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Search for the Higgs Boson
The Inter-University Centre for Astronomy and Astrophysics 24 December 2008
Ashutosh KotwalDuke University
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Why Build Accelerators? From Atoms to Quarks
� Scattering of probe particles off matter to investigate substructure, i.e. “look inside”
� Rutherford did it, shooting α particles at a gold foil, to tell us the structure of the atom (1911)
� Quantum mechanics: ∆r ~ h / ∆p
atom
nucleus
proton, neutron
quarks
Radius Accelerator energy
10-10 m
10-15 m
10-18 m
<10-18 m
10 electron-volts (eV)
106 eV (MeV)
109 eV (GeV)
> GeV
.αAu
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A Century of Particle Physics: Standard Model
� Quark constituents of nucleons established in high energy electron scattering experiments at Stanford Linear Accelerator Center (SLAC), 1966-1978
� Point-like particles explain high scattering rate at large energy and angle
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A Century of Particle Physics: Standard Model
� Success # 1: discovery of 6 quarks and 6 leptons
� 12 fundamental matter particles (and their antimatter counterparts) fit neatly into an elegant mathematical framework
But note theintriguing pattern ofmass values; not explained:
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A Century of Particle Physics: Standard Model
� The “charm quark” (c) discovered at SLAC in 1974
�
The heaviest lepton, “τ” was also discovered at SLAC in 1975
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A Century of Particle Physics: Standard Model
� The heaviest “top quark” (t) discovered at Fermilab in 1995
� The next heaviest, “bottom quark” (b) was also discovered at Fermilab in 1977
� Appearance of τ lepton in ντ beam established at Fermilab
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A Century of Particle Physics: Standard Model
� Success # 2: a really elegant framework for predicting the nature of fundamental forces
� matter particles (quarks and leptons) transform in curved internal spaces
� The equations of motion predict terms that describe particle interactions with force fields
Analogous to the Coriolis and Centrifugal forces generated in rotating frames of reference
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A Century of Particle Physics: Standard Model
Notion of symmetry of equations under “gauge transformations” not just a theoretical success: beautifully confirmed by large amount of experimental particle physics measurements, for
� Electromagnetic force ψ(x) eiφ (x) ψ(x)
� Weak force (radioactivity)
� Strong (nuclear) force: internal space is 3D spherical surface
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The “Problem”, thus Excitement, of Particle Physics
� This highly successful theory predicts that particles should be massless!
Obviously not true in nature
� Not just “Dark Matter”, we do not know the origin of “Visible Matter”
� Theory rescued by postulating a new “Higgs” field, which permeates all space
� A sticky field, particles moving through space scatter off the Higgs field, thereby appearing to be massive
[ Image proposed by David Miller, University College London ]
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The “Problem”, thus Excitement, of Particle Physics
� Proof of the concept: superconductivity
� Normally massless photon (quantum of electromagnetic force) becomes massive in a superconductor
� Conclusion: our vacuum is not a true vacuum
� Its a “false vacuum”, behaving like a superconductor
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Crossing the Energy Threshold for Discoveries“Critical Temperature” for superconducting vacuum ~ 1 TeV
Accelerators at Fermilab (running now with 2 TeV energy) and CERN (start running in 2007 with 14 TeV energy) are at the energyat which the “Higgs Boson” is expected to show up
CERN, Switzerland
FERMILAB
Search for Higgs boson is a key mission of the High Energy Physics program
Large Hadron Collider
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Collider Detector at Fermilab (CDF)
Centralhadroniccalorimeter
Muondetector
Centraloutertracker(COT)
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Particle Detection
Drift chamber (COT):reconstuct particletrajectory by sensingionization in gason high voltage wires
Electromagnetic(EM) calorimeter:lead sheets causee/γ shower, senselight in alternatingscintillator sheets
Hadronic calorimeter:steel sheetscause hadronicshowers, sensescintillator light
Muon chambers:detect penetratingparticles behindshielding
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CDF Tracking Chamber
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Standard Model Higgs Boson Production and Decay
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Higgs Boson Production and Decay
� Different production and decay mechanisms expected for Higgs boson are exploited for its search
MH (GeV/c2)
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Light Higgs Boson Production and Decay
W, Z decay to electrons, muons, τ, and/or neutrinos
Higgs boson decays to bottom quarks
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Simulated Higgs Signal on Expected Backgrounds
Key requirements for observing signal:
Excellent lepton identification, good calorimeters for jet and Missing ET reconstruction, excellent silicon detectors for b jet identification
Good reconstruction of decay particle momentum vectors
Good simulation of signal and background events
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Tagging of b-quark jets
DØ: Neural Network tagger with multiple operating points
CDF: Secondary Vertex tagger, jet probability tagger, and Neural Network flavor separators
50-70% Efficient with 0.3-5% mistag rate
“ ��� �� � ���
� "! # � $ %'& ( #
)! * �! +
“ ,�- ./ 0 1�2
3 4"5 6 . 7 8'9 : 6 4
;5 < .5 =
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Multivariate Techniques for Signal/Background Discrimination
> Likelihood discriminants: Often using Standard Model Matrix Elements to compute differential probability distributions for kinematics
? Artificial Neural Networks: construct non-linear function of kinematics
@ Decision trees: event classification using sequential cuts
Decision Tree
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SM Higgs: ZH→llbb
Z+2jets Neural Network Output ZH
Top quark backgroundsuppressed by anotherneural network
Z + 2 jets background dominant
Part of my research groupactivity
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SM Higgs: VH→ννbb
W (-> lν) + Higgs with lepton undetected alsoincluded in signal
Key issue: modelling the shape of QCD background
Higgs x10
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SM Higgs: WH→lνbb
1.7
2.7
2.7
Lum (fb-1)
Obs. Limit
Exp. Limit
Higgs Events
Analysis
9.38.57.5DØ NN
5.75.67.8CDF ME+BDT
5.05.88.3CDF NN
ACB DE FG D HG I J K L LM N B OQP RM ST U UV I V G D W XY
Key issue: shape of W+bbbackground
obtained from simulation,with normalization fromdata control regions
most sensitive channelfor low-mass Higgs atTevatron
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Heavy Higgs Boson Production and Decay
H
νW-
W+
e-
ν
W-
W+
E+
Spin correlation: Charged leptons
go in the same direction
Most sensitive channel at the Tevatron
Key issue: maximizing lepton acceptance
Higgs
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Heavy Higgs Boson Production and Decay
Most sensitive channel at the Tevatron
Key issue: maximizing lepton acceptance
3.0
3.0
Lum (fb-1)
Obs. Limit
Exp. Limit
Higgs Events
Analysis
2.01.915.6DØ NN
1.61.617.2CDF ME+NN
ZC[ \] ^_ \` _ a bdc e fg h [ iQj k g lm n no a o _ \ p qr
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SM Higgs Boson Production LimitsComparison of Higgs boson production cross section upper limit to the theoretical expectation
Expected Limits on ratio: 1.2 @ 165, 1.4 @ 170 GeV
Observed Limit 1.0 @ 170 GeV
Tevatron excludes at 95% C.L. the production of a SM Higgs boson of 170 GeV
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SM Higgs Boson Production LimitsComparison of Higgs boson production cross section upper limit to the theoretical expectation
s Low mass combination difficult due to ~70 channels
t Expected sensitivity of CDF/DØ combined: <3.0xSM @ 115GeV
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Tevatron Higgs Search Projections
u Improvements for low-mass Higgs in progress
v Dijet mass resolution, increased lepton acceptance and b-tagging efficiency
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Precision Standard Model Measurements Constraining the Higgs and New Physics
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w Radiative corrections due to heavy quark and Higgs loops and exotica
Precision Measurements of W boson and top quark masses
x Top quark mass and W boson mass constrain the mass of the Higgs boson, and possibly new particles beyond the standard model
y My research focuses on the precise mass measurements of these two particles
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Top Quark Mass Measurement
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Top Quark Production at the Tevatron
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Progress on Mtop at the Tevatron
Reconstructed top mass in 680 pb-1of CDF data, fit withsimulated lineshape
CDF Run 2
CDF Run 2
Improved top mass precision dueto in-situ calibration of jet energyusing W->jj decays in the sameevents
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Measurement of Mtop in the dilepton channel
Neural Network for optimized event selection
Matrix-element-based likelihood fitting in dilepton channel
Part of my research activities
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Progress on Mtop at the Tevatron
CDF lepton+jets systematics(preliminary)
Dominant systematic uncertaintiescan be reduced with improved understanding of the data and generator models
δMtop < 1 GeV may be possible
¤
¤ contribution of my research group
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W Boson Mass Measurement
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z SM Higgs fit: MH = 84+34-26 GeV (LEPEWWG & TeVEWWG)
{ LEPII direct searches: MH > 114.4 GeV @ 95% CL (PLB 565, 61)
Motivation for MW measurement
?MW
GF
Sin2θW
Mtop MZ
In addition to the Higgs, is there another missing piece in this puzzle?
( AFBb vs ALR: 3.2σ )
Must continue improvingprecision of MW , Mtop ...
other precision measurementsconstrain Higgs, equivalent to δMW ~ 20 MeV
Motivate direct measurement of MW at the 20 MeV level
νN
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Standard Model Higgs Constraint
∆χ2
MW and leptonic measurements of sin2θ prefer low SM Higgs mass,
hadronic (heavy flavor) measurements of sin2θ prefer higher SM Higgs mass (Ab
FB prefers ~ 500 GeV Higgs)
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Fitting for the W Boson Mass
MW = 80 GeV
MW = 81 GeVMonte Carlo template
Muons DataSimulation
Perform fits to kinematic distributions sensitive to theW boson mass
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Comparisons
The CDF Run 2 result is the most precise single measurement of the W mass(PRL 99:151801, 2007; Phys. Rev. D 77:112001, 2008)
... and factor of 10 more data being analyzed now by my research group
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MW vs Mtop
Supersymmetry theory
Current theory
Lightest neutral supersymmetric particle could be dark matter candidate!
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Preliminary MW Studies of 2.4 fb-1 Data from Tevatron
∆MW~20 MeV
2 fb-1
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The Future – Large Hadron Collider at CERN
LHC will start collecting data with 14 TeV collisions in 2009
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ATLAS Experiment at Large Hadron ColliderA simulated Higgs boson production event, with Higgs decaying to two Z bosons
I am a member of ATLAS Collaboration and will be studying the Higgs boson with ATLAS data
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Summary
| CDF and D0 experiments at the Fermilab Tevatron in pursuit of the mass-generating mechanism:
} Are closing in on the Higgs boson using direct searches
~ Higgs boson excluded at 170 GeV @ 95% CL
� Are constraining the Higgs boson mass by making the most precise measurements of the top quark and W boson masses
� Searching for new fundamental symmetries of nature
� Supersymmetry
� Substructure of particles
� New forces
� Additional spatial dimensions
� Fermilab continues to collect and analyze x3 more data in the next few years – nature may reveal more of its secrets!
� Large Hadron Collider at CERN is likely to reveal additional hidden symmetries