The Measurement of the W The Measurement of the W massmassat LEPat LEP

XXXIX Recontres de Moriond,

April 2004

Ann Moutoussi, CERN

OutlineOutline

Introduction: the Standard Model and MW

Measurement of MW: Direct reconstruction

Systematic errors

QCD related errors

Results and conclusions

Mw within the Standard ModelMw within the Standard Model

Mw can be computed

at Born level from , Mz , GF

Higher order radiative corrections involve Mt, MH:

e.g

O(, sMz , GF , Mtop, Mhiggs)

Precision measurements of Mw check the prediction

If consistent > SM still OK,

Use measurements to predict Mhiggs

If not consistent > Hints for New Physics?

tW W

HW W

The LEP goal for MThe LEP goal for MWW

PP Colliders 80.454 ±0.059GeV/c2

EW fits (LEP/SLD) 80.373±0.033GeV/c2

EW Fits (LEP/SLD)

with Mtop 80.378±0.023GeV/c2

LEP Goal:precision of ~40 MeV …Very difficult task..

First phase optimise statistical power of analysis

Last years fight known and new systematics!

No update since last winter

ee++ee- - WW++WW--

W decay modes: Leptonic: W l(32%) Hadronic: W qq

(67%)

q

q

q

q

q q

Semileptonic (qqSemileptonic (qqll)) Hadronic (4q) Hadronic (4q) LeptonicLeptonic

44% 46%Low Mw sensitivity

~40K events in total

MW measurementMW measurement

Identify and best reconstruct leptons (e,m,tau) Best Cluster jets and measure energy and direction

Event-by-event reconstruction of the invariant masses of W decay products

Statistical sensitivity limited by resolution of jet/lepton energies and momenta

Can improve resolutions using the knowledge of ECM and

Energy-Momentum conservation

optionally: equal W mass constraint

WW

4

1

4

1

,0 ,

mm

psEi

ii

i

Mreco

Mw Reconstruction(1) qqqqMw Reconstruction(1) qqqq

Minus :− Particle to Jet association

mixing between jets from different Ws smearing of Mw distribution

Statistics: Optimise clustering algorithms

− Jet-Jet association to a W Wrong pairLoss of all Mw information

Statistics: Optimise pairing algorithms(~85% correct pairing)

Plus:

No unmeasured particles,

Fully constrained system

Minus :− Neutrino

3 3 unknowns only 2 constrains fit

Plus:

Only two jets

no loss of information due to particle mixing or combinatorial bkg

Considered golden channel

Mw Reconstruction(2) qq Mw Reconstruction(2) qq l

Reconstructed MwReconstructed Mw

Mreco still far from underlying Mw distribution

After the kinematic Fit: Mreco

True Mw

Mjet

Mreco

qq l

W Mass extractionW Mass extraction

Assume MC events are identical to data, except from Mw!

Discrepancies between data and MC are sources of systematic errors

In practice, only one MC sample is generated, at a reference value

MWref. Predictions at other values of MW are obtained by re-weighting

the events

To relate Mreco to Mw use Monte Carlo events•Fit Mreco with analytical function(eg BW) and then correct it using MC

or

•Compare Mreco distribution to MC predictions at different Mw values

Systematics

Systematics(largest)Systematics(largest)

Source Currently/MeV LEP Energy determination 17

Detector Simulation Jet & Leptons energy/direction 15

QCD simulation

Jet Fragmentation 18

Jet-Jet interactions(4q) 93

(Expected final statistical error for LEP 25 MeV)

Unacceptable!!

parton shower (large Q2, pQCD)

Fragmentation (quarks hadrons):

Simulation of a MC event(1)Simulation of a MC event(1)

hadronisation (phenomenological)

e+

e-

W+

W-

q

_qq

_q

Available models: Jetset, Herwig, Ariadne.

All models: need to be tuned to data (generally Z qq, LEP1). Simulate Data ~as well/bad!

Jetset globaly better

used as Reference MC from all LEP experiments

parton shower (large Q2, pQCD)

Fragmentation (quarks hadrons):

Simulation of a MC event(2)Simulation of a MC event(2)

Hard process: e+e-4q

e+

e-

W+

W-

q

_qq

_q

Interconnection effects

Bose-Einstein correlations: momenta of identical bosons tend to be correlated.

d~0.1 fm

Colour reconnection: hadronic interaction between W decays

•d(W+,W-) < 1 fm

hadronisation (phenomenological)

Not included in reference MC

Bose-Einstein Correlations Bose-Einstein Correlations (BEC)(BEC)

Intra-W :BEI not relevant for Mreco

W1

W2

Bettwen-W’s:BEB: could cause wrong particle-dijet association

Mw shifts ~ 35 MeV(LUBOEI)

Main Observable: distance in momentum space

between pairs of charged pions: Q2=(pi-pj)2

Any evidence for such effects?

Look for BE in data

Observation BEC in WObservation BEC in W++WW-- eventsevents

Inter W, BEI confirmed

Between W’s, BEB, disfavoured

MW down from ~35 to ~15 MeV

Final

BEBBEI

eg

CR modelsCR models

Based on the JETSET string model: SK1:

• it has a free parameter I controlling

the reconnection probability P

Based on Ariadne, AR2: Based on HERWIG (Herwig-CR)

P=1 MW ~400MeV

P=0.5 MW ~115MeV

P=0.3 MW ~ 50MeV

MW ~ 40MeV

MW ~ 70MeV

MW Far too large!

Any evidence for such effects/models?

Look for CR effects in data

The particle flow analysisThe particle flow analysis Most CR models predict a

modified particle flow in W+W- events:

CR:

No CR:

W-

W+

W-

W+

The ratio of particle flow between the inter and intra-W regions is built:

(A + B) / (C + D)

A

B

C

D

•Data

-SK1(extreme parameter)-Jetset

Measurement sensitive only to extreme scenarios, i.e SK1 with high CR probability and not so to Herwig, Ariadne

LEP results from particle flowLEP results from particle flow

preferred value: kI=1.18,

P~0.5 kI value excluded at 1

value used for CR studies and MW evaluation= ~100 MeV!

Do something to make analyses more robust!

Fit LEP measurement

for free parameter k (CR P)

(CR P)

Towards a less CR sensitive analysis:

The logic The logic Interconnection effects mainly occur in the

inter-W region and between soft particles

Proposed solution: modify clustering algorithm to dismiss information from those particles. “purer” information

loss of statistical precision

Many variations of jet algorithms (cones, pcuts) have been considered

aiming for the best combination of

Robustness against reconnection effects with minimal information loss

Reduction of Reduction of MMW W

MW (MeV)

Model Standard R=0.5rad

SK1, kI~2 ~115 ~50

Herwig ~40 ~15

Good reduction factors

for all available models!

e.g for R=0.5, 2.3-2.6 smaller MMWW

with ~25% increase of stat. error:

Algorithms simple and intuitive

measurement less sensitive to CR

independent of specific model implementation

A by-product: Measure CR?A by-product: Measure CR?

The difference between MW measured with cone/pcut

and standard analyses (MC-S) is sensitive to CR effects:

DELPHI, Cone algorithm R=0.5

e.g

DELPHI preliminary:

Exclude extreme scenarios.

Minimum at ~1.3, P~0.5

Results

ResultsResults

80.411±0.032(stat) ±0.030(syst)GeV/c2

80.420±0.035(stat) ±0.101(syst)GeV/c2

(Weight of qqqq in combination: 0.09%)

qq qq l

qqqqqqqq

Mw=80.412±0.042 GeV/c2LEP Combination

Mw GeV

±

±

±

±

±

±

80.426±0.034

80.378±0.023

Mw and Mtop, MHiggsMw and Mtop, MHiggs

mH

MwMtop GeV

Mw wants a low Higgs Mass...

After all this work….After all this work….

Ongoing LEP efforts to find optimal jet clustering and make qqqq measurement robust against CR

If all experiments use them Total error in hadronic channel: ~110 ~60 MeV. Total error from ~42 to ~39 MeV Weight of hadronic channel in combination: 0.09% 0.29%.*Learn something about Final State Interactions too...*

Detector Systematics still an issue after all these years..

Final values for Summer?!?!

ResultsResults

Mw=80.420±0.035(stat) ±0.101(syst)GeV/c2

Mw=80.411±0.032(stat) ±0.030(syst)GeV/c2

Combined: Mw=80.412±0.042 GeV/c2

qq qq l qqqq qqqq

Weight of qqqq in combination: 0.05%

Detector SimulationDetector Simulation

As measurement is calibrated using MC Systematic errors related to the detector arise from discrepancies in the detector simulation.

Most effort devoted to Jet Energy, Mass and Direction:

Jet Energy (mass, multiplicity,etc) calibrated, checked

and MC tuned using Z qq events:Clean enviroment, Ebeam~Ejet, Jets back to back well separated

e.g Compare Ejet/Ebeam

as a function of polar angle , for Data and MC (ratio) for total energy, Ejet

And for individual types of particles (Echarged, Ephotons, etc)

Jet Energy Simulation Jet Energy Simulation

(Ejet/Ebeam)Data/MC

cos

2000 publication Preliminary results Towards final results

Better: simulation of Calorimeter endcaps, photon energy calibration,

treatment of small calorimeter measurements ,etc etc

Small changes on Mw ~ size of calorimeter systematic

(Ejet/Ebeam)Data/MC

cos

Data-MC=

-0.024±0.007

rad

Data

MC

qq qq e

Jet Direction simulation Jet Direction simulation Test done with W events:

Compare Data and MC = neutral-Chargerd , being the dijet angle Jet1-Jet2

Charged1

•Collecting the full statistics allowed

relevant sensitivity

•qq qq e bad surprise

Data different from MC

by 24mrad

neutChar

Jet1

Jet2Charged2

The electron channel: qq The electron channel: qq e

Angle to lepton/degrees

# Data

MC

Particles associated to a jet

qq qq e

What could make neutral dijet Q be more open in Data than in MC?

Look near the electron…..

•EM shower of v.energetic electrons

not well simulated.

•Existing algorithm to collect electrons cloud not adequate.

New electron reconstruction

Mw from qq qq e moved by ~100 MeV…

WW production at LEPWW production at LEP

1. Theoretical precision ~0.5% Thanks to 2000 calculations RACOONWW, YFSWW with improved O(a)

corrections

2. LEP measurement precision ~1%

Very good agreement

Example: Jet Mass and Baryon Example: Jet Mass and Baryon ## Jet Mass

Jetset Herwig Data

Identical W 2q events, Hadronised with Jetset/Herwig

Study Mjet1, Mjet12, Mreco Vs (No of neutrons)

Jet Mass enters into Dijet-Mass (Mjet12) and also

shows some discrepancy between Data and MC

0 2 4 6 8 10

Mjet(12)/GeV

neutron

8

4

0

0 2 4 6 8 10

Effect much smaller

but ~20MeV

neutron

Mreco/GeV

0 2 4 6 8 10

8

4

0

Mjet1/GeV

neutron

5

2

0

LEP EnergyLEP Energy

At LEP2:Error mainly from extrapolation.

Ebeam~20MeV (E/E~10-4!)

mW~17MeVmW~17MeV

Ebeam measured from total bending fieldtotal bending field

Calibrated with resonant depolarizationresonant depolarization:spin precession freq Ebeam

intrinsic resolution ~ 200keV !!

only works up to 60GeV extrapolation

Kinematic fit the absolute energy/momentum scale is calibrated by the LEP beam energy measurement

…and will stay ~there

ALEPH: Energy resolutionALEPH: Energy resolution Energy resolution for a

calorimeter object adding ECAL + HCAL is:

E/GeV0.6E)(

E/GeV2.1)E( = 6 GeV

Peak=90.5GeV

Total Visible Energy (GeV)

@ 91GeV ecm Z qq

Take into account particle ID to:•use momentum measurement of

tracks pointing to calorimeter objects

•avoid double counting of energy.

•apply specific calibrations.

build new objects with:

ALEPH: Jet DirectionALEPH: Jet Direction

Jet and resolution

=18mrad

=19mrad

Jet direction information is based on tracks,

addition of neutral objects improves resolution by 15%

Jet Direction simulation(1)Jet Direction simulation(1)

Still at the Z pole:

Difficult, as no refference (like Ebeam)

Tests rely on “correct” position of tracks and check calorimeter objects

by comparing neutral toChargerd as a function of jet

neut

Charged

Z axis

cos

mra

d

Charged -neutral ) Data/MC

No significant effect,

small systematic error

But..

The electron channel: qq The electron channel: qq e

Particles near an electron

Angle to lepton/degrees

ee++ee- - ee++ee--#

What is all this stuff there? Look near the electron…..at bhabha events..

QCD models at LEPQCD models at LEP

Model Parton Shower Hadronisation

JETSET abcString

ARIADNE CDM

HERWIG abc Cluster

All models: need to be tuned to data

(generally Z qq, LEP1). Simulate Data ~as well/bad!

Available Models:

Jetset somewhat better

used as Reference MC from all LEP experiments

Specific systematics for Specific systematics for cones?cones? Cone and standard analysis can have different

sensitivity to fragmentation:cone could be more sensitive to angular distribution of

particles inside jet

Use Z qq events from LEP1.

Data and MC Energy and multiplicity distributions were compared as a function of angle to jet axis

No indications of new sources of systematics

Data/JETSETHERWIG/JETSET

HERWIG

DataJETSET

Angular distributionsAngular distributions

Jet Energy: Velocity:

HERWIG

DataJETSET

Data/JETSETHERWIG/JETSET

Inter-jet angle in WInter-jet angle in W++WW-- events events

M212 ~ 2E1E2(1 - cos )

Z qq events too different semileptonic W+W- events used. independent sample free from CR effects

DataJETSET

Variable checked:

SC

No indications of new sources of systematics

S - C

For Data and Jetset

Conclusions(2)Conclusions(2)

Statistical errors exceeded all expectations

(analyses really pushed to the limit!)

Systematic errors dominantA lot of effort invested to fight against the larger known

(eg Colour Recconection) lead to more understanding of the causes and the design of promisingly more robust analyses

Detector Systematics. The precision required from Mw exceeds this of all previous analyses. Jets and the simulation (especially of neutral part) cannot rely on LEP1, more detail needed (10MeV!)

Effort put on guessing those unexpected systematics!

FragmentationFragmentation

“Traditionally”: Compare different models: (various X)pass them through full analysis : Max Mw ~20 MeV (Jetset-Herwig)

Latest work:

But.. Mw is due to X between Data & reference MC(Jetset)

1. Identify fragmentation variable, X, with significant dMw/dx

2. Estimate X(Data-MC) at some control sample, eg Z events

3. Propagate dX(Data-MC) in refference MC Mass

distribution Mw

Method for MW

measurement

Introduction

The Standard Model and

Mw

QCD effects on MW

FragmentationFragmentation If all particles are detected and associated to Ws perfectly,

discrepancies in fragmentation do not bias MW measurement.

Biases come from interplays:

E, p spectra

Baryon rates (e.g

n,p)

Thresholds

charged ->

mneutrals ->

m

Angular size of jetsAcceptance

Jet algorithms

Discrepancies

MC-reality on fragmentation x

Detector

fD (X)

Reconstructio

n fA (XE)

e.g

W W event selectionW W event selection Semileptonic channel (qql

2 jets 1 isolated lepton, 1 neutrino: missing E&P

Efficiency ~70% Purity ~90-95% main bkg We, qq(

Hadronic channel (qqqq) ( 4 jets

• large multiplicity• spherical topology

low missing E&P Efficiency ~80% Purity ~85% main bkg qq(

Statistics: Use multivariable analyses (e.g neural networks, even for qql events!)