1. High Pt physics with TOF ALICE B.V.Zagreev Bologna - 22.01.2008

2. Outlook • High Pt physics motivation • Why TOF is relevant detector for this physics? • RHIC results • Conclusion • ITEP group activity

3. gluon radiation Motivation • Investigation of early stage of hot & dense matter evolution • 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

4. TOF PID performance • At first glance it is impossible to study high Pt with TOF

5. Single inclusive hadron distribution vs x Medium effects introduced at parton splitting Hump-backed plateau N. Borghini & U. Wiedemann Hep-ph/0506218 z = phadron/Ejet • Quenching effect: decreases of the particles at high z (low x) & increases of the particles at low z (high x) =ln(EJet/phadron) • Fragmentation strongly modified at phadron~1-5 GeV/c even for the highest energy jets • ALICE should be well dedicated to test this x range (tracking down to 100 MeV/c) • EMCal => improves Ejet determination M.E. - ALICE PWG4 meeting - CERN January 15. 2008 - 3

6. How to use TOF ALICE for high Pt physics? • 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 (TOF) range • From RHIC data the p/π~1 at high Pt => we can enlarge TOF PID range • Actually we have got additional parameter – leading particle momentum vector and now can measure identified particle with respect to this direction (and reaction plane).

7. 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.

8. Jet quenching

9. Enhancement of barion production

10. Enhancement in strange barion production

11. Azimuthal correlations

12. 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… • PID in this range (few GeV) can clarify situation => we have wide field for activity

13. Resonances • Resonances properties (yields, spectra, width, mass) could be different in medium • The resonances which are unaffected by the hadronic medium have to be used φ – meson is of particular interest because • in case of QGP strong enhancement is expected • small cross section of φ interaction with hadron gas • possible bright effect of double mass peak TOF can identify φ up to Pt=4-5 GeV/c

14. ΦProduction K+K- and e+e- e+e- K+K- • The leptonic channel yield is a little higher than hadronic channel • More accurate measurement is required to confirm whether there is branch ratio modification

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

16. φ – meson azimuthal correlations

17. What should be done? • Fragmentation calculations and measurements • relative to leading particle energy z’ = phadron/Eleading particle or ’=ln(Eleadingparticle/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. • Estimation of background from underlying events

18. 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.

19. ITEP group activity • Simulation of residual correlations in strange particles femptoscopy with TOF (ALIFEMTO package) (Mikhailov, Stavinsky) • Short living resonances feasibility studies (Kiselev) • Direct (thermal & prompt) photons generator for ALIROOT (Kiselev)

20. Backup