Transcript Searching for the Higgs Boson: Challenges for the Coming Years

Searching for the
Higgs Boson:
Challenges for the
Coming Years
S. Dawson, BNL
December, 2007
Helmholtz Kick-Off Workshop:
Physics at the Terascale
DESY
Apologies
For all the topics I won’t cover
Especially experimental aspects of Higgs
physics and NLO Monte Carlos
For all the important work I won’t reference
Much of it done by people in this room!
Question???
• Question: Why is the Higgs so important?
• Answer: Discovering (or definitively excluding)
the Higgs will fundamentally change our
understanding
– It’s a win-win combination
– Single Higgs boson may or may
not be Standard Model-like
– There may be many new particles
associated with the symmetry
breaking
– Higgs sector probes a large
variety of new phenomena
On Very General Grounds…..
We expect a Higgs boson or something like it….
2

Mh 
GF E


 2

A WL WL  Z L Z L  
2
8 2  E  M h 

Unitarity

2
Light Higgs: Mh < 800 GeV
No Higgs:
c ~ 1.2 TeV
Unitarity violation
LHC expects to discover Higgs
or New Physics below 1 TeV
The Chimney
Extrapolate Higgs potential to high scale 
V=-22+4
Forbidden
Allowed
Forbidden
Standard Model is only
consistent to GUT scale
for small range of Higgs
masses
Heavy Higgs implies
new physics at some low
scale
Standard Model is Incomplete Without
Something Like a Higgs Boson
•Need physical, scalar particle, h, with unknown mass
Mh is ONLY unknown parameter of EW sector
•Observables predicted in terms of:
MZ, GF, , Mh
•Higgs and top quark masses give loop corrections:
 Mt2, log (Mh)
Everything is calculable….testable theory
Precision Measurements Limit Higgs Mass
 LEP EWWG (July, 2007):
2007
– Mt=170.9  1.8 GeV
– Mh=76+36-24 GeV
– Mh < 144 GeV (one-sided
95% cl)
– Mh < 182 GeV (Precision
measurements plus direct
search limit)
Best fit in region excluded from direct searches
Understanding Higgs Limit
Theory: Input MZ, GF, 
→ Predict MW, limit Mh
Run II
2009
Jan, 07
Higgs Mass Limits ASSUME Standard
Model
 It’s easy to construct models
which evade Higgs mass
limits
 All you need is large =
T
 Explicit models typically have
other new particles…..
Example: Evade Higgs Limits
Fourth Generation can have heavy Higgs by tuning
masses to get cancellations
Many other examples (Little Higgs with T parity…)
Nc 
M U2 
1  2Y ln 2 
S 
6 
MD 
Nc (M U  M D )2
T 
12sW2
M W2
(a) MU=310 GeV
MD=260 GeV
(b) MU=320 GeV
MD=260 GeV
(c) MU=330 GeV
MD=260 GeV
Kribs, Plehn, Spannowsky,Tait, hep-ph/0706.3718
MSSM Fits Precision Measurements
2
 Slight preference for MSSM over SM
Mh(GeV)
Ellis, Heinemeyer, Olive, Weber, Weiglein, hep-ph/0706.0652
Includes WMAP, B
physics observables
SM Higgs Searches at Tevatron
LP07
Most likely region for Tevatron Higgs discovery:
Can use gg→h →WW* channel
CDF/DO Projections
FNAL PAC, Nov. 07
Tevatron MSSM limits from bg→bh (1 fb-1)
LP, 2007
The LHC Higgs Challenge
 Precise predictions for Higgs production &
backgrounds
 Understanding uncertainties on predictions
 PDFs, scale uncertainties, model dependence
 Implementing NLO/NNLO in useful Monte
Carlo programs
 Can we distinguish the Standard Model
Higgs from all other possibilities?
SM Production Mechanisms at LHC
Total cross sections for all
important channels
calculated to NLO or NNLO
Gluon fusion is dominant
channel for entire Mh range
Bands are scale
dependence
Dawson, Jackson, Reina, Wackeroth
SM Higgs, CMS 2007
Includes radiative corrections
No ATLAS plot with radiative
corrections
Improvement in  channel
At low Mh, gg→h→ and
vector boson fusion are most
important channels
Note: no tth discovery channel
Initial LHC running
 Standard Model Higgs
could be discovered with
5σ significance with 5 fb-1
 1fb-1 could exclude a
Standard Model Higgs
boson at 95% confidence
level
Assumes detector is
well understood
Gluon Fusion
Higgs coupling is proportional to top quark mass
For heavy fermion masses,
 s2 (  ) 2 
 ( gg  h) 
M h  s  M h2 
2
576v

NLO corrections increase rate by 80-100%
NNLO corrections known in large mt limit
Soft gluon resummation increases rate by 6%
EW two loop effects increase rate 5-8%
h
Good shape theoretically
But…Production Can Be Very Different….
Add dimension 6 operator:
L6 g 
fg
2
  G G  a
a
 Expand around vacuum:  0 
 Generate interaction
L6 g 
h
(h  v)
2
fgv
2
hG G  a
a
 New operator is just arbitrary enhancement or
suppression of ggh production rate
 36  v  2 
   SM 1 
fg   

s
   

Manohar and Wise, hep-ph/0601212
4th generation:
 f gv2   s
 2 
   6


Need to go beyond Total Cross Sections
Our estimates of
scale dependence
are inadequate
Anastasiou, Dixon, & Melnikov, hep-ph/0211141
Beyond Total Cross Sections
 gg→h→ total cross section at NNLO with arbitrary cuts
(FEHIP)
 New: NNLO MC for gg→h→ (also h→WW)
Photons isolated:
Total energy in
cone of R=.3 is
less than 6 GeV
Catani & Grazzini, hep-ph/0703012
Anastasiou, Melnikov, & Petriello, hep-ph/0501130
Understanding Transverse Momentum
Spectrum for gg→h
 At small transverse
momentum, qT << Mh, large
logarithms of the form:
snln2n (Mh2/qT2)
 Logarithms summed to all
orders in perturbation theory
 Resummed calculation at
low qT matched to fixed
order at large qT
Pythia peak still softer
Challenge: Understand low momentum region
Bozzi, Catani, deFlorian, Grazzini
New Experimental Studies of gg→h→
 What’s new:
–
–
–
–
More contributions to background
Realistic detector material
More realistic K factors (for signal and background)
Reducible backgrounds (gj and jj) considered
Signal Significance for Mh = 130 GeV and 30 fb-1
ATLAS
LO (TDR, 1999)
NLO (update, cut based)
NLO (likelihood methods)
3.9 
6.3 
8.7 
CMS
NLO (cut based, TDR-2006)
NLO (neural net, TDR-2006)
6.0 
8.2 
K. Jacobs, BNL Forum, 2007
Not Hard to Construct Models With Suppressed
h Rate
 2 Higgs doublet model where only 1 couples to
fermions (tan =v1/v2)
/SM
2HD, Mh=140 GeV
 Little Higgs models, models with radion/Higgs mixing
also tend to have suppressed ggh rate
Phalen, Thomas, Wells, hep-ph/0612219
Vector Boson Fusion
 Two energetic jets with large rapidity interval
 VBF important for Higgs coupling measurements; discovery
 hWW, : At least one W/ decays leptonically
 Backgrounds: tt, Wt, WW+jets, /Z*+jets
 NLO corrections increase rate by 5-10%
 Implemented (along with many backgrounds and
decays) in NLO MC
Zeppenfeld et al, http://www-itp.phsik.uni-karlsruhe.de/~vbfnloweb
Higgs plus Jets at NLO
 Higgs + 1 jet, Higgs + 2 jet known at NLO in large mt limit
 Higgs + 2 jets process is background to vector boson
fusion process
Higgs + 2 jets
d/d
h
h
|(jet)|
Campbell, Ellis, Zanderighi, hep-ph/0608194; DelDuca, Kilgore, Oleari, Schmidt, Zeppenfeld, hep-ph/0301013
Vector Boson Fusion and EW Corrections
 Electroweak corrections to
vector boson fusion are of
similar size as QCD
corrections (-4% , -7%)
 Partial cancellation between
EW & QCD
 EW corrections change shape
of distributions
Ciccolini, Denner, Dittmaier, hep-ph/0710.4749
EW
QCD
’s and VBF
CMS, VBF, h->->l+jet, TDR
full simulation
ATLAS, VBF, h(2l)
+2jets
l+jet
30 fb-1
Mh=120 GeV
30 fb-1
CMS:
Nikitenko, Les Houches 2007
Challenge: Backgrounds to NLO
 Need backgrounds to
NLO/NNLO
 Need theory calculations for
distributions and in format
useful to experimentalists
 Monte Carlos with NLO
calculations necessary!
 New calculational techniques
make these calculations
possible
Les Houches 07
wishlist for NLO
calculations:
pp  VV  jet
pp  tt bb
pp  tt  jet  jet
pp  WWW
pp  VVbb
pp  VV  jet  jet
pp  V  3 jets
pp  bb bb
pp  4 jets
gg  WW *
tt @ NNLO
Z /   jet @ NNLO
V=Z,W,
Example of importance of Backgrounds:
h → WW* → ℓ ℓ
 BR(h  WW*) large for Mh >140 GeV
 Neutrinos in final state → no mass peak
 Large backgrounds: WW, Wt, tt
Higgs
Need accurate understanding
of backgrounds
WW
K. Jacobs, BNL Forum 2007
Importance of Backgrounds (Cont.)
NLO results change shape of distributions
pp→WW → l l
 (gg  WW):
~ 5% of WW before cuts,
~ 30% of WW after cuts
T. Binoth, M. Cicciolini, N. Kauer. M. Krämer, hep-ph/0611170
Is tth Channel Observable?
 h→bb, t →bjj, t →bl
 Large combinatoric backgrounds,
also tt jet jet, W jets, WW jets….
 Challenge is understanding shape
of background
S/Sqrt(B+B2)
 Important channel for measuring
tth Yukawa coupling
 Final state with 4 b’s:
Uncertainty on background, B/B
Challenge: Figure out how to observe this channel!
Higgs Production can be very different in
MSSM
 For large tan , dominant production is with b’s
 bbh can be 10x’s SM Higgs rate for large tan 
LHC
SUSY Higgs are produced with b’s!
Spira
Do we understand predictions in non-SM?
 MSSM is best studied case
 FeynHiggs includes higher order MSSM effects in
calculation of Higgs masses/couplings
Mass of
lightest
Higgs
(GeV)
tan =15
tan =5
CP Violating Phase
Hahn, Heinemeyer, Hollik, Rzehak, Weiglein, www.feynhiggs.de, hep-ph/0710.4891
tan 
Need to find Multiple Higgs of MSSM
5 contours
h,A,H,H
h,A,H
H,H
h,H
h (SM -like)
h,H
h,H,H
h,A,H,H
MA(GeV
h,H
Decoupling regime (the wedge):
Only one Higgs boson observable
with SM like properties
In general, non-standard models of electroweak
symmetry breaking have new particles in addition to
Higgs boson
Suppose the LHC finds a Higgs….
The SM predicts production/decay rates
 We need to understand the uncertainties on these
predictions
 Making good progress with NLO, NNLO calculations
Spin/parity
 Is it a scalar or a pseudoscalar?
Higgs can’t be heavier than  200 GeV in SM
Minimal SM has no extra scalar particles
 Spectroscopy of new states in non-minimal models
important
Is it the Higgs?
 Measure couplings to fermions & gauge bosons
 ( h  bb )
mb

3
2
(h     )
m
2
 Measure spin/parity
J
PC
0

 Measure self interactions
2
V
2
2
Mh 2 Mh 3 Mh 4
h 
h  2 h
2
2v
8v
Need good
ideas here!
Higgs Couplings Difficult to Measure at LHC
Ratios of
couplings easier
Extraction of
couplings requires
understanding NLO
QCD corrections for
signal & background
Challenge: Can we do better?
Duhrssen, Heinemeyer, Logan, Rainwater, Weiglein, Zeppenfeld, hep-ph/0407190
Is the Higgs a Scalar?
 Weak boson fusion sensitive to tensor
structure of HVV coupling
 New structures from higher dimension operators




T   c1 g   c2 p1  p2 g   p1 p2  c3  p1 p2 
SM
CP even
CP odd
Loop induced
Need to Measure CP of Higgs
 Azimuthal angle between tagged jets sensitive to c2, c3
Not so pretty if
admixture of CP
even/odd
Challenge: Pursue other
ideas/techniques for
determining spin/parity of
Higgs
Figy, Hankele, Klamke, Zeppenfeld, hep-ph/0609075
What if we don’t see the Higgs?
 Maybe we just missed it
 Production cross sections are smaller than SM
 Higgs BRs are smaller than SM
 Easy to arrange -- Example: add gauge singlet Higgs
 Higgs decayed invisibly
 Maybe Mh>800 GeV → New resonances in WW, ZZ
channels
Standard Model is Effective Low Energy Theory
• We don’t know what’s happening at high energy
• Effective theory approach:
L  LSM  i f i
Oi
 ...
2
• Compute deviations from SM due to new operators
and compare with experimental data
LHC job is to probe physics
which generates these operators
New physics with light Higgs:
gg ' 
 BW  
2

O ,1  D    D    
OBW  



v2
S  e 2 f BW

v2
T   2 f  ,1
2
2
  5 TeV
What if we don’t find a Higgs?
 Electroweak symmetry breaking is strongly interacting
 Unitarity considerations imply effects grow with E2
 Difficult to implement in specific models
 Technicolor, Extra-D
 Effective Lagrangian approach:
L  LSM  i f i Oi
Operators don’t involve Higgs Boson
 Gives Non SM VV, VVV, VVVV couplings
 Couplings contribute to electroweak observables:
O  gg ' Tr B  W 
S  161
v
O  Tr  ( gW  g ' B 
T  2
4
1

 3
2
2
1
1
3

Chivukula, Simmons, Matsuzaki, Tanabashi, hep-ph/0702218, Dawson & Jackson, hepph/0703299, Alam, Dawson, Szalapski, hep-ph/9706542
3

No light Higgs Scenario
 No resonance
 Expect effects of effective
Lagrangian couplings which grow
with energy
 Counting experiments
 Most explicit models have TeV scale
resonances (Example: Extra
dimension Higgsless models)
 Very hard!
Challenge: This type of scenario
needs more work to determine what
is observable
Eboli, Gonzales-Garcia, Mizukoshi, hep-ph/0606118
VBF, WWjj→ejj
tt,ttj,ttjj, SM
backgrounds
Signal with
effective WW
couplings
Conclusion
• Keep computing higher order corrections
to signal and backgrounds
• Expect the unexpected
– Prejudice is dangerous!
– Theorists can construct models with enhanced
or suppressed Higgs production rates and with
or without SM-like branching ratios
• We need to measure:
– BR for as many production/decay chains as
possible
– Spin/parity
– Spectroscopy of new particles associated with
symmetry breaking
Thanks!
 Thanks to the organizers for a
superb scientific program
 Thanks to the secretariat for
wonderful organization and
hospitality
 I expect great physics from the
Helmholtz initiative!
Discoveries!