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Search for the Higgs Search for the Higgs Boson at the DBoson at the DØ Ø
ExperimentExperiment
Alex Melnitchouk University of Mississippi
Olemiss Colloquium October 30, 2007
OUTLINE
• Standard Model (SM) of Elementary Particles
• Why Look for HiggsWhat is Mass ? Where does it come from ?Electroweak Symmetry Breaking
• Higgs Production and Decay Modes
• Tevatron proton-antiproton collider
• DØ Detector
• Selected Examples of Higgs Searches at DØ non-SM Higgs SM Higgs
• Combined SM Higgs Limits
• Summary
Standard Model of Elementary Particles
Higgs Boson
• Standard Model is a relativistic quantum field theory based on SU(3) SU(2) U(1) gauge group
• SM contains: Spin-1/2 fermions, spin-1 bosons, spin-0 boson
Bound states structures in the Universe
Matter and Energy
In 1717 Isaac Newton wrote “Are not the gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition ? ”
special relativity: E = mc2
Particle Physics: elementary particle (massive or massless) is an excitation of a quantum field above its ground state (vacuum)
• Massive Structures (atoms, biological cells, living beings, planets)
• Light (pure energy)
QUESTIONS:
• What is the difference between the two ?
• What is mass anyway ?
What Do We Know About Mass?
• Measure of Inertia Galileo: speed of falling objects
does not depend on mass
Newton: a = F/m
• Massive particles behave also as wavesDouble-slit QM experiment: electrons (particles of well defined and measured mass) form interference patterns
• Mass is equivalent to energy: E = mc2
• Mass and Spin – two fundamental quantities
V. Bargman and E.P.Wigner: all relativistic wave equations (i.e. particles) can be classified by mass and spin (e.g. massive fermions, massless bosons etc.)
• Mass and Space-Time are connected distribution of mass in the Universe affects
the geometry of space-time (General Relativity)
• Where does mass come from ? Standard Model of elementary particles suggests that mass is not an intrinsic property of a particle but rather comes from the interaction with the HIGGS FIELD
Gauge Symmetries and Interactions
• Existence and properties of force carriers follow from the requirement of the local gauge invariance on the fermion field (Dirac) Lagrangian.
• Gauge groups Interactions: U(1): Electromagnetic SU(2): Weak SU(3): Strong
• e.g. U(1) Photon (Electromagnetic interaction)
• Dirac Lagrangian
is not invariant under
• To preserve the invariance need to introduce additional vector field A ( photon field)
• Photon field is massless
• How do we explain massive W and Z gauge bosons ? Mass terms break the local gauge invariance and make the theory non-renormalizable
)()( )( xex xi Ψ→Ψ α
νν
γγ FFAemiL4
1)( −ΨΨ+Ψ−∂Ψ=
υυν AAF ∂−∂=
Electroweak Theory. Higgs Mechanism
• Electromagnetic and weak interactions are unified under SU(2) U(1) gauge group
• Introduce complex scalar (Higgs) field doublet
• Its Lagrangian is invariant under SU(2) U(1)
• But a choice of particular ground state e.g. • 1=0, 2=0, 4=0, 3
2=-/=v2
breaks the symmetry in such a way that massive gauge bosons appear
W1
W2
W3 B
Massless weak and electromagnetic mediators
⎟⎟⎠
⎞⎜⎜⎝
⎛++
=⎟⎟⎠
⎞⎜⎜⎝
⎛
=43
21
2
1i
i
β
α
22 )()()( −−∂∂= ××× L
Higgs Mechanism. EW Symmetry Breaking
• Symmetry breaking reveals three extra degrees of freedom (in the unbroken theory they correspond to zero-energy excitations along the ground state surface)
vev
Singlet illustration of spontaneous symmetry
breaking
1
2
V()
which get absorbed as additional (longitudinal) polarizations of W,Z
)( WWW ìì2
1ì
±±±≡ m
€
0
Z ≡−μB Wsinθ +
μ
3
W Wcosθ
€
A≡μB Wcosθ +
μ
3
W Wsinθ
- Weak gauge bosons acquire mass
- Photon remains massless W photonmass = 0
mass = 80.4 GeV
Higgs Boson
• Unstable weakly interacting massive spin 0 particle Higgs boson (Higgs field excitation) is also predicted – need to find it to verify Higgs hypothesis (1960’s)
P.W. Higgs, Phys. Rev. Lett. 12 508 (1964); F. Englert and R. Brout, Phys. Rev. Lett. 13 321 (1964); G.S. Guralnik, C.R. Hagen, and T.W.B. Kibble, Phys. Rev. Lett. 13 585 (1964).
Higgs Field Parameters
• There are three parameters that describe the Higgs field :
, , and v (vacuum expectation value)
• v can be expressed in terms of Fermi coupling constant GF (which has been determined from muon lifetime measurement)
v = (2 GF ) –1/2 = 246 GeV and related to the other parameters via v 2 = - 2 /
• There remains a single independent parameter, which can not be determined without experimental information about the Higgs boson
• This parameter can be rewritten as the Higgs boson mass mH = (-2 2) 1/2
22 )()()( −−∂∂= ××× L
Looking for Higgs
• Where do we find it ?Not in natural phenomena (heavy particle)Not in cosmic rays (unstable particle)
• Produce it in the high energy collision in a particle colliderHow can it be produced ?How does it decay ?
How do we detect its decays ?
Tevatron Collider and Detectors
Main Injector & Recycler
p source
Booster
DØ
DØDØ
p p
Tevatron
Batavia, Illinois
Chicago
Run I 1992-95Run II 2001-10(?)100 larger dataset at increased energy s =1.96 TeV ; t = 396 ns
CDF
CDF
Proton-Antiproton Collision
Interaction of proton (antiproton) constituents Center-of-mass energy is not fixed Energy balance can not be used use balance of transverse energy
UnderlyingEvent
u
u
d
gq
q u
u
d
Hard Scatter
p p
Leading SM Higgs Production Processes at Tevatron
80 100 120 140 160
0.01
0.1
1.0
10.0
Higgs Mass, GeV
Cross-Section, pb
s = 2 TeV
gluon fusion : cross-section ~ m2 the top-quark loop is dominant
(Z*)
(Z)W/Z associated
W/Z fusion
quark-antiquark fusion cross-section is small :
• Higgs-fermion coupling ~ mf
• Masses of u,d quarks are small
Higgs Decay Modes
low masses (< ~135 GeV) : bb
high masses (> ~135 GeV ) : WW
r-z View of the DØ Detector
-10 -5 0 5 10 (m)
5
0
5
Tracking System Calorimeter
Muon System
protons anti-protons
A Slice of the DØ Detector
Hadronic
layers
Tracking system
Magnetized volume
Calorimeter Induces shower
in dense material
Innermost tracking layers
use silicon
Muon
detector
Absorber material
EM layersfine sampling
Interactionpoint
Jet
Electron
Photon
EM showers developing via e+e- pair production and bremsstrahlung
The DØ detector was built and is operated by an international collaboration of ~ 670 physicists from ~80 universities and laboratories in 18 nations
DØ detector.
The work of many people…
Four Examples of Higgs Searches at DØ
H γγ H++ ++
H bb
H W+W-
pick decays to particles from each
group in the SM table
tree-level decay
decays via loops
Four Examples of Higgs Searches (Cont’d)
H γγ H++ ++
H bb
H W+W-
Non-SM Higgs Searches
SM Higgs Searches
Particle Mass, in M(proton)
M(photon) = 0
M(muon) 0.1 x M(proton)
M(b-quark) 4 x M(proton)
M(W) 90 x M(proton)
Go through the four analyses in the order of decreasing mass of Higgs decay products
starting with the non-SM Hγγ search
Hγγ Decays
• no tree-level Hγγ coupling (Higgs is neutral)
• diphoton decays happen via W or top-quark loop:
• Hγγ Standard Model branching fraction is small: ~ 10-3-10-4
• however many extensions of the SM predict enhanced γγ decay rate of the Higgs
• There are several theories (extra dimensions, SUSY, generic two-doublet, strong dynamics) with the same underlying idea for γγ enhancement
How Can Hγγ Decays be Enhanced
• In the Standard Model (SM), Higgs holds a monopoly on producing bare mass: Gauge boson masses (EWSB†)Fermion masses
• The mass terms in the SM Lagrangian have a common factor of (1/v††)
• The relative strength of Higgs couplings to W, Z, and Fermions is fixed
• A more general scenario would allow different mechanisms for EWSB and fermion masses => couplings can vary independently => branchings enhanced
† EWSB = Electroweak symmetry breaking †† v = Higgs field vacuum expectation value
(1/v) 2m2WW+W-H (1/v)
m2ZZZH (1/v) mf H
Hγγ Analysis Overview
• Inclusive γγ X search for NON-SM Higgs
• Main variable: diphoton invariant mass
• General strategy:
understand invariant mass spectrum of di-photon candidate sample
look for a “bump”
• Main backgrounds: real photons or/and misidentified jets from QCD
processes
Two types of backgrounds to hγγ signal:
both photons are real (physics background=irreducible background)
at least one photon is a quark (or gluon) jet, misidentified for a photon (instrumental background=reducible background)
Main Backgrounds
Comparing Background Predictions of Different Methods
Question: how many photon candidates
observed in our data sample are real photons ?
Employ several independent methods to answer this question
How many real photons in our data? Answers from DØ Calorimeter,
Preshower Detector, and Simulation
preshower detector
Photon Jet
Photon Jet
charge in
liquid argon
light in scintillator
strips
Monte Carlo Simulation
Photon
unit cell
Estimated Fraction of Real Photons
Control Region Signal Region
-- Preshower info
-- Calorimeter info
-- Theory
the methods are un-correlated to a large extent
Di-Photon Invariant Mass
Event Displays of
γγ Candidate Event
Mass = 125.8 GeV
• 14
Hγγ Results
“Fermiophobic” Higgs is excluded for the mass values below 92 GeV
• Double charged Higgs appears in left-right symmetric models, Higgs triplet models, Little Higgs models.
• Search for pair production of doubly-charged Higgs in pp H++H-- ++--
• Can also look for Higgs decays to electrons and taus (as well as mixed lepton flavor decays)
• Currently focus on ++-- final state assuming B()=100%
Doubly-Charged Higgs Decaying to Muons
• Select events with at three isolated muons
• Match muons measured by the muon system to the tracks measured by the central tracker
• To reduce Z+- background, require smaller azimuthal separation between the muons: <0.8
Doubly-Charged Higgs Decaying to Muons (Cont’d)
Doubly-Charged Higgs. Invariant Mass of Muon Pairs
Doubly-Charged Higgs. Results
What do we know about SM Higgs Mass so far
• Electro-weak precision measurements : mH < 144 GeV
• LEP* direct searches : mH > 114 GeV
Well defined target !
LEP* = Large Electron-Positron Collider at CERN
SM Higgs Search Strategies
• Light Mass Region (M<~140 GeV) Look for H bb Use qqW/Z+H(bb)
• High Mass Region (M>~140 GeV) Look for HW+W-
• gluon fusion
• W associated production
• Z +-, e+e- , νν
• Weν , ν
• Plenty of final states !
ZHe+e-bb (or +-bb) Analysis. Introduction
• Higgs produced in association with the Z boson look for Higgs decays into bb,
while Z decays into +- or e+e- (7% of the time) • Event Selection
at least two jets• two loosely b-tagged jets or one tightly b-tagged jets
• Neural Network b-tagging algorithm
two electrons or two muons with PT above 15 GeV
• invariant mass of the pair of electrons (or muons) should be consistent with the Z boson mass: 70 GeV < Minv<110 GeV ( M(Z)90 GeV )
• Main backgrounds are Z+jets, especially Z+bb• Instead of searching for a resonance in the di-jet mass
distribution, use a multivariate Neural Net M(bb), PT(jet1), PT (jet2) , angular separation between
two electrons (muons), …, total transverse energy in the event
ZHe+e-bb (or +-bb) Analysis (Cont’d)
Invariant Mass of two most
energetic jets
Neural Net Output
Set limit on ZZ (~5 times larger than SM prediction)
ZZ e+e-bb (or +-bb) background looks very much like Higgs signal
ZHe+e-bb (or +-bb) Analysis. Results
WHWWW Analysis. Introduction
• Higgs produced in association with the W boson and decays into a pair of Ws Higgs decays mostly two WW pairs
for Higgs masses above 135 GeV three Ws in the event W decays to electron and neutrino (or muon and
neutrino) 20% of the time• Require two isolated like-sign electrons or muons
with transverse momentum above 15 GeV
• WHWWW is advantageous over HWW in HWW look for opposite side leptons large physics backgrounds from
Z/γ*+-, WWe+νe-ν • Look for excess of events over predicted SM
background• Physics backgrounds are small• Instrumental backgrounds
“charge flips” e.g. in Z/γ*+- events mis-identified electrons or muons
WHWWW Analysis. Invariant Mass of ee, ,
e
ee
e
19 events
5 events
15 events
WHWW Analysis. Results
Combining SM Higgs Limits
• Searches in 15 final states, each designed to isolate particular Higgs production and decay mode
• Some analyses use two datasets: before and after the 2006 DØ Detector upgrade (RunIIa and RunIIb)
• Total of 21 individual analyses
• Luminosities ranging from 0.9 to fb-1 1.7 fb-1
Combined SM Higgs Limits from DØ Experiment
Combined SM Higgs Limits from Tevatron (DØ + CDF)
SUMMARY
• Origin of mass is one of the most exciting topics of modern physics
• Standard Model is incomplete without a mechanism for electroweak symmetry breaking
• Higgs mechanism is a simple and elegant solution
• Higgs searches are well underway at the Tevatron proton-antiproton collider
• Many final states have been studied by the DØ experiment
• No signal yet
• More data is coming
• We may see the Higgs soon
• If you are a student considering different research avenues – DØ experiment is a great place to do physics !
Back Up Slides Start Here
SUSY Higgs
• Supersymmetry (SUSY) is a symmetry between spin degrees of freedom any ordinary particle has a (much heavier) supersymmetric partner particle (to be discovered yet)
• SUSY Higgs sector consists of more than one Higgs particle
• e.g. Minimal Supersymmetric Model (MSSM) : two complex scalar Higgs doublets two VEV’s v1 and v2 (tan=v1/v2) 5 Higgs particles : h0, H0, A0, H+, H-
• Searches targeting SM-like h0 or H+(H-)
CPS Templates
Fitting Method . Signal Region
Examples of Enhancement of hγγ decays
hγγ Branching Fraction
Higgs Mass, GeV
Standard Model
no couplings to fermions (Fermiophobic Higgs)
no couplings to down-type fermions
in general we should be prepared for any hγγ branching fraction ( up to 1.0 ) due to new physics
S.Mrenna, J.Wells, Phys. Rev. D63, 015006 (2001)
no couplings to top,bottom quarks
DØ Tracking System
Silicon Tracker
(0,0,0)
• Central Fiber Tracker
• Silicon Microstrip Tracker
• Focus on Silicon Tracker
Identification of a Photon Shower. Isolation
Hadronic
point
Photon-induced shower is smaller than quark/gluon shower both transversely and
longitudinally
Photon ID Tools (Monte Carlo Distributions)
EM fraction
Isolation (previous slide)
multi-variable shower shape tool
γQCD jet misidentified as γ
ratio of EM cluster energy deposited in EM calorimeter and total energy
measure of cluster narrowness
- layer energy fractions -width at shower maximum
Definitions of some Kinematic Variables
pT = psin
y
x
z
Pseudorapidity = - log (tan /2)
r
Tevatron RunI (0.1 fb-1) 1992-1996
DØ : γγ2 jets analysis mass limit of 78.5 GeV at 95% C.L. B.Abbot et al. Phys. Rev. Lett. 82, 2244 (1999 )
CDF : γγ2 jets; γγe, , MissingEt analysis mass limit of 82 GeV at 95% C.L. F.Abe et al. Phys. Rev. D59, 092002 (1999)
LEP limit : 108.2 GeV at 95% CL hep-ex/0107035 (2001)
Current Status of hγγ Searches
Limits set for “Benchmark Fermiophobic Higgs”:
-- all Higgs-fermion couplings are turned off
-- Higgs production cross-section (W/Z associated, W/Z fusion) is the same as in the Standard Model
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