rutgers university september 7 2005 search for higgs amitabh lath rutgers the state university of nj

Rutgers UniversitySeptember 7 2005

Search for Higgs

Amitabh LathRutgers

The State University of NJ

Rutgers UniversitySeptember 7,2005

What is the Higgs?

Short answer: Field/Particle responsible for mass.

Longer answer: Completes the Standard Model of Particles and Fields.

So what is this Standard Model of P & F?


Our Periodic Table


Our Periodic Table

+ Higgs!(not yet found)

Q=2/3

-1/3

0

1

Quark st

uff: hadro

ns.

3q

bary

on

q-a

nti(q)

meso

n


Experimentalist View: Prologue

J.J. Thomson, 1897

Einstein, photoelectric effect

Street and Stevenson, 1937 cloud chamber


Experimentalist View: Weak Effects

Reines and CowanReactor (beta decay)

Lederman, Schwartz and Steinberger group using AGS pion beam


Experimentalist View: Quarks

Kendall, Friedman, Taylor groupat SLAC.

Gell-Mann’s quarksFeynman’s “partons”



Gell-Mann predictsOmega- particle.

Found by bubble chamber group at BNL (Samios et al)



The strange particle decay, KL not as prolific as predicted

Glashow, Iliopoulos, and Maiani posit a “new” particle. (GIM Mechanism)Diagrams with “charm” destructively interfere. Found by Ting, Richter groups (almost

simultaneously) at BNL, SLAC.


Experimentalist View: 3rd Generation

Beautiful analysis by Martin Perl at SLAC’s SPEAR

Fermilab fixed-target experiment (Lederman)

Fermilab collider program(cast of hundreds)

Note: q for each generation = 0


Experimentalist View: Weak Bosons

Glashow, Weinberg and Salaam introduce two primordial fields:• Weak Isospin• Weak Hyperchargethat mix (weak mixing angle)to give W,Z and

They also introduce a scalar field (Higgs).

+ Higgs!

W, Z discovered by UA2


Electro-Weak Unification

Weak Isospin (3-component) W1 + iW2 = W+

W1 – iW2 = W-

Weak Hypercharge (1-component)

cosw -sinw W3

Z sinw cosw B0

LeftHandedInt. only!

=

Weak Mixing Angle


Higgs and ElectroWeak Unification

Adding scalar field to Lagrangian gives mass-like terms, leading to

Masses of bosons determined by couplings, and parameters of the scalar field potential (v,).

Mhiggs is a free parameter.

MW/MZ = cosWAll t

hree

things

in this

equation

can be

measured

by

experiments.


What is Higgs? Scalar Field. Introduced

in: A MODEL OF LEPTONS Steven Weinberg,

Phys.Rev.Lett.19:12641266,1967 . Gives mass to everything, but Higgs boson not seen.

Fermion masses? Need (arbitrary) couplings. Called Gi in Halzen & Martin.

MH < 246 GeV/c2

(fit of all EW data and top mass)


Different Types of Higgs

SM Higgs One (complex) doublet. Cross section for direct production at

Tevatron depressingly low. Theoretical problems.

Beyond SM Higgs Different models fix (some) SM problems. Tevatron can compete in some parts of

parameter space.


Problems with Higgs

Fundamental Scalar Field has problems.

What are the corrections to its mass?

Two basic types of solutions: Higgs is a composite (Technicolor) Other diagrams cancel problems.

(SUSY)


Beyond SM Higgs (Technicolor)

“pion” like composite Higgs. Techniquarks make

technipions QCD-like strong force

(ergo “technicolor”). Ruled out!

The electron-positron asymmetries at the Z pole were not kind to this theory.

However, a lot of physicists have a soft spot for compositeness.

S = Isospin cons. terms

T =

isos

pin

brea

king

ter

ms


Beyond SM Higgs (Supersymmetry)

The “GIM Mechanism” solution to the Higgs problem. Got a problem diagram? Add

another to cancel it out. Fermion sFermion Boson Bosino

A very expensive theory. Doubles the particle spectrum. Consolation prizes: LSPDark

Matter? Unification of couplings?


The MSSM Higgs Minimal

SuperSymmetric Standard Model. Simplest realistic SUSY

theory. 3 neutral(h,A,H),

2 charged Higgs Bosons (masses related).

tan ratio of up/down type couplings.

The LEP experiment (e+e- collider) covered a large swath of parameter space by ruling out higgs up to 90 GeV.


MSSM Higgs Production

At large tan, both gg and bb production contribute. rises like tanb2.

A, and h/H are produced simultaneously.

~100 pb! For inclusive mode

decay is very interesting.

Double this!

~SM Higgs


Higgs Decays

Higgs decays to bb 90% and 10%

(Higgs couples to mass) higgs bb? No hope for inclusive. higgs ? Yes!

Decays to WW, ZZ take over if Higgs is heavy enough.


Higgs Decays Most Higgs searches look for b-quarks.

SM, SUSY higgses (ask SVS). Problem: “real” b-quarks from hadronic

interaction swamp those from Higgs. About 6 orders of magnitude!

Usual solution: Look at “associated” stuff: (Higgs+W/Z, Higgs+extra b-quarks)

Our solution: Forget b. Look for Give up (some) sensitivity to SM Higgs. Gain a clean channel for MSSM Higgs.


The Tevatron Accelerator

Main Injector

Tevatron

CDF

p-pbar collisions at 1.96 TeV C.M. Energy


CDF Collaboration

Missing Conway…


The CDF Detector

beampipe

Collision point

Superconducting Magnet


Particle Detection• Electron

track, contained cluster, E/P~1 , no track

• Quark matter () - track, extended (hadron) cluster

• Muon penetrating track

• Weak, no charge () Missing momenta


Particle Detection• Electron

track, contained cluster, E/P~1 , no track

• Quark matter () - track, extended (hadron) cluster

• Muon penetrating track

• Weak, no charge () Missing momenta

~nb

~mb

~nb

HIGGS? ~pb


The tau lepton.

3rd generation lepton. Heavy. Mass = 1.77

GeV/c2

Lots of decays Leptonic: e,

(17% each) Hadronic: t , , ,

(64%)

Shorthand: e, , h


Looking for Higgs

H

, quark matter

e or

Backgrounds?•Z (irreducible) •We, + “jets” (quark stuff including )

Signal:• One goes to electron or muon.

• We know how to detect e,• Second goes to pions


Detecting taus? Taus are leptons that can decay to “jets” of pions. jets from q,g are fatter.

Signal cone.

Isolation cone (annulus). Up to 30o. Veto tracks, 0…

Should get: Characteristic 1,3 track enhancement. |Q| = 1 m < 1.8

e ,

Standard cuts


Problem with taus.

Fakes from jets (as with almost everything at hadron machines).

h IS a jet; albeit a narrow one (pencil jet).

Jet production 100’s b. Compare to 100’s of pb for production.

Don’t know enough about JETS need fake rate from data. This is where the problems start. Different sources of jets give very different

fake estimates.


Fake Rates. Prologue.

CDF Run 1 (U.C.

Berkeley)

Jet triggered samples give lower fake rates than lepton triggered ones.

Fake rates rising with jet ET.

What is going on? Huge difference!


Solution: Multi-Parameter.

Instead of parameterizing fake rate in one variable, we now use several.

Big discrepancy in rates disappears.

Is this the right thing to do?

Why bother?

“Relative” fake rates


Check of Fake Rate Predictor

Newfangled multi-parameter fake rate function is scary. Jet variables (energy, direction, etc)

BLACK BOX probability of a fake .

Let’s test it on different samples of jets. Jet triggered sample (assume one jet is “lepton”) Photon+jet (use photon as lepton) W+jet. MC Jets.

Let’s look at different distributions, not just absolute numbers.


Checking Jet Fake Rates

PredictorPure sample ofjets

Fake rate prediction

reconstPure sample ofjets

Fake rate measured


Checking Fake Rates (QCD)

This is a sanity check at best, since we built our “black box” -- fake predictor-- using Jet Triggered data.


Checking Fake Rates (+jet)

This is the most heartening set of plots.


Checking Fake Rate (W+jet)


signature

• Both e h and h are included in these plots

• Ntracks = 1,3 and |Q| = 1 not applied to the left plot to demonstrate tau signature

Classic

Hadronic tau

signature


Expected vs. Observed Events

source he hµ all

Z → τ τ 215.8±3.9 186.5±3.4 402.4±5.3

Z → ll 6.5±0.5 8.3±0.8 14.8±1.0

tt, VV 1.0±0.1 0.9±0.1 1.9±0.1

jet → τ 44.6±0.2 30.0±0.4 73.2±0.5

total bg 268.9±3.9 225.7±3.7 492.3±5.4

Observed 260 229 489

Errors

are stat.

only

No big whopping discrepancy…


Higgs-Z Separation.Define p(ET) as (EX,EY, 0 ,ET)

Define mvis = m(p(1) + p(2) + p(ET))

Use these shapes to fit the data,

Is there a “Higgs bump”?

Total acceptance ~1.5% (from simulations).


Mvis Plot

We use this “mass-like” variable to get Higgs/Z separation.

Nice fit! Note: Z does not

peak at MZ!


More on Higgs Fit•Fix Z, other bg by known and luminosity.

•Fit A shapes from simulation

• We don’t see a higgs signal.

?


Events in high mass tailTau (3-prong)

Electron

These look like good Z/* events (Or higgs signal?)More data coming soon!


Expected and Observed Limits


Expected and Observed Limits

fluctuated low

near MZfluctuated high

around m=130

GeV


Compare to Run 1 Limits(914 exp, 700 obs)* (BR to ) = 82.3 exp, 63 obs

(836 exp, 643 obs)* (BR to ) = 75.2 exp, 57.9 obs

We’re a factor o

f 8

better, w

ith approx

three tim

es the

luminosity.

• Get the low pT leptons (trig)• Reconstruct .• Understand bg (esp W+jet)• Fit, don’t count.

m=120

m=140


Extracting MSSM Parameters

We need production cross-section for inclusive A/H/h production. We only require two taus, no extra jets,

b-quarks, leptons, neutrinos…nothing. Most of the recent work has

concentrated on Higgs production associated with “spectator” b-quarks.

Why? Nobody thought could compete.

We need bb A and ggA.


Getting the Cross Section Not trivial! Consensus

emerging slowly among theorists. These results have

spurred some activity. The method (as of June

2005): Get bbhiggs and

gghiggs for SM. Multiply by a

MSSM/SM factor. This ensures corrections

apply properly.This

is fo

r tan=

30


The Exclusion Plot

We did not see a higgs signal, but we can put limits on mass/coupling (tan).

The exclusion depends in “scenarios” chosen by theorists points in the SUSY parameter space that are considered good benchmarks.


Exclusion

benchmark parameters


Exclusion vs. the Parameter.

•The higgs bb search (D0) is sensitive to the SUSY parameter .•This parameter controls the higgs/bb coupling. •As higgs/bb coupling goes up (down), higgs/ coupling goes down (up).


The near future

tan=40 is an interesting benchmark.


Conclusions Taus are the new b.

Lower BR offset by (much) lower bg. Not thought possible at start of Run2. Bonus: insensitive to higgs radiative

corrections. More sensitivity soon.

3x data. Add h-h and -e channels. Smarter fitting. May find ways to be sensitive to SM Higgs.

Taus may be only way to see light higgs at LHC (SM or SUSY).


Afterword: The SM Higgs Current results from DØ and CDF:

WH->lbb ZH->bb WW->ll WWW->l±l± + X

Standard Model predictions much lower than current analyses

Can we close the gap? Maybe.


Backup slides


Big Problem: W+quark “jet”

W gives electron +… …Jet fakes hadronic . OLD (Run 1):

require n to “go along” Now (Run 2)

Geometrical cut

bisector of 2 tau directions

Pvis = sum of the projections of

visible products (onto axis)

P = sum of the projections of visible products and missing transverse energy


Removing W+jet

P > 1.6 Pvis – 10

Suppresses W+jet background without hurting signal


Checking Fake Rate (MC

W+jet)

Maybe Pythia jets aren’t as wrong as we all supposed…


Systematic Errors for Higgs Search

Huge, b,g come from sea

Comes from W

Large error on small bg

This hurts everybody

Source Uncertainty(%) Event cuts 0.9 Electron ID / Trigger 1.9 Muon ID / Trigger 4.6 Tau ID / Trigger /

Hadronic scale 3.6 QCD estimate

(on predicted fakes) 20 Z cross section 2.1 PDF (Z,W) 3.0 PDF (Higgs) 5.7 Luminosity 6.0

Price you have

to pay at h

adron

mach

ines.


Compare to Run 1

Run 1, ~100 pb-1

rutgers university september 7 2005 search for higgs amitabh lath rutgers the state university of nj

Documents

rutgers university

mass slide

state university of

w weak hypercharge

ua2 slide

component cos w sin

component w

higgs amitabh lath rutgers