Brown University Providence, RI

Alex Melnitchouk

Ph.D Thesis DefenseSeptember XX , 2003

Alex Melnitchouk

Search for the Higgs Search for the Higgs BosonBoson

Brown University

University of Mississippi

Oxford, MS March 25, 2004

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 !!!

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)

Z

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 Zee events in Run I

• Number of Interactions (observed) = Cross Section Integrated Luminosity Geometrical Detector Coverage Fraction

Detector Efficiency 10,000 of observed Zee 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 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

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

Fermions Interact via Gauge Boson Exchange

• electron-electron (Möller) scattering

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

e

e

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

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

cosèWsinèBZ W

3

W

0

ììì +−≡

sinèWcosèB W

3

W ìì +≡A

- 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

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

Main Injector & Recycler

p source

Booster

CDF

CDF

DØ

DØDØ

p p

Tevatron

Batavia, Illinois

Chicago

Run I 1992-95Run II 2001-09(?)100 larger dataset at increased energy s =1.96 TeV ; t = 396 ns

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

DØ detector.

The work of many people…

Coordinate System

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

UnderlyingEvent

u

u

d

gq

q u

u

d

Hard Scatter

y

x

z

Pseudorapidity = - log (tan /2)

p p

r

r-z View of the DØ Detector

-10 -5 0 5 10 (m)

5

0

5

Tracking System Calorimeter

Muon System

protons anti-protons

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

why

very clean experimental signature

decays can be enhanced

Examples of Enhancement of h decays

h 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 h 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

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?

p p

s =1.96 TeV

h : ~ 100 GeV (1011 eV)

Mh=120 GeV

2.0 16.5 31.0 45.0 60.0

Energy, keV

The Scale of Photon

Energies

Atomic Spectra: ~ eV

-rays: ~ MeV (106 eV)

X -rays: ~ keV (103 eV)

Higgs

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

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)

(0,0,0)

EM

Had

ron

ic

EM Hadronic

Several unit cells = readout cell

Using Cell information – reconstruct clusters of deposited energy to identify photons

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

QCD jet misidentified as

ratio of EM cluster energy deposited in EM calorimeter and total energy

measure of cluster narrowness

- layer energy fractions -width at shower maximum

DØ Tracking System

Silicon Tracker

(0,0,0)

• Central Fiber Tracker

• Silicon Microstrip Tracker

• Focus on Silicon Tracker

Silicon Tracker. Longitudinal View

6 Barrels12 F-Disks and 4 H-Disks

North South1/2-cylinder

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

50 cm

Barrel/Disk Design:

Silicon Tracker. x-y View

SMT Outer support structure

a ladder

beam

line

a track

Barrel x-y view

a hit

Ladders Installed in Barrels

barrel with ladders

cooling system outlets

cabling

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

Mass = 125.8 GeV

Topcolor h event is generally expected to look like this one

• 14

Major Backgrounds : Drell-Yan and QCD

• Z/e+e- with

e+e- misidentified as photons (lost tracks)

e.g.

3. two hadronic jets misidentified as photons

1. two photons

2. a photon and a hadronic jet misidentified as photon

• QCD processes that in the final state contain :

e.g.

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 qqW/Z+H(bb)

For ggH(bb) QCD background is very large !

• High Mass Region (M>~140 GeV) Use inclusive production

Look for HWW

Low Mass Region: (DØ) Study SM

backgrounds to WH(Weν, Hbb)

Weν + two or more

quark/gluon jets (no b-quark jet requirement)

Weν + two b-quark jets:

Expect: 5.5 1.6 events

Observe: 3 events

Consistent with SM background

Low Mass Region: WH(We()ν, Hbb) search at CDF

• We()ν + 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,e,) (DØ)

Missing Et in dimuon events

(ee)

Dielectron Mass in WW(ee) events (DØ)

Dielectron Invariant Mass

DØ B(HWW) 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

Invariant mass spectrum for >=3 jets (three b-tagged)

Higgs signal at the exclusion limit

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-- ++--

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

Recent Tevatron Higgs Sensitivity Study

• Earlier estimates were not over-optimistic

• Improvement due to sophisticated analysis techniques

The Large Hadron Collider (LHC)

Lake Geneva

Main CERN site

SPS

p p 14 TeV

CMS

ATLAS

ATLAS

CMS

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”

Both LHC detectors have invested heavily in precision EM calorimetry and muon systems in order to exploit these channels

H for mH = 120 GeV, 100fb-1, CMS

H ZZ(*) 4l, for mH = 300 GeV, 10fb-1, ATLAS

( 1 fb-1 =1000 pb-1 )

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 !