Associated Λ/Σ0 electroproduction with the Kaos spectrometer at MAMI

P. Achenbacha on behalf of the A1 Collaboration

Institut fur Kernphysik, Johannes Gutenberg-Universitat, J.-J.-Becherweg 45, D-55099 Mainz.

Abstract. An instrument of central importance for the strangeness photo- and electroproduction at the 1.5 GeVelectron beam of the MAMI accelerator at the Institut fur Kernphysik in Mainz, Germany, is the newly installedmagnetic spectrometer Kaos that is operated by the A1 collaboration in (e, e′K) reactions on the proton or lightnuclei. Its compact design and its capability to detect negative and positive charged particles simultaneouslycomplements the existing spectrometers. The strangeness program performed with Kaos in 2008–9 is address-ing some important issues in the field of elementary kaon photo- and electroproduction reactions. Althoughrecent measurements have been performed at Jefferson Lab, there are still a number of open problems in the in-terpretation of the data and the description of the elementary process using phenomenological models. With theidentification ofΛ andΣ0 hyperons in the missing mass spectra from kaon production off a liquid hydrogen targetit is demonstrated that the extended facility at MAMI is capable to perform strangeness electroproduction spec-troscopy at low momentum transfersQ2 < 0.5 (GeV/c)2. The covered kinematics and systematic uncertainties inthe cross-section extraction from the data are discussed.

1 Introduction

Analogous to the successful description of pion photopro-duction in the∆ resonance region orη photoproductionin the second resonance region, phenomenological mod-els can describe the electromagnetic kaon production am-plitudes associated toΛ/Σ0 hyperons. Many experimen-tal data for kaon production has been collected sofar byJefferson Lab, ELSA, SPring8, GRAAL, and Tohoku. Al-though recent measurements with high statistics have beenperformed at Jefferson Lab [1–3], there are still a numberof open problems in the interpretation of the data and thedescription of the elementary process using phenomeno-logical models, see Ref. [4] for a recent discussion.

Theoretical groups have developed a particular typeof effective Lagrangian model, commonly referred to asisobar approach, in which the reaction amplitude is con-structed froms-, t-, or u-channel exchange diagrams. Mostof the models use single-channel approaches, in which asingle hadron is exchanged. Since several resonances maycontribute in this channel, models disagree on their relativeimportance, and many free parameters have to be fixed. Ex-perimental set-ups used to study the strangeness produc-tion channels are missing forward angle acceptance lead-ing to a strong variation of the models in the forward re-gion. Concerning the data published after 1990 discrepan-cies between the CLAS [5] and SAPHIR [6] experimentsdo not allow for a unique description of the process. Thissituation clearly indicates that more experimental and theo-retical work is needed in order to provide a comprehensiveunderstanding of the elementary reaction. A number of

a e-mail:patrick@kph.uni-mainz.de

new experiments are now addressing these issues, amongthem the charged kaon electroproduction program with theKaos spectrometer at MAMI.

2 Kinematics and experimental setup

The accelerator facility MAMI at the Institut fur Kern-physik in Mainz, Germany, can now be used to study ele-mentary strangeness processes as the threshold energy wascrossed by the 1508 MeV end-point energy of stage C com-pared to the 855 MeV end-point energy of stage B. Thekinematic regions accessible in electroproduction by theelectron accelerator facility are shown in Fig. 1.

In a typical two-arm strangeness electroproduction ex-periment a charged kaon is detected in coincidence witha scattered electron and the recoiling hadronic system re-mains unobserved. The incident electron of energyE scat-ters by radiating a virtual photon,qµ. The scattered electronof energyE′ is emitted at a polar angleθ with respect tothe direction of the incident beam. The virtual photon car-ries momentum,q, and energy,q0, defining the momentumtransfer squaredQ2 = −qµqµ.

The goals of the first measurements at MAMI were todetermine the angular dependence of the virtual photopro-duction cross-section at very lowQ2. This cross-sectionmay be related to the five-fold differential electroproduc-tion cross-section via:

d5σ

dE′dΩedΩ∗K= Γv

d2σ

dΩ∗K(W,Q2, ǫ, θ∗K , φ

∗K) , (1)

where the kaon anglesθ∗K andφ∗K are given in spherical co-ordinates in the hadronic center-of-mass system and the

EPJ Web of Conferences 3, 07010 (2010) DOI:10.1051/epjconf/20100307010 © Owned by the authors, published by EDP Sciences, 2010

This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.

Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20100307010

EPJ Web of Conferences

Invariant Energy W (GeV)1 1.2 1.4 1.6 1.8 2

2 (

GeV

/c)

2F

our−

Mom

entu

m T

rans

fer

Q

0

0.5

1

1.5

2

2.5

MAMI−C

ΛK

ΣKNωNρ

NφMAMI−B

Nπ

Nη

Fig. 1. Kinematic regions accessible in electroproduction bythe electron accelerator facility. The end-point energy of stageMAMI-B is 855 MeV and of stage MAMI-C is 1508 MeV (as ofSummer 2009). The threshold energies forπ/η/ρ/ω andφ pro-duction offthe nucleon and the associated strangeness channelsKΛ and KΣ are indicated; the kinematic regions for pion andkaon production are shaded. Only at MAMI-C the electroproduc-tion of open strangeness is possible.

flux factor of virtual photons per scattered electron intodE′dΩe is given by:

Γv =α

2π2E′

E|q|Q2

11− ǫ

, (2)

andthe transverse polarization factor of the virtual photonis denoted byǫ:

ǫ =(

1+ 2|q|2

Q2tan2 θ

2

)−1. (3)

First strangeness production experiments were carriedout at the spectrometer facility of the A1 Collaboration [7].An instrument of central importance for this program is thenewly installed magnetic spectrometer Kaos, dedicated tothe detection of charged kaons. Its compact design and itscapability to detect negative and positive charged particlessimultaneously complements the existing spectrometers.

Fig. 2 shows a schematic drawing of the Kaosspec-trometer as realized in the spectrometer hall. Kaosis op-erated in a single-dipole configuration. The first-order fo-cusing is achieved with a bending of the central trajec-tory by ∼ 45 with a momentum dispersion of 2.2 cm/%.The dispersion to magnification ratio of the Kaoshadronarm is D/Mx = 1.2 cm/% which leads to a first-order re-solving power,R1 = D/(Mxσx), of 2 400 for a beam-spotsize,σ0, of 0.5 mm. The first order momentum resolution,

∆p/p =√

(σ2x + M2

xσ20)/D2, is determined from these val-

ues to be of the order of∼ 10−3, where the spatial reso-lution in the focal plane,σx, was approximated with theanode wire distance of 2 mm in the MWPC.

Detailed measurements with electron beams elasticallyscattered off12C and181Ta foils have been performed tostudy the properties of the magnet optics. To select welldetermined angles in both planes a sieve collimator, madeof 20 mm thick lead with 16 holes, 5 mm in diameter each,was installed in a distance of 1000 mm from the target in

Fig. 2. Overview of the first spectrometer set-up for strangenesselectroproductionat the Mainz Microtron MAMI. Ray-tracedparticle trajectories through the Kaosspectrometer are shown byfull lines. The instrumentation of the hadron arm is indicated.Both spectrometers were positioned as close to the exit beam-lineas mechanically possible.

Cou

nts

0

500

1000

1500

2000

2500

3000

3500

= 1.90 mrad1σ = 2.24 mrad2σ = 2.50 mrad3σ = 2.72 mrad4σ = 2.39 mrad5σ = 2.06 mrad6σ

(mrad)0ΘHorizontal angle -150 -100 -50 0 50 100

Fig. 3. Preliminary analysis of elastically scattered electrons of450MeV energy, back-traced from the detector system of theKaos spectrometer to the target, before applying any correctionsof the aberrations. The sieve-slit collimator was made of a 20 mmthick lead block with holes of angular width ofσ ∼ 2.4 mrad at1000 mm distance to the target. The peaks in the projection on theaxis of the target angleθ0 in the dispersive plane were fitted byGaussian distributions, resulting in widths ofσ ∼ 1.9–2.7 mrad.

front of the magnet. For each hole the sieve collimatordefines primary angle distributions with a width ofσ =2.45 mrad. Electrons passing different holes are clearly sep-arated in the reconstructed coordinates. The deviations tothe nominal hole coordinates quantify the uncorrected op-tical aberrations of the system. The magnitude of the nec-essary correction of the transfer matrix elements is of theorder of 10 mrad or more, as can be deduced from the devi-ations of the measured hole positions to the nominal posi-tions. The projections of the events to the target angleθo inthe dispersive plane produced peaked distributions widthwidths ofσ ∼ 1.9–2.7 mrad, as shown in Fig. 3. A finer col-limator was prepared in 2009 for an improved determina-

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Table 1. Kinematics of the 2009 beam-time on kaon elementaryelectroproduction at the Mainz Microtron.

initial electron energy,E (MeV) 1508final electron energy,E′ (MeV) 327electron scattering angle,θ (deg) 15.5degree of transverse polarization,ǫ 0.406total energy incm system,W (MeV) 1745virtual photon momentum transfer,Q2 (GeV2/c2) .036virtual photon flux factor,Γv (ph./GeV/sr) .004

tion of the transfer matrix elements. It contained 53 holesin 5 different horizontal lines along the dispersive plane.The calibrations are ongoing and no final results from theexperiments can be provided unless the calibration runs arefully analyzed.

In 2008 and 2009 strangeness production offthe protonwas measured at two different kinematic settings with thisexperimental set-up. The beam from MAMI-C impingedwith an energy of 1507 MeV on a liquid-hydrogen target.Positive kaons were detected in the Kaosspectrometer ben-efiting from the large in-plane angular acceptance at scat-tering anglesθK = 21–43, and of the large momentumacceptance atp = 400–700 MeV/c. The scattered elec-trons from the H(e,e′K) reaction were detected in the high-resolution magnetic spectrometer B (SpekB) that was keptfixed at an angle ofθ ≈ -15 during the experiments.

The luminosity was calculated from the density andthickness of the target, and the beam current, and is cor-rected for the DAQ dead-time. The collected data in 2008corresponds to effectively 3 days of beam-time at 1–4µAbeam current for each kinematic point with a total inte-grated luminosity of

∫

Ldt ∼ 284 fbarn−1. One setting witha central momentum in Kaosof 530 MeV/cwas continuedby a longer data-taking campaign on kaon elementary elec-troproduction in 2009 for a statistically and systematicallyimproved data-set. In Table 1 a summary on this particularkinematics is given. The current was held constant at 2, re-spectively 4µA. With a run-time of 265 h using 2µA beamcurrent (at 13 % dead-time) and 40 h using 4µA beam cur-rent (at 44 % dead-time) the accumulated and corrected lu-minosity for the 2009 data-taking campaign was

∫

Ldt ∼3000 fbarn−1.

3 Particle tracking and identification withKaos

The tracking of particles through Kaosis performed bymeans of two large MWPC with a total of 2× 310 ana-logue channels. Five cathode wires are connected togetherand are brought to one charge sensitive preamplifier fol-lowed by an ADC card. The transputer-based read-out sys-tem is connected to a multi-link card of a front-end com-puter. To determine the particle track the measured chargedistributions are analyzed by the center-of-gravity method.

In the subsequent analysis of the data not only the par-ticle track is used, but a large set of track quality factors.

For example, the correlation that exists between the in-duced charge measured in one cathode plane to the inducedcharge of the perpendicular plane allows in many cases acorrect pairing of clusters. Valid tracks are bound to an-gular limits, given by the acceptance of the spectrometer.Especially the relation between vertical hit positions of thetwo chambers provides a powerful criterion for the trackfinding. The multi-hit capability of the MWPC is impor-tant to allow for beam currents above 1µA. As long astwo tracks are sufficiently separated in space to induce dis-tinct charge distributions on both cathode planes, the dou-ble track ambiguity can be resolved with high efficiency.

In 2009 two small scintillating detectors of dimensions∼ 2 × 3 cm2 with 5 mm thickness were installed in frontof each MWPC to determine the tracking and detectionefficiency of the hadron arm. The detectors can easily bemoved out of the spectrometer’s acceptance when not inuse.

Particle identification in the Kaosspectrometer is basedon the particle’s time-of-flight and its specific energy loss.A segmented scintillator wall with 30 paddles read outat both ends by fast photomultipliers is located near thefocal-surface and measures the arrival time. A second wallwith 30 paddles is used to discriminate valid tracks againstbackground events. A top–bottom mean timing for deriv-ing the trigger is performed by summing the analogue sig-nals. The signal amplitudes were corrected for the parti-cle’s pathlength through the scintillator bulk material andthe light absorption inside the paddle. Because of aging ofthe material the absorption of the scintillation light insidethe scintillator is strong.

The time spectrum is systematically broadened by thepropagation time dispersion inside the scintillator, the timedifferences between different scintillator paddles and theirassociated electronic channels, and by the variation of thetime-of-flight, being proportional to the pathlength throughthe spectrometer. The Gaussian width of the (e′, π) peakafter corrections is∆tFWHM = 1.07 ns, which is a typicalinter-spectrometer time resolution.

From the point of view of background events in thedetectors the set-up of the Kaosspectrometer at forwardangles is problematic. Shielding walls were installed com-prising neutron shielding material (borated polyethylene)and lead for the electromagnetic shielding. In order to copewith the remaining high background rates Kaoshas beenequipped with a modern trigger system. The correlationbetween the paddles in both walls corresponding to validtrajectories was employed for the trigger decision. The rel-atively high complexity of the valid patterns and the intrin-sic necessity of a flexible trigger logic for a spectrometerthat faces a wide experimental program demanded a pro-grammable logic system which was realized in so-calledVUPROM modules. Each so called VME Universal Pro-cessing Module (VUPROM) has 256 I/O channels in a1-unit-wide VME 6U form-factor. The digital signals aretransmitted in high speed differential LVDS standard andare combined in 8 groups of 32 channels each, where onegroup is reserved for output, three groups are allocatedto input, and the remaining four groups are freely config-urable as input or output. Such a module is equipped with

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)2Missing mass (MeV/c1060 1080 1100 1120 1140 1160 1180 1200 1220

Co

un

ts/b

in a

fter

PID

cu

ts

0

50

100

150

200

Fig. 4. Preliminary missing mass spectrum in the H(e, e′K+) reac-tion from the 2009 data-taking campaign as discussed in the text.The mass resolution is limited by the errors in the estimated trans-fer matrix that was not yet corrected for the aberrations seen incalibration measurements. The broad structure at 1170 MeV/c2 isbelieved to be caused by particles scattered inside the spectrom-eter. The background in two averaged (e′,K) coincidence timeside-bands was subtracted with the appropriate weight.

one Virtex-4 FPGA, capable of operation at 400 MHz, andone 1 GHz DSP. The coincidence trigger is based on time-of-flight and is therefore generated from the VUPROMlogic and from SpekB scintillator signals.

Some more selected properties of the optics of the Kaosspectrometer, its kaon identification capabilities, and exist-ing or planned modifications to operate Kaosas a double-arm spectrometer under zero degree scattering angle canbe found in Ref. [8].

4 Data analysis

Kaon identification is based on specific energy-loss andtime-of-flight, electron identification on a signal in a gasCherenkov counter. The flight time difference between pro-tons and kaons is 10–15 ns, between pions and kaons 5–10 ns. The energy-loss separation between kaons and pi-ons is small, whereas the separation between kaons andprotons is of the order of 2–5 MeV/cm. The following cutswere applied to a limited data set taken with 2µA beamcurrent in 2009:

Electron identification: Finite sum of Cherenkov counteramplitudes to reject pions in the electron arm.

Spectrometer fiducial volume:θ0 > -10.5 and θ0 < 8

andθ0 < 0.475× (∆p+20 %) to restrict the acceptanceto a region where agreement between the Monte Carlocode and the analyzed data was excellent.

MWPC track reconstruction: MWPC quality> 0.0001 en-suring good quality tracks in the hadron arm.

Particle identification: Positive squared missing mass fora proper missing mass reconstruction, andpKaos < 1.8× pcentral for a proper momentum reconstruction, andβKaos > 0.4 to reject erroneous timing values.

0.00 0.20 0.40 0.60 0.80 0

100

200

300

400

500

hype

ron

yiel

d

kaon c−m scattering angle cos θ*K

countsbackgroundK−MAIDyield

Λ(1115)

0.00 0.20 0.40 0.60 0.80 0

100

200

300

400

500

hype

ron

yiel

d

kaon c−m scattering angle cos θ*K

countsbackgroundK−MAIDyield

Σ(1192)

Fig. 5. Preliminary yield of identified H(e, e′K+)Λ, Σ0 reactionsas a function of the center-of-mass kaon scattering angle. Themeasured events (open round symbols) are corrected for back-ground events (black round symbols) to obtain the hyperon yield(blue round symbols). The data is compared to a Monte Carlosimulation using the K-Maid model [10] as an input for the cross-sections and a generalized acceptance function for the detectordescription. The background subtraction is still dependent on thecalibration.

Kaon identification:|tKF,G | < 1.8 ns, being the measured

TOF corrected for the expected kaon flight time, and|∆EK

F,G | < 400 keV, being the measured specific en-ergy loss corrected for the expected kaon energy loss.A small tK indicates that the correct mass hypothesishas been made while a small∆EK reduces the randombackground events.

Reaction channel identification: For theΛ andΣ0 hyper-ons 1110 MeV/c2 < MX < 1140 MeV/c2, respectively1190 MeV/c2 < MX < 1205 MeV/c2.

After electron and kaon identification the measured mo-menta, in magnitude and direction with respect to the in-coming beam, allow for a full reconstruction of the miss-ing energy and missing momentum of the recoiling sys-tem. After application of all cuts on the data, the remainingevents are used to extract the kaon yield in the two hyperonchannels. A cut was placed on the missing mass in orderto separate theΛ andΣ0 reaction channels. Because theΣ0

peak resides on the tail of theΛ peak, the latter has to beaccounted for in the analysis of theΣ0-region of the mea-sured spectrum. A preliminary missing mass distributionis shown in Fig. 4. The mass resolution is limited by the

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19th International IUPAP Conference on Few-Body Problems in Physics

errorsin the estimated transfer matrix that was not yet cor-rected by the aberrations seen in calibration measurements.Since the Kaosspectrometer is operated as a single dipolewith open yoke geometry a significant fraction of detectedparticles have scattered inside the spectrometer and needto be treated accordingly.

Random background events in the missing mass spec-tra were subtracted by two averaged (e′,K) coincidencetime side-bands with the appropriate weights. The prelim-inary yields of identified H(e,e′K+)Λ, Σ0 reactions as afunction of the center-of-mass kaon scattering angle areshown in Fig. 5. The background subtraction is still depen-dent on the calibration. The data is compared to a MonteCarlo simulation using the K-Maid model [10] as an inputfor the cross-sections and a generalized acceptance func-tion for the detector description.

The number of kaons that were actually detected wasless than the true number of kaons produced in the reac-tion, and corrections for their decay had to be taken intoaccount. The kaon survival fraction varied between 0.2 and0.35 for the range of momenta detected.

5 Extraction of cr oss-sections

The further analysis required a detailed Monte Carlo sim-ulation of the experiment in order to extract cross-sectioninformation from the data. The experimental yield can berelated to the cross-section by

Y = L ×∫

Γ(Q2,W)d2σ

dΩ∗KA(d5V)R(d5V) dQ2dWdφedΩ∗K ,

(4)whereL is the experimental luminosity including globalefficiencies such as dead-times,A is the acceptance func-tion of the coincidence spectrometer set-up including mo-mentum dependent corrections such as the tracking effi-ciency, andR is the correction due to radiative or energylosses.

In the Monte Carlo for the integral is evaluated in thephase-space∆V = ∆Q2∆W∆φe∆Ω

∗K with limits that ex-

tended beyond the physical acceptances of the spectrom-eters. The simulation is giving the phase-space acceptedby the spectrometers according to a chosen kinematics to-gether with the radiative corrections, absorption and decaylosses as well as detector inefficiencies. By virtue of theMonte Carlo technique used to perform this integral thequantitiesA and R are not available separately or on anevent-by-event basis. In Fig. 6 the phase-space in the tar-get angle vs. momentum plane for kaon acceptance in theKaos spectrometer simulated by a Monte Carlo integrationmethod is compared to the data measured during the 2009beam-time. The kaon data was extracted after particle iden-tification and missing mass cuts. The agreement inside thefiducial volume is excellent.

The general acceptance can be written in terms of thecross-section at a given point (d2σ/dΩ∗K)0,θ∗K , called thescaling point, at〈Q2〉, 〈W〉, 〈φe〉, and〈φ∗K〉, when studying

−50% 0% 50% −15°

−10°

−5°

0°

5°

10°

15°

Rel

ativ

e ka

on a

ngle

Θ0

Relative kaon momentum ∆p

−50% 0% 50% −15°

−10°

−5°

0°

5°

10°

15°

Rel

ativ

e ka

on a

ngle

Θ0

Relative kaon momentum ∆p

Fig. 6. Phase-space in the target angle vs. momentum plane forkaonacceptance in the Kaosspectrometer simulated by aGeant4Monte Carlo integration (top) and measured during the 2009beam-time (bottom). The kaon data was extracted after particleidentification and missing mass cuts. The simulation includes ra-diative corrections and energy-loss in the target.

the dependence on the remaining variable cosθ∗K :

Y = L ×

(

d2σ

dΩ∗K

)

0,θ∗K

× (5)

∫

Γ(Q2,W)

d2σdΩ∗K

(

d2σdΩ∗K

)

0,θ∗K

A(d5V)R(d5V) dQ2dWdφedΩ∗K

Then, the behavior of the cross-section across the accep-tance needs to be implemented according to a given the-oretical description. We applied the K-Maid [10] isobarmodel, however, we could as well compare to the full (SL)and the simplified version (SLA) of the so-called Saclay-Lyon model [9]. The scaling point for the data set from2009 is at〈Q2〉 = 0.036 (GeV/c)2, 〈W〉 = 1.750 GeV,〈ǫ〉 =0.4. Finally, the cross-section is evaluated dividing the yieldby the luminosity and the evaluated integral. An importantobservable to be investigated is the stability of the resultsas a function of the applied cuts and its influence on the ex-tracted cross-section. Up to now, systematic uncertaintiesbeing discussed in the next section hinder the extraction offinal values.

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Position in MWPC L (mm)600 800 1000 1200 1400 1600 1800

Tra

ck a

ng

le (

deg

)

30

35

40

45

50

55

60

65

70

Position in MWPC L (mm)600 800 1000 1200 1400 1600 1800

Tra

ck a

ng

le (

deg

)

30

35

40

45

50

55

60

65

70

Fig. 7. Top: Projection of simulated tracks for particles with mo-mentaof 350 MeV up to 600 MeV in steps of 25 MeV, over thefull angular acceptance, and for particles of central target anglesover the full momentum acceptance, in the coordinate system ofthe tracking chambers. In the regions with strong curvature themomentum depends on many higher-order terms of the trans-fer matrix. Bottom: Simulated geometrical scintillator wall inef-ficiencies in the same coordinate system. Both effects contributeto systematic uncertainties in the cross-section extraction.

6 Systematic uncertainties

The total efficiency for the detection of kaons with theKaos spectrometer can be factored into the intrinsic wirechamber (MWPC) detection efficiency, the analysis cut ef-ficiency, the trigger efficiency, and the tracking efficiency.In order to investigate the MWPC, tracking, and triggerefficiencies dedicated measurements have been performedin 2008 and 2009. The efficiency of the MWPC for theidentification of minimum ionizing particles is measuredto be better 98 % with only small variations over time. Onthe contrast, the tracking efficiency is dependent on the hitmultiplicities in the wire chambers leading to ghost tracksthat are classified according to a likelihood method and thatare matched to scintillator hits. The track reconstruction inthe wire chambers represents a primary source of ineffi-ciency for beam currents of severalµA. For pions trackingefficiencies ranged between 76 and 45 % corresponding tobeam currents of 1–4µA, for protons the efficiencies werebetween 92 and 77 %.

In an ideal spectrometer the position along the focalsurface at which a particle is detected is directly relatedto its momentum. In practice, however, due to kinematicbroadening and spectrometer aberrations this position willalso depend on the angle of the particle trajectory withthe central ray. Fig. 7 (top) shows the projection of sim-ulated tracks with momenta of 350 MeV up to 600 MeVover the full angular acceptance in the coordinate systemof the tracking chambers. Particles with given momenta butvarying target angles leave traces in this coordinate systemthat are lines of constant slopes in the central region of theacceptance and are strongly curved at the edges. The plotincludes also the trace for particles of central target anglesover the full momentum acceptance. In the regions withstrong curvature the momentum depends on many higher-order terms of the transfer matrix, which need to be pre-cisely determined.

The trigger was generated by a combination of hits inthe two scintillator walls. Particular momentum and trackangle combinations allow the particle to cross the gapsbetween the scintillator paddles giving rise to significantdetector inefficiencies. In Fig. 7 (bottom) a simulation ofthese geometrical inefficiencies are shown in the trackingcoordinate system. Momentum and track angle dependenttrigger efficiency corrections need to be applied to the gen-eralized acceptance.

7 Summary

Using the recently installed magnetic spectrometer Kaosand a high-resolution spectrometer in coincidence it waspossible to measure elementary kaon electroproduction atMAMI in a kinematic regime sofar not covered by Jeffer-son Lab experiments. At different settings we have con-firmed the particle identification power and tracking capa-bilities of the Kaos spectrometer’s hadron arm detectionsystem. The large background rates and inefficiencies en-countered in the 2008–9 data-taking campaigns are cor-rected using dedicated calibration data and simulation pro-cedures. The systematic uncertainties in the cross-sectionextraction are under study.

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5. K. H. Glanderet al. (SAPHIR Collaboration), Eur.Phys. J.A 19, (2004) 251.

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