1. VEPP-2000 plans for the study of the nucleon form factors
Budker Institute of Nuclear Physics, Novosibirsk
VEPP-2000 plans for the study of the nucleon form factors
Sergey Serednyakov
Nucleon 05 Workshop
12-14 October, 2005
12-14 October, 2005
N Nbar at VEPP-2000

2. NNbar cross section and formfactor data
OUTLINE
VEPP-2000 collider
SND and CMD-3 detectors
NNbar cross section and formfactor data
Prospects of VEPP-2000 for NNbar measurements
Conclusions
12-14 October, 2005
N Nbar at VEPP-2000

3. VEPP-2000 Complex 2Emax=2000 МeV L=1031сm-2s-1 at E=510 МeV
Refs for VEPP-200
1. In: Proc.Frascati Phys.Series,v XVI, p.393,Nov.16-19,1999
2. In: Proc. 7-th Europ.Part.Accel.Conf.,EPAC 2000, p.439, Vienna,2000
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4. VEPP-2000 e+e- collider CMD-3 Start of VEPP-2000 project –2000
SND
Start of VEPP-2000 project –2000
Two collider detectors:
Energy range 2E= GeV
SND and CMD-3
VEPP-2000 parameters:
perimeter – 24.4 m
collision time – 82 nsec
beam current – 0.2 A
bunch length – 3.3 cm
energy spread – 0.7 MeV
x≃ z =6.3 cm
L ≃ 1032 at 2E=2.0 GeV
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5. SND detector for VEPP-2000
Ref.: NIM A449 (2000)
1 – VEPP-2000 beam pipe, 2 – tracking system, 3 – aerogel cherenkov counter, 4 – NaI(Tl) counters, 5 – vacuum phototriodes, 6 – absorber, 7-9 – muon system, 10 – VEPP-2000 s.c focusing solenoids.
12-14 October, 2005
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6. CMD-3 detector
1 – beam pipe, 2 – drift chamber, 3 – BGO, 4 – Z – chamber,
5 – s.c. solenoid, 6 – LXe, 7 – CSI, 8 – yoke , 9 – VEPP s.c. solenoid
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7. Physical program at VEPP-2000
1. Precise measurement of the quantity
R=(e+e-- > hadrons)/ (e+e-->+--)
2. Study of hadronic channels:
e+e-- > 2h, 3h, 4h …, h= ,K,
3. Study of ‘excited’ vector mesons: ’, ’’, ’, ’,..
4. CVC tests: comparison of e+e-- > hadr. (T=1)
cross section with -decay spectra
5. Study of nucleon-antinucleon pair production –
nucleon electromagnetic form factors,
search for NNbar resonances, ..
6. Hadron production in ‘radiative return’
(ISR) processes
7. Two photon physics
8. Test of the QED high order processes 2->4,5
12-14 October, 2005
N Nbar at VEPP-2000

8. N Nbar production cross section
For e+ep pbar:
C~1 at Tkin. ~1 MeV
At the threshold we have s=4MN2 and GE=GM,
if GE =GM=0.5, then
Rad. correction:
For T=1MeV: e-n=0.62;
For T=50 MeV: e-n=0.82;
12-14 October, 2005
N Nbar at VEPP-2000

9. Present data Cross section e+e- -> N Nbar Time like form factor
e+e-->p pbar
e+e-->n nbar
Cross section
e+e- -> N Nbar
Neutron FF
Proton FF
Time like
form factor
Dm=5MeV
In VEPP-2000
Dm <= 1 MeV !
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N Nbar at VEPP-2000

10. Study of p pbar production at VEPP-2000
Picture of expected event
Goal – measurement of proton e.m. FF at threshold,
1 – significant improvement of accuracy,
2 – separation between GM and GE ,
3 – confirm rise of both FFs at threshold,
4 – search of resonant structure
GEANT p pbar DC trigger efficiency
P pbar z-distribution (MC)
Estimates of statistics
at threshold :
Instant luminosity –
- 0.1/(nb.sec)
Time – 107 sec
Integrated luminosity - 1/fb
Cross section nb
Detection efficiency – 0.15
Number of events: 105
GE /GM – with 5% of
statistical accuracy, 10 bins
12-14 October, 2005
N Nbar at VEPP-2000

11. Detection of antineutrons at VEPP-2000
Goal – measurement of neutron e.m. FF at threshold,
1 – significant improvement of accuracy,
2 – separation between GM and GE ,
3 – confirm rise of both FFs at threshold,
4 – search of resonant structure
Picture of expected event
MC accuracy of angle measurement, s~7-10o
Estimates of statistics
at threshold :
Instant luminosity –
- 0.1/(nb.sec)
Time – 107 sec
Integrated luminosity - 1/fb
Cross section nb
Detection efficiency – 0.15
Number of events: 105
GE /GM – with 5% of
statistical accuracy, 10 bins
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12. Antineutron interaction with detector material
General comments:
- the antineutron annihilation
length ~ 15 cm is ~calorimeter
thickness 35 cm
- neutrons and antineutrons
have similar interaction
length the detection efficiency
of n nbar events is close to
100%
Detection efficiency =a Tr Rec ,
where a = annihilation probability,
Tr = trigger efficiency 0.8
Rec = reconstruction eff.  0.5
The detection efficiency at T=10 MeV:
 
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N Nbar at VEPP-2000

13. e+e- -> n nbar, MC distribution over cos q. Extraction of GM , GE
GM gives (1+ cos 2q ),
GE gives (1- cos 2q ).
With T =10MeV, N=3000 ev.,
ratio GE /GM is measured
with 8% accuracy
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N Nbar at VEPP-2000

14. Detection of antineutrons at VEPP-2000, time measurements
Time-amplitude correlation
Time resolution
s=3 nsec
Monte Carlo time spectra
of n nbar events in SND
calorimeter
Time measurements with NaI(Tl)
counter, DE=30 MeV, s=3 nsec,
phototriode readout
Conclusion: Time resolution of the whole NaI(Tl) calorimeter
s<1 nsec at DE~0.5 GeV is expected.
12-14 October, 2005
N Nbar at VEPP-2000

15. Background in n nbar process
Physical background
e+ e--> KSKL, 0.1 nb
e+ e--> KSKL0, 1 nb
  e+ e--> 0->00, 0.1 nb
  e+ e--> , ->neutrals,10 pb
  e+ e--> hadrons->neutrals,<0.1
  e+ e--> 4,5, (QED), 0.1 nb
For comparison e+e-->n nbar
cross section 1 nb
The most physical background comes from the reactions with production of KL.
KL interactions and decays in flight look similar to nbar because they give ‘stars’ outside the detector center.
3 types of background:
Cosmic background:
- they are suppressed by cosmic
veto system;
the survived events can fake
the n nbar events;
time measurements can separate
the effect from cosmic background
Beam background:
is inverse proportional to the beam
life time
can be monitored by measurements
with one beam in the ring;
can be suppressed by nbar
annihilation time measurements
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16. Example of suppression of physical background for n nbar in SND (MC)
e+e->n nbar
Event momentum
cut
e+e->KsKL
Conclusion: effective suppression of physical
background by ~ 2 orders is possible using event momentum vs
event energy distribution
e+e->KsKlp0
e+e->wp0
Event energy
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17. Schedule VEPP-2000 -- beam -- 2006 -- luminosity -- 2006
-- running
SND upgrade
-- nbar trigger
-- data taking
-- FF results
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18. Conclusions 1. VEPP-2000 can produce ~105 ppbar and nnbar events
per year at the threshold region 2E<2000 MeV. This exceeds by
2-3 orders the world statistics of e+en nbar process.
2. The ppbar events can be detected by pbar annihilation star in
beam pipe. The GE/GM ratio can be obtained from the star
z-coordinate distribution
3. SND calorimeter can be used as efficient antineutron detector.
Time measurements in NaI(Tl) option is considered to improve
events selection and suppress background (physical, beam
and cosmic) .
12-14 October, 2005
N Nbar at VEPP-2000