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### gg physics at the LC

Giulia Pancheri - INFN-Frascati

AdA: where e+e- collisions were born

Where e+e- collisions were born

G. Pancheri - gamma gamma at LC

Photon-photon processes

- Formation of neutral resonances with other quantum numbers than in e+e- , with C=+ and J=0,2
- 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)

G. Pancheri - gamma gamma at LC

Normal mode of operation

ECM approx up to 1/2 ee

Non-monochromatic

Can access JP=0±

QCD and some top

Total cross-sections

Photon Collider

ECM ~ 0.8 EeeCM

Lum ~ 0.2 ee

Higgs and top physics

EWSB, SUSY are accessible and interesting

photon-photonG. Pancheri - gamma gamma at LC

Photon Collider

Energy is transferred to

the photon up to almost

80% of initial electron’s

- Compton Back Scattering of laser light

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

s=Bs-h + Ase+ Cse1

- Fit3

C≠ 0 e=0.093

e1=0.418

- Fit 1

C=0 e=0.250

- Fit2

C=0 e=0.093 as in pp

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

A realistic QCD model should relate the fit to QCD phenomenological inputs quantities like densities etc.

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

The hard cross-section

- Mini-jet cross-section

S∫ densities ∫dpt ds/dpt

G. Pancheri - gamma gamma at LC

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 ptmin=1~ 2 GeV

g g g g

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

Maximum soft gluon energy

- q1 and q2 : any two partons
- X : the 2-jet final state
- Q2≥4 p2tmin
- qmax depends on x1,x2
- We average it over densities

G. Pancheri - gamma gamma at LC

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)

G. Pancheri - gamma gamma at LC

Soft gluons give b-distributions

In eikonal representation

stot≈2∫d2b [1-e-n(b,s)/2]

- n(b,s)=average # of collisions at distance b, at energy √s
- b-distribution is needed

Our ansatz:

b-distribution =

Fourier transform of soft gluon Kt distribution

G. Pancheri - gamma gamma at LC

How the model works

- Choose ptmin = 1÷2 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 stot≈2∫d2b [1-e-n(b,s)/2]

G. Pancheri - gamma gamma at LC

qmax for ptmin=1.15 geV

G. Pancheri - gamma gamma at LC

b-distribution for hard collisions from soft gluon resummation for ptmin=1.15 GeV for singular as

G. Pancheri - gamma gamma at LC

Example of Eikonalized proton-antiproton total cross-section for ptmin=1.15

G. Pancheri - gamma gamma at LC

Comparison with proton data

R.Godbole,

Grau

R. Hedge

G. Pancheri

Y. Srivastava

Les Houches 2005

GGPS PRD 2005

G. Pancheri - gamma gamma at LC

photon-photon : eikonal minijet and BN model

G. Pancheri - gamma gamma at LC

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

G. Pancheri - gamma gamma at LC

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