gluons in the proton and exclusive hard diffraction

Gluons in the proton and exclusive hard diffractionAharon LevyTel Aviv University and MPI Introduction soft, hard interactions gluons data on exclusive vector meson electroproduction sizes of gluon cloud sizes of photon configurations effective Pomeron trajectory comparison to theory

Aharon Levy - Hosza seminar

LHeC


Deep Inelastic kinematicsSpin[20 fb-1 /point]


McAllister, HofstadterEe=188 MeVrmin=0.4 fmBloom et al. 10 GeV 0.05 fmCERN, FNAL fixed target 500 GeV 0.007 fmHERA 50 TeV 0.0007 fmHERA KinematicsEe=27.5 GeVEP=920 GeVs=(k+P)2 = (320 GeV)2Transverse distance scale:Impact parameter:where t is the square of the 4-momentumtransferred to the proton


Proton momentum framePartons frozen during time of interaction. Virtual photon samples the quark distribution.Assume that partons form incoherent beam. The parton density distributions are meant to be universal quantities.


Proton rest-framesoft and hard studied by W (or x~1/W2) dependence of the cross section.


soft

high energy behavior tot s0.08Donnachie and Lanshoff universal behavior of total hadron-hadron cross section :


Regge trajectories


hardThe rise of F2 with decreasing x is strongly dependent on Q2.DIS:


soft hardBelow Q2 0.5 GeV2, see same energy dependence as observed in hadron-hadron interactions. Start to resolve the partons.


QCD based fits can follow the data accurately, yield parton densities. BUT: many free parameters (18-30) (only know how parton densities evolve)

form of parameterisation fixed by hand (not given by theory)

F2 parton densities. * sees partons. parton density increases with decreasing x.


From Pumplin, DIS05There are signs that DGLAP (Q2 evolution) may be in trouble at small x (negative gluons, high 2 for fits).

Need better data to test whether our parton densities are reasonable. The structure function FL will provide an important test. all is not well Can also get information on gluon density from exclusive hard processes.


Exclusive VM electroproduction(V0 = DVCS)


soft to hard transition Expect to increase from soft (~0.2, from soft Pomeron value) to hard (~0.8, from xg(x,Q2)2) Expect b to decrease from soft (~10 GeV-2) to hard (~4-5 GeV-2)


ingredientsUse QED for photon wave function. Study properties of V-meson wf and the gluon density in the proton.


Mass distributions


Photoproduction process becomes hard as scale (mass) becomes larger.


(W) 0 Fix mass change Q2


(W) - , J/,


(Q2+M2) - VM


(Q2)Fit to whole Q2 range gives bad 2/df (~70)

VMncomments2.440.09Q2>10 GeV22.750.13 0.07Q2>10 GeV2J/2.4860.0800.068All Q21.540.09 0.04Q2>3 GeV2


b(Q2) 0,


b(Q2+M2) - VM


Frankfurt - Strikman


Information on L and T Use 0 decay angular distribution to get r0400 density matrix element - ratio of longitudinal- to transverse-photon fluxes ( = 0.996)using SCHC


R=L/T (Q2)When r0004 close to 1, error on R large and asymmetric advantageous to use r0004 rather than R.


Photon configuration - sizesT: large size small size strong color forces color screening large cross section small cross section*: *T, *L*T both sizes, *L small size


L/tot(W)


L/tot(t)


(W) - DVCSFinal state is real Tusing SCHC initial * is *Tbut W dep of steep large *T configurations suppressed


Effective Pomeron trajectory0 photoproduction


Effective Pomeron trajectory0 electroproduction


Comparison to theory All theories use dipole picture Use QED for photon wave function Use models for VM wave function usually take a Gaussian shape Use gluon density in the proton Some use saturation model, others take sum of nonperturbative + pQCD calculation, and some just start at higher Q2 Most work in configuration space, MRT works in momentum space. Configuration space puts emphasis on VM wave function. Momentum space on the gluon distribution. W dependence information on the gluon Q2 and R properties of the wave function


0 data - Comparison to theoryMartin-Ryskin-Teubner (MRT) work in momentum space, use parton-hadron duality, put emphasis on gluon density determination. Phys. Rev. D 62, 014022 (2000).Forshaw-Sandapen-Shaw (FSS) improved understanding of VM wf. Try Gaussian and DGKP (2-dim Gaussian with light-cone variables). Phys. Rev. D 69, 094013 (2004).Kowalski-Motyka-Watt (KMW) add impact parameter dependence, Q2 evolution DGLAP. Phys. Rev. D 74, 074016 (2006).Dosch-Ferreira (DF) focusing on the dipole cross section using Wilson loops. Use soft+hard Pomeron for an effective evolution. Eur. Phys. J. C 51, 83 (2007).


Q2KMW good for Q2>2GeV2 miss Q2=0DF miss most Q2FSS Gauss better than DGKP


Q2Data seem to preferMRST99 and CTEQ6.5M


W dependenceKMW - closeFSS:Sat-Gauss right W-dep. wrong norm. MRT:CTEQ6.5M slightly better in W-dep.


L/tot(Q2)


L/tot(W)All models have mild W dependence. None describes all kinematic regions.


Summary and conclusions

HERA data shows transition from soft to hard interactions.The cross section is rising with W and its logarithmic derivative in W, , increases with Q2.The exponential slope of the t distribution decreases with Q2 and levels off at about b = 5 GeV-2. Transverse size of gluon density (0.6 fm) inside the charge radius of the proton (0.8 fm).The ratio of cross sections induced by longitudinally and transversely polarised virtual photons increases with Q2, but is independent of W and t. The large configurations of the transversely polarised photon are suppressed.The effective Pomeron trajectory has a larger intercept and smaller slope than those extracted from soft interactions.All these features are compatible with expectations of perturbative QCD.None of the models which have been compared to the measurements are able to reproduce all the features of the data.Precision measurements of exclusive vector meson electroproduction can help determine the gluon density in the proton.


Fixed Q2>>0 to read Q2~0 (May 16,2005)

gluons in the proton and exclusive hard diffraction

Documents

aharon levy hosza seminarqcd

hard transition

exclusive hard processes

hard interactions gluons

parton density distributions

theoryf2 parton densities

gluon density

hadronhadron interactions