The Origin of Mass - Exploring the Higgs Sector at the Large Hadron Collider with ATLAS

The Origin of MassThe Origin of Mass

--

Exploring the Higgs Sector at the Exploring the Higgs Sector at the

Large Hadron Collider with ATLASLarge Hadron Collider with ATLAS

Markus Schumacher, Universität Bonn

Seminar über Teilchenphysik, Universität Heidelberg, May 2006

Markus Schumacher The Origin Of Mass - Exploring the Higgs Sector at the LHC 2

Outline

Which mass? Why? The mass problem and the SM solution.

Higgs boson phenomenology and experimental environment

Discovery potential for a SM like Higgs boson

Investigation of the Higgs boson profile

Discovery potential for MSSM Higgs bosons

Conclusions

only ATLAS results. CMS TDR will only become public this Friday.


Which mass?

Unknown Form of

Dark Matter

Unknown Formof

Dark Energy60%

40%

20%

80%

100%

0%

neutrinos protons/neutrons in stars, dust,etc.

The nucleon mass

O(%) due to Mu~5 MeV

and Md ~10 MeV

Rest: kinetic energy

of partons +

other QCD effects


Particle masses and their relevance

electron mass: def. length scale of our world, Bohr radius a=1/em me

me = 0 no atomic binding

me = 0.02MeV human giants 45 m, visible light in infrared

me = 105 MeV nucleon capture pen energetically possible only helium, n + different universe

no/small W mass: fusion in stars: p+pD e+ GF~ (MW )-2

short burning time of sun at lower temperature no humans on earth

mass values of e, u, d, W and their fine tuning are

essential for creation and development of our universe

principle of mass generation Higgs mechanism origin of mass values even no theoretical explanation yet

quarks massless or mu=md proton mass > neutron mass proton decay pnepossible modified nucleosynthesis different universe


Gauge symmetries of the SM and particele masses

consistent description of nature based on gauge symmetries

electroweak SU(2)LxU(1)Y symmetry forbids „ad hoc“ masses for

gauge bosons: W and Z fermions: (l = doublet, r = singlet) „ad hoc“ mass terms destroy

renormalisibility no precision prediction for observables

high energy behaviour of theory e.g. WLWL scattering


High energy behaviour of in WWWW

violates unitarity

at ECM ~ 1.2 TeV

massive gauge bosons: 1 longitudinal + 2 transverse d.o.f.

massless gauge bosons: only 2 transverse d.o.f.

scalar boson Hrestores unitarity, if

gHWW ~ MW

gHff ~ Mf

and MH < 1TeV

const=f(MH)


The Higgs Kibble mechanism

The „standard“ solution:

one new doublet of complex scalar fields (4 degrees of freedom)

with appropriately chosen potential V

V = -2 || + ||2

2, > 0

minimum of V not at =0 spontaneous symmetry breaking

3 massless excitations along valley 3 longitudinal d.o.f for W+- and Z

1 massive exciation out of valley 1 d.o.f for „physical“ Higgs boson

Higgs field has two „components“

1) omnipresent, constant background condensate v= 247 GeV (from GF)

2) Higgs boson H with unknown mass MH ~ ~ v


Mass generation and Higgs Boson couplings: = v + H

v =247 GeVx

Fermion

gf

MV ~ g v gauge coupling

mf ~ gf v Yukawa coupling

introduced „ad hoc“

x x

W/Z boson

g gauge

interaction with „ether“ v=247 GeV

fermions: gf ~ mf / v

W/Z bosons: gV ~ MV / v = g2 v

interaction with Higgs boson H

fermion

gf

x

W/Z boson

g gauge

Higgs

Higgs v

2

2VVH coupling ~ vev

only existent after EWSB

hint towards background condensate

1 unknown parameter in SM: MH

2


50 100 200 100010-3

10-2

10-1

100

bb cc tt gg WW ZZ

Bra

nchi

ng r

atio (Higgs

)

mH (GeV)

bb

WW

ZZ

tt

ccgg

Decays of the Higgs boson in the SM

for M<135 GeV: H bb, dominant

for M>135 GeV: H WW, ZZ dominant

tiny: H also important

HDECAY: Djouadi, Spira et al.


… mt + … ln(MH)

What is the mass of the Higgs boson ?

MW(Phys) = MW(Born) +

2

WW W

W

WHt

b

theory: unitarity in WW scattering MH < 1 TeV

direct search at LEP: MH<114.4 GeV excluded with 95% CL

indirect prediction in SM, e.g.

MH < 186 GeV (mtop=172.7 GeV)

with 95% CL

Standard Model prefers a light Higgs boson

S. Roth


Status of SM Higgs boson searches at TEVATRON

Expected sensitivity:

95% CL exclusion up to 130 GeV with 4fb-1 per experiment

3 sigma evidence up to 130 GeV with 8fb-1 per experiment

Current sensitivity::

Cross section limits at level of

~ 5 to 20 x SM cross section


The Large Hadron Collider LHC

proton proton collisions at ECM of 14 TeV, start in 2007

initial luminosity: (2)x1033 cm-2s-1 10 to 20 fb-1/year

design luminosity: 1034 cm-2s-1 100 fb-1/year


A Toroidal LHC ApparatuS

MC studies with fast simulation of ATLAS detector key performance numbers from full sim.: b/tau/jet/el.// identification, isolation criteria, jet veto, mass resolutions, trigger efficiencies, …


Production of the SM Higgs Boson at LHC

gluon fusion dominant for all masses

VBF roughly one order of magnitude smaller

HW, HZ,H tt only relevant for small MH


QCD corrections and knowledge of cross sections

K = NNLO/LO~2

= 15% from scale variations

error from PDF uncertainty ~10%

caveat: scale variations may underestimate the uncertainties!

ttH: K ~ 1.2, ~15% WH/ZH: K~1.3 ~7% VBF: K ~ 1.1, ~4%

but: rarely MC at NLO avaiable (except gluon gluon fusion)

background: NLO calculations often not avaiable

need background estimate from data

ATLAS policy: use K=1 for signal and background

remark: NLO calculation for total cross section is not

NLO calculation for additional jet needs NNLO or NLO+(N)LL

e.g.: gluon gluon fusion

Harlander et al.


Cross sections for background processes

Higgs 150 GeV: S/B <= 10-10

3 level trigger system on

leptons, photons, missing energy

provides reduction by 10 000 000

overwhelming background: mainly QCD driven

signal: often electroweak interaction

photons, leptons, … in final state

no access to fully hadronic events

e.g. GGF, VBF with Hbb


An event at the LHC

+ ~23 overlayed pp interactions per bunch crossing at high luminosity

~109 proton proton collisions / second ~1600 charged particles enter detector per event

+ effects from „pile up“: read out time > t btw. bunch crossings

„hard“ collision

+ ISR,FSR

+ „underlying

event“


The challenge of event reconstruction

low lumiosity high luminosity


Which channels may provide discovery?

Status 2001discovery channels:

inclusive:

H 2 photons

ZZ 4 leptons

WW ll

exclusive:

ttH, Hbb

VBF, HZZ,WW for large M

efficient trigger no hadronic final states: e.g. GGF, VBF: Hbb

Higgs boson mass reconstructable? which mass resolution?

background reducible and controllable?

- good signal-to-background ratio

- small uncertainty on BG, estimation from data itself possible?


H 2 Photons

signature: two high Pt photons

background: irreducible pp +x

reducible pp j, jj, …

exp. issues (mainly for ECAL):

- , jet separation (Eff=80%, Reject. ~ few 1000)

- energy scale, angular resolution

- conversions/dead material

mass resolution M: ~1 to 1.5%

precise background estimate

from sidebands (O(0.1%))

S/BG ~ 1/20

preliminary NLO study:

increase of S/B by 50%

ATLAS 100fb-1


H ZZ(*) 4 leptons

reducible BG:

tt, Zbb 4 leptons

lepton isolation and

veto against b-jets

good mass resolution M ~1% muon spectrometer + tracking detectors

small and flat background easy estimate from sidebands no Monte Carlo needed preliminary NLO study indicates significance increase by 25%

signal: 4 iso. leptons

1(2) dilepton mass = MZ

irreducible BG:

ZZ 4 leptons

four lepton mass


signal:

- 2 leptons + missing ET

- lepton spin corrleations

- no mass peak transverse mass

H WW l l

ATLAS

M=170GeV

30fb-1

transverse mass BG: WW, WZ, tt

lepton iso., missing E resolution

jet (b-jet) veto against tt

BG estimate in data from ll : 5%

normalisation from sideband

shape from MC

NLO effect on spin corr.

ggWW contribution signal like

Dührssen, prel.

ll


ttH with Hbb

mass resolution M: ~ 15%

50% correct bb pairings

difficult background estimate from

data with exp. uncertainty ~ O(10%)

normalisation from side band

shape from „re-tagged“ ttjj sample

reducible BG: tt+jets, W+jets b-tagging

irreducible BG: ttbb reconstruct mass peak

exp. issue: full reconstruction of ttH final state combinatorics !!! need good b-tagging + jet / missing energy performance

S/BG ~ 1/6

30 fb-1

only channel to see Hbb

ATLAS

signature: 1 lepton, missing energy

6 jets of which 4 b-tagged


Vektor boson fusion VBF: ppqqH

signature: 2 forward jets

with large rapidity gap only Higgs decays

in central part of dector=-ln tan(/2)

ATLASATLAS

Jet

Jet

Forward tagging jets

Higgs Decay


VBF: Challenges

pT>20GeV

reconstruction of taggings jets

influence of - „underlying event“ (UE) ? - overlapping events (OE) ? - „pile up“ (PU) ?

so far only low lumi considered

ATLAS

central jet veto:

influence of UE, OE, PU?

efficiency of jet veto at NLO?

but: no NLO MC-Generator yet

now: study started using SHERPA

Zeppenfeld et al.


VBF: H ll 4 signature: tagging jets + 2 leptons + large missing tranvsere energy

background: QCD processes tt,Zjj central jet veto

reconstruction of m

M /M ~ 10% dominated by Emiss

He

ATLAS30 fb-1

expected BG ~ 5 to 10%

for MH > 125 GeV: side band

for MH < 125 normalisation from Z-peak, shape from Z

collinear approximation


Idea: jjZandjjZwith identical topology

muons are MIPS same energy deposition in calorimeters

only difference: momentum spectra of muons

Method: select Z events

„randomise“ -momenta according to Z MC

apply „usual“ selection and mass reconstruction

VBF, H: determination of background from data

shape of background

can be extracted precisely

from data itself

(M. Schmitz, Diplomarbeit BN 2006)


Weak Boson Fusion: HWWll (lqq)

HWWe

ATLAS

10 fb-1

HWWll: VBF versus inclusive channel

ATLAS

M=170GeV

30fb-1

S/BG ~ 3.6 Signal = 82.4 S/BG ~ 0.7 Signal = 144

smaller rate larger sig-to-BG ratio smaller K-factor

more challenging for detector understanding

order of significance depends on channel and Higgs mass

VBF with respect to gluon fusion


excl

ud

ed b

y L

EP

Discovery potential in SM 10 fb-1

VBF dominates discovery potential for low mass (at least at LO)

with 15 fb-1 and combination of channels: discovery from LEP to 1TeV

prel. NLO studies: increase of signifcance up to 50% for incl. channels

so far: cut based improvement with multivariate techniques

30 fb-1

excl

ud

ed b

y L

EP


“Indirect” from transverse mass spectrum: HWWllWHWWWlll

Measurement of Higgs boson mass

ATLAS

M/M: 0.1% to 1%

Direct from mass peak: HHbb HZZ4l (energy scale 0.1 (0.02)% for l,,1% for jets)

300 fb-1

Higgs boson mass

- determines Higgs sector in the SM

- is precision observable of the SM

S. Roth


Determination of Higgs boson couplings

coupling in production Hx= const x Hx and decay BR(Hyy)= Hy / tot

Hx x BR ~ HX Hy

tot

Prod. Decay

Partial width: Hz ~ gHz2

goal: - disentangle contribution from production and decay

- determine total width tot

H WW used as reference as most precise determination for MH>120 GeV

model independent:

only ratio of partial width

13 final states in global fit

(including various syst. uncertainties)


for MH<200 GeV, tot<< mass resolution

no direct determination

indirect limits needed

Absolute Couplings with gV < gVSM

coupling to W, Z, , b, t

Dührssen et al.

lower bounds from observed rates:

tot > W+Z+t+g+....

upper bound with theoretical input

weak assumption: gV<gVSM

(true in all models with only doublets and singlets)

tot< Rate(VBF,HWW)/(V2 in SM)

300 fb-1


MH2 = 2 =MPlanck

Motivation for Supersymmetry from Higgs sector

Higgs boson mass problem in SM: 2

“solves” hierarchy problem: why v=246 GeV << MPl=1019GeV ?

MH2 = (MSM-MSUSY)

2 2

W W+ HH natural value ~ MPl

vs electroweak fit MH~O(100GeV)

SUSY solution: - partner with spin difference by ½ cancel divergence exactly if same M - SUSY broken in nature, but hierarchy still fine if MSUSY~1 TeV

H H

-

SUSY breaking in MSSM: parametrised by 105 additional parameters too many constrained MSSM with O(5) additional parameters


The Higgs sector of the MSSM

SUSY needs two Higgs doublets 2 vevs: v1,v2 5 Higgs Bosons: h,H,A,H+,H-

SUSY “solves” hierarchy problem: why MH, v << MPl=1019GeV ?

2 parameters determine Higg sector at Born level: tanv2/v1, MA

+ ~100 additional, which are fixed in benchmark scenarios

lightest Higgs Boson: Mh<135 GeV (for mtop=175GeV)

modified couplings

w.r.t. SM

Is at least one Higgs boson observable at LHC?

Discrimination SM MSSM ? - several Higgs bosons observable

- characteristics of h differ from SM


Discovery potential for light Higgs boson h

large area covered by many channels

stable discovery and parameter

determination possible

small area uncovered @ mh~95 GeV

300 fb-130 fb-1

VBF dominates observation

small area from bbh,h

for small Mh

ATLAS preliminary

ATLAS preliminary

VBF, H covers whole plane

via observation of h or H

with 30fb-1


Overall discovery potential: 300 fb-1

at least one Higgs boson

observable for all parameters

in all CPC benchmark scenarios

significant area where only lightest Higgs boson h is observable

can H SUSY decays or

Higgs from SUSY decays

provide observation?

discrimination via h profile

determination?

similar results in other benchmark scenarios

VBF channels , H/Aonly used with 30fb-1

300 fb-1

ATLAS preliminary


ATLASprel.

300 fb-1

SM or extended Higgs Sector e.g. Minimale SUSY ?

discrimination via VBF

R = BR(h WW) BR(h )

assumption: Mh well known no systematic uncertainties

comparison of expected

determination of R in MSSM

with SM prediction for same MH

=|RMSSM-RSM|exp =|RMSSM-RSM|exp


The Higgs sector in the CP violating MSSM

mass eigenstates H1, H2, H3

<> CP eigenstates h,A,H

at Born level: CP symmetry conserved in Higgs sector

complex SUSY breaking parameters (,At) introduce new CP phases

mixing between neutral CP eigenstates

no a priori reason for real SUSY parameters baryogenesis: 3 Sacharov conditions

B violation : via sphaleron processes

CP violation : SM too less, CPV MSSM new sources fine

No therm. Equ. : SM no strong 1st order electroweak phase transition

CPV MSSM still fine (even better NMSSM)

no absolute limit on mass of H1 from LEP

Why consider such scenarios?


Discovery potential in CP violating MSSM

MH1: < 50 GeV, MH2: 105 to 115 GeV, MH3: 140 to 180 GeV, M H+-:130 to 170 GeV

most promising channel: tt bW bH+, H+W H1, H1bb

final state: 4b 2j l same as ttH, Hbb

revised studies for H2/3H1H1 also interesting


Conclusions our universe needs masses of elementary particles

Higgs Kibble mechanism allows their consistent description

LHC: - discovery of Higgs boson in SM with 15fb-1 well understood data

- determination of mass, width, spin, CP

- ratio of partial width, absolute couplings only with theo. input

- CPC MSSM: at least one Higgs boson observable

discrimination from SM seems promising

- CPV MSSM: so far uncovered area at low MH as not studied yet

promising channels are investigated now

now: prepare for data taking

- determine background, trigger eff., id eff./rejection from data

- improve reconstruction and MC simulation (mis. calibration, allignment)

- perform NLO studies and investigate other „exotic“ scenarios


Higgs Physics is golden (J.Lykken 2000)

Thanks for your attention!

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The Origin of Mass - Exploring the Higgs Sector at the Large Hadron Collider with ATLAS

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