) 2 (dN/dt) [Pl P2 ] (dN/dt) [l~+~r-)l P2 ]

0.15 -0.275 (1.49 ± 0.07) X 10 -2 (1.76 ± 0.18) × 10 -2 1.18 ± 0.14 0.275-0.40 (3.44 ± 0.20) × 10 -2 (4.18 ± 0.52) × 10 -2 1.21 ± 0.18 0.40 -0.525 (10.32 ± 0.80) × 10 -2 (10.46 ± 1.34) × 10 -2 1.02 ± 0.15

the stabili ty o f the ratio B/A against d i f ferent fiducial

cuts. The ratios A and B :~4 increase marked ly wi th

increasing Itl implying that double dissociat ion has a

f lat ter t -dependence than single dissociation. This is to

be expec ted f rom fac tor iza t ion [eq. (3)] for different

values o f t, since single dissociat ion, excep t for very

low masses, has a less sharp t -dependence than elastic

scattering, and fur ther conf i rms that the non-col l inear

events are real dissociations and are not the tails o f

coll inear events.

,4 The ratios are small, in spite of the fact that single disso- ciation has a cross-section comparable to elastic scattering, because of the small aperture of the arm 2 chambers.

We thank C. Rubbia for his cont r ibu t ions to the

exper iment .

References

[1] M.G. Albrow et al., Nucl. Plays. B51 (1973) 388; B54 (1973) 6; Phys. Lett. 42B (1972) 279.

[2] L. Baksay et al., Aachen-UCLA-CERN-Collaboration, paper presented at the 17th Intern. Conf. on High-energy physics, London, 1974.

[3] R. Webb et al., Phys. Lett. B55 (1975) 331.

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