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Preparation for and measurement of new physics processes using the ATLAS detector at the LHC

Preparation for and measurement of new physics processes using the ATLAS detector at the LHC. Claire Gwenlan PPARC Postdoctoral Fellow University of Oxford.  Introduction  “New Physics” with Jets and Missing Energy  Preparation for new physics measurements

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Preparation for and measurement of new physics processes using the ATLAS detector at the LHC

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  1. Preparation for and measurement of new physics processes using the ATLAS detector at the LHC Claire Gwenlan PPARC Postdoctoral Fellow University of Oxford  Introduction  “New Physics” with Jets and Missing Energy  Preparation for new physics measurements - The importance of proton structure - Controlling the SM backgrounds - Hadronic reconstruction and energy flow  Summary

  2.  Introducing “new physics”, beyond the SM, at the TeV scale can provide solutions to problems such as: - what is the nature of dark matter? - why is the Higgs mass at the electroweak scale (hierarchy problem)? (- where does gravity fit in)? Introduction _______________________________________  The LHC will provide proton-proton collisions at CM 14 TeV -> if new physics is accessible, ATLAS will provide wealth of data allowing us to explore far beyond current understanding -> both LHC and ATLAS on-target for data-taking in summer 2007  New physics can manifest itself through several final state signatures of proton-(anti-)proton collisions involving involving e.g. large missing energy, jets, leptons, photons, …  Will require good knowledge of: -> proton structure, SM backgrounds, experimental uncertainties  I will focus on signatures of “Jets” and “Jets+Missing Energy” -> sensitive to large variety of new physics models including: SUSY, extra spatial dimensions, leptoquarks, … 2

  3. SUperSYmmetry (e.g. R-parity, gravity mediated)  Postulates symmetry between bosons and fermions - each SM particle has SUSY partner (spins differ by 1/2) - symmetry not exact (SM/SUSY particles have different masses) Signatures of new physics at the LHC ____________________________________  SUSY: - can unify the gauge couplings -> - may provide dark matter candidate - needed in string theory  Gluinos/squarks copiously produced in high-energy proton-proton collisions - cascade decays follow soon after -> - lightest SUSY particle stable and weakly interacting -> missing-ET  “Jets+missing-ET” is key signature for SUSY discovery 3

  4. G our world (3+1dimensions) Large Extra Dimensions (ADD) (N. Arkani-Hamed, S. Dimopoulos, G. Dvali)  SM particles confined to brane (our 3 spatial dimensional world)  Gravity propagates in all spatial dimensions extra dimensions Signatures of new physics at the LHC ____________________________________  Detectable through direct graviton emission into extra dimensions, (via ggGg, qgGq, qqbarGg) giving a “jet+missing-ET” signature brane PLUS many other new physics scenarios can be probed with “jets” and “jets+missing-ET” (e.g. other SUSY/extra dimensions models, leptoquarks,…) Discovery and future precision measurements need understanding/ control of major uncertainties, especially: 1. proton structure (PDFs), 2. SM backgrounds, 3. experimental uncerts. (hadronic reconstruction) I will address these issues (integral part of this will be measurements of SM jets with early ATLAS data) 4

  5.  Knowledge of proton structure embodied in proton parton distribution functions (PDFs) -> probability of finding parton of particular type carrying fraction x of proton momentum The importance of proton structure at the LHC _______________________________________  At hadron colliders, cross sections written as convolution of partonic cross section with PDFs abX:partonic cross section -> process dependent, pertubatively calculable fi:PDF -> cannot be calculated, but is process independent (universal) -> determined in fits to data and once measured at one experiment can be used to make predictions for other processes  At the LHC, PDFs needed to make predictions for both: - new physics processes (SUSY, Higgs, …) - SM processes, which are also backgrounds to new physics Fits and evaluation of PDF uncertainties performed by several groups - CTEQ, MRST, ZEUS, …, (ATLAS !!!) 5

  6.  I am a leading member of the ZEUS QCD fitting group -> use data, mainly from HERA (ep  eX), and the most up-to-date QCD calculations to determine PDFs and their consequences  Many reasons for this: - understand proton structure - one of the best tests of QCD - good determination of strong coupling (important in unification) - needed for discovery of new physics (Tevatron, LHC) !!! The ZEUS QCD fit _______________________________________ My latest ZEUS fit (to HERA -> inclusive DIS data + jet data) with evaluation of uncertainties ZEUS coll., Eur. Phys. J. C42,1 6

  7. An example: inclusive jets  Jet cross sections sensitive to wide variety of new physics (e.g. SUSY, extra dimensions, quark compositeness) Impact of PDF uncertainties on discovery physics _______________________________________ My PDF Fit  I have recently shown that PDFs are likely to be the dominant source of uncertainty on jet cross sections at the LHC …  and that the high-x gluon forms the dominant contributionto total PDF uncertainty This could hide signs of new physics!!! NLO scale uncertainties: 10-15% (not shown) What can be done? 7

  8. Improving the high-x gluon ____________________________________ Most important aspect of my recent work is the rigorous inclusion, for the first time, of jet data in a QCD fit -> significantly improved knowledge of high-x gluon -> precisely where further constraints needed for new physics discovery at the LHC [ also allowed first competitive extraction of s(MZ) using only HERA data: s(MZ)=0.1183±0.0028(exp.)± 0.0008(model)±0.0050(scale) ] 8

  9. LHC start-up PDFs_______________________________________ I have also shown that further improvements can be expected from future HERA data before LHC start-up (C. Gwenlan et al.,hep-ph/0509220)

  10. LHC start-up PDFs _______________________________________ I have also shown that further improvements can be expected from future HERA data before LHC start-up (C. Gwenlan et al.,hep-ph/0509220) … and that this will significantly improve uncertainties on LHC jet cross section predictions Improvements at high jet-ET Such improvements will require new jet measurements from HERA (work I am performing with a PhD student at UCL) I will include any relevant new data in the fits when available in order to extract PDFs (and strong coupling)

  11. ZEUS PDF before including “data” ZEUS PDF after including “data” e+ “data” e+ “data” Next steps:_______________________________________  Include ATLAS data in fits -> important additional constraints - Drell-Yan (including W production) - QCD Jet production - others (direct-photon, W/Z+jets, …) <- I will measure with early ATLAS data !!!  Work already initiated at Oxford on W production (qq’Wl) - addition of simulated data in ZEUS QCD fit -> - low-x gluon shape: xg(x) ~ xb - BEFORE: b = 0.199±0.046 - AFTER: b=0.181±0.030 (35% reduction in uncertainty) A.Tricoli,A.Cooper-Sarkar C.Gwenlan, hep-ex/0509002  I am now in process of investigating impact of LHC jet data for constraints on high-x gluon -> first results using simulated ATLAS inclusive jet data expected soon -> will naturally lead to inclusion of real data (including my own SM jet measurements) in the PDF fits

  12. SM backgrounds to new physics_______________________________________  Backgrounds must be well understood/controlled/modelled - “Jets”: QCD jets - “Jets+missing-ET”: tt+jets, W+jets, Z+jets, QCD multijets example: SUSY (jets+missing-ET) S. Asai Reduction (“Jets+missing-ET”)  established methods exist for reducing/controlling backgrounds in similar searches at the Tevatron -> NEED to optimise for LHC Understanding/Modelling (“Jets” and “Jets+missing-ET”)  Particular challenge is modelling events with many jets-> cannot be calculated in NLO QCD Standard approach: matrix element+parton showers Monte Carlo (MC) -> many tunable parameters (UE, parton shower matching criteria,…) meff 12

  13. Best tuning to HERA 4-jets (stars) Best tuned model (stars) Untuned model (solid) Untuned model (solid) SM backgrounds to new physics_______________________________________ I have worked previously on MC “tuning” -> establish best parameters to describe wide variety of data from HERA, LEP, Tevatron Best global models found by tuning to my HERA four-jet measurements (led to best simultaneous description of LEP, HERA, Tevatron data) Angular variable “Jets and energy flow in photoproduction at HERA”, C. Gwenlan, UCL-THESIS 13

  14. SM backgrounds to new physics_______________________________________ A major consideration:  events with many high-pT jets -> parton showers (PS) don’t do good job -> need matrix elements (ME)  New state-of-the-art MC (e.g. ALPGEN, Sherpa) - generate multi-parton final states using LO matrix elements - treat soft/collinear region with parton showers - “match” ME-PS to avoid double counting Parton showers (PYTHIA) Matrix elements (ALPGEN) soft  hard PS Additional tunable parameters in matching criteria -> ME Available models will need to be tuned/validated against data -> including my early measurements of SM jets (especially multi-jets) double count region 14

  15. Hadronic reconstruction and energy flow_______________________________________  Processes with “jets” and “jets+missing-ET” require excellent reconstruction of hadronic final state -> dominant experimental uncertainties from hadronic energy scale and jet-pT/missing-ET resolutions  Several collider experiments have successfully employed combined energy flow techniques to improve reconstruction quality Calorimeter Isolated track Inner Detector Charged Deposit Neutral Deposit 15 New physics with ATLAS at the LHC

  16. Hadronic reconstruction and energy flow_______________________________________  Processes with “jets” and “jets+missing-ET” require excellent reconstruction of hadronic final state -> dominant experimental uncertainties from hadronic energy scale and jet-pT/missing-ET resolutions  Several collider experiments have successfully employed combined energy flow techniques to improve reconstruction quality MC: CALORIMETER cells example: ZEUS energy flow algorithm  I have previously implemented ZEUS energy-flow reconstruction in standard analysis framework and validated its performance in a range of jet topologies e.g. multi-jets -> MC: Energy-Flow (combined track+CAL) 16 New physics with ATLAS at the LHC

  17. Hadronic reconstruction and energy flow_______________________________________  Processes with “jets” and “jets+missing-ET” require excellent reconstruction of hadronic final state -> dominant experimental uncertainties from hadronic energy scale and jet-pT/missing-ET resolutions  Several collider experiments have successfully employed combined energy flow techniques to improve reconstruction quality Four-jet invariant mass: data (solid points) compared to PYTHIA MC using ZEUS energy flow algorithm for hadronic final state recon. example: ZEUS energy flow algorithm  I have previously implemented ZEUS energy-flow reconstruction in standard analysis framework and validated its performance in a range of jet topologies e.g. multi-jets ->  Used in full data analysis of multi-jets (->) and rapidity-gaps-between-jets at HERA 17 New physics with ATLAS at the LHC

  18. Validating and improving the ATLAS energy-flow _______________________________________  A new energy-flow algorithm now being developed within ATLAS -> needs full validation I have begun working with development team (Sheffield) to validate algorithm using the wide range of jet topologies I will measure (validate now with simulated data, later with my real measurements of inclusive jet, dijets and multi-jets) -> first tests/validation of algorithm performance in specific analysis chains  Initial tests suggest algorithm promising for low-pT jets Need to investigate to what extent algorithm can be used at high scales and whether performance can be improved -> I have suggested a new idea that further calibration might be possible by employing momentum balance of high-scale jets against multiple lower-ET jets-> needs investigation High-pT Jet Lower-pT Jets 18

  19. Summary_______________________________________  Much excitement about “new physics” beyond the SM  I want to focus on searches using signatures of “jets” and “jets+missing-ET” -> sensitive to wide variety of new physics models  Discovery and future precision measurement relies on good understanding of experimental and theoretical uncertainties -> have identified major limiting sources of uncertainty -> have developed plan to understand/reduce and control (already started work in several areas) -> much of my work will also be of direct and significant benefit to ATLAS community as a whole  Wealth of prospective publications from this work - early SM jet cross sections (some of the first ATLAS publications) - PDF fits including ATLAS data - results of high scale “jets” and “jets+missing-ET” searches Looking forward to 2007 !!! 19

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