# Presentation: 325734

## On the Trail of the Standard Model Higgs Boson

- Jason Nielsen
- Lawrence Berkeley National Lab.

- VERTEX 2004

- Fermilab, February 7, 2006

## A Trail Map for the Search

- Physical Higgs boson is concrete prediction of electroweak symmetry breaking mechanism in the standard model
- Experiments suggest Higgs boson mass likely between 114 and 200 Gev/c2
- Isolating Higgs boson production at Tevatron requires advanced experimental techniques and theoretical calculations

## Central Challenges in High-Energy Physics

- Particle collisions at the energy frontier enable us to
- pursue these and other questions about nature

- Unification of the basic forces
- 	and the origin of mass for the
- fundamental particles
- Unexpected new physics
- 	or extra dimensions not
- included in Standard Model
- Unknown new physics
- 	(forces or particles)
- hinted at by cosmology

## Motivation for a Higgs Boson

- A single weakly-coupled Higgs boson:
-  remnant of EWSB which gives mass to W±, Z bosons
-  has well-predicted production and decay rates

- But that also:
-  leaves problem of quadratically divergent corrections to MH
-  which leads in turn to supersymmetric theories

- This talk focuses exclusively on the Standard Model Higgs boson.

- Expect “new” physics below 1 TeV

- Something must break symmetry
- and give mass to weak gauge bosons

- Weak interaction acts at short-range

## Results from Recent Electroweak Fits

- mH < 186 GeV/c2 at 95% CL  (or 219 including LEP search)

- Updated summer 2005 with
- new Tevatron mt=172.7±2.8 GeV

- _Effects of the Higgs boson are felt via loop interactions_

- Precision measurements
- are sensitive to the Higgs mass

## Higgs Searches in e+e- Collisions

- 3 excess

- (Phys. Lett B495 (2000) 1.)

- ALEPH

- mH > 114.4 GeV at 95% CL

- Combined ALEPH/DELPHI/L3/OPAL results (April 2003)

## Tevatron Run 2 at Fermilab

- Record inst. luminosity 1.8E32cm-2 s-1
- 24 pb-1 integrated in a single week

- _Tevatron achievements in 2005:_

- delivered
- to tape

- results shown today use
- data through Aug 2004

- These CDF results use integrated
- luminosity 320±20 pb-1
- (cf. 110 pb-1 from Run 1)

**Notes:**
By Mar. 1, we may have 2E32 inst lum
FY06 goal is ~800 delivered
We will double the data for APS

## Higgs Production and Decay in pp Collider

- WH, ZH are good compromise
-  lepton cuts QCD background

- Hbb is decay of choice
-  but non-res. bb too high

## CDF II Detector at the Tevatron

**Notes:**
Physics in the transverse plane
See E_T and p_T instead of E, p

## Run 2 CDF Silicon Detectors

- 8 layers, 704 ladders, 722432 channels
- Total sensor area: 6 m2
- Largest operational silicon detector

- Three different detector geometries
- fill the silicon tracking volume
-  3-dimensional space points
-  2-D closest to beams

- Solid-state detectors:
-  e-h pairs along particle track
-  spatial resolution <10m

## Modeling CDF Silicon Detector Changes

-  Realistic simulation uses
-  Detector configuration
-  Lists of dead/noisy strips

- Overall detector status:
- Holding steady at 85% good
- Offline noise calibration
- used to improve clustering

## Tevatron Cross Section Hierarchy

- In proton-antiproton collisions at s = 1.96 TeV:

- b-jet pairs from QCD

- high-energy leptons

- Particle production
- rates vary widely:
- the Higgs is the
- “needle in the haystack!”

## Standard Model Higgs Boson Searches

- Given ggHbb is difficult, what channels are promising?

-  WHlbb  and  ZH bb
-  Trigger on leptons or missing energy from 
-  Look for resonant bb decay in mass distribution

-  ggHWW  and WH WWW
- Trigger on leptonic decay of W bosons
-  Distinctive angular spectrum between leptons

- For Higgs mass < 140 GeV

- For Higgs mass > 140 GeV

## Promising Channels for Higgs at Tevatron

- Update indicated
- workshop results
- are feasible

- Benchmark with
- current physics
- analyses using
- 350 pb-1

- Studies before Run 2 highlighted low-mass SM Higgs search
-  Follow up on possible LEP excess at 115 GeV
-  Extend reach toward 130 GeV by combining CDF and D0

## WHlbb Channel Overview

- WHlbb Channel Overview

- Large Hbb branching fraction

- Trigger on high-ET lepton:
-  factor of 5 less than ggH

- Key is understanding composition of lepton + 2 jets sample

-  Challenge of physics objects: leptons, jets, missing ET
-  Constrain W+jets production, including W + heavy flavor
-  Understand fakes from detector and algorithms
-  Overlap with top quark sample, especially single top

- To be published in PRL

## b Tagging Tool: Secondary Vertexing

**B hadrons in WH signal events are long-lived **_**and massive**_

- Vertex of displaced tracks

- Event is tagged if _any_ jet is tagged

- Form vertex of displaced tracks
-  Sec vtx err: 190 microns
- Cut on decay length significance

- Also check efficiency in data
- events vs. efficiency in simulation

## Jet Energy Measurement and Calibration

- Current resolution is 17 GeV
-  defines mass window
-  direct factor in acceptance
-  luminosity equivalence

- Working to improve resolution
-  track + calorimetry (H1)
-  lookup tables (hyperball)
-  neural network function

- Important handle in Higgs search: bb pair mass resonance

## Validation with Top Quark Physics

_*“*__*Yesterday’s discovery is today’s tool”*_

- Pair production at Tevatron

- But also:
-  only quark at electroweak symmetry breaking scale
-  constrains Higgs mass; fertile ground for new physics

- Develop and validate techniques and calculations in W+jets

**Notes:**
High-energy quark jets similar to jets in Higgs events

## Top Quark Pair Production Cross Section

- Search for top production in WbWblvbjjb sample:
- test b-tagging algorithms and efficiency
- test W reconstruction requirements (lepton, neutrino)
- test calculation of other physics with same final signature

- PRD 71, 052003 (2005)
- PRL in preparation with more data

## WHlbb Event Selection at CDF

- Rejection of top pair events with dilepton signature:
-  Veto events with a second isolated track pT> 20 GeV

- W boson

-  Exactly one isolated e/with ET>20 GeV
-  Total missing ET>20 GeV

- Jets

-  Exactly two jets with ET>15 GeV and ||<2.0
-  At least one jet must be tagged with secondary vertex
-  No extra jets with 8 < ET < 15 GeV or with ||>2.0

##  WHlbb Signal Acceptance

- Significant uncertainties:
-  b-tagging efficiency (8%)
-  ISR,FSR modeling (5%)
-  jet energies (3%)

- Includes effects from:
-  all selection criteria
-  simulation/data difference
-  trigger efficiencies

- Calculated using PYTHIA Monte Carlo and full simulation

- lepton ID: 30%
- kinematic: 40%
- b-tagging: 45%

## Backgrounds to WH Production

- Each background estimate is a miniature analysis unto itself.
- Techniques can be spun off to measure other physics processes.

| non-W QCD | false isolated leptons or false missing energy |
| --- | --- |
| top quark production | physics background |
| mistags in W events | false b-tags w/ lepton + MET |
| W + heavy flavor | physics background |

- Presented in rough order of _increasing_ contribution
- to the vertex-tagged lepton + jets sample:

## False tags in W+jets events

- Fraction of light flavor jets are “mistagged” with displaced vertex

- Light flavor Lxy distribution is roughly symmetric about 0
-  Use negative Lxy distribution to predict positive side “mistags”

- Correction factor (27 ± 13)% accounts for material interactions,
- long-lived light flavor particles (KS, )

## Physics Background: W + heavy flavor

- Real W + real tagged heavy flavor = physics background!

- Since Run 1, little confidence in absolute normalization from
- W+HF jets Monte Carlo calculations
- Use W+jets data for normalization
- Calculate fraction of W + heavy flavor using Monte Carlo

- Use ALPGEN Monte Carlo:
- Matrix element LO calculation
- Includes W+HF/W+ n jets processes
- Feed to HERWIG parton shower

- _Viable NLO W + heavy flavor MC is not yet available_

## Monte Carlo Matching Prescriptions

- Matrix element describes hard, separated partons
- 	Parton shower models emission of soft gluons

**Problem**: PS radiation in W + n jets process results in
- events which are also produced by ME in W + (n+1) process

**Solution:** Mangano’s prescription requires each ME parton
- match a final state particle-level “jet” and no extra jets;
- otherwise reject event

**Notes:**
Mention MADCUP project at Wisconsin

## Estimate of W+HF Contribution

-  Calibrate calculations using jet processes without W boson
-  Gluon splitting in 3-jet sample similar to W+HF diagrams

- Calibrated correction factor 1.5±0.4 consistent with phenomenological K-factor

- Final calculation of true W+HF backgrounds is then:

- Additional factor 1.2 from fit
- to W+1-jet data (normalization)

- (PRD 71, 052003 (2005))

## Summary of Background Contributions 

- Use W+1-jet bin to
- constrain W+HF bkgd

- Top pair cross section
- Measured from the
- W+3,4-jets events

- Number of W+2-jet
- Events consistent with
- Background predictions

## bb Dijet Invariant Mass Distribution

## CDF WH Production Limits

## Cross-Check with Double-Tagged Events

- Fluctuation at 100 GeV not
- present in this data sample

- Slightly worse expected limits
- due to reduced acceptance

## Search for ZHbb Production

- Search for ZHbb Production

- Selection for bb:
-  at least 2 jets (>25 GeV)
-  missing ET > 70 GeV
-  at least 1 b-tagged jet

- Check background estimates in control regions: leptons, MET-jet

- Major background:

- Powerful search in low-mass Higgs regions; ID missing energy

## CDF ZHbb Candidate Event

- Two b-tagged jets:
-   Jet1   ET= 100.3 GeV
-   Jet2   ET= 54.7 GeV

- Dijet mass mjj= 82 GeV

- Could be example of ZZ

- Missing ET=145 GeV

## ZHbb Search Results

- Includes sliding mass window
- dependent on Higgs mass

- Slightly more sensitive
- than WH at low mass

## Search for ggHWW 

-  Interesting angular correlation
- due to scalar Higgs
-  Different from SM W+W- bkgd
- decay angular correlation!
-  Use angle between dileptons
- as final discriminant

- Select events with
-  leptons 20, 10 GeV
-  missing ET > 50 (CDF), 20 (DØ)
-  low-energy jets allowed

- Most useful for mH > 135 GeV

## ggHWW Search Results

- _Sliding cuts for different m__H_
-  Example result for mH=160
-  W+W- still dominant background
-  Fit  for signal cross section

- Exclusion similar to WH, ZH
-  extends to higher masses
-  still limited by statistics
-  work to extend acceptance

## Summary of Tevatron SM Higgs Searches

## Summary of Tevatron SM Higgs Searches

## Improvements Underway

| Improvement | WHlbb | ZHbb |
| --- | --- | --- |
| Mass resolution | 1.7 | 1.7 |
| Continuous b-tag (NN) | 1.5 | 1.5 |
| Forward b-tag | 1.1 | 1.1 |
| Forward leptons | 1.3 | 1.0 |
| Track-only leptons | 1.4 | 1.0 |
| NN Selection | 1.75 | 1.75 |
| WH signal in ZH | 1.0 | 2.7 |
| CDF+DØ combination | 2.0 | 2.0 |

- Tools common
- to CDF analyses

- Analysis-specific
- improvements

- Identified potential to approach SM cross section exclusions

- Challenge to develop and apply improvements to Higgs searches

- _And don’t forget factor of 10 more data!_

## Neural Network Event Selection Studies

- Two advantages over selection with simple rectangular cuts:
- Use new event variables with weak or correlated effect
- Exploit correlations between event variables

- Using fit to NN output distro gives additional improvement
- DØ collaboration estimated 75% improvement in ZH channel

- Use NN selection

- Standard selection

- Run 1 CDF study of expected WHlbb limits:

- =12.6 pb
- L=90 pb-1

- =13.0 pb
- L=95 pb-1

- =9.3 pb
- L=150 pb-1

- =10.0pb
- L=95pb-1

## Large Hadron Collider at CERN

**Italy**

- p

- p

**14 TeV**

- Next generation collider: startup scheduled for _2007 _

- ATLAS

- CMS

- Luminosity target: 1034cm-2 s-1

- Increased production of heavy
- particles like Higgs, top quark

**More **particles at **higher **energy
- requires new detector design
- and technology

## Prospects for SM Higgs at LHC

- Should discover SM Higgs regardless of mass value!

- _Low-mass Higgs channels:_
-  count on H (m =1.5 GeV/c2)
-  add WBF HWW, H
-  ttH needs ttjj background study

- _High-mass Higgs channels:_
-  golden mode 4e/ opens >2mZ

**Experimental techniques: Learn from Tevatron results!**

## How the Tevatron Informs LHC Higgs

- Real-world Tevatron experience and detailed LHC MC studies
-  utilize the best of both worlds! Some examples:

- Selection of weak boson fusion
- depends on central jet veto
-  check efficiency using top events?

- Tools for matrix element-parton shower matching
-  NLO vs LO comparison

- Techniques for measuring b-tag eff,
-  ID in hadron collider environment

## Summary

- Expect Standard Model Higgs boson mass to be below 200 GeV/c2
- Increased Tevatron luminosity and upgraded detectors offer possibility to probe Higgs boson masses above 114 GeV LEP exclusion
- First analyses highlight challenges and provide benchmark for sensitivity estimates
- Identified to-do list for improving search strategies
- Chance to explore interesting low-mass region before LHC turn-on

## Slide 44

## Calibrating the b-tagging Efficiency

- Select soft (pT>8 GeV) electron events to enrich HF,
- then require tag to reduce charm contributions

- “Scale factor” measured on this sample is applied
- to other samples, even if the efficiencies differ

- Measure ratio of single- and double-tagged events (in both data and simulation)

## Jet Events without W Bosons

- Fake isolated (I<0.1) lepton along with fake missing ET
- for QCD background these fakes are uncorrelated

- Signal region D predicted by

- Model event kinematics from sideband

## Even Zbb is Hard!

## Calculating Heavy Flavor Fractions

- Processes we are interested in: Wbb, Wcc, Wc

- -

- -

- Use ALPGEN + HERWIG samples stitched together after MLM matching prescription,
- ex. Wc + 0,1,2 p

- Back to reality:
- _Does this MC calculation_
- _really reflect the data?_

- Not sensitive to
- matching criteria

## One Provocative Candidate Event

- 3 NN b-tagged jets

- ECM=206.7 GeV

- HZ  bbbb selection

- Reconstructed mH = 110 ± 3 GeV/c2

## Theoretical Constraints on Higgs Mass

- Finding a Higgs boson with mH~120 GeV is evidence for TeV-scale New Physics