Brown University Providence, RI

1
Search for the Higgs Boson
Brown University Providence, RI
Alex Melnitchouk
Ph.D Thesis Defense September XX , 2003
2
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

Fresh results !!!
3

• 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
• How many collisions happened
• Intrinsic properties of
  Z-boson, proton, antiproton
  (that are
  independent of the number of collisions)

4
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

5
Units

• Use h c 1 convention
• Use GeV (10 9 eV) units
  for Energy,
  Momentum, and Mass

6
Theory Experiment.
One-Slide Review of Basics

• Theoretical description needs to (be)
• Quantum (small distances 1015 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

7
Matter and Energy

• Massive Structures
  (atoms, biological cells, living
  beings, planets)
• Light (pure energy)
• QUESTIONS
• What is the difference between the two ?
• What is mass anyway ?

8
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

9
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
10
Fermions Interact via
Gauge Boson Exchange

• electron-electron (Möller) scattering
• Attraction between the nucleus and atomic
  electron that leads to a bound state (atom)

?
11
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 Am ( 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

12
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.
• ?10, ?20, ?40,
  ?32-m2/?v2
  breaks the symmetry in such a way that massive
  gauge bosons appear

W1? W2? W3?
B?
Massless weak and electromagnetic mediators
13
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)

Singlet illustration of spontaneous symmetry
breaking
V(?)
which get absorbed as additional
(longitudinal) polarizations of W,Z
?1
?2

- Weak gauge bosons
acquire mass
vev
- Photon remains massless
14
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
  (1960s)

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).
15
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

16
What have we found out about mH from the
experiments so far

• Electro-weak precision
  measurements mH lt
  211 GeV
• LEP direct searches mH gt 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

17
Tevatron Collider and Detectors
18
The DØ detector was built and is operated by an
international collaboration of 670 physicists
from 80 universities and laboratories in 19
nationsgt 50 non-USA 120 graduate students
DØ detector.
The work
of many people
19
Coordinate System
r
p
p

• Center-of-mass energy is not fixed
• Energy balance can not be used
• ? use pT psin ?

20
r-z View of the DØ Detector
Muon System
5 0 5
Tracking System
Calorimeter
-10 -5 0
5 10 (m)
protons anti-protons
21
Leading SM Higgs Production Processes at Tevatron
gluon fusion
cross-section m2
? the top-quark loop is dominant
Cross-Section, pb
10.0
W/Z associated
1.0
(Z)
(Z)
0.1
0.01
80 100 120 140 160
W/Z fusion
Higgs Mass, GeV

• quark-antiquark fusion
  cross-section is small
• Higgs-fermion coupling mf
• Masses of u,d quarks are small

22
Higgs Decay Modes

• why gg ?
• very clean experimental signature
• gg decays can be enhanced

23
Examples of Enhancement of h?gg decays
h?gg 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?gg
branching fraction ( up to 1.0 ) due to new
physics
24
h?gg Search Strategy
Focus on 2 Scenarios

• Fermiophobic Higgs (does not couple to fermions)
• Production W/Z associated W/Z fusion
• Main signature with diphotons gg 2jets
• Topcolor Higgs (of all fermions couples only to
  top)
• Production all three leading processes
• Main signature with diphotons gg
• 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(gg) 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?
25
The Scale of Photon Energies
Atomic Spectra eV
X -rays keV (103 eV)
? -rays MeV (106 eV)
h?? ? 100 GeV (1011 eV)
Mh120 GeV
?
Higgs
?
26
A Slice of the DØ Detector
Electron
EM showers developing via ee- pair
production and bremsstrahlung
Photon
Experimental signature of a Photon
EM-like shower in the
calorimeter NO
associated track
27
DØ Calorimeter

• Uranium/Liquid Argon Sampling Calorimeter
• Three modules -- Central Calorimeter (CC)
  -- Two End Calorimeters (EC)

Unit cell
28
DØ Calorimeter (Contd)
Several unit cells
readout cell
?
?
Hadronic
EM
(0,0,0)
EM
Hadronic
Using Cell information reconstruct clusters of
deposited energy to identify photons
29
Identification of a Photon Shower. Isolation
Photon-induced shower is smaller than quark/gluon
shower both transversely and longitudinally
30
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
31
DØ Tracking System

• Central Fiber Tracker
• Silicon Microstrip Tracker

(0,0,0)
Silicon Tracker

• Focus on Silicon Tracker

32
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 z0 with ? 30 cm
? Barrel/Disk Design
? 50 cm
33
Silicon Tracker. x-y View
Barrel x-y view
beam line
a hit
SMT Outer support structure
a track
a ladder
34
Ladders Installed in Barrels
barrel with ladders
cabling
cooling system outlets
35
Selection
of gg Candidate Events

• Trigger
  di-EM high pT
  trigger
• Offline (on both objects)
• Kinematic cuts pT gt 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)
  

36
Event Displays of gg Candidate

• 14

Mass 125.8 GeV Topcolor h?gg event is
generally expected to look like this one
37
Major Backgrounds Drell-Yan and
QCD
e.g.

• 3. two hadronic jets misidentified as photons
  

e.g.
38
Observed gg Events and Predicted Backgrounds
Spring-Summer 2003(?Ldt52pb-1)
39
(?Ldt52pb-1) Results. No B(h?gg)
limits yet ?
Fermiophobic
Topcolor
40
(?Ldt ? 190pb-1) Results(end of last week !)
Diphoton PT cut
41
(?Ldt? 190pb-1) B(h?gg) Limits (end of last week
!)
42
SM Higgs Search Strategy

• Light Mass Region (Mlt140 GeV)
• Use qq?W/ZH(?bb)
• For gg?H(?bb)
  QCD background is very large !
• High Mass Region (Mgt140 GeV)
• Use inclusive production
• Look for H?WW

43
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
44
Low Mass Region WH(W?e(?)?,
H?bb) search at CDF

• W?e(?)? 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

45
High Mass RegionLook for Excess in WW(ee,e?,??)
(DØ)
Missing Et in dimuon events
??(ee)
46
Dielectron Mass in
WW(ee) events (DØ)
Dielectron Invariant Mass
47
DØ ??B(H?WW) Limits (end of
last week !)
48
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 VEVs v1 and v2 (tan?v1/v2)
• 5 Higgs particles h0, H0, A0, H, H-

49
DØ Search for Neutral SUSY Higgs Bosons (h,A,H)

• Production cross section (tan?)2
• High tan? (gt30) 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

50
DØ Search for Neutral SUSY Higgs Bosons (Contd)
Invariant mass spectrum for
gt 4 jets (two
b-tagged) . Backgrounds
Higgs signal at the exclusion
limit
Invariant mass spectrum for
gt3 jets (three b-tagged)
51
DØ Neutral SUSY Higgs Limits ?Ldt ? 130 pb-1( tan
? vs. mA ) (end of last week !)
52
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 ? HH--? ???-?-

53
Doubly-Charged Higgs Limits

• Assuming B(H? ??)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

54
Recent Tevatron Higgs Sensitivity Study

• Earlier estimates were not over-optimistic
• Improvement due to sophisticated analysis
  techniques

55
The Large Hadron Collider (LHC)
56
Higgs at LHC

• Production cross section and luminosity both
  10 times higher at LHC than at Tevatron
• Can use rarer decay modes of Higgs

57
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

58
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 dont see it ?
• still important result
• most probable mass range (lt125 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 !