1. High Multiplicity Events in p+p Collisions at LHC Energy
Masahiro Konno for the ALICE Collaboration
(University of Tsukuba)
Yamagata, 9/21/2008

2. p+p Collisions at LHC
LHC: Proton+Proton collider at maximum energy √s = 14 TeV
(Luminosity: 1030 – 1034 cm-2s-1 => Min. Bias rate: 50 kHz – 500 MHz)
First circulating beam performed on Sep. 10, 2008
=> First p+p collisions will be soon...
Physics topics: Higgs, SUSY, low-x physics, reference data for heavy ion
First Pb+Pb beam expected by end of 2009
What else can one do with p+p data while waiting for Pb+Pb?
=> Heavy Ion Physics with p+p collisions at LHC energy
Particle density dNch/dη ~ can be reached even in p+p
(central SPS, mid-central RHIC)
Predictions p+p 14 TeV
<Nch>
Ntot
dN/dη(η=0)
PYTHIA6.214
91.0
158.4
6.8
PHOJET1.12
69.6
115.1
5.1
Multiplicity distributions
(PYTHIA, 14 TeV p+p)

3. High Multiplicity Events in p+p Collisions
In high multiplicity events in p+p collisions, one expects secondary collisions
of particles to take place – rescattering. Rescattering may occur among partons
early in the collisions, and also among hadrons later in the collisions. Both kinds
of rescattering can lead to collective behavior among the particles.
- Hadronic FS interactions
- Multiple parton interactions => Collective flow, Thermal behavior
=> Small QGP? (not new, explored even at Tevatron...)
Increasing the center-of-mass energy increases the parton fluxes in p+p collisions.
At very high energies, multiple-parton interactions become important.
Energy density vs. Npart
Upper: Central
Lower: MB

4. Particle Rapidity Density
p+p (p+pbar)
RHIC
arXiv:
High multiplicity
events in p+p
Particle density dNch/dη increases with √s.
This is due to the increase of the rate of
multiple parton scatterings.
dNch/dη/(Npart/2) shows different trends for
p+p (p+pbar) and heavy ion data – effective
energy for transverse particle production.
LHC
energy

5. ALICE Experiment A dedicated experiment for heavy ion physics at LHC
Point 2 in LHC ring
ALICE detector
A dedicated experiment for heavy ion physics at LHC
Sensitivity to low pT (cutoff ~0.2 GeV/c) thanks to low magnetic field (0.5 T)
and small amount of material (X/X0=7%, ITS) before TPC.
- Particle identification capability: dE/dx (ITS/TPC), TRD, TOF, HMPID, PHOS

6. ALICE Detector TPC Time Projection Chamber Silicon Pixel Detector SPD
510 cm
88µs
560 cm
Main tracking device (|η|<0.9)
dE/dx measurement for PID
Drift time: ~88 μs
Silicon Pixel Detector
Inner two layers of ITS (|η|<2.0, 1.4)
1200 chips, 9.8 M pixels
Tracking, Multiplicity, Vertex, Trigger
SPD
Sector: 4 (outer) + 2 (inner) staves
Half-Stave: 10 chips
SPD: 10 sectors (1200 chips)
Beam-gas event

7. Triggers – Min. bias, High multiplicity
Minimum bias trigger is realized using signals from V0 and SPD detectors.
The trigger is based on the number of fired chips in two layers of SPD (Fast-OR).
Trigger efficiency: >99% for non. diff events with minimized background (beam-gas).
In ALICE, max luminosity: <5x1030 cm-2s-1 (~200 kHz) to avoid unacceptable pile-up.
ALICE can collect a large sample of p+p events (~109) in the first year.
High multiplicity triggers should be employed to record events with large multiplicities
to study multiple parton interactions, parton saturation effects, etc.
The trigger thresholds on the number of fired SPD chips select high-multiplicity events.
~few % order

8. Multiplicity Measurement
Multiplicity can be measured by counting tracks with SPD. The tracklets are
extracted by association of clusters in the two layers and the primary vertex.
The detector response matrix can be obtained from simulation studies.
Correction by taking into account the geometrical acceptance depending on zvertex.
ALICE PPR II
(PYTHIA 14 TeV p+p)
σ/M vs. M
Mrec vs. Mgen
σ/M ~ 5-10%
(|η|<0.5)
acceptance
correction
dNch/dη vs. η

9. Separation of Soft and Hard Events
Event Selection in p+p collisions
- Min. bias events: usually NSD in experiments. soft is dominant.
- Jet events: particle showers from hard scattering
- High multiplicity events: jets (hard), not-jets (soft)
We usually focus on the hard scattering in p+p, but should not forget
the underlying event or minimum bias events.
The identification of soft and hard interactions is largely a matter of definition.
Can one separate the ‘soft’ from the ‘hard’?
=> Veto on jets (In CDF, ‘soft’ events contain no cluster with ET > 1.1 GeV
based on calorimeter. All other events are ‘hard’)
In ALICE, only charged jets can be reconstructed with tracking system.
(EMCAL will be added later to enhance the trigger capability.)
Ref: PRD (2002)
PRD (2002)
Nch vs. pT(jet)

10. Multiplicity Distributions
PRD (2004)
Multiplicity distribution is a most basic
observable to study the particle production.
At lower energies, the data follow KNO scaling:
At higher energies, the scaling is found to be
broken. This is explained as follows:
(1) contributions from jets
(2) multiple parton interactions
CDF report: in soft events KNO scaling is valid.
Even in a single parton scattering, there are high
multiplicity events.
Thus, multiplicity measurements at LHC will bring
a first indication of the importance of MPI and
soft-hard interplay. Will be a first paper in ALICE.
E735, UA5
1800 GeV
900 GeV
546 GeV
200 GeV
charged multiplicity distributions

11. pT Spectra Transverse momentum of charged particles
can be measured with TPC and ITS.
In 14 TeV p+p collisions, according to PYTHIA,
12% of Min. bias events have pT(hard)>10 GeV/c.
(c.f. 1% at 1.8 TeV)
For 14 TeV p+p, a considerable part of collisions
is expected to have more than one hard (or soft )
scattering.
- CDF report: in soft events, pT distribution is only
determined by multiplicity, and is energy invariant.
In hard events, pT distributions rise with √s.
Data points: CDF
Lines: PYTHIA
14 TeV
1800 GeV
630 GeV
pT resolution vs. pT
(ALICE PPR II)

12. Mean pT vs. Multiplicity
The correlation between <pT> and multiplicity is
attributed to (a) the increase of jet contribution,
(b) radial flow effect (in A+A).
- Since high multiplicity jets have a high mean pT,
it could be expected that high multiplicity events
have higher mean pT.
- CDF report: <pT> also increases with multiplicity
even in soft events. K π
PRD (2002)
soft
hard
CDF data
<pT> follows the same trend on
multiplicity dNch/dh in both p+p
and A+A for pi/K/p.

13. Elliptic Flow Heavy Ion Physics (SPS, RHIC):
v2, ε/2 vs. Centrality
Heavy Ion Physics (SPS, RHIC):
Elliptic flow is sensitive to early parton
dynamics, and HBT is instead affected by
late hadronic interactions and freeze-out.
Does size and shape (parton spatial distribution)
matter for a proton?
=> Measure elliptic flow, HBT in p+p collisions.
- Centrality dependence of v2
PHENIX
Au+Au at √sNN = 200 GeV
Ref: arXiv:
Higher v2 can be obtained in peripheral p+p,
but the multiplicity is lower, the measurement
of v2 is not so easy. Also selecting soft events
is required to distinguish jet contribution.
p+p at LHC (prediction)

14. HBT Measurement Momentum correlations can be measured at ALICE.
The Radii (STAR) scale with charged multiplicity dNch/dη
including various collisions systems (p+p, Au+Au, etc).
The radii measured in p+pbar (E735) increase with charged
multiplicity. The observed behavior may be related to the
space-time evolution of jet hadronization.
C2 vs. qs (ALICE MC)
R vs. dNch/dη (E735)
STAR
Ref: PRD (1993)

15. Summary LHC is Proton+Proton collider, but also heavy ion collider.
What can one do with p+p data while waiting for Pb+Pb?
=> Heavy ion physics with p+p collisions at LHC energy
(... not just reference to heavy ion data)
=> High multiplicity events in p+p collisions
Since the particle density dNch/dη ~ can be reached
even in p+p, it is comparable to mid-central Cu+Cu collisions
at RHIC (√sNN = 200 GeV).
In ALICE experiment, min. bias and high multiplicity triggers are available.
Multiplicity measurement can be done with silicon detector.
A possible way : Selection between soft and hard events a la CDF/STAR
-- with/without charged jets reconstructed with TPC
- Observables: multiplicity, pT spectra, <pT>, elliptic flow, HBT, etc.