1 / 27

Hall C Summer Meeting 4 August 2008

Meson Form Factors and Reaction Mechanism. Tanja Horn. Hall C Summer Meeting 4 August 2008. The Fundamental Issue. Confinement occurs at an intermediate distance scale Lattice QCD and phenomenological models give insight into the hadron structure at the confinement scale.

deborah-lancaster
Download Presentation

Hall C Summer Meeting 4 August 2008

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Meson Form Factors and Reaction Mechanism Tanja Horn Hall C Summer Meeting 4 August 2008

  2. The Fundamental Issue • Confinement occurs at an intermediate distance scale • Lattice QCD and phenomenological models give insight into the hadron structure at the confinement scale • Need experimental observables of the fundamental degrees of freedom of QCD in coordinate space • Forward parton distributions do not resolve partons in space • Form Factors measure spatial distributions, but the resolution cannot be selected independent of momentum transfer • Need a combination of both

  3. Exclusive Processes and GPDs • Increasing the virtuality of the photon (Q2) allows one to probe short distances • Sensitivity to partonic degrees of freedom • At sufficiently high Q2, the process should be understandable in terms of the “handbag” diagram • Incoming virtual photon scatters off one quark • interaction can be calculated in perturbative QCD • The non-perturbative (soft) physics is represented by the GPDs • Shown to factorize from QCD perturbative processes for longitudinal photons [Collins, Frankfurt, Strikman, 1997] t-channel process handbag

  4. GPDs fromExclusive Meson Production • Interest: spin/flavor structure of quark GPDs – mesons select spin • Requires L/T separation to facilitate interpretation, which is complicated by convolution with meson distribution amplitude (DA) • Vector mesons (r,w,f) allow for transverse imaging of the nucleon From: Diehl, Kugler, Schaefer, CW 2005

  5. Q2 dependence of σL and σT Hall C data at 6 GeV • The Q-6QCD scaling prediction is reasonably consistent with recent JLab π+σL data Q2=2.7-3.9 GeV2 Q2=1.4-2.2 GeV2 • To access physics contained in GPDs, one is limited to the kinematic regime where hard-soft factorization applies σL σT T. Horn et al., arXiv:0707.1794 (2007)

  6. Fπ - a factorization puzzle? T. Horn et al., Phys. Rev. Lett. 97 (2006) 192001. • Fπ has a simple prediction in perturbative QCD • The Q2 dependence of Fπ is also consistent with hard-soft factorization prediction (Q-2) already at values Q2>1 GeV2 • But the observed magnitude of Fπ is larger than the hard QCD prediction • Could be due to QCD factorization not being applicable in this regime • Or insufficient knowledge about additional soft contributions from the meson wave function T. Horn et al., arXiv:0707.1794 (2007). A.P. Bakulev et al, Phys. Rev. D70 (2004)] H.J. Kwee and R.F. Lebed, arXiv:0708:4054 (2007) H.R.Grigoryan and A.V.Radyushkin, arXiv:0709.0500 (2007)

  7. Strangeness in GPDs and exclusive processes • Kaon production probes polarized GPDs analogous to pions • High –t meson production to learn about the reaction mechanism • QCD factorization • Kaon pole term is expected to be prominent • Kaon form factor measurements • Relatively model independent pole dominance test through

  8. Q2 dependence of σKaon • Many measurements of exclusive p(e,e’K+)Λ(Σ°) exist, but contribution of σT unknown at higher energies • Difficult to draw a conclusion about the reaction mechanism • Limited Q2 range • Significant uncertainty due to scaling in xB and -t p(e,e’K+)Λ p(e,e’K+)Σ° W=1.84 GeV W=1.84 GeV

  9. K+ Form Factor at 6 GeV • JLAB experiment E93-018 extracted –t dependence of σLK+ near Q2=1 GeV2 • Trial Kaon FF extraction was attempted using a simple Chew-Low extrapolation technique gKLN poorly known Q2=1.0 GeV2 W=1.84 GeV Q2=0.75 GeV2 t=mK2 (kaon pole) -t dependence shows some “pole-like” behavior

  10. Motivation Summary Studies of kaon electroproduction provide a way to determine if scaling behavior observed in Fπ would manifest itself in a similar system Direct comparison of the scaling properties of σL provides another important tool in the search of the onset of factorization σL is expected to evolve towards Q-6 scaling at sufficiently large Q2 Transverse contributions are suppressed by an additional factor of Q-2 xB dependence of σL in Σ° production could provide information about pole and non-pole contributions

  11. Experiment Goals • To meet motivation requirements perform the measurement above the resonance region – first time for W>2.5 GeV • Allows for meaningful studies of the Q2 dependence of σL and better extraction of the kaon form factor • Measure the Q2 dependence of the p(e,e’K+)Λ(Σ°) cross section at fixed xB and –t to search for evidence of hard-soft factorization • Separate the cross section components: L, T, LT, TT • The highest Q2 for any L/T separation in K+ electroproduction • Also measure the Q2 dependence of the kaon form factor to shed new light on the apparent pion form factor scaling puzzle

  12. Experiment Overview • Planned proposal for PAC34 • T. Horn, P. Markowitz, G. Huber • Measure separated cross sections for the p(e,e’K+)Λ(Σ°) reaction at two values of xB • Near parallel kinematics to separate L,T,LT,TT • Measure the separated cross sections at varying –t allows for extraction of kaon ff (W>2.5 GeV)

  13. Cross Section Separation • The virtual photon cross section can be written in terms of contributions from transversely and longitudinally polarized photons. • Separate σL, σT, σLT, and σTT by simultaneous fit using measured azimuthal angle (φK) and knowledge of photon polarization (ε)

  14. Separation in a Multi-Dimensional Phase Space Low ε • Cuts are placed on the data to equalize the Q2-W range measured at the different ε-settings High ε • Multiple SHMS settings (±2° left and right of the q vector) are used to obtain good φ coverage over a range of –t • Measuring 0<φ<2π allows to determine L, T, LT and TT • Determine LT, TT for xB=0.25 only • For xB=0.40 apply a “parallel” cut on θK SHMS+2° SHMS-2° High ε Radial coordinate (-t), Azimuthal coordinate (φ)

  15. Kaon PID E93-108 Kaon PID • Aerogel Cerenkov is essential for proper kaon identification at lower momenta as time-of-flight alone is not sufficient TOF Aerogel Cerenkov Discrimination power Kaon 12 GeV Kinematics Heavy Gas Cerenkov Momentum (GeV/c)

  16. Expected Missing Mass Resolution Λ • Missing mass resolution is very good • Acceptance allows for simultaneous studies of both Λ and Σ° channels • Kinematic dependences of the ratio Σ° Simulation at Q2=2.0 GeV2 , W=3.0 and high ε

  17. Predictions for the Q2 dependence of R=σL/σT • VGL/Regge parameterization was used for the L/T ratio • Projected Δ(L/T)=30-50% for typical kinematics • Future predictions may indicate larger values of R, and thus lower uncertainties • Reaching Q2=8 possible (still under study) VGL/Regge Fπ param

  18. Projected Uncertainties for Q-n scaling • QCD scaling predicts σL~Q-6and σT~Q-8 • Projected uncertainties use R as determined from VGL/Regge p(e,e’K+)Λ xB=0.25 1/Q4 1/Q6 1/Q8 • Data will provide important information about the onset of factorization in 12 GeV kinematics and may provide a way to study effects related to SU(3)

  19. Projected Uncertainties for the Kaon FF Q-2 dependence p(e,e’K+)Λ • First measurement of FK well above the resonance region • Measure form factor to Q2=3 GeV2 with good overlap with elastic scattering data W>2.5 GeV • Limited by t<0.2 GeV2 requirement to minimize non-pole contributions • Data will provide important information the apparent scaling puzzle observed in the pion ff For VGL/Regge calculation, assume Λ2K=0.67 GeV2, andΛ2K*=1.5 GeV2,

  20. Summary • First measurement of kaon electroproduction above the resonance region • Meaningful studies of the Q2 dependence of the cross section and kaon ff extractions • L/T separated K+ cross sections will be essential for our understanding of the reaction mechanism at 12 GeV • determine if scaling behavior observed in pion production would manifest itself in a similar system • Direct comparison of the scaling properties of σL over a wide kinematic range provides another important tool in the search of the onset of factorization • σL is expected to evolve towards Q-6 scaling at sufficiently large Q2 • Transverse contributions are suppressed by an additional factor of Q-2

  21. Projected Uncertainties for σL at constant Q2 • xB scan at Q2=3 GeV2 • Expect significant x-dependence is non-pole contributes • Provides information about non-pole contributions Axial only Pion pole only Axial and pole

  22. Uncertainty in σL • Assuming equal correlated sytematic uncertainties at each ε • Due to amplification by 1/Δε, uncertainty in σL is dominated by uncorrelated systematic uncertainty • If R more favorable, precision in σL improves even for small Δε

  23. Fit: 1/Qn Q-n scaling after the Jlab Upgrade • E12-07-105 (T. Horn et al) approved for 42 days in Hall C • QCD scaling predicts σL~Q-6and σT~Q-8 • Projected uncertainties for σL are improved by a factor of more than two compared to 6 GeV • Data will provide important information about feasibility of GPD experiments at JLab 12 GeV kinematics

  24. Fπ after the JLab Upgrade • Experiment (E12-06-101) approved for 55 days in Hall C • The 11 GeV electron beam and the SHMS in Hall C with θ=5.5º allows for precision data up to Q2=6 GeV2 • May expect to see the onset of perturbative regime

  25. Tests of the Handbag Dominance • To access physics contained in GPDs, one is limited to the kinematic regime where hard-soft factorization applies • No single criterion for the applicability, but tests of necessary conditions can provide evidence that the Q2 scaling regime (partonic picture) has been reached • One of the most stringent tests of factorization is the Q2 dependence of the πelectroproduction cross section • σL scales to leading order as Q-6 • σT scales as Q-8 • As Q2 becomes large: σL >> σT Factorization Q2 ? • Factorization theorems for meson electroproduction have been proven rigorously only for longitudinal photons [Collins, Frankfurt, Strikman, 1997]

  26. Pion/Kaon ratio studies • Overlap with Fpi-3 at Q2=6 GeV2 • Additional info about reaction mechanism through π+/K+ ratios • QCD Factorization

More Related