Differentiating Quark and Gluon Jets Why and How ?

Rene Bellwied (WSU)

STAR high pt retreat

Marin County, Jan 17-19, 2004

Quark and gluon jet differences

Difference between quark and gluon jets because of the greater color charge carried by the gluon.

Gluon jets should have higher multiplicities, softer fragmentation and broader angular distribution than quark jets (even without medium modifications).

Parton distribution functions (hep-ex/0305109)

Modification of fragmentation functions (e..g.hep-ph/0005044)

What about flavor ? (and not just heavy quarks)

• Quark jet has less energy loss than gluon jet

• Different energy loss for particles and antiparticles ?

• Fragmentation functions for baryons and mesons are different

• Fragmentation functions for strange baryons and non-strange baryons are different

• We need predictions for identified particle jets

Fragmentation functions for

different baryons

Bourelly & Soffer hep-ph/0305070

How to measure gluon jets ?

• All analyses at LEP are based on three jet events as the contain exactly one gluon jet.– Jet analysis: look at single jets in three jet

event, identify gluon jets via b-quark anti-tagging

– Hemisphere analysis: hard gluon leading jet, tag subleading jets as b-quark jets.

– Symmetric event analysis: all three jets are investigated.

OPAL study of three jet events

9

Helio Takai for ATLAS Heavy Ion Programmeat QM 2004: THREE JET EVENTS?

In 3jet events the “third” jet is a hard radiated gluon. Because the Quark-gluon plasma is a colored object, the gluon should couple much stronger to the media and therefore quenching should be larger.

What should be observed ? Perhaps an enhancement in the ratio of 2 to 3 jet events?(preliminary calculations by Urs Wiedemann, hep-ph/0310274)

obviously a LEP event!

Another way : direct photon jetsdirect photon jets are interesting because there are only two processes contributing to direct photon production:

Also direct photons do not fragment therefore they carry off the total momentum of the hard scatter.

Different x-distributionsa.) the beam will introduce quarks at the valence level, e.g. a pion introduces a q-qbar pair at the valence level, a proton only gives quarks, the qbar have to come from the sea level.b.) valence quarks have the hardest x-distribution followed by gluons and sea quarksc.) e.g. in Compton process we will need an initial state gluon. This will more likely happen at lower pt in pion induced interactions. In proton induced interactions the gluons will be at higher pt than the sea anti-quarks.d.) with an incident pion beam there will be an increasingly larger percentage of recoil gluon jets at larger values of direct photon pt, whereas with proton (nuclear) beams there will be a larger percentage of gluon jets at lower direct photon pt.

Difference: pi and proton beams

Quark and gluon jet properties at LEP(this is our reference which has to be confirmed in pp reactions first)

• Good summary: M.Seibel, hep-ex/9911018• In QCD three fundamental vertices for 3g and 4g

coupling:– Radiation of gluon by a quark q -> qg– Radiation of gluon by gluon g -> gg– Splitting of gluon into q-pair g -> q qbar

• Probability of process is proportional to the color factors CF, CA, TF, respectively

• Main contribution to particle production in hadronic events is due to gluon radiation. Therefore difference in properties of quark jets to gluon jets should be according to CF/CA.

Results (I)

• Gluon bremsstrahlung is expected to be higher in gluon than in quark jets by CA/CF (= 2.25). Measurement shows that the ratio is slightly lower at lower pt and reaches the expected value at higher pt.

• Possible reason: higher order corrections

Results (II)

CA/CF = 2.25 (Brodsky (1976)).

Difference in <Nch> between q and g jets

Scaling violation in quark and gluon jets

Identified particle studies in q- and g-jets

• The following particles have been studied:+, -, K+,K-, p, pbar, 0, , K*,

• The and the are especially interesting because they are isospin singlets like the gluons, but no overproduction of these particles has been found.

• Significant excess if protons in gluon jets has been found

Strangeness in quark and gluon jets

1.) Strange quarks must come from the sea, i.e. gluon splitting. Gluon jets should show harder strange particle fragmentation functions than quark jets, because strange quark-antiquark pair could be created sooner in the gluon initiated fragmentation process, because the quark jet would first have to emit a gluon before it could subsequently split into a a s-sbar pair.

2.) For a proton or neutron beam (heavy ions) the Anti-lambda has to be produced by sea-quarks or gluons, i.e. softer fragmentation functions and parton distribution function, which means the Anti-Lambda leading particle (and potentially the whole jet) should be softer, than a Lambda or a charged hadron which has significant valence quark contributions.

X.N. Wang simulations

No evidence for pQCDdrop due to q- and g-jetsyet. pt too low ? Soft + quench does well.

OPAL simulations and data

STAR difference function results:Trigger pT bins :2.5 - 2.8 GeV2.8 - 3.2 GeV3.2 - 3.7 GeV

For Lambda, Anti Lambda, K0 short3.7-10.0 GeV

For Primary:3.7 - 4.5 GeV4.5 – 5.4 GeV5.4 – 6.5 GeV6.5 – 10.0 GeV

pT trigger

pT trigger

Nsame - Nback

Nsame - Nback

Same side and back side correlation results:

Trigger PT Trigger PT

same side correlation after background subtraction

back side correlation after background subtraction

Is this due to difference between quark and gluon jets ? No !Anti-lambda/Lambda is constant over that pt range, so it’s not the trigger particle but rather the associated particles that cause the difference. The associated particles are predominantly in the intermediate pt-range (2-6 GeV/c), therefore maybe recombination could cause the effect. Early qualitative studies by Fries and Hwa show that two particle correlations can be caused by recombination and that the Anti-lambda trigger particle could cause a different correlation functions than the Lambda trigger particle in particular in the soft-hard recombination picture.

Summary

• In AA collisions it will be very difficult to distinguish between quark and gluon jets. Can we measure 3-jet events with the EMC ?

• Even direct photon jets will only allow a relative distinction (at the 10% level) as a function of the gamma pt.

• There are many phenomena in the particle composition of quark and gluon jets that are not well understood, but that have been measured in elementary collisions.

• These effects are expected to change due to medium modifications in the case of a QGP.

• We might be able to infer quark and gluon jet behavior in identified particle jets.

• We need high pt pp reference measurements (large stats) and we need to particle identify beyond the recombination regime (pt > 6 GeV/c)