Recent results from SND detector

Transcription

1 Recent results from SND detector Dmitry Shtol Budker Institute of Nuclear Physics, Novosibirsk State University, Novosibirsk, Russia HADRON September, 2015

2 VEPP-2000 e + e - collider center-of-mass energy E= GeV circumference 24.4 m round beam optics beam energy spread 0.6 МэВ at E=1.8 GeV L = cm -2 sec -1 Year Energy(GeV) L(pb -1 ) Total

3 VEPP-2000 e + e - collider During data taking period the luminosity was limited by the deficit of positrons Currently the complex VEPP-2000 is being upgraded. The maximum BEP energy will be increased up to 1 GeV. The injection system will be changed. Electrons and positrons will be transported to BEP from the VEPP-5 injection complex through 250 m beamline. Experiments at upgraded VEPP-2000 is expected to be started in

4 SND detector 3D view of electromagnetic calorimeter 1 beam pipe, 2 tracking system, 3 aerogel Cherenkov counter, 4 NaI(Tl) crystals, 5 phototriodes, 6 iron absorber, 7-9 muon detector, 10 focusing solenoids. 4

5 Physics program 1. Measurement of exclusive hadronic cross sections below 2 GeV. The goal is to obtain the total cross section for e + e hadrons, which used for calculation HVP contribition to the muon (g-2) and the running QED. 2. Study of dynamics of hadron production, i.e. separation between different intermediate states, for example,,, a 0 etc. in the reaction e + e + 0. This is needed for understanding hadronization mechanisms. 3. Hadron spectroscopy: study of light-vector-meson exitations., in particular, search for their radiative decays. 4. Search for rare and forbidden decays of the,, and mesons. 5. Study of nucleon-antinucleon pair production, extraction of the proton and neutron electromagnetic form-factors. 6. Two-photon physics, in particular, measurement of the photonmeson transition form factors for 0,,. 7. Search for production of C-even resonances: e + e,, f 1,f 2,a 2 8. Using radiative return technique as alternative method for measurement of hadronic cross sections. 9. Test of high-order QED: 2 4,5. 5

6 e + e + 0 JETP 148, 34(2015) Our results are in agreement with the SND measurements at VEPP- 2M and the BABAR data, but disagree with DM2 This is the most precise measurement to date. The two maxima in the cross section corresponds to the (1420) and (1650) resonances. Their amplitudes interfere with the tails of (782) and (1020) resonances, and with each other. e + e + 0 is the only process, in which the (1420) resonance is clearly seen. Above 1.8 GeV the cross-section energy dependence cannot be described by contributions of known resonances. 6

7 e + e + Phys. Rev. D 91, (2015) results in mode. Results in 3 0 mode are preliminary. It was assumed that the dominant reaction mechanism is (770). We observe contribution of other mechanism, presumably (1450). Our result agrees with the BABAR measurement, but has better accuracy. The (1450) contribution dominates. The (1700) contribution is small. 7

8 e + e + The product B( e + e )B( + ) = ( ) 10-7 is obtained from the fit (here is (1450)). From comparison with measurements in other decay modes we extract the ratio B( ) : B( + ) : B( + ) = ( ) : 1 : ( ), which can be compared with predictions (7-8) : 1 : (4-10). e + e + - о Using CVC hypothesis we calculate B CVC ( о ) = ( )%, which is in agreement with the measured value B exp ( о ) = ( ). 8

9 e + e K + K (preliminary) It is assumed that the dominant reaction mechanism is φ(1680) φ(1020). This hypothesis is in agreement with the data. Our result agrees with the BABAR measurement. Having much more statistics (after VEPP-2000 upgrade) we ll be able to get better precision. 9

10 e + e K + K Our results are in agreement with BABAR measurement and have similar accuracy. Both isoscalar and isovector resonances contribute into the cross section. Due to their interference the cross section has complex energy behavior. 10

11 e + e The fit includes the contributions of the (1680) and (1420) resonances. Zero (within errors) cross-section for E cm >1.8 GeV is due to resonances interference. 11

12 e + e 0 Our previous result based on data has been updated using the full SND data set. The bug has been fixed in the radiative-correction calculation. The cross section is described by a sum of the (770), (1450), and (1700) contributions. From the measured cross section we have extracted the transition form factor. It has been found that the VMD model cannot describe simultaneously our data and data obtained from the 0 + decay. 12

13 e + e p anti-p Preliminary GM s 1 d C ( ) ( cos ) 2 4mp 2, d 4s 2 GE ( s) sin s s 4E b G E /G M =1,64 ± 0,26 The cross section is constant, through it is natural to expect its decrease as =(1-4m p2 /s) 1/2 when approaching threshold. We confirm the BABAR result, that G E /G M near threshold strongly differs from unity. This was unexpected result, because G E = G M at threshold. 13

14 e + e n anti-n The e + e n anti-n cross section is constant and coincides within the errors with that for proton anti-proton (pqcd p / n = 4). p = n Either isoscalar or isovector amplitude dominates in e + e N anti-n. I. Subthreshold resonance II. Final state interaction V. F.Dmitriev, A.I.Milstein, S.G.Salnikov Phys.Atom.Nucl. 77 (2014) 1173: Paris N anti-n optical potential: I=0 attraction, I=1 repulsion the isovector form factor dominates. 14

15 Cross section (nb) The e + e 6 cross section near N anti-n threshold pp pp 6 nn nn 6 A.E. Obrazovsky, S.I. Serednyakov JETP Lett. 99 (2014) 363 In the total e + e hadrons cross section, the appearance of the e + e N anti-n processes is fully compensated by the dip in the cross section for the isovector processes e + e 3( + )+2( + 0 ). In other cross sections near N anti-n threshold, any features, comparable in magnitude with that for e + e 6, are not observed. 15

16 Phys. Rev. D 91, (2015) Search for e + e decay The rare decay e + e is predicted to have a branching fraction of (1-2) Its value is sensitive to the transition form factor. The strictest limit on the decay branching fraction B( e + e ) < at 90%CL was set in 2014 by CMD-3 at VEPP The decay is searched for using the inverse reaction e + e. About 2.9 fb -1 was accumulated at the center of mass energy of MeV. The collider energy spread (FWHM=0.590 MeV) is significantly larger than the width = MeV. The radiative correction and energy spread leads to reduction of the e + e cross section compared to the Born one by a factor of four. 16

17 Search for e + e decay The process e + e has been searched for in five decay chains: + - with and 3 0, and 0 0 with + - 0,, 3 0. No data events have been observed. The upper limit has been obtained to be B( e + e ) < at 90%CL The combined SND+CMD-3 limit is B( e + e ) < at 90%CL. 17

18 Conclusion Cross-sections of the following processes: 1. e + e + 0 ; 2. e + e + ; 3. e + e K + K ; 4. e + e ; 5. e + e 0 ; 6. e + e p anti-p; 7. e + e n anti-n; are measured. Precision of most of results is similar or better then worldwide. For e + e decay an upper limit of branching fraction was set. 18

19 We also plan to perform search for e + e decay. The inverse reaction e + e is proposed for this search. arxiv: VEPP-2000 parameters at E=m c MeV: Luminosity cm -2 sec -1. Accuracy of the energy setting 60 kev. Energy spread E =150 kev ( = kev). We have analyzed a data sample with an integrated luminosity of 108 nb -1 collected at E= MeV and found no background events for the reaction e + e in the decay mode In the absence of background, a sensitivity to B( e + e ) of 10-6 can be reached during two weeks of VEPP-2000 operation. Such a sensitivity is better than the current upper limit 19 (HADES Collaboration, 2014) by a factor of 2.3.

20 20

21 Two central charged tracks; Two clusters without tracks in EMC (photons), energy ; 0.3<E dep / s<0.8, E ch / s<0.6; After kinematic fit: 30 <θ ch <150 ; χ 2 vtx<40, χ 2 π+π- γγ <30; e + e MeV<m γγ <200 MeV. 21

22 e + e + Two central charged tracks; Two clusters without tracks in EMC (photons); 0.4<E dep / s<0.9, E ch / s<0.6; After kinematic fit: χ 2 vtx<200, χ 2 π+π- γγ <60; 350 MeV<m γγ <750 MeV; 36 <θ γ <

23 e + e K + K selection Two central charged tracks; Selecting kaons by combined de/dx and threshold Cherenkov counter; Two ore more clusters without tracks in EMC (photons); 0.4<E dep / s<0.9; After kinematic fit: χ 2 vtx<200, χ 2 K+K- γγ <30; 350 MeV<m γγ <750 MeV; 36 <θ γ <

24 e + e K + K Two central charged collinear tracks; At least one particle get into Cherenkov counter working region without hit; de/dx < 1.5(dE/dx) bhabha (p anti-p suppress). 24

25 e + e Two or more central charged tracks; Four or more clusters without tracks in EMC (photons); E dep >300 MeV. Kinematic fit: Search of common vertex, selecting track pare with minimum χ 2 vtx ; χ 2 π+ π 4γ <200 or χ 2 π+ π 4γ(γ) <200 ; m 1 γγ-m π0 <65 MeV; 400 MeV<m 2 γγ <700 MeV. 25

26 e + e 0 No charged tracks in DC; At least five clusters in EMC; E dep / s>0.5; After kinematic fit: χ 2 5γ <30 for s<1.7 GeV and χ 2 5γ <15 for s 1.7 GeV; χ 2 π0π0γ- χ 2 5γ<10; m π0γ -M ω <200 MeV/c 2 at least for one π 0 γ combination; 26

27 e + e p anti-p s>960 MeV: Two charged central tracks with maximum de/dx; de/dx>1.3(de/dx) bhabha for both particles; Energy of one of particles < 200 MeV; 36 <Θ ch <144 ; s<960 MeV: Muon system veto; 20 <Θ ch <160 ; Tracks with common vertex; 0.65 GeV <E dep <1.4 GeV; Two particles with maximum energy are collinear; 27

28 e + e n anti-n Two clusters in EMC without charged tracks; 950 MeV<E dep <1500 MeV; P EMC >0.5E beam (P EMC is a total event momentum, calculated by calorimeter); Three calorimeter layers fired; Energy deposition fraction in small polar angle area (36 < θ or θ>144 ) is no more then 0.6; No more then one non-central charged track; Muon system veto; No cosmic track in calorimeter; 28

29 Processes: e + e decay 1. η + with γγ and 3 0 decay channels; 2. η + with γγ, and 3 0 decay channels; 29