ELSEVIER Nuclear Physics B (Proc. Suppl.) 99 (2001) 47-49 SUPPLEMENTS www.elseviei.nl/locate/npe

The Two-Component Non-Perturbative Pomeron and the G-Universality

Basarab Nicolescua

aLaboratoire de Physique Nucleaire et de Hautes Energies (Unite de Recherche des Universites Paris 6 et Paris 7, Associee au CNRS.) - LPTPE, Tour 12 E3 - 4, Place Jussieu 75252 Paris Cedex 05, Prance e-mail: nicolesc@in2p3.fr

In this communication we present a generalization of the Donnachie-Landshoff model inspired by the recent discovery of a 2-component Pomeron in LLA-QCD by Bartels, Lipatov and Vacca. In particular, we explore a new property, not present in the usual Regge theory - the G-Universality - which signifies the independence of one of the Pomeron components on the nature of the initial and final hadrons. The best description of the pp, pp, r*p, K&p, yy and yp forward data is obtained when G-universality is imposed. Moreover, the en’s behaviour of the hadron amplitude, first established by Heisenberg, is clearly favoured by the data.

The Donnachie-Landshoff model [l] - denoted as Xs’ in the following, is very successful in describing CTT and forward (t = 0) p data for pp, pp, n*p, K*p, yy and yp scatterings: X2/dof = 1.020 for 16 parameters, 383 data points and fi _> 9 GeV [2].

In the present communication I will explore a &CD-inspired generalization of this model. The results are obtained in collaboration with P. Gau-

ron [3]. Recently, Bartels, Lipatov and Vacca [4] dis-

covered the existence of a 2-component Pomeron in LLA. The first component is associated with 2- gluon exchanges and corresponds to an intercept

@ 2 1. (I)

The second component is associated with 3-gluon exchanges with C = +l and corresponds to an intercept

39 - ap -1. (2)

This last component is exchange-degenerate with the 3-gluon C = -1 Odderon. It is therefore useful to explore possible 2-component Pomeron generalizations of the l-component Xs’ Pomeron

QAB(S) = ZAB + XAB(S) + YA+Bs~+-’

fY~&--1, (3)

where GAB are total cross-sections,

XAs(s) = X,@aP-l (4)

= XAB!?n s (5)

= xAB[e,2($)-;], (6)

and cyp, cu+ and cy_ are Reggeon intercepts;

ZAB, XAB, Y;B, SO are constants. The + sign in front of the YiB term in eq. (3) corresponds to {A = p, r-, K- , B = p} and the - sign to {A = p, T+, K+, B = p}. If A = y in eqs. (3)- (6), then B = y, p and YyB = 0. An implicit scale factor of 1 (GeV)2 is present in the Reggeon and en s terms.

The first model in eqs. (4)-(6) - denoted as 2 + Xs’ in the following - corresponds to a gener- alized Donnachie - Landshoff approach [5,6]; the second - denoted as Z + Xen s - to the well known dipole approach [7]; the third - denoted as 2 + Xln2 s - to the Heisenberg - F’roissart - Mar- tin form first considered in 1952 by W. Heisen- berg [B]. The pparameter is calculated from (3) by using the known s -+ sebiKi2 crossing rule.

We study, in particular, the following proper- ties:

1. The G-universality [5,9] (“G” from “gluon”) expressed by (see eqs. (3)-(6))

xAB(s) = x(s), (7)

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48 B. Nicolescu/Nuclear Physics B (Proc. Sup@.) 99 (2001) 47-49

Table 1 Results of the fits of ffT data. The symbol = in the Q + column means weak exchange-degeneracy (a+ = a_).

G- exchange

Model Universality degeneracy “+ cY_ N Par x2 ldof

Xs’

Z+Xs’

z+xens

2 +xen2s

Yes No Yes No

X X

X X

X X

X X

X X

X X

X X

X X

X X

X X

X X

X X

x X

X X

X X

0.66f 0.02 0.45f 0.02 16 0.931

III 0.48 15 1.009

0.618f0.021 0.465f0.021 17 0.936

= 0.491f0.023 16 0.980

0.526f0.029 0.479f0.023 17 0.835

= 0.487f0.023 16 0.836

0.826f0.013 0.468f0.022 16 0.865

= 0.586f0.019 15 1.281

0.658f0.007 0.485f0.022 16 1.066

IZ 0.610f0.016 15 1.286

0.653*0.026 0.465f0.022 17 0.939

ZZ 0.491f0.023 16 0.990

0.583f0.077 0.476f0.023 17 0.822

= 0.478f0.024 16 0.822

= 0.48 15 0.819

The results for the simultaneous description of f’jp, pp, &p, K&p, yy and yp reactions are given in Tables I (fits of 0~ data only) and II (fits of 0~ and p data). This means independence of XAB on A and B (A, B= hadrons only), a property not present in the usual Regge theory.

2. The weak exchange-degeneracy

a+ = CL, yA+B # y/&s.

It can be seen from Tables I-II that the G- universality leads to a clear improvement of the description of all the considered data. Moreover, the G-universality leads to a mild violation of the weak exchange-degeneracy (Q+ - (Y_ N O.l), in contrast with the non-universality cases. These two independent features could hardly be consid- ered as numerical accidents. It is therefore impor- tant to explore the validity of the 2-component G-

universal Pomeron in all the other (non-forward) existing data.

A remarkable result is the fact that the for- ward data clearly favour the maximal Heisenberg- Froissart-Martin en’s behaviour of the hadron scattering amplitude [8]: the absolute mini- mum of x2/dof is precisely obtained for the G-universal en23 form of the amplitude. Our x2/dof is better than that given in the last edi- tion of “Review of Particle Physics” [2].

Let us also note that the dipole model, cor- responding to a en s behaviour of the scatter- ing amplitude, has a serious pathology: the first component of the Pomeron ZAB has a negatiwe contribution to the total cross-sections. There- fore this en s fit has to be dismissed. The above pathological feature of the Cn s model was already remarked in J.R. Cudell et al. [2], but it was omit- ted from the “Review of Particle Physics” [2].

The theoretical and numerical details will be presented elsewhere [3].

Table 2

B. Nicolescu/Nuclear Physics B (Proc. Suppl.) 99 (2001) 47-49

Results of the fits of (TT and p data. The symbol = has the same meaning as in Table 1.

Model

G- exchange

Universality degeneracy

Yes No Yes No

‘u+ a_ N Par x2/W

XS’ X X 0.66f 0.02 0.45f 0.02 16 1.020

X X = 0.48 15 1.320

X X 0.641%0.012 0.440f0.015 17 1.024

z+xse X X = 0.494&0.013 16 1.203

X X 0.602f0.014 0.458f0.016 17 0.986

X X = 0.500f0.013 16 1.092

X X 0.816f0.001 0.450f0.012 16 0.941

2+X&s X X = 0.569f0.001 15 1.769

X X 0.691f0.005 0.465f0.015 16 1.250

X X = 0.592f0.008 15 1.944

X X 0.651f0.017 0.442f0.016 17 1.015

2 + xenss X X = 0.475*0.014 16 1.142

X X