multiple parton interactions (mpi) and high multiplicity (hm) final states at hadron and heavy ion...

Multiple Parton Interactions (MPI) and High Multiplicity (HM) final states

at hadron and heavy ion colliders

Paolo Bartalini (Central China Normal University)

CCNU October 8 2015

NOTA BENE:Multiple Parton Interactions in the same collision vertex!

i.e. MPI is not referring to the trivial pile-up effects

Thanks to Sarah Porteboeuf-Houssais, Fiorella Fionda, Antonio Ortiz Velasquez,Andreas Morsch, Michele Floris, Peter Skands, Leif Lönblad, Markus Diehl, etc. etc. etc.

See also the review paper published in Mod.Phys.Lett. A28 (2013) 1330010.

MPI@LHC: pictorial view

Scale of primary scatter

Scal

e of

sec

onda

ry s

catt

er(s

)pp, pA

, AA

Credits: Picture in background by Ellie Dobson

UE Measurementspp results from all LHC experiments

MB

High Multiplicity

pp. pA. AA resultsRUN 2 higher

energy. More Stat

DPS 2 low scales

4 (mini)jets g + 3 (mini) jets

Multiple HF

DPS 1 high scale

W/Z + (mini)jetsW/Z Multiple HF

CMS, ATLAS, LHCbhave pp results

DPS 2 high scales

W+W, W+Z, Z+Z

High Luminosity LHC

Double parton sc

attering (D

PS)

Soft MPI

Paolo Bartalini - CCNU October 8 2015

RADIATION, SPECTATORS…not enough to account forthe observed multiplicities& pT spectra

The Pythia solution: [T. Sjöstrand et al. PRD 36 (1987) 2019]

Multiple Parton Interactions (MPI)(now available in other general purpose MCs:

Herwig/Jimmy, Sherpa, etc.) Inspired by early observation ofdouble high PT scatterings

pQCD MPI Models

< NMPI > = sparton-parton /sproton-proton

Main Parameter: PT cut-off PT0

Cross Section Regularization for PT 0. PT0 can be interpreted as inverse of effective colour screening length. Controls the number of interactions:

(dampening)

“post HERA” PDFs have increased color screening at low x x g(x,Q2) x-/2 for x 0 PT0

s’ = PT0s (√s’ / √s)e

3

Perturbative description! PT0 ≈ few GeV

Typical event: one leading interaction accompanied by several soft MPI at PT ≈ PT0.More rare events with extra high-pT interactions may even show up.Scale of leading interaction influence NMPI (Pedestal effect, relevant for Underlying Event).Apart from the bias of the leading interaction linear relationship between <Nch> and <NMPI> Paolo Bartalini - CCNU October 8 2015

MPI in soft QCD measurements

4Paolo Bartalini - CCNU October 8 2015

Koba-Nielsen-Oleson (KNO) scaling [Nucl. Rev. B40 (1972) 371].Charged Multiplicity & KNO @ ISR

Scaling variable

ch

ch

N

Nz

Evolution of the charged particle multiplicity distribution in proton-proton collisions P(Nch) with √s follows KNO-scaling with

and )()( chch zNNP Energy independent function

Up to √s≈100 GeV, it works pretty well!

NSD events in full phase space measured by the SFM (Split Field Magnet) at ISR energiesCompilation from J. Phys. G 37 (2010) 083001

Different multiplicity distributions When self normalized : KNO scaling


Koba-Nielsen-Oleson (KNO) scaling

KNO @ SPS & Tevatron Need for MPI

6

At energies greater than √s=200 GeV in pp and pp collisions,

NSD events in full phase space Compilation from J. Phys. G 37 (2010) 083001

Violation of KNO-scaling (√s > 200 GeV) Phys. Lett. B 167 (1986) 476

Deviation from KNO-scaling increases with √s

Can be interpreted as a consequence of particle production through (soft) MPIPhys. Rev. D 84 (2011) 034026 Hep-ph/1106.4959 Phys. Rept. 349 (2001) 301 Hep-ph/0004215 J. Phys. G 37 (2010) 083001


Soft MPI model in PythiaInspired by DPS observation (AFS)

and KNO violation (UA5)In the years ‘80

CMS: Violation of the KNO scaling at the LHC

CMS confirms violation for |h|<2.4. Sensitive effect in the tails (large z = Nch /<Nch>).

Ψ(z

) =

<N

ch>

P(N

ch)

Ψ(z

) =

<N

ch>

P(N

ch)

z = Nch /<Nch> z = Nch /<Nch>

|η| < 2.4 |η| < 0.5

77Phys. Rev. Lett. 105 (2010) 022002Paolo Bartalini - CCNU October 8 2015

No evidence of violation for |h|<0.5.

http://dx.doi.org/10.1103/PhysRevLett.105.022002

Normalized order-q moments Cq = <Nch>q/<Nchq>

8

√s dependence of the normalized moments Cq of the multiplicity distribution for: (a) |η| < 2.4 and (b) |η| < 0.5

(a)ln(s) linear fits(b)constant fits

Violation sensitive for |h|< 2.4, i.e. large pseudo-rapidity.No clear violation for |h|< 0.5 at least up to the order 4.

Phys. Rev. Lett. 105 (2010) 022002Paolo Bartalini - CCNU October 8 2015

If KNO scaling holds Cq are independent from √s.

http://dx.doi.org/10.1103/PhysRevLett.105.022002

Herwig++: Impact of color reconnection on dNch/d h

dNch/dh

ATLAS, pp √s = 0.9 TeV, charged particles with pT > 0.5 GeV & |h| < 2.5

Color reconnection model by Röhr, Siodmok and Gieseke based on momentum structure, implemented from Herwig++ 2.5.

9arxiv 1102.1672

events with Nch ≥ 6

Color reconnection unavoidable to describe basic observables like the pseudo-rapidity distribution of charged tracks

[M. Seymour, 5th MPI@LHC]


http://arxiv.org/pdf/1102.1672.pdf

dNch/dh

10

Naïve picture: if MPIs uncorrelated expect flat <pT> vs Nch

p-p: Color Reconnection in MPI unavoidable to describe <pT> vs Nch and dNch/dh. √s scaling properties driven by Nch

p-Pb: EPOS OK, however shape of dNch/d h qualitatively similar to the Pythia 8 predictions for tune 4C CR (i.e. with Color Reconections).

Pb-Pb: Bad description by HI MCs, shape of in agreement with Pythia 8 predictions for tune 4C NOCR (i.e. with no Color Reconnections).

ALICE, Phys. Lett.B B727 (2013) 371


ALICE: charged particles: <pT> vs multiplicity

– A reminder of the LHC Legacy (see also the back-up slides)• soft QCD measurements with charged tracks in pp inelastic and not single

diffractive events at different √s (0.9 TeV, 2.36 TeV, 7 TeV, 8 TeV).– <Nch>, Nch dNch/dh vs h, <pT> vs Nch. – LHC Tracker detectors turn out to be essential tools for these measurements: full

charged track reconstruction from pT as low as 100 MeV.

• Fast increase of average multiplicities vs √s (> ln (s)) confirmed by LHC data, described by few TH predictions and by tuned pQCD MPI models.

• pQCD MPI models also essential to describe the Nch tails & KNO violation.– KNO scaling seems to apply when restricting to small pseudorapidity range (|h|<0.5).

• <pT> vs Nch studied at different √s– It smoothly rises with Nch and scales with √s!– Interpretation in the context of the pQCD MPI models:

» the deviation from flatness indicate correlations in MPI. » the √s scaling means that all the properties are driven by Nch (NMPI).

Bottom Line on Soft QCD measurements with charged tracks


Multiplicity dependent analyses focusing on High Multiplicity final states

Also triggered by the observation of long range - h fcorrelations in High Multiplicity events (ridge)


13

ALICE: di-hadron azimuthal correlations in p-Pb

Two particle correlation function: Trigger particle → unidentified hadron Associated particle → unidentified hadron 0.7 < pT,ass < pT,trigg < 5 GeV/c

Double “ridge” like structures observed → in order to study the jet-like component, the ridge structures have been subtracted

5% more central 5% more peripheral

Short range (|Δη|<1.2) near (away) side at Δφ=0 (Δφ=π)

Long range (1.2<|Δη|<1.8) near side (Δφ=0) and away side (Δφ=π) simmetrized

Subtraction: short range – long range (symmetrized) correlations

Number of associated particles in the near (<Nass,nearside>) and away (<Nass,awayside>) side calculated by integrating the subtracted Δφ projection

Phys. Lett. B 741 (2015) 38-50

Jet-like Ridge-like


14

ALICE: Number of uncorrelated seeds

increases almost linearly with multiplicity (deviation observed at low multiplicity) → there is no evident saturation of nMPIs at high multiplicities in p-Pb

In pp there is an indication of a limit in the increasing of the MPIs

provides the number of independent source of particle production → in PYTHIA the uncorrelated seeds are found to be proportional to the number of the MPIs

FIRST

INTRODUCED

BY ALICE!

p-Pb data p-p data

PYTHIA 6 pp

JHEP 1309 (2013) 049 Phys. Lett. B 741 (2015) 38-50


15

ALICE: D and J/ψ yields vs charged multiplicity

In pp D meson (2<pT<4 GeV/c) and J/ψ (pT>0) yields (both inclusive and non prompt) show a similar increase with charged particle multiplicity within the current statistical and systematic uncertainties possible to fit with identity! Interpretation driven by pQCD/MPI models: all the yields (in particular Nch and NHF) are proportional to NMPI pp reproduced by Pythia 8, implementing HF production in MPI.

Different magnitude between D mesons and J/ψ observed in p-Pb at large multiplicities however different y and pT ranges


See also Phys. Lett. B712: 165-175, 2012.

16

CMS: Υ(nS) yields self-normalized to their integrated values as a function of particle multiplicity at mid rapidity normalized to the average number

Y(nS) yields increase with multiplicity: similar/linear patterns observed in the three examined systems

CMS: Heavy-flavour vs multiplicity(pp / p-Pb / Pb-Pb)

JHEP 04 (2014) 103

Υ(1S) Υ(2S) Υ(3S)


ALICE: Transverse Sphericity (ST) analysis

17Paolo Bartalini - CCNU October 8 2015 Eur.Phys.J. C72 (2012) 2124

Where l1 and l2 are theeigenvalues of Sxy:

ST ≈ 0 jetty eventsST ≈ 1 isotropic events 3<Nch<10

|h|<.8 pT>0.5 GeV 10<Nch<20

20<Nch<30 30<Nch

Evidence that large multiplicity events are less jetty than expected: no model reproduces the ALICE observations for Nch > 30

Average Transverse Sphericity grows with Nch, as expected.

Sphericity observables may provide additional handles to study large multiplicity features.

CMS: jet pT vs Nch

• Similarly to the centrality classification of events in nuclear collisions, events are sorted according to their charged particle multiplicity:

• Let’s first of all have a look to jets:1. In each event, jets are found with anti-kT algorithm

using charged particles only. The charged particles falling within a jet are further called “intra-jet” particles.

18Eur.Phys.J. C73 (2013) 2674

[M. Azarkin, MPI@ LHC 2013, Antwerpen]

MPI clearly needed to describe jet pT vs Nch

Pythia 6&8 with status of the art tunes OK up to Nch≈ 80However too many hard jets in the highest multiplicity rangeHerwig++ has always less hard jets than data.

50 < Nch < 80

110 < Nch < 140


19

ALICE: Identified hadrons at low-pT vs charged multiplicity (p-Pb)

Multiplicity dependence of <pT> for identified particles:clear mass ordering → indication for a collective expansion with a common velocity field.

The same kind of mass ordering is also qualitatively expected from colour re-connections [A. Ortiz Velasquez et al. Phys. Rev. Lett. 111 (2013) 4, 042001]

Similar evolution of the blast–wave parameters with increasing

multiplicity in p-Pb and Pb-Pb

PYTHIA8 pp events (no hydrodynamic evolution) also show the same trend (albeit at a 30% smaller Tkin)

MPI + Color Reconnection causes similar effect as radial flow

ALICE, Phys. Lett.B 728 (2014) 25

Increasing multiplicity

ALICE: Blast-wave model fit (thermal+collective)

ALICE, Phys. Lett.B 728 (2014) 25


– ALICE: Primary Seeds Experimental progress in direct detection of soft MPI• Hints of saturation of Number of Primary Seeds vs Nch in pp, no saturation in pPb.• Linear evolution of event yields in pp and pPb (ALICE, CMS)

– ALICE: Transverse Sphericity analysis (pp).• Events look more spherical with respect to the predictions of the QCD models. • confirmed by the CMS systematic analysis of pp High multiplicity events, that turn out to be

much less jetty than predicted by Pythia. • In the context of the pQCD MPI models HM events can be regarded as the result of several

soft MPI instead of being contributed by fewer hard MPI (ending up producing jets).

– ALICE: Multiplicity dependence of <pT> of identified particles• indication for a collective expansion with a common velocity field, • Pattern reproduced qualitatively by Pythia MPI with Color Reconnections.

– Several pending issues, exciting progress perspectives in the LHC RUN II, in particular for what concerns the data collected by the ALICE high multiplicity triggers.

20

Bottom line on Multiplicity dependent analayses


The Double Parton Scattering (DPS)

Detecting patterns of two (or more) High-pT Multiple Parton Interactions


MPI@LHC: pictorial view


Scal

e of

sec

onda

ry s

catt

er(s

)pp, pA

, AA


UE Measurementspp results from all LHC experiments

MB

High Multiplicity


energy. More Stat

DPS 2 low scales


Multiple HF

DPS 1 high scale



DPS 2 high scales

W+W, W+Z, Z+Z

High Luminosity LHC

Double parton sc

attering (D

PS)

Soft MPI


• sDPS(A+B+X) = m * s(A+X) * s(B+X) / seff – m = ½ for identical interactions, m = 1 otherwise.– Probabilistic interpretation: P(B|A) = P(B) * (sINEL/seff).– Formalism applies to inclusive processes only.– σeff can be regarded as a hadronic form factor.– Huge ongoing TH effort to understand correlations: IP, Flavour, Spin, Color, …

• Under the assumption of no correlations: – σeff ≈ geometrical quantity, in principle scale and √s independent. [D.Treleani et al.]

– Prediction: seff = 36÷41 mb. [M.Diehl et al.]

• Measurements use the relationship in the following way: – seff = m * s(A+X) * s(B+X) / sDPS(A+B+X).– Need an accurate Single Parton Scattering (SPS) background.– Checking Scale and √s independency is in the EXP TODO list.– Statistics often limits the possibility to extract seff in a differential way.

23

Effective cross section seff


The Double Parton Scattering (a)

in final states with jets


pT(jet 1)pT(jet 2)

pT(jet 4)

pT(jet 3)

Disentangle double-parton-scattering from bremsstrahlung

• No correlation (DPS) vs Strong correlation (SPS)After PAIRING, one can define different correlation angles between jet pairs:

AFS solution:• Study Δφ between pT1 - pT2 and pT3 - pT4

CDF solution:• Study Δφ between pT1 + pT2 and pT3 + pT4 (CDF nomenclature: ΔS)

pT(jet j)

pT(jet i)pT(jet l)

pT(jet k)

Double Parton Scattering in “objects” topologies


Measurement of DPS @ Tevatron (3jet + g)

26

Double high PT interactions observed by AFS, UA2 and CDF in 4jets topologies.

CDF and D0 use also 3jet + gIMPROVED PAIRING!

seff

~ 14 mb

DPSD0: seff

~ 16 mb [Phys.Rev. D81 (2010) 052012]

+ recent update with HF jets [arXiv:1402.1550] Are the SIGNAL and BACKGROUND templates used in these analyses reliable?- A lot of emphasis in the definition of a data oriented methodology for the SIGNAL- However the big issue is the BACKGROUND modeling: Single Parton Scattering (SPS) with direct photon and three extra jets is not trivial at all (modern ME tools badly needed but these tools were not available in the last century…)

Effect of triple interactions:

seff

~ 11 mb

[Treleani et al., PRD76:076006,2007]

(based on the CDF paper)

[CDF Collab, Phys. Rev. Lett. 79, 584 (1997)]


The Double Parton Scattering (b)

looking for extra di-jets in events with heavy bosons

(the W+2jets benchmark)


CMS: DPS in W mn + 2 jets

Event selection:– Exactly one m– with pT > 35 GeV, |h| < 2.1 – Required to be isolated and to pass tight ID criteria– particle flow Missing Transverse Energy, MET > 30 GeV – transverse mass of (m and MET) > 50 GeV– Exactly 2 anti-KT jets with pT > 20 GeV and |η| < 2.0(not inclusive, need correction to use the seff formalism)

Data: Collision data at √s = 7 TeV, Single Muon data streams with integrated luminosity of ~ 5 fb-1

28

Unfolding

High stat.SPS: DPS:

Along the lines of the ATLAS experience[New J.Phys. 15 (2013) 033038] with more MCs, Unfolding, higher stat, more observables, etc.

DPS signal fractions from fit to templates

Drel pT = Relative pT balance of the di-jet system.DS = Angle between total momenta of paired objects (mn, di-jet) projected in the transverse plane.

JHEP 1403 (2014) 032

– Signal at small DS (DPS is flat while SPS is peaked at p) and small Drel pT (back-to-back di-jet in transverse plane).– Signal Template combining W+0jets and di-jet samples.– Backg. Template Madgraph+Pythia 8 (no jets from MPI).– No double counting or phase space gaps.– DPS signal fraction from simultaneous fit to Drel pT and DS. – Extract seff from signal fraction.


[S.Bansal, 5th MPI@LHC]

CMS: W ln + 2 jets – Data vs Models (particle level)

29JHEP 1403 (2014) 032 Paolo Bartalini - CCNU October 8 2015

• First results on 4jets already 30 years ago: AFS , UA2: seff < 10 mb.

• Tevatron measurements from the years nineties + updates (4jets, 3jet+ ): g σeff ≈ 10÷15 mb.– Insufficient effort on background

modeling in early measurements.

• LHC (W+2jet, etc.) seff ≈ 15÷20 mb.• NEW: compatible with the seff figures

from MC tuning relying on soft QCD Measurements at the LHC.

• Still well below the trivial TH prediction under the no correlations assumption: seff = 36÷41 mb.

30

Effective cross section seff

30

Time

Large systematics frombackground modeling


DPS in pA collisions: W(ln)+2jet

Enhanced shoulder at ≈ 40 GeV in pA interactions

[D.Treleani, 5th MPI@LHC]

LHC Analysis feasible in RUN II ?

31DPS/SPS ratios turn out to be enhanced of a factor A1/3 in pA with respect to the corresponding ones in pp Paolo Bartalini - CCNU October 8 2015

The Double Parton Scattering (c)

the charm laboratory


DPS & Open charm production at the LHC


DPS essential to describe HF x-sections (in particular for multiple HF production) Double HF rates can be used to quote DPS:

R.Maciula and A.SzczurekPhys. Rev. D87, 074039 (2013)

LHCb collab. Phys.Lett. B707 (2012), JHEP 1206 (2012) and

J.Phys. G41 (2014) 11, 115002

P(DPS) = (P(SPS))2 * (sINEL/2seff)

LHCb: Z + D

LHCb-PAPER-2013-062 34

[A.Bursche, 5th MPI@LHC]

pp collisions at √s = 7 TeV1 fb-1

VERY FEW EVENTS, HOWEVERSPS BACKGROUND SMALL!


– First results from the year eighties– Huge recent TH and EXP progress

• Improved MPI formalism to connect TH to EXP• Better comprehension of systematic effects, usage of ME tools to

describe the SPS backgrounds.• Redundant Measurements of seff in different final states and at

different √s– Still large uncertainties mostly arising from background modeling– No differential measurements yet– Consistency with UE measurements (MPI Universality)

• Important Spin-offs: HF production at the LHC, background to searches etc.

3535

Bottom Line: Double Parton Scattering


The Underlying Event

Impact on isolations, jet pedestals, vertex reco etc.

Actually UE is interesting per se: handle on soft MPI and beam remnants.

36

Measuring the complementary activity in the presence of a hard scattering

Relying on charged Tracks is very convenientTracker detectors @ LHC allow to reconstruct

Tracks with pT as low as ≈ 100 MeV

Leading object (i.e. leading track or jet or DY, etc.)


ALICE UE activity (pp @ 7 TeV)

Transverse Region UE Measurement: Fast rise followed by plateau. Indication of two different regimes (two scale picture). MPI rise dominates at low pT, initial- and final- state radiation matter at higher pT.

Leading charged particle analysis, the same effects seen also on jet analysis.

37

Parti

cle

Den

sity

P

P

PP

Lower average transverse dimension of gluons with respect to quarks also

explains UE in DY < UE in Jets [next slide]

GPDF Analysis “Inter-parton correlations and MPIs”. [M.Strikman, Phys. Rev. D83 (2011) 054012]

JHEP 07:116, 2012.


),(),(

11LTpN

LTpN TchTev

CMS: UE activity in Jets and Drell-Yan Z(mm) eventsTransverse

UE Measurements in (track) Jets: along the lines of the ALICE measurement.UE Measurements in Drell-Yan:

MPI saturated. Radiative increase of UE activity with pT di-lepton. Constant vs Mdi-lepton. Min activity around 80% with respect to the plateau in jet events.

81 GeV < Mmm < 101 GeV

38Eur.Phys.J. C72 (2012) 2080.

In the GPDF framework this is explained as the lower average transverse dimension of gluons with respect to quarks


– Two scale picture in the case of jet events: rise at low pT + plateau at a rather modest energy dependent pT (O(few GeV)).

high pT jets select central collisions hence large MPI multiplicity.– Single scale picture (plateau) in the case of DY.

DY events always select central collisions hence large MPI multiplicity. First MC tunes describing at the same time UE and DPS measurements are now on

the market: seff ≈ 20 mb. Clearly we are dealing with the Same physics! (MPI)

3939

Bottom Line: Underlying Event Measurements


ALICE HIGH MULTIPLICITY TASK FORCE (HMTF)

• Among the LHC collaborations, ALICE has an unique opportunity to collect a large sample of low pile-up High Multiplicity pp data in RUN II

• The corner of the phase space studied in the CMS “ridge” (7 TeV data) analysis corresponds to multiplicities ten times higher than the average: – dN/dη|h|<0.5 ≈ 55 ≈ 10 x〈 dN/dη|h|<0.5〉

• In terms of cross sections this corresponds to ≈ 10-5 x sINEL

• Goal of 1.6 pb-1 in 2015

40

Events (1.6 pb-1) Rejection factor z=dN/dh / <dN/h>

100 M 10-3 6.5

10 M 10-4 8

1 M 10-5 10

High Multiplicity

Trigger Requirements


The HM triggered data are collected by requiring a large charge in the forward scintillators (V0 detector) or a large number of fired FO Silicon Pixel Detector chips above the threshold values.

A reduction factor of ~10-3 is currently used online on the SPD and V0 trigger to collect HM events.

The plot shows the integrated luminosity for the data collected by SPD and V0 HM triggers in RUN II.The Di-muon trigger is also shown (not relevant here).

Latest HM triggered data collected by SPD and V0 HM triggers

[Prabhakar Palni, QM2015]

ALICE HMTF: Example of Expected Physics Performances

[Prabhakar Palni, QM2015]

• Variation of the Cascades/Pion yield ratio with respect to the charged multiplicity density in |eta|<0.5.

We expect to extend the measurement in this region using 13 TeV pp data

– Unique sample of High Multiplicity – Low Pile-up data in the LHC RUN II.

– RUN I Lessons.• Trigger / Background Rejection.

– Promising early performances– Confident to extend the z = dN/dh / <dN/h> reach

• 1M events @ z=10

– Relevant for the study of small systems & MPI@LHC

43

ALICE HMTF Bottom Line


MPI@ALICE: bottom line


Scal

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sec

onda

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catt

er(s

)pp, pA

, AA


UE MeasurementsFor now pp only

MB

High Multiplicity


energy. More Stat

DPS 2 low scales


Multiple HF

For now pp only

DPS 1 high scale



DPS 2 high scales

W+W, W+Z, Z+Z

High Luminosity LHC

Double parton sc

attering (D

PS)

Soft MPI


BACKUP


46

ALICE Soft QCD & MPI references (p-p data)

• Underlying event.– JHEP 07:116, 2012.

• Multiplicity distributions and spectra.– Eur. Phys. J. C68: 345-354, 2010; Eur. Phys. J. C68: 89-108, 2010; Phys. Lett. B693:

53-68, 2010.

• Average transverse momentum vs. Nch.– Phys. Lett. B727: 371-380, 2013.

• Average transverse sphericity (ST) vs. Nch.– Eur.Phys.J. C72 (2012) 2124.

• Uncorrelated seeds vs. Nch.– JHEP 1309 (2013) 049

• J/ψ (and open charm) production vs. Nch.– Phys. Lett. B712: 165-175, 2012; (+Preliminary)


47

ALICE Soft QCD & MPI - references (p-Pb data)

• Underlying event.• Multiplicity distributions and spectra.

– PRL 110, 032301, 2013; ALICE, Phys. Lett.B 728 (2014) 25.

• Average transverse momentum vs. Nch.– Phys. Lett. B727: 371-380, 2013.

• Average transverse sphericity (ST) vs. Nch.

• Uncorrelated seeds vs. Nch.– Phys. Lett. B741: 38-50, 2015.

• J/ψ (and open charm) production vs. Nch.– (+Preliminary)


48

Diffraction - References

• Measurement of inelastic, single- and double-diffraction cross sections in proton--proton collisions– Eur. Phys. J. C (2013) 73:2456

Ambitious plans for diffraction measurements in RUN II thanks to the installation of dedicated scintillator counters extending the pseudorapidity coverage up to |h| ≈ 7.


ALICE performances


D mesons D0-> K-π+

D+->K-π+π+

D*+-> D0π+

Ds+ -> φπ+->K-K+π+

|ƞ| < 0.9ITS: tracking, vertexingTPC: tracking PIDTOF: PID

HF decay electrons, J/Ψ and non-prompt J/ΨD, B, Λc , … -> e + XJ/Ψ -> e+e-

B->J/Ψ -> e+e-

|ƞ| < 0.9ITS : tracking, vertexingTPC : tracking PIDTOF, EMCAL, TRD : e-ID

HF decay muons and J/ΨD, B, Λc , … -> μ+ XJ/Ψ -> μ +μ-

-4 < ƞ < -2.5Muon Spectrometer : trigger and μ -ID

Trigger,Centrality determinationVZERO

ALICE

50

extremely low-mass tracker e.g. low material budget particle identification down to pT ~ 100 MeV/c

vertexingHMPID

ITS TPC

TRD

TOF

ALICE Particle identification


Other results


DPS


ATLAS: W ln + 2 jets

54

Single partonscattering

- Measure fraction of W0 + 2jDPI in the W+2jet sample - use difference in kinematics (pT, …)

- Extract seffW selectionSingle lepton trigger1 lepton (e, μ) pT > 20 GeV, η < 2.5MET > 25 GeV, mT > 40 GeV

Jet selection(Minimum bias trigger used to measure di-jet x-section alone)2 jets, pT > 20 GeV, |h| < 2.8

W+2jets W+MPI

New J.Phys. 15 (2013) 033038.

double parton scattering


ATLAS: W ln + 2 jets : DPS Rate

55

- Focusing on the jet pT balance in the transverse plane: Dnjets = Ιpjet1+pjet2Ι/(Ιpjet1Ι+Ιpjet2Ι)

- Two different MC sets used to describe the inclusive W production:Alpgen+Herwig+Jimmy (A+H+J) reported on the left plot and Sherpa reported on the right plot.

- Notice that inclusive W production contains both the SPS and the DPS components- Of course other (i.e. non W) Standard Model backgrounds need to be taken into account.

New J.Phys. 15 (2013) 033038.

DPS signal expected at small Dn jets

(back-to-back di-jet in transverse plane).


ATLAS: W ln + 2 jets : DPS Rate

56

- Subtraction of other (i.e. non W) Standard Model Backgrounds.- Extraction of fR

DP using fit to data with two templates:- Template A (SPS sample): both jets originate from the primary scatter (MC relying on ME tools)- Template B (DPS sample): both jets originate from the DPS scatter (from data).

New J.Phys. 15 (2013) 033038.

Signal at small Dn jets

(back-to-back di-jet in transverse plane).

(1- f(D)DP)*A + f(D)

DP*B


Bottom line on ATLAS W ln + 2 jets

57

Low luminosity. Good for pile-up rejection.

Exclusive W+jets: no ambiguity in the choice of the DPS-jets candidates.

Data oriented signal.

Usage of modern ME tools for the background description (SHERPA etc.)

First DPS result at the LHC (consistent with previous measurements at lower energies).

The measurement is not inclusive however the appropriate formalism is used to compute seff

Hybrid SPS (from MC) vs DPS (from DATA) makes hard to guarantee the unitarity requirement.

The analysis relies on just one DPS observable.

Statistical uncertainty is significant (comparable to the sum of systematic uncertainties). Integrated luminosity 36 pb-1 only.

New J.Phys. 15 (2013) 033038.Paolo Bartalini - CCNU October 8 2015

Using different observables may bring to significant differences in seff extraction

(see the Monte Carlo test reported in the next slides)

• Df (also called Sf(2jets))– Angle between the momenta of the extra-jets projected in the transverse plane.

• Drel pT (also called Dnjets and SpT(2jets))

– Ιpjet1+pjet2Ι/(Ιpjet1Ι+Ιpjet2Ι) where pjet1 and pjet2 are the jet momenta projected in the transverse plane.

• D pT (also called Djets)– Ιpjet1+pjet2Ι where pjet1 and pjet2 are the jet momenta projected in the transverse

plane.• DS

– Angle between total momenta of paired objects projected in the transverse plane.– Widely used in published DPS phenomenology (3jet+g analyses)

58

DPS Observables in W ln + 2 jets + X analysis


Pseudo-data experiment (pseudo-data = Madgraph W+njets matched to Pythia 6 with MPI on)

MC = Sherpa W+njets with MPI on, Extra-DPS-Signal = Pythia 8 W+2jets DPSFitted DPS Signal fraction and reduced χ2 reported in the table

59

Df DS

[R.Kumar, 4th MPI@LHC]

Drel pT MC


Pseudo-data experiment (data = Madgraph with MPI on)

MC = Sherpa W+njets with MPI on, Extra-DPS-Signal = Pythia 8 W+2jets DPSFitted DPS Signal fraction and reduced χ2 reported in the table

60

Df DS

[R.Kumar, 4th MPI@LHC]

Drel pT MC

Conclusions:●Uncertainties and bad fits seen for W+0jet, W+1jet indicate that we can trust only ME tools having at least 2 extra emissions general purpose MCs ruled out.●Identical results in rows for W+2jet and W+3jet indicate that adding the 3rd emissiondoes not affect the results in a significant way.●Fitted signal fraction significantly different from 0% means that Sherpa and Madgraph tunes have different intrinsic DPS content MadGraph+PS has more DPS than Sherpa.●The choice of the observable influence the quoted DPS fraction.


LHCb: Z + D

LHCb-PAPER-2013-062 61

[A.Bursche, 5th MPI@LHC]

pp collisions at √s = 7 TeV1 fb-1

VERY FEW EVENTS, HOWEVERSPS BACKGROUND SMALL!


In this study from ATLAS the opposite approach is adopted

- seff from W+2jets is assumed in order to estimate the DPS contribution to W+J/y

ATLAS: W + prompt J/ y

SPS predictions still below the DPS subtracted data. DPS higher than expected?

62

[C.Melachrinos, 5th MPI@LHC]

ATLAS-CONF-2013-042Paolo Bartalini - CCNU October 8 2015

LHCb: Double J/y Production

Phys. Lett. B 707 (2011) 52.

pTm > 650 MeV ( + - m m channel)

3.0 < m + -m m < 3.2 GeV, 2 < yJ/y < 4.5, pT

J/y < 10 GeV

2×J/y : fit one mm-mass in bins of another mm-pair mass: 141±19 events

63

sJ/yJ/y = 5.1 +- 1.0 (stat) +- 1.1 (syst) nbi.e. around 20% higher than the SPS predictions contribution from DPS?

SPS Prediction for mJ/yJ/ y (in orange) includes direct production and feed down from (y 2S).

See [S.P. Baranov, A.M. Snigirev, N.P. Zotov, Phys. Lett. B 705 (2011) 116–119]


http://dx.doi.org/10.1016/j.physletb.2011.12.015



64

CMS: Double J/ y production

However SPS at large Dy is expected to be very suppressed. Although seff is not quoted, to date, this is one of the most clear evidences of DPS production at colliders. [D.Ciangottini, 5th MPI@LHC]

DPS

CMS PAS, BPH-11-021

LHCb

CMS

Computations of Double-quarkonium production via SPS at the LHC not developed for CMS acceptance e.g. assume dominance of color singlet productIon.‐(P.Ko et al., Novoselov et al., Berezhnoy, et al., etc.) Differences expected in Dy and pT J/yJ/y distributions.


UE


√s = 0.9 TeV √s = 2.76 TeV √s = 7 TeV

Energy deposited in CASTOR (5.2 < |h| < 6.6) for events with a charged particle jet in the central pseudorapidity region |hjet| < 2, as a function of charged particle jet transverse momentum pT (normalized to the average energy

in inclusive events) pT evolution of observable changes trend with √s (decreasing at low √s, increasing at high √s)Interpretation: at low √s drag fragmentation effects, at high √s the major role of the MPI is restored. pQCD models adopting pT-ordered showers and tuned@LHC in the central region are favored by forward data (agreement within 5-10%) Good agreement also for EPOS 1.99, QGSJET01, QGSJETII-03, SIBYLL 2.1 (within 20%)

CMS: UE as Energy flow in the VERY forward region

JHEP 1304, 072 (2013). 6666Paolo Bartalini - CCNU October 8 2015

COLOR RECO


Herwig++: Impact of color reconnection on dNch/d h

dNch/dh

ATLAS, pp √s = 0.9 TeV, charged particles with pT > 0.5 GeV & |h| < 2.5

<pT> vs Nch

Color reconnection model by Röhr, Siodmok and Gieseke based on momentum structure, implemented from Herwig++ 2.5.

68arxiv 1102.1672

events with Nch ≥ 6

Color reconnection unavoidable to describes the shapes of pseudo-rapidity and <pT> vs Nch. [M. Seymour, 5th MPI@LHC 2013

http://arxiv.org/pdf/1102.1672.pdf

MPI in soft QCD measurements


ALICE & CMS: Single charged particle spectra: dNch/dhThe very first LHC paper was the The dNch/dh measurement reported by ALICE!

The dNch/dh distributions are corrected for Not Single Diffractive (NSD) events.

At 7 TeV: dNch/dh|(|h|<0.5) = 5.78 ± 0.01 (stat.) ± 0.23 (syst.) for non-single-diffractive events.Relative increase from √s = 0.9 to 7 TeV = (66.1 ± 1.0 (stat.) ± 4.2 (syst.))%.LHC measurements clearly confirm trend to have a rise stronger than ln (s).


71

Underlying event measurement – Number density

JHEP 1207 (2012) 116

Comparison of number density in the plateau of the Transverse region (underlying event)And dNch/dƞ in minimum bias event

UE number density grows logarithmically and faster than MB dNch/dƞ


72

• Average transverse momentum vs Nch.• Underlying event.• Average transverse sphericity (ST) vMini-jet

analysis vs Nch.

• J/ψ production vs Nch.

Eur. Phys. J. C68: 345-354, 2010. Eur. Phys. J. C68: 89-108, 2010.

ALICE: Charged Multiplicity

The number of events used in this analysis corresponds to

about 0.15 M, 0.04 M and 0.3 M interactions for the 0.9, 2.36 and 7 TeV data, respectively.


Long Range Correlations


Definition of di-hadron correlation functionBackground distribution:Signal distribution:

Particle pairs from same event

Pairs frommixed events

Ratio Signal/Background = Associated 2D Yields:

Can specify selected pair of trigger (pT

trig ) and associated (pTassoc )

particle pT ranges.

bins


Long and short range correlations

Bose-Einstein correlations: (Δφ,Δη) ~ (0,0)

pp minimum bias events √s = 7 TeV

Momentum conservation:~ cos(Δφ)

“Away-side” jet correlations:Correlation of particles between

back-to-back jets

“Near-side”, Δφ~ 0 jet peak:Correlation of particles

within a single jet

Short-range correlationsResonances, string or cluster fragmentation


CMS: Associated 2D Yields for Particles with PT > 0.1 GeV

The jet peak is cut for better visibility of the correlations. Jet peak correlations with away-side – stronger in the high multiplicity events No significant “new” structure seen in the high multiplicity events.

Special trigger developed to collect these rare O(10-5) events. It doesn’t rely on jet triggers!

MinBiasHigh Multiplicity (N>110)pp interactions at 7 TeV

76J. High Energy Phys. 09 (2010) 091Paolo Bartalini - CCNU October 8 2015

http://dx.doi.org/10.1007/JHEP09(2010)091

CMS: Associated 2D Yields for Particles with 1.0 < PT < 3 GeV

MinBias High Multiplicity (N>110)

Limiting pT of particles to 1<pT<3 GeV Gives a pronounced structure at large Δη around ΔΦ=0 in the high multiplicity events.


77J. High Energy Phys. 09 (2010) 091.Paolo Bartalini - CCNU October 8 2015



CMS: Associated 1D yields in bins of pT and Nch

Incr

easi

ng m

ultip

licity

Increasing pTLong range:Project 2 < |Δη| < 4.8 onto Δφ:

Ridge most pronounced for high multiplicity events and at 1 < pT < 3 GeV. No ridge seen in tested MC models (Pythia 8, Pythia6, Herwig++, etc.) Several interpretations proposed forthis HI-like effect in pp interactions. Clear major role of Multiple Parton Interactions.[S. Alderweireldt, P.Mechelen arXiv:1203.2048]


78J. High Energy Phys. 09 (2010) 091.Paolo Bartalini - CCNU October 8 2015

http://arxiv.org/abs/1203.2048



Same approach as in pp ridge paper for “apples-apples” comparison

CMS: Ridge in p-Pb interactions

79Phys. Lett. B718 (2013) 795-814.

Average over ridge region(2<|Δη|<4)

Same qualitative conclusions.From a quantitative point of view the effect is much larger in p-Pb than in pp. Why???

Neither HIJING nor the tested hydro model do describe the ridge

More successful models: AMPT, EPOS etc. See the talk of Klaus Werner at QM2014

Average over |Df|<1.2


80

ALICE reports double “ridge” structure in p-Pb interactions at √sNN = 5.02 TeV.

Phys. Lett. B 719 (2013) 29.

ALICE: Double Ridge in p-Pb interactions

Correlation profile for lower multiplicity data (60%-100% lowest) is subtracted from the one for higher multiplicity (20% highest), revealing a second ridge at ≈ Df p identical to the first one at ≈ 0.Df corroborating and extending the results previously reported by CMS.


81

0-20% 60-100% 0-20% – 60-100%

Only significant contribution from second Fourier coefficient v2 → see next slide

First coefficient smaller w.r.t. the case without subtraction (up to ~10 times smaller)

Third coefficient still small

Phys.Lett. B726 (2013) 164-177

ALICE: h-(π, K, p) long range correlations in p-Pb

Two particle correlation function: Trigger particle → unidentified hadron Associated particle → identified hadron (π, K, p) 0.7 < pT,ass < pT,trigg < 5 GeV/c

Double “ridge” like structures observed → in order to study the jet-like component, the ridge structures have been subtracted

Jet-like Ridge-like


82

Similar behaviour as in Pb-Pb collisions → mass ordering at low pT qualitatively consistent with hydro models

MPI + Color Reconnection also at the origin of flow-like pattern in p-Pb ? → still open question

See S. Alderweireldt and P. Van Mechelen, arXiv:1203.2048 [hep-ph]

p-Pb Pb-Pb 10-20%

Two particle correlation function: Trigger particle → unidentified hadron Associated particle → identified hadron (π, K, p) Same p

T interval for trigger / associated particles

Ridge like component isolated by subtracting low multiplicity correlations (60-100%) from high multiplicity correlations(0-20%):

Mostly jet contribution (i.e. no significant ridge) in low multiplicity p-Pb events

h-(π, K, p) long range correlations in p-Pb


Summary and Conclusions• Long-range correlations:

– Significant ridge structures are observed in high multiplicity pp (√s = 2.76 and 7 TeV), p-Pb (√sNN = 5.02 TeV) and Pb-Pb (√sNN = 2.76 TeV) collisions.

– effect showing up in the intermediate momentum range: pT = 1-4 GeV– strong mechanism to produce particles in a plane.– Pb-Pb expected from the elliptic flow.– p-Pb and pp observations still miss an agreed interpretation.– The size of the effect is huge in p-Pb.– Second ridge structure also detected in p-Pb.Interpretation: Large multiplicities without pronounced jetty structures point to an important role played by Multiple Parton Interactions.

– Angular momentum conservation?– Color reconnections?– AMPT is successful however it also relies on pQCD MPI for the description of the initial state.

• Reminder: Short-range (BE) correlations: – The radius of effective emission region (r) grows with Nch

– r vs Nch scales with √s– More info in the back-up slides

Correlations: bottom line


UE


85

ALICE Underlying event measurement

Energy scale given by the leading particle(approximation to the original outgoing parton

momentum)

pp collision description inspired by pQCD models with Multiple Parton Interactions

1 hard scattering on top of underlying event (UE) activity

UE = fragmentation of beam remnants+ MPI+ initial and final state radiation (ISR/FSR)

Toward: |Δϕ|<π/3

Transverse: π/3<|Δϕ|<2π/3

Away: |Δϕ|>2π/3

Study of charged particle in the three regions gives insight into the UE behavior

√s=0.9 and 7 TeV, pT threshold: 0.15, 0.5 and 1 GeV/c


86

Underlying event measurement – Δϕ-correlation

d

dN

LTpNch

Tev ),(

11Δϕ-correlation between tracks and the leading track

JHEP 07:116, 2012. Paolo Bartalini - CCNU October 8 2015

87

Underlying event measurement – Number density√s = 0.9 TeV √s = 7 TeV

Toward and Away

Toward Toward

Transverse Transverse

Away Away

Monotonic increase with leading pT

regions dominated by jet fragmentation, ↑ leading pT: ↑ hadronic activity

Transverse

Plateau at ~4 GeV/c : production independent of hard scale

Increase interpreted in term of MPI

),(),(

11LTpN

LTpN TchTev

Average charged particle density vs. leading track transverse momentum pT,LT

In PYTHIA: higher pT biases towards more central collisions (higher MPI probabilities)

JHEP 07:116, 2012. Paolo Bartalini - CCNU October 8 2015

SPHERICITY/SPHEROCITY

89

Using Event Shapes:Exercising with the existing data


Example of MPI Spin-off

Centrality determination in pPb


91

Centrality determination in ALICE

Centrality classes are defined as percentiles of the multiplicity/summed-amplitude distributions

For a given centrality class the information from the Glauber MC in the corresponding generated distribution is used to calculate the mean number of participants <N

part>, the mean number of

collisions <Ncoll

>, and the average nuclear overlap

function < TpA

> (< TAA

>)

Bias observed in p-Pb collisions! (→ next slides)

Similar approach used in Pb-Pb and p-Pb in ALICE: multiplicity distribution of a given “estimator” (i.e. V0A multiplicity) fitted by Negative Binomial Distribution(NDB)(*) + Glauber MC. Ingredients:

Glauber MC: given the σNN and assuming dP/db ~ b → this gives Npart, Ncoll, TpA (TAA) event-by-

event basis (b randomly changed and NN interaction happens if bNN < √σNN/π )

NBD function used to represent the multiplicity distribution for the “estimator” (e.g. V0A) for a given Npart

convolution Npart from Glauber + NBD → used to fit the reconstructed multiplicity distribution

(e.g. VZERO amplitude in Pb-Pb) Phys. Rev. C 88 (2013) 044909


92

ALICE: biases on centrality determination in p-Pb

Much smaller bias in Pb-Pb

arXiv:1412.6828v1

<M

ult>

/<N

part>

(est

imat

or) /

<M

ult>

/<N

part>

(b)arXiv:1412.6828v1

Multiplicity bias: compared to Pb-Pb collisions, in p-Pb collisions the correlation between the centrality estimator and Ncoll is very loose: Same Npart (Ncoll) can contribute to several adjacent centrality classes

Geometric bias: for a given p-A impact parameter (b), the mean number number of hard scatterings <nhard> depends on the average p-n impact parameter (bNN). This is mainly important for peripheral collisions


(first studies in Jiangyong Jia, Phys.Lett. B681 (2009) 320–325, arXiv:0907.4175 [nucl-th].)

93

Q pA ( pT ; cent )=dN pA /dpT

N collGlauber⋅dN pp /dpT

centrality classes determined using energy deposit in ZNA (Pb-going side) calorimeter

Number of binary collisions <Ncoll

> determined by studying correlation

of various pairs of observables, in ZNA centrality classes, that are expected to scale linearly with Ncoll or Npart

RpPb

consistent with unity at high pT

Cronin enhancement clearly visible (stronger in more central collisions)

Nuclear Modification Factor in p-PbMinimum Bias (data)

arXiv:1412.6828v1

ALICE, Eur. Phys. J. C74 (2014)


RELEVANT GENERATOR LEVEL STUDIES

95

NMPI vs Multiplicityin QCD models

Pythia (pQCD based model) EPOS (Gribov-Regge multiple scattering framework)

<NMPI

> as a function of

charged particle multiplicity

PYTHIA simulation(P.Skands)

EPOS simulation

u: number of multiple scatterings <u>(MB) << 10

Larger number of multiple scatterings ≡ High event multiplicity

K. Werner WPCF 2011, Tokyo, Japan


Saturationat largeNinteractions

96

Pythia (pQCD based model)

<NMPI

> as a function of

charged particle multiplicity

PYTHIA simulation(P.Skands)

Larger number of multiple scatterings ≡ High event multiplicity


Saturationat largeNMPI

NMPI

[Guy Paic QM 2015]

NMPI vs MultiplicityPythia

Color reconnection and flow-like patterns in pp


pp interaction simulated with Pythia 8 Tune 4C don’t know about flow, however, qualitatively, the /L KS

0 ratio in different Nch ranges evolve as the /L KS0

ratio in different centrality ranges in Pb-Pb interactions (measured by ALICE).

Color reconnection matters. Flat shapes otherwise.

[G.Paic, 5th MPI@LHC]See also arXiv:1404.2372

http://arxiv.org/abs/arXiv:1404.2372

98

Using event shapes as additional handle on large multiplicity events

Jetty events

Isotropic events

3 M events in this multiplicity bin. But the effect at low pT is seen with 0.1 M events.

Tune 4C

http://arxiv.org/abs/1503.03129


99

Using event shapes

Jetty events

Isotropic events

This analysis would be accessible using the first MB data: 100-200M events.

Growing collaboration and visit exchanges CCUN UNAM (Mexico) on this topic.Paolo Bartalini - CCNU October 8 2015

• HMTF

Central-Forward correlations & connection to MPI

[D.D. Chinellato]

VOM = V0A + V0C (4.3 wide h range)

Event selections corresponding tolarger pseudorapidity ranges are more sensitive to the number of MPI

To decrease the biases central measurements may use forward trigger and vice-versa

multiple parton interactions (mpi) and high multiplicity (hm) final states at hadron and heavy ion...

Documents

multiplicity kno

pt pt0

extra highpt interactions

paolo bartalini ccnu

violation of kno

knoscaling increases

scaling s

high scales w w