Finding the Missing Piece The Detectors searching for Higgs Boson

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Finding the Missing PieceThe Detectors searching
for Higgs Boson

• Becket Hui
• Physics Department
• University of Texas at Austin

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Overview

• Background
• Production Methods
• Detection Methods
• Results and Conclusion

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Motivation Why Bother?

• Higgs Mechanism unifies electromagnetic and weak
  interaction
• still havent found it yet
• physicists are just ambitious people
• good chance of winning 10 million Krona

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BackgroundWhat is Higgs boson?

• doublet in a complex SU(2) space
• after spontaneous symmetry breaking
• 3 become Goldstone bosons
• 1 scalar field boson remains ? Higgs
• no electric or color charge, spin zero
• only weakly interact with
• Z, W vector bosons, fermions

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Coupling StrengthHow weak could that be?

• weak interaction gw 10-310-1
• electromagnetism gem 0.3
• strong interaction gs gt 1

• Higgs coupling strength in tree level

where g 2MW/v

• for fermions gH mf /v 10-6 10-3
• for vector bosons gH 2m2V /v 20 30
• big for vector bosons, small for fermions

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Coupling StrengthHow weak could that be?

• coupling strength is roughly proportional to mass
  of coupled fermions/ vector bosons
• strong for high mass fermions
• t, heavy quarks
• very strong for vector bosons

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ProcedureRecipe of finding Higgs

• find a way to produce Higgs
• find the decay modes of Higgs
• write a Monte Carlo
• collide and get the data
• filter out noise by different selection rules
• compare filtered data with Monte Carlo
• results depend on mH

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Production type 1How to make Higgs?

• by Higgsstrahlung process

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Production type 1Whos cooking it this way?

• Large Electron-Positron Collider

• operated from 1989 to 2000
• 27 km circumference
• center of mass energy 209GeV

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Production type 2How to make Higgs?

• by hadron collision
• various modes from q-q interaction

• gluon gluon fusion
• Higgstrahlung process
• heavy quark fusion
• vector boson fusion

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Production type 2Whos cooking it this way?

• Tevatron in Fermi National Accelerator Laboratory

• operated from 1983, 6.3 km circumference
• now, center of mass energy is up to 1.96TeV

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Production type 2Who else?

• Large Hadron Collider at CERN

• will start run in 2007, 27 km circumference
• center of mass energy is expected to be 7TeV

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Decay ModesWhat exactly are we looking for?

• table of branching ratio

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Decay ModesWhat exactly are we looking for?

• the branching ratio depends on mH
• if mH lt 130GeV, decay dominated by a pair of
  bottom quark
• if 130GeV lt mH lt 180GeV, decay dominated by a
  pair of W bosons
• if 180GeV lt mH, decay to pair of Z bosons starts
  to increase in probability

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Decay ModesWhat exactly are we looking for?

• for LEP, there are 4 channels to look at
• 4 jet channel
• missing energy channel
• tau channel
• e µ channel

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Decay ModesWhat exactly are we looking for?

• for decays led to pair of W
• its very likely to involve a bottom quarks pair
• it may decay to qqantibbanti
• it may decay to lepton neutrino pairs
• for decays led to pair of Z
• any quark-anti quark/ lepton-anti lepton pair(s)
• predominantly bbanti

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Selection RuleNow what?

• b quark tagging!!
• to identify jet with b quark decay
• may tag b quarks in 3 independent ways
• lifetime
• high momentum leptons
• kinematic tagging

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Selection RuleNow what?

• to minimize the signal of background leptons
• energy cut
• open angle
• angle correlations
• rapidity cut

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LEPWhat have they archived?

• sensitivity of Higgs mass is up to 115GeV
• couldnt find any data to support the existence
  of Higgs within 95 confidence level
• set the new lower bound for Higgs mass at 114.4GeV

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TevatronWhat are they doing?

• able to sense mH up to 180GeV
• develop good QCD and NLO corrections so that the
  theoretical background and Higgs decay has very
  small uncertainties
• did not have enough luminosity in Run 1
• still analyzing Run 2?
• waiting to have Run 2b in 2008

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LHCWhat will they do?

• able to detect mH up to 650GeV
• if H exists, good tool to determine different
  properties, coupling modes of H
• have already developed numerous amount of monte
  carlo models

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Future in this FieldWhat is its potential value?

• continuous efforts on the search
• new efforts on the search of Minimal
  Super-symmetric Standard Model (MSSM) Higgs
• experimentally study Higgs properties
• if it could not be found
• we can sit down and develop a new theory from
  scratch