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Risultati dell’esperimento ATLAS dopo il run 1 di LHC

Risultati dell’esperimento ATLAS dopo il run 1 di LHC C. Gemme (INFN Genova), F. Parodi (INFN/University Genova ) Genova , 28 Maggio 2013. LHC physics. Standard Model is a gauge theory based on the following “internal” symmetries: SU(3) c × SU(2) I × U(1) Y

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Risultati dell’esperimento ATLAS dopo il run 1 di LHC

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  1. Risultati dell’esperimento ATLAS dopo il run 1 di LHC C. Gemme (INFN Genova), F. Parodi (INFN/University Genova) Genova, 28 Maggio 2013

  2. LHC physics • Standard Model is a gauge theory based on the following “internal” symmetries: SU(3)c × SU(2)I × U(1)Y • Matter is build of fermions - quarks and leptons, three families of each, with corresponding antiparticles; quarks come in three colors, leptons are color singlets, do not couple to gluons. • Bosons are carriers of interactions: 8 massless gluons, 3 heavy weak bosons (W,Z) and 1 massless photon. • A neutral scalar Higgs field permeates the Universe and is (in some way) responsible for masses of all particles (their masses originate from couplings to Higgs field). • LHC big questions: • Test the Standard Model, hopefully find “physics beyond SM” • Find clues to the Electroweak symmetry breaking - Higgs(ses) C. Gemme - F. Parodi - Atlas results

  3. LHC physics • Single neutral Higgs scalar – the only missing particle in Standard Model, escaping detection for 50 years, at least until July 4th, 2013 C. Gemme - F. Parodi - Atlas results

  4. CMS LHCb ALICE ATLAS 7 anni di costruzione nel tunnel gia‘ utilizzato da LEP: 1989-2000 LHC Tunnel LHC: 27 km di circonferenza 4 4 C. Gemme - F. Parodi - Atlas results Leonardo Rossi

  5. LHC • The key parameters of an accelerator are the c.m.s. energy (√s) and the of collisions that can be generated (L). • Higher energy means possibility to generate with larger cross-section high mass particles. • High luminosity gives the opportunity to access rare (small cross-sections) events. • N x = ∫s xL (t) dt • LHCis app collider, designed for √s = 14 TeV and maximum design Lmax = 1034 cm-2s-1 • Run at √s = 7 TeV in 2010 and 2011, and at √s = 8 TeV in 2012 and Lmax = 8 1033 cm-2s-1 • Upgrading at √s = 13 TeV in 2015 C. Gemme - F. Parodi - Atlas results

  6. A collider particle detector • Tracking systems to reconstruct trajectories and momenta of charged particles • EM/hadroniccalorimeters to measure energy of particles and missing energy • Muon Spectrometers to precisely measure muonmomenta • Efficient Trigger system to reduce the huge collision rate C. Gemme - F. Parodi - Atlas results

  7. Inner Detector Genova http://www.atlas.ch/multimedia/atlas-built-1-minute.html C. Gemme - F. Parodi - Atlas results

  8. Pixel Detector • It is the innermost detector, crucial for the tracks parameters and vertex reconstruction. • 1744 pixel modules in the detector. • 2 End-caps (16% of the detector) built in US. • 3 cylindrical barrel Layers built in Europe (half in Genova) EndCap@Cern Integration around the beampipe Installation in the ID Lowering in the pit C. Gemme - F. Parodi - Atlas results

  9. Trigger • The interesting events are only few hundreds every second out of the 20 MHz of interactions frequency. • It would be impossible to transfer out of the detector such a huge amount of data (each event is ~ few MB) • The trigger system is designed to select the interesting events, based on their signatures, in a short time. • The ATLAS trigger system has a 3-levels structure: • Each level analyzes only events accepted by the previous step, the algorithms being more and more complex, requiring more information and more time to take a decision. Tracking at L2 is Genova responsability C. Gemme - F. Parodi - Atlas results

  10. ATLAS Data Taking ATLAS Integrated Luminosity ATLAS Peak Instantaneous Luminosity 7.7 1033 3.6 1033 2012 @8TeV 2011 @7TeV 2 1032 2010 @7TeV • LHC pp Run 1 ended (2010-2012), now preparing for next run from 2015. • ATLAS recorded: • 45 pb-1in 2010  ~1.5M Z, ~220 H@125GeV • 5.3 fb-1in 2011 ~160M Z, ~92k H@125GeV • 22 fb-1 in 2012  ~830M Z, ~490k H@125GeV • Excellent data-taking efficiency (>90%) and detector performance • % of not operative channels typically 0.5%, max 4% Integrated Luminosity: delivered vs recorded C. Gemme - F. Parodi - Atlas results

  11. Typical run conditions ... • LHC providing very stable beam conditions for several hours, ATLAS recording and use on average ~ 90% of the delivered luminosity. • Bunch spacing 50 ns ( vs 25 ns nominal) and p/bunch up to 1.7 1011 (vs ~1.1 1011 nominal) Transverse beam position stable in ~ 2 mm ATLAS Instantaneous Luminosity Transverse beam width ~15 mm slightly increasing along the run. Inefficiencies in data taking, mainly synchronizations C. Gemme - F. Parodi - Atlas results

  12. The Challenge in 2012: Pile-up • Pile-up: number of minimum bias collisions properly distributed in time and overlaying the physics collision Event in ATLAS with 2 reconstructed vertices in 2011 at 7 TeV : Display with track pT threshold of 0.4 GeV and all tracks are required to have at least 3 Pixel and 6 SCT hits Z → μμ event in ATLAS with 25 reconstructed vertices: Display with track pT threshold of 0.4 GeV and all tracks are required to have at least 3 Pixel and 6 SCT hits C. Gemme - F. Parodi - Atlas results

  13. The Challenge in 2012: Pile-up • Running with 50 ns bunch spacing (rather than 25 ns) results in 2x larger pile-up for the same instantaneous luminosity • On average ~20 interactions per bunch-crossing • Up to 40 interactions at peak luminosity • Huge effort to minimize physics impact • Biggest impact for calorimeter, trigger rates and computing. Peak interactions per BC nominal@25ns nominal@25ns Event size linear with pile-up C. Gemme - F. Parodi - Atlas results

  14. Physics observables • Data selection and analysis are based on physics observables in the final state; they are introduced in the next slides: • Leptons: electrons, muons • Photons • Hadrons (jets) • b-jet, tau • Neutrinos (missing energy) C. Gemme - F. Parodi - Atlas results

  15. Electrons/photons • Electrons and photons are completely absorbed by the EM calorimeter, creating a typical shower shape in the 4 layers of the Pb/LAr calorimeter. • According to the EM shower shape, to the association to a track, and to a secondary vertex, calorimeters deposits are associated to electrons, photons or converted photons. Calorimeters +Tracker C. Gemme - F. Parodi - Atlas results

  16. Electrons/photons • Electrons and photons are selected at L1 based on energy deposit in trigger towers (rough granularity) . • Projective towers such as to select primary particles. • Following trigger levels and offline selection use the full calorimeters granularity and depth and the tracker information. • Rejection with respect to hadrons is achieved using mainly the shower shape and leakage veto in the hadroniccalo. Calorimeters +Tracker C. Gemme - F. Parodi - Atlas results

  17. Muons • Muons are reconstructed as charged particles not being absorbed by the calorimeters. • Several algorithms in place to identify muons, exploiting all the detectors to get the maximum coverage. • Main algorithm combines tracker and muon spectrometer. • Magnetic fields (solenoidal in the tracker, toroidal in the muon spectrometer) allow the momentum measurement. • Muons required to be isolated to suppress background in many analyses. Muon Spectrometer + Tracker C. Gemme - F. Parodi - Atlas results

  18. Muons • Momentum resolution is highly improved at low momentum by using the Tracker information. • /pT < 10% up to 1 TeV in |h| <2.7 Muon Momentum Resolution Muon Spectrometer + Tracker Dominant at low p Dominant at high p C. Gemme - F. Parodi - Atlas results

  19. Jet • Jets are generated by the hadronization of quarks and gluons. • Calorimeters are heavily sensitive to pile-up • In-time PU estimated via the number of primary vertices • Out-of time PU (as read-out time is rather long) • The energy deposits are measured starting from Topo-Clusters: Group of calorimeter cells topologically connected optimized for electronic noise and pile-up suppression. Calorimeters + Tracker Da pensare ancora C. Gemme - F. Parodi - Atlas results

  20. b-jet • The b-tagging is the capability to identify jets coming from b-quark fragmentation. It is based on the relatively long lifetime of b-hadrons (t~1.5 ps, bgct ~ 3 mm for pT ~50 GeV). • Several b-tagging algorithms, exploiting: tracks impact parameters (JetProb, IP3D), reconstruction of the secondary vertex (SV1), topological structure of b and c-hadron decays inside the jet (JetFitter). • Different algorithm combinations for improved performance, quantified in light jet rejection vs b-tagging efficiency : IP3D+SV1, JetFitterCOMBNN, MV1. Calorimeters + Tracker C. Gemme - F. Parodi - Atlas results

  21. Tau • Tau is the heaviest lepton and is not stable, t~1.5 ps, bgct ~ 3 mm for pT ~50 GeV. METTERE VALORI tau • It decays hadronically in 65% generating a rather collimated jet of hadrons. • Tau hadronic reconstruction is seeded by jets • Requiring combined information from calorimeter and tracking • Input to multivariate algorithms Calorimeters + Tracker W tau v C. Gemme - F. Parodi - Atlas results

  22. Tau • Tau’s are identified thanks to some peculiar characteristics: • Collimated decay products, no gluon radiation, low invariant mass, lifetime  provide discrimination against jets; • EM energy fraction , EM component from pi0, transition radiation  provide discrimination against electrons. Calorimeters + Tracker C. Gemme - F. Parodi - Atlas results

  23. Missing energy • To detect particles that escape detection (mainly n’s, but also beyond SM low interacting particles), a balance of the event energy is done. • Missing Transverse momentum is a complex event quantity: • Adding significant signals from all detectors • Asking for momentum conservation in the transverse plane • ETmiss (in particular its resolution) is highly affected by pile‐up. • Using tracks not associated to physics objects and matched to PV to provide a reliable estimate of pile conditions and correct for it (Soft term vertex fraction). Full Detector! Dependence on pile-up almost flat Events with Etmiss, Good agreement data/MC C. Gemme - F. Parodi - Atlas results

  24. L1: ~ 65 kHz L2: ~ 5 kHz EF: ~ 400Hz C. Gemme - F. Parodi - Atlas results

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