Andrey Korytov, UF 12 October 2006, Boston University 1 Higgs Physics at LHC (gearing up for discovery) Andrey Korytov

Andrey Korytov, UF 12 October 2006, Boston University 1

Higgs Physics at LHC(gearing up for discovery)

Andrey Korytov


Outline

• Introductory remarks (what we know)

• LHC, ATLAS, and CMS

• Gold-plated channel HZZ4 at CMS (in full scrutiny)

• Other SM Higgs discovery channels

• MSSM Higgs discovery channels


SM Higgs Trivia

• 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 to remember: • 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

f f

0v h


What we know: theory

After renormalization• (Q)• If mH were small at 1 TeV,

runs down, flips sign at some scale Q, and vacuum breaks loose

• If mH were large at 1 TeV, runs up, coupling 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 (apparently 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 (example on MH=110 GeV):• SM Higgs exclusion at 95% CL was expected at L=1.2

fb-1

• Now at L~300 pb-1, the excluded x-section should’ve been a factor of two above the SM x-section

• The actual difference is a factor of ten


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

• TDR is out for print last month

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

Z/+Z/ 4 with spectacular peak at m4=mZ

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

t-channel s-channel


HZZ4analysis strategy

Peak 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 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


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 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 derated

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


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...


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 ~ enhances coupling to “down”

fermions: b and are very important!~ suppresses coupling to Z and W

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 }


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

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?


MSSM Higgs or SM Higgs?

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


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…


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

Andrey Korytov, UF 12 October 2006, Boston University 1 Higgs Physics at LHC (gearing up for discovery) Andrey Korytov