Risultati dell’esperimento ATLAS dopo il Run 1 di LHCgemmec/talks/ATLAS_FirstPart_Final.pdf ·...

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


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


• 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


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


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)


EndCap@Cern Integration around the beampipe

Lowering in the pit

Installation in the ID

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

ATLAS Data Taking


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


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.


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)


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.


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.


Calo

rim

ete

rs +

Tracker

Similar Trigger logic for others objects

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 S

pectr

om

ete

r + T

racker

Muons


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)


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.


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


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.


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).


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


L1: ~ 65 kHz

L2: ~ 5 kHz

EF: ~ 400Hz

Risultati dell’esperimento ATLAS dopo il Run 1 di LHCgemmec/talks/ATLAS_FirstPart_Final.pdf ·...

Documents

Transcript of Risultati dell’esperimento ATLAS dopo il Run 1 di LHCgemmec/talks/ATLAS_FirstPart_Final.pdf ·...