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A very biased look at
Higgs Physics at LHC
Jeff Forshaw
University of Manchester
WHEPP8, Mumbai, January 2004
Standard Model Higgs
Discovery potential at LHC
Heavy Higgs
i.e. lightest Higgs much heavier than Standard Model upper limit
Real triplet model
Light Higgs
i.e. lightest Higgs much lighter than Standard Model lower limit
MSSM with explicit CP violation
Invisible Higgs
No Higgs
i.e. lightest Higgs has significant branching into invisible particles
i.e. strong dynamics
WW scattering
In collaboration with Douglas Ross (Southampton), Agustin Sabio-Vera
(Cambridge), Ben White (Manchester), Jon Butterworth (UCL/ZEUS/ATLAS),
Brian Cox (Manchester/H1/ATLAS), Jae Sik Lee (Manchester), James Monk
(Manchester), Apostolos Pilaftsis (Manchester).
Precise data (0.1%) from LEP, SLC and Tevatron imply a
light Higgs boson when interpreted within the Standard Model
52
33
mh  81
GeV
See lepewwg.web.cern.ch
Similar conclusions in
minimal SUSY extensions,
i.e. < 135 GeV
Higgs production at LHC
from Keith Ellis
Discovery potential for a SM Higgs
Note new role of
VBF for “low” mass
Higgs
e.g. see LHC Higgs working Group meetings
(Cranmer, Mellado, Quayle and Wu: 5σ in each of dilepton and l+jet channels and room for improvement!)
Is it possible to improve even more in the
<140 GeV region?
 De Roeck, Khoze, Martin and Ryskin propose to sidestep
pileup even at high luminosity by using tracking
information. Could then allow to use H->bb decay
channel. Identify primary vertex and cut on tracks emanating from there,
e.g. no tracks above some threshold ~1 GeV between tag jets and b-jets.
 Needs experimental study.
Another possible contribution in the
VBF channel?
 “Two loop” QCD contribution not so far considered..
Leading order (one loop) jet+Higgs+jet is ~10% of VBF for typical cuts [del Duca et
al]
q
Same colour topology
as VBF – so will survive
minijet veto
q
 But can be estimated by computing imaginary part
Heavy Higgs
Introduce a real triplet (Y=0):
[Lynn & Nardi, Blank & Hollik]
v  246 GeV
M W2
1

2
2
M Z cos W cos 2 
m  1v
2
h
2
m  m  4 v / 
2
0
2

2
Tree level violation of
custodial symmetry
(a) 4 ~ 1  decoupling limit   +  0 
(b) 4 ~   all Higgses below 500 GeV
m  m  m0 ~ v
Oblique Quantum Corrections
i.e. not vertex or box
  (q)  g  (q 2 )  

(m)  (m2 )  (0)
[Lynn, Stuart; Peskin, Takeuchi]


S, T and U are gauge invariant and finite:
   SM (mh )  ASM S ({mi })  BSM T ({mi })  CSM U ({mi })
General observable
New physics enters only via S, T and U
In the triplet model we work to lowest order in

Quantum corrections are expected to be small:
S  0
T  0
No hypercharge
Custodial symmetry
To lowest order in
 quantum loops give (for no mixing in neutral sector)
Expected since Y=0
Expected since approximately custodial
m  m0  m
is the mass difference between the neutral and
charged (mainly) triplet higgses.
Hence quantum corrections are small
But tree level corrections are interesting:
• Direct correction to W mass since
MW2  M Z2 cos2W (1   2 )
• Indirect correction to all observables since
GF  GFSM (1   2 )
tree level
   SM (mh )  ASM  STM (mk , m )  BSM ( TTM (mk , m )   tree )
 CSM  UTM (mk , m )
 2
For SM contribution
Use ZFITTER:
13 observables
GF  1.6639 x 10-5 GeV -2
 s  0.119
Tree Level Effects
95% CL
contour
mtop  5.1 GeV
( 5)
 had
 6.5x10 -4 GeV
Standard Model
mhref  100 GeV
Quantum Level Effects
95% CL
contour
mhref  100 GeV
Some general remarks
[Peskin & Wells]
There are other ways to accommodate a heavier higgs boson:
•S<0
extra SU(2)xSU(2) multiplets [Dugan & Randall]
new singlet majorana fermions [Gates & Terning]
•T>0
4th generation [e.g. Dobrescu & Hill]
2 higgs doublets [Chankowski et al]
• New vector bosons [e.g. Casalbuoni et al, extra dimensions]
would be seen at LHC
If NO new particles at LHC/FLC then the crucial information
could come from even more precise electroweak measurements:
GigaZ (~1 month of linear collider) and/or M W  15 MeV (LHC).
[Figure from Peskin & Wells]
Conclusion: Triplet Model
 The data do not force us into a light Higgs
 Real triplet model: tree level mechanism “naturally”
produces*
T  0 and S  0
“Quasi-decoupling”: lightest Higgs can have mass
up to strong dynamics scale (500 GeV) without any
other consequences.
 May be very helpful to improve precision (GigaZ)
*
Quantum mechanism also viable
Light Higgs
MSSM with CP Violation
 Higgs sector CP violation natural
Since the soft SUSY breaking trilinear couplings and gaugino
masses can be complex
(h, H , A)  ( H1 , H 2 , H 3 )
 Easy to arrange for lightest Higgs to have
weak coupling to Z.
Hence it may not have been seen at CERN
 Light Higgs scenario is more general*, e.g.
if Higgs mixes with anything with a reduced
coupling to the Z (as occurs with the radion in Randall-Sundrum).
[Pilaftsis; Carena, Ellis, Pilaftsis, Wagner]
*See also 2HDM
CPX Scenario
 Mixing of h, H and A via top and bottom squark loops
at one-loop (and gluino loops at two-loop).
 EDM constraints can be avoided without fine-tuning.
 Benchmark scenario (CPX):
M squark  500 GeV; | M gluino | 1 TeV;
 =2 TeV; |At ,b | 1 TeV;
arg( At ,b )  arg( M gluino )   CP
Carena, Ellis,
Pilaftsis, Wagner
 Difficulties in detecting such a light Higgs at Tevatron
and LHC via conventional search channels.
 Dedicated LEP analysis underway in an attempt to
exclude the low mass regions.
 Possibility to look in diffraction?
KMR predict 3 fb for a 115 GeV higgs
includes gap survival factor 1/50
LHC:
100 fb 1
[Khoze, Martin & Ryskin]
Decay to b quarks viable since QCD background is
heavily suppressed.
m  1 GeV (tagged protons needed)
S/B > 1 anticipated at LHC
(de Roeck et al)
Crucial to measure “exclusive” dijets at Run II to ascertain the
reliability of the theoretical calculations which do at present suffer
from large uncertainties.
R = ratio of CPX ggH coupling to that in SM
Diffractive Higgs production cross-section at LHC (solid) and
Tevatron (dotted). MSSM parameters chosen to lie in the region not
currently excluded by LEP, i.e.
3  tan   5
mH1  60 GeV
Conclusions: Light Higgs
 Rate may well be high enough to be explored at
the LHC – especially in tau decay channel (but a detailed study of S/B
needed).
 Central production with tagged protons may well
be a useful tool which is able to complement more
traditional search strategies.
 Need for suitable forward detectors.
Invisible Higgs
 Not hard to imagine models where the Higgs decays
invisibly, e.g. to a pair of neutralinos.
 Zeppenfeld & Eboli propose that such a Higgs could be
discovered in VBF: “gaps between jets with missing ET”
signature.
 Would need to work in low lumi phase.
 ATLAS & CMS have supported the original idea with
detailed studies: 5 sigma discovery with 30/fb for a wide
range of parameter space.
Possible “fly in the ointment”:
q
q
• Cut on missing ET>100 GeV helps
kill QCD background.
• Signal has tag jets close in
azimuth and this is also effective
in reducing QCD background
• Absence of QCD radiation in
interjet region suppresses LO
QCD jj but not QCD singlet exchange.
q
q
Large momentum transfer colour singlet
May need to fall back on ZH->dilepton+missing ET…
Godbole, Guchait, Mazumbar, Moretti, Roy
No Higgs
What if there is no new “light”
physics?
 Suppose new physics is at a scale Q >> E (E = energy
of experiment) [e.g. could be a heavy Standard Model Higgs or some new strong
dynamics]
 Can use effective field theory approach to parametrize
ignorance of the new physics
 Q cannot be much beyond 1 TeV since we know that
the Standard Model without a Higgs breaks down at
around 1 TeV (WW -> WW violates unitarity)
Electroweak chiral lagrangian
Global:
SU (2) L  SU (2) R  SU (2)C
Local:
SU (2) L  U (1)Y  U (1)Q
Goldstone (“pion”) bosons of the
broken chiral symmetry.
The only dimension two
operator allowed by gauge
invariance and custodial symmetry
Longhitano;
Appelquist & Bernard
Can relax the assumption of custodial symmetry
without any problem – just more parameters.
Actually we need to do this if we are to get
away with no light higgs boson (i.e. in light of
precision data). Bagger, Falk & Swarz
Focus on quartic couplings of vector bosons:
One-loop amplitude for WW->WW. Calculated using the equivalence
theorem. Physical WW->WW amplitudes obtained by crossing and isospin symmetry
Unitarization
 Effective theory only valid for E<<Q
 Can try to extrapolate to higher E by insisting that
partial waves are unitary [considerable ambiguity]
Elastic unitarity

Pade approximation.
Matches one-loop perturbation
theory result
 Pade (and N/D) schemes have been implemented into
PYTHIA
Resonance Map in Pade scheme
A = 900 GeV scalar
B = 1300 GeV vector
C = 1800 GeV vector
D = 800 GeV scalar and
1300 GeV vector
E = no resonances
SM = 1 TeV Higgs
TC = Re-scaled QCD
See Dobado et al
What can LHC do in 1 year in scenarios A-E?
Probing the new physics at LHC
Jet
Focus on semi-leptonic
decay channel
Jet(s)
Missing ET + e or 
Background is many
orders of magnitude
bigger than signal…
Jet
Analysis: Leptonic W
• Combine all charged
leptons 1 by 1 with
neutrino (missing pT)
to give W 4-vector up
to 2- fold ambiguity
(mass fixed to central
value)
• Cut at 320 GeV on
W transverse
momentum (plot (d))
[Distributions are area normalised]
Analysis: Hadronic W
(a) and (b) : pT and h of
highest pT jet
(c) mass of highest pT
jet in region | h | < 4
(hadronic W reconstructed
as 1 jet 98% of time)
• Cut at pT > 320 GeV and
70 GeV < MJ < 90 GeV
Subjet method for identifying
energetic W bosons
Use kt algorithm to
find scale at which
W-jet candidate
resolves into two
subjets.
For a genuine W, we
expect the scale at
which the subjets are
resolved (i.e. ypT2) to
be of order MW2
Cut: 1.6 < log(pTy1/2) < 2.0
The Hadronic Environment I
(a) Top quark veto :
Rejection Cut:
130 GeV < MWJ < 240 GeV
(b) Tag jets :
Must have forward and
backward tag jets satisfying
Cut: pT > 20 GeV, E > 300 GeV,
4.5 > |h| > 2
Divide event into 3 regions :
“forward” of most forward W
“backward” of most backward W
“central” (contains both W candidates)
Forward (backward) tag jet is highest pT jet
in forward (backward) region
The Hadronic Environment II
(c) Hard pT: pT of 2 tag jets + 2
W’s = 0 for signal
Cut: pT < 50 GeV
(d) Minijet veto: no colour
exchanged between
protons
Minijets: all jets (apart from W
candidate) with |h| < 2
Cut: Nminijets (pT > 15 GeV) < 2
WW mass and cos * distributions
A = 900 GeV
scalar
B = 1300 GeV
vector
C = 1800 GeV
vector
D = 800 GeV
scalar and 1300
GeV vector
E = no
resonances
Simulated measurement 100 fb-1
Underlying Event
The minijet cut is sensitive to underlying event (and pileup) although the
approach developed here is less sensitive than in previous analyses
(a) Hadronic W width
affected greatly by
underlying event model
(b) Subjet analysis
insensitive to models
(c) and (d) : The minijet
distributions are sensitive,
particularly below 20 GeV.
The 15 GeV threshold we use
is marginal …
but measurement of the
underlying event in data
should allow tuning of models
and cut
Conclusions (WW Scattering)
 It may be that there is no new physics until we enter
the TeV region (LHC). In which case we may well want a ~2 TeV linear collider.
 WW scattering is an excellent place to look for evidence
of the new physics:
With 1 year high luminosity should be able to measure
WW cross-section differential in WW mass and hence
the mass of any new resonances (up to about 1.5 TeV).
In some scenarios, it may be possible to measure
the spin of the resonance(s) too.