Global properties of proton Global properties of proton - - proton events at LHC: proton events at LHC: prospects for a minimum bias physics programme prospects for a minimum bias physics programme Introduction on the motivations for a pp minimum bias physics programme (also in ALICE) Some features of ALICE important for pp runs A review of measurements of global observables from past and present collider experiments, and the expectations at 14 TeV: •Multiplicity Distributions, correlation <p t > vs N ch •Some remarks on the Underlying Event properties Correlations: forward/backward, HBT, intermittency Particle abundances: strangeness Catania, Italy 11-12 Jan 2005 I Convegno Nazionale sulla Fisica di ALICE Marco Monteno – INFN Torino
Transcript
Global properties of protonGlobal properties of proton--proton events at LHC:proton events at LHC:
prospects for a minimum bias physics programme prospects for a minimum bias physics programme
� Introduction on the motivations for a pp minimum bias physics
programme (also in ALICE)
� Some features of ALICE important for pp runs
� A review of measurements of global observables from past and
present collider experiments, and the expectations at 14 TeV:
•Multiplicity Distributions, correlation <pt> vs Nch
Catania, Italy 11-12 Jan 2005 I Convegno Nazionale sulla Fisica di ALICE
Marco Monteno – INFN Torino
o A complete understanding of the physics of collisions between protons at LHC, will require the study of all processes, included those with big cross sections (ranging around 100 mb(σtot), 60 mb(σinel.) and 12 mb(σdiff.) ), which represent the majority of the events. They are generally characterized by the production of particles with low PT.
o At the start-up of LHC, when the luminosity will be still rather low for ATLAS and CMS (1033 cm–2 s–1), ATLAS, CMS, LHCb and ALICE will study minimum bias eventsin the new energy domain at 14 TeV.
o Studies of minimum bias proton-proton events will be necessary also to establish a reference for the measurements with heavy ion collisions (strangeness enhancement, J/ψ and Υ suppression, jet-quenching), and to estimate the background to signals at high-PT
(Higgs, SUSY, etc.) PILE-UP !!!
Therefore a proton-proton programme has
to be considered integral part of the
ALICE experiment
Introduction on pp physics at LHC
Magnetic field (T)
PT cutoff
(GeV/c)
Material thickness: X/X0 (%)
ALICE 02.-0.5 0.1–0.25 7
ATLAS 2.0 0.5 30
CMS 4.0 0.75 20
LHCb 4Tm 0.1* 3.2
o ALICE is sensitive down to very low PT
o Moreover ALICE has remarkable capabilities of particle identification
• ππππ, K, p identified in large acceptance (2π * 1.8 units η) via a combination of dE/dx
in Si and TPC and TOF from ~100 MeV to 2 (p/K) - 3.5 (K/p) GeV/c
•Electrons identified from 100 MeV/c to 100 GeV/c (with varying efficiency)
combining Si+TPC+TOF with a dedicated TRD
•In small acceptance HMPID extends PID to ~5 GeV
•Photons measured with high resolution in PHOS, counting in PMD
ALICE: a very good soft particle tracker
�In ALICE the Minimum Bias trigger is provided by a coincidence between the V0 counters, scintillator layers that cover the pseudorapidity interval –5<η<-3.2 and 1.6<η<4.8. That corresponds to a visible inelastic cross section of ~60 mb.
�The charged particle multiplicity is measured over 8.8 rapidity units,whereas the momentum is measured in the TPC and in the Inner Tracking System (ITS) over 1.8 rapidity units with optimal resolution, and up to 3 units in total.
2 4 6-2-4-6η
0
Multiplicity
measurement
Multiplicity
measurement
Momentum
& Multiplicity
measurement
Arb
itrary
units
2
4
6
8
10
12
TPC
ITS Pixel
FMD FMD
Acceptance of multiplicity measurements in ALICE
The data from the experiments at the CERN ISR collider (< 63 GeV) and those of fixed target experiments show that the multiplicitydistributions (MD) follow the approximate KNO-scaling:
<n> Pn(z,s) = Ψ(z) z= n/<n>
Then, it was discovered in the UA5 experiment (at the SppS collider) that KNO scaling breaks down for c.m. energy ≥ 200 GeV
Up to ISR energies, NSD charged multiplicity distributions in full phase space are fitted remarkably well by Negative Binomial Distributions (NBD) having two parameters: k and <n> , where k can be seen as the number of sources of the observed <n> particles.
A shoulder structure in the tail of the multiplicity distribution starts to appear at the energies of UA5, at the SppS collider (200-900 GeV).
Multiplicity distributions from CERN and Fermilab colliders
n
Pn
0 50 100 150 200 25010-7
10-6
10-5
10-4
10-3
10-2
0.1
1
10
102
E735 1800 GeV
E735 1000 GeV
E735 546 GeV
E735 300 GeV
UA5 900 GeV
UA5 546 GeV
UA5 200 GeV
Multiplicity distributions at SppS and Tevatron collidersin full phase space
UA5 data fall below E735 at high multiplicities, whereE735 data sample are statistically more reliable.
But E735 data are less accurate at low multiplicity
Both sets of data show a shoulder structure, but the Tevatron distributions are somehow wider.
The general behaviour of the Multiplicity DistributionsIn full phase space is uncertain!Extrapolation to higher energyor to full phase space is inaccurate
However, the peculiarities of the multiplicity distributions can be explained in a
multi-component scenario, by alternative approaches:
� The multiladder Pomeron-exchange in the Dual Parton Model� Multiple parton interactions
Walker (2004) analyzed the charged MD arising from pp and ppbar collisions over
the c.m. energy range 30-1800 GeV (from ISR to Tevatron energies), including the
most recent E735 data (at Tevatron collider). He finds that:
�a portion of each distribution does obey KNO scaling (obeyed at ISR energies)
�Those parts of the distributions that do not scale are the result of multiple parton
collisions. They seem to account for essentially all the increase in the NSD
inelastic cross section as the energy grows.
Proton AntiProton
Multiple Parton Interactions
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying EventUnderlying Event
Multi-component models for multiplicity distributions
It has been proposed to describe the observed shoulder structure as the weighted superposition of two classes of events: soft events (without minijets) and
semi-hard events (with minijets), the MD of each component being of NBD type
(Fuglesang, Giovannini-Ugoccioni).
Decomposition of
the multiplicity
distributions for the
1, 2 and 3
parton-parton
collisions
Plot of the partial cross section
For 1, 2 and 3 parton-parton
collisions versus c.m. energy
σ2 at 1800 GeV agrees very well With the value measured by CDFIn hard scattering processes (3jets+ γ) :
14.5 + 1.7 +1.7-2.3 mb
Walker (2004)
c.m. energy (GeV)
<n
ch>
100 1000 100000
10
20
30
40
50
60
70
80
f.p.s.
|η| < 3.0
|η| < 2.0
|η| < 1.0
CDF
E735
UA5
ISR
NA22
Average charged multiplicity versus energy
A simple scaling law for the √s dependence of the particle multiplicity was proposed by Feynman, who predicted a linear rise with ln s. However, th best fit to pp and ppbar data is given by a quadratic polynomial in ln s.
This non-linear dependence suggests that the Feynman scaling is only approximately valid.
The ln2 s term can be
interpreted as the effectof the sharp increase of theminijet production and/or theoccurrence of multiple parton collisions, as
it is also suggested by the rapid increase of <pt> withmultiplicity.
Behaviour of <nch> in the TeV
region based on UA5 data is compared with the E735results (extrapol. to fullphase space with MC): clear discrepancies!
Also the rapid increase of <pT> with multiplicity, firstly observed by UA1 and then confirmed by
UA5, suggests (as the ln2s increase of <nch>) the effect of the sharp increase of the minijet
production and/or double (or even triple) parton-parton collisions.
Correlation of <pT> with charged multiplicity
E735 at Tevatron measured this correlation also separately for identified particles.The correlation appeared very different for π, K and protons.
< p
t > (
GeV
/c)
1800 Gev
630 Gev
Min. Bias
Charged Multiplicity
Rat
io(6
30/1
800)
CDF analysis of Minimum bias events in the two-component scenario (1)
The minimum bias sample was divided
Into two classes
o soft events (without minijets)
o hard events (presence of minijets)
A hard event has been defined as an
event with at least a calorimeter cluster
In |η|<2.4 (a cluster being a seed
calorimeter tower with Et>1 GeV, and at
least a contiguous tower with Et > 0.1 GeV.
Data at two different energies:
630 GeV, 1800 GeV
compared to search for any scaling
properties (KNO scaling of the
multiplicity distributions, or the scaling of
<pT> at fixed multiplicity.
< p
t > (
GeV
/c)
1800 Gev
630 Gev
Soft
Charged Multiplicity
Rat
io(6
30
/18
00
)
< p
t > (
GeV
/c)
1800 Gev
630 Gev
Hard
Charged Multiplicity
Rat
io(6
30
/18
00
)
CDF analysis of Minimum bias events in the two-component scenario (2)
� The soft component is found to satisfy KNO scaling, while the hard one does not
� The <pT> distribution scales at fixed multiplicity in the soft component, and not in
the hard one.
0 5 10 15-0.1
-0.05
0
0.05
0.1200 GeV
q
Hq
0 5 10 15-0.1
-0.05
0
0.05
0.1540 GeV
q
Hq
0 5 10 15-0.1
-0.05
0
0.05
0.1900 GeV
q
Hq
Sign oscillations of higher order momentsof the multiplicity distributions
Hq = Fq/Kq
ratio of factorial cumulants over factorial moments
Fq = ∑ n(n-1)…(n-q+1) Pn
Kq = Fq - ∑ i=1 (q-1
i) K q-1 Fi
Hq vs q oscillations have been explained as the effect of the
weighted superposition of different event classes, each class
being described by a NBD with different parameters.
�in hadron-hadron collisions in the GeV region:
superposition of soft and semihard events
� in e+e- annihilation at LEP energies:superposition of two NBD associated to 2-jet and multi-jet
production (hard gluon radiation)
The observed shoulder structure in the final charged particle
multiplicity distributions and the Hq vs q oscillations have
in this framework same oriigin.
�Most of the the events in a Min.Bias sample are produced by soft collisions of the
beam projectiles, featuring lower multiplicity in the hadronic final state (and showing
KNO-scaling of multiplicity distributions and of the correlation <pT> vs multiplicity (CDF
analysis by Rimondi et al.)
� As energy increases, a larger fraction of p-p collisions is due to hard interactions,
where the scattered energetic partons undergo a fragmentation process that produces
clusters of hadrons called jets (or mini-jets, for lower energy scale of the hard process)
Proton AntiProton
“Hard” Scattering
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying Event Underlying Event
Initial-State
Radiation
Final-State
Radiation
The “underlying event” consists of
hard initial & final-state radiation
plus the “beam-beam remnants”
and possible multiple parton
interactions.
Underlying Event is all
the rest of the event
other than the hard scatter
Multiplicity in Min.Bias and Underlying Event
Charged Jet #1
Direction
∆φ∆φ∆φ∆φ
“Transverse” “Transverse”
“Toward”
“Away”
“Toward-Side” Jet
“Away-Side” Jet
• Look at charged particle correlations in the azimuthal angle ∆φ ∆φ ∆φ ∆φ relative to the leading charged particle jet.
• Define |∆φ∆φ∆φ∆φ| < 60o
as “Toward”, 60o
< |∆φ∆φ∆φ∆φ| < 120o
as “Transverse”,”, and |∆φ∆φ∆φ∆φ| > 120o
as “Away”.
• All three regions have the same size in ηηηη-φφφφ space, ∆η∆η∆η∆ηx∆φ∆φ∆φ∆φ = 2x120o
= 4ππππ/3.
Charged Jet #1
Direction
∆φ∆φ∆φ∆φ
“Toward”
“Transverse” “Transverse”
“Away”
Charged Particle ∆φ∆φ∆φ∆φ Correlations PT >
0.5 GeV/c |ηηηη| < 1
Perpendicular to the plane of the
2-to-2 hard scattering
“Transverse” region is
very sensitive to the
“underlying event”!
-1 +1
φφφφ
2ππππ
0
ηηηη
Leading
ChgJet
Toward Region
Transverse
Region
Transverse
Region
Away Region
Away Region
Look at the charged
particle density in the
“transverse” region!
R.Field - CDF“Underlying Event”as defined by “Charged particle Jets”
Average “transverse” charge particle density (|η|<1, PT>0.5 GeV) versus PT(leading charged jet)
The Underlying Event has an higher multiplicity than an average MB
event. Normalized multiplicity in the Transverse region is twice that
of the ordinary MB events.
Min-Bias Mean Charge density= 0.24
PT distribution of the “transverse” charged particle densities
Charged Particle Density
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0 2 4 6 8 10 12 14
PT(charged) (GeV/c)
Ch
arg
ed
Den
sit
y d
N/d
ηη ηηd
φφ φφdP
T (
1/G
eV
/c)
CDF Run 1data uncorrected
theory corrected
1.8 TeV |ηηηη|<1 PT>0.5 GeV/c
CDF Min-Bias
"Transverse"
PT(chgjet#1) > 5 GeV/c
"Transverse"
PT(chgjet#1) > 30 GeV/c
PYTHIA 6.206 Set A
CTEQ5L
�The Underlying Event has
a harder pT spectrum than a
average MB event
� The harder is the hard
scattering, the more
energy enters into the
Underlying Event
outside the hard scattering
�Minimum bias and underlying event data from the SppS and the Tevatron
have been compared to PYTHIA and PHOJET simulations (A.Moraes et al,
ATLAS).
�The data have been used to tune the PYTHIA 6.214 multiparton collision
model, and good agreement is found. The main parameter of the multiparton
model is the pT cutoff , used in the calculation of the QCD 2 →2 cross section.
� PHOJET 1.12 (an implementation of the Dual Parton Model) also gives
good agreement.
� Predictions are made also for the multiplicities in the minimum bias and
underlying events at the LHC.
�PYTHIA 6.214-tuned and PHOJET 1.12 generate LHC predictions
with ~30% difference for Min.Bias events;
� When extrapolating from Tevatron to LHC energy PYTHIA 6.214
predicts an increase of a factor ~3 in the UE activity, whereas
PHOJET 1.12 suggests more modest increase by a factor 1.5
MC tuning to collider data and extrapolation to LHC
But… effect of partonic saturation at the LHC energies?
D. Kharzeev, E. Levin, M.Nardi“COLOR GLASS CONDENSATE AT THE LHC: HADRONMULTIPLICITIES IN PP, PA AND AA COLLISIONS",hep-ph/0408050, published in Nucl.Phys.A747:609-629,2005
At LHC new regime of QCD: At LHC new regime of QCD: ααss is small,is small,
but perturbative theory is not valid, due but perturbative theory is not valid, due
to to strong non-linear effects at low x.
Parton saturation is expected also in pp.
Color Glass CondensateClassical effective theory (high density
limit of QCD)
The valence quarks of the hadrons (fast
partons) are treated as a source for a
classical color field representing the
small-x (slow) gluons.
However, rather large uncertainties on the
energy dependence of the saturation scale.
LHC data will be a test for the idea of saturation !!!
Forward-backward correlations (1)Cov(nB,nF)
bFB = ----------------------- nB = a + b nF
√Var(nB)Var(nF)
In hadron-hadron b is rather large with respect to e+e- annihilation (no color flow),
and is increasing with c.m. energy. The correlation of nB with nF is reproduced (from
ISR to UA5 energies) by the model of weighted superposition of different event classes.
Forward-backward correlations (2)Prediction for LHC energy 14 TeV
Bose-Einstein correlations (1)
BE correlations are measured in terms of the second order normalized
i.e. the ratio of the two-particle inclusive cross-section over the product of the
single particle (a and b) inclusive cross sections.
R2 is directly related to the Fourier transform of the space-time distribution
of particle production points. Accordingly space distribution and lifetime of
boson sources can be measured. In the standard Goldhaber parameterization:
R2 (Q) = λ exp (- G2 Q2)
λ is the strength of the effect, and G is the measure of the source size.
The effect depends strongly on the masses of the particles used for the
analysis (higher mass particles come from most internal shells)
Bose-Einstein correlations (2)The strength parameter λ of BE effect depends also on Pt intervals, and on related
multiplicity densities, as measured by UA1 at 630 GeV.
Low pt sample: dependence of λ on multiplicity is important to test
theoretical models
High pt sample: the cumulant exceeds 1(which would indicate full coherence in pure
BE correlations). Hence we conclude thatthis data sample is dominated by processes
other than BE correlations.
αq
1 2 3 4 5 60
0.1
0.2
hh, UA1
hh, NA22
order q
1 2 3 4 5 60
0.1
0.2
O+AgBr, KLM
S+AgBr, KLM
Intermittency
Presence of very short range correlations as local fluctuations of multiplicity
distributions in momentum space.
Intermittency:
correlation at all scales
which implies a power-law
dependence of the
normalized factorial moments
Fq(δ) on the size δ of the
phase-space bins
Large particle concentrations in small rapidity intervals observed in JACEE
(cosmic rays) and in NA22. Possible explanation of these spikes related to an
Intermittent behaviour (non statistical, self-similar fluctuations of the particle
density from bin to bin in the phase space)
The intermittency slope αq
was shown to increase,
both in h-h and in A-A.
Fq(δ)/ <n(δ)>q = δ -αq
λS = 2<ss>/(<uu>+<dd>) Wroblewski Factor
pp (√ s = 20 – 1000 GeV) λS ≈0.25 fairly constant
Strangeness content λS
Most recent analyses based on a statistical model of hadronization(Becattini et al) are very accurate in reproducing particle abundances: they show no rise in λs
Open question:
Will λS increase in pp at LHC or not?
K/π ratio
Remarkable stability of the ratio at midrapidity between√s = 27 -1800 GeV
Mild increase of the ratio in full phase space.
K/ π increases with multiplicity
when only low pt data are taken into account, but it is flat if the whole spectrum is analyzed
These behaviours, and the expectation that higher <dN/dη> is linked to the
production of heavier particles can be checked at LHC by ALICE with accurate measurementsof event composition.
Summary
ALICE will offer a unique opportunity to study low pt physics
and minimum bias events, and consequently to hunt substructures in strong interactions, in order to identify the
interface between perturbative and non-perturbative regimes.
Particle multiplicity distributions and particle correlations will
be measured. A unified description will be needed to understand the underlying dynamics of high energy hadronic
