VEPP-2000 plans for the study of the nucleon form factors

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


OUTLINE VEPP-2000 collider

SND and CMD-3 detectors NNbar cross section and formfactor data Prospects of VEPP-2000 for NNbar measurements Conclusions


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


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


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 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 !


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)


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


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


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 ).


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.


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:


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


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


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)

.

VEPP-2000 plans for the study of the nucleon form factors

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