1. On the Trail of the Higgs Boson
Meenakshi Narain

2. Outline Higgs Primer How and where have we looked for the Higgs
Why do we think there is a Higgs Boson ?
How and where have we looked for the Higgs
Discussion of Higgs Search Results at LEP
How and where will we look in the future
LHC and the Tevatron
Search Strategies at the Tevatron:
Standard Model Higgs
Low and High Mass regions
Top/Higgs associated production
SUSY Higgs
Conclusions

3. The Standard Model Quarks Leptons
Interactions described by local gauge theories:
strong electroweak
SU(3)color £ SU(2)L £ U(1)Y
8 gluons W§ Z0 
Adding mass terms for W§, Z bosons, fermions breaks local gauge invariance

4. B (Hfermions) / fermion mass
The Higgs Mechanism
Breaks symmetry while maintaining local gauge invariance (renormalizability)
Add complex weak isospin doublet with “mexican hat” potential
3 components of  form longitudinal components of W§, Z (massive)
1 component  real scalar particle
Higgs boson
Couple fermion fields to   fermion mass terms
B (Hfermions) / fermion mass

5. Theoretical Limits on Higgs Mass
updated EW precision
updated direct limit
M Planck,gravity
MH too large:
Higgs self coupling
blows up at some scale 
MH too small:
for scalar field
values O() the Higgs potential
becomes unstable
If SM is valid up to  ¼ Planck Scale
130 . MH GeV
e.g. Riseelmann, hep-ph/

6. Experimental Limits on Higgs Mass
Indirect
Higher order corrections link SM parameters
e.g. MW = Mtree
Measure MW, mt (or others)  constrain MH
LEP,TeV,NuTeV,SLC
global fit: MH < % CL
(LEPEWWG Summer 2002)
Direct
LEP: e+e-ZH
MH > % CL
(LHWG Note/ ) W t b W
Higgs

7. Experimental Limits on Higgs Mass
(LEPEWW Summer 2002

8. Is there anything beyond the SM?
Problems of the SM
Many free parameters
Hierarchy: Planck scale vs ewk scale
large corrections to scalar masses (MH)
fine tuning required to keep MH light
Triviality:
self couplings of scalars blow up at high energies
Gravity not included
SM can only be the low energy limit of a more comprehensive theory

9. Supersymmetry Symmetry between fermions and bosons
Natural solution to hierarchy problem
Additional corrections to MH precisely cancel divergences
More complicated Higgs sector
¸ 2 Higgs doublets 5 physical scalar particles:
CP-even: h0, H0, CP-odd: A0, charged: H§
MSSM:
Mh GeV
SUSY with gauge coupling unification:
Mh GeV (Quiros&Espinosa hep-ph/ )

10. Can the Higgs be heavy? Global fit to electroweak data  MH<193 GeV
Assumes no physics beyond SM
If Higgs heavier, there must be new physics at some scale 
Peskin, Wells PRD 64, (2001)
e.g. topcolor-seesaw model
positive contributions to T
allows MH. 450 GeV
Chivukula, Hölbling, hep/ph
Delta T = weak isotriplett contributions to W, Z loop diagrams

11. Higgs Searches Very low mass LEP: e+e- collider, Two different Eras
LEP1: ps = Mz
LEP2: ps above Mz upto 209 GeV

12. The LEP Detectors
Being dismantled ( )

13. Higgs Production at LEP
Dominant Production Process:
Bjorken Process
``Higgsstrahlung’’
Higgs Production Cross Section
LEP1
(pb)
s = mZ
LEP2
Center of mass energy (GeV)
s  mH+mZ

14. Very little background expected
Higgs Search at LEP1
For mH<2mb:
H gg, cc, 
0.0  mH  65 GeV/c2
Excluded at 95% C.L.
Very little background expected
Events
expected
at LEP
among 2x107 Z
For mH<2mW : H bb (~80%)
For mH<2m :
H ee, 
For mH<2mW :
H bb (~80%)

15. Decay Branching Ratios Higgs Decay modes (mH=115 GeV)
Higgs Searches at LEP2
Cross Sections
 L ~200 pb-1 5s 3s
s ~ 0.04 pb
Extending the reach
to s-MZ.
Decay Branching Ratios
Higgs Decay modes (mH=115 GeV)
73.6%
7.2%
6.6%
8.1%

16. Event Signatures Defined by the Z decay mode: “4-jets” “leptons”
“missing E”
“leptons”

17. SM background processes
At LEP SM processes well understood and measured.

18. SM Four Jet Channel Topologies: Backgrounds: 4b: Hbb/Zbb and
ZZ: Dominant for mH ~ 90 GeV/c2 when Z bb
Most important for 4b channel at all masses
WW:A priori reducible (no b-quark jets, apart from Vub ).
Tedious if jets are mistagged.
Quark pair production (QCD processes):
Important near threshold production for mH>mZ
b’s and gluons back-to-back,
reconstructed mass = maximum possible
4b: Hbb/Zbb and
2b: Hbb/Zqq (where q= [u,d,s,c])
Different backgrounds and hence performance

19. Event Selection Discrimination from event shape & kinematics
Correlated quantities  Likelihood or Neural Network Analyses
B-tagging critical in enhancing signal contribution
Thrust
Emin
min (di-jet) mass
Log Y34

20. precise tracking silicon microstrip detectors
Identifying b-jets
1) Semileptonic decays of the b-quark
B(b  + X)  20%  detect  in jets
2) life time  1.5 ps  c  0.5 mm
Flight Length ~ few mm
Collision
Impact Parameter
Decay
Vertex
B Decay Products
e+e-bbqq
precise tracking silicon microstrip detectors

21. Silicon Vertex Detector
After full alignement, hit
precisions are :
~10 m in R
~15 m in Rz
in the central part of the detector.
Ex: DELPHI
~3 double-sided layers
~0.5 X0
More on this later in the week

22. Tuning b-tagging performance
Simulated IP distributions and resolutions tuned on data.
 data/simulation agree within 5%
before tuning
after tuning
R significances

23. B-tagging Performance checked on control samples:
Data and predictions agree within 5%
Excellent WW rejection reached, but control of tail is critical!

24. Mass Reconstruction Need to understand all significant distributions
Pre-selection level
ALEPH
The mass reconstruction depends heavily on good calibration of the detectors (tracking, calorimetry..) and on software techniques…

25. pairing & mass reconstruction2 4 1 3
pairing & mass reconstruction
six possible pairings:
H dijet
(1,2)
M=97
B=5.7
(1,3)
(1,4)
(2,3)
M=113
B=3.4
(2,4)
(3,4)
Z dijet
(3,4)
M=MZ
B=-0.5
(2,4)
(2,3)
(1,4)
M=MZ
B=2.0
(1,3)
(1,2)
For each pairing:
make a 5C fit with Mij =MZ
& build a likelihood including the probability that the two other jets are b-tagged coming from the Higgs decay.
A unique mass value is selected from the most likely combination
from Jesus Marco, Budapest

26. Fully Hadronic WW control sample
Reconstructing mass
Typical resolution: 3 GeV Preselection sample
ALEPH – 4C fit and
DELPHI & OPAL: 5C fit
L3: 6C fit with mH
mH=mpair1+mpair2-mZ
Fully Hadronic WW control sample
Incorrect Pairing
Correct pairing mREC~2mW-mZ
WW events selected with the Aleph (Cut) analysis where none of the jets are
b-tagged

27. Fishing out the signal:
The baits:
Ingredients for the first likelihood variable L1

28. The discriminant D = L1 x L2
Fishing Cont’d
Variables used for the second likelihood L2
The discriminant D = L1 x L2

29. One of the 3 Aleph events ZZ hyp. mZ=97 GeV mZ=94 GeV
4 b cand.
HZ hyp.
mH=114.4 GeV/c2
NN = 0.997
jet b-tag: Z H
0.999
ELEP=206.7
ZZ hyp.
mZ=97 GeV
mZ=94 GeV
A 22 GeV shower in SICAL that was giving Evis = 252 GeV
is rejected by a better algorithm : mH= > mH=114.4

30. Missing Energy Channel
Any quark WW
s=192GeV
MH (GeV)
Cross Section
Fusion fb
Cross-section ~ few fb at 115:
Not accessible with ½ fb-1 per experiment

31. The “Missing Energy Channel”
Preselect events, use Likehood or Neural Nets
Btags Emiss, Cosmiss, Acollinearity
double ISR, and bb events (when the neutrinos take most of the energy), give collinear topologies; for an event at rest, the mass recoiling to a Z is pushed to s-MZ. While, due to the Z width, even in the Higgstrahlung close to the kinematical limit, the H is not usually produced at rest.
To improve on signal mass reconstruction: kinematic fits with E,p conservation and the Z mass constraint.
Mass resolution: 7 GeV

32. L3 Hnn characteristics
Mass and neural network output both at signal peak
Used for discriminator

33. Hnn: irreducible background, ee->bb
The signal is not collinear !
for acollinearity < 5o
5% of Higgs s ~ pb
30% of qq(g) s ~ 80 pb
The collinear events are:
-Zgg double radiative events to the Z with
(visible mass ~M(Z))
-qq where the energy is lost in n or
for detector problems,
(high visible mass)
at 206 GeV 6x ee -> qq
5x ee -> bb
loose > 60 GeV in neutrinos: 
In for L= 220 pb-1, every exp.
has ~10 events ee ->bb
that loose more than 60 GeV in neutrinos
Of these 10 evts , 80% have pT>10 GeV
WPHACT
Montecarlo for Higgs 115 GeV/c2, Ecm=206 GeV

34. The L3 event

35. but BR = 3 % for each flavor...
The lepton channel
the golden candidates!
but BR = 3 % for each flavor... m g
If the g is included in the jet:
a very high di-jet mass ->
good high mass Higgs candidate !
If the g is associated
to the muon ->
a perfect ZZ
but…radiated photons
Background
llqq
L3 eeqq, Ecm=206 GeV
M(ee)= 89 GeV/c2
M(qq)= 108 GeV/c2

36. SM Higgs combined results
3 selections with increasing purity for a 115 GeV/c2 signal
Data 17
Bkg
Sig
S/N for m(H) > 109 GeV/c2
> > >2
Data 5
Bkg
Sig
Data 4
Bkg
Sig
All channels contribute
Hqq,H,Hll,qq
event selection based on a discriminant variable and mass

37. Interpreting the Results
LEP HIGGS WG
Combine all channels in a 2-D space:
reconstructed Higgs mass MHrec
discriminant variable G (b-tag, kinematical info..)
In each bin of MHrec and G:
Background (MC) bi
Signal (MC) si
Num. of candidates Ni
For each “test mass” mH
Construct a parameter Q to order experimental outcomes:
Does the experiment look signal like or background like?

38. Combined LEP Test Statistic
Likelihood:
The Final LEP Result
1, 2 bands
from background only
Data consistent with a Higgs signal of MH=115.6 GeV/c2

39. The results of each experiment

40. The results per channel

41. Confidence Level Estimation
MH=110
MH=120
obs b
MH=116
s+b
CLs+b
1-CLb
Prob of seeing the candidates (or more) if they were background:
1 -CLb = P(Q>Qobs|background)
1 -CLb x x10-7
1  

42. Compatibility with the background
1-CLb @ MH=116 GeV/c2
ALEPH x 10-3
DELPHI L
OPAL
4-jets x 10-3
l+n+t
Neutrinos
Leptons
Taus
LEP

43. Confidence Levels… Probability of missing the signal if it is there:
MH=116
1-CLb
CLs+b
s+b b
obs
Probability of missing the signal if it is there:
CLs+b = P(QQobs|signal+background)
CLs = CL s+b / CLb gives the lower bound on Higgs mass

44. The combined limit LEP 115.3 114.4 Exp Obs ALEPH 113.5 111.5
DELPHI L
OPAL
4-jets
l+n+t
LEP

45. The first 4 events maintain the highest weight in the final analyses
The events
The first 4 events maintain
the highest weight in the final analyses

46. from end of 2000 to the final results 2002…
Higgs discovery?
from end of to the final results 2002…
Jul 2001
final
Nov 2000
4.2 x 10-3
mH>113.5
(115.3 expected)
~ 0.09 mH>114.4
~0.03 mH>114.1
(115.4 expected)
Changes: % final Ebeam
final detector calibrations
Aleph 2D correlation in Hqq
Delphi Hee optimized New MC for sig and bkg
L more MC
OPAL better bgk estim New analyses, new mass rec.
Background Probabilities 1-CLb (mH=115)
Nov ® July ® final
ALEPH: ® ®
DELPHI: ® ®
L3: ® ®
OPAL: ® ®
Earlier analyses suggested hints of signal which generated much
excitement. Even now, the result is consistent with background only at 9% level.

47. improve the sensitivity and/or better control the background
The “final” results
All 4 experiments implemented various modifications in order to
improve the sensitivity and/or better control the background
Full data processing: final detector calibration, alignment, b-tagging…
New MC generators (DELPHI) , more MC statistics (all)
Precise knowledge of the LEP cm energy (all)
Upgrades for some analyses:
- New analyses with better sensitivity (OPAL):
new jet pairing (4- jet), and L -> NN(miss.ener)
- Better rejection of beam-related background (ALEPH)
- Extension of analyses down to bb threshold (DELPHI)
L3: final result already last year :
Few candidate events compatible with the Higgs hypothesis
ALEPH: Excess of events compared to what is expected from SM background, suggesting a Higgs boson with mass mH~114 GeV/c2
DELPHI: No evidence for any Higgs signal, limit set to mH> GeV/c2
OPAL: No evidence for any Higgs signal, limit set to mH> GeV/c2

48. The Far Future: LHC Large Hadron collider (p-p, ps » 14TeV)
Reach for Higgs upto 1TeV

49. Meanwhile in the Prairie
The Fermilab Tevatron Collider
(p anti-p):
Main Injector
(new)
Tevatron DØ
CDF
Chicago 
p source
Booster
Run 1: 100pb-1, 1.8TeV
(?)
Run 2: ~15fb-1, 1.96TeV
CDF DØ

50. The D experiment muon detector calorimeter tracker 5000 tons
500+ physicists
Run I:
100 pb-1
80 publications
100 Ph.D.s
beam pipe

51. D Tracker Solenoid (2 T) Fiber Tracker Silicon Tracker H-disks (4)
20cm
50cm
1.3m
Fiber Tracker
et=1.7
Silicon Tracker
H-disks (4)
Barrels (6)
F-disks (12)
800,000 channels
1400 silicon elements
area : 4.7 m2
10 m single hit resolution

52. The CDF experiment
muon detector
calorimeter
tracker

53. SM Higgs Production at Tevatron
Gluon fusion
Associated Production
s[pb] (mH=120 GeV)
typical production cross-sections
gg H WH ZH
0.7
0.17
0.1 WZ
Wbb
3.2 11 tt
tb+tq+tbq
7.5
3.4
QCD
O(106)
M. Spira, hep-ph/
WZ/ZH production is cleanest

54. SM Higgs Decays and BRs Divide into two regions Low Mass High Mass
H!bb domintaes
gg!H precluded by QCD background
High Mass
Gauge Boson decays dominate
H!WW becomes promising
Less sensitivity in cross over region _
M. Spira, hep-ph/

55.    Low Mass Higgs Search 1 2 3
Higgs couples most strongly to massive particles:
Focus on associated production (WH/ZH)
Best Prospects: leptonic W/Z decays
QCD background large for hadronic channels
SM Background processes:
Sensitivity will depend on
b-jet tagging
dijet mass resolution   1  2 3

56. SM Higgs: Leptonic Channel (1)
Typical Selection:
Main backgrounds:
Event selection optimized to maximize S/B

57. Expected Events and Sensitivity
Sensitivity crucially depends on dijet mass resolutions

58. Sensitivity crucially depends on dijet mass resolutions
CDF RunI
“Calibration”
for Higgs Search
Problem is not intrinsic jet resolution
In 2 jet WH events, mass resolution ~ 10%
(but, costs 30-70% in efficiency)
With 2 jet requirement relaxed,
mass resolution is about 15%

59. Multivariate Analysis Techniques
Further Improvements from use of Neural Networks, Grid Search, Likelihood methods.
Significant gains, compare S/B with and without neural nets

60. SM Higgs: Leptonic Channel (2)
Main backgrounds:
Event selection optimized to maximize S/B
Typical Selection:

61. Some distributions:

62. Use Neural Networks to optimize analysis:
use different networks
one for signal
4 different ones for bkg

63. SM Higgs: Leptonic Channel (3)
Small rate but good S/B
Main backgrounds:
Typical Selection:

64. Neural Network Analysis:
signal
Backgrounds
(4 different networks)
Kinematic fit may enhance sensitivity
Add Taus?

65. Low Mass Higgs Search It’s going to be challenging…
A 120 GeV Higgs signal
Total Background

66. High Mass Higgs Search Around MH=130 GeV, H0! bb takes a plunge
For MH>130 GeV, H0! WW* rises
Exploit the large gg! H0 cross section
Identify topologies with small SM cross sections
Focus on leptonic Gauge Boson Decays
Stange, Marciano, Willenbrock
(PRD49, 1994);
Gunion & Han (PRD51, 1995);
Mrenna & Kane (hep-ph/ )

67. High Mass Higgs Search (1)
Dileptons, Missing Energy signature
Leading contribution from gluon fusion
SM backgrounds:
Bkg¼ 25pb
S/B ¼ 4x10-4

68. Continuum vs. at threshold production via spin 0 resonance
Typical Selection:
Exploits difference in kinematics for WW prod
Continuum vs. at threshold production via spin 0 resonance
Utilize angular correlations between leptons & ET

69. Angular correlations between leptons:
Due to spin correlation of Higgs Boson decay products, the two charged leptons tend to move in parallel

70. Add likelihood analysis based on variables
Define “Cluster Mass” MC to sharpen mass
Likelihood analysis give a factor of 40 reduction for dominant WW background. (MH=170 GeV)
S/pB for 30fb-1 » 3.8
Likelihood analysis
Kinematic cuts
Bkg: WW

71. High Mass Higgs Search (2)
Distinct Signature:
Same sign dileptons + jets
Contributions from process:
Backgrounds to consider:
Di-boson (WZ, ZZ), Tri-Boson, top pair production…

72. Selection criterion: Dominant bkg: WZ, ZZ Good S/B, but low rates
(WZ)=0.15fb
W/Zjjj = 0.15fb (fake j! e)
 (top, di/tri-boson) = 0.18fb
Good S/B, but low rates
Limits significance
Control of systematics challenging
Fine tuning of cuts

73. High Mass Higgs Search Results for MH > 130 GeV:
Require large luminosity » 30fb-1

74. Combined Significance
Combine all SM channels
Use Neural Network Analyses for low mass
Assume 10% resolution for mass reconstruction
Systematic error: Min of 10% or 1/p(sLdt £ B)
Band) 30% effect from mbb, b, bkgs

75. Goldstein et. al., hep-ph/0006311
Striking topologies:
Small  £ B
key: Multijet reconstruction
Critical: b-jet tagging
New NLO corrections k=0.8
Backgrounds:
Goldstein et. al., hep-ph/

76. Low Mass High Mass Lepton+6 jets Reconstruct top For 3 or more jets:
require least 3 or 4 b-tagged jets
Reconstruct top
“Know” jet assignments
For 3 or more jets:
For 1fb-1:
S=0.4, B=0.3 and S/pB=0.7
For 15 fb-1 data
Expect 5 or 6 signal events
2.8(4.1) for 1(2) experiment(s)
Assumes 70% b-tagging efficiency.
High Mass
Both lepton+jets and dilepton channels
Like sign dileptons: 3-6 signal events, less than 1 bkg
Trilepton: signal events, less than 1 bkg

77. SUSY Higgs Search Neutral Higgs
Translate SM results into SUSY (tan, MA) parameter space according to SUSY couplings
Vh, VH can be Standard Model-like
reinterpret the SM analysis
In addition: enhanced at large tan
mixing angle between CP even h and H

78. MSSM Higgs Search
Exploit enhanced Yukawa coupling due to large bb cross section at the Tevtraon:
Event selection:
Trigger on 4 jets
At least 3 jets b-tagged
Mass dependent jet ET cuts
e.g. leading jet pT> M/2 -5 GeV
Reconstruct mass with b-tagged jets pairs
Backgrounds:
Limited by trigger efficiency, hard to trigger on jets
Impact parameter triggers will help
Require excellent b-tagging efficiency
Adding ! will help

79. Sensitivity: tan vs MA plane
With 2 fb-1 data, sensitivity for MA » 125 GeV
(for interesting region for tan» mt/mb» 35)
Relatively modest integrated luminosity required !
Exclusion
Discovery

80. MSSM Higgs: V Channel Model Independent interpretation: Exclusion
Apply to any “New Physics” model
Compare the above R (for experiments) to theoretical expectations R for “SUSY”, assuming some typical values of SUSY parameters (tan, mixing angle , etc.)
Exclusion
Discovery

81. Sensitivity for MSSM Higgs
LEP
V, 5fb-1
Exclusion
V and bb:
Consider different scenarios
Minimal Mixing
At each point Left-Right Stop mixing= 0
Maximal Left-Right Stop Mixing
Large A and  ) Suppression of H(bb)
5 discovery
Maximal Mixing
Minimal Mixing
30fb-1
With 15 to 20 fb-1,
can cover most of the
MSSM parameter space.
10fb-1
20fb-1
10fb-1
15fb-1

82. References
The Tevatron studies described were performed by the RunII SUSY/Higgs working group at the Tevatron. The details of the study can be found in hep-ph/
A comprehensive note – with multitude of theoretical and experimental references.
ttH production at the Tevatron: hep-ph/
Latest LEP results:
For Higgs from LHWG Note/
For Electroweak Global Fits: LEP EWWG/
LHC:
ATLAS and CMS TDR

83. Conclusion Does the Higgs Boson Exist? LEP… a “scent” of the Higgs?
…lots of theoretical motivation and
…mounting indirect evidence
LEP… a “scent” of the Higgs?
Tevatron Run II:
no single discovery channel, combine all modes
If NO SM Higgs: exclude upto 190 GeV with 10fb-1 data
If SM Higgs exists: a 3 to 5 discovery upto 190 GeV with 30fb-1 data
Can rule out 115 GeV Higgs with 2fb-1 data per experiment
Verification at 3 will require 5-6 fb-1.
Other models – beyond the SM, .eg. MSSM, can cover a large parameter space with 15 to 20 fb-1 data
LHC: In the far future, potential to cover the entire range
Tractricious, designed by Wilson and constructed by members of the Technical Support Section, sits in front of the Industrial Complex. The structure is comprised of 16 stainless steel outer tubes, made from scrap cryostat tubes from Tevatron magnets, and 16 inner pipes from old well casings. Each tube is free standing and designed to withstand winds up to 80 mph.

84. Onward to uncharted Territories….

86. Cross Sections

87. Mass Resolutions: cont’d
Signal significance depends on bb mass resolution
For RunII aim for 10% mass resolution
30% better than in the previous Run
WHlnbb
CDF RunI
“Calibration” for Higgs Search

88. Mass Resolutions: cont’d
Compare Run I Jet ET resolution to Fast MC
Optimize b-jet reconstruction and corrections
corrections (partly for b’s):
b/light-q jet calibration
Improvement due to increased +jets statistics
Significant sample of Z bb
Correct for  in bl
Correct for  in jets
Can get 12% at M=120 GeV
If only 12% mass resolution
Required luminosity increases by 20%
WHlnbb

89. Mass Resolution Issues
Problem is not intrinsic jet resolution
In 2 jet WH events, Mjj is close to gaussian
Mass resolution is about 10% (but, costs 30-70% in efficiency)
With 2 jet requirement relaxed,
Mass resolution is about 15%
3rd jet must be judiciously used!

90. More Improvements – b-tagging
b-jet tagging: Will it be good enough?
Displaced Vertices
3-D vs 2D vertexing possible
Improved impact parameter resolution
(Extrapolation from CDF Run I eff.)
Semileptonic tags do
primary
vtx
secondary
Lxy
secondary vtx  2 tracks
tagged if Lxy/s Lxy >3.0
e or m in jet b

91. can we improve? b-tagging For bb backgrounds:
Relative Luminosity goes as
Eff increase from 60%  65% would result in the same signal significance for 20% less integrated luminosity.
LEP2, S.Jin
PHENO2000

92. Low Mass Higgs Searches
Channels:

93. WH: Leptonic Channels
Distributions

94. bb mass reconstruction
the extracted signal significance depends on input dijet mass resolution
optimized b-jet reconstruction+corrections
WHlnbb
E. Barberis
improvement from use of tracking and preshower
in jet reconstruction? (also, different algorithms?)
corrections (partly specific to b’s):
- corrections for n into jets (bln)
- corrections for m into jets
- b/light-q jet calibration
- b/light-q parton corrections
and...
-effect of extra interactions
on jet reconstruction

95. b-tagging fakes: displaced vertices: RunI SVX algorithms on RunII
detector (3D Si, large h)
~55-60% do
primary
vtx
secondary
Lxy
secondary vtx  2 tracks
tagged if Lxy/s Lxy >3.0
M.Roco
M.Roco
+ soft lepton tagging (~10%)
e or m in jet b
DØ used only m‘s in top analyses

96. Collisions and particle identification
Hadron collisions: a proton (anti-proton) beam is a broad-
band beam of partons
hard scattering p _
- total energy: unknown
- total longitudinal momentum: unknown
- total transverse momentum: zero
heavy quarks and leptons decay via weak interactions to
lighter counterparts:

97. Particle identification
an energetic quark or gluon appears in a detector as a jet
of colorless hadrons:
the total transverse energy of invisible particles (e.g. n) can
be inferred from the energy of the visible particles:

98. A generic detector neutrinos detector cross section around
the collision point 

99. Beyond the Standard Model
in the window mH~ GeV: SM valid up to mPlank
quadratic divergencies in scalar masses
but:
10-38 !
needs adjustment to the 38 decimal place: violates concept of naturalness
SM valid to EW scale
EWSB
Fundamental th. to higher scales
Higgs sector:
SM, further problems, ad hoc
couplings, mixings..
Extended, MSSM?
Composite, TC?

100. Higgs in MSSM
the simplest minimal supersymmetric standard model (MSSM)
supersymmetric ) SM
recipe ) an extra Higgs doublet
3) supersymmetric partners
CP odd
5 physical Higgs bosons
CP even SM
mh<130 GeV
theory limit
on lightest Higgs
SUSY
fermion  boson symmetry
for every SM spin degree of freedom there is
a supersymmetric spin degree of freedom

101. LHC Higgs Search
SM discovery modes:
Higgs Mass Reach

102. LHC: Higgs Coupling Measure production rate and couplings
Large top-Higgs coupling  cross section for ttH associated production is sizeable
Also an additional discovery mode and test of SM couplings
With 300fb-1: g yt H

103. Neutrinos Neutrinos do not interact with the detector.
Momentum conservation tells us:
But don’t measure pz. Therefore
invisible

104. Identifying Muons

105. b-tagging Exploit long lifetime of the b-jet: b» 1.6ps
HZ Candidate, ALEPH Experiment
e+e-bbbb
s = 206.7
Mrec=1103 GeV/c2
Three well b-tagged jets with two reconstructed displaced vertices

106. Collinearity of Hnn We cannot add criteria in the light of the data.
Otherwise classical statistical analysis is impossible.
Four fermion backgrounds acollinear
Two fermion collinear
Total well modelled
L3 candidate

107. Identifying particles
Dijet event
W! e