1. AdA: where e+e- collisions were born
gg physics at the LC
Giulia Pancheri - INFN-Frascati
Where e+e- collisions were born
AdA: where e+e- collisions were born
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

2. G. Pancheri - gamma gamma at LC
For same
cm energy 
xsections
are
larger
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

3. 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
LNF-25th october, 2006

4. G. Pancheri - gamma gamma at LC
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 JP=0±
QCD and some top
Total cross-sections
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

5. G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

6. G. 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
LNF-25th october, 2006

7. G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

8. G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

9. G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

10. G. Pancheri - gamma gamma at LC
QCD tests
Pomeron?
Jets
Photon densities
Models for total cross-section
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

11. 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
LNF-25th october, 2006

12. 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
LNF-25th october, 2006

13. 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
LNF-25th october, 2006

14. 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
LNF-25th october, 2006

15. 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
LNF-25th october, 2006

16. 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
LNF-25th october, 2006

17. The hard cross-section
Mini-jet cross-section
S∫ densities ∫dpt ds/dpt
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

18. 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
LNF-25th october, 2006

19. 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
LNF-25th october, 2006

20. 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
LNF-25th october, 2006

21. 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
LNF-25th october, 2006

22. 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
LNF-25th october, 2006

23. 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
LNF-25th october, 2006

24. 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
LNF-25th october, 2006

25. G. Pancheri - gamma gamma at LC
qmax for ptmin=1.15 geV
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

26. G. Pancheri - gamma gamma at LC
sjet for ptmin=1.15 GeV
green band MRST
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

27. 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
LNF-25th october, 2006

28. G. Pancheri - gamma gamma at LC
Example of Eikonalized proton-antiproton total cross-section for ptmin=1.15
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

29. 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
LNF-25th october, 2006

30. photon-photon : eikonal minijet and BN model
G. Pancheri - gamma gamma at LC
LNF-25th october, 2006

31. 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
LNF-25th october, 2006