Holographic QCD Nordita , 25 July 2019

1. Searching for quark matter inside neutron starsAleksi VuorinenUniversity of Helsinki & Helsinki Institute of Physics Holographic QCD Nordita, 25 July 2019 High energy physics: looking into the smallest and largest in Nature High energy theory in Helsinki: presentation by research area A few research highlights Some facts and figures Final thoughts Main references: Annala, Gorda, Kurkela, AV, PRL 120 (2018), 1711.02644 Gorda, Kurkela, Romatschke, Säppi, AV, PRL 121 (2018), 1807.04120 Annala, Gorda, Kurkela, Nättilä, AV, 1903.09121

2. Prediction from the basic structure of QCD: Due to asymptotic freedom, at high enough energy density one enters a deconfined phase, where quarks and gluons replace hadrons as the degrees of freedom of the system Shuryak, Phys. Rept. 61 (1980)

3. Fundamental prediction from the structure of QCD: Due to asymptotic freedom, at high enough energy densities one enters the deconfined phase, with quarks and gluons as the degrees of freedom ?

4. Underlying question: • Can we determine the material properties of dense QCD matter using only first principles field theory tools and robust observational data about neutron stars? GR description via Tolman-Oppenheimer-Volkoveqs: Ozel et al., ApJ 820 (2016)

5. Central challenge: Find Equation of State of QCD matter in the limit • Electricalneutrality: • Betaequilibrium:+ Hebeler et al., ApJ 773 (2013) Main questions to beaddressed in thistalk: • Can QCD theorypredicttheEoS and the NS MR-curve? • Can weinfertheEoSfromastrophysicalobservations? • Doesdeconfinedmatterexist inside thestars?

6. NS matter EoS – limits from theory • Observational information: From mass measurements to gravitational waves • EoS: Interpolation as a way to combine theory and observations • Quark matter inside NSs? • Conclusions

7. EoS – theoretical limits

8. ? • Proceeding inwards from the crust: • increases gradually, starting from • Baryon/massdensityincreasefrom 0 to beyond • Composition changesfromnuclei to neutrons/quarks

9. Low-density behavior of EoS well known from nuclear theory side. Challenges begin close to saturation density: • At , current errors in Chiral Effective Theory EoS - mostly due to uncertainties in effective theory parameters • State-of-the-art EoS NNNLO in chiral perturbation theory power counting [Tews et al., PRL 110 (2013), Hebeler et al., ApJ 772 (2013)]

10. Asymptotic freedom of QCD High-density limit from a non-interacting theory. However,… • At interesting densities system strongly interacting but no nonperturbative methods available • Naïve expectation: Weak coupling methods only useful at very high densities

11. Recent improvement: First part of four-loop pressure at derived: [Gorda, Kurkela, Romatschke, Säppi, Vuorinen, PRL 121 (2018), 1807.04120] Linear log term also “almost there”

12. Three-loop result with nonzero quark masses [Kurkela, Romatschke, Vuorinen, PRD 81 (2009)] • Uncertainty of result at level around • Main uncertaintyfromrenormalizationscaledependence • Pairingcontributions to EoSsubdominant at relevant densities (see, however, also: Cherman, Sen, Yaffe, 1808.04827)

13. Conclusion: Sizable no man’s land extending from outer core to densities not realized inside physical neutron stars Options: Use models, holography (next two talks), orinterpolate between the limits using observational data • Conclusion: Sizable no man’s land extending from outer core to densities not realized inside physical neutron stars • Options: Use models, holography (next two talks), or interpolate between the limits using observational data

14. What do we know from observations?

15. By now, two accurate Shapiro delay measurements of two- solar-mass stars: Demorest et al., Nature 467 (2010) Antoniadis et al., Science 340 (2013) Fig: J. Lattimer

16. Radius measurementsmoreproblematic, butprogressthroughobservation of X-ray emission: • Cooling of thermonuclear X-rayburstsprovideradii to m [Nättilä et al., Astronomy & Astrophysics 608 (2017), …] • With NICER mission, launched 3 June 2017, X-raypulseprofiling Radius of a single star (perhaps) to m

17. Radius measurementsmoreproblematic, butprogressthroughobservation of X-ray emission: • Cooling of thermonuclear X-rayburstsprovideradii to m [Nättilä et al., Astronomy & Astrophysics 608 (2017), …] • With NICER mission, launched 3 June 2017, X-raypulseprofiling Radius of a single star (perhaps) to m

18. Radius measurementsmoreproblematic, butprogressthroughobservation of X-ray emission: • Cooling of thermonuclear X-rayburstsprovideradii to m [Nättilä et al., Astronomy & Astrophysics 608 (2017), …] • With NICER mission, launched 3 June 2017, X-raypulseprofiling Radius of a single star (perhaps) to m

19. Gravitational wavebreakthrough: First observation of a NS merger by LIGO and Virgo in 2017! Three types of potential inputs: Tidal deformabilities of the NSs during inspiral – good measure of stellar compactness EM signatures – present if no immediate collapse to a BH Ringdown pattern – sensitive to EoS (also at ), butfreq. toohigh for LIGO Gravitationalwavebreakthrough: First observation of a NS merger by LIGO and Virgo in 2017! Three types of potential inputs: Tidal deformabilities of the NSs during inspiral – good measure of stellar compactness EM signatures – present if no immediate collapse to a BH Ringdown pattern – sensitive to EoS (also at ), butfreq. toohigh for LIGO LIGO and Virgocollaborations, PRL 119 (2017), PRL 121 (2018)

20. Tidal deformability: How large a quadrupolarmoment a star’sgravitationalfielddevelopsdue to an externalquadrupolarfield Tidal deformability: How large a quadrupolarmoment a star’sgravitationalfielddevelopsdue to an externalquadrupolarfield Substantial effect on observed GW waveform during inspiral phase Read et al., PRD 88 (2013)

21. Tidal deformability: How large a quadrupolarmoment a star’sgravitationalfielddevelopsdue to an externalquadrupolarfield Recent LIGO bound at 90% credence using low spin prior [LIGO and Virgo, PRL 121 (2018)]

22. Interpolation – or how to optimally combine theoretical and observational insights

23. Interpolate EoS using piecewise polytropic form, , varying all parameters (, ) Require: • Smooth matching to nuclear and quark matter EoSs • Continuity of and – with at most one exception (1st order transition) • Subluminality • Optional: astrophysical constraints [Kurkela et al., ApJ 789 (2014)]

24. Implement then two-solar-mass constraint: Accept only EoSs that fulfill Assumption here and in the following: All stars considered main seq. NSs • Excluded: twin stars[e.g. Alvarez-Castillo, Blaschke, PRC96 (2017)], strange quark stars[e.g. Weber et al., IAU 291 (2013)] Quadrutropic interpolation, using close to 200.000 randomly generated EoSs Figures mostly fromAnnala, Gorda, Kurkela, Vuorinen, PRL 120 (2018)

26. Next, determine tidal deformabilities for each EoS and compare to LIGO results for stars: • at 90% credence

29. Recent result of Nättilä et al., Astronomy & Astrophysics 608 (2017): km with 1 credence Caveat: mass measurement of the star in question still rather uncertain

31. Lesson 1: and pQCD constraints force EoS to be hard at low density and soft at high density Efficient bracketing of km

32. Lesson 2: Stringent limits from only one tidal deformab. measurement: EoS cannot be overly stiff at low densitykm

33. Lesson 3: Accurate radius measurements of stars with well-known (large) masses very valuable for EoS determination especially at low densities

34. How about quark matter?

35. Recent work: Implementinterpolationstartingfromspeed of sound, and classifyresults in terms of max and thestrength of thedeconfinement transition[Annala, Gorda, Kurkela, Nättilä, Vuorinen, 1903.09121]

36. Recent work: Implementinterpolationstartingfromspeed of sound, and classifyresults in terms of max and thestrength of thedeconfinement transition[Annala, Gorda, Kurkela, Nättilä, Vuorinen, 1903.09121] Interestingbecause of tension betweenstandardlore in nuclearphysics and experiencefromothercontexts

37. Setting nontrivial upperlimits for speed of sound leads to increasinglyconstrainedresults; however, evensub-conformalEoSsviable Low-EoSssuggesttwo-phasestructure of theEoSband

38. Recent simultaneous MR-measurements [1] and limits drawn from EM counterparts of GW170817 [2] in excellent agreement with low- EoSs [1] Nättilä et al., Astronomy & Astrophysics 608 (2017) [2] Margalit and Metzger, Astrophys. Journal 850 (2017); Radice and Dai, Eur. Phys. J. A55 (2019)

39. Extrapolation of low- and high-density EoSs further high-lights two-phase structure, critical region similar to QGP Distinguishing feature between phases: slope in nearlyconformal QM, 2.5 in sub-nuclearmatter

40. Obvious questions: Is the two-phase structure only a property of the band, or does it persist more differentially – and for larger values of ? Where do the centers of the (most massive) NSs lie, i.e. can does quark matter exist inside NSs?

41. Procedure: Generate a large ensemble of EoSs, allowing for 1st order transitions with arbitrary latent heats and requiring smoothness (continuity of elsewhere Study distribution of polytropic index at the centers of maximal mass stars and typical pulsars: QM iff at all Categorize results in terms of the maximal value reaches at any density for given EoS

42. Centers of 1.44starsalways in hadronicphase • Centers of 2stars in QM phaseif • Centers of stars in QM phaseif or if MeV/(!)

43. In maximal mass stars, QM core present in vast majority of stars – and always sizable if • In 2 stars, QM coresizableif; on the other hand, an observation of a single NS with would rule out QM cores in 2stars

44. Final thoughts

45. Major open questions: • Can QCD theorypredicttheEoS and the NS MR-curve? • Can weinfertheEoSfromastrophysicalobservations? • Doesdeconfinedmatterexist inside thestars? Major open questions: • Can QCD theorypredicttheEoS and the NS MR-curve? • Notthereyet – needfundamentallynewmachinery • Can weinfertheEoSfromastrophysicalobservations? • Fastprogressrecently, uncertaintiesreducing • Doesdeconfinedmatterexist inside thestars? • Lookspromising, thoughmanydetailsstillunclear