# Presentation: 658004

## Discovering the Higgs at the Tevatron

- Pushpalatha C. Bhat
- Fermi National Accelerator Laboratory

## Outline

- Introduction
- Why do we care about the Higgs?
- Finding the Higgs at the Tevatron in Run II (WH,ZH channels)
- Optimal Event Selection
- Event Simulation
- Results
- Conclusions

## The Standard Model

- The prevailing theory of matter and forces
- Had spectacular success in explaining all 	  experimental data, so far!
- But, some deficiencies:
- Seemingly random masses for quarks & leptons.
- The electroweak gauge bosons are massive!
-  Electroweak symmetry is somehow broken!

## Spontaneous Symmetry Breaking Ferromagnet

## The Mysterious Mr. Higgs

- Interaction of a postulated scalar “Higgs field” with the massless fields of the theory causes the Electroweak Symmetry Breaking (EWSB)
- This “Higgs mechanism” endows the  W, Z gauge bosons with mass.
- There remains a massive spin-zero particle, the Higgs boson

## Why do we care about the Higgs?

- The Higgs field, which is the agency of electro-weak symmetry breaking (EWSB), is a crucial feature of the Standard Model. Without the Higgs field the theory would not make sense.
- However, we do not know whether the Higgs field is fundamental or whether it merely models a deeper theory of EWSB.
- Therefore, a major goal of particle physics during the next few years is to determine, if, in broad outline, the current picture of EWSB is correct. A necessary, though by no means sufficient, step towards that goal is to determine whether or not the Higgs boson exists.

## What do we know about the Higgs? 

- There is a soft upper bound from the Standard Model. The theory remains mathematically viable if
- MH < 1000 GeV/c2
- There is a lower bound on its mass from LEP
- MH > 106 GeV/c2 @ 95% CL http://l3www.cern.ch/conferences/talks99.html
- The bound could reach 114 GeV/c2 in the near future.
- There are also strong hints from
- Global analyses of precision LEP and Tevatron data
- Supersymmetric versions of the Standard Model.

## Hints from Analysis of Precision Data

- The standard Model predicts a relationship
- from which one can infer something about the Higgs boson mass.

## Hints from Analysis of Precision Data

- LEP Electroweak Group, http://www.cern.ch/LEPEWWG/plots/summer99

- MH =              GeV/c2 MH < 215 GeV/c2  at 95% C.L.

## Susy, the Elegant Lady

- Supersymmetry (Susy) is a beautiful & elegant theory, or so say our theorist friends!
- If nature is supersymmetric every fermion has a bosonic partner (the squarks and the sleptons) and every boson has a fermionic partner (gluinos, winos, zino, photino,...)
- Solves many problems of the Standard Model including some in the Higgs Mechanism
- But, the price to pay is the doubling of the particle spectrum and zillions of parameters!

## Higgs Meets Susy

- The Minimal Supersymmetric extension of the Standard Model (MSSM) predicts 5 Higgs bosons (h0, H0, A0, H±)
- In a broad class of Susy models, MH < 150 GeV/c2  if the Susy particle masses are less than 1 TeV/c2
- In the MSSM,
- If MA >> MZ,
-          H0, A0, H±  become very heavy and degenerate 	in mass
- 	
- 	 So, can we find HSM or h0 at the Tevatron in
- 	     Run II ?

- Mh < 130 GeV/c2

- h0  behaves very much like the SM Higgs boson

## SM Higgs Production and Decay

- The SM Higgs couples to a particle with a strength proportional to the particle’s mass. Therefore, the main decay mode is
- which has a branching ratio of about 85%.

- The dominant process is gg **** H. Unfortunately, it is swamped by di-jet production. The more promising channels are WH and ZH.

- (pb)

## WH, ZH Production

***BR(H************bb) = 85%***

- P (+1)

- b (-1/3)

- b (+1/3)

- P (-1)

- P (+1)

- b (-1/3)

- b (+1/3)

- P (-1)

## Finding a Needle in a Haystack 

**N = L******
- N	Number of interactions
- L	Integrated luminosity
- 	(particles/unit area)
- 	Cross section (effective
- 	area of interactions)

- In order to get a single WH interaction we
- need a total flux of **2x10****37** particles cm-2.
- Assuming a luminosity of **10****31** particles
- cm-2 s-1, about one WH event would be
- produced every three weeks, compared to
- a trillion ordinary interactions in the same
- period.

## Low Mass (MH < 130 GeV/c2 )

- Signals:
- Backgrounds

- P.C. Bhat, R. Gilmartin, H.B. Prosper, hep-ph/0001152

## Cross Sections

## Slide 17

## The DØ Detector

## Tagging b-Quarks

- Double b-tagging efficiency  45%

***Tag with soft ************ ***
***  ******b************ c************, b ************ c ************ s*********

***Tag with displaced vertex***

***Collision vertex***

***Secondary vertex***

- b

## Slide 20

## Di-Jet Mass Resolution

- Used
- m/m = 10%

## Optimal Event Selection

- Conventional cuts

## Discriminant Approximation with Neural Networks 

- A feed forward neural network is a non-linear function that approximates the Bayesian posterior probability *p(s|x,y).*

## More on Neural Networks

- Neural Networks (NN) are mathematical, adaptive systems (algorithms).
- The “hidden” transformation functions, g, adapt themselves to the data as part of the training process.  The number of such functions need to grow only as the complexity of the problem grows.
- NN estimates a mapping function without requiring a mathematical description of how the output formally depends on the input.

## Calculating the Discriminant 

- Consider the sum

- Where
- 	di  	= 1 for signal
- 		= 0 for background
- 	****	= vector of parameters
- Then

- in the limit of large data samples and provided that the
- function *n(x,y,********)* is flexible enough.

## A Toy Example for NN Classification

- P(s|x,y) is the probability the event is a signal, given that it is characterized by *(x,y).*

## Run II Higgs Studies

## Event Simulation

- Event generation
- WH, ZH, ZZ and Top with PYTHIA
- Wbb, Zbb with CompHEP, fragmentation with PYTHIA
- Detector modeling
- SHW (http://www.physics.rutgers.edu/~jconway/soft/shw/shw.html)
- Trigger
- Tracking
- Jet-finding
- b-tagging (double b-tag efficiency ~ 45%)
- Di-jet mass resolution ~ 14%

## WH  lbb                       Single Lepton Channel

**Signal**
- WH
**Backgrounds**
- Wbb
- WZ
- tqb
- tb
- Tt
**Detector Simulation**
- SHW
**Networks**
- Jetnet (v3.0)
- 3 Networks/MH
- WH vs WZ
- WH vs Wbb
- WH vs tt
- 3 cuts

-  (7,9,1)
- Network

## WH(100 GeV/c2) vs. Wbb

## WH(100 GeV/c2) vs. WZ

## WH(100 GeV/c2) vs. tt

## WH(100 GeV/c2) NN Distributions

- WH(100 GeV/c2) NN Distributions

## WH(110 GeV/c2) NN Distributions

- WH(110 GeV/c2) NN Distributions

## Results (WH)Was It Worth It?

## Single Lepton Channel

## Results, Standard vs. NN

- Comparison of           achievable with conventional and neural network cuts..

## Results

## Results, Standard vs. NN

## Results

- Combined results of all three channels.

## Conclusions

- The origin of mass is one of the deepest mysteries of  Nature. The Higgs boson may be the key to unraveling this mystery.
- There is a tantalizing possibility that we can discover a low-mass Higgs boson at the Fermilab Tevatron within the next five years!
- Run II holds promise of great discoveries and perhaps some surprises!
- But to make it all happen we need to use the most optimal methods of analysis.

## SUSY Higgs (MSSM)

## A Little Bit of Susy

- The Standard Model suffers from a horrible problem: The correction to the (mass)2  parameter in the Higgs potential
- 	is too large by more than 30 orders of magnitude!
- However, if Nature is supersymmetric every fermion has a bosonic partner and every boson a fermionic partner. In a supersymmetric theory each pair supplies equal and opposite corrections to the Higgs (mass)2 parameter. The perfect cancellation yields sensible corrections.
- Susy theories explain why the (mass)2 can become negative at the electro-weak scale and hence <H>vac.

## A Bit More Susy

- The Minimal Supersymmetric extension of the Standard Model (MSSM) predicts the existence of 5 Higgs bosons:
- 2 neutral CP-even scalars (h0,H0)
- 1 neutral CP-odd pseudo-scalar (A0)
- 2 charged scalars (H±)
- In a broad class of Susy models mh < 150 GeV/c2, provided that the masses of the supersymmetric particles are < 1 TeV.
- In the MSSM the bound is lowered to mh < 130 GeV/c2.
- Moreover, if mA >> mZ, the bosons A0, H0, H±, become heavy (and nearly degenerate in mass), while h0 behaves very much like the SM Higgs boson.

## High Mass (MH >130 GeV/c2 )

- For MH > 130 GeV/c2 , can look at:

- T. Han, A.S. Turcot and R. Zhang, Phys. Rev. D **59**, 093001 (1999)

## h -> W*W* Channels 

- Cuts on lots of variables
- Simple Variables
- Intelligent Variables
- Genius Class Variables !

- T. Han, A.S. Turcot and R. Zhang, Phys. Rev. D **59**, 093001 (1999)

## b-Jet Tagging Efficiency

## WH(100 GeV/c2) NN Distributions