Risultati dell esperimento ATLAS dopo il run 1 di LHC C. Gemme (INFN Genova), F. Parodi (INFN/University Genova) Genova, 28 Maggio 2013 1
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) 28/5/2013 C. Gemme - F. Parodi - Atlas results 2
LHC physics Single neutral Higgs scalar the only missing particle in Standard Model, escaping detection for 50 years, at least until July 4 th, 2013 28/5/2013 C. Gemme - F. Parodi - Atlas results 3
7 anni di costruzione nel tunnel gia utilizzato da LEP: 1989-2000 LHC Tunnel LHC: 27 km di circonferenza CMS LHCb ALICE 4 28/5/2013 C. Gemme - F. Parodi - Atlas results Leonardo Rossi ATLAS 4
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 x L (t) dt LHC is a pp collider, designed for s = 14 TeV and maximum design L max = 10 34 cm - 2 s -1 Run at s = 7 TeV in 2010 and 2011, and at s = 8 TeV in 2012 and L max = 8 10 33 cm -2 s -1 Upgrading at s = 13 TeV in 2015 28/5/2013 C. Gemme - F. Parodi - Atlas results 5
A collider particle detector Tracking systems to reconstruct trajectories and momenta of charged particles EM/hadronic calorimeters to measure energy of particles and missing energy Muon Spectrometers to precisely measure muon momenta Efficient Trigger system to reduce the huge collision rate 28/5/2013 C. Gemme - F. Parodi - Atlas results 6
Inner Detector Genova http://www.atlas.ch/multimedia/atlas-built-1-minute.html 28/5/2013 C. Gemme - F. Parodi - Atlas results 7
Pixel Detector It is the innermost detector, crucial for the tracks parameters and vertex reconstruction. 1744 pixel modules in the detector. o 2 End-caps (16% of the detector) built in US. o 3 cylindrical barrel Layers built in Europe (half in Genova) EndCap@Cern Integration around the beampipe Installation in the ID Lowering in the pit 28/5/2013 C. Gemme - F. Parodi - Atlas results 8
Trigger Tracking at L2 is Genova responsability 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. 28/5/2013 C. Gemme - F. Parodi - Atlas results 9
ATLAS Data Taking ATLAS Integrated Luminosity ATLAS Peak Instantaneous Luminosity 2012 @8TeV 2011 @7TeV 2010 @7TeV 2 10 32 3.6 10 33 7.7 10 33 LHC pp Run 1 ended (2010-2012), now preparing for next run from 2015. ATLAS recorded: 45 pb -1 in 2010 ~1.5M Z, ~220 H@125GeV 5.3 fb -1 in 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 28/5/2013 C. Gemme - F. Parodi - Atlas results 10
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 10 11 (vs ~1.1 10 11 nominal) Transverse beam position stable in ~ 2 mm ATLAS Instantaneous Luminosity Inefficiencies in data taking, mainly synchronizations Transverse beam width ~15 mm slightly increasing along the run. 28/5/2013 C. Gemme - F. Parodi - Atlas results 11
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 28/5/2013 C. Gemme - F. Parodi - Atlas results 12
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 28/5/2013 C. Gemme - F. Parodi - Atlas results 13
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) 28/5/2013 C. Gemme - F. Parodi - Atlas results 14
Calorimeters +Tracker 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. 28/5/2013 C. Gemme - F. Parodi - Atlas results 15
Calorimeters +Tracker 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 hadronic calo. 28/5/2013 C. Gemme - F. Parodi - Atlas results 16
Muon Spectrometer + Tracker 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. 28/5/2013 C. Gemme - F. Parodi - Atlas results 17
Muon Spectrometer + Tracker Muons Momentum resolution is highly improved at low momentum by using the Tracker information. s/p T < 10% up to 1 TeV in h <2.7 Muon Momentum Resolution Dominant at low p Dominant at high p 28/5/2013 C. Gemme - F. Parodi - Atlas results 18
Calorimeters + Tracker 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. 28/5/2013 C. Gemme - F. Parodi - Atlas results 19
Calorimeters + Tracker 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 p T ~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. 28/5/2013 C. Gemme - F. Parodi - Atlas results 20
Calorimeters + Tracker Tau Tau is the heaviest lepton and is not stable, t~1.5 ps, bgct ~ 3 mm for p T ~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 W tau v 28/5/2013 C. Gemme - F. Parodi - Atlas results 21
Calorimeters + Tracker 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. 28/5/2013 C. Gemme - F. Parodi - Atlas results 22
Full Detector! 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). Dependence on pile-up almost flat 28/5/2013 Events with Etmiss, Good agreement data/mc C. Gemme - F. Parodi - Atlas results 23
L1: ~ 65 khz L2: ~ 5 khz EF: ~ 400Hz 28/5/2013 C. Gemme - F. Parodi - Atlas results 24