1 Part 1: Free the quarks! Part 2: Measuring dimuons in heavy-ion collisions Part 3: “The dog that didn’t bark” and other scenes from the particle zoo; including some illuminating examples of how scientific knowledge evolves From High-Energy Heavy-Ion Collisions to Deconfined Quark Matter Carlos Lourenço, CERN PH-EP

2 The little bang according to the Scientific American QGP

3 Not easy to peek inside the QGP Question: How can we probe the properties of the hot and dense matter system produced in high-energy nuclear collisions? Answer: Use hard probes, like jets or heavy quarkonia (J/ , etc) Questions: What is a jet? What is “jet tomography”? What is jet quenching? Can jet production be accurately calculated? With which ingredients? Are they well known? Only in pp or also in A-A? Answers: See next slides Question: What should we deduce from the fact that the dog does not bark? “Is there any point to which you would wish to draw my attention?” “To the curious incident of the dog in the night-time.” “The dog did nothing in the night-time.” “That was the curious incident,” remarked Sherlock Holmes Answer: Sometimes, the most important is what is not said...

4 What is a jet? A “blast” of particles, all going in roughly the same direction Calorimeter View Tracking View

5 What is “jet tomography”? Tomography: Uses a calibrated probe, and a well understood interaction, to derive the 3-D density profile of the medium from the absorption profile of the probe. In heavy-ion collisions: The suppression of the jets gives the density profile of the matter they cross

6 Can jet production be calculated in pQCD? Yes, through calculations of processes like this one: The data points seem to agree with the pQCD calculation, over 11 orders of magnitude! But... look again, at the high E T tail... on a linear scale, as (data-theory) / theory E T (GeV) ?  A clear indication of quark substructure (compositeness) ! Really ??? g g g g

7 What are protons made of? pQCD calculates partonic processes, like qq→qq, qg→qg, gg→gg, etc But our beams are made of protons, antiprotons, neutrons... not of quarks and gluons! The probability that we find quarks, anti-quarks or gluons inside a proton depends on their fractional momenta and on the “resolution” of our probe: f(x,Q 2 ) gluons sea quarks valence quarks parton distribution functions, PDFs ?

8 How do we know what the parton densities are? Parton Density Functions Hard Scatter Calculation Cross-Section Calculation Measurement 5 experiments e-e- e-e- qq  DIS l+l+ l-l-  Drell-Yan

9 Each class of experiments (DIS, Drell-Yan, etc) gets part of the story No single experiment measures the entirety of the proton (it’s like the tale of the blind men and the elephant) The results from each experiment go into a global fit Not all experiments agree – there is an art to “average” them together Two principal groups do this: → Martin, Roberts and Stirling (MRS) → CTEQ (Coordinated Theoretical-Experimental project on QCD) What means MRST, CTEQ6M, etc? “Laws are like sausages. It’s better not to see them being made.” – Otto von Bismark PDFs

10 Back to the CDF high-E T jet excess New sets of Parton Distribution Functions were calculated, including the CDF data The excess is gone! The quarks do not have substructure after all... Level of belief that this was “new physics” Graduate student doing analysis Postdoc Graduate student’s advisor Other faculty on experiment Spokesperson Average theorist Big-shot theorist

11 PDFs pQCD D(z) PDFs D(z) Fragmentation into the final state hadrons We need to convolute the pQCD hard interaction with (initial state) parton densities and (final state) fragmentation functions, which define how the quarks and gluons hadronise. People build particle detectors, not parton detectors... The PDFs and the fragmentation functions are (supposed to be) universal: they should be the same for all processes. hadrons partons

12 What is jet quenching? In pp, expect two back-to-back jets In the QGP, expect mono jets... The “away-side” jet gets absorbed by the dense QCD medium

13 Jet quenching: setting the stage with pp collisions Azimuthal correlations show strong back-to-back peaks Azimuthal Angular Correlations

14 Jet quenching: discovery mode with central Au-Au collisions Azimuthal correlations show that the “away-side jet” has disappeared... if we only detect high-p T particles, p T > 2 GeV/c Azimuthal Angular Correlations

15 Jet quenching: where is the energy gone? The “away-side jet” energy is distributed over many soft particles, no longer collimated into jets, which are seen when the p T threshold is lowered to 200 MeV/c Azimuthal Angular Correlations

16 Jet quenching through two-particle azimuthal correlations Two-particle azimuthal correlations show back-to-back jets in pp and d-Au collisions The jet opposite to the high-p T trigger particle disappears in central Au-Au collisions Possible interpretation: the produced hard partons (probe) radiate gluons while traversing the dense colored medium created in central Au+Au collisions pp d-Au central Au-Au

17 Jet quenching: latest result with much higher statistics The availability of very high statistics reveals a very curious double-peak structure in the azimuthal distribution of the away-side peak, in central Au+Au collisions

18 Jet quenching through the “nuclear modification factor” The nuclear modification factor, versus p T, shows a very significant suppression of high-p T hadrons in central Au+Au collisions, with respect to the extrapolation of pp measurements. No suppression is seen for photon production.

19 Back to the PDFs Is a free proton the same as a proton inside a nucleus? No! There are “nuclear effects” modifying the parton distribution functions. The probabilities of finding partons of given x change when the proton is inside a nucleus. The “EKS 98 model” provides the ratio between the PDFs in a proton of a nucleus A and in a free proton: “Shadowing” or “anti-shadowing”: decrease or increase the parton’s density in a particular kinematic range Shadowing Anti-shadowing EKS 98 for Pb

20 Is this relevant? This implies a ~20% higher charm production cross-section in Pb-Pb collisions at SPS energies, and a ~40% reduction at the LHC, as compared to the linear extrapolation from pp collisions. Remarks: For a given collision energy and a given mass produced, the values of x depend on the rapidity range where the measurement is made. If the pp and Pb-Pb collisions are collected at different energies, the corrections for the nuclear effects are particularly tricky. We cannot directly compare heavy-ion and pp data. Has implications in the analysis of the SPS J/  suppression data.

21 The first time the J/  was suppressed years ago The real dimuon spectra (...) amounted to some 4% of the in-time data sample. The real effect varied with dimuon mass from 2% at 1.5 GeV/c 2 to 40% at 5 GeV/c 2. Of course, with such a small signal-to-noise ratio, the data were extremely sensitive to systematical effects that would distort the subtraction procedure. 1 nb 1 pb p-U   at 29.5 GeV Phys. Rev. D8 (73) 2016 ?

22 Leon Lederman was a very careful person... and was not desperately looking for a job

23 1 nb 1 pb p-U   at 29.5 GeV Phys. Rev. D8 (73) 2016 ? The paper gives plenty of detailed information, including all the numerical values

24 NA50 p-A 400 GeV We can fit the data to the sum of an exponential continuum and a Gaussian “peak”. It works quite well, with a “resonance” centered at 3.2 GeV and with ~600 MeV mass resolution. The “excess” to the left of the peak is due to the use of a Gaussian: it underestimates multiple scattering and energy loss effects.

25 In 1974, to be on the safe side, the same particle was discovered twice! At BNL, Sam Ting et al. observed the J At SLAC, Burt Richter et al. discovered the  Phys. Rev. Lett. 33 (74) 1404 Phys. Rev. Lett. 33 (74) 1406 From the observation to the measurement It was called J/ ...

26 The Matsui and Satz paper years ago In 1986, Matsui and Satz proposed J/  suppression as a signal of the QCD phase transition from confined hadronic matter to a deconfined partonic plasma. “we thus conclude that: there appears to be no mechanism for J/  suppression in a nuclear collision except the formation of a plasma and if such a plasma is produced, there seems to be no way to avoid J/  suppression”

27 Theoretical prediction: in the presence of new physics (formation of a QCD medium with deconfined quarks and gluons) the centrality dependence of quarkonia production yields will be very significantly affected → we have a “signature” Above certain consecutive thresholds, the  ’, the  c and the J/  resonances (besides the Upsilon states) will “dissolve” in the formed medium → we have more than a simple signature; we have a “smoking gun”... However,... What happens to the charmonia states in the presence of “old physics”? What are the basic properties of J/  and  ’ production in pp and p-A collisions? Do we have a robust and well understood baseline, in A-A collisions, with respect to which we can clearly and unambiguously extract patterns specific to the high density medium produced in high-energy nuclear collisions? What should we really expect in the absence of a deconfined QCD medium but accounting for all the other “standard” aspects of nuclear collisions? → We need accurate p-A data and a robust model to turn the p-A patterns into solid A-A expectations... Quarkonia studies: why and how

28 Studies of J/  and  ’ production, in p-nucleus and Pb-Pb collisions, have been published in the last few years by the NA50 collaboration. Charmonia production yields have been presented either in relative terms, with respect to the yield of high-mass Drell-Yan dimuons, or as absolute production cross-sections per target nucleon. Charmonia studies made by NA50 J/  Pb-Pb 158 GeV p-Pb 400 GeV 400 GeV  J/  ~ 105 MeV

29 The J/  and  ’ are absorbed in p-nucleus collisions... The J/  and  ’ production cross-sections scale less than linearly with the number of target nucleons (contrary to what happens with high-mass Drell-Yan dimuons).  2 /ndf = 0.7  2 /ndf = 1.4 From a global fit to the 400 and 450 GeV p-A data (16 independent measurements), NA50 determined the following absorption cross-sections: from production cross-sections:  abs (J/  = 4.5 ± 0.5 mb ;  abs (  ’) = 8.3 ± 0.9 mb from cross-section ratios (  /DY):  abs (J/  = 4.2 ± 0.5 mb ;  abs (  ’) = 7.7 ± 0.9 mb

30... as a function of the mass number or of L L is the “path length” which the J/  and  ’ states traverse in the target nucleus, from the production point of the ccbar pair up to the nuclear “surface” the “  L parametrization” exp(-  L  abs ) is a good approximation of the full Glauber calculation J/J/ L Projectile Target ’’ J/  p-A 400 GeV The solid lines are the result of Glauber calculations, assuming that the reduction of the production cross-section per target nucleon is exclusively due to final state absorption of the charmonia states in the cold nuclear matter it crosses.

31 J/  production from p-A to Pb-Pb collisions In S-U and peripheral Pb-Pb collisions, the data points seem to follow the normal nuclear absorption, which scales with the length of nuclear matter crossed by the J/ . In central Pb-Pb collisions the L scaling is broken and an “anomalous suppression” sets in. J/  normal nuclear absorption curve exp(-  L  abs ) Central Pb-Pb collisions are not easy to understand without hands-on tutorials

32 The J/  production yield measured in Pb-Pb collisions, relative to the DY yield, is suppressed from peripheral to central collisions... at low transverse momentum  Only the low p T J/  mesons get suppressed ! The transverse momentum is an important observable R i = (N J/  / N DY ) (E Ti ) (N J/  / N DY ) (E T1 ) 〈 p T 2 〉 (GeV/c) 2 J/  p-A 400 GeV S-U 200 GeV Pb-Pb 158 GeV L (fm) The J/  “central over peripheral ratio” strongly depends on p T (at the SPS)... Part of this behaviour is due to the “Cronin effect”, and part to the melting of low momentum ccbar states in the QCD plasma

33 The  ’ suppression pattern in S-U and in Pb-Pb shows a significantly stronger drop than expected from the Glauber extrapolation of the p-A data  abs = 8 ± 1 mb  abs ~ 20 mb ’’ The “change of slope” is quite significant and looks very abrupt... The  ’ is also suppressed from p-nucleus to nucleus-nucleus  2 /ndf = 1.4

34 Could it be because of melting in the QGP? Yes, it could be... But it is very unfortunate that the “drop” happens between p-A and S-U/Pb-Pb, when we change collision systems and collision energies. If the extra (strong)  ’ suppression is due to the dissolution of the bound cc state by the QGP, Lattice QCD says that this would indicate that T c sits in the most peripheral S-U or Pb-Pb collisions at SPS energies... ’’ “Anomalous”  ’ suppression from p-nucleus to S-U and Pb-Pb

35 We have seen anomalous J/  and  ’ suppression in central heavy-ion collisions, as expected in the case of QGP formation ’’ Charmonia suppression at the SPS J/  Can we conclude that a system of hot and dense deconfined QCD matter has been produced at the CERN SPS? There is a BIG difference between “the measurements are compatible with...” and “the measurements show beyond reasonable doubt that...”