Studies of the jet fragmentation in p+p

collisions in STAR

Elena Bruna

Yale University

STAR Collaboration meeting, June 16-21 2008

OUTLINE

Jets in p+p at STAR Jet reconstruction:

Jet Finding Algorithm Theoretical and Experimental Issues in Jet Finding Performance

Fragmentation functions on p+p events Conclusions

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HIGH-pT AT RHIC

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p+p collisions

JETS IN p+p COLLISIONS

Hard probes early times Calculable in pQCD: factorization theorem

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p pa, xa b, xbσab

c, x c

d, x d

D

D

Jet cross section:

JET RECONSTRUCTION Jet = collimated spray of high energy hadrons Interplay between theory and experiment:

THEORY: “calculate” the real jet EXPERIMENT: measure the jet

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Why reconstruct jets?• Full knowledge of jet properties: jet shape, fragmentation

functions, energy, …

IDEA: going from tracks and EMC towers to jets Jet Finding

• Theoretical and experimental issues in Jet Finding

• Jet Finding Algorithms Cone algorithms

KT algorithms

THEORETICAL ISSUES Required THEORETICAL features in a

jet finding algorithm:

Collinear safety: the algorithm should be insensitive to any collinear radiation.

Example A: if the energy is split among soft particles, and each tower is under a threshold, the jet is lost

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OK BAD: 2 jets are merged in one

A

Example B: if the energy of a parton is split in two towers, and the algorithm starts with the particles with highest E, a different jet may be found

B

Infrared safety: the algorithm should not be sensitive to soft radiation

EXPERIMENTAL ISSUES

Required EXPERIMENTAL features in a jet finding algorithm: Detector independence: the performance of the

jet algorithm should not be dependent on detector segmentation, energy resolution, …

Stability with luminosity: jet finding should not be strongly affected by multiple hard scatterings at high beam luminosities.

Fast Efficient: the jet algorithm should find as many

physically interesting jets as possible

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CONE ALGORITHM

A ‘seed’ defines the approximate jet direction seed = track with E>Ethreshold

Tracks which are within a radius of R<Rcone are taken (R=√(ΔΦ2+Δη2))

The centroid of the cone is given by summing the momenta of the particles inside the cone

The centroid becomes the new seed : procedure iterated until the seed position is stable

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Rcone

seed

Rcone

centroid = new seed

tracks or towers

PART I: searching midpoint Search for missing jets using the midpoint of all the pairs of found

jets as seed

PART II: splitting/merging This stage starts once stable cones have been found (see previous

slide) IDEA: disentangle jets which share common towers in the

calorimeter

MIDPOINT CONE ALGORITHM

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midpoint

JET #1 pTjet1>pTjet2

JET #2

1. Take the lower-pT jet (#2)2. f=Eshared/Ejet#2

3. if f>50% then MERGE jet#1 and jet#2 else SPLIT the jets

KT JET ALGORITHM Start with a list of preclusters, i.e. 4-vectors of tracks, and

calorimeter towers. Each precluster is defined by: E, p, y. Calculate:

For each precluster i: For each pair (i,j) of preclusters: (D is a parameter of the jet algorithm)

Find the minimum of all the di and dij and label it dmin

If dmin is a dij, remove preclusters i and j from the list and replace them with a new merged precluster

If dmin is a di, the precluster i is not “mergeable” and it can be added to the list of jets.

Repeat the procedure until the list of preclusters is empty, i.e. all the jets have been found

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RECENT RESULTS AND PERSPECTIVES

Inclusive differential cross section for p+pjet + X measured by STAR with polarized proton beams.

Increased L in 2006: High-pT jets PID of jet fragments

GOALS for STAR:

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2003-2004 data

Study of the fragmentation functions for particles inside jets in p+p for different jet energies and opening angles

Measure jets in Au+Au Study the hadrochemical modifications of jets in the

nuclear medium

MIDPOINT CONE JET FINDING IN p+p IN STAR

Performance study DATA: p+p PYTHIA events (2006) Jet Finder applied to:

PYTHIA particles PYTHIA Jets (no detector effects) Reconstructed tracks and calorimeter towers RECO Jets

(detector effects) SETUP for the Jet Finder:

R=0.7 (ϑc~0.49 rad), |ηjet|<0.3

R=0.5 (ϑc~0.35 rad), |ηjet|<0.5

R=0.4 (ϑc~0.28 rad), |ηjet|<0.6

seed: ET>0.5 GeV

PYTHIA Jets vs RECO Jets Only the leading RECO Jets are considered 12

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η=-1 η=+1

JET

z

R=0.7

jet=0.3

ENERGY RESOLUTION (1 of 2)

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10<E(PYTHIA)<10.3 GeV 20<E(PYTHIA)<20.5 GeV

30<E(PYTHIA)<30.5 GeV

BLACK = RECO jetRED = PYTHIA jet

BLACK = RECO jetRED = PYTHIA jet

BLACK = RECO jetRED = PYTHIA jetR=0.7

ENERGY RESOLUTION (2 of 2)

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R=0.7

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10<E(PYTHIA)<10.3 GeV 20<E(PYTHIA)<20.5 GeV

30<E(PYTHIA)<30.5 GeV

MULTIPLICITY OF JET FRAGMENTS (1 of 2)

BLACK = RECO jetRED = PYTHIA jet

R=0.7

MULTIPLICITY OF JET FRAGMENTS (2 of 2)

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10<E(PYTHIA)<10.3 GeV

BLACK = RECO jetRED = PYTHIA jet

R=0.7

all particles

charged particles

neutral particles

JETS IN VACUUM

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MLLA (modified leading logarithmic approximation) formalism provides a good description of fragmentation functions in e+e- and ppbar collisions. e+e-√s=29

GeV

H. Aihara et al. (TPC/2 coll.), PRL 52, 577 (1984)

STAR p+p 2006 data: Measure fragmentation functions in p+p at 200 GeV as

baseline for Au+Au test pQCD models (MLLA, …)

ξξ pphh

11 5.4 GeV/c5.4 GeV/c

22 2.0 Gev/c2.0 Gev/c

33 0.7 GeV/c0.7 GeV/c

44 0.25 GeV/c0.25 GeV/c

55 0.1 GeV/c0.1 GeV/c

€

ξ =lnE jet

pch

JET QUENCHING IN HOT NUCLEAR MATTER

Signatures: Modification of jet energy distributions Modification of jet fragmentation functions Modification of the hadrochemical composition of

the jet fragments [Sapeta, Wiedemann arXiv:0707.3494]

Medium-modified MLLA (includes hadrochemistry predictions): IDEA: in-medium gluon radiation implies an

enhancement of the parton splitting MODEL: the parton splitting functions are enhanced

by a common factor [Sapeta, Wiedemann arXiv:0707.3494]

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MODEL PREDICTIONS

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[Sapeta, Wiedemann arXiv:0707.3494]

Full jet reconstruction and PID inside jets in both p+p and A-A is required

JETS ON REAL DATA: p+p (2006)

p+p 2006 data set: Luminosity ~8.7 pb-1

8.3 M Jet Patch events STAR Triggers:

MinBias: Beam-Beam-Counter (BBC)

High Tower: BBC + 1 tower (0.05 x 0.05 with ET>5.4 GeV

Jet Patch: BBC+ 20x20 towers (patch, 1 x 1

) withET>8 GeV

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ξDISTRIBUTIONS FOR CHARGED HADRONS (1 of

2) 2 jet energies:

30<Ejet<40 GeV

40<Ejet<50 GeV

ξ distributions compared with PYTHIA simulations

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Very good agreement between data and PYTHIA

ξ FOR CHARGED HADRONS (1 of 2)

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SUMMARY AND OUTLOOK Full jet reconstruction in p+p at RHIC is needed as

a baseline to study hadrochemical modifications of jets in Au+Au collisions

The standard jet finding algorithm (midpoint cone) has been tested on PYTHIA events with different settings of the parameters (seed, Radius) Test other algorithms: KT, …

Analysis on p+p (run 2006): in progress Fragmentation functions: charged particles, p, K, π, e,

Λ, …

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EXTRA SLIDES

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TRIGGER BIAS: JET PATCH VS HIGH TOWER

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