Higgs Hunting: The Large Hadron Collider vs' the Linear Collider

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Higgs HuntingThe Large Hadron Collider vs.
the Linear Collider

• Lashkar Kashif
• Harvard University HEPL
• BTSM
• April 30, 2004

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Outline

• Higgs search where we are now
• - very brief review of LEPII results and
  Tevatron expectations
• Brief intro to the LHC
• SM Higgs production and decay modes at the LHC
• Higgs detection in various mass ranges
• Higgs measurements at the LHC
• Intro to the LC
• SM Higgs production at the LC
• Higgs detection and measurements at the LC
• SM vs. MSSM Higgs
• Conversation between the LHC and the LC

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Current Status of Higgs Search

• LEP II ended its very successful run in 2000
• Sadly, success doesnt include a Higgs, but came
  pretty close
• Data from four detectors hint at Higgs mass
  GeV, but with 2.9s significance
• (3s -gt evidence, 5s or greater -gt discovery)
• 2.9s not enough to claim Higgs discovery, or even
  evidence
• Lower limit of mh gt 114.6 GeV set at 95 CL
• Until 2008, ball in Tevatrons court

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• Golden Higgs event from Delphi at LEP II
• h -gt 4b-like event, 4-jet topology
• Higgs hypothesis gave mh 114.4 GeV

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Higgs Search at the Tevatron

• Tevatron running at 2 TeV CM energy
• Higgs mass reach 100 GeV lt mh lt 190 GeV
• 5s discovery possible for mh 115 GeV with 15
  fb-1 in integrated luminosity
• 3s evidence achievable for mh 175 GeV
• High hopes for American glory, but Tevatron
  luminosity goal not reached -gt lab director
  bailing out
• 10 fb-1 of data possible by 2008
• With 10 fb-1, CDF and D0 can exclude SM Higgs up
  to 185 GeV
• Even if a Higgs is seen, only very rough profile
  possible

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The integrated luminosity required per experiment
at the Tevatron to either exclude a SM Higgs
boson at 95 CL or to discover it at the 3s or 5s
level, as a function of the Higgs mass.
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Enter the LHC
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The Large Hadron Collider

• Proton-proton collisions at center-of-mass energy
  14 TeV
• Accelerator to be commissioned in April 2007,
  collisions in fall 2007
• Initial instantaneous luminosity 1033 cm-2 s-1,
  design luminosity 1034 cm-2 s-1

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SM Higgs production in pp collisions
gluon fusion

• Dominant Higgs production mechanism for all mh
• Loop dominated by the top owing to the large
  top-Higgs coupling
• Cross-section sensitive to (heavier) 4th
  generation quarks
• -gt Can show evidence for 4th gen quarks even if
  they are too heavy for direct detection

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SM Higgs production in pp collisions
vector boson fusion

• Important for high mh where couplings to the W, Z
  are strong
• Use effective W/Z approximation to calculate
  cross-section
• Cross-section smaller than gluon fusion for all
  mh, but -
• very clean signature consisting of two
  forward-going jets due to q3,q4

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SM Higgs production in pp collisions
associative production

• Bosonic and fermionic hW, hZ,
• Small x-section, but final states have clean
  signature
• Useful in intermediate Higgs mass ranges
• Associative production x-sections change
  substantially for SUSY Higgs sector
• -gt Helps distinguish lightest SUSY Higgs from
  SM Higgs

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Production cross-section of the SM Higgs as a
function of mh. Note that gluon fusion is the
dominant mechanism across the entire range.
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SM Higgs decay modes decays to fermions and
vector boson pairs

• For mh lt 135 GeV, main decay mode is h -gt
• h -gt and h -gt also present
• For mh gt 135 GeV, decays to vector boson pairs
  dominate, particularly h -gt WW()
• For mh gt 2mt, branching ratio for h -gt
  increases sharply and reaches max value 20

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SM Higgs decay modes two-photon and
two-gluon decays

• Since the photon is massless, no direct hg
  coupling
• Coupling through bosonic or fermionic loops
• Clean 2g signature
• Two-gluon decay signature swamped by QCD
  background unimportant at the LHC

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• (a) (b)
• SM Higgs branching ratios (a) 80 250 GeV, (b)
  250 1000 GeV. Note the mess for mh lt 135 GeV in
  (a).

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SM Higgs Detection at the LHC Light Higgs (mZlt
mh lt 2mZ)

• h -gt dominant, but b-jet signal swamped by
  di-jet background also hard to trigger on
• Better detection possible with associative
  production with vector boson or top pair
• - Identify tight leptons
• - Tag b from top decays
• h -gt mode also convenient small x-section,
  but also small background, mainly direct
  di-photons
• For 150 GeV lt mh lt 180 GeV, h -gt ZZ -gt ll-ll-
  is a convenient channel can impose mass
  constraint on one lepton pair
• Main backgrounds are direct ZZ, Zg, and

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SM Higgs Detection at the LHC intermediate mass
Higgs (2mZlt mh lt 650 GeV)

• For mh gt 2mZ , the all-important h -gt ZZ -gt
  ll-ll- channel turns on (Golden channel)
• Almost all backgrounds can be eliminated by
  requiring Z mass constraint on both lepton pairs
• Upper mass reach of this channel 650 GeV due to
  decreasing rate with increasing mh
• For mh gt 300 GeV, h -gt WW -gt lnjj and h -gt ZZ -gt
  llnn are also excellent discovery channels (next
  slide)
• These channels allow detection above mh 650 GeV
  , and indeed beyond the 1 TeV range of an SM Higgs

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SM Higgs Detection at the LHC heavy Higgs (mh gt
650 GeV)

• Decays to vector bosons completely dominate
• Only detection channels are those in which at
  least one boson decays to neutrinos or jets
• In h -gt WW -gt lnjj, can reduce background through
  lepton ID and missing ET
• In h -gt ZZ -gt llnn, reduce background by imposing
  Z mass constraint on lepton pair and requiring
  large missing ET
• h -gt ZZ -gt lljj also possible, but missing ET
  requirement cannot be imposed, hence
  uninteresting
• Same problem with h -gt ZZ -gt 4j and h -gt WW -gt 4j

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• Expected luminosity requirements for SM Higgs
  discovery at 5s for each LHC experiment. Note the
  variation in required luminosity in the Golden
  channel (red).

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LHC discovery potential for MSSM Higgs

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Bottom line

• ATLAS and CMS have been designed to discover SM
  Higgs at gt10s regardless of mass
• Non-SM Higgs reach is few TeV
• If there is an elementary Higgs, fair bet that it
  will be found by 2009
• But -

• Discovery only means we have found some particle
  predicted by precision EW data
• Must measure Higgs mass, quantum numbers and
  couplings very accurately to establish it as the
  mechanism for EWSB

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Things We Want to Measure

• Higgs properties
• - Mass
• - Production rate and lifetime
• - Spin
• Couplings to
• - Fermions ghff
• - Gauge bosons ghWW , ghZZ
• - self-coupling l
• Using measured fermionic couplings, calculate mf
  and compare with known values
• Using measured l, calculate mh and compare with
  measured mh

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What we can measure at the LHC (not much)

• Higgs mass
• - Better than 0.1 precision for mh lt 400 GeV,
  0.1 -1 precision for 400 GeV lt mh lt 700 GeV
• Lifetime and widths
• - Total width determinable only for mh gt 250 GeV
  in the 4l mass peak
• - GW / GZ, GW / Gt and a few other ratios of
  partial widths can be measured crudely, but not
  many
• Spin
• - Angular distribution in h -gt ZZ -gt 4l channel
  sensitive to spin and CP eigenvalue -gt measure
  DCS
• Self-coupling No significant measurement
  possible at the LHC

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How to make precision measurements?

• Hard way SuperLHC (also super-messy)
• - Instantaneous luminosity 1035 cm-2 s-1, 6000
  fb-1 of data
• Easier way Linear ee- collider
• - 500 GeV center-of-mass energy, upgradeable to
  a TeV
• - Instantaneous luminosity 51034 cm-2 s-1
• - Integrated luminosity 200-300 fb-1 yr-1 at
  500 GeV, 350-550 fb-1 yr-1 at 1 TeV
• Theory and precision EW measurements favor a
  light Higgs
• At the LC, expect
• - 105 light Higgs per year
• - 104 intermediate mass Higgs per year
• - Fewer heavy Higgs

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Schematic of TESLA Tera-electronvolt
Superconducting Linear Accelerator

• The world has made a case for an LC. Problem is,
  it has really made three cases for the LC, namely
  the NLC (or FLC) in the US, the JLC in Japan, and
  the TESLA in Germany.
• Rift over what technology to use for
  accelerating cavities
• Lets concentrate on the physics

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SM Higgs Production at the LC

• Two main processes
• Higgstrahlung e and e- produce virtual Z,
  which decays to real Z and Higgs
• - Can reconstruct Higgs mass in spectrum of
  missing mass recoiling against Z
• Vector boson fusion e and e- each emit a
  virtual vector boson, which fuse to yield a Higgs
  and a lepton pair
• - Important at higher energies

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Cross sections for Higgsstrahlung (ee- ? Zh) and
Higgs production via WW- fusion (ee- ? ??h) and
ZZ fusion (ee- ? ee-h) as a function of mh for
two center-of-mass energies, vs 500 and 800
GeV.
s (fb)
mh (GeV)
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Subdominant Production Modes

• Associated production with a quark pair,
  predominantly with a top pair
• - Can be used to determine the top-Higgs
  coupling
• Double-Higgs production small cross-section but-
• With sufficient luminosity, can use this mode to
  extract the self-coupling l

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SM Higgs Physics at the LC

• Higgs decay modes basically same as at the LHC
  decays to
• - vector boson pairs
• - fermions
• - two photons
• But detection techniques are very different
• The LC will have
• - Clean events
• - Little QCD background
• - Low event rates
• - Beam-energy constraint crucial advantage over
  hadron machines
• These factors will enable employing energy-flow
  algorithm -gt reconstruction of 4-vectors of all
  particles in an event

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Higgs mass measurement at the LC

• For Higgs production in the Higgstrahlung
  channel, can use Z recoil mass to reconstruct
  Higgs signal
• - Cleanest for Z -gt ee- or mm-
• - For hadronic Z decays, can see Higgs signal in
  jet-jet invariant mass plot, enabled by
  energy-flow (another talk)
• - These reconstructions yield Higgs mass
  independent of Higgs decay mode
• - Can see invisible Higgs decays (into
  neutralinos, massive neutrinos etc), not possible
  at the LHC
• As in the LHC case, leptonic decay modes fall off
  for heavy Higgs, and hadronic ones must be
  considered

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LC detector simulation results (a) invariant
mass of the jets recoiling from a Z reconstructed
hadronically, for mh 115 GeV, precision 2 GeV
(b) recoil mass Mrecoil of a lepton pair for
different Higgs masses. precision 140 MeV
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Higgs coupling measurements at the LC light Higgs

• For mh lt 2mW, ghZZ and ghWW are best measured
  using Higgstrahlung and WW fusion cross-sections
  respectively
• (remember that the LHC cannot measure these
  couplings with any precision)
• Measure s(Z -gt Zh) using the recoil mass method
• With efficient b-tagging, can separate ee- -gt
  WW- -gt -gt events from Zh
  -gt -gt events, and measure
  s(WW -gtnnh)
• Absolute branching fractions to all quarks and
  vector bosons can be measured with good accuracy
  (by contrast, the LHC can give only branching
  ratios in a limited number of cases)

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Higgs coupling measurements at the LC
intermediate mass and heavy Higgs

• In this range, W, Z couplings can be measured
  more precisely than the LHC due both to greater
  Higgs production rates and larger branching
  ratios into these modes (no gluon fusion)
• Cross-sections measured as before
• In addition to leptonic decay modes of Z,
  hadronic modes can also be used now
• To measure BR(h -gt ZZ), must separate hadronic Z
  decays from hadronic W decays
• For a heavy Higgs, the h -gt tt mode is observable
  at the LC, while this mode will be swamped by
  large QCD tt background at the LHC

• In this range, W, Z couplings can be measured
  more precisely than the LHC due both to greater
  Higgs production rates and larger branching
  ratios into these modes (no gluon fusion)
• Cross-sections measured as before
• In addition to leptonic decay modes of Z,
  hadronic modes can also be used now
• To measure BR(h -gt ZZ), must separate hadronic Z
  decays from hadronic W decays
• For a heavy Higgs, the h -gt tt mode is observable
  at the LC, while this mode will be swamped by
  large QCD tt background at the LHC

• In this range, W, Z couplings can be measured
  more precisely than the LHC due both to greater
  Higgs production rates and larger branching
  ratios into these modes (no gluon fusion)
• Cross-sections measured as before
• In addition to leptonic decay modes of Z,
  hadronic modes can also be used now
• To measure BR(h -gt ZZ), must separate hadronic Z
  decays from hadronic W decays
• For a heavy Higgs, the h -gt tt mode is observable
  at the LC, while this mode will be swamped by
  large QCD tt background at the LHC

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Higgs self-coupling

• The LC can determine l from double Higgstrahlung
  and h -gt hh events
• 23 precision is possible
• (the LHC will not be able to determine this
  coupling at all)
• In the MSSM, a variety of double Higgs production
  processes are required to determine ghhh, gAhh
  etc.

h
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Distinguishing an SM Higgs from an MSSM Higgs

• Suppose ATLAS finds one light Higgs
• In the decoupling limit, SM Higgs and lightest
  CP-even MSSM Higgs have very similar
  phenomenology
• - Couplings to WW, ZZ, tt and cc nearly
  identical, but
• - Couplings to t t - and bb measurably
  different
• - There will also be SUSY loop contributions,
  e.g., to h0 -gt gg
• Key measurements are widths and branching ratios
• Can the LHC determine these parameters with
  sufficient resolution?
• The LC will have enough sensitivity to these
  differences to make the distinction feasible
  (e.g. hep-ph/0106116)

No.
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So, when do we want an LC?

• Old school after the LHC finds at least one
  Higgs and measures its mass
• New school now! It is tomfoolery to wait for the
  LHC
• What if the any Higgs turns out to be too heavy
  to produce at the LC, and what if there is no
  Higgs?
• 1. Any model that uses current precision EW data
  and predicts mh gt 500 GeV has new physics below
  500 GeV
• 2. Even if you dont believe in the Higgs, or in
  SUSY, it is essentially impossible to construct a
  model where there is no new physics at the TeV
  scale
• 3. Whatever new physics there is, precision
  measurements will be essential to elucidate its
  nature

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Feedback between the LC and the LHC

• The two machines will play a symbiotic role
• Discoveries (Higgs and other) at the LHC can be
  analyzed at the LC, which will in turn provide
  feedback to the LHC for further exploration,
  i.e., determine its running program
• - A non-Higgs example the measurement of
  neutralino mass at the LC, and feed in to the LHC
  to yield greatly improved measurement of
  sparticle masses at the LHC
• In short, overlap between the LHC and the LC
  running times can have unexpected, unthought-of
  consequences
• If the LC proposal is accepted by the end of
  2004, the machine can be commissioned in 2015,
  when the LHC Run I has ended and an upgradation
  is under way. That would be perfect, and any
  delay now will hurt physics

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Happy Higgs Hunting!!(Copy of PDB page for the
SM Higgs, borrowed from Eilam Gross)