Preparing to enter the new "Energy Frontier"

with the ATLAS experiment

Mark Hodgkinson

Sheffield Monday Physics Seminar

17 March 2008

Contents

• Overview of Particle Physics

• Introduction to Tevatron and LHC

• Current State of ATLAS

• What we have to do to measure Jets in ATLAS

• Conclusions

What is the world made of?• Everything is made from atoms • Atoms are made from a nucleus

and have particles called electrons surrounding the nucleus

• The nucleus is made from particles called protons and neutrons

• Protons and neutrons are made out of yet smaller particles called quarks• As far as anyone knows quarks and the Electrons are not built from smaller particles

• As we probe deeper we find further substructure until we penetrate no further - everything made from electrons + quarks

Yet more particles!

• Early 20th century - discover unknown particles entering the atmosphere from outer space

• Using particle detectors the properties could be measured - it was found they were not the same particles present in atoms

• We now know many years later there are hundreds of different types of subatomic particles

• We have a mathematical model (The Standard Model) that describes how all these particles behave towards each other

• All the fundamental particles (except one) predicted by this model have been discovered - the top quark was the last particle discovered in 1995 at FermiLab in USA

• One last particle predicted by this model is yet to be found - called the Higgs Boson particle.

• Sheffield one of 164 universities working on the ATLAS experiment

• I am one of 2000 physicists who work on ATLAS

• Rival experiment is called CMS

• Two other experiments (ALICE and LHCB) study other types of physics

• 27 km proton beam pipe underneathFrance/Switzerland• 2 beams circulate in opposite directions• Beams consist of bunches of protons with gaps between bunches

• 4 Collision points• 40 million bunch crossingper second at each point• ATLAS will filter this to ~200 events recorded every second

The Energy Frontier

• Tevatron collides protons and anti-protons at 1.96 TeV centre of mass energy

• One eV is amount of energy an electron gains when accelarated across a potential difference of 1 volt = 1.6 10-19 J

• Tevatron is the current Energy Frontier• LHC will collide protons with protons at 14 TeV centre of mass

energy• Know Standard Model does not work above 1.2 TeV without a

Higgs or other new physics - without something new unitarity is violated in WW scattering.

CDF Fermilab

D0CDF

The Standard Model

• 3 known generations of fermions• Accounts for all known forces (electromagnetic,

weak nuclear and strong nuclear) except gravity• Complete agreement with experiment so far -

but no observed mechanism to generate particle masses

Fermions

e e up strange top

down charm bottom

Gauge Bosons

Photon ()

W±, Z

Gluon

ATLAS• In 1992 there were 4 proposals

for detectors to search for the Higgs on the LHC: CMS, EAGLE, ASCOT and L3P

• EAGLE and ASCOT became the ATLAS collaboration.

• L3P was dropped, leaving ATLAS and CMS

• Detect charged particles with the tracking system - curvature of trajectory in magnetic field gives you the momentum of the particle

• Calorimeters surround this and can detect all particle types if they interact in the detector

• Thirdly we have a dedicated muon detection system outside the calorimeters - muons penetrate a long way through our detectors

Status of ATLAS• Last components were lowered into

the pit on February 29• We are currently in the detector

commissioning phase• Series of “MileStone” week long runs• Data used is cosmic rays (no

collisions yet!)

• Milestone 4 run• No tracking available..• Can clearly see deposits in calorimeter and muon systems• Many problems found and fixed - e.gno-one noticed calorimeter data was useless for 5 days…now we have much improved data quality monitoring procedures

Milestone Week 6

More Commisioning• We have to commission the software and computing

infrastructure, not just the detector• Raw data is electronic signals - this has to be turned into

software objects like electrons or muons to be used in the actual data analysis

• Need to calibrate the measured energies to real energies (e.g. energy deposited in inactive material in detector)

• Nearly all data thrown awayvia hardware triggers Then it has to be: Calibrated Reconstructed at Tier 0/1 Shipped to Tier 2• Finally we make histograms for publication at Tier 4

Jet Physics

QCD• Quantum ChromoDynamics (QCD) is similar to

Quantum ElectroDynamics except: Photon -> Gluon1 Electric Charge -> 3 Colour ChargesPhoton Charge = 0 -> Gluon Charge != 0 Inverse Square Law -> Linear Law• Lund String Model (other models exist):

VI

V2

V2 > V1

Hadronisation• All these quarks combine into

composite particles - pions, kaons, protons, neutrons

• 20 - 60 stable particles perJet

Charged and neutral components seen in calorimeter

Charged componentseen in tracker

Jet Algorithms• Need to associate composite particles to correct quark or gluon

• Many and varied - main varieties are cone based (geometric cone search) and KT (search in momentum space)

• Only mention Cone algorithm here

• Use coordinate system of r, , • defined such that Lorentz Invariant - called pesudorapidity

€

=−ln tanθ

2

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⎝ ⎜

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⎝ ⎜

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• All objects with ET > threshold are seeds• Build cone of size around seeds• Add particles in cone to jet and recalculate jet axis• Iterate until find stable jet axis

€

R = η 2 + φ2

Interlude on Simulation

• First of all we simulate the way in which particles interact with each other - this is mostly independent of any experiment

• Code maintained by phenomenologists with no allegiance to any experiment - mostly FORTRAN, some C++

• Models tuned on results released by many particle experiments• Some further tuning can happen “in-house” by the experimenters• We call this the Generator Level Monte Carlo

Physics Simulation

Detector Simulation

• We use GEANT 4 toolkit - again this is independent of any experiment

• Toolkit, not program!

• We use the tools to build a simulation of the ATLAS detector so we know how it responds to different particles

• C++, in ATLAS we configure at run time with python scripts

Why We Need a Calibration• Hadronic Showers complex:- Visible electromagnetic energy (electrons, photons, 0

decays) ~50%- Visible energy from ionisation ~25%- Invisible energy from nuclear interactions (excitation,

break up) ~25%- Escaped energy (e.g. neutrinos) ~2%• Additional problem - not all visible energy can be detected.

ATLAS uses non-compensating hadronic calorimeters

• Jet response varies over the detector - e.g. in crack regions many particles cannot be detected• Charged particles with low pT bent out of cone in calorimeter

Hadronic Scale

Particle Jet Scale

Parton Scale

• Energy not included in reconstructed jet, that doescome from the hadronisation• Energy from underlying event included in reconstructed jet

Towers• Towers are fixed grid on the entire calorimeter of 0.1 x 0.1 in eta

and phi• Calorimeter is actually many detectors called cells - size of cell

depends on which part of calorimeter cell is in• Tower can have E < 0 (noise)

2 minimisation :

• Where

• Corrects to particle scale€

2 =E e − E truth

e

E truthe

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e

∑2

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E e = wiE ie

Vi

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i= cells

∑ E ie

TopoClusters• Define any CaloCells with |E| > as seed cells• Add any neighbouring cells (3D) with |E| > to seed• Repeat with new neighbours, until no neighbours pass• Noise suppression built in• Search for local maxima to decide if cluster needs splitting• Can also use H1 weights technique, but in addition Local

Hadron Calibration is studied

€

4σ noise

€

2σ noise

Local Hadron Calibration• We know how much energy an x GeV pion deposits

in each cell in the calorimeter…• …this is checked with real data in the Test Beam• Thus calibrating to the hadron scale using validated

detector simulation potentially more reliable than H1 weights

• Also allows to factorise all steps to understand the errors on every step in the calibration

TRT LAr

Tilecal

MDT-RPC BOSMDT-RPC BOS

Tilecal

LArTRT

Pixel & SCT

•Thus have corrections for: Invisible particles (neutrinos) Energy deposited in dead material Energy not clustered

In-Situ Calibration

• Pick jet in part of detector where jet is well measured

• Use di-jet events• Beam pipe goes through page!• Makes jet calibration uniform in eta

€

PT = 0∑

• Can also use balance of photon and jet - photon is wellmeasured in all eta and pt (relative to jet)• For very high pt jets probablity of event with 2 jets or event withone photon and one jet is small• Balance high pT jet against many low pT jets

Jet Performance in ATLAS

• H1 weights give best linearity and best resolution using Monte Carlo..

• Futher approach involves trying to subtract energy from charged particles from calorimeter to be replaced with measurement in tracker

• Energy Flow is work in progress (Myself, D.Tovey, R.Duxfield)

= 0 single ± TDR:Tracking: p

T/p

T 0.036%p

T1.3%

Calo: E/E 50%/E

What do we require with Jets?• We need good resolution (sigma/mean of

Gaussian distribution) and accurate energy scale (mean)

• How good depends what you wish to measure…• In fact most stringent requirement is to get the

jet energy scale at 1% level eventually• After 10 years Tevatron got to 4% level…• Estimates place ATLAS at 5% level initially after

a lot of hard work to understand the detector performance once we have collisions

Top Mass

• Invariant Mass

• Top decays so fast no time to hadronise• Also have fully hadronic and fully

leptonic events €

M = P∑( )2

− E∑( )2

• Jet Energy Scale error will be significant contribution to error on top mass at LHC• LHC produces larger top sample in 1 week than

tevatron has in 10 years

ATLASTOP 2006

Jet Resolution

• Many beyond the Standard Model particles decay to two quarks

• Can look in invariant mass spectrum for bumps above expected Standard Model prediction

• Better jet resolution = sharper bump

• 20% improvement in jet energy resolution means 40% less data to discover Higgs in specific model - we do not know which model nature chooses of course… • Obviously these numbers depend on your choice of new physics model

Conclusions• Standard Model incomplete - no particle mass

generation mechanism observed so far• LHC allows access to a brand new energy frontier in

particle physics• Has to be new physics at these energy scales• Getting ATLAS up and running is a huge task

requiring many people expert in different areas (software engineering, hardware engineering, calibration, data analysis, Grid technology, etc, etc)

• Showed example of Jets - similar chain of processes required to be able to measure properties of electrons, muons etc

• Once we are up and running will measure well understood processes

• Then we can search for the new physics

14 May 2 pm Media Room

The ATLAS Experiment

Craig Buttar

University of Glasgow