gg physics at the LC

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gg physics at the LC
Giulia Pancheri - INFN-Frascati
Where ee- collisions were born

• AdA where ee- collisions were born

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• For same
• cm energy
• ??
• xsections
• are
• larger

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Photon-photon processes

• Formation of neutral resonances with other
  quantum numbers than in ee- , with C and J0,2
• Higher mass reach since particles can be produced
  individually (not necessarily in pairs)
• The ggH loop is sensitive to all charged
  fundamental particles of the theory
• Direct pair production of new fermions, charged
  scalars, charged vectors
• Pair production of neutral scalars, vectors (in
  loop, also gg gg
• Measurement of parton densities of real real
  photons (in eg mode)

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photon-photon

• Photon Collider
• ECM 0.8 EeeCM
• Lum 0.2 ee
• Higgs and top physics
• EWSB, SUSY are accessible and interesting

• Normal mode of operation
• ECM approx up to 1/2 ee
• Non-monochromatic
• Can access JP0
• QCD and some top
• Total cross-sections

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Photon Collider
Energy is transferred to the photon up to
almost 80 of initial electrons

• Compton Back Scattering of laser light

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QCD tests

• Pomeron?
• Jets
• Photon densities
• Models for total cross-section

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Models for total x-sections

• Interest lies in QCD role
• What is the Pomeron? The Reggeon?
• Are these concepts universal?
• Or do they just phenomenologically describe our
  ignorance?
• How can ILC help ?

A.de Roeck, R. Godbole, A. Grau, G. Pancheri,
JHEP 2003
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sBs-h Ase Cse1

• Fit3
• C? 0 e0.093
• e10.418
• Fit 1
• C0 e0.250
• Fit2
• C0 e0.093 as in pp

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se e controls the rise

• Could e be the same for all hadronic
  cross-sections?
• The total gg x-section is extracted from data
  which do not include all the phase space
• MC simulations are based on minijets models
  which indeed are the best in simulating L3 data

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se e controls the rise

• Should e be the same for all hadronic
  cross-sections?
• Yes if the model
• is based on Regge poles and a universal Pomeron
  pole exchange or
• On Gribov factorization alone
• Not necessarily if
• The model has some connection with QCD and photon
  densities play a role

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A realistic QCD model should relate the fit to
QCD phenomenological inputs quantities like
densities etc.
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The BN Eikonal Minijet model includes kt
resummation
R.Godbole, A. Grau, G.Pancheri, Y.Srivastava PRD
2005 A. Corsetti, A. Grau, G.Pancheri, Y.
Srivastava PLB 1996

• Multiple parton interactions optical theorem
  and eikonal representation for Tel(s,t)
• Hard scattering to drive the rise due to 1/x
• Soft gluons down to zero momentum to tame the
  rise

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The hard cross-section

• Mini-jet cross-section
• S? densities ?dpt ds/dpt

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The hard scattering part

• qq,qg and mostly
• Minijet cross-section depends upon
• parton densities
• GRV, MRST, CTEQ for protons
• GRS, CJK for photons
• pt cutoff ptmin1 2 GeV

g g g g
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Soft resummation

• Probablity of total KT from infinite of soft
  gluons
• ? d2b eiKTb exp-?d3n(k)1-e-iktb
  
• depends upon single gluon energy
• maximum use Kinematics
• minimum 0 if Bloch-Nordsieck states

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Role of resummation

• An infinite number of soft quanta
• down to zero momentum but how?
• next slides
• Up to an energy dependent limit qmax
• Higher hadron energy possibility of
  more small x partons with high energy (1-2
  GeV) higher qmax

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Maximum soft gluon energy

• q1 and q2 any two partons
• X the 2-jet final state
• Q24 p2tmin
• qmax depends on x1,x2
• We average it over densities

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Zero momentum quanta

• Soft gluons need to be resummed if they are
  indeed soft 1/k
• Resummation implies integration over dkt
• What matters will be ?as(kt )dkt f(kt)
• and not as(0)

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Soft gluons give b-distributions

• In eikonal representation
• stot2?d2b 1-e-n(b,s)/2
• n(b,s)average of collisions at distance b, at
  energy vs
• b-distribution is needed
• Our ansatz

b-distribution Fourier transform of soft gluon
Kt distribution
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How the model works

• Choose ptmin 12 GeV for mini-jets
• Choose parton densities
• Calculate minijet x-section
• Calculate qmax for soft gluons
• Calculate A(b,s) for given qmax
• Calculate nhard (b,s)A(b,s) sjet(ptmin,s)
• Parametrize nsoft
• Evaluate n(b,s) nsoft nhard
• Eikonalize stot2?d2b 1-e-n(b,s)/2

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qmax for ptmin1.15 geV
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sjet for ptmin1.15 GeV
green band MRST
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b-distribution for hard collisions from soft
gluon resummation for ptmin1.15 GeV for
singular as
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Example of Eikonalized proton-antiproton total
cross-section for ptmin1.15
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Comparison with proton data

• R.Godbole,
• Grau
• R. Hedge
• G. Pancheri
• Y. Srivastava
• Les Houches 2005
• GGPS PRD 2005

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photon-photon eikonal minijet and BN model
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Conclusions

• gg in Photon Collider Mode would be very
  interesting for Higgs, WW, unknown resonances,
  etc.
• In regular mode, QCD and total cross-sections
  measurements can give insight on details of low
  x physics complementary to LHC