High Pt physics with TOF ALICE B.V.Zagreev ITEP -16.06.2008.

High Pt physics with TOF ALICE

B.V.Zagreev

ITEP -16.06.2008

main RHIC finding (QM2008 – Shuryak)

• Strong radial and elliptic flows are very well described by ideal hydro => ”the most perfect liquid known”

• Strong jet quenching, well beyond pQCD gluon radiation rate, same for heavy charm quarks (b coming)

• Jets destroyed and their energy goes into hydrodynamical ”conical flow” in athimuthal correlations

Two of three - jet (high Pt) physics!!!

Motivation

• Initial production at high-pT is calculable in perturbative QCD and can be calibrated by reference measurements

• These partons will first travel through a dense color medium. They are expected to lose energy through collision energy loss and medium induced gluon radiation, “jet quenching”.

• The magnitude of the energy loss depends on the gluon density of the

medium and on the path length

gluon radiation

Use jets and high-pT particles to probe the medium

Goal: measure medium properties• Density, temperature

Number of degrees of freedom• Dynamical properties

e.g. viscosity

However, we still need to calibrate our probe:• Fragmentation, hadronisation in the vacuum• … and in the medium• Calibrate/constrain energy loss mechanism• Check initial production rates

What we know from RHIC?

• Usually people distinct three Pt regions:

– bulk (Pt < 2 GeV) – seems to be driven by thermal properties of the matter.

– high Pt > 6 GeV – measured particle spectra are well described by pQCD calculations (except jet quenching effect). One can use them as hard trigger.

– intermediate region – most interesting effects of hard particles (partons) interactions with media. Different theoretical models (jets + recombination/coalescence mechanism), situation is not clear.

TOF PID performance

• At first glance it is impossible to study high Pt with TOF, but for quenched jets it is not the case

Single inclusive hadron distribution vs Single inclusive hadron distribution vs

N. Borghini & U. WiedemannHep-ph/0506218

=ln(EJet/phadron)

Quenching effect: decreases of the particles at high z (low ) & increases of the particles at low z (high )

z = phadron/Ejet

Hump-backed plateau

Medium effects introduced at parton splitting

M.E. - ALICE PWG4 meeting - CERN January 15. 2008 - 3

• ALICE should be well dedicated to test this range (tracking down to 100 MeV/c)• EMCal => improves Ejet determination

Fragmentation strongly modified at phadron~1-5 GeV/c even for the highest energy jets

PID with TOF can help to study this fragmentation modification.

• We can use high Pt (even not identified) charged particle or photon as a trigger and study accompanying particles!

• Fragmentation strongly modified at phadron~1-5 GeV/c even for the highest energy jets.– We even don’t need jet reconstructions: instead of z we can use

z’ = phadron/Eleading particle (need theoretical predictions!)– Fragmentation distributions should also depend on particle type.

(need theoretical predictions!)

=>we need PID in this range to study jet chemical composition. (From RHIC data the p/π~1 at high Pt => we can even enlarge TOF PID range)

Azimuthal correlations

Azimuthal correlations

• Lot of theoretical explanations of double away-side peak: deflected jet, large gluon radiation, shock waves (Mach cones), Cerenkov radiation

• Long-range Δη correlation on the near-side (ridge): coupling of induced radiation to the longitudinal flow, turbulent color fields, anisotropic plasma, interplay of jet-quenching and strong radial flow…

• Chemical composition of away side jet is different compare with trigger jet (fragmentation in vacuum)

Athimuthal correlation at RHIC for baryons and mesons.

• TOF PID allows us to study this effect in details – identification of π, K, p, φ, Λ…• φ – meson is of particular interest (TOF can identify φ up to Pt=4-5 GeV/c)

• in case of QGP strong enhancement is expected• small cross section of φ interaction with hadron gas• possible bright effect of double mass peak

Heavy ion generators

• Low & intermediate energies: RQMD, UrQMD etc.

• High energies (up to LHC): HIJING, FRITIOF, VENUS, LUCIAE, DPMJET-III, PSM, NEXUS etc.

• PYQUEN – reproduces mainly jet quenching effect at RHIC

PYQUEN vs. RHIC data

PYQUEN vs. RHIC data

Double away-side peak?

Fragmentation function at ALICE Pb-Pb 5.5 TeV (Et>100 GeV)

PYQUEN – Pt spectraEt>50 GeV Et>5 GeV

PYQUEN – barion/meson ratioEt>50 GeV Et>5 GeV

PYQUEN - athimutal correlationsEt>50 GeV Et>5 GeV

What should be done?

• PYQUEN development: double peak, barion/meson ratio,…

• Estimation of background from underlying events + AliRoot simulation

• Fragmentation calculations and measurements – relative to leading particle energy z’ = phadron/Eleading particle or ’=ln(Eleading particle/phadron)

– for different types of particles (π, K, p, φ…)

• Different angular correlations of different types of particles, with respect to jet direction, reaction plane etc.

Conclusion

• The jets at intermediate Pt of few GeV have been shown to be significantly modified in the both their particle composition and their angular and fragmentation distributions compare to p+p collisions.

• High Pt trigger particle provides additional parameter (direction and momentum of this particle) for such investigations of interactions between hard scattered partons and the medium.

• ALICE TOF is the relevant detector for this high Pt physics.

• We need both theoretical and experimental researches in this area.

As compared to jet physics at RHIC, there are two fundamentally new features in central Pb–Pb collisions at the LHC:

1)The multi-jet production per event is not restricted to the minijet region Et < 2 GeV but extends to about 20 GeV 2)Jet rates are high at energies at which jets can be distinguished from the background energy of the underlying event.

Hence, event-by-event reconstruction of jets with reasonable energy resolution will be possible.

Backup slides

Jet quenching

Enhancement of barion production

Enhancement in strange barion production

φ – meson angle correlations•Such effects probably are enhanced in jet production, as soon as this is a trigger on early stage of reaction.

φ – meson azimuthal correlations