Search for Neutral Higgs Bosons in Multi-jet Events at DØ

Andy Haas

Columbia University

on behalf of the
DØ Collaboration

DPF 2006 - Hawaii

Oct. 31, 2006

MSSM Higgs Theory

There are two ways to calculate the signal production at a ppbar collider:

ppbar->bbh (4-flavor scheme)

ppbar->bh (5-flavor scheme) :
use b-PDF

Both methods now agree at NLO

Kinematics and cross-sections are weighted to the NLO distributions
(from MCFM)

bbh

bh

MSSM Higgs Phenomenology

The Tevatron at Fermilab

DØ

88% average efficiency

Run I:

100pb-1

4-8 fb-1 expected by end of 2009

This talk:

950pb-1

Electronics

Tracker

Solenoid Magnet

3 Layer
Muon

System

Preshowers

The Run II DØ Detector

Recently upgraded!

Calorimeter
clustering

in Level 1 trigger!

b-Jet Tagging

Several mature algorithms used:

3 main categories:

- Soft-lepton tagging

- Impact Parameter based

- Secondary Vertex reconstruction

Combine in Neural Network:

vertex mass

vertex number of tracks

vertex decay length significance

chi2/DOF of vertex

number of vertices

two methods of combined track impact parameter significances

TIGHT
(eff=48%, fake=0.3%)

Triggering

A very difficult signature to trigger on!

No high-pT leptons, or large mET

Trigger on jets

3 calorimeter towers / clusters at Level1/2

3 jets with pT>25,25,15 GeV at L3

And use the fact that they are b jets, at Level3

Reconstruct central tracks

Compute a combined event probability based on the impact parameter (IP) of the tracks

More b's -> more high IP tracks -> smaller non-b probability

Require the event's non-b probability < 5%

Measure trigger efficiencies in data

model the trigger effects in the Monte Carlo

L1

L2

L3

Event Selection / Analysis Method

About 75 million events in initial data sample

Jets considered in |eta|<2.5
Primary Vertex |Z| < 35cm
(b-tag acceptance)

Require at least 3 jets with pT>40,25,15 GeV

No more than 5 jets with pT>15 GeV

Require at least 3 b-tagged jets

Signal:

Invariant mass of leading jets is
peaked at mA

Backgrounds (determined from data):

Shape estimated from the
double b-tagged data sample
(taking into account the kinematic bias from requiring a 3rd b-tag)

Normalized outside the “signal region”
(for each candidate mA and tanB)

Events with >=3
b-tagged jets

“Signal window”

Triple b-tagged Background

Double b-tagged
data sample

Calculate TRF

Apply TRF to
un-b-tagged jets

Triple b-tagged
background shape

Double b-tagged
data sample

Tag-rate-function (TRF)

Triple b-tagged
background

Fit outside the
signal region to data

Data vs. Background

Data agrees well with the predicted background from the TRF method

Will set upper tanB limits on various Higgs masses using the CLs (LEP Higgs) method

Acceptance systematics:
(Total: ~17%)

Trigger efficiency

Kinematic cuts

Modeling of NLO effects

b-Tagging efficiency

Background systematics:
(Total: ~2.5%)

Statistics of TRF

Shape of TRF

(mA=160, tanB=60)

Limits

Monte Carlo Cross-checks

Use ALPGEN+Pythia to produce simulated background samples

Cross-section uncertainties are 50-100% due to renormalization/factorization scale dependence at leading-order

Useful for understanding the background shapes and physics composition of the background

Can check the ratio of observed triple and double b-tagged events with what is expected from MC

Checks the modeling of the trigger turn-ons

All distributions roughly agree between data / MC

bb

Double b-tagged

Double b-tagged

Triple b-tagged

Conclusions

New limits in the mA / tanB plane from the search for neutral Higgs bosons in the MSSM

Many new techniques used, such as:

Level3 event b-tagging:
needed to remain within bandwidth limitations as luminosity increased

Neural Net b-tagging (offline): light-jet backgrounds reduced by a factor of ~3

State-of-the-art MC modeling of backgrounds used as a cross-check

This analysis will be extended using new data

Expect up to 8 fb-1 by 2009

Will use the newly commissioned Layer0 of silicon -> better b-tagging!

Use of new L1 calorimeter trigger will enable efficient collection of multi-jet events at high luminosity

Possible to exclude up to mA~250 GeV for high tanB, and down to tanB~20 for low mA by the end of Run II

Will continue to combine with
A->tautau and bA->btautau channels

Will eventually combine with CDF

Dmitri Denisov, ICHEP04, August 21 2004

Andy Haas

2004-07-07T15:59:13