vector boson + jets at nlo vs. lhc datatpp.ph.unimelb.edu.au/lhcworkshop2012/talks/dixon.pdfl. dixon...
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Vector boson + jets at NLO
vs. LHC data
Lance Dixon (SLAC)
for the BlackHat collaboration Z. Bern, LD, G. Diana, F. Febres Cordero,
S. Höche, H. Ita, D. Kosower, D. Maître, K. Ozeren
LHC Theory Workshop University of Melbourne - July 4, 2012
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 2
“New physics at the LHC is a riddle,
wrapped in a mystery, inside an enigma;
but perhaps there is a key.” -W. Churchill
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 3
LHC Data Dominated by Jets
new physics
• Every process shown
also with one more jet
at ~ 1/5 the rate
• Need accurate
production rates for
X + 1,2,3,… jets in Standard Model
Jets from quarks
and gluons.
• q,g from decay of
new particles?
• Or from old QCD?
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 4
Classic SUSY dark matter signature
Heavy colored particles decay rapidly to stable
Weakly Interacting Massive Particle (WIMP = LSP)
plus jets
Missing transverse energy
MET + 4 jets
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 5
Irreducible Standard Model Background
• MET + 4 jets from
pp Z + 4 jets,
Z neutrinos
Neutrinos escape detector State of art for Z + 4 jets
was based on
Leading Order (LO)
approximation in QCD
normalization uncertain
n n LO
Now available at
Next to Leading Order,
greatly reducing
theoretical uncertainties
n n NLO
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 6
NLO = Loops
• NLO also needs tree-level amplitudes with one more parton
• Both terms infinite – combine in dim. reg. to get a finite result
tree + 1 parton 1 loop
NLO
NLO cross section needs Feynman diagrams
with exactly one closed loop
Textbook methods quickly fail, even with very powerful computers
• One-loop amplitudes were the bottleneck for a long time
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 7
A Better Way to Compute?
• Want loop-level scattering amplitudes for
many ultra-relativistic (“massless”) particles
– especially quarks and gluons of QCD
• Feynman diagrams, while very general and powerful, are not optimized for these processes
• There are much more efficient methods
8
Just one QCD loop can be a challenge
pp W + n jets (just amplitudes with most gluons)
# of jets # 1-loop Feynman diagrams
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012
Current limit with
Feynman diagrams
Current limit with
on-shell methods
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Branch cut information
Generalized Unitarity (One-loop Plasticity)
Ordinary unitarity: put 2 particles on shell
Generalized unitarity: put 3 or 4 particles on shell
Trees recycled into loops!
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 10
One-Loop Amplitude Decomposition
rational part; from D-dimensional trees, or recursively
Bern, LD, Dunbar, Kosower (1994)
Known functions (integrals),
same for all amplitudes
coefficients can be determined from products
of trees using (generalized) unitarity
Missing from the old, nonperturbative analytic S-matrix
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 11
Blackhat+Sherpa: Berger, Bern, LD, Diana, Febres Cordero, Forde,
Gleisberg, Höche, Ita, Kosower, Maître, Ozeren
W + 3 jets 0902.2760, 0907.1984
Z + 3 jets 1004.1659
W + 4 jets 1009.2338
Z + 4 jets 1108.2229
W + 5 jets 12mm.nnnn
Recent progress on V + jets at NLO
Rocket: W + 3 jets Ellis, Melnikov, Zanderighi, 0901.4101, 0906.1445
MCFM: V + 0,1,2 jets Campbell, Ellis, hep-ph/0202176
• Could try to use such predictions directly for backgrounds
to experimental searches.
• However, it is generally safer to use data-driven techniques
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 12
NLO pp W + 4 jets Berger et al., 1009.2338
First hadron collider process known at NLO with 5 objects in final state.
Also important SUSY background.
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 13
NLO pp Z + 4 jets, and ratio to W±
Ita et al.
1108.2229
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 14
NLO pp W + 5 jets also feasible
+ + …
PRELIMINARY
leading color
BH+Sherpa
vs. LHC data
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L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 16
NLO pp Z + 1,2,3,4 jets vs. ATLAS 2010 data
Analysis in progress with
full 2011 data set:
> 100 times the 2010 data
ATLAS
1111.2690
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 17
NLO pp W + 1,2,3,4 jets vs. ATLAS 2010 data
NLO undershoots badly for W + 1 jet –
production dominated by W + 2 parton
configurations. Theory can be improved here: Rubin, Salam, Sapeta 1006.2144
ATLAS
1201.1276
Issues with forward jets?
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 18
ATLAS 1201.1276
Large x gluon
too large?
Also forward jet issues at Tevatron
L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 19
D0 @ PLHC Vancouver
• Note that probability
of emitting another jet
seems OK
• Important experimentally
for assessing effects of
central jet veto on
backgrounds, in search
for Higgs in vector boson
fusion
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Pure QCD: pp 4 jets vs. ATLAS data
1112.3940
4 jet events might
hide pair production
of 2 colored particles,
each decaying to a
pair of jets [e.g. talk
by B. Dobrescu]
Detailed study of
multi-jet QCD
dynamics may help
understand other
channels
ATLAS 1107.2092
Pure QCD Jet Production Ratio
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3/2 ratio
agrees well with
ATLAS data
even in regions
where it’s
quite large
Looking forward
to measurement
of 4/3 ratio
ATLAS 1107.2092
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Fixed order vs. Monte Carlo
• Previous plots NLO but fixed-order, few partons: no model of long-distance effects included;
cannot pass through a detector simulation
• Methods available for matching NLO
parton-level results to parton showers, with NLO accuracy:
– MC@NLO Frixione, Webber (2002) + SHERPA implementation
– POWHEG Nason (2004); Frixione, Nason, Oleari (2007)
• Recently implemented for increasingly complex final states
NLO MC for W + 1,2,3 jets vs. ATLAS data
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Höche et al., 1201.5882
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Conclusions
• Thanks to new methods for computing loop amplitudes, lots of NLO (fixed order, parton-level) results now available for comparison with LHC data, in particular: W/Z + 1,2,3,4,5 jets 4 jets
• Except for some issues at large jet rapidity, agrees quite well with 2010 LHC data – ATLAS analyses of both W and Z + 1,2,3,4 jets, & pure QCD 4 jets
• So far, the NLO results for the most complex processes are only at parton level, not yet embedded in a full Monte Carlo, like MC@NLO, POWHEG, or SHERPA. But this will happen before long – already done up to W + 3 jets!
• Looking forward to learning even more from comparisons to 2011 LHC data from both CMS and ATLAS