In the experiments at the Mainz Microtron, MAMI, ∆0 particles in the electron scattering reactions 12C (e, e ′∆0 ) 11C have been produced in nuclear medium. The three high-resolution magnetic spectrometers of the A1 collaboration have been used in triple coincidence measurements 12C (e, e ′p π −) 11C. Indications for substantial changes of delta properties in the nuclear medium have been obtained. Because of low statistics, these results cannot yet be taken as conclusive. A new setup for measurements at MAMI with silicon detectors for proton detection, which subtend larger solid angle, is described.U eksperimentima s mikrotronom u Mainzu, MAMI, proizvodili smo u nuklearnoj sredini čestice ∆0 reakcijom 12C (e, e ′∆0 ) 11C. U okviru suradnje A1, rabili smo tri magnetska spektrometra visokog razlučivanja za trosudesna mjerenja 12C (e, e ′p π −) 11C. Podaci ukazuju na promjene svojstava delta čestica u nuklearnoj sredini. Zbog slabe statistike, ti se ishodi još ne mogu smatrati konačnima. Opisuje se i nov mjerni sustav za detekciju protona sastavljen od silicijskih detektora koji zahvaća veći prostorni kut

Bosanr (for the A1 collaboration at MAMI), Damir

Makek, Mihael

English

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ISSN1330–0016CODEN FIZBE7MODIFICATIONS OF DELTA IN NUCLEAR MEDIUMDAMIR BOSNAR and MIHAEL MAKEKfor the A1 collaboration at MAMIDepartment of Physics, Faculty of Science, P.O. Box 331, HR-10 002 Zagreb, CroatiaReceived 10 December 2003; Accepted 30 August 2004Online 14 November 2004In the experiments at the Mainz Microtron, MAMI, ∆0 particles in the electronscattering reactions 12C(e, e′∆0) 11C have been produced in nuclear medium. Thethree high-resolution magnetic spectrometers of the A1 collaboration have beenused in triple coincidence measurements 12C(e, e′pπ−) 11C. Indications for sub-stantial changes of delta properties in the nuclear medium have been obtained.Because of low statistics, these results cannot yet be taken as conclusive. A newsetup for measurements at MAMI with silicon detectors for proton detection, whichsubtend larger solid angle, is described.PACS numbers: 14.20.-c, 25.30.Dh,Fj UDC 539.12Keywords: ∆0 particles in nuclear medium, electron scattering reaction,1. IntroductionHadrons are composite objects consisting of quarks and gluons that occupysome volume in space. When packed close together, e.g. in the nucleus, one wouldexpect that their properties would change due to mutual interactions and influ-ences. Surprisingly, there is no conclusive experimental evidence of such changes.Because of that, experimental investigations of hadron properites in the nuclearmedium are among the most interesting in hadron physics. One line of these inves-tigations is the production of hadrons in nuclei in scattering reactions. In the past,several experiments, using hadron probes, have reported hints of changes of hadronproperties in the nuclear medium. But, they all suffered either from low statisticsor their overall resolution was not sufficient to provide conclusive evidence of suchchanges. For a summary of previous experiments see, e.g., Ref. [1].In the nineties, the invention of electron accelerators with continuous electronbeams made possible the investigations with electromagnetic probes. The electro-magnetic interactions are theoretically well described and suitable choices of scatter-ing reactions in investigations of hadron production in the nuclear medium can giveFIZIKA B 13 (2004) 2, 507–512 507bosnar and makek: modifications of delta in nuclear mediumsome answers about their properties and possible changes. The A1-collaboration [2]at MAMI has been using electron scattering on the nucleus and nuclei to exploretheir properties [3]. And electron scattering on the 12C nuclei has been used toproduce delta particles and to explore their properties in nuclear medium [1]. Priorto this measurement, only one exclusive delta-electroproduction experiment at andabove delta production energies has been done [4].In the next section, a short description of the beam properties of MAMI will begiven, the third section is devoted to the description of the magnetic spectrometersand additional detectors of the A1-collaboration, the fourth section describes thechoice of kinematics, and last section will give an overview of the obtained resultsand plans for future measurements.2. Electron accelerator MAMIThe electron accelerator MAMI, Mainz microtron, is located at the campus ofthe Johannes Gutenberg University in Mainz, Germany. It is an electron acceleratorwith a 100% duty factor, and consists of an injector linac followed by a cascade ofthree microtrons. It delivers a continues electron beam with the maximum energy of885 MeV, and the maximum current of about 100 µA. A polarized electron beamis available as well, with the maximum polarization of 80% and the maximumcurrent of about 15 µA [5]. The energy of the outgoing beam has fluctuationsof less than 10−6, and in any mode of operation, MAMI is characterized by itsvery high stability and reliability. The upgrading of the accelerator is currentlybeing undertaken. The existing third stage of the microtron will be used as theinput to the double sided-microtron. After this upgrade, which will be completedin 2005, the maximum electron energy will be 1.5 GeV. The ongoing activities willbe continued at these higher energies and with the production of the strangenessin the nuclei new possibilities will be opened.3. Three magnetic spectrometers of the A1-collaborationand additional detectorsThe A1 collaboration at MAMI has built three high-resolution magnetic spec-trometers [6]. The magnetic spectrometers, named A, B, C, have solid angles of28 msr, 5.6 msr and 28 msr, respectively. They can detect electrons, pions, pro-tons and other charged particles with the maximum momentum 665 MeV/c, 810MeV/c, and 490 MeV/c, and have acceptance of 20%, 15%, 25%, respectively. De-tector packages in all three spectrometers are built in the same way and consistof two double-planes of the vertical drift chambers (VDCs), two layers (3 mm and10 mm thick) of the scintillator counters and the Cherenkov counter. In the spec-trometer A, a proton polarimeter [7] can be put in place of the Cherenkov counter.It can determine the polarization of protons detected in the spectrometer A. Inaddition, the spectormeter B can be taken out of the plane to the extend of 10◦.The magnetic spectrometers provide information regarding the momentum of the508 FIZIKA B 13 (2004) 2, 507–512bosnar and makek: modifications of delta in nuclear mediumoutgoing charged particles with a resolution better than 10−4, the angles of theoutgoing charged particles with a resolution better than 3 mrad at the target, andinformation regarding the type of charged particles. Single measurements, as wellas double and triple coincidence measurements are possible. Software packages in-cluding hardware control, data acquistion, and data analysis have been developed[8]. For specific measurements, additional detectors can be used together with themagnetic spectrometers. E.g., a neutron detector consisting of a wall of scintillationdetetectors for neutron detection has been used in investigations of the nucleon-nucleon correlations in the nucleus. A neutron polarimeter for neutron detectionand determination of their polarization has been used in the measurements of theneutron electric form factor [9]. The forth magnetic spectrometer, named shortorbit spectrometer, has recently been completed and succesfully tested [10]. It willbe used to detect charged pions right above the treshold of pion production. In thefuture investigations of the hadron properties in the nuclear medium, a telescopemade of silicium detectors, described below, will be used for proton detection.4. KinematicsWe have investigated properties of delta particles produced in the electron scat-tering reactions 12C(e, e′∆0) 11C. We have measured 12C(e, e′pπ−) 11C and wehave detected protons and pions, produced in the decay of delta particles, with themagnetic spectrometers A and C, respectively, along with scattered electrons in thespectrometer B. Delta particles in these reactions can be produced in two differentways, which can be separated kinematically. The first possibility is the so-calledquasi free-production. Delta is produced on a neutron which is knocked out of the12C nucleus, while the rest of the nucleus does not participate in this reaction. Thisdelta takes all the momentum of a virtual photon, and the pion and the protonproduced in the delta decay are boosted in its direction. The second possibility isthe so-called bound delta production. Delta is produced on a neutron which re-mains bound in 12C nucleus, and the whole 12C∆ nucleus takes the momentum ofa virtual photon. The pion and the proton produced in the delta decay are emittedinto the back-to-back direction in the rest frame of the ∆ particle. The producedproton and pion are not so much boosted in the virtual photon direction, in thesecond case as compared to the first case, due to the mass ratio of the ∆ and 12Cnucleus.In order to determine the corresponding kinematical regions, Monte Carlo sim-ulations of these reactions have been performed [1]. Using these simulations, kine-matics in which the bound delta production can clearly be separated from thequasi-free delta production has been selected, and a series of measurements for var-ious kinematics has been performed [1,11]. In this report, we will show the resultsof the measurements for a kinematical setting in the bound delta production region(see Table 1).FIZIKA B 13 (2004) 2, 507–512 509bosnar and makek: modifications of delta in nuclear mediumTABLE 1. Kinematical setting in the bound delta region.Ee (MeV) θe′ q2 (GeV/c)2 pp (MeV/c) θp pπ (MeV/c) θπ Nevents855 18◦ −0.047 222.0 90.8◦ 203.7 63.3◦ 2525. Results of the measurements and discussionSince we have detected e′, p and π− in the reaction 12C(e, e′pπ−) 11C, we candetermine their 4-momenta and calculate the missing mass of the reaction, whichcorresponds to the mass of residual 11CMmiss =√(ω + M(12C) − Ep − Eπ)2 − (~q − ~pp − ~pπ)2 , (1)where Ep, Eπ and ~pp, ~pπ are the energies and momenta of the proton and thepion respectively, ω and ~q are the energy and momentum of the virtual photon andM(12C) is the mass of the 12C nucleus. In the analysis, we have concentrated onthe reactions in which the residual 11C nuclei have remained in the ground state,Mmiss = M(11Cg.s.), and which could be separated form the excited states. Theexcellent overall resolution achieved in the measurements provided a reconstructionof various physical quantities needed to study the delta production and its proper-ties in the nucleus. One of the most interesting is the excitation energy spectrumof the 12C∆ nucleus, ω̂ = W − M(12C), where W is the invariant total mass.In the previous measurements [1] of the bound delta kinematics, two peaks withthe widths of several MeV in the spectrum of the ω̂ have been noticed at the energiesof 282 and 296 MeV (see Fig. 5 in Ref. [1]). These peaks have been interpretedas a substantial change of delta properties in the nuclear medium, in particularof the width of the delta particle, and a simple model has explained observedenergies of the peaks. Later on, a better quantitative theoretical justification hasbeen developed [12]. −20  −10    0   10   20  0 20 40 60 80CountsExc.En.11C  [MeV]200 250 300 350 400  0  5 10 15 20CountsW − m12C  [MeV]Fig. 1. Energy spectrum of residual 11C nucleus obtained from Mmiss − M(11C),left, and excitation energies ω̂ of the 12C∆ nucleus, right, after a cut on the groundstate of 11C.510 FIZIKA B 13 (2004) 2, 507–512bosnar and makek: modifications of delta in nuclear mediumIn order to explore the properties of the delta particle in the nuclear medium inmore detail, and to determine the width of the delta particle in nuclear medium,a dedicated measurement for only one selected kinematics in the bound delta pro-duction region (see Table 1) has been performed. As in previous measurements,an excellent resolution of excitation energies of the residual 11C nucleus has beenachieved, as is shown in Fig. 1, left. This spectrum has been obtained after thecut on the timing in the three spectrometers, which selects the triple coincidenceevents. The cut around zero in this histogram selects events in which the 11C wasleft in the ground state. However, previously obsereved peaks at the 282 and 296MeV in the excitation energies ω̂ of the 12C∆ were not clearly observed in thismeasurement, Fig. 1, right. No hardware or software problem has been detected inthis measurement, and the question of possible changes of the delta properties inthe nuclear medium remains open.One serious difficulty encountered in these triple coincidence measurements islow statistics caused by small solid angles of the magnetic spectrometers, as canbe seen from the last column of Table 1, which shows the total number of goodevents after all cuts. To collect higher statistics, one possibility is to do very longmeasurements, or to enlarge the available solid angles of the detectors. In orderto further examine properties of delta particles in the nuclear medium, we haveadopted the second approach.6. Future measurementsIn our future measurements, we intend to use a telescope made of silicon de-tectors for proton detection and the magnetic spectrometer A (28 msr) for thedetection of electrons, instead of the previously used spectrometer B (5.6 msr).The spectrometer C (28 msr) will be used, as previously, for the detection of pions.The telescope made of silicon detectors consists of double-sided strip detector, 300µm thick, with dimensions 24 × 24 mm2 with the strip pitch 1 mm wide (detectortype BB2 from Micron Semiconductor [13]). The strips on the opposite sides ofthe detector are at 90◦ and that provides angular information in both θ and φ di-rections. Behind the strip detector there are 5 silicon detectors, (MSX09 detectorsfrom the same producer) each 1 mm thick and with dimensions 30× 30 mm2. Thecombination of these detectors will provide a good angle and energy informationof detected protons together with the ∆E − E information necessary for particleseparation. The detectors will be placed in the scattering chamber at the distanceof 8.2 cm from the target and at this distance they will subtend the solid angle of90 msr (see Fig. 2).The described telescope made of silicon detectors for proton detection, in con-juction with the magnetic spectrometers should provide a satisfactory overall res-olution, and due to a larger solid angle, improved statistics.FIZIKA B 13 (2004) 2, 507–512 511bosnar and makek: modifications of delta in nuclear medium82.0 mmA1 Scattering ChamberChamber CoverTargetRotating MotorSi DetectorFig. 2. Telescopeof the BB2 andfive MSX09 detec-tors, left; Holderof the telescopeof silicon detec-tors with the ro-tating motor inthe A1-scatteringchamber, right.References[1] P. Bartsch et al., Eur. Phys. J. A 4 (1999) 209.[2] wwwa1.kph.uni-mainz.de/A1[3] wwwa1.kph.uni-mainz.de/A1/publications[4] L. Elouadrhiri et al., Phys. Rev. C 50 (1994) R2266.[5] K. Aulenbacher et al., Nucl. Instr. Meth. A 391 (1997) 498.[6] K. I. Blomqvist et al., Nulc. Instr. Meth. A 403 (1998) 263.[7] Th. Pospischil et al., Nucl. Instr. Meth. A 483 (2002) 713.[8] M. O. Distler et al., Proc. 12th IEEE Real Time Congress on Nuclear and PlasmaSciences, Valencia, June 2001.[9] M. Seimetz, Messung des elektrischen Formfaktors des Neutrons bei Q2 = 0.3 and 0.6(GeV/c)2, Ph.D. thesis, Mainz University (2003).[10] M. Ding, Ph.D. thesis, Mainz University, in preparation.[11] A. Wagner, Ph.D. thesis, Mainz University, in prepartion.[12] Th. Walcher, Phys. Rev. C 62 (2001) 064605.[13] www.micronsemiconductor.co.ukPROMJENE SVOJSTAVA DELTA U NUKLEARNOJ SREDINIU eksperimentima s mikrotronom u Mainzu, MAMI, proizvodili smo u nuk-learnoj sredini čestice ∆0 reakcijom 12C(e, e′∆0) 11C. U okviru suradnje A1, ra-bili smo tri magnetska spektrometra visokog razlučivanja za trosudesna mjerenja12C(e, e′pπ−) 11C. Podaci ukazuju na promjene svojstava delta čestica u nuklearnojsredini. Zbog slabe statistike, ti se ishodi još ne mogu smatrati konačnima. Opisujese i nov mjerni sustav za detekciju protona sastavljen od silicijskih detektora kojizahvaća veći prostorni kut.512 FIZIKA B 13 (2004) 2, 507–512

Promjene svojstava Delta u nuklearnoj sredini

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