Jets and QCD resummation Albrecht Kyrieleis n n

Jets and QCD resummation Albrecht Kyrieleis

n n BFKL at NLO Gaps between Jets

BFKL at NLO n n n BFKL enables use of p. QCD at very high energy s (NLO) BFKL resums: ( s ) ( s ln s)n * * total cross section: first completely perturb. Test of NLO BFKL

The NLO photon impact factor (J. Bartels, G. Chachamis, C. F. Ciao, S. Gieseke, A. K. ) (1) n n (0) (1) (0) Analytic calculation of Feynman diagrams Exclusion of LO BFKL Cancel IR-divergences phase space integration (0)

2005: first results for the impact factor

Gaps between jets n n n Wide spectrum of processes : rapidity gap between jets e. g. : pp ! H+2 j Good understanding needed to test SM Challenge to perturb. QCD since (e. g. ) resummation is needed

Higgs via WBF: n n n GF: WBF crucial for discovery/couplings of the Higgs at LHC Veto on interjet activity to suppress GF High precision of WBF signal need to undestand interjet radiation !

dijets

gap definition & problematic regions gap event n n ( trans. energy in gap )

In 2005: n n n we have combined these two resummations Started to resum ( s. L) in H+2 j and compare to HERWIG / PYTHIA but a problem came up. .

Let’s consider dijet production For Q 0¿ Q : expect most dominant contribution from ( s L)n

Resumming : primary gluons Assume: real/virtual gluons equal and opposite only contribution: ordered virtual gluons

Resumming n n : secondary gluons outside gap (with subsequent emission inside gap) also contribute these terms have been resummed only for in large limit full resummation for pp not yet feasable we compute the full contribution to pp from one gluon outside the gap

one gluon outside the gap n Any number of k? ordered gluons one being outside gap

Main problem: soft emission fom qqg n n n amplitude A is 4 -dim vector in color space add soft gluon , multiplty by 4 x 4 Matrix we have calculated sum over all numbers of soft gluons = Exp(- S s dk? / k? ) A Diagonalize : Exp can be evaluated

Surprising result We calculated: Check collinear limits n Final state: g(out) || q’: finite n Initial state: g(out) || q : divergence which cannot be absorbed in pdf’s ! divergent contribution from arbitrar. forward gluon with non-zero p?

From partons to hadrons n n divergence absent but n+1 n we find new super-leading logs ( s L ) instead These logs are more important than all leading logs ( s L) do they cancel ? ?

Possible cancellation? At fixed order in s : n 1, 2, … gluons outside gap possible n Each gluon outside gives one additional L at maximum n Maximal super-leading log from maximum number of gluons outside cannot be canceled Cancellation amoung diff. numbers of gluons outside gap does not seem possible

Plans for 2006 n Try to cancel the super-leading logs n Resum the ( s L)n in H+2 j to account for interjet radiation and enhance precision of WBF signal Extend this resummation to the background processes * * total cross section in NLO BFKL n n