1 c. mills (edinburgh) 12 january 2015 higgs physics at atlas from the ww perspective corrinne mills...

1c. mills (Edinburgh)12 January 2015

Higgs physics at ATLAS Higgs physics at ATLAS from the WW from the WW

perspectiveperspectivecorrinne mills

(University of Edinburgh)

Presentation at UPenn12 January 2015


OverviewOverview• Study of the Higgs boson discovered at 125 GeV in 2012 has been a

fruitful enterprise So far, everything (interactions, quantum numbers) is in excellent agreement

with the Standard Model predictions

• H WW is a powerful channel for rate measurements Balance between large event yield and manageable S/B

• Final Run I results in WW channel are out! Substantial improvement since Moriond ‘13 (same dataset)

40% improvement in observed significance (4.3 6.1) and 65% in expected (3.5 5.8)

Reimagined almost every aspect of the analysis

Objects, background estimates, signal categorization Can’t cover an 80 page paper here, picking a few favorite things

Working on this measurement since mid-2011 and convener of the WW Higgs analysis group since March 2014

• Knowledge and tools gained apply to some of the most interesting measurements in the next LHC runs


The MeasurementsThe Measurements


The Standard Model The Standard Model Higgs BosonHiggs Boson

H

V

V

H

f

f

H

H

H

tree-level (LO) interactions:

Standard Model makes a very specific set of predictions for the Higgs bosonSpin-zero scalar particleResponsible for electroweak symmetry breaking (masses of W and Z bosons)Posited to give mass to all elementary particlesInteractions completely determined by the mass of the Higgs boson, the mass of the other particle, and the vacuum expecation value v

Standard Model makes a very specific set of predictions for the Higgs bosonSpin-zero scalar particleResponsible for electroweak symmetry breaking (masses of W and Z bosons)Posited to give mass to all elementary particlesInteractions completely determined by the mass of the Higgs boson, the mass of the other particle, and the vacuum expecation value v


The Large Hadron The Large Hadron Collider, CERNCollider, CERN

The AlpsThe Alps

Lac LemanLac Leman

CERNCERN

GenèveGenève

airplanes go hereairplanes go here

world’s highest energy world’s highest energy particle colliderparticle colliderpp collisions at pp collisions at sqrt(s) = 8 TeVsqrt(s) = 8 TeV


SM Higgs Boson SM Higgs Boson ProductionProduction

• LHC collisions are qq, gg, qg, qqbar (energetic protons gluons ⇒and sea quarks matter)

• H is colorless, gluon is massless no (direct) strong production⇒

• direct qqbar production suppressed by mq2/v

• LHC collisions are qq, gg, qg, qqbar (energetic protons gluons ⇒and sea quarks matter)

• H is colorless, gluon is massless no (direct) strong production⇒

• direct qqbar production suppressed by mq2/v

g

g

H

q’ q’’

q q’

W+

W–

H

ggFggF

VBFVBF

t, b, etc…

HW/Z(*)

W/Z

q

q

VHVH

ttHttH

g

gt

t

H


SM Higgs Boson SM Higgs Boson ProductionProduction

g

g

H

q’ q’’

q q’

W+

W–

H

ggFggF

VBFVBF

t, b, etc…

HW/Z(*)

W/Z

q

q

VHVH

ttHttH

g

gt

t

H

process 8 TeV 13 TeV

ggF gluon-gluon fusion 19 pb 44 pb

VBF vector-boson fusion 1.6 pb 3.7 pb

VH associated production 1.1 pb 2.2 pb

ttH associated production 0.13 pb 0.51 pb

Cross sections for mH=125 GeV:


H → diphoton (H → diphoton ()) • : good sensitivity at low mH, full mH

reconstruction from photon 4-vectors• Nothing else in the SM gives you a

resonant peak in the diphoton spectrum

• : good sensitivity at low mH, full mH reconstruction from photon 4-vectors

• Nothing else in the SM gives you a resonant peak in the diphoton spectrum

t, W, …H


H → ZZ → 4lH → ZZ → 4lZZ: 4l channel (with Z → ee, Z → ) has excellent S/B, best resolution for mH reconstruction, but low branching ratio to 4l

ZZ: 4l channel (with Z → ee, Z → ) has excellent S/B, best resolution for mH reconstruction, but low branching ratio to 4l

Z

Z(*)H


Detection: Fermionic Detection: Fermionic decaysdecaysbb and : large BRs at low mH, full mass reconstruction (but worse resolution than leptons or s), huge continuum background

bb and : large BRs at low mH, full mass reconstruction (but worse resolution than leptons or s), huge continuum background

f

f

H


H → WW → lH → WW → lllW+

W–

’

125

~22%H

WW channel well-suited to exploring rare production modes and searching at high-mass

Large branching ratio and good S/B from clean dilepton signature

WW channel well-suited to exploring rare production modes and searching at high-mass

Large branching ratio and good S/B from clean dilepton signature

channel(pp→H)@ 8 TeV BR (H→VV) BR(VV→4l)

evt. yield in 20 fb-1

S/B (approx.)

H →

22.3 pb

0.0023 - 1000 0.03 – 0.5

H → ZZ → 4l 0.026 0.0011 130 2

H → WW → ll 0.22 0.047 4700 0.1 – 0.4


The ATLAS DetectorThe ATLAS Detector

Inner Detector (tracking):Pixels (silicon)SCT (silicon strips)TRT (straw tubes / ionization)

Electromagnetic calorimeterLiquid Argon (LAr) with lead absorber

Hadronic calorimeter (jets, hadrons)Steel absorber + scintillatorLAr with copper/tungsten absorber

Solenoid

Toroids

Muon Detectors: large radii for precise momentum measurement Precision: Drift tubes (MDT) and Cathode Strip Chambers (CSC)Trigger: Resistive Plate Chamber (RPC) and Thin Gap Chamber (TGC)


Hadron Collider Hadron Collider KinematicsKinematics

beam axisz

x


Jets and b-taggingJets and b-tagging

Final state quarks and gluons fragment to produce additional quarks and gluons and hadronize into colorless particles (p, n, , , etc.)

Identified as a collimated “jet” of particles in tracker and calorimeter

Final state quarks and gluons fragment to produce additional quarks and gluons and hadronize into colorless particles (p, n, , , etc.)

Identified as a collimated “jet” of particles in tracker and calorimeter

Hadrons containing heavy quarks (b, c) travel some distance in the detector and can be identified by the resulting displaced tracks and decay vertices

“tagging” of b-jets: we use 85% efficient operating point

Hadrons containing heavy quarks (b, c) travel some distance in the detector and can be identified by the resulting displaced tracks and decay vertices

“tagging” of b-jets: we use 85% efficient operating point


Electrons and MuonsElectrons and Muons• A pair of charged leptons is the foundation of the event selection

Guiding principle: reasonable efficiency and ruthless suppression of background from fake and non-prompt leptons

Why do we care? W+jets, that’s why:

(pp H WW ll) = 0.23 pb vs (pp W l) = 12,000 pb

• Muons: combined track from inner detector and muon spectrometer

• Electrons: cluster in EM calorimeter combined with inner detector track

• Isolation requirements check that there is not substantial activity in a small cone around the lepton direction (as from a jet)

Use both calorimeter and tracking information

• Impact parameter requirements reject leptons from heavy flavor decay


Missing Transverse Missing Transverse EnergyEnergy

Detect neutrinos by summing up all of the other objects in the event•fully reconstructed objects (leptons, jets, photons) and “soft” stuff

invoke conservation of momentum in the plane transverse to the beam:

“Missing Transverse Energy”

corresponds to the transverse energy of the neutrinos in the event

Detect neutrinos by summing up all of the other objects in the event•fully reconstructed objects (leptons, jets, photons) and “soft” stuff

invoke conservation of momentum in the plane transverse to the beam:

“Missing Transverse Energy”

corresponds to the transverse energy of the neutrinos in the event

… but we have two neutral leptons – the neutrinos – to account for as well:


TrackMETTrackMET• ETmiss from resolved objects (leptons, jets,

photons) and “soft term” of not-otherwise-identified objects, usually hadrons

• Soft term from tracks gives better resolution than hadron clusters for high pileup


The WW analysisThe WW analysis


The H The H WW signature WW signature

W

W*

Final-state signature: 2 charged leptons + 2 neutrinos

Off-shell W (mH < 2mW) means one lepton is soft

Final-state signature: 2 charged leptons + 2 neutrinos

Off-shell W (mH < 2mW) means one lepton is soft

(soft)

jets depend on production mode(VBF has two at LO)

jets depend on production mode(VBF has two at LO)

spin zero initial state influences final-state kinematics

spin zero initial state influences final-state kinematics

from 1201.3084v1

MT0 mH

Transverse mass MT: Like invariant mass, but drop missing pz information signal discriminant

Transverse mass MT: Like invariant mass, but drop missing pz information signal discriminant

g

g

H

t, b, …


Begin from the beginning: two charged leptons (e or ) and missing transverse energy*:

Stacked plots show signal and background, compared to the data

[Yep, signal is so small as to be (almost) invisible here.]

e

ee/

Event selection vs. Event selection vs. backgroundsbackgrounds

* no missing ET cut applied in e VBF channel


The backgroundsThe backgrounds

g

g

t

t

t W+

W–

b

b

W+

W–

q

q

Z/*

WW: quasi-irreducible

top: has jetsthe “easy” ones:

Z/* → → ll(not illustrated)

cake diagrams?


The backgroundsThe backgrounds

q

q’

W±

g

W+jets: fake leptons

the tricky ones:

q

q

Z/*

Z/*: no neutrinos

W(*): soft/lost/merged leptons, q

W±

q *

split in two,show next to plots


Event Selection & Higgs Event Selection & Higgs topologytopology

• Reject dominant BG like WW and Z and by using the spin correlations

Leptons tend to not be back-to-back, and one lepton is soft because of the off-shell W (mH < 2mW)

• Importance of pink (VV) and light blue (W+jets) grows

• Reject dominant BG like WW and Z and by using the spin correlations

Leptons tend to not be back-to-back, and one lepton is soft because of the off-shell W (mH < 2mW)

• Importance of pink (VV) and light blue (W+jets) grows


Event selection vs. Event selection vs. backgroundsbackgrounds

• We start from the basics: two leptons and (usually) missing transverse energy, and then we split the data by the number of jets and by the flavor of the lepton pair

• Then we apply additional criteria to improve the signal-to-background ratio

• For most backgrounds, we can use one or two key event properties to reject them while keeping most signal

• The events that fail that requirement are typically enriched in the target background. We use these to normalize the background in question

reduces the uncertainties associated with theoretical cross sections with event requirements, particularly exclusive jet requirements

• Textbook example: irreducible background from continuum WW


WW background and mWW background and mllllIn the zero-jet categories the dominant background is WW

Normalize to data in signal-depleted “control regions” (CR)

In the zero-jet categories the dominant background is WW

Normalize to data in signal-depleted “control regions” (CR)

55 110

extrapolate using MC simulation

Normalize to data subtract other

processes

validation region

signal region

control region

Dilepton invariant mass mll is a good signal discriminant because the spin-0 of the Higgs boson, combined with V-A structure of W decay correlates lepton directions

Dilepton invariant mass mll is a good signal discriminant because the spin-0 of the Higgs boson, combined with V-A structure of W decay correlates lepton directions

Estimated background

even

ts /

10 G

eV


Better Living through Better Living through CategoriesCategories

• Back to ggF: categorize to separate signal(s) and backgrounds Njet – Dilepton invariant mass mll – pT of subleading lepton


Better Living through Better Living through CategoriesCategories

• Categorize to separate signal(s) and backgrounds Njet – Dilepton invariant mass mll – pT of subleading lepton


The Low pThe Low pTT Bin Binthe “low-pT” data (10 < pT

sublead < 15 GeV) add ~20% signal acceptance but challenging backgrounds

the “low-pT” data (10 < pTsublead < 15 GeV) add ~20% signal

acceptance but challenging backgrounds

(mH ~ 125 GeV) < 2 x (mW ~ 80 GeV) Signal produces one W off-shell, leptons produced by

one W are soft

Previous results did not have this bin (pTsublead > 15)

(mH ~ 125 GeV) < 2 x (mW ~ 80 GeV) Signal produces one W off-shell, leptons produced by

one W are soft

Previous results did not have this bin (pTsublead > 15)


New Technology for New Technology for W+jetsW+jets

• W+jets normalized using isolated + non-isolated dilepton pair

• Extrapolate to SR using fake rate measured in jet-enriched sample

• Stringent lepton ID cuts these events are typically actually heavy-flavor meson decays

• Jet composition matters: tune sample in which we derive the fake rate pick Z+jets



• Stringent lepton ID cuts these events are typically actually heavy-flavor meson decays

• Jet composition matters: tune sample in which we derive the fake rate pick Z+jets

s

c

W±

g


New Technology for New Technology for W+jetsW+jets



• Previously: large systematics (40%) from difference of extrapolation factors between multijet vs. W+jets

• Now: Use Z+jets data, a better analog (20% systematic, + stat uncertainty will improve!)



• Previously: large systematics (40%) from difference of extrapolation factors between multijet vs. W+jets

• Now: Use Z+jets data, a better analog (20% systematic, + stat uncertainty will improve!)

s

c

W±

g


And a new control region And a new control region • “Other dibosons” = “Not WW”

• WZ, W*, W: Z, , *

• Produce lepton with same or opposite charge as W with equal probability

• Long-time pipe dream in dilepton WW: normalize “non-WW’’ using same-sign data

Need robust W+jets prediction

• And now we have one!!

• In this control region, extrapolate from same-sign to opposite-sign

• “Other dibosons” = “Not WW”

• WZ, W*, W: Z, , *

• Produce lepton with same or opposite charge as W with equal probability

• Long-time pipe dream in dilepton WW: normalize “non-WW’’ using same-sign data

Need robust W+jets prediction

• And now we have one!!

• In this control region, extrapolate from same-sign to opposite-sign


ggF ≥2 jet categoryggF ≥2 jet category• Add acceptance by taking events from the complement of the

VBF category in events with ≥ 2 jets

• Closes the system of jet migrations between categories restores the total signal yield to being an inclusive measurement

• anticorrelations among signal yields in different categories results in improved total uncertainty


VBF categoryVBF category• Numerous variables to distinguish the VBF channel from ggF

and backgrounds

Hig

gs

bo

son

kin

em

ati

cs

VB

F je

t k

ine

ma

tic

s

(ll)

mT

Y(jj)

m(jj)


BDT VBFBDT VBF• Boosted Decision Tree (BDT)

event discriminant Automated cut sequence to optimize

expected signal-background separation

Repeat, giving higher weight to background events passing (and signal events failing)

Output a “forest” of decision trees

• Trained to select VBF signal events and reject background (including ggF H boson production)

-1 = background-like

1 = signal-like

• Rebin to manageable 4 bins 0th bin is “background-like”, not used

(ee/ is used as well, but is not shown)

e data, Njet ≥ 2 after preselection

S/B(B includes ggF)

0.110.01 2.10.6


Top background and the Top background and the b-vetob-veto

g

g

t

t

t W+

W–

b

b

≥ 2-jet data after a veto on b-tagged jets: still top-dominated

But these jets are not the same:

1)Different kinematics (large opening angle, see left)

• Reflected in the BDT

2) t → Wb always, and we can tag b-jets using impact parameter + secondary vertex information

≥ 2-jet data after a veto on b-tagged jets: still top-dominated

But these jets are not the same:

1)Different kinematics (large opening angle, see left)

• Reflected in the BDT

2) t → Wb always, and we can tag b-jets using impact parameter + secondary vertex information

VBF signal

top

q’ q’’

q q’

W+

W–

HW+

W–

’


Top quark BG in the VBF Top quark BG in the VBF channelchannel

• Simple extrapolation (from tagged to btag-vetoed) of a complex variable (the BDT discriminant)

• Check modeling of key variables (mjj shown) and the BDT discriminant itself

• Theory uncertainties from standardized program of QCD scale, generator and parton shower comparison, PDF dependence

• Simple extrapolation (from tagged to btag-vetoed) of a complex variable (the BDT discriminant)

• Check modeling of key variables (mjj shown) and the BDT discriminant itself

• Theory uncertainties from standardized program of QCD scale, generator and parton shower comparison, PDF dependence

(theory) uncertainty on yield in signal region

uncertainty on control region


The ResultsThe Results


Observation of HObservation of HWWWWProbability that the observed data are produced by the background alone:

(CMS: 4.3 observed (5.8 expected) from 1312.1129 [hep-ex])

z0 = 6.1 standard deviations(5.8 expected)

e data, Njet ≥ 2


Signal strength: the Signal strength: the global pictureglobal picture

• Signal strength “” = (obs)/(pred(SM))

0 is bg-only, 1 is SM

• WW has best single-channel signal strength: 20% precision

CMS: 0.72 ± 0.12 (stat) ± 0.12 (expt.) +0.12-0.10 (theo.)


VBF vs. ggF: testing the VBF vs. ggF: testing the SMSM

Different production mechanisms, can separate through different signal regions

Test for new physics in the loops

Different production mechanisms, can separate through different signal regions

Test for new physics in the loops

g

g

H

t,b,???

q’ q’

q q

W,Z

W,Z

H Best-fit values from WW → ll

ggF = 1.01± 0.19 (stat.) +0.22-0.18 (syst.)

VBF = 1.27 +0.44-0.40 (stat.) +0.29

-0.21 (syst.)

Best-fit values from WW → ll

ggF = 1.01± 0.19 (stat.) +0.22-0.18 (syst.)

VBF = 1.27 +0.44-0.40 (stat.) +0.29

-0.21 (syst.)


• Use ggF signal regions as “control regions”, test for VBF signal

• Equivalently, test for a nonzero ratio VBF/ggF

likelihood curve: similar to a chi2

minimum is most probable value of tested quantity

significance from value at zero-crossing2.7 expected3.2 observedEvidence for VBF production

likelihood curve: similar to a chi2

minimum is most probable value of tested quantity

significance from value at zero-crossing2.7 expected3.2 observedEvidence for VBF production

VBF evidenceVBF evidence


Vector and fermion Vector and fermion couplingscouplings

Simple scaling of production and decay modes involving vector boson (V) and fermions (f)


~75% ~25%

HW+

W–

g

g

H

quarks

q’ q’

q q

W,Z

W,Z

H

F2

V2

V2

Partial width scales with (V

2)Partial width scales with (V

2)

Branching ratio is the partial width divided by the total width, which appears in the denominator above

Branching ratio is the partial width divided by the total width, which appears in the denominator above


Vector and fermion Vector and fermion couplingscouplings



The road aheadThe road ahead


The search path: high-The search path: high-mass Hmass H

• LHC Run II: centre-of-mass energy increase 8 13 TeV Correponding ability to produce higher-mass particles

• Newly-accessible particles will be quickly visible in the next run

from https://twiki.cern.ch/twiki/bin/view/LHCPhysics/HiggsEuropeanStrategy2012

ggF: x6 increase

VBF: x3 increase

for mH = 1 TeV:


A second Higgs boson at A second Higgs boson at high mass?high mass?

• Q: Why would there be a high-mass copy of our friend at 125 GeV?

• A: Why not? Nature seems to like copies (leptons, quarks)

• A’: Predicted by numerous well-motivated models which also solve other problems

Supersymmetry “stabilizes” mH, gives a dark matter candidate

Two-higgs-doublet model Many variants; one is basically SUSY with decoupled superpartners

Type II Seesaw models Seesaw models give a “natural” mechanism for light neutrino masses

explains gap between neutrino and other fermion masses

Any of these can allow enough CP violation for a matter universe to arise from a matter-antimatter initial state


High-mass H search with High-mass H search with WWWW

• WW possible or even favored decay mode at high mH

• ljj experimental signature Higher branching ratio than ll Single isolated high-pT lepton

(provides trigger)

Missing transverse energy

Two jets (sometimes merged for mH > 1 TeV)

• Bump hunting in m(ljj) Reconstruct m(WW) up to twofold

ambiguity in neutrino pZ

• Different background composition but similar challenges (top quark rejection, non-prompt leptons)

Apply tools from dilepton analysis

Starting with QCD

from 1206.6074 (7 TeV paper)


Precision tests: ttHPrecision tests: ttH• Direct searches are not the only way

to look for BSM physics at the LHC

• There could well be new heavy particles hiding in the quantum loops of ggF production

Need to know the dominant ttH contribution

• ttH Yukawa coupling measurement Fundamental physics

Leading quantum correction to Higgs boson mass (“naturalness problem”)

Experimentally interesting (see right)

• These will steadily benefit from more data (complementary to searches)

Populate sidebands / control regions

Rare decay modes with good S/B become accessible

g

g t

t

H


ttH multilepton ttH multilepton signaturessignatures

• At 13 TeV cross section increases by factor 5, faster than any other Higgs production mode multilepton decays become accessible

• Experimental and theoretical challenges Multi-object, complex final states with heavy flavor push limits of MC

generator technology

Bring experience from WW in data-driven fake lepton BGs

g

gt

t

H W+

W–

W+

W–

b

b


ConclusionConclusion• Final Run I WW paper is a waypoint along a long but profitable

road Beyond cut and count: categorization of events improves precision by

separating signal and background

Improved background estimation and fit model result in the most powerful single channel on ATLAS for rate measurements

• Last details of Run I WW measurements being finalized for publication…

Spin/CP, high-mass search, VH production mode, differential cross section, off-shell couplings

• …and Run II is coming up fast

6.1 observation of H WW decay3.2 evidence for VBF production


backupbackup


W+jets systematicsW+jets systematics


Control region yieldsControl region yields


BDT VBFBDT VBF• Results (BDT discriminant and dilepton transverse mass mT)


BDT schematicBDT schematic

http://tmva.sourceforge.net/docu/TMVAUsersGuide.pdf


Drell Yan (Z/Drell Yan (Z/*)*)Measure missing transverse energy only with tracks: more robust against extra interactions (pileup)

Measure missing transverse energy only with tracks: more robust against extra interactions (pileup)

• Left: additional rejection after initial ET

miss requirement

First inclusion of ee+ channels in 2012 WW analysis

• Left: additional rejection after initial ET

miss requirement

First inclusion of ee+ channels in 2012 WW analysis

45


The ATLAS DetectorThe ATLAS Detector

Inner Detector (tracking):Pixels (silicon)SCT (silicon strips)TRT (straw tubes / ionization)

Electromagnetic calorimeterLiquid Argon (LAr) with lead

absorber

Hadronic calorimeter (jets, hadrons)Steel absorber + scintillatorLAr with copper/tungsten absorber

Solenoid

Toroids

Muon Detectors: large radii for precise momentum measurement Precision: Drift tubes (MDT) and Cathode Strip Chambers (CSC)Trigger: Resistive Plate Chamber (RPC) and Thin Gap Chamber

(TGC)


llll combined signal combined signal strengthstrength

Combined WW → ll signal strength = /SM (2011+2012, all Njet):


Everybody loves the Everybody loves the banana plotbanana plot

• measured signal strength ((Nobs-B)/(BRSM x SM)) vs. mH


One last pOne last p00 plot for old plot for old times’ saketimes’ sake• Probability that data can be produced by the background alone

• Slight mass sensitivity because mT binning evolves with mH


Now it has friendsNow it has friends• “mango” (no BR in denom) and “papaya” (no in denom) plots

Residual sensitivity to mass through cross section (mango) and acceptance (papaya)

mango papaya


Top background and the Top background and the b-vetob-veto

Therefore we reject events by selecting on the unique VBF topology Event discriminant: “boosted decision tree” (BDT) separates signal from background

(0 = background-like 3 = most signal-like)

And reject events in which a jet is b-tagged Use precision tracking to reconstruct b hadron decay vertex

And use the events with one b-tagged jet to normalize the top background

Therefore we reject events by selecting on the unique VBF topology Event discriminant: “boosted decision tree” (BDT) separates signal from background

(0 = background-like 3 = most signal-like)

And reject events in which a jet is b-tagged Use precision tracking to reconstruct b hadron decay vertex

And use the events with one b-tagged jet to normalize the top background

b-tagb-veto signal region


The Englert-Brout-Higgs The Englert-Brout-Higgs MechanismMechanism

Gauge invariance does not allow fundamental particle masses: instead, generate masses through interaction with a scalar field

Electroweak Symmetry Breaking

Choice of ground state is arbitrary but sets the relationship between W and Z masses, renders photon massless

Interaction with nonzero field permeating space generates mass

Structure of the electroweak interactions appears to reflect this mechanism – what is its nature?

“vacuum expectation value”

Reduce four degrees of freedom to one:

1-d analog: choice of phase


The Higgs BosonThe Higgs BosonFor every field, a particle

Particles represent excitations of a quantum field

Photons for the electromagnetic field

Matter (think electrons) for corresponding fermionic fields

[Gravitons for gravitational fields]

Higgs boson for the Higgs field

This last prediction led to an almost 40-year search

1 c. mills (edinburgh) 12 january 2015 higgs physics at atlas from the ww perspective corrinne mills...

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