The project to measure the nucleon form factors at VEPP-2000

Febr.28-March 2, 2006

N Nbar project at VEPP-2000 1

The project to measure the nucleon form factors at VEPP-2000

Workshop - e+e- collisions from phi to psi ,

February 27 – March 2, 2006

Sergey Serednyakov

Budker Institute of Nuclear Physics, Novosibirsk

Updated version of the talk givenat Nucleon form factor workshop in Frascati in Octob.14-16, 2005



OUTLINE1- VEPP-2000 collider, SND and CMD-3 detectors

2- NNbar cross section and formfactor data 3- Prospects of VEPP-2000 for NNbar measurements



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-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



VEPP-2000 e+e- collider

CMD-3

SND

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

Febr.15, 2006



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



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



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



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;



Present data – region near thresholde+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 !



Study of p pbar production at VEPP-2000 with SND

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



Detection of antineutrons at VEPP-2000 with SND

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



Detection of antineutrons at VEPP-2000 with SND,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.



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 detection

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:



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



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+en 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..

The project to measure the nucleon form factors at VEPP-2000

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