Measurements of the Proton and Kaon Form Factors via ISR at BABAR

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1 Measurements of the Proton and Kaon Form Factors via ISR at BABAR Fabio Anulli INFN Sezione di Roma on behalf of the BABAR Collaboration HADRON 015 XVI International Conference on Hadron Spectroscopy September, 015 Newport News, VA

2 x = Initial State Radiation at B-factories E * γ s dσ e + e fγ (s, m f ) dm f dcosθ γ = m f s W (s, x,θ γ ) σ e + e f ( m f ) m f = s " =s(1 x) The hadronic cross section e + e - f can be extracted from the ISR cross section e + e - γ f. W(s,x) is calculated in QED with accuracy better than 1% level Ø Quantum numbers at production vertex J PC =1 Ø Continuous ISR spectrum: Access a large energy range from threshold up to s ~5-8 GeV Ø α em suppression compensated by the huge luminosity Ø Comparable or better sensitivity than previous measurements based on energy scan ISR studies at the ϒ(4S) yield the same observables as low energy e + e - experiments! Precise measurements of exclusive e + e - cross sections at low energies Hadronic contribution to (g-) µ see Evgeny Solodov talk Light uark spectroscopy (Tue 4:15pm Session 3) Heavy uark spectroscopy Time-Like Form Factors (this talk) 09/14/015, Newport News F. Anulli

Large-Angle vs Small-Angle The ISR photon is emitted predominantly along the e + e - collision axis. The hadrons are boosted in the direction opposite to the photon.

3 Large-Angle vs Small-Angle The ISR photon is emitted predominantly along the e + e - collision axis. The hadrons are boosted in the direction opposite to the photon. Two different experimental approaches: Large-Angle (tagged) ISR analysis Small-Angle (untagged) ISR analysis Detection efficiency for simulated ISR pp events ISR-γ detected: 0 ο < θ γ * < 160 ο ( ~10% acceptance). Optimal for m f <3 GeV/ c ISR-γ along the beam: ~90% acceptance. Suitable for m f >3-4 GeV/ c Above 3 GeV the statistics can be significantly increased using the small-angle analysis 09/14/015, Newport News F. Anulli 3

4 Large-Angle vs Small-Angle Analysis ANALYSIS STRATEGY Ø Full reconstruction of hadronic final state Ø π/k/p discrimination using de/dx and Cherenkov angle Ø Reuire a hard γ with E γ* >3 GeV Ø Kinematic fit under different mass hypothesis, reuiring p and E conservation Ø Fit χ -values used for signal selection and background subtraction Ø Main backgrounds: Ø Similar ISR processes Ø e + e -, with final state containing π 0 s Ø M miss < 1 GeV /c 4 Ø P T < 0.15 GeV/c Ø Main backgrounds: Ø Similar ISR processes Ø Two-photon: e + e - e + e - 09/14/015, Newport News F. Anulli 4

5 PEP-II and BABAR Asymmetric-energy collider 9.0 GeV e- vs 3.1 GeV e+ βγ 0.56 Ten years of operation from 1999 to 008 Total integrated luminosity: ~530 fb -1 Analyses presented here based on the full data set 09/14/015, Newport News F. Anulli 5

6 e + e - γ ISR pp 09/14/015, Newport News F. Anulli 6

+ e Time-like e The Nucleon Form Factor in the Time-like region µ γ γ * ( ) = s µ Γ ( ) > 0 N N dσ (

" # G M " ( ) G M ( ) + 1 τ sin θ G E 1+ cos θ # $ + 1 τ G E $ % & % & ' τ 4M y C= 1 e ; y β = 1 1 τ y

M = G E ) Expectations from QCD Ø by definition, at threshold: G E (4m p ) = G M (4m p ) Ø perturbative

7 + e Time-like e The Nucleon Form Factor in the Time-like region µ γ γ * ( ) = s µ Γ ( ) > 0 N N dσ ( dω,θ) = α β C 4 σ = 4πα βc 3m pp e + e - pp cross section! " # G M " ( ) G M ( ) + 1 τ sin θ G E 1+ cos θ # $ + 1 τ G E $ % & % & ' τ 4M y C= 1 e ; y β = 1 1 τ y = παm β F = G M + 1 τ G E 1+1/ τ Effective Form Factor F (it corresponds to G M under the assumption G M = G E ) Expectations from QCD Ø by definition, at threshold: G E (4m p ) = G M (4m p ) Ø perturbative QCD constrains the FF asymptotic behavior pqcd + analyticity Ø ± GE, M ( ) = GE, M ( ) 09/14/015, Newport News F. Anulli 7 G E, M 4 constant ln Λ QCD

8 e + e - pp cross section LA analysis: PRD 87, (013) SA analysis: PRD 88, (013) Cross section (pb) BES CLEO NU BABAR (LA ISR) BABAR (SA ISR) 10 - Low energy region M pp (GeV/c ) 09/14/015, Newport News F. Anulli Ø BABAR is the only experiment to have measured the cross section from threshold up to 6 GeV Ø Results consistent and generally more precise than previous experiments Ø Cross section nearly constant for ~00 MeV above threshold Ø Possible structure at.9 GeV/c Ø New measurements from BESIII and CMD-3 (not reported here) available and consistent with BABAR data

9 e + e - pp: form factor (LA) Low energy region BABAR: PRD 87, (013) Full energy region with LA analysis Ø Many measurements both from e + e - pp and pp e + e - experiments Ø BABAR data consistent with both pp and e + e - previous results Ø FF ~ 1 at threshold (like a point-like particle?) Ø Steep drop of the FF after threshold seen by both PS170 (pp) and BABAR (e + e - ) Ø Fit to data above M pp >3 GeV/c of the asymptotic prediction C F pp M 4 ln M / Λ ( ) 09/14/015, Newport News F. Anulli 9

10 G M 10 - e + e - pp: form factor (SA) BABAR: PRD 88, (013) BES CLEO NU E835 E760 BABAR (LA ISR) BABAR (SA ISR) SLAC M pp (GeV/c ) Ø BABAR data consistent with previous data (and LA BABAR results) below 4 GeV Ø Data above ~4 GeV have a tendency to decrease more rapidly than pqcd asymptotic prediction and to approach space-like data (SLAC 1993: G M from electron-proton scattering) 09/14/015, Newport News F. Anulli 10

11 e + e - γ ISR Κ + Κ - 09/14/015, Newport News F. Anulli 11

e + e - Κ + Κ - The cross section depends on one form factor Asymptotic perturbative-qcd prediction F K ( m) = 8πα s f K m The fit to the data up to 5 GeV follow the pqcd behavior, but

12 e + e - Κ + Κ - The cross section depends on one form factor Asymptotic perturbative-qcd prediction F K ( m) = 8πα s f K m The fit to the data up to 5 GeV follow the pqcd behavior, but are above the absolute asymptotic prediction by a factor of ~4. BABAR: PRD 88, (013) BABAR PRD 88, (013) LA analysis (for.5 < s < 5 GeV) 09/14/015, Newport News F. Anulli 1

13 e + e - Κ + Κ - : Kaon Form Factor (SA) The SA analysis extends the energy range up to 7.5 GeV, and allows a better check of the asymptotic behavior BABAR: arxiv: preliminary BABAR preliminary Comparison of LA and SA analyses results in the common energy range The BABAR results are consistent each other, as well as with CLEO results The fit confirm a tendency of the data to approach the asymptotic limit, reducing the gap seen with the previous analysis New precise data (also at higher energies) are needed to see when the asymptotic limit is reached. Asy NLO: Brodsky and Lepage, PLB87, 359 (1979) Asy NLO: Melic, Nizic and Passek, PRD60, (1999) CZ: Chernyak and Zhitnisky Phys.Rept.11, 173 (1984) 09/14/015, Newport News F. Anulli 13

14 BABAR: arxiv: BABAR preliminary J/ψ Κ + Κ - and ψ(s) Κ + Κ - N Γ ee xb*(ev) B* 10 4 B 10 4 world ave. J/ψ 46±8 1.86±0.11± ±0.0±0.1.43±0.5 ψ(s) 66± ±0.035± ±0.15± ±0.05 BABAR preliminary Ø The measured Branching Fractions B* do not account for interference between resonant and non-resonant amplitudes in e + e - Κ + Κ -. Ø The world averages are obtained from e + e - Κ + Κ - processes only. Ø There is ameasurement of B(J/ψ Κ + Κ - ) = (.86±0.1)x10-4, obtained from J/ψ produced in ψ(s) J/ψπ + π - [PRD85,09007] ==> no interference with continuum Κ + Κ - production 09/14/015, Newport News F. Anulli 14

J/ψ Κ + Κ - and ψ(s) Κ + Κ - A s A γ A NR + + Relative phase between A γ and A s Interference between resonant and non resonant amplitudes ==> shift of B(ψ Κ + Κ - ) κ = κ determined from data (DM1,

15 J/ψ Κ + Κ - and ψ(s) Κ + Κ - A s A γ A NR + + Relative phase between A γ and A s Interference between resonant and non resonant amplitudes ==> shift of B(ψ Κ + Κ - ) κ = κ determined from data (DM1, BABAR) A γ is calculated from the measured kaon form factors at the mass of the resonance A s, φ and db are then extracted from these euations in a model independent way. Previous determinations of the relative phase were obtained under the assumptions F K =F π, and κ=0 09/14/015, Newport News F. Anulli 15

16 J/ψ Κ + Κ - and ψ(s) Κ + Κ - The sign of φ cannot be determined calculations performed either with sinφ > 0 or sinφ < 0. Two inconsistent measurements of B(J/ψ Κ S Κ L ) are available: BES: (1.8±0.14) 10-4 ; Seth et al. (.6±0.1) 10-4 => calculation for the J/ψ performed for both values and then averaged J/ψ BABAR preliminary : arxiv: ψ(s) BABAR preliminary: arxiv: /14/015, Newport News F. Anulli 16

17 J/ψ Κ + Κ - and ψ(s) Κ + Κ - Corrected branching fractions: BABAR preliminary: arxiv: < The db shifts are of the order of ±5% for the J/ψ and ±15% for the ψ(s) The corrected B(J/ψ Κ + Κ - ) can be directly compared to Seth et al. measurement obtained from the decay chain ψ(s) J/ψ Κ + Κ -, that is: B(J/ψ Κ + Κ - ) = (.86 ± 0.1) 10-4 [PRD 85, (01)]. The difference is ~1σ for sinφ > 0 and ~σ for sinφ < 0. A more precise measurement of both BFs could help to determine sinφ 09/14/015, Newport News F. Anulli 17

18 Conclusions Ø The ISR method has been proven to be very effective to study low-energy e + e - annihilation processes at BABAR Ø With the combination of LA and SA analyses, each reaction can be studied in the energy region from threshold up to more than 7 GeV Ø Studies of resonance production and perturbative QCD predictions are possible Ø The e + e - pp cross section and the proton form factor have been measured from threshold to 6 GeV Ø Near the threshold the proton FF is close to the point-like value Ø The e + e - pp cross section is nearly constant from the threshold up to. GeV, and the FF shows a steep rise toward threshold. Ø Beginning from =5 GeV the G M reveals the tendency to approach the QCD prediction F( ) = F(- ). Ø The e + e - Κ + Κ - cross section and the kaon form factor have been measured from threshold up to 7.5 GeV Ø Our data clearly indicate that F K (m) tend to approach the leading-twist pqcd predictions. Ø The BFs for J/ψ Κ + Κ - and ψ(s) Κ + Κ - have been measured, together with the BF-shifts due to interference and the relative phase between threegluon and 1-photon decays of the ψ Κ + Κ - decays. 09/14/015, Newport News F. Anulli 18

Conclusions Ø The ISR method has been proven to be very effective to study low-energy e + e - annihilation processes at BABAR Ø With the combination of LA and SA analyses, each reaction can be

19 Conclusions Ø The ISR method has been proven to be very effective to study low-energy e + e - annihilation processes at BABAR Ø With the combination of LA and SA analyses, each reaction can be studied in the energy region from threshold up to more than 7 GeV Ø Studies of resonance production and perturbative QCD predictions are possible Ø The e + e - pp cross section and the proton form factor have been measured from threshold to 6 GeV Ø Near the threshold the proton FF is close to the point-like value Ø The e + e - pp cross section is nearly constant from the threshold up to. GeV, and the FF shows a steep rise toward threshold. Ø Beginning from =5 GeV the G M reveals the tendency to approach the QCD prediction F( ) = F(- ). Ø The e + e - Κ + Κ - cross section and the kaon form factor have been measured from threshold up to 7.5 GeV Ø Our data clearly indicate that FK(m) tend to approach the leading-twist pqcd predictions. Ø The BFs for J/ψ Κ + Κ - and ψ(s) Κ + Κ - have been measured, together with the BF-shifts due to interference and the relative phase between threegluon and 1-photon decays of the ψ Κ + Κ - decays. 09/14/015, Newport News F. Anulli 19

20 BACKUP slides 09/14/015, Newport News F. Anulli 0

21 Outline Ø Initial State Radiation method at BABAR l Large angle vs small angle ISR analyses Ø Proton Form Factors: l Definitions and main properties l Cross section and effective form factor at low and high energies Ø Kaon Form Factor: l F K at low and high energies l J/ψ Κ + Κ - and ψ(s) Κ + Κ - branching fractions Ø Conclusions 09/14/015, Newport News F. Anulli 1

22 ISR cross section at Born approximation x dσ e + e fγ (s, m f ) dm f dcosθ γ E = s * γ Radiator function: = m f s ISR method in a nutshell W (s, x,θ γ ) σ e + e f m f = =s(1 x) W ( s, x, θ ) γ ( m f ) θ γ* : ISR photon polar angle in the e + e - c.m. α x + x π x sin θγ = x e + e - γ ISR m pp = s(1-x) x = E γ E b f e + e f cross section extracted from the measured mass spectrum σ 0 (m i ) = ΔN(m i) Δm reconstruction efficiency 1 ( )dl(m i ) /dm ε( s,m i ) 1+ δ rad radiative corrections ISR luminosity ISR differential luminosity dl dm = m s α π x & ( x + x ' ( ) log 1+ C " Obtained from integration of the radiator function over θ γ * " Known at <1% level 1 C x ) C+ L * ee * cosθ min Detector angular acceptance Luminosity integrated by the collider 09/14/015, Newport News F. Anulli

23 09/14/015, Newport News F. Anulli 3 FF: Space-like and Time-like region Ø Dirac (F 1 ) and Pauli (F ) F.F. Time-like + e e N N ) ( * γ 0 > = s ( ) µ Γ µ γ Space-like e N N ) ( * γ e < 0 ( ) µ Γ µ γ ( ) ( ) ( ) 1 F M i F N ν µν µ µ σ =γ + Γ ( ) ( ) ( ) ( ) ( ) ( ) F F G F M F G M E + + Ø Sachs FF: Electric (G E ), Magnetic (G M ) = = = = ) ( ) ( ) ( ) ( n n p p F F F F

24 FF: Space-like and Time-like region e p e θ p Ø Elastic scattering σ M θ ( θ ) = G τ 1+ ( 1 τ ) tan G dσ dω 1 τ α E 'cos θ/ σ M = 3 4 4E sin θ /, E M ( ) ( ) τ 4M e p p θ + e dσ dω α β C 1 (, θ ) = ( 1+ cos θ ) + sin θ 4 1 y β = 1 C= τ 1 e ; y G M G E τ y = παm β C: correction for Coulomb interaction at threshold. C=1 for neutral baryon production: e + e - nn, ΛΛ, 09/14/015, Newport News F. Anulli 4

25 Time-like G E /G M measurements dσ ( G M ) d cosθ ~1+ d ( G ) cos E θ p d cosθ σ ~sin θ p cos θ p Ø BABAR measured angular distribution in 6 energy bins from threshold up to ~3 GeV/c 1.877<m pp <1.950 GeV cos θ p.05<m pp <.100 GeV 1.950<m pp <.05 GeV cos θ p.100<m pp <.00 GeV Ø Observed maximum at m GeV/c (G E dominance after threshold) Ø Ø Inconsistent with PS170 measurements at LEAR Strong point in favour of ISR method: very weak angular dependence of detection efficiency cos θ p.00<m pp <.400 GeV.400<m pp <3.000 GeV cos θ p cos θ p 09/14/015, Newport News F. Anulli 5 cos θ p cos θ p

26 e + e - γ ISR pp : Analysis strategy (LA vs SA) Ø Monte Carlo simulations used for detector acceptances, selection efficiencies and estimates of different background sources: Ø ISR generators based on: H.Czyz et al, Eur. Phys. J. C 35(004)57 Ø multiple ISR soft photons: M.Caffo et al, N. C. 110A(1997)515 Ø final state radiation: (PHOTOS) E. Barberio et al, Comp. Phys Comm. 66(1991)115 09/14/015, Newport News F. Anulli 6

MEASUREMENT OF THE PROTON ELECTROMAGNETIC FORM FACTORS AT BABAR arxiv: v1 [hep-ex] 29 Nov 2013

MEASUREMENT OF THE PROTON ELECTROMAGNETIC FORM FACTORS AT BABAR arxiv:1311.7517v1 [hep-ex] 29 Nov 213 V. P. Druzhinin, on behalf of the BABAR Collaboration Budker Institute of Nuclear Physics SB RAS, Novosibirsk