Improving ATLAS hard diffraction measurements with the STEP award Hardeep Bansil University of Birmingham 18/10/2013

Introduction to Diffraction Feature of hadron-hadron interactions: Elastic and diffractive processes account for 50% of total proton-proton cross section Diffractive interaction mediated by a colourless exchange with vacuum quantum numbers Historically known as the Pomeron ( IP ) Final-state particles separated by large polar angle (or pseudorapidity, η=-ln[tan(θ/2)]) Large Rapidity Gap (LRG) 2 pp → Xp Single Diffractive (SD) pp → XY Double Diffractive (DD) pp → pp Elastic pp → X Non Diffractive (ND) IP

Main topic of thesis IP content in terms of quarks and gluons still not well known Study of diffractive dijets using early ATLAS 2010 data Compared to Monte Carlo simulations of ND, SD and DD events (PYTHIA8) Measurement (with full systematic treatment) performed as function of: Δ η F - largest forward gap from either side of ATLAS detector acceptance ξ - fraction of momentum of proton transferred to pomeron Transverse momentum (p T ) and pseudorapidity (η) of jets Data shape described by mix of SD+DD and ND jets after large gap MC normalisation inadequate → needs further investigation 3 IP X

Improvement 1: Monte Carlo production Analysis uses only one set of Monte Carlo samples generated using PYTHIA8 Need samples created by other generators Want model independent conclusions Unfold additional variables (z IP ) PYTHIA8 diffractive samples use unconventional model in generation Plans: PYTHIA8 - correct existing samples Generate samples with completely different MCs Expected time for generation: 2 months 4

Improvement 2: Unfolding Correct data for experimental effects (finite resolution, limited acceptance) Correction process uses RooUnfold software to unfold individual distributions separately but there are two exponentially falling distributions in this analysis ∆η F and jet p T Both produce a net migration to larger rapidity gaps Improve by using simultaneous unfolding procedure for both variables Could also be used to unfold variables (e.g. jet p T ) as function of gap size Currently investigating new software and time/difficulty to implement Expected time for updating unfolding process – 1 month 5

Improvement 3: Trigger Current trigger strategy based on 2010 ATLAS inclusive dijet analysis Uses jet-based triggers as required to analyse entire 2010 data range Ideas to optimise trigger for early 2010 data Use MBTS based trigger (2 counters over threshold) Less biased and fully efficient Higher trigger rate Smaller systematic uncertainty relative to jet-based triggers Expected time for changes: 1-2 month(s) 6 MBTS

Paper to publish + conference Aim is to get analysis published in e.g. Physics Review Letters or Physics Letters B Present results to working group and get editorial board as soon as possible Paper (+ supporting material) should be condensed form of analysis covered in thesis Realistic timescale: Expecting editorial review process to take up to 3 months (common for most ATLAS analyses) Accounts for rewriting text and potential corrections / additional studies Invited to speak at “Multiple Partonic LHC 2013”, Antwerp, Dec MC Production Unfolding Trigger Write paper Review process Conference

Future projects Projects to contribute to in parallel (but not top priority) with ATLAS colleagues Aim not to complete analyses but to lay groundwork for other people to continue Both result in better understanding of pomeron structure Diffractive charm Can work along Birmingham colleagues involved with previous charm meson studies Code for charm meson reconstruction already available ALFA Ensure proton intact for single diffractive events by tagging proton ATLAS results for elastic process ( pp → pp ) are close to completion, can work closely with experts involved 8