Higgs Boson at LHC: gearing up for discovery

Andrey Korytov, UF 7 February 2007, UCLA 1

Higgs Boson at LHC: gearing up for discovery

Andrey Korytov


Outline

• Introductory remarks: what we already know

• LHC, ATLAS, and CMS

• Gold-plated channel SM HZZ4 at CMS

• Other SM Higgs discovery channels

• A few words on MSSM Higgs (and more if time permits)


SM Higgs Trivia: intelligent design

• Start from scalar fieldo doublet pseudo-scalar in SM

• Require local gauge invarianceo need massless gauge fields Ao lagrangian acquires terms

• Mexican hat potentialo min V() is not at =0 o non-zero vacuum expectation value v0—ether of 21 centuryo expand around minimumo effective mass terms for gauge bosonso effective mass for h-field itself

• Free lunch: o force interact with fermions with ad hoc couplings f o effective fermion masses (within the P-violation framework!)

Two important points: • Higgs boson mass is the only free parameter• (Higgs-particle coupling) ~ (mass of particle)

o Production mechanisms: first one needs to produce heavy particleso Decay channels: higgs likes to decay to heaviest particles it can

decay to

( )x

( ) ( ) ( )x U x x

g AA2 42( )

2V

2 20~ vhm

2 20~ vAm g

2 20~ vf fm

0v h


What we know: theory

After renormalization• (Q)

• If mH were small at 1 TeV, runs down with Q, flips sign at some scale Q, and vacuum breaks loose

• If mH were large at 1 TeV, runs up with Q, explodes at some scale, theory becomes non-perturbative, and theorists can retire

SM Higgs has a very narrow window of opportunity to be self-sufficient due to a fine-tuned accidental cancellation of large correction factors

non-p

ert

urb

ati

ve

un

sta

ble

vacu

um

N

ew

Ph

ysic

s E

ner

gy

Sca

le

(G

eV

)

103

1

06

1

09

10

12

1

01

5

10

18

0 200 400 600

Higgs mass MH (GeV)


What we know: direct search at LEP

}MJJ=MH =?

}MJJ=MZ=91 GeVZ0

H bb

jet (b-tagged)

jet (b-tagged)

q

q jet

jet

e+

e-

Z0

LEP Energy209 GeV


What we know: direct search at LEP

MH (GeV/c2)

Points—dataDashed line—expected background (no-Higgs processes)

ALEPH Collaboration data - 2000

small excess?Formally, it looked like 4 effect!If it was Higgs, they saw too many...

LEP was let run longer to get more data

Tight Cuts

After taking more data and combining results of all 4 experiments,

the final word from LEP:

No discovery...

Consistency with background: ~1.7

Limit on Higgs mass:

MH > 114.4 GeV @95% CL

Phys. Lett. B565 (2003) 61


What we know: direct search at Tevatron

Some lessons: MH=110 GeV (the best expectations)

• SM Higgs exclusion at 95% CL was expected at L=1.2 fb-1

• Now at L~1 fb-1, the excluded limit is almost a factor of 10 worse

MH=160 GeV (the best current limit)• Disparity between the current limit and projected expectations is a factor of 2

This is not to discredit the excellent ongoing effort (very nice solid analyses!)

It just shows that the reality can be not as rosy as projections---something to remember as we expect the LHC turn-on…


What we know: circumstantial evidence

Presence of too light or two heavy Higgs in loops would make various SM precision measurements less self-consistent

• mH<166 GeV at 95% CL

• mH<199 GeV at 95% CL, if the direct search limit mH>114 GeV is included

LEP EW Working Group July 2006 WW

H


Large Hadron Collider

2007 (Dec)• hardware commissioning run• sqrt(s)=900 GeV• Lint ~ 100 nb-1 (0.0001 fb-1)

2008• first physics run• sqrt(s)=14 TeV• Lint ~ 0.1-1 fb-1

2009• sqrt(s)=14 TeV• Lint ~ 10 fb-1

2010• sqrt(s)=14 TeV• Lint ~ 20-100 fb-1

Switzerland

France

Geneva airport

6 miles


ATLAS


Compact Muon Solenoid


CMS Endcap Muon Chambers

3.3 m

1.5 m

muon is detected with ~100 m precision ~ 4 ns time resolution

We need 500 of them to cover ~1000 m2

President of France J. Chirac is observing live muons detected by the Endcap Muon Chambers.


CMS Physics Technical Design Report

Physics TDR

• Comprehensive/up-to-date overview of CMS physics reach

• First part of TDR is devoted to 11 in-depth (showcase) analyses; HZZ4 is one of them

• Published last fall

650 pages308 figures 207 tables1.50 kg


SM Higgs: discovery signatures at L=30 fb-1

Colored cells = { detailed studies available }YES = { sure discovery in the appropriate range of masses at L=30 fb-1 }

Hbb H H HWW HZZ

inclusive YES YES YES

qqH YES YES YES

W/Z+H

ttH


HZZ4dominant 4 backgrounds

tt Wb + Wb BX + BX X + X 4X

Zbb + BB + X + 2(X) + X 4X

ZZ

tt 4 + X

Zbb 4 + X

Higgs signal H 4

ZZ 4 with spectacular peak at m4=mZ

(this s-channel contribution was overlooked in all previous studies)

t-channel s-channel


HZZ4analysis strategyPeak in mdistribution Cut variables: • muon isolation: 2 muons in tt and Zbb appear in B-decays, i.e. within jets• displaced vertex: 2 muons in tt and Zbb appear in B-decays• missing energy: tt will have hard neutrinos• kinematics: muons in Zbb and tt tend to be softer• NOT USED:

o pT() for Higgs is larger than for ZZ, but the non-zero pT appears only at NLO, which is not accounted for in the current MC simulation

o number of jets for Higgs (ggHZZ) is larger than for ZZ (qqZZ), but this effect of hard jets is again NLO…

Cut optimization• mH-dependent (read m4-dependent)• identify most important and not-correlated cuts

o isolation cut on the least isolated muon (i.e., the same cut for all muons)o muon pT cut for the 3rd softest muon

• and produce smooth cut(m4) functionsThis strategy makes the search automatically optimized for any mass at which Higgs boson may chose to show up

Peak search:• Include statistics and systematics into significance evaluation

Final probabilistic interpretation (significance dilution due to scanning)


HZZ4understanding ZZ bkgd

Knlo(m4)

Box-diagram

Control samples

QCD scale uncertainties

PDF scale uncertainties

Isolation cut uncertainties

Muon efficiency uncertainty

2+ + x + …

2


~20% over LO

Zecher, Matsuura, van der Bijhep-ph/9404295

HZZ4understanding ZZ bkgd

Formally (by counting vertices), NNLOHowever, - it is the LO for ggZZ and - contribution is large due to large gg “luminosity”

Knlo(m4)

Box-diagram

Control samples






HZZ4ZZ bkgd

Knlo(m4)

Box-diagram

Control samples:• qq Z 2

o very similar origin to ZZ bkgdo huge statistics

• ZZ 4 sidebandso would be perfect, if not for rather complicated shape o and very limited statistics





L=10 fb-1

Total 8 eventsExp bkgd 0.8 evtsScL = 4.7


HZZ4ZZ bkgd

Knlo(m4)

Box-diagram

Control samples

QCD scale uncertainties• estimate of higher-order

contributions




Normalization to Z2Normalization to Z2


HZZ4ZZ bkgd

Knlo(m4)

Box-diagram

Control samples



Isolation cut uncertainties • Underlying Event is the main source for energy flow in vicinity of

muons in the irreducible ZZ-bkgd; but UE activity is poorly predicted…• Use data to calibrate UE activity:

o UE activity in Z must be very similar to that in ZZ (qq …)o MC studies confirm this statement

Muon efficiency uncertainty: use data

three colors: different UE models— ZZ events- - Z events (random cones)


HZZ4ZZ background

Knlo(m4)

Box-diagram

Control samples



Isolation cut uncertainties: use data

Muon efficiency uncertainty: use data

• single muon trigger; well reconstructed muon 0

• take advantage of muon being measured twice: in Tracker and Stand Alone Muon system

• find Z-peak three times…• (efficiency) ~ 1%

0

1 0

( )in

t k Z

v

r

M trk

N N

0

3 0

( )

GR

GRM SAM TRK

inv

M

GRM

ZN N

M

0

2 0

( )in AM

Z

v S

SAM

M

N N


HZZ4Higgs signal over ZZ bkgd

Peak search results: • Significance:

o Counting Experimento LLR for m spectrum

• Luminosity needed• Including systematics



Peak search results: • Significance• Luminosity needed• Including systematics



Peak search results: • Significance• Luminosity needed• Including systematics

o significance must be de-rated

o effect depends on how we define the control sample: Z2 peak vs ZZ4 sidebands

Calculations done for luminosities, at which the expected significance would be 5, if there were not systematic errors


HZZ4lcombining four channels

new


HZZ4word of caution

Search in a broad range of parameter phase space

mH=115-600 GeV

Probability of finding a local excess somewhere is much higher than naïve statistical significance might imply: e.g. S=3 is almost meaningless

A priori assumptions must be clearly defined

Background-onlypseudo experiment

Search for Higgs peak

— actual probability

- - probability implied by local statistical significance


SM Higgs: discovery signatures at L=30 fb-1

Colored cells = { detailed studies available }YES = { sure discovery in the appropriate range of masses at L=30 fb-1 }

Hbb H H HWW HZZ

inclusive YES YES YES

qqH YES YES YES

W/Z+H

ttH


Standard Model Higgs: H

Backgrounds:• prompt • prompt + jet(brem , )• dijet

Analysis:• Cut-based

o PT, isolation, M

o events sorted by “EM shower profile quality”

• Optimizedo loose cuts and sortingo event-by-event kinematical Likelihood Ratioo bkgd pdf from sidebands, signal pdf from MC

• Systematic errors folded in

CMSM < 1%

new

CMS


Standard Model Higgs: HWW2l2

Backgrounds:• WW, tt, Wt(b), WZ, ZZ • ggWW (box)

Analysis:• KNLO(pT

WW)

• cuts: o e/ kinematics, isolation, jet veto,

MET

• counting experiment, no peak• background from a control sample:

o signal: 12<mll<40 GeV

o control sample: me>60 GeV

• reduce syst. errors; pay stat. penalty

• systematic errors are folded in

Signal Region Control Sample

new

CMS


Standard Model Higgs: qqH, HWW2l2

Backgrounds:• tt, WWjj, Wt

Analysis:• 2 high pT leptons + MET• 2 forward jets (b-jet veto)• central jet veto• counting experiment, no peak:• background from data:

o Signal: all cutso Control sample: no lepton cuts

Result• better than inclusive WW (!!!)

jet

jet

ATLAS

ATLASMH=160 GeVHWWe

Signal Region Control Sample


Standard Model Higgs: qqH, H

Backgrounds:• Zjj, tt

Analysis:• two forward jets, central jet

veto• two leptons (e, , -jet)+MET

l+ l l+ -jet

• mass(l; l or -jet; pTmis)

o despite 3 or 4 ’s present, works quite well in collinear approximation

He

ATLAS 30 fb-1

ATLAS

H

pT

mis


Difficult (impossible) channel: ttH, Hbb

30 fb-1

ATLAS

SM Higgs: ttH, Hbb

CMS: • careful study of systematic errors in the Physics TDR• syst error control at sub-percent level is needed: not

feasible...

if higgs boson is light, can we use Hbb?


Standard Model Higgs: Summary

Benchmark luminosities:• 0.2 fb-1: exclusion limits will start carving into SM Higgs x-

section• 1 fb-1: discoveries become possible if MH~170 GeV• 10 fb-1: SM Higgs is discovered (or excluded) in full range

NLO cross sectionsSystematic errors included

new


mtop=174.3 GeV

MSSM Higgs bosons: h, H, A, H±

• SUSY stabilizes Higgs mass• 2 Higgs field doublets needed• Physical scalar particles: h, H, A, H±

• Properties at tree levelo fully defined by 2 free parameters: MA, tano CP-even h and H are almost SM-like in vicinity

of their mass limits vs MA: hmax and Hmin

o large tan ~ suppresses coupling to Z and W ~ enhances coupling to “down”

fermions: b and are very important!o CP-odd A never couples to Z and W:

~ decays: bb, (and tt for small tan)o H± strongly couples to tb and o all Higgs bosons are narrow (<10 GeV)

• Loop corrections o gives sensitivity to other SUSY parameterso mh

max scenario = { most conservative LEP limits }


ATLASL=300 fb-1

MSSM Higgs or SM Higgs?

SM-like h only:• considerable area…• even at L=300 fb-1

Any handles?• decays to SUSY

particles?• SUSY particle decays?• measure branching

ratios?


Summary

Standard Model Higgs:• expect to start excluding SM Higgs at L~0.1 fb-1

• discoveries may be expected already at L~1 fb-1

• SM Higgs, if that’s all we have, is expected to be discovered by the time we reach L~10 fb-1

MSSM Higgs:• nearly full (M, tan) plane is expected to be covered at

L~30 fb-1

• there is a serious chance to see only a SM-like Higgs…


Backup: SUSY Higgs Plots


MSSM Higgs boson: h, H, A production

• x-sections are large, often much larger than SM (dotted line)

• bb(h/H/A) production is very important

h H A

h H A

tan=3

tan=30


MSSM Higgs: SM-like signatures

CMS:• better detector

simulation• systematics included• contours recessed…

ATLAS:• no systematics

included

CMS 2003 CMS 2006

ATLASnew


MSSM Higgs: heavy neutral H, A• production in association with bb (especially good at large

tan)• bb-decay mode (~80%) is overwhelmed with QCD background• -decay mode (~20%) is the next best• -decays (~0.1%) allow for direct measurement of • better detector simulation (i.e. more realistic) • systematics included

• contours recessed (low MA band, qqH, moved to SM-like Higgs plot)

ATLAS

CMS 2003 CMS 2006

new


MSSM Higgs: H±

Heavy H± (M>mt):• production via gg tbH± bjj+b and gb tH±

bjj+o H± (H± tb overwhelmed by

bkgd)o tWbjjb

• backgrounds: tt, Wt, W+jets

Light H± (M<mt):• production via gg/qq tt

b+blo t H±b, H± o tWblb

• backgrounds: tt, Wt, Wjjj

new


Difficult (impossible) channels…

MSSM Higgs: bb(H/A), (H/A)bb MSSM Higgs: H±tb


MSSM Higgs bosons: h, H, A, H±

Loop corrections give sensitivity to the rest of SUSY sector, more specifically to:• stop quark mixing Xt

• squark masses MSUSY

• gluino mass Mg

• SU(2) gaugino mass M2

• higgsino mass parameter

*Suggested by Carena et al. , Eur.Phys.J.C26,601(2003)

Special benchmark points*:• max stop mixing (mhmax):

o mh < 133 GeVo MSUSY~1 TeVo most conservative LEP limits

• no mixing: o mh < 119 GeVo MSUSY~1 TeV

• gluophobic h o ggh is suppressed

(top+stop loop cancellation)o mh < 119 GeVo MSUSY~350 GeV

• small eff (mix h/H):o and bb-decays

suppressed even for large tan

o mh < 123 GeVo MSUSY~800 GeV


MSSM Higgs: other benchmark points?

ATLAS studies:• preliminary (no

syst)

• vector boson fusion:o qq(h/H)o h/H, WW,

• caveat for small eff: decoupling from is compensated by WW enhancement

• all four special points are well covered at L=30 fb-1


ATLASL=300 fb-1


SM-like h only:• considerable area…• even at L=300 fb-1

Any handles?• decays to SUSY

particles?• SUSY particle decays?• measure branching

ratios?



Decays to SUSY:

• h22

(2l1)+(2l1

)

• Signature:o Four leptonso Large MET

Msleptons=250 GeV ATLAS300 fb-1

BR for different channels: • R = BR(hWW) /

BR(h)• =|RMSSM-RSM|/expimental


MSSM Higgs: yet another twist

CP-violation in Higgs sector

• complex couplings:o mass eigenstates H1, H2, H3

are mixtures of h, H, Ao production/decay modes

change

• new benchmark point CPX (maximum effect) suggested by Carena et al., Phys.Lett B495 (2000) 155

• new parameterization: MH± ; tan

• uncovered holes remain• more studies needed

ATLAS preliminary

o qqH, HWW, o bbH, Ho tbH± and tH±, H±o …

ATLASL=30 fb-1

not excludedat LEP


Backup: ATLAS/CMS Summary Plots


SM Higgs

newCMS 2003 CMS 2006


MSSM SM-like Higgs

ATLAS

CMS 2003 CMS 2006 new


MSSM H and A

new

ATLAS

CMS 2003 CMS 2006


ATLAS

MSSM H± (old/new plots)

newCMS 2003 CMS 2006

Documents

Higgs Boson at LHC: gearing up for discovery