Transcript Измерение адронных сечений на ВЭПП-2М

Budker Institute of Nuclear Physics,
Siberian Branch of the Russian Academy of Sciences
CMD-2 and SND results on the
r, w and f.
M.N. Achasov
International Workshop «e+e- Collisions
from f to Y»
February 27 – March 2, 2006,
BINP,
Novosibirsk, Russia
Cross section e+e → hadrons
Process e+e → hadrons
(√s < 1,4 GeV)
V – vector meson, IG(JPC) = 1+(1 ) и 0(1 )
V = r, w, f , excited states.
(at √s < 1,4 GeV the vector mesons dominance model describes the
e+e → hadrons cross section with accuracy of about 1%)
Importance of the e+e → hadrons processes
studies in the energy region √s < 1,4 GeV
• Determination of the parameters of the
light vector mesons r, w, f, ...
• R(s)=s(e+e → hadrons )/ s(e+e → m+m)
( calculation of the hadronic contribution to the
anomalous magnetic moment of the muon (g-2)/2; …)
• Check of the CVC
(comparison of the isovector part of the e+e →
hadrons cross section with spectral function in
t → n hadrons decays)
Experimental situation in the energy region
√s < 2 GeV.
• VEPP-2M (√s <1,4 ГэВ, luminosity up to
3 × 1030 cm-2s-1) finished its operation and now
is beening reconstructed as VEPP-2000 (energy
region up to 2 ГэВ, luminosity up to 1032 cm-2s-1)
• ffactory DAFNE (luminosity up to 1033 cm-2s-1)
nowadays produce experimental data at the f
meson mass (1020 MeV).
• B-factory at SLAC the e+e → hadrons cross
section at low energies is measured with
radiative return method :
VEPP-2M
Center of mass energy region 360 – 1400 MeV
Luminosity at 1000 MeV
3 × 1030 cm-2s-1
CRYOGENIC MAGNETIC DETECTOR - 2 (CMD-2)
SPHERICAL NEUTRAL DETECTOR (SND)
1 – beam pipeline, 2 – drift chamber,
3 – Z-chamber, 4 – superconducting
solenoid, 5 – compensating magnets,
6 – BGO end cup calorimeter, 7 –
CsI(Tl,Na) calorimeter, 8 – muon
system, 9 – magnet yoke.
1 – beam pipeline, 2 – drift chambers,
3 – scintillation counter, 4 – light
guides, 5 – PMT, 6 – NaI(Tl) crystals, 7
– VPT, 8 – iron absorber, 9 – stream
tubes, 10 – iron plates, 11 –
scintillation counters, 12 – magnetic
lens, 13 – bonding magnets.
Integrated luminosity
1995 - 2000
Total luminosity per detector is 30 pb-1.
Integrated luminosity
accumulation
dynamics
Integrated luminosity
distribution over energy
region.
The cross sections e+e → hadrons
measurement method
•The experimental data is accumulated using √s energy
scan method and then analyzed ;
•The e+e → hadrons cross sections measurement:
s
N
ILd rad d spr
N – number of the e+e → hadrons events;
IL – integrated luminosity (measured using e+e → e+e and gg );
 - detection efficiency (obtained from MC simulation);
drad – radiative correction;
dspr – correction due to the beam energy spread.
Analysis of the
e+e → hadrons cross sections
In order to obtain the vector mesons V = r,w,f,...
parameters the measured cross section of the
process e+e → X is fitted with theoretical
expression:
V m
4
s X ( s)  3 / 2 PX ( s) 
s
V  r ,w ,f ,...
3
V
mV s (V  X )
DV ( s)
ifV
e
PX (mV )
The following parameters can be obtained from
the fit: mV - mass, V – width, fV – phase and
12B(V  X ) B(V  e+ e )
s (V  X ) 
mV2
2
3
Rare decays of r, w, f (Br<10 ).
Measurement of already known
decays
First observation
r → 0g, hg
r → 00g, ++, +0
w → hg, 00g, 0e+e
f → +, he+e
f → f0g, a0g, h/g, w0, ++,
0e+e
Results of the r, w, f mesons rare decays studies with
CMD-2 and SND detectors at VEPP-2M.
The e+e → + cross section
The e+e→+ process cross
section in the VEPP-2M energy
region.
The value  (r → e+e)
obtained in various
experiments. Yellow area
shows the average value
For data description the r, w, r/ mesons are required.
At √s  780 МэВ the r  w interference is seen.
The rmeson parameters (mass and width)
Here are shown the mr and r measured in e+e
experiments in processes:
e+e → r0 → + (SND-05, CMD-2-02, OLYA-85),
e+e →r → + 0 (SND-05, KLOE-03),
t  → r  n →   0n (ALEPH-05, CLOE-2-00)
Yellow area shows the average value.
The e+e → +0 cross section
The e+e→ +0
cross section
measured by SND.
Syst. error 3,4–5,4%.
The curve is the
result of the fit
taking into account
w, r, f, w/, w//.
The e+e →r → +0 transition dominates in this reaction.
At VEPP-2M the e+e → w → +0 transition was observed.
The intermediate state different from these two was observed
by KLOE and CMD-2 in the f-meson energy region (maybe
e+e → r/  → +0 ).
The e+e → 0g cross section
The e+e → 0g process
cross section.
Syst. error:
SND - 3%, CMD-2 - 6 %.
Curve is the result of the
fit taking into account
r, w, f resonances.
(Dots are SND data)
The wmeson parameters (mass and width)
In measurements CMD-2-04 and SND-03 the w-meson
parameters were obtained using e+e → +0
process, while in CMD-2-05 - using e+e → 0g reaction.
Yellow area shows the average value.
The wmeson main decays modes
Yellow area shows the
average value.
The
e+e
+

→ K K and KSKL cross sections
For data description the f, r, w, f
resonances are required.
/
e+e

The
→ K K cross section in the f –
+ 
meson energy region.
f → K K decay.
+
Systematic errors: SND – 7%, CMD-2 – 3-4% (preliminary)
Yellow area shows the average value.
The e+e → KSKL cross section in the f –
meson energy region. f → KSKL decay.
The cross section
measurements were
based on the KS → 00
and + decays.
Systematic errors:
SND – 3,3%, CMD-2 – 1,7%
Yellow area shows the
average value.
The e+e → hg cross section
The cross section was measured using h → 000,
+0, gg decays. For data /description the following
states are required f, r, w, r . The systematic errors in
the vicinity of the fmeson peak are 3-7%
The f → +0 and hg decays.
Comparison of the SND and CMD-2 measurements
with the other experimental results.
Yellow area shows the average value.
The fmeson parameters (mass and width)
Comparison of the SND and CMD-2 measurements
with the other experimental results.
Yellow area shows the average value.
Probability of the f → e+e decay.
Yellow area shows
the VEPP-2M
average.
Deviation from KLOE
result is 3 s.
CMD-2-05 – using the main f → hadrons decays (preliminary).
KLOE-05 – using m+m cross section and e+e forward-backward asymmetry.
СНД-01 – using the main f → hadrons decays and m+m cross section.
CMD-2-95 – using the main f → hadrons decays
Resonances with IG(JPC) = 1+(1 ) and 0(1 )
above f meson.
The e+e → +-+- cross section
The e+e → +0 cross section
These resonances exist. Their parameters are not well established and
nature is not clear. The number of them is unknown.
Some important results from SND and CMD-2:
• The clear experimental evidence of the w/ resonance;
• Observation of the a1 dominance in the process e+e → ++
/
Estimation of the w and w
//
parameters
The parameters of the w/, w/ / were estimated from combined analysis
of the e+e → +0 process cross section measured by SND and
DM-2 data on the e+e → w reaction
mV, MeV V , MeV
s(V→ 3), nb (V→ee),eV
1400
870
5
600
w// 1770
500
5,5
860
w/
The ratios of the wave functions at the origin are more then 1 :
Yw / (0)
S
Yw (0)
S
2
Yw // (0)
S
~4
Yw (0)
S
2
while
~5
Y
S
 (2S )
S
 (1S )
Y
(0)
(0)
2
Y ( 2 S ) (0)
S
 0,44
Y
S
J /
(0)
2
 0,6
Conclusions
• In the 1995-2000 the experiments with CMD-2
and SND detectors at VEPP-2M were fulfilled.
• The cross sections of the e+e annihilation in
hadrons were measured in the energy region
√s = 0,36 – 1,38 GeV.
• Results of these experiments determine
nowadays the accuracy of the light vector
mesons parameters determination. They are
one of the main source of information about
particle physics at low energies.