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Hot and Cold QCD Matter

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Hot and Cold QCD Matter 2. Quantum Chromodynamics: The Fundamental Description of the Heart of Visible Matter Sidebar 2.5: Jetting through the Quark-Gluon Plasma Understanding how quark-gluon plasma (QGP) works enabled by upgrades including the sPHENIX microscope requires new microscopy using energetic quark probes at RHIC. called “jets,” generated in the initial interaction of the A century ago, Ernest Rutherford discovered atomic colliding beams. These high-energy quarks are initially able to “see” the very short distance structure of the nuclei by aiming a beam of alpha particles at a gold foil medium they traverse. As they propagate, they rapidly and observing that they were sometimes scattered at shed energy by splitting of lower energy partons and, large angles. The simplest way to “see” pointlike quarks as this happens, the length scale that they “see” grows and gluons within QGP is, as Rutherford would have rapidly. The combination of all these partons eventually understood, to look for evidence of jets, or partons forms the hadrons that together make up a jet. The within jets, scattering of individual quarks and gluons as they plow through QGP. As the top-right panel illustrates, curves in the top-left panel illustrate how the resolving power (inverse of length scale) of jets at the LHC and partons that can resolve the microscopic structure of RHIC decreases (symbolically, from green to yellow to QGP are more likely to be deflected by larger angles orange) as they propagate and as the QGP in which they than the partons with less resolving power that only see are propagating cools. The highest energy jets at the the nearly perfect liquid. First exploratory measurements of the jet deflection angle are now being carried out LHC probe very short wavelengths, where they should resolve the individual weakly coupled “bare” quarks at the LHC (lower-right, where the sharp peak at the and gluons (green). A key area is the lowest energy right-hand edge of the plot corresponds to undeflected jets, optimally measured at RHIC, that probe longer jets) and at RHIC. Full exploitation of Rutherford-like scattering experiments requires the capabilities of wavelengths toward the scale of the nearly perfect liquid itself (orange). The curves are heavier in the regime sPHENIX at RHIC as well as upgrades to the LHC and its where the resolving power of the jets is determined detectors. largely by the medium itself. The bottom-left panel Understanding the evolution of the microscopic shows the momentum range, related to theHot resolving substructureand of QGP Cold as a function of scale QCD will complete Matter power, of many jet observables in current measurements the connection between the fundamental laws of nature, (muted red and blue) and the enormously increased QCD, and the emergent300 phenomena discovered at RHIC. reach at both RHIC (bright red) and the LHC (bright blue) LHC The Phases of QCD RHIC 250 6 SIS 100/FA IR Quark-Gluon Plasma J SHINE -PAR 200 C SIS NIC 18 A-MPD NIC /H RHIC B A-F AD ES-II R XT E HIC FX S values according to the modified thickness functions. In sity profile150 for the constituentT quark st 1 Ord er Ph the IP-Glasma framework the additional parameter m ase Tra 100 Critical ns controls the infrared physics and thus a↵ects the spa- itio Point? n Temperature (MeV) 1 ˜ tial size of the gluon distribution. Because of this the b/ColorBq 50 HadronTq(b )=Gas e− Superconductor, (22) Nuclear8⇡ B˜3 values for the parameters Bqc and Bq in both models Vacuum Matter q 0 cannot be directly compared. Examples of the proton 0 200 400 600 800 1000 1200 1400 1600 Baryon Chemical Potential μ (MeV) density profiles obtained from the IP-Glasma model with and sample the constituent quark locationsB from a three- the parametrization used in this work are illustrated in b/B˜qc Fig. 4 by showing 1 Re Tr V (x)/N . dimensional exponential distribution e− .The − c sampled quarks are then projected on⇠ the transverse plane. We note that the resulting transverse density pro- 30 kT file is not exactly exponential. 1 xp bT 0 [fm] y B. Stringy proton 1 − In order to explore the dependence on the model de- tails we also implement the geometric fluctuations using a color string inspired picture. Here, the idea is that based on quenched lattice QCD calculations, the con- James Dunlopstituent quarks are connected via gluon fields that merge at the Fermat point2 of the quark triangle [99] (see also Brookhaven National LaboratoryRef. [56]). We are not aware of calculations beyond the quenched approximation, which would be a more appro- priate input to our model. We implement this picture by sampling the constituent quark positions from a three dimensional Gaussian dis- tribution with width Bt. Then, the density profile is ob- FIG. 4: Illustration of the proton density profile (1.0 tained by connecting the constituent quarks to the Fer- − mat point of the triangle by tubes whose transverse shape Re Tr V (x, y)/Nc) obtained from the IP-Glasma framework at 3 2 2 x 10− with parameters Bqc =3.0GeV− ,Bq =0.3GeV− is Gaussian with width Br. The 2-dimensional density ⇡ and m =0.4GeV. profile of the proton Tp(b) is then obtained by integrat- ing over the longitudinal direction. In this picture the total gluonic content of the pro- The total photon-proton cross section, and the pro- ton also fluctuates event-by-event, as when the quarks ton structure functions, are proportional to the integral are sampled to be further away from each other, the flux of the dipole amplitude over impact parameter. As the tubes are longer at a constant density, leading to more modification (21) is done in the exponent and the im- gluons in the proton. This adds normalization fluctu- pact parameter dependence factorizes only in the dilute ations to the picture, which are similar to those intro- region, the replacement (21)a↵ects the overall normal- duced by saturation scale fluctuations (see the following ization of, for example, F2. In practice, including geo- section). The overall normalization factor, which con- 2 2 metric fluctuations (Bqc =3.3 GeV− ,Bq =0.7 GeV− ) trols the energy density of the tube, is fixed by requiring 3 2 2 decreases F2 at x 10− ,Q 10 GeV by approxi- that the proton structure function F2 calculated from the ⇠ ⇠ 2 2 3 mately 8%. The di↵ractive cross section changes more, stringy proton at Q = 10 GeV , x = 10− is the same as it is proportional to the squared amplitude. Ideally as that from the original IPsat parametrization without one should perform a new fit to HERA DIS data with fluctuations. Example density profiles (integrated over geometric fluctuations included, but this is beyond the the longitudinal direction) are shown in Fig. 5. The pa- scope of this work. However, this normalization uncer- rameters Bt and Br are fixed by requiring a good de- tainty is similar for both coherent and incoherent cross scription of HERA coherent and incoherent di↵ractive sections and will not a↵ect our conclusions about the re- J/ production measurements [100]. quired amount of geometric fluctuations in the proton wave function. To determine the sensitivity on the details of the as- 2 The Fermat point of a triangle is defined such that the total sumed proton shape we will also calculate the di↵ractive distance from that point to the vertices of the triangle is the cross sections using a three-dimensional exponential den- smallest possible. Hot QCD Matter 300 LHC The Phases of QCD RHIC 250 SIS 100/FA IR Quark-Gluon Plasma J SHINE -PAR 200 C SIS NIC 18 A-MPD NIC /H RHIC B A-F AD ES-II R XT E HIC FX S 150 T st 1 Ord er Ph ase Tra 100 Critical ns itio Point? n Temperature (MeV) Color 50 Superconductor Hadron GasNuclear Vacuum Matter 0 0 200 400 600 800 1000 1200 1400 1600 Baryon Chemical Potential μB(MeV) 8/3/2017 Dunlop Hot and Cold QCD 2 RHIC: A Flexible Machine To do condensed matter at the femto-scale, need to tune matter properties over a wide range 8/3/2017 Dunlop Hot and Cold QCD 3 The Most Vortical Fluid Nature 548, 62-64 (03 August 2017) BBC ˆ Jsys !* * pp θ BBC quark-gluon plasma Λ forward-going beam fragment Figure 3: A sketch of the immediate aftermath of a Au+Au collision. The vorticity of fluid created at midrapidity is suggested. The average vorticity points along the direction of the angular momen- tum of the collision, Jˆsys. This direction is estimated experimentally by measuring the sidewards deflection of the forward- and backward-going fragments and particles in the BBC detectors. L STAR: Lambda spinhyperons is arepolarized depicted as spinning tops; with see text for respect details. Obviously, elements to inreaction this depiction plane Apparently strongestare at not drawn lowest to scale: the energies fluid and the beam fragments have sizes of a few femtometers, whereas Consistent with vorticitythe radius of each(9 BBC ± is about1) onex meter. 1021s-1, far greater than frame, then previously observed in any systemdN 1 ~ = 2 1 + aH PH cosq⇤ . (1) d cosq⇤ | | ⇣ ⌘ The subscript H denotes L or L, and the decay parameter a = a = 0.642 0.01317. The Potential: measurement of late-time magneticL − L field± angle q is indicated in figure 3, in which L hyperons are depicted as tops spinning about their 8/3/2017 ⇤ Dunlop Hot and Cold QCD 4 polarization direction.
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