Rappold, C.

Saito, T.

GSI Repository

Improved particle identification with probabilistic approach for the HypHIprojectC.Rappold∗1,2 and T.R. Saito1,3,41GSI, Darmstadt, Germany; 2Justus-Liebig-Universita¨t Giessen, Germany; 3Johannes Gutenberg-Universita¨t, Mainz,Germany; 4The Helmholtz Institute Mainz, Mainz, GermanyThe first experiment of the HypHI collaboration aimedto demonstrate the feasibility of the hypernuclear spec-troscopy by means of heavy ion beam induced reactions.The phase 0 experiment was performed with a 6Li beamat 2AGeV impinged on a stable 12C target material. Themain goal of the experiment was to produce, reconstructand identify decay vertexes of Λ particle and 3ΛH, 4ΛH [?].With the finalized data analysis of Phase 0 experiment, thefirst results show that the experimental method is viable forthe study of hypernuclei [1, 2, 3].Possible improvement on the particle identification isnecessary in order to improve the event reconstruction ofthe Λ and hypernuclear candidates. During the track re-construction the particle identification has to define whichtrack corresponds which particle or fragment. The idea forimproving the identification is to perform a maximum like-lihood ratio test for each particle hypothesis. From thetrack reconstruction, the kinematics of the track is deter-mine and a partial particle identification is associated to thetrack thanks to the time-of-flight measurement.For each track, its velocity βmeasured is calculated fromthe measured time-of-flight and path length, then a proba-bility of being pi, K , proton, deuteron, triton for the case ofcharge Z = 1 species is given. Theoretically, we know thatβ = p/√p2 +m2, where p and m correspond to the mo-mentum and mass of the particle, therefore for a measuredmomentum pmeasured and a given mass we can calculatethat should be the velocity βlikelihood . The particle identi-fication are usually defined by a side band as explained in[1]. Following the same PID cut than in [1], the probabil-ity of being a given particle P ((βmeasured, pmeasured)|id)is then defined by a Gaussian probability density functionat βmeasured. The mean value and standard deviation ofthe Gaussian function are βtheory and βtheory ± βsideband.This probability of identification is given for each parti-cle hypothesis and correspond how close or how far is themeasured momentum and measured velocity of the trackfrom each hypothesis. Finally, the maximum likelihoodratio test is performed for the each hypothesis, P (a) =P ((β, p)|a)/∑id(P ((β, p)|id) where a corresponds to thetested species (pi, K , proton, deuteron, triton ) and id cor-responds for all the species. The particle identification isthen decided by the highest ratio.The probability of identification can also use to improvethe selectivity of the good track candidates. This new pro-cedure of the particle identification was tested on the ex-∗ c.rappold@gsi.deperimental data of the phase 0 experiment. Figure 1 showsthe mass spectrum of the track reconstruction calculatedfrom the m = p ∗√1./β2 − 1. with a first p-value cut atp > 0.05. The black line represents the mass spectrumfrom the current situation of the final results published in[1]. The red line shows under the same conditions that theblack line with a additional cut on the probability of identi-fication. A clear separation is now possible with this proba-bilistic particle identification. A huge background suppres-sion of bad track reconstruction can be seen, and all speciescan be identified directly in the mass spectrum instead ofthe β-momentum correlation plot.The clear improvement of the particle identificationshown for the phase 0 experiment data will help to the cur-rent data analysis of the phase 0.5 experiment of 20Ne+12C.Mass (GeV)0 1 2 3 4Counts / 20 MeV0100020003000Figure 1: Improved particle identification in the mass spec-trum. Initial mass spectrum after the track reconstructionand p-value cut at pv > 0.05 is shown in black line. Thered line shows the improved mass spectrum of the track re-construction when the probability of identification is usedby setting a minimum probability.References[1] C. Rappold et al., Nucl. Phys. A 913, 170 (2013).[2] C. Rappold et al., Phys. Lett. B 728, 543 (2014).[3] C. Rappold et al., Phys. Rev. C 88, 041001 (2013).[4] T.R. Saito et al., Letter Of Intent of HypHI project.MU-HADRONS-03 GSI SCIENTIFIC REPORT 2014110 DOI:10.15120/GR-2015-1-MU-HADRONS-03

Improved particle identification with probabilistic approach for the HypHI project

http://repository.gsi.de/record/183876/files/MU-HADRONS-03.pdf

Improved particle identification with probabilistic approach for the HypHI project

Abstract

Similar works

Full text

Available Versions

GSI Repository