santiago ian hinchliffe 05/29/07 1 physics with atlas. ian hinchliffe part 3: standard model issues:...

Santiago Ian Hinchliffe 05/29/07 1

Physics with ATLAS

.

Ian Hinchliffe

Part 3: Standard model issues: confirming our expectations:on to new physics


Jet calibration: back to the quarks/gluons?

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Phase 3: absolute energy measurement of parton energy. Goal precision 1%....

• correct for energy losses out of jet clustering

• correct for energy physics effect such as: underlying event, ISR, FSR

•This is not very well defined and ultimately rather arbitrary

•Needed, for example, for W to q qbar to jets


Jet calibration: How to use the data

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Need a sample of events with a well defined mass/

Events from ttbar with 1 lepton used to trigger. Other W jj : Use known W mass. Maximum Energy 200 GeV, jets overlap as energy of W increases. Does not work very well for K_t

Events with Z(e+e-,+-) + jet: pT balance or Etmiss projection method. Calibrates jets relative to electrons or muons. Very clean smamples. Useable for light and b jets, about 5% of the total event rate. pT range ~ 40-400 GeV.

+j : pT balance or Etmiss projection method. Higher statistics but high QCD background. More on next slides…

You must use a theory (actually a Monte Carlo simulation) to get back to quarks

Using Erec/Eparton correction.


Jet calibrations

.

particleTjet

partonT

recT

particleTjetrec

TT E

E

E

EEE 1/Kptcl

Cone 0.5 < 1.5 Cone 0.7 < 1.5

•Differences quark-gluon jets calibration are 5% level for ET=40 GeV

• What is the right cone to use? q-jets for cone 0.7 negligible corrections

Get this from monte carlo


Jet calibration

.

Parton levelParticle level KtReconstruction level Kt

Parton levelParticle level Cone 0.7Reconstruction level Cone 0.7

Biases on pT balance MOP for the different jet algorithms:

In order to understand differences among cone0.7 cone 0.4 and kT, the underlying event subtraction algorithm and the introduced biases are under study using transverse interaction region.

pT

ba

lan

ce

pT

ba

lan

ce

(pTγ+pTparton)/2 (GeV)

Too close to the generation cut

-1 - 0%-1 - 0%-1 - 0%Parton level

10 - 2%-15 - -7%-2 - 0%Recon level

7 - 1%-7 - -3%1 - 0%Particle level

KtCone 0.4Cone 0.7Algorithms Selection gives <1% bias

(pTγ+pTparton)/2 (GeV)


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Rest of the event: Models and underlying Events

The beam remnants are important. These must be measured.

↓d ↑probability of hard-scattering

d

4/

2

2int

s

t

t

dpdp

dn ~ σint

↓pt0 ↑n pt0

PHOJETPYTHIA

Multiple Pomeron exchanges

…more models and new ideas available: e.g. JIMMY etc


.

Predictions for LHC for Underlying Events

Agreement with CDF data, but

different predictions in region

transverse to the leading jet !

Tra

nsvers

e <

Nch

g >

PYTHIA6.214 - tuned

PHOJET1.12

Pt (leading jet in GeV)

x 3

x1.5

Tevatron (CDF data)

LHC

Moraes, Buttar, Dawson

(see also work of R. Field)

After comprehensive study and tuning:

Must measure this with early data

Low pt jets are harder to find if there this underlying event is large


Pileup

.

Other energy can come from “underlying event” or energy from other events

in same bunch crossing: “Pile up”

At LHC expect dNCh

/d~ 6.5 for min bias: 23 interactions per bunch crossing at full

luminosity

Haze of additional particles at low PT

Makes pattern recognition difficult

Degrades calorimeter resolution

Probability of a second hard scatter event very small, so all pile up events are same

type

Can significantly effect measurements where we sum over a large number of

detector cells (eg Total Energy in Calorimeter): Jets affected

Reduce sensitivity by requiring a minimum energy per cell

Forward jets

Higgs Decay


QCD and Higgs

.

Two high PT jets with large d separation

Strong discovery potential for low Higgs

mass

Jet

Jet

This is one of the Higgs productionmechanisms.

Look for forward tagging jets

How easy this is depends on the soft QCD

Will rethink strategy after early QCD data


QCD: jets

.

Must measure this across entire kinematic range

Cannot accept all data

Must define a trigger strategy

Trigger on a localized clusterof transverse energy

30 inverse fb


QCD: jets

.

1400 GeV

200300 GeV

3000170 GeV

12,000130 GeV

72,00090 GeV

360,00065 GeV

1,440,00050 GeV

5,760,00035 GeV

17,280,00025 GeV

PrescaleThreshold

Jets must be prescaled to get reasonable total rate

Prescales set so each prescaled jet threshold will record approximately the same number of jets.

This gives 10 hz at 10**33


QCD: jet measurements

.

L = 30 fb-1

Main systematic errors:

calorimeter response (jet energy

scale),

jet trigger efficiency,

luminosity (dominant uncertainty ~5% )

the underlying event.

-test of pQCD in an energy regime never probed!

- first measurement to validate our understanding of pQCD at high momentum

transfers: Do not expect any surprises here!

- αS(MZ) measurement with 10% accuracy


QCD: multijets

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1110 GeV

50080 GeV

150065 GeV

750050 GeV

75,00035 GeV

1,125,000

25 GeV

PrescaleThreshold(4 jets above)

More complex jet states are more difficult to predict

4 jet rates are enormous

2.5 Hz at 1033 for pt>1110 Ge

Also need prescaling triggers to measure over full kinematic range


QCD: multijets

.

Multi-jet/particle production is important for several physics studies:

- tt production with hadronic final state

- Higgs production in association with tt and bb

- Some SUSY has multijet final state

LHC: large centre-of-mass energy → large phase space, many particles in the final state

→ Need to go beyond PYTHIA/HERWIG showering approximation

Many important progress over past few years:

N-parton/particle ME event generators:

automatic LO-ME generation up to 2 → 6 processes + phase space integration, PS interface

e.g SHERPA, MADEVENT etc.

NLO parton level generators:

2 →2: for most processes, see e.g. MCFM, MNR, NLOJET++

2 →3: first processes available:

…ideas for automated calculations upcoming !

MC@NLO: full event generator using NLO ME/PS/hadronisation model

Hard emissions are treated with NLO ME while soft/collinear emissions by PS

HQ, Higgs, Drell-Yan, W/Z-pairs …extendable

NNLO:

first results start being available for

tot and rapidity distributions of Higgs and Drell-Yan

jetttppH,tt pp jets, pp 2jets, W/Z pp jets,3 pp


QCD: “predictions”

.

Example:Richardson 2003

Herwig

Full ME + PS

0 jets component1 jet component

2 jet component

3 jet component

4 jets component

TeV 14s XWpp

Are the HERWIG/PYTHIA

estimates for finding SUSY

In multi-jet channels ok ?

Look at events with W and jets

Predictions for 4th highest pt jet


Fakes and triggers

.

We have seen that sometimes an electron or tau and a jet are similar

In an analysis there are many ways to distinguish and much time to do it

At fixed pt the there are many more jets than electrons and photons

In trigger this is much harder. Trigger must be

FAST: time scale is set by 25 ns bunch crossing

EFFICIENT; want to keep all real electrons

PROVIDE LARGE REJECTION: output rate is fixed, do not want to retain events that

are useless in analyis

Electrons: Use shower shape in calorimeter: both longitudinal and transverse

Photons, similar to electrons with no track, and symmetric shape in etaxphi


Back to leptons

.

Electrons and muons are straightforward

What about tau?

New physics may favor third generation

tau expected to couple strongly to Higgs

Observing tau

Can only see decay products: neutrino lost, tau energy cannot be measured

Tau decay to electron or muon not very useful

More energy lost (two neutrinos) How do you know e or mu came from tau

Must use hadronic decays


Tau properties

.

Unlike e and , decay to hadrons

Look like narrow jets in calorimeter

1 or 3 charged tracks

May have EM energy (0)


W/Z to tau

.

Low luminosity runs (L=0.5 1030 - 1033 cm-2s-1) trigger early on large samples of SM candles :

Z→ℓℓ W→ℓν

ℓ = e/μ but also τ (hadronic τ + ETmiss trigger )

• Energy and momentum scale calibration from Z→ℓℓ (ℓ = e/μ )

• ETmiss calibration from W→ℓν

• Understand W+jets and Z+jets:– important background for tt and

searches

• Measure the W→τν cross section:– validation of τ-id needed for

searches

Signal evidence through Ntrack spectrum


Taus

.

20 < Pt < 3040 < Pt < 5070 < Pt < 130Tau-jetsQCD-jets

R<0.4

R<0.1

~90% of energy are depositedin R<0.1. -> narrow jet

ET(R<0.1)ET(R<0.4)

These distributions depend on luminosity due to the pile-up.

ATLAS, preliminary


Taus

.


Top quarks

. tt final states (LHC,10 fb-1)

• Full hadronic (3.7 M) : 6 jets

• Semileptonic (2.5 M) : ℓ + + 4jets

• Dileptonic (0.4 M) : 2ℓ + 2 + 2jets

Weqq

Golden channel

(early physics, precision meas.)

10%

90% σtt(LHC) ~ 830 ± 100 pb

millions of top quarks produced at LHC

Cross section LHC = 100 x TevatronBackground LHC = 10 x Tevatron


Top quarks

.

Event selection:

• 4 jets pT> 40 GeV

• Isolated lepton : pT> 20 GeV

• missing ET > 20 GeV

Event toplogy:3 jets with highest ∑ pT

Mjjj (GeV)

L=100 pb-1

(1 day @ 1033 cm-2s-1)

Signal (MC@NLO)

W+n jets (Alpgen) + combinatorial

Full simulationEve

nts

…without b-tag – b-tag might not be good on day 1

• Feedback on detector performance (JES, b-tagging, …) and on MC description• Top events will be used to calibrate the calorimeter jet scale (W→jj from t→bW)• With 30pb-1 data, δmtop ~ 3.2 GeV (sys. Error dominated: FSR, b-jet scale)


Top quarks

Selection:

• 1 Lepton

• missing ET

• 4 (high-PT)-jets (2 b-jets) signal efficiency few % very small SM background

S/B=O(100)

Top signal

W+jets background

Top mass (GeV)

Nu

mb

er

of

Even

ts• ‘Standard’ Top physics at the LHC: - b-tag is important in selection; - most measurements limited by systematic uncertainties

• ‘Early’ top physics at the LHC: - cross-section measurement (~ 20%) - decay properties

…with b-tag

Can use this to get clean sample of b-jets to calibrateb-tagging efficiency


Now for something new

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Easy new physics : resonances

.

c=1

c=0.5c=0.1

c=0.05

c=0.01

Drell-Yan production of a 1.5 TeV Gn and its

subsequent tower states

pp Gn ll

• Di-Lepton search:• Generally good channel

for searching new physics with low luminosity data!

• Resonance could be– New gauge boson

– Graviton resonance

• Few 100pb-1 enough to discover for mZ’~1 TeV

• Limits rapidly exceed Tevatron


Why Supersymmetry

.

Required by gravity (string theory)

Can “explain” why Higgs is so light, if there are new SUSY particles below 1 TeV

SUSY models naturally predict a stable, neutral weakly interacting particle

Properties are just right for this to be dark matter! Best motivated “beyond standard model” theory?

Focus Point

Coannihilation

Bulk


.

Very many particles

SUSY Particle summary


.

Example Mass spectrum


.

Production rates at LHC

LHC produces mostly the heavier states

Then we get a cascade


.

Production rates at LHC Squark and gluino rates given by QCD: only mass counts Slepton and gaugino given by EW: some dependence on

couplings

Very large rates. Dominated by squarks and gluinos unless they are very

heavy Everything is produced at one, so we need a model!


Supersymmetry

.

• Strongly interacting sparticles (squarks, gluinos) dominate production• ~100 events per day (for squark/gluino masses of ~1TeV)• Discovery possible with only 1 fb-1

• Heavier than sleptons, gauginos etc. → cascade decays to LSP.• Long decay chains and large mass differences between SUSY states

– Many high pT objects observed (leptons, jets, b-jets).

• If R-Parity conserved LSP (lightest neutralino in mSUGRA) stable and sparticles pair produced.– Large ET

miss signature (c.f. W→ℓ).

• Closest equivalent SM signature t→Wb. • Biggest physics background is neutrino emission (eg Z→)

lqql

g~ q~ l~

~

~p p


Supersymmetry

.

Some previous predictions made with jets from parton e.g. boson production from parton shower only

Or boson + 1 jet in M.E. Cover high kT region of

phase space badly Need high kT jets for

SUSY analysis Use newer M.E. Monte

Carlos

Look at hardest jets/leptons

Meff=pTi| + ET

miss

Distribution peaked at ~ twice SUSY mass scale for signal events.

ATLAS 10 fb-1


Supersymmetry: inclusive searches


Supersymmetry: inclusive searches

Adding leptons improves S/B

Note scale: event rates are huge


Supersymmetry: inclusive searches Map of discovery potential corresponding to a 5σ excess above background in the mSUGRA parameter space

Best channel: jets + ETmiss channel

(no lepton requirement)

Many complementary channelsCan reach 2 TeV masses

A factor two increase or decrease in total background cross-section results in small effect on overall discovery potential (few tens of GeV).

Similar reach in gluino and squarks mass should apply to any model in which they decay into an invisible and relatively light LSP.

If R parity is violated the presence of additional leptons make the discovery easier!

ll01

~For example:

lqq01

~


SUSY mass measurements

20.6 fb-1

MC Truth, lRMC Truth, lLMC Reconstructed

ATLAS Preliminary

Full sim.

01,

02

~ llll LR 264 154 , 255 137

~ ~

GeV


Supersymmetry masses

lqql

g~ q~ l~

~

~p p

.

llq edge1% error(100 fb-1)

lq edge1% error(100 fb1)

ll edge llq threshold

By finding the jets and leptons in this chain we can constrain all the masses


Microscopic Black hole

• Development of a Monte Carlo generator: CHARYBDIS (richardson et al)

- evaporation and time evolution

- “grey body” factors (transmission of particles through curved space-time outside horizon)

- Planck phase: few hard jets

• Simulation in ATLAS: This is a geometric cross section. Huge rate

- cut on the event shape (sphericity)

- mBH reconstructed for each event

- #XD deduced from TH, mBH and MP (Hawking formula)

For n+3 dim., ( )

13 1

28 ( )1

2

n nBH

PS

P

n MM nM

R

V pbTe 2 (1 )~ 0~ 0P SM R O ~1Hz

LHC is a Black Hole (BH) factory


Spectacular signature

Micro black hole decaying via

Hawking radiation Photons + Jets + …

We will certainly know something is happening

Large multiplicities Large ET

Large missing ET

Highly spherical compared to BGs

Theory uncertainty limits interpretation

Geometrical information difficult to disentangle

No perturbative physics

Micro black hole decaying via Hawking radiation

Photons + Jets + … We will certainly know something

is happening Large multiplicities Large ET

Large missing ET

Highly spherical compared to BGs

Theory uncertainty limits interpretation

Geometrical information difficult to disentangle

No perturbative physics


Some conclusions and cautions

New physics for realists

If Tevatron did not see it and it's inside their mass reach, apply strong health warning

If it looks “totally crazy”, it probably is Beware of “counting experiments” until SM is calibrated

or S/B is huge Beware 4peaks in expected places (150 GeV Higgs?) Beware “old men in a hurry”


Some conclusions and cautions

An abstract that I would like to see in 2009

The CMATLAS experiment operating at LHC has observed anexcess of 9 dimuon and 11 dielectron events in events selected to have4 jets with pt>50 GeV. The invariant mass of the lepton pairs is below 109 GeV and has no peak. These events are inconsistent with the standard model expectation of 2 events. They are consistent with the cascade decay of a two or more new particles. This signal could due for example, to SUSY or UED.


Perhaps we might find the Grail(Higgs)?

Come back for the 2010/2011 school when one of you will be showing real LHC data

But we hope for something more revolutionarBut we hope for something more revolutionary

santiago ian hinchliffe 05/29/07 1 physics with atlas. ian hinchliffe part 3: standard model issues:...

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