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CTEQ SS02J. Huston
QCD Backgrounds to New Physics II
J. Huston
…thanks to Weiming Yao, John Campbell, Bruce KnutesonNigel Glover for transparencies
CTEQ SS02J. Huston
but first some commercials
CTEQ SS02J. Huston
Run 2 Monte Carlo Workshop
Transparencies, videolinks to individual talksand links to programscan all be found athttp://www-theory.fnal.gov/runiimc/
I’ll be referring to someof these programs inthe course of my talk
CTEQ SS02J. Huston
Les Houches
Two workshops on “Physics at TeVColliders” have been held so far, in1999 and 2001 (May 21-June 1)
Working groups on QCD/SM, Higgs,Beyond Standard Model
See web page:
http://wwwlapp.in2p3.fr/conferences/LesHouches/Houches2001/
especially for links to writeups from 1999and 2001
QCD 1999 writeup (hep-ph/0005114)is an excellent pedagogical reviewfor new students
QCD 2001 writeup (hep-ph/0204316)is a good treatment of the state ofthe art for pdfs, NLO calculations,Monte Carlos
Les Houches 2003 will have more ofa concentration on EW/top physics
CTEQ SS02J. Huston
Les Houches 2001 Writeups
The QCD/SM Working Group: SummaryReport
◆ hep-ph/0204316
The Higgs Working Group: Summary Report(2001)
◆ hep-ph/0203056
The Beyond the Standard Model WorkingGroup: Summary Report
◆ hep-ph/0204031
CTEQ SS02J. Huston
Les Houches 2001
200th bottle of wineconsumed at theworkshop
CTEQ SS02J. Huston
Other useful references (for pdfs)
LHC Guide to Parton Distributions and Cross Sections, J. Huston;http://www.pa.msu.edu/~huston/lhc/lhc_pdfnote.ps
The QCD and Standard Model Working Group: Summary Report from LesHouches; hep-ph/0005114
Parton Distributions Working Group, Tevatron Run 2 Workshop; hep-ph/0006300
A QCD Analysis of HERA and fixed target structure functiondata, M. Botje; hep-ph/9912439
Global fit to the charged leptons DIS data…, S. Alekhin; hep-ph/0011002
Walter Giele’s presentation to the QCD group on Jan. 12; http://www-cdf.fnal.gov/internal/physics/qcd/qcd99_internal_meetings.html
Uncertainties of Predictions from Parton Distributions I: the Lagrange MultiplierMethod, D. Stump, J. Huston et al.;hep-ph/0101051
Uncertainties of Predictions from Parton Distributions II: the Hessian Method, J.Pumplin, J. Huston et al.; hep-ph/0101032
Error Estimates on Parton Density Distributions, M. Botje; hep-ph/0110123
New Generation of Parton Distributions with Uncertainties from GlobalQCD Analysis (CTEQ6); hep-ph/0201195
CTEQ SS02J. Huston
Other references…
…to the ability to calculate QCD backgrounds …to the need to do so
CTEQ SS02J. Huston
Monojets in UA1
UA1 monojets (1983-1984)
◆ Possible signature of new physics(SUSY, etc)
◆ A number of backgrounds wereidentified, but each was noted asbeing too small to account for theobserved signal
▲ pp->Z + jets
|_ νν
▲ pp->W + jets
|_ τ + ν
|_ hadrons + ν
▲ pp->W + jets
|_ l + ν
▲ pp->W + jets
|_ τ + ν
|_ l + ν
jet
…but the sum was not
◆ “The sum of many small things is abig thing.” G. Altarelli
Can calculate from first principles orcalibrate to observed cross sectionsfor Z->e+e- and W->eν
Ellis, Kleiss, Stirling PL 167B, 1986.
CTEQ SS02J. Huston
Signatures of New Physics
Ws, jets, γs, b quarks, ET
…pretty much the same as signatures for SM physics
How do we find new physics? By showing that its not old physics.
◆ can be modifications to the rate of production
◆ …or modification to the kinematics, e.g.angular distributions
Crucial to understand the QCD dynamics and normalization of bothbackgrounds to any new physics and to the new physics itself
Some backgrounds can be measured in situ
◆ …but may still want to predict in advance, e.g. QCD backgrounds toH->γγ
For some backgrounds, need to rely on theoretical calculations, e.g. ttbbbackgrounds to ttH
CTEQ SS02J. Huston
Theoretical Predictions for New (Old) Physics
There are a variety of programs available forcomparison of data to theory and/or predictions.
◆ Tree level
◆ Leading log Monte Carlo
◆ NnLO
◆ Resummed
Important to know strengths/weaknesses of each.
In general, agree quite well…but beforeyou appeal to new physics, check theME. (for example using Comphep)Can have ME corrections to MC or MC corrections to ME. (in CDF->HERPRT)
Perhaps biggest effort…include NLO MEcorrections in Monte Carlo programs…correct normalizations. Correct shapes. NnLO needed for precision physics.
Resummed description describes soft gluoneffects (better than MC’s)…has correctnormalization (but need HO to get it); resummedpredictions include non-perturbative effectscorrectly…may have to be put in by hand in MC’sthreshold kT
W,Z, Higgsdijet, direct γ
b space(ResBos)
qt space
Where possible, normalize to existing data.…in addition, worry about pdf, fragmentation uncertainties
CTEQ SS02J. Huston
W + Jet(s) at the Tevatron
Good testing ground for parton showers,matrix elements, NLO
Background for new physics
◆ or old physics (top production)
Reasonable agreement for the leadingorder comparisons using VECBOS (butlarge scale dependence)
-1.5
-1
-0.5
0
0.5
1
0 10 20 30 40 50 60 70 80 90 100
Systematic uncertainties
Leading Jet ET (GeV)
dσ
(W +
≥1
jets
) / d
ET
(D
ata −
QC
D)
/ QC
D
CDF PRELIMINARY
CDF Data (108 pb-1)0.4 Jet Cones, |ηd| < 2.4
DYRAD NLO QCD Predictionswith jet smearingMRSA/
Qr2 = Qf
2 = (0.5 MW)2
Qr2 = Qf
2 = MW2
Qr2 = Qf
2 = (2.0 MW)2
Good agreement with NLO (and smaller scale dependence) for W + >= 1 jet
CTEQ SS02J. Huston
W + jets
For W + >=n jet production, typicallyuse Herwig (Herprt) for additionalgluon radiation and for hadronization
Can also start off with n-1 jets andgenerate additional jets using Herwig
10-1
1
10
10 2
10 3
0 20 40 60 80 100 120 140 160 180 2
(a)
(b)
(c)
+ CDF Preliminary Data (108 pb-1)
− VECBOS + HERWIG Fragmentation+ Detector Simulation
CTEQ3M, Q2 = < pT >2
(Normalized to data)
Jet ET (GeV)
Eve
nts
/ 5 G
eV
(a) ET2 (W + ≥2 Jets Data)
VECBOS W + 1 Jet
(b) ET3 (W + ≥3 Jets Data)
VECBOS W + 2 Jets
(c) ET4 (W + ≥4 Jets Data)
VECBOS W + 3 Jets
10-1
1
10
10 2
10 3
0 20 40 60 80 100 120 140 160 180 200
(a)
(b)
(c)
(d)
+ CDF Preliminary Data (108 pb-1)
− VECBOS + HERWIG Fragmentation+ Detector Simulation
CTEQ3M, Q2 = < pT >2
(Normalized to data)
Jet ET (GeV)
Eve
nts
/ 5 G
eV
(a) ET1 (W + ≥1 Jet)
(b) ET2 (W + ≥2 Jets)
(c) ET3 (W + ≥3 Jets)
(d) ET4 (W + ≥4 Jets)
CTEQ SS02J. Huston
More Comparisons (VECBOS and HERWIG)
Start with W + n jets from VECBOS
0
100
200
300
400
500
0 1 2 3 4 5
+ CDF PreliminaryData (108 pb-1)
− VECBOS + HERWIG+ Detector SimulationCTEQ3M, Q2 = < pT >(Normalized to data)
Eve
nts
/ 0.5
W + ≥2 Jets DataVECBOS W + 1 Jet
0
20
40
60
80
100
0 1 2 3 4 5∆Rjj of Leading Two Jets
Eve
nts
/ 0.5
W + ≥3 Jets DataVECBOS W + 2 Jet
0
100
200
300
400
500
0 1 2 3 4 5 6
+ CDF PreliminaryData (108 pb-1)
− VECBOS + HERWIG+ Detector SimulationCTEQ3M, Q2 = < pT >2
(Normalized to data)
Eve
nts
/ 0.5
W + ≥2 Jets
0
20
40
60
80
100
0 1 2 3 4 5 6∆Rjj of Leading Two Jets
Eve
nts
/ 0.5
W + ≥3 Jets
•Start with W + (n-1) jets from VECBOS
CTEQ SS02J. Huston
More Comparisons
Start with W + n jets from VECBOS Start with W + (n-1) jets fromVECBOS
1
10
10 2
10 3
0 50 100 150 200 250 300 350 400
+ CDF Preliminary Data (108 pb-1)− VECBOS + HERWIG Fragmentation
+ Detector SimulationCTEQ3M, Q2 = < pT >2
(Normalized to data)
Eve
nts
/ (10
GeV
/c2 )
W + ≥2 Jets DataVECBOS W + 1 Jet
10-1
1
10
10 2
0 50 100 150 200 250 300 350 400Mjj of Leading Two Jets (GeV/c2)
Eve
nts
/ (10
GeV
/c2 )
W + ≥3 Jets DataVECBOS W + 2 Jets
1
10
10 2
10 3
0 50 100 150 200 250 300 350 400
+ CDF Preliminary Data (108 pb-1)− VECBOS + HERWIG Fragmentation
+ Detector SimulationCTEQ3M, Q2 = < pT >2
(Normalized to data)
Eve
nts
/ (10
GeV
/c2 )
W + ≥2 Jets
10-1
1
10
10 2
0 50 100 150 200 250 300 350 400Mjj of Leading Two Jets (GeV/c2)
Eve
nts
/ (10
GeV
/c2 )
W + ≥3 Jets
CTEQ SS02J. Huston
When good Monte Carlos go bad
Consider W + jet(s) sample
Compare data (Run 0 CDF) toVECBOS+HERPRT (Herwigradiation+hadronization interfaceto VECBOS) normalized to W+Xjets
Starting with W + 1 jet rate indata, Herwig predicts 1 W+ >4 jetevents; in data observe 10
…factor of ~2 every jet
◆ very dependent on kinematicsituation, though
▲ jet ET cuts
▲ center-of-mass energy
▲ etc
#events >1 jet >2 jets >3 jets >4 jet
pT>10 GeV/c
Data 920 213 42 10
VECBOS + HERPRT (Q=<pT>)
W + 1jet 920 178 21 1
W + 2jet ----- 213 43 6
W + 3jet ----- ----- 42 10
VECBOS+HERPRT(Q=mW)
W + 1jet 920 176 24 2
W + 2jet ---- 213 46 6
W + 3 jet ---- ----- 42 7
CTEQ SS02J. Huston
Factors get worse at the LHC
CTEQ SS02J. Huston
Why?
Some reasons given by the experts (Mangano, Yuan,Ilyin…)◆ Herwig (any Monte Carlo) only has collinear part of matrix element for gluon
emission; underestimate for the wide angle emission that leads to widelyseparated jets
◆ phase space: Herwig has ordering in virtualities for gluon emission while thisis not present in exact matrix element calculations; more phase space forgluon emission in exact matrix element calculations
◆ in case of exact matrix element, there are interferences among all of thedifferent diagrams; these interferences become large when emissions takeplace at large angles (don’t know a priori whether interference is positive ornegative)
◆ unitarity of Herwig evolution: multijet events in Herwig will always be afraction of the 2 jet rate, since multijet events all start from the 2-jet hardprocess
▲ all “K-factors” from higher order are missed.
CTEQ SS02J. Huston
Tree Level Calculations
Leading order matrixelement calculationsdescribe multi-bodyconfigurations betterthan parton showers
Many programs existfor calculation of multi-body final states attree-level
◆ References: see Dieterstalk; see Run 2 MCworkshop
CompHep
◆ includes SM Lagrangian and several othermodels, including MSSM
◆ deals with matrix elements squared
◆ calculates leading order 2->4-6 in the finalstate taking into account all of QCD andEW diagrams
◆ color flow information; interface exits toPythia
◆ great user interface
Grace
◆ similar to CompHep
Madgraph
◆ SM + MSSM
◆ deals with helicity amplitudes
◆ “unlimited” external particles (12?)
◆ color flow information
◆ not much user interfacing yet
Alpha + O’Mega
◆ does not use Feynman diagrams
◆ gg->10 g (5,348,843,500 diagrams)
CTEQ SS02J. Huston
Monte Carlo Interfaces
To obtain full predictability for atheoretical calculation, would liketo interface to a Monte Carloprogram (Herwig, Pythia, Isajet)
◆ parton showering (additional jets)
◆ hadronization
◆ detector simulation
Some interfaces already exist
◆ VECBOS->Herwig (HERPRT)
◆ CompHep->Pythia
A general interface accord wasreached at the 2001 LesHouches workshop
All of the matrix elementprograms mentioned will output4-vector and color flowinformation in such a way as tobe universally readable by allMonte Carlo programs
CompHep, Grace, Madgraph,Alpha, etc, etc
->Herwig, Pythia, Isajet
CTEQ SS02J. Huston
Les Houches accords
Les Houches accord #1 (ME->MC)
◆ accord implemented in Pythia 6.2
◆ accord implemented in CompHEP
▲ CDF top dilepton group hasbeen generating ttbar eventswith CompHEP/Madgraph +Pythia
◆ accord implemented in Wbbgen (notyet released)
◆ accord implemented in Madgraph
▲ MADCUP:http://pheno.physics.wisc.edu/Software/MadCUP/}.
▲ MADGRAPH 2: within a fewweeks
◆ work proceeding on Herwig; inrelease 6.5 June 2002
◆ work proceeding on Grace
◆ In AcerMC:hep-ph/0201302
Les Houches accord #2 (pdfs inME/MC)
◆ version of pdf interface has beendeveloped
▲ writeup available now;website will be publicallyavailable next week(http://pdf.fnal.gov)
◆ commitment for beingimplemented in MCFM
CTEQ SS02J. Huston
Les Houches accord #2
Using the interface isas easy as usingPDFLIB (and mucheasier to update)
First version will haveCTEQ6M, CTEQ6L, allof CTEQ6 error pdfsand MRST2001 pdfs
See pdf.fnal.gov
call InitiPDFset(name)
◆ called once at the beginningof the code; name is the filename of external PDF file thatdefines PDF set
call InitPDF(mem)
◆ mem specifies individualmember of pdf set
call evolvePDF(x,Q,f)
◆ returns pdf momentumdensities for flavor f atmomentum fraction x andscale Q
CTEQ SS02J. Huston
Reminder: the big idea:◆ The Les Houches accords will be
implemented in all ME/MC programsthat experimentalists/theorists use
◆ They will make it easy to generatethe multi-parton final states crucial tomuch of the Run 2/HERA/LHCphysics program and to compare theresults from different programs
◆ experimentalists/theorists can allshare common MC data sets
◆ They will make it possible togenerate the pdf uncertainties for anycross sections
CTEQ SS02J. Huston
Les Houches accord
Example: hadronic tt production
1
2
35
t 8b
9W
4t
6b
7W
501
502502
503
503
I ISTUP(I) IDUP(I) MOTHUP(1,I) MOTHUP(2,I) ICOLUP(1,I) ICOLUP(2,I)1 -1 21 (g) 501 5022 -1 21 (g) 503 5013 +2 21 (g) 1 2 503 5024 +2 -6 (t) 3 3 0 5025 +2 6 (t) 3 3 503 06 +1 -5 (b) 4 4 0 5027 +1 -24 (W−) 4 4 0 08 +1 5 (b) 5 5 503 09 +1 24 (W+) 5 5 0 0
The Z0, t, t are given ISTUP=+2, which informs the showering and hadronization generatorto preserve their invariant masses when showering and hadronizing the event.
The definition of a line as ‘color’ or ‘anti-color’ depends on the orientation of the graph.This ambiguity is resolved by defining color and anti-color according to the physical timeorder. A quark will always have its color tag ICOLUP(1,I) filled, but never its anti-color tagICOLUP(2,I). The reverse is true for an anti-quark, and a gluon will always have informationin both ICOLUP(1,I) and ICOLUP(2,I) tags.
Note the difference in the treatment by the parton shower of the above example, and anidentical final state, where the intermediate particles are not specified:
I ISTUP(I) IDUP(I) MOTHUP(1,I) MOTHUP(2,I) ICOLUP(1,I) ICOLUP(2,I)1 -1 21 (g) 501 5022 -1 21 (g) 503 5013 +1 -5 (b) 1 2 0 5024 +1 -24 (W−) 1 2 0 05 +1 5 (b) 1 2 503 06 +1 24 (W+) 1 2 0 0
6
Generic User Process Interface for Event Generators
E. Boos, M. Dobbs, W. Giele, I. Hinchliffe, J. Huston, V. Ilyin,J. Kanzaki, K. Kato, Y. Kurihara, L. Lonnblad, M. Mangano, S. Mrenna,
F. Paige, E. Richter-Was, M. Seymour, T. Sjostrand, B. Webber, D. Zeppenfeld
September 7, 2001
Abstract
Generic Fortran common blocks are presented for use by High Energy Physicsevent generators for the transfer of event configurations from parton level generatorsto showering and hadronization event generators.
Modularization of High Energy Particle Physics event generation is becoming increasinglyuseful as the complexity of Monte Carlo programs grows. To accommodate this trend, severalauthors of popular Monte Carlo and matrix element programs attending the Physics at TeVColliders Workshop in Les Houches, 2001 have agreed on a generic format for the transfer ofparton level event configurations from matrix element event generators (MEG) to showeringand hadronization event generators (SHG).
CompHEP [1]Grace [2] Herwig [6]
MadGraph [3] ⇒ Isajet [7]VecBos [4] Pythia [8]
WbbGen [5] . . .. . .
Events generated this way are customarily called user (or user-defined) processes, todistinguish them from the internal processes that come with the SHG. Specific solutionsare already in use, including an interface of WbbGen with Herwig [9] and an interface ofCompHEP with Pythia [10]—that experience is exploited here.
An implementation of the user process interface has been included in Pythia 6.2, described(with an example) in Ref. [11].
The user process interface discussed here is not intended as a replacement for HEPEVT [12],which is the standard Fortran common block for interface between generators and analy-sis/detector simulation. The user process common blocks address the communication be-tween two event generators only, a MEG one and a SHG one, and not the communicationof event generators with the outside world.
In the course of a normal event generation run, this communication occurs at two stages:(1) At initilization, to establish the basic parameters of the run as a whole. (2) For eachnew event that is to be transferred from the MEG to the SHG. Each of these two stages
1
hep-ph/0109068
CTEQ SS02J. Huston
Parton Showering
Determination of the Higgs signalrequires an understanding of the Higgs pTdistribution at both LHC and Tevatron
◆ for example, for gg->HX->γγX, the shape of thesignal pT distribution is harder than that of the γγbackground; this can be used to advantage
To reliably predict the Higgs pTdistribution, especially for low to mediumpT region, have to include effects of softgluon radiation
◆ can either use parton showering a la Herwig,Pythia, ISAJET or kT resummation a la ResBos
◆ parton showering resums primarily the (universal)leading logs while an analytic kT resummation canresum all logs with Q2/pT
2 in their arguments; butexpect predictions to be similar and Monte Carlosoffer a more useful format
Where possible it’s best to compare pTpredictions to a similar data set to insurecorrectness of formalism; if data is notavailable, compare MC’s to a resummedcalculation or at least to another MonteCarlo
◆ all parton showers are notequal
Note the large difference between PYTHIA versions5.7 and 6.1. Which one is correct?
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50 60 70 80 90 1
gg → H + X at LHCmH = 150 GeV, CTEQ4M, √s = 14 TeV
pT (GeV
dσ/d
p T (
pb/G
eV)
PYTHIA 5.7 default
PYTHIA 6.122 default
PYTHIA 6.122 Q2max=s
10-3
10-2
10-1
0 20 40 60 80 100 120 140 160 180 2
pT (GeV
dσ/d
p T (
pb/G
eV)
CTEQ SS02J. Huston
Change in PYTHIA
Older version of PYTHIA has more events atmoderate pT
Two changes from 5.7 to 6.1
◆ A cut has been placed on the combination of zand Q2 values in a branching: u=Q2-s(1-z)<0where s refers to the subsystem of hardscattering plus shower partons
▲ corner of emissions that do not respect thisrequirement occurs when Q2 value of space-likeemitting parton is little changed and z value ofbranching is close to unity
▲ necessary if matrix element corrections are to bemade to process
▲ net result is substantial reduction in amount of gluonradiation
▲ In principle affects all processes; in practice onlygg initial states
◆ Parameter for minimum gluon energy emitted inspace-like showers is modified by extra factorcorresponding to 1/γ factor for boost to hard
subprocess frame
▲ result is increase in gluon radiation
The above are choices, not bugs; which versionis more correct?
->Compare to ResBos
u^
QT=√(1-z) Q2
u^ cut
no u^ cut
s^/MW
2
t^ /MW2
s^/MW
2
0
1000
2000
3000
4000
5000
6000
7000
-10000 -5000 0 50000
500
1000
1500
2000
2500
3000
0 20 40 60 80
-1.5
-1.25
-1
-0.75
-0.5
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1 1.5 2 2.5 3-1.5
-1.25
-1
-0.75
-0.5
-0.25
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0.25
0.5
1 1.5 2 2.5 3
S. Mrenna80 GeV Higgs generated at the Tevatron with Pythia
CTEQ SS02J. Huston
Comparison of PYTHIA and ResBos for Higgs Production at LHC
ResBos agrees much better with themore recent version of PYTHIA
◆ Suppression of gluon radiationleading to a decrease in the averagepT of the produced Higgs
◆ Affects the ability of CMS to chooseto the correct vertex to associate withthe diphoton pair
Note that PYTHIA does not describethe high pT end well unless Qmax
2 isset to s (14 TeV)
◆ Again, ResBos has the correct matrixelement matching at high pT; settingQmax
2=s allows enough additionalgluon radiation to mimic the matrixelement
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50 60 70 80 90 100
gg → H + X at LHCmH = 150 GeV, CTEQ4M, √s = 14 TeV
ResBos 98.07.14
pT (GeV)
dσ/d
p T (
pb/G
eV)
PYTHIA 5.7 default
PYTHIA 6.122 default
PYTHIA 6.122 Q2max=s
10-3
10-2
10-1
0 20 40 60 80 100 120 140 160 180 200pT (GeV)
dσ/d
p T (
pb/G
eV)
CTEQ SS02J. Huston
Comparisons with Herwig at the LHC
HERWIG (v5.6) similar in shape inPYTHIA 6.1 (and perhaps even moresimilar in shape to ResBos)
Is there something similar to the u-hat cut that regulates the HERWIGbehavior?
◆ Herwig treatment of colorcoherence? 0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50 60 70 80 90 10
gg → H + X at LHCmH = 150 GeV, CTEQ4M, √s = 14 TeV
ResBos 98.07.14
pT (GeV)
dσ/d
p T (
pb/G
eV)
PYTHIA 6.122
HERWIG 5.6
10-3
10-2
10-1
0 20 40 60 80 100 120 140 160 180pT (GeV)
dσ/d
p T (
pb/G
eV)
CTEQ SS02J. Huston
Logs that we know and love
A1, B1 and (a bit of) A2 areeffectively in Monte Carlos(especially Herwig)
A1,A2 and B1 for Higgsproduction are in currentoff-the-shelf version ofResBos
◆ …as are C0 and C1which control the NLOnormalization
The B2 term has recentlybeen calculated for gg->H
C. Balazs, Higgs production with soft-gluon effects, les Houches ’99 14
CTEQ SS02J. Huston
Study of gg->Higgs for different masses anddifferent energies
Outgrowth of previous LesHouches work
◆ C. Balazs, J. Huston, I.Puljak; Phys. Rev. D63(2001) 014021; hep-ph/0002032.
Probe Higgs production fordifferent kinematic regions
◆ most difficult case for partonshowering (gg)
◆ important process
Try to understand whetherimprovement in Pythia isuniversal and what theunderlying
0 10 20 30 40 50 60 70 80 90 1000
0.005
0.01
0.015
0.02
0.025
0.03
pT (GeV)0 10 20 30 40 50 60 70 80 90 100
dp
T (
pb
/Gev
)⁄
dσ
0
0.005
0.01
0.015
0.02
0.025
0.03
0 20 40 60 80 100 120 140 160 180 20010
-6
10-5
10-4
10-3
10-2
pT (GeV)0 10 20 30 40 50 60 70 80 90 100
dp
T (
pb
/Gev
)⁄
dσ
0
0.005
0.01
0.015
0.02
0.025
0.03
gg → H + X at TevatronmH = 125 GeV, CTEQ5M, √s = 1.96 TeVcurves: ResBos NLL: B(1,2), A(1,2), C(0,1) LL: B(1), A(1), C(0)histograms: PYTHIA 5.7 PYTHIA 6.136 PYTHIA 6.203 HERWIG 6.3
pT (GeV)0 20 40 60 80 100 120 140 160 180 200
dp
T (
pb
/Gev
)⁄
dσ
10-6
10-5
10-4
10-3
10-2
CTEQ SS02J. Huston
mH=125 GeV at 14 and 40 TeV
0 10 20 30 40 50 60 70 80 90 1000
0.5
1
1.5
2
2.5
3
3.5
pT (GeV)0 10 20 30 40 50 60 70 80 90 100
dp
T (
pb
/Gev
)⁄
dσ
0
0.5
1
1.5
2
2.5
3
3.5
0 20 40 60 80 100 120 140 160 180 20010
-3
10-2
10-1
1
pT (GeV)0 10 20 30 40 50 60 70 80 90 100
dp
T (
pb
/Gev
)⁄
dσ
0
0.5
1
1.5
2
2.5
3
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CTEQ SS02J. Huston
Rescale to make up for lost high pT cross section
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3.5gg → H + X at LHCmH = 125 GeV, CTEQ5M, √s = 40 TeVcurves: ResBos NLL: B(1,2), A(1,2), C(0,1) LL: B(1), A(1), C(0)histograms: PYTHIA 5.7 PYTHIA 6.136 PYTHIA 6.203 HERWIG 6.3
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•Herwig agrees almost exactly with ResBos LL
CTEQ SS02J. Huston
Absolute normalizations
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gg → H + X at TevatronmH = 125 GeV, CTEQ5M, √s = 1.96 TeVcurves: ResBos NLL: B(1,2), A(1,2), C(0,1) LL: B(1), A(1), C(0)histograms: PYTHIA 5.7 PYTHIA 6.136 PYTHIA 6.203 HERWIG 6.3
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CTEQ SS02J. Huston
mH = 500 GeV
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gg → H + X at LHCmH = 500 GeV, CTEQ5M, √s = 14 TeVcurves: ResBos NLL: B(1,2), A(1,2), C(0,1) LL: B(1), A(1), C(0)histograms: PYTHIA 5.7 6.136 6.203
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gg → H + X at LHCmH = 500 GeV, CTEQ5M, √s = 40 TeVcurves: ResBos NLL: B(1,2), A(1,2), C(0,1) LL: B(1), A(1), C(0)histograms: PYTHIA 5.7 6.136 6.203
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CTEQ SS02J. Huston
The need for higher order…
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LONLO
NNLO
arX
iv:h
ep-p
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0606
96
Jun
2001
CTEQ SS02J. Huston
What would we like?
Bruce Knuteson’s wishlist from the Run 2 Monte Carlo workshop
…all at NLO
CTEQ SS02J. Huston
What are we likely to get?
CTEQ SS02J. Huston
MCFM (Monte Carlo for FemtobarnProcesses) J. Campbell and K.Ellis
Goal is to provide a unifieddescription of processes involvingheavy quarks, leptons and missingenergy at NLO accuracy
There have so far been three main applicationsof this Monte Carlo, each associated with adifferent paper.
◆ Calculation of the Wbb background to a WHsignal at the Tevatron.
R.K.Ellis, Sinisa Veseli, Phys. Rev. D60:011501(1999), hep-ph/9810489.
◆ Vector boson pair production at the Tevatron,including all spin correlations of the boson decayproducts.
J.M.Campbell, R.K.Ellis, Phys. Rev. D60:113006(1999), hep-ph/9905386.
◆ Calculation of the Zbb and other backgrounds to aZH signal at the Tevatron.
J.M.Campbell, R.K.Ellis, FERMILAB-PUB-00-145-T, June 2000, hep-ph/0006304.
The last of these references contains the mostdetails of our method.
CTEQ SS02J. Huston
Higgs backgrounds using MCFM
CTEQ SS02J. Huston
Wbbar and Zbbar
0
0.5
1
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2
2.5
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dσ /
dMbb
[fb/
GeV
]
Mbb [GeV] (with Gaussian smearing of 10 GeV)
pp− → νe e+ bb− + X
√s=2 TeV MRS98
NLO
LO
CTEQ SS02J. Huston
Recent example of data vs Monte Carlo
There is a discovery potential atthe Tevatron during Run 2 for arelatively light Higgs (especially ifHiggs mass is 115 GeV)
…but small signal to backgroundratio makes understanding ofbackgrounds very important
CDF and ATLAS recently wentthrough similar exercisesregarding this background
◆ CDF using Run 1 data
◆ ATLAS using Monte Carlopredictions
ν
b
b-
ν
CTEQ SS02J. Huston
Data vs Monte Carlo
CTEQ SS02J. Huston
Sleuth strategy
Consider recent major discoveries inhep
◆ W,Z bosonsCERN 1983
◆ top quark Fermilab 1995
◆ tau neutrino Fermilab 2000
◆ Higgs Boson? CERN 2000
In all cases, predictions weredefinite, aside from mass
Plethora of models that appear dailyon hep-ph
Is it possible to perform a “generic”search?
Transparencies from Bruce Knuteson talk at Moriond 2001
CTEQ SS02J. Huston
W2j
We consider exclusive final statesWe consider exclusive final states
We assume the existence of standard object definitions
These define e, _, ττττ, γγγγ, j, b, ET, W, and Z fi
All events that contain the same numbers of each of theseobjects belong to the same final state
Step 1: Exclusive final statesSleuth Bruce Knuteson
e_ET
Z4j
eET jj eE
T 3jW3j eeγγγγeγγγγγγγγ
ZγγγγWγγγγγγγγ
__jj e_ET j
γγγγγγγγγγγγ ___eee
CTEQ SS02J. Huston
Results
Results agree well with expectationNo evidence of new physics is observed
DØ data
Search for regions of excess (more data Search for regions of excess (more data events than expected from background)events than expected from background) within that variable space within that variable space
probability to be SM
CTEQ SS02J. Huston
Fragmentation Uncertainties: Higgs->γγ and
Backgrounds
One of the most useful searchmodes for the discovery of theHiggs in the 100-150 GeV massrange at the LHC is in the twophoton mode
Higgs->γγ has very large
backgrounds from QCD sources
◆ Diphoton production
◆ γπo and πoπo production; jetsfragmenting into very high z πo’s
With excellent diphoton massresolution, can try to resolveHiggs “bump”
Still important to understand levelof background
10000
12500
15000
17500
20000
105 120 135
mγγ (GeV)
Eve
nts
/ 2 G
eV
0
500
1000
1500
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mγγ (GeV)
Sign
al-b
ackg
roun
d, e
vent
s / 2
GeV
CTEQ SS02J. Huston
Diphoton Backgrounds in ATLAS
Again, for a H->γγ search at the
LHC, face irreducible backgrounds
from QCD γγ and reducible
backgrounds from γπo and πoπo
◆ in range from 70 to 170 GeV
jet-jet cross section is estimated
to be a factor of 2E6 times the γγ
cross section and γ-jet a factor
of 8E2 larger
◆ Need rejection factors of 2E7 and
8E3 respectively
◆ PYTHIA results seem to indicate
that reducible backgrounds are
comfortably less than reducible
ones
◆ …but how to normalize PYTHIApredictions for very high z fragmentation ofjets; fragmentation not known well at highz and certainly not for gluon jets
ATLAS detector and physics performance Volume IITechnical Design Report 25 May 1999
from higher-order corrections and from uncertainties in jet fragmentation. In order to reducethese backgrounds to a level well below that of the irreducible γγ continuum, rejection factors of2x107 and 8x103 are required.
These rejection factors have been realisticallyevaluated by using large samples of fully sim-ulated two-jet events, as described inSection 7.6. The rejection factors were then ap-plied to estimate the cross-sections for the re-ducible jet-jet and γ-jet backgrounds relative tothe irreducible γγ-background. The results areshown in Figure 19-2 as a function of the two-photon invariant mass mγγ. After applying thefull photon identification cuts from the calo-rimeter and the Inner Detector, the residualjet-jet and γ-jet backgrounds are found to be atthe level of approximately 15% and 20%, re-spectively, of the irreducible γγ backgroundover the mass range relevant to theH → γγ search.
Z → ee background
h
Figure 19-2 Expected ratios of the residual reduciblejet-jet and γ-jet backgrounds to the irreducible γγ-con-tinuum background as a function of the invariant massof the pair of photon candidates at high luminosity.
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CTEQ SS02J. Huston
Different models predict different high z fragmentation
Backgrounds to γγ production inHiggs mass region arise fromfragmentation of jets to high z πo’s
◆ Pythia and Herwig predict verydifferent rates for high z
◆ all fragmentation is not equal
Example of a background that canbe measured in situ, but nice to beable to predict the environmentbeforehand
DIPHOX (see Run 2 MC workshop)program can calculate γγ, γπo, andπoπo cross sections to NLO
◆ comparisons underway to Tevatrondata
gg->γγ and qqbar->γγ at NNLO maybe available soon
10-3
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1
10
10 2
(1/N
Jet)
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h)
Y topology
Quark jetsGluon jets
DELPHI
00.20.40.60.8
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xE(ch)
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on/Q
uark
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xE(ch)
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on/Q
uark
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Jet)
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Ariadne 4.08
00.20.40.60.8
11.21.4
0 0.2 0.4 0.6 0.8
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xE(ch)
Glu
on/Q
uark
10-3
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(1/N
Jet)
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x E(c
h)
Herwig 5.8C
00.20.40.60.8
11.21.4
0 0.2 0.4 0.6 0.8
Gluon / Quark
xE(ch)
Glu
on/Q
uark
B. Webber, hep-ph/9912399
CTEQ SS02J. Huston
PDF Uncertainties
What’s unknown aboutPDF’s
◆ the gluon distribution
◆ strange and anti-strangequarks
◆ details in the {u,d} quarksector; up/downdifferences and ratios
◆ heavy quarkdistributions
Σ of quark distributions (q + qbar) is well-
determined over wide range of x and Q2
◆ Quark distributions primarily determinedfrom DIS and DY data sets which havelarge statistics and systematic errors infew percent range (±3% for 10-4<x<0.75)
◆ Individual quark flavors, though may haveuncertainties larger than that on the sum;important, for example, for W asymmetry
information on dbar and ubar comes atsmall x from HERA and at medium x fromfixed target DY production on H2 and D2
targets
◆ Note dbar≠ubar
strange quark sea determined fromdimuon production in ν DIS (CCFR)
d/u at large x comes from FT DYproduction on H2 and D2 and leptonasymmetry in W production
CTEQ SS02J. Huston
Example:Jets at the Tevatron
Both experiments compare to NLO QCDcalculations
◆ D0: JETRAD, modified Snowmassclustering(Rsep=1.3, µF=µR=ETmax/2
◆ CDF: EKS, Snowmass clustering(Rsep=1.3 (2.0 in some previouscomparisons), µF=µR=ETjet/2
•In Run 1a, CDF observed an excess in thejet cross section at high ET, outside the range of the theoretical uncertainties shown
CTEQ SS02J. Huston
Similar excess observed in Run 1B
CTEQ SS02J. Huston
Exotic explanations
Transverse Energy
Cross section for various compositeness scales ΛC
1
1.5
2
2.5
3
3.5
4
4.5
5
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Rat
io
Λ1300/Λ∞
Λ1400/Λ∞
Λ1500/Λ∞
Λ1600/Λ∞
Λ1700/Λ∞
Data 92/Λ∞
CDF Preliminary
Only statistical errors are plotted
CTEQ SS02J. Huston
Non-exotic explanations
Modify the gluon distribution at high x
CTEQ SS02J. Huston
Tevatron Jets and the high x gluon
Best fit to CDF and D0 central jet cross sections provided byCTEQ5HJ pdf’s
50 100 150 200 250 300 350 400pT
0
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1.2
1.4
Incl. Jet : pt7 * dσ/dpt
(Error bars: statistical only)
14% < Corr. Sys. Err. < 27%
Ratio: Prel. data / NLO QCD (CTEQ5M | CTEQ5HJ)
norm. facor :CTEQ5HJ: 1.04CTEQ5M : 1.00
CDF
Data / CTEQ5MCTEQ5HJ / CTEQ5MCDF Data ( Prel. )CTEQ5HJCTEQ5M
100 200 300 400 pT
0
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1.4
Incl. Jet : pt7 * dσ/dpt
Error bars: statistical only
8% < Corr. Sys. Err. < 30%
Ratio: Data / NLO QCD (CTEQ5M | CTEQ5HJ)
χ2 = norm. factor :CTEQ5HJ: 25/24 1.08CTEQ5M : 24/24 1.04
D0
Data / CTEQ5MCTEQ5HJ / CTEQ5MD0 dataCTEQ5HJCTEQ5M
CTEQ SS02J. Huston
D0 jet cross section as function of rapidity
10-1
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10 8
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ET (GeV)
⟨⟨⟨⟨ dσσσσ /
/// dE
T dηηηη⟩⟩⟩⟩
(fb/
GeV
) 0.0 ≤≤≤≤ ||||ηηηη|||| <<<< 0.5 0.5 ≤≤≤≤ ||||ηηηη|||| <<<< 1.0 1.0 ≤≤≤≤ ||||ηηηη|||| <<<< 1.5 1.5 ≤≤≤≤ ||||ηηηη|||| <<<< 2.0 2.0 ≤≤≤≤ ||||ηηηη|||| <<<< 3.0
DØ PreliminaryDØ Preliminary√√√√↵↵↵↵��������
���� ====
−−−−
����1
pb 92 Ldt √√√√↵↵↵↵��������
���� ====
−−−−
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pb 92 LdtRun 1BRun 1B
Nominal cross sections & statistical errors only
-1
0
1
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0
1
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JETRADµ=ET
max/2
CTEQ4HJ provides bestdescription of data
How reliable is NLO theory inthis region?K-factors?
CTEQ SS02J. Huston
Chisquares for recent pdf’s
PDF χχχχ2222 χχχχ2222//// dof Prob
CTEQ3M 121.56 1.35 0.01
CTEQ4M 92.46 1.03 0.41
CTEQ4HJ 59.38 0.66 0.99
MRST 113.78 1.26 0.05
MRSTgD 155.52 1.73 <0.01
MRSTgU 85.09 0.95 0.631
10
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10 4
10 5
10 6
10 7
50 100 150 200 250 300 350 400 450 500
0.0 ≤ |η| < 0.50.5 ≤ |η| < 1.01.0 ≤ |η| < 1.51.5 ≤ |η| < 2.02.0 ≤ |η| < 3.0
•For 90 data points, are the chisquaresfor CTEQ4M and MRSTgU “good”?•Compared to CTEQ4HJ?
CTEQ SS02J. Huston
D0 jet cross section
CTEQ4 and CTEQ5 had CDFand D0 central jet cross sectionsin fit
Statistical power not greatenough to strongly influence highx gluon
◆ CTEQ4HJ/5HJ required aspecial emphasis to be given tohigh ET data points
Central fit for CTEQ6 is naturallyHJ-like
χ2 for CDF+D0 jet data is 113 for
123 data points
dataΗ theory theory versus pT GeVerror bars Η stat.Η syst.
100 200 300 400 500
Η 0.5
0
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1
1.52ΗΗΗ3
100 200 300 400 500
Η 0.5
0
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1
1.51.5ΗΗΗ2
100 200 300 400 500
Η 0.5
0
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1.51ΗΗΗ1.5
100 200 300 400 500
Η 0.5
0
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1.50.5ΗΗΗ1
100 200 300 400 500
Η 0.5
0
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1
1.50ΗΗΗ0.5
Figure 19: Comparison between theory and the D0 jet data. The error bars are statistical
and systematic errors combined.
33
CTEQ SS02J. Huston
PDF Uncertainties: included in CTEQ6M sets inLHAPDF
Use Hessian technique (T=10)
CTEQ SS02J. Huston
Gluon Uncertainty
Gluon is fairly well-constrained up to an x-value of 0.3
New gluon is stiffer thanCTEQ5M; not quite as stiffas CTEQ5HJ
CTEQ SS02J. Huston
Luminosity function uncertainties at the Tevatron
102
√s (GeV)
Luminosity function at TeV RunII
Frac
tiona
l Unc
erta
inty
0.1
0.1
0.1
-0.1
-0.1
-0.1
0.1
Q-G --> Z
0
0
0
^
≈
≈ ≈
≈
200 405020
-0.1
0
≈
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Q-G --> W-
Q-G --> γ*
≈
102
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Luminosity function at TeV RunII
Frac
tiona
l Unc
erta
inty
0.1
0.1
0.1
-0.1
-0.1
-0.2
0.2
Q-Q --> W+ (W-)
Q-Q --> γ* (Z)
G-G
0
0
0
^
≈
≈ ≈
≈−
−
0.3
0.4
200 4005020
±
CTEQ SS02J. Huston
Luminosity Function Uncertainties at the LHC
102 10
√s (GeV)
Luminosity function at LHC
Frac
tiona
l Unc
erta
inty
0.1
0.1
0.1
-0.1
-0.1
-0.1
0.1
0
0
0
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-0.1
Q-G --> W+
Q-G --> W-
Q-G --> Z
Q-G --> γ*
^50 500200
≈ ≈
≈≈
≈≈
102 103
√s (GeV)
Luminosity function at LHC
Frac
tiona
l Unc
erta
inty
0.1
0.1
0.1
-0.1
-0.1
-0.2
0.2
Q-Q --> W+
Q-Q --> W-
G-G
0
0
0
≈
≈ ≈
≈−
−
±
50 200 500
^
CTEQ SS02J. Huston
Effective use of pdf uncertainties
PDF uncertainties are important both for precision measurements (W/Z crosssections) as well as for studies of potential new physics (a la jet cross sections athigh ET)
Most Monte Carlo/matrix element programs have “central” pdf’s built in, or caneasily interface to PDFLIB
Determining the pdf uncertainty for a particular cross section/distribution mightrequire the use of many pdf’s
◆ CTEQ Hessian pdf errors require using 33 pdf’s
◆ GKK on the order of 100
Too clumsy to attempt to includes grids for calculation of all of these pdf’s withthe MC programs
->Les Houches accord #2◆ Each pdf can be specified by a few lines of information, if MC programs can perform
the evolution
◆ Fast evolution routine will be included in new releases to construct grids for each pdf
NB: pdf uncertainties make most sense in the context of NLOcalculations; current MC programs are basically leading order and LOpdfs should be used when available
◆ NNB: CTEQ6L is a leading order fit to the data but using the 2-loop αs, since somehigher order corrections are in MC programs like Pythia, Herwig, etc
CTEQ SS02J. Huston
Conclusions
Great opportunity at Run 2 at theTevatron for discovery of new physics;even better opportunity when the LHCturns on
In order to be believeable, we mustunderstand the QCD backgrounds to anynew physics
◆ Don’t rely totally on Monte Carlosand certainly not on one Monte Carloalone
◆ In the words of Ronald Reagan,“Trust but Verify”, if possible,theoretical predictions/formalismswith data
▲ existing Run 1 data/Run 2 data
▲ background” data to be taken at theLHC
If no data, then verify with more completetheoretical treatments
Many new tools/links between old toolsare now being developed to make this jobeasier for experimenters
Hopefully, we’ll find many more of thetype of the event on the right to try them
t