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Λ hyper nuclear spectroscopic experiment
+
by the (e,e’K ) reaction at Jefferson Lab
Graduate school of science, Tohoku University
Toshiyuki Gogami for HES-HKS collaboration
1.Introduction
2.Experimental Setup (JLab E05-115)
We have been performing Λ hypernuclear
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 Λ hypernuclear structures can be
investigated by the (e,e’K+) experiment.
7 He, 9 Li, 10 Be, 12 B,28 Al and 52 V were
Λ
Λ
Λ
Λ
Λ
Λ
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
The (e,e’K+) experiment is a coincidence experiment between
HES
HKS
scattered electron and generated kaon. The cross section of
(e,e’K+) are larger at forward scattered angles of these two
-4
-4
Δp/p
~2×10
~2×10
particles. Therefore, both of these two particles need to be
detected at forward angles at the same time. To do so, a dipole
Momentum 0.844 ± 0.144
1.20± 0.15
magnet (splitter magnet) was set just after the target to separate
[GeV/c]
scattered electron and kaon into different directions.
Angle
3.0 – 9.0
1.0 – 13.0
Figure.1 shows the experimental setup of JLab E05-115. There
[degree]
are HES (High resolution Electron Spectrometer) and HKS (High
2.344
resolution Kaon Spectrometer) to measure momenta of scattered Beam energy
[GeV]
electron and kaon associated with (e,e’K+) reaction, respectively.
7Li , 9Be , 10B , 12C , 52Cr (,CH ,H O)
Target
Both HES and HKS consist of QQD magnets.
2 2
(Hypernuclei) (7ΛHe, 9ΛLi, 10ΛBe, 12ΛB, 52ΛV) (,Λ,Λ)
3.Particle identification
4.New tracking code for high multiplicity data
Before Cherenkov cut
Figure.3 : Picture of HKS detector package
K+
p, π+
1 [m]
e , e+
After Cherenkov cut
K+
Mass square [GeV/c2]2
Target
Figure.4 : Mass square distribution
Figure.6 : Background simulation of HKS
There is not only kaon in 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 :
𝑚2 = 𝑝2(1 𝛽2 − 1)
NPE
Aerogel (n=1.05)
π+
K+
NPE
Drift chambers
Cherenkov detectors -AC,WC-KDC1,KDC2σ ≈ 250 [μm]
• Aerogel (n=1.05)
TOF walls -2X,1Y,1X• Water (n=1.33)
(Plastic scintillators)
TOF σ ≈ 170 [ps]
π+
p
Water (n=1.33)
K+
p
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).
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.
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).
where, 𝑝 is a momentum reconstructed by a transfer matrix of HKS, and 𝛽 (= 𝑣 𝑐 ) is derived by TOF
measurement. Figure.5 is showing NPE of Cherenkov detectors vs. mass square. We can distinguish pion, kaon
and 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.9 shows a coincidence time between HES and HKS after
the kaon selection. It is calculated by the following equation :
𝑇𝑐𝑜𝑖𝑛 = 𝑇𝐻𝐾𝑆 − 𝑇𝐻𝐸𝑆
Figure.9 : Coincidence time between HES and HKS.
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
+
p(e,e’K )Λ
( ~2ns bunch structure) can be seen, and a peak on the center is a
~4 MeV/c2 (FWHM)
bunch which includes real coincidence events.
Figure.10 shows a missing mass spectrum of CH2 target. When the
0 clearly can be
+
0
real
coincidence
events
are
chosen,
peaks
of
Λ
and
Σ
p(e,e’K )Σ
seen on the Quasi-free Λ events which come from 12C and
accidental background events. These peaks are used for the energy
12
QF from C
scale calibration. Also, thick tungsten alloy sieve slits which are set
Accidental b.g.
just before the Q-magnet of each spectrometer are used for
Figure.10 : Missing mass spectrum of CH2 target
calibration of angular component.
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.
After this development, the number of
kaons is increased by ~130%, and also
the analysis time is decreased by ~30%
for 52Cr target.
6.Summary
• 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.