Production of η and η mesons via the quasi-free proton-neutron interaction

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1 Productio of η ad η mesos via the quasi-free roto-eutro iteractio P. Moskal 1, H.-H. Adam, A. Budzaowski, R. Czyżykiewicz, D. Grzoka, M. Jausz, L. Jarczyk, T. Johasso, B. Kamys, A. Khoukaz, K. Kilia, P. Kowia, W. Oelert, C. Piskor-Igatowicz, J. Przerwa, T. Rożek, R. Sato, G. Scheers, T. Sefzick, M. Siemaszko, J. Smyrski, A. Strzałkowski, A. Täscher, P. Witer, M. Wolke 2, P. Wüster ad W. Zier IKP, Forschugszetrum Jülich, D Jülich, Germay IKP, Westfälische Wilhelms Uiversität, D Müster, Germay Istitute of Nuclear Physics, PL Cracow, Polad M. Smoluchowski Istitute of Physics, Jagelloia Uiversity, PL-3-59 Cracow, Polad Usala Uiversity, S Usala, Swede ZEL, Forschugszetrum Jülich, D Jülich, Germay Istitute of Physics, Uiversity of Silesia, PL-4-7 Katowice, Polad Abstract. A comariso of the close-to-threshold total cross sectios for the η meso roductio i both the η ad η reactios should rovide isight ito the flavour-siglet (erhas also ito gluoium) cotet of the η meso ad the relevace of quark-gluo or hadroic degrees of freedom i the creatio rocess. The excitatio fuctio for the reactio η has bee already established. At reset, exerimetal ivestigatios of the quasi-free X reactios are carried out at the COSY-11 facility usig a beam of stochastically cooled rotos ad the deutero cluster target. A method of measuremet ad relimiary results from the test exerimets of the η reactio are reseted i this reort. INTRODUCTION Close-to-threshold roductio of η ad η mesos i the ucleo-ucleo iteractio requires a large mometum trasfer betwee the ucleos ad occur at distaces i the order of.3 fm. This imlies that the quark-gluo degrees of freedom may lay a sigificat role i the roductio dyamics of these mesos. Therefore, additioally to the mechaisms associated with meso exchages it is ossible that the η meso is created from excited glue i the iteractio regio of the collidig ucleos [1, 2], which coule to the η meso directly via its gluoic comoet or through its SU(3)- flavour-siglet admixture. The roductio through the colour-siglet object as suggested i referece [1] is isosi ideedet ad should lead to the same roductio yield of the η meso i the η ad η reactios after correctig for the fial 1 .moskal@fz-juelich.de 2 reset address: The Svedberg Laboratory, Thumbergsvåge 5A, Box 533, S Usala, Swede.

2 ad iitial state iteractio betwee the ucleos. Ivestigatios of the η-meso roductio i collisios of ucleos allowed to coclude that, close to the kiematical threshold, the creatio of η meso from isosi I = exceeds the roductio with I = 1 by about a factor of 12. This was derived from the measured ratio of the total cross sectios for the reactios η ad η σ η (R η σ η ), which was determied to be R η 65 i the excess eergy rage betwee 16 MeV ad 19 MeV [3]. The large differece of the total cross sectio betwee the isosi chaels suggests the domiace of isovector meso (π ad ρ) exchage i the creatio of η i ucleo-ucleo collisios [4, 3]. Sice the quark structure of η ad η mesos is very similar we ca by aalogy to the η meso roductio exect that i the case of domiat isovector meso exchage the ratio R η should also be about 6.5. If, however, the η meso was roduced via its flavour-blid gluoic comoet from the colour siglet glue excited i the iteractio regio, the ratio R η should aroach uity after correctios for the iteractios betwee the articiatig baryos. Figure 1 demostrates qualitatively the fact that the roductio of mesos i the roto-eutro collisios is more robable tha i the roto-roto iteractio if it is drive by the isovector meso exchages oly. This is because i the case of the rotoeutro collisios there are always more ossibilities to realise the exchage or fusio of the isovector mesos tha i the case of the reactio of rotos. π ο ο,ρ,... π ο ο,ρ,... π,ρ, π,ρ,... ρ ρ ρ ρ ρ+ ρ a) b) c) d) a) b) c) η,ω,... η,ω,... ω ω ω ω e) f) d) e) FIGURE 1. (left) Examle of diagrams with the isovector ad isoscalar meso exchage leadig to the creatio of the meso η i the roto-roto ad roto-eutro collisios. (right) Fusio of the virtual ω ad ρ mesos emitted from the collidig ucleos. The close-to-threshold excitatio fuctio for the η reactio has bee already determied [5, 6, 7, 8] whereas the total cross sectio for the η meso roductio i the roto-eutro iteractio remais ukow. As a first ste towards the determiatio of the value of R η the feasibility of the measuremet of the η reactio by meas of the COSY-11 facility was studied usig a Mote-Carlo method [9, 1]. As a secod ste, a test exerimet of the η reactio susected to have by at least a factor of 3 larger cross sectio tha the oe for the η reactio was erformed. I this test measuremet, usig a beam of rotos ad a deutero cluster target, we have rove the ability of the COSY-11 facility to study the quasi-free creatio of mesos via the X reactio. Araisals of simulatios ad relimiary results of the measuremets of the quasi-free η reactio erformed usig the ewly

3 exteded COSY-11 facility [9, 1] will be reseted i the ext sectio. TEST MEASUREMENT OF THE PN PNη REACTION As a geeral commissioig of the exteded COSY-11 facility to ivestigate quasi-free X reactios, we have erformed a measuremet of the η rocess at a beam mometum of 2.75 GeV/c. The exerimet, carried out i Jue 22, had bee receded by the istallatio of a sectator [11] ad eutro detectors, ad by a series of thorough simulatios erformed i order to determie the best coditios for measurig quasi-free η ad η reactios [9, 1]. Figure 2 resets the COSY-11 detectio facility with suerimosed tracks of rotos ad eutro origiatig from the quasi-free X reactio iduced by a roto beam [12] imigig o a deutero target [13]. The idetificatio of the η reactio is based o the measuremet of the four-mometum vectors of the outgoig ucleos ad the η meso is idetified via the missig mass techique. The slow roto stoed i the first layer of the ositio sesitive silico detector (Si sec ) is, i the aalysis, cosidered as a sectator without iteractio with the bombardig article ad it is movig with the Fermi mometum as ossessed at the momet of the collisio. Sisec S4 Si mo DIPOLE V N roto B eutro BEAM VACUUM CHAMBER CLUSTER TARGET EXIT WINDOW D1 D2 S1 S2 roto S3 FIGURE 2. Schematic view of the COSY-11 detectio setu [14]. Oly detectors eeded for the measuremets of the reactio d s ηη are show. D1, D2 deote the drift chambers; S1, S2, S3, S4 ad V the scitillatio detectors; N the eutro detector ad Si mo ad Si sec silico stri detectors to detect elastically scattered ad sectator rotos, resectively. From the measuremet of the mometum vector of the sectator roto s oe ca ifer the mometum vector of the struck eutro s at the time of the reactio ad hece calculate the total eergy of the collidig ucleos for each evet. I the

4 couts Exerimet T sec [ MeV ] couts Mote-Carlo T sec [ MeV ] FIGURE 3. Distributios of the kietic eergy of the sectator rotos. (a) Exerimet, (b) Mote-Carlo simulatios takig ito accout the accetace of the COSY-11 detectio system ad a aalytical arametrizatio of the deutero wave fuctio [15] calculated from the PARIS otetial [16]. aroximatio that the struck eutro is treated as a free article we ca assume that the matrix elemet for quasi free meso roductio off a boud eutro is idetical to that for the free Meso reactio 3. I figures 3a ad 3b the measured ad exected distributio of the kietic eergy of the sectator roto is reseted. Though still very rough eergy calibratio of the detector uits oe recogizes a substatial similarity i the shae of both distributios Mote-Carlo couts 15 Exerimet couts Q cm [ MeV ] Q cm [ MeV ] FIGURE 4. Distributios of the excess eergy Q CM for the quasi-free X reactio, determied with resect to the η threshold. (a) Exerimet. (b) Simulatio. Figure 4 shows sectra of the excess eergy i resect to the η system as obtaied i the exerimet (4a) ad the simulatio (4b) for the η reactio. The remarkable differece betwee the distributios comes from the fact that i reality additioally to the η reactio also the multi-io roductio is registered. The η ad multiio roductio caot be distiguished from each other o the evet-by-evet basis by 3 For more comrehesive discussio of this issue the reader is referred to referece [17]

5 couts X above threshold of η below threshold of η couts η SIMULATION EXPERIMENT PRLIMINARY m X [ GeV / c 2 ] m X [ GeV / c 2 ] FIGURE 5. Missig mass sectra as obtaied durig the Jue 2 ru: a) Evet distributio for Q ( black lie) ad for Q (gray lie). b) Histogram reresets the differece betwee umber of evets above ad below threshold for the η reactio, ad the lie corresods to the Mote-Carlo simulatio. meas of the missig mass techique. However, we ca determie the umber of the registered η reactios from the multi-io backgroud comarig the missig mass distributios for Q values larger ad smaller tha zero. Kowig that egative values of Q ca oly be assiged to the multi-io evets we ca derive the shae of the missig mass ditributio corresodig to these evets. This is show as the solid lie i figure 5. A thorough evaluatio of the backgroud is i rogress, however, rough comariso of evets for ositive ad egative Q yields the romissig results with a clear sigal from the η reactios, as ca be deduced by isectio of figures 5a ad 5b. REFERENCES 1. S. D. Bass, Phys. Lett. B 463 (1999) S. D. Bass, e-prit Archive: he h/ H. Calé et al., Phys. Rev. C 58 (1998) G. Faldt, T. Johasso, C. Wilki, Phys. Scrita T 99 (22) P. Moskal et al., Phys. Rev. Lett. 8 (1998) P. Moskal et al., Phys. Lett. B 474 (2) F. Hibou et al., Phys. Lett. B 438 (1998) F. Balestra et al., Phys. Lett. B 491 (2) P. Moskal, Schrifte des FZ-Jülich: Matter & Material 11 (22) 27; e-prit Archive: ucl-ex/ R. Czyżykiewicz, Diloma Thesis, Jagelloia Uiversity (22), Berichte des FZ-Jülich, Jül R. Bilger et al., Nucl. Istr. & Meth. A 457 (21) D. Prasuh et al., Nucl. Istr. & Meth. A 441 (2) H. Dombrowski et al., Nucl. Istr. & Meth. A 386 (1997) S. Brauksiee et al., Nucl. Istr. & Meth. A 376 (1996) M. Lacombe et al., Phys. Lett. 11B (1981) M. Lacombe et al., Phys. Rev. C 21 (198) P. Moskal, M. Wolke, A. Khoukaz, W. Oelert, Prog. Part. Nucl. Phys. 49 (22) 1, he-h/282.