Update: Hard QCD jets in Pythia with ATLAS detector simulation Oliver Harris, 15/09/06 (updated 22/09/06)

Recent work with Pythia/Atlfast: Analysis being done in , to match ATLAS CSC study Using CTEQ6M PDF to match order (i.e. NLO) of ZEUS-JETS Generated 9 sets of data for each PDF, using same p T bins as CSC study Created inclusive p T spectra in three eta bins Started looking at dijet final states – possibility for new physics study (compositeness)

Inclusive jet cross-section in nine p T regimes, weighted by total cross-section for each regime and added to create a continuous spectrum: some possible statistics issues at pT > ~2TeV

A similar effect is seen in the ZEUS-JETS data

Compositeness searches at the LHC One possibility for new physics at the LHC is compositeness – quarks are not elementary, but made of ‘preons’ Compositeness becomes apparent above some energy scale, Λ If CoM energy of interaction < Λ, propagator effectively shrinks to zero size and we have a contact interaction  c.f. e-  interactions at energies << m W

Experimental signatures Analogous situation to Rutherford scattering With elementary quarks, most collisions are ‘glancing blows’ (low-Q 2 t-channel gluon exchanges) With quarks composite at energies above Λ, we expect more large-angle collisions Therefore more low-|η|/high-p T dijets, i.e. more pairs of jets in central region

Rutherford scattering:Quark compositeness:  -particles Expected result with uniformly distributed charges inside nucleus ‘Plum pudding’ atom Actual results -> nucleus ‘test’ quarks from other proton q qbar proton (Lorentz contracted) Composite quarks preons Preons presumably ‘quarkify’ before quarks hadronise

Experimental signature of quark compositeness is therefore an excess of dijets in central region: Ratio of dijet η distributions is dependent on energy scale Λ: bigger Λ -> smaller quark compositeness scale -> harder to detect CMS

Progress since last week: Attended ATLAS UK 06 meeting Generated AODs with compositeness turned on, for Λ = 5, 10, 15 TeV Analysed these and produced CMS-style ratio plot for two η regions  however, minimum dijet mass is too low, or stats too low, or both, for much to be seen Next week: look at ways to improve dijet invariant mass plots – possibly need to refine Pythia jobOptions

Progress since last slide: Started production-line generation of events with ZEUS-JETS PDF Now have several 10^5 AOD and AAN events Dijet pseudorapidity study underway:  results so far too statistics-limited to show much: expect preliminary results next week (need to combine jobs of 1k events into larger samples for analysis)