Low energy e+e– hadronsin Novosibirsk

I. LogashenkoOn behalf of the CMD-2 and SND Collaborations

Budker Institure of Nuclear Physics (Novosibirsk, Russia)

Boston University (Boston, USA)

10th International Workshop On Tau Lepton Physics

Novosibirsk, Russia, Sep 22 – 25, 2008

Cross-section e+e- hadrons

Measurement of the cross-section e+e- hadrons in VEPP-2M energy range is interesting for:

• measurement of R(s)

• measurement of parameters of light vector mesons ρ, ω, φ, ρ’, ρ’’, ω’, ω’’ • comparison with spectral functions of the hadronic tau decays

At low s R(s) has to be measured. The value and the error of the hadronic contribution to muon’s (g-2) are dominated by low

energy R(s).

(0) *

(0)( )

e e hadronsR s

e e

R(s) measurements at low s

VEPP-2M

Babar/Belle (ISR)

KLOE (ISR)

VEPP-2000

At VEPP-2M the cross-sections of each final state are measured exclusively

VEPP-2M collider

• VEPP-2M collider: 0.36-1.4 GeV in c.m., L1030 1/cm2s at 1 GeV

• Detectors CMD-2 and SND: 70 pb-1 collected in 1993-

2000

CMD-2 detector

SND detector

1 - beam pipe, 2 - drift chambers, 3 - coincidence counter, 4 - fibre light guide, 5 - PMTs, 6 - NaI(Tl) crystals, 7 – phototriodes, 8 - iron absorber, 9 - muon tubes, 10 - 1cm iron plate, 11 - muon counters, 12 - magnetic lenses, 13 - bending magnets

Example of CMD-2 and SND events

e+e-π+π- in CMD-2 e+e-K+K- in SND

Overview of the results

How cross-section is measured

(1 )

H bgN Ne e H

L

• Luminosity L is measured using Bhabha scattering at large angles

• Efficiency is calculated via Monte Carlo + corrections for imperfect detector

• Radiative correction accounts for ISR effects only

All modes except 2

2 2

2 2

(1 )

(point-like ) (1 )ee ee

ee

NF

N

• Ratio N(2)/N(ee) is measured directly detector inefficiencies are cancelled out

• Virtually no background

• Analysis does not rely on simulation (sort of)

• Radiative corrections account for ISR and FSR effects

• Formfactor is measured to better precision than L (1-2%)

2

e+e-π+π-

Experimental data

96

95,98

97

96,98

98,2000

CMD-2

SND

Events signature:two back-to-back tracks, originated near the interaction point

Data sample includes: e+e-, μ+μ-, π+π-, cosmic muons

There is almost no background at √s <1 GeV

Data were taken in 6 separate runs between 1994 and 2000

Event separation (CMD-2)

ee

• e// separation using particles momentum• can measure N()/N(ee) and compare to QED

<0.6 GeV

Momentum

Energy

ee,

>0.6 GeV

• e// separation using energy deposition• N()/N(ee) is fixed according to QED

Momentum

Energy

ln , ,

, , ,cosmic

a aevents a

L N f E E

a e e

Likelihood minimization:

Event separation (SND)Event separation is based on neural network:

• 7 input parameters: energy deposition in each layer for both clusters and polar angle• 2 hidden layers 20 neurons each• 1 output parameter – Re/π • Trained on simulated events• Checked on experimental 3π and e+e- events

E1(1

,2,3

), E

2(1

,2,3

),Θ

Re/π

Distribution by separation parameter

Misidentification ~ 0.5--1%

Radiative corrections

ISR+FSR

ISR+FSR+VP

Radiation terms

Vacuum polarization

• CMD-2 uses custom Monte-Carlo generator to calculate RC

ee, , final states: 1 at large angle, multiple ’s along initial or final particles (≤0.2%)

• CMD-2 calculation is consistent with independent calculations (BHWIDE, KKMC)

• SND uses BHWIDE for ee final state and CMD-2 generator for , final states

Comparison with other calculations of the radiative corrections

( )e e e e ( )e e

Comparison with BHWIDE Comparison with KKMC

Reconstruction efficiency correction (CMD-2)

All selected final states produce very similar signal in the drift chamber.

In the first order, the reconstruction efficiency is cancelled out from the formfactor calculation

Due to drift chamber malfunction, the correction for reconstruction efficiency reach up to 4% percent for 1998 data set. Measured ratio εππ/εee

Pion formfactor - results

SND 3.2% 1.3%

CMD2 0.7% 0.6% (95)/ 0.8% (98) 1.2-4.2%

SND o

nly

CMD2 only

Internal Cross-checks

Δ(95-98)≈0.7%±0.5%

CM

D2,

2 in

dependent

scans

of

rho-r

egio

nC

MD

2 v

s SN

D√

s<0.5

2 G

eV

Δ(SND-CMD2)≈1.2%±3.6%

CM

D2 v

s SN

D0.6

<√

s<1.0

GeV

Δ(SND-CMD2)≈-0.53%±0.34%

( )1

( )

( 2.0 1.3 0.7)%

meas

QED

CMD2√s<0.52 GeV

Other modes

“Narrow” resonances

(782) (1020)Mass and width are measured to ≈0.1 MeV, Γee to ≈2-3%

Cross-section e+e- 4π

Systematic error:Systematic error ≈5-7%

Efficiency determination gives main contribution to the systematic error

SND = 8%CMD2= 15% (discrepancy 15-25%)problem with efficiency determination! CMD2-reanalysis preliminary = 8%

ee 0 0e e

Cross-section e+e- 3π

0e e

Systematic error:

≈6% (>1GeV)1-2% on omega2.5% on phi

Fit: , , , ,

Cross-section e+e- 2K

CMD2

SND

Systematic error ≈5-8% Systematic error ≈3-9%

Systematic error is ≈2-3% at φ resonance

CMD2(prel.)

SND

e Ke K S Lee KK

Overview of the results

1.0%~0.6-0.7% 0.6% 1.5 -- 3.5 % 1.5%

1.5%~ 6 -- 1% 1--2% 2.5 -- 3.5 % 2.0%

Systematic error:

Total error:Error of R(s)

Future measurements at VEPP-2000

VEPP-2000 storage ring

• Up to 2 GeV c.m. energy

• Factor >10 in luminosity

L=1031 cm-2c-1, √s=1.0 GeV

L=1032 cm-2c-1, √s=2.0 GeV

≈100 1/pb per detector per year

Status:

construction is finished

had first beams and collisions

2009 – start of experiments

CMD-3 Detector

1 – vacuum tube, 2 – drift chamber, 3 – calorimeter BGO (680 crystals), 4 – Z–chamber, 5 – CMD-3 superconducting solenoid, 6 – calorimeter LXe (400 liters), 7 – calorimeter CsI (1152 crystals), 8 – magnet yoke, 9 – solenoids of VEPP-2000

Advantages compared to CMD-2:

• new drift chamber with x2 better resolution

better tracking• thicker barrel calorimeter

better separation• LXe calorimeter

- much better spatial resolution for γ’s- shower profile

SND 2000

1 – beam pipe2 – tracking system3 – aerogel4 – NaI(Tl) crystals5 – phototriodes6 – muon absorber7–9 – muon detector10 – focusing solenoid

Advantages compared to “old” SND:

• new system - cherenkov counter (n=1.05, 1.13)e/π separation E<450 MeVπ/K separation E<1 GeV

• new drift chamberbetter trackingbetter determination of solid angle

How can we improve measurement of R?

We expect to:

• measure 2 mode to 0.3-0.4%, 4 mode to 2%

• overall improvement in R precision by factor 2-3

We expect to get the following improvements:

• high statistics! (x10-x100) – current measurement is still statistics-limited

• radiative corrections to 0.1% - add photon jet + large angle γ

• measure radiative tails and compare them to calculations (ISR?)

• luminosity to 0.5%, use γγ in addition to Bhabha for cross-check

• much better separation (LXe @CMD-3, Cerenkov @SND) – smaller systematic error, try to measure e+e-μ+μ-

• precise trigger efficiency monitoring

• better drift chambers – higher resolution, efficiencies

Examples of improvements (CMD-3)

CM

D-3

CM

D-2

Separation by energy deposition Separation by momentum

Because of better resolution of the drift chamber, the separation by momentum should work up to √s ≈ 0.65 GeV

CMD-22*0.26 GeV

CMD-32*0.32 MeV

4σ

√s/2, GeV

Conclusion

• CMD-2 and SND data analyses are nearly complete. Cross sections of all major modes of e+e- hadrons are measured at energy range √s < 1.4 GeV . These are the best direct (energy-scan) measurements at the moment.

• There is good agreement between Novosibirsk results, in particular: pion formfactor CMD-2 (94,95) vs CMD-2 (98), CMD-2 vs SND.

• Over the last few years new indirect high precision measurements of R were performed: new tau-decay data and ISR. The question of agreement between different methods is still open.

New more precise measurement of R is scheduled at VEPP-2000.

Backup slides

Pion formfactor calculation

2 1

1 1 1

Bee ee ee

bg corrBN Dee

NF

N

Master formula:

• σB – Born cross-section (Fπ=1)

• δ – radiative correction

• ε – reconstruction efficiency

• ΔN – correction for nuclear interactions

• ΔD – correction for decay in flight

• Δbg – correction for e+e-3π,4π,2K background

• Δcorr – correction for E+E- correlation

Nππ/Nee is measured, other values are calculated:

What is really measured?

Definition of (e+e-hadrons) depends on the application

• Hadron spectroscopy: vacuum polarization (VP) is the part of the cross-section (“dressed”), final state radiation (FSR) is not• “Bare” cross-section used in R: vice versa – FSR is the part of the cross-section, VP is not • Measured number of events include VP and part of FSR allowed by the event selection

CMD-2 published 2 cross-sections e+e-2:radiative correction take into account part of FSR, allowed by the event selection (thus remove FSR completely from the measured cross-section); VP is left untouched. Used to get rho-meson mass, width, …

VP is removed, all FSR is added. Used for R calculation

0( )

20( ) 1 ( ) 1 ( )s s

VP FSR

Systematic errors

Source of error CMD-2√s<1 GeV

SND CMD-2√s>1.0

GeV

Event separation 0.2-0.4% 0.5% 0.2-1.5%

Fiducial volume 0.2% 0.8% 0.2-0.5%

Energy calibration 0.1-0.3% 0.3% 0.7-1.1%

Efficiency correction 0.2%-0.5%

0.6% 0.5-2.0%

Pion losses (decay, NI) 0.2% 0.2% 0.2%

Other 0.2% 0.5% 0.6-2.2%

Radiative corrections 0.3-0.4% 0.2% 0.5-2.0%

Total 0.6-0.8%

1.3% 1.2-4.2%