Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Improving ATLAS hard diffraction measurements with the STEP award Hardeep Bansil University of Birmingham 18/10/2013 Diffractive Dijets at ATLAS Look for single diffractive events (pp → pX) Involves a large rapidity gap due to colourless exchange with vacuum quantum numbers: “pomeron ( IP )” Search for diffraction with a hard scale set by 2 jets Described by diffractive PDFs + pQCD cross-sections IP Key variables ΔηF - largest forward gap from either side of ATLAS detector acceptance ξ - fraction of momentum of proton transferred to pomeron Transverse momentum (pT) and pseudorapidity (η) of jets Quantify amount of rapidity gap survival probability Work presented in thesis includes full systematic treatment and represents first measurement at ATLAS Still could benefit from improvements before being put forward as official ATLAS measurement for publishing 2 Diffractive Dijets at ATLAS Rescatter with p? Look for single diffractive events (pp → pX) Involves a large rapidity gap due to colourless exchange with vacuum quantum numbers: “pomeron ( IP )” X Search for diffraction with a hard scale set by 2 jets Described by diffractive PDFs + pQCD cross-sections Key variables ΔηF - largest forward gap from either side of ATLAS detector acceptance ξ - fraction of momentum of proton transferred to pomeron Transverse momentum (pT) and pseudorapidity (η) of jets IP Quantify amount of rapidity gap survival probability Work presented in thesis includes full systematic treatment and represents first measurement at ATLAS Still could benefit from improvements before being put forward as official ATLAS measurement for publishing 3 Improvement 1: Monte Carlo production Current analysis only uses one set of Monte Carlo samples, generated using PYTHIA8, to provide description of non-diffractive and diffractive dijets Initial plan was to use two different models (PYTHIA8 and HERWIG++) Test effect of parton showering / hadronisation models on rapidity gap distributions and gap survival probability HERWIG++ could not be used: Problems with underlying event model produce events with very large gaps PYTHIA8 diffractive samples use unconventional pomeron flux parametrisation (black) based on past experiments (blue/green) PYTHIA8 - generate new samples or correct existing diffractive samples Generate new HERWIG++ samples with improved underlying event Allow rapidity gap survival probability to be measured Also generate new samples using other generators e.g. POMWIG Further investigate model dependencies, allow zIP to be unfolded Expected time for generation: 2 months 4 Improvement 2: Unfolding Transforms data to underlying true distributions by correcting for effects of finite resolution and limited acceptance of the detector Correction process uses RooUnfold software to unfold individual distributions separately but there are two exponentially falling distributions in this analysis, ∆ηF and jet pT Both produce a net migration to larger rapidity gaps Currently controlled by optimising jet pT scale for current analysis Improve by using simultaneous unfolding procedure for both variables Migrations will be better controlled and there should not be a necessity to correct the jet pT scale as was done in this analysis Currently investigating modified version of RooUnfold used by other analyses and Imagiro software to perform this Speaking to authors of both software packages Expected time for updating unfolding process - 1 month 5 Improvement 3: Trigger Current trigger method selects jets using a hardware based jet trigger (L1_J5) and minimum bias trigger (L1_MBTS_1) and per-jet strategy to check whether both jets are capable of triggering event Selection via jet trigger dependent on pT and η of jet - allowed to drop down to 80% to increase statistics but reduces precision of analysis Potentially effects measurement L1_MBTS_1 fully efficient but prescaled (accepts 1 in every 50 events triggered) Analysis uses multiple data streams – overlap removal applied but potentially loses useful events Trialing new trigger strategies Simplify overlap removal / per-jet triggering strategy to single event level trigger Keep existing strategy but select jets up to 100% with J5 Use different minimum bias trigger only (L1_MBTS_2) Unprescaled, fully efficient and has highest trigger rate for all data used Expected time for changes: 1-2 month(s) 6 Paper to publish + conference Aim is to get analysis published in e.g. Physics Review Letters Present results to Soft QCD Working group (21/10/2013) and then to ATLAS Standard Model working group at earliest opportunity Get editorial board as soon as possible – even if analysis not fully complete (typical for other analyses) Paper should be relatively quick to write now that thesis has been written as it will be more condensed form of this analysis Expecting paper to be completed over space of 1 month Expecting editorial review process to take up to 3 months (common for most ATLAS analyses) Accounts for rewrites and potential corrections / additional studies Can start writing a lot of this already with plan to start editorial review process in January 2014 Additional aim is to represent ATLAS at international conference Currently targeting “Multiple Partonic Interactions @ LHC”, Antwerp, Dec 2-6 Otherwise will look to conferences in early 2014 7 Future projects Projects to work on in parallel with improving current analysis Aim not to complete analyses but to lay groundwork for future studies Diffractive charm Based on previous work done at HERA (ep collider) Can work along Birmingham colleagues involved with previous charm studies Code for D* meson production already available Need to retrieve files for analysis to do first feasibility studies (2 weeks) ALFA Roman pots located 240m from centre of detector used in dedicated runs Remove ambiguity for single diffractive events and allow for pomeron structure studies, similar to previous experiments Data from 2011/2012 needs to be reprocessed to include jets (maximum 2 weeks) ATLAS results for elastic process (pp → pp) are close to completion, can work closely with experts involved 8