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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 describing QCD and ElectroWeak interactions 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.
The SM predicts a neutral scalar Higgs field that permeates the Universe and is (in some way) responsible for masses of all particles (their masses originate from couplings to Higgs field).
28/5/2013 C. Gemme - F. Parodi - Atlas results 2
LHC big questions:
Test the Standard Model, hopefully find “physics beyond SM”
Find clues to the Electroweak symmetry breaking - Higgs(ses)
LHC physics
Single neutral Higgs scalar – the only missing particle in Standard Model, escaping detection for 50 years, at least until July 4th, 2012
28/5/2013 C. Gemme - F. Parodi - Atlas results 3
LHC
28/5/2013 C. Gemme - F. Parodi - Atlas results 4 Leonardo Rossi
4
Tunnel LHC: 27 km di
circonferenza
CMS
ALICE
LHCb
ATLAS
7 anni di costruzione nel
tunnel gia‘ utilizzato da LEP:
1989-2000
LHC
28/5/2013 C. Gemme - F. Parodi - Atlas results 5
• The key parameters of an accelerator are the c.m.s. energy (√s) and the frequency 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
• LHC is a pp 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
• Bunch spacing 50 ns ( vs 25 ns nominal) and p/bunch up to 1.7 1011 (vs ~1.1 1011 nominal)
A collider particle detector
28/5/2013 C. Gemme - F. Parodi - Atlas results 6
Tracking systems to reconstruct trajectories and momenta of charged particles in solenoid magnet.
EM/hadronic calorimeters to measure energy of particles and missing energy
Muon Spectrometers in toroid magnets to precisely measure muon momenta.
Efficient Trigger system to reduce the huge collision rate
28/5/2013 C. Gemme - F. Parodi - Atlas results 7
http://www.atlas.ch/multimedia/atlas-built-1-minute.html
Inner Detector
Genova
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)
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EndCap@Cern Integration around the beampipe
Lowering in the pit
Installation in the ID
Trigger
28/5/2013 C. Gemme - F. Parodi - Atlas results 9
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
ATLAS Data Taking
28/5/2013 C. Gemme - F. Parodi - Atlas results 10
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%
2012 @8TeV
2011 @7TeV
2010 @7TeV
7.7 1033
3.6 1033
2 1032
ATLAS Integrated Luminosity ATLAS Peak Instantaneous Luminosity
Integrated Luminosity: delivered vs recorded
The Challenge in 2012: Pile-up
Pile-up: number of p-p interactions, minimum bias, overlaying the physics collision
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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
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
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.
28/5/2013 C. Gemme - F. Parodi - Atlas results 12
nominal@25ns
Event size linear with pile-up
Peak interactions per BC
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)
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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, energy deposits in the calorimeters are associated to electrons, photons or converted photons.
28/5/2013 C. Gemme - F. Parodi - Atlas results 14
Calo
rim
ete
rs +
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 15
Calo
rim
ete
rs +
Tracker
Similar Trigger logic for others objects
Muons
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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 S
pectr
om
ete
r + T
racker
Muons
28/5/2013 C. Gemme - F. Parodi - Atlas results 17
Muon S
pectr
om
ete
r + T
racker
Dominant at low p
Dominant at high p
Momentum resolution is highly improved at low momentum by using the Tracker information.
• s/pT < 10% up to 1 TeV in |h| <2.7
Muon Momentum Resolution
Jet
Collimated sprays of energetic hadrons, called jets, are the dominant feature of high energy proton-proton interactions. Jets are produced via the fragmentation of quarks and gluons.
Hadronic jets used for ATLAS physics analyses are reconstructed by a jet algorithm clustering the energy depositions of electromagnetic and hadronic showers in the calorimeters.
• Charged particles associated to a jet are exploited for pile-up suppression (JetVertexFraction)
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Calo
rim
ete
rs +
Tra
cker
Longitudinal view of the highest mass dijet event in 2010
<Njet> dependence on pile-up almost flat
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 ~ 4.5 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.
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Calo
rim
ete
rs +
Tra
cker
Genova
Tau
Tau is the heaviest lepton and is not stable, t~0.3 ps, bgct ~ 2.5 mm for pT ~50 GeV.
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
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Calo
rim
ete
rs +
Tra
cker
W tau v
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 21
Calo
rim
ete
rs +
Tra
cker
dijet
tau
An Example Variable: Calorimetric Radius
tau
electrons
An Example Variable: EM track function
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 energy 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).
28/5/2013 C. Gemme - F. Parodi - Atlas results 22
ETmiss Distribution, Good agreement data/MC
ETmiss Resolution
dependence on pile-up almost flat
Full D
ete
cto
r!
Physics objects: trigger selection Trigger menu optimized for a luminosity
of 8 1033 cm-2s-1 • Peak Level-1 rate ~70 kHz
• Peak Event Filter rate ~ 1 kHz
Streaming at Event Filter • Average (prompt) physics output rate ~400 Hz
~ equally shared between E/g, Muons, Jet/tau/ET
miss
• In addition ~200 Hz of delayed physics events
• Delayed events reconstructed during shutdown
28/5/2013 C. Gemme - F. Parodi - Atlas results 23
L1: ~ 65 kHz
L2: ~ 5 kHz
EF: ~ 400Hz