1 / 159

Strangeness and Heavy Flavour (Episode 1)

Strangeness and Heavy Flavour (Episode 1). Federico Antinori (INFN Padova & CERN). Tutorial: kinematic variables collision centrality invariant cross-section. Rapidity. Four-momentum : ( c = 1 , z coordinate along collision axis). Addition of velocities along z :. (Galileo).

mari
Download Presentation

Strangeness and Heavy Flavour (Episode 1)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Strangeness and Heavy Flavour(Episode 1) Federico Antinori (INFN Padova & CERN)

  2. Tutorial:kinematic variablescollision centralityinvariant cross-section

  3. Rapidity Four-momentum: (c = 1 , z coordinate along collision axis) Addition of velocities along z: (Galileo) (relativistic) “rapidity” Federico Antinori - Trento - May 2008

  4. so under a Lorentz transformation to a frame moving with velocity b along z : (rapidities “add-up”) • therefore, rapidity distributions are boost-invariant: compare • e.g. at SPS: 0 1 2 3 4 5 6 ylab Federico Antinori - Trento - May 2008

  5. in the non-relativistic limit: Exercise: prove this • it can be shown that: Exercise: prove this Federico Antinori - Trento - May 2008

  6. Transverse variables • Transverse momentum: • Transverse mass: • Transverse energy: Exercise: prove these qi = angle w.r.t. beam direction Federico Antinori - Trento - May 2008

  7. Pseudorapidity rapidity pseudorapidity • in the ultrarelativistic limit: p ~ E  h ~ y • for the transverse variables: rapidity pseudorapidity Federico Antinori - Trento - May 2008

  8. it can be shown that: Exercise: prove this • so for ultra-relativistic particles (y ~ h) • rapidity only depends on emission angle Federico Antinori - Trento - May 2008

  9. Collision centrality participants spectators b y=0 spectators rapidity • How far do the centers of the two colliding nuclei pass one another? • Usually expressed in terms of: • b (impact parameter) • number of participants Npart(b) • [sometimes one speaks of “number of wounded nucleons”: NW(b) ] • cross section s(b) Federico Antinori - Trento - May 2008

  10. Experimentally, the centrality is evaluated by measuring one or more of these variables: • Nch: number of charged particles produced in a given rapidity interval (near mid-rapidity) • increases (~ linearly) with Npart • ET: transverse energy = SEi sin qi • increases (~ linearly) with Npart • EZDC: energy collected in a “zero degree” calorimeter • increases (~ linearly) with Nspectators Federico Antinori - Trento - May 2008

  11. e.g.: NA57 experiment: the centrality is evaluated from the charged particle multiplicity in the pseudorapidity range 2 < h < 4 Pb-Pb events are divided into multiplicity classes The distribution of the number of participants for the events in each class is evaluated Federico Antinori - Trento - May 2008

  12. Invariant cross-section • A + B  a1 + a2 + … + an • we may be interested only in the production of a specific type of particle e.g.: Pb + Pb W- + X • Differential cross section: • but is not Lorentz invariant; is used instead: • Lorentz-invariant for a boost along z Exercise: prove this Federico Antinori - Trento - May 2008

  13. Invariant cross-section: • in terms of pT: • integrating over j: Federico Antinori - Trento - May 2008

  14. End of tutorial

  15. Transverse Mass Spectra Apparent Temperature Thermal Freeze-out Federico Antinori - Trento - May 2008

  16. Transverse mass distributions Usually fitted to thermal distributions: T = “inverse slope” or “apparent temperature” or “mT slope” What does T mean? R.Stock Federico Antinori - Trento - May 2008

  17. Thermal freeze-out • In nucleus-nucleus collision we form a strongly interacting “fireball” which expands and cools down • When finally the system is so dilute (i.e. the mean free path is so large) that interactions among the collision products cease, we have “thermal freeze out” • From then on the collision products just stream out towards the detector Federico Antinori - Trento - May 2008

  18. Transverse flow • The temperature of the mT spectra is modified by the presence of a collective transverse flow FormT < 2m : apparent temperature transv. flow velocity freeze-out temperature In practice, it is a complicated business to disentangle the thermal and flow contributions. But additional information (e.g. from HBT interferometry) can be used Federico Antinori - Trento - May 2008

  19. Strangeness and Heavy Flavour(Episode 2) Federico Antinori (INFN Padova & CERN)

  20. Tutorial kinematic variables collision centrality invariant cross-section Transverse Mass Distributions transverse flow Summary of Episode 1 Federico Antinori - Trento - May 2008

  21. Strangeness Enhancement Federico Antinori - Trento - May 2008

  22. Historic QGP predictions K+ s s s s s s s s d d u u d X- u p- u d d d d d d d d d d d s u u u u u u u u s s u d d d d p+ u u s u u u u p u u d d d s W+ u s s u u d d u s d L • restoration of csymmetry -> increased production of s • mass of strange quark in QGP expected to go back to current value • mS ~ 150 MeV ~ Tc • copious production of ss pairs, mostly by gg fusion [Rafelski: Phys. Rep. 88 (1982) 331] [Rafelski-Müller: P. R. Lett. 48 (1982) 1066] • deconfinement  stronger effect for multi-strange • can be built recombining s quarks • strangeness enhancement increasing with strangeness content [Koch, Müller & Rafelski: Phys. Rep. 142 (1986) 167] Federico Antinori - Trento - May 2008

  23. E(W-) > E(X-) > E(L) (sss) (ssd) (sud) |s| = 3 |s| = 2 |s| = 1 • The QGP strangeness abundance is enhanced • As the QGP cools down, eventually the quarks recombine into hadrons (“hadronization”) • The abundance of strange hadrons should also be enhanced • The enhancement should be larger for particles of higher strangeness content, e.g.: Federico Antinori - Trento - May 2008

  24. Strangeness in a hadronic system E(W-) < E(X-) < E(L) (sss) (ssd) (sud) |s| = 3 |s| = 2 |s| = 1 • If a relatively long-lived strongly interacting hadronic system is formed in the collision, a certain amount of enhancement of the abundance of strange particles could be expected even in the absence of QGP • e.g.: • such processes are relatively easy (= fast on the collision timescale) for kaons and L, but are progressively harder (= slow on the collision timescale) for particles of higher strangeness • in this case, one expects: • The production of multistrange baryons such as X and W is therefore expected to be particularly sensitive to deconfinement Federico Antinori - Trento - May 2008

  25. Strange baryons (hyperons) beam p L p- • There are 35 strange baryons listed in the PDG summary tables • Only 6 decay weakly (ct ~ cm’s  separate decay vertex from event interaction vertex): L, S+, S- (sqq) X0, X-(ssq) W- (sss) • Only 3 of them can decay into final state with only charged particles Federico Antinori - Trento - May 2008

  26. Example: WA97 / NA57 • Aim: study the production of multi-strange particles in Pb-Pb collisions • Experimental technique: • high granularity silicon pixel tracker at central rapidity ycm ~ 0 • detect Ks0, L, X, W, by reconstructing weak decay topologies Federico Antinori - Trento - May 2008

  27. Hyperon signals • From NA57, Pb-Pb collisions at 158 A GeV/c Federico Antinori - Trento - May 2008

  28. Yield, Enhancement • Yield: multiplicity per event e.g.: # of W- / event in y1 < y < y2 : • Enhancement: yield per participant (i.e. wounded) nucleon relative to yield per participant nucleon in p-Be e.g.: W- enhancement: Federico Antinori - Trento - May 2008

  29. Strangeness enhancement pattern • Enhancement relative to pBe for pPb and 5 centrality classes in PbPb: (particles/event/participant) / (particles/event/participant) in pBe Federico Antinori - Trento - May 2008

  30. Strangeness enhancement pattern • Enhancement relative to p-Be Enhancement is larger for particles of higher strangeness content (QGP prediction!) up to a factor ~ 20 for W So far, no hadronic model has reproduced these observations (try harder!) Actually, the most reliable hadronic models predicted an opposite behaviour of enhancement vs strangeness Federico Antinori - Trento - May 2008

  31. Chemical Equilibrium Thermal Fits Chemical Freeze-out Federico Antinori - Trento - May 2008

  32. Chemical equilibrium [P.Braun-Munzinger, I.Heppe, J.Stachel, Phys. Lett. B465 (1999), 15] • The relative particle abundances measured in Pb-Pb collisions are close to the thermodynamical (chemical) equilibrium values (maximum entropy) corresponding to a temperature of ~ 170 MeV (“chemical freeze-out temperature”) this would be a natural outcome of statistical hadronization of uncorrelated quarks “chemical freeze out”: the moment when elastic interactions cease Federico Antinori - Trento - May 2008

  33. Hyperon enhancements @ RHIC • similar picture • open: NA57 @ SPS • closed: STAR @ RHIC Federico Antinori - Trento - May 2008

  34. Thermal fits again doing well Chemical equilibrium @ RHIC 200 GeV 62.4 GeV Jun Takahashi (STAR), SQM`07 Federico Antinori - Trento - May 2008

  35. remarkable regularity in freeze-out systematics Jean Cleymans, SQM`07 Federico Antinori - Trento - May 2008

  36. T vs µB systematics • the extracted freeze-out points at SPS and RHIC lay very close to the predicted QGP phase boundary

  37. A departure from equilibrium? The K+/π+ “horn”

  38. K abundance vs sNN • K+/p+ shows a sharp maximum at sNN ~8 GeV • K-/p- does not... • What’s this? • K+(us) very sensitive to baryon density, which decreases with energy • K+ peak indicates phase transition? (M.Gazdzicki) • new experiments under preparation Federico Antinori - Trento - May 2008

  39. More on strangeness from RHIC Nuclear modification factors Elliptic flow

  40. Participants Scaling vs Binary Scaling e.g.: Npart (or Nwound) = 7 “participants” Nbin (or Ncoll) = 12 “binary collisions” • “Soft”, large cross-section processes expected to scale like Npart • “Hard”, low cross-section processes expected to scale like Nbin Federico Antinori - Trento - May 2008

  41. Rcp, RAA, RdAu Yield/collision in central collisions Yield/collision in peripheral collisions Yield/collision in nucleus-nucleus Yield/collision in proton-proton Yield/collision in deuteron-nucleus Yield/collision in proton-proton Federico Antinori - Trento - May 2008

  42. High pT suppression • High pT particle production expected to scale with number of binary NN collisions if no medium effects • Clearly does not work for more central collisions • Interpreted as due to parton energy loss Federico Antinori - Trento - May 2008

  43. Baryon puzzle @ RHIC • Central Au-Au: as many p- (K-) as p (L) at pT ~ 1.5  2.5 GeV • e+e-jet (SLD) • very few baryons from fragmentation! p K p Federico Antinori - Trento - May 2008 H.Huang @ SQM 2004

  44. Rcp • strange particles come to rescue! • if loss is partonic, shouldn’t it affect p and p in the same way? Federico Antinori - Trento - May 2008

  45. Quark Recombination K+ s s s s s s s s d d u u d X- u p- u d d d d d d d d d d d s u u u u u u u u s s u d d d d p+ u u s u u u u p u u d d d s W+ u s s u u d d u s d L • if hadrons are formed by recombination, features of the parton spectrum are shifted to higher pT in the hadron spectrum, in a different way for mesons and baryons  constituent quark counting S.Bass @ SQM`04 Federico Antinori - Trento - May 2008

  46. Elliptic Flow • Non-central collisions are azimuthally asymmetric • The transfer of this asymmetry to momentum space provides a measure of the strength of collective phenomena • Large mean free path • particles stream out isotropically, no memory of the asymmetry • extreme: ideal gas (infinite mean free path) • Small mean free path • larger density gradient -> larger pressure gradient -> larger momentum • extreme: ideal liquid (zero mean free path, hydrodynamic limit)

  47. Azimuthal Asymmetry • at low pT: azimuthal asymmetry as large as expected at hydro limit! • very far from “ideal gas” picture of plasma Federico Antinori - Trento - May 2008

  48. elliptic flow v2 STAR Preliminary • Recombination also offers an explanation for the v2 baryon puzzle... scaled with n(quarks) Federico Antinori - Trento - May 2008

  49. A few hiccups... • Recombination provides a natural explanation for the hyperon enhancement pattern and for the RCP, v2 behaviours, but has a few theoretical problems... • it violates 2nd law of thermodynamics • reduction of the number of particles  lower disorder  entropy decreased • it actually also violates the 1st... • impossible to conserve energy and momentum simultaneously • what happens to gluons? • must break down at low pT (where hydrodynamical behaviour is expected to dominate) Federico Antinori - Trento - May 2008

  50. Kinetic Transverse Energy (KET) Scaling Phys. Rev Lett. 98, 162301 (2007) • impressive scaling also in softest region if KET used instead than pT (as expected in hydrodynamics-inspired models) Federico Antinori - Trento - May 2008

More Related