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Gli adroni ad alto P T sono prodotti da partoni con Hard-scattering iniziale. I partoni hanno bisogno di un tempo finito per uscire dalla zone della collisione,

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Presentation on theme: "Gli adroni ad alto P T sono prodotti da partoni con Hard-scattering iniziale. I partoni hanno bisogno di un tempo finito per uscire dalla zone della collisione,"— Presentation transcript:

1 Gli adroni ad alto P T sono prodotti da partoni con Hard-scattering iniziale. I partoni hanno bisogno di un tempo finito per uscire dalla zone della collisione, durante il quale si forma il dense medium durante questo tempo subiscono Interazione forte col mezzo e quindi costituiscono una sonda per studiare il mezzo stesso. Ci sono due variabile che si possono studiare: 1)Rapporto degli adroni emessi in collisioni tra nuclei rispetto a p-p ( corretto per effetti geometrici e scalando col numero di collisioni) ; 2) Disappearance del jet back to back nellangolo azimuthale ( jet quenching); QGP (II) E. Scapparone 24 Maggio, 2010

2 The results are consistent with the effects of parton energy loss in traversing dense medium, predicted before the data were available Nessun effetto in d-Au effetto associato a interazione tra ioni pesanti. PHENIX

3 Per ripdodurre lintensita della soppressione, questi MC devono assumere Una densita di gluoni 30 volte maggiore di quella della materia fredda e Confinata ( densita di energia 100 volte maggiore). centralità 90% 50% 0%


5 Altra possible normalizzazione: I fotoni

6 As one increases the collision energy in nucleus nucleus collisions, the produced plasma reaches higher energy and particle densities, the system stays longer in the QGP phase, and correspondingly the traversing partons are more quenched.

7 Il meccanismo dominante di perdita di energia dei partoni nel QGP e la radiazione di gluoni (gluonstrahlung). Near side: dove seleziono la particella trigger Away side: lato opposto Maggiori effetti del mezzo

8 Select a High Pt particle as trigger and measure the Δ = trig angle

9 Shape simile a p-p + soppressione Evoluzione da low to high pt PHENIX(2008) Medium Fragmentation

10 Experimentally, the centrality is evaluated by measuring one or more of these variables: –N ch : number of charged particles produced in a given rapidity interval (near mid-rapidity) increases (~ linearly) with N part –E T : transverse energy = E i sin i increases (~ linearly) with N part –E ZDC : energy collected in a zero degree calorimeter increases (~ linearly) with N spectators

11 Bjorkens formula To have an estimate of the energy density reached in the initial stages of the collisions, we can project back in time the energy carried by the collision products (Bjorkens estimate) In the center-of-mass frame v=0 at the center of the slab dv = c d = (c/2) dy (non rel.: y = ) dz = 2 dv = c dy dV = S dz = S c dy dE = dV Consider a thin cylindrical slab of transverse dimension S of expanding matter contained within a thickness dz at time. v=0-dvdv dz dE Bjorkens formula

12 Initial energy density Bjorkens formula Transverse dimension S : Initial time : usually taken to be ~ 1 fm/c i.e.: equal to the formation time: the time it takes for the energy initially stored in the field to materialize into hadrons Estimate for central (head-on) Pb-Pb collisions at the SPS ~ (400/160) GeV/fm 3 ~ 2.5 GeV/fm 3 Enough for deconfinement! Published estimate from NA49: = GeV/fm 3 [Phys. Rev. Lett. 75 (1995), 3814] RHIC ~ 5 GeV/fm3

13 Strangeness enhancement restoration of symmetry -> increased production of s –mass of strange quark in QGP expected to go back to current value m S ~ 150 MeV ~ Tc copious production of ss pairs, mostly by gg fusion [Rafelski: Phys. Rep. 88 (1982) 331] [Rafelski-Müller: P. R. Lett. 48 (1982) 1066] deconfinement stronger effect for multi-strange –can be built recombining uncorrelated s quarks produced in independent microscopic reactions strangeness enhancement increasing with strangeness content [Koch, Müller & Rafelski: Phys. Rep. 142 (1986) 167] s u s d ud u u u u u u u u u u u u u u d d d d d d d d dd d d d d s s ss s s s s s s s s s s dd d d d d d u u u u u u u u d K+K+ u p -

14 Charmonium as a Probe of QGP Matsui and Satz predicted J/ production suppression in Quark Gluon Plasma because of color screening

15 Charmonium suppression QGP signature proposed by Matsui and Satz, 1986 In the plasma phase the interaction potential is expected to be screened beyond the Debye length D (analogous to e.m. Debye screening): Charmonium (cc) and bottonium (bb) states with r > D will not bind; their production will be suppressed D, and therefore which onium states will be suppressed, depends on the temperature

16 and, using: 1 MeV -1 = fm: D 0.15 fm Debye screening with n 0 = density of electrons in the plasma In a QGP, the colour field is likewise going to be screened. In order to have a back-of-envelope estimate the screening length, one can take the above formula, and substitute: n 0 n = 3.6 T 3 (Stefan-Boltzmann law for QGP) e 2 (Gauss system) QCD ~ 1 kT ~ 200 MeV getting:n = MeV 3 In an electromagnetic plasma, the potential of a charge is screened by the field of the electrons that surround it [see e.g.: Jackson p. 494] :

17 c c c c c V= kr - / R V = - e – r/ d R QGP NO QGP

18 Come si identifica una risonanza (esempio J/ ) ? - Identificazione dei leptoni ( canale a 3 molto difficile); - misura del loro momento; - Calcolo della massa invariante e selezione. m inv = sqrt( (E 1 +E 2 ) 2 – (p 1 +p 2 ) 2 )

19 NA38

20 Dimuon Spectrum The measured dimuon spectrum is fitted to a source cocktail in order to extract the J/, and Drell-Yan contributions NA50 dimuon spectrum (Pb-Pb, 158 A GeV/c) Quarkonium production is usually normalised to Drell-Yan production (which is not influenced by strong interactions)

21 Why do we keep using Drell-Yan ? Drell-Yan (muon pairs) is a well known computable process, proportional to the # of elementary nucleon-nucleon collisions, with the following priceless advantages : identical experimental biases identical inefficiencies identical selection criteria identical cuts as J/ Therefore the corrections cancel out in the ratio (J/ ) (DY) which is insensitive to normalization factors/uncertainties PUNTI NEGATIVI : 1) statistica DY << statistica J/ 2) normalizzazione isospin

22 Nuclear absorption There is a normal suppression of the production of J/, observed already in pA and lighter ion collisions and attributed to nuclear absorpion The Pb-Pb point falls below the nuclear absorption curve (anomalous suppression) Branching to muons

23 From a fit of experimental p-A data (NA38,NA50): abs = mb ( hep-ex/ ) In S-U collisions the same suppression is observed The normal suppression is interpreted as the absorption, in the nuclear environment, of the c-cbar pair before the J/ (or or ) formation : preresonance absorption.


25 The CERN Pb ion programme Started in 1994 Pb nuclei accelerated to 158 A GeV/c (40 A GeV/c in 1999) collide on fixed targets (typically 4-6 weeks/year) 7 experiments: –NA44 (single arm spectrometer: particle spectra, interferometry, particle correlations) –NA45 (e+e- spectrometer: low mass lepton pairs) –NA49 (large acceptance TPC: particle spectra, strangeness production, interferometry, …) –NA50 (dimuon spectrometer: high mass lepton pairs, J/ production) –NA52(focussing spectrometer: strangelet search, particle production) –WA97/NA57 (silicon pixel telescope spectrometer: production of strange and multiply strange particles) –WA98 (photon and hadron spectrometer: photon and hadron production)

26 A Pb-Pb collision at the SPS Busy events! (thousands of produced particles) High granularity detectors are employed (TPC, Si Pixels,...)

27 NA38 first results O+U at 200 GeV/c: Factor 2 suppression… but… including: normal nuclear absorption !!! IMR charm-like excess !!! (fit starts from 1.7 GeV/c 2 !!)

28 Experiment NA50 Aim: study the production of J/ in Pb-Pb collisions Experimental technique: –absorb all charged particles produced in the collision except muons –detect J/ by reconstructing the decays J/ (B.R. 5.9 %)

29 Anomalous J/ suppression J/ normalized to Drell-Yan as a function of the transverse energy (i.e. centrality) The data points deviate from the solid curve, which indicates the prediction for nuclear absorption The deviation increases with increasing collision centrality

30 Attempts at describing the NA50 data within purely hadronic models without deconfinement –dissociation of the J/ in final state hadronic interactions with comovers try harder...

31 J / L Projectile Target

32 NA60: stesso rivelatore di NA50 con aggiunta di rivelatori al Silicio (tracking). Alcune delle motivazioni dellesperimento: If the J/psi suppression pattern in Pb-Pb collisions indicates that central Pb-Pb collisions produce a state of matter where colour is no longer confined, we should move on to the detailed understanding of how deconfinement sets in, and what physics variable governs the threshold behaviour of charmonia ( c ) suppression: (local) energy density, density of wounded nucleons, density of percolation clusters, etc. This requires collecting data with smaller nuclear systems like In-In. If the ' suppression is due to Debye screening, its suppression pattern could provide a clear measurement of T_c. However, the hadronic "comovers" produced in S-U collisions may "absorb" the psi' mesons, since its binding energy is only around 40 MeV. What mechanism is responsible for the ' suppression? The presently existing results are not clear in what concerns the onset and pattern of the psi' suppression. A new measurement is needed, with improved mass resolution to have a cleaner separation of the psi peak with respect to the J/ shoulder, and which scans the energy density region from the p-U to the S-U data. In-In collisions are also well placed for this study. The J/psi data collected in central Pb-Pb collisions indicate that we are already beyond the point where the phase transition takes place, but do not provide any information on the value of the critical temperature. Finite temperature lattice QCD tells us that the strongly bound J/psi ccbar state should be screened when the medium reaches temperatures % higher than T_c, while the large and more loosely bound psi' state should melt near T_c. The NA38 experiment has shown that the psi' is significantly suppressed when going from p-U to peripheral S-U collisions. We need to see if this suppression follows a smooth pattern or a sudden transition, within a single collision system rather than comparing p-U to S-U data.

33 NA60s detector concept MUON FILTER BEAM TRACKER TARGET BOX VERTEX TELESCOPE Dipole field 2.5 T BEAM IC not to scale Origin of muons can be accurately determined Improved dimuon mass resolution Matching in coordinate and in momentum space ZDC allows studies vs. collision centrality Idea: place a high granularity and radiation-hard silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering and energy loss in the absorber beam ~ 1m Muon Spectrometer MWPCs Trigger Hodoscopes Toroidal Magnet Iron wall Hadron absorber ZDC Target area

34 Study the J/ suppression pattern as a function of different centrality variables, including data from different collision systems Study collisions between other systems, such as Indium-Indium Which is the variable driving the suppression? Is the anomalous suppression also present in lighter nuclear systems? Study the nuclear dependence of c production in p-A collisions What is the impact of the c feed-down on the observed J/ suppression pattern? Specific questions that remain open Study J/ production in p-A collisions at 158 GeV What is the normal nuclear absorption cross-section at the energy of the heavy ion data? New and accurate measurements are needed to answer these questions S-U In-In Pb-Pb N part L (fm) pure Glauber calculation

35 The normal absorption curve is based on the NA50 results. Its uncertainty (~ 8%) at 158 GeV is dominated by the (model dependent) extrapolation from the 400 and 450 GeV data Comparison with previous results An anomalous suppression is present already in Indium-Indium

36 Spiegazioni alternativa al QGP -Comovers model. La J/ puo interagire con gli adroni comovers. La sezione durto e molto difficile da stimare. Na50 J/ suppression can be reproduced by DPM with absorption by comovers. The number of comovers in Capella model is proportional to number of participants and also to number of collisions. A. Capella, D. Sousa, nucl-th/ ) Comovers

37 Suppression by produced hadrons (comovers) 158 GeV The model takes into account nuclear absorption and comovers interaction with co = 0.65 mb (Capella-Ferreiro) J/ NColl nuclear absorption comover + nuclear absorption 158 GeV (E. Ferreiro, private communication) The smeared form (dashed line) is obtained taking into account the resolution on N Part due to our experimental resolution NA60 In-In 158 GeV

38 [First works: Baym, Physica (Amsterdam) 96A, 131 (1979) Celik et al., Phys. Lett. 97B (1980) 128] Forma di deconfinamento geometrica, di pre-equilibrio. Pre-requisito al deconfinamento Vero e proprio, applicabile ai sistemi finiti. Se il condensato di partoni contiene partoni abbastanza hard, puo dissociare la J/ R Superficie di area R 2 N dischi di raggio r<< R Densita n= N/ R 2 Aumentanto la densita si trovano cluster di area sempre maggiore Quando N,R e n finito, la cluster size diverge a n= 1.13/ R 2. Per N,R finiti si ha Percolazione quando il cluster piu largo Riempie tutta la superficie. =n(r/R) 2 Percolation probability r/R=1/100 2) Percolation

39 A causa delloverlap, alla soglia di percolation, solo 2/3 dellarea e riempita. Local Percolation: Hard probe, come gli stati c-cbar risentono del mezzo localmente. Si ottiene a 1.72/ r2

40 3) Regeneration regeneration suppression L. Grandchamp and R. Rapp, Phys. Lett. B (2001) Regeneration models[2,3,4] predict an enhanced production of hidden charm states for sufficiently high charm densities This would imply thermalization of charm quarks, and by extension, the light quarks that comprise the QGP Osservazione sperimentale (Gazdzicki and Gorenstein) Il rapporto di J/ / - = cost a dominant fraction of the Jc mesons produced in hadronic and nuclear collisions at CERN SPS energies is created at hadronization according to the available hadronic phase space


42 Two component model: suppression in hadronic and QGP phase + statistical production at hadronization

43 J/ at SPS J/ in NA60 poorly reproduced by models which fit NA50 data Satz, Digal, Fortunato Rapp, Grandchamp, Brown Capella, Ferreiro

44 - Lattice Gauge calculations now indicate that the J/ψ remains bound up to 1.5 to 2 times the deconfinement temperature (the J/ψ is very small in radius, and would reasonably require a higher density to become unbound due to screening). The maximum temperature reached in central Au+Au collisions at RHIC is thought to be ~ 2 times the transition temperature, so it is now not clear if the J/psi is expected to melt at RHIC. - some higher states that feed down to the J/ψ are expected to melt just above the deconfinement temperature; - the large charm quark production cross section at RHIC leads to predictions that J/ψ will be formed by random coalescence of unrelated charm pairs in central Au+Au collisions, even if the initial group of forming J/ψ is destroyed in deconfined matter; - modifications of gluon densities at low momentum-fraction in heavy nuclei are expected to start to be significant at RHIC and perhaps modify the initial charm anti-charm production cross sections. A RHIC tutto piu complicato

45 R AuAu (y~0) ~ R AuAu (SPS) Lower rapidity R AA look surprisingly similar, while there are obvious differences: –Cold nuclear matter effects (x Bjorken,…) –Energy density –… ±12% global syst ±7% global syst ±11% global syst

46 R AuAu (y~0) > R AuAu (y~1.7) More suppression at forward rapidity ! ±12% global syst ±7% global syst R AA (y~1.7) R AA (y~0) 60%

47 Quick comparison to SPS At mid-rapidity, the amount of surviving RHIC is compatible with SPS (~60%) but depends a lot on CNM (and pp references)… At forward rapidity, RHIC anomalous suppression is much stronger ! ±11% global systematics ±35% global systematics ±30% global systematics J/ψ survival beyond CNM 44± 23% 25±12%

48 J/ψ R AA over CNM in Cu+Cu and Au+Au 48 Calculations by M.J. Leitch using break-up cross-section and errors estimated from 2008 data Differences between mid and forward rapidity measurement is washed out. Suppression beyond cold nuclear matter effects is observed, consistent with de- confinement

49 Conclusions Two qualitative possible scenarios 1.Large melting + some regeneration 2.Initial effects (CGC) + melting (of ψ, χ c ?) Need better handle of CNM Need better open charm measurements ! Smoking gun would have been a J/ψ rise … v 2 could become the smoking gun –(maybe run7 with 4 x run4 and reaction plane detector) Data is young, new ideas may arise …

50 Johanna Stachel

51 Model prediction

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