High energy gg interactions at the LHC

1. Krzysztof Piotrzkowski UniversiteCatholique de Louvain, CP3 Center High energy gg interactions at the LHC • Introduction: LHC as a gg collider • Two-photon exclusive pair production • Summary/Outlook Electromagnetic and Diffractive Processes at the LHC ECT*, Trento, 4-8 January, 2010

2. LHC as a High Energy gg Collider p p Phys. Rev. D63 (2001) 071502(R) hep-ex/0201027 Observation: Provided efficient measurement of very forward-scattered protons one can study high-energy gg collisions at the LHC • Highlights: • gg CM energy W up to/beyond 1 TeV (and under control) • Large photon flux F therefore significant gg luminosity • Complementary (and clean) physics to pp interactions, eg studies of exclusive production of heavy particles might be possible opens new field high energy gg (and gp) physics K. Piotrzkowski

3. DISCLAIMER: This is NOT meant for studying all photon interactions at the LHC but those for which the QCD background can be strongly suppressed, as for example in the exclusive production of pairs of charged particles. This IS meant for studying production of selected final states in photon interactions at the LHC. Note: At Tevatron available energy too small for EW physics (but enough for lepton pairs – CDF recently published measurement of exclusive two-photon production of ee pairs) Initial inspiration: K. Piotrzkowski

4. How measure these events? HPS p scattered p p p beam Measure (gg) X in the CMS or ATLAS detector and scattered protons using very forwarddetectors(thanks to proton energy loss) Very forward detectors needed – capable of running at high luminosity, installed as far (> 100 m) from IP and as close to the beam (2 mm) as possible – expected photon energy resolution of HPS is 2-5GeV ! K. Piotrzkowski

5. LHC as a gg collider Equivalent photon approximation (EPA) …introduced to major event generators as Madgraph, Pythia, Sherpa, Calchep K. Piotrzkowski

6. Kinematics/gg Luminosity VirtualityQ2 of colliding photons vary between kinematical minimum = Mp2x2/(1-x) where x is fraction of proton momentum carried by a photon, and Q2max ~ 1/proton radius2 Phys. Rev. D63 (2001) 071502(R) W2 = s x1 x2 Photon flux 1/Q2 Q2 - Q2min sq2/4 for x>0.0007, Q2<2GeV2 protons scattered at `zero-degree’ angle Use EPA à la Budnev et al.* * error found in the elastic (Q2 integrated) g flux for protons! dWSgg =‘gg : pp luminosity’ Note: it’s few times larger if one of protons is allowed to break up K. Piotrzkowski

7. Problem: Same signature (one or two very forward protons) has also central diffraction (i.e. pomeron-pomeron scattering) in strong interactions Both processes weakly interfere, and transverse momentum of the scattered protons are in average much softer in two-photon case Phys. Rev. D63 (2001) 071502(R) Q2 < 0.01 GeV2 a) `true’ distributions; b) distributions smeared due to beam intrinsic pT; all plots normalized for pT2 < 2 GeV2 pT gives powerful separation handle provided that size of gg and pomeron-pomeron cross-sections are not too different... …unless special high-b running then even total gg cross-section measurement could be contemplated… Assuming ultimate pT resolution  100 MeV; i.e. neglecting detector effects K. Piotrzkowski

8. Also HE photoproduction ! K. Piotrzkowski

9. LHC as a gg collider arXiv:0908.2020 HECTOR K. Piotrzkowski

10. Detector acceptance Note: CEP of WW is < 1 fb K. Piotrzkowski

11. Muons: A Dimuon Event at 2.36 TeV pT(m1) = 3.6 GeV/c, pT(m2) = 2.6 GeV/c, m(mm)= 3.03 GeV/c2 K. Piotrzkowski * From Jim Virdee’s Council talk Friday December 18, 2009

12. Lagrangian for aQGCs K. Piotrzkowski

13. Setting limits on aQGCs K. Piotrzkowski

14. Setting limits on aQGCs K. Piotrzkowski

15. Setting limits on aQGCs K. Piotrzkowski

16. Setting limits on aQGCs K. Piotrzkowski

17. Unitarity bounds So far no constrains on W were applied  need to watch unitarity bounds! K. Piotrzkowski

18. Unitarity bounds K. Piotrzkowski

19. Unitarity bounds Improvement of ~4000 K. Piotrzkowski

20. Differential distributions Definitely a lot of space for improvements, but can it be done at very high luminosity? K. Piotrzkowski

21. Event pileup In preceding discussion no event pileup was assumed so full exclusivity condition could be applied: Apart from two leptons, no activity in central detectors (including calorimeters) Very powerful condition, leaves no non-exclusive backgrounds (beautifully applied at Tevatron: see 5 papers by CDF) BUT cannot be used if two events occur in the same bunch crossing  large loss of luminosity by requesting single event BX… Solution 1: Use track-based exclusivity, allow only two leptons and NO more tracks from the vertex (Note: only charged particlers and ±2.5 rapidity units vs ±5 for calorimeters) Works well for medium luminosities, up to 2.1033 cm-2s-1 Solution 1+2: Apply track exclusivity and require detection in the forward detectors + z-vertex matching from time difference measurements in ToF detectors (GasToF) See next talks K. Piotrzkowski

22. Summary/Outlook • LHC is a powerful photon-photon collider (1% pp luminosity available for gg collisions at energies above 200 GeV) • Searches of new massive charged particles in exclusive two-photon pair production (MSSM for example) • Exclusive two-photon WW and ZZ production provides stringent tests if SM • • Exclusive dilepton events from leptonic W and Z decays will allow to improve LEP quartic gauge couplings limits by orders of magnitudes already with small integrated LHC luminosity • • To be able to study the exclusive production at nominal LHC luminosity novel, very forward proton detectors are mandatory! • • Wide set of other models will be studied (SU(2) gauge symmetric, Higgsless scenarios, etc.) • Stay Tuned! K. Piotrzkowski