Slide 1

Diffractive structure functions in e-A scattering Cyrille Marquet Columbia University based on C. Marquet, Phys. Rev. D 76 (2007) 094017 + paper in preparation with Henri Kowalski, Tuomas Lappi and Raju Venugopalan Slide 2 Motivations - diffraction probes QCD in a different way than inclusive measurements colorless t-channel exchanges, Pomerons, , the large fraction of events (10 %) observed at HERA was not expected a challenge for QCD - proposal for an electron-ion collider (at BNL or Jlab) by the EIC collaboration an important part of the physics program is diffraction: a first measurement of the nuclear diffractive structure functions - this talk: predictions in the Color Glass Condensate (CGC) framework Nikolaev, Schaefer, Zakharov and Zoller (2002) Frankfurt, Guzey and Strikman (2004) with nuclear DPDFs (leading-twist shadowing) an effective theory for QCD at high partonic density where the leading-twist approximation is not justified the non-linear weakly coupled regime of QCD - other existing predictions: for diffractive dijets Slide 3 Diffractive deep inelastic scattering ~ momentum fraction of the struck parton with respect to the Pomeron x pom ~ momentum fraction of the Pomeron with respect to the hadron/nucleus k k p p eh center-of-mass energy S = (k+p) 2 *h center-of-mass energy W 2 = (k-k+p) 2 photon virtuality Q 2 = - (k-k) 2 > 0 momentum transfer t = (p-p) 2 < 0 diffractive mass of the final state M X 2 = (p-p+k-k) 2 x ~ momentum fraction of the struck parton with respect to the hadron/nucleus Slide 4 Collinear factorization in the limit Q with x fixed perturbative non perturbative for inclusive DIS a = quarks, gluons Dokshitzer-Gribov-Lipatov-Altarelli-Parisi perturbative evolution of with Q 2 : not valid if x is too small when the hadron becomes a dense system of partons for diffractive DIS another set of pdfs, same Q evolution higher twists ~ Slide 5 Dipole factorization deep inelastic scattering (DIS): in the limit x 0 with Q fixed the photon split into a dipole (QED wavefunction (r,Q) ) the dipole then interacts with the target at small x, the dipole cross-section is comparable to that of a pion, even though r ~ 1/Q Hard diffraction on nuclei in progress with Kowalski, Lappi and Venugopalan the ratios F A D / F p D for each contributions: quark-antiquark-gluon quark-antiquark (T) quark-antiquark (L) > 1 and ~ const. > 1 and decreases with < 1 and ~ const. for Au nucleus, without breakup the decrease with (decreasing ) of is slower for a nucleus than for a proton as a function of : the quark-antiquark contributions for values at which they dominate: the decrease (with increasing Q 2 ) of the diffractive cross-section is slower for a nucleus than for a proton as a function of Q 2 : Slide 10 The ratio F 2 D,A / F 2 D,p the quark-antiquark contribution dominates the quark-antiquark-gluon contribution is important only for very small values of , the ratio gets constant and decreases with A decreases with A the ratio of the structure functions: for Au nucleus, one gets a 15 % bigger structure function when allowing breakup into nucleons comparison breakup / no breakup: next step: compute Slide 11 Conclusions - CGC (saturation) phenomenology is very successful at both HERA and RHIC - the same dipole scattering amplitude describes inclusive and diffractive DIS global description with very few parameters - after fitting a few parameters on inclusive data, the parameter free predictions for diffraction agree very well with the HERA data - the model also describes vector meson (, , J/) production (total cross- sections and t-spectra) with 2 additional parameters Marquet, Peschanski and Soyez (2007) Why do these high-energy QCD computations work so well at HERA ? we would understand better if this could be tested with a future EIC depending on the energy options, one could reach Qs ~ 2 GeV