production of nucleon resonances by single diffraction dissociation at the cern isr

$: Production of nucleon resonances by single diffraction dissociation at the CERN ISR$
Volume 55B, number 3 PHYSICS LETTERS 17 February 1975

P R O D U C T I O N O F N U C L E O N R E S O N A N C E S B Y S I N G 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 T T H E C E R N I S R

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

T. MEYER, W. MARSH, W. LOCKMAN, J. LAYTER, A. KERNAN, M. HANSROUL, S.-Y. FUNG, H. FOETH, R. ELLIS,

A. DEREVSHIKOV .3 , M. BOZZO .4 , A. BOHM and L. BAKSAY IlL Physikalische Institut der Technischen Hochschule *s, Aachen, Germany Department of Physics, University oje California .6, Los Angeles, Calif., USA

Department of Physics, University of California* 7, Riverside, Calif, USA CERN, Geneva, Switzerland

Received 14 January 1975

The single diffraction dissociation process pp ~ (l~+Ir-) p has been studied at the CERN ISR at x/~ = 45 GeV and 0.1 < - t < 0.6 GeV 2. The reaction is dominated by nucleon resonance production: pp ~ pN(1520) and pp ~ pN(1688) with cross-sections (0.25 ± 0.08) mb and (0.56 ± 0.19) mb respectively.

Recent studies of the inclusive reaction p + p ~- p + X have established that diffractive excitation of one nucleon into the mass range 1 ~ M X ~ 10 GeV occurs with a cross-section of approximately 5 mb at ISR energies [ 1 ]. For the same reaction at incident laboratory momenta 6 < Plab < 30 GeV/c the missing- mass spectrurn exhibits a fine structure which is ascribed to production of nucleon resonances [2, 3]. The question of how resonance production extrapo- lates to very high energies is extremely important for the understanding of the nature of the Pomeron and of the diffraction dissociation process. This question, however, cannot be investigated in experiments such as those of ref. [1 ] because of large uncertainty in the missing-mass measurement, AM X (full width at half maximum), ~ (4.5/Mx) GeV at V~ = 31 GeV. Recently, evidence has been reported for nucleon

, l Guggenheim Fellow, on sabbatical leave from the Univer- sity of California, Berkeley, USA.

.2 Permanent address: University of Chicago, USA.

.3 Visitor from IHEP, Serpukhov, USSR.

.4 Istituto di Fisica delrUniversita, INFN, Sezione di Geneva, Italy.

.s Supported by the Bundesministedum fiir Forschung und Technologic, Germany.

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

.7 Supported by the US Atomic Energy Commission.

resonance production in the inclusive reaction p + p (p+rt++lr - ) + X [4] at V~ = 35 GeV and in the ex-

clusive reaction p + p ~ p + n + rt + [5] at x/s = 53 GeV.

In the first study at the CERN intersecting storage rings (ISR) of the exclusive reaction

p + p -~ (p+Tr + +, r - ) + p (1)

we have observed nucleon resonance production:

p + p -~ p + N(1520) (2)

p + p -~ p + N(1688) (3)

with cross-sections at x/~-= 45 GeV (equivalent Plab = 1000 GeV/c), which are comparable to those at Plab = 30 GeV/c [2, 3]. For this experiment the multi-particle magnetic spectrometer used in the experiment of ref. [4] was supplemented by a system of multiwire proportional chambers (MWPC) in the opposite arm. The momenta of the dissociation products pn+rr - are measured in the spectrometer, while the direction of the scattered proton is measured in the MWPC telescope. Good mass resolution, increasing from 0.04 GeV to 0.08 GeV (FWHM) over the mass interval 1.5 GeV to 2.0 GeV, is achieved by measuring the effec- tive mass of the decay particles pTr+rr - from the excited nucleon state.

The experimental set-up is shown in fig. 1. The

331


Top view

Septum Mognets

i W ,' S U2

W UI t X..--~--I U2 '3'~ / I

ARM t ARM 2 $i~e view

Fig. 1. The experimental set-up at ISR intersection I6. The symbol W is used to indicate wire chambers. Scintillation counters in coincidence are assigned the same symbol.

multi-particle magnetic spectrometer system in arm 1 is composed of two identical septum magnets, each with aperture 62 X 80 cm 2 and fBdl of 4.7 kG X 2.6 m, placed above and below the beam pipe in arm 1 downstream of the intersection region I6. Particle trajectories before and after each magnet are determined by pairs of MWPC modules. Each module contains four planes: x, y planes with 2 mm wire spacing and u, o planes with 2x/2 mm spacing oriented at 0 °, 90 °, +45 °, and - 4 5 ° from the vertical, respectively. The angular aperture of these magnets with respect to the beam direction is +-(15-100) mrad in the vertical plane and -+ 60 mrad in the horizontal plane. The upstream half of each magnet contains a four-element gas (~erenkov counter f'llled with freon 22 at atmo- spheric pressure. Thresholds for 7r, K, and p are 3.6, 12.7 and 24.1 GeV/c, respectively. In arm 2, four 95 X 95 mm 2 MWPCs with 1 mm wire spacing (of special construction in order to be sensitive at close distance from the ISR beam), are arranged as two x, y pairs at 2.7 m and 3.6 m from the intersection region, and subtend an angular range of 12 to 35 mrad with respect to the beam direction. Two scintillation counter telescopes, U1 and D1, cover the apertures of the arm 1 spectrometers, and a third telescope S is associated with the MWPCs in arm 2. The intersection region is surrounded by a barrel of 12 counters X, which cover angles greater than 400 mrad with respect to both incident beams. The two ends of the barrel X are partially closed by the counter hodoscopes Y1 and Y2, leaving a circular opening of 0 ~ 100 mrad which approximately matches the aperture of the

arm 1 spectrometer. A detailed description of the en- tire experimental set-up is given in a separate commu- nication [6].

The coincidence trigger used to initiate read-out of the wire chambers, U1 "DI • S ' 2 , selects preferentially the diffraction dissociation process p + p --* p' + p, with p' decaying into two or more charged particles in the arm 1 spectrometer, and p traversing the wire chambers in arm 2. Reactions with charged particles at angles greater than 400 mrad relative to the beam direction are vetoed by the barrel counter X. A total of 2.3 X 106 events were recorded at 22/22 GeV/c colliding beam momenta at a rate of ~ 100 events/sec.

Track reconstruction through the spectrometer re- quires an average of 15 msec per event on the CDC 7600 and gave 127000 events with three tracks in arm 1. Fig. 2 shows the distribution ofPtot , the total momentum in arm 1, for three-particle systems satis- fying the following criteria: i) the charge configura- tion is + + - ; ii) the minimum root-mean-square distance of the three tracks from a common vertex in the intersection region is less than 12 mm; iii) no plane in arm 2 has more than one wire hit, and at least three of the four planes have a hit within a fi- ducial region having greater than 95% efficiency. The four-hit events define a unique track in arm 2. For the three-hit events the vertex of the arm 1 tracks is needed to reconstruct the single track in arm 2.

To select events of reaction (1) we have applied the additional criterion that the arm 1 and arm 2 momenta be collinear in the c.m. system. The collinearity requirement is 160xl ~< 3 mrad and IGOyl ~< 3 mrad, where 60 x and 60y are the projections of the devia- tion from collinearity on the horizontal and vertical planes. A scatter plot of GOy versus fox, along with the GO x projection for events with IGOyl <~ 3 mrad is shown in the inset in fig. 2. The shaded histogram in fig. 2 shows Ptot for the collinear events. The peak centred on the beam momentum Pbeam corresponds to dissociation of the incident arm 1 proton into a low mass p~r+n - system; the width of the peak (7% FWHM) reflects the resolution of the spectrometer. The data analysis described below was performed on the 6955 coUinear events with Ptot within Pbeam + 2 GeV/c. From the symmetry of the peak it is estimated that the background from events with more than three outgoing particles is less than 2%. Identification of particles in arm 1 is accomplished with the t~eren-

332


(_9 tr) OJ 0

E > ¢D

:::3

180C

1 4 O O

I 0 0 0

6 0 0

200

pp ---* (prr+rr -) p

[ ] All events

[ ] Collinear events

10

| v OI

b f m 0

i i i I I ~ . ~ . . . . , , , r . f r / / / i / i / i / A o ' / / / / / / / / / / / , ¢ ~ ' . ~ _ i t

6 I0 14 18 22 26 50 0 =~ -~0 -10 0 I0

Ptotal ( G e V / c ) BO= t=wt

. : : : . . : . . ;" t ; ~ . ; . : ~.:.~ ." ... : , ~ . . . ~ ; . , ' ~ ,.':." . ~ : . . "

• ....~,.,., -:,,::;.. ,';...:.. . • " : . ~ : ' . . . . ' . . " ~ . " - . . " C : . : ,

.'.'..:~ :...;'~T~r'~.,..s. :. • :., 2 r . , . . , . ~ : ' : . ' - - , ..

• : . . ; - . . . . ~ . . ' : -7.. : . . " . • . . ~ . ~ . ; ~ ,

... - ~.. r.::.~.,T.~c ...... . • ; . . . a t . - , . • ... ...-.-..,...?.. : ' ; . . . . . .

, • . • , . . ' . . " . . % • . .

• , r , ~ • o . . • . ' . . . . , . .

# " 2 1

2 0

Fig. 2. The total momentum distribution for the three particles in arm 1. The shaded histogram shows events which satisfy the criteria IfOxl ~ 3 mrad and If0yl < 3 mrad. The inset shows the scatter plot of roy versus f o x , and the fO x projection for events with IfOyl < 3 mrad.

kov hodoscope. The positive pion can be distinguished from the proton by the ~erenkov pulse-height infor- mation in 80% of the events. For the remainder it was assumed that the higher momentum positive particle is the proton, since this is found to be the case for 85% of the identified events.

Fig. 3 shows the experimental distribution in invariant mass, M(plr+~r-), of these arm 1 particles for dif- ferent t-intervals. The M(pTr+zr - ) distribution is dominated by a pronounced peak at 1.7 GeV, with a lesser structure at 1.5 GeV. Qualitatively, these features are similar to those observed in studies of reaction (1) at incident laboratory momenta 1 0 - 3 0 G e V / c . Spin- parity analyses [e.g. 7] of diffractively excited N~r and Nrrrr systems in the incident momentum region 1 0 - 3 0 GeV/c have led to the interpretation of the 1.5 GeV and 1.7 GeV peaks as being dominantly N(1520) and N(1688) of spin-parity 3 / 2 - and 5/2 ÷, respectively. Dalitz plots show that I t -A++(1236) dominates the p~r+n - system for M(prr+rr - ) below 1.8 GeV. Also shown in fig. 3 is the geometrical acceptance of our detector, calculated for i) the t-dependence for pp -+ p 'p measured in this experiment, and

ii) a three-body invariant phase-space description for p ' ~ p~r÷rr-. The acceptance was also calculated assum- ing p' ~ 7r-A++(1236) ~ Ir-lr+p, with isotropic production and decay of A ++. For M(prr+Tr - ) below 2 GeV this acceptance is unchanged from that o f fig. 3; above 2 GeV the acceptance for p' ~ n - A ++ is about 10% lower than in fig. 3 ~.

The acceptance-corrected invariant mass distribution has been fitted with an 11-parameter form of two Breit-Wigner amplitudes and a fourth-order poly- nomial in M(pTr+n - ) added incoherently• To take ac- count of the proximity of the rrA threshold to the mass interval being fitted, the width r in the Breit- Wigner amplitude was parametrized as F = r o p / p o,

where p (Po) is the momentum in the 7rA system of total energy M ( M o ) . The resonance parameters and errors given by the fit are M = (1500+-8) MeV, F o = (150+ 50) MeV for the lower peak, and M = (1678-+4)

* The acceptance for Pbeam of 26 GoV/c extends to consider- ably higher masses M(pn+n-) than at 22 GoV/c. In order to check acceptance calculations we have compared the mass spectra corrected by phase-space acceptance to those from a 26/26 GeV/c run and found excellent agreement.

333


350

280

210

140

70

20C

IGC

t20

80

40

0

t50

120

9O

60

50

0 1.2

~ 0,1~-1<0,6 GeV z

6731 events

02 .< - t < 0.4 GeV 2

1.4 1,6 Le Z0 2.2 2.4 2.6 2B

10

5

0 ~0

M ( p T * T - ) (GeV)

Fig. 3. Experimental distribution in M(plr+Ir -) for: a) 0.1 < - t < 0.6 GeV2; b) 0.1 < - t < 0.2 GeV2; c) 0.2 < - t 0.4 GeV 2. The smooth curves are the corresponding accept- ances. The broken curves in (b) show the contributions of N(1520) and N(1688).

MeV, Po = (148-+ 16) MeV for the higher mass peak; indicating that these structures are consistent with the N(1520) and N(1688). The dominance of ~rA in the 1.4-1 .8 GeV M(prr+~r - ) mass range is compatible with the observed decay modes of N(1520) and N(1688),

and the absence of a significant ~r+A ° signal is consistent with isotopic spin assignment of 1/2.

Fig. 4 shows the distribution in four-momentum transfer between the incident beam-1 proton and the outgoing pTr+zr - system for the M(prr+rr - ) intervals: a) 1 .4-1.6 GeV; b) 1 .6-1.8 GeV; and c) 1.8-2.1 GeV. The measured t-distributions have been weighted by the acceptance calculated for the experimentally observed mass distributions. The cross-sections were determined by comparing the pp ~ prr+rr-p rate with the rate for elastic scatters pp ~ pp, recorded with the trigger (U1 +D1)"S "2. We estimate an over-all nor- malization uncertainty of +20%; the errors shown in fig. 4 are statistical only. Within the t-interval 0 . 1 4 - 0.5 GeV 2 the differential cross-sections for the mass intervals 1 .6-1.8 GeV and 1.8-2.1 GeV can be fitted by an exponential form A e bt with slopes of 6.3 +- 0.4 GeV -2 and 7.1 + 0.4 GeV -2 , respectively. For the mass interval 1 .4-1 .6 GeV the data are suggestive of a structure at - t ~ 0.2 GeV 2. The fitted slope for 0.2 < - t < 0.5 GeV 2 is 4.8 + 0.5 GeV -2 . These cross-sections have similar features to those observed in the reaction p + p = p + (n+rr +) at x/~ = 53 GeV [5]. At 24 GeV/c the slope for pp ~ pN(1688) is 5.12+0.08 GeV -2 [3] indicating that the differential cross-section for this reaction is shrinking with increasing s.

Extrapolating the observed differential cross-sections to t = 0 ~, and considering both possibilities of proton dissociation, gives a cross-section o f 0.33 +- 0.1 mb at Plab of 1000 GeV/c for reaction (1) with M(prr+rr - ) < 2.5 GeV. From a similar analysis of data taken at x/f = 53 GeV we obtain 0.34 + 0.1 mb. Also, allowing for the production of masses beyond 2.5 GeV, a comparison with cross-sections in the momentum interval 10 < Plab < 205 GeV/c [8], confirms the trend, noted at FNAL energies, of a decreasing momentum dependence of the cross-section with increasing s. This is furthermore consistent with the re- suits of ref. [4], where it has been shown that in the X/~-range from 35 GeV to 53 GeV, the total cross-section for the inclusive reaction p + p ~ (plr+*r - ) + X is invariant.

From the fits to the mass distributions in fig. 3 the cross-sections for the structures at 1500 and 1700 MeV in the reaction p + p ~ (prr+Ir - ) + p at Plab =

A steepening of the differential cross-section at smaller momentum transfers (see ref. [4]) results in a shift of these values which has been allowed for in the error~ assignment.

334


(..9

E

b "o

0.1

0.01

1.4 ~< M (p~+" r r - ) < 1,6 GeV

+ ++

.l-., I- ÷%÷

1.6~ M (p'n"+'rr -) ,::1.8 GeV .I. +

+

+t~.H

tt..,_l,-+

i I i I i i I I i i

0.2 0.4 0.6 0,2 0,4 0.6

q-

1,8-. < M (p rr+'rr - ) < 2.1GeV

++ ÷÷ ***" t tttti +t

i I I I i

0.2 0,4 0.6

- t ( G e V z )

Fig. 4. Differential cross-sections for pp --* l~+n-p in the M(p~r+n -) ranges 1.4-1.6, 1.6-1.8, and 1.8-2.1 GeV.

1000 GeV/c are found to be (0.063-+0.017) mb and (0.19-+0.06) mb, respectively with the assumption that these are N(1520) and N(1688) and allowing for the other decay modes of these resonances [9], the total cross-sections for reactions (2) and (3) are estimated to be (0.25-+0.08) mb and (0.56-+0.19) mb, respectively. These cross-sections are significantly higher than would be expected from a simple power law extrapolation (Plag, with n = 0.56 -+ 0.06 and n = 0.34 + 0.06, respectively [2]) of lower energy data, and are virtually unchanged from Plab = 30 C, eV/c.

We wish to thank C. Rubbia for his contribution to the design of the spectrometer and in the execution of the experiment. We also thank K. Bussman, R. Hammarstr~Sm, P. Poggi, and H. Rigoni for technical assistance. Discussions with G.L. Kane and P. Stevens were extremely helpful. One of us (G.T.) wishes to express his thanks for the hospitality extended to him by the NP Di'vision at CERN. The cooperation of B. Couchman, the ISR Experimental Support Group, and the ISR staff has been indispensable.

References

[1] M.G. Albrow et al., CERN-HoUand-Lancaster-Manehester Collaboration, Nucl. Phys. B51 (1973) 388; Nucl. Phys. B54 (1973) 6; A. Bohm et al., Aaehen-CERN-Harvard-Genova-Torino Collaboration, Nucl. Phys. B78 (1974) 1; J.C. Sens, Recent advances in particle physics (New York Academy of Sciences 1974) p. 155.

[2] R.M. Edelstein et al., Phys. Rev. D5 (1972) 1073. [3] J.V. Allaby et al., Nucl. Phys. B52 (1973) 316. [4] L. Baksay et al., Phys. Lett. 53B (1974) 484. [5] E. Nagy et al., CERN-Hamburg-Orsay-Vienna Collabora-

tion, Contribution to the 17th Intern. Conf. on High- energy physics, London, July 1974

[6] L. Baksay et al., Multiwire proportional chamber spectrometer for the CERN ISR, Aaehen-CERN-Genova- Harvard-UCLA Collaboration, to be submitted to Nucl. Instrum. Methods.

[71 D.C. Colley et al., Phys. Lett. 42B (1972) 497. [8] M. Derrick, B. Musgrave, P. Schreiner and H. Yuta, Phys.

Rev. D9 (1974) 1215; E. Bracci, J.P. Droulez, E. Flaminio, J.D. Hansen and D. R.O. Morrison, Compilation of cross-sections, III: p and p induced reactions, CERN/HERA 73-1 (1973); J.G. Rushbrooke et al., Phys. Rev. D4 (1971) 3273; U. Idschok et al., Bonn-DESY-Hamburg-Miinchen Collab- oration, Nucl. Phys. B53 (1973) 282.

[9] Particle Data Group, Review of particle properties, Phys. Lett. 50B (1974) 1.

335

production of nucleon resonances by single diffraction dissociation at the cern isr

Documents