Study of e + e annihilation to a proton-antiproton pair. Vladimir Druzhinin BINP, Novosibirsk

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1 Study of e + e annihilation to a proton-antiproton pair Vladimir Druzhinin BINP, Novoibirk

2 e + e - pp cro ection b E p M E G m G C 4, ) ( ) ( 3 ) ( 4 ) ( C i the Coulomb factor. The cro ection i nonzero at threhold. G E and G M are the electric and magnetic form factor, G E (0)=1, G M (0)=.79. in ) ( 4 ) co (1 ) ( 4 ) ( ), ( G m G C d d E p M At threhold G E (4m p ) = G M (4m p ). m G m G F p E p M p 1 ) ( ) ( ) ( ) ( From the meaured cro ection, a combination of the quared form factor can be extracted. We define the effective form factor: The ratio of the form factor G E /G M can be determined from the analyi of the proton polar-angle ditribution. The G E term decreae with energy increae a 1/E.

3 Previou e + e - experiment Below 3. GeV: ADONE73 (1973), DM1(1979), DM(1983,1990), FENICE(1994), BES(005). Firt meaurement of the proton timelike form factor Statitical accuracy of thee meaurement i (0-30)%. Limited tatitic do not allow to extract the G E /G M ratio from the analyi of the angular ditribution. The detection efficiency wa calculated auming G E = G M With modern knowledge on the G E /G M ratio the model uncertainty i etimated to be 5-7% depending on detector acceptance. 3

4 Proton-antiproton colliion Near threhold data were obtained in PS170 experiment at the antiproton torage ring LEAR (CERN): teep growth of the form factor near therhold, the ratio G E /G M, meaured with 30% accuracy in five energy point, agree with unity. Above 3 GeV meaurement were performed at FNAL (E835 and E760). The trong decreae of the form factor wa oberved which agree with the dependence (m) /m 4 predicted by QCD for aymptotic proton form. 4

5 Space-like form factor (e-p cattering) arxiv:hep-ph/ arxiv: Q G D q (1 Q 1 ( GeV ) / 0.71) Two method to determine G E /G M : Roenbluth: from angular ditribution Polarization tranfer: polarized electron beam + meaurement of the final-proton polarization. 5

6 Meaurement G E /G M ratio We ue the method cloe to Rothenbluth to meaure the ratio of the time-like form factor. I our meaurement correct? A poible explanation of the Roenbluth-polarization difference i a contribution of higher-order diagram, in particular, two-photon exchange. Space-like: G E /G M (0)=1/.79 No charge ymmetry in ep cattering the interference of one- and two-photon amplitude contribute to the cro ection Time-like: G E /G M (m p )=1 Annihilation i chargeymmetric the interference doe not contribute to the total cro ection The interference can be een a aymmetry in angular ditribution 6

7 Size of the two-photon contribution From A.Gramolin talk at the workhop Scattering and annihilation electromagnetic procee Trento, February 18-,

8 Recent and new meaurement Progre in meaurement became poible with developing high luminoity е + е -collider: direct meaurement: CESR (CLEO), BEPC-II (BESIII), VEPP-000 (SND and CMD-3) ISR method: PEP-II (BABAR), BEPC-II (BESIII) The point marked NU (Phy. Rev. Lett. 110 (013) 000) wa obtained on CLEO data (~1 fb), collected at 3.77 and 4.17 GeV. The accuracy of the cro ection meaurement i about 10%. 8

9 ISR method The ma pectrum of the protonantiproton ytem in the reaction e + e - pp i related to the cro ection for the nonradiative proce e + e - pp. d e dm d e pp co m W (, x, ) e e pp ( m), x E 1 m The function W(,x,) i calculated in QED. It decribe angular (1/in at >> m e /) and energy (1/x) ditribution of the ISR photon. 9

10 tagged (LA) ISR ISR method Untagged (SA) ISR p untagged e + p p e e + p e tagged The ISR photon i emitted predominantly along the beam axi. The produced hadronic ytem i booted againt the ISR photon. Due to limited detector acceptance the ma region below 3 GeV can be tudied only with detected photon (about 10% of ISR event). However, above 3 GeV tatitic can be ignificantly increaed uing mall-angle ISR. 10

11 Advantage of LA ISR method A wide energy region i tudied in a ingle experiment. The effective ISR luminoity (pb -1 /GeV) increae with ma, partly compenating a decreae of the meaured cro ection. A low dependence of the detection efficiency on hadron invariant ma. Meaurement near and above threhold with the ame election criteria. A low dependence of the detection efficiency on hadron angular ditribution (in the hadron ret frame). For proton thi ignificantly increae enitivity for meaurement of the G E /G M ratio. 11

12 BABAR detector Detector of Internally Recflected Cherenkov Light (DIRC) 1.5 T Solenoid Electromagnetic Calorimeter (EMC) Drift Chamber (DCH) Intrumented Flux Return (IFR) Silicon Vertex Tracker (SVT) arxiv: The full data ample collected at and near (4S): 469 fb -1. The previou analyi (Phy. Rev. D 73, (005) 01005) wa baed on a half of the data: 31 fb -1. 1

13 Event election All final particle mut be detected and well recontructed. track with oppoite charge, which are required to originate from the interaction region and be identified a proton. They mut be in the DIRC polar angle range: 5.8 < < A photon candidate with E c.m. > 3 GeV in the well undertood calorimeter region: 0 < < A ignificant fraction of data event contain beam-generated background photon and charged track. Any number of extra photon and charged track are allowed in an event. Backround from e+e- +-, +-, K+K- exceed ignal by -3 order of magnitude. Thee background are uppreed by the PID requirement by a factor of for pion and muon, and 10 4 for kaon with a lo of about 30% of the ignal event. 13

14 proton Event election Further background uppreion i baed on kinematic fitting. A kinematic fit to the e + e - h + h hypothei (h=p,k) i performed with requirement of energy and momentum conervation. p 30, K 30 kaon 898 event The requirement provide additional background uppreion by a factor of 50 for pion and muon, and 30 for kaon, with a lo of 5% of ignal event. 14

15 e + e - pp 0 background K MC The main background arie from e + e - pp 0 with an undetected low-energy photon or with merged photon from the 0 decay. Thee event have ditribution peaked at low value and cannot be eparated from ignal event. The e + e - pp 0 event are elected in data uing of the kinematic fit to the e + e - pp hypothei and requirement on the two-photon invariant ma. The ma pectrum of pp 0 event paing the pp election i determined a K MC (dn/dm) data. The cale factor K MC found from imulation i about 3. M pp < >4.5 N data N pp

16 e + e - pp 0 background The cro ection for e + e - pp 0 at c.m. energy of 10.6 GeV i about 5 fb and eem too large. At low energie thi proce wa tudied in our 005 analyi. It dominant mechanim wa found to be pn(1440). So, we expect the QCD caling () 4 / 5, imilar to e + e - pp. The etimated cro ection at 10.6 GeV i about 0.0 fb. At high energy mechanim i different: the pp pair ha low invariant ma, i.e. the pion and pp pair are produced back-to back. The ma pectrum i reminicent of ISR pp ma pectrum. 0 pp pair 16

17 Background ubtraction pp 0 ISR and e + e pp data N N =N /N ISR and e + e column how JETSET prediction for background from procee other than e + e - pp 0 : e + e - pp, pp 0, pp 0, pp 0 N 1 N To ubtract ISR and e + e background, we ue the method baed on difference of the ditribution for ignal and background event: N ig N N 1 βbkg, Nbkg N 1 N 1 β β ppγ The ISR and e + e background i about 1% (110 30). bkg ig. 17

18 Angular ditribution The ditribution over the angle between the proton momentum in the pp ret frame and the momentum of pp ytem in the e + e - c.m. frame i fitted by a um of hitogram, obtained from two imulated event ample, one with G E =0 and other with G M =0. Thee ditribution are cloe to 1+co p and in p. 18

19 Aymmetry in the angular ditribution GeV/c The aymmetry i abent in lowet order (pp). It arie from higher-order contribution (oft extra ISR and FSR interference, two-photon exchange). Meauring the aymmetry we control the higher-order contribution. Our imulation ue a model with one-photon exchange. The aymmetry in the imulated ditribution i due to an aymmetry in the detection efficiency. We analyze the difference between the meaured and fitted ditribution. The lope i The integral aymmetry (co p A (co p 0) (co 0) (co p p 0) 0)

20 Detection efficiency The detection efficiency i determined uing MC imulation with 0 < < 160. The model dependence due to uncertainty in the G E /G M ratio i about 1% for M pp <3 GeV/c, and 4% for higher mae, where we ue G E = G M aumption. Thi MC efficiency mut be corrected to account for data-mc difference in detector repone: MC ( 1 i ) i 0

21 Efficiency correction (%) Effect 1.9 GeV/c 3.0 GeV/c 4.5 GeV/c cut Track recontruction Track overlap Nuclear interaction PID Photon inefficiency Trigger and filter Total

22 Photon detection efficiency Calorimeter inefficiency i tudied uing e + e - + event elected with the requirement of two identified muon with zero recoil ma and without any condition on photon parameter. Photon converion i tudied uing e + e - + event with four track (two muon and two cloe electron). -( )%. -( )%. -( )%.

23 Nuclear interaction MC proton antiproton data proton antiproton In MC imulation the nuclear interaction lead to about 6% event lo. To check correctne of the imulation we compare track loe in SVT in data and MC uing proton and antiproton from p decay. Data and MC are in a good agreement for proton. For antiproton the imulation overetimate the annihilation cro ection by a factor of Thi lead to ( )% efficiency correction. 3

24 PID efficiency Data-MC difference in proton ID i tudied with the ue of J/pp event elected with one and two identified proton. The correction i found to be about (1.9.0)%. 4

25 J/ and (S) decay J/ dσ d coθ γ 1 (V e m σ( 0 θγ 160 ) e )B(V V N εrl, f ) W(, x 0, x 0 1 ) m V N,% R ee B pp, ev J/ (S) (S) PDG B( J / B( (S) pp) ( ) 10 pp) ( ) BESIII: 5

26 Radiative correction Cro ection σ eepp (m) (dn/dm) εr dl/dm corr With choen width of the ma bin the reolution correction inignificantly change the hape of ma pectrum but lead to about 0% increae in the error and their correlation., dl dm α πx x x 1 C log 1 C x C m L, pb -1 x 1 m, C coθ 0, θ

27 Meaured cro ection Agreement with the 005 reult. Improvement in accuracy by a factor of about 1.5. Step-like ma dependence near.,.55, 3.0 GeV. 7

28 Meaured cro ection In the ma region under tudy the cro ection change by about 4 order of magnitude. Reaonable agreement with previou e + e - meaurement 8

29 Effective proton form factor A teep falloff and GE/GM 1 near threhold are explained by nucleon final tate interaction. A complex tep-like behavior far of threhold ( or even 3 tep at.,.55, 3 GeV/c ) i not decribed by exiting model. 9

30 Near threhold enhancement V.F.Dmitriev and A.I.Miltein arxiv: nucl-th/ The Pari nucleon-antinucleon potential with an aborptive part i ued. The quetion: Can uch a model decribe the dip oberved in e + e - 6? arxiv: e + e - 3( + - ) Similar to Coulomb interaction, nuclear interaction of proton and antiproton may modify the cro ection and the G E /G M ratio near threhold. BaBar CMD-3 pp threhold 30

31 Example of the form-factor model Exiting form-factor model (baed on VMD) decribe reaonably well the pace-like form factor, but are not capable of reproducing the complex ma dependence of the time-like form factor. E.L.Lomon and S.Pacetti, Phy. Rev. D 85, (01) There i a hypothei that the tep in the cro ection arie becaue of opening new channel:, p (J.L.Roner, Phy.Rev. D74, (006)), But there i no even qualitative explanation of a poible mechanim. or eat away the proton cro ection? 31

32 QCD-motivated fit QCD prediction for the aymptotic proton form factor: F α pp (m ) m All the data above 3 GeV except the two NU point are decribed well by thi function. Adding the NU point change the fit / from 9/16 to 41/18. The NU reult indicate that the form factor near 4 GeV decreae lowly than predicted by QCD. In the ma region from 3 to 4.5 GeV the time-like form factor i about two time larger the the pace-like one. The aymptotic value of the pace- and time-like form factor are expected to be the ame. 4 3

33 Summary on the BABAR meaurement The e + e - pp cro ection and the proton effective form factor have been meaured from threhold up to 4.5 GeV uing the full BABAR data ample. The form factor have complex ma dependence. There are a near-threhold teep falloff and a tep-like behavior at higher mae. The G E /G M ratio ha been meaured from threhold to 3 GeV/c. A large deviation of thi ratio from unity i oberved below. GeV/c. Aymmetry in the proton angular ditribution have been meaured. The branching fraction for J/ and (S) decay to pp have been meaured. We plan to meaure the cro ection in the ma range 3-7 GeV uing the ame data, but mall angle ISR. 33

34 VEPP-000 reult The polar angle range ued by CMD-3 i co < 0.7. A ignificant (~100) increae of data i expected after VEPP- 000 upgrade, in

35 ISR, tagged and untagged: 10 fb -1 at 3.77 GeV From Critina Morale talk at Mainz-Oray joint meeting on EM phyic at PANDA, Oray, January 01.9 fb -1 tagged BABAR BES-III plan Direct can (014?) E L N BES (50%) N BABAR / 100 MeV Due to the retricted polar angle range ( co <0.8, a ignificant improvement in G E /G M accuracy i not expected. 35

36 PANDA and SuperKEKB PANDA at the HESR antiproton beam at FAIR (018): pp e + e - при q от 5 до 5 ГэВ Belle II at SuperKEKB BABAR100 data are expected Each point fb -1, 4 month E N PANDA (50%) SuperKEKB N/(bin GeV) / / / / /0. 36