- Email: [email protected]
Detector KEDR tagger
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120 www.elsevier.com/locate/npbps
Detector KEDR tagger. V.N.Zhilich for the KEDR collaboration.a,∗ a
Budker Institute of Nuclear Physics, Novosibirsk, Russia; Novosibirsk State University.
Abstract The KEDR Tagging System is designed to enhance the detector ability to study the two-photon processes e+ e− →e+ e− X. The collider magnetic elements (dipoles and lenses) form a magnetic spectrometer for the scattered electrons and positrons. Its energies are measured with 8 blocks of the drift tube hodoscope which are placed beside of the vacuum chamber. This allows to determine an invariant mass of the system X with resolution 3-15 Mev for Winv = 300 ÷ 3000 MeV at the beam energies Eb = 1.5 − 5.0 GeV. Recently the TS was upgraded with triple-GEM 2-dimensional detectors and a laser Compton scattering calibration system. The energy resolution for the scattered e± at the level 3 · 10−4 was demonstrated. Keywords: Two-photon physics, Photon tagging, GEM, Compton scattering, KEDR detector 1. INTRODUCTION The KEDR detector at VEPP-4M electron-positron collider [1, 2] contains a unique tagging system (TS) that is intended for study of two-photon interactions e+ e− →e+ e− X. The C-even system X is originated from two virtual photons emitted by colliding electron and positron [3, 4, 5]. The reaction kinematics is fixed by the final electron and positron which are called as Scattered Electrons (SE). The detection of both SEs (”double–tag” experiment) determines the parameters of the system X independently of from the detector central part. The detection of one SE (”single–tag” experiment) provides a useful constraint and allows to suppress background. At present time KEDR collected about 15 pb−1 for Eb = 1.5 − 1.9 GeV. A valuable statistics is related to W < 0.8 GeV. The physics problems for the investigation with the help of the TS are: ∗ Corresponding
author Email address: [email protected] (V.N.Zhilich for the KEDR collaboration.)
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.02.025
• The cross section measurements of γγ → γγ resonance (π0 ) and non–resonance channels. The only direct measurements of π0 two-photon width of π0 was done at the Crystal Ball detector at 1986 year [6]. • The present results for γγ → π0 π0 , π+ π− are shown in Fig. 1 [7]. We plan to measure γγ → π+ π− cross section near threshold in the W range of 0.3-0.8 GeV. • With assumptions about reaction dynamics the total cross section of the process σ(γγ → hadrons) is approximated with a formula σ = A + B/W. The parameters A ≈ 240 nbn and B ≈ 270 nbn·GeV are related to the γN → Any NN → Any total cross sections, where N is any hadron or nucleon. Parameters A, B can be unfolded only in the range W < 2 GeV. All present experiments (see Fig. 2) are related to W > 1.5 GeV and have rather poor accuracy (10–20%) [8, 9, 10].
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
elec
tron
11 00 00 11 00Sc 11
beam
eleatter ctr ed on
Compton photons
000 111 111 000 000 111
11 00 00 11 00 11
TS 1
111 000 000 111 000 111
TS 2
P−4M
TS 3
VEP
TS 4
116
KEDR
Luminosity Monitor
Laser
laser beam
KEDR
Figure 3: The electron arm of the KEDR tagging system. TS1–TS4 are coordinate stations. The collider lenses and dipoles form a focusing magnetic spectrometer. The laser is used for the TS energy calibration.
2. Detector and system layout
Figure 1: The present experimental and theoretical results for processes γγ → π0 π0 , π+ π− . The measurements for W < 0.6GeV was done about 20 years ago.
TPC/2G SLAC
Figure 2: The Wγγ dependence of sigma(γγ− > hadrons) from the detector TPC/2γ(SLAC) 1980-1986 years (upper plot) and the detector MD-1(BINP) 1983-1985 years (lower plot).
The main idea of this tagging system(TS) is to use collider magnetic structure as a spectrometer as shown in Fig. 3. The dipole magnets allow to register SE leaving interaction point at zero angle. After emission of the virtual photon ω the SE with energy E = Eb − ω is taken away from equilibrium orbit with a help of vertical magnetic field and then is detected in one of the TS stations. The quadrupole lenses focus SEs in such a way that their transverse coordinates at the place of the detection almost do not depend on their initial angle. Thus measuring the coordinate one can determine the SE energy. Four blocks of the TS are placed along vacuum chambers at distances of 9–17 m from interaction point which allows to cover energy range ES E = (0.4 ÷ 0.97)·Eb . Three blocks have width of 90 mm and one of 180 mm. The TS was designed to work with a large background of the single Bremsstrahlung scattering (few MHz). It should provide high radiation tolerance and a reliable multitrack recognition. A drift tube hodoscope was selected (Fig. 4). The drift tubes with radii of 3 mm are a fast, stable and independent detectors for the passing electron. The radiation stable gaseous mixture CF4 + 10%iC4 H10 guaranties at least 5 years of the stable operation with a maximum luminosity. The coordinate resolution of a tube is 0.3-0.5 mm. The steel walls of 90 μm provide a good defence against synchrotron radiation photons and contributes about 0.15 mm to the coordinate resolution due to the multiple scattering. The track coordinate resolution at the block center is about 0.25–0.35 mm for the low rate condition. For a real experimental conditions this resolution is about 0.4 mm due to the pileup and pickup noise. In order to improve spatial resolution (Fig. 7) and to add the possibility of rejection of the background from single Bremsstrahlung the upgrade of the TS system was performed [11]. Each TS station was supplemented in front of it with high resolution two-
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
Figure 4: A drift tube hodoscope. 1 — a drift tube, 2 — an anode pin, 3 — an anode wire, 4 — a flange. The tube radii are 3 mm and the maximum drift time is about 30 ns. A single tube resolution is 0.3– 0.5 mm. Six double layers provide 6–12 points per track. The track coordinate resolution at the center of the block is 0.25–0.35 mm.
117
Figure 6: The averaged difference between a track coordinate at the GEM and its hodoscope extrapolation is presented. At the insertion the ΔX distribution is shown. The 90% of events are placed within σ = 0.45mm.
coordinate detector based on cascaded Gas Electron Multipliers (GEM) [12]. The configuration of the chosen triple-GEM detectors is shown in Fig. 5, upper plot. The amplitude information is read from the 2 layers strips that are shown schematically in Fig. 5, lower plot. The vertical coordinate is calculated with a difference of stereo and horizontal channel numbers. The inclination of stereo strips is variable. Such layout provides better spatial resolution in vertical direction in the orbit plane but keeps a good multitrack reconstruction capability. With a cosmic tests the GEM detector resolutions was found to be 75 μm for horizontal and 220 μm for vertical coordinates. The 2D reconstruction efficiency is 92-95%. The differrence between track coordinates at the GEM and its hodoscope extrapolation is shown in Fig. 6. The ΔX distribution can be described with a sum of two Gaussians. The Gaussian with σ = 0.45mm contains 90% of events. This GEM system was included to the KEDR DAQ at 2010 year. The detailed description of the GEM one can find elsewhere [13]. This is the first GEM-detector at the experiments in the e+e- colliders.
Figure 5: Schematic view of the triple-GEM detector and its channels structure.
3. Energy resolution of the tagging system. Our Institute have experience to work with a similar TS at the MD-1 detector [10] where collider dipoles
118
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
zero scattering angle is Xtr = a
Eb Eb + b + X0 = + b, ES E E˜ S E
E˜ S E is the true SE energy, a, b are geometrical parameters. ES E is the calculated SE energy. If E˜ S E is close to focusing energy then the scattering angle influence is suppressed. The photon energy is calculated as ω = E0 − E˜ S E ≈ Eb − ES E ,
Figure 7: The issues to the virtual photon energy resolution for one of TS modules. 1 — beam energy spread, 2,3 — track accuracy 0.1 and 0.3 mm, 4 — SE angle spread, 5 — the resulting resolution.
Figure 8: The scattered electron spectra of from the laser calibration system. The edge width characterizes the TS energy resolution. Using of the GEM improve resolution 2 times. The coordinate resolution 0.3 mm corresponds to the SE energy resolution of 2.5 · 10−4 .
formed a simple magnetic spectrometer. In the present TS the quadrupoles focus SE in such a way that their transverse coordinates at the detection place depend weakly on their initial angle. The transverse coordinate Xtr of the SE originated at the collision point X0 with a
where E0 is an initial SE energy. It is differ from Eb due to the beam energy spread. For the VEPP-4 optics there is a strong correlation between X0 and (Eb − E0 ). With that a (ES E − E˜ S E ) shift is partially compensated by the (E0 − Eb ) shift. The Fig 7 illustrates the different contributions to the virtual photon energy resolution. A real photons of the single Bremsstrahlung process have a smaller angular spread which results in the better energy resolution. To realize an advantage of the high energy resolution all TS blocks should be adjusted with an accuracy about 0.1 mm. It is very difficult to realize if one take into account the possible beam shifts after the collider tuning. To realize an accurate calibration of the tagging system the laser calibration system was designed. The laser photons with energy 1.17(2.34) eV are scattered by the beam electron to the angle of 180◦ . The energy of photon increases about 4(Eb /me )2 times while the beam electron lose an essential part of its energy and hits the TS. Both a photon and SE spectra have a very sharp edge. The edge position is used for absolute energy calibration of the TS while its width is a measure for the TS resolution. The examples of such spectra is presented in Fig 8. The GEM demonstrates 2 times better energy resolution. The σ = 0.34mm correspond to SE energy resolution about 2.5 · 10−4 . With changing of the Eb energy from 1.8 to 4.0 GeV we have calibrated the TS. The results of calibration are shown in Fig. 9. The line shows the result of the TS simulation. There obsirved difference requires some tuning of the simulation model. The second plot in Fig. 9 shows the expected energy resolution for the virtual an real photons tagged with the TS. For the real photon one can get the energy resolution about 0.3%. The expected double tag efficiency and invariant mass resolutin is shown in Fig. 10. To compare with the MD-1 tagging system the efficiency increase 2 times and the resolution about 10 times.
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
Figure 9: The expected tagging system energy resolution for Eb =1800 MeV. Upper plot — for scattering electrons σ(ES E )/Eb ·103 . Points show the experimental resolution recently measured with the laser calibration system. Lower plot — for photons σ(ω)/ω · 102 . The line 1 is for virtual photons from two-photon processes, The line 2 is for real photons from the single Bremsstrahlung process.
4. Some issues of the TS to the physical analysis. The accurate measurement of SE energies provides a useful constraints which allows to improve quality of the central part reconstruction. There are two examples of such applications. The decay gammas of the e+ e− →e+ e− π0 , η process are detected in the calorimeter endcaps. The photon energy resolution is degraded due to the beampipe and connection gaps. The invariant mass of two gammas also can be calculated with the SE energy and gamma’s angles. The expected signals are shown in Fig. 11. The invariant mass resolution is improved about 2 times. Another example is for a double tag mode. The
119
Figure 10: The tagging system efficiency (upper plot) and an invariant mass resolution (lower plot) the for double-tagged events at the collider energy 1800 and 5000 MeV.
events of γγ→3, 4π was selected. The selection requires at least one track in the central part and some more acollinear tracks or clusters. The missing mass was calculates for two SE and 1 track. The signal should have Mmis ≥ mπ . The results are shown in Fig 12. The is a clear signal of 2 charge events mostly γγ→e+ e− which can be easily rejected. 5. Acknowledgments We acknowledge the staff of VEPP-4 for their hard work during the experiment. This experiment was supported in part by the Ministry of Education and Science of the Russian Federation and by Russian Foundation for Basic Research under grants 09-02-01143-a and 1002-00871-a.
120
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
References [1] V.V. Anashin et al. Nucl.Instrum.Meth. A 478 (2002) 420. [2] V.Smaluk for the VEPP-4 team Proc. of EPAC 2004, Luzern, 2004, p. 749. [3] Budnev V. M. et al. Physics Reports. 1975. V. 15. P.181-282. [4] Brodsky S. J. Proc. XIth. Int. Workshop on Gamma-Gamma collisions, Egmond aan Zee. Netherlands, 10-15 May 1997. P. 197. [5] Urner D. Review of two photon interactions // AIP Conf. Proc. 2004. 698:566-570. [6] D. Williams et al. By Crystal Ball Collaboration. Phys.Rev.D38:1365,1988. [7] Berger et al. Zeit. Phys. C 26, 199 ,1985 Boyer et al. Pahys. Rev. D 42, 1350, 1990 [8] Berger et al. Phys. Lett. 149B, 421, 1984. [9] Aihara et al. Phys. Rev. D41, 2667, 1990 [10] Baru S. E. et al. Physics Reports. 1996. V. 267. P. 71-159. [11] Aulchenko V. M. et al. Nucl. Instr. Meth. A. 2002. V. 494. P. 241245. [12] Sauli F. Nucl. Instr. Meth. A. 1997. V. 386. P. 531-534. [13] Aulchenko V. M. et. al. Nucl. Instr. Meth. A. 2009. V. 598. P. 112115. [14] Aulchenko V. M. et al. 2011 J. of Instr. 6 P07001. Figure 11: The expected signal of γγ → π0 , η → γγ for 10 pb−1 of the MC simulation. Open histogram — an invariant mass is calculated with 2 photons in the endcap calorimeter. Hatched histogram — an invariant mass is calculated with cluster angles and the SE energy. The resolution improves about 2 times.
Figure 12: Experiment, 3 pb−1. The γγ→3, 4π was selected with double tagging. Missing mass is calculated for e, + e+ &1 track. The histogram shows the distribution for Δ =
2 −m2 Mmis π
m2π
, where Mmis
is a missing mass. Open histogramm — selected events, horizontaly hatched — simulated γγ→hadrons, cross hatched — γγ→ee, μμ. There is a clear peak of γγ → 2charged background.
Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120 www.elsevier.com/locate/npbps
Detector KEDR tagger. V.N.Zhilich for the KEDR collaboration.a,∗ a
Budker Institute of Nuclear Physics, Novosibirsk, Russia; Novosibirsk State University.
Abstract The KEDR Tagging System is designed to enhance the detector ability to study the two-photon processes e+ e− →e+ e− X. The collider magnetic elements (dipoles and lenses) form a magnetic spectrometer for the scattered electrons and positrons. Its energies are measured with 8 blocks of the drift tube hodoscope which are placed beside of the vacuum chamber. This allows to determine an invariant mass of the system X with resolution 3-15 Mev for Winv = 300 ÷ 3000 MeV at the beam energies Eb = 1.5 − 5.0 GeV. Recently the TS was upgraded with triple-GEM 2-dimensional detectors and a laser Compton scattering calibration system. The energy resolution for the scattered e± at the level 3 · 10−4 was demonstrated. Keywords: Two-photon physics, Photon tagging, GEM, Compton scattering, KEDR detector 1. INTRODUCTION The KEDR detector at VEPP-4M electron-positron collider [1, 2] contains a unique tagging system (TS) that is intended for study of two-photon interactions e+ e− →e+ e− X. The C-even system X is originated from two virtual photons emitted by colliding electron and positron [3, 4, 5]. The reaction kinematics is fixed by the final electron and positron which are called as Scattered Electrons (SE). The detection of both SEs (”double–tag” experiment) determines the parameters of the system X independently of from the detector central part. The detection of one SE (”single–tag” experiment) provides a useful constraint and allows to suppress background. At present time KEDR collected about 15 pb−1 for Eb = 1.5 − 1.9 GeV. A valuable statistics is related to W < 0.8 GeV. The physics problems for the investigation with the help of the TS are: ∗ Corresponding
author Email address: [email protected] (V.N.Zhilich for the KEDR collaboration.)
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.02.025
• The cross section measurements of γγ → γγ resonance (π0 ) and non–resonance channels. The only direct measurements of π0 two-photon width of π0 was done at the Crystal Ball detector at 1986 year [6]. • The present results for γγ → π0 π0 , π+ π− are shown in Fig. 1 [7]. We plan to measure γγ → π+ π− cross section near threshold in the W range of 0.3-0.8 GeV. • With assumptions about reaction dynamics the total cross section of the process σ(γγ → hadrons) is approximated with a formula σ = A + B/W. The parameters A ≈ 240 nbn and B ≈ 270 nbn·GeV are related to the γN → Any NN → Any total cross sections, where N is any hadron or nucleon. Parameters A, B can be unfolded only in the range W < 2 GeV. All present experiments (see Fig. 2) are related to W > 1.5 GeV and have rather poor accuracy (10–20%) [8, 9, 10].
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
elec
tron
11 00 00 11 00Sc 11
beam
eleatter ctr ed on
Compton photons
000 111 111 000 000 111
11 00 00 11 00 11
TS 1
111 000 000 111 000 111
TS 2
P−4M
TS 3
VEP
TS 4
116
KEDR
Luminosity Monitor
Laser
laser beam
KEDR
Figure 3: The electron arm of the KEDR tagging system. TS1–TS4 are coordinate stations. The collider lenses and dipoles form a focusing magnetic spectrometer. The laser is used for the TS energy calibration.
2. Detector and system layout
Figure 1: The present experimental and theoretical results for processes γγ → π0 π0 , π+ π− . The measurements for W < 0.6GeV was done about 20 years ago.
TPC/2G SLAC
Figure 2: The Wγγ dependence of sigma(γγ− > hadrons) from the detector TPC/2γ(SLAC) 1980-1986 years (upper plot) and the detector MD-1(BINP) 1983-1985 years (lower plot).
The main idea of this tagging system(TS) is to use collider magnetic structure as a spectrometer as shown in Fig. 3. The dipole magnets allow to register SE leaving interaction point at zero angle. After emission of the virtual photon ω the SE with energy E = Eb − ω is taken away from equilibrium orbit with a help of vertical magnetic field and then is detected in one of the TS stations. The quadrupole lenses focus SEs in such a way that their transverse coordinates at the place of the detection almost do not depend on their initial angle. Thus measuring the coordinate one can determine the SE energy. Four blocks of the TS are placed along vacuum chambers at distances of 9–17 m from interaction point which allows to cover energy range ES E = (0.4 ÷ 0.97)·Eb . Three blocks have width of 90 mm and one of 180 mm. The TS was designed to work with a large background of the single Bremsstrahlung scattering (few MHz). It should provide high radiation tolerance and a reliable multitrack recognition. A drift tube hodoscope was selected (Fig. 4). The drift tubes with radii of 3 mm are a fast, stable and independent detectors for the passing electron. The radiation stable gaseous mixture CF4 + 10%iC4 H10 guaranties at least 5 years of the stable operation with a maximum luminosity. The coordinate resolution of a tube is 0.3-0.5 mm. The steel walls of 90 μm provide a good defence against synchrotron radiation photons and contributes about 0.15 mm to the coordinate resolution due to the multiple scattering. The track coordinate resolution at the block center is about 0.25–0.35 mm for the low rate condition. For a real experimental conditions this resolution is about 0.4 mm due to the pileup and pickup noise. In order to improve spatial resolution (Fig. 7) and to add the possibility of rejection of the background from single Bremsstrahlung the upgrade of the TS system was performed [11]. Each TS station was supplemented in front of it with high resolution two-
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
Figure 4: A drift tube hodoscope. 1 — a drift tube, 2 — an anode pin, 3 — an anode wire, 4 — a flange. The tube radii are 3 mm and the maximum drift time is about 30 ns. A single tube resolution is 0.3– 0.5 mm. Six double layers provide 6–12 points per track. The track coordinate resolution at the center of the block is 0.25–0.35 mm.
117
Figure 6: The averaged difference between a track coordinate at the GEM and its hodoscope extrapolation is presented. At the insertion the ΔX distribution is shown. The 90% of events are placed within σ = 0.45mm.
coordinate detector based on cascaded Gas Electron Multipliers (GEM) [12]. The configuration of the chosen triple-GEM detectors is shown in Fig. 5, upper plot. The amplitude information is read from the 2 layers strips that are shown schematically in Fig. 5, lower plot. The vertical coordinate is calculated with a difference of stereo and horizontal channel numbers. The inclination of stereo strips is variable. Such layout provides better spatial resolution in vertical direction in the orbit plane but keeps a good multitrack reconstruction capability. With a cosmic tests the GEM detector resolutions was found to be 75 μm for horizontal and 220 μm for vertical coordinates. The 2D reconstruction efficiency is 92-95%. The differrence between track coordinates at the GEM and its hodoscope extrapolation is shown in Fig. 6. The ΔX distribution can be described with a sum of two Gaussians. The Gaussian with σ = 0.45mm contains 90% of events. This GEM system was included to the KEDR DAQ at 2010 year. The detailed description of the GEM one can find elsewhere [13]. This is the first GEM-detector at the experiments in the e+e- colliders.
Figure 5: Schematic view of the triple-GEM detector and its channels structure.
3. Energy resolution of the tagging system. Our Institute have experience to work with a similar TS at the MD-1 detector [10] where collider dipoles
118
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
zero scattering angle is Xtr = a
Eb Eb + b + X0 = + b, ES E E˜ S E
E˜ S E is the true SE energy, a, b are geometrical parameters. ES E is the calculated SE energy. If E˜ S E is close to focusing energy then the scattering angle influence is suppressed. The photon energy is calculated as ω = E0 − E˜ S E ≈ Eb − ES E ,
Figure 7: The issues to the virtual photon energy resolution for one of TS modules. 1 — beam energy spread, 2,3 — track accuracy 0.1 and 0.3 mm, 4 — SE angle spread, 5 — the resulting resolution.
Figure 8: The scattered electron spectra of from the laser calibration system. The edge width characterizes the TS energy resolution. Using of the GEM improve resolution 2 times. The coordinate resolution 0.3 mm corresponds to the SE energy resolution of 2.5 · 10−4 .
formed a simple magnetic spectrometer. In the present TS the quadrupoles focus SE in such a way that their transverse coordinates at the detection place depend weakly on their initial angle. The transverse coordinate Xtr of the SE originated at the collision point X0 with a
where E0 is an initial SE energy. It is differ from Eb due to the beam energy spread. For the VEPP-4 optics there is a strong correlation between X0 and (Eb − E0 ). With that a (ES E − E˜ S E ) shift is partially compensated by the (E0 − Eb ) shift. The Fig 7 illustrates the different contributions to the virtual photon energy resolution. A real photons of the single Bremsstrahlung process have a smaller angular spread which results in the better energy resolution. To realize an advantage of the high energy resolution all TS blocks should be adjusted with an accuracy about 0.1 mm. It is very difficult to realize if one take into account the possible beam shifts after the collider tuning. To realize an accurate calibration of the tagging system the laser calibration system was designed. The laser photons with energy 1.17(2.34) eV are scattered by the beam electron to the angle of 180◦ . The energy of photon increases about 4(Eb /me )2 times while the beam electron lose an essential part of its energy and hits the TS. Both a photon and SE spectra have a very sharp edge. The edge position is used for absolute energy calibration of the TS while its width is a measure for the TS resolution. The examples of such spectra is presented in Fig 8. The GEM demonstrates 2 times better energy resolution. The σ = 0.34mm correspond to SE energy resolution about 2.5 · 10−4 . With changing of the Eb energy from 1.8 to 4.0 GeV we have calibrated the TS. The results of calibration are shown in Fig. 9. The line shows the result of the TS simulation. There obsirved difference requires some tuning of the simulation model. The second plot in Fig. 9 shows the expected energy resolution for the virtual an real photons tagged with the TS. For the real photon one can get the energy resolution about 0.3%. The expected double tag efficiency and invariant mass resolutin is shown in Fig. 10. To compare with the MD-1 tagging system the efficiency increase 2 times and the resolution about 10 times.
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
Figure 9: The expected tagging system energy resolution for Eb =1800 MeV. Upper plot — for scattering electrons σ(ES E )/Eb ·103 . Points show the experimental resolution recently measured with the laser calibration system. Lower plot — for photons σ(ω)/ω · 102 . The line 1 is for virtual photons from two-photon processes, The line 2 is for real photons from the single Bremsstrahlung process.
4. Some issues of the TS to the physical analysis. The accurate measurement of SE energies provides a useful constraints which allows to improve quality of the central part reconstruction. There are two examples of such applications. The decay gammas of the e+ e− →e+ e− π0 , η process are detected in the calorimeter endcaps. The photon energy resolution is degraded due to the beampipe and connection gaps. The invariant mass of two gammas also can be calculated with the SE energy and gamma’s angles. The expected signals are shown in Fig. 11. The invariant mass resolution is improved about 2 times. Another example is for a double tag mode. The
119
Figure 10: The tagging system efficiency (upper plot) and an invariant mass resolution (lower plot) the for double-tagged events at the collider energy 1800 and 5000 MeV.
events of γγ→3, 4π was selected. The selection requires at least one track in the central part and some more acollinear tracks or clusters. The missing mass was calculates for two SE and 1 track. The signal should have Mmis ≥ mπ . The results are shown in Fig 12. The is a clear signal of 2 charge events mostly γγ→e+ e− which can be easily rejected. 5. Acknowledgments We acknowledge the staff of VEPP-4 for their hard work during the experiment. This experiment was supported in part by the Ministry of Education and Science of the Russian Federation and by Russian Foundation for Basic Research under grants 09-02-01143-a and 1002-00871-a.
120
V.N. Zhilich / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 115–120
References [1] V.V. Anashin et al. Nucl.Instrum.Meth. A 478 (2002) 420. [2] V.Smaluk for the VEPP-4 team Proc. of EPAC 2004, Luzern, 2004, p. 749. [3] Budnev V. M. et al. Physics Reports. 1975. V. 15. P.181-282. [4] Brodsky S. J. Proc. XIth. Int. Workshop on Gamma-Gamma collisions, Egmond aan Zee. Netherlands, 10-15 May 1997. P. 197. [5] Urner D. Review of two photon interactions // AIP Conf. Proc. 2004. 698:566-570. [6] D. Williams et al. By Crystal Ball Collaboration. Phys.Rev.D38:1365,1988. [7] Berger et al. Zeit. Phys. C 26, 199 ,1985 Boyer et al. Pahys. Rev. D 42, 1350, 1990 [8] Berger et al. Phys. Lett. 149B, 421, 1984. [9] Aihara et al. Phys. Rev. D41, 2667, 1990 [10] Baru S. E. et al. Physics Reports. 1996. V. 267. P. 71-159. [11] Aulchenko V. M. et al. Nucl. Instr. Meth. A. 2002. V. 494. P. 241245. [12] Sauli F. Nucl. Instr. Meth. A. 1997. V. 386. P. 531-534. [13] Aulchenko V. M. et. al. Nucl. Instr. Meth. A. 2009. V. 598. P. 112115. [14] Aulchenko V. M. et al. 2011 J. of Instr. 6 P07001. Figure 11: The expected signal of γγ → π0 , η → γγ for 10 pb−1 of the MC simulation. Open histogram — an invariant mass is calculated with 2 photons in the endcap calorimeter. Hatched histogram — an invariant mass is calculated with cluster angles and the SE energy. The resolution improves about 2 times.
Figure 12: Experiment, 3 pb−1. The γγ→3, 4π was selected with double tagging. Missing mass is calculated for e, + e+ &1 track. The histogram shows the distribution for Δ =
2 −m2 Mmis π
m2π
, where Mmis
is a missing mass. Open histogramm — selected events, horizontaly hatched — simulated γγ→hadrons, cross hatched — γγ→ee, μμ. There is a clear peak of γγ → 2charged background.