Higgs Boson at CMS: gearing up for discovery

Andrey Korytov, UF 19 February 2008, Cambridge 1

Higgs Boson at CMS: gearing up for discovery

Andrey Korytov


Outline

• Introductory remarks: what we already know

• LHC, CMS

• SM Higgs frontrunners: o H WW2l2o HZZ4lo H 2

• A few other SM Higgs production/decay channels

• A few words on MSSM Higgs

• Concluding remarks


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

Tight Cuts The final word:No discovery...Consistency with background: ~1.7

MH > 114.4 GeV @95% CL

Phys. Lett. B565 (2003) 61


What we know: direct search at Tevatron

At MH= 115 Expected: 3.8xSM Observed: 4.2xSM

At MH= 160 Expected: 1.9xSM Observed: 1.1xSM

Z/W+H, Hbb

HWW


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<144 GeV at 95% CL (unconstrained by the LEP 114-GeV limit)mH<182 GeV at 95% CL (if combined with the LEP 114-GeV limit)

LEP EWK Working Group

W

H


Large Hadron Collider

2008: first collisions (Lint ~ 10+ pb-1 ?)

2009: Lint ~ 1+ fb-1

2010: Lint ~ 10+ fb-1

Beyond: Lint ~ 100 fb-1 / yr

Switzerland

France

Geneva airport

6 miles


Compact Muon Solenoid


CMS: Conceptual Design (cf. ATLAS)

Large Si Tracker in a Large Magnetic Field:• high momentum measurement precision: ~1% up to 100 GeV

Precision PbWO4 EM Calorimeter:• very good energy resolution for photons/electrons: <1% above 30 GeV

Hadron calorimeter has moderate energy resolution: ~10% above 100 GeV Muon system: muon trigger and identification (also important for measuring TeV muons)


CMS: all major components are underground

Endcap Muon Disks go down Tracker Insertion (Dec’07)


CMS commissioning

Underground Muons, Fall 2007


CMS commissioning

Underground Muons, Fall 2007


CMS Physics Technical Design Report

Physics TDR published in 2006

• Comprehensive/up-to-date overview of CMS physics reach650 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


CMS: SM Higgs search frontrunners

NLO cross sectionsSystematic errors included

NLO cross sectionsSystematic errors included

Benchmark luminosities:• 0.1 fb-1: exclusion limits will start carving into SM Higgs x-section

• 1 fb-1: discoveries become possible around MH~165 GeV

• 10 fb-1:SM Higgs is discovered (or excluded) in full range


So-called golden channel

I will use this channel to illustrate the level of scrutiny all major search channels now undergo• strategy for data driven background control• strategy for measuring all efficiencies directly from data

HZZ4l


tt 4 + X

Zbb 4 + X

Higgs signal H 4

ZZ 4 with spectacular peak at m4=mZ

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

HZZ4dominant 4 backgrounds

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

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

ZZ (dominant after cuts)

t-channel s-channel


HZZ4analysis strategy

Peak in mdistribution

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

m4 mass windowo isolation cut on the least isolated muon (i.e., the same cut for all

muons)o muon pT cut for the 3rd softest muono after applying these cuts, others do not help anymore

• 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 must be properly de-rated due to a search being conducted in a wide range of masses)

Higgs signal H 4


HZZ4understanding ZZ bkgd

Knlo(m4)

Box-diagram

Control samples

QCD scale uncertainties

PDF scale uncertainties

Isolation cut uncertainties

Muon efficiency uncertainty

2+ + * + …

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 background

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 background

Knlo(m4)

Box-diagram

Control samples

QCD scale uncertainties• estimate of higher-order

contributions




Normalization to Z2Normalization to Z2


HZZ4ZZ background

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 (qq Z) must be very similar to that in ZZ (qq ZZ)o MC studies (random cone and tag-and-probe techniques) 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


ZZ is a dominant bkgd

HZZ4: signal and background after cuts


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

systematic errors are under control

L=10 fb-1

Total 8 eventsExp bkgd 0.8 evtsScL = 4.7


HZZ4lcombining four channels

new

electrons are difficult systematic errors are under control


HZZ4lsignificance de-rating

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


Standard Model Higgs: H



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

Analysis:• Cut-based

o PT, , isolation, M

o events sorted by photon quality (EM shower profile quality)

• Optimized (NN)o loose cuts and sortingo event-by-event kinematical Likelihood Ratioo bkgd pdf from sidebands, signal pdf from MC

• Systematic errors folded in

L=1 fb-1

Signal x 10



L=7.7 fb-1, Signal x 10(best category of events)

Di-photon mass alone does not provide the full power

CMS

Events sorted by their individual LLR’s

L=7.7 fb-1, Signal not scaled(all categories of events)

CMS



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



CMS

NOT YET PUBLIC

Discovery Exclusion


VBF channels qqH

qqH, H -- is it better than inclusive H?qqH, HWW -- is it better than inclusive

HWWl?CMS and ATLAS do not seem to agree…


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?


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…


P.S. Word of caution from Tevatron

MH=110 GeV (the best expectations in 2003)• SM Higgs exclusion at 95% CL was expected at L=1.2 fb-1

• Now at L~2 fb-1, the excluded limit is a factor of 6 behind the early expectations

MH=160 GeV (the best current limit)• It took enormous effort (well beyond of what was contemplated in 2003) to bring

the current limit where it is now and approx in agreement with the earlier expectations

The reality may be not as rosy as projections---something to remember as we gear up for the Higgs search at LHC…


Backup slides


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


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

ATLAS


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

CMS 2003 CMS 2006


MSSM SM-like Higgs

ATLAS

CMS 2003 CMS 2006


MSSM H and A

ATLAS

CMS 2003 CMS 2006


ATLAS

MSSM H± (old/new plots)

CMS 2003 CMS 2006


ATLAS

Documents

Higgs Boson at CMS: gearing up for discovery