xlvi th rencontres de moriond qcd and high energy interactions theoretical summary
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XLVI TH 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 - PowerPoint PPT PresentationTRANSCRIPT
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XLVITH Rencontres de MoriondQCD and High Energy Interactions
Theoretical SummaryLance Dixon (CERN & SLAC)
La Thuile, 26 March 2011
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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
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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
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New Physics the big picture
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H. Murayama
S. Todorova
But still lots to do in understanding “old physics”
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Where is Higgs?
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 5
• 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
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SM Higgs at Tevatron
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• Expect 10-12 fb-1 total by September shutdown.• Could “fail to exclude” at 2s • Hard to get to 3s evidence in SM unless analysisimprovements are greater than projected
M. Cooke
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SM Higgs at Tevatron (cont.)
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• 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
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SM Higgs at Tevatron (cont.)
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New high mass combination of CDF and D0
J. Hays
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But Wait!
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 9
• Theoretical uncertainties leave (high mass)Higgs in a “half dead, half alive state” - A. Djouadi
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Which uncertainties?
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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.
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But Wait!
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• 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
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How to improve sH uncertainty?
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• Very large logs due to soft-gluons. To go beyondNNLO, can resum “eikonal logs”: NNLL Catani,de Florian, Grazzini, Nasonalso 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)
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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 forfinal predictions.
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T. Becher
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Where is SUSY?
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 14
• 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
?
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“Generic” SUSY Cascade Searches at LHC
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• Already surpassed Tevatron in many cases!
• Will discover or really constrain overall mass scale over next two years
N. Barlow
M. Chiorboli
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Many more specialized SUSY Searches
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• Just one example: top squark might be much lighter than• other quarks, because of running effects, large tL-tR splitting):
Tevatron, Ph. Gris
~ ~
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SUSY Higgs
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• 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
• However, if MA large – decoupling limit – effect shuts off
~ tanb
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Direct SUSY Higgs Search
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Cross section tan2β enhanced Tevatron, St. Denis
(roughly, due to SUSY radiative corrections!)
watchthis space?
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SUSY in Flavor
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• 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
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Bs m+m- (cont.)
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• Exciting because LHCb has already reached ~ Tevatron sensitivity:
M.-O. Bettler
• Will cover much of remaining distance to SM in next year or two
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Theory input for Bs m+m-
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R. Fleischer
Predict using “U spin” s d
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U spin for CP Asymmetries too
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R. Fleischer
Enough information
to solve for g
At LHCb
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Other New Physics Theories
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• 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
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Strings at the LHC?
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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
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pQCD for colliders
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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.
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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
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(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
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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
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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.
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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
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Top is here
Based on
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 31
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
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Top mostly looks normal so far
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S. Amerio
M. CristinzianiJ. Gruschke
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Except for maybe AFB
• In SM, AFB = 0 at O(as
2). 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]
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 33
_
O. Brandt
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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
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Bai, Hewett, Kaplan, Rizzo, 1101.5203
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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]
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A. Koshelkin
A. Tang
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• 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-perturbativedistance scales r ~ 1/Q, reflected in dependence on as freezing point used
Extreme QCD (cont.)
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B. Zakharov
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• 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
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Finite temperature black hole present(Hawking radiation)
• Not really QCD, but maybe it is “close enough” at strong coupling
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AdS/CFT & bulk flow
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• 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
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AdS/CFT & jet quenching
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• 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
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g + c as energy loss probe
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F. Arleo
Study, e.g.
Preliminary computation using a perturbative energy loss framework
bc
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• Can use a different model to put in quarks Sugimoto
AdS/CFT for light hadron spectrum
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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
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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
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bq
T. Mannel
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Nonperturbative QCD in B’s (cont.)
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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
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Nonperturbative QCD in B’s (cont.)
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G. Paz
• CP asymmetry in B sg once thought to beperturbative, precise SM test.• But now long-distance effects have beenestimated using SCET, found to be large
vs.exp
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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 systemMoriond QCD 26 March 2011L. Dixon Theoretical Summary 45
_
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Want new particles?
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D. Liventsev
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What are they?
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 47
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
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Soft & diffractive physics• New implementation of a helical variation on the Lund
string model
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S. Todorova
Proposed test for it:
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The underlying event at LHC• Harder than expected• The “hard tail” of theunderlying event is doubleparton scattering.• Suggests that DPSshould be visible at LHC soon,
whether in bbjj E. Berger gjjj as at Tevatron N. Skachkov
or maybe even Wjj or Zjj
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C.-E. Wulz
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Soft diffraction at LHC
• MC strategy proposed to remedy this. K. Goulianos
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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
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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!
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Extra Slides
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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…
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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”
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Li S.-Y.
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General Uncertainty Issues
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• 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?