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Electroweak Symmetry Breaking: experimental investigations. The question The tools : accelerators and detectors The status from precision electroweak measurements The status of direct searches The near future (Tevatron, LHC) The Susy factory: ILC The Higgs factory: muon collider

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Electroweak Symmetry Breaking:

experimental investigations

  • The question
  • The tools : accelerators and detectors
  • The status from precision electroweak measurements
  • The status of direct searches
  • The near future (Tevatron, LHC)
  • The Susy factory: ILC
  • The Higgs factory: muon collider
  • Conclusions

The Question: Why do W’s have mass

(and photons dont)?

The Standard Model answer:

a complex doublet of self coupling scalars with weak isospin ½

splits off into

W+L W-L W0L (additional degrees of freedom of massive particle) and h0

Furthermore, W0 and B field mix by angle qw to give Z and g


mg = 0

This important test of the model (or is it?) is verified with high precision

speed of em radiation is independent of wavelength

residual energy carriedby vector potential (Arhonov-Boehm effect)

Magneto hydrodynamics of solar plasma mg< 6 10 –17eV(new, PDG 2004)

and of course is respected by em gauge invariance



ran around Z peak

Z mass and width

then up to 209 GeV

collected 20MZ,& 80 kW


Slac Linear Collider

92 GeV polarized e+e- collider

ran at Z peak (500kZ)

observed first Z event

polarized beam (~77%)

very small vertex

 excellent b, c tag



2 TeV proton-antiproton collider

WZ event in D0




These are real magnets now!



measure Z and W masses

measure W

check relation mW= mZcosW

see that it is affected by Electroweak Radiative corrections

use these to predict top quark mass

find the top and check its mass

use mass to refine Higgs boson mass from EWRCs

try to find a physical h particle

what if not? verify properties of W and Z, WW, WZ, ZZ scattering

If yes, identify its properties, Susy or not – other Higgses




relations to the well measured


at first order:

Dr = a /p (mtop/mZ)2

- a /4p log (mh/mZ)2

e3 = cos2qwa /9p log (mh/mZ)2

dnb=20/13 a /p (mtop/mZ)2

complete formulae at 2d order

including strong corrections

are available in fitting codes


parameters of the sm m z 2
Parameters of the SM: (Mz2)

Using the latest experimental data from BESII:

5hadron = 0.02761 0.00036

(Burkhardt and Pietrzyk 2001)

5hadron = 0.02755  0.00023

(Hagiwara et al. 2003)

These data has also confirm the validity

of extending the use of perturbative QCD

in the calculation of 5hadron.

The most precise of these theory-driven

calculations gives,

5hadron = 0.02747  0.00012

(Troconiz and Yndurain 2001)

using CMD-2 and KLOE

latest data, seem to cancel out

using CMD-2 latest data

 is not anymore the limiting factor in the SM fits…

thanks BES !!!


parameters of the sm m 2
Parameters of the SM: (~M2)











(11658472.07± 0.11)10-10

(692.4 to 694.4 ± 7)10-10 [e+e- -based 04]

(12.0 ± 3.5)10-10 [Melnikov & Vainshtein 03]


BNL01 m-

(0.7 ppm)

BNL00 m+

(0.7 ppm)

New -data collected in 2001, confirms previous measurements using +

(a+ - 11659000)x 10-10 = 203 ± (6 stat.  5 syst.)

(a- - 11659000) x 10-10 = 214 ± (6 stat.  5 syst.)

(a - 11659000)exp x 10-10 = 208 ± (5 stat.  4 syst.)

(a - 11659000)th x 10-10 = 183 ± 7[e+e-] DEHZ04

including KLOE

2.7 from prediction

(was 1.9 before inclusion of 2001 data)


luminosity measurement to 6 10-4!

LEP: N = 2.9841 0.0083

Ginv (new)< 2.1 MeV

NB this is 2s low


energy resolution (resonant depolarization)

+-200 keV! variations due to tides,




mZ= 91187.5 +-2.1 MeV


Note relative insensitivity of Gz to

Higgs mass.

Was the dominant new factor

in 1994 when results from the 1993 scan

(with res. dep. on each point)

 GZ = 2494.8 +- 2.5 MeV

mtop = 174 +- 12 +- 18 GeV

Bolek Pietrzyk Moriond March 1994


mtop = 174 +- 16 GeV

CDF may 1994



Measuring sin2qWeff (mZ)

sin2qWeff  ¼ (1- gV/gA)

gV = gL + gR

gA = gL - gR


e+e-  mn qq

W mass

ALEPH evts

e+e-  q1 q2 q3 q4

WZ event in D0



SM: combination of and yields mW and sin2qWeff(Q2)

experiment expresses result in terms of   sin2qW  = 1 – m2W/m2Z

which is strictly and obviously equivalent to mW once mZ is so well measured.

beyond SM: sensitivity to unexpted Q2 dep. of couplings and or propagators (Z’)

Trivial problems: predictions are sensitive to assumptions about

isospin symmetry violations

is u(x) in neutron strictly equal to d(x) in proton? charm production?


Measuring masses with JETs

  • which may not be independent
  • LEP mW from the 4quark channel
  • W  qq gives two dependent jets
  • (in JETSET language these jets are part
  • of one single string) but they form a
  • WW qq qq gives two COLOR SINGLETS
  • which in principle shouldnt talk to each other
  • Is this true? It has been suspected that
  • there may be some O(as2) correction leading
  • for example to
  • Bose Einstein correlations between (BEB)
  • the two systems
  • Color reconnection effects
  • there has been some progress in trying to
  • see such effects in data.
  • This can affect WW 4q mass by > 100 MeV
bec in w w events
BEC in W+W- events



fraction of model seen

BEC effects experimentally established in Z jets at LEP1

Inter-W BEC? Analyses performed in 4 LEP experiments to search/limit them

Observable: distance in p-space between pairs of charged pions:


0 1 Q(GeV)

  • Inter-W BEC correlations disfavoured
  • Limit on systematic: dMW ~ 15 MeV
the particle flow analysis
The particle flow analysis
  • Data
  • - SK1 (extreme parameter)

No CR:



- Jetset








Observable: ratio of particle flow between the inter and intra-W regions:

(A + B) / (C + D)

CR models predict a modified particle flow in W+W- events:

results from particle flow
Results from particle flow

CR Prob

‘Asymmetry’ from experiments combined in a c2

For SK1:

Preferred value of the parameter

(0.5 + 0.2 - 0.3)

corresponds to dMW ~ 100 MeV!!

reduction of d m w
Reduction of dMW


idea is to reduce effects by excluding particles situated outside

angular cones around the jets.

Some resolution is lost but systematic error is reduced.

Good reduction factors are obtained for all available models

Example: Cone (R=0.5 rad), with a statistical loss of ~ 25%:

MW (GeV)


SK1 k=2.13

Cone radius (rad)

m w from direct reconstruction
mW from direct reconstruction



Results in CERN-EP/2003-091, LEPEWWG/2003-02

still with standard jet algorithms

DmW = 22 ± 43 MeV


 errors as of summer 2003

errors expected for

summer ‘05 conferences:

there will also be an

improvement on the

beam energy error

due to usage of LEP


lots of hard work, and improved understanding … but diminishing returns

physics processes at lep2
Physics processes at LEP2

~100k evts

~10k evts

~1k evts

~100 evts

w pair cross sections
W-pair cross sections

n exchange t channel ONLY


agreement to 0.6+-0.9%

Clear proof of SU(2)xU(1) gauge couplings !

NB this is really non trivial. W3= Z cosqW + B sinqW


LEPII (and LC) energy calibration

Alas, beam polarization vanishes at LEP above E=65 GeV

res. dep. will not work for linear collider

idea: use e+e-  Z g to measure Ebeam given that mZ is so well known



Jets that are boosted

lead to non trivial systematics!

Tesla TDR  mW +- 6 MeV … hmmmm …

the calorimeter and tracker

will have to be very carefully designed,

and full identification of final state

hadrons (incl. neutrons, L and K)

will be needed.

This method gives a statistical error that

matches that of the W mass measurement

in the lvqq channel.

using muons instead would require

20 times more stats.

Systematic uncertainties

Similar results by L3, OPAL


TOP mass measurement


Status as of Moriond 2005

Method similar to mw at LEP II: form ‘estimator’ and compare measured distribution

to templates with different top masses as input.

(this cannot be done by rescaling since top is too narrow)

Progress was noted when a ‘likelihood’ was built including event by event error estimate

(D0, CDF)

There is a flurry of new measurements and measurement techniques at RUNII.

In most cases the limitation comes from the JET ENERGY CORRECTIONS.

d0 run i top mass analysis using me method
D0 Run I - Top Mass Analysis Using ME Method

Top Mass determined using maximum likelihood

  • 91 candidate tt events
  • 77 with exactly 4 jets selected
  • 22 passing cut on background probability (Pbkg < 10-11)

Expected statistical error


  • Nature429, 638-642 (2004)

Expected 5.4 GeV

Observed 3.6 GeV

Jet energy scale syst: 3.3 GeV/c2

Mtop = 180.1 ± 3.6 (stat) ± 3.9 (sys) GeV/c2

Comparable precision to all previous measurements combined

(some luck involved!)

m top measurements
Mtop Measurements
  • Combined RunI mass:
  • mt=178.0 ± 4.3 GeV/c2
      • was: 174.3 ± 5.1 GeV/c2
  • Run II measurements
    • Systematic uncertainty largely dominated by jet energy correction: will be reduced
    • RunII goal is dm~2-3 GeV/c2

error bars: red=stat, blue=total

measuring mtop
Measuring Mtop


LO ME final state:

  • Lepton+jets
    • Undetected neutrino
      • Px and Py from Et conservation
      • 2 solutions for Pz from MW=Mln
    • Leading 4 jets combinatorics
      • 12 possible jet-parton assignments
      • 6 with 1 b-tag
      • 2 with 2 b-tags
    • ISR + FSR
  • Dileptons
    • Less statistics
    • 2 undetected neutrinos
    • Less combinatorics: 2 jets

CDF sees:

Largest uncertainty: Jet Energy Measurement

jet energy corrections
Jet Energy Corrections

Determine true “particle”, “parton” E,p from measured jet E, q


  • Non-linear response
  • Uninstrumented regions
  • Response to different particles
  • Out of cone E loss
  • Spectator interactions
  • Underlying event

but: top is NOT a color singlet, nor is tt pair.

This method requires that the effect on the mass reconstructed using

a specific jet rec. algorithm is perfectly modelled by the MC

in a situation where there is no conservation law to prevent large effects.

* There is no calibration of this! *

(At LEP a light quark typically acquires 5-10 GeV due to fragmentation.

This is not particularly well modelled in qqbar situation. But what about ppbar?)


Color flow must be broken, but where?



W (color singlet)


W (color singlet)


and why not this?



W (color singlet)


W (color singlet)


top mass outlook

Tevatron aims at measuring mtop with a precision of 2-3 GeV.

This would be a remarkable achievement and progress.

LHC hopes to be able to reach 1 GeV

ATLAS note (SN-ATLAS-2004-040) mentions testing top mass

against varying the jet cuts.

Because of all the gluons around this may be a very sticky business!


ELECTROWEAK fits (as of Moriond 2005)

this in fact is a verification

of the validity of the relation

mW = mZ cosqW at tree level.

(up to corrections due to mHiggs

and any new physics cancellation)


ELECTROWEAK fits (as of Moriond 2005)

these plots show the fact that

sin2qeffW i the most sensitive

estimator of the Higgs mass,

but the limitation will soon come

from the top mass meast

consistency with the sm
Consistency with the SM

SM fits:

with a 2/d.o.f. = 15.8/13 and

a 67% correlation between

mtop and log(mHiggs).

The largest contribution to the

2 is AbFBwith 2.4. It pulls for

a large mHiggs in opposition to l,

mW and leptonic asymmetries.

5hadron = 0.02769 0.00035

s(mZ) = 0.1186  0.0027

mtop = 178.2  3.9 GeV

log(mHiggs) = 2.06  0.21

constraints on m higgs
Constraints on mHiggs

MH= 126+73-48 GeV

MH  280 GeV @ 95% C.L.

constraints on m higgs1
Constraints on mHiggs

Is there any chance to improve this constraints?

[log(mHiggs)]2 = [exp]2 + [mt]2 + []2 + [s]2

Z asymmetries,sin2eff :[0.22]2 = [0.15]2 +[0.12]2+ [0.10]2 + [0.01]2

all high Q2 data:[0.21]2 = [0.12]2 +[0.13]2 + [0.10]2 + [0.04]2

[0.03] if theory-driven

The reduction in mtop (5.1  4.3 GeV) has reduced the uncertainty on mHiggs , but still the TOP priority is to reduce the uncertainty on mtop ,which is limited by systematic uncertainties!

search for the sm higgs boson
Search for the SM Higgs Boson
  • Mass determines Higgs boson profile:

@ 114 GeV : s ~ 0.1 pb

BR(Hbb) ~ 74% BR(Htt) ~ 7%

  • SM searches exploited b-tagging extensively

ALEPH 4-Jet candidate

Mbb=114.3 GeV

two b-tags


SM Higgs: the final word from LEP

Consistency with BG

only hypothesis:

Mass limit via


Mass spectrum after tight selection cuts

Consistency with:

  • background only: 1-CLB = 0.09 @ 115 GeV (1.7s excess)
  • signal + background: CLS+B= 0.15 @ 115 GeV

Observed Limit: 114.4 GeVExpected Limit: 115.3 GeV

Phy. Lett. B565 (2003) 61


Higgs at Tevatron?

Ldt (fb-1)


Updated in 2003 in the low Higgs mass region

W(Z)Hln(nn,ll)bb to include VBF

 better detector understanding

 optimization of analysis


Tevatron will begin sensitivity to LEP Higgs limit (or signal?) when >2.5 fb-1

will have been accumulated … it could be quite soon (Moriond 2007?)



CMS note 03 033 ATLAS SN-ATLAS-2003-024

more on-going


H →WW (*)→ℓ ν ℓ ν

NB in this channel,

it is easy to determine

the spin of the Higgs!



striking now: there is aways at least two channels of which at least one allows

determination of spin of Higgs and, if mH<160 GeV

the ratio of couplings to bosons vs fermions.



The standard Model has been verified in many ways experimentally

(boson couplings, masses properties)

its structure is still mysterious, and the mechanism by which

masses are given is still unclear.

It all works as if there was a Higgs, although one could not help notice that

the radiative corrections assocaited to it as consistent with

log (mH/mZ)=0 ….

If the Higgs is indeed lower in mass than 280 GeV it will be discovered at

LHC rather rapidly, and thanks to the realization of the importance of VBF

we should be able if it is not of mass higher than 2 mW to measure its

mass spin and parity

Precision physics with jets is delicate (color reconnection) and will reserve much

fun in the near future.

we are living in exciting times!