Transcript СОСТОЯНИЕ РАБОТ ПО ДЕТЕКТОРУ СНД

Budker Institute of Nuclear Physics, Novosibirsk
The project to measure the nucleon
form factors at VEPP-2000
Sergey Serednyakov
Workshop - e+e- collisions from phi to psi ,
February 27 – March 2, 2006
Updated version of the talk given
at Nucleon form factor workshop
in Frascati in Octob.14-16, 2005
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N Nbar project at VEPP-2000
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OUTLINE
1- VEPP-2000 collider, SND and CMD-3 detectors
2- NNbar cross section and formfactor data
3- Prospects of VEPP-2000 for NNbar
measurements
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N Nbar project at VEPP-2000
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VEPP-2000 Complex
2E = 400 - 2000 МeV
L=1031 сm-2s-1 at E=510 МeV
L=1032 сm-2s-1 at E=1000 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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N Nbar project at VEPP-2000
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VEPP-2000 e+e- collider
CMD-3
SND
Start of VEPP-2000 project –2000
Two collider detectors:
Energy range 2E=0.4-2.0 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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Febr.15, 2006
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SND detector for VEPP-2000
Ref.: NIM A449 (2000) 125-139
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.
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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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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
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N Nbar production cross section
e  e   pp, nn
dσ
α 2βC
4 M 2N
2
2

{| G M (s)| (1  cos  ) 
| G E (s)|2 si n2  }
dΩ
4s
s
42 C
2M 2 N
2

{| G M (s) | 
| G E (s ) | 2 }
3s
s



/(1  e  ) C~1 at Tkin. ~1 MeV
For e+ep pbar: C 

At the threshold we have s=4MN2 and GE=GM,
2 2C
2
2
|
G
(
4
M
)
|
 0.7nb
if GE =GM=0.5, then  
N
E
2
4M N
Rad. correction: d  d 0e n , n 
For T=1MeV: e-n=0.62;
For T=50 MeV: e-n=0.82;
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4
E
E
ln
ln
,

me Tkin
N Nbar project at VEPP-2000
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Present data – region near threshold
e+e-->p pbar
e+e-->n nbar
Cross section
e+e- -> N Nbar
Proton FF
Neutron FF
Time like
form factor
Dm=5MeV
In VEPP-2000
Dm <= 1 MeV !
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Study of p pbar production at VEPP-2000 with SND
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
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Picture of expected event
Estimates of statistics
at threshold :
Instant luminosity –
- 0.1/(nb.sec)
Time – 107 sec
Integrated luminosity - 1/fb
Cross section - 0.7 nb
Detection efficiency – 0.15
Number of events: 105
GE /GM – with 5% of
statistical accuracy, 1010bins
Detection of antineutrons at VEPP-2000 with SND
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
MC accuracy of angle measurement,
Picture of expected event
~7-10o
Estimates of statistics
at threshold :
Instant luminosity –
- 0.1/(nb.sec)
Time – 107 sec
Integrated luminosity - 1/fb
Cross section - 0.7 nb
Detection efficiency – 0.15
Number of events: 105
GE /GM – with 5% of
statistical accuracy, 10 bins
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Detection of antineutrons at VEPP-2000 with SND,
time measurements
Time-amplitude correlation
Monte Carlo time spectra
of n nbar events in SND
calorimeter
Time resolution
3 nsec
Time measurements with NaI(Tl)
counter, DE=30 MeV, =3 nsec,
phototriode readout
Conclusion: Time resolution of the whole NaI(Tl) calorimeter
<1 nsec at DE~0.5 GeV is expected.
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Background in n nbar detection
Physical background
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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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.
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Example of suppression of physical background for n nbar in SND (MC)
Event momentum
e+e->n nbar
cut
e+e->KsKL
e+e->KsKl0
e+e->0
Conclusion: effective
suppression of physical
background by ~ 2
orders is possible using
event momentum vs
event energy
distribution
Event energy
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Conclusions
1. VEPP-2000 can produce ~105 ppbar and nnbar events
per year in the range 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 for T<20 MeV and by “thick” tracks in DC for
T>30 MeV. The GE/GM ratio can be measured with
10% accuracy.
.
3. SND and CMD-3 calorimeters can be used as efficient
antineutron detector. Time of flight measurements will
improve events selection and background suppression.
The GE/GM ratio can be measured as well.
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