1. Higgs Maxwell Workshop: Current Status and Future Prospects “Theory Forward Look”
James Stirling
Cambridge University
Higgs-Maxwell 2009

2. current issues and challenges looking forwards to 2020
… and briefly backwards to 1999
Note: only a brief and highly subjective survey!
Higgs-Maxwell 2009

3. Standard Model “Theory of Everything”?
6 quarks* (u,d,s,c,b,t)
6 leptons (e,,,e,,)
gauge bosons (,W,Z,g)
Higgs boson
~10-18m
D=4
+ supersymmetry?
particle  sparticle
bottom up
top down
dark matter?
bottom up
+ string, brane theory?
M-Theory?
*quarks and gluons
confined in hadrons:
baryons (p,n), mesons ()
~10-35m
D=11?
“Theory of
Everything”?

4. the key questions particle physics LHC  expts.
What is the origin of mass? Is it the Higgs boson?
the good news: the answer is likely to be at or below the 1 TeV scale
What is the origin of the matter-antimatter asymmetry in the universe?
CKM by itself cannot explain the cosmological asymmetry, therefore need to look for flavour ,CP violation beyond the SM
Is there unification of particles and forces including gravity?
in simplest models, this will be at ~1016GeV, mass and coupling measurements at lower energies give hints
What is the dark matter? And the dark energy?
LSP or axion or …?
What is the quantum theory of gravity?
best candidate is (super)string theory, with extra dimensions and supersymmetry, but at what scale?
What are the properties of neutrinos?
may also be a window on unification, matter asymmetry, …
LHC
 expts.
Higgs-Maxwell 2009

5. Higgs-Maxwell 2009

6. the 3 pillars of the Standard Model
MW = cosw MZ  [ 1 + α F(mt,MH,SUSY,..)+ …]
I – CKM
II – EW
Higgs-Maxwell 2009

7. the 3 pillars of the Standard Model
I – CKM
II – EW
Higgs-Maxwell 2009

8. III – QCD S DGLAP thrust@NNLO S(MZ)=0.118±0.002 HERASFWG
Gehrmann-De Ridder
Gehrmann
Glover
Heinrich
PDG2008

9. (SM) Higgs production at LHC
arXiv:  [hep-ph]
small
print!
Higgs-Maxwell 2009

10. direct Higgs searches
Tevatron excludes at 95% C.L. the production of a 170 GeV SM Higgs boson!
Higgs-Maxwell 2009

11. supersymmetry The argument for TeV scale supersymmetry:
solves naturalness/hierarchy problem
allows coupling constant unification
predicts light Higgs mass
provides candidate (LSP) for dark matter
Problem: to say anything more quantitative requires model assumptions, e.g. masses, SUSY breaking mechanism,… and even the most general MSSM has > 100 free parameters!
Hence phenomenological studies focus on “constrained, minimal” models etc.
Higgs-Maxwell 2009

12. e.g. CMSSM five parameters: tan = v2/v1 m0, the common scalar mass
m1/2, the common gaugino mass
A0, the common trilinear coupling
sign() = ±1, Higgs potential parameter
… at MGUT,then radiative corrections
accelerator and non-accelerator experiments already provide reasonably strong constraints on models like CMSSM
Higgs-Maxwell 2009

13. WMAP cosmological constraint from DMh2
allowed by g - 2
WMAP cosmological constraint from DMh2
excluded by b→s
LSP = charged stau
excluded by LEP
Ellis
Olive
Santoso
Spanos
Higgs-Maxwell 2009

14. prospects for sparticle discovery at LHC
SUSY

15. fine-tuning of parameters
alternatives to SUSY?
Challenges for alternative theories for non-SUSY TeV scale physics:
if based on strong dynamics, issues of theoretical consistency and predictivity
strong EW precision constraints from experiment, need to avoid large corrections which would spoil agreement
no compelling single obvious alternative
different models can have similar phenomenology
in general
fine-tuning of parameters
complexity
of models

16. Higgs or no Higgs… Randall-Sundrum I Holographic PNGB Higgs Higgsless
Technicolour
Little Higgs
???
See for example, “Little Higgs, Non-standard Higgs, No Higgs and All That”, Hsin-Chia Cheng, arXiv: [hep-ph]
Higgs-Maxwell 2009

17. a Theory of Everything?
String Theory: everything we want in one consistent framework…
Quantum Mechanics
Standard Model-like gauge theory
General Relativity
Cosmology (inflation)
Bonus: new theoretical toolbox with applications to e.g. perturbative QCD calculations, ….

18. too much of a good thing?
consistent string theories require N=10 dimensions
but string unification is easier if one of these dimensions is smaller than the GUT scale → ‘large extra dimension’ scenarios
possibility of signatures at LHC:
Kaluza-Klein excitations of gravitons
missing energy leaking into extra dimensions
microscopic short-lived black hole production …
but there appear to be an enormous number of models with SM-like behaviour → estimated D vacua!
Higgs-Maxwell 2009

19. M-theory In fact, the 5 possible string theories
are all thought to be related as different aspects of a single theory:
M-theory
Higgs-Maxwell 2009

20. Higgs-Maxwell 2009

21. string model-building
classify all possible compactifications/constructions that give rise to a low-energy theory resembling as much as possible the SM or MSSM
if completely realistic models are found, this proves that string theory may be a unified theory of all particles and interactions
identify general patterns (e.g. symmetries, extra particle content etc.) which could be present in large classes of realistic vacua
obtain, if possible, predictions that could be tested experimentally (e.g. SUSY-breaking MSSM soft terms, dark matter,..)
what information on possible string compactifications may be extracted from LHC data?
if only very few classes (if any) of compactifications were able to fit all experimental data, what other new testable predictions could be derived from these?
─ a formidable task!

22. Luis Ibanez, ATM08
Higgs-Maxwell 2009

23. Luis Ibanez, ATM08
Higgs-Maxwell 2009

24. progress in theory calculations
driven by high-precision measurements at the LEP, SLC, Tevatron, HERA, … colliders, theorists have made corresponding progress in refining the theoretical predictions
in practice
precision electroweak
precision pQCD
…and including BSM etc contributions in loops
precision npQCD on the lattice: key tool in e.g. flavour physics
gg→ttbb background only at LO, difficult to distinguish from signal

25. precision pQCD in the LHC era
fine-tuned event simulation MCs, interfaced with NLO hard scattering ^ 
LO for any n-particle hard scattering final state
NLO for a wide range of processes, esp. with multijets, interfaced with parton shower MCs
NNLO for a restricted range of ‘quasi-inclusive’ processes
supplemented by NnLL improvements, EW corrections, …
few % precision on pdfs up to NNLO, ‘good enough’ in most cases

26. recent progress… …
on-shell LO tree-amplitudes, arbitrary # of external legs: automated brute force (MADGRAPH,…) or recursion (MHV-BCFW, ..)
rough estimates of multiparticle scattering cross sections, e.g. p + p →n jets + X
two-loops (NNLO),  4 legs
high-precision for Z, H, … inclusive cross sections at LHC etc.
one-loop (NLO),  5 legs
precision multiparticle scattering cross sections, especially LHC backgrounds involving W,Z, jets, top, …
Higgs-Maxwell 2009

27. Anastasiou, Dixon, Melnikov, Petriello, 2004
the impact of NNLO …
Anastasiou, Dixon, Melnikov, Petriello, 2004

28. progress in one-loop calculations
~1948 (Schwinger)
~2008 (Bern, Dixon, Kosower; Dixon, Kunszt, Signer; Campbell, Ellis; Febres, Cordero, Reina , Wackeroth)
electron anomalous magnetic moment
Higgs-Maxwell 2009

29. feynman diagrams complexity…
consider
Passarino-Veltmann
reduction !

30. 5 legs generally OK > 5 legs ???
Higgs-Maxwell 2009

31. recent one-loop progress…*
pp→ WW + 2j via VBF Jager, Oleari, Zeppenfeld, Bozzi
pp→ WW + 1j Campbell, Ellis, Zanderighi; Dittmaier, Kallweit, Uwer
pp → H + 2j Campbell, Ellis, Zanderighi; Ciccolini, Denner, Dittmaier
pp → ZZZ Lazopoulos, Petriello, Melnikov
pp → WWZ Hankele, Zeppenfeld
pp → VVV Binoth, Ossola, Papadopoulos, Pittau
pp → ttH, ttZ Lazopoulos, Petriello, Melnikov, McElmurry
pp → Wbb, Zbb Febres Cordero, Reina, Wackeroth
pp → t t + 1j Dittmaier, Uwer, Weinzierl
pp → t t + bb (qqbar only) Denner, Bredenstein, Dittmaier, Pozzorini
uu~ → ss~cc~ Binoth, Heinrich et al
Higgs-Maxwell 2009
*relevant for LHC

32. (Durham-Edinburgh-Glasgow contributors in red)
Perhaps the most important recent development has been the appearance of automated programmes for one-loop, multi-leg amplitudes, either based on
traditional or numerical Feynman approaches (Golem95, …)
on-shell methods based on unitarity method + on-shell recursion (BlackHat, CutTools, Rocket, …)
real hope of addressing all the experimenters’ wishlist!
See for example the “NLO multileg working group: summary report” at the Workshop "Physics at TeV Colliders", Les Houches, 2007, arXiv: [hep-ph] with contributors:
Bern, Bernicot, Binoth, Boudjema, Britto, Campbell, Czakon, Denner, Dissertori, Dittmaier, Dixon, Duplancic, Ellis, Frederix, Gehrmann, Gehrmann-De Ridder, Giele, Glover, Guillet, Heinrich, Kallweit, Karg, Kauer, Kosower, Krauss, Kunszt, Le, Mastrolia, Mitov, Moch, Odaka, Ossola, Papadopoulos, Pilon, Pittau, Reiter, Sanguinetti, Schumann, Schwinn, Skands, Soper, Stenzel, Uwer, Weinzierl, Zanderighi
(Durham-Edinburgh-Glasgow contributors in red)

33. interfacing NnLO and parton showers+
POWHEG …
Benefits of both:
NnLO correct overall rate, hard scattering kinematics, reduced scale dep.
PS complete event picture, correct treatment of collinear logs to all orders
Example: Frixione, Webber
processes include: pp  WW,WZ,ZZ,bb,tt,t,tW,H,W,Z/,HW/Z
pT distribution of tt at Tevatron

34. see e.g. Martin et al, arXiv:0901.002 [hep-ph]
LHC
most new physics samples pdfs in a region of x where they are already well known
low-mass forward production (e.g. Drell-Yan) might provide new information on small-x partons
SUSY,
Higgs,
W,Z, … *
see e.g. Martin et al, arXiv: [hep-ph]

35. Burton Richter, summarising the 1999 Lepton-Photon Conference:
back in 1999 …
Burton Richter, summarising the 1999 Lepton-Photon Conference:
experimenters (and phenomenologists) need to be more concerned about systematic errors and the tails on error-distribution functions
experimenters should learn more theory
all theorists should have a required course in statistics before receiving their PhD
we all hope for new things from LEP2 and the Tevatron, although the chances seem small
we have big hopes for B-factories – the SM’s CP-violation is not enough and new directions may become clear from the factories
neutrino physics is in ferment: more data should help make things clear but it will take 4-5 years       
Higgs-Maxwell 2009

36. 
LHC starts up in 2005 and we all hope to find out what is beyond our SM
an e+e- collider of TeV is a necessary companion to the LHC; it will only come about if we all get behind it and push it as an international and regional program
non-accelerator experiments in space and on the ground will be of increasing importance; the HEP community should not be too parochial
string theorists are doing great things – I hope they justify or eliminate supersymmetry and think up an experimental test
there is an exciting future: the work will be difficult, expensive and rewarding… the young generation, with support from governments, can and will do it.      
Higgs-Maxwell 2009

37. looking forward to 2020…
what will we be working on?!
WJS2009

38. decreasing likelihood
looking forward to 2020…
the UK theory community will be strong and active
there will be a significant improvement in our ability to perform hard calculations in perturbative field theory
the LHC will have collected a large amount of data at 14 TeV
the Higgs boson will be discovered at the LHC
supersymmetry will be discovered at the LHC
… and we will know the origin of the dark matter
we will understand the matter-antimatter asymmetry in the Universe
we will understand the origin of the “dark energy”
string theory will constrain the many parameters of the SM and SUSY
large extra dimensions will be discovered at the LHC
the LHC will discover something completely unexpected
decreasing likelihood

39. 1970
1980
1990
2000
2020
2010 t c b
W,Z
H, SUSY,ED,…?
The End!