Opportunities with diffraction

Transcription

1 Opportunities with diffraction Krzysztof Golec-Biernat Institute of Nuclear Physics in Kraków IWHSS17, Cortona, 2 5 April 2017 Krzysztof Golec-Biernat Opportunities with diffraction 1 / 29

2 Plan Diffraction and the pomeron Hera perspective Exclusive diffractive processes - from Hera to LHC Shock wave approach to diffraction Krzysztof Golec-Biernat Opportunities with diffraction 2 / 29

3 Diffractive processes s L R X Y s t, mx 2, my 2 single diffraction SD double dissociation DD vacuum quantum number exchange s Q 2 DIS: s Q 2, t, m 2 p P X Y semihard process Λ QCD Q 2 s => x = Q2 s 1 perturbative QCD applicable Krzysztof Golec-Biernat Opportunities with diffraction 3 / 29

4 Diffraction and pomeron Diffraction is about the structure of pomeron - vacuum quantum number exchange. Regge theory - soft pomeron trajectory with intercept above one α(t) = α IP (0) + α IP t = (0.25 GeV 2 ) t Gives asymptotic behaviour when s A(s, t) iβ(t) s α(t) => σ tot s α IP (0) 1 QCD - two gluon color singlet exchange, BFKL pomeron - α IP (0) = 1.3 Krzysztof Golec-Biernat Opportunities with diffraction 4 / 29

5 Diffraction at HERA Large rapidity gaps in DIS pomeron momentum fraction x IP = Q2 + M 2 Q 2 + W 2 1 Diffractive structure functions F D 2,L(x, Q 2 ; x IP, t) More exclusive measurements, e.g. dijets, vector mesons, heavy quarks, γ Krzysztof Golec-Biernat Opportunities with diffraction 5 / 29

6 Soft pomeron exchange Collinear factorization approach Soft pomeron with partonic structure - Ingelman-Schlein model ( 80) F2 D = f IP (x IP, t) { } β q IP (β, Q 2 ) + q IP (β, Q 2 ) q β = x/x IP is a pomeron momentum fraction carried by a parton. Pomeron flux f IP x 1 2α IP (t) IP and pomeron PDFs {q IP, q IP, g} Krzysztof Golec-Biernat Opportunities with diffraction 6 / 29

7 Pomeron PDFs Pomeron PDFs evolved with DGLAP equations fitted to diffractive data. Pomeron is gluon dominated (in comparison to normal PDFs). Krzysztof Golec-Biernat Opportunities with diffraction 7 / 29

8 Soft color interaction (SCI) model Soft gluon exchanges neutralize color but do not change momenta. (A. Edin, G. Ingelman, J. Rathsman, Phys.Lett. B366 (1996) 371) e e e e p 3 p 3 To check this mechanism studying W ± production asymmetry in rapidity A(y) = dσ W + dσ W dσ W + + dσ W (KGB, C. Royon, L. Schoeffel, R. Staszewski, Phys.Rev. D84 (2011) ) Krzysztof Golec-Biernat Opportunities with diffraction 8 / 29

9 Diffractive W ± production asymmetry In LO W ± from fusion of two quarks (ud or du) In SCI model quarks from the proton - asymmetry A(y) 0 Quark distributions in the pomeron are flavour blind - A(y) = 0 Study also the ratio R = σ W ±/σ Z. Krzysztof Golec-Biernat Opportunities with diffraction 9 / 29

10 BFKL pomeron and its unitarization DIS at small x (high energy) can be viewed as a quark dipole interaction. Two gluons σ γ p const BFKL pomeron σ γ p x 0.3 Unitarized pomeron σ γ p ln(1/x) Krzysztof Golec-Biernat Opportunities with diffraction 10 / 29

11 Approaches to unitarization γ* G V G G N Color dipoles of Mueller and Kovchegov Shock wave approach of Balitstky Color Glass Condensate and McLerran and Venugopalan QCD reggeon field theory of Bartels and Lipatov Krzysztof Golec-Biernat Opportunities with diffraction 11 / 29

12 Saturation model A quark dipole of transverse size r interacting with the proton Dipole cross section with saturation scale Q s 1 GeV in perturb. domain } ˆσ dip (r, x) = σ 0 {1 exp( r 2 Qs 2 (x)) Q s(x) = Q 0 x λ Red parameters fitted DIS data on F 2 for x 10 2 F 2(x, Q 2 ) d 2 r Ψ γ qq(r, Q 2 )) 2 ˆσ dip (r, x) (KGB, M. Wüsthoff, PRD 59 (1998) ) Krzysztof Golec-Biernat Opportunities with diffraction 12 / 29

13 Physical interpretation σ(mb) r(fm) For a given dipole size cross section saturates when x 0 DIPOL x=10 2 x=10 5 Parton saturation confirmed by the Balitsky-Kovchegov equation. (I. Balitsky, Nucl.Phys. B463 (1996) 99; Y. Kovchegov, PRD 60 (1999) ) Krzysztof Golec-Biernat Opportunities with diffraction 13 / 29

14 Saturation in diffraction Two diffractive states: qq and qqg interacting with ˆσ dip = d 2 b N(b) F D 2 = F qq T + F qq L + F qqg T Can also be applied to diffractive vector meson production Krzysztof Golec-Biernat Opportunities with diffraction 14 / 29

15 Comparison with HERA diffractive data x P F 2 D(3) 0.05 Q 2 =8 GeV Tqqg Sum Tqq 0.01 Lqq β The three contributions to diffractive structure function. F qq L is higher twist which dominates for β = Q2 Q 2 +M 2 1. Krzysztof Golec-Biernat Opportunities with diffraction 15 / 29

16 Constant ratio σ diff /σ γ p σ diff /σ tot Q 2 = 8 GeV 2 Q 2 = 27 GeV 2 Q 2 = 14 GeV 2 Q 2 = 60 GeV 2 M X < 3 GeV < M X < 7.5 GeV < M X < 15 GeV W(GeV) Explained by the saturation model (KGB, M. Wüsthoff, PRD 60 (1999) ) Krzysztof Golec-Biernat Opportunities with diffraction 16 / 29

17 Vector meson production (H. Kowalski, L. Motyka, G. Watt, PRD 74 (2006) ) Scattering amplitude A(γ + p V + p) = (Ψ V ) N dip (x, r, b) Ψ γ New element - impact parameter b (or t) dependence Improved dipole scattering amplitude N(x, r, b) = 1 exp{ r 2 Q s(x, b)} b-dependent saturation scale Q s (b CGC model) Krzysztof Golec-Biernat Opportunities with diffraction 17 / 29

18 VM production - energy dependence Change of energy dependence with VM and Q 2 and a slight dependence on the choice of the VM wave function. Krzysztof Golec-Biernat Opportunities with diffraction 18 / 29

19 VM production - t dependence Eikonal form of dipole scattering amplitude with saturation scale Q S (x, b) crucial for these results. Krzysztof Golec-Biernat Opportunities with diffraction 19 / 29

20 Exclusive diffractive dijet production Two gluon exchange in k T -factorization versus collinear approach (Bartels, Ewertz, Lotter, Wüsthoff,Jung,...) Look at azimuthal dependence: dσ (1 + A cos φ) dφ Krzysztof Golec-Biernat Opportunities with diffraction 20 / 29

21 k T factorization vs collinear factorization Two gluon exchange model works better then resolved pomeron model. Krzysztof Golec-Biernat Opportunities with diffraction 21 / 29

22 LHC perspective Such processes can also be measured at the LHC in pp, pa or ultraperipheral AA collisions. ep pp, pa, AA Details of the description in ep collisions transferred to hadronic collisions. Krzysztof Golec-Biernat Opportunities with diffraction 22 / 29

23 Central diffractive production production at the LHC (LHC forward physics, J.Phys. G43 (2016) , arxiv: [hep-ph]) Program to build very forward detectors to tag protons at small angles Double pomeron exchange Central exclusive production Absorptive factors which reduce cross sections are important - gap survival factors. Krzysztof Golec-Biernat Opportunities with diffraction 23 / 29

24 Systematic studies within shock wave approach (I. Balistky, Nucl.Phys. B463 (1996) 99) At high energy particles move along straight lines - Wilson lines { } Û(x ) = exp ig dx + Â (x +, x ) High energy scattering amplitude with factorization parameter η A(s) = d 2 x d 2 y I A (x, y ; η) B Tr[ Û η(x }{{} )Û η(y )] N c B }{{} Impact factor dipole operator Krzysztof Golec-Biernat Opportunities with diffraction 24 / 29

25 Balitsky-Kovchegov equation Balitsky-JIMWLK equations evolve dipole operators into multidipoles Ûη 12 η Û η 13Ûη 32 η = αsnc 2π 2 =... d 2 z 12 2 z 3 z 2 13 z 32 2 } {Ûη 13 + Ûη 32 Ûη 12 Ûη 13Ûη 32 Kovchegov equation for dipole operator only in large N c limit Û η 12 = αsnc η 2π 2 d 2 z 12 2 z 3 z 2 13 z 32 2 { Ûη Ûη Ûη Ûη Ûη } BFKL/BKP eq saturation Krzysztof Golec-Biernat Opportunities with diffraction 25 / 29

26 qqg impact factors in NLO approximation (R. Boussarie, A. V. Grabovsky, S. Wallon, L. Szymanowski, D. Yu. Ivanov, JHEP 409 (2014) 026, JHEP 1611 (2016) 149, arxiv: [hep-ph]) NLO qq production graphs plus LO real gluon emission Krzysztof Golec-Biernat Opportunities with diffraction 26 / 29

27 Application to diffractive production exclusive diffractive dijet electroproduction with merged gluon using jet cone algorithm exclusive diffractive dijet photoproduction (Q 2 = 0) with jet k or M J as hard scale non-exclusive diffractive dijet production - trijet production photoproduction of open charm or charmonium production Krzysztof Golec-Biernat Opportunities with diffraction 27 / 29

28 Applications to VM production Exclusive diffractive production of light vector mesons: γ p ρp Additional collinear factorization with distribution amplitude φ (x) A = 1 0 dx H(x,...) φ (x, µ), µ = Q 2, t Amplitude infrared finite for both longitudinal and transverse photons, and also in photoproduction limit (no end point sigularity). Krzysztof Golec-Biernat Opportunities with diffraction 28 / 29

29 Summary Diffractive processes with a hard scale probe the QCD nature of the pomeron: resolved pomeron BFKL pomeron parton saturation as unitarization mechanism. VM production supports saturation mechanism. Natural applications to pp, pa and ultraperipheral AA collisions Shock wave approximation program is theoretically sound and promissing. Krzysztof Golec-Biernat Opportunities with diffraction 29 / 29