12-14 October, 2005 N Nbar at VEPP-2000 1
VEPP-2000 plans for the study
of the nucleon form factors
Nucleon 05 Workshop12-14 October, 2005
Sergey Serednyakov
Budker Institute of Nuclear Physics, Novosibirsk
12-14 October, 2005 N Nbar at VEPP-2000 2
OUTLINE VEPP-2000 collider
SND and CMD-3 detectors NNbar cross section and formfactor data Prospects of VEPP-2000 for NNbar measurements Conclusions
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VEPP-2000 Complex
2Emax=2000 МeV L=1031сm-2s-1 at E=510 МeV L=1032сm-2s-1 at E=1000 МeV
Refs for VEPP-2001. In: Proc.Frascati Phys.Series,v XVI, p.393,Nov.16-19,19992. In: Proc. 7-th Europ.Part.Accel.Conf.,EPAC 2000, p.439, Vienna,2000
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VEPP-2000 e+e- collider
CMD-3
SND
14.06.2005
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
Start of VEPP-2000 project –2000 Two collider detectors: Energy range 2E=0.4-2.0 GeV SND and CMD-3
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SND detector for VEPP-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.
Ref.: NIM A449 (2000) 125-139
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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 spectra5. Study of nucleon-antinucleon pair production – nucleon electromagnetic form factors, search for NNbar resonances, ..6. Hadron production in ‘radiative return’ (ISR) processes7. Two photon physics8. Test of the QED high order processes 2->4,5
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N Nbar production cross section
}sin|(s)G|s
M4)cos(1|(s)G{|
s4
βCα
dΩ
dσ 222
222
EN
M
nnppee ,
)e1/(C
For e+ep pbar:
At the threshold we have s=4MN2 and
GE=GM,
}|)s(G|s
M2|)s(G{|
s3
C4 2E
N2
2M
2
nbMGM
CNE
N
7.0|)4(|4
2 222
2
if GE =GM=0.5,
then
C~1 at Tkin. ~1 MeV
Rad. correction: ,T
Eln
m
Eln
4n,edd
kine
n0
For T=1MeV: e-n=0.62;
For T=50 MeV: e-n=0.82;
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Present datae+e-->p pbar
e+e-->n nbar
Proton FFNeutron FF
Cross section
e+e- -> N Nbar
Time likeform factor
m=5MeV
In VEPP-2000m <= 1 MeV !
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Study of p pbar production at VEPP-2000
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
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
Picture of expected event
GEANT p pbar DC trigger efficiency
P pbar z-distribution (MC)
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Detection of antineutrons at VEPP-2000
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
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, ~7-10o
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Antineutron interaction with detector material
General comments:- the antineutron annihilation length ~ 15 cm is ~calorimeterthickness 35 cm- neutrons and antineutrons have similar interaction length the detection efficiencyof 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:
0.3 - 0.4
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e+e- -> n nbar, MC distribution over cos Extraction of GM , GE
With T =10MeV, N=3000 ev., ratio GE /GM is measured with 8% accuracy
GM gives (1+ cos 2 ),
GE gives (1- cos 2 ).
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Detection of antineutrons at VEPP-2000,time measurements
Monte Carlo time spectraof n nbar events in SNDcalorimeter
Time-amplitude correlation
Time resolution 3 nsec
Time measurements with NaI(Tl)counter, E=30 MeV, =3 nsec,phototriode readout
Conclusion: Time resolution of the whole NaI(Tl) calorimeter<1 nsec at E~0.5 GeV is expected.
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Physical background e+ e--> KSKL, 0.1 nb e+ e--> KSKL0, 1 nb e+ e--> 0->00, 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.
Background in n nbar process
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
3 types of background:
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Example of suppression of physical background for n nbar in SND (MC)
Conclusion: effective suppression of physicalbackground by ~ 2
orders is possible using event momentum vsevent energy distribution
Even
t m
om
entu
m
Event energy
e+e->n nbar
e+e->KsKL
e+e->KsKl
e+e->
cut
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Schedule
VEPP-2000 -- beam -- 2006 -- luminosity -- 2006 -- running -- 2007--2012
SND -- upgrade -- 2004-2006 -- nbar trigger -- 2006 -- data taking -- 2007-2012 -- FF results -- 2008-2010
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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+en 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)
.