Toward an Understanding of HadronHadron Collisions

1
Toward an Understanding of Hadron-Hadron
Collisions

• Lecture 1 From Field-Feynman to the Tevatron.

• From 7 GeV/c p0s to 600 GeV/c Jets!

• QCD Monte-Carlo Models (PYTHIA Tune A).

• Min-Bias Collisions at the Tevatron.

? extrapolations to the LHC!

• Lecture 2 A Detailed Study of the Underlying
  Event at the Tevatron.

• QCD Monte-Carlo Models tunes at the Tevatron.

? extrapolations to the LHC!
UEMB_at_CMS
Florida-Perugia
CMS at the LHC
University of Perugia
2
The Fermilab Tevatron

• Fermi National Laboratory (Fermilab) is near
  Chicago, Illinois. CDF and DØ are the the two
  collider detector experiments at Fermilab.

• Protons collide with antiprotons at a
  center-of-mass energy of almost 2 TeV (actually
  1.96 TeV).

3
Feynman-Field Phenomenology
1973-1980
Feynman-Field Jet Model

• FF1 Quark Elastic Scattering as a Source of
  High Transverse Momentum Mesons, R. D. Field
  and R. P. Feynman, Phys. Rev. D15, 2590-2616
  (1977).
• FFF1 Correlations Among Particles and Jets
  Produced with Large Transverse Momenta, R. P.
  Feynman, R. D. Field and G. C. Fox, Nucl. Phys.
  B128, 1-65 (1977).
• FF2 A Parameterization of the properties of
  Quark Jets, R. D. Field and R. P. Feynman,
  Nucl. Phys. B136, 1-76 (1978).
• F1 Can Existing High Transverse Momentum Hadron
  Experiments be Interpreted by Contemporary
  Quantum Chromodynamics Ideas?, R. D. Field,
  Phys. Rev. Letters 40, 997-1000 (1978).
• FFF2 A Quantum Chromodynamic Approach for the
  Large Transverse Momentum Production of Particles
  and Jets, R. P. Feynman, R. D. Field and G. C.
  Fox, Phys. Rev. D18, 3320-3343 (1978).

• FW1 A QCD Model for ee- Annihilation, R. D.
  Field and S. Wolfram, Nucl. Phys. B213, 65-84
  (1983).

My 1st graduate student!
4
Hadron-Hadron Collisions
FF1 1977 (preQCD)

• What happens when two hadrons collide at high
  energy?

Feynman quote from FF1 The model we shall choose
is not a popular one, so that we will not
duplicate too much of the work of others who are
similarly analyzing various models (e.g.
constituent interchange model, multiperipheral
models, etc.). We shall assume that the high PT
particles arise from direct hard collisions
between constituent quarks in the incoming
particles, which fragment or cascade down into
several hadrons.

• Most of the time the hadrons ooze through each
  other and fall apart (i.e. no hard scattering).
  The outgoing particles continue in roughly the
  same direction as initial proton and antiproton.

• Occasionally there will be a large transverse
  momentum meson. Question Where did it come from?

• We assumed it came from quark-quark elastic
  scattering, but we did not know how to calculate
  it!

Black-Box Model
5
Quark-QuarkBlack-Box Model
No gluons!
FF1 1977 (preQCD)
Quark Distribution Functions determined from
deep-inelastic lepton-hadron collisions
Feynman quote from FF1 Because of the incomplete
knowledge of our functions some things can be
predicted with more certainty than others.
Those experimental results that are not well
predicted can be used up to determine these
functions in greater detail to permit better
predictions of further experiments. Our papers
will be a bit long because we wish to discuss
this interplay in detail.
Quark Fragmentation Functions determined from
ee- annihilations
Quark-Quark Cross-Section Unknown! Deteremined
from hadron-hadron collisions.
6
Quark-QuarkBlack-Box Model
Predict particle ratios
FF1 1977 (preQCD)
Predict increase with increasing CM energy W
Beam-Beam Remnants
Predict overall event topology (FFF1 paper 1977)
7 GeV/c p0s!
7
Telagram from Feynman
July 1976
SAW CRONIN AM NOW CONVINCED WERE RIGHT TRACK
QUICK WRITE FEYNMAN
8
Letter from Feynman
July 1976
9
Letter from Feynmanpage 1
Spelling?
10
Letter from Feynmanpage 3
It is fun!
Onward!
11
Feynman Talk at Coral Gables in December 1976
1st transparency
Last transparency
Feynman-Field Jet Model
12
QCD ApproachQuarks Gluons
Quark Gluon Fragmentation Functions Q2
dependence predicted from QCD
FFF2 1978
Feynman quote from FFF2 We investigate whether
the present experimental behavior of mesons with
large transverse momentum in hadron-hadron
collisions is consistent with the theory of
quantum-chromodynamics (QCD) with asymptotic
freedom, at least as the theory is now partially
understood.
Parton Distribution Functions Q2 dependence
predicted from QCD
Quark Gluon Cross-Sections Calculated from QCD
13
High PT Jets
CDF (2006)
Feynman, Field, Fox (1978)
Predict large jet cross-section
30 GeV/c!
Feynman quote from FFF At the time of this
writing, there is still no sharp quantitative
test of QCD. An important test will come in
connection with the phenomena of high PT
discussed here.
600 GeV/c Jets!
14
A Parameterization of the Properties of Jets
Field-Feynman 1978
Secondary Mesons (after decay)

• Assumed that jets could be analyzed on a
  recursive principle.

• Let f(h)dh be the probability that the rank 1
  meson leaves fractional momentum h to the
  remaining cascade, leaving quark b with
  momentum P1 h1P0.

Rank 2
Rank 1

• Assume that the mesons originating from quark b
  are distributed in presisely the same way as the
  mesons which came from quark a (i.e. same
  function f(h)), leaving quark c with momentum
  P2 h2P1 h2h1P0.

Primary Mesons
continue

• Add in flavor dependence by letting bu
  probabliity of producing u-ubar pair, bd
  probability of producing d-dbar pair, etc.

Calculate F(z) from f(h) and bi!

• Let F(z)dz be the probability of finding a meson
  (independent of rank) with fractional mementum z
  of the original quark a within the jet.

Original quark with flavor a and momentum P0
15
A Parameterization of the Properties of Jets
R. P. Feynman ISMD, Kaysersberg, France, June
12, 1977
Feynman quote from FF2 The predictions of the
model are reasonable enough physically that we
expect it may be close enough to reality to be
useful in designing future experiments and to
serve as a reasonable approximation to compare
to data. We do not think of the model as a
sound physical theory, ....
16
Monte-Carlo Simulationof Hadron-Hadron Collisions
FF1-FFF1 (1977) Black-Box Model
FF2 (1978) Monte-Carlo simulation of jets
F1-FFF2 (1978) QCD Approach
FFFW FieldJet (1980) QCD leading-log order
simulation of hadron-hadron collisions
FF or FW Fragmentation
the past
ISAJET (FF Fragmentation)
HERWIG (FW Fragmentation)
PYTHIA
today
tomorrow
SHERPA
PYTHIA 6.3
17
Monte-Carlo Simulationof Hadron-Hadron Collisions

• Color singlet proton collides with a color
  singlet antiproton.

• At short times (small distances) the color forces
  are weak and the outgoing partons move away from
  the beam-beam remnants.

• A red quark gets knocked out of the proton and a
  blue antiquark gets knocked out of the antiproton.

• The resulting event consists of hadrons and
  leptons in the form of two large transverse
  momentum outgoing jets plus the beam-beam
  remnants.

• At long times (large distances) the color forces
  become strong and quark-antiquark pairs are
  pulled out of the vacuum and hadrons are formed.

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Monte-Carlo Simulationof Quark and Gluon Jets

• ISAJET Evolve the parton-shower from Q2 (virtual
  photon invariant mass) to Qmin 5 GeV. Use a
  complicated fragmentation model to evolve from
  Qmin to outgoing hadrons.

• HERWIG Evolve the parton-shower from Q2 (virtual
  photon invariant mass) to Qmin 1 GeV. Form
  color singlet clusters which decay into hadrons
  according to 2-particle phase space.

• MLLA Evolve the parton-shower from Q2 (virtual
  photon invariant mass) to Qmin 230 MeV. Assume
  that the charged particles behave the same as the
  partons with Nchg/Nparton 0.56!

Q2
MLLA Curve!
200 MeV
5 GeV
1 GeV
19
Collider Coordinates

• The z-axis is defined to be the beam axis with
  the xy-plane being the transverse plane.

• qcm is the center-of-mass scattering angle and f
  is the azimuthal angle. The transverse
  momentum of a particle is given by PT P
  cos(qcm).

• Use h and f to determine the direction of an
  outgoing particle, where h is the
  pseudo-rapidity defined by h -log(tan(qcm/2)).

• The rapidity is defined by y
  log((Epz)/(E-pz))/2 and is equal to h in the
  limit E gtgt mc2.

20
Quark Gluon Jets

• The CDF calorimeter measures energy deposited in
  a cell of size DhxDf 0.11x15o, whch is
  converted into transverse energy, ET E cos(qcm).

• Jets are defined to be clusters of transverse
  energy with a radius R in h-f space. A jet is
  the representation in the detector of an outgoing
  parton (quark or gluon).
• The sum of the ET of the cells within a jet
  corresponds roughly to the ET of the outgoing
  parton and the position of the cluster in the
  grid gives the partons direction.

Can also construct jets from the charged
particles!
Calorimeter Jets
21
CDF Run 2 DiJet EventJuly 2002
ETjet1 403 GeV ETjet2 322 GeV
Raw ET values!!
22
Proton-AntiProton Collisionsat the Tevatron
The CDF Min-Bias trigger picks up most of the
hard core cross-section plus a small amount of
single double diffraction.
stot sEL sIN
stot sEL sSD sDD sHC
1.8 TeV 78mb 18mb 9mb
(4-7)mb (47-44)mb
CDF Min-Bias trigger 1 charged particle in
forward BBC AND 1 charged particle in backward BBC
The hard core component contains both hard
and soft collisions.
Beam-Beam Counters 3.2 lt h lt 5.9
23
Proton-AntiProton Collisions at the Tevatron

• Hard core does not imply that a hard
  parton-parton collision has occured?

• 90 of hard core collisions are soft hard
  core and the proton and antiproton ooze through
  each other and fall apart (i.e. no hard
  scattering, PT(hard) lt 5 GeV/c). The outgoing
  particles continue in roughly the same direction
  as initial proton and antiproton.

• 10 of the hard core collisions arise from a
  hard parton-parton collision (PT(hard) gt 5
  GeV/c) resulting in large transverse momentum
  outgoing partons.

• About 0.3 of all parton-parton collisions
  produce a b-bbar quark pair (about 1/1,000 of all
  interactions).

24
CDF Min-Bias DataCharged Particle Density
ltdNchg/dhgt 4.2
ltdNchg/dhdfgt 0.67

• Shows CDF Min-Bias data on the number of
  charged particles per unit pseudo-rapidity at 630
  and 1,800 GeV. There are about 4.2 charged
  particles per unit h in Min-Bias collisions at
  1.8 TeV (h lt 1, all PT).

• Convert to charged particle density, dNchg/dhdf,
  by dividing by 2p. There are about 0.67 charged
  particles per unit h-f in Min-Bias collisions
  at 1.8 TeV (h lt 1, all PT).

25
Particle Densities
Charged Particles pT gt 0.5 GeV/c h lt 1
CDF Run 2 Min-Bias
DhDf 4p 12.6

• Study the charged particles (pT gt 0.5 GeV/c, h
  lt 1) and form the charged particle density,
  dNchg/dhdf, and the charged scalar pT sum
  density, dPTsum/dhdf.

26
CDF Min-Bias DataEnergy Dependence
LHC?

• Shows the center-of-mass energy dependence of the
  charged particle density, dNchg/dhdf, for
  Min-Bias collisions at h 0. Also show a log
  fit (Fit 1) and a (log)2 fit (Fit 2) to the CDF
  plus UA5 data.

• What should we expect for the LHC?

27
Herwig Soft Min-Bias
Can we believe HERWIG soft Min-Bias?
Can we believe HERWIG soft Min-Bias? No!
LHC?

• Shows the center-of-mass energy dependence of the
  charged particle density, dNchg/dhdf, for
  Min-Bias collisions compared with the HERWIG
  Soft Min-Bias Monte-Carlo model. Note there
  is no hard scattering in HERWIG Soft Min-Bias.

• HERWIG Soft Min-Bias contains no hard
  parton-parton interactions and describes fairly
  well the charged particle density, dNchg/dhdf, in
  Min-Bias collisions.

• HERWIG Soft Min-Bias predicts a 45 rise in
  dNchg/dhdf at h 0 in going from the Tevatron
  (1.8 TeV) to the LHC (14 TeV). 4 charged
  particles per unit h becomes 6.

28
CDF Min-Bias DataPT Dependence
Lots of hard scattering in Min-Bias!

• Shows the energy dependence of the charged
  particle density, dNchg/dhdf, for Min-Bias
  collisions compared with HERWIG Soft Min-Bias.

• Shows the PT dependence of the charged particle
  density, dNchg/dhdfdPT, for Min-Bias collisions
  at 1.8 TeV collisions compared with HERWIG Soft
  Min-Bias.

• HERWIG Soft Min-Bias does not describe the
  Min-Bias data! The Min-Bias data contain a
  lot of hard parton-parton collisions which
  results in many more particles at large PT than
  are produces by any soft model.

29
Min-Bias CombiningHard and Soft Collisions
HERWIG diverges!
sHC
PYTHIA cuts off the divergence. Can run
PT(hard)gt0!

• HERWIG hard QCD with PT(hard) gt 3 GeV/c
  describes well the high PT tail but produces too
  many charged particles overall. Not all of the
  Min-Bias collisions have a hard scattering with
  PT(hard) gt 3 GeV/c!

HERWIG soft Min-Bias does not fit the
Min-Bias data!

• One cannot run the HERWIG hard QCD Monte-Carlo
  with PT(hard) lt 3 GeV/c because the perturbative
  2-to-2 cross-sections diverge like 1/PT(hard)4?

30
Monte-Carlo Simulationof Hadron-Hadron Collisions
Maybe not all soft!

• The underlying event in a hard scattering process
  has a hard component (particles that arise from
  initial final-state radiation and from the
  outgoing hard scattered partons) and a soft?
  component (beam-beam remnants).

• Clearly? the underlying event in a hard
  scattering process should not look like a
  Min-Bias event because of the hard component
  (i.e. initial final-state radiation).

• However, perhaps Min-Bias collisions are a good
  model for the beam-beam remnant component of
  the underlying event.

Are these the same?

• The beam-beam remnant component is, however,
  color connected to the hard component so this
  comparison is (at best) an approximation.

31
MPI Multiple PartonInteractions

• PYTHIA models the soft component of the
  underlying event with color string fragmentation,
  but in addition includes a contribution arising
  from multiple parton interactions (MPI) in which
  one interaction is hard and the other is
  semi-hard.

• The probability that a hard scattering events
  also contains a semi-hard multiple parton
  interaction can be varied but adjusting the
  cut-off for the MPI.
• One can also adjust whether the probability of a
  MPI depends on the PT of the hard scattering,
  PT(hard) (constant cross section or varying with
  impact parameter).
• One can adjust the color connections and flavor
  of the MPI (singlet or nearest neighbor, q-qbar
  or glue-glue).
• Also, one can adjust how the probability of a MPI
  depends on PT(hard) (single or double Gaussian
  matter distribution).

32
PYTHIA 6.2 Multiple PartonInteraction Parameters
and now HERWIG!
Pythia uses multiple parton interactions to
enhance the underlying event.
Jimmy MPI J. M. Butterworth J. R. Forshaw M. H.
Seymour
Multiple parton interaction more likely in a hard
(central) collision!
Same parameter that cuts-off the hard 2-to-2
parton cross sections!
Hard Core
33
Tuning PYTHIA 6.2Multiple Parton Interaction
Parameters
Hard Core
Determine by comparing with 630 GeV data!
Affects the amount of initial-state radiation!
Take E0 1.8 TeV
Reference point at 1.8 TeV
34
Tuned PYTHIA 6.206
CDF Default!
PYTHIA 6.206 CTEQ5L
Run 1 Analysis

• Plot shows the Transverse charged particle
  density versus PT(chgjet1) compared to the QCD
  hard scattering predictions of two tuned versions
  of PYTHIA 6.206 (CTEQ5L, Set B (PARP(67)1) and
  Set A (PARP(67)4)).

Old PYTHIA default (more initial-state radiation)
Old PYTHIA default (more initial-state radiation)
New PYTHIA default (less initial-state radiation)
New PYTHIA default (less initial-state radiation)
35
PYTHIA Min-BiasSoft Hard
Tuned to fit the underlying event!
PYTHIA Tune A CDF Run 2 Default
12 of Min-Bias events have PT(hard) gt 5 GeV/c!
1 of Min-Bias events have PT(hard) gt 10 GeV/c!

• PYTHIA regulates the perturbative 2-to-2
  parton-parton cross sections with cut-off
  parameters which allows one to run with PT(hard)
  gt 0. One can simulate both hard and soft
  collisions in one program.

Lots of hard scattering in Min-Bias!

• The relative amount of hard versus soft
  depends on the cut-off and can be tuned.

• This PYTHIA fit predicts that 12 of all
  Min-Bias events are a result of a hard 2-to-2
  parton-parton scattering with PT(hard) gt 5 GeV/c
  (1 with PT(hard) gt 10 GeV/c)!

36
Min-Bias AssociatedCharged Particle Density
Associated densities do not include PTmax!
Highest pT charged particle!

• Use the maximum pT charged particle in the event,
  PTmax, to define a direction and look at the the
  associated density, dNchg/dhdf, in min-bias
  collisions (pT gt 0.5 GeV/c, h lt 1).

It is more probable to find a particle
accompanying PTmax than it is to find a particle
in the central region!

• Shows the data on the Df dependence of the
  associated charged particle density,
  dNchg/dhdf, for charged particles (pT gt 0.5
  GeV/c, h lt 1, not including PTmax) relative to
  PTmax (rotated to 180o) for min-bias events.
  Also shown is the average charged particle
  density, dNchg/dhdf, for min-bias events.

37
Min-Bias AssociatedCharged Particle Density
Rapid rise in the particle density in the
transverse region as PTmax increases!
PTmax gt 2.0 GeV/c
Transverse Region
Transverse Region
Ave Min-Bias 0.25 per unit h-f
PTmax gt 0.5 GeV/c

• Shows the data on the Df dependence of the
  associated charged particle density,
  dNchg/dhdf, for charged particles (pT gt 0.5
  GeV/c, h lt 1, not including PTmax) relative to
  PTmax (rotated to 180o) for min-bias events
  with PTmax gt 0.5, 1.0, and 2.0 GeV/c.

• Shows jet structure in min-bias collisions
  (i.e. the birth of the leading two jets!).

38
Min-Bias AssociatedCharged PTsum Density
Associated densities do not include PTmax!
Highest pT charged particle!

• Use the maximum pT charged particle in the event,
  PTmax, to define a direction and look at the the
  associated PTsum density, dPTsum/dhdf.
• Shows the data on the Df dependence of the
  associated charged PTsum density, dPTsum/dhdf,
  for charged particles (pT gt 0.5 GeV/c, h lt 1,
  not including PTmax) relative to PTmax (rotated
  to 180o) for min-bias events. Also shown is
  the average charged particle density,
  dPTsum/dhdf, for min-bias events.

39
Min-Bias AssociatedCharged PTsum Density
Rapid rise in the PTsum density in the
transverse region as PTmax increases!
Transverse Region
Transverse Region
Ave Min-Bias 0.24 GeV/c per unit h-f

• Shows the data on the Df dependence of the
  associated charged PTsum density, dPTsum/dhdf,
  for charged particles (pT gt 0.5 GeV/c, h lt 1,
  not including PTmax) relative to PTmax (rotated
  to 180o) for min-bias events with PTmax gt 0.5,
  1.0, and 2.0 GeV/c.

• Shows jet structure in min-bias collisions
  (i.e. the birth of the leading two jets!).

40
Min-Bias AssociatedCharged Particle Density
PY Tune A
PTmax gt 2.0 GeV/c
Transverse Region
Transverse Region
PTmax gt 0.5 GeV/c

• Shows the data on the Df dependence of the
  associated charged particle density,
  dNchg/dhdf, for charged particles (pT gt 0.5
  GeV/c, h lt 1, not including PTmax) relative to
  PTmax (rotated to 180o) for min-bias events
  with PTmax gt 0.5 GeV/c and PTmax gt 2.0 GeV/c
  compared with PYTHIA Tune A (after CDFSIM).

• PYTHIA Tune A predicts a larger correlation than
  is seen in the min-bias data (i.e. Tune A
  min-bias is a bit too jetty).

41
Min-Bias AssociatedCharged PTsum Density
PY Tune A
PTmax gt 2.0 GeV/c
Transverse Region
Transverse Region
PTmax gt 0.5 GeV/c

• Shows the data on the Df dependence of the
  associated charged PTsum density, dPTsum/dhdf,
  for charged particles (pT gt 0.5 GeV/c, h lt 1,
  not including PTmax) relative to PTmax (rotated
  to 180o) for min-bias events with PTmax gt 0.5
  GeV/c and PTmax gt 2.0 GeV/c compared with PYTHIA
  Tune A (after CDFSIM).

• PYTHIA Tune A predicts a larger correlation than
  is seen in the min-bias data (i.e. Tune A
  min-bias is a bit too jetty).

42
PYTHIA Tune ALHC Predictions
LHC?

• Shows the center-of-mass energy dependence of the
  charged particle density, dNchg/dhdf, for
  Min-Bias collisions compared with PYTHIA Tune A
  with PT(hard) gt 0.

• PYTHIA was tuned to fit the underlying event in
  hard-scattering processes at 1.8 TeV and 630 GeV.
  

• PYTHIA Tune A predicts a 42 rise in dNchg/dhdf
  at h 0 in going from the Tevatron (1.8 TeV) to
  the LHC (14 TeV). Similar to HERWIG soft
  min-bias, 4 charged particles per unit h becomes
  6.

43
PYTHIA Tune ALHC Predictions
12 of Min-Bias events have PT(hard) gt 10 GeV/c!
LHC?

• Shows the center-of-mass energy dependence of the
  charged particle density, dNchg/dhdfdPT, for
  Min-Bias collisions compared with PYTHIA Tune A
  with PT(hard) gt 0.

1 of Min-Bias events have PT(hard) gt 10 GeV/c!

• PYTHIA Tune A predicts that 1 of all Min-Bias
  events at 1.8 TeV are a result of a hard 2-to-2
  parton-parton scattering with PT(hard) gt 10 GeV/c
  which increases to 12 at 14 TeV!

44
LHC Min-Bias Predictions
Tevatron LHC
12 times more likely to find a 10 GeV jet in
Min-Bias at the LHC!

• Both HERWIG and the tuned PYTHIA Tune A predict a
  40-45 rise in dNchg/dhdf at h 0 in going from
  the Tevatron (1.8 TeV) to the LHC (14 TeV). 4
  charged particles per unit h at the Tevatron
  becomes 6 per unit h at the LHC.

• The tuned PYTHIA Tune A predicts that 1 of all
  Min-Bias events at the Tevatron (1.8 TeV) are
  the result of a hard 2-to-2 parton-parton
  scattering with PT(hard) gt 10 GeV/c which
  increases to 12 at LHC (14 TeV)!