Excitation of the (1232) isobar in deuteron charge exchange on hydrogen at 1.6, 1.8 and 2.3 GeV

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1 Excitation of the (13) isobar in euteron charge exchange on hyrogen at 1.6, 1.8 an.3 GeV Davi Mchelishvili for the ANKE collaboration High Energy Physics Institute, Tbilisi State University, 186 Tbilisi, Georgia Institut für Kernhysik, Forschungszentrum Jülich, D-545 Jülich, Germany Deuteron charge-exchange break-u {}n, where the final {} iroton system is at very low excitation energy an hence in the 1 S state, is a owerful tool to robe the sin-fli terms in the roton-neutron charge-exchange reaction. Recent measurements with the ANKE sectrometer at the COSY storage ring at 1.6, 1.8, an.3 GeV have extene this stuy into the ion-rouction regime in orer to investigate the excitation of the (13) isobar in the {} reaction. Values of the ifferential cross section an two euteron tensor analysing owers, A xx an A yy, have been extracte in terms of the iroton rouction angle or invariant mass. These ata can be interrete in terms of the sin-longituinal or sin-transverse contributions to the elementary n rocess. The results resente are comare to those obtaine PoS(STORI11)4 with the SPES-4 sectrometer at Saclay at GeV, where only a single combination of A xx an A yy was measure. 8th International Conference on Nuclear Physics at Storage Rings-Stori11, October 9-14, 11 INFN, Laboratori Nazionali i Frascati, Italy Seaker. c Coyright owne by the author(s) uner the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. htt://os.sissa.it/

2 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili 1. Introuction A goo unerstaning of the Nucleon Nucleon interaction (NN) remains one of the most imortant goals of nuclear an haronic hysics. Aart from their intrinsic imortance for the stuy of nuclear forces, NN ata are necessary ingreients in the moeling of meson rouction an other nuclear reactions at intermeiate energies. It was emhasise many years ago that quasi free (,n) or (n, ) reactions on the euteron can act, in suitable kinematic regions, as a sin filter that selects the sin eenent contribution to the n elastic cross section [1]. The comarison of this reaction with free backwar elastic scattering on a nucleon target might allow a irect reconstruction of the n backwar amlitues []. Theory suggeste that much information on the n charge-exchange amlitues coul be extracte by stuying the euteron charge-exchange break-u reaction, {}X. Two channels are of interest here: X = n an X =. By selecting the two final rotons with low excitation energy, tyically E < 3 MeV, the emerging iroton is ominantly in the 1 S state. In imulse aroximation these reactions can be consiere as n n or n scattering with a sectator roton. The imulse aroximation moel [3] has been imlemente in etail for the neutron channel (Fig. 1) an it can reict analysing owers, sin correlation coefficients an cross section for this reaction [4]. In the 1 S limit, the {} s n reaction observables are irectly relate to the n sin eenent amlitues [3]. n n n + π π N PoS(STORI11)4 Figure 1: Deuteron charge exchange break-u iagram for the neutron channel. Figure : The simlest imlementation of irect rouction in the euteron charge exchange break-u reaction. Since, the SAID n ata base has significant ambiguities above 8 MeV nucleon energy [5], the euteron charge exchange break-u reaction with low excite iroton system becomes a owerful tool to robe the sin-fli terms in the roton-neutron charge exchange reaction. The ANKE collaboration is involve in the measurement of the ifferential cross section, analysing owers, an sin correlation coefficients of the euteron charge exchange break-u reaction, {} s n. The aim is to euce the energy eenence of the sin eenent n elastic amlitues. The methoology has been checke at T = 1.17 GeV euteron beam energy where the n amlitues are reasonably well known [6]. The results resente there are in a goo agreement with imulse aroximation reictions. The success of this technique encourages its alication at higher energies, where more recise n ata are neee.

3 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili However, recent measurements at ANKE/COSY at high energies clearly show the ossibility to exten this stuy into the ion-rouction regime in orer to investigate the excitation of the (13) isobar. It was emonstrate many years ago at Saclay that at T =. GeV the (13) isobar can be excite in the charge exchange reaction {} [7]. The simlest interretation of irect rouction through a one-ion-exchange mechanism is shown in Fig.. Within this framework, such measurements woul correson to a sin transfer from the initial neutron to final roton in the n rocess, an this woul give valuable information about the sin structure in the excitation of the isobar.. The exerimental setu Two exeriments have been erforme at the COoler SYnchrotron (COSY) of the Forschungszentrum Jülich using olarise euteron beams at T = 1., 1.6, 1.8 (in 5) an 1.,.7 GeV (in 6) an unolarise hyrogen cluster target. This machine is caable of accelerating an storing olarise an unolarise rotons an euterons with momenta u to 3.7 GeV/c. The forwar art (FD) of the ANKE magnetic sectrometer [8] is use for the euteron charge exchange reaction stuies. The etaile escrition of the FD an the reaction ientification roceure can be foun in Ref. [9]. 3. Deuteron beam olarimetry The first ste when stuying the sin observables of the charge exchange reaction is to establish the olarimetry stanars using the scattering asymmetries in a suitable nuclear reaction with known analysing owers. Polarisation calibration stanars escribe in the revious stuy [1] are few an exist only at iscrete energies. But, if one avois eolarising resonances in the machine, the beam olarisation can be conserve when raming u or own the beam energy [11]. Since there are no eolarising resonances for euterons in the COSY energy region, this makes things easier. This olarisation exort technique, which has been checke in ractice [9], is a useful tool for the olarisation exeriments at any available energy at COSY. The ata on T = 1.6 GeV, 1.8 GeV an.7 GeV energy were taken using a COSY suer cycle that inclue the T = 1. GeV flat-to to rovie the calibration stanar. The following reactions were use in our analysis in orer to etermine the olarisation of the euteron beam at T = 1. GeV, where the analysing owers are well known: quasi free n π for the vector comonent (P z ) an {}n for the tensor (P zz ) comonent. In orer to minimise systematic errors, several configurations of the ion source (with ifferent vector an tensor olarisations) were emloye an the beam olarisation ha to be etermine searately for each state. In orer to achieve this, the relative luminosities C n of each state with resect to the unolarise moe ha to be establishe so that one coul then use: PoS(STORI11)4 N ol /N = C n [ P zz [A xx (q)(1 cosϕ) + A yy (q)(1 + cosϕ)] ], (3.1) where N ol an N are the numbers of olarise an unolarise counts, resectively. Details on the count calibration an the full roceure for the beam olarisation etermination can be foun in Ref. [9]. 3

4 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili 4. Luminosity etermination The cross section etermination requires a recise normalisation to obtain absolute values. Generally, the luminosity of the exeriment can be fixe using any reaction with a well known cross section. Current analyses use the quasi-free n π reaction for this urose since it is clearly ientifie at ANKE forwar etector. Furthermore, the cross section of the π + rocess is known from SAID [5] an this is larger than that for n π by an isosin factor of two. An aitional avantage of this reaction is that the shaowing effect in the euteron (where one nucleon hies behin the other) largely cancels out between the {}X an s π reactions. The count rates of the reaction nees corrections for several factors, such as DAQ ea time, track reconstruction an roortional chamber efficiency, etc., but the most imortant one is the etector geometric accetance. A Monte Carlo simulation was use at all energies to estimate the geometric accetance of the ANKE forwar etector an make aroriate corrections. 5. Results 5.1 Differential cross section The missing mass sectra of the {}X at three ifferent energies are resente in Fig. 3 (note: for clarity of resentation the high mass region is scale by factor of eight). At higher M x, above the πn threshol, there is a lot of strength that must be associate with the rouction of a single ion. It is therefore temting to interret the ata in a form that is comletely analogous to that use for the {} s n case. For examle, if for simlicity one assumes one-ionexchange then, for the excitation of the (13) isobar, we are looking rather at the iagram of Fig.. It shoul be note that this inclues the same triangle loo integration at the bottom as for the {} s n reaction, i.e., it eens on the same tye of {} s form factor. However, if we take a simle one-ion-exchange moel for the n amlitue (we use the one of Dmitriev, quote in [1]), the shae of the corresoning cross section reictions is wrong at low M x, as can be seen in Fig. 4. There is some flexibility with the normalisation, because of uncertainty in the vertex functions but, if the moel is ajuste to fit on the right, it is too low on the left. This roblem is, of course, much more general than Dmitriev s imlementation of the moel. Since the is a -wave πn resonance, there can be little strength at low mass. Exactly the same roblem was note in the ioneering exeriments at Saclay [1], where the one-ion-exchange reiction also agrees with the ata at high M x but vastly unerestimate them at low M x. However, at Saclay they also measure the same reaction with a euterium target. It shoul be note here that the cross section for n {} s shoul be three times bigger than that for {} s. After taking shaowing into account, the authors ivie their euterium target ata by a factor of four to comare with the hyrogen ata. This works very well inee at high M x but fails miserably near threshol. This means that the excess of events at low M x must be mainly associate with isosin I = 1 so they are not comatible with the irect rouction as shown in Fig.. In an attemt to maintain this aroach, an attemt has been mae to estimate the s-wave πn contribution to irect rouction. For this urose, Dmitriev s moel reictions were moifie in PoS(STORI11)4 4

5 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili Number of events 4 a) T = 1.6 GeV )] σ/m [µb/(mev/c.4. a) T = 1.6 GeV b) T = 1.8 GeV c) T =.7 GeV [GeV/c Figure 3: The missing mass M x istribution for the reaction {} S X at three euteron beam energies. In aition to the neutron eak, one sees clear evience for the excitation of the isobar. the following way: M X ( ) σ m s wave ] b) T = 1.8 GeV c) T =.7 GeV M x [GeV/c ] Figure 4: Differential cross section for the {}X reaction for M x > M N + M π at three euteron beam energies. Curves correson to one-ion-exchange reictions [1] ( ) σ σ(s 11) + σ(s 31 ) m wave σ(p 33 ) (5.1) where σ(s 11 ), σ(s 31 ) an σ(p 33 ) are SAID reictions for πn elastic scattering, an an are the real an virtual ion momenta, resectively. As shown in Fig. 5, it gives a small extra strength at low M x. This s-wave contribution woul have to be increase by orers of magnitue to agree with the ata. The roblem that we are face with here is very analogous to the search for the excitation of the I = 1 Roer resonance in inclusive X or α αx measurements [13]. Although the X state here must have I = 1, it oes not nee to be a N resonance. These measurements show the largest strength at very low values of M x, with only a small enhancement connecte with the N (144). The ominant backgroun is connecte with the ossibility of exciting the (13) insie the rojectile or α, as mentione in Ref. [13]. This means that the ion an nucleon that make u the state X are rouce at ifferent vertices. The corresoning iagram for the {} s X reaction is shown in Fig. 6. The only real ifference between this an the stanar imulse aroximation of Fig. is an interchange of the two final nucleons, which means that the evaluation of the corresoning amlitues require the same basic inut. The evaluation of the cross section an analysing owers PoS(STORI11)4 5

6 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili ] σ/m [µb/(mev/c π (π ) n () π (π - ) [(GeV/c) Figure 5: Differential cross section reictions for the {} reaction at T =.7 GeV. Simle estimation of s-wave contribution (ashe) using SAID amlitues gives little aitional effect over the -wave (soli). M X ] n Figure 6: excitation in the incient euteron. This may be the ominant mechanism at the low M x. for this mechanism is currently in rogress. Note that the state X here no longer has to have isosin I = 3 because it oes not come from the ecay of the. 5. Tensor analysing owers The fact that we have two ifferent mass regions, where ifferent mechanisms are ominant, is also reflecte in the tensor analysing ower behaviour shown in Fig. 7. Here the sum an ifference of euteron Cartesian tensor analysing owers A xx an A yy are resente as functions of the missing mass M x. [These quantities are roortional to the sherical tensor comonents T an T.] The first thing to note is the minimum in A xx + A yy for M x 1.15 GeV/c. This is recisely the region where there is the biggest iscreancy with the cross section reictions in Fig. 4. The secon oint to notice is that the values of A xx + A yy are remarkably stable an seem to show a universal behaviour, ineenent of beam energy. Hence, whatever the mechanism is riving the reaction, it seems to be similar at all energies. It shoul be note that the comarison between ANKE results for the tensor analysing owers with Saturne ata shown in Ref. [9] is encouraging. Until the relative contributions of the two riving mechanisms (an their ossible interferences) is sorte out, one can only assume that at high M x the irect rouction ominates. These are shown in Fig. 8 as a function of the transverse momentum transfer q t. In the forwar irection, q t = an one must then have A xx = A yy because there is no way of searating the x an y irections. The behaviour of both observables is similar at all three energies. However, it is imortant to note the ifferences from the charge-exchange with neutron channel: the signs are oosite to those of the {} s n reaction [14] an they ten to be very small at q t =. These will rove to be valuable constraints on the moelling of the n amlitues, once we have ientifie the relative contributions of the two riving mechanisms. PoS(STORI11)4 6

7 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili + A yy )/ xx (A.4. T = 1.6 GeV T = 1.8 GeV T =.7 GeV Analysing owers A yy A xx a) T = 1.6 GeV.5 A yy b) T = 1.8 GeV -. - A yy A xx M x [GeV/c ] Figure 7: The sum an ifference of the Cartesian tensor analysing owers at ifferent beam energies. 6. Summary an outlook A xx A yy A xx c) T =.7 GeV q [MeV/c] t Figure 8: A xx an A yy tensor analysing owers at three euteron beam energies. Only high mass ata (1.19 < M x < 1.35 GeV/c ) are use. ANKE ata on euteron charge exchange allows one to investigate the {}X reaction in region. In the simlest interretation these measurements woul correson to the sin transfer from an initial neutron to a final roton in the elementary n rocess. PoS(STORI11)4 Theoretical work is neee to quantify the secon contributory mechanism (Fig. 6). A large amount of ata was successfully obtaine from the first ouble olarise n scattering exeriment at T =.7 GeV at ANKE [15]. It will be use for sin-correlation stuies. The rouction will also be stuie in the near future in the {} channel at energies u to T =.88 GeV by using a olarise euterium target. Acknowlegements I am grateful to other members of the ANKE Collaboration for their hel with the exeriments. This work has been suorte by the JCHP FFE, an the Shota Rustaveli National Science Founation (SRNSF grant ). 7

8 {}s at T = 1.6, 1.8 an.7 GeV Davi Mchelishvili References [1] N. W. Dean, Symmetrization Effects in Sectator Momentum Distributions; Phys. Rev. D 5, 1661 (197). N. W. Dean Inelastic Scattering from Deuterium in the Imulse Aroximation; Phys. Rev. D 5, 83 (197). [] F. Lehar an C. Wilkin, Nucleon charge exchange on the euteron: A critical review; Eur. Phys. J. A 37, 143 (8). [3] D. V. Bugg an C. Wilkin, Polarisation in the (, ) reaction at intermeiate energies; Nucl. Phys. A 467, 575 (1987). [4] J. Carbonell, M. B. Barbaro an C. Wilkin, Deuteron analysing owers in the charge exchange reaction ()n; Nucl. Phys. A 59, 653 (1991). [5] R. A. Arnt, I. I. Strakovsky an R. L. Workman, Nucleon-nucleon elastic scattering to 3 GeV; Phys. Rev. C 6, 345 (). htt://gwac.hys.gwu.eu [6] D. Chilaze et al., The n reaction as a metho to stuy neutron-roton charge-exchange amlitues; Eur. Phys. J. A 4, 3 (9). [7] C. Ellegaar et al., Sin structure of the excitation; Phys. Lett. B 31, 365 (1989). [8] S. Barsov et al., ANKE, a new facility for meium energy haron hysics at COSY-Jülich; Nucl. Instr. Methos Phys. Res. A 46, 364 (1). [9] D. Mchelishvili an D. Chilaze, Recent results from the euteron charge exchange on hyrogen rogramme at ANKE/COSY; J. Phys.: Conf. Ser. 95, 199 (11). [1] D. Chilaze et al., Determination of euteron beam olarizations at COSY; Phys. Rev. ST Accel. Beams 9, 511 (6). [11] R. E. Pollock et al., Calibration of the olarization of a beam of arbitrary energy in a storage ring; Phys. Rev. E 55, 766 (1997). [1] C. Ellegaar et al., The (3He,t) ++ reaction; Phys. Lett. B 154, 11 (1985). [13] P. Fernánez e Córoba et al., Projectile elta excitation in alha-roton scattering; Nucl. Phys. A 586, 586 (1995). [14] D. Chilaze et al., Vector an tensor analysing owers in euteron roton breaku reactions at intermeiate energies; Phys. Lett. B 637, 17 (6). [15] A. Kacharava et al., Measurement of the {}n charge exchange reaction with olarise beam an target; COSY roosal 17 (7). PoS(STORI11)4 8