DVCS and exclusive vector meson production in DIS Christoffer Flensburg (continuing Emil Avsars work) PhD under Leif Lönnblad and Gösta Gustafson DESY September 08

Contents The need for Fluctuations How our dipole model accounts for it Comparison with experiments Future plans and ideas

Eikonal cross sections Fluctuations are important in the cascade and interaction!

Dipole model in x-space Initial state dipoles in impact parameter space Can account for fluctuations Easier to account for saturation Easier to account for multiple interactions

Evolution in our model Mueller dipoles in impact parameter space Evolve in rapidity from initial wavefunction: 1 to 2 splitting Energy conservation Running coupling constant Confinement 2 to 2 swing Saturation The most important things to remember. Impact parameter space. Not momentum space for the transeverse coordinates, but the actual x-space. Only initial state and total cross section. We are not, at the moment, predicting any final states, only total cross sections. Covers many effects. Saturation, runing alpha s, e cons.

Life of a Dipole So let’s show off our program. Hihi.  this is a typical dipole from a virtual photon. Units are length in inverse GeV. Rapidity 0 means that no emission has taken place.

Life of a Dipole Bad luck first 2 units of rapidity...

Life of a Dipole Typical emission.

Life of a Dipole Each dipole splits again. Forms a chain of dipoles.

Life of a Dipole More splittings. Note that small dipoles still are favoured, even though energy conservation dampens the effect. Tends to clump up around qqbar.

Life of a Dipole The occasional statistical fluke that makes the dipole actually grow in transverse size. This sort of emission allows interaction at larger distance, ie collisions at high b (impact parameter). Also not e that it is no longer a pure chain of dipoles. Two loops has disconnected from the original chain. From ”the dipole swing” which is connected to saturation.

Life of a Dipole More emissions, more loops...

Life of a Dipole The crouching swan?

Life of a Dipole ...

Life of a Dipole Ok, lets stop here... Note the ”hot spots” where the original qqbar were. Also the first emission can be seen. So that is what photons do before coliding. 

Collision in our model Interaction probability Sum over all pairs of dipoles, and unitarise: Average and integrate over b.

What xsecs can we calculate? Xsec for any high energy collision where the initial states can be modeled by dipoles. Total, diffractive, elastic pp Total γ*p as function of W and Q. Quasielastic γ*p γ* --> real photons. (DVCS) γ* --> vector mesons.

What do we need? Two parameters in evolution Dipole Wavefunctions. ΛQCD Confinement scale rmax Dipole Wavefunctions. Photon WF calculated perturbatively. VMD corrections needed for small Q2. Proton and Mesons need to be modeled. Proton modeled by dipole triangle. Different meson models are tested. Nämn jeff o Co om mesonmodel testandet.

Proton results Tuning of evolution and proton parameters. (Note, energy dependence tune independent) (As is large -t xsec)

Photon results VMD correction in photon WF.

Vector Meson Results (rho) Similar results for phi and J/Psi. J/Psi more model dependent.

Exclusive final states. Future plans. Exclusive final states.

Problems Need to decide which dipoles interact. Need to decide which dipoles to keep, and which to reabsorb. Need to decide exactly how to reabsorb virtual dipoles. Virtual dipole emiossions.

Example xT Send to FSR. y

Summary Fluctuations in cascade and interaction needed. Can be done in an initial state, impact parameter dipole model. Accounts for many other effects. Can be applied to many processes. Good results Working on final states.

Thank you for being invited. Thank you for your attention. Christoffer Flensburg (continuing Emil Avsars work) PhD under Leif Lönnblad and Gösta Gustafson DESY September 08