1 / 19

Open questions in QCD at high parton density (e+A, p+A, …)

Open questions in QCD at high parton density (e+A, p+A, …). Cyrille Marquet. University of Santiago de Compostela and CERN - Theory Division. Contents. gluon saturation: status and open questions - what we know - what we would like to know

min
Download Presentation

Open questions in QCD at high parton density (e+A, p+A, …)

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. Open questions in QCD at high parton density (e+A, p+A, …) Cyrille Marquet University of Santiago de CompostelaandCERN - Theory Division

  2. Contents • gluon saturation: status and open questions - what we know - what we would like to know - what questions can(not) be answered with e+A (p+A) - why QGP physicists should care • e+A measurements: highlights - structure functions F2 and FL - hard diffraction - di-hadron production - exclusive vector meson production - cold nuclear matter effects

  3. Gluon saturation:status and open questions

  4. What we know for sure • fundamental consequence of QCD dynamics: at asymptotically small x: - QCD evolution becomes non-linear - particle production becomes non-linear - QCD stays weakly coupled • the Color Glass Condensate (CGC) has emerged as the best candidateto approximate QCD in the saturationregime both in terms of practical applicability and phenomenological success see student lecture by Albacete • the energy dependence of the saturation scale, and more generally of observables, can be computed from first principles although in practice, the predictivity will depend on the level of accuracy of the calculation (LO vs NLO, amount of non-perturbative inputs needed, …)

  5. A big open question • is this relevant at today’s colliders ? in other words: can we get away with using such a gluon distribution (with ad hoc cutoff if necessary) ? or do we need to properly take into account the QCD dynamics at kT ~ QS and below ? the CGC phenomenology is successful for every collider process that involves small-x partons and kT ~ QS , i.e. for a broad range for high-energy observables: multiplicities in p+p, d+Au, Au+Au and Pb+Pb; forward spectra and correlations in p+p and d+Au; total, diffractive and exclusive cross sections in e+p and e+A, … • the CGC is not widely accepted because - for each of these observables, there are alternatives explanations - the applicability of the theory can be questioned when values of QS start to drop below 1 GeV (e.g. p+p and peripheral d+Au at RHIC)

  6. What EICs can do EICs = EIC stage 1, EIC stage 2, LHeC see plenary talk by Deshpande • provide golden measurements the kind that will prove non-linear QCD evolution to be indispensable, or irrelevant • twice one thought one had found such observables modification of particle production at forward rapidities in p+A versus p+p single inclusive di-hadron correlations see parallel talk by Xiao see parallel talks by Jalilian-Marian, Lappi, Li EICs would provide smoking guns for saturation, something that very likely cannot be done with p+A (let alone A+A)

  7. Bigger open questions (I) EICs would also provide data that can help us address the following questions • the impact parameter dependence of the gluon density and of QS this has always been the main non-perturbative input in CGC calculations in the case of a proton, using an impact-parameter averaged saturation scale is enough most of the time, but in the case of a nucleus it is not modeling what is done in the most advanced CGC phenomenological studies, is to treat the nucleus as a collection of Woods-Saxon distributed CGCs, and to evolve (down in x) the resulting gluon density at different impact parameters independently but is this good enough ? (in principle not)

  8. Bigger open questions (II) • the transition from the saturation to the high-pT (leading-twist) regime rcBK evolution (down in x) does not contain the DGLAP limit, hence after some evolution (at forward rapidities), RpA predictions reach unity only at unrealistically large values of pT Albacete, Dumitru how RpA goes back towards unity at high-pT ? • the transition from the saturation regime to confinement how does it happen ? does the coupling run with Qs ? are classical fields still the right degrees of freedom ? • universality properties of the saturation regime p+A and e+A collisions offer special opportunities to explore this many-body system of strongly-correlated gluons in this talk, I focus on what is unique to e+A, p+A provides great possibilities as well, already presented see student lecture by Armesto, plenary talk by Dumitru

  9. Why QGP physicists should care • bulk observables in heavy-ion collisions reflect the properties of the • initial state as much as those of the hydro evolution of the QGP see plenary talks by Dusling, Song, Wiedemann • the main source of error in the extraction of medium parameters • (e.g. η/s) is our insufficient understanding of initial state fluctuations see parallel talks by Moreland, Schenke new sources of uncertainties keep emerging, for instance even two CGC models predict different eccentricities QGP properties cannot be precisely extracted from data without a proper understanding of the initial state; e+A collisions: access to a precise picture

  10. e+A measurements:highlights see also: - student lecture by Armesto - plenary talk by Deshpande - parallel talks by Lee, Stasto - poster by Lamont

  11. Deep inelastic scattering (DIS) *A center-of-mass energy W2 = (q+p)2 photon virtuality Q2 = - (k-k’)2 = - q2 > 0 e+A @ EIC e+Pb @ LHeC NOT all processes require Q2 ~ QS2 in order to probe saturation effects

  12. Inclusive structure functions measures quark distributions gluon distribution precisely measuring FL is crucial, and this requires an e+A energy ( ) scan Albacete, Ullrich can NLO DGLAP simultaneously accommodate F2 and FL data if saturation sets in according to current models ?

  13. Hard diffraction in DIS a surprising QCD feature at HERA: a proton in its rest frame hit by a 25 TeV electron remains intact 15% of the time Guzey, Lamont, CM observable subject to strong non-linear effects even with Q2 values significantly bigger than QS2 at HERA the NLO DGLAP description breaks down already at Q2 ~ 8 GeV2 this enhancement is specific to e+A(there is no equivalent in p+A) clean and unambiguous signal of saturation, already at EIC stage-1

  14. Exclusive Vector Meson production @ LHeC energydependence @ EIC Newman, Watt momentum transfer dependence through a Fourier transformation, one can extract the spatial gluon distribution (and correlations), this is not feasible in p+A Toll, Ullrich

  15. Di-hadrons in DIS • directly sensitive to the kT dependence of the gluon distribution at the qualitative level: similar effects as in p+A Lee, Xiao, Zheng but at the qualitative level, this process involves a different unintegrated gluon distribution see parallel talk by Yuan unique access to Weizsacker-Williams gluon distribution (a different operator definition is involved in p+A)

  16. Cold nuclear matter effects • hard probes (esp. jets) in heavy-ion collisions need calibration see plenary talk by Milhano what is the effect of cold nuclear matter on parton branching ? on hadronization ? what is the x,Q2 dependence of nuclear quarks and gluons? answering these questions can help “establish the probe” • the complementarity of e+A with respect to p+A can be of help especially when cold matter effects in p+A collisions are “stranger than expected” see plenary talk by Wysocki

  17. Nuclear parton distributions EICs can reveal the nuclear structure throughout the (x,Q2) plane, from gluon saturation at low x to the gluon EMC effect and itsQ2 evolution at high x kinematical reach of EIC uncertainties on nuclear gluons Ullrich advantage of LHeC : kinematical reach; advantage of EIC: the wide range of nuclei the EICs have constraining power, they will be to nuclei what HERA is to the proton p+A will already do a lot here, complementarity of EIC/LHeC: better handle on kinematics, systematics, and A coverage

  18. In-medium fragmentation • unprecedented ν range large v : in-medium parton propagation - energy loss and pT-broadening - modifications of jet shapes semi-inclusive DIS small v : in-medium hadronization - dynamics of confinement - stages of hadronization and their time scales Brooks, Qiu Wang • first time access to heavy quarks

  19. Conclusions • all detailed studies can be found in - the INT report on the Physics case for the Electron-Ion Collider, arXiv:1108.1713 e+A conveners : A. Accardi, M. Lamont and CM - the upcoming EIC white paper e+A conveners: Y. Kovchegov and T. Ullrich - the LHeC Conceptual Design Report, arXiv:1206.2913 small-x conveners: N. Armesto, B. Cole, P. Newman and A. Stasto • thanks to the EIC task forces at Brookhaven and Jefferson labs • thanks to the LHeC small-x working group

More Related