Search for the Higgs Boson
Alex Melnitchouk
Brown University
University of
Mississippi
Oxford, MS
March 25, 2004
Brown University
Providence, RI
Alex Melnitchouk
Ph.D Thesis Defense
September XX , 2003
OUTLINE
• Brief Overview of some Particle Physics Basics
 Luminosity and Cross Section
 Units
 Connection between theory and experiment
• Why Look for Higgs
 What is Mass ? Where does it come from ?
 Standard Model of Elementary Particles
 Electroweak Symmetry Breaking
• What have we learned experimentally
about Higgs so far ?
• Tevatron proton-antiproton collider.
 Higgs Production and Decay Modes
 DØ Detector
h search at DØ.
 Overview of current Higgs analyses
• Beyond the Tevatron
• Conclusions
Z
AN EXAMPLE:
Collide bunches of protons and antiprotons
at certain (high) energy to produce, e.g.,
Z-bosons
At the end of the day the number of
Z-bosons produced will depend on:
1. How many collisions happened
2. Intrinsic properties of
Z-boson, proton, antiproton
(that are independent of the number of collisions)
Luminosity and Cross Section
• Integrated Luminosity Ldt (total number of collisions)
Measured in Inverse Picobarns (pb-1),
e.g. DØ experiment
at Fermi National Accelerator Laboratory (Fermilab)
collected 100 pb-1 of proton-antiproton collisions data
during Run I (1992-1996)
• Cross Section  (interaction probability)
Measured in Picobarns (pb)
e.g  (pp  Z(ee)+X) 200 pb
for collision energy of 1.8 TeV
• Number of Interactions (that happened) =
Cross Section  Integrated Luminosity
e.g 20,000 of Zee events in Run I
• Number of Interactions (observed) =
Cross Section  Integrated Luminosity
 Geometrical Detector Coverage Fraction
 Detector Efficiency
  10,000 of observed Zee events in Run I
Units
• Use h = c = 1 convention
• Use GeV (10 9 eV) units
for Energy, Momentum, and Mass
Theory
Experiment.
One-Slide Review of Basics
• Theoretical description needs to (be):
 Quantum (small distances ~ 10–15 cm)
 Relativistic (speeds close to c)
 Accommodate transformations (production, decays) of particles
 Realtivistic Quantum Field Theory
• Definitions
 A field = system with infinite number
of degrees of freedom
 An elementary particle =
excitation of the field above its ground state(vacuum)
 Lagrangian (total energy) expressed
as a function of fields and their couplings
• To relate Theory to Experiment:
 Perturbative expansion of the Lagrangian
(in terms of coupling constant)
 Calculate expansion terms
(Feynman diagrams)
 Derive Experimentally Measurable Quantities:
•
Cross Sections, Lifetimes
Matter and Energy
1. Massive Structures
(atoms, biological cells, living beings, planets)
2. 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 waves
Double-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
Standard Model of
Elementary Particles
• 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
Higgs
Boson
Bound states  structures
in the Universe
Fermions Interact
via Gauge Boson Exchange
• electron-electron (Möller) scattering
e

e
• Attraction between the nucleus and atomic
electron that leads to a bound state (atom)

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
L  i  m  m   m
m
)
(i 
(mx
m) 
)
ei( x0
 ( x)
is not invariant under
• To preserve the invariance need to introduce additional
vector field Am ( photon field)
1
L   (i  m  m)  e Am   Fm F m
4
m
m
• Photon field is massless
F m   m A   Am
• How do we explain massive W and Z gauge bosons ?
Mass terms break the local gauge invariance and make
the theory non-renormalizable
Electroweak Theory.
Higgs Mechanism
• Electromagnetic and weak interactions are unified
under SU(2)  U(1) gauge group
W1m W2m W3m
Bm
Massless weak and electromagnetic mediators
•
Introduce complex scalar (Higgs) field doublet
 
  
 




1  1  i 2 


2   3  i 4 
• Its Lagrangian is invariant under SU(2)  U(1)
L  ( m ) ( m )  m 2   () 2
• But a choice of particular ground state e.g.
•
1=0, 2=0, 4=0, 32=-m2/=v2
breaks the symmetry in such a way that massive
gauge bosons appear
Higgs Mechanism.
EW Symmetry Breaking
• Symmetry breaking reveals
three extra degrees of freedom Singlet illustration of
(in the unbroken theory they
spontaneous symmetry
breaking
correspond to zero-energy
excitations along the
V()
ground state surface)
which get absorbed
as additional
(longitudinal)
polarizations of W,Z
1
2
vev
- Weak gauge bosons acquire mass



Wμ  2 ( Wμ  Wμ )
Zμ   Bμ sinθ  Wμ cosθ
1
3
0
W
W
mass = 80.4 GeV
- Photon remains massless
A  Bμ cosθ  Wμ sinθ
3
W
W
W
photon
mass = 0
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 :
L  ( m ) ( m )  m 2   ()2
m, , 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
v2=-m2/
• 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 m 2) 1/2
What have we found out about
mH from the experiments so far
• Electro-weak precision
measurements :
mH < 211 GeV
• LEP* direct searches : mH > 114 GeV
Well defined target !
• Summer and Autumn 2000: Hints of a Higgs?
 the LEP data may be giving some indication of a Higgs
with mass 115 GeV (right at the limit of sensitivity)
 despite these hints, CERN management decided to shut
off LEP operations in order to expedite construction of the
LHC†
Before LHC turns on (end of this decade)
the place to look for Higgs is Tevatron** !!!
LEP* = Large Electron-Positron Collider at CERN
LHC† = Large (proton-proton) Hadron Collider at CERN
Tevatron** = Proton-antiproton collider at Fermilab
Tevatron Collider
and Detectors
Batavia, Illinois
Run I 1992-95
Run II 2001-09(?)
100  larger dataset at
increased energy
s =1.96 TeV ; t = 396 ns
Chicago
CDF
DØ
DØ
CDF
DØ
Booster
Tevatron
`p
p
`p source
Main Injector
& Recycler
DØ detector.
The work of many people…
The DØ detector was built and is operated by an
international collaboration of ~ 670 physicists
from 80 universities and laboratories in 19 nations
> 50% non-USA
~ 120 graduate students
Coordinate System
y
Pseudorapidity
 = - log (tan /2)
x
r

z
Underlying
Event
p
d
u
u
g
q
q
d
u
u
Hard Scatter
 Center-of-mass energy is not fixed
 Energy balance can not be used

 use pT = psin 
p
r-z View of the DØ Detector
Muon System
5
0
5
Tracking System
-10
-5
Calorimeter
0
5
(m)
protons
anti-protons
10
Leading SM Higgs Production
Processes at Tevatron
gluon fusion: cross-section ~ m2
 the top-quark loop is dominant
Cross-Section, pb
10.0
s = 2 TeV
W/Z associated
(Z)
1.0
(Z*)
0.1
0.01
W/Z fusion
80 100 120 140 160
Higgs Mass, GeV
quark-antiquark fusion
cross-section is small :
• Higgs-fermion coupling ~ mf
• Masses of u,d quarks are small
Higgs Decay Modes
why  ?
 very clean experimental signature
  decays can be enhanced
Examples of Enhancement
of h decays
h Branching Fraction
no couplings to fermions
(Fermiophobic Higgs)
no couplings to
top,bottom quarks
no couplings to
down-type fermions
Standard Model
Higgs Mass, GeV
S.Mrenna, J.Wells, Phys. Rev. D63, 015006 (2001)
in general we should be prepared for any h
branching fraction ( up to 1.0 ) due to new physics
h Search Strategy
Focus on 2 Scenarios
1
•
•
Fermiophobic Higgs (does not couple to fermions)
Production: W/Z associated + W/Z fusion
Main signature with diphotons :  + 2jets
2
•
•
Topcolor Higgs (of all fermions couples only to top)
Production: all three leading processes
Main signature with diphotons : 
3
Remaining models would give similar signal to
one of the two scenarios:
e.g. no couplings to down-type fermions  topcolor
no t, b couplings  fermiophobic;
Goal :
setting limits on Cross-Section  B()
for both scenarios assuming SM couplings
to W/Z and top-quark (in case of Topcolor)
NEXT QUESTION :
How do we identify
photons in the D0 detector?
The Scale of
Photon
Energies
Atomic
Spectra: ~ eV
X -rays: ~ keV
(103 eV)
Energy, keV
2.0
 -rays: ~ MeV
16.5
31.0
45.0 60.0
(106 eV)
h  : ~ 100 GeV
(1011 eV)
Higgs


`p
p
s =1.96 TeV
Mh=120 GeV
A Slice of the DØ Detector
Interaction
point
Muon
detector
Calorimeter
Tracking system Induces shower
Magnetized
volume
Innermost
tracking layers
use silicon
in dense
material
EM layers
fine sampling Hadronic
layers
Absorber
material
Jet
Electron
Photon
EM showers
developing via
e+e- pair production
and bremsstrahlung
Experimental signature of a Photon :
EM-like shower in the calorimeter
+ NO associated track
DØ Calorimeter
• Uranium/Liquid Argon Sampling Calorimeter
• Three modules: -- Central Calorimeter (CC)
-- Two End Calorimeters (EC)
Unit cell
DØ Calorimeter (Cont’d)
Several unit cells = readout cell

Hadronic

EM
(0,0,0)
EM
Hadronic
Using Cell information – reconstruct clusters of
deposited energy to identify photons
Identification of a Photon
Shower. Isolation
Photon-induced shower is smaller than quark/gluon
shower both transversely and longitudinally
Hadronic
point
Photon ID Tools
(Monte Carlo Distributions)
EM fraction
ratio of EM cluster
energy deposited in
EM calorimeter and
total energy
Isolation
(previous slide)
measure of
cluster
narrowness
multi-variable
shower shape tool
- layer energy
fractions
-width at shower
maximum

QCD jet
misidentified
as 
DØ Tracking System
• Central Fiber Tracker
• Silicon Microstrip Tracker
(0,0,0)
Silicon Tracker
• Focus on Silicon Tracker
Silicon Tracker.
Longitudinal View
In z-coordinate
large region has to be covered -protons and antiprotons collide in bunches:
interaction point is Gaussian-distributed
about z=0 with  = 30 cm
 Barrel/Disk Design:
North
South
1/2-cylinder
 50 cm
6 Barrels
12 F-Disks and 4 H-Disks
Silicon Tracker. x-y View
Barrel
x-y view
beam
line
a hit
SMT Outer
support structure
a ladder
a track
Ladders Installed in Barrels
cabling
barrel with
ladders
cooling
system
outlets
Selection
of  Candidate Events
• Trigger:
di-EM* high pT trigger
• Offline: (on both objects)
•
•
•
Kinematic cuts: pT > 25GeV
Acceptance cuts: Central or End Cap
Calorimeter up to ||=2.4
Photon ID: - shower shape consistent
with EM* shape (EMfraction,
Isolation, H-matrix 2)
- track veto
• *EM = Electromagnetic Object (Photon or Electron)
Event Displays of 
Candidate
•
14
Mass = 125.8 GeV
Topcolor h event
is generally expected
to look like this one
Major Backgrounds :
Drell-Yan and QCD
•
•
Z/e+e- with
e+e- misidentified e.g.
as photons
(lost tracks)
QCD processes that in the final state contain :
1. two
photons
2. a photon and a hadronic jet
misidentified as photon
e.g.
3. two hadronic jets misidentified as photons
e.g.
Observed  Events and
Predicted Backgrounds
Spring-Summer 2003(Ldt=52pb-1)
(Ldt=52pb-1) Results.
No B(h) limits yet 
Fermiophobic
Topcolor
(Ldt  190pb-1) Results
(end of last week !)
Diphoton PT cut
(Ldt 190pb-1) B(h) Limits
(end of last week !)
SM Higgs Search Strategy
• Light Mass Region (M<~140 GeV)
 Use qqW/Z+H(bb)
 For ggH(bb)
QCD background is very large !
• High Mass Region (M>~140 GeV)
 Use inclusive production
 Look for
HWW
Low Mass Region: (DØ) Study SM
backgrounds to WH(We, Hbb)
We
+ two or more
quark/gluon jets
(no b-quark jet
requirement)
We
+ two b-quark jets:
Expect:
5.5 1.6 events
Observe: 3 events
Consistent with
SM background
Low Mass Region:
WH(We(m), Hbb) search at CDF
• We(m) + at least one b-tagged jet
• use 162 pb-1
• Improved limits
on the Cross Section  Branching Fraction
over Run I but sensitivity of current search is
still limited by statistics
High Mass Region:Look for
Excess in WW(ee,em,mm) (DØ)
Missing Et in
dimuon events
(ee)
Dielectron Mass
in WW(ee) events (DØ)
Dielectron Invariant Mass
DØ B(HWW) Limits
(end of last week !)
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-
DØ Search for Neutral SUSY
Higgs Bosons (h,A,H)
• Production cross section ~ (tan)2
• High tan (>~30) models
are motivated by Grand Unification
Neutral Higgs Production can be enhanced
• look for a signal in the invariant mass spectrum of
the two jets with the highest transverse energy in
triple b-tagged multi-jet events
DØ Search for Neutral SUSY
Higgs Bosons (Cont’d)
Invariant mass spectrum for
> = 4 jets (two b-tagged) . Backgrounds
Higgs signal at the
exclusion limit
Invariant mass spectrum for
>=3 jets (three b-tagged)
DØ Neutral SUSY Higgs Limits
Ldt  130 pb-1( tan  vs. mA )
(end of last week !)
Doubly-Charged Higgs (DØ)
• Double charged Higgs appears e.g. in left-right
symmetric models, in Higgs triplet models
• Search for pair production of doubly-charged
Higgs in pp  H++H-- m+m+m-m-
Doubly-Charged Higgs Limits
• Assuming B(H mm)=1.0 DØ set 95% CL limits
of 118.4 GeV and 98.2 GeV for left-handed and
right-handed doubly-charged Higgs boson
• CDF performed similar search and set limits of
135 / 113 GeV
Recent Tevatron Higgs
Sensitivity Study
• Earlier estimates were not over-optimistic
• Improvement due to sophisticated
analysis techniques
The Large Hadron
Collider (LHC)
Lake Geneva

CMS
SPS
ATLAS
Main CERN site
p
p
14 TeV
CMS
ATLAS
Higgs at LHC
• Production cross section and luminosity
both
~ 10 times higher at LHC than at
Tevatron
 Can use rarer decay modes of Higgs
LHC “Precision Channels”
H   for mH = 120 GeV, 100fb-1, CMS
( 1 fb-1 =1000 pb-1 )
H  ZZ(*)  4l, for mH = 300 GeV, 10fb-1,
ATLAS
Both LHC detectors
have invested heavily
in precision EM
calorimetry and muon
systems in order to
exploit these channels
Conclusions
• CDF and DØ are taking physics quality data
and working on many Higgs searches
• Tevatron performance is being improved
• We can see the Higgs in next couple years !
• What if we don’t see it ?
 still important result
 most probable mass range (<125 GeV)
can be excluded with ~5fb-1
 almost all allowed range
can be excluded with ~10fb-1
 In case of MSSM Higgs
almost all parameter space can be excluded
with ~5-10 fb-1
• Stay tuned !