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
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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.
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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))
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Scattered
electrons
ηmax
Inclusive DIS process
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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
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Moriond QCD, 8-15 March 2008
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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.
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Moriond QCD, 8-15 March 2008
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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.
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Moriond QCD, 8-15 March 2008
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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
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Moriond QCD, 8-15 March 2008
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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
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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.
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Moriond QCD, 8-15 March 2008
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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.
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Moriond QCD, 8-15 March 2008
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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
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Moriond QCD, 8-15 March 2008
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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.
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• 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
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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)
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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.
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Moriond QCD, 8-15 March 2008
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