Status of EW symmetry breaking

Four seas conference

17 April 2002 1

Vanina Ruhlmann-KleiderCEA/DAPNIA/SPP

Status of EW symmetry breaking

1) Introduction

2) The SM Higgs boson

3) Other scenarios

4) Conclusions

V.Ruhlmann-Kleider Four seas conference 2

1)The Standard Model of particle physics : the ingredients

12 elementary constituents

6 leptons 6 quarks

e-

-

-

e u

c

t

d

s

b


3 interactions propagated by intermediate bosons of spin 1: (massless) electromagnetic interaction W et Z (massive) weak interaction 8 gluons (massless) strong interaction

one example :

The and Z fields are linear combinations of two vector fields which do not know the difference between the em and weak interactions : before mass generation that is, at high energy

ElectroWeak symmetry

Fundamental interactions


is assumed to be spontaneous i.e. due to a non-zero v.e.v. is therefore responsible for the generation of the particle

masses :

M = 0 MZ ~ 91 GeV MW ~ 80 GeV

confirmed when discovering the W,Z at CERN in the 80’s

what is the exact mechanism of the

breaking ?

the SM minimal solution: Higgs mechanism with one doublet of scalar fields with a non-zero v.e.v.

EW symmetry breaking


the SM minimal solution: Higgs mechanism with one doublet of scalar fields acquiring a non-zero v.e.v. one Higgs boson all properties predicted

its mass, which is poorly constrained by theory: 0 MH 1000 GeV

search ALL experimental clues to such a Higgs boson

Other more complicated scenarios exist, too …

EW symmetry breaking


2) Status of the search for the SM Higgs boson

Over the past decade, the search strategy was twofold: Direct search for a Higgs boson actually produced in collisions:

LEP sensitivity to low masses

Indirect constraints from precise EW measurements sensitive to the quantum corrections due to loops with the Higgs boson:

LEP, SLC, TeVatron sensitivity to low and high masses

see W.Adam’s talk


Direct search at LEP: the environment Initial S/B ratios:

at LEP 1 :

MH = 10 GeV: 10-3

MH = 60 GeV: 10-5

at LEP 2 :

MH = 60 GeV: 10-2

MH = 115 GeV: 10-3

LEP 1 result:

Before LEP:

MH > 100 MeV (95%CL)

MH > 60 GeV (95% CL)


Direct search at LEP: experimental signature

LEP provided the ideal experimental environment to search for a light Higgs boson, i.e. with mass MH S – MZ :

the dominant production process:

main decay is H bb (80% at 100 GeV) clean signature

Z boson easy to tag: mass, decays:

Z hadrons 70% Z charged leptons 10% Z neutrinos 20%


Tools for the direct search at LEP: b-tagging

Step 1: a silicon vertex detectorAfter full alignement, hit precisions are :

~10 m in R ~15 m in Rzin the central part of the detector.

Ex: DELPHI

~3 double-sided layers

~0.5 X0



Step 2: impact parameters B hadron lifetimes <>= 1.6 ps

flight distances ~3 mm

impact parameters ~c ~400 m

Experimental resolutions:

R: = 20 60/p sin3/2 m

Rz: = 39 71/p m ( ~90o)

IP/ (= significances)as basic inputs of b-tagging

PV

SV

IP


Step 2: secondary vertices B hadron decays lead to tracks originating from secondary

vertices information from reconstructed SV add more discrimination between b quarks and other flavours

more powerful b-tagging:

e.g. SV masses

c quarks

b quarks




Step 3: tuning and control of performance Simulated IP distributions and resolutions tuned on data.

data/simulation agree within 5%

R significances

before tuning after tuning


Step 3: control of performance b-tagging performance checked on control samples:

Z data

Z data

b-tagging variable

data/simulation agree within 5%



To reach the best S/B discrimination: multidimensional analyses (NN, likelihood …)

L3

Hqq channel

likelihood

To improve on signal mass reconstructions: kinematic fits with E,p conservation and the Z mass constraint

H channel

Tools: multidimensional analyses and kinematic fits


Tools… : statistical interpretation of the results

To make an unbiased and powerful statistical analysis of the search results:

stop selection of signal-like events at a loose level test compatibility of data with B-only and S+B hypotheses rates and 2d

pdf’s (H mass vs a second variable such as b-tagging, NN …)

e.g. Hqq channel, S = 206.5 GeV, DELPHI

Tools : likelihood ratio test-statistics (-2lnQ) and confidence levels (CLs,CLb)


The HZ production cross-section rises fast once the kinematic threshold is crossed a few pb-1 are enough to test a given MH

hypothesis as soon as SMH+MZ

e.g. july 2000: hypothesisMH=110 GeV is excluded

LEP 2 result :

MH > 114.1 GeV (95% CL)

Direct search for the SM Higgs boson at LEP: results


A possible signal ?

likel

ihoo

d ra

tio te

st-s

tatis

tics

expected behaviour from background only (mean, ±1

and ±2 bands)

expected behaviour from a a 115 GeV signal + bkg

data: consistent with a signal of mass:

compatibility with the hypothesis of a background fluctuation: 3.4%compatibility with the hypothesis of a 115.6 GeV signal: 44.%a handful of events makes most of the effect

mass hypothesis

MH = 115.6 ± 0.8 GeV

Direct search for the SM Higgs boson at LEP: results


Final state: e+e- HZ qq bb

Comparing signal and background probabilities:

ln(1+s/b) = 1.73

4 jets of particles2 b-jets

Reconstructed mass: MH = 114.3 3 GeV

One event consistent with the SM Higgs boson production


EW precise measurements:(LEP,SLC,TeVatron..)

(95% CL)

Direct searches (LEP):

(95% CL)

MH 196 GeV

MH 114.1 GeV

MH = 115.6 GeV ?

Summary about the SM Higgs boson:


3) Other more complicated scenarios

One Higgs boson with non-standard properties: Same decays but different cross-section:

MH 105 GeV: BR /SM 20%

Non-b hadronic decays and different cross-section:

MH 105 GeV: BR /SM 30%

114.1 GeV

112.9 GeV


Invisible decays and different cross-section:

MH 105 GeV: BR /SM

25%

Photonic decays and different cross-section:

MH 105 GeV: BR /SM

5%

114.4 GeV

115. GeV

One Higgs boson with non-standard properties:


two representative scenarios: results in the (mh, tan) plane

Mh 91.5 GeV , MA 92.2 GeV (95% CL)0.7 tan 10.5 excluded (95% CL)

Mh 91.0 GeV , MA 91.9 GeV (95% CL)0.5 tan 2.4 excluded (95% CL)

ratio

of

the

tw

o H

igg

s d

ou

ble

t v.

e.v

.’s e+e- h A

e+e- h Z

More Higgs bosons: h, A, H, H+, H- from SUperSYmmetry


the same two scenarios: results in the (mA, tan) plane

the mass limits in the representative scenarios have been checked to be valid in most scenarios Large mu scenario (h, A decoupled from b’s): completely excluded reinterpretation of the analyses in models with explicit CP violation : under progress

More Higgs bosons: h, A, H, H+, H- from supersymmetry


charged Higgs bosons:

New decays open at high mass (H+ W A) : dedicated analyses under progress

assuming : Br(H± ) + Br(H± cs) = 1

MH 78.6 GeV (95% CL)

More Higgs bosons: h, A, H, H+, H- in general 2HD models


neutral Higgs bosons: masses and couplings no longer constrained as in SUSY Models, more final states to be expected and hence analysed, e.g.

More general analyses of LEP data to cover less constrainedtopologies than in SM or SUSY-driven analyses

is forbidden in SUSY models(Mh ~ MA when cos(-) is large)

but allowed in 2HD models

is negligible in the SM and experimentally excluded in SUSY models

but possible in 2HD models

More Higgs bosons: h, A, H, H+, H- in type II 2HD models


Other results: analyses of other final states (non-b hadrons, 4’s), reinterpretation of the existing analyses in models with two doublets and a singlet …

reduction factor 1

reduction factor 0.1

Z bb h/A bb

enhancement factor of the bb h/A couplings

Neutral Higgs bosons in type II 2HD models: examples of results


EW symmetry breaking due to condensation in the vacuum of strongly-interacting fermions (technifermions):

most models are disfavoured by EW precision constraints some models fulfill them direct searches

MT 79.8 GeV (95% CL) MT 206.7GeV (95%CL)

No elementary Higgs bosons: technicolor models

J.Ellis et al., Phys. Lett. B343 (1995) 282.

SMdata

TC models


Conclusions The past decade did open the era of the search for

the exact EW symmetry breaking mechanism with both the precise EW measurements and the direct searches (LEP, SLC, TeVatron)

SM Higgs boson:

Many other scenarios have also been investigated The main question : is the EW symmetry breaking

due to doublet(s) of scalar fields or not ?

TeVatron run II, LHC, LC

MH < 196 GeV MH > 114 GeV MH = 115.6 GeV ?

Documents

Status of EW symmetry breaking