ATLAS Physics Preparations

ATLAS Physics Preparations Ariel Schwartzman SLAC Annual Program Review

Outline LHC: the new energy frontier Physics plans* : - Searches for long lived particles - Supersymmetry searches in b+MET events Physics signature reconstruction - Jet/MET improvements - Hadronic flavor tagging SLAC as a west coast analysis center * Focus on two main analysis, rather than covering all topics. SLAC Annual Program Review

The next energy frontier: the LHC The LHC will open the Tera-scale to exploration, entering a new era in particle physics, and holding the key to the answers of some of the most important questions of particle physics today: - electroweak symmetry breaking - dark matter - are there extra dimensions of space? LHC: 7xTevatron center of mass energy 100xTevatron luminosity New physics expected at the TeV scale with large production cross-sections. Precision Standard Model measurements: top quark properties. SLAC Annual Program Review

Supersymmetry One of the most attractive extensions of the Standard Model: - hierarchy problem - unification of the gauge forces - candidate for the dark matter in the Universe Large cross sections for SUSY particles with masses bellow 1TeV: ~5000 events in 100pb-1 of data. Several SUSY models predict different experimental signatures: If Rp is conserved, the LSP is stable: multi-jets, leptons, and missing ET. If Rp is violated the LSP decays, given rise to displaced vertex signatures. Search for full SUSY parameter space, rather than for a particular model. SLAC Annual Program Review

c LSP l q l* l’ q Long lived particle signatures Ignacio Aracena, Keith Bechtol, Claus Horn, Tim Nelson, Dan Silverstein, Su Dong Several Standard Model extensions predict neutral, weakly-coupled, unstable particles: Rp violating SUSY, Hidden Valley models, gauge-mediated extensions of MSSM. The experimental signature of Rp violating SUSY events, depends on the LSP lifetime. Decays in muon system Decays in inner detector Decays in calorimeter SLAC Annual Program Review

Long lived particle search challenges LSP decays in the calorimeter: - jets without tracks. - high EHAD/EEM ratio. - multiple muon hits. LSP decays within the tracker detector: - Dominant decay for most couplings. - Search for high mass, high multiplicity displaced vertices. - Matches well SLAC expertise. New effort within ATLAS, complementary to work performed by other groups focused on calorimeter/muon decays. - Unique experimental challenges: - tracking and vertexing for very high displaced vertices. - high level trigger. - track-jets from displaced vertices. - non-prompt track reconstruction. - non-pointing electrons: calorimeter tracking into TRT. SLAC Annual Program Review

Rp conserved SUSY signals at the LHC Squarks and gluinos, if they exist, will be copiously produced at the LHC via the strong interaction. Squark and gluino production leads to signatures with multi-jets through cascade decays and the neutral LSP, which escapes detection. Jet+MET is the dominant signature, with the highest discovery reach. Several other distinct signatures involving final states with leptons. Mo (Gev) SLAC Annual Program Review

Probing for SUSY in b-jet + MET final states • The requirement of b-jets in events with large MET and multi-jets is well • motivated both theoretically and experimentally: • QCD, and W/Z+(light quark) jets backgrounds are significantly reduced. • Top background is irreducible, but b-tagging provides additional handles to control and • measure it. • Reducing non-top SM backgrounds by requiring b-tags could be one • of the keys to the early discovery of SUSY at the LHC. • In many scenarios, where the third-generation of squarks are expected • to be light, most new physics events will contain two or four b- or t-quarks. • Even if SUSY is first discovered in inclusive jet + MET or lepton events, • understanding the production of third-generation squarks will be critical • to answer important physics questions. SLAC Annual Program Review

Physics questions addressed in b-jet + MET events Is new physics related to Dark Matter? since b-tags reduce the QCD multi-jet background, looser MET cuts can open up the possibility to study the shape of the MET, to determine if the missing energy originates from a massive object. Quadratic divergences in the top sector: third-generation squarks (stops, sbottoms) are responsible for the largest quantum corrections that stabilize the weak scale. b-tagging can help determine the mass scale for third-generation partners. Origin of the electroweak symmetry breaking: if third-generation partners are light, higgs particles can be produced in decay chains starting with stops and sbottoms, as well as in gluino decays. A light Higgs could be first discovered in this channel. SLAC Annual Program Review

b-jet + MET inclusive events Standard Model Measurements/background SUSY signature topologies Top pair production cross section Top quark mass Wbb cross section Strategy for b-jet + MET searches Research program starting, coordinated with Philip Schuster and Natalia Toro (SLAC/Stanford theory group) Signature almost unexplored in ATLAS: b-tag and MET are among the most challenging and complex techniques. Expertise in multiple areas is required. Matches well SLAC strengths. Verifying top x section and mass and probing for SUSY at the same time. Separate 0- and 1-lepton channels. Also (untagged) inclusive jet+MET sample. SLAC Annual Program Review

Experimental challenges Efficient b-tagtrigger at level 2 to afford lower MET and jet pT requirements. Jet energy scale and resolution, and effect of multiple interactions. (much broader scope beyond SUSY searches: SM precision measurements) MET clean up, calibration, significance. In particular, in the presence of b-jets. b-tagging: - b-tag background estimation at high tag multiplicities. - Estimation of b production from gluon splitting. - b/c flavor separation. In August 2007, SLAC initiated the Jet/Met/b-tag analysis group to work on understanding the experimental issues in the b-jet+MET channel, and developing new tools and improvements to enhance the LHC physics potential SLAC Annual Program Review

Physics signature reconstruction: current activities within the Jet/Met/b-tag group High Level Trigger: L2 b-tag: chi2 probability, secondary vertex (A. Schwartzman/SLAC) L2 MET, mHT, Jet (I. Aracena/SLAC) Jet/MET reconstruction and energy scale: Tower-jet reconstruction/noise suppression (D. Miller/Stanford) Jet energy resolution measurement (G. Romeo/U. Buenos Aires) Semileptonic b-jet energy scale (D. Lopez/Columbia) Missing ET significance (B. Butler/Stanford and K. Perez/Columbia) Jet/MET improvements using tracks: Track-based jet energy corrections (Z. Marshall/Columbia) Jet-Vertex Association, pile-up corrections (D. Miller/Stanford) MET fake rejection using tracks (S. Majewski/BNL) Hadronic flavor tagging: Gluon bb tagging (A. Schwartzman/SLAC) Quark/gluon tagging (A. Schwartzman/SLAC) b/c flavor separation (A. Schwartzman/SLAC) • 2007/2008 accomplishments: • - 3 internal ATLAS notes. • 5 contributions to public • ATLAS notes. • 11 contributions to ATLAS • workshops. • - 2 conference posters. • 2 APS talks. • several new official • ATLAS software packages. SLAC Annual Program Review

Jet energy scale and resolution Until very recently, ATLAS jet calibration plan, was based on cell weighting techniques using Monte Carlo truth information. SLAC has been actively involved in the development of an ATLAS-wide strategy for jet energy scale calibration based on data. This plan has now the support of the ATLAS management and a new ATLAS sub-group is being setup to organize this work with SLAC continuing to play a significant role: through participation on major aspects, and with one of its members co-coordinating this group. SLAC Annual Program Review

muons calo-clusters (b-jet energy measurement) Track multiplicity Charged E fraction Track-Jet width Pt leading track (improved energy resolution, hadronic flavor tagging) Jet-Vertex E Fraction (pile-up energy correction, vertex selection) Tracking input to Jet reconstruction SLAC research program to use tracking information to improve calorimeter-jet energy measurements: New approach, conceptually different from energy flow techniques: Use tracks to extract information abut jet topology and fragmentation, and correct jet response as a function of (track)jet particle composition. SLAC is leading the development of jet reconstruction algorithms combining tracking with calorimeter information: Improved energy resolution measurement and hadronic flavor tagging. SLAC Annual Program Review

Improving Jet resolution with tracks New approach in collider experiments Jet energy scale calibrates the average energy response Rjet(pT,) = 1 Use tracking information to reduce some of the sources of jet-by-jet energy fluctuations and Improve the energy resolution. Implemented in ATLAS software. Presented at APS conference, 04/08. Being studied in several ATLAS physics working groups. SLAC Annual Program Review

Jet-Vertex association • At the LHC high luminosities, additional interactions in the same beam • crossing can give rise to extra jets, and distort kinematics distributions. • Use tracks and vertices to: • identify and reject jets originating • from additional interactions. • - subtract energy from additional • interactions particles contributing to • hard-scatter jets. (New approach in hadron colliders) Technique presented at APS conference 04/08. Now part of the official ATLAS jet software. SLAC Annual Program Review

Tagging gluon bb One of the dominant Standard Model backgrounds to SUSY in the b+MET channel is W boson production in association with heavy flavor quarks originating from gluon splitting. Tagging b-jets from gluon splitting can reduce b-tag backgrounds to new physics and improve its estimation from data. (New idea, not explored at the Tevatron) Developed neural network to separate gbb from b jets using track, vertex, and calorimeter information. Performance in the simulation: 50% b-jet efficiency @ 90% gbb rejection 90% b-jet efficiency @ 45% gbb rejection SLAC Annual Program Review

Track-Jets 3D: (eta,phi,z) Calorimeter Jets Tower-jets noise suppression energy scale Hard scatter vertex selection Physics Jets JVF(vtx) R(ftrk,ntrk) Offset(jvf) q/g, b/c, gbb MET significance METsig likelihood Overview of Jet and METimprovements in ATLAS Coherent use of tracks, vertices, and calorimeter information to improve jets, MET, b-tagging, and physics signature reconstruction at high luminosity Improved Jet resolution SLAC Annual Program Review

SLAC as a west coast center for ATLAS With the startup of the LHC, we enter a new environment for accelerator-based particle physics in which one machine is supplying data to a world-wide community. In order for U.S. groups to successfully contribute to the LHC physics program, they will need local centers that provide intellectual support. SLAC can contribute to the support of the US ATLAS community in several ways: - serving as a center of experts on physics analysis and detector issues. - hosting of workshops on ATLAS physics analysis. - coordinating different university groups working on similar topics in relatively isolated efforts. Example: jet energy calibration. - providing a local center for US students and postdocs - providing support to US university physicist at CERN SLAC Annual Program Review

SLAC as a center of expertise (I) The SLAC experimental group in ATLAS is involved in many of the key areas for LHC physics, including jet energy scale and resolution, missing energy, tracking, and b-tagging. Strong involvement in the commissioning of the pixel detector, high level trigger, simulation, and data distribution through the Tier 2 centers. Starting upgrade projects. Strong theory group, with experts in the full range of specialties needed for LHC physics. Leading experts in new physics models, computational QCD, and members of the development teams for widely-used event generators. SLAC has already begun organizing working groups that connect this expertise to the U.S. university community. SLAC Annual Program Review

SLAC as a center of expertise (II) Jet/Met/b-tag group: - Students and postdocs from Stanford,Columbia, BNL, UCSC, Buenos Aires. - Coordination of work in the development of physics signature tools. - Jet resolution improvements using tracks, MET, pile-up, energy scale. - Significant contributions to ATLAS: new reconstruction algorithms, ATLAS notes, contributions to ATLAS workshops and HEP conferences. Starting new working group on to look at new particle searches in jet+MET topologies and data-driven background estimation in the early LHC data. Joint SLAC/ATLAS experimental/theory seminar: - Made experimental group aware of new developments in QCD and new types of physics models. - Regular contributions from other U.S. west coast institutions. Theory meetings on LHC physics: - West Coast LHC Theory Network. - Regional meetings on more specialized topics with UC Davis, Oregon, and Washington. SLAC Annual Program Review

SLAC organization of physics analysis workshops Within the U.S. ATLAS organization, ANL, BNL, and LBNL are the analysis support centers that provide support to the user community: - organization of analysis jamborees and workshop to provide the basic technical training for US ATLAS collaborators. - grid computing, tracking software, physics analysis tools: foundation for students and postdocs to begin analysis in ATLAS. There is another type of workshop that also needs to be provided to the U.S. ATLAS community: - Key physics analysis problems: intensive work on specific, well-defined set of problems. - These workshops must arise from the need of physics, and all the major groups in ATLAS should identify useful topics and create the framework in which a broader group of members can address them. - Example: ATLAS hadronic final state analysis forum held at SLAC in January 08 (overlapping with the LHC West coast theory meeting) where ATLAS collaborators worked on concreted analysis topics involving jets and MET. Some of the participants continue to work with our group, for instance in the Jet/Met/b-tag analysis group. SLAC Annual Program Review

SLAC hosting of students and postdocs SLAC can host graduate students and postdocs on short or long term assignments from university groups. Short-term visits to SLAC will allow new students and postdocs towork directly with members of the local analysis signature and reconstruction groups. Opportunities for students and postdocs based in the US to become involved in ATLAS detector monitoring, hardware upgrade projects, and trigger algorithms. Academic setting with lectures and seminars on LHC physics: - LHC physics course at Stanford. - paper-reading meetings. New ATLAS members can study physics issues and train to be more effective members of the collaboration. Particularly important for young physicists who will eventually be based at CERN to begin their work in ATLAS in this more supportive environment. SLAC Annual Program Review

SLAC support to US university physicists at CERN SLAC will maintain a strong presence at CERN, in both the detector and physics projects. We intend that this group be supportive to physicists from other U.S. institutions. Students and postdocs from small university groups can find it difficult to become Integrated into ATLAS efforts at CERN: - physics projects are not centrally organized at CERN. - young people at CERN often do not have regular supervision of their work. SLAC has already been providing support to students and postdocs from several University groups: - Jet/Met/b-tag analysis group - Iowa postdocs working in the pixel detector - hosting summer students from Fresno State University SLAC is building up a large strong CERN-based group, well integrated into the ATLAS working groups and closely connected to the ATLAS physics analysis organization at SLAC. SLAC Annual Program Review

Summary With the start of the LHC, we enter a new era of scientific exploration. Early data may fundamentally change our understanding of the Universe. SLAC is participating in two broad topology-based searches for new physics: long lived particles, and b+jet+MET analyses, and in most of its key experimental issues: trigger, tracking, vertexing, b-tagging, jet energy calibration, tracking Input to jet reconstruction, etc. SLAC has made a significant impact in ATLAS during the last year: new jet software, trigger algorithms, publications, roadmap for jet energy calibration. SLAC has started organizing physics analysis activities between U.S. physicists with common interests, and workshops on specific physics analysis topics. We are committed to help and support U.S. university groups and make the new way of working, with data distributed to a worldwide community, succeed. SLAC Annual Program Review

ATLAS Physics Preparations