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

XLVITH Rencontres de MoriondQCD 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

Moriond QCD 26 March 2011L. Dixon Theoretical Summary 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


New Physics the big picture


H. Murayama

S. Todorova

But still lots to do in understanding “old physics”

Where is Higgs?


• Been looking for him for decades• Know everything but Higgs mass• Search really heating up now, bothfrom last stages of Tevatron operation,first stages of LHC

Precisionelectroweak

LEP2directsearch Look for now mostly in WW, ZZ,

and bb (with associated W or Z)

M. Cooke

SM Higgs at Tevatron


• Expect 10-12 fb-1 total by September shutdown.• Could “fail to exclude” at 2s • Hard to get to 3 s evidence in SM unless analysisimprovements are greater than projected

M. Cooke

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, …

M. Cookenot yet updated for new channels

• High mass region also difficult, many channels, but most sensitive is

• Now CDF/D0 each can exclude a range of masses

gg H WW lnln

R. Lysak

SM Higgs at Tevatron (cont.)


New high mass combination of CDF and D0

J. Hays

But Wait!


• Theoretical uncertainties leave (high mass)Higgs in a “half dead, half alive state” - A. Djouadi

Which uncertainties?


Biggest ones:1) gg H has large higher order corrections

A. Djouadi

2) gg H strongly dependent on g(x) for x~0.1. Different pdf sets give quitedifferent cross sections: - MSTW, CTEQ, NNPDF have Tevatron jet data, g(x) larger- ABKM09, HERAPDF do not, g(x) smaller.

However, partial N3LO results and NNLL resummations suggest (to me) that final cross section will not dip to lowest value in k=3 range.

But Wait!


• Brand new NNLO fit to HERA data: HERAPDF1.5• Gives large x gluon much closer to MSTW!

V. Radescu

• NNLO Higgs cross sectionnot yet shown, but clearly itwill rise a lot closer to thatusing MSTW.

High Mass

How to improve sH uncertainty?


• Very large logs due to soft-gluons. To go beyondNNLO, can resum “eikonal logs”: NNLL Catani,de Florian, Grazzini, Nason

also Ahrens et al., arXiv:1008.3162

C. White

H

• Maybe go to next-to-eikonal approximation before long.

• Would still want at least one N3LO hard term: a3 d(x)

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.

• Subtle “collinear anomaly” in SCET applied to

Drell-Yan qT resummation (where there is a ton of data).

Stay tuned for

final predictions.Moriond QCD 26 March 2011L. Dixon Theoretical Summary 13

T. Becher

Where is SUSY?


• Been looking for her for decades• Know nothing (theoretically) about any masses (no upperbounds, anyway, except for lightest Higgs)• Search really heating up now, from first stages of LHC,but also Tevatron, progress in flavor physics too

?

“Generic” SUSY Cascade Searches at LHC


• Already surpassed Tevatron in many cases!

• Will discover or really constrain overall mass scale over next two years

N. Barlow

M. Chiorboli

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

~ ~

SUSY Higgs


• Relatively simple sector: only depends on ~ masses of heavy

Higgses, H, A, H±

• Light Higgs h has mass < MZ cos b + radiative corrections• LEP2 lower bound want tan b somewhat large• Unification suggests very large tan ~ b mt/mb ~ 40• If MA small, coupling of h to b,s,d, ,t m,e all enhanced by

tanb

• However, if MA large – decoupling limit – effect shuts off

~ tanb

Direct SUSY Higgs Search


Cross section tan2β enhanced Tevatron, St. Denis

(roughly, due to SUSY radiative corrections!)

watchthis space?

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

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

Theory input for Bs m+m-


R. Fleischer

Predict using

“U spin” s d

U spin for CP Asymmetries too


R. Fleischer

Enough information

to solve for g

At LHCb

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?)• 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

TaylorStrogolasHarel ThompsonChoBrandt

Strings at the LHC?


T. Taylor

Possibly, if there are also large extra dimensions, and stringmass scale M is also very low.

gg g* ggqg q* qg …

BIG di-jet bump from

So big that

pQCD for colliders


New physics Higgs physics

LHC

@ 7

TeV • Every process shown

also occurs with one more jetat ~ 1/5 the rate• Very important to understand

not only Standard Model

production of X but also of

X + n jetswhere

X = W, Z, tt, WW, H, … n = 1,2,3,…

_

If you don’t know exactly what to look for, need to understand all major SM processes.

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


(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


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


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.


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


Top is here

Based on


M. Cristinziani

O. Brandt

Parameter in PYTHIA

• Yukawa coupling is parameter in Lagrangian (related to MS mass)• Given this exceptional precision,theorists need to do a better jobof relating the two masses (via pole mass, presumably)

___

Z. Ye

Z. Ye

Helps, but now onlysensitive at 5 GeV level

Top mostly looks normal so far


S. Amerio

M. CristinzianiJ. Gruschke

Except for maybe AFB

• In SM,

AFB = 0 at O(as2). MC is O(as

3), 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]Moriond QCD 26 March 2011L. Dixon Theoretical Summary 33

_

O. Brandt

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 interactionssame sign tops, especially at LHC


Bai, Hewett, Kaplan, Rizzo, 1101.5203

QCD in extreme or nonperturbative environments

• 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]


A. Koshelkin

A. Tang

• 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

Extreme QCD (cont.)


B. Zakharov

• 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

AdS/CFT


Finite temperature black hole present(Hawking radiation)

• Not really QCD, but maybe it is “close enough” at strong coupling

AdS/CFT & bulk flow


• Very strong coupling very small viscosity: h/s = 1/4p 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

M. Nicassio , ALICE

• Comparison with RHIC results 30 % higher

Hydrodynamic model predictions with viscous corrections in agreement with observed increase

|h|<0.8 0.2<pt<5GeV/c

Chen

• Third moment, v3, recently measured at RHIC, and in perfect agreement too! Chen

AdS/CFT & jet quenching


• Energy loss in strongly-coupled plasma is large – so largeit 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

parton

hot and dense mediumA. Mischke

g + c as energy loss probe


F. Arleo

Study, e.g.

Preliminary computation using a perturbative energy loss framework

b

c

• Can use a different model to put in quarks Sugimoto

AdS/CFT for light hadron spectrum


Closed strings

Open strings on D8

D4 wrapped onD4-brane

D8-brane

Nc strings are attached

glueballs

mesons

baryons

Meson & baryonmasses, form factors, etc., agree surprisinglywell with data,with only 2 freeparameters

Nonperturbative QCD in B Physics

• Into the brown muck

• For mQ >> LQCD, a heavy quark is almost

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


b

q

T. Mannel

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

Nonperturbative QCD in B’s (cont.)


G. Paz

• CP asymmetry in B s g once thought to beperturbative, precise SM test.• But now long-distance effects have beenestimated using SCET, found to be large

vs.exp

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


_

_

Want new particles?


D. Liventsev

What are they?


D. Liventsev

or baryon-antibaryon?? C. Qiao

dd

Compute two-pion exchange (loop level)potential between baryons. Might be abound state, but answerdepends on cutoff ineffective theory, soinconclusive

Soft & diffractive physics

• New implementation of a helical variation on the Lund string model


S. Todorova

Proposed test for it:

The underlying event at LHC

• 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


C.-E. Wulz

Soft diffraction at LHC

• MC strategy proposed to remedy this. K. Goulianos


Hard diffraction at LHC

• New results for p0p0

• 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.Moriond QCD 26 March 2011L. Dixon Theoretical Summary 51

L. Harland-Lang

g

g

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!


Extra Slides


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…


Extreme QCD (cont.)

• 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”


Li S.-Y.

General Uncertainty Issues


• Should one ever add independent sources of errorlinearly, even if theoretical? No! Implies 100% correlation.

A. Djouadi, J. Hays

• What is the “right” distributionfor theoretical uncertainties?Flat or Gaussian?• I would say neither:- pQCD theorist rarely has any idea which part of her/his bandis preferred (I know I don’t)- won’t swear that answer won’tever fall outside band (I wouldn’t).• Envelope of Flat+Gaussian?

Documents

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