PIONIZATION in Pp INTERACTIONS at 12 and 24 Gev/C H

Volume 39B, number 2 PHYSICS LETTERS 17 April 1972

PIONIZATION IN pp INTERACTIONS AT 12 AND 24 GeV/c H. J. MUCK, M. SCHACHTER, F. SELONKE, and B. WESSELS University of Bonn, Germany

V. BLOBEL, A. BRANDT, G. DREWS, H. FESEFELDT, B. HELLWIG, D. M(SNKEMEYER, and P. SODING DESY and University of Hamburg, Germany

G.W. BRANDENBURG *, H. FRANZ, P. FREUND, D. LOERS, W. RICHTER, and W. SCHRANKE L Max-Planck-Institut flit Physik und Astrophysik, Mi~chen, Germany Received 1 March 1972

Inclusive 7r- and 7r+ distributions in the regioh of small longitudinal c.m. momenta, from pp interactions at 12 and 24 GeV/c, are presented and compared with CERN-ISR data. Interesting scaling and fac- torization properties are found. The extent to which the observed behavior can be interpreted as evidence for pionization is discussed.

One of the problems in high-energy interactions currently much discussed is pionization. It d3a 1 d2(r is a crucial feature of inclusive reactions in the d-3-p/E = ~ dy.dp2T ~f(pw)>0, (1) multiperipheral model [1], the parton model [2], and the field-theoretical treatment [3] of high- with f(P~v) independent of y* and s (apart perhaps energy collisions. However, comparatively little from lo~s terms), and of the quantum numbers of detailed data from the central cr pionization re- the incident particles. This behavior corresponds gion of inclusive reactions have been published to double-pomeron exchange in a double-peripher- so far [e.g. 4, 5]. al model. If the exchanged quantum numbers are In this letter we present double-differential pure C = +1 and I = 0, the functions f(PT) will be cross sections for the central region in the re- equal for ~r+, ~r- and 7ro. actions p+p~ rr± + (anything) at incident laboratory The data we present come from 130 000 events momenta of 12 and 24 GeV/c, and compare them (54 000 events) observed in the CERN 2 m hydro- with CERN-ISR measurements at higher momenta. gen bubble chamber which was ex0osed to a p We find remarkable regularities when we express beam of 12 GeV/c (24 GeV/c). Events of all topo- these cross sections as functions of y*, p~, and logies were measured on flyingspot digitizers, s. The use of the c.m. longitudinal rapidit~ which also gave ionization information. Using y* = arsiluh(p~/p2T +m2fl/2)(where p~ and PT are kinematics and ionization it was possible to sep- the longitudinal and transverse c.m. momenta), in- arately obtain the distributions for pions and pro- stead of the more conventional variable x=P*/½~fs), tons in the whole backward c.m hemisphere J'. is crucial because these regularities in the central Kaons were not distinguished from pions and are region are not evident in the variables x and PT" therefore included in the distributions given. The central (or pionization ) region is some- This is irrelevant in the context of the investi- what arbitrarily definedby] y* ]< ]y ~'ncI-A, where gations described here. The overall normaliza- Y ~nc is the c. m. rapidity of the incident particles tion of the cross sections was checked by compar- (1.62at12 GeV/c and 1.97 at 24GeV/c')andh is a constant of the order of 1. We recall that for asymp- At 24 GeV/c a pion at rest in the c.m. system has a totic s the prediction [1-3] for the Lorentz-invar- laboratory momentum of 0.49 GeV/b. There fore the iant single-particle distributions in the central 16% of~" /p ambiguities we found occurred only at transverse momentap T > 0.4 GeV/c. They were re- region (for unpolarized incident particles) is solved on a statistical basis, exploiting the forward/ *Present address: Stanford Linear Accelerator Center, backward symmetry in the c.m. system. The proced- Stanford, Calif. USA. ure will be described in detail elsewhere.

303 Volume 39B, number 2 PHYSICS LETTERS 17 April 1972

I , I I I I ' I "'T I I , 1 I " "~ ?, --~X ol2OeV/c ~a~_-t_~, o 12GeV/c 50 e24GeV/c "~--~-~ . .24GeV/c PT=0 I ....+'~-'I 3eric -lu {IO

\ i; 0"05I t 0"02I (b) (a) I i I I I I 0.0 1.0 2.0 (~o 1.o 2.0 y~

I 1 , r "pp ~ lz'X PT=0.2GeWc :pp_...:x ' ' ' 50 ' 1500 GeV/c 7 ~-~. T- .~ 11oo Gev/c I. s 20 !o .. 500 G~VIc 10 o o. 10 12GeV/c ~kOeV/c ~ 225GeV/c ,2 v,c \ 5 lY~ncl for 12 2/, 50011001500GeV/c Iv~"cl-m for 12 2/, 225 500 11001500 C-eV/c (d} , ; ~ ,* ~ t~" (ci 1 2 3 4 5 2 4 y* y*

Fig. 1. (a), (b) Invariant inclusive ?r- and ?r + differentia[ cross sections 7r-ld2(~/dy ~dP2 at 12 and 24 GeV/c, as a function of the c.m. rapidity y *. The curves connect data points at equal s andpT, to quz}te the eye. (c), (d) Comparison of our 12 and 24 GeV/c data with CERN-ISR data ~tPT = 0.2 GeV/cand equivalent laboratory momenta between 225 and 1500 GeV/c. The dashed curves are drawn by us to guide the eye.

304 Volume 39B, number 2 PHYSICS LETTERS 17 April 1972 ison of our atot(PP) with values measured in pp~ff'X pp -..¢*X{without 2 pro~ e',nmts) counter experiments [6]; they agreed within 2%. In figs.la and lb we show the ~r- and y+ inclus- 12 c~v~ t ! z ive distributions ~r-1 d2a/dy* dp2T atPT = 0 and , ! t Ip?,o.~ for various regions of p T' at 12 and 24 GeV/c. [C~Z) z i[[lilltt|t t ll[|llt || The values atPT = 0 have been obtained by a 8 slight extrapolation of our measured complete p2 -distributions.A by fitting the function 4 a exp(b~ 2 + cp~) to the data in the region p~? < 0~0 i = i = = [ z I / 0.3(GeV~c) 2. For the n+ distribution the 2-prong 0.0 y. z.O ID y 2.0 events (i.e. final states with only two charged particles) were excluded; these events are a 20I bac~ ground to pionization because the total charge ~7.~ 16 24~V/c 24 oev/¢ t of all pionization products should be zero. We observe the following features: (1) The distributions tend to become independ- [ltli|l| ½111111 ½ ent of y* at small lY* ] for allPT , but particular- 8 ly so for the smaller values ofp T. $ (2) The flat regions in y* show a tendency to become wider with increasing s. (3) The cross sections are s- , t ~ ~ 2.0 dependent, i.e. we are not yet in a pure scaling O0 ' ' ' 1~0 ....y* z.O oo ' ' t'o ' ' ' region. (4) The ~r+ cross sect ions are larger than Fig.2. Slopes of the p2 distributions (for constan, y *) as the rr- cross sections, but the ~r+/~r- ration de- a function of the c.m. rapidity y*, at two fixed values of creases towards 1 with increasing s. p2for 7r- and 7r+ from inclusive 12 and 24 GeV/c pp in- We now compare, in figs. lc and ld, our 12 teractions. ~.Thepre~se definition of the slope is and 2,4 GeV/c data (represented b3 the curves) -(a/ap~in(dZa/dy*dt~); it was determined by a fit of with y+ and y- inclusive data measured at the tl),e func~ioona exp(b/;2*+ cp 4) to the measured o CERN pp Intersecting Storage Rings, at c.m. en- d~/dy*dp~ in fixed, small intervals ofy * and p,~. ) ergies equivalent to incident laboratory momenta between 225 GeV/c and 1500 GeV/c [7, 8]. (Note 0.2 GeV/c, y* = 0 and equivalent laboratory inci- the change in abscissa scale with respect to dent momenta of 500-1500 GeV/c, as indicated figs. la and lb. ) We show only the data for the in the figures. It is then seen that the y* distrik- smallest available transverse momentum PT = utions are consistent with an expanding plateau 0.2 GeV/c; for the larger PT the compari son if the numbers of pions of each charge become comes out similar. To give an idea of how fast equal in the central region at large s. A log s in- the central region is expanding with increasing crease of the cross section in the region of small energy, the values of ( y~ I are indicated at the [ Y*t may persist at ISR energies. bottom of the figures. ~mCe the ISR ~+ data are Although the absolute cross section in the cen- essentially outside the central region, we also tral region still depends significantly on s and consider the inclusive photon spectra measured on the charge of the pion, the form of./(pT) (see [9] at the ISR, which extend to smaller l Y*[" eq. (1)) nevertheless shows a remarkable limiting From these we calculate, assuming two observed property already in the 12- 24 GeV/c region. photons per ~o, a value of ~r-ld2a/dy*dF2,~ ~ 44 This is seen from fig.2 which shows the logar- mb(Gev/c) -2 for inclusive ~o production ~t PT = ithmic derivative with respect to/~ (i. e. the n slope n ) of the PT-distributions,2 as a function of One might suspect that th~ flatness of the inclusive y*. As thep2T distributions are not simple expon- y* distributions at small |y*| could be only a trivial entials (they change gradually from a steeper consequence of the symmetry of the pp reaction in slope at smallp~ to a flatter one at largerp~), the c.m. system, together with the assumption of we show the slopes at the two fixed ~ values of smoothness of the cross section at Y* = 0. That this is not necessarily so can be seen from processes to 0.1 and 1 (GeV/c) 2. Fig.2 gives direc~t evidence which pionization does not contribute, like p+p~ #+ that the form of the transverse momentum dis- + (one charged) + {anything neutral)(2-prongs); or in tribution is already becoming independent of the which pic:'nization is suppressed, like p+p--*.p+?r++/r- pion charge, and very nearly independent of s, +p (dominated by bayron resonance production). In at 12-24 GeV/c in the central region. Further- these reactions we observe a stronger variation of do'/dy * at small ]y* [ ; in fact they show a valley at more at 24 GeV/c the form of thePT distribution y* = O. is seen to be independent of y* up to about [y* [ =

305 Volume 39B, number 2 PHYSICS LETTERS 17 April 1972

1, i.e. the y* and PT dependence factorize in a K. B(~ckmann, V.P. Kenney, E. Lohrmann, H.H. region of Ay* ~ 2 around y* = 0. Nagel, B. Nellen, N. Schmitz, B.M. Schwarzschild, The data presented strongly suggest that and M.W° Teucher for advice, help, criticism, features characteristic of pionization (see eq. (1)) and active support. already begin to show up in the 12-24 GeV/c region. A problem is , of course, the kinematic overlap at these incident momenta between pion- References ization on the one hand, and fragmentation or []] D. Amati, A. Stanghetiini and S. Fubini, Nuovo Cim- excitation of the projectile and target protons on ento 26 (1962) 896; the other. Therefore we can not observe pioniza- K.G. Wiison, Acta Phys. Austr. 17 (1963) 37; tion here as an isolated phenomenon free of back- C.E.De Tar, Phvs. Rev. D3 (1971) 128. ground. However, assuming only proton fragmenta- [2] R. P. Feynman, in High energy collision, eds. C. N. Yang eta[. (New York, 1969) p. 237; Phys. Rev. ation and excitation a convincing explanation for Letters 23 (1969) 1415. the remarkable regularities of the pion distribu- [3] H. Cheng,and T. T. Wu, Phys. Rev. Letters 23 (1969) tions in the central region may be hard to find. 1311; 24 (1970) 1456. From this point of view these regularities would [4] L. Michejda, Nucl. Phys. B35 {1971) 287; rather seem accidental. J. H. Friedman, C. Bisk and D. B. Smith, Phys. Pev. Letters 28 (1972) 191; L. von Lindern et al., l:hys. Rev. Letters 27 (1971) We are indebted to the CERN operations crew 1745. of the PS and of the 2m bubble chamber for their [5] L. Van Hove, in Proc. Colloquium on Multiparticle excellent cooperation. We wish to particularly dynamics, Helsinki (1971) D. 227. thank H. H. Nagel and J. Sey,~rlein, who have been [6] Particle Data Group, UCRL-20 000 NN (1970). [7] A. Bertin eta[., Bologna preprint. responsible for the development of our flying- [8] L. G. Ratner eta[., Phys. Rev. Letters 27 (1971) 86, spot digitizers. Finally we thank T. T. Wu for and to be published. stimulatilg discussions and suggestions, and [9] G. Neuhofer eta[., Phys. Letters 38B (1972) 51.

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