Transcript 슬라이드 1
Diffraction at HERA 43rd Rencontres de Moriond : QCD and High Energy Interactions 8-15 March 2008, La Thuile, Italy Heuijin Lim (ANL) on behalf of H1 and ZEUS collaborations Measurements of inclusive diffraction Diffractive Parton Densities Test of factorization in diffractive Final States Heuijin Lim Moriond QCD, 8-15 March 2008 1 Inclusive Diffraction in Deep Inelastic Scattering Inclusive DIS process DIS : Probe structure of proton Diffractive DIS : Probe structure of colorless system, Pomeron, exchange Kinematics of ep eXp Q (k k) 2 2 W (p q)2 y s x IP Diffractive DIS process N (MN) Heuijin Lim Q 2 M2X Q2 W 2 t (p p) 2 x Q2 (2q p) MX (k k p p) Q2 x β 2 MX Q 2 x IP 2 xL pZ Ep 1 x IP In pQCD, Pomeron exchange can be described Moriond QCD, 8-15 March 2008 by two-gluon exchange. 2 2 Event selection MX method LPS method Diffractive DIS process Mass of X MX xL=1-xIP=p⁄Z/pZ Leading Proton Spectrometer Outgoing proton Large Rapidity Gap LRG method η=-ln(tan(θ/2)) Heuijin Lim Scattered electrons ηmax Inclusive DIS process Moriond QCD, 8-15 March 2008 3 Measurements of inclusive diffractive DIS Diffractive cross-section : dσ(MX,W,Q2)/dMX Comparison of diffractive and total γ*p scattering Q2 dependence of αIP(0) and rdiff(=σdiff/σtot) Diffractive structure function : xIPF2D(3)(β,xIP,Q2), xIPF2D(4)(β,xIP,Q2, t) Q2 dependence and β dependence Comparison with MX, LRG and LPS measurements Diffractive Parton Densities Extract DPDFs from QCD fit to inclusive diffractive DIS Combined QCD fit to inclusive diffractive and Dijets Test of factorization in diffractive Final States Dijets in DIS and Photoproduction Heuijin Lim Moriond QCD, 8-15 March 2008 4 Diffractive Cross Section with MX method FPCI (98-99) : 4.2 pb-1 2.2 < Q2 < 80 GeV2 Nucl. Phys. B713, 3(2005) FPCII (99-00) : 52.4 pb-1 20 < Q2 < 450 GeV2 Submitted to Nucl. Phys. B. (arXiv:0802.3017) • Within systematic errors, good agreement between FPCI and FPCII. Heuijin Lim Moriond QCD, 8-15 March 2008 5 Comparison of diffractive and total γ*p scattering tot σ γ p d2 σ diff γ p dMX 4π 2 α 2 Q (1 x) 2 tot 2 αIP (0) 1 F2 (x, Q ) ~ (W ) diff 2 2( αIP (0) 1) ~ (W ) • For Q2 < 20 GeV 2, αIPdiff(0) is consistent with soft Pomeron. • For Q2 > 30 GeV2, data suggest increase with Q2. Regge factorisation breaking. • αIPdiff(0) is lower than αIPtot(0) : Different Pomeron trajectory! r = σdiff(0.28 < MX < 35 GeV)/σtot • Within the errors of the measurements, r is independent on W. • At W=220 GeV, r ~ 15.8 % at Q2= 4 GeV2 r ~ 5.0 % at Q2= 190 GeV2 r shows a logarithmic dependence on Q2. Heuijin Lim Moriond QCD, 8-15 March 2008 6 Q2 dependence of xIPF2D(3) diff 1 Q2 (Q2 M2X ) dσ γ p D(3) x IPF2 2 4π α 2MX dMX Probability to find a quark carrying x=βxIP in a proton undergoing a diffractive process. x = β xIP < 0.001 • For β≥ 0.9 (dominated by events with MX<2 GeV), Constant or slowly decreasing with Q2 Expect higher twist effect from (qq )L • For x=β xIP < 0.001, xIPF2D(3) increases with increasing Q2. Positive scaling violations. Suggest perturbative effects such as gluon emission • For fixed β, Q2 dependence of xIPF2D(3) changes with xIP. Inconsistent with the Regge factorisation hypothesis Heuijin Lim Moriond QCD, 8-15 March 2008 7 Diffractive structure function with LPS method F2D(4) ( , Q2 , xIP , t) fIP (xIP , t) F2IP ( , Q2 ) nIR fIR (xIP , t) F2IR ( , Q2 ) The parameterization of xIP dependence gives the intercept of αIP. 0.01 0.040 αIP (0) 1.1 0.02(stat.) - 0.02 (syst.) 0.02(model) αIP (0) 1.114 0.018(stat.) 0.012(syst.)- 0.020 (model) 2 BIP 7.2 0.7(stat.)1.4 0.7 (syst.) GeV 2 BIP 5.5 2.0 0.7 GeV Not Soft! Soft 2 0.04 2 0.19 2 α (0) ~ 0.25 GeV αIP (0) 0.03 0.07(stat. ) (syst.)GeV α (0) 0.06 GeV IP 0.08 IP March 2008 0.06 Heuijin Lim Moriond QCD, 8-15 8 Comparison of LRG with LPS measurements ZEUS LRG 00 (prel.), ZEUS LPS 00 (prel.) (LPS/LRG) (LPS/LRG) H1 LRG with MN<1.6 GeV ZEUS LRG with MN < 2.3 GeV • Ratio between LRG and LPS is independent of Q2, β and xIP within errors. It shows 23~25 % of proton dissociation contribution in LRG data. For xIP > 0.01, their constant ratio tells that the LRG data also contain the reggeon contribution. Heuijin Lim Moriond QCD, 8-15 March 2008 9 Factorization in Diffractive DIS Diffractive DIS, like inclusive DIS, is factorizable : D * σ (γ p Xp) D 2 2 fi p (z, Q , x IP , t) σ γ*i (z, Q ) Diffractive parton distribution function Universal partonic cross section fi/pD(z,Q2,xIP,t) : Probability to find in a proton, with a probe of resolution Q2, parton i with momentum fraction z, under the condition that the proton remains intact and emerges with small energy loss, xIP, and momentum transfer, t. Obey DGLAP universal for diffractive ep DIS (inclusive, Dijet, Cham) Assumption : Proton vertex factorization Shape of diffractive PDFs independent of xIP and t. D 2 D 2 fi p (z, Q , xIP , t) fIP p (xIP , t) fi p (z, Q ) Regge motivated IP flux At large xIP, a separately factorizable sub-leading exchange (IR), with different xIP dependence and partonic composition. Heuijin Lim Moriond QCD, 8-15 March 2008 10 H1 2006 DPDF Fits • Fit H1 LRG data with Q2≥8.5 GeV2, MX>2 GeV, β≤0.8) • Parameterize the parton density 2 B z (z, Q0 ) A qz q (1 z) 2 z g (z, Q 0 ) A g (1 z) Cq Cg χ2 ~ 158/183 d.o.f., Q02=1.75 GeV2 • Effective Pomeron trajectory (Fit A) : αIP 1.118 0.008(exp.) 0.029 0.010 (model) ~70 % gluons in diffractive exchange • Due to lack of sensitivity for the gluon density at high z. Fit B : using zg(z,Q02)=Ag χ2 ~ 164/184 d.o.f. , Q02=2.5 GeV2 Heuijin Lim Moriond QCD, 8-15 March 2008 11 Diffractive dijets in DIS H1 2007 Jets DPDF Fit 2006 - B H1 2007 Jets DPDF zIP= longitudinal momentum fraction of color singlet carried by parton entering hard sub-process zIP distribution is sensitive to gluon DPDF. Heuijin Lim • H1 Dijet data are in better agreement with H1 Fit B. • ZEUS Dijet data consistent with H1 Fit B and MRW 2006 fit. • Combined Fit to inclusive and Dijet data better constraint to quark and gluon densities, to H1 Moriond QCD, close 8-15 March 2008Fit B. Fit 2006 – A Fit 2006 -- B 12 Diffractive dijets in Photoproduction (Q2 ~ 0 GeV2) xγ = 1 xγ < 1 Interactions destroy gap for resolved PHP. xγ = fraction of photon momentum in hard scattering. • Dijets are suppressed by a factor~ 0.5 for direct and resolved γ. • Dijets are weakly suppressed by a factor ~ 0.87 for direct and resolved γ. (big difference: ETjet1 (ETjet2) > 5 (4) GeV) (ETjet1 (ETjet2) > 7.5 (6.5) GeV) Heuijin Lim Moriond QCD, 8-15 March 2008 ET distribution may not be well reproduced by NLO. Still work in progress. 13 Conclusion Inclusive diffraction was measured with high statistics for a wide kinematic range from low to high Q2. Good agreement with the results measured by MX, LPS and LRG methods. Diffractive DIS and total γ*p process cannot be described by an unique pomeron and r=σdiff(0.28 < MX < 35 GeV)/σtot decreases ~ ln(1+Q2). Indication for Regge factorisation breaking was seen in Q2 dependence of αIPdiff(0) Q2 dependence of xIPF2D(3) for fixed β and fixed xIP Evidence for pQCD evolution with Q2 as xIP0 or β 0. Combined QCD fit to inclusive diffractive DIS and Dijets It improved the gluon structure at high z. Gluon dominated: ~ 70 %. Diffractive charm and Dijets DIS data are consistent with NLO predictions based on DPDFs from inclusive data. Diffractive Dijets in photoproduction ZEUS : agreement with NLO prediction and no large suppression(~0.87). H1 : suppression(~0.5) ETjet dependence is investigated. Heuijin Lim Moriond QCD, 8-15 March 2008 14