Difrakce na experimentu H1
a detektor VFPS
1.část
Alice Valkárová, ÚČJF
1.12.2005
Difrakce – hadronová fyzika
Hadronová fyzika: Difrakce je jev, kdy částice (nebo soubor částic) po
interakci má stejná kvantová čísla jako počáteční částice.
dσ/dt≃dσ/dt| (t=0) (1-B|t|), |t|∝θ²při vys.energiích, B ∝R²,kde R je
poloměr hadronu terčíku.
Podobný obrazec rozptylu jako v optice
Analogie s optikou není však zdaleka úplná...
Pojem difrakční hadronové fyziky se objevil v 50-tých letech (Landau,
Pomerančuk, Feinberg a dal.) a většina interakcí, která se tehdy a brzo
poté studovala, byla difrakčního typu.
To ale není ta fyzika,co nás zajímá!!!!
1.12.2005
Diffractive scattering
Large fractions of events (∼ 30% of  tot ) in which:
• beam particle emerge intact (elastic) or dissociate into low
mass states X, Y (MX, MY ≪ √s)
• there is a t-channel exchange of a colourless object
• emerging systems hadronize independently ⇨
2
y

1
2
(ln
s
M
Large Rapidity Gap (LRG) if s is large enough:
X)
1.12.2005
From hadrons to partons
So far, we discussed hadron degrees of freedom,
the soft interactions.
We need to describe phenomena in terms of hadronic subcomponents
and quantum field theories, i.e. in terms of QCD ⇒
need hard scale to apply perturbative methods!
1984 – hard diffraction predicted by Ingelman& Schlein
G.Ingelman,P.Schlein,Phys.Lett.B152,256(1985), 583 citations!!!!!
1987 – beginning of the age of hard diffraction –
UA8 experiment – first measurement of diffractive
jet production
UA8 collaboration,Evidence for transverse jets in high mass diffraction
Phys.Lett.B211,239,(1988), 163 citations
Since that time – hard diffraction measured by Tevatron and
HERA!
1.12.2005
• HERA II: rok 2005 ∼ 115 pb-1
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Kinematics of ep diffraction
Photoproduction – Q2 ≃0, DIS scattering Q2>5 GeV2
W 2  (q  p)2   Q 2  2 pq
Q 2  q 2  (e  e' )2
M Q
xP 
W  Q2
2
X
2
2
xP= fraction of proton momentum
carried by singlet (pomeron)
Q2
Q2
Q2

 2
 2
2
2 ( p  p' ) q M X  Q  t M X  Q 2
p'
xF   1
p
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t  ( p  p' )2
β= fraction of exchanged singlet
(pomeron) momentum carried by
struck quark
Experimental Techniques
We are sure - it is the diffraction!
1.12.2005
Some diffractive dissociation background
can be still present
Models for hard diffraction
1.12.2005
QCD factorisation
get PDFs from inclusive diffraction ⇨ predict cross sections for exclusive diffraction
inclusive
dijet
hard scattering QCD
matrix element, perturbatively
calculated, process dependent
Universal diffractive parton densities
identical for all processes
 ( p  Xp) 
D
*
2
 *i
2
f
(
x
,
Q
,
x
,
t
)


(
x
,
Q
)
i
IP
D
parton_ i
  *i universal hard scattering cross section (same as in inclusive DIS)
D
f i diffractive parton distribution functions → obey DGLAP
universal for diffractive ep DIS (inclusive, di-jets, charm)
1.12.2005
Results from inclusive diffraction
Regge factorisation is an additional assumption, β
there is no PROOF!!
D
IP
fi ( x, Q 2 , xIP , t )  f IP / p ( xIP , t )  fi (   x / xIP , Q 2 )
pomeron flux factor
pomeron PDF
σdiff = flux(xP) · object (β,Q2)
Reduced cross section from
inclusive diffractive data
•
get diffractive PDFs from a NLO (LO)
DGLAP QCD Fit to inclusive data from
6.5 GeV2 to 120 GeV2
• extrapolation of the Fit
to lower Q2
to higher Q2
gives a reasonably good description of
inclusive data from ∼3.5 GeV2 –1600 GeV2
1.12.2005
Q2
Diffractive Parton Densities
• determined from NLO QCD
analysis of diffractive structure
function
• more sensitive to quarks
• gluons from scaling violation,
poorer constraint
• gluon carries about 75% of pomeron momentum
• large uncertainty at large zP
Assuming factorisation holds,
the jet and HQ cross sections give
better constraint on the gluon density
1.12.2005
Jet and HQ production
Hard scale is ET of the jet or HQ mass
Can reconstruct zP in dijet events
• tests of universality of PDF’s
(=QCD factorisation)
• test of DGLAP evolution
1.12.2005
Direct access to gluon
density
Charm cross section (DIS)
Good agreement within experimental & theoretical
uncertainties.
NLO calculations with PDFs from inclusive diffraction
NLO calculations HVQDIS
(Harris & Smith)
Good description of diffractive D* production in DIS
(2GeV2 <Q2<100 GeV2)
1.12.2005
Dijets in DIS
NLO calculations = diffractive extension of
DISENT Catani&Seymour (Nucl.Phys.B485 (1997) 29),
interfaced to diffr.PDFs of H1
Hadronisation corrections – RAPGAP MC
1.12.2005
Dijets in DIS
• NLO corrections to LO
are significant – factor 1.9
• excess at high xγ is
kinematically connected
with the lack of events with
ηlab of jets < -0.4 in
comparison with NLO
Good agreement with
NLO within exp.&theor.
uncertainties
1.12.2005
γ*p

pp
At Tevatron HERA PDF’s
do not work….????
Exporting PDFs from HERA to
the Tevatron.........
CDF Tevatron data:
Dijet cross section factor 5-10 lower
than the QCD calculation using HERA
PDFs
?
1.12.2005
Direct and resolved processes at HERA
xγ - fraction of photon’s momentum
in hard subprocess
x  x
OBS
(E  p )


z
jets
( E  pz ) hadrons
DIS (Q2>5GeV2) and direct photoproduction (Q2≃0):
• photon directly involved in hard scattering
• xγ=1
unsuppressed!
?
Resolved photoproduction:
• photon fluctuates into hadronic system, which
takes part in hadronic scattering
• dominant at Q2≃0
• xγ<1
1.12.2005
suppressed!
?
Photoproduction as hadronic process
HERA resolved photoproduction
Secondary interactions
between spectators
Typical models that describe suppression at Tevatron assume secondary
interactions of spectators as the cause:
resolved contribution expected to be suppressed by factor 0.34
(Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61)
1.12.2005
Dijets in photoproduction
The same kinematical region as for DIS
• NLO overestimates the cross section by
factor ∼2
• both direct and resolved are suppressed
• RAPGAP LO – good description
1.12.2005
Ratio:data over NLO prediction
• no suppression observed for DIS
• overall suppression factor of
about 2 observed for both
resolved and direct
components in photoproduction
• suppression is independent of the
cms energy W
1.12.2005
Summary of 1st part
 Dijets in DIS & D* cross section:
• agree with the NLO prediction with the H1 2002 diffractive pDFs
• factorisation holds (assuming PDF is correct)
 Dijets in photoproduction:
• to investigate the puzzle of disagreement of HERA/Tevatron data
(expectation: resolved will be suppressed and direct not)
• data are half of NLO prediction – both resolved and direct are
suppressed ⇨ conflict with the theoretical expectation
 More ideas?
1.12.2005
Hádanka zatím nerozřešena..........