24/10/2006Prof. dr hab. Elżbieta Richter-Wąs Physics Program of the experiments at L arge H adron C ollider Lecture 3

24/10/2006Prof. dr hab. Elżbieta Richter-Wąs

Physics Program of the experiments at

Large HadronCollider

Lecture 3


The LHC physics programme

•Search for Standard Model Higgs boson over ~ 115 < mH < 1000 GeV.

• Search for Supersymmetry and other physics beyond the SM (q/l compositness, leptoquarks, W’/Z’, heavy q/l, unpredicted ? ….) up to masses of ~ 5 TeV

• Precise measurements :

-- W mass, WW, WWZ Triple Gauge Couplings

-- top mass, couplings and decay properties

-- Higgs mass, spin, couplings (if Higgs found)

-- B-physics: CP violation, rare decays, B0 oscillations (ATLAS, CMS, LHCb)

-- QCD jet cross-section and s

-- etc. ….

• Study of phase transition at high density from hadronic matter to plasma of deconfined quarks

and gluons. Transition plasma hadronic matter happened in universe ~ 10-5 s after Big Bang

(ALICE)


Outline of this lecture

Physics programme: precise measurement of the W and top-quark mass detector comissioning with top physics


Keyword: large event statistics

Expected event rates in ATLAS/CMS for representative known physics processes at low luminosity (L=1033 cm-2 s-1)

LHC is a B-factory, top factory, W/Z factory,.... also possible Higgs factory, SUSY factory, ….

Process Events/s Events/year Other machines

(total statistics)

W e 15 108 104 LEP / 107 Tev.

Z ee 1.5 107 107 LEP

0.8 107 104 Tevatron

105 1012 108 Belle/BaBar

QCD jets 102 109 107 TevatronpT > 200 GeV

tt

bb


Precise measurements: motivation

W mass and top mass are fundamentalparameters of the Standard Model

since GF, EM, sinW are known with high precision, precise measurements of mtop and mW constrain radiative corrections and Higgs mass (weakly because of logarithmic dependence)

radiative correctionsr ~ f (mtop

2, log mH)r 3%

rW

EM

1 sin

1

G 2

m

1/2

F

W

Fermi constant measured in muondecay

Weinberg anglemeasured at LEP/SLC

Electromagnetic constantmeasured in atomic transitions, e+e- machines, etc.


Precision of direct measurements

1983: UA1 5 GeV

1989: UA1 2.9 GeV

1990: UA2 900 MeV

1995: CDF 180 MeV

2000: DO 91 MeV

combined Run I 59 MeV

( L =120pb-1)

2004: LEP 42 MeV

Tevatron, Run II data :

CDF: 79 MeV

D0: 84 MeVpreliminary results from winter/spring conferences

integrated luminosity ~ 200 fb-1 (in June 2005 Tevatron declared collected 1fb-1)


Why we need precise measurement?

For equal contribution to the Higgs mass uncertaintityMw ~ 0.7 10-2 mt

LHC will measure top masswith 1 GeV precision to match it the 7 MeV erroron the wish list ?!

-> test for the Standard Model consistency-> prediction for the Higgs mass


Measurement of W mass

Method used at hadron colliders different from e+e- colliders

• W jet jet : cannot be extracted from QCD jet-jet production cannot be used

• W : since + X , too many undetected neutrinos cannot be used

only W e and W decays are used to measure mW at hadron colliders


W production at LHC

q’

qW e,

Ex.

e, (pp W + X) 30 nb

~ 300 106 events produced~ 60 106 events selected after analysis cuts

one year atlow L, perexperiment

~ 50 times larger statistics than at Tevatron

~ 6000 times larger statistics than WW at LEP


W boson production at LHC


W boson decay


W boson production at Tevatron


Transverse momentum of the charged lepton


Transverse momentum of the charged lepton


Transverse mass of the W boson (mT)


Transverse mass of the W boson (mT)


W mass measurement at Tevatron


Uncertainties on mW

Come mainly from capability of Monte Carlo prediction to reproduce real life, that is:• detector performance: energy resolution, energy scale, etc. • physics: pT

W, W,W, backgrounds, etc.

Dominant error (today at Tevatron, most likely also at LHC): knowledge of lepton energy scale of the detector:if measurement of lepton energy wrong by 1%, then measured mW wrong by 1%


Calibration of detector energy scale

Example : EM

CALOe- beam

E = 100 GeV Emeasured

• if Emeasured = 100.000 GeV calorimeter is perfectly calibrated• if Emeasured = 99, 101 GeV energy scale known to 1%

• to measure mW to ~ 20 MeV need to know energy scale to 0.2 ‰ , i.e.

if E electron = 100 GeV then 99.98 GeV < Emeasured < 100.02 GeV


Calibration strategy

• detectors equipped with calibration systems which inject known pulses:

• calorimeter modules calibrated with test beams of known energy set the energy scale

• inside LHC detectors: calorimeter sits behind inner detector electrons lose energy in material of inner detector need a final calibration “ in situ ” by using physics samples: e.g. Z e+ e- decays 1/sec at low L constrain mee = mZ

reconstructedknown to 10-5

from LEP

cell out

inin

check that all cells give same response: if not correct


Use Z events to calibrate recoil in W events


Expected precision on mW at LHC

Source of uncertainty mW

Statistical error << 2 MeVPhysics uncertainties ~ 15 MeV(pT

W, W, W, …)Detector performance < 10 MeV(energy resolution, lepton identification, etc,)Energy scale 15 MeVTotal(per experiment, per channel) ~ 25 MeV

Combining both channels (e) and both experiments (ATLAS, CMS), mW 15 MeV should be achieved.However: very difficult measurement


Recent (summer 2005) results from CDF


Measurement of mtop

• Top is most intriguing fermion: -- mtop 174 GeV clues about origin of particle masses ?

• Discovered in ‘94 at Tevatron precise measurements of mass, couplings, etc. just started

Top mass spectrum from CDF

tt bl bjj events

S+BS+B

BB

-- u c t d s b

m (t-b) 170 GeV radiative corrections


Why is top quark so important


Top production at LHC

e.g.g

g

t

t

t

t

q

q

(pp +X ) 800 pbtt

107 t tbar pairs produced in one year at low L

~ 102 times more than at Tevatron

measure mtop, tt, BR, Vtb, single top, rare decays (e.g. t Zc), resonances, etc.

production is the main background to new physics (SUSY, Higgs)

tt


Top decays

t W

b

BR 100% in SM

-- hadronic channel: both W jj 6 jet final states. BR 50 % but large QCD multijet background.

-- leptonic channel: both W l 2 jets + 2l + ET

miss final states. BR 10 %. Little kinematic constraints to reconstruct mass.

-- semileptonic channel: one W jj , one W l 4 jets + 1l + ET

miss final states. BR 40 %. If l = e, gold-plated channel for mass measurement at hadron colliders.

In all cases two jets are b-jets displaced vertices in the inner detector


Tagging a b-quark


Example from D0: tt Wb Wb b bjj event


Expected precision on mtop at LHC

Source of uncertainty mtop

Statistical error << 100 MeVPhysics uncertainties ~ 1.3 GeV(background, FSR, ISR, fragmentation, etc. ) Jet scale (b-jets, light-quark jets) ~ 0.8 GeV

Total ~ 1.5 GeV(per experiment, per channel)

• Uncertainty dominated by the knowledge of physics and not of detector. • By combining both experiments and all channels : mtop ~ 1 GeV at LHC

From mtop ~ 1 GeV, mW ~ 15 MeV indirect measurement mH/mH ~ 25% (today ~ 50%)


Detector commissioning with top physics

Top one of ‘easier’ bread and butter

Cross section 830±100 pb

1.Used as calibration tool

2.Rich in ‘precision and new physics’

Top mass Mt, cross section σt

What are we going to do with the first month of data?

Many detector-level checks (tracking, calorimetry etc)

Try to see large cross section known physics signals

But to ultimately get to interesting physics, also need to calibrate many higher level reconstruction concepts such as jet energy scales, b-tagging and missing energy


Learning from ttbar production

t

t

Abundant clean source of b jets2 out of 4 jets in event are b jets

O(50%) a priori purity(need to be careful with ISR

and jet reconstruction)Remaining 2 jets can be kinematically

identified (should form W mass) possibility for further purification



Abundant source of W decays into light jetsInvariant mass of jets should add up to well known W mass

Suitable for light jet energy scale calibration (target prec. 1%)

Caveat: should not use W mass in jetassignment for calibration purposeto avoid bias

If (limited) b-tagging is available,W jet assignment combinatoricsgreatly reduced

t

t



t

t

Known amount of missing energy4-momentum of single neutrino in eachevent can be constrained from eventkinematics

Inputs in calculation: m(top) from Tevatron, b-jet energy scale and lepton energy scale

Calibration of missingenergy relevant forall SUSY and mostexotics!



t

t

Two ways to reconstruct the top massInitially mostly useful in event selection,as energy scale calibrations must be understood before quality measurementcan be made

Ultimately determine m(top)from kinematic fit to complete event

Needs understanding of bias and resolutions of all quantities

Not a day 1 topic


Various scenarios under study

pp collisionsWhat variations in predictions of t-tbar – which generator to use?

Underlying event parameterization

Background estimation from MC

Aim to be as independent from MC as possible.

Detector pessimistic scenariosPartly or non-working b-tagging at startup

Dead regions in the LArg

Jet energy scale

Get good ‘feel’ for important systematic uncertainties – use data to check data

Estimate realistic potential for top physics during the first few months of running


UE / min-bias determination

LHC?

<nch> at = 0 Mtop for various UE models

Top peak for standard analysis using 2 b-tags

Difference can be as large as 5 GeV

Extremely difficult to predict the magnitude of the UE at LHC

Will have to learn much more from Tevatron before startup

Energy dependence of dN/d?

Really need data to check data on UE – Only few thousand events required


QCD Multijet Background

Not possible to realistically generate this background

Crucially depends on Atlas’ capabilities to minimize mis-identification and increase e/ separation of 10-5

This background has to be obtained from data itself

E.g. method developed by CDF during run-1:

e-,0

Use missing ET vs lepton isolation to define 4 regions:

A. Low lepton quality and small missing ET

Mostly non-W events (i.e. QCD background)

B. High lepton quality and small missing ET

Observation of reduction in QCD background by isolation cut

C. Low lepton quality and high missing ET

W enriched sample with a fraction of QCD background

D. High lepton quality and high missing ET

W enriched sampleThe QCD reduction factor B/A can be applied to the “W enriched sample “ (region C and D). The non-W candidate in D will therefore be (B/A)xC. Therefore, the fraction of non-W events in the region D will be: (B.C)/(A.D)


Top Mass: physics TDR reconstruction

Gold-plated channel : single lepton

• pT (lep) > 20 GeV

• pTmiss > 20 GeV

• ≥ 4 jets with pT > 40 GeV

• ≥ 2 b-tagged jets

• | mjjb-<mjjb>|< 20 GeV

Uncertainty on light jet scale: Hadronic 1% Mt < 0.7 GeV10% M = 3 GeV

B-tagging essential,

JES dominate

Uncertainty On b-jet scale: Hadronic

1% Mt = 0.7 GeV5% Mt = 3.5 GeV10% Mt = 7.0 GeV


Pessimistic scenario: LArg dead Regions

EM clusters

Jets

mtop(without ) – mtop(with)

ATLAS Preliminary

Argon gap (width ~ 4 mm) is split in two half gaps by the electrode

~ 33 / 1024 sectors where we may be unable to set the HV on one half gap

multiply energy by 2 to recover

Simulated 100 000 tt events (~ 1.5 days at LHC at low L)

If 33 weak HV sectors die (very pessimistic), effects on the top mass measurement, after a

crude recalibration, are:

Loss of signal: < 8 %

Displacement of the peak of the mass distribution: -0.2 GeV

(Increase in background not studied)


Pessimistic scenarios: b-tagging & JES

Algorithms benefiting from early top-sample for calibration

B-tagging

Identify jets originating from b quarks from their topology

Select a pure t-tbar sample with tight kinematical cuts

Compare 0 vs 1 vs 2 b-tagged jets in top events

Can expect the b-tagging efficiency different in data from MC

Jet energy scale calibration

Relate energy of reconstructed jet to energy of parton

Dependent of flavor of initial quark need to measure separately for b jets

Observation of hadronic W for calibrating JES

In most pessimistic scenario b-tag is absent at start

Can we observe the top without b-tagging?


Top analysis w/o b-tag

• First apply selection cuts

• Assign jets to W, top decays

1 lepton PT > 20 GeV

Missing ET > 20 GeV

4 jets(R=0.4) PT > 40 GeV

Selection efficiency = 5.3%

TOP CANDIDATE

1 Hadronic top:Three jets with highest vector-sum pT as the decay products of the top

2 W boson:Two jets in hadronic top with highest momentum in reconstructed jjj C.M. frame.

W CANDIDATE


Signal-only distributions

MW = 78.1±0.8 GeVmtop = 162.7±0.8 GeV

S/B = 1.20 S/B = 0.5

S

B

m(tophad) m(Whad)

TOP CANDIDATE

W CANDIDATE

• Clear top, W mass peaks visible• Background due to mis-assignment of jets

– Easier to get top assignment right than to get W assignment right

• Masses shifted somewhat low– Effect of (imperfect) energy calibration

Jet energy scalecalibration possible fromshift in m(W)

L=300 pb-1

(~1 week of running)


Consistency checks

m(t)Subset of events where

chosen 3-jet combination does not line up with top

quark

Empirical background shape describes combinatoric

background well under peak


Signal + Wjets background

S/B = 0.45 S/B = 0.27

S

B

m(tophad) m(Whad)

TOP CANDIDATE

W CANDIDATE • Plots now include W+jets background– Background level roughly triples– Signal still well visible– Caveat: bkg. cross section quite

uncertain

Jet energy scalecalibration possible fromshift in m(W)

L=300 pb-1




TOP CANDIDATE

W CANDIDATE

• Now also exploit correlation between m(tophad) and m(Whad)

– Show m(tophad) only for events with |m(jj)-m(W)|<10 GeV

m(tophad) m(tophad)

B

S

S/B = 0.45

S/B = 1.77

m(Whad)L=300 pb-1




TOP CANDIDATE

• Can also clean up sample by with requirement on m(jl) [semi-leptonic top]– NB: There are two m(top) solutions for

each candidate due to ambiguity in reconstruction of pZ of neutrino

• Also clean signal quite a bit– m(W) cut not applied here

m(tophad) m(tophad)

B

S

S/B = 0.45 S/B = 1.11

SEMI LEPTONIC TOP CANDIDATE

|m(jl)-mt|<30 GeV

L=300 pb-1



Exploiting ttbar for JES calibration

• Use the W mass constraint to set the JES

– Rescale jet E and angles to parton energy = Eparton / Ejet

– Takes into account variation of rescaling parameter with energy and correlation between energies and opening angle

M E E Mjj j j j j W 2 11 2 1 2( cos )

jeti

parti

iWPDG

WE

EwithMM 21

before

after

2

,,

2,1

2

2χ

EX

W

Wjj

X

iXi

EiXMm


Exploiting ttbar as b-jet sample

TOP CANDIDATE

W CANDIDATE

• Simple demonstration use of ttbar sample to provide b enriched jet sample– Cut on m(Whad) and m(tophad) masses– Look at b-jet prob for 4th jet (must be b-jet if

all assignments are correct)

W+jets (background)‘random jet’,

no b enhancement expected

Clear enhancementobserved!

ttbar (signal)‘always b jet if all jet assignment are OK’

b enrichment expected and observed


Use b-tag of 4th jet

ttbar

W+jets

Standard analysis (for comparison)

Cut on b signal probability > 0.90 on 4th jet

ttbar

W+jets

Use b-tag of 4th jet to clean up hadronic top


How well we understand W+jets bgd.?

• We know that we underestimate the level of background– Only generating W + 4 partons now, but W + 3,5

partons may also result in W + 4 jet final state due to splitting/merging

– MLM matching prescription to explicit elimination of double counting.

W l W l W l

W + 4 partons(32 pb*)

W + 3 partons (80 pb*)

W + 5 partons(15 pb*)

parton is reconstructed as 2 jets

2 parton reconstructed as single jets

* These are the cross sections with the analysis cuts on lepton and jet pT applied at the truth level


Summary on the top mass

Understand the interplay between using the top signal as tool to improve the understanding of the detector (b-tagging, jet E scale, ID, etc..) and top precision measurementsCan reconstruct top and W signal after ~ one week of data taking without using b tagging

Can progressively clean up signal with use of b-tag, ET-miss, event topology

Many useful spinoffsHadronic W sample light quark jet energy scale calibrationKinematically identified b jets useful for b-tag calibration

Continue to improve realism of study and quality of analysisImportant improvement in W+jets estimate underwayIncorporate and estimate trigger efficiency to few (%)Also continue to improve jet assignment algorithms

Expect estimate of (ttbar) with error ~20% in first running periodOne of the first physics measurements of LHC?

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24/10/2006Prof. dr hab. Elżbieta Richter-Wąs Physics Program of the experiments at L arge H adron C ollider Lecture 3