Monte Carlo simulations were made for a possible DIRC at the WASA detector at
COSY. A statistical method for pattern recognition is presented and the
possible angle resolution and velocity precision achieved are discussed.Comment: Minor changes in text. Figures updated. accepted by JINS

H Machner

J Ritman

K. Kaufmann

R Siudak

R. Brun

V Kyryanchuk

WASA-at-COSY collaboration

English

arXiv

Monte Carlo simulations were performed to investigate the possibility to add a DIRC detector to the WASA detector at the COSY accelerator. A statistical method for pattern recognition is presented and the possible angle resolution and velocity precision achieved are discussed

Kyryanchuk, V.

Machner, H.

Siudak, R.

Ritman, J.

Juelich Shared Electronic Resources

arXiv:0906.5547v3  [physics.data-an]  8 Sep 2009Event Reconstruction for a DIRCV. Kyryanchuk a 1 , H. Machner a,b 2, R. Siudak a3 , J. RitmanaaInstitut fu¨r Kernphysik and Ju¨lich Centre for Hadron Physics,Forschungszentrum Ju¨lich, 52425 Ju¨lich, GermanybFachbereich Physik, University Duisburg-Essen, Lotharstr. 1,47048 Duisburg, GermanyAbstractMonte Carlo simulations were performed to investigate the possibility to add aDIRC detector to the WASA detector at the COSY accelerator. A statisticalmethod for pattern recognition is presented and the possible angle resolution andvelocity precision achieved are discussed.1 IntroductionIt is planned to measure rare decays of η′ mesons with the WASA at COSYdetector [1], [2]. The foreseen production reaction isp+ p→ η′ + p+ p (1)where the two protons will be measured and the η′ will be identified viamissing mass technique. The cross section for this process is small, becauseno nucleon resonance couples strongly to the η′p channel, in contrary to ηproduction. The η′ peak will be on top of a huge multi-pion background.The only chance to identify the η′ is to measure the four momentum vectorsof the two protons with high accuracy. Although their emission angles(θ, φ) are measured with sufficient precision, their kinetic energies arepoorly determined by the present setup. A possible option to achievesufficient resolutions seems to be a DIRC detector (Detection [of] InternallyReflected Cherenkov [light]) [3] in the forward detection system of WASA.Cherenkov detectors are widely used in high energy physics. They can havedifferent applications: particle identification, energy measuring, thresholddetector for detecting specific type of particles in a definite energy range.1Our aim is to measure the velocity of protons in the kinetic energy rangefrom T = 400 MeV to T = 1000 MeV with a precision better than∆β/β = 0.5%2 DIRC DesignWith the Monte Carlo package GEANT3 [4] different radiator materialswere studied: LiF, CaF2, quartz (fused silica) and plexiglas. All materialshave different refractive indices which are smooth functions of the wavelength in the visible range (see Fig. 1). Quartz has the largest values forthe refraction index n at any given wavelength λ compared to the two othermaterials. n varies from 1.60 in the UV range to 1.45 in the IR range. Thetransmittances are rather independent from the wavelength above a certain“threshold” wavelength as is shown in Fig. 2 (data taken from Ref. [5]and [6]). The radiator should have a large refractive index and hightransmittance to measure particles with “low” kinetic energy. LiF hashigher transmittance in the UV range compared to CaF2 and quartz. Thisgives the possibility to build a Cherenkov detector with LiF as radiator anda proper photocathode for detecting photons in the UV range. However, inthe visible range, where the transmittance of LiF and quartz arecompatible, quartz has the larger refractive index. We therefore omit CaF2and LiF in the further studies.1.401.441.481.521.561.60refractive index n0 100 200 300 400 500 600 700 800 900 1000wavelengt λ (nm)quartzCaF2LiFFigure 1: The refractive index n as function of the wavelength for the indi-cated materials (from Refs. [5, 6]).20.00.20.40.60.81.0quantum efficiencytransmittance0 100 200 300 400 500 600 700 800 900 1000wavelength  λ (nm)LiFCH4Quartz (UV)CaF2Quartz (IR)CsI (st)CsTeKCsSbGaAsPCsI (refl.)Figure 2: Radiator transmittances (solid curves) [5] and quantum efficienciesof photocathodes (dotted curves) [7–10] as function of the wave length. Thebulk transmittances are given for 10 mm thickness and reflection losses atthe surface result in an additional reduction of the transmittance by 10%.The quantum efficiencies for different photocathodes are also shown in Fig.2 (from Refs. [7–10]). CsI and CsTe are used for photon detecting in theUV range. Bi-alkali (KCsSb) and multi-alkali (GaAsP) photocathodes areused for the near UV and the visible light range. These types ofphotocathodes have larger and broader distributions of the correspondingquantum efficiency function. This could compensate a lower number ofemitted photons in this range of wavelengths. In a first step, simulationswere performed for a RICH type Cherenkov detector with differentradiators and photon detectors. However, a DIRC type detector was foundto be favorable due to following reasons: energy losses in the detector, thebroad range of velocities to be measured in the proximity to the Cherenkovthreshold and the availability of very fast photon detectors. Based on theabove discussed properties of the materials we limit ourselves in thesimulations of DIRC detectors to fused silica and UV transparent plexiglasas radiator and photomultipliers with bialkali photocathodes.We will now concentrate on the DIRC detector. The design differs from theBabar DIRC [3,11]. It consists of radiator bars with dimensions 2x6x50cm3 and a focusing element with a cylindrical mirror (Figs. 3 and 4). Thedimensions were for fused silica whereas for plexiglas an increased thicknessof 4 cm was assumed.The individual modules are arranged in two planes,one above and one below the beam axis. The normal axis to each plane is3tilted by 20◦ away from the beam axis (see Fig. 3). Emitted Cherenkovphotons create an image on a pixelised screen. Each pixel is a bi-alkaliphotocathode with dimension 5x5 mm2.protonbeam50 cm23 cm70oFigure 3: Schematic side view of the detector. The radiator bars are inclinedby 70 degree with respect to the beam axis. A simulated proton is indicatedwith an arrow, the Cherenkov light as the zigzag and arrow. The focussingbar (mirror) is on top.3 Monte Carlo ResultsA single event image consists of two arcs, which are the results of theinternal reflection from the left and right side of the radiator bar. Theshape of the arcs depend on the kinetic energy and particle species. Therelative positions of the arcs depend on the horizontal angle of the detectedparticle track: the larger the horizontal angle, the larger the distancebetween arc centers [3].We have simulated Cherenkov images for protons with 600 MeV and 1000MeV kinetic energy. We applied a left handed coordinate system with the46 cm50 cmFigure 4: Assembly of several bars to one module. Two such modules areforeseen, one above and one below the beam.z-axis in the beam direction, x− z defines the horizontal plane and y isvertical to this plane. The angle between the ejectile momentum and the(x-z) plane is θvert and the angle between the momentum projected in theplane and the beam direction is θhoriz. Thus θhoriz = arctan(zx/pz) andθvert = arcsin(py/p). Simulations were performed for track directionsθhoriz ∼ 8.5◦ fixed and θvert varied and vice versa: θvert ∼ 8.5◦ fixed and θhorizvaried. They are shown in Fig. 5.Figure 5: Left: Scatter plot of the position of detected photons from 100simulated events for protons with kinetic energy T= 600 MeV. The abscissaand ordinate are the horizontal and vertical position numbers on the photo-cathode. Right: Same as left but for T=1000 MeVThe average number of emitted photons depends on the path through theradiator end the kinetic energy. Fig. 6 shows the dependence of the numberof detected photons on the horizontal track direction for protons with5kinetic energy T = 600 MeV. The number of detected photons dropssharply when the particle passes through the edges of the radiator bars.This effect results from the detector construction.0102030405060number of ph otons0 2 4 6 8 10 12 14θhor  (O)Figure 6: The average number of the detected photons as function of thehorizontal angle for protons with T = 600 MeV.Furthermore, image distortion is also observed. Smeared arcs or even fourarcs could be observed when the particle goes through the edges of twoneighboring bars.4 Event ReconstructionThe following method is used for event reconstruction. A collection ofevents with different particle species as well as energies and directions issimulated. Then a second set of simulated data named detected are“compared” to the simulated data. The comparison is realized by thefollowing function F (2)F (k, (E, P )) =∑i,jdet(i, j) · sim(i, j). (2)Here k denotes the particle species, E, P the energy and momentum of theparticle, det(i, j) is the number of detected photons in (i, j)-th6photoelement in the experiment and sim(i, j) the simulated averagenumber of detected photons in (i, j)-th photoelement. The expression is amaximum when the experimental image on the screen is similar to thesimulated one. This method provides an opportunity to determine β andthe direction of a particle if the particle type is known. The particle speciesinformation has to come from other sources.The possibility of track reconstruction for protons with T=600 MeV isdemonstrated in Figs. 7 and 8. The particle direction is extracted underthe assumption that the particle species and its energy are known.Horizontal and vertical angles relative to the beam direction arereconstructed given that one of both is fixed. The small widths (σ) ofGaussians fitted to the data of ∼0.3◦ and ∼0.5◦ indicate the sensitivity ofthe method. Similar results were obtained for protons with T=1000 MeV.100200300400500F(p,E,P )2 4 6 8 10 12 14 16θver ( 0)θhor=8.5310100200300400500F(p,E,P )0 4 8 12 16θhor ( 0)θver=8.5310Figure 7: Track reconstruction. Left: F as function of the vertical angle andfor fixed horizontal angle. The results (dots) are connected by lines to guidethe eye. The smooth curve is a fit with a Gaussian plus a constant. Right:Same as left but horizontal and vertical angles are interchanged.Fig. 8 shows the β-resolution for protons. Here we have assumed that theangle measurement is performed with other detectors. The method gives aresolution (σβ/β) for protons better than 0.3%. The results obtained canbe transformed into the missing mass resolution. For an incident protonbeam with 3.3 GeV/c momentum a missing mass resolution of σmm ≈ 0.570.00150.00200.00250.0030σβ/β0.75 0.80 0.85 0.90 0.95βFigure 8: Relative β-resolution for protons as function of β. The results areshown by dots, the solid line is a linear fit to the dots.MeV is achieved.5 SummaryWe have designed a DIRC detector to measure protons with kinetic energyfrom 400 to 1000 MeV. Monte Carlo simulations for such a device havebeen performed. A method is discussed to deduce four momentum vectorsof the protons. The method gives reliable information for the eventreconstruction: velocity-resolution for protons is better than 0.3%. Theroutine is able to reconstruct tracks and identify particles when additionalinformation is provided by complementary detectors. The large data baseof simulated events is a disadvantage of this method; however, optimizationof the algorithm will increase the speed.References[1] WASA-at-COSY Collaboration, H.H. Adam, et al.,arXiv:nucl-ex/0411038 (2004).[2] WASA-at-COSY Collaboration, C. Adolph, et al., Phys. lett. B 677, 24(2009).8[3] P. Coyle et al., Nucl. Instr. and Meth. in Phys. Research A 343, 292(1994).[4] R. Brun, F. Bruyant, M. Maire, A.C. McPherson, P. Zanarini, Programpackage GEANT3, cernlib@cern.ch (1987).[5] SCHOTT optical glass data sheets, www.schott.com.[6] Korth Kristalle, www.korth.de/de/503728952d091450d/index.htm[7] G. R. Carruthers, Applied Optics 8, 633 (1969).[8] A.Breskin, Nucl. Instr. and Methods in Phys. Research A 371, 116(1996).[9] K. Kaufmann, http://www.photonics.com/Content/ReadArticle.aspx?ArticleID=25145.[10] A. Braem, C. Joram, F. Piuz, E. Schyns, J. Se´guinot, Nucl. Instr. andMeth. in Phys. Research A 502, 205 (2003).[11] J. Cohen-Tanugi, M. Convery, B. Ratcliff, X. Sarazin, J. Schwiening, J.Va’vra, Nucl. Instr. and Meth. in Phys. Research A 515 680 (2003).292-299.9

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