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