1. Summary of Higgs session: For the Higgs Working group
TeV4LHC Workshop Fermilab, 22 October 2005
Summary of Higgs session:
Experimental part
Ia Iashvili
SUNY at Buffalo
For the Higgs Working group
Outline
Tevatron Higgs searches
Experience gained
What can Tevatron achieve before LHC
Example of Tevatron data being used for testing LHC predictions
Some LHC Higgs studies
Summary and outlook
Only selected topics. Contributions at earlier TeV4LHC meetings won’t be covered. These will be documented in the workshop write-up.

2. Introduction s(pp H + X) [pb] √s = 2 TeV MH [GeV] H bb H  WW(*)
ggH HW HZ
Hqq
Htt
Hbb
s(pp H + X) [pb]
√s = 2 TeV
MH [GeV]
H bb
H  WW(*)
Dominant decay modes
Goal:
Achieve above
Help to achieve this (similar sensitivity in Atlas)

3. WHlbb searches at Tevatron
WH/ZH, Hbb is best for light Higgs search
At MH=115GeV
ZHnnbb … 15 events/fb-1
ZHllbb … 2 events/fb-1
WHlnbb … 14 events/fb–1
WHlbb: high pT isolated lepton, missing ET and two b-jets
Background
non-W QCD
false isolated leptons or
false missing energy
top quark production
physics background
mistags in W events
false b-tags with
lepton + MET
W + heavy flavor
 Should be estimated entirely from data
 Strong motivation for measuring top production cross section precisely!
 Mistagging rate estimated in data
 Flavor composition from MC (Alpgen) and cross checked in data
Each background estimate is a miniature analysis unto itself.
Techniques can be spun off to measure other physics processes.

4. WHlbb searches at Tevatron
Checking event kinematics
Checking event counts
B-tagging performance
Check efficiency in data
events vs. efficiency in
simulation:
need scale factor of 0.91±0.06

5. WHlbb searches at Tevatron

6. ZHbb searches at Tevatron
Missing ET
b-jet
120o y x
Signal has a distinctive topology
Large missing transverse energy
two high pT b-jets
No isolated leptons
Jets are acoplanar
But diffical to trigger – no high-pT lepton in the event
trigger on missing ET and jets
Suffers from large background
“physics” backgrounds W+jets, Z+jets, top, ZZ, and WZ
 Can be estimated from MC
“instrumental” backgrounds QCD multijet events with mismeasurement of jets  Estimated from data
Need to find smart variable to separate signal from background

7. ZHbb searches at Tevatron
Control Region 2 – EWK
Require min. 1 lepton
Missing ET and 2nd leading jets are not parallel
Optimized cuts are tested in this region before looking at the real data in the Signal Region
Extended Signal Region (no optimization)
Veto events with leptons
Missing ET and 2nd leading jet are not parallel
Cut optimization is performed in this region based on MC simulation before looking at the data
Control Region 1 – QCD h.f.
Veto events with identified leptons
Missing ET and 2nd leading jet are parallel
For the 120 GeV Higgs mass in ±20 GeV mass window around the expected reconstructed peak value (Dijet mass resolution is ~17 %):
SM background prediction: 4.36  1.02 events
QCD (11.4%), Top (20.5%), EWK (18.2%), Light flavor mistag(50%)
Observed: 6 events.

8. ZHbb searches at Tevatron
Instrumental bkgd
from sidebands
sideband
signal
sideband
Asym = (ET-HT)/(ET+HT)
Rtrk = |PTtrk-PT,2trk|/PTtrk
Physics bkgd
from MC
Mass
Window
105GeV
[70,120]
115GeV
[80,130]
125GeV
[90,140]
135GeV
[100,150]
Data 4 3 2
Acc (%)
0.29  0.07
0.33  0.08
0.35  0.09
0.34  0.09
Total BKG
2.75  0.88
2.19  0.72
1.93  0.66
1.71  0.57
Wjj/Wbb 32%, Zjj/Zbb 31%, Instr. 16%,
Top 15%, WZ/ZZ 6%

9. HW+W-ll searches at Tevatron
Two high pT isolated lepton and large missing ET and no hard jets
Clean and easily triggerable signature
Sensitive to tau channels with leptonic decay
One of the most promissing channels at LHC as well
Backgrounds:
WW, WZ, ZZ, DY, ttbar estimated from PYTHIA and normalized NLO cross section calculations.
W+jets(jete/m) estimated (at least partially) from data
multijets events estimated from data
Efficiencies:
Lepton triggering, reconstruction and identification efficiencies all have to be determined in data and factorized in MC for signal rate estimation
Precise estimation is importrant since no “bump” can be observed from Higgs
WW production is the dominant background compulsory to first measure the WW production cross-section
(W+W-)= (stat)+1.2 (sys)±0.9(lumi) pb
[PRL 94, (2005)] (DØ)
(W+W-)= (stat)+1.8 (sys)±0.9(lumi) pb
[PRL 94, (2005)] (CDF)
Consistent with thepry prediction of pb (J.Ohnemus, J.M. Campbell, R.K.Ellis)

10. HW+W-ll searches at Tevatron
Exploit spin correlations W- W+ e+ e- 
Leptons tend to be parallel small (ℓ, ℓ)
Neutrinos go parallel -- typically larger missing energy than WW
Small di-lepton invariant mass

11. Tevatron SM Higgs Sensitivity: expectations two years ago
Prospects updated in 2003 in the low Higgs mass region
W(Z) H ln(nn,ll) bb
 better detector understanding
 optimization of analysis
Sensitivity in the mass region above LEP limit (114.4 GeV ) starts at ~2 fb-1
With 8 fb-1: exclusion GeV & GeV,
sigma 115 – 130 GeV
Meanwhile
 understanding detectors better, optimizing analysis techniques  measuring SM backgrounds (Zb, WW, Wbb)
 Placing first Higgs limits which can be compared to the prospects

12. Sensitivity with existing Tevatron analyses
Cross-Section times branching fraction limit
as a multiple of the SM rate
CDF DØ
Work in progress
Work in progress
the “kink” at around
140 GeV goes away
We should be around 6 at low masses, not around with the current lumi (0.3 fb-1).
Where can we gain ?

13. So how do we get there? DØ Step 1 (for early 2006) WH/ZH:
Optimize b-tagging (Looser)
Combine single and double tag
S/sqrt(B) is 40-50% in single tag compared to double tag. Equivalent to 20% more lumi than double tag alone.
WH(e): include Phi-cracks
WH(): combine single- and +jets trigger
ZH : optimize Selection
Step 2
WH/ZH:
include WHWWW and Z l+l- channel ! (*1.3)
use Neural Net Tagger (*1.34*1.34)
use Neural Net Selection (*1.8)
use TrackCalJets mass resolution (*1.3)
WH(e): include End-Cap calorimeter
WH(): improve QCD rejection  loosen b-tag
WH : include W   (*1.4)
All these improvements will bring us to the expected level of sensitivity

14. So how do we get there? Start with existing channels, add in ideas
CDF
Improvement
WHlbb
ZHbb
ZHllbb
Mass resolution
1.7
Continuous b-tag (NN)
1.5
Forward b-tag
1.1
Forward leptons
1.3
1.0
1.6
Track-only leptons
1.4
NN Selection
1.75
WH signal in ZH
2.7
Product of above
8.9
13.3
7.2
CDF+DØ combination
2.0
All combined
17.8
26.6
14.4
Start with existing channels, add in ideas
with latest knowledge
of how well they work.
Expect a factor of ~10 luminosity improvement per
channel, and a factor of 2 from CDF+DØ Combination

15. Expected Signal Significance CDF+DØ vs Luminosity
Work in progress
Work in progress
per experiment
per experiment
mH=115 GeV assumed

16. Improving Jet Energy Resolution Using Tracks
Idea
Reconstruct calorimeter-based jets (0.5 cone)
Use track momentum measurements to set an accurate scale for hadron response for each hadron in the jet.
Proposed in CMS
CMS Note 2004/015, O.Kodolova et al. “Jet energy correction with charged particle tracks in CMS”
Tevatron provides an excellent opportunity to test and optimize this technique on real collider data.
Propagate tracks to the calorimeter surface.
dca(xy) < 0.5cm, dca(z) < 1.0 cm.
Classify tracks:
DR(vtx)<0.5, DR(cal)<0.5 : IN jet
DR(vtx)<0.5, DR(cal)>0.5 : Out-of-cone
For each IN-jet track: Etrkjet=Ecaljet +(1-F)Etrk
For each Out-of-cone track: Etrkjet=Ecaljet +Etrk
where F is a single pion calorimeter response
Out-Jet
In-Jet

17. Improving Jet Energy Resolution Using Tracks
12% improvement
in mass resolution.
12%  20% by optimization of the TrackCalJet algorithm
Performance in Zbbbar MC events
10-20% jet resolution improvement
in data at 40 GeV.
Higher improvement at lower pT.
In CMS MC studies: ~40% improvement
at the same energies
Performance in +jet data events

18. QCD Higher Order Corrections in H + 1jet at the LHC
Low mass Higgs searches with H in association with high PT jets are crucial at the LHC
NLO QCD corrections for VBF signal and Z+jets in H+2jet analysis have been considered in the past
QCD Higher order corrections have not been evaluated within the H+1jet analysis neither for signal nor for the Z+jets background
NLO corrections are evaluated here with MCFM
Also address the impact of Z+2-3jet tree level ME on Z+jets using ALPGEN/SHERPA
QCD HO corrections are large in the
region of the phase space where the signal-to-background is optimal for searches
QCD Z+1j is enhanced by a factor of 2
Signal, H+1j is enhanced by a factor 1.75
Need to re-optimize the analysis
Signal significance does not decrease
Tag jet
Not Tagged
Large PTH & MHJ
Tag jet

19. Summary and outlook
Much has been learned from Tevatron Higgs searches on various challenges and issues faced at hadron collider environment
We have also learned how important current experience is
to actually achieve expected performance
Tevatron will provide important information on Higgs sector before LHC. For low mass Higgs searched Tevatron is complementary to LHC.
We are able to test LHC predictions using Tevatron data and provide important feadback
The plan is to document our experience and findings in the Workshop proceeding (beginning of next year)