Looking Inside Hadron Collisionsskands/slides/rochester_050405.pdf · ³: hadron-hadron collisions. ³¶µ * · ¸ T Q ¸ , 6: partonŒparton collisions. * · ¸ T Q ¹ ¸ , 6 º

Rochester — HEP Seminar — Apr 05 2005Rochester — HEP Seminar — Apr 05 2005

Looking Inside Hadron CollisionsPeter Skands (Fermilab)

Butch Cassidy and the Sundance Kid. Copyright: Twentieth Century Fox Films Inc.

Looking Inside...

Matrix Elements and Parton showersThe Underlying EventBeam Remnants and Hadronization

Peter Skands, Looking Inside Hadron Collisions – p.1/42

Why Study Supernovae?Why Study Supernovae?

They are the highest energy explosions in the universe

They give us clues to other physicsType Ia = large-distance standard candles� distance/redshift relation� �

problem

SN1987a� neutrino physics,Cooling � limits on light/weak particles+ much much more ...

Price: extremely complicated dynamics � they arenow almost making them explode in simulations...

Much can be done even in complex environments.More if the complex dynamics can be understood and modeled


The Near (Accelerator) Future is Hadron CollisionsThe Near (Accelerator) Future is Hadron Collisions

Tevatron

expect 2 – 10 by LHC turn-onLarge , , and samples (including hard tails !)

Reduction of and mass uncertainties by

Potential discoveries...

LHC

Explore EWSB / Probe New Physics up to TeV

1 year more than , , events

Improved Systematics — jet energy scales, luminosity —from high–statistics ’standard candles’

Large discovery potential + percent level physics!



Tevatron

expect 2 – 10

��

by LHC turn-on �

Large

�

,

�

, and

samples (including hard tails !)

Reduction of and

�

mass uncertainties by ��


LHC

Explore EWSB / Probe New Physics up to TeV

1 year more than , , events


Large discovery potential + percent level physics!



Tevatron

expect 2 – 10

��

by LHC turn-on �

Large

�

,

�

, and

samples (including hard tails !)

Reduction of and

�

mass uncertainties by ��


LHC

Explore EWSB / Probe New Physics up to ��

TeV

1 year � more than� � � �

,

�, events � ��


� � Large discovery potential + percent level physics!


But Hadrons Grant Nothing Witout Hard WorkBut Hadrons Grant Nothing Witout Hard Work

Not all discovery channels producedramatic signatures � Needtheoretical control of shapes,backgrounds, uncertainties, ...

Scattering at LHC

� � “rescaled”scattering at Tevatron. (smaller �,more intensive BGs, UE,...)

Aiming for percent levelmeasurements, PDFs, luminosities,jets etc � � solid understanding ofQCD in hadron collisions, bothperturbative and non–perturbative, iscrucial

State-of-the art is wide range of FO,PS, Res, hadr, ... results & tools.


Matrix Elements and Parton ShowersMatrix Elements and Parton Showers

Matrix Elements — Fast Forward� FO �: Exact (interference, helicity, loops, ...)� Limited Applicability: LEP � multiple soft gluons significant inbuilding full event structure� Phase space for soft gluons larger at HE� PT expansion better behaved at HE ( � � smaller)� Few, well–separated partons in final state!

For full event structure, need to go beyond FO.

Parton Showers — Fast Forward

� Approximate in wide–angle hard region� Universal� exponentiate � arbitrary number of FS partons� Match to hadronization

Marriage desireable � (L)CKKW, MLM, MC@NLO, ...


What I am Talking AboutWhat I am Talking About

Focus of this talk:Parton Showers & Underlying EventsBeam Remnants & Hadronization

Not the focus of this talk:Resummation approachesFixed–Order approachesParton Shower / (born-level) Matrix Elementmatching & mergingParton Shower / NLO Matrix Element matching &merging


Parton Showers: the basicsParton Showers: the basics

Today, basically 2 approaches to showers:Parton Showers (e.g. HERWIG, PYTHIA)and Dipole Showers (e.g. ARIADNE).







Basic formalism: Sudakov (DGLAP) evolution:

" #$ % &')( * &+ ,+ , -/. 0 + ,1243 5( 6 798 0;: 1 &: <= > ?

@BA CD@ E E E

@

: some measure of ‘resolution’, F: energy sharingGIH J KML N F O : collinear limit (

P 6 Q) of ME (can include R SUT Q

effects).

Correctly resums Leading Logs + some NLL effects (momentum conservation,running VXW etc).

Big boon: universal and amenable to iteration 6 algorithm can give you fullyexclusive (=‘resolved’) final states 6 match to hadronization




Essential difference: ordering variables.consider e.g. gluon emission off a YZ Y\[ system.

]

^\_ ` a;b

]

^\_ ` a;b

]

^\_ ` ab

PYTHIA/JETSET HERWIG ARIADNEc d ( e c d for ISR) f g dh d i dkj

High–virtuality ems. first. Large–angle ems. first. Large– i j ems. first.


Parton Showers:Parton Showers:


Another important difference is the way recoils are as-signed, i.e. how the on–shell kinematics prior to the branch-ing is reinterpreted to include the virtual (branching) leg.e.g. ISR:

1

2

l4mn ,

o p

, q

3 n rs l mn ,

p � p

Matrix Element (1st) Correction1 and 2 on shell 3 and 2’ now on shellt uvw � xzy � u � � � � u y t uvw � �|{ � u y � }~� � u y


New Parton Showers: Why Bother?New Parton Showers: Why Bother?


Each has pros and cons, e.g.:In PYTHIA, ME merging is easy, and emissions are ordered in some measure of(Lorentz invariant) hardness, but angular ordering has to be imposed by hand,and kinematics are somewhat messy.

HERWIG has inherent angular ordering, but also has the (in)famous “deadzone” problem, is not Lorentz invariant and has quite messy kinematics.

ARIADNE has inherent angular ordering, simple kinematics, and is ordered ina (Lorentz Invariant) measure of hardness, but is primarily a tool for FSR, withsomewhat primitive modeling of ISR and hadron collisions, and � 6� Y is’artificial’ in dipole formalism.

Finally, while all of these describe LEP data very well, none are perfect.

Possible to combine the virtues of each of these ap-proaches while avoiding the vices?


Underlying Event: the basicsUnderlying Event: the basics

Why multiple perturbative interactions?

Consider perturbative QCD

� �

scattering:&��&�� p�

�� p � ��

� � ‘2-jet’ cross sect

� u� u �� u��

�� u� � ��

while total ¡ ¡ cross sect:� ¢ ¢ y £B¤ £¥0.01

0.1

1

10

100

1000

10000

0 5 10 15 20 25 30 35 40 45 50

sigm

a (m

b)

pTmin (GeV)

Integrated cross section above pTmin for pp at 14 TeV

jet cross sectiontotal cross section

¦ [ J [ Nz§¨ A ©«ª O¬ ¦®¯ ®

for §¨ A © ª° ±�²

GeV


What’s going on?What’s going on?

1. Multiple interactions (MI)!

Must exist (hadrons are composite!)�´³¶µ ³ : hadron-hadron collisions. � ³¶µ ³ * ·¸ T Q � ¸

� , 6 ,: parton–parton collisions. � , 6 , * ·¸ T Q ¹ � ¸

� , 6 , º �´³ µ ³ »¼ ½ ¹ ¾ º �2. Breakdown of pQCD, colour screening.

�\¿ Q À 2

GeVPeter Skands, Looking Inside Hadron Collisions – p.9/42

(Multiple Interactions — Direct Evidence)(Multiple Interactions — Direct Evidence)

Basic idea : expect four pair-wise balancing jets in double partonscattering (DPS) but not in double bremsstrahlung emission.

AFS : 4-jet events at

Á¿ Â Ã

GeV in 1.8 units of Ä. Projectout 2 pairs of jets and study imbalancing variable,ÅÇÆ È ,¿ n É È ,¿ , . Excess of events with small

Å.

CDF : Extraction by comparingdouble parton scattering (DPS) toa mix of two separate scatterings.Sample: 14000 Ê ËÍÌ Q É ÎÏ

events.Strong signal observed, 53% DPS

(Note: only plot made was com-parison to PYTHIA with MI switchedoff !)


(Multiple Interactions — Indirect Verifications)(Multiple Interactions — Indirect Verifications)

Basic idea :Hadronization alone produces roughly Poissonianfluctuations in multiplicity.

Additional soft interactions +

Ð

dependence Ñ largerfluctuations.

UA5 : (900 GeV)½ ¹/Ò Ó ¾ *35.6,� ¸ÕÔ Ö *19.6.

0.001

0.01

0.1

0 20 40 60 80 100

Pro

babi

lity

ncharged

Charged multiplicity distribution at 900 GeV

UA5 dataTune A double Gaussian

at most one hard int.

+ forward–backward correlations ( UA5 , E735 ), pedestal

effect ( UA1 , CDF , H1 ), R. Field’s studies ( CDF ), ...Peter Skands, Looking Inside Hadron Collisions – p.11/42

UE: Present StatusUE: Present Status

Available tools:

Soft UE model (min-bias) (HERWIG)

Soft+semi-hard UE (DTU) (ISAJET, DTUJET)

Multiple Interactions (PYTHIA, JIMMY)

Of these, the PYTHIA model (from 1987) was probably themost sophisticated; tunes like ’Tune A’ can simultaneouslyreproduce a large part of Tevatron min–bias and UE data,as well as data from other colliders.

[T. Sjöstrand, M. van Zijl, “A Multiple Interaction Model For The Event Structure In Hadron Colli-sions”, Phys. Rev. D 36 (1987) 2019.]

[R.D. Field, presentations available at www.phys.ufl.edu/ ×rfield/cdf/]


New UE Model: Why Bother?New UE Model: Why Bother?

QCD point of view: hadron collisions are complex.Present models are not.More detail Ñ more insight Ñ more precision

LHC point of view: reliable extrapolations require suchinsight.Simple parametrizations are not sufficient.

New Physics and precision point of view: random andsystematic fluctuations in the underlying activity willimpact cuts/measurements:More reliable understanding is needed.

Practical point of view: Tevatron (and RHIC, HERA?)data is (will be?) available to test new developments:a great topic for phenomenology right now!


Completing the pictureCompleting the picture

p

Ø ÙÚ

ÚÙÛ

to

hard

int.

beam

remn.

How are the hard scattering initiators and beam remnantpartons correlated:

+ In impact parameter?+ In flavour?+ In longitudinal momentum?+ In colour?+ In (primordial) transverse momentum?+ What does the beam remnant look like?+ (How) are the showers correlated / intertwined?


Completing the pictureCompleting the picture

p

Ø ÙÚ

ÚÙÛ

to

hard

int.

beam

remn.

How are the hard scattering initiators and beam remnantpartons correlated:

+ In impact parameter?+ In flavour?+ In longitudinal momentum?+ In colour?+ In (primordial) transverse momentum?+ What does the beam remnant look like?+ (How) are the showers correlated / intertwined?


(...) Hadronization.(...) Hadronization.

Imagine placing a stick o’ dynamite inside a proton, impartingthe 3 valence quarks with large momenta relative to each other.

‘Ordinary’ colour topology ‘Baryonic’ colour topology

(e.g.

Ü Q;Ý Þ ßÞ): (e.g. ):

Þ ßÞÞn

Þ ,

Þà

How does such a system fragment? How to draw the strings?


So...So...

All this was just to argue:

There is no such thing as ‘a simple hadron collision’!

or: If a model is simple, it is wrong.


We therefore proceeded to complicate matters...


News in PYTHIA 6.3News in PYTHIA 6.3

PYTHIA 6.3 includes a new model for multipleparton–parton interactions, including correlations in

Ð,

flavour, á, colour, and

âäã ,+ beam remnants, + stringhadronization (extended to baryonic string topologies).

It also contains new ISR and FSR parton showers, basedon a � ¿ -ordered sequence of

� Ñ 2parton splittings inside

dipoles.

Further, the description of parton showers and theunderlying event has been unified in a common � ¿ -ordered‘interleaved evolution’ of the event as a whole.

(The old PYTHIA shower and underlying-event frameworkremains in PYEVNT, while the new options are obtained byusing PYEVNW instead.)


Unifying PS and UE: Interleaved EvolutionUnifying PS and UE: Interleaved Evolution

The new picture: start at the most inclusive level,

åÝ å.

Add exclusivity progressively by evolving everything downwards.

int.number

æç

hard int.

1 2 3 4

æçè éêæçè ëì

æçíæçî

æçïæç î ï

æçð

ISR

ISR

ISR

ISR

æ ñçí

interleavedmult. int.

interleavedmult. int.

interleavedmult int.Intertwined?

òÇóôevolution

õöõX÷ø ùú õöüûý õX÷ø þ õöý ÿ�õ ÷ø þ õö �ý õX÷ø � �

�� õöüû ý õ ÷ �ø þ õöý ÿ�õX÷ �ø þ õö �ý õ ÷ �ø� õX÷ �ø �

� “Finegraining”


THE NEW FRAMEWORKTHE NEW FRAMEWORK

Multiple Interactions

T. Sjostrand+PS, JHEP 0403 (2004) 053T.Sjostrand+PS, EPJ C39 (2005) 129


Correlations in flavour and ��Correlations in flavour and ��

Consider a hadron,

�

:

?

MI context: need PDFs for finding partons

� n�� ¸ withmomenta á n � � � á ¸ in �

probed at scales

o n � � � o ¸

� �� "! �$# # # ! %& p� # # # p % '

But experimentally, all we got is ¹ * �

.Global fits: CTEQ MRST

DIS fits: Alekhin H1 ZEUS

Other PDF: GRV ...

� � �� "! � & p� '

So we make a theoretical cocktail...Peter Skands, Looking Inside Hadron Collisions – p.20/42

Correlated PDF’s in flavour and ��Correlated PDF’s in flavour and ��Q: What are the pdf’s for a proton with 1 valence quark, 2sea quarks, and 5 gluons knocked out of it?

1. Overall momentum conservation (‘trivial’):Starting point: simple scaling ansatz in á.

For the ¹’th scattering:

á ( )*,+ + -/. + * � ?¸10 n

2 á 2 *¼ 3 ¸ 0 á1 À �+ 3 Q á+

x

xu(x

)

X=0.7

X=1

CTEQ6.1

0

1

0 0.5 1 x

xd(x

) CTEQ6.1

0

1

0 0.5 1 x

xg(x

) CTEQ6.1

0

1

0 0.5 1Peter Skands, Looking Inside Hadron Collisions – p.21/42


Normalization and shape:

G If valence quark knocked out.Ý Impose valence counting rule:@

Q Þ 4 5 67 ¸ 8:9<; = ,?> @9 Æ A 4 5 67 ¸.G If sea quark knocked out.Ý Postulate “companion antiquark”:

n 0 BWQ ÞÒC D7 8:9E 9GF > @9 Æ HJI

G But then momentum sum rule would be violated:

Assume sea+gluon fluctuates up when a valence quark isremoved and down when a companion quark is added.



Normalization and shape:

G If valence quark knocked out.Ý Impose valence counting rule:@

Q Þ 4 5 67 ¸ 8:9<; = ,?> @9 Æ A 4 5 67 ¸.G If sea quark knocked out.Ý Postulate “companion antiquark”:

n 0 BWQ ÞÒC D7 8:9E 9GF > @9 Æ HJI

G But then momentum sum rule would be violated:@Q 9 K7 Þ 7 ¸ 89 ; = , > ÉML ¸ 8:9<; = ,> N @9 OÆ P

Ý Assume sea+gluon fluctuates up when a valence quark isremoved and down when a companion quark is added.


Correlated PDF’s in flavour and ��Correlated PDF’s in flavour and ��Remnant PDFs

QR ST UWVX YZ?[ \^] _ ` abc

d e f g hZ?[ef g hZi Yf g hZi j ] b k l m npo q Y rs gZi j ]t k l m no u Y vwxyZi j ]t z ]|{y n }~

Y vwxZi \ ] z ]�{ _ ` � �� \] o ]|{ _] o ]�{ �� ]|{] o ]�{ z �� i Y vwxZi \] z ]{ _� ] ` a

� � R �� VX �[ \^] _ ` qt �i j ]t k l m n

q ` a�� ?� ^¡ � � ��¢ £ � ��¥¤ u ^¡ vwx y�¢ £

¦�� ¢ ^¡ � � ��¢ £ 10-3

10-2

10-1

1

10-4

10-3

10-2

10-1

xs = 0.001p=4p=0

xs = 0.1p=4p=0

x

xqc(

x;x s)

Companion Distributions

Used to select §©¨ -ordered

ª « ª

scatterings, and to performbackwards DGLAP shower evolution.



+ showers

T.Sjostrand+PS, EPJ C39 (2005) 129T.Sjostrand, hep-ph/0401061.


p ¬–ordered showers: Simple Kinematicsp ¬–ordered showers: Simple Kinematics

Consider branching ® ¯^° in lightcone coordinates ± ²´³ µ¶ ±¸·

± ¹º³ » ± ¹¼± ¹½³ ¾¿ À » Á ± ¹¼± Â1Ã ÄÅ ÆÇÈ É Ê Ë Ì ÄÅÍÎÏÎÑÐ

ÎÑÎÏÒ³ Ó Ô Õ ¼³ Ô Õ º Ö ± Õ¨» Ö Ô Õ ½ Ö ± Õ¨¿ À »

Timelike branching:

× ØÚÙ Û ØÝÜ Þ ß Ûà Ù ßÛ©á Ù ß

âãâã ä åçæ è é êë ì é íî å

Spacelike branching:

Û Ü Ù ß × ØÚÙï Û Øà Þ ß

Û©á Ù ßâã

âã ä åçæ è ê ë ì é í î å

Guideline, not final ± ¨ !


p ¬–ordered showers: General Strategy (1)p ¬–ordered showers: General Strategy (1)

1) Define

ð Õ¨�ñ ò ó ô³ » ¾ ¿ À » Áõ Õ

for FSR

ð Õ¨�ñ ò ó ô³ ¾ ¿ À » Áõ Õ

for ISR ö ÷ø

2) Evolve all radiators downwards in ð ¨�ñ ò ó ô from common ±¨ ù úû

üý ¼³ ü ð Õ¨�ñ ò ó ôð Õ¨�ñ ò ó ôþÿ ¾ ð Õ¨�ñ ò ó ô Á� � � ¼ � º ½ ¾ » Á ü » Ç � ð À � ��

� ��

üý º³ ü ð Õ¨ñ ò ó ôð Õ¨�ñ ò ó ôþÿ ¾ ð Õ¨�ñ ò ó ô Á� � � �� ¼ ¾ � �� ð Õ¨ñ ò ó ô Á�� º ¾ �� ð Õ¨�ñ ò ó ô Á� ¼ � º ½ ¾ » Á ü » Ç � ð ¾ À � � � Á

3) Then Derive

õ Õ õ Õ³ ð Õ¨�ñ ò ó ô � » ¾¿ À » Á for FSRõ Õ³ ð Õ¨�ñ ò ó ô � ¾¿ À » Á for ISR



4) Interpret � as Lorentz invariant energy fraction (not lightcone)(

� �� "! # $ % & ' (*) + � (! #), straightforward match to matrix elements by reweighting!

5) Construct exclusive kinematics based on- (

and �, assuming yetunbranched partons on-shell

6) Continue summing those logs.... One combined sequence /0 # 12 3 /04 3 /0 ( 365 5 5 3 /0 # 798

Tevatron: ttbar + 1 jet

Jet pT (GeV)

dσ/d

p T (p

b/G

eV)

pT2 (power shower)

pT2 (wimpy shower)

10-5

10-4

10-3

10-2

10-1

0 25 50 75 100 125 150 175 200 225 250 275 300

NB: Choice of ±¨ ù ú û non-trivial and very important for hard jet tail: wimpy vs power showers...



4) Interpret � as Lorentz invariant energy fraction (not lightcone)(

� �� "! # $ % & ' (*) + � (! #), straightforward match to matrix elements by reweighting!

5) Construct exclusive kinematics based on- (

and �, assuming yetunbranched partons on-shell

6) Continue summing those logs.... One combined sequence /0 # 12 3 /04 3 /0 ( 365 5 5 3 /0 # 798Tevatron: ttbar + 1 jet

Jet pT (GeV)

dσ/d

p T (p

b/G

eV)

pT2 (power shower)

pT2 (wimpy shower)

10-5

10-4

10-3

10-2

10-1

0 25 50 75 100 125 150 175 200 225 250 275 300

NB: Choice of ±¨ ù ú û non-trivial and very important for hard jet tail: wimpy vs power showers...


(p ¬–ordered showers: Some Details)(p ¬–ordered showers: Some Details)

FSR Evolution:Massive quarks: ð Õ¨�ñ ò ó ô³ » ¾ ¿ À » Á ¾ Ô Õ À Ô Õ*; Á

Ó Ô Õ ® Ô Õ<; when ð Õ¨�ñ ò ó ô ® =

.

Special treatment of narrow resonances (e.g. top).

ISR Evolution:Massive quarks: ð Õ¨ñ ò ó ô³ ¾¿ À » Á ¾õ Õ Ö Ô Õ<; Á ³ Ô Õ<; Ö ð Õ¨> ?Ó Light–Cone ð Õ¨ > ? ® =

when ð Õ¨�ñ ò ó ô ® Ô Õ<; .Backwards evolution uses correlated pdf’s at scaleswhere more than 1 interaction is resolved.

Both ISR and FSR:ME merging by veto for many SM+MSSM processes.Gluon polarization ® asymmetric @ distribution.



+ remnants+ (string) hadronization

T. Sjostrand+PS, NPB 659 (2003) 243T. Sjostrand+PS, JHEP 0403 (2004) 053


Confinement, primordial

A ¬, and BDC EF sharing.Confinement, primordial

A ¬, and BGC EF sharing.

Confined wavefunctions « H¨ I J KML � N OQP ?R .Empirically, one notes a need for larger values.S Õ�T

S HVU S HVW XY Â Z �� [\ �] ; ^ \ ]_ `ñ a ^cb dfe gh `ñ a ]i ¼ jlk e ^\ ]_ d `ñ a ^cb _ `ñ a ]m n o ^\ ]qp r ^b Õ `ñ a ]s tu ¼ v kw x ^« Fitted approx. shape y z{ | I ª~} � { K z� � { |GeV

Recoils : along colour neighbours (or chain ofneighbours) or onto all initiators and beam remnantpartons equally. (

Z�� rescaled to maintain energy conservation.)

Lightcone fractions ��f� Z in remnants: use remnant pdf’sand fragmentation functions (with

z� � § | conserved).

Composite BR systems possible (diquarks, mesons,w. pion/gluon clouds?) « larger �?


Intermezzo: now it gets tougherIntermezzo: now it gets tougher

We have arrived at:A set of ±¨ -ordered interactions, with showers, taking intoaccount non-zero primordial

�¥¨ effects.

A set of partons (possibly diquarks etc) left behind in thebeam remnants, whose flavours are known and whosekinematics have been worked out (i.e. � and

��¨ ).

But life grants nothing to us mortals without hard work

How are initiator and remnant partons correlated in colour?

How do remnant systems hadronize?


Hadronization: String JunctionsHadronization: String Junctions

Fundamental properties of QCD vacuum suggest stringpicture still applicable.

Baryon wavefunction building and string energyminimization ³ Ó picture of 3 string pieces meeting at a‘string junction’.

junction

(Warning: This picture was drawn in a “pedagogical projection” where distances close tothe center are greatly exaggerated!)


(Junction Fragmentation)(Junction Fragmentation)

How does the junction move?

A junction is a topological feature of the string confinementfield:

� ¾�� Á ³ �� . Each string piece acts on the other twowith a constant force, � ��k .

³ Ó in junction rest frame (JRF) the angle is 120

�

betweenthe string pieces.

Or better, ‘pull vectors’ lie at 120�

:

± �� ô ô³ �� _ � � ± �� Â � �� ¡ �¢

(since soft gluons ‘eaten’ by string)

Note: the junction motion also determines the baryonnumber flow!)


Junction FragmentationJunction Fragmentation

How does the system fragment?

£¤£¥

£¦ £§¤ £¨ £©¨ £ £©ª £ £ª £« £« ¬

£¥£®£©®£¯£°¯±³²

£¦£´£´±qµ

NB: Other topologies also possible (junction–junctionstrings, junction–junction annihilation).


Colour Correlations:Colour Correlations:

Currently, this is the biggest question.

Tune A depends on VERY high degree of (brute force)colour correlation in the final state.

Several physical possibilities for colour flow orderinginvestigated with new model. So far it has not beenpossible to obtain similarly extreme correlations.

This may be telling us interesting things!

More studies are still needed... in progress.

Fortunately, this is not a showstopper. Mostly relevantfor soft details (parton ¶ hadron multiplicity etc).


Model TestsModel Tests

Whole framework.3 rough tunes were made to ‘Tune A’ at the Tevatron,using charged multiplicity distribution and

· ± ¨ ¸ ¾�¹»º ¼ Á , thelatter being highly sensitive to the colour correlations.Similar overall results are achieved (not shown here), but· ±¨ ¸ ¾ ¹�º ¼ Á still difficult.Anyway, these were only rough tunes...

10-5

10-4

10-3

10-2

10-1

0 50 100 150nch

P(n

ch)

Tevatron Run II: charged multiplicity

Tune ARapSharp ISRLow FSRHigh FSR

0.3

0.4

0.5

0.6

50 100 150nch

<p⊥>

Tevatron Run II: <p⊥>(nch)

Tune ARapSharp ISRLow FSRHigh FSR


Model Tests: FSR AlgorithmModel Tests: FSR Algorithm

Tested on ALEPH data (courtesy G. Rudolph).½¿¾ À

of modelDistribution nb.of PY6.3 PY6.1of interv. ÁÂ -ord. mass-ord.Sphericity 23 25 16Aplanarity 16 23 168ÃÅÄ Thrust 21 60 8Thrust Æ ÇÉÈ ÊË 18 26 139jet res. ÌÍ (D) 20 10 22ÎÏ Ð Á ÑÒfÓ Æ 46 207 151ÁÂ ÇÉÈ 25 99 170ÁÂ ÊÔ Õ Ö ×ÙØ Ú

GeV 7 29 24ÁÂ ÊÔ Õ (19) (590) (1560)Î(B) 19 20 68

sum

ÛÝÜ Ê Þ Ï 190 497 765

(Also, generator is not perfect. Adding 1% to errors Óß Õ³ �à á. i.e. generator is ‘correct’ to â1%)


Model Tests: ISR AlgorithmModel Tests: ISR Algorithm

Less easy to test. We looked at ± ¨ of

ã d

at Tevatron.

Compared “Tune A” with an ‘intermediate scenario’ (“Rap”),and three rough tunes of the new framework.

Description is improved (but there is still a need for a largeprimordial

�¨ ).

0

5

10

15

20

25

30

0 5 10 15 20

dσ/d

p TZ

(pb/

GeV

)

pTZ (GeV)

Tune ARap

Sharp ISRLow FSRHigh FSRCDF data

0.001

0.01

0.1

1

10

0 50 100 150 200

dσ/d

p TZ

(pb/

GeV

)

pTZ (GeV)

Tune ARap

Sharp ISRLow FSRHigh FSRCDF data

ä More studies ongoing (e.g. looking at åæ of ç ç)...Peter Skands, Looking Inside Hadron Collisions – p.37/42

OutlookOutlook

To fully exploit expected experimental precision, needgood understanding of (all aspects of) hadroncollisions.

We’ve developed a new UE/PS model including:åæ –ordered interleaved parton showers and multiple interactions,correlated remnant parton distributions, impact parameterdependence, extended (junction) string fragmentation model, etc.

We even made it available! « PYTHIA 6.3

Good overall performance, though still only primitivestudies/tunes carried out (except for FSR).

Colour correlations still a headache.

Conclusion: still a long way to go for LHC.

But hey, we’re still way ahead of the Supernova crowd!



OutlookOutlook

To fully exploit expected experimental precision, needgood understanding of (all aspects of) hadroncollisions.

We’ve developed a new UE/PS model including:åæ –ordered interleaved parton showers and multiple interactions,correlated remnant parton distributions, impact parameterdependence, extended (junction) string fragmentation model, etc.

We even made it available! « PYTHIA 6.3

Good overall performance, though still only primitivestudies/tunes carried out (except for FSR).

Colour correlations still a headache.

Conclusion: still a long way to go for LHC.

But hey, we’re still way ahead of the Supernova crowd!



PYTHIA 6.3OVERVIEW OF RELEVANT

PARAMETERS


PYTHIA 6.3 Parameter Overview: SwitchesPYTHIA 6.3 Parameter Overview: Switches

MSTP(61) Master switch for initial–state radiation. Default is on.

MSTP(71) Master switch for final–state radiation. Default is on.

MSTP(81) Master switch for multiple interactions and beam remnant framework.

MSTP(70) Selects regularization scheme for ISR when èé ê ë. Default is sharp

cutoff at the regularization scale used for MI.

MSTP(72) Selects maximum scale for radiation off FSR dipoles stretched be-tween ISR partons. Default is èé scale of radiating parton.

MSTP(82) Selects which functional form to assume for the impact-parameter de-pendence of the matter overlap between two beam particles.

MSTP(84) Selects whether initial–state radiation is turned on or off for subse-quent interactions (i.e. interactions after the main one). Default is on.

MSTP(85) Selects whether final–state radiation is turned on or off for subsequentinteractions (i.e. interactions after the main one). Default is on.

MSTP(89) Controls how initial–state parton shower initiators are colour–connected to each other. Default is to assume a rapidity ordering.

MSTP(95) Selects whether colour reconnections are allowed or not. Default ison.


PYTHIA 6.3 Parameter Overview: ParametersPYTHIA 6.3 Parameter Overview: Parameters

PARP(82) Regularization scale, ìéí , for multiple interactions, at referenceenergy PARP(89). Default is 2 GeV.

PARP(89) Reference energy for energy rescaling of ìéí cutoff, i.e. the en-ergy scale at which ìéí is equal to PARP(82). Default is 1800GeV.

PARP(90) Power of energy rescaling used to determine the value of ìéí atscales different from the reference scale PARP(89).

PARP(83:84) Shape parameters, controlling the assumed matter distribution oroverlap profile, as applicable (i.e. depending on MSTP(82)).

PARP(78) Controls the amount of colour reconnection in the final state.

PARP(79) Enhancement factor for î values of composite systems (e.g. di-quarks) in the beam remnant.

PARP(80) Suppression factor for initial–state colour connections that wouldbreak up the beam remnant.


More information on PYTHIA 6.3More information on PYTHIA 6.3

The PYTHIA 6.3 manual: hep-ph/0308153

“Notes on using PYTHIA 6.3”: on my homepage:http://home.fnal.gov/ Nskands/Physics descriptions of the new ISR/FSR/MIframework:

TS+PS, “Transverse-Momentum-Ordered Showers andInterleaved Multiple Interactions”, hep-ph/0408302.

TS, “New Showers with transverse-momentum-ordering”,hep-ph/0401061.

TS+PS, “Multiple Interactions and the Structure of BeamRemnants”, JHEP 0403 (2004) 053.

+ Slides like these.(See “Slides/Talks” on my homepage for a complete list)


Documents

Looking Inside Hadron Collisionsskands/slides/rochester_050405.pdf · ³: hadron-hadron collisions. ³¶µ * · ¸ T Q ¸ , 6: partonŒparton collisions. * · ¸ T Q ¹ ¸ , 6 º