BaBar Form Factors Measurements

Transcription

1 BaBar Form Factors Measurements Evgeny Kozyrev on behalf of the BaBar collaboration Novosibirsk State University Budker INP SB RAS, Novosibirsk, Russia PhiPsi 015, Hefei, China

2 Outline Kaon Form Factors Proton Form Factors Collins asymmetries in inclusive charged KK and Kπ pairs Conclusion Asymmetric energy collider 9 Gev for electrons and 3.1 GeV for positrons βγ = 0.56 Total integrated luminosity ~ 530 fb-1 (469 fb-1 is used in presented analyses) 1

3 e+e- K+K- γisr The produced hadronic system is boosted against the ISR photon. Due to limited detector acceptance the mass region below.5 GeV can be studied only with detected photon (about 10% of ISR events). Above 3 GeV we can significantly increase statistics using small-angle ISR.

4 e+e- K+K- γisr Two BABAR works dedicated to kaon form factor measurement: 1 - Phys.Rev.D (013) mkk < 5 GeV/c - LA - (subm. to Phys. Rev. D).6 < mkk < 8 GeV/c - SA 3 kaons in P-wave π α β + σ ( s)= F (s) one form factor to describe K K γ vertex 3s pqcd predicts 8 π αs f K FK = s α s C ln ( s / Λ ) GF Γ ( K l ν )= f K m K (1 ( ml / m K ) ) V qs 8π f K =156 MeV From LA ISR study data are higher the QCD prediction by 4 times CZ: Z.Ph. C4 569 (1989) Does the difference between the prediction and data decrease with mass increase? 3

5 e+e- K+K- γisr SA ISR Main selection criteria: MC two tracks K+, K- with common vertex (1) MC e+e-γ μ+μ-γ MC () removes LA ISR suppresses KKπ < θ <.4 rad p\bar pγ, photon events Ptrans. < 0.15 GeV/c Mmiss < 1 GeV/c (3) J/psi ψ(s) χc0 μ+μ-γ pt,k+k- (GeV/c) Mmiss (GeV/c4) neither kaon candidate is identified as a muon Main backgrounds: 1. Two photon e+e- e+e- K+K-, ~3%. ISR with π0: e+e- γk+k-π0, ~5% 3. ISR misid: e+e- γμ+μ-, ~5% Systematics ~ 3% MКК (GeV/c4) 4

6 e+e- K+K- γisr SA ISR Detection efficiency & results Our results are in agreement with the BABAR large-angle ISR measurements The correction for the conditions Mmiss < 1 GeV/c4 and pt<0.15 GeV/c was studied using J/psi events selected with looser cuts and found to be zero. The nonmonotonic behavior of the efficiency above 5.5 GeV is explained by the inefficiency of background filters. 5

7 e+e- K+K- γisr Scaled form factor The mass dependence is approximated by a smooth function. One of the NU points lies below the curve by 3.5 sigmas. Since this point is obtained in the maximum of ψ(3770), the deviation may be result of interference resonant and nonresonant amplitudes. NU - K.K. Seth, et al., Phys. Rev. Lett. 110 (013) 000 Asy (NLO) B. Melic, et. al. PRD 60, (1999) Our data clearly indicates that the form factor decreases faster than αs/m approaching the pqcd predictions. 6

8 CONCLUSION The e+e- ->K+K- cross section and the kaon form factor have been measured from 3.0 to 7.5 GeV using small-angle ISR. This is the first measurement of the kaon form factor above 5 GeV. Our data clearly indicate that the difference between the measured form factor and pqcd predictions decreases with mass increase. 7

9 e+e- p+p- γisr Phys. Rev. D 87, (013) Phys. Rev. D 88, (013) mpp < 4 GeV (LA ISR) 3 < mpp < 8 GeV (SA ISR) μν iσ Γ μ (q)=γ F1 (q )+ q ν F (q ) M N μ It can be redefined using electric and magnetic form factors: q G E (q )=F 1 (q )+ F (q ) 4M G M (q )=F1 (q )+F (q ) d σ α βc 4M = ((1+cos θ) G + sin θ G M E ) dω 4 q q 4 π α βc M σ= ( G + sin θ G M E )= 3M q 4 π α βc M (1+ ) F 3M q 8

10 e+e- p+p- γisr Angular distribution LA G dn = A ( H M (cos θ p, M pp )+ E H E (cos θ p, M pp )) dcos θ p GM Fitted variables in 6 Mpp bins from MC HM syst. err ~ 4-5% (dominant contribution is due to the uncertainty in the pbarpπ0 background) HE The cosθp distribution for the mass region from threshold to 3 GeV/c The result significantly larger than PS170 The ratio limits to unity with Mpp increasing 9

11 e+e- p+p- γisr cross section LA The detection efficiency from MC in the model with GE = GM Systematic is about 4% Results consistent and generally more precise than other experiments Cross section nearly constant for ~ 00 MeV above threshold 10

12 e+e- p+p- γisr cross section SA Assuming GE = GM : Fit according to pqcd expectation ΛQCD = 0.3 GeV χ/ndf = 17.4 The profile is described by pqcd. SLAC 1993 data corresponds to measurement in space-like region. In the region [3-4.5] GeV/c the timelike FF about two times higher than the space-like FF. Systematic uncertainty decreases from 16% at 3 GeV to 6% at 4.5 GeV. Expectations from QCD: by definition, at threshold: perturbative QCD constrains the FF asymptotic behavior QCD + analyticity G E(4mp) = GM(4mp) q : G E, M (q ) const 4 q ln ( q / Λ QCD ) q ± : G E, M (q )=G E, M ( q ) 11

13 CONCLUSION The e+e- ->K+K- cross section and the kaon form factor have been measured from 3.0 to 7.5 GeV using small-angle ISR. This is the first measurement of the kaon form factor above 5 GeV. Our data clearly indicate that the difference between the measured form factor and leading-twist pqcd predictions decreases with mass increase. Using the ISR method in BaBar the proton FF has been measured from the threshold up to 6 GeV The e+e- ->pp cross section is nearly constant from the threshold up to. GeV. A resonance structure in proton FF is seen in the region -3 GeV Beginning from q=5 GeV the proton FF reveals the tendency to approach to the QCD prediction F(q)=F(-q). GE/GM measured via angular distribution for q < 9 GeV GE/GM > 1 just above threshold (disagreement with previous results ) 1

14 Collins asymmetries in inclusive charged KK and Kπ pairs Our understanding of the hadronic physics depends strongly on what we know about the parton distributions functions (PDFs) and fragmentation functions (FFs) universal non-pertubative objects Collins FF (H 1) [NPB 396, 161 (1993)]: chiral-odd function used to describe the fragmentation of a transversely polarized quark (q ) into a spinless hadron: q hx : D q 1 P _ _ ( z, P _ _ ; s q )= D ( z, P _ _ )+ H 1_ _ q ( z, P _ _ ) s q ( k q P _ _ ) zm h q 1 Unpolarized FF Collins FF The introduction of this function requires to take into account the quark's tranverse momentum Transverse Momentum Dependent (TMD) FFs Physics motivation: understand better the fragmentation process evolution of TMD objects in SIDIS experiments, H couples with the transversity parton 13 distribution function (PDF)

15 Collins asymmetries in inclusive charged KK and Kπ pairs In e + e - q q spins unknown, but sq sq exploit this correlation by using hadrons in opposite jets + + define favored (u π,d π, s K )and disfavored + +/(u π, d π, s( s ) π...) (1) σ cos(ϕ1 +ϕ ) H _ _ (h 1) 1 H _ _(h ) 1 () Collins effect for KK never measured in electron-positron annihilation 14

16 Collins asymmetries in inclusive charged KK and Kπ pairs EVENT SELECTION Number of charged tracks > Selection of two jets topology: thrust > 0.8 cosθthrust < 0.6 Visible energy Evis > 11 GeV Most energetic photon Eγ < GeV TRACK SELECTION Electrons and muons veto K and π in the DIRC acceptance region K/π fractional energy z: 0.15 < z < 0.9 Opening angle θh-thrust of hadron with respect to the thrust axis < 45 Qt < 3.5 GeV, where Qt is the transverse momentum of the virtual photon in the two hadrons center-of-mass energy 15

17 Collins asymmetries in inclusive charged KK and Kπ pairs N (φ1+φ)/<n> 16

18 Collins asymmetries in inclusive charged KK and Kπ pairs 17

19 Collins asymmetries in inclusive charged KK and Kπ pairs 18

20 Extracting uds Asymmetry 19

21 CONCLUSION The e+e- ->K+K- cross section and the kaon form factor have been measured from 3.0 to 7.5 GeV using small-angle ISR. This is the first measurement of the kaon form factor above 5 GeV. Our data clearly indicate that the difference between the measured form factor and leading-twist pqcd predictions decreases with mass increase. Using the ISR method in BaBar the proton FF has been measured from the threshold up to 6 GeV Near the threshold the proton FF is close to the point-like value The e+e- ->pp cross section is nearly constant from the threshold up to. GeV. A resonance structure in proton FF is seen in the region -3 GeV Beginning from q=5 GeV the proton FF reveals the tendency to approach to the QCD prediction F(q)=F(-q). GE/GM measured via angular distribution for q < 9 GeV GE/GM > 1 just above threshold (disagreement with previous results ) Simultaneous extraction of AKK, AKπ, and Aππ Collins asymmetry AUL asymmetry for KK are slightly larger than ππ AUC asymmetry for KK are slightly lower than ππ 0

22 Back up

23 Collins asymmetries. Double ratios. 15

24 Collins asymmetries in inclusive charged KK and Kπ pairs

25 Extracting uds Asymmetry 18

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