Transcript XLVITH Rencontres de Moriond QCD and High Energy Interactions Theoretical Summary Lance Dixon (CERN & SLAC) La Thuile, 26 March 2011

XLVITH Rencontres de Moriond
QCD and High Energy Interactions
Theoretical Summary
Lance Dixon (CERN & SLAC)
La Thuile, 26 March 2011
Lots to Review!
• We heard an amazing number of excellent talks on
different theoretical ideas and approaches
• And, especially with the advent of the LHC, a flood of
new data from the energy frontier, which heavily
impacts many theories of new physics at the
electroweak scale
• Can’t possibly cover everything
• Apologies in advance if I missed your favorite topic or
talk, or if I distorted it
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
2
Some General Themes
• Searching for new physics – new terms in the
Lagrangian of Nature – in many different channels
• Searching for, or further exploring, new phenomena in
QCD, e.g. QGP in heavy ion collisions, or diffraction in
new kinematical regimes
• Improving the precision of theoretical predictions in
systematic approximations like pQCD, SCET, other
resummations
• Establishing new frameworks/models/pictures where
no fully systematic approximation exists
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
3
New Physics
the big picture
But still lots to do in understanding “old physics”
S. Todorova
L. Dixon
Theoretical Summary
Moriond QCD
H. Murayama
26 March 2011
4
Where is Higgs?
• Been looking for him for decades
• Know everything but Higgs mass
• Search really heating up now, both
from last stages of Tevatron operation,
first stages of LHC
LEP2
direct
search
L. Dixon
M. Cooke
Precision
electroweak
Theoretical Summary
Look for now mostly in WW, ZZ,
and bb (with associated W or Z)
Moriond QCD
26 March 2011
5
SM Higgs at Tevatron
• Expect 10-12 fb-1 total by September shutdown.
• Could “fail to exclude” at 2s
• Hard to get to 3s evidence in SM unless analysis
improvements are greater than projected
M. Cooke
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
6
SM Higgs at Tevatron (cont.)
• Low mass region more
difficult, at ~ 1.5-2 x SM
now, with 5.9 fb-1, even
with many channels,
improved acceptances,
hadronic t’s, MVAs, …
• High mass region also
difficult, many channels,
but most sensitive is
M. Cooke
not yet updated for new channels
gg  H  WW  lnln
• Now CDF/D0 each can
exclude a range of masses
L. Dixon
Theoretical Summary
R. Lysak
Moriond QCD
26 March 2011
7
SM Higgs at Tevatron (cont.)
New high mass combination of CDF and D0
J. Hays
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
8
But Wait!
• Theoretical uncertainties leave (high mass)
Higgs in a “half dead, half alive state” - A. Djouadi
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
9
Which uncertainties?
Biggest ones:
1) gg  H has large higher order corrections
A. Djouadi
However, partial N3LO results and NNLL resummations suggest (to
me) that final cross section will not dip to lowest value in k=3 range.
2) gg  H strongly dependent on g(x)
for x~0.1. Different pdf sets give quite
different cross sections:
-MSTW, CTEQ, NNPDF have Tevatron jet data,
 g(x) larger
- ABKM09, HERAPDF do not, g(x) smaller.
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
10
But Wait!
V. Radescu
• Brand new NNLO fit to HERA data: HERAPDF1.5
• Gives large x gluon much closer to MSTW!
• NNLO Higgs cross section
not yet shown, but clearly it
will rise a lot closer to that
using MSTW.
High Mass
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
11
How to improve sH uncertainty?
• Very large logs due to soft-gluons. To go beyond
NNLO, can resum “eikonal logs”:
NNLL Catani,de Florian, Grazzini, Nason
also Ahrens et al., arXiv:1008.3162
• Maybe
go to next-to-eikonal approximation before long.
C. White
H
• Would still want at least one N3LO hard term: a3 d(x)
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
12
Improve uncertainties from cuts
• Effect of jet vetos. Need to resum large logs here,
~ ln(mH/qT), from soft and collinear ISR. Done at
Tevatron (as I understand it) by reweighting MC to an
NNLL resummation of Higgs qT spectrum
(HQT, Bozzi et al.; and/or RESBOS).
• SCET recently applied to resum another variable, “beam
thrust”. Reweight to this, check consistency… Berger et al.
T. Becher
• Subtle “collinear anomaly” in SCET applied to
Drell-Yan qT resummation (where there is a ton of data).
Stay tuned for
final predictions.
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
13
Where is SUSY?
?
• Been looking for her for decades
• Know nothing (theoretically) about any masses (no upper
bounds, anyway, except for lightest Higgs)
• Search really heating up now, from first stages of LHC,
but also Tevatron, progress in flavor physics too
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
14
“Generic” SUSY Cascade
Searches at LHC
• Already
surpassed Tevatron in many cases!
N. Barlow
M. Chiorboli
• Will discover or really constrain overall mass scale over next two years
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
15
Many more specialized
SUSY Searches
• Just one example: top squark might be much lighter than
~ ~
•other quarks, because of running effects, large tL-tR splitting):
Tevatron, Ph. Gris
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
16
SUSY Higgs
• Relatively simple sector: only depends on
~ masses of heavy
Higgses, H, A, H±
• Light Higgs h has mass < MZ cosb + radiative corrections
• LEP2 lower bound  want tanb somewhat large
• Unification suggests very large tanb ~ mt/mb ~ 40
• If MA small, coupling of h to b,s,d,t,m,e all enhanced by tanb
~ tanb
• However, if MA large – decoupling limit – effect shuts off
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
17
Direct SUSY Higgs Search
Cross section tan2β enhanced
Tevatron, St. Denis
(roughly, due to SUSY
radiative corrections!)
watch
this
space?
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
18
SUSY in Flavor
• Muon g-2 anomaly likes large tanβ
• Rare B decays can also get huge enhancements
• However, both also depend on masses,
flavor mixings of other superpartners
Bs,d  m+m-
M.-O. Bettler, R. Fleischer
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
19
Bs  m+m- (cont.)
• Exciting because LHCb has already reached
~ Tevatron sensitivity:
M.-O. Bettler
• Will cover much of remaining
distance to SM in next year or two
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
20
Theory input for Bs  m+mR. Fleischer
Predict using
“U spin”
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
s  d
21
U spin for CP Asymmetries too
R. Fleischer
Enough information
to solve for
g
At LHCb
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
22
Other New Physics Theories
• 4th generation
• New W’s or Z’s
• Vector-like quarks
• Axial-like gluons
• Universal (Flat) Extra Dimensions (UED)
• Warped Extra Dimensions (RS)
• String Excitations
• Black Holes (only the safe kind?)
Taylor
Strogolas
Harel
Thompson
Cho
Brandt
• Many searched for in di-lepton, di-photon, or di-jets
 Particle masses excluded up to around 1 TeV
• Other searches often harder, more model specific
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
23
Strings at the LHC?
T. Taylor
Possibly, if there are also large extra dimensions, and string
mass scale M is also very low.
BIG di-jet bump from
gg  g*  gg
qg  q*  qg
…
So big that
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
24
pQCD for colliders
LHC @ 7 TeV
If you don’t know exactly what to look for,
need to understand all major SM processes.
• Every process shown
also occurs with one more jet
at ~ 1/5 the rate
• Very important to understand
not only Standard Model
X but also of
X + n jets
production of
_
where
X = W, Z, tt, WW, H, …
n = 1,2,3,…
 New physics
 Higgs physics
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
25
pQCD for hadron colliders
• Great advancement of last 5 years:
• Monte Carlo programs that match leading order QCD exact matrix
elements to parton showers (LO ME+PS)
– ALPGEN, MadGraph, SHERPA.
• Describe event “shapes” pretty well
• But, we need to do better, especially with normalizations.
• For this we need NLO QCD.
• Even better would be NLO QCD matched to a parton shower
(NLOPS).
• Even better than that, merging samples with different numbers of
jets, as in LO ME+PS but at NLO  MENLOPS
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
26
(Approximate) MENLOPS
• Merging samples with different numbers of jets at NLO = MENLOPS
• Can get very close with a recipe combining available NLOPS
(POWHEG) and LO ME+PS tools, at least for Drell-Yan (W,Z) and
also top-quark pair production
K. Hamilton
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
27
New NLO methods and results
• Unitarity-based method applied to one-loop 6-point amplitudes
entering
pp  W+W+ + 2 jets
T. Melia
• Same sign dilepton background
• Preparation for full pp  WW + 2 jets,
QCD background to WW electroweak scattering
• Unitarity recycles trees into loops
• Greatly improved theory uncertainty 
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
28
New NLO methods (cont.)
• All NLO calculations need one-loop amplitudes, but they also need to
integrate tree cross sections with one extra parton over a singular
phase space region.
• Catani-Seymour dipole subtraction method for dsA is standard, but
requires lots of different terms – may be computationally limiting step.
• Method proposed by Nagy-Soper recently implemented. T. Robens
• Promising for more complex processes if integrals of subtraction
terms dsA can be tabulated numerically.
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
29
Approximate (N)NLO method
• LoopSim – method to approximate higher order corrections
– like an approximate Sudakov factor
S. Sapeta
• Works best when leading-order (LO) QCD is anomalously small –
i.e. when there are “giant K factors”, as in Z + 1 jet production at LHC
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
30
Top is here
O. Brandt
Based on
Z. Ye
Parameter in PYTHIA
• Yukawa coupling is parameter
in
___
Lagrangian (related to MS mass)
• Given this exceptional precision,
theorists need to do a better job
of relating the two masses
(via pole mass, presumably)
L. Dixon
Theoretical Summary
M. Cristinziani
Z. Ye
Helps, but
now only
sensitive at
5 GeV level
Moriond QCD
26 March 2011
31
Top mostly looks normal so far
S. Amerio
M. Cristinziani
L. Dixon
Theoretical Summary
J. Gruschke
Moriond QCD
26 March 2011
32
Except for maybe AFB
O. Brandt
• In SM,
AFB = 0 at O(as2). MC is O(as3), first order in which AFB ≠ 0
• Would like O(as4), to be sure no large corrections.
_
• People working on it, in conjunction with s(t t) at NNLO.
• What about measuring AFB with an extra jet?
• Known at second order already [Dittmaier, Uwer, Weinzierl]
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
33
New physics for AFB?
• s-channel  heavy color-octet,
Lorentz vector state
• Candidates: top-color, technicolor,
universal and warped extra dimensions, chiral color,
axi-gluons, …  dijet and Mtt bumps
• t-channel: flavor-changing interactions
 same sign tops, especially at LHC
Bai, Hewett, Kaplan, Rizzo, 1101.5203
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
34
QCD in extreme or
nonperturbative environments
A. Tang
A. Koshelkin
• Self-consistent nonperturbative solution to coupled
gluon-quark quasi-classical equations: currents
generated by fermions and gauge field exactly compensate
each other
• Proposed application to heavy ion collisions as
alternative to Glasma model
[McLerran ,Venugopalan]
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
35
Extreme QCD (cont.)
• Study jet quenching via (quasi)perturbative approach,
Bethe-Heitler gluon radiation + collisional losses
off scattering centers in quark-gluon plasma,
• Technically challenging calculation.
• At the end, very sensitive to non-perturbative
distance scales r ~ 1/Q, reflected in dependence on
as freezing point used
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
B. Zakharov
36
AdS/CFT
• Another approach to understanding jet quenching,
also the bulk properties of quark-gluon plasma.
• AdS/CFT maps a strongly-coupled 3+1 (conformal) gauge theory
“holographically” to a semi-classical 4+1 gravitational theory in
Anti-de Sitter space
• Not really QCD, but
maybe it is “close
enough” at strong
coupling
Finite temperature
black hole present
(Hawking radiation)
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
37
AdS/CFT & bulk flow
• Very
strong coupling  very small viscosity:
h/s = 1/4p
Chen
Kovtun, Starinets, Son
• Using this value in hydro codes gives second moment of azimuthal
particle correlations, v2, consistent with observations first at RHIC, and
now at LHC
• Comparison with RHIC results
M. Nicassio , ALICE
 30 % higher
 Hydrodynamic model predictions with
viscous corrections in agreement
with observed increase
|h|<0.8
0.2<pt<5GeV/c
•Third moment, v3, recently
measured at RHIC, and in perfect
agreement too!
Chen
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
38
AdS/CFT & jet quenching
• Energy loss in strongly-coupled plasma is large – so large
it can’t be reliably computed for light quark – need heavy one
• But that’s harder to measure, hasn’t been done (quite) yet,
though open charm has now been found in heavy ions 
hot and dense medium
parton
L. Dixon
Theoretical Summary
A. Mischke
Moriond QCD
26 March 2011
39
g + c as energy loss probe
F. Arleo
Study, e.g.
c
b
Preliminary computation
using a perturbative
energy loss framework
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
40
AdS/CFT for light hadron spectrum
• Can use a different model to put in quarks
Closed strings
glueballs
Open strings on D8
mesons
D8-brane
D4 wrapped on
baryons
D4-brane
Nc strings are attached
L. Dixon
Theoretical Summary
Moriond QCD
Sugimoto
Meson & baryon
masses,
form factors, etc.,
agree surprisingly
well with data,
with only 2 free
parameters
26 March 2011
41
Nonperturbative QCD in B Physics
q
• Into the brown muck
• For mQ >> LQCD, a heavy quark is almost
b
static within its meson, and its spin decouples
• Nowadays, want to go to high order in LQCD/ mQ
• Have to classify all local operators at a given order
T. Mannel
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
42
Nonperturbative QCD in B’s (cont.)
T. Mannel
Relates matrix elements to better known
matrix elements of lower dimension operators.
• Apply method to inclusive B  c l n decay width G

 inclusive
Also reviewed new lattice value
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
43
Nonperturbative QCD in B’s (cont.)
• CP asymmetry in B  sg once thought to be
G. Paz
perturbative, precise SM test.
• But now long-distance effects have been
estimated using SCET, found to be large
exp
vs.
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
44
From b to c
• Life gets more difficult for theorists:
LQCD/ mc is not << 1.
• More model dependence, e.g. pole dominance model for
the various topological amplitudes entering D decays
C.-D. Lu
• Life (sometimes) gets easier for experimentalists:
• Particle yields can be much larger
– consider for example
_
the rich cornucopia of exotic c-c containing hadrons from
3872 to 4660 GeV
D. Liventsev
_
• No analogs discovered yet in the b-b system
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
45
Want new particles?
D. Liventsev
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
46
What are they?
D. Liventsev
d
or baryon-antibaryon??
L. Dixon
Theoretical Summary
d
C. Qiao
Moriond QCD
Compute two-pion
exchange (loop level)
potential between
baryons. Might be a
bound state, but answer
depends on cutoff in
effective theory, so
inconclusive
26 March 2011
47
Soft & diffractive physics
• New implementation of a helical variation on the Lund
string model
S. Todorova
Proposed test for it:
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
48
The underlying event at LHC
C.-E. Wulz
• Harder than expected
• The “hard tail” of the
underlying event is double
parton scattering.
• Suggests that DPS
should be visible at LHC soon,
whether in bbjj E. Berger 
gjjj as at Tevatron N. Skachkov
or maybe even Wjj or Zjj
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
49
Soft diffraction at LHC
• MC strategy proposed to remedy this.
L. Dixon
Theoretical Summary
Moriond QCD
K. Goulianos
26 March 2011
50
Hard diffraction at LHC
L. Harland-Lang
• New results for p0p0
g
g
• Two curious features:
1) Amplitude preferred by `Jz=0 selection rule’,
g+g+  p0p0, vanishes exactly at LO.
2) Other amplitude, g+g-  p0p0, has a radiation zero at LO.
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
51
Conclusions
• Standard Model is still holding up, though some tensions need
follow-up: D0 like-sign muon asymmetry, Tevatron top-quark
forward backward asymmetry, muonic hydrogen Lamb shift
measurement, …
• Theory is coming along nicely in multiple regimes: pQCD for
colliders, HQET in B physics, SCET now for colliders, AdS/CFT as
a qualitative model for heavy ion strongly coupled quark-gluon
plasma and hadron spectra
• Some areas are still difficult and may need new frameworks: for
example X, Y, Z spectroscopy
• All in all a very exciting week to be in Moriond/La Thuile!
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
52
Extra Slides
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
53
AdS/CFT & gluon scattering
• Alday, Maldacena showed how to compute gluon scattering at
strong coupling as a minimal area problem for a string world-sheet
with a polygonal boundary (strong-coupling Wilson loop)
• Can also compute it at weak coupling. Numerical results are
remarkably similar, for no good reason (at present):
V. Del Duca
•
Can this circle ideas or techniques help with NNLO QCD? Time will tell…
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
54
Extreme QCD (cont.)
Li S.-Y.
• Unitary exclusive quark combination models. Also
applied to heavy-ion collisions – J/y pT spectrum
(after fitting model to electron pT spectrum)
• Basic principles:
- ALL quarks goes into hadrons (confinement)
- Unitarity (without gluons!)
“If your model isn’t unitary, I can’t help you”
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
55
General Uncertainty Issues
A. Djouadi, J. Hays
• Should one ever add independent sources of error
linearly, even if theoretical? No! Implies 100% correlation.
• What is the “right” distribution
for theoretical uncertainties?
Flat or Gaussian?
• I would say neither:
- pQCD theorist rarely has any idea
which part of her/his band
is preferred (I know I don’t)
- won’t swear that answer won’t
ever fall outside band (I wouldn’t).
• Envelope of Flat+Gaussian?
L. Dixon
Theoretical Summary
Moriond QCD
26 March 2011
56