Recent results from Relativistic Heavy Ion Collider Bedanga Mohanty Variable Energy Cyclotron Centre, Kolkata Outline : Motivation RHIC experiments Results Summary Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Motivation Aim : To create and study QGP – a state of deconfined, thermalized quarks and gluons over a large volume predicted by QCD at high energy density. Lattice QCD prediction: F. Karsch, Prog. Theor. Phys. Suppl. 153, 106 (2004) TC≈1708 MeV, eC≈1 GeV/fm3 Achieved by colliding heavy ions at high energy and comparing the results to reference systems such as nucleon-nucleon collisions and nucleon-nucleus collisions Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 RHIC experiments Collisions : Au+Au, Cu+Cu, d+Au and p+p Energy : 19.6, 56, 62.4, 130 and 200 GeV Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

What happens when 2 heavy ions collide Courtesy of S. Bass In this talk we will address : Is QGP formed at RHIC ? When is the particle composition (which change via inelastic collisions) decided ? What is the freeze-out (after elastic collisions among hadrons have ceased) conditions at RHIC ? Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Observables in RHIC experiments Energy, momentum, spatial position and multiplicity Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Final stable particles identified at RHIC Identify the particle type Mostly 5 charged particles and 1 neutral hadron are stable enough to reach the detectors We detect photons also Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Freeze-out conditions at RHIC W X L The particle spectra are fitted to hydrodynamics motivated functional forms to extract the freeze-out Temperature (random aspect) and radial flow velocity (collective aspect) Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Freeze-out conditions at RHIC Tch p,K,p,L: Tkin decreases, b increases with centrality X, W (low hadronic x-sections): higher Tkin and significant radial flow. Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Chemical Freeze-out conditions at RHIC Tch gs Chemical freeze-out temperature ~ 165 MeV Rare particles chemical and kinetic freeze-out at same temperature Strangeness is chemically equilibrated Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Penetrating probes History tells us they are the best to probe and establish features/properties of matter Two examples from past : Discovery of the atomic nucleus Discovery of quark structure of proton Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Discovery of the atomic nucleus the clever experiment the (surprise!) result Penetrating probe : a particle Medium: Au - nucleus It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you! Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Discovery of the quark structure of proton Penetrating probe : electron Medium : proton Can we do a similar thing to discover QGP ? Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Jets/heavy particles QGP penetrating beam (jets or heavy particles) absorption or scattering pattern Probe QGP created in Au-Au collisions as transient state after ~ 1 fm Penetrating beams created by parton scattering before QGP is formed High transverse momentum particles  jets Heavy particles  open and hidden charm or bottom Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

What to expect from penetrating probes ? Help establish the creation of a dense matter (QGP type) - jets - high pT particles stopped or production suppressed? - Modifies jet shape ? - heavy particles - are particles with heavy quark quarks ? - do they melt J/y and regenerates them ? Help establish the creation of hot matter (QGP type) - photons will provide information on temperature Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Is high pT particle production suppressed ? PRL 91 (2003) 72301 Nuclear modification factor: High pT particle production suppressed by factor ~ 5 Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Is high pT particle production suppressed ? Binary Collision scaling All experiments agree! Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Suppression is due to energy loss in dense medium ? Due to saturation of gluon density in the gold nucleus at this high energy of collisions The observed suppression can be due to energy loss of partons while traversing a dense medium Initial state effect? Gluon saturation Final state effect? Partonic energy loss Au Au d How to resolve this : Solution: Perform a control experiment by turning off final state effects: d+Au collisions Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Suppression is due to energy loss in dense medium ? Production rate of high pT pions suppressed in Au-Au and NOT in d-Au Recoil peak in back-to-back jets at 180o is absent in Au-Au data but present in p-p and d-Au data Controlled experiment successful in showing the suppression is a final state effect due to energy loss in dense medium Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Suppression is due to energy loss in dense medium ? All experiments agree! Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Can we prove that it is at partonic level ? Let us look at particle dependence : p0, h and direct g If partonic : direct g should not be suppressed, p0 and h should show similar suppression Energy density > 15 GeV/fm3 Effect seems to be at partonic level Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

What about strange particles ? Strange baryons do not show suppression RAA increases with strangeness content number of s-quark Rcp for baryons and mesons different at intermediate pT and then becomes similar at higher pT Recombination + Fragmentation: R. J. Fries, S. A. Bass, et. al. nucl-th/0306027 Medium allows for recombination at partonic level Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

What about heavy particles ? Coupling of heavy quarks to medium is less due to mass p If energy loss is due to radiative process then RAA for electrons from heavy flavour > RAA for light hadrons Medium so dense – even heavy particles are stopped ! Results indicate energy loss mechanism still to be understood Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 What about J/y ? The J/y’s may melt in hot medium and the charm and anti-charm become unbound, and may combine with light quarks to emerge as “open charm” mesons. These quarks effectively cannot “see” each other! T=0 T=200 aeff 0.52 0.20 0.41 fm 1.07 fm ∞ 0.59 fm From Introduction to High-Energy Heavy-Ion Collisions, C.Y. Wong 1994 Matsui & Satz (Phys. Lett. B178 (1986) 416): In the QGP the screening radius could become smaller than the J/Y radius, effectively screening the quarks from each other! J/y production will be suppressed if QGP is formed Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

J/y suppression at RHIC ? dAu mm 200 GeV/c AuAu mm 200 GeV/c CuCu mm 200 GeV/c AuAu ee 200 GeV/c CuCu ee 200 GeV/c CuCu mm 62 GeV/c Factor of 3 suppression in J/y production Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

What more does J/y suppression tell ? Models that explain data at SPS over predicts suppression at RHIC data are consistent with the predicted suppression + re-generation at the energy density of RHIC collisions Recent lattice calculations show J/Ys remaining bound as high as 1.5Tc (Petreczky et al, hep-lat/0309012) The matter may melt and also regenerate J/y’s Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Temperature of the initial system Tinitial > Tcritical - puts QGP discovery on firm footing How : By measuring the pT spectra of direct photons Why direct photons : The thermal photons produced from the hot QGP phase do not interact with the medium and hence carry information of the system at initial stage Decayphotons hard: thermal: Disadvantage : photons from decay and choose pT region where direct photons dominate Advantage : At RHIC decay photons background suppressed due to suppression in p0 production Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Direct photons at RHIC First direct measurement of initial temperature of the system formed at RHIC Tave ~ 265 MeV Ttrue ~ 215 MeV J. Alam et al., nucl-th/0508043 Tinitial (~ 400 MeV) > Tcritical (~ 175 MeV) Matter formed is very hot, initial temperature much higher than predicted by Lattice QCD Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

What more on direct photons at RHIC ? Model independent confirmation : p0 suppression at high pT Direct g scales with Ncoll : GAA = RAA R. J. Fries, B. Muller, D. K. Srivastava PRC 72 (2005) 041902 ® Jet-photon conversion have significant contribution top high pT photon production Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Summary There is compelling experimental evidence that heavy-ion collisions at RHIC has produced a state of matter characterized by Very high initial energy density (> 15 GeV/fm3) Very high initial temperature (> 400 MeV >> 1012 K) Density of unscreened color charges 10 times that of nucleon A matter so strongly coupled that even heavy quarks flow A matter so dense that heavy quarks and 20 GeV hadrons are stopped A matter so hot that melts J/y and regenerates them Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Summary This state of matter is not describable in terms of ordinary color-neutral hadrons, because there is no known self-consistent theory of matter composed of ordinary hadrons at the measured densities The most economical description of this matter is in terms of the underlying quark and gluon degrees of freedom References : BRAHMS Collaboration, I Arsene et al., Nucl. Phys. A 757: 1, 2005 PHOBOS Collaboration, B.B. Back et al., Nucl. Phys. A 757: 28, 2005 STAR Collaboration, J. Adams et al., Nucl. Phys. A 757: 102, 2005 PHENIX Collaboration, K. Adcox et al., Nucl. Phys. A 757: 184, 2005 Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Summary As we move from the initial discovery phase of dense partonic matter to the next phase of probing the properties Of the matter created at RHIC, two important questions Remain to be resolved : Do the quarks and gluons in this matter reach thermal equilibrium with one another Before they become confined within hadrons ? Whether chiral symmetry is restored ? References : BRAHMS Collaboration, I Arsene et al., Nucl. Phys. A 757: 1, 2005 PHOBOS Collaboration, B.B. Back et al., Nucl. Phys. A 757: 28, 2005 STAR Collaboration, J. Adams et al., Nucl. Phys. A 757: 102, 2005 PHENIX Collaboration, K. Adcox et al., Nucl. Phys. A 757: 184, 2005 Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005 Thanks Thank you for inviting me To present results from here At Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Jets and signatures What are jets : Signals in heavy ion collisions (indirect) Observation of the high pT leading particle and Azimuthal angular correlation q hadrons leading particle leading particle schematic view of jet production What happens when such jets pass through dense matter as QGP ? Scattered parton expected to loose energy leading to suppression in high pT particle production and suppression in angular correlation All these should have a strong dependence on the path length traveled (centrality or angle to reaction plane) Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005

Suppression is due to energy loss in dense medium ? The observed suppression can be due to energy loss of partons while traversing a dense medium (Final state effect) Or Due to saturation of gluon density in the gold nucleus at this high energy of collisions (Initial state effect) How to resolve this : controlled experiment Initial state effect Gluon density saturated in incoming gold nucleus Deuteron shows no or little saturation Expect suppression of jet yield, but with reduced magnitude Final state effect Medium created in d-Au has small volume No suppression of jet yield expected in d-Au Bedanga Mohanty DAE-BRNS 50th Symposium on Nuclear Physics , December 15th 2005