QCD Backgrounds to New Physics II

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

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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

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10 3

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(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

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(a)

(b)

(c)

(d)

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− VECBOS + HERWIG Fragmentation+ Detector Simulation

CTEQ3M, Q2 = < pT >2


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

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Eve

nts

/ 0.5


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+ CDF PreliminaryData (108 pb-1)

− VECBOS + HERWIG+ Detector SimulationCTEQ3M, Q2 = < pT >2


Eve

nts

/ 0.5

W + ≥2 Jets

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Eve

nts

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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

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10 2

10 3

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+ CDF Preliminary Data (108 pb-1)− VECBOS + HERWIG Fragmentation

+ Detector SimulationCTEQ3M, Q2 = < pT >2


Eve

nts

/ (10

GeV

/c2 )


10-1

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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

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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


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

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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

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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

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1000

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7000

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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

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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

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10-1

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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

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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

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dp

T (

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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

pT (GeV)0 20 40 60 80 100 120 140 160 180 200

dp

T (

pb

/Gev

)⁄

dσ

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CTEQ SS02J. Huston

mH=125 GeV at 14 and 40 TeV

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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

pT (GeV)0 20 40 60 80 100 120 140 160 180 200

dp

T (

pb

/Gev

)⁄

dσ

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CTEQ SS02J. Huston

Rescale to make up for lost high pT cross section

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•Herwig agrees almost exactly with ResBos LL

CTEQ SS02J. Huston

Absolute normalizations

0 10 20 30 40 50 60 70 80 90 1000

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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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1 gg → H + X at LHCmH = 125 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 PYTHIA 6.136 PYTHIA 6.203 HERWIG 6.3

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CTEQ SS02J. Huston

mH = 500 GeV

0 20 40 60 80 100 120 140 160 180 2000

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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…

0

0.2

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1

0 0.5 1 1.5 2

LONLO

NNLO

arX

iv:h

ep-p

h/01

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

1.5

2

2.5

0 20 40 60 80 100 120 140 160

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.

0

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jet-jet/γγ

mγγ (GeV)

Rat

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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

11.21.4

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xE(ch)

Glu

on/Q

uark

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h)

Jetset 7.4

00.20.40.60.8

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0 0.2 0.4 0.6 0.8

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xE(ch)

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on/Q

uark

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(1/N

Jet)

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x E(c

h)

Ariadne 4.08

00.20.40.60.8

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0 0.2 0.4 0.6 0.8

Gluon / Quark

xE(ch)

Glu

on/Q

uark

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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

0 50 100 150 200 250 300 350 400 45

Rat

io

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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

0.2

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0.6

0.8

1

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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0.6

0.8

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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

1

10

10 2

10 3

10 4

10 5

10 6

10 7

10 8

0 100 200 300 400 500

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 √√√√↵↵↵↵��

�� ====

−−−−

��1

pb 92 LdtRun 1BRun 1B

Nominal cross sections & statistical errors only

-1

0

1

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0

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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

10 2

10 3

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

0.5

1

1.52ΗΗΗ3

100 200 300 400 500

Η 0.5

0

0.5

1

1.51.5ΗΗΗ2

100 200 300 400 500

Η 0.5

0

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1

1.51ΗΗΗ1.5

100 200 300 400 500

Η 0.5

0

0.5

1

1.50.5ΗΗΗ1

100 200 300 400 500

Η 0.5

0

0.5

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

≈

Q-G --> W+

Q-G --> W-

Q-G --> γ*

≈

102

√s (GeV)

Luminosity function at TeV RunII

Frac

tiona

l Unc

erta

inty

0.1

0.1

0.1

-0.1

-0.1

-0.2

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

0

-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

Documents

QCD Backgrounds to New Physics II