July 7, 2008SLAC Annual Program ReviewPage 1 New Approaches to Hadronic Final State Reconstruction D. W. Miller SLAC ATLAS

July 7, 2008SLAC Annual Program ReviewPage 2 Overview Extending jet algorithm functionality and flexibility for first data New approaches to jet reconstruction and missing energy through use of tracking information Jet-vertex association for high luminosity jet identification, jet-energy corrections and primary vertex selection Semileptonic b-jet energy corrections Missing E T significance

Extending jet algorithms for first data Legacy jet algorithms Tower algorithm (TowerJets) –Built from geometrical arrangements of calorimeter cells –No noise suppression is applied Topological clusters (TopoJets) –Built from topological clusters of calorimeter cells –Noise suppression is applied –Clusters size can grow non- linearly in presence of pile-up New approach in ATLAS Use the most desirable aspects of each algorithm –Towers are finite, well-defined objects –Clusters efficiently select cells above a dynamic noise threshold Build towers using only cells used in topological clusters –Noise suppression –Well-behaved at high- luminosity –Overlap studies between TopoJets and TowerJets now possible July 7, 2008SLAC Annual Program ReviewPage 3 (DWM, SLAC)

Advances in jet resolution using tracking information July 7, 2008SLAC Annual Program ReviewPage 4 Jet structure Jet response depends on f trk Improvements with f trk correction (Marshall, Columbia U.)

Jets & tracks: improvements and implementation ftrk response corrections Jet resolution –Corrections applied in bins of f trk –Jet-by-jet fluctuations are reduced –Improvement in overall jet energy resolution Jet resolution –Corrections applied in bins of f trk –Jet-by-jet fluctuations are reduced –Improvement in overall jet energy resolution Improvements to jet-resolution: 10% at 40 GeV Official ATLAS software package integrated into full reconstruction Available to all ATLAS users for first data Documentation in ATLAS internal note –ATL-COM-PHYS Used in Higss Z/W  jjγγ analysis July 7, 2008SLAC Annual Program ReviewPage 5 (Marshall, Columbia U.)

Missing energy applications July 7, 2008SLAC Annual Program ReviewPage 6 Improvements to jet-energy has implications for missing energy measurements –Integral for both standard model measurements and new physics searches with b-jets Improvements to jet-energy has implications for missing energy measurements –Integral for both standard model measurements and new physics searches with b-jets Missing energy using f trk Artificial missing energy from p T imbalance is corrected following jet energy corrections (Schwartzman, SLAC)

Tracks-jets as complementary to calorimeter jets Calorimeter jets can incorporate particles from multiple interactions Using track-jets with Z information, can separate interactions July 7, 2008SLAC Annual Program ReviewPage 7 New approach in ATLAS Build “jets” from tracks in Inner Detector tracker –Inherently 3D objects (Z, η, φ) unluke calo jets New approach in ATLAS Build “jets” from tracks in Inner Detector tracker –Inherently 3D objects (Z, η, φ) unluke calo jets (Schwartzman, SLAC)

Improved jet-finding efficiency with track-jets Limited by tracker acceptanceImproved efficiency for low-p T July 7, 2008SLAC Annual Program ReviewPage 8 The use of tracks provides complementary information to calorimeters Track-jets allow for the measurement of jet reconstruction efficiency Energy losses in the inner detector and cryostat decrease calo efficiency but do not affect track-jets The use of tracks provides complementary information to calorimeters Track-jets allow for the measurement of jet reconstruction efficiency Energy losses in the inner detector and cryostat decrease calo efficiency but do not affect track-jets

Identification of fake missing energy with track-jets Unmeasured calorimeter jets results in artificial missing energy Calibration of hadronic energy incorrect Using track-jets to pin-point un- reconstructed jets, events with fake missing energy can be removed July 7, 2008SLAC Annual Program ReviewPage 9 (Majewski, BNL)

Jet-vertex association (JVF) Associate standard calorimeter jets to primary vertices using tracks Jet-by-jet energy correction for pile-up and jet-selection criterion. Improve jet-energy reconstruction, missing energy resolution and primary vertex (PV) selection using this jet-vertex fraction, or JVF. July 7, 2008SLAC Annual Program ReviewPage 10 JVF measures the fraction of charged particle transverse (track) momentum in each jet from each identified primary vertex in the event. ATLAS detector with pileup

New approach to jet identification in ATLAS Using JVF to select hard-scatter jets in events with pile-up July 7, 2008SLAC Annual Program ReviewPage 11 Using JVF we recover the flat jet-multiplicity distribution without raising the jet p T threshold In official ATLAS software and available to all ATLAS users for first data Fully documented in internal ATLAS note: ATL-COM-PHYS Using JVF we recover the flat jet-multiplicity distribution without raising the jet p T threshold In official ATLAS software and available to all ATLAS users for first data Fully documented in internal ATLAS note: ATL-COM-PHYS ttbar events: cm -2 s -1 ttbar events: 2x10 33 cm -2 s -1 (DWM, SLAC)

Jet-by-jet energy corrections for pile-up contributions Jet energy response vs. JVF: & 2x10 33 cm -2 s -1 (ttbar) Jet energy response vs. p T and JVF (ttbar) July 7, 2008SLAC Annual Program ReviewPage 12 Using JVF to correct jet energies in a luminosity independent manner per jet basis Derive jet-energy correction similar to that already done for f trk to correct jets for pileup contributions on a per jet basis Using JVF to correct jet energies in a luminosity independent manner per jet basis Derive jet-energy correction similar to that already done for f trk to correct jets for pileup contributions on a per jet basis (DWM, SLAC)

Using jet-vertex association to select primary vertices Improving primary vertex selection in events with multiple interactions The high luminsotiy environment in ATLAS will oresent challenges to correct hard-scatter vertex identification By using information from reconstructed objects in the event, this selection can be improved –Extremely important for b-tagging New approach in ATLAS Measure efficiencies in real data with high-p T lepton tagging July 7, 2008SLAC Annual Program ReviewPage 13 Muons and JVF tagged correct PV Jet & μ -tag Default μ jet (DWM, SLAC)

Semileptonic b-jet corrections b-jet response in ttbar events July 7, 2008SLAC Annual Program ReviewPage 14 Correcting for missing neutrino energy Jets with constituent ν’s have energies systematically underestimated by 10% –Use p T jet and p T μ to parameterize response –Apply correction in barrel and end cap regions In official ATLAS jet reconstruction software Available to all users for first data Documented in internal ATLAS note: –ATL-COM-PHYS (Mateos, Columbia U.)

Use of semileptonic b-jet correction in ATLAS Data-driven methods for estimating corrections under way July 7, 2008SLAC Annual Program ReviewPage 15 Not only will the jet energy scale of b-jets occupy an integral role in many standard model measurements in early data (such as ttbar) but the preponderance of b-jet channels in SUSY and Higgs searches renders it even more important Technique applied in Higgs Working Group H(120 GeV)  bbbar analysis (Mateos, Columbia U.)

Missing energy significance Combine known jet resolutions with measured jet energy and missing E T to define a likelihood function that jet energy fluctuations contribute to the missing E T –Separate true missing E T from fake missing E T July 7, 2008SLAC Annual Program ReviewPage 16 QCD Events (fake missing E T ) W+jets (real missing E T ) (Perez, Columbia U.)

Summary SLAC is providing a coherent yet wide range of contributions to ATLAS Jet/MET performance and physics groups –New approaches to combined detector performance using tracking and calorimetry –Novel techniques for coping with and correcting for pile-up in the LHC environment –Coherent approach to maximizing precision jet-energy measurements for standard model measurements and for new physics searches –Applications to many physics analyses –Extensive documentation in internal ATLAS notes –All methods are available to the ATLAS community for first data Several active areas of research and techniques not covered! –Quark/gluon tagging: SUSY jet+MET background reduction –b/c quark-jet separation: b-jet background reduction –Gluon splitting (gluon  bbbar) removal: large source of background for SUSY b-jet+MET –Di-jet resolution measurements in first data July 7, 2008SLAC Annual Program ReviewPage 17

Outlook and conclusions This work has provided many important tools and techniques for understanding the first physics data from ATLAS –Roadmap for jet-energy scale with first data is being planned with this work at the forefront –Expect to be heavily involved (lead??) the data-driven approaches to measuring the jet-energy scale and applying these new techniques –New approach to jet-tower building is expected to aid greatly in understanding the jet response in first data The tools and methodologies being developed by the SLAC ATLAS group provide users with the capability to select, improve and reject events in his or her own analysis and are already proving useful in both standard model studies as well as physics searches. July 7, 2008SLAC Annual Program ReviewPage 18