# Presentation: 674574

## Search for Standard Model Higgsin ZH  l+lbb channel at DØ

- Shaohua Fu
- Fermilab
*For the D**Ø Collaboration*
- DPF 2006, Oct. 29 – Nov. 3
- Honolulu, Hawaii

**Notes:**
Search for associated production of Z and Higgs bosons in leptonic final states at D0.

## Tevatron and DØ Detector

- Run II goal: 4 ~ 8 fb1 in 2009
- Analyses presented here used 840-920 pb1

- DØ detector in Run IIa
- Run IIb: Layer0 for SMT, trigger upgrade

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

**Notes:**
Data: April 2002 to March 2006 (before shutdown).
D0 detector: well understood and highly efficient. Precise tracking with silicon vertex detector. Excellent calorimeters and muon chambers coverage.

## Constraints on SM Higgs

- Standard Model Higgs is the key to Electro-Weak symmetry breaking and gives masses to elementary particles, with its own mass unpredicted
- Limit from direct searches at LEP2: mH > 114.4 GeV at 95% C.L.
- Indirect limit from fits to precision EW measurements from LEP, SLC, and Tevatron: mH < 166 GeV at 95% C.L. (< 199 GeV if LEP2 limit included)
- Indirect best fit value: 85 +3928 GeV at 68% C.L.
- A light Higgs is favored

- LEPEWWG July 2006

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## SM Higgs Production and Decay

- SM Higgs production at Tevatron
- Gluon fusion  ~ 0.8-0.2 pb (mH 115-180 GeV)
- Associated production with a *W* or *Z* boson  ~ 0.1-0.03 pb
- Dominant decays
- Low mass (mH<135 GeV): *H**bb*, high mass (mH>135 GeV): *H**W**+**W***
- This analysis: *ZH*  *l**+**l****bb*, in *e**+**e*** and* ****+***** channels

**Excluded at LEP**

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Data Selection

- Integrated luminosity = 920 (840) pb–1 for *e**+**e*** (***+*****) sample
- Using all EM (Muon) triggers with efficiency ~100% (75%) for *e**+**e*** (***+*****) sample
*e**+**e*** sample
- 2 electrons pT > 15 GeV, || < 1.1 or 1.5 < || < 2.5, central track match
*Z* candidate: 65 GeV < Mee < 115 GeV
***+***** sample
- 2 muons pT > 15 GeV, central track matched (|| < 2.5), and isolated
*Z* candidate: 70 GeV < M < 110 GeV, *Z* pT > 20 GeV
- At least 2 jets
- pT > 15 GeV, || < 2.5
*b*-tagging
- Secondary vertex
- Large impact parameter of the tracks
- Neural Net tagging algorithm
- ~60% efficiency and ~3% light-jet fake rate
- 	(*b*-tagging and taggability)

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Backgrounds

*Z*(* l**+**l***) + jets
- Including *Z*(* l**+**l***) + light-parton jets and *Z*(* l**+**l***) + *c* jets
*Z*(* l**+**l***) + *b* jets
- Hard to reduce *Z*+*bb* background, which will pass *b*-tagging just like signal
- Top pair, *WZ*, *ZZ*, etc.
- Small contribution, e.g. *tt* * l**+**l****bb*** is reduced by *l**+**l*** invariant mass cut
- MC Normalization
*Z*(* l**+**l***) + jets normalized to #data under *Z* peak, other MC normalized to *Z*+jets using NLO cross sections
- Multijet background – QCD process
*e**+**e*** channel: selecting QCD enhanced sample from data by inverting the electron shower shape and track-matching requirement, then fitting Mee distribution to normalize QCD sample.
***+***** channel: fitting M distribution of data with a Gaussian (*Z*) and an exponential (Drell-Yan + QCD) to get the fraction of QCD, depending on jet multiplicity and *b*-tagging.

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Data and Backgrounds

*Z* peak for *Z*(* e**+**e***) + 2 jets (before *b*-tagging)
- 2900 events within 65 GeV < Mee < 115 GeV
- 102 events estimated for QCD background
- Total background estimated to be 2860  470 events
- Signal *ZH* (mH=115 GeV): 0.78  0.03 events

- Mee

*Z* pT

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Data and Backgrounds

*Z*(* l**+**l***) + 2 jets (before *b*-tagging): *e**+**e*** and ***+***** combined
- 5386 events observed in data
- 5610  930 events expected as total background
- Signal *ZH* (mH=115 GeV): 1.50  0.06 events
- Leading and second lepton pT distributions:

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Data and Backgrounds

*Z*(* l**+**l***) + 2 jets (before *b*-tagging): *e**+**e*** and ***+***** combined
- Leading and second jet pT distributions:
- Then apply *b*-tagging

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## With 0 b-tagged jet

| 0 b-tagged jet (Mjj 60~110 GeV) |  |  |
| --- | --- | --- |
|  | e+e | + |
| Data | 832 | 647 |
| Bckg. | 808 134 | 671 111 |
| QCD
Z + jets
Z + bb
tt
ZZ
WZ | 28.5
740
19.3
1.0
10.9
8.7 | 6.71
662
21.2
0.9
6.9
6.6 |
| ZH 115 | 0.111 0.004 | 0.11 0.004 |

- #events in di-jet range 60 (70) ~ 110 GeV for *e**+**e**** *(***+*****) channel

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Exclusive Single b-tagging

| 1 b-tagged jet (Mjj 60~110 GeV) |  |  |
| --- | --- | --- |
|  | e+e | + |
| Data | 126 | 99 |
| Bckg. | 111 22 | 88 18 |
| QCD
Z + jets
Z + bb
tt
ZZ
WZ | 4.4
82.4
17.8
2.0
2.2
2.3 | 0.4
75.5
16.0
1.8
1.4
2.0 |
| ZH 115 | 0.243 0.011 | 0.21 0.009 |

- #events in di-jet range 60 (70) ~ 110 GeV for *e**+**e**** *(***+*****) channel

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Inclusive Double b-tagging

| 2 b-tagged jets (Mjj 60~110 GeV) |  |  |
| --- | --- | --- |
|  | e+e | + |
| Data | 8 | 10 |
| Bckg. | 9.8 3.4 | 8.7 3.1 |
| QCD
Z + jets
Z + bb
tt
ZZ
WZ | 0.25
3.7
3.8
1.3
0.11
0.75 | 1.8
3.7
3.4
1.2
0.07
0.65 |
| ZH 115 | 0.169 0.014 | 0.14 0.012 |

- #events in di-jet range 60 (70) ~ 110 GeV for *e**+**e**** *(***+*****) channel

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Systematic Uncertainties

- Sources of uncertainties
- Lepton identification efficiency uncertainty: 4%
- Jet energy scale correction uncertainty: 1-7% for backgrounds, and 1-2% for *ZH* signals
*b*-tagging uncertainty (double *b*-tagging): 7-8% for backgrounds and signals
- NLO cross section uncertainty: 15% for *Z*+jets, 50% for *Z*+bb, 6-8% for other backgrounds
- QCD normalization uncertainty: 30%
- Total uncertainties
- Correlations among each background uncertainty taken into account
- For double *b*-tagging: 35% on total background, 9% on *ZH* signals

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Cross Section Limits

*e**+**e*** and ***+***** combined
- No excess observed, so set cross section limits
- Modified frequentist approach (CLS), using di-jet mass distribution
- 95% C.L. upper limits on *ZH* cross section: 3.3-1.6 pb for mH=105-155 GeV

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu

## Summary

*ZH ** l**+**l****bb* searches performed at DØ in *e**+**e*** and* ****+***** final states, using about 1 fb–1 integrated luminosity
- No excess of events is observed, thus upper limits on *ZH* cross section are set as 3.3-1.6 pb for mH=105-155 GeV
*ZH *mH=115 GeV cross section limit is about 30 times of the SM production
- To improve sensitivity
- Optimization techniques
- 	(e.g. Neural Net selection)
- Better detector understanding
- Layer0 silicon detector
- 	 better *b*-tagging
- Combine all channels
- More data!

- Shaohua Fu, Fermilab

- DPF 2006, Honolulu