Post on 13-Jan-2016
St. Andrews, ScotlandJ. Huston
QCD Backgrounds to New Physics IJ. Huston
…thanks to Weiming Yao, John Campbell, Bruce KnutesonNigel Glover for transparencies
St. Andrews, ScotlandJ. Huston
Home again
Houston Renfrewshire
A large village located 6 miles (9.6 km) north-west of Paisley, the name derives from 'Hew's town', a 13th century village around the castle of Hugo de Kilpeter. In 1781 the castle was partly demolished and a New Town of 35 houses was erected. The Crosslee Mill (1793, demolished 1986) was used for cotton spinning but in 1916-17 a new factory was constructed for 'spinning cordite fuses'. The Mercat Cross has a sundial from the early 18th century and a shaft which may be 14th century; Houston House (17th century) and Barochan House (17th century and
1896) are other notable buildings.
don’t worry about the extra o
St. Andrews, ScotlandJ. Huston
Run 2 Monte Carlo Workshop
Transparencies, video links to individual talks and links to programs can all be found at http://www-theory.fnal.gov/runiimc/
I’ll be referring to some of these programs in the course of my talk
St. Andrews, ScotlandJ. Huston
Les Houches Two workshops on “Physics at
TeV Colliders” have been held so far, in 1999 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 1999
QCD writeup (hep-ph/0005114) is an excellent pedagogical review for many of the issues discussed here and at this SS
St. Andrews, ScotlandJ. Huston
Other useful references (for pdf uncertainties)
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 Les Houches; hep-ph/0005114
Parton Distributions Working Group, Tevatron Run 2 Workshop; hep-ph/0006300
A QCD Analysis of HERA and fixed target structure function data, 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
Multiplier Method, 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
St. Andrews, ScotlandJ. Huston
Other references…
…to the ability to calculate QCD backgrounds …to the need to do so
St. Andrews, ScotlandJ. Huston
One person was more offended by the book than Carlo Rubbia
“Stirling was a dapper Englishman, in his mid-thirties…”
p. 228
St. Andrews, ScotlandJ. Huston
Monojets in UA1 UA1 monojets (1983-1984)
Possible signature of new physics (SUSY, etc)
A number of backgrounds were identified, but each was noted as being too small to account for the observed signal
pp->Z + jets
|_ pp->W + jets
|_ |_ hadrons +
pp->W + jets
|_ l + pp->W + jets
|_ |_ l +
jet
…but the sum was not “The sum of many small things is a
big thing.” G. Altarelli
Can calculate from first principles or calibrate to observed cross sections for Z->e+e- and W->e
Ellis, Kleiss, Stirling PL 167B, 1986.
St. Andrews, ScotlandJ. Huston
Signatures of New Physics W’s, jets, ’s, b quarks, ET
…pretty much the same as signatures for SM physics How do we find new physics? By showing that it’s 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 of both backgrounds 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 to
H-> For some backgrounds, need to rely on theoretical calculations, e.g. ttbb
backgrounds to ttH
St. Andrews, ScotlandJ. Huston
Theoretical Predictions for New (Old) Physics
There are a variety of programs available for comparison 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 before you 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 correct normalization (but need HO to get it); resummed predictions include non-perturbative effects correctly…may have to be put in by hand in MC’sthreshold kT
W,Z, Higgsdijet, direct
b space(ResBos)
qt space
Where possible, normalize to existing data.…in addition, worry about pdf, fragmentation uncertainties
St. Andrews, ScotlandJ. 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 leading order comparisons using VECBOS (but large scale dependence)
•Good agreement with NLO (and smaller scale dependence) for W + >= 1 jet
St. Andrews, ScotlandJ. Huston
W + jets For W + >=n jet production, typically
use Herwig (Herprt) for additional gluon radiation and for hadronization
Can also start off with n-1 jets and generate additional jets using Herwig
St. Andrews, ScotlandJ. Huston
More Comparisons (VECBOS and HERWIG)
Start with W + n jets from VECBOS •Start with W + (n-1) jets from VECBOS
St. Andrews, ScotlandJ. Huston
More Comparisons Start with W + n jets from VECBOS Start with W + (n-1) jets from
VECBOS
St. Andrews, ScotlandJ. Huston
When good Monte Carlos go bad
Consider W + jet(s) sample Compare data (Run 0 CDF) to
VECBOS+HERPRT (Herwig radiation+hadronization interface to VECBOS) normalized to W+X jets
Starting with W + 1 jet rate in data, Herwig predicts 1 W+ >4 jet events; in data observe 10
…factor of ~2 every jet very dependent on kinematic
situation, 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 4210
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
St. Andrews, ScotlandJ. Huston
Factors get worse at the LHC
St. Andrews, ScotlandJ. 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 widely separated jets
phase space: Herwig has ordering in virtualities for gluon emission while this is not present in exact matrix element calculations; more phase space for gluon emission in exact matrix element calculations
in case of exact matrix element, there are interferences among all of the different diagrams; these interferences become large when emissions take place at large angles (don’t know a priori whether interference is positive or negative)
unitarity of Herwig evolution: multijet events in Herwig will always be a fraction of the 2 jet rate, since multijet events all start from the 2-jet hard process
all “K-factors” from higher order are missed.
St. Andrews, ScotlandJ. Huston
Tree Level Calculations
Leading order matrix element calculations describe multi-body configurations better than parton showers
Many programs exist for calculation of multi-body final states at tree-level
references at beginning of talk; see Run 2 MC workshop
CompHep includes SM Lagrangian and several other
models, including MSSM deals with matrix elements squared calculates leading order 2->4-6 in the final
state taking into account all of QCD and EW diagrams
color flow information; interface exits to Pythia
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)
St. Andrews, ScotlandJ. Huston
An example using CompHep
St. Andrews, ScotlandJ. Huston
Monte Carlo Interfaces To obtain full predictability for a
theoretical calculation, would like to interface to a Monte Carlo program (Herwig, Pythia, Isajet)
parton showering (additional jets) hadronization detector simulation
Some interfaces already exist VECBOS->Herwig (HERPRT) CompHep->Pythia
A general interface accord was reached at the 2001 Les Houches workshop
All of the matrix element programs mentioned will output 4-vector and color flow information in such a way as to be universally readable by all Monte Carlo programs
CompHep, Grace, Madgraph, Alpha, etc, etc
->Herwig, Pythia, Isajet
St. Andrews, ScotlandJ. Huston
Les Houches accord (draft)
St. Andrews, ScotlandJ. Huston
Parton Showering Determination of the Higgs signal
requires an understanding of the Higgs pT distribution at both LHC and Tevatron
for example, for gg->HX->X, the shape of the signal pT distribution is harder than that of the background; this can be used to advantage
To reliably predict the Higgs pT distribution, especially for low to medium pT region, have to include effects of soft gluon 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 can resum all logs with Q2/pT
2 in their arguments; but expect predictions to be similar and Monte Carlos offer a more useful format
Where possible it’s best to compare pT predictions to a similar data set to insure correctness of formalism; if data is not available, compare MC’s to a resummed calculation or at least to another Monte Carlo
all parton showers are not equal
Note the large difference between PYTHIA versions5.7 and 6.1. Which one is correct?
St. Andrews, ScotlandJ. Huston
Change in PYTHIA Older version of PYTHIA has more events at
moderate pT
Two changes from 5.7 to 6.1 A cut has been placed on the combination of z
and Q2 values in a branching: u=Q2-s(1-z)<0 where s refers to the subsystem of hard scattering plus shower partons
corner of emissions that do not respect this requirement occurs when Q2 value of space-like emitting parton is little changed and z value of branching is close to unity
necessary if matrix element corrections are to be made to process
net result is substantial reduction in amount of gluon radiation
In principle affects all processes; in practice only gg initial states
Parameter for minimum gluon energy emitted in space-like showers is modified by extra factor corresponding to 1/ factor for boost to hard subprocess frame
result is increase in gluon radiation
The above are choices, not bugs; which version is more correct?
->Compare to ResBos
S. Mrenna80 GeV Higgs generated at the Tevatron with Pythia
St. Andrews, ScotlandJ. Huston
Comparison of PYTHIA and ResBos for Higgs Production at LHC
ResBos agrees much better with the more recent version of PYTHIA
Suppression of gluon radiation leading to a decrease in the average pT of the produced Higgs
Affects the ability of CMS to choose to the correct vertex to associate with the diphoton pair
Note that PYTHIA does not describe the high pT end well unless Qmax
2 is set to s (14 TeV)
Again, ResBos has the correct matrix element matching at high pT; setting Qmax
2=s allows enough additional gluon radiation to mimic the matrix element
St. Andrews, ScotlandJ. Huston
Comparisons with Herwig at the LHC
HERWIG (v5.6) similar in shape in PYTHIA 6.1 (and perhaps even more similar in shape to ResBos)
Is there something similar to the u-hat cut that regulates the HERWIG behavior?
Herwig treatment of color coherence? (study in progress)
St. Andrews, ScotlandJ. Huston
The need for higher order…
St. Andrews, ScotlandJ. Huston
What would we like?
Bruce Knuteson’s wishlist from the Run 2 Monte Carlo workshop
…all at NLO
St. Andrews, ScotlandJ. Huston
What are we likely to get?
St. Andrews, ScotlandJ. Huston
MCFM (Monte Carlo for Femtobarn Processes) J. Campbell and K.Ellis
Goal is to provide a unified description of processes involving heavy quarks, leptons and missing energy at NLO accuracy
There have so far been three main applications of this Monte Carlo, each associated with a different paper.
Calculation of the Wbb background to a WH signal 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 decay products.
J.M.Campbell, R.K.Ellis, Phys. Rev. D60:113006 (1999), hep-ph/9905386.
Calculation of the Zbb and other backgrounds to a ZH 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 most
details of our method.
St. Andrews, ScotlandJ. Huston
Higgs backgrounds using MCFM
St. Andrews, ScotlandJ. Huston
Wbbar and Zbbar
St. Andrews, ScotlandJ. Huston
Recent example of data vs Monte Carlo
There is a discovery potential at the Tevatron during Run 2 for a relatively light Higgs (especially if Higgs mass is 115 GeV)
…but small signal to background ratio makes understanding of backgrounds very important
CDF and ATLAS recently went through similar exercises regarding this background
CDF using Run 1 data ATLAS using Monte Carlo
predictions
St. Andrews, ScotlandJ. Huston
Data vs Monte Carlo
St. Andrews, ScotlandJ. Huston
Sleuth strategy Consider recent major discoveries in
hep W,Z bosons CERN 1983 top quark Fermilab 1995 tau neutrino Fermilab 2000 Higgs Boson? CERN 2000
In all cases, predictions were definite, aside from mass
Plethora of models that appear daily on hep-ph
Is it possible to perform a “generic” search?
Transparencies from Bruce Knuteson talk at Moriond 2001
St. Andrews, ScotlandJ. Huston
W2j
We consider exclusive final statesWe consider exclusive final states
We assume the existence of standard object definitions
These define e, μ, , , j, b, ET, W, and Z fi
All events that contain the same numbers of each of these objects belong to the same final state
Step 1: Exclusive final statesSleuth Bruce Knuteson
Steps:Steps:
1)1)
eμET
Z4j
eET jj eE
T 3j
W3jeee
ZWμμjj eμE
T j
μμμeee
St. Andrews, ScotlandJ. Huston
Step 2: VariablesSleuth Bruce Knuteson
2) 2) Define variablesDefine variables
What is it we’re looking for?
The physics responsible for EWSB
What do we know about it?
Its natural scale is a few hundred GeV
What characteristics will such events have?
Final state objects with large transverse momentum
What variables do we want to look at?
pT’s
St. Andrews, ScotlandJ. Huston
If the final state contains Then consider the variable
1 or more lepton
1 or more /W/Z
1 or more jet
missing ET
∑ lTp
∑ ZWTp
//γ
TE
∑ jTp
(adjust slightly for idiosyncrasies of each experiment)
Sleuth Bruce Knuteson Step 2: Variables
St. Andrews, ScotlandJ. Huston
Input: 1 data file, estimated backgrounds
transform variables into the unit box define regions about sets of data points
Voronoi diagrams define the “interestingness” of an arbitrary region
the probability that the background within that region fluctuates up to or beyond the observed number of events
search the data to find the most interesting region, R determine P, the fraction of hypothetical similar experiments (hse’s) in
which you would see something more interesting than R Take account of the fact that we have looked in many different places
For each final state . . .
Output: R, P
3) 3) Search for regions of excess (more data events than expected from Search for regions of excess (more data events than expected from background) within that variable spacebackground) within that variable space
Step 3: Search for regions of excessSleuth Bruce Knuteson
St. Andrews, ScotlandJ. Huston
If the data contain no new physics, Sleuth will find to be random in (0,1)
If we find small, we have something interesting
If the data contain new physics, Sleuth will hopefully find to be small
If we find large, is there no new physics in our data?
or have we just missed it?
How sensitive is Sleuth to new physics?
Impossible to answer, in general
(Sensitive to what new physics?)
But we can provide an answer for specific cases
SensitivitySleuth Bruce Knuteson
St. Andrews, ScotlandJ. Huston
tt provides a reasonable sensitivity check [cf. DØ PRL (1997, 125 pb-1)]
in eμET 2j: find > 2 in 25% of an ensemble of mock experiments
[cf. dedicated search: 2.75 (3 events with 0.2 expected)]
in W 4j: find > 3 in 25% of an ensemble of mock experiments
[cf. dedicated search: 2.6 (19 events with 8.7 expected) w/o b-tag]
[cf. dedicated search: 3.6 (11 events with 2.5 expected) w/ b-tag]
Would we have “discovered” top with Sleuth? No. But results are nonetheless encouraging.
Lessons: b-tagging, combination of channels important for top
other sensitivity checks (WW, leptoquarks) give similarly sensible results
SensitivitySleuth Bruce Knuteson
St. Andrews, ScotlandJ. Huston
Results
Results agree well with expectationNo evidence of new physics is observed
DØ data
St. Andrews, ScotlandJ. Huston
Sleuth is a quasi-model-independent search strategy for new high pT physics
Defines final states and variables
Systematically searches for and quantifies regions of excess
Sleuth allows an a posteriori analysis of interesting events
Sleuth appears sensitive to new physics
SleuthSleuth finds no evidence of new physics in DØ data finds no evidence of new physics in DØ data
Sleuth has the potential for being a very useful tool Looking forward to Run IILooking forward to Run II hep-ex/0006011 PRD
hep-ex/0011067 PRDhep-ex/0011071 PRL
ConclusionsConclusionsDD
St. Andrews, ScotlandJ. Huston
Fragmentation: Higgs-> and Backgrounds
One of the most useful search modes for the discovery of the Higgs in the 100-150 GeV mass range at the LHC is in the two photon mode
Higgs-> has very large backgrounds from QCD sources
Diphoton production o and oo production; jets
fragmenting into very high z o’s
With excellent diphoton mass resolution, can try to resolve Higgs “bump”
Still important to understand level of background
St. Andrews, ScotlandJ. Huston
Diphoton Backgrounds in ATLAS Again, for a H-> search at the
LHC, face irreducible backgrounds
from QCD and reducible
backgrounds from o and oo
in range from 70 to 170 GeV
jet-jet cross section is estimated
to be a factor of 2E6 times the cross section and -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 PYTHIA
predictions for very high z fragmentation of jets; fragmentation not known well at high z and certainly not for gluon jets
St. Andrews, ScotlandJ. Huston
Different models predict different high z fragmentation
Backgrounds to production in Higgs mass region arise from fragmentation of jets to high z o’s
Pythia and Herwig predict very different rates for high z
all fragmentation is not equal Example of a background that can
be measured in situ, but nice to be able to predict the environment beforehand
DIPHOX (see Run 2 MC workshop) program can calculate , o, and oo cross sections to NLO
comparisons underway to Tevatron data
gg-> and qqbar-> at NNLO may be available soon
B. Webber, hep-ph/9912399
St. Andrews, ScotlandJ. Huston
PDF Uncertainties
What’s unknown about PDF’s
the gluon distribution strange and anti-strange
quarks details in the {u,d} quark
sector; up/down differences and ratios
heavy quark distributions
of quark distributions (q + qbar) is well-determined over wide range of x and Q2
Quark distributions primarily determined from DIS and DY data sets which have large statistics and systematic errors in few percent range (±3% for 10-4<x<0.75)
Individual quark flavors, though may have uncertainties larger than that on the sum; important, for example, for W asymmetry
information on dbar and ubar comes at small x from HERA and at medium x from fixed target DY production on H2 and D2 targets
Note dbar≠ubar
strange quark sea determined from dimuon production in DIS (CCFR)
d/u at large x comes from FT DY production on H2 and D2 and lepton asymmetry in W production
St. Andrews, ScotlandJ. Huston
Example:Jets at the Tevatron Both experiments compare to NLO QCD
calculations D0: JETRAD, modified Snowmass
clustering(Rsep=1.3, F=R=ETmax/2 CDF: EKS, Snowmass clustering
(Rsep=1.3 (2.0 in some previous
comparisons),F=R=ETjet/2
•In Run 1a, CDF observed an excess in thejet cross section at high ET, outside the range of the theoretical uncertainties shown
St. Andrews, ScotlandJ. Huston
Similar excess observed in Run 1B
St. Andrews, ScotlandJ. Huston
Exotic explanations
St. Andrews, ScotlandJ. Huston
Non-exotic explanationsModify the gluon distribution at high x
St. Andrews, ScotlandJ. Huston
Tevatron Jets and the high x gluon
Best fit to CDF and D0 central jet cross sections provided by
CTEQ5HJ pdf’s
St. Andrews, ScotlandJ. Huston
D0 jet cross section as function of rapidity
ET (GeV)
d2
dE
T d
(
fb/G
eV) 0.0 0.5
0.5 1.0 1.0 1.5 1.5 2.0 2.0 3.0
DØ PreliminaryDØ Preliminary√↵
=
−
1
pb 92 Ldt √↵
=
−
1
pb 92 LdtRun 1BRun 1B
Nominal cross sections & statistical errors only
JETRAD=ET
max/2
CTEQ4HJ provides bestdescription of data
How reliable is NLO theory inthis region?K-factors?
St. Andrews, ScotlandJ. Huston
Chisquares for recent pdf’s
PDF χ2 χ2/ dof ProbCTEQ3M 121.56 1.35 0.01CTEQ4M 92.46 1.03 0.41CTEQ4HJ 59.38 0.66 0.99MRST 113.78 1.26 0.05MRSTgD 155.52 1.73 <0.01MRSTgU 85.09 0.95 0.63
•For 90 data points, are the chisquaresfor CTEQ4M and MRSTgU “good”?•Compared to CTEQ4HJ?
St. Andrews, ScotlandJ. Huston
A new tool PDF uncertainties are important both for precision measurements (W/Z
cross sections) as well as for studies of potential new physics (a la jet cross sections at high ET)
Most Monte Carlo/matrix element programs have “central” pdf’s built in, or can easily interface to PDFLIB
Determining the pdf uncertainty for a particular cross section/distribution might require 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 with the 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
St. Andrews, ScotlandJ. Huston
Conclusions Great opportunity at Run 2 at the Tevatron
for discovery of new physics; even better opportunity when the LHC turns on
In order to be believeable, we must understand the QCD backgrounds to any new physics
Don’t rely totally on Monte Carlos and certainly not on one Monte Carlo alone
In the words of Ronald Reagan, “Trust but Verify”, if possible, theoretical predictions/formalisms with data
existing Run 1 data/Run 2 data background” data to be taken at the
LHC If no data, then verify with more complete
theoretical treatments Many new tools/links between old tools are
now being developed to make this job easier for experimenters
Hopefully, we’ll find many more of the type of the event on the right to try them out on