Monte Carlo Tuning and More In View of the LHC

1
Monte Carlo Tuning (and More) In View of the LHC

• Paolo Bartalini
• EP Division, CERN
• IRES, March 28 - 2003

(Thanks to M.L.Mangano, A.Moraes, N.Brook,
C.Mesropian, V.Tano etc.)
2
OUTLINE

• Monte Carlo Tools for the LHC
• Cross Section Evaluators
• Parton Level MCs
• Shower MCs
• The Underlying Event
• Multiplicity Tuning of Pythia
• Recent CDF Results

3
Final States at the LHC
High Luminosity Golden Channel
4
Final states at the LHC
Goal of MC development for the LHC is to provide
a description as accurate as possible of these
events (and more), as well as of the features of
new physics processes rates, distributions, fine
details of the final states (overall
multiplicities, heavy-quark content)
5
Use and Abuse of MC Simulation

• Use
• benchmarks for the design of detectors, trigger
  and analysis strategies
• tests and measurements of SM
• study of properties of new particles (masses,
  cross-sections, couplings)
• Abuse claims of discoveries!
• top and SUSY discovery in UA1
• Rb at LEP
• quark compositeness at CDF
• Only the benchmarking against real data can turn
  MC simulation into powerful study tools

6
Example H?bb in qq ?Hqq

• bbjj bg is 102 times the signal, but can be
  extracted from data (smooth behaviour under the
  signal peak)
• bg from multiple collisions (jjb ? jjb)
  signal, but peak under the signal! Much more
  sensitive to MC simulation uncertainties!

M.L.Mangano et al., Phys. Lett. B556 (2003) 50.
7
Factorization Theorem

• transition from partonic final state to the
  hadronic observable (hadronization, fragm.
  function, jet definition, etc)
• Sum over all histories with X in them

• sum over all initial state histories leading, at
  the scale Q, to

8
Cross-section Evaluators

• Only some component of the final state is singled
  out for the measurement, all the rest being
  ignored (i.e. integrated over). E.g. pp?ee- X
• No events are generated, only cross-sections
  are evaluated
• Experimental selection criteria (e.g. jet
  definition or acceptance) are applied on
  parton-level quantities. Provided these are
  infrared/collinear finite, it therefore doesnt
  matter what F(X) is, as we assume (fact. theorem)
  that
• Thanks to the inclusiveness of the result, it is
  straightforward to include higher-order
  corrections, as well as to resum classes of
  dominant and subdominant logs

9
State of the Art

• NLO available for
• jet and heavy quarks production
• prompt photon production
• gauge boson pairs
• most new physics processes (e.g. SUSY)
• NNLO available for
• W/Z/DY production
• Higgs production

10
Parton Level (Matrix Element) Mcs

• Parton level configurations (i.E. Sets of quarks
  and gluons) are generated, with probability
  proportional to the respective perturbative M.E.
• Transition function between a final-state parton
  and the observed object (jet, missing energy,
  lepton, etc) is unity
• No need to expand f(x) or F(X) in terms of
  histories, since they all lead to the same
  observable
• Experimentally, equivalent to assuming
• Smart jet clustering (parton ? jet)
• Linear detector response

11
Codes Available For

• W/Z/gamma N jets (N?6)
• W/Z/gamma Q Qbar N jets (N?4)
• Q Qbar N jets (N?4)
• Q Qbar Q Qbar N jets (N?2)
• Q Qbar H N jets (N?3)
• nW mZ kH N jets (nmkN ?8, N?2)
• N jets (N?5)

12
Shower Monte Carlo

• After the generation of a given parton-level
  configuration (typically LO, 2?1 or 2?2) , each
  possible IS and FS parton-level history
  (shower) is generated, with probability defined
  by the shower algorithm (unitary evolution).
• Algorithm numerical, Markov-like evolution,
  implementing within a given appoximation scheme
  the QCD dynamics
• branching probabilities
• infrared cutoff scheme
• hadronization model
• Herwig, Pythia, Isajet

13
Complementarity of the 3 Approaches
14
Brief Guide to Shower MCs

• Evaluate parton-level probability, from Feynman
  rules phase space. E.g.

• As a result of acceleration, q will emit
  radiation
• The probability that radiation will (or will
  not) be emitted is evaluated as a function of the
  acceleration of the colour charges

15
Sudakov

• Generate ?? ?1 ??
• If ?1 lt P(Q , Q0) ? no radiation, q goes
  directly on-shell at scale Q0GeV
• Else
• calculate Q1 / P(Q1,Q0) ?1
• emission at scale Q1
• Go back to 1) and reiterate, until shower stops
  in 2). At each step the probability of emission
  gets smaller and smaller

prob. of no radiation between Q and Q0
1
?2
?1
P
Q
Q0
Q1
Q2
16
Problems Quantum Coherence

?
17
Solution (Angular Ordering)
?(???1?


?(???2?
Drawbacks
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Coherence is Essential to Describe CDF Multijet
Data

• 3 jet distributions in hadronic collisions

Full Coherence
No Coherence
Soft emissions know about beam line (large Y)
Partial Coherence
Pseudorapidity of Gluon Jet
F.Abe et al., PRD 50 (1994) 5562
19
The Programs (Pythia/Isajet/Herwig/Ariadne)

• Isajet
• Q2 ordering with no coherence
• large range of hard processes
• Pythia
• Q2 ordering with veto of non-ordered emissions
• large range of hard processes
• Herwig
• complete color coherence NLO evolution for
  large x
• smaller range of hard processes
• Ariadne
• complete color dipole model (best fit to HERA
  data)
• interfaced to PYTHIA/LEPTO for hard processes

20
Hadronization
At the end of the perturbative evolution, the
final state consists of quarks and gluons,
forming, as a result of angular-ordering,
low-mass clusters of colour-singlet pairs
21
Bottom Line

• Implementation of quantum coherence in shower
  MCs is possible, in the limit of large-Nc and
  for soft and collinear emission.
• Large-angle, hard emission cannot be described
  accurately
• Possible cure requires starting the shower with
  seed multi-parton configurations, evaluated
  using exact (possibly tree-level only) matrix
  elements.
• Potential problems, however, due to double
  counting for extra jet emission

22
What is Minimum Bias ?

• Experimental Definition depends on the Trigger !
  Minimum Bias is usually associated to
  non-single diffractive events (NSD)
• Constitutes unavoidable background prediction of
  radiation levels detector damage occupancy
  etc.
• Theoretical Definition the most popular models
  associate Minimum Bias events to
  non-diffractive inelastic interactions (n.dif)
• stot selas ss.dif sd.dif sn.dif

A.Moraes, MC at Hadron Collider Workshop,
Durham, January 2003.
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The Underlying Event
Contributions from soft events need
modeling (Pythia, Phojet, Herwig)
Minimum Bias events are dominated by soft
interactions, although theres also some
contribution from hard scattering.
The Underlying Event is everything except the
hard component (Jets). ? Contributions from beam
remnant and initialfinal state radiation
The Pythia solution Multiple Parton interactions
T. Sjostrand et al. PRD 36 (1987) 2019
Multiple hard interactions observed by AFS, UA2,
CDF!
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Multiple Parton Interactions in Pythia

• Scenario needed to describe the high multiplicity
  observed at hadron colliders several
  parton-parton interactions within a single
  hadron-hadron collision
• Main free parameters
• Choice of multiple interaction model
• Default model with all hadron collisions
  equivalent (MSTP(82)1)
• Varying impact parameter between the colliding
  hadrons hadronic matter described by a Gaussian
  (MSTP(82)3)
• Minimum PT of hard parton-parton scattering
• ? CONTROLS NUMBER OF INTERACTIONS AND HENCE
  MULTIPLICITY

25
Varying impact parameter model (3) versus default
model (1)

• Varying impact parameter model invented to
  describe the shape of UA5 multiplicity
  distribution PLB 138 (1984) 304

Pythia 6.134 including double diffraction
with Minimum PT tuned to reproduce the mean
multiplicity
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Mean Charged Multiplicity at ? 0

• UA5 data at ?s 53, 200, 546, 900 GeV Z. Phys.
  C 33 (1986) 1
• CDF data at ?s 630, 1800 GeV
  PRD 41 (1989) 2330

dN/d? measured at ? 0 versus ?s CDF fit is
superimposed
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Minimum PT versus? s

• Pythia 6.134, double diffraction included
• For each set of parton distribution functions and
  for each value of ?s, the minimum PT is adjusted
  to reproduce the measured multiplicity
• P. Bartalini, O. Schneider
• CERN 2000-004, pgg 293-300

28
The Multiple Interactions Tuning

• In the context of the Pythia Multiple
  Interactions framework, due to the increasing
  color screening at small x, the post-Hera pdfs
  imply a running PT min

OUR FITS
Pythia defaults for M.I. Model gt 1
Older pdfs ? Regge-like x dependency xg(x,Q2)
const for x 0
New pdfs ? Lipatov-like x dependency xg(x,Q2)
x-? for x 0
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KNO Scaling Koba, Nielsen, Olesen, Nucl. Rev.
B40 (1972) 371
z nch/ltnchgt F(z) ltnchgt?n/(?n ?n)

• The rise of multiple interactions at low x can
  also be used to interpretate the KNO scaling
  violations observed by UA5
• A.Moraes, MC at Hadron Collider Workshop,
  Durham, January 2003

30
Pseudorapidity distribution of charged tracks at
the LHC

• Comparison between prdictions from
• CTEQ4L M.I. Model 3 (tuned)
• CTEQ4L M.I. Model 3 (untuned)
• Predicted value at ?0 with tuned parameters is
  in agreement with phenomenological fit of lower
  energy data

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Minimum Bias Event Shapes at LHCb
No large differences in PT distributions
Neutrals included
32
Minimum Bias Event Shapes at LHCb (b)
b events at LHCb
Differences in ?nch? same change for
b-events More noticeble difference in PT
distributions
33
Underlying Event Herwig versus Pythia
34
Multiple Interactions in Herwig (JIMMY)

• In principle MI not available within HERWIG
• In practice, interface program (JIMMY
  Butterworth, Forshaw Walker) allows MI
  hep-ph/9601371
• Also available, ad-hoc modelling of the soft
  underlying event (SUE based on UA5 model)
• Parameter available for tuning in both JIMMY and
  SUE options.

35
Jimmy versus UA5 Data
- essentially one free parameter the min. pT of
the hard scatt.
As pTmin ? the of scatters decrease
predictions approach UA5 data. Failed to find a
setting that could describe the data. No further
study presented here.
36
Herwig UA5 Minimum Bias Model

• Mean event charged multiplicity chosen according
  to
• 1/k in negative binomial given by
• The mass spectrum of soft clusters derived from
• Soft cluster pT spectra

37
The Underlying Event Studies at CDF
Examines jet event structure from 1 GeV to 50 GeV
looking at toward, away, transverse regions in f
for central rapidities
T.Affolder et al. PRD 65 (2003) 092002
Cone 1
use charged tracks (SpTtracks) and compare
results to MC predictions
38
The Charged Jet Evolution in pp Collisions at
CDF(Toward Region)
_

• Study only the charged particle components of
  jets charged particle jets
• Data Minimum-Bias and Jet20 data
• Examine the properties of the leading charged
  particle jet
• and compare with Monte-Carlo models HERWIG,
  ISAJET, and PYTHIA
• Study the growth and development (evolution) from
  PT1 0.5 GeV/c to 50 GeV/c

Use simple, non-standard, jet definition with
R0.7

• Assign all charged particles (PTgt 0.5 GeV/c) and
  hlt1) to a jet
• Jets contain particles from the underlying event
  as well
• as from outgoing partons
• Even one charged particle can be jet

6 particles 5 jets
39
The Toward Region
QCD hard scattering models agree well with
leading jet observables
Evidence for charged particle clusters in the
MinBias data apparent around PT1 of 2 GeV/c
Jet20 data connects on smoothly to Min-Bias data
The charged particle jets in the Min-Bias data
are continuation of the high transverse momentum
charged jets observed in the Jet20 data.
40
The Transverse Region
Rapid growth and then constant plateau for
PT1gt5GeV/c
Comparison of data with the QCD Monte-Carlo
predictions of HERWIG, ISAJET, and PYTHIA
41
Improving the Pythia Tuning in the Transverse
Region
Mean number of charged tracks in the Transverse
region vs PT of the leading jet compared to MC
results
Good agreement with tuned Pythia 6.206
42
The Transverse Region (PT)
Sum of PT of charged tracks in the Transverse
region vs PT of the leading jet compared to MC
results
43
Pythia - CDF Tuning
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Pythia - Charged Multiplicity at LHCb
45
Pythia - Charged Multiplicity at LHCb (b)
b events at LHCb
46
New Fitting/Tuning Tool

• JetWeb based on HERA HZTOOL package being
  updated to include Minimum Bias data
• J.M.Butterworth and S.Butterworth
  hep-ph/0210404
• also submitted to Comput. Phys. Commun.
• Web page - http//jetweb.hep.ucl.ac.uk/
• Database of data, MC and comparisons
• Web interface allows access to DB and submission
  of jobs to generate MC plots

47
What is JetWeb for?

• Final state in (esp. hadron-hadron) collisions
  poorly understood.
• Hadronization not calculable in perturbative QCD.
• Monte Carlo generators (e.g. Pythia, Herwig) are
  valuable, but have many free parameters.
• How do we know which predictions to trust when
  planning for future colliders?
• Tune to existing data, but which data?Different
  models (fail to) describe different measurements.
• Automate procedure allow comparison of new MC
  (or set of parameters) with experimental results
  stored in a database.

48
HzTool

• Developed in HERA Workshop to enable comparison
  of data with existing and future MC generators.
• Routine written in Fortran for each analysis
  fills HBOOK histograms from generated events to
  compare with measurements.
• Range of data already included H1, ZEUS, UA5,
  OPAL, CDF, D0. Contributing authors also from
  ATLAS and Linear Collider.
• Still need more analyses from more experiments to
  be included.
• Longer-term move to OO framework?

49
What does JetWeb add to HZTOOL?

• Easier access via WWW interface.
• Expanding database of existing data, predictions
  and comparisons.
• Reduces duplication of effort and computing
  resources.
• Scalable design to keep up with new data and
  models.

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JetWeb home page
51
Final Remarks (high-Q2)

• A lot of progress has taken place in the recent
  years, but...
• 30 yrs after QCD, still a lot of work to be done
  to achieve a satisfactory description of all
  high-Q2 processes accessible at LHC
• Most of the key conceptual difficulties have been
  recently, or are being, solved, and their
  implementation into concrete MC schemes should be
  achievable in the next 5 years
• Forthcoming data from Tevatron will help
  improving our tools, but the final test will need
  real LHC data

52
Conclusions (The Underlying Event)

• Comparisons between Pythia and experimental data
  (UA5, CDF) demonstrate that Multiple Interaction
  models are successful in reproducing the charged
  track multiplicity spectrum.
• M.I. varying impact parameter models have to be
  adopted.
• Running PT cut-off in Multiple Interactions is
  mandatory, predictions made at larger energies
  (ex. LHC) with fixed PT cut-off are most likely
  to overestimate the multiplicity observables.
• running PT cut-off is even more important if a
  post HERA set of parton distribution functions
  is used.
• But cannot relay on just one model!

53
Workshop on MCs for the LHC (MC4LHC), july 7 -
aug 2 2003, at CERN

• Web page http//lhc-monte-carlo.web.cern.ch/lhc-mo
  nte-carlo
• Authors of all main programs are expected to come
  for several weeks to CERN, give tutorials,
  discuss new developments, absorb requirements as
  discussed with the users, etc.
• Working groups on special topics, so far there
  are groups defined on
• W production processes
• Tuning of minimum bias events and the underlying
  event
• COMMON CODE REPOSITORIES
• COMMON TOOLS
• COMMON EVENT FILES
• TUNING AND VALIDATION OF EVENT GENERATORS (see
  JetWeb)

54
Backup (Theory)
55
Use of the Fact. Theorem

• The possible histories of initial and final
  state, and their relative probabilities, are in
  principle independent of the hard process (they
  only depend on the flavours of partons involved
  and on the scales Q)
• Once an algorithm is developed to describe IS and
  FS evolution, it can be applied to partonic IS
  and FS arising from the calculation of an
  arbitrary hard process
• Depending on the extent to which different
  possible FS and IS histories affect the value of
  the observable X, different realizations of the
  factorization theorem can be used

56
Power corrections

• Classes of non-perturbative effects linked to
  the dominant power-like (1/Q) corrections can be
  parametrised in terms of a single quantity,
  formally given by

Their effect is expected to be very large even at
the Tevatron, and in general for LHC events with
jets in the few-hundred GeV energy range.

• In the case of 1st moments of shape variables,
  for example
• FFPT Fnon-PT
• Fnon-PTcF P, with
• and
• PP0 ?0(?)-?0(? S) ? /?S

• The impact of these effects at LEP is very
  large, and their understanding is essential for
  any quantitative QCD study

57
M(ontecarlo) o(f) E(verything)
Matrix Element MCs
Cross-Section Evaluators
Shower MCs

• Better treatment of radiation off heavy quarks
• Full treatment of spin correlations in
  production and decay
• Better description of underlying event
• Better decay tables
• ..

58
Backup (Herwig)
59
UA5 Model versus CDF data
current ad-hoc phenomenological model from UA5
CDF data (ltnchgt at ? 0) Good description with
UA5 model
60

• HERWIG Minimum bias model versus CDF
• current ad-hoc phenomenological model from UA5

W A R N I N G No single diffractive events in
HERWIG though CDF claim UA5 model doesnt
describe underlying event in hard scatter Cant
generate b-events from min bias model need to
generate hard 2 ? 2 process
61
UA5 Model versus UA1 data
pmbm1 0.1 pmbm2 9.0 pmbn1 8.0 pmbn2
0.12 pmbn3 -7.0 pmbk1 0.03 pmbk2
0.12 pmbp1 6.5 pmbp2 5.0 pmbp3 7.5
UA5 data at different ?s
62
HERWIG single diffraction
Implement UA5 single diffraction model into HERWIG
Implementation complete need comparisons with
data
63

• HERWIG new developments
• New min bias model MI eikonal model similar to
  PYTHIA
• New model contain the JIMMY hard part but a new
  soft component
• Authors claim reproducibility of the data
• New model should be in next release of HERWIG
• Claimed a solid weeks work for it to be properly
  integrated

64
Backup (CDF)
65
The Underlying Event Studies at CDF
Underlying Event energy beam-beam
remnants part of initial/final state
radiation must be subtracted from jet
energies for comparison with NLO QCD
predictions
Largest uncertainty for low ET
66
The Underlying Event Studies at CDF
Complimentary Analyses
Examines jet event structure from 1 GeV to 50 GeV
looking at toward, away, transverse regions in f
for central rapidities
2p
Cone 1
Examines jet events from 50 GeV to 300 GeV
looking in 2 cones at same h as leading jet, and
at 90o in f away in same central h
Leading Jet
f
Cone 2
0
-1 1
h
Both analyses use charged tracks (SpTtracks) and
compare results to MC predictions
67
Backup (JetWeb)
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JetWeb

• A WWW Interface and Database for Monte Carlo
  Tuning and Validation
• See J. M. Butterworth and S. Butterworth,hep-ph/0
  210404, also submitted to Comput. Phys. Commun.
• Based on HzTool (J. Bromley et al., Future
  Physics at HERA, vol 1, 611-612)
• Database of data, MC and comparisons
• Web interface allows access to DB and submission
  of jobs to generate more MC plots
• http//jetweb.hep.ucl.ac.uk

69
JetWeb Search Form
70
JetWeb Search Results
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A JetWeb Fit
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JetWeb Plots
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The JetWeb Server

• Java object model
• Java servlets running in Tomcat container
• Data underlying model stored in MySQL database.
• MC Model, Logparms, Logfile
• Data Paper, Plot, DataPoint
• Comparison Fit

74
The JetWeb Server
75
JetWeb on the Grid

• Processing power
• Currently submit jobs to separate batch farms at
  Manchester and UCL.
• CPU intensive as use increases, need more power.
• Grid should enable transparent access to a wider
  range of resources.
• Small(ish) self-contained executable run almost
  anywhere.
• Users could submit jobs, using their own
  certificates, to any resource they are entitled
  to use.
• Storage
• Make database accessible as a resource in its own
  right.
• Use Grid mechanisms to mirror data for faster and
  more reliable access.

76
The Story so Far

• Writes out Grid scripts as well as PBS/NQS.
• Semi-automatic procedure
• shell script submits jobs (sometimes)
• output retrieved by hand
• Limited success
• teething troubles with scripts
• frequent failures of Grid components (RB, LB, VO
  server)
• difficult to configure Grid node (CE, SE)
  correctly
• Four jobs run so far on GridPP testbed via IC and
  CERN RBs. (At UCL, Manchester, Oxford thanks!)
  Many more to come.

77
The Future

• More automated job submission and output
  retrieval
• Running jobs with user-provided proxy
  certificates
• Something similar done in GUIDO?
• Grid storage and database access
• Spitfire?
• OGSA-DAI?
• Combine JetWeb DB with more general DB of results
  (Durham)

78
Conclusions

• Gathering useful experience, but progress is
  slow.
• Hard work getting anything to run
• lack of documentation ? hard for non-expert to
  use
• failure of Grid components (but more stable now)
• Need more (wo)manpower, more powerful web and DB
  servers, more expertise!
• Well, it is a TESTbed, and things should become
  easier as we move towards a production Grid.