Predicting MinBias and the Underlying Event at the LHC

1
Predicting Min-Bias and the Underlying Event
at the LHC
Early Physics Measurements
Rick Field University of Florida
Outline of Talk

• Min-Bias at 900 GeV, 2.2 TeV, 7 TeV, 10 TeV,
  and 14 TeV.

• New CDF charged multiplicity distribution and
  comparisons with the QCD Monte-Carlo tunes.

CERN November 6, 2009

• Relationship between the underlying event in a
  hard scattering process and min-bias collisions.

CDF Run 2

• The underlying event at 900 GeV, 7 TeV, 10 TeV,
  and 14 teV.

• The underlying event in Drell-Yan production at
  7 TeV,

UEMB_at_CMS
CMS at the LHC

• Summary Conclusions.

2
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
The hard core component contains both hard
and soft collisions.
CDF Min-Bias trigger 1 charged particle in
forward BBC AND 1 charged particle in backward BBC
Inelastic Non-Diffractive Component
Beam-Beam Counters 3.2 lt h lt 5.9
3
Inelastic Non-Diffractive Cross-Section
My guess!
Lots of events!
Linear scale!
Log scale!
stot sEL sSD sDD sHC

• The inelastic non-diffractive cross section
  versus center-of-mass energy from PYTHIA (1.2).

• sHC varies slowly. Only a 13 increase between 7
  TeV ( 58 mb) and 14 teV ( 66 mb). Linear on a
  log scale!

4
Charged Particle Density dN/dh

• Charged particle (all pT) pseudo-rapidity
  distribution, dNchg/dhdf, at 1.96 TeV for
  inelastic non-diffractive collisions from PYTHIA
  Tune A, Tune DW, Tune S320, and Tune P324.

• Charged particle (pTgt0.5 GeV/c) pseudo-rapidity
  distribution, dNchg/dhdf, at 1.96 TeV for
  inelastic non-diffractive collisions from PYTHIA
  Tune A, Tune DW, Tune S320, and Tune P324.

5
Charged Particle Density dN/dh
RDF LHC Prediction!
Tevatron
LHC

• Charged particle (all pT) pseudo-rapidity
  distribution, dNchg/dhdf, at 1.96 TeV for
  inelastic non-diffractive collisions from PYTHIA
  Tune A, Tune DW, Tune S320, and Tune P324.

• Extrapolations (all pT) of PYTHIA Tune A, Tune
  DW, Tune S320, Tune P324. and ATLAS to the LHC.

6
Charged Particle Density dN/dh
RDF LHC Prediction!
Tevatron
LHC

• Charged particle (pT gt 0.5 GeV/c) pseudo-rapidity
  distribution, dNchg/dhdf, at 1.96 TeV for
  inelastic non-diffractive collisions from PYTHIA
  Tune A, Tune DW, Tune S320, and Tune P324.

• Extrapolations (pT gt 0.5 GeV/c) of PYTHIA Tune A,
  Tune DW, Tune S320, Tune P324. and ATLAS to the
  LHC.

7
Min-Bias Charged Particle Density
LHC14
LHC7
Tevatron
900 GeV
RHIC
1.96 TeV ? 14 TeV (dN/dh increase 1.58 times)
0.2 TeV ? 1.96 TeV (dN/dh increase 1.63 times)
LHC
Tevatron
RHIC

• Shows the min-bias charged particle density,
  dN/dh, for charged particles (pT gt 0.5 GeV/c) for
  at 0.2 TeV, 0.9 TeV, 1.96 TeV and 14 TeV
  predicted by PYTHIA Tune DW at the particle level
  (i.e. generator level).

Linear scale!
8
Min-Bias Charged Particle Density
LHC14
LHC10
LHC7
Tevatron
900 GeV
RHIC
7 TeV ? 14 TeV (dN/dh 19 increase)
Log scale!
Linear on a log plot!
LHC7
LHC14

• Shows the min-bias charged particle density,
  dN/dh, for charged particles (pT gt 0.5 GeV/c) for
  at 0.2 TeV, 0.9 TeV, 1.96 TeV and 14 TeV
  predicted by PYTHIA Tune DW at the particle level
  (i.e. generator level).

9
Charged Particle Multiplicity
New
Tune A!
No MPI!
7 decades!

• Data at 1.96 TeV on the charged particle
  multiplicity (pT gt 0.4 GeV/c, h lt 1) for
  min-bias collisions at CDF Run 2.

• The data are compared with PYTHIA Tune A and Tune
  A without multiple parton interactions
  (pyAnoMPI).

10
Charged Particle Multiplicity
Tune A!
No MPI!
Tune S320!

• Data at 1.96 TeV on the charged particle
  multiplicity (pT gt 0.4 GeV/c, h lt 1) for
  min-bias collisions at CDF Run 2.

• The data are compared with PYTHIA Tune A and Tune
  A without multiple parton interactions
  (pyAnoMPI).

11
The Underlying Event
Select inelastic non-diffractive events that
contain a hard scattering
Hard parton-parton collisions is hard (pT gt 2
GeV/c)
Semi-hard parton-parton collision (pT lt 2
GeV/c)
The underlying-event (UE)!



Given that you have one hard scattering it is
more probable to have MPI! Hence, the UE has
more activity than min-bias.
Multiple-parton interactions (MPI)!
12
The Inelastic Non-Diffractive Cross-Section
Occasionally one of the parton-parton collisions
is hard (pT gt 2 GeV/c)
Majority of min-bias events!
Semi-hard parton-parton collision (pT lt 2
GeV/c)




Multiple-parton interactions (MPI)!
13
The Underlying Event
Select inelastic non-diffractive events that
contain a hard scattering
Hard parton-parton collisions is hard (pT gt 2
GeV/c)
Semi-hard parton-parton collision (pT lt 2
GeV/c)
The underlying-event (UE)!



Given that you have one hard scattering it is
more probable to have MPI! Hence, the UE has
more activity than min-bias.
Multiple-parton interactions (MPI)!
14
Min-Bias Correlations
New

• Data at 1.96 TeV on the average pT of charged
  particles versus the number of charged particles
  (pT gt 0.4 GeV/c, h lt 1) for min-bias
  collisions at CDF Run 2. The data are corrected
  to the particle level and are compared with
  PYTHIA Tune A at the particle level (i.e.
  generator level).

15
Min-Bias Average PT versus Nchg

• Beam-beam remnants (i.e. soft hard core) produces
  low multiplicity and small ltpTgt with ltpTgt
  independent of the multiplicity.

• Hard scattering (with no MPI) produces large
  multiplicity and large ltpTgt.

• Hard scattering (with MPI) produces large
  multiplicity and medium ltpTgt.

This observable is sensitive to the MPI tuning!



The CDF min-bias trigger picks up most of the
hard core component!
16
Charged Particle Multiplicity
The charged multiplicity distribution does not
change between 1.96 and 2.2 TeV and
proton-proton is the same as proton-antiproton!
Tune A prediction at 900 GeV!
Tune A prediction at 2.2 TeV!
Tune A!
No MPI!

• Data at 1.96 TeV on the charged particle
  multiplicity (pT gt 0.4 GeV/c, h lt 1) for
  min-bias collisions at CDF Run 2. The data are
  compared with PYTHIA Tune A and Tune A without
  multiple parton interactions (pyAnoMPI).

• Prediction from PYTHIA Tune A for proton-proton
  collisions at 900 GeV and 2.2 TeV.

17
LHC Predictions 900 GeV

• Charged multiplicity distributions for
  proton-proton collisions at 900 GeV (h lt 2)
  from PYTHIA Tune A, Tune DW, Tune S320, and Tune
  P329.

18
LHC Predictions 900 GeV
If we get 10,000 HC collisions we could measure
ltNchggt BUT we also want to measure the activity
in the underlying event which would take
2,000,000 HC collisions!
1,000 events L 24/mb!
100 events L 2.4/mb!
10 events L 0.24/mb!

• Charged multiplicity distributions for
  proton-proton collisions at 900 GeV (pT gt 0.5
  GeV/c, h lt 2) from PYTHIA Tune A, Tune DW, Tune
  DWT, Tune S320, and Tune P329.

19
Transverse Charged Density

• Shows the charged particle density in the
  transverse region for charged particles (pT gt
  0.5 GeV/c, h lt 1) at 7 TeV as defined by PTmax,
  PT(chgjet1), and PT(muon-pair) from PYTHIA Tune
  DW at the particle level (i.e. generator level).
  Charged particle jets are constructed using the
  Anti-KT algorithm with d 0.5.

20
Min-Bias AssociatedCharged Particle Density
LHC14
LHC10
LHC7
Tevatron
900 GeV
RHIC
0.2 TeV ? 1.96 TeV (UE increase 2.7 times)
1.96 TeV ? 14 TeV (UE increase 1.9 times)
RHIC
LHC
Tevatron

• Shows the associated charged particle density
  in the transverse region as a function of PTmax
  for charged particles (pT gt 0.5 GeV/c, h lt 1,
  not including PTmax) for min-bias events at 0.2
  TeV, 0.9 TeV, 1.96 TeV, 7 TeV, 10 TeV, 14 TeV
  predicted by PYTHIA Tune DW at the particle level
  (i.e. generator level).

Linear scale!
21
Min-Bias AssociatedCharged Particle Density
LHC14
LHC10
LHC7
Tevatron
900 GeV
RHIC
7 TeV ? 14 TeV (UE increase 20)
LHC7
LHC14
Linear on a log plot!

• Shows the associated charged particle density
  in the transverse region as a function of PTmax
  for charged particles (pT gt 0.5 GeV/c, h lt 1,
  not including PTmax) for min-bias events at 0.2
  TeV, 0.9 TeV, 1.96 TeV, 7 TeV, 10 TeV, 14 TeV
  predicted by PYTHIA Tune DW at the particle level
  (i.e. generator level).

Log scale!
22
sHC PTmax gt 5 GeV/c
Still lots of events!
Log scale!
Linear scale!
stot sEL sSD sDD sHC
In 1,000,000 HC collisions at 900 GeV you get 940
with PTmax gt 5 GeV/c!

• The inelastic non-diffractive PTmax gt 5 GeV/c
  cross section versus center-of-mass energy from
  PYTHIA (1.2).

• sHC(PTmax gt 5 GeV/c) varies more rapidly. Factor
  of 2.3 increase between 7 TeV ( 0.56 mb) and 14
  teV ( 1.3 mb). Linear on a linear scale!

23
Transverse Charged Density
With 1/nb of min-bias data at 7 TeV we could
study the UE out to PTmax 25 GeV/c or
PT(chgjet1) 45 GeV/c !

• Shows the charged particle density in the
  transverse region for charged particles (pT gt
  0.5 GeV/c, h lt 1) at 7 TeV as defined by PTmax
  and PT(chgjet1) from PYTHIA Tune DW at the
  particle level (i.e. generator level). Charged
  particle jet are constructed using the Anti-KT
  algorithm with d 0.5.

• Shows the leading charged particle jet, chgjet1,
  and the leading charged particle, PTmax,
  differential cross section, ds/dPT (pT gt 0.5
  GeV/c, h lt 1) from PYTHIA Tune DW at the
  particle level (i.e. generator level). Charged
  particle jet are constructed using the Anti-KT
  algorithm with d 0.5.

24
Transverse Charged Density

• Shows the charged particle density in the
  transverse region for charged particles (pT gt
  0.5 GeV/c) at 7 TeV for h lt 1 and h lt 2 as
  defined PT(chgjet1) from PYTHIA Tune DW at the
  particle level (i.e. generator level). Charged
  particle jet are constructed using the Anti-KT
  algorithm with d 0.5.

• Shows the charged particle density in the
  transMAX and transMIN region for charged
  particles (pT gt 0.5 GeV/c) at 7 TeV for h lt 1
  and h lt 2 as defined PT(chgjet1) from PYTHIA
  Tune DW at the particle level (i.e. generator
  level). Charged particle jet are constructed
  using the Anti-KT algorithm with d 0.5.

25
QCD Monte-Carlo ModelsHigh Transverse Momentum
Jets
Underlying Event

• Start with the perturbative 2-to-2 (or sometimes
  2-to-3) parton-parton scattering and add initial
  and final-state gluon radiation (in the leading
  log approximation or modified leading log
  approximation).

• The underlying event consists of the beam-beam
  remnants and from particles arising from soft or
  semi-soft multiple parton interactions (MPI).

The underlying event is an unavoidable
background to most collider observables and
having good understand of it leads to more
precise collider measurements!

• Of course the outgoing colored partons fragment
  into hadron jet and inevitably underlying
  event observables receive contributions from
  initial and final-state radiation.

26
Drell-Yan Muon-Pair Cross-Section
Linear scale!

• The Drell-Yan muon-pair cross section 70 lt
  M(pair) lt 110 GeV versus center-of-mass energy
  from PYTHIA (1.3).

• The Drell-Yan cross-section varies rapidly.
  Factor of 2.2 increase between 7 TeV ( 0.9 nb)
  and 14 teV ( 2 nb). Linear on a linear scale!

Note nb not mb!
27
Drell-Yan Muon-Pair Cross-Section
Linear scale!
4,700 events in 10/pb!
CMS acceptance!

• The Drell-Yan muon-pair cross section 70 lt
  M(pair) lt 110 GeV (h(m) lt 2.4, pT(m) gt 5 GeV/c)
  versus center-of-mass energy from PYTHIA (1.3).

• The CMS Drell-Yan cross-section varies rapidly.
  Factor of 1.9 increase between 7 TeV ( 0.5 nb)
  and 14 TeV ( 0.9 nb). Linear on a linear scale!

28
Transverse Charged Density
Note at CMS min-bias is pre-scaled by a factor
of 5,000 so this really corresponds to 5/pb
delivered !
With 10/pb of data at 7 TeV we could study the UE
in Drell-Yan production out to PT(pair) 15
GeV/c !

• Shows the charged particle density in the
  transverse region for charged particles (pT gt
  0.5 GeV/c, h lt 1) at 7 TeV as defined by PTmax,
  PT(chgjet1), and PT(muon-pair) for PYTHIA Tune
  DW at the particle level (i.e. generator level).
  Charged particle jet are constructed using the
  Anti-KT algorithm with d 0.5.

• Shows the leading charged particle jet, chgjet1,
  and the leading charged particle, PTmax,
  differential cross section, ds/dPT (pT gt 0.5
  GeV/c, h lt 1), and the Drell-Yan differential
  cross-section (70 lt M(pair) lt 110 GeV) from
  PYTHIA Tune DW at the particle level (i.e.
  generator level). Charged particle jet are
  constructed using the Anti-KT algorithm with d
  0.5.

29
Z-Boson Towards Region
RDF LHC Prediction!
Tevatron
LHC

• Data at 1.96 TeV on the density of charged
  particles, dN/dhdf, with pT gt 0.5 GeV/c and h lt
  1 for Z-Boson events as a function of PT(Z) for
  the toward region from PYTHIA Tune AW, Tune DW,
  Tune S320, and Tune P329 at the particle level
  (i.e. generator level).

• Extrapolations of PYTHIA Tune AW, Tune DW, Tune
  DWT, Tune S320, and Tune P329, and pyATLAS to the
  LHC.

30
Drell-Yan Charged Multiplicity
It would be nice to have 2/pb at 7 TeV
(acquired) which might mean 3/pb (delivered)!
10 events L 21/nb!
100 events L 210/nb!
1,000 events L 2.1/pb!

• Prediction from PYTHIA Tune DW, Tune S320, and
  Tune P329 for Drell-Yan muon-pair production (70
  lt M(pair) lt 110 GeV) for proton-proton collisions
  at 7 TeV for the number of charged particles with
  pT gt 0.5 GeV and h lt 2 (excluding the
  lepton-pair).

31
Drell-Yan Charged PT PTsum

• Prediction from PYTHIA Tune DW, Tune S320 for
  Drell-Yan muon-pair production (70 lt M(pair) lt
  110 GeV) for proton-proton collisions at 7 TeV
  for the charged pT distribution h lt 2
  (excluding the lepton-pair). The plot shows the
  ltNchggt per unit pT versus pT.

• Prediction from PYTHIA Tune DW, Tune S320 for
  Drell-Yan muon-pair production (70 lt M(pair) lt
  110 GeV) for proton-proton collisions at 7 TeV
  for the PTsum distributions with pT gt 0.5 GeV and
  h lt 2 (excluding the lepton-pair). The plot
  shows the probability of having a given PTsum in
  the event.

32
Early LHC Measurements

• The amount of activity in min-bias collisions
  (multiplicity distribution, pT distribution,
  PTsum distribution, dNchg/dh).

• The amount of activity in the underlying event
  in hard scattering events (transverse Nchg
  distribution, transverse pT distribution,
  transverse PTsum distribution for events with
  PTmax gt 5 GeV/c).

• The amount of activity in the underlying event
  in Drell-Yan events (Nchg distribution, pT
  distribution, PTsum distribution, excluding the
  lepton-pair),

33
Summary Conclusions
It is very important to measure BOTH
min-bias and the underlying event at 900 GeV!
I hope we get enough events.

• We are making good progress in understanding and
  modeling the underlying event. RHIC data at
  200 GeV are very important!

• The new Pythia pT ordered tunes (py64 S320 and
  py64 P329) are very similar to Tune A, Tune AW,
  and Tune DW. At present the new tunes do not fit
  the data better than Tune AW and Tune DW.
  However, the new tune are theoretically
  preferred!

Py64 S320 LHC Reference Tune!

• It is clear now that the default value PARP(90)
  0.16 is not correct and the value should be
  closer to the Tune A value of 0.25.

• The new and old PYTHIA tunes are beginning to
  converge and I believe we are finally in a
  position to make some legitimate predictions at
  the LHC!

• All tunes with the default value PARP(90) 0.16
  are wrong and are overestimating the activity of
  min-bias and the underlying event at the LHC!
  This includes all my T tunes and the old ATLAS
  tunes!

UEMB_at_CMS

• Need to measure Min-Bias and the underlying
  event at the LHC as soon as possible to see if
  there is new QCD physics to be learned!