1. Low mass SM Higgs boson Introduction
J.-F. Grivaz
LAL-Orsay
Ph.D students
J.-F. Grivaz
D0-France October 2008

2. High mass getting “under control”
Why “low mass” ?
LEP direct searches:
M_H >114 GeV
Precision EW measurements: M_H < 154 GeV at 95% C.L (K. Mönig)
High mass getting “under control”
(G. Bernardi)
Preferred by SUSY
(M. Carena)
J.-F. Grivaz
D0-France October 2008

3. Profile of a low mass (mH < 135 GeV) Higgs boson
0.2
0.1 pb
Excluded
at LEP
Hbb
Dominant
(~73% at 115 GeV)
To disentangle Higgs final states from the QCD
background, one needs leptons and/or missing ET
in the production or in the decay.
HWW*
Increasingly useful with
increasing mass
For Hbb, this means
no gluon-gluon fusion
Higgstrahlung with W/Z leptonic decays
H
may add some sensitivity
(not yet fully exploited)
J.-F. Grivaz
D0-France October 2008

4. Putting it all together:
Three main channels:
WHebb 14.1 fb
WHbb “”
ZH bb 15.6 fb
In spite of its lower cross section:
ZH (ee/)bb 22.6 fb
is also powerful because
of its very distinct signature
The French groups have launched a three-year coordinated effort
(the ANR “HiggsTeV” project) on all these channels:
WH at LPNHE and IPHC
ZH in bb at LAL and CPPM
ZH in llbb at CPPM
and also on common tools:
triggering, b-tagging, jet energy resolution, advanced analysis techniques
(cf. A. Duperrin, S. Greder)
J.-F. Grivaz
D0-France October 2008

5. On the way to the Higgs: Done (F. Déliot) times the branching30
Done
(F. Déliot)
0.1
times the
branching
fractions
Similar final state
Lower cross section, but Mbbconstraint + more channels + 2 expts
J.-F. Grivaz
D0-France October 2008

6. The main backgrounds: lepton misidentification, fake missing ET
QCD multijets (specifically with b-jets: times the signal)
lepton misidentification, fake missing ET
estimated from data at DØ, analysis dependent
W/Z + jets
the main background before b-tagging
generic studies in the “V+jets” group at DØ
W/Z + bb
Non-peaking irreducible background (103 times the signal)
t-tbar, single top
Also a source of W bosons and of b-jets
Dibosons, specifically (W/Z)(Zbb)
Neighboring peaking background
J.-F. Grivaz
D0-France October 2008

7. V+jets studies Selections: The main goal is to validate the simulation
Well understood triggers
single EM, single muon, diEM, di-muon
Clean, high pT, isolated leptons
Jets within a well understood acceptance
Energy scale corrected
For the simulation:
Trigger turn-ons
Lepton identification scale factors
Energy scale and resolution adjustments for jets
The main goal is to validate the simulation
J.-F. Grivaz
D0-France October 2008

8. Generators for V+jets:
Mostly ALPGEN with MLM matching
interfaced with PYTHIA for parton showering and hadronization
Alternative generators for comparisons:
PYTHIA
ALPGEN interfaced with HERWIG
SHERPA with CKKW matching
ALPGEN generations:
(W/Z)+(0n)-lp
(all exclusive except n-lp inclusive)
Heavy flavors are generated separately:
(W/Z)cc and (W/Z)bb + (0n)-lp
HF appropriately removed from W/Z + n-lp
Real zero bias events overlaid according to the data luminosity profile
J.-F. Grivaz
D0-France October 2008

9. “A priori” corrections to ALPGEN:
The inclusive W and Z production cross sections are normalized to theory at NNLO
The HF contributions are further increased based on NLO K-factors from MCFM
The Z pT distribution is reweighted to match the DØ measurement (unfolded)
Good agreement
between data
and RESBOS
at low Z pT
“Rescaled” NNLO
OK at high pT
ALPGEN reweighted
to data at high Z pT
ALPGEN reweighted
to RESBOS at low Z pT
J.-F. Grivaz
D0-France October 2008

10. 1st and 2nd jet pT well modeled
Basic checks in Z events
After Z pT reweighting:
No additional scale factor
needed in the inclusive sample
Jet multiplicity improved
No additional scale factor needed for  2 jets ( 1.2 before reweighting)
Z pT  OK
(muon channel)
1st and 2nd jet pT well modeled
(muon channel)
C. Ochando
J.-F. Grivaz
D0-France October 2008

11. Hopefully improved once W pT reweighting is available
Underway: W pT reweighting
No measurement with similar precision as for Z  ee
 Rely on theory for the W pT / Z pT ratio
Verified that OK at NNLO for a few pT values (Melnikov-Petriello)
At the moment, an additional scale factor is needed for W + 2 jets (~ 1.25)
Hopefully improved once W pT reweighting is available
J.-F. Grivaz
D0-France October 2008

12. W/Z + heavy flavors:
Theoretical expectations for HF predictions are on a less solid ground,
e.g., no massive quark calculation with MCFM at NLO.
Determine an additional scale factor from data.
This is done, for a given process, e.g., W+2-jets, and within some
analysis cuts by comparing the numbers of events with 0-tag, 1-tag,
2-tags, given the known tag rates for b and c jets, and the mistag rate.
The results are consistent with unity, but with still large statistical
and systematic uncertainties.
In the end, the W/Z+HF cross sections are affected
by uncertainties of ~ 30%.
J.-F. Grivaz
D0-France October 2008

13. Further data/MC comparisons: Z  ee / Jet pT > 15 GeV / Detector level
PYTHIA
PYTHIA v6.314
SHERPA v1.0.6
PYTHIA
PYTHIA too soft
(As expected)
SHERPA
SHERPA
SHERPA
a bit too hard
J.-F. Grivaz
D0-France October 2008

14. Z  /  1 jet / Jet pT > 20 GeV / Unfolded
PYTHIA v6.418
ALPGEN v2.13
+PYTHIA v6.323
+HERWIG v6.510
SHERPA v1.1.1
(native showering)
Ratios to
ALPGEN
 + PYTHIA
+HERWIG 
Z pT: Data in between ALPGEN+PYTHIA and ALPGEN+HERWIG…
(No Z pT reweighting applied here)
J.-F. Grivaz
D0-France October 2008

15. Z  /  1 jet / Jet pT > 20 GeV / Unfolded
Ratios to
ALPGEN
+HERWIG
Jet-1 pT and : ALPGEN+HERWIG in fairly good agreement with data
J.-F. Grivaz
D0-France October 2008

16. W  e /  2 jets / Jet-1(2) pT > 30(20) GeV / Detector level
ALPGEN
+PYTHIA
Work in progress
Good description of jet pT’s
Jet ’s broader than MC
(better with SHERPA)
J.-F. Grivaz
D0-France October 2008

17. W + b-jets: W (e/)  + 1 or 2 jets with pT > 20 GeV
“Ultratight” secondary vertex
tagging for high b purity.
Fit the vertex mass distribution
to b + charm + light templates
Measured cross section
3.5 times bigger than
ALPGEN prediction…
(Investigations are underway)
Similar, but less dramatic
result in Z+b-jets.
J.-F. Grivaz
D0-France October 2008

18. The next step: (We/ )(W/Zqq)
Similar topology to WH  e/  bb
including a mass peak (can’t separate W and Z)
except for the presence of b jets
Small S/B : 4 pb vs 500 pb for W+2 jets
28 fb for WH vs 20 pb for Wbb
Similar need for multivariate discriminants
CDF analysis in 1.2 fb1:
One central lepton with pT>20 GeV
MET > 25 GeV
MT > 30 GeV
2 jets with pT>15 GeV
(j1,j2) < 2.5
NN trained with 6 angular and shape variables
(little correlation with the dijet mass)
Cut on 6v-NN to improve the significance
Signal fraction enhanced by 50%
J.-F. Grivaz
D0-France October 2008

19. The ultimate benchmarks
3-parameter fit to the dijet mass:
2 for the background
1 for the signal fraction
Main systematics:
Background shape,
Jet energy scale and resolution
Background subtracted signal
Significance: 1.7
The next steps:
achieve a 3-5 significance in this channel
move to (We/ )(Zbb)
and possibly (Z ee, , ) (Zbb)
The ultimate benchmarks
J.-F. Grivaz
D0-France October 2008

20. 2 high pT, isolated, same flavor, consistent with the Z mass
Search for (Zee/)(Hbb)
2 high pT, isolated, same flavor,
opposite sign leptons
consistent with the Z mass
2 high pT jets, b-tagged
B. Calpas
Very clean signal, but low cross section: 2  2.6 fb
Maximize lepton acceptance (loose lepton ID criteria)
Maximize b-tagging efficiency (single and double tag)
Maximize the information used (multivariate discriminants)
Backgrounds:
Multijets with two misidentified leptons and with real or fake b jets
ends up to be very small in spite of huge initial cross section
Z/* + jets, more specifically Z/* + bb (2  1.4 pb  B/S ~ 500)
top pairs, mostly (t  b l )(t  b l ) (2  70 fb  B/S ~ 30)
dibosons, more specifically (Zll)(Zbb) (2  14 fb  B/S ~ 5)
J.-F. Grivaz
D0-France October 2008

21. Acceptance for mH = 120 GeV: 10.8%
The CDF search in 1 fb1 (arXiv: )
1st e/: pT > 18 GeV and || < 1
2nd e: pT > 10 GeV if || < 1 or >18 GeV if 1<|| < 2.4
2nd : pT > 10 GeV
Isolated within R<0.4
76 < Mee or M < 106 GeV
Opposite sign (for electrons: only in CC)
Jet1 ET > 25 GeV
Jet2 ET > 15 GeV
Both with || < 2
One or both b-tagged
Fake lepton background from data
Fake b-tags from udsg from data
Acceptance for mH = 120 GeV: 10.8%
J.-F. Grivaz
D0-France October 2008

22. Anti Z+jets NN before b-tag
No real missing ET expected  Correct the jet energies
with a NN which uses the MET projections onto those jets
The dijet mass resolution is improved from 18% to 11%.
Separation of signal from backgrounds with a 2D-NN: ZH vs Zbb and ZH vs tt
8 input variables:  (j1,j2,l1,l2)pT, MET, Mjj, R(j1,Z), R(j2,Z), R(j1,j2), Sph., j2 
Main systematic uncertainties:
Background cross sections (40% for Zbb, 20% for tt)
Zbb shape from PYTHIA vs. ALPGEN
Signal shape: ISR/FSR, PDF, JES
Luminosity, b-tagging (8% for b-jets) 1
Anti Z+jets NN before b-tag
after a cut on Anti tt NN
J.-F. Grivaz
D0-France October 2008

23. Results of the search in 1 fb1
Systematic uncertainties increase the expected limit by 14% (12% from b-tagging)
J.-F. Grivaz
D0-France October 2008

24. Improvements with 2.4 fb1 Larger integrated luminosity
Looser diEM trigger
(no track requirement)
Looser lepton identification
(isolated tracks for electrons in the gap)
Improved NN for jet energy correction
Loose b-tag in addition to tight
Better Z+jets simulation
2D-NN re-optimization
Most signal-like event
J.-F. Grivaz
D0-France October 2008

25. Electron channel:10v-NN
The DØ search in 2.3 fb1
Some differences wrt CDF:
Lepton pT > 15 GeV and || < 2
Jet pT > 15 GeV in || < 2.5
2-loose or 1-tight b-tags
Distributions at pre-tag level
Electron channel:10v-NN
Muon channel: 23v-BDT
ICHEP08 results
Analysis
Lum (fb-1)
Higgs Events
Exp. Limit
Obs. Limit
CDF NN
2.4
1.8
11.8
11.6
DØ NN,BDT
2.3
2.0
12.3
11.0
J.-F. Grivaz
D0-France October 2008

26. For the other low-mass Higgs search channels, see:
A. Duperrin (Z)(Hbb)
G. Bernardi (We/ )(Hbb) + other less important channels
J.-F. Grivaz
D0-France October 2008