Λ hyper nuclear spectroscopic experiment by the ( e,e’K + ) reaction at Jefferson Lab

1. 1.Introduction 2.Experimental Setup (JLab E05-115) We have been performing Λhypernuclear spectroscopic experiment by the (e,e’K+) reaction since 2000 at Thomas Jefferson National Accelerator Facility (JLab). The (e,e’K+) can achieve 100 keV (FWHM) energy resolution compared to a few MeV (FWHM) by the (K-,π-) and (π+,K+) experiments (Table.1). Therefore, more precise Λhypernuclear structures can be investigated by the (e,e’K+) experiment. 7ΛHe, 9ΛLi, 10ΛBe, 12ΛB,28ΛAl and 52ΛV were measured in the experiment at JLab Hall C. In addition, 9ΛLi, 12ΛB, 16ΛN were measured in the experiment at JLab Hall A. Figure.1 : HES-HKS group photo in the experimental hall C in JLab (2009). Figure.2 : The experimental setup of JLab E05-115 (2009) Table.1 : The features of (e,e’K+) reaction comparing to (π+,K+) and (K-,π-) reactions. • Λ binding energy • Cross section Missing Mass : M2HY = (Ee + MT - EK+ - Ee’)2 - ( pe- pK+- pe’ )2 Measure with spectrometers Λhyper nuclear spectroscopic experiment by the (e,e’K+) reaction at Jefferson Lab The (e,e’K+) experiment is a coincidence experiment between scattered electron and generated kaon. The cross section of (e,e’K+) are larger at forward scattered angles of these two particles. Therefore, both of these two particles need to be detected at forward angles at the same time. To do so, a dipole magnet (splitter magnet) was set just after the target to separate scattered electron and kaon into different directions. Figure.1 shows the experimental setup of JLab E05-115. There are HES (High resolution Electron Spectrometer) and HKS (High resolution Kaon Spectrometer) to measure momenta of scattered electron and kaon associated with (e,e’K+) reaction, respectively. Both HES and HKS consist of QQD magnets. 3.Particle identification 4.New tracking code for high multiplicity data Before Cherenkov cut Figure.3 : Picture of HKS detector package It is getting harder to perform the (e,e’K+) experiment as the proton number of the target, Ztar is larger. This is because that the background events which are caused by electromagnetic processes roughly proportional to Z2tar. Mainly, the electron arm suffers from these backgrounds. However, the hadron arm (HKS) also suffers from them which are not on the HKS optics . A positron generated in the target by pair creation process hit the vacuum chamber which is just after the HKS dipole magnet, and generate background events such as positron and electron in the HKS detectors (Fig.6). These background events make the singles rate of HKS be higher ( ~30MHz/plane, 8μA beam on 52Cr target). 1 [m] K+ p, π+ e , e+ After Cherenkov cut K+ SIMULATION Mass square [GeV/c2]2 Target Figure.4 : Mass square distribution Graduate school of science, Tohoku University Toshiyuki Gogami for HES-HKS collaboration Figure.6 : Background simulation of HKS Drift chambers -KDC1,KDC2- Aerogel (n=1.05) • Cherenkov detectors -AC,WC- • Aerogel (n=1.05) • Water (n=1.33) σ ≈ 250 [μm] TOF walls -2X,1Y,1X- (Plastic scintillators) NPE π+ We used two planar-type drift chambers which have 6-layers (uu’xx’vv’) in each chamber for HKS tracking. Figure.7 shows the multiplicity of the typical layer of the drift cambers. The multiplicity for 52Cr target (~5) is much higher than that for CH2 target (~2) as you can see the figure. The conventional tracking code that we used in JLab hall C cannot handle high multiplicity data efficiently. Therefore, we lose events for high multiplicity data in the tracking stage. To deal with the high multiplicity data, a new tracking code need to be developed. TOF σ ≈ 170 [ps] p K+ There is not only kaonin HKS but also proton and pion as background particles. To measure kaon efficiently, HKS has two drift chambers(KDC1,KDC2) for tracking, three scintillator walls(KTOF1X,1Y,2X) for TOF measurement and two type of Cherenkov detectors(WC1,WC2,AC1,AC2,AC3) for proton and pion rejection in the stages of trigger and offline analysis (Fig.3). Figure.4 shows mass square distribution which can be calculated by the following equation : Water (n=1.33) π+ K+ p NPE Figure.7 : Multiplicity of the typical layer of HKS drift chambers. Mass square [GeV/c2]2 Figure.5 : NPE of Cherenkov detector vs. mass square A new event selection routine is developed and implemented to increase the tracking efficiency for the high multiplicity data. Before the pattern recognition which is in the first stage of the tracking, hit-wires to use for tracking are selected by the combination of the TOF detectors considering HKS optics (Fig.8). After this development, the number of kaons is increased by ~130%, and also the analysis time is decreased by ~30% for 52Cr target. where, is a momentum reconstructed by a transfer matrix of HKS, and () is derived by TOF measurement. Figure.5 is showing NPE of Cherenkov detectors vs. mass square. We can distinguish pion, kaonand proton with mass square and Cherenkov detector information as you can see in the figure. When the Cherenkov and mass square cut are applied to survive ~90% kaon in the total event, <2% proton and <1% pion are contaminated in the kaon. 5.Energy scale calibration One of the large advantages of the (e,e’K+) reaction is that the absolute energy calibration can be done with data of Λ and Σ0 converted from a proton target such as CH2 and H2O target.We measure both positions and angles of electron and kaon at the reference planes. Then, those information are converted to momentum vectors at the target by transfer matrices to calculate the missing mass. Therefore, the tuning of the transfer matrices is a heart part to measure hypernuclei with better energy resolution. Figure.8 : Event display of the developed hit-wire selection for tracking. Blue squares represent hit TOF segments, green regions are selective regions determined by the TOF combinations, red markers are selected hit-wires for tracking and black markers are hit-wires which are not used for tracking. 6.Summary Figure.9 shows a coincidence time between HES and HKS after the kaon selection. It is calculated by the following equation : where, THKS and THES are the times at the target which are simply calculated by path length from the focal planes to the target and β of the particles. In the figure, the beam structure of the CEBAF ( ~2ns bunch structure) can be seen, and a peak on the center is a bunch which includes real coincidence events. • We performed the (e,e’K+) experiment at JLab Hall-C in 2009 (JLab E05-115), and successfully took data of 7ΛHe, 9ΛLi, 10ΛBe, 12ΛB and 52ΛV. • Kaon identification • When the cut applied to survive ~90%, <2% proton and <1% pion contaminate in the kaon. • New tracking code for high multiplicity data ( 52Cr , H2O target ) • The number of kaon is increased by ~130% • The analysis time is decreased by ~30% • Energy scale calibration is in progress. Figure.9 : Coincidence time between HES and HKS. p(e,e’K+)Λ ~4 MeV/c2 (FWHM) Figure.10 shows a missing mass spectrum of CH2 target. When the real coincidence events are chosen, peaks of Λ and Σ0 clearly can be seen on the Quasi-free Λ events which come from 12C and accidental background events. These peaks are used for the energy scale calibration. Also, thick tungsten alloy sieve slits which are set just before the Q-magnet of each spectrometer are used for calibration of angular component. Very Preliminary p(e,e’K+)Σ0 QF from 12C Accidental b.g. Figure.10 : Missing mass spectrum of CH2 target