HVP CONTRIBUTION TO MUON
(G-2) AND RECENT RESULTS
FROM VEPP-2000
Logashenko Ivan
Budker Institute of Nuclear Physics
Novosibirsk State University
Fundamental Constants Meeting 2015
The SM value of ππ: today
β’ QED: Kinoshita et al., 2012: up to 5 loops (12672 diagrams). 0.7 ppb
β’ EW: 2 loops, now Higgs mass is known. 9 ppb
β’ Hadronic
LBL: model-dependent calculations; improvement is expected from lattice calculations
HVP: the value is based on the hadronic cross-section π+πβ data; there are effort to get it via lattice calculations.
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370 ppb 10 ppb 220 ppb
New experiment at FNAL: 140 ppb
The lowest-order hadronic contribution
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0.01 ppm 0.22 ppm
The hadronic contribution is
calculated by integrating experimental
cross-section π π+πβ β βππππππ .
Weighting function ~1/π , therefore
lower energies contribute the most.
Starting from π ~2 πΊππ the pQCD
estimation of π π+πβ β βππππππ is
used. At lower energies only the
experimental data are used.
Many sources of data:
β’ Novosibirsk: CMD-2 and SND
(VEPP-2M), CMD-3 and SND
(VEPP-2000)
β’ Factories: Babar, KLOE
β’ BES
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Inclusive vs exclusive measurements
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Exclusive approach:
β’ measure each final state separately and calculate the sum
VEPP-2M, VEPP-2000, Babar, KLOE
Inclusive approach:
β’ select events with any hadron(s) in the final state
BES, KEDR (now)
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Inclusive measurement at BES
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πΉ =π π+πβ β πππ ππππ
π π+πβ β π+πβ
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There is ongoing R measurement with KEDR detector at VEPP-4M (Novosibirsk)
ISR approach
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e
e
hadrons
s sβ
2( hadrons ) ( , ) ( hadrons)d e e H Q d e e
Main idea: cross-section
is measured in the wide
energy range, using
events with hard photon,
emitted by initial
particles.
New approach to measurement of the hadronic cross-sections was fully developed
over last decade: ISR (Initial State Radiation), mainly by BaBar and KLOE.
BaBar
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π+πβ β π+πβ and the hadronic
contribution
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In units of hadronic contribution:
πΏπππ»ππ = 0.6%
Ξaπ exp β theory β 4.0% Β± 1.1%
Estimated accuracy of Fermilab
experiment
πΏππ = 0.25%
Energy range π < 2 GeV contribute
>90% of the πππ»ππ value
π+πβ β π+πβ alone contribute 73% of
the value and 0.45% out of 0.6% of
the error
VEPP-2000 goal: measure π(π+πβ βπ+πβ) with systematic accuracy of
~0.3% and small statistical errors
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Contribution to the value and error of
ππβππ,πΏπ
from different energy ranges
Do existing data agree?
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Cross section π+πβ β π +π β
CMD-2 fit
0.35%
VEPP-2M data ISR data
β’ In integral, there is reasonable agreement between existing data sets
β’ But there are local disagreements well beyond claimed syst.errors
VEPP-2000 and the world
FCM2015
VEPP-2M
Babar/Belle (ISR)
KLOE (ISR)
VEPP-2000
Tau decays
ΠΠΠΠ
BESBES (ISR)
VEPP-2000: direct exclusive measurement of π π+πβ β βππππππ World-best luminosity below 2 GeV (1 GeV excluded)
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There was VEPP-2M
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Energy range: 0.36 β 1.4 GeV
Luminosity up to 5*1030 1/cm2s
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Cross-section measurements at VEPP-2M
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Hadronic cross-section measurements with precision from <1% to ~5%
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From VEPP-2M to VEPP-2000
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2001
2010
Main VEPP-2000 advantages:
β’ maximum energy up to 2 GeV
β’ higher luminosity
2001 VEPP-2M decommissioned
2010 first engineering run at
VEPP-2000 collider with 2
new detectors: CMD-3 and
SND
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VEPP-2000 (2010-2013)
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ILU3 MeVLinac
B-3M250 MeVsynchro-betatron
BEPe+,e
booster
825 MeV SND
CMD-3
e e+
converter
2 m2 m
VEPP-2000
Maximum c.m. energy is 2 GeV, project luminosity is πΏ = 10321/ππ2π at π = 2 GeV
Unique optics, βround beamsβ, allows to reach higher luminosity
Experiments with two detectors, CMD-3 and SND, started by the end of 2010
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Energy measurement
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Starting from 2012, energy is monitored continuously using compton backscattering
MeV
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Detector CMD-3
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DC
ZC
LXe
CsIBGO
TOF
Mu Advantages compared to CMD-2:
β’ new drift chamber with two times
better resolution, higher B field
better tracking
better momentum resolution
β’ thicker barrel calorimeter
(8.3π0 β 13.4 π0)
better particle separation
β’ LXe calorimeter
measurement of conversion
point for Ξ³βs
measurement of shower profile
β’ TOF system
particle id (mainly π, π)
Detector SND
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1 β beam pipe
2 β tracking system
3 β aerogel
4 β NaI(Tl) crystals
5 β phototriodes
6 β muon absorber
7β9 β muon detector
10 β focusing solenoid
Advantages compared to previous SND:
β’ new system - Cherenkov counter (n=1.05, 1.13)
e/Ο separation E<450 MeV
Ο/K separation E<1 GeV
β’ new drift chamber
better tracking
better determination of solid angle
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Collected luminosity
FCM2015
Currently the luminosity is limited by a deficit
of positrons (from πΈ > 650 MeV) and limited
energy of the booster (from πΈ > 825 MeV).
After upgrade (ongoing) we expect luminosity
increase by up to factor 10 at maximum
energy.
Beam energy, 2E, MeV
Lum
inosi
ty, 1/c
m2s
Energy ramping
Limited e+
production
CMD-3 data, average per run
About 60 pb-1 collected per detector
π(782) 8.3 1/ππ
2πΈ < 1 GeV (except π) 9.4 1/ππ
π(1019) 8.4 1/ππ
2πΈ > 1.04 GeV 34.5 1/ππ
BaBar
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VEPP-2000 after upgrade (2015-)
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BEPe+,e
booster
1000 MeV
SND
CMD-3
VEPP-2000
250 m
beamline
π+ πβ source
β’ New positron source β
no luminosity limitation
due to lack of π+
β’ Booster energy
increased to 1 Gev β no
deadtime due to energy
ramping
Physics program
FCM2015
1. Precision measurement of π = (π+πβ β βππππππ )/ π(π+πβ β π+πβ)
exclusive approach, up to <1% for major modes
2. Study of hadronic final states:
π+πβ β 2β, 3β, 4β, β¦ β = π, πΎ, π
3. Study of vector mesons and theirs excitations:
β,ββ,β,β, β¦
4. Comparison of cross-sections π+πβ β βππππππ (π = 1) with spectral
functions of π-decays
5. Study of nucleon electromagnetic formfactor at threshold
π+πβ β π π, π π
6. Measurement of the cross-sections using ISR
7. Study of higher order QED processes
Overall, we plan to collect 0.5 Γ· 1 1/fb
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π+πβ β π+πβ cross section
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1. Select final state with 2 back-to-back
charged particles
Cuts: pavr, Ξπ, ΞΞ, Ξπ
Fiducial volume:
Ξ0 β€ Ξππ£π β€ π β Ξ0 , Ξ0 = 0.9β¦1.1
2. Identify π+πβ, π+πβ, π+πβ and background
3. Use βmasterβ formula:
πΉπ2 =
ππππππ
ππππ΅ 1 + πΏππ νππ
ππππ΅ 1 + πΏππ νππ
π π+πβ β π+πβ =ππΌ2
3π 1 β
4ππ2
π
3/2
πΉπ2
Ξ
π+ πβ
π+
πβ
πππ΅ - βBornβ cross-section π+πβ β π, point-like pions; πΏπ - radiative correction;
νπ - detection efficiency (not including acceptance)
π+πβ β π+πβ: π, π, π separation
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Energy deposition, MeV, -
En
erg
y d
ep
ositio
n, M
eV
, +
Momentum, MeV/c, -
Mom
entu
m, M
eV
/c, +
π = 500 πππ
π = 500 πππ
660 πππ
660 πππ
960 πππ
960 πππ
Simulated muons
π
π
π
π
π , π
Data
Ca
lorim
ete
rD
rift C
ham
ber
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π+πβ β π+πβ: VERY preliminary results
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πΉπ2
π π π separation
by momentum by energy deposition
2013 data
CMD-3
πππ πππ ππ₯π
πππ πππ ππΈπ·
Work in
progress
π+πβ β π+πβ: statistics and systematics
FCM2015
Main sources of systematics:
β’ π π π separation β 0.2%
multiple ways to get detector
response from data itself
β’ fiducial volume β 0.1%
2 independent systems, which can
be used to determine fiducial
volume
β’ beam energy β 0.1%
constant monitoring with Compton
backscattering
β’ radiative corrections β 0.1%
proof from data
Many systematic studies rely on
high statistics
Expected statistical error for 2013 data
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π+πβ β πΎ+πΎβ
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CMD-3, @π 1020
There is discrepancy
between CMD-2 and
BABAR
SND, above π 1020
Complicated π(π ) due to
interference of excited vector
resonances
Aerogel Cherenkov counters
provide kaons ID
π+πβ β π+πβπΎπΎ πΎπΎ = π0, π
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Preliminary results from SND, using about 50% of available data.
Estimated systematic error ~5%.
There is ongoing analysis to measure 3π cross section below π
π+πβ β 4π
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Ec.m (GeV)
CMD-3
π +π βπ +π βSND
π +π βπ ππ π
Need to measure these channels to few %.
The dominant source of systematic error is the model uncertainty (evaluation of
the detector acceptance). High statistics allows for more accurate study of the
dynamics.
PRELIMINARY
π+πβ β πΎ+πΎβπ+πβ
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Many intermediate states: πΎβπΎπ, ππΎπΎ, πππ, πΎβπΎβ, β¦
PRELIMINARY
π+πβ β 5π
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Dominated by intermediate π and π
π and π are observed in several final
states, e.g. π β 3π and π β 2πΎ
Expected systematic is 10%
Cross section π+πβ β ππ+πβ
π β 3π
π β 2πΎ
Invariant mass of 3π
π
π
π+πβ β 6π
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π πthreshold
Systematic error is 6%, main source is model dependence. High statistics
will help to reduce this error.
Preliminary studies of dynamics of π+πβ β 3(π+πβ):
β’ Main production mode: π 770 + 4π (phase space or π0(1370))
β’ Hint of energy dependent dynamics in 1.7-1.9 GeV energy range
Phys.Lett.
B723 (2013)
82-89
π+πβ β π(π +π β)
π+πβ β π π
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ππ
πΞ©=πΌ2π½πΆ
4π πΊπ π 2 1 + cos2 Ξ +
4π2
π πΊπΈ π 2 sin2 Ξ
π(π+πβ β π π)
π π π πππ π―
PID by dE/dx, secondaries
Angular distribution allows to
measure | πΊπΈ πΊπ |
CMD3
Need more statistics
π+πβ β π π at SND
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Phys. Rev. D 90, 112007 (2014)
πΉ 2 =πΊπ
2 + πΊπΈ2 2π
1 + 1 2π, π =
π
4ππ2
Effective formfactor
π(π+πβ β π π)
Conclusion
β’ In 2011-2013 CMD-3 and SND have collected 60-70 1/pb per
detector in the whole energy range 0.32 β€ π β€ 2.0 GeV,
available at VEPP-2000. Collected integral is similar to the total
integral available before.
β’ Data analysis of many inclusive modes of π+πβ β βππππππ is in
progress. First results (ππ0, 6π, ππΎ, πβ²) have been published.
β’ After VEPP-2000 upgrade (scheduled to be finished in 2015)
we plan to resume data taking with the ultimate goal of 1 1/fb
β’ We expect to produce new precise measurements of hadron
production R(s), to improve the precision of the hadronic
contribution to muon (g-2)
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