CMD-3 measurement CMD-3 measurement of e+e- π+π- of e+e- π+π- → → Fedor Ignatov Fedor Ignatov BINP, Novosibirsk BINP, Novosibirsk PhiPsi17, Mainz PhiPsi17, Mainz
R(s), e+e- → hadrons R(s), e+e- → hadrons measurement of R(s) : R s = 0 e e − ∗ hadrons 0 e e − ∗ − R(s) is one of the fundamental quantities in high energy physics: its reflects number of quarks and colors; used for pQCD tests; QCD sum rules provide a method of extracting from R(s): quark masses,quark and gluon condensates, Λ QCD Through dispersion relations it is essential for the interpretation of precision measurements of: muon (g-2) - good test of SM α QED (M Z ) - necessary for precise electroweak predictions The value and the error of the hadronic contribution to muon (g-2) are dominated by low energy R(s) (<2GeV gives 93% of the value). 2 π + π − gives the main contribution (73%) to a μ and its precision 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
50 years of hadron production at colliders 50 years of hadron production at colliders 1 September 1967 Start of e+e- hadrons measurements → Phys.Lett. 25B (1967) no.6, 433-435 VEPP-2, Novosibirsk e+e- → ρ ππ → Detector was made from different layers of Spark chambers, readouts by photo camera 3 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Rho meson today Rho meson today Before 1985 1967: Low statistical precision 1972: 1975: Systematic >10% 1980: NA7 A few points with >1-5% 1981: 1984: 1979-1984: 1985 - VEPP-2M 1984: 1985: with more detailed scan 1989: OLYA systematic 4% 2005: CMD 2% 2004: 2005: 2004-2009: 2004 with CMD2 at VEPP-2M 2011: 2009: was boost to systematic: 0.6% 2016: (near same total statistic) The uncertainty in a µ (had) was improved by factor 3 as the result of VEPP-2M measurements New ISR method e+e- → γ + hadron: New g-2 experiments and future e+e- as ILC KLOE: 0.8% require average precision ~0.2% BaBar: 0.5% BES: 0.9% 4 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Published cross section e+ e− → π+ π− Published cross section e+ e− → π+ π− Relative to CMD-2 fit, yellow band – systematic value Points, red band: only statistical error Systematic B.Malaescu, Moriond 2014 Uncertainties In integral, there is reasonable agreement (ρ-region) between existing data sets CMD2: 0.6-0.8% SND: 1.5% But there are local inconsistencies larger than BABAR :0.5% claimed systematic errors additional scale → KLOE: 0.8% factor for error of integral value BES: 0.9% 5 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
VEPP-2000 e+e- collider (2E<2 GeV) VEPP-2000 e+e- collider (2E<2 GeV) 250 m (2010-2013,2016-) beamline e+/e- source ✗ New positron source from 2016 CMD-3 (no luminosity limitation due to lack of e+) before after upgrade 2×10 7 3×10 8 e + /sec VEPP-2000 BEP 10 9 10 11 e − /sec BEP E max , МэВ 825 1000 e + ,e − booster See on: Thursday, afternoon SND See on: Thursday, afternoon 1000 MeV Dmitry SHWARTZ Dmitry SHWARTZ “Overview of the BINP “Overview of the BINP accelerator complex” accelerator complex” Maximum c.m. energy is 2 GeV, project luminosity is L = 10 32 cm -2 s -1 at 2E= 2 GeV Unique optics, “round beams”, allows to reach higher luminosity Experiments with two detectors, CMD-3 and SND, started by the end of 2010 6 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
VEPP-2000 collider ring SND CMD-3 7 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
CMD-3 Detector CMD-3 Detector Mu Advantages compared to previous CMD-2: ✗ new drift chamber with x2 better spatial resolution, higher B field better efficiency better momentum resolution LXe ✗ thicker barrel calorimeter, BGO 8.3 X 0 13.4 X 180cm → 0 better particle separation DC ZC ✗ Unique LXe calorimeter with 7 ionization layers with strip readout TOF ~2mm measurement of CsI conversion point, tracking capability, shower profile (from 7 layers + CsI) ✗ TOF system particle id (mainly p, n) 8 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
e+e- -> π+π- by CMD3 e+e- -> π+π- by CMD3 Very challenging channel as needs to be measured at best systematic precision ~ a few per mil But... Clean topology of collinear events (mostly without physical background) Overall corrections at the level of a few percent Plans to reduce systematic error from 0.6-0.8% (by CMD2) -> 0.35% (CMD3) 3 Key components for this precise measurement: 1) PID - particle separation 2) Acceptance determination spatial angle of detection 3) Radiative correction, MC generators ... efficiencies ... beam energy precision Many systematic studies rely on high statistics 9 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Event selection Event selection Simple event signature with 2 back-to-back ● Two charged collinear tracks: Q 1 + Q 2 = 0 |Δ ϕ|< 0.15, |Δθ|< 0.25 charged particles ● Vertex position close to interaction point: π - ρ average < 0.3см, | Z average |< 5см |Δρ|< 0.3см, |Δ Z |< 5см ● Fiducial volume inside good region of DCh: e - θ e + 1. <(π+θ − )/ 2 <π− 1. + −θ ● Quality of selected tracks: 2 / ndf < 10,N hits ≥ 10 χ π + ● Filtration of low momentum and cosmic background: 0.45E beam < p + ,p – < E beam + 100MeV / c Data sample includes events with: e+e-, μ+μ-, π+π-, cosmic muons Almost no other background at √s <1 GeV 10 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Event separation Event separation E beam =460 MeV E beam =250 MeV Particle ID can be done by momentum or energy deposition Momentum At low energies momentum resolution πμ e e πμ of DCh enough to π μ μ e e π separate different types At higher energies π π Electron shower in calorimeter far away from MIPs Energy deposition Both methods can be used separately for cross-check μ e e μ π μ μ e e π Nμμ can be fixed (or 11 not) from QED 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Event separation by momentum Event separation by momentum from MC generator e+e- For particle separation: As input: momentum spectra for ee,ππ,μμ events from MC generator (in applied selection criteria) + cosmic,3π background from data(MC) Generated distributions are convolved with detector response function which includes (with mostly all free parameters in it): ✗ momentum resolution, π+π- ✗ bremsstrahlung of electron on vacuum tube, ✗ pion decay in flight Nππ/Nee obtained as result of binned likelihood minimization 12 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Fit result by momentum Fit result by momentum Projection to one charge with different slices over another E = 252.8 MeV E = 391.48 MeV e- e- μ- μ- π- π- 13 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Event separation by energy deposition Event separation by energy deposition At this moment: Full energy deposition in LXe+CsI calorimeter is used for particle separation Pion from As input: PDF distributions are taken mostly from data φ 3π → itself (fitted by analytical function, and used with some free parameters) ✗ Electron - described by mostly free function ✗ Muons – taken from data cosmic ✗ Pions - from φ 3π , ω 3π events → → After fit ✗ Cosmic - from data itself (events are selected by vertex position) Nππ/Nee obtained as result of binned likelihood minimization μ As plans: to exploit information about shower profile (energy deposition in 7 layers of LXe, + CsI) Neural net can be used for event classification 14 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Precision of fiducial volume Precision of fiducial volume Polar angle measured by e+ ZC chamber DC chamber θ multiwire chamber with help of charge with 2 layers and with strip division method readout along Z coordinate (Z resolution ~ 2mm), Unstable, depends on strip size: 6mm calibration and thermal Z coordinate resolution ~ stability of electronic 0.7 mm (for θ track ~ 1 rad) Calibration done relative to ZC (LXe) LXe calorimeter ionization collected in 7 layers with cathode strip readout, combined strip size: 10-15 mm Coordinate resolution ~ 2mm Both subsystem with strip precision < 100 μm 15 give <0.1% in Luminosity determination 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
Precision of fiducial volume Precision of fiducial volume Monitoring of z-measurement between ZC vs LXe RHO2013 scan ±0.1% Luminosity determination at θ>1rad Variation because of DCh instability, different B field, ZC noise level 16 26 June 2017, PHIPSI17, Mainz CMD-3 Collaboration
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