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(The Uncertainties Of). Higgs Discovery at the LHC. OR Hunting the (non-SUPAsymmetric) Higgs. Tony Doyle. OR The Iceman Cometh. Outline. Ingredients Motivation Constraints Dependencies Collider Detector Grid Analysis. Discovery? When?. Introduction.

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the uncertainties of

(The Uncertainties Of)

Higgs Discovery at the LHC

ORHunting the (non-SUPAsymmetric) Higgs

Tony Doyle

ORThe Iceman Cometh

Particle Physics Theme Launch

  • Ingredients
  • Motivation
  • Constraints
  • Dependencies
    • Collider
    • Detector
    • Grid
  • Analysis

Discovery? When?

Particle Physics Theme Launch

  • The Iceman Cometh? exposes the human need for illusion as an antidote to despair..
  • The Iceman Cometh exposes the physicist need for a goal as an antidote to building a machine/detector/computer system/simulation *

*delete as applicable

  • Apologies to Eugene O’Neill
  • Thanks to Peter Higgs..

Particle Physics Theme Launch


Where Theory meets Experiment

Particle Physics Theme Launch


The Higgs Mechanism

Higgs potential:

V()= -2+ + (+)2 (>0)

=exp(ia(x)T’’a) |0 . (0 + H(x))

mH = 2 (a free parameter)

Theory: vacuum potential

The ultimate green field site


Ready to interact

Particle Physics Theme Launch

lhc motivation higgs production in pp collisions
LHC Motivation: Higgs Production in pp Collisions












unitarity arguments

mH ≤ 1 TeV

(Lee et al, 1977)

MH ≤ 1000 GeV

EW ≥ 500 GeV

E1 ≥ 1000 GeV

E2 >≥ 1000 GeV

Parton luminosities


 Proton Proton Collider with Ep ≥ 7 TeV

Particle Physics Theme Launch


LHC Motivation: Higgs Discovery Potential

For 30 fb-1, the Standard Model Higgs can be discovered over the full allowed mass range  End of talk?

Particle Physics Theme Launch

latest electroweak fits

logarithmic sensitivity to mH:

  • Measured mt=172.7  2.9 GeV

3 GeVshift in measured mt causes ~20% shift in mH

  • Preferred mH = 91 GeV
  • Incl. theory uncertainty gives mH < 186 GeV (95% CL)
  • Does not incl. LEP2 direct search limit: mH > 114.4 GeV (95% CL)
  • Renormalise probability (mH >114.4 GeV) to 100%, then mH <219 GeV (95% CL)
Latest Electroweak Fits

Greater focus on the light Higgs

Particle Physics Theme Launch

tevatron direct search
Tevatron direct search
  • Tevatron may be able to exclude a light Higgs (mH<130 GeV)
  • Given sufficient luminosity, evidence for a light Higgs still possible

L [fb-1 ]

Combined CDF and D0

Updated in 2003 in the low Higgs mass region

W(Z)Hln(nn,ll)bb to include

1. better detector understanding

2. optimised analysis

10 fb-1/expt

4 fb-1/expt

Particle Physics Theme Launch

a when cometh the lhc
A. When cometh the LHC?

Particle Physics Theme Launch

a when cometh the lhc1
A. When cometh the LHC?
  • Stages:
  • • Establish injection conditions
  • • Fill
  • • Ramp up
  • • Squeeze
  • • Prepare physics
  • • Physics
  • • Standard beam dump
  • Non-trivial

7+7 = 14 TeV

Ultimate design luminosity (SUPA-LHC?)

L = 1034 cm-2s-1 = 10 events nb-1 s-1

or 100 events fb-1 yr-1(continuous running)

Realistically expect?

Particle Physics Theme Launch


LHC Startup Planning

“Main objectives are to terminate installation in February 2007 and enable first collisions in summer 2007” Lyn Evans

Particle Physics Theme Launch

lhc commissioning in 4 stages
LHC Commissioning(in 4 stages)

Stage 1: Initial commissioning to pilot physics

Stage 2: 75 ns (Nominal emittance, full crossing angle)

Stage 3: 25 ns (Limited intensity)

Stage 4: 25 ns (Performance)

Particle Physics Theme Launch

a when cometh the lhc2
A. When cometh the LHC?
  • Stages:
  • • Establish injection conditions
  • • Fill
  • • Ramp up
  • • Squeeze
  • • Prepare physics
  • • Physics
  • • Standard beam dump
  • Non-trivial

7+7 = 14 TeV

Ultimate design luminosity (SUPA-LHC?)

L = 1034 cm-2s-1 = 10 events nb-1 s-1

or 100 events fb-1 yr-1(continuous running)

Realistically expect?

2007 2008 2009

collisions ~2fb-1 ~10fb-1

Particle Physics Theme Launch

bat 40
Bat 40

B. When cometh ATLAS?

not where.. the detector will end up

Particle Physics Theme Launch


The ATLAS Detector

ATLAS = large international collaborationto find the Higgs

(and much more) in the range 0.1TeV < mH < 1TeV

The ATLAS experiment is 26m long, stands 20m high and weighs 7000 tons

SCT alone has 6.2 million read-out channels

Particle Physics Theme Launch

atlas status
ATLAS Status



All four completed SCT barrel

cylinders have been integrated

in their thermal enclosure

The barrel LAr and Tile calorimeters are in their ‘garage position’.

LAr end-cap C on its way

A cosmic muon registered in the

barrel Tile calorimeter

Integrated end-cap TRT wheels

Particle Physics Theme Launch


B. When Cometh ATLAS?

“with good will and great efforts from everybody we can be confident that the Technical Coordination Team will manage to have ATLAS installed by June 2007”

Particle Physics Theme Launch


C. When Cometh the Grid?

  • Aim: by 2008 (full year’s data taking)
  • CPU ~100MSi2k (100,000 CPUs)
  • Storage ~80PB
  • - Involving >100 institutes worldwide
  • Build on complex middleware being developed in advanced Grid technology projects, both in Europe (Glite) and in the USA (VDT)
  • Prototype went live in September 2003 in 12 countries
  • Extensively tested by the LHC experiments in September 2004
  • Currently 197 sites, 13,797 CPUs, 5PB storage in September 2005

Particle Physics Theme Launch

a rare phenomena huge background

What is the LHC Computing Challenge?

a. Rare Phenomena Huge Background

b. Complexity

All interactions

9 orders of magnitude

The Higgs

  • “one in a billion events”
  • “millions of readout channels”

Particle Physics Theme Launch

meeting analysis requirements
Meeting Analysis Requirements…
  • Requirements = Pledges
  • Compare
  • (a) Bottom up
  • (b) Top down

Particle Physics Theme Launch

c when cometh the grid
C. When Cometh the Grid?
  • Service Challenges – UK deployment plans
  • End point

April ’07

  • Context:

the virtual




Particle Physics Theme Launch

production modes e cm 4x 1 x 2 e 1 e 2
Production ModesEcm=(4x1x2.E1E2)






Hbb (large backgrounds)

HZ (low significance)

Particle Physics Theme Launch













Decay Modes

large irreducible backgrounds for tt, cc and gg decay modes, Zg

Particle Physics Theme Launch

light higgs
Light Higgs
  • Various decay modes:
    • mH<130 GeV: tt Hl X bb
    • mH<150 GeV: H
    • mH<150 GeV: Htt
    • 130< mH<2 mZ:HWW(*)l l
    • 130< mH<700 GeV:HZZ(*)l l l l
    • 700< mH<1 TeV:HZZll HWWl jj
  • “(no K-factors)” = conservative

[ATLAS 04]


Particle Physics Theme Launch


Higgs Production Corrections and Uncertainties

PDF Comparison: CTEQ6, MRST2002, Alekhin

  • example: σ(mH=120 GeV) @ LHC

The HO pQCD corrections to (gg→H) are large (more diagrams, more colour)

can improve NNLO precision slightly by resumming additional soft/collinear higher-order logarithms

Particle Physics Theme Launch









Higgs Production Corrections and Uncertainties

Djouadi & Ferrag,


*Increase of cross section compared to lower order (correction - not an uncertainty)

Not just LO, NLO, NNLO… also resummations

threshold logs


 σtheory  ± 9%

Catani et al,


Particle Physics Theme Launch



30 fb-1

Analysis Methods e.g. Hbb, via ttH

  • Complex decay channel:
    • One lepton (trigger)
    • 4 b-jets + 2 jets

 Need excellent b-tagging

    • Fully reconstructed t
      • High jet multiplicity  combinatorials
  • Background:
    • Reducible: tt+jj, W+jets
      • Extracted from data
    • Irreducible: ttbb, combinatorials
  • Discovery potential with 30 fb-1:

MH < 125 GeV

Particle Physics Theme Launch


NLO Background Example


30 fb-1





the more external coloured particles, the more difficult the NLO pQCD calculation


pp →ttbb + X

bkgd. to ttH

the more particles, the more difficult the whole analysis

the leading order O(αS4) cross section has a large renormalisation scale dependence

Particle Physics Theme Launch

analysis methods e g h gg
Analysis Methods e.g. H  gg

1. Radiation lengths 3. NLO corrections

2. Primary vertex 4. inc. recoil jet

(all/unconverted/converted g’s)

An “easy”

decay mode

(30 fb-1)

“easy” is relative

Particle Physics Theme Launch

combined analysis

Two or more channels in complete light mass range

  • Enhance early Higgs discovery?
  • ATL-PHYS-2004-034 (2004)
Combined Analysis
  • Requires an understanding of (all of) the individual signals, backgrounds and the relative systematic uncertainties…






  • Invert the problem
  • “Luminosity required for discovery?”
  • Assume the LHC luminosity guesstimates are correct…

Particle Physics Theme Launch

a light summary
A (Light) Summary

Priors indicate a Higgs mass 114.4 < mH [GeV] <219

A Grid service will be launched in April 2007

The ATLAS detector will be complete in June 2007

The LHC will provide first collisions in Summer 2007

These will enable data analyses such that the Higgs will be discovered on May 29th 2009

probably… large corrections

“Of course I like it when they give seminars on the search for the Higgs and so on…”

Particle Physics Theme Launch


How the Higgs will feel on his 80th birthday?

what have I done?

Of course I like it when they give seminars on the search for the Higgs and so on. And of course I think it's important to look for. But you want to know the truth, when I consider the huge sums going for this, the lifetimes spent on the search, I can't help but think: “Good heavens, what have I done?” Peter Higgs

The last Barrel Toroid coil was moved into position on 25th August

and the structure was released from the external supports on 29th September

Particle Physics Theme Launch