Post on 22-Dec-2015
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?
Rutgers UniversitySeptember 7,2005
Our Periodic Table
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
Experimentalist View: Prologue
J.J. Thomson, 1897
Einstein, photoelectric effect
Street and Stevenson, 1937 cloud chamber
Rutgers UniversitySeptember 7,2005
Experimentalist View: Weak Effects
Reines and CowanReactor (beta decay)
Lederman, Schwartz and Steinberger group using AGS pion beam
Rutgers UniversitySeptember 7,2005
Experimentalist View: Quarks
Kendall, Friedman, Taylor groupat SLAC.
Gell-Mann’s quarksFeynman’s “partons”
Rutgers UniversitySeptember 7,2005
Experimentalist View: Quarks
Gell-Mann predictsOmega- particle.
Found by bubble chamber group at BNL (Samios et al)
Rutgers UniversitySeptember 7,2005
Experimentalist View: Quarks
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.
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
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)
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
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)
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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?
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
The Tevatron Accelerator
Main Injector
Tevatron
CDF
p-pbar collisions at 1.96 TeV C.M. Energy
Rutgers UniversitySeptember 7,2005
CDF Collaboration
Missing Conway…
Rutgers UniversitySeptember 7,2005
The CDF Detector
beampipe
Collision point
Superconducting Magnet
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
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!
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
Checking Jet Fake Rates
PredictorPure sample ofjets
Fake rate prediction
reconstPure sample ofjets
Fake rate measured
Rutgers UniversitySeptember 7,2005
Checking Fake Rates (QCD)
This is a sanity check at best, since we built our “black box” -- fake predictor-- using Jet Triggered data.
Rutgers UniversitySeptember 7,2005
Checking Fake Rates (+jet)
This is the most heartening set of plots.
Rutgers UniversitySeptember 7,2005
Checking Fake Rate (W+jet)
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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…
Rutgers UniversitySeptember 7,2005
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).
Rutgers UniversitySeptember 7,2005
Mvis Plot
We use this “mass-like” variable to get Higgs/Z separation.
Nice fit! Note: Z does not
peak at MZ!
Rutgers UniversitySeptember 7,2005
More on Higgs Fit•Fix Z, other bg by known and luminosity.
•Fit A shapes from simulation
• We don’t see a higgs signal.
?
Rutgers UniversitySeptember 7,2005
Events in high mass tailTau (3-prong)
Electron
These look like good Z/* events (Or higgs signal?)More data coming soon!
Rutgers UniversitySeptember 7,2005
Expected and Observed Limits
Rutgers UniversitySeptember 7,2005
Expected and Observed Limits
fluctuated low
near MZfluctuated high
around m=130
GeV
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
Exclusion
benchmark parameters
Rutgers UniversitySeptember 7,2005
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).
Rutgers UniversitySeptember 7,2005
The near future
tan=40 is an interesting benchmark.
Rutgers UniversitySeptember 7,2005
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).
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
Backup slides
Rutgers UniversitySeptember 7,2005
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
Rutgers UniversitySeptember 7,2005
Removing W+jet
P > 1.6 Pvis – 10
Suppresses W+jet background without hurting signal
Rutgers UniversitySeptember 7,2005
Checking Fake Rate (MC
W+jet)
Maybe Pythia jets aren’t as wrong as we all supposed…
Rutgers UniversitySeptember 7,2005
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.
Rutgers UniversitySeptember 7,2005
Compare to Run 1
Run 1, ~100 pb-1