BE, CR and cone jet finders

1
The Measurement of the W massat LEP
XXXIX Recontres de Moriond, April 2004 Ann
Moutoussi, CERN
2
Outline

• Introduction the Standard Model and MW
• Measurement of MW
• Direct reconstruction
• Systematic errors
• QCD related errors
• Results and conclusions

3
Mw within the Standard Model

• Mw can be computed
• at Born level from a, Mz , GF
• Higher order radiative corrections involve Mt,
  MH
• 
  e.g
• O(a, as ,Mz , GF , Mtop, Mhiggs)

Precision measurements of Mw check the
prediction If consistent gt SM still OK,
Use measurements to
predict Mhiggs If not consistent gt Hints for New
Physics?
4
The LEP goal for MW
PP Colliders 80.454 0.059GeV/c2
EW fits (LEP/SLD) 80.3730.033GeV/c2
EW Fits (LEP/SLD) with Mtop
80.3780.023GeV/c2
LEP Goalprecision of 40 MeV Very difficult
task..

• First phase optimise statistical power of
  analysis
• Last years fight known and new systematics!
  

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ee- WW-
40K events in total
W decay modes Leptonic W ? ln (32) Hadronic
W ? qq (67)
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MW measurement
Event-by-event reconstruction of the invariant
masses of W decay products

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

• 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

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Mw Reconstruction(1) qqqq
Plus No unmeasured particles, Fully constrained
system
8
Mw Reconstruction(2) qq ln

• Plus
• Only two jets
• no loss of information due to particle mixing or
  combinatorial bkg
• Considered golden channel

• Minus 
• Neutrino
• 3 unknowns
• only 2 constrains fit

9
Reconstructed Mw
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W Mass extraction

• 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

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
? Assume MC events are identical to data, except
from Mw! ? Discrepancies between data and MC are
sources of systematic errors
11
Systematics
12
Systematics(largest)
(Expected final statistical error for LEP ? 25
MeV)

• Source Currently/MeV
• LEP Energy determination 17
  
• Detector Simulation
• Jet Leptons energy/direction 15
• QCD simulation
• Jet Fragmentation 18
• Jet-Jet interactions(4q) 93
  

Unacceptable!!
13
Simulation of a MC event(1)
Fragmentation (quarks ? hadrons)

• parton shower (large Q2, pQCD)

• hadronisation (phenomenological)

• 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
14
Simulation of a MC event(2)

• Fragmentation (quarks ? hadrons)

• Interconnection effects

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

Not included in reference MC
15
Bose-Einstein Correlations (BEC)

• Intra-W BEI not relevant for Mreco

• Bettwen-WsBEB 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
16
Observation BEC in WW- events

• Inter W, BEI confirmed
• Between Ws, BEB, disfavoured

17
CR models

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

DMW 70MeV
DMW 40MeV

• Based on the JETSET string model SK1
• it has a free parameter kI controlling the
  reconnection probability P

P1 DMW 400MeV P0.5 DMW
115MeV P0.3 DMW 50MeV
DMW Far too large! Any evidence for such
effects/models? Look for CR effects in data
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The particle flow analysis

• Most CR models predict a modified particle flow
  in WW- events

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

19
LEP results from particle flow
Fit LEP measurement for free parameter k (CR P)
20
Towards a less CR sensitive analysis
21
The logic

• Interconnection effects mainly occur in the
  inter-W region and between soft particles

Many variations of jet algorithms (cones, pcuts)
have been considered aiming for the best
combination of
Robustness against reconnection effects with
minimal information loss
22
Reduction of DMW
Good reduction factors for all available models!
e.g for R0.5, 2.3-2.6 smaller DMW with 25
increase of stat. error
DMW (MeV) DMW (MeV)
Model Standard R0.5rad
SK1, kI2 115 50
Herwig 40 15
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A by-product Measure CR?

• The difference between MW measured with cone/pcut
  and standard analyses (DMC-S) is sensitive to CR
  effects

24
Results
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Results
qq ln
80.4110.032(stat) 0.030(syst)GeV/c2
qqqq
80.4200.035(stat) 0.101(syst)GeV/c2
(Weight of qqqq in combination 0.09)
26
Mw and Mtop, MHiggs
Mw wants a low Higgs Mass...
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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?!?!

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Results
qq ln
qqqq
Mw80.4200.035(stat)
0.101(syst)GeV/c2
Mw80.4110.032(stat)
0.030(syst)GeV/c2
Combined Mw80.4120.042 GeV/c2
Weight of qqqq in combination 0.05 ?
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Detector 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, EbeamEjet,
• Jets back to back ? well separated

e.g Compare Ejet/Ebeam as a function of polar
angle q , for Data and MC (ratio) for total
energy, Ejet And for individual types of
particles (Echarged, Ephotons, etc)
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Jet Energy Simulation
2000 publication Preliminary results
Towards final results
Better simulation of Calorimeter endcaps,
photon energy calibration, treatment of small Q
calorimeter measurements ,etc etc
Small changes on Mw size of calorimeter
systematic
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Jet Direction simulation
Test done with W events Compare Data and MC DQ
qneutral- qChargerd , q being the dijet angle
Jet1-Jet2
Charged1

• Collecting the full statistics allowed
• relevant sensitivity
• qq en ? bad surprise
• Data different from MC
• by 24mrad

32
The electron channel qq en
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 en moved by 100 MeV
33
WW production at LEP

• Theoretical precision 0.5
• Thanks to 2000 calculations
• RACOONWW, YFSWW with improved O(a)
  corrections
• LEP measurement precision 1

Very good agreement
34
Example Jet Mass and Baryon
Jet Mass enters into Dijet-Mass (Mjet12) and also
shows some discrepancy between Data and MC
Identical W ? 2q events, Hadronised with
Jetset/Herwig Study DMjet1, DMjet12, DMreco Vs D
(No of neutrons)
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LEP Energy
Kinematic fit ? the absolute energy/momentum
scale is calibrated by the LEP beam energy
measurement

• Ebeam measured from total bending field
• Calibrated with resonant depolarization
• spin precession freq ? Ebeam
• intrinsic resolution 200keV !!
• only works up to 60GeV ? extrapolation

• At LEP2
• Error mainly from extrapolation.
• ?Ebeam20MeV (?E/E10-4!) ?

?mW17MeV
and will stay there
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ALEPH Energy resolution

• Energy resolution for a calorimeter object
  adding ECAL HCAL is

• 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

Total Visible Energy (GeV)
37
ALEPH Jet Direction
Jet direction information is based on tracks,
addition of neutral objects improves resolution
by 15
Jet Dq and Df resolution
38
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 qneutral to
qChargerd as a function of qjet
No significant effect, small systematic
error But..
39
The electron channel qq en
What is all this stuff there? Look near the
electron..at bhabha events..
Particles near an electron

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QCD models at LEP
Available Models
Model Parton Shower Hadronisation
JETSET a?bc String
ARIADNE CDM String
HERWIG a?bc Cluster

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

Jetset somewhat better used as Reference MC
from all LEP experiments
41
Specific systematics for cones?

• Cone and standard analysis can have different
  sensitivity to fragmentation
• cone could be more sensitive to angular
  distribution of particles inside jet

No indications of new sources of
systematics
42
Angular distributions

• Jet Energy

• Velocity

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Inter-jet angle in WW- events

• M212 2E1E2(1 - cos q12)

• Z ? qq events too different ? semileptonic WW-
  events used.
• independent sample
• free from CR effects

• Variable checked

For Data and Jetset
No indications of new sources of
systematics
44
Conclusions(2)

• Statistical errors exceeded all expectations
• (analyses really pushed to the limit!)
• Systematic errors dominant
• A 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!

45
Fragmentation

• Traditionally
• Compare different models (various dX)
• pass them through full analysis Max DMw 20 MeV
  (Jetset-Herwig)

But.. DMw is due to dX between Data reference
MC(Jetset)
Latest work

1. Identify fragmentation variable, X, with
   significant dMw/dx
2. Estimate dX(Data-MC) at some control sample, eg
   Z events
3. Propagate dX(Data-MC) in refference MC Mass
   distribution ? DMw

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Method for MW measurement
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Introduction The Standard Model and Mw
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QCD effects on MW
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Fragmentation

• If all particles are detected and associated to
  Ws perfectly, discrepancies in fragmentation do
  not bias MW measurement.
• Biases come from interplays

Discrepancies MC-reality on fragmentation dx Detector fD (X) Reconstruction fA (XE)
E, p spectra Baryon rates (e.g n,p) Thresholds charged -gt mp neutrals -gt mg
Angular size of jets Acceptance
Angular size of jets Jet algorithms
e.g
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W W event selection

• Semileptonic channel (qqln) (44 )
• 2 jets
• 1 isolated lepton, 1 neutrino missing EP
• Efficiency 70
• Purity 90-95
• main bkg Wen,
  qq(g)
• Hadronic channel (qqqq) (36 )
• 4 jets
• large multiplicity
• spherical topology
• low missing EP
• Efficiency 80
• Purity 85
• main bkg qq(g), ZZ

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