Les Houches SM and NLO multi-leg group: experimental introduction and
chargeG. Heinrich, J. Huston, J. Maestre, D. Maitre, R. Pittau,
G. Soyez (jet liason)
Understanding cross sections at the LHC
PDF’s, PDF luminositiesand PDF uncertainties
Sudakov form factorsunderlying eventand minimumbias events
LO, NLO and NNLO calculations K-factors
jet algorithms and jet reconstruction
benchmark cross sections and pdfcorrelations
We’ll be dealing with all of these topics in this session,in the NLM group, in the Tools/MC group and in overlap.
Understanding cross sections at the LHC
We’re all looking for BSM physics at the LHC
Before we publish BSM discoveries from the early running of the LHC, we want to make sure that we measure/understand SM cross sections detector and reconstruction
algorithms operating properly SM physics understood
properly especially the effects of
higher order corrections SM backgrounds to BSM
physics correctly taken into account
This is the first Les Houches at which we have LHC data to test in addition to a plethora of
data from the Tevatron, including some mysteries
List of topics (from web page)
Higher order calculations and techniques->Roberto’s talk Public computational tools/templates->(mostly) Roberto’s talk NLO/PS matching (joint with Tools/MC WG)->(mostly) Roberto’s
talk Jetology
jet observables boosted object tagging connections/differences between LO, NLO and MC jet
clustering for complex n-parton final states variations in NLO multi-parton cross sections with jet
algorithms, jet sizes and scales Higgs observables To this, I would add PDFs (also in parallel with Tools/MC group I
would say)
We had 3 evo pre-meetings
May 6: HO computations and techniquesMay 13: JetologyMay 20: Higgs observables
Start with the wishlist Began in 2005, added to in 2007 and
2009 only process 12 left among NLO
Are there other motivated needs for NLO multi-parton final states? from dedicated calculation or
automatic calculation? one thing we promised to do last
Les Houches is provide a table of the needed accuracy for each final state
Should we move on to expanding the NNLO list?
There’s also the issue of how experimentalists can use these calculations aMC@NLO: but what is the
learning curve to get to say W + 3,4 jets at NLO
ntuples more practical for immediate future, i.e. before next Les Houches?
CalculationsOnce we have the
calculations, how do we (experimentalists) use them?
If a theoretical calculation is done, but it can not be used by any experimentalists, does it make a sound?
We need public programs and/or public ntuples
Example: Blackhat+Sherpa ntuples
Born loop: lc and fmlc real
vsub
so this is not Sherpa the parton shower,but Sherpa usedas a (very efficient) fixed order matrix elementgenerator
How it’s put together
Born loop: lc and fmlc real
vsub
for W+3 jets,W+3 parton tree-levelmatrix elements
all of the virtual terms, both leading color and full-minus-leading color; the latter is typically a few % effect, but much of the complexity of the calculation
all of the real emission terms,(W+4 partons for W + 3 jets), modified by the dipole subtraction terms; divergences are gone
the dipole subtraction termsevaluated in n-body phase space; to make matters more complex,vsub can be either + or -,compensated by otherterms in the total crosssection; note the sum over all quarks andantiquarks; makes mattersmore complex when coming to scale uncertainties
ROOT ntuples More complex to use than MCFM
no manual for example and you don’t produce the events
yourself my student Brian Martin and I are
the beta users ntuples produced separately by
Blackhat + Sherpa for No jet clustering has been performed;
that’s up to the user a difference from MCFM, where
the program has to be re-run for each jet size/algorithm
What algorithms/jet sizes that can be run depends on how the files were generated i.e. whether the right counter-
events are present For the files on the right at 7 TeV (for
W+ + 3 jets), one can use kT, antikT, siscone (f=0.75) for jet sizes of 0.4, 0.5, 0.6 and 0.7
bornLO (stands alone for pure LO comparisons; not to be added with other contributions below) 20 files, 5M events/file, 780
MB/file Born
18 files, 5M events/file, 750 MB/file
loop-lc (leading color loop corrections) 398 files, 100K events/file, 19
MB/file loop-fmlc (needed for full color loop
corrections) 399 files, 15K events/file, 3
MB/file real (real emission terms)
169 files, 2.5 M event/file, 5 GB/file
vsub (subtraction terms) 18 files, 10M events/file, 2.8
GB/file
Jet Clustering For jet clustering, we use
SpartyJet, and store the jet results in SJ ntuples and they tend to be big
since we store the results for multiple jet algorithms/sizes
Then we friend the Blackhat+Sherpa ntuples with the SpartyJet ntuples producing analysis ntuples (histograms with cuts) for each of the event categories
Add all event category histograms together to get the plots of relevant physical observables
http://projects.hepforge.org/spartyjet/
If interested, please [email protected]
LogisticsSo total file disk space is quite large, multi-TB
(and there are many events to be processed) I bought a 20TB disk specifically for this
purposeBut they’re divided into few GB files
(Blackhat+SJ)So we can make our analysis parallel using
350 nodes at MSUPossible to run through W + 3 jet NLO analysis
in few days (much faster without the scale variations) somewhat longer with more variations included
…so for exampleW+ + 3 jets at 7 TeV for
standard cuts (plus for electron cuts) |ym|<2.4 pT
m>20 GeV/c pT
n> 25 GeV/c PT
jet>20 GeV/c |yjet|<2.8 mT(m,n)>40 GeV
New cuts or histograms means re-running through the ntuples
For antikT4 born: 22.69 pb loop-lc: -0.69 pb loop-fmlc: 0.39 pb vsub: 27.16 pb real: -17.34 pb Total: 32.21 pb
Predictions
From Blackhat+Sherpa, we have ntuples (in same format) for W + 1,2, 3,4 jets
Makes it easy to make plots for different jet multiplicities and/or combined jet multiplicities including PDF uncertainties including scale uncertainties
would like to explore a CKKW-like scale at NLO at Les Houches
examining dependence on jet size/algorithm
Lead jet pT
Scale dependenceFactorization and
renormalization scale dependence for any cross section can be calculated (relatively easily) independent of the evaluation of the full matrix element, if you’re careful to collect the relevant terms
In new version of Blackhat+Sherpa ntuples, they were careful to collect the relevant terms
Reweighting
can reweight each event tonew
-PDF-factorization scale-renormalization scale-as (tied to the relevant
PDFs)
based on weights stored in ntuple (and linking with LHAPDF)
so, for example, the events were generated with CTEQ6,and were re-weighted to CTEQ6.6
Reweighting, cont.
complex:carry both single and doublelogs
we run into thesum over quarksand antiquarksagain
9
PDF Errors
Better than what is done in MCFM (as far as disk space is concerned); PDF errors aregenerated on-the-fly through calls to LHAPDF. But then don’t store information for individual eigenvectors.
Example scale/PDF uncertainty
LO at thispoint for 4 jets
…calculated using ntuples
LO/NLO predictions for jet cross sections
Don’t believe (fixed) LO predictions for jet cross sections
Let’s look at predictions for W+ + 3 jets for two different jet algorithms as a function of jet size at the LHC (7 TeV)
At LO, both antikT and SISCone show a marked decrease in cross section as the jet size increases because of the log(1/DR)
terms But at NLO, the two cross
sections show little dependence on the jet size, and are similar to each other due to addition of extra gluon
in jet possible at NLO You’ll see the same thing in
ATLAS Monte Carlo
note NLO~LO because a scale of HT
has been used; if a scale like mW
2+pTW2 is used K-factor <<1
Blackhat + Sherpa
Predictions for jet cross sections
Compare to ATLAS ALPGEN+PYTHIA samples for jet sizes of 0.7
At parton level, antikT is ~25%higher than SISCone (same aswe observe here at LO)
At topocluster level, antikT is~2% higher than SISCone (not the 7% observed here)
Why 2%, not 7%? Some of the W + 3 partonevents reconstructed as 2 jets at the parton level forSISCone are reconstructed as3 jets at the hadron. The crosssection for 3 jets increases.
Try this out in ATLAS/CMS Monte Carlo
Take W + 2 parton events (ALPGEN+PYTHIA), run SISCone 0.7 algorithm on parton level, hadron level (not shown) and topocluster level
Plot the probability for the two sub-jets to merge as a function of the separation of the original two partons in DR
Color code: red: high probability for merging blue: low probability for merging everything for DR<0.7 is merged
for SISCone (and antikT) Parton level reconstruction agrees
with naïve expectation Topocluster level reconstruction
agrees with need for Rsep
I’d like to come to some resolution/better understanding on this issue at Les Houches, using a standardized file of W + jets events
Choosing jet sizeExperimentally
in complex final states, such as W + n jets, it is useful to have jet sizes smaller so as to be able to resolve the n jet structure
this can also reduce the impact of pileup/underlying event
Theoretically hadronization effects
become larger as R decreases
for small R, the ln R perturbative terms referred to previously can become noticeable
this restriction in the gluon phase space can affect the scale dependence, i.e. the scale uncertainty for an n-jet final state can depend on the jet size,
…to be investigated
Another motivation for the use of multiple jet algorithms/parameters in LHC analyses. Can we explore this further?
Jet sizes and scale uncertainties: the Goldilocks theorm
Take inclusive jet production at the LHC for transverse momenta of the order of 50 GeV
Look at the theory uncertainty due to scale dependence as a function of jet size
It appears to be a minimum for cone sizes of the order of 0.7 i.e. if you use a cone size of 0.4, there are residual un-
cancelled virtual effects if you use a cone size of 1.0, you are adding too much tree
level information with its intrinsically larger scale uncertaintyThis effect becomes smaller for jet pT values on the
order of 100 GeV/c how does it translate for multi-parton final states? …good subject for investigation here
Scale choices Take inclusive jet production at
the LHC Canonical scale choice is
mr=mf=1.0*pT
Close to saddle point for low pT
But saddle point moves down for higher pT
Can we think about recommendations for scale choices (and ranges) for the LHC?
I know there is worry about typical scale choices that can lead to negative cross sections, for example at very forward rapidities
Rather than look for some magic formula, we should try to understand what is going on the kinematic/scale point-of-view
R=0.4antikT
Scale dependence also depends on jet size
R=0.4antikT
R=0.6antikT
One scheme F. Olness and D. Soper,
arXiv:0907.5052 Define x1 and x2
Make a circle of radius |x|=2 around a central scale (could be saddle point, or could be some canonical scale) and evaluate the scale uncertainty
AJ and MJK carry information on thescale dependence beyond NLO
col
Fred is here,so maybewe can explore thisfurther, comparingto the LHCdata
Another scheme
Higgs Cross Section Working GrouparXiv:1101.0593
Scale dependence: jet algorithms
Look at results for SISCone/antikT; antikT cross sections larger than SISCone, smaller scale dependence?
H. Ita, SLAC Hadronic Final State Forum
Z + 3 jets: scale dependence
Note that peak cross sections are actually quite close; the cross sections just peakat different scales.
1004.1659Can we understand/quantify this better? For LHC cross sections.
Scales: CKKW and NLO Applying a CKKW-like scale at LO also leads to better agreement
for shapes of kinematic distributions (Partially) investigated at last Les Houches; needs more work at
this Les Houches
0910.3671 Melnikov, ZanderighiSee review of W + 3 jets in Les Houches2009 NLM proceedings
Jet vetos For some cross sections, the scale dependence improves with a jet veto,
and in others the scale dependence worsens I think it would be worthwhile to collect this information And of course, these conclusions are drawn from using fixed order
predictions only
WWjet tTbB
Uncertainties for Higgs production with jet binning
…large logs result fromjet vetoing
naïve scalevariation may provide toosmall an estimate of scale uncertainty
have to resum these logs;can re-weight MC@NLOor Powheg using thisinformation
maybe we can generalizeto other processes at the LHC
F. Tackmann May 20 evo
CDF Wjj Potentially an important
discovery, but are current tools capable of modelling the W + jets background precisely enough
Session on Saturday afternoon
You know that it’s important when it makes it to prime-time TV
…if you paid close attention
CDFWjjanalysiscuts
LHC jets ATLAS and CMS are both
using an IR-safe jet algorithm (anti-kT)
Unfortunately no common sizes 0.4 and 0.6 for ATLAS 0.5 and 0.7 for CMS
It would be nice to have at least one common
jet size exploit any capability to
perform analyses with multiple jet sizes/algorithms
ATLAS topoclusters have the potential to allow for more flexibility in jet analyses
Should be similar potential in CMS with particle flow, etc
UE/pileup corrections: Jet areas
note that the kT
algorithm hasthe largest jet areas, SISConethe smallest and anti-kT the most regular; one of the reasons we like the antikt
determined byclustering ghostparticles of vanishing energy;see jet references
Jets: area-based correction: Cacciari/Salam/Soyez
Used by both ATLAS and CMS. Can we understand what works/what needs improvement in the light of LHC data with significant pileup?
Aside: Photon isolation at the LHC
From a theoretical perspective, it’s best to apply a Frixione-style isolation criterion, in which the amount of energy allowed depends on the distance from the photon; this has the advantage of removing the fragmentation contribution for photon production, as well as discriminating against backgrounds from jet fragmentation
But most of the energy in an isolation cone is from underlying event/pileup At Les Houches, we started to develop (being continued by Mike Hance, Brian,…in
ATLAS): (1) an implementation of the Frixione isolation appropriate for segmented
calorimeters (2) a hybrid technique that separates the UE/pileup energy from fragmentation
contributions using the jet density approach
more developmentat this Les Houches?
Jets at parton level and in (NLO) MC
…from Jet Pair Production in Powheg, arXiv:1012.3380
note that theory/datahas a slope notevident with fixedorder comparisons(NLO corrected byUE/hadronization)
also observed inATLAS comparisons;differences observedwhen using Pythia asshower instead of Herwig
an effect we need tounderstand; this will affect all global PDF fits, for example; Les Houches is a good place to do it
PDFs We’ve learned a lot from the PDF4LHC exercises In particular, we’ve seen where the PDFs agree and where they don’t The exercise was at NLO; now we are in a position to continue it at NNLO
Plots by G. Watt
…as well as to start adding LHC data
…and alreadyseeing differences between Experiments
Note that resummed predictions areimportant
PDF correlations Consider a cross section X(a), a
function of the Hessian eigenvectors ith component of gradient of X is
Now take 2 cross sections X and Y or one or both can be pdf’s
Consider the projection of gradients of X and Y onto a circle of radius 1 in the plane of the gradients in the parton parameter space
The circle maps onto an ellipse in the XY plane
The angle f between the gradients of X and Y is given by
The ellipse itself is given by
• If two cross sections are verycorrelated, then cosf~1• …uncorrelated, then cosf~0• …anti-correlated, then cosf~-1
…from PDF4LHC report
Correlations, continued…
one interesting angle to calculateis the angle between the gradientfor a particular physics processand the hyperplane formed by thefirst n eigenvectors
take gg->Higgs (120 GeV)
eigenvector cos f =1 0.028<=2 0.077<=3 0.077<=4 0.534 <=5 0.551<=6 0.553<=7 0.602<=8 0.604<=9 0.609<=10 0.808<=11 0.808
so very strong correlation (0.8) between the Higgs crosssection and the hyperplane formed by the first 11 (of 22)eigenvectors in CTEQ6.6
low number eigenvectors have quadratic c2 behavior
Being used by Higgs combination groups
Can we extend this use?
SummaryDue to lack of time, haven’t mentioned
boosted jets/analyses, but clearly this is an important aspect of this workshop some people are coming straight from
BOOST2011 There are a lot of interesting physics
topics at this Les Houches, as well as LHC data (for the first time) and greatly improved NLO technology
It should be an interesting week and a half
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