Last Results From VEPP2M CMD2 and SND

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Prospect of accuracy improvements
of the hadronic cross sections
measurements to the level 10-3
experimental and theoretical problems
G.Fedotovich Budker
Institute of Nuclear Physics
Novosibirsk, Russia
LNF, Frascati 7 - 10 April 2008
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Outline

• Current status of the accuracy of the hadronic
  cross sections measurements. Inspection of the
  last generation experiments - CMD-2, SND KLOE
• 2. Main sources of systematic errors
• Accelerator
• Detector
• Theory
• 3. What can we expect in the nearest future
• 4. Usage of space-like data to calculate VP
  operator ?(-q2)
• 5. Conclusion

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Some features of CMD-2, SND and KLOE
experiments

• Large data sample due to high integrated
  luminosity and large detectors acceptance
  (calorimeter covers about 0.9?4?). Every detector
  collected several millions ??- events
• Multiple scan (up and down) of the same energy
  range to avoid possible systematic in energy
  determination step ?(2E) 10 MeV in the
  continuum and about 1 MeV near ? and ? peaks
  (CMD-2 and SND)
• Absolute calibration of beam energy using the
  resonance depolarization method (better than
  10-4) ? negligible systematic error comes due to
  energy uncertainty (CMD-2 and SND)
• Good space resolution resulting in perfect
  momentum resolution (?p/p 0.4, KLOE) ?
  powerful instrument for charged PID
• Excellent energy resolution at a few percent
  level (?E/E 4, SND KLOE) leads to small
  background helps to separate events

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Some features of CMD-2, SND and KLOE experiments

• Detection efficiencies and calorimeter
  response were studied
• using pure experimental data rather than MC
  events (2?106 ?
• and ? meson decays have been used, CMD-2
  SND)
• Charged and neutral triggers for the same data
  sample possibility to measure and monitor
  triggers efficiencies (CMD-2 SND)
• Changing events selection criteria to check
  cross section stability.
• All detectors carefully studied this item
• Cross check possibility unstable particles
  detected via different decay modes (?º?2?, ee-?
  ??2?, ??-?0, 3?0)
• MC generators based on differential cross
  sections with precise RC
• for the processes of ee- annihilation were
  developed (CMD-2 KLOE)

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How cross sections are measured to
understand the main factors giving dominant
contributions to systematic uncertainty for
hadronic cross sections
All modes except 2?
2? mode

• Efficiency ? is calculated via Monte Carlo
  corrections for detector imperfections
• Integrated luminosity L is measured using LAB
  events
• RC ? accounts for ISR effects only
• VP effects are included in cross section
  properties

• Ratio N(2?)/N(ee) is measured directly ?
  detection inefficien-cies are cancelled out in
  part
• RC account for ISR and FSR effects
• Events separation procedure analysis dont
  rely on simulation
• Form factor is measured to better precision than
  L

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Luminosity measurement

• Precision of luminosity measurement will be
  improved significantly due to better extraction
  of Bhabha events, increasing detection efficiency
  and more accurate calculation of the radiative
  corrections.
• Alternative method to measure luminosity based
  on the process ee- ? ? ?. In that case
  Feynman graph does not contain VP effects.
  Powerful instrument to understand systematic.

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R measurement at CMD-2, SND and KLOE
(for vslt1GeV 2pi dominant channel)
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R measurement at CMD-3 (systematic errors
review)
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Derivative dF?(E)²/dE/F?(E)²x ?E/E Energy
determination
(?E/E 10-3)
Derivative jumps up and down inside corridor ?1.
Near ? and ? mesons reach values
?6. Conservative upper estimation is 1 - energy
region gives the main hadronic contribution to aµ
(g 2)/2 Very important task for machine
physicists to determine beam energy with
relative accuracy ?E/E ? 10-4 or even better
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p/µ/e separation based on charged particle
momentum

• DC resolution will be 2.5 times better (already
  achieved)
• Magnetic field will be increased 1.5 times
  (already achieved)
• As a result p/µ/e separation based on momentum
  will be possible up to 2320 MeV - practically
  to the ? peak

Vertical axis the number of standard deviations
between average momentum of pions and muons
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p/µ/e separation based on energy deposition

• Energy resolution of barrel part will be
  improved (8X0 ? 15X0)
• p/e separation will be considerably better
• Part pions looks like muons will be suppress to
  the level 10 (was 25 at CMD-2) . We can try p/µ
  separation based on energy deposition
• Information of energy deposition in depth of
  calorimeter provides additional factor for p/µ
  separation

p/µ/e ?????????? ?? ??????????????? ??? ?????????
??????
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Fiducial volume

• Z-chamber In first approach we will have the
  same z-coordinate resolution ? the accuracy of
  the fiducial volume measurement will not change.
  At polar angles ? 60 ?z 0.7 mm
  system. shift is smaller 0.1 mm. For LAB
  events it leads to acceptance uncertainty about
  0.2
• LXe calorimeter For normal incident particles ?
  90 ?z 0.9 mm. The acceptance will
  be determined with the same accuracy. The
  capability for cross check will be in hand. Very
  possible we improve the measurement accuracy of
  the detector acceptance by factor of 1.5
  
• We assume that fiducial volume will be
  determined at least with the same accuracy
  (or better) as we had at CMD-2
• 3. Huge statistics Help to study systematic of
  z-coordinate determination in DC to improve the
  accuracy of DC calibration procedure. Study in
  detail angular distributions of multi hadrons
  events to choose model for simulation

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Radiative corrections
What we have currently and what we can expect in
the nearest future - theoretical
aspects. For all three detectors MC generators
with precise RC were developed. Channel
ee- ? ee- BHWIDE (LEP, 0.5), MCGPJ (CMD-2,
0.2) photon jet radiation in collinear region,
BabaYaga (KLOE, 0.5 ? 0.1) used parton shower
approach. We plan to include NLO corrections and
increase the accuracy of our MCGPJ to 0.1 level
(for all other channels too). This work is in
progress with Dubna, E.Kuraev et.al. Channel
ee- ? ??-, ??- KKMC used in LEP experiments
adopted for low energies, 0.1, BUT for VP
effects smooth approximation was applied.
Resulting systematic accuracy is not better than
1 (out of ? ? energy region).
MCGPJ (CMD-2, 0.2). Quite possible
that in our case systematic uncertainty is better
than 0.1. B.Smith, M.Voloshin PL B 324 all
enhanced second order corrections contribute not
more than 0.02 and quickly decrease with energy
increase
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Radiative corrections Channel ee-
? ?? MCGPJ (CMD-2, 0.2). This process has a big
cross section. Very important channel for
luminosity measurement cross check possible.
ISR only. Feynman graph does not contain VP
effects. Channel ee- ? ??-, KK- MCGPJ
(CMD-2, 0.2). ISR FSR are taken into account.
Unfortunately there are no other MC generator
with similar accuracy for comparison.
Experimental and theoretical evidences are
required to prove validity of sQED application
for pions kaons Channels with neutral
particles in FS ee- ? KLKS, ?0?, ??, ??0. ISR
are taken into account. MCGPJ (CMD-2, 0.2 or
better). Very possible that accuracy is better
than 0.1 VP effects currently are
calculated with accuracy better than 0.05 (on ?
with 0.5 on ? - 0.15) and do not contribute
to final systematic error
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Trigger reconstruction efficiencies

• ? Trigger efficiency close to 100. Charged
  neutral triggers for the same data sample
  powerful instrument to monitor trigger stability
  and its real efficiency (CMD-2, KLOE CMD-3)
• Efficiency of track reconstruction in DC will be
  better than 98 with uncertainty lt0.1 (CMD-3
  new SND)
• Bremsstrahlung of electrons (positrons) on the
  wall of the machine vacuum chamber. At CMD-2 and
  SND we had correction about (0.5 ? 0.05)
  (?slt1GeV). We ( SND) hope to have the same
  accuracy in experiments at VEPP-2000.
• Optimization of selection criteria for collinear
  events Polar angle compromise
  for every detector,
  Threshold on transverse momentum of charged
  particles in DC, Choice of optimal
  acollinearity angle between tracks in DC
  Choice threshold on energy deposition in
  calorimeters and so on
• ?? reconstruction main source of systematic
  error for processes with ??in FS. LXe calorimeter
  will significantly push down this error

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Another way for aµ calculation

• Special experiment is necessary to measure cross
  sections of
• ONLY THREE PURE QED PROCESSES ee- ? ??, for
  luminosity measurement (no VP effects, accuracy lt
  0.1). BUT special calorim. is required to detect
  photon conversion point (LXe in CMD-3)
• ? ee- ? ??- direct cross section measurement
  to extract 1 ?(s)²
  (accuracy lt 0.1).
  VP effects must be removed from RC. Effects of
  FSR and CI must be included
  into RC
• ? SCAN EXPERIMENT Luminosity 1032 cm-2 s-1 ,
  100 energy points with number of muon events 108
  /per year (statistical accuracy about 0.1 in
  every point). Cross section has
  practically isotropic distribution vs polar angle
• ? ee- ? ee- process to extract ?(-q²) from
  t-channel in space-like region (accuracy lt0.1)

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Contribution to aµ

Time-like region
Space-like region
t -s(1-cos?)/2 t ? -0.04 GeV² x ? 0.75
Red lines resonance contributions
x lt 0.7 analytical approximation

• ? Cross section of all three QED processes can be
  measured in one direct scan experiment
• Accuracy of RC calculation will not contribute to
  final systematic error ?(-q²)
• We hope to achieve experimental systematic error
  for LAB better than 0.3 with CMD-3 at VEPP-2000.

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Analytical behavior of ?(q²) in space-like
region
This interval of q² variations corresponds to x
changing from 0.01 to 0.99 Very ghostly chance
to measure ?(q²) in this region with pro mille
accuracy
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Conclusions

• Despite decades of experiments, precise studies
  of ee?
• annihilation into hadrons at low energies are
  still interesting
• and can provide a lot of important information
• ? In a few years new precision data from CMD-3
  and SND working
• at VEPP-2000 as well as with ISR at DAFNE and
  Bfactories and
• BELLE are expected
• ? Progress is particularly expected for the
  channel ee-???-, where
• systematic uncertainty 0.3 or even better
  will be achieved
• ? MC generators were done
• e?e? ? e?e?, BHWIDE (0.5), MCGPJ (0.2?!) and
  BabaYaga (0.1)
• e?e? ? ????,???-, KKMC (0.1 adopted for low
  ener.), MCGPJ (0.2)
• e?e? ? ???? and KK-, MCGPJ (0.2?!)
• Only ISR is taken into account for processes with
  neutral particles
• in final state e?e? ? ??, KlKs, ?0?,??,??,
  ??0, MCGPJ(0.2) We can expect
  progress in theoretical improvement of the
  accuracy of RC calculation with pro mille
  accuracy

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? Measurement of beam energy with relative
accuracy better than 10-4 are extremely needed
(resonance depolarization techniques only) ? To
have enough statistic 106 at every energy point
(100) machine with luminosity greater than 1032
cm-2s-1 in ? meson energy region is required ?
To illuminate possible systematic error in
hadronic cross sections more accurate and
independent measurements (CMD-3 SND) are
necessitated ? Efforts of theorists are required
to build models to describe in detail energy
dependence of cross sections with 4 more pions
in FS ? CMD-3 and SND will measure hadronic
cross sections with accuracy close to pro mille.
DAFNE HEPr can provide independent measurement of
the hadronic cross sections - valuable
information for cross check accuracy with CMD-3
SND ? Luminosity and trigger efficiency must be
measured in different channels at the same data
sample to arrange cross check for better study
of systematic ? Usage of space-like data can
help in understanding of systematic in hadronic
contribution to aµ (in far future, JLAB)
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VEPP-2000

• circumference 24.4 m
• revolution time 82 nsec
• beam current 0.2 A
• beam length 3.3 cm
• energy spread 0.7 MeV
• ?x ?z 6.3 cm
• L 1032 cm-2s-1 at 2E2.0 GeV
• L 1031 cm-2s-1 2E1.0 GeV

SND
Total integrated luminosity with all detectors on
VEPP-2M 70 pb-1
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Luminosity measurement
Bhabha scattering events are preferable for
normalize purpose to calculate cross
sections with collinear events in FS
?vis(e?e? ? e?e?)
?vis(e?e? ? x?x?) -----
Main sources of systematic errors in previous
experiments
were Quality of events separation Nee
, Nxx typical value 0.4 - 5
Accuracy of beam energy measurement
typical value 0.3 - 1 Events detection
reconstruction efficiencies typical value
0.2 - 2 Systematic error of RC calculation
typical value 0.3 - 1 Very soon
accuracy of RC calculation with systematic error
less than 0.1 will be required for future
forthcoming experiments (for example
CMD-3 experiments at VEPP-2000)
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• Dominant channel
  ee- ? ??-
• energy interval
  energy interval
• 390 520 MeV
  600 960 MeV
• Group aµ(??), 10-10
  
  aµ(??), 10-10
• Old 48.72 1.45 1.12
  (1.83) 374.8 4.1 8.5
  (9.4)
• CMD-2 46.17 0.98 0.32 (1.03)
  377.1 1.9 2.7 (3.3)
• SND 47.80 1.73 0.69 (1.86)
  376.8 1.3 4.7 (4.8)
• CMD-2/SND 46.55 0.85 0.29 (0.90)
  KLOE 375.6 0.8 4.9 (5.0)
• Average 46.97 0.73 0.45 (0.86)
  376.5 0.6 1.5 (1.7)
• energy
  interval 1040 1380 MeV
• Group
  aµ(??), 10-10
• OLYA
  7.49 0.18 0.83 (0.83)
• CMD-2
  7.01 0.10 0.16 (0.19)
• Average
  7.03 0.09 0.16 (0.18)

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p/µ separation
440 MeV ?
480 MeV ?
320 MeV ?
360 MeV ?
230 MeV ?
280 MeV ?
Iron yoke
Iron yoke
130 MeV ?
170 MeV ?
CSI
CSI
LXE
LXE
Vertical axis - radius (in cm) where pions and
muons penetrate

• At any energies p and µ escape LXE it is bad
• At energies above f meson muon range system
  will serve to suppress cosmic events, below
  to mark muon events
• At energies below 2400 MeV p /µ does not escape
  CsI calorimeter it is fine

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Pion formfactor (CMD-2)
9?105 ??- events
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Event separation (CMD2)
gt 0.6 GeV
lt 0.6 GeV
ee
??
??

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

• e/?/? separation is based on energy deposition
  in calorimeter
• N(??)/N(ee) is fixed according to QED