POLARIZATION IN K-p INTERACTIONS

HAL Id: jpa-00224549

https://hal.archives-ouvertes.fr/jpa-00224549

Submitted on 1 Jan 1985

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished 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.

POLARIZATION IN K-p INTERACTIONS

T. Haupt

To cite this version:

T. Haupt. POLARIZATION IN K-p INTERACTIONS. Journal de Physique Colloques, 1985, 46

(C2), pp.C2-295-C2-301. �10.1051/jphyscol:1985235�. �jpa-00224549�

JOURNAL DE PHYSIQUE

Colloque C2, supplement au n°2, Tome 46, fevrier 1985

page C2-295

POLARIZATION IN K P INTERACTIONS

Aachen-Berlin-Bombay-CERN-Cracow-Innsbruck-Jammu-London-Vienna-Warsaw Collaboration

Presented by : T. Haupt

Institute of Nuclear Physics, Cracow, Poland

Résumé - La polarisation des hypérons Ap, mesurée dans une expérience portant sur la réaction K~p à 110 GeV/c croît avec p^ dans l'hémisphère du proton, mais ne dépend guère de Xp. Les résultats sont comparés avec ceux d'autres expériences : K~p à plus basse énergie, ainsi que pp, pp, irip et K+p.

Abstract - The analysis of the polarization of A° hyperons produced in K p interactions at 110 GeV/c shows a strong dependence of the polarization on the transverse momentum of A" in the proton hemisphere, but no significant depen- dence on xF. These results are compared with those obtained in other K~

experiments at lower energies as well as with the polarization from the re- actions induced by p, p, ir- and K+.

The data presented in this paper come from the exposure of BEBC to a tagged 110 GeV/c RF separated negative beam (mostly K~) from the CERN SPS with subsequent measurement and geometrical reconstruction of tracks in the events . No particle identification was made except for slow protons and for V 's, where the selection of events

(1) was based on the kinematical fits of V . After standard corrections, these procedures yielded a sample of 985 unambiguous A 's (the overall microbarn equivalent was 0.39 ev/ub).

The polarization P was measured by using the mean value of the

o . -*

cosine of the angle 9 between the normal to the A production plane z and the decay proton momentum in the A rest frame:

(2) where z is defined as

(3)

and a is the asymmetry parameter for the A -» pir decay (o = 0.65).

As in most experiments, the polarization values are underestimated as it was not possible to distinguish between the directly produced lambdas Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985235

C2-296

JOURNAL

DE

PHYSIQUE

and those arising from the decay of E O or baryon resonances (E(1385).

. . ,

etc.).

Before the presentation of the h0 polarization data it should be pointed out (as s h o w earlier2)) that there is a significant difference between the polarization of A0 produced in the following channels:

K-p + A O

+

pions K - ~ + A O K E

+

^pions

i.e. in the hypercharge annihilation and associated production channels, respectively (fig. 1). The relative yields of these channels change with energy. At low energies the A0 inclusive cross section is dominated by the hypercharge annihilation channel (4). while at 110 GeV/c it dies off leaving the associated production channel (5) as

Fig. 1 A0 polarization in the hypercharge annihilation (0) and associated production (W) channels at 4.2 GeV/c (from ref. 2).

dominant (c.f. fig. 2). Moreover, in the channel ( 5 ) , h0 are produced predominantly in the proton hemisphere (fig. 31 and constitute almost the whole backward sample at 110 GeV/c. As a consequence, in order to compare the data with those obtained at lower energies the do

polarization data is examined in the three following subprocesses:

Fig. 2 The energy dependence of the relative yield of the associated production channel in the K-p interactions. The hand-drawn line is to guide the eye.

Fig.

3

Invariant

xp

distribution

of

A" and A" with seen K o at 110

GeV/c

(from ref.

1).

(a) Hypercharge annihilation in the proton hemisphere K - ~

+

' A + ^pions - low energy data only (cf. (4)) (b) Association production in the proton hemisphere

KWp

9

A ' K ~ + ^pions at 4.2 GeV/c

KWp

+

A0Kzeen + anything at 10, 16 and 32 GeV/c K - ~

9

h0 + anything at 110 GeV/c (cf.

_{( 5 ) )}

( c )

Inclusive h0 production in the kaon hemisphere

K-p

+

,lo

t

anything at

3.93

- ^{110 GeV/c}

(in the kaon hemisphere no data on separated channels are available except at 4.2 GeV/c).

The results on A0 polarization at 110 GeVlc are presented in

table

1 and compared with the other datas) in figs 4-6.

A,

strong dependence of the polarization on the transverse momentum pT of

AO

is observed. The absolute value of the polarization increases with increasing pT. The polarization is negative for large pT in the proton hemisphere and positive (with a large statistical error) for large p in the kaon

T

hemisphere. Within statistical errors, no significant dependence on

xF = 2p / f E

is observed, where pL is the longitudinal momentum of A and

L

the total energy available in c.m.s.

JOURNAL

DE

PHYSIQUE

TABLE 1 Polarization at 110 GeV/c

Error

GeV/c I ^{P Error}

In fig.

4

the results in the proton hemisphere are compared with other K-p data obtained at lower energies. For the associated

production channel

1 5 )

the data show a clear increase in negative polarization with increasing pT, independent of the primary energy

(fig. 4(a)). For the hypercharge annihilation channel (4) the situation is different: the 4.2 GeV/c data show a definite increase in positive polarization with increasing pT, while the 10 and 16 GeV/c data are consistent with zero (fig. 4(b)).

PROTON HEMISPHERE

K'p--

AOK~?: +

anything K - p d A O

+

pions

1.0

0.8 0.6 0.4

a l r [ l i i l

-

-

4

la) ^-

^- _-

- - -

-0.8

- -

-1.0

A

-

+ +

^-

+

¹

y + ' t t _- -

-

-

- -

I I I I I I I

I I I I I ( I I * =

- -

lb)

-

-

- -

a

3

z 0 0.4 0.8 1.2 1.6 0 0.4 0.8 1.2 1.6 ~ i4 p o l a r i z a t i o n of ~ . 0 I-

a p~ l G e V / c ) produced i n t h e proton he-

E misphere i n t h e K-p i n t e r -

LL

4

1.0 a c t i o n s a t 3 . 9 3 ( x ) , 4.2 ( w ) ,

o n 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0

-1.0 -0.8 -0 6 -0.4 -0.2 0 -1.0 -0.8 -0.6 -0.4 -0.2 0

XF XF

I I I I l I I [ I

-

(d) -

It ^-

-i**i. .+

⁺ _,

-

( w o ( o o t ' - c h a n n e l

-

10 and 16 (l)

,

32

(v)

^and

110 (A) (GeV/c) a s a f u n c t i o n - o f pT i n t h e channel (5)

( f i g . 4 ( a ) ) and t h e channel - ( 4 ) (fig. 4 ( b ) ) , and a s a

f u n c t i o n o f x ~ i n t h e chan-

.-riel

(5) ( f i g . (4) ( f i g . 4 ( c ) ) 4 ( d ) ) . and t h e

-

-

- -

I I I I I I I I .

In fig. 4(c.d) the AO polarization is presented as a function of xF in the proton hemisphere. In spite of the large scatter of experimental points, the following conclusions emerge. For small Ix

I

values, say lxFl < 0.6, the polarization is consistent with

F

zero in both channels (4.5) while for large

I x I

region it approaches F

small negative values in channel (5) and positive values in channel (4).

Fig. 5 shows the polarization o f A0 produced in the reactions induced by p,

p,

nf and K+ in the proton fragmentation regiong). Both the pT

-

(fig. 5(a)) and xF - (fig. S(b)) dependences of the polarization seem to be consistent with those observed in channel (5) in the K - ~ interactions (except for the 6 and 19 GeV/c pp data in fig. 51b) which show a large positive polarization for I x

I

> 0.8).

F

F ~ R . 5 Polarization of A0 produced in the proton hemisphere

in

the reactions induced by non-strange projectiles and K+ as a function of p~ (fig. 5(a)) and x~ (fig. 5(b)). The hand-drawn solid lines indicate the trend of the K-p data as it follows from figs 4Ca.c).

PROTON HEMISPHERE p.jj<n:n;

K- INDUCED

REACTONS

o pp 6 GeV/c e pp 19 "

0 PP 2 4

;;

P P 28 o K'p ₁₃ "

v K'p 32 "

A K ' p 7 0 "

fr n'p 18.5 "

NON EXOTIC p i _{3.6 &} 5.7

rn pp 12 GeV/c

A n - p 15 "

*

^{n-p 18.5 "}

0.8 0.4

4 0

dl

^{0.8 1.2 ;4 1.8}

^{* n-P}

1004200.300

!Y m PT (GeV/c)

z!

a d a

-1.0 -0.8 -0.6 -0.4 -0.2 0

XF

I I I I

- -

- -

-

-

-

- -

\ ^-

z -0.8 - -

0 I

I-

JOURNAL DE PHYSIQUE

In the K- hemisphere the presence of the s-quark in the projectile suggest the possibility of a polarization different from that in the proton hemisphere. Indeed, some differences are observed. An indication of an energy dependence may be seen in fig. 6(a) which shows the polarization plotted against pT for the kaon hemisphere. For large pT, say pT > 0.4, the polarization is negative at low energies while at 110 GeV/c the data give a positive value (though with large statistical uncertainty). The xF dependence of the polarization in this hemisphere is presented in fig. 6(b). An increase in negative polarization with increasing xF is observed. The absolute values of polarization are greater than those in the proton hemisphere for corresponding xF intervals (cf. figs. 4(c,d) and 6(b)).

KAON HEMISPHERE K-p--, Ao+anything

Fit?. 6 Polarization of A0 produced in the kaon hemisphere in the K-p interactions at 3.93-110 GeV/c as a function of p~ (fig. 6(a)) and xp (fig. 6(b)).

The observations of the present experiment combined with the compilation of the previous data can be sununari-zed as follows. In the associated production channel ( 5 ) in the proton hemisphere, the data show a strong p -dependence, a weak x -dependence and no energy

T F

dependence of the AO polarization. This behaviour is analogous to that observed in the reactions induced by non-strange projectiles (or K

' )

. In the hypercharge annihilation channel ( 4 ) in the proton hemisphere the polarization has an opposite sign to that in channel

( 5 ) . Finally, in the

K-

hemisphere an indication appears for an energy dependence of the polarization for the large pT region and for a clear x -dependence. The absolute values of the polarization are

F

greater than in the proton hemisphere for corresponding xF intervals.

A

fuller account of the experiment, as well as a comparison with theoretical models. will be aublished elsewhere4).

References

1) P.R.S. Wright et al., Nucl. Phys. 8189 (1981) 421.

2 ) S.N. Ganguli et al., N. Cim. 4 4 A (1978) 345.

3) S.N. Ganguli and D.P. Roy, Phys. Rep. 67 (1980) 201 and refs in

it.

4)

T.

Haupt et al., Zeit. fiir Phys. C, in the press.