1

Gems from OPAL:Highlights from a LEP experiment

Dean KarlenCarleton UniversityOttawa, Canada

2

Outline• OPAL at LEP 1989 - 1999

– Z0 properties• the mass and widths

– b hadron lifetimes• longer lived, species specific

– W + W − production• a new twist to likelihood selections

• LEP 2000– what lies ahead in our final year

OPAL

OPAL

6

First Z0 at LEP: August 13, 1989• OPAL caught the first one at 23:16

7

November 7, 1999: √s = 204 GeV

8

Data recorded by OPAL

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 20000

50

100

150

200

250

80

100

120

140

160

180

200

220

year

Inte

grat

ed lu

min

osity

(pb−1

)

Cen

tre o

f mas

s en

ergy

(GeV

)

9

Z0 properties

precision beyond expectation

10

Situation before LEP turn-on• two PRL articles on August 14, 1989:

– best mZ at pp: 90.9 ± 0.3 ± 0.2 GeV (CDF)

• expectations for LEP experiments:δmZ, δΓZ ≈ 20 MeV (beam energy stability)δNν ≈ 0.2 (∆ε/ε ≈ 1%, ∆L/L ≈ 2%)

Mark II at SLC:mZ = 91.11 ± 0.23 GeVΓZ = 1.61 GeVNν = 3.8 ± 1.4

Mark II at SLC:mZ = 91.11 ± 0.23 GeVΓZ = 1.61 GeVNν = 3.8 ± 1.4

+ 0.60− 0.43

11

First OPAL results• 1.3 pb−1 Z0 scan Sept. - Dec. 1989• DPF’90 at Rice University (January 1990)

mZ = 91,154 ± 21 ± 30 MeVΓZ = 2536 ± 45 MeVNν = 2.73 ± 0.26

mZ = 91,154 ± 21 ± 30 MeVΓZ = 2536 ± 45 MeVNν = 2.73 ± 0.26

expectations nearlyattained after onlya few months

12

The other way to count neutrinos...• Using 5.3 pb−1 of data collected in 1990,

the cross section for was measured for the first time:

γνν→−+ee

Nν = 3.0 ± 0.4 ± 0.2

13

Precision beyond expectation• In the following years, systematic errors

were reduced well below expectations:∆L/L (systematic) 0.06 %∆ε/ε (systematic) 0.1 - 0.2 %∆Ebeam contributes less than 2 MeV to mZ, ΓZ

• LEP experiments combine measurements

– Recent combination:mZ = 91,187.2 ± 2.1 MeVΓZ = 2494.4 ± 2.4 MeVNν = 2.9835 ± 0.0083

mZ = 91,187.2 ± 2.1 MeVΓZ = 2494.4 ± 2.4 MeVNν = 2.9835 ± 0.0083

14

b hadron lifetimes

Long live the B

15

Situation before LEP turn-on• Average b hadron lifetime (RPP ‘90):

• smallest error: Mark II at PEP: 0.98 ± 0.12 ± 0.13 ps

• Expectations for LEP experiments:δτ/τ ≈ 10 % for average b hadron lifetimesδτ/τ ≈ 10 % for exclusive b hadron lifetimes

– not clear that differences between species will be measurable; b baryon thought most difficult

τB = 1.18 ± 0.11 ps

16

First results (1990 data)• average lifetime from lepton impact

parameters

pt (GeV)

impa

ct p

aram

eter

reso

lutio

n (µ

m)

(only DELPHI had silicon strip vertex detector)

OPAL used precision gas vertex detector:

τB = 1.37 ± 0.07 ± 0.07τB = 1.37 ± 0.07 ± 0.07

expectation attainedafter first full year

17

and later...• All LEP experiments added silicon strip

vertex detectors (with smaller beam pipes)– primary benefit is in efficiency

• (two track separation not absolute resolution)– especially useful for exclusive lifetimes

• Evolution of average B hadron lifetime• Big surprise: short b baryon lifetime

18

Evolution of B hadron lifetime

RPP edition1988 1990 1992 1994 1996 1998 2000

Aver

age

B h

adro

n lif

etim

e (p

s)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

τB = 1.564 ± 0.014τB = 1.564 ± 0.014

19

Measuring the b baryon lifetime• b baryons first established through excess

in right sign Λ lepton pairs• shortly afterwards, b baryon lifetime

measured by vertexing the two• still not understood why

lifetime should beso short

20

Evolution of b baryon lifetime

1.0) - 0.9 :(theory 04.074.0)B()( b ±=τΛτ +

21

W + W − production

A new twist to likelihood selections

22

Challenging event selection• All hadronic mode has large background

• use a sophisticated discriminant?

23

Growth of neural networks

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 19990

10

20

30

40

50

total neural network

24

Growth of Likelihood selections

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 19990

10

20

30

40

50

total likelihood

25

Review of Likelihood selections• Given a set of observables, x, the optimal

selection takes events having the largest value of:

• This is equivalent to taking the events with the largest (Bayesian) probability of being signal:

)background |(signal) |( xpxpR rr=

)back()back|()sig()sig|()sig()sig|()|sig(pxppxp

pxpxp rr

rr

+=

26

Review of Likelihood selections• A method using such a discriminant is

called a “likelihood selection”.• Problem: the two likelihoods,

are not usually known analytically.• Common practice: use simulated samples,

look at projections, form approximation:

– ie. ignore correlations

)background |( signal), |( xpxp rr

sig)|(sig)|(sig)|()sig|(~21 nxpxpxpxp L

r=

27

Review of Likelihood selections• Problems with this approach:

– By ignoring correlations, the selection is no longer optimal

– Evaluating systematic uncertainties (due to possible incorrect simulation of correlations) rather difficult

• suffers from the “black box” problem of neural networks

– one can adjust the input projections but there are no handles to study correlations

28

A new twist• An improved approximation that accounts

for linear correlations:– “Projection and Correlation Approximation”

• Two step approach:– transform variables so that each projection

follows a standard Gaussian– approximate the transformed pdf as a multi-

dimensional Gaussian• works well for a large class of pdfs, but not all

29

A new twist• Step 1: Transform projections to Gaussian

– why?• smoothes out edges from cuts on projections• offers a simple goodness of fit test for the overall

approximation• allows efficient generation of random numbers that

follow approximate pdfs– how?

[ ]1)(2erf2)( 1 −= − xFxy

30

A new twist• Step 2: Approximate transformed pdf as

multidimensional Gaussian:– determine covariance matrix, V

– in terms of the original projections,

)exp()2()( 1212/11 yVyVyG T rrr −−− −π=

[ ] ∏=

−− −−=n

iii

T xpyIVyVxP1

1212/1 )( )(exp)( rrr

31

An example of the PCA method• Signal

– the pdf is a ratio of polynomials

32

An example of the PCA method• background

– the pdf is a ratio of polynomials

signal

background

34

An example of the PCA method • check goodness of fit with weight

distribution in the transformed space:

35

An example of the PCA method• Problem:

– with a mix:– select a

sample enriched in signal events

– estimate total number of signal events

60% signal40% background

36

Simple 1D cut

• efficiency: 85% , purity 71%

37

Simple 2D cut

• efficiency: 91% , purity 64%

38

Likelihood cut

• efficiency: 89% , purity 79%

39

Likelihood selection:• likelihood selection improves the purity and

efficiency, but selected region is complicated– ragged edges due to limited control sample– may be more sensitive to simulation errors

than simpler cut methods

40

PCA method• Advantages of this method

– somewhat better performance– ability to study correlations– goodness of fit test for approximate pdf -

indicates presence of non-linear correlations– efficient simulation according to approximate

pdf (useful for generating ensembles of experimental data)

41

Application to W + W − selection• Original OPAL W+ W− selection (172 GeV):

– 7 variables, traditional likelihood selection– little attention paid to correlations

42

W + W − selection• New W + W − selection (183 GeV onwards)

– reduced number of likelihood variables– variables with simple correlations chosen– better performance (efficiency, purity)

• prior to likelihood, preselection is applied to remove events from the Z0/γ → qq process– invariant mass, visible energy, hard photons– removes 96% of Z0/γ but still 93% efficient

43

4 likelihood variables

44

Correlations amongst variablescheck fit for all combinations of 2D transformed projections

ρ = - 0.56,good fit (χ2 = 51/49)

lw420

sphe

lw420

ly45

ρ = - 0.07,poor fit (χ2 = 166/49)

45

Likelihood output• Good agreement

– likelihood cut chosen to maximize επ– efficiency = 86%, purity = 79% (at 189 GeV)

46

W + W − cross section

• in good agreement with SM

σ(W

+W

−)

(pb)

47

W + W − angular distribution

• in good agreement with SM

SM∆g1

Z= + 0.5∆g1

Z= − 0.5

√s = 189 GeV√s = 189 GeV

48

Triple gauge couplings• OPAL preliminary

√s = 189 GeV√s = 189 GeV95% C.L. contours95% C.L. contours

49

W mass• 1998,9 result (≥ 189 GeV) not quite ready:

– expected error: ± 50 MeV (stat)± 40 MeV (syst)

• LEP goal:– ± 30 MeV

√s = 189 GeV√s = 189 GeV

reconstructed mass (GeV)reconstructed mass (GeV)

even

ts /

GeV

even

ts /

GeV

50

Other recent results from OPAL

• ZZ production also in good agreement

σ(Z

0 Z0

) (p

b)σ

(Z0 Z

0)

(pb)

51

Z0Z0 is a background to...

• mH0 > 102 GeV(95% CL)

even

ts /

4 G

eV

reconstructed mass (GeV)

52

tail of the Z0

hadrons →−+ee

85.0/

hadrons

>′

→−+

ss

ee

53

The invisible tail...cr

oss

sect

ion

(pb)

center of mass energy (GeV)

At 189 GeV:Nν = 2.67 ± 0.13

± 0.11

At 189 GeV:Nν = 2.67 ± 0.13

± 0.11

xT > 0.0515° < θ < 165 °

54

LEP 2000

Final run is about to begin

55

constraints

95 96 97 98 99 100 101 102 103 104 1050123456789

10

Beam energy (GeV)

Max

imal

cur

rent

(mA

)

1999

2000 1999 2000

RF limit

Cryo limit

56

LEP plans for 2000• need spare RF capacity to allow recovery from

RF trips (without losing beam)• can increase energy as currents decline within a

fill (mini-ramps, detectors remain on)• one option: optimize running strategy according

to Higgs sensitivity– eg. Ebeam = 103 GeV (only 1 Klystron spare)

and Lumi = 90 pb-1, then expected Higgs limit would be 112 GeV (at 95% CL, compared to 109 GeV expected at end of 1999)

57

LEP plans for 2000• other searches (need limited luminosity at highest

energy if crossing threshold)• running lower energy (101 GeV and 200 pb-1,

giving 2 klystron margin, longer fills) would improve δmW somewhat, but Higgs limit only extended to 110.5 GeV

• mW precision can be improved by better beam energy determination: – magnetic field extrapolation (NMR): δEbeam ≈ 20 MeV– with dedicated spectrometer: δEbeam ≈ 10-15 MeV

58

LEP plans for 2000• Achievable energies still under study:

– 102-103 GeV may be possible with 2 klystrons to spare (0.7 GeV gained for every klystron)… so 104 GeV is possible (with no spare klystron)

• LEPC meeting in July - review LEP combined results to decide on possible extension of LEP run (few weeks)– decision will likely be based on only 50 pb-1 per

experiment at 102 GeV

59

Summary• LEP has been a wonderful success

– Standard model put to the test– Many important non-discoveries

• no 4th generation light neutrino• no SM Higgs boson below 102 GeV

• it has been a real privilege to be a part of the LEP experience