EXOTIC EXCHANGE

(1)

HAL Id: jpa-00215220

https://hal.archives-ouvertes.fr/jpa-00215220

Submitted on 1 Jan 1973

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EXOTIC EXCHANGE

C. Akerlof

To cite this version:

C. Akerlof. EXOTIC EXCHANGE. Journal de Physique Colloques, 1973, 34 (C1), pp.C1-303-C1-304.

�10.1051/jphyscol:1973140�. �jpa-00215220�

(2)

E X O T I C EXCHANGE

G.W. AKERLOF

Physics Department, University of Michigan, Ann Arbor

Up u n t i l now t h e idea of e x o t i c exchange has been regarded a s perhaps somewhat immoral by t h e physics community and thus f a r few experimental groups have been tempted t o explore a l a r g e c l a s s of reactions so studiously ignored by t h e o r e t i c i a n s . I hope t o e l e v a t e the standing of those e x o t i c exchange chan- n e l s by squashing some of t h e previously held supers- t i t i o n s and w i l l t r y t o show t h a t new data has some relevance t o our models of normal exchange processes.

As of l a s t year t h e most s i g n i f i c a n t data on e x o t i c exchanges had been measured by Chanbaud e t a l . [ I ] on t h e channels

detecting these r e a c t i o n s not l e a s t of which i s t h e smallness of t h e cross sections. For counter experi- ments where t h e E- i s not seen, a d d i t i o n a l problems a r i s e because a missing mass technique must revolve a f from t h e t h r e e body i n e l a s t i c threshold only 58 MeV away.

Figure 1 shows t h e missing mass spectrum f o r n-p + K'X- a t 5 GeV/c. The presence of a c l e a r f

s i g n a l i s seen, adjacent t o the rapidly r i s i n g i n e - l a s t i c continuum.

and by Atherton e t a l . [2] who found evidence f o r - ²¹⁰⁰

t h e r e a c t i o n pp ^-t^X-E-. Reference [ I ] , combined with w data from lower energies, indicated t h a t a t 5 GeV/c

t h e t h r e e e x o t i c baryon exchange r e a c t i o n s were continuing t o f a l l rapidly with energy, with something l i k e an s-' dependence. The l a t t e r work [2] demons- t r a t e d a much l e s s severe energy dependence of t h e

- ⁴

order of s f o r a r e a c t i o n which could proceed by e x o t i c exchange.

Two contributions t o t h i s Conference provide data f o r another s e t of reactions, which would be induced by e x o t i c meson exchange. Our group a t t h e Universi- t y of Michigan [3] has explored t h e energy dependence of t h e d i f f e r e n t i a l cross s e c t i o n s f o r t h e r e a c t i o n s

n-p + K'E-

+ -

K-p ^-t K C

from 1.9 t o 5.0 GeV/c while t h e Amsterdam-Nijmegen group [4] have submitted data a t 4.25 GeV/c on t h e r e a c t i o n s

C -

K - ~ ⁺ n~

+ ^{m -}

and K-p + n Y

There a r e a number of experimental problems i n

P x - I

Threshold

z-

The energy dependence of t h i s r e a c t i o n i s p l o t t e d i n f i g u r e 2. There i s a marked change i n t h e slope of t h e data a s t h e energy i n c r e a s e s from 1.9 t o 5.0 GeV/c. We a t t r i b u t e t h e rapid cross s e c t i o n decrease a t low energy t o t h e diminishing s t r e n g t h of s- channel resonances-an e f f e c t observed i n such d i -

- + -

v e r s e r e a c t i o n s such a s n p -, cpn, pp -t K n and K-p + pK-. Above 3 GeV/c the cross s e c t i o n drops much more slowly, a s would be expected from the gra-

dual domination of exchange forces.

I f t h e cross s e c t i o n i s described by an s-" beha- viour, then between 4 and 5 GeV/c t h e n-p

-

^K+=-

data has a n value between 1 and 2 with a standard

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973140

(3)

C1-304 C.W. AKERLOF

error of 2 1 . This must be compared with Regge cut The Amsterdam-Nijmegen point at 4.25 GeV/c agrees models which expect an n value between 2.5 and 3.0. within the large statistical errors with our point

f -

at 4 GeV. By extrapolation the K - ~ + K E and K - ~ + K+=- cross sections will interest around 6 GeV/c, consistent with the notion that exchange forces dominate above 5 GeV/c at a level of about 80 nanobarns/GeV 2

.

The Amsterdam-Ni jmegen K - ~

-

^rrff

data also indicates that the f angular distribution is significantly broader than would be expected from a Regge cut model.

- + - ^.

In sununary we see that the K p -t K c ^dlfferen-

tial cross section exhibits the symptoms of beco- ming dominated by exchange forces at high energies.

The nature of the exchange force is not yet clear but it is quite possible that the asymptotic energy

- 2

dependence may be like s which would rule out a Regge cut phenomenology. The apparent smoothness of the energy dependence also should not encourage those favouring statistical models.

The smallness of the exotic differential cross sections should be emphasized. At 5.0 GeV the ratio The K - ~ + ?f data is shown in figure 3. The data _4--

of the forbidden K - ~ + K E reaction to the allowed appears to have a shoulder around 3 GeV/c before _K+ _p

-r K+E+ reaction is 0.0002. This number is not continuing its steep decrease at high energies.

easily reconcilable with any current phenomenology but at least presents the hope that some ultimately successful theoretical model of allowed reactions will contain only small exotic exchange amplitudes.

Finally the results at the Amsterdam-Nijmegen group for the reaction rt-p + K+Y*- (1385) show that no selection rule operates to favour strongly the production of baryon decuplet rather than baryon octets. This should lay to rest an idea proposed by Dahl et al. [5] some years ago.

REFERENCES

[I] V. CHABAUD et al., Phys. Letters 38B (1972) 445- [3] C.W. AKERLOF et al., paper 370

449 ; 41B (1972) 209 [4] Amsterdam-Nijmegen collaboration, paper 225 [2] H.W:ATHERTON et al., Phys. Letters 42B (1972) [5] 0.1. DAHL et al., Phys. W v . 163 (1967) 1430

522