# Presentation: 936555

## Search for the Standard Model Higgs Boson at DØ

- Michele Petteni
- Imperial College London
- On behalf of the DØ Collaboration

## Talk Outline

- What do we expect from the Standard Model?
- Higgs production and decay at the Tevatron
- Expectations for the Tevatron
- Brief description of DØ and the Tevatron
- Analysis channels
- Outlook

## The Higgs Boson

- Theoretical limits:
- WLWL scattering,    mH< 1 TeV
- Other limits depend on the scale of new physics

- Higgs boson needed for Electroweak symmetry breaking:
- Allows for difference in W,Z and photon mass
- Generates masses for fermions
- Neutral, scalar particle

## Experimental Constraints

- Higgs couples to mass :
- W and Top mass measurements constrain mass of Higgs via effect of radiative corrections.
- Global Electroweak fits give indication of Higgs mass
- Recently updated for new top mass measurement from Tevatron.
- Direct Searches
- This has not changed from LEP limit, 114.4 GeV

## Higgs Production and Decay

- Two search strategies:
- Below 135 GeV look for associated production with 2 bjets from the Higgs decay.
- Above this limit exploit gluon fusion and reconstruct the two gauge bosons.
- Possibility of third strategy in the intermediate mass range?
- More on this later

- Excluded by LEP

## Why the Tevatron?

- Sensitivity studies performed :
- Higgs Working Group Collaboration,**hep-ph/0010338**
- Tevatron Higgs Sensitivity Study Group, **FERMILAB-PUB-03/320-E**
- Studies have some inherent assumptions
- For low mass Higgs :
- Excellent b-tagging to identify jets from the Higgs
- Lepton-id to identify the Z/W
- Jet resolution is essential to disentangle the mass peak
- For high mass Higgs :
- Hinges around lepton identification

- Sensitivity studies highlight important facts
- Not one golden channel
- Combined results from both Tevatron experiments
- Evidence for light mass Higgs is feasible at the Tevatron

- 8 fb-1 for Run IIb

## The DØ Detector

## Operations Report

- Luminosity progressing steadily and in a regular manner
-  1 fb-1 delivered
- DØ operating well and is stable
- Analysis presented taken with data up to latest shutdown

- Stable at 88% efficiency

- Peak luminosity > 1.2x1032

- Data used in analysis

## Experimental Results

- Low mass Higgs :
- Z/* ( e+e-) + ≥ n jets x-section
- WH  e  b b
- ZH    b b
- Intermediate mass Higgs :
- WH  WWW*
- High Mass Higgs
- H  WW*

## Z/* ( e+e-) + ≥ n jets

- Z/W + n jets main background to light mass higgs searches
- Main cuts:
- Two leading electrons pT > 25 GeV, consistent with Z mass
- Jet pT > 20 GeV, || < 2.5
- Results normalised to inclusive cross-section:
- Jet multiplicity figure compared with NLO MC and to matrix-element - parton shower treatment (ME-PS)
- Jet pT spectrum compared to Alpgen + Pythia normalised to data

## Next step measure Z+n bjet cross-sections.
Higgs limit 

- Higgs limit

- Cross-section Ratios

## WH Searches; W(e)+jets

- Results are an update on the analysis published in PRL 94, 091802(2005).
- Includes data shown in the PRL paper (174 pb-1)
- WH process has a higher x-section than ZH and benefits as only one lepton to identify
- Main backgrounds are Wjj (before b-tagging) and subsequently tt, single top, and WZ and the ever present QCD

## WH Searches; W(e)+jets (2)

- Main cuts:
- Isolated electron, pT > 20 GeV, || < 1.1
- Missing ET > 25
- Two jets, pT > 20 GeV,     || < 2.5
- After basic selections 3844 events, expect 3256 W/Z+jets events
- Good agreement with MC

## WH Searches; W(e)+jets (3)

- After b-tagging see 13 events expect 10.2
- B-tagging efficiency major source of uncertainty
- Set limit using a mass window for 4 Higgs mass values

## ZH bb

- Due to the large branching ratio for Z decay this channel has about the same contribution as all the W channels combined
- However :
- No “easy” handle to trigger the events
- trigger on missing ET from the jets and the presence of jets
- Highly sensitive to any mis-measurements
- Significant QCD background
- large cross-section
- difficult to estimate
- Main non-QCD backgrounds are Zbb, ZZ, ZW, Wbb and top (after b-tagging)

## Topology Cuts

- Require two acoplanar jets, missing ET and veto any isolated leptons
- In order to reduce the QCD/instrumental background, exploit correlation between missing energy from calorimeter cells, jets and tracks
- They should be aligned
- Form asymmetries and cut on this value (peaks at ~0 for signal)
- Cut on angles between jets and missing ET

## Estimation of Instrumental Background

**Estimation of Instrumental Background**

- Physics bkgd. from MC

- sideband

- sideband

## ZH bb Results

-  Expect 2125, see 2140 before b-tagging.
-  After b-tagging observe 9 expect 6.4
-  Apply 50 GeV mass window
-  Signal acceptance 0.3%
-  Zbb and Wbb major backgrounds
-  Instrumental half of these (same as top)

## WHWWW*llqq

- Exploits like-sign leptons in final state
- Low backgrounds
- Clear signal
- Require 2 like sign leptons (veto 3rd)
- Missing ET > 20 GeV
- Main backgrounds
- Physics, WZlll
- Charge flips from Zll, WWll,ttll, etc.
- Instrumental from “real” like sign leptons from semileptonic decays,  conversions...

## WHWWW*llqq (2)

|  | ee | e |  |
| --- | --- | --- | --- |
| Observed | 1 | 3 | 2 |
| WZ | 0.43±0.03 | 0.33±0.03 | 0.16±0.03 |
| Charge Flips | 0.20±0.06 | 0.05±0.01 | 3.40±0.73 |
| W/QCD | 0.07±0.04 | 3.94±0.23 | 0.16±0.18 |
| Total | 0.70±0.08 | 4.32±0.23 | 3.72±0.75 |

-   Different signals have different background contributions
-   Observed limit covers nicely the intermediate Higgs mass region

## HWWll

- Look in ee,  and e channels
- Require:
- Two opposite sign leptons
- Missing ET > 20 GeV
- Cut out events with potentially large contribution to the missing energy from jet mis-measurement
- Veto low mass resonances and reduce Z contribution

- Excellent agreement  after basic cuts

## WHWWll (2)

- In order to further suppress background use angular correlations:
- Spin of Ws are correlated
- Leptons from Higgs are collinear, back-to-back for background
- Cut on opening angle between ll
- Mass dependent cuts

- Selection acceptance varies from 16.7% to 3.9%
- Combine all channels using likelihood functions
- Numbers of events depends on Higgs mass, for mH= 160 GeV expect 17.7±1.0 and see 20.

## Summary

- Preliminary results look good
- Good description of background processes and the understanding of environment has improved dramatically
- Analysis (always) need more work, striving to reach sensitivity outlined by sensitivity study
- Need advanced analysis techniques
- Tevatron has already delivered 1 fb-1 (0.8 fb-1 on tape), by the end of the year we will have 1 fb-1 to tape
- With this luminosity getting close to excluding at 95% a Higgs mass close to 115 GeV

## Backup Slide

## Comparison of WH published Results with Sensitivity Prospective Study

**Prospective Study assumed:****  Larger ECAL coverage (+30%), improved em-id  (+40%), extended b-tag efficiency (+50%, 2 tags)  and  30% less backgd (better dijet mass resolution)**
****** ****Factor 2 in  S/********B****    ********  2.4/ 2 = 1.2 difference (only) in sensitivity**
**Apply Advanced techniques**
**Missing Factors can be recovered**

| 

Dijet mass window | DØ Analysis
(PRL ‘05)
174 pb-1
WH ebb
[85,135] | Prospective Study (‘03)  normalized to  174 pb-1 and to WH bbe
  [100,136] |    Ratio 

 Prospective
  DØ Analysis
R=0.72    |
| --- | --- | --- | --- |
| Dijet mass resolution  | 14 +/- 1 % | 10 % | R=0.71 |
| Signal events (S) | 0.049  | 0.145 | R=3.0 |
| Background evts (B) | 1. 07 | 1.76 | R=1.6 |
| S/B | 0.045   | 0.11 | R=2.4 |
| S/B | 0.046  | 0.082 | R=1.8 |

**Comparison of WH published Results ****with Sensitivity Prospective Study**