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Heavy Ion Physics in the Future
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Nuclear Physics A 956 (2016) 248–255 www.elsevier.com/locate/nuclphysa
Heavy Ion Physics in the Future Shoji Nagamiya RIKEN, 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan KEK, 1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, Japan
Abstract Bevalac started its operation in 1970, which was an initiation of this field. Then, 15 years later (in 1985) both AGS and SPS started their operations. After further 15 or 25 years, RHIC (in 2000) and then LHC (in 2010) started operating. Present highlights are centered at these two facilities. However, how about the future direction in this field? Here, firstly the past achievements of RHIC and LHC are reviewed, and then the current and future issues are considered. Finally, the scope on the lower-energy region, in which I am currently involved, is described. Keywords: RHIC, LHC, Collider, Fixed Target. High Temperature, High density.
1. RHIC and LHC In the well-known phase diagram in the plane of temperature (T ) and density (ρ), RHIC and LHC cover high-temperature region at ρ 0. In order to compare the high-temperature region with the lowtemperature region, most of us have used, in so far, the comparison between a) the most central collision (for high T ) and b) either peripheral collisions or pp or pA collisions (for low T ). However, we recently discovered a surprisingly similar behavior at both RHIC and LHC between heavy-ion central collisions and high-multiplicity pp or pA collisions, as discussed at this conference. Therefore, I would like to review first all the historical data by noting what types of data remain as strong implications on the formation of bulk quark-gluon plasma (hereafter called QGP) and what types of data are required further considerations. 1.1. Initial Data from RHIC Immediately after the RHIC started its operation, two major discoveries were announced. First one is the high-pT suppression [1] of particle production, as shown in Figure 1, which strongly suggested that particle energy would be lost when partons traverse through a very dense matter like QGP. Secondly, a strong elliptic flow v2 was observed, which agreed very much with a hydrodynamical flow with very low viscosity, as shown in Figure 2 [2]. Namely, dense and low viscous fluid seems to be created in nuclear collisions at RHIC. These two are called jet quenching and a strong elliptic flow and both of them strongly suggested the formation of bulk QGP. However, in recent studies, a behavior to v2 observed in heavy-ion collisions is similarly detected in light systems like pA collisions [3]. Therefore, the observed behavior as shown in Figure 2 might not be http://dx.doi.org/10.1016/j.nuclphysa.2016.03.003 0375-9474/© 2016 Elsevier B.V. All rights reserved.
S. Nagamiya / Nuclear Physics A 956 (2016) 248–255
Fig. 1. Jet quenching observed at RHIC [1].
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Fig. 2. Strong elliptic flow observed at RHIC [2].
strong evidence on the formation of QGP, or perhaps, a small-system QGP might have been formed. On the other hand, the high-pT suppression still remains in AA collisions alone (perhaps, because the path length is shorter in pA or pp collisions). This unique high-pT suppression is discussed first. At LHC, the pT suppression has been observed also, by covering a much broader range up to 100 GeV. From these results, a theory group called JET Collaboration extracted the jet transport parameters qˆ [4], which are summarized below qˆ = 1.2 ± 0.3 GeV2 /fm at 370 MeV (RHIC) qˆ = 1.9 ± 0.7 GeV2 /fm at 470 MeV (LHC)
(1) (2)
Here, qˆ is the rate of pT broadening per unit length, which is proportional to p2T . Since the radiative energy loss, dE/dx, is ∝ to p2T , the energy loss for parton to traverse the length L is ∝ to qˆ × L. 1.2. Data from LHC When LHC started its operation, the very interesting result, a large jet ET asymmetry, was reported from the ATLAS group [5], as shown in Figure 1.3 (top). This asymmetry is direct evidence of the parton energy loss in the bulk QGP, similarly to high-pT suppression observed at both RHIC and LHC. In addition, if one looks at the lost jet energy distribution on the other side, which was observed by the CMS group [6], it is distributed very widely. Namely, if one looks at the in-cone distribution, it is suppressed less than 1, while if one looks as the out-of-cone distribution, it is enhanced more than 1, as shown in Figure 1.3 (bottom). At this conference, many reports on the di-jet asymmetries were presented [7] and it was one of major highlights at this conference. The implication of this di-jet asymmetry is worth to consider in the future. 1.3. Notable Data on the Flow At RHIC, if all the available v2 values are normalized by the number of quarks nq , namely, 2 for mesons and 3 for baryons, then, the entire plots of v2 /nq vs KET /nq seem to be on the same curve [8], where KET = (m2 + p2T )1/2 − m. This is called the quark number scaling and, again, this fact seems to give a strong implication on the formation of bulk QGP. On the other hand, as described before, an entire issue on the flow is now open for discussions [3] [9], as people observed this phenomenon in all pA, dA and AA systems. In addition, at LHC, a slight deviation from quark number scaling is found by the ALICE group [10]. At LHC there might be an effect from the radial flow. To my surprise, higher order flows, not only v2 but up to v5 , have also been measured at both RHIC and LHC. This trend has never been measured in light systems like pA, so that these measurements could still be a unique signal for heavy-ion collisions (Or, one must try to measure v2 to v5 in pA collisions).
S. Nagamiya / Nuclear Physics A 956 (2016) 248–255
250
Asymmetric di-jets and even mono-jets (!)
ATLAS Aj =
ET1 – ET2 ET1 + ET2
'I Lost jet energy distributed very widely
CMS
Fig. 3. ET asymmetry observed at ATLAS [5] (top) and the lost jet energy distribution on the other side at CMS [6] (bottom).
An interesting theoretical summary on the flow is shown in Figure 1.3 [11]. The values, of course, depend strongly on the initial condition. According to their calculations, the value of η/s at LHC is 0.2 and that at RHIC is 0.12, and both values are larger but very close to the non-viscous fluid limit of η/s = 1/4π = 0.08, by fitting all v2 to v5 values. The result was amazing, because at both RHIC and LHC, η/s is close to 0.08. On the other hand, LHC seems to create slightly more viscous fluid than RHIC does. Perhaps, at the higher temperature stage, the matter initially contains a certain gas phase, since, in a gas, η/s is very large. Note 1 that a gas phase would be created at LHC more than at RHIC due to higher temperature at LHC.
Fig. 4. Current understanding for v2 to v5 [11].
1.4. Direct Photons Regarding photons, a direct photon has been measured from electron pairs with effective mass equals to zero. The temperature extracted from these direct photon measurements is about 220 MeV at RHIC [12] and 300 MeV at LHC [13] (see Figure 1.4). More recently, the temperature from RHIC has been measured to 240 MeV [14] but both values agree within errors (namely, at RHIC, 220 ± 19 ± 19 MeV and 240 ± 25± 7 MeV). Both temperatures at RHIC and LHC are higher than the critical temperature for the QGP phase transition T C , which is 160 to 170 MeV. This is very natural, because photons observe an earlier stage of the collision than T C . Note that the critical temperature T C is also very close to the hadronization temperature, T had , which has been studied for a long time by Stachel et al. [15]. What is the real initial temperature? It should be much higher than the observed values by photons, most likely, 30 to 40% higher than the values observed here.
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Fig. 5. Temperatures measured by direct photons at RHIC (left) [12] and at LHC (right) [13].
1.5. Sequential Melting of Quarkonia
Fig. 6. Sequential melting of quaorkonia observed by CMS [17].
I personally have been interested very much in the J/Ψ suppression that was predicted well before the RHIC started its operation [16], because this could be a direct evidence for the deconfinement of QGP. The results on Figure 6 from CMS [17], made me very excited, since this could be direct evidence of sequential melting of quaokonia. Namely, it could be the first observation of deconfinement in heavy ion collisions. The measurements are being continued at LHC.
1.6. New Question in p+p and p+A
Fig. 7. An example of a big question on the similarity between pA collisions and heavy-ion collisions.
A new question, which is one of serious topics at this conference, is that several phenomena observed in heavy-ion collisions at both RHIC and LHC show up in high-multiplicity p+A or even in p+p collisions. This is a great surprise not only to me but also most of the people. Here one example from CMS is shown in Figure 7. In the jet studies it is indicated that a small system like a pA collision behaves very similarly to a big system like an AA collision. There was a special session on the comparison between pA and AA at this conference on Friday. Still, many unsolved questions remain.
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Fig. 8. Summary of various parameters obtained at RHIC and LHC.
Fig. 9. Near future plans at RHIC and LHC.
2. What Do We Expect in the Near Future Next I summarize the knowledge about results on qˆ , η/s, initial temperature, and transition temperature in Figure 8, although other theoretical predictions may exist. Both RHIC and LHC presented their 10 yearplans, as shown in Figure 9. RHIC plans heavy-flavor run, Beam Energy Scan II (BES II) and s-PHENIX with a new configuration. LHC already started the energy recovery run and, then, LHC and ALICE Upgrades are planned for the purpose of the high luminosity runs at LHC. 2.1. Futures of RHIC and LHC It will definitely allow us >10 years to conduct future programs at LHC. Here, studies on the properties of QGP up to T ∼ 3T C must be conducted. The subjects are a) sequential quarkonia melting, b) v2 to v5 or even higher order flows, c) jet-jet correlations, in particular on jet anomalies, d) heavy flavor, e) J/Ψ melting versus recombination, f) chiral magnetic effects, g) recovery of chiral symmetries, and so on. On the other hand, RHIC will complete its scientific mission by the middle 2020s, but many issues similar to those at LHC must be clarified there near T C up to ∼ 2T C . Then, the Beam Energy Scans to search for critical points, and sPHENIX to study QGP via jets and Upsilon probes will start. In addition, pA and pp questions must be definitely solved. Finally, eRHIC, and lower energy heavy-ion collisions to probe a high-density region might have to be done, which I describe after this. 2.2. Comparison between Collider and Fixed Target Experiments In order to study details on the lower energy region, collider versus fixed target experiments are compared. At RHIC and LHC, the luminosity is 1027−28 cm−2 s−1 . This 1027−28 corresponds to 7-70 kHz collision rate for Au + Au collisions. Currently, 50 kHz data rate is a future big challenge at both RHIC and LHC. Now, compare with this number from a fixed target experiment. Presently, 109 per bunch beams are available at AGS for RHIC. If this beam was bombarded to the 1% target, then, it is already 107 interactions per bunch. In addition, if 5 bunches form one pulse and one pulse is stretched to 5 seconds, then, per bunch is almost equals to per second. Therefore, if one can accelerate 1010−11 beams/s on a fixed target, the 1% target is equivalent to the 100 MHz or even 1 GHz collision rate. Therefore, a new physics might come out with high-intensity heavy-ion beams on a fixed target. In the well-known phase diagram, RHIC and LHC will cover high-temperature regions, as described before (Figure 10). On the other hand, STAR s (and also other s) beam energy scan will cover the region into higher density. The on-going FAIR and/or, perhaps, NICA and J-PARC, will cover much higher densities. Hereafter, I describe these high-density regions, including its relation to neutron star. Of course, STAR Collaboration has already reported interesting results [18], including the third-order and/or fourthorder fluctuation that are sensitive to a critical point and phase boundaries [19]. Future data are extremely interesting toward a high-density region.
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Fig.10. Famous phase diagram and the possible coverage by many methods.
3. Lower Energy Scope At the AGS energy of 15 GeV per nucleon, the colliding nuclei stop each other, and the Lorenz factor becomes real, as shown in Figure 11. At the AGS energies, I estimated that the density up to 6 times the normal nuclear matter density can be achieved. Recently, many other predictions have been reported, in particular, up to which densities the colliding nuclei can achieve. In addition to the high-density formation, another interest would be multi-strangeness production. The merit here is that the production rates reach the maximum at around 10 AGeV on a fixed target. For example, I estimated that the production of S (strangeness) ≥ 3 hypernuclei is possible but marginal, according to the statistical model [20]. At a full intensity beam of 1011 /s, we can have 10 MHz central collisions, which leads to 0.3 events/hour, that is marginal but not impossible. In relation to both high-density and multi-strangeness production, studies are in progress in terms of neutron stars, as shown in Figure 12. This nice slide was provided by H. Tamura at Tohoku University [21]. Density attained by neutron stars is about 3 to 10 times the normal nuclear matter density. Below or around the normal density, which appears near the surface, the nuclear pasta and/or the pure nucleon star exist. On the other hand, in the interior of the neutron star, where a high-density matter is created, there are Fig. 11. Density reached at 15 AGeV. presently two speculations; one is a normal strange hadronic matter with strong three-body forces, as shown on the left-side of Figure 12 or a quark matter as shown on the right-side. These two are currently actively discussed [22]. Regardless of the hadronic or quark matter, a strangeness-rich object could be created (called the strangelet). Furthermore, this strangelet might be stable. At AGS, there was a search of this object [23]. We must repeat it with much higher intensity heavy-ion beams. In addition, high-density formation may help to create much higher multi-lambda production than predicted by the statistical model described before.
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Fig. 13. Several possible experiments under discussions. Fig. 12. Summary of current understanding for neutron stars.
Several experiments are under discussions. The first example is to have a strong magnet behind the target. Detectors are installed in between the primary neutron flux and the primary beams for a stable object like strangelet. Or, they are installed to measure lambda decays to study multi-strangeness nuclei. The second example is to detect lepton pairs for the measurement of high-density matter. See Figure 13. 3.1. FAIR, NICA? and J-PARC? The first realistic approach for 10-20 AGeV energy on a fixed target came from the FAIR at GSI, as shown in Figure 14. Here, the beam intensity up to 1010−11 /s can be achieved in heavy-ion collisions. Five major experiments are on the table. A lepton pair experiment is among them. This project is well known among the community. However, the energy around or above 10 AGeV has a certain delay, by waiting a future expansion of the FAIR. Therefore, projects other than FAIR have also been proposed to cover around or over 10 AGeV region, although they are quite preliminary. The first proposal came from Russia, called the NICA project. I glanced the letter of intent published this year (2015). A very strong message was presented there. The second approach came from our J-PARC. To achieve this energy, a linac and a booster ring have to be added. The merit here is that two major rings, 3 GeV and 50 GeV, as indicated in Figure 15, have already exFig. 14. Current plan at FAIR. isted and they are working. Therefore, only the left-side part must be constructed. We may obtain up to 1010−11 /s for heavy-ions above 10 GeV per nucleon, according to the estimate of the accelerator group. In fact, proton beams up to 5 times 1013 /s or 1014 /s were already accelerated there. The heavy-ion linac and the booster ring fit within 50 m2 . This is very tiny as compared to the J-PARC linac (330 m) or 3 GeV (diameter is 150 m). The injection point to the 3 GeV has already been decided but we must wait for the funding approval. 3.2. Summary toward High Density Approaches In this section I would like to summarize lower energy issues. FAIR at GSI is a real project and it is in progress. The NICA proposal was written in 2015. The J-PARC’s letters of intent will be submitted in 2016. High-density studies via rare processes like electron pairs must be conducted. Search for strangelets or
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Fig. 15. Acceleration scheme under discussions at J-PARC.
multi-strangeness states can/shall be done if the production rate larger than 10−11 for central AA collisions. Also, if hypernuclei are produced in the projectile frame, then the lifetime is elongated by a βγ factor that corresponds to about 30-60 cm. It is interesting to study hyperon mixing in neutron-star. Studies of fluctuation by the STAR BES, as shown in Figure 9, are very interesting, too. 4. Summary of My Talk RHIC has played an extremely important role for the discovery and properties of quark-gluon plasma. RHIC will complete its scientific mission by mid-2020’s. LHC, on the other hand, has added more surprises and will have, at least, ten years of runs for the future unexpected discoveries. Questions in pA, dA and pp as compared with AA collisions, as heavily discussed at this conference, must be solved. Finally, it would be useful to explore the high-density region with very high-intensity heavy ion beams on the order of 1011 particles per second at selected facilities. I appreciate for the support by Y. Akiba, K. Shigaki, T. Gunji, et al. (and Y. Watanabe for English corrections) when I prepared this manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
PHENIX Collaboration, Phys. Rev. Letters 88, 022301 (2002). STAR Collaboration, Phys. Rev. Letters 88, 402 (2001). P. Bo˙zek, presented at this Conference (2015). JET Collaboration, Phys. Rev C90, 014909 (2014). ATLAS Collaboration, Phys. Rev C90, 014909 CMS Collaboration, Phys. Rev C90, 014909. For example, A. Angelami, presented at this Conference (2015). PHENIX Collaboration, Phys. Rev. Letters 91, 182301 (2003); STAR Collaboration, Phys. Rev. Letters 92, 052302 (2004). PHENIX Collaboration, Phys. Rev. Letters, 114, 192301 (2015). T. Gunji, presented at this Conference (2015). C. Gale, S. Jeon, B. Schenke, P. Tribedy, and R. Venugopalan, Phys. Rev. Letters 110, 012302 (2013). PHENIX Collaboration, Phys. Rev Letters 104, 123391 (2010). ALICE Collaboration, Nucl. Phys. A904-905, 573c (2013). A. Drees, presented at this Conference (2015). J. Stachel, A. Andronic, P. Braun-Munzinger, K. Redlich, J. Phys. Conf. Ser. 509 012019 (2014). T. Matsui and H. Satz, Phys. Letters B178, 416 (1986). CMS Collaboration, Phys. Rev. Letters 109, 222301 (2012). M. Mustafa, presented at this Conference (2015). STAR Collaboration, Phys. Rev. Letters 112, 032302 (2014). A. Andronic, P. Braun-Munzinger, J. Stachel, H. Stoecker, Phys. Letters B697, 203 (2011). H. Tamura, private communication (2015). K. Masuda, T. Hatsuda and T. Takatsuka, Astrophys. J., 764, 12 (2013). T. A. Armsyrong, et al. (E886 Collaboration), Phys. Rev. 63, 054903 (2001).
Nuclear Physics A 956 (2016) 248–255 www.elsevier.com/locate/nuclphysa
Heavy Ion Physics in the Future Shoji Nagamiya RIKEN, 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan KEK, 1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, Japan
Abstract Bevalac started its operation in 1970, which was an initiation of this field. Then, 15 years later (in 1985) both AGS and SPS started their operations. After further 15 or 25 years, RHIC (in 2000) and then LHC (in 2010) started operating. Present highlights are centered at these two facilities. However, how about the future direction in this field? Here, firstly the past achievements of RHIC and LHC are reviewed, and then the current and future issues are considered. Finally, the scope on the lower-energy region, in which I am currently involved, is described. Keywords: RHIC, LHC, Collider, Fixed Target. High Temperature, High density.
1. RHIC and LHC In the well-known phase diagram in the plane of temperature (T ) and density (ρ), RHIC and LHC cover high-temperature region at ρ 0. In order to compare the high-temperature region with the lowtemperature region, most of us have used, in so far, the comparison between a) the most central collision (for high T ) and b) either peripheral collisions or pp or pA collisions (for low T ). However, we recently discovered a surprisingly similar behavior at both RHIC and LHC between heavy-ion central collisions and high-multiplicity pp or pA collisions, as discussed at this conference. Therefore, I would like to review first all the historical data by noting what types of data remain as strong implications on the formation of bulk quark-gluon plasma (hereafter called QGP) and what types of data are required further considerations. 1.1. Initial Data from RHIC Immediately after the RHIC started its operation, two major discoveries were announced. First one is the high-pT suppression [1] of particle production, as shown in Figure 1, which strongly suggested that particle energy would be lost when partons traverse through a very dense matter like QGP. Secondly, a strong elliptic flow v2 was observed, which agreed very much with a hydrodynamical flow with very low viscosity, as shown in Figure 2 [2]. Namely, dense and low viscous fluid seems to be created in nuclear collisions at RHIC. These two are called jet quenching and a strong elliptic flow and both of them strongly suggested the formation of bulk QGP. However, in recent studies, a behavior to v2 observed in heavy-ion collisions is similarly detected in light systems like pA collisions [3]. Therefore, the observed behavior as shown in Figure 2 might not be http://dx.doi.org/10.1016/j.nuclphysa.2016.03.003 0375-9474/© 2016 Elsevier B.V. All rights reserved.
S. Nagamiya / Nuclear Physics A 956 (2016) 248–255
Fig. 1. Jet quenching observed at RHIC [1].
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Fig. 2. Strong elliptic flow observed at RHIC [2].
strong evidence on the formation of QGP, or perhaps, a small-system QGP might have been formed. On the other hand, the high-pT suppression still remains in AA collisions alone (perhaps, because the path length is shorter in pA or pp collisions). This unique high-pT suppression is discussed first. At LHC, the pT suppression has been observed also, by covering a much broader range up to 100 GeV. From these results, a theory group called JET Collaboration extracted the jet transport parameters qˆ [4], which are summarized below qˆ = 1.2 ± 0.3 GeV2 /fm at 370 MeV (RHIC) qˆ = 1.9 ± 0.7 GeV2 /fm at 470 MeV (LHC)
(1) (2)
Here, qˆ is the rate of pT broadening per unit length, which is proportional to p2T . Since the radiative energy loss, dE/dx, is ∝ to p2T , the energy loss for parton to traverse the length L is ∝ to qˆ × L. 1.2. Data from LHC When LHC started its operation, the very interesting result, a large jet ET asymmetry, was reported from the ATLAS group [5], as shown in Figure 1.3 (top). This asymmetry is direct evidence of the parton energy loss in the bulk QGP, similarly to high-pT suppression observed at both RHIC and LHC. In addition, if one looks at the lost jet energy distribution on the other side, which was observed by the CMS group [6], it is distributed very widely. Namely, if one looks at the in-cone distribution, it is suppressed less than 1, while if one looks as the out-of-cone distribution, it is enhanced more than 1, as shown in Figure 1.3 (bottom). At this conference, many reports on the di-jet asymmetries were presented [7] and it was one of major highlights at this conference. The implication of this di-jet asymmetry is worth to consider in the future. 1.3. Notable Data on the Flow At RHIC, if all the available v2 values are normalized by the number of quarks nq , namely, 2 for mesons and 3 for baryons, then, the entire plots of v2 /nq vs KET /nq seem to be on the same curve [8], where KET = (m2 + p2T )1/2 − m. This is called the quark number scaling and, again, this fact seems to give a strong implication on the formation of bulk QGP. On the other hand, as described before, an entire issue on the flow is now open for discussions [3] [9], as people observed this phenomenon in all pA, dA and AA systems. In addition, at LHC, a slight deviation from quark number scaling is found by the ALICE group [10]. At LHC there might be an effect from the radial flow. To my surprise, higher order flows, not only v2 but up to v5 , have also been measured at both RHIC and LHC. This trend has never been measured in light systems like pA, so that these measurements could still be a unique signal for heavy-ion collisions (Or, one must try to measure v2 to v5 in pA collisions).
S. Nagamiya / Nuclear Physics A 956 (2016) 248–255
250
Asymmetric di-jets and even mono-jets (!)
ATLAS Aj =
ET1 – ET2 ET1 + ET2
'I Lost jet energy distributed very widely
CMS
Fig. 3. ET asymmetry observed at ATLAS [5] (top) and the lost jet energy distribution on the other side at CMS [6] (bottom).
An interesting theoretical summary on the flow is shown in Figure 1.3 [11]. The values, of course, depend strongly on the initial condition. According to their calculations, the value of η/s at LHC is 0.2 and that at RHIC is 0.12, and both values are larger but very close to the non-viscous fluid limit of η/s = 1/4π = 0.08, by fitting all v2 to v5 values. The result was amazing, because at both RHIC and LHC, η/s is close to 0.08. On the other hand, LHC seems to create slightly more viscous fluid than RHIC does. Perhaps, at the higher temperature stage, the matter initially contains a certain gas phase, since, in a gas, η/s is very large. Note 1 that a gas phase would be created at LHC more than at RHIC due to higher temperature at LHC.
Fig. 4. Current understanding for v2 to v5 [11].
1.4. Direct Photons Regarding photons, a direct photon has been measured from electron pairs with effective mass equals to zero. The temperature extracted from these direct photon measurements is about 220 MeV at RHIC [12] and 300 MeV at LHC [13] (see Figure 1.4). More recently, the temperature from RHIC has been measured to 240 MeV [14] but both values agree within errors (namely, at RHIC, 220 ± 19 ± 19 MeV and 240 ± 25± 7 MeV). Both temperatures at RHIC and LHC are higher than the critical temperature for the QGP phase transition T C , which is 160 to 170 MeV. This is very natural, because photons observe an earlier stage of the collision than T C . Note that the critical temperature T C is also very close to the hadronization temperature, T had , which has been studied for a long time by Stachel et al. [15]. What is the real initial temperature? It should be much higher than the observed values by photons, most likely, 30 to 40% higher than the values observed here.
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Fig. 5. Temperatures measured by direct photons at RHIC (left) [12] and at LHC (right) [13].
1.5. Sequential Melting of Quarkonia
Fig. 6. Sequential melting of quaorkonia observed by CMS [17].
I personally have been interested very much in the J/Ψ suppression that was predicted well before the RHIC started its operation [16], because this could be a direct evidence for the deconfinement of QGP. The results on Figure 6 from CMS [17], made me very excited, since this could be direct evidence of sequential melting of quaokonia. Namely, it could be the first observation of deconfinement in heavy ion collisions. The measurements are being continued at LHC.
1.6. New Question in p+p and p+A
Fig. 7. An example of a big question on the similarity between pA collisions and heavy-ion collisions.
A new question, which is one of serious topics at this conference, is that several phenomena observed in heavy-ion collisions at both RHIC and LHC show up in high-multiplicity p+A or even in p+p collisions. This is a great surprise not only to me but also most of the people. Here one example from CMS is shown in Figure 7. In the jet studies it is indicated that a small system like a pA collision behaves very similarly to a big system like an AA collision. There was a special session on the comparison between pA and AA at this conference on Friday. Still, many unsolved questions remain.
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Fig. 8. Summary of various parameters obtained at RHIC and LHC.
Fig. 9. Near future plans at RHIC and LHC.
2. What Do We Expect in the Near Future Next I summarize the knowledge about results on qˆ , η/s, initial temperature, and transition temperature in Figure 8, although other theoretical predictions may exist. Both RHIC and LHC presented their 10 yearplans, as shown in Figure 9. RHIC plans heavy-flavor run, Beam Energy Scan II (BES II) and s-PHENIX with a new configuration. LHC already started the energy recovery run and, then, LHC and ALICE Upgrades are planned for the purpose of the high luminosity runs at LHC. 2.1. Futures of RHIC and LHC It will definitely allow us >10 years to conduct future programs at LHC. Here, studies on the properties of QGP up to T ∼ 3T C must be conducted. The subjects are a) sequential quarkonia melting, b) v2 to v5 or even higher order flows, c) jet-jet correlations, in particular on jet anomalies, d) heavy flavor, e) J/Ψ melting versus recombination, f) chiral magnetic effects, g) recovery of chiral symmetries, and so on. On the other hand, RHIC will complete its scientific mission by the middle 2020s, but many issues similar to those at LHC must be clarified there near T C up to ∼ 2T C . Then, the Beam Energy Scans to search for critical points, and sPHENIX to study QGP via jets and Upsilon probes will start. In addition, pA and pp questions must be definitely solved. Finally, eRHIC, and lower energy heavy-ion collisions to probe a high-density region might have to be done, which I describe after this. 2.2. Comparison between Collider and Fixed Target Experiments In order to study details on the lower energy region, collider versus fixed target experiments are compared. At RHIC and LHC, the luminosity is 1027−28 cm−2 s−1 . This 1027−28 corresponds to 7-70 kHz collision rate for Au + Au collisions. Currently, 50 kHz data rate is a future big challenge at both RHIC and LHC. Now, compare with this number from a fixed target experiment. Presently, 109 per bunch beams are available at AGS for RHIC. If this beam was bombarded to the 1% target, then, it is already 107 interactions per bunch. In addition, if 5 bunches form one pulse and one pulse is stretched to 5 seconds, then, per bunch is almost equals to per second. Therefore, if one can accelerate 1010−11 beams/s on a fixed target, the 1% target is equivalent to the 100 MHz or even 1 GHz collision rate. Therefore, a new physics might come out with high-intensity heavy-ion beams on a fixed target. In the well-known phase diagram, RHIC and LHC will cover high-temperature regions, as described before (Figure 10). On the other hand, STAR s (and also other s) beam energy scan will cover the region into higher density. The on-going FAIR and/or, perhaps, NICA and J-PARC, will cover much higher densities. Hereafter, I describe these high-density regions, including its relation to neutron star. Of course, STAR Collaboration has already reported interesting results [18], including the third-order and/or fourthorder fluctuation that are sensitive to a critical point and phase boundaries [19]. Future data are extremely interesting toward a high-density region.
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Fig.10. Famous phase diagram and the possible coverage by many methods.
3. Lower Energy Scope At the AGS energy of 15 GeV per nucleon, the colliding nuclei stop each other, and the Lorenz factor becomes real, as shown in Figure 11. At the AGS energies, I estimated that the density up to 6 times the normal nuclear matter density can be achieved. Recently, many other predictions have been reported, in particular, up to which densities the colliding nuclei can achieve. In addition to the high-density formation, another interest would be multi-strangeness production. The merit here is that the production rates reach the maximum at around 10 AGeV on a fixed target. For example, I estimated that the production of S (strangeness) ≥ 3 hypernuclei is possible but marginal, according to the statistical model [20]. At a full intensity beam of 1011 /s, we can have 10 MHz central collisions, which leads to 0.3 events/hour, that is marginal but not impossible. In relation to both high-density and multi-strangeness production, studies are in progress in terms of neutron stars, as shown in Figure 12. This nice slide was provided by H. Tamura at Tohoku University [21]. Density attained by neutron stars is about 3 to 10 times the normal nuclear matter density. Below or around the normal density, which appears near the surface, the nuclear pasta and/or the pure nucleon star exist. On the other hand, in the interior of the neutron star, where a high-density matter is created, there are Fig. 11. Density reached at 15 AGeV. presently two speculations; one is a normal strange hadronic matter with strong three-body forces, as shown on the left-side of Figure 12 or a quark matter as shown on the right-side. These two are currently actively discussed [22]. Regardless of the hadronic or quark matter, a strangeness-rich object could be created (called the strangelet). Furthermore, this strangelet might be stable. At AGS, there was a search of this object [23]. We must repeat it with much higher intensity heavy-ion beams. In addition, high-density formation may help to create much higher multi-lambda production than predicted by the statistical model described before.
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Fig. 13. Several possible experiments under discussions. Fig. 12. Summary of current understanding for neutron stars.
Several experiments are under discussions. The first example is to have a strong magnet behind the target. Detectors are installed in between the primary neutron flux and the primary beams for a stable object like strangelet. Or, they are installed to measure lambda decays to study multi-strangeness nuclei. The second example is to detect lepton pairs for the measurement of high-density matter. See Figure 13. 3.1. FAIR, NICA? and J-PARC? The first realistic approach for 10-20 AGeV energy on a fixed target came from the FAIR at GSI, as shown in Figure 14. Here, the beam intensity up to 1010−11 /s can be achieved in heavy-ion collisions. Five major experiments are on the table. A lepton pair experiment is among them. This project is well known among the community. However, the energy around or above 10 AGeV has a certain delay, by waiting a future expansion of the FAIR. Therefore, projects other than FAIR have also been proposed to cover around or over 10 AGeV region, although they are quite preliminary. The first proposal came from Russia, called the NICA project. I glanced the letter of intent published this year (2015). A very strong message was presented there. The second approach came from our J-PARC. To achieve this energy, a linac and a booster ring have to be added. The merit here is that two major rings, 3 GeV and 50 GeV, as indicated in Figure 15, have already exFig. 14. Current plan at FAIR. isted and they are working. Therefore, only the left-side part must be constructed. We may obtain up to 1010−11 /s for heavy-ions above 10 GeV per nucleon, according to the estimate of the accelerator group. In fact, proton beams up to 5 times 1013 /s or 1014 /s were already accelerated there. The heavy-ion linac and the booster ring fit within 50 m2 . This is very tiny as compared to the J-PARC linac (330 m) or 3 GeV (diameter is 150 m). The injection point to the 3 GeV has already been decided but we must wait for the funding approval. 3.2. Summary toward High Density Approaches In this section I would like to summarize lower energy issues. FAIR at GSI is a real project and it is in progress. The NICA proposal was written in 2015. The J-PARC’s letters of intent will be submitted in 2016. High-density studies via rare processes like electron pairs must be conducted. Search for strangelets or
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Fig. 15. Acceleration scheme under discussions at J-PARC.
multi-strangeness states can/shall be done if the production rate larger than 10−11 for central AA collisions. Also, if hypernuclei are produced in the projectile frame, then the lifetime is elongated by a βγ factor that corresponds to about 30-60 cm. It is interesting to study hyperon mixing in neutron-star. Studies of fluctuation by the STAR BES, as shown in Figure 9, are very interesting, too. 4. Summary of My Talk RHIC has played an extremely important role for the discovery and properties of quark-gluon plasma. RHIC will complete its scientific mission by mid-2020’s. LHC, on the other hand, has added more surprises and will have, at least, ten years of runs for the future unexpected discoveries. Questions in pA, dA and pp as compared with AA collisions, as heavily discussed at this conference, must be solved. Finally, it would be useful to explore the high-density region with very high-intensity heavy ion beams on the order of 1011 particles per second at selected facilities. I appreciate for the support by Y. Akiba, K. Shigaki, T. Gunji, et al. (and Y. Watanabe for English corrections) when I prepared this manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
PHENIX Collaboration, Phys. Rev. Letters 88, 022301 (2002). STAR Collaboration, Phys. Rev. Letters 88, 402 (2001). P. Bo˙zek, presented at this Conference (2015). JET Collaboration, Phys. Rev C90, 014909 (2014). ATLAS Collaboration, Phys. Rev C90, 014909 CMS Collaboration, Phys. Rev C90, 014909. For example, A. Angelami, presented at this Conference (2015). PHENIX Collaboration, Phys. Rev. Letters 91, 182301 (2003); STAR Collaboration, Phys. Rev. Letters 92, 052302 (2004). PHENIX Collaboration, Phys. Rev. Letters, 114, 192301 (2015). T. Gunji, presented at this Conference (2015). C. Gale, S. Jeon, B. Schenke, P. Tribedy, and R. Venugopalan, Phys. Rev. Letters 110, 012302 (2013). PHENIX Collaboration, Phys. Rev Letters 104, 123391 (2010). ALICE Collaboration, Nucl. Phys. A904-905, 573c (2013). A. Drees, presented at this Conference (2015). J. Stachel, A. Andronic, P. Braun-Munzinger, K. Redlich, J. Phys. Conf. Ser. 509 012019 (2014). T. Matsui and H. Satz, Phys. Letters B178, 416 (1986). CMS Collaboration, Phys. Rev. Letters 109, 222301 (2012). M. Mustafa, presented at this Conference (2015). STAR Collaboration, Phys. Rev. Letters 112, 032302 (2014). A. Andronic, P. Braun-Munzinger, J. Stachel, H. Stoecker, Phys. Letters B697, 203 (2011). H. Tamura, private communication (2015). K. Masuda, T. Hatsuda and T. Takatsuka, Astrophys. J., 764, 12 (2013). T. A. Armsyrong, et al. (E886 Collaboration), Phys. Rev. 63, 054903 (2001).