Diffractive dijet photoproduction in UPCs at the LHC

Transcription

1 Diffractive dijet photoproduction in UPCs at the LHC V. Guzey Petersburg Nuclear Physics Institute (PNPI), National Research Center Kurchatov Institute, Gatchina, Russia Outline: l Diffractive dijet photoproduction in lepton-proton scattering l NLO QCD predictions for cross sections of diffractive dijet photoproduction in pp, pa, AA UPCs at the LHC, Guzey, Klasen, JHEP 26 (26) 29 l Summary Workshop INT-7-65W Probing QCD in Photon-Nucleus Interactions at RHIC and LHC: the Path to EIC, INT, Seattle, Feb 3-7, 27

2 Inclusive diffraction in ep scattering at HERA Program to measure inclusive diffraction in hard diffractive processes, ZEUS, H inclusive diffractive DIS diffractive production of open charm diffractive production of two Main Results: - inclusive diffractive constitutes -5% of the total γ*p cross section and scales with Q 2 - Collinear QCD factorization (Collins 998) holds for inclusive diffractive DIS, electro- and photoproduction of open charm and electroproduction of cross sections of these processes are described using diffractive PDFs determined from inclusive diffractive DIS. - QCD factorization is strongly violated in diffractive photoproduction of, the mechanism is unknown. A. Bruni, 26th Rencontres de Blois (24) 2

3 Diffractive dijet photoproduction at HERA Theory of this process in QCD is well-established: - cross section in NLO of perturbative QCD - PDFs of the photon from e+e- data - proton diffractive PDFs from inclusive diffraction NLO calculations overestimate the data by factor ~2 and offer two scenarios of factorization breaking, Klasen, Kramer, EPJ C7 (2) 9 Global suppression R=.5 ep e+2+x +Y Resolved-only suppression with R(res.)=.4 ep e+2+x +Y ep e+2+x +Y H NLO, R=.5 NLO, R=.4 (res) 8 8 obs dσ/dx γ [pb] H 99- NLO, R=. NLO, R= obs x γ obs dσ/dz IP [pb] H 99- NLO, R=. NLO, R= obs z IP ep e+2+x +Y ep e+2+x +Y ep e+2+x +Y obs dσ/dx γ [pb] 6 4 NLO, R=.37 (res+dir-is) xγ = photon momentum fraction participating in the hard process obs x γ dσ/dlog (x IP ) [pb] 6 obs dσ/dz IP [pb]

4 Factorization breaking in diffraction Factorization breaking in diffractive processes occurs due to soft inelastic interactions, which strongly suppress rapidity gap(s) in the final state. Well-established in pp scattering rapidity gap survival S.: - HERA diffractive PDFs overestimate hard dijet production in p-antip scattering at Tevatron by factor, CDF, PRL 84 (2) included in analysis of soft and hard diffraction at Tevatron and the LHC, Khoze, Martin, Ryskin, EPJ C 8 (2) 67; Frankfurt, Hyde, Strikman, Weiss, PRD 75 (27) included in analysis of pp UPCs at the LHC, Jones, Martin, Ryskin, Teubner, JHEP (23) 85 In our case, the photon interacts via fluctuations/configurations that have different transverse sizes (cross sections) talk by L. Frankfurt - direct photon = small-size fluctuations no suppression - resolved photon = large-size fluctuations suppression due screening with cross section ~σρn - First suggested to explain the HERA data, Kaidalov, Khoze, Martin, Ryskin, PLB 567 (23) 6. - Fresh look & implementation, Guzey, Klasen, EPJC 76 (26) 467 4

5 Diffractive dijet photoproduction in UPCs So far UPC program focused on photoproduction of light (ρ, ɸ) and heavy (J/ψ, ψ(2s), Υ) vector mesons + first Run 2 results on inclusive jet photoproduction, talk by A. Angerami We propose: diffractive dijet photoproduction, Guzey, Klasen, JHEP 26 (23) 29 A IP γ A Jet Jet Remnant A IP γ A Remnant Jet Jet Remnant These events are characterized by: - absence of hadronic activity along beam directions (rapidity gaps) - two with large pt - remnants from Pomeron and γ B (a) direct photon B B B (b) resolved photon Studies of this process in UPCs at the LHC may allow to: - improve understanding of QCD factorization breaking in diffraction - for the first time determine nuclear diffractive PDFs - improve determination of proton diffractive PDFs 5

6 Diffractive dijet photoproduction in UPC (2) In direct analogy with calculations for the ep case: - photon flux from electron photon flux from proton/nucleus - additional suppression of strong inelastic interaction for small b (in calculation of photon flux) - proton diffractive PDFs nuclear diffractive PDFs - model for factorization breaking for resolved photon for nuclear case Two contributions for right/left moving ions: d (AA! A X + A) =d (AA! A X + A) (+) + d (AA! A X + A) ( ) Cross section of dijet photoproduction in UPCs: d (AA! A X + A) (+) = X a,b Z tmin t cut dt Z x max P x min P Z Z ymax dx P dz P dy y min Z dx f /A (y)f a/ (x,µ 2 )f D(4) b/a (x P,z P,t,µ 2 )dˆ(n) ab! photon flux (including suppression of strong interaction at small b) PDF of the photon (includes direct component and effect of factorization breaking) nuclear diffractive PDF (including the effect of nuclear shadowing) elementary parton cros section 6

7 Factorization breaking for resolved photon Suppression factor for resolved component for γa case: R d 2 N /2T b e A (b) 2 inel N T e A (b) R(res.) = R d2 b e N /2T A (b) 2 Nuclear optical density Probability of coherent ρ photoproduction Probability not to have inelastic interactions R(res.).8.6 Proton proton R(res.)..8.6 Nucleus nucleus W, GeV R(res).4 agrees with analysis of HERA data, Klasen, Kramer (2). Follows Kaidalov, Khoze, Martin, Ryskin,PLB 567 (23) W, GeV RA(res). Rp(res) since it is much easier to break up nuclei. 7

8 Leading twist nuclear shadowing model Combination of Gribov-Glauber NS model with QCD factorization theorems for inclusive and diffractive DIS shadowing for individual partons j, Frankfurt, Strikman (999) + Interaction with 2 nucleons: model-indep via diffractive PDFs: j 2 (x) = 6 xf j/n (x, µ 2 ) Z. x dx P f D(4) j/n (x, µ2,x P,t= ) Interaction with 3 nucleons: via soft hadronic fluctuations of γ*: P(σ) probability to interact with cross section σ In quasi-eikonal approximation in low-x limit, Frankfurt, Guzey, Strikman, Phys. Rept. 52 (22) 255 xf j/a (x, µ 2 )=Af j/n (x, µ 2 ) 2 j 2 f j/n(x, µ 2 ) [ j soft (x)]2 Z d 2 b e 2 j soft (x)t A(b) + j soft (x) 2 T A (b)! 8

9 Leading twist nuclear shadowing model (2) Predicts nuclear PDFs at µ 2 =3-4 GeV 2. Subsequent Q 2 dependence by DGLAP evolution. Magnitude of shadowing is determined by proton diffractive PDFs, ZEUS, H 26 naturally predicts large shadowing for ga(x,µ 2 ) confirmed by ALICE UPC data Presents alternative to small-x extrapolation of npdfs from global fits. Contains free parameter: soft(x) = R d P ( ) 3 R d P ( ) 2 Estimated two plausible models of cross section fluctuations in the photon: - like in the pion, Blattel et al, like in the dipole model, McDermott, Frankfurt, Guzey, Strikman, 2 Unique features of leading twist nuclear shadowing model: - Predictions for impact parameter dependent nuclear PDFs ga(x,b,q 2 ) shift of t- dependence of γa J/ψA cross section; oscillations of beam-spin nuclear DVCS asymmetry at EIC. - Predictions for nuclear diffractive PDFs diffractive photoproduction in UPCs; inclusive diffraction in γ*a at EIC. 9

10 Nuclear diffractive parton distributions Leading twist nuclear shadowing model can be applied to inclusive diffraction in γ*a: a b c + Z f D(3) j/a (x, µ2,x P ) = 6 f D(4) j/n (x, µ2,x P,t= ) d 2 b e 2 j soft (x)t A(b) j soft (x) Predicted large probability of hard diffraction on nuclei and nuclear diffractive PDFs:! 2 P g Q 2 =4 GeV 2 Pb-28 proton x Can be measured in inclusive γ*a diffraction at LHeC/EIC and hard diffraction in γa, e.g., diffractive photoproduction of di in UPCs@LHC, Guzey, Klasen 26 g A D(3) (x,xp )/Ag p D(3) (x,xp ) Pb-28, Q 2 =4 GeV 2, β=. x P

11 Predictions for pp UPCs dσ/dx γ, pb pp UPCs at 3 TeV dσ/dx P, pb R(glob.)=.5 R(res.)=.4 R= x γ xγ=photon momentum fraction z P zp=pomeron momentum fraction Cross sections are as large as at HERA: O(nb) Sensitivity to small zp is larger than HERA new constraints on proton gp D W is times larger that HERA dσ/dw, pb/gev. Sensitivity to scheme of factorization breaking is small W, GeV W=γp energy

12 dσ/dx γ, pb pp UPCs at 3 TeV x γ dσ/dx P, pb R(glob.)=.5 R(res.)=.4 R= z P Predictions for pp UPCs: xγ, zp, ET, W, <η>, Δη, M2, MX dσ/de T jet, pb/gev jet E T, GeV 5 dσ/dw, pb/gev W, GeV d σ/d<η >, pb d σ/d η, pb <η > η dσ/dm 2, pb/gev M 2, GeV dσ/dm X, pb/gev M X, GeV 2

13 Predictions for pa UPCs dσ/dx γ, nb pa UPCs at 8.6 TeV dσ/dx P, nb R(glob.) R(res.) R= x γ z P dσ/de T jet, pb/gev jet E T, GeV d σ/d<η >, pb <η > ET=transverse momentum <η>=average rapidity 3

14 Predictions for AA UPCs dσ/dx γ, nb AA UPCs at 5. TeV dσ/dx P, nb R(glob.)=. R(res.)=.4 R= x γ z P Cross sections are enhanced by Z 2 7 due to photon flux and A 4/3 2 due to nuclear diffractive PDFs large: O(microbarns) Without LT nuclear shadowing cross section increases by factor 7 Large qualitative sensitivity to scheme of factorization breaking: red and blue lines cross over! 4

15 xγ-dependence of factorization breaking xγ=the most sensitive observable to scenario of factorization breaking UPCs with nuclei give a principle possibility to distinguish the global suppression and resolved-only suppression scenarios. dσ/dx γ, nb AA, R(glob.) AA, R(res.) pa, R(glob.) pa, R(res.) pp, R(res.) Run dσ/dx γ, nb Run x γ x γ 5

16 Summary l In framework of collinear factorization and NLO pqcd, we calculated cross sections of diffractive dijet photoproduction in pp, pa and AA UPCs. l This process allows for the first time to measure nuclear diffractive PDFs, further constrain proton diffractive PDFs and might shed light on mechanism of QCD factorization breaking in diffraction. The key observable is dependence on photon momentum fraction xγ. l Dijet photoproduction in UPCs in the color dipole model with saturation dynamics talk by A. Rezaeian l Dijet photoproduction in UPCs is complimentary to diffractive production of two in pp scattering at the LHC, ATLAS, PLB 754 (26) 24. 6