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Phenomenological applications of QCD threshold resummation Werner Vogelsang Univ. Tbingen GGI Firenze, 27/09/2011 QCD threshold resummation: Important applications at LHC: precision QCD (see talks of previous weeks) Today:

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Phenomenological applications of QCD threshold resummation

Werner Vogelsang

  • Univ. Tübingen

GGI Firenze, 27/09/2011

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  • Important applications at LHC: “precision QCD”

(see talks of previous weeks)

QCD threshold resummation:

  • Today: discuss a few phenomenological

applications towards lower energies: Tevatron, RHIC, fixed target

  • Here, focus is to achieve quantitative

description of observables

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  • Introduction
  • W boson production at RHIC
  • Drell-Yan process in πN scattering
  • Hadron pair production in pp collisions
  • Top quark charge asymmetry at the Tevatron

Outline:

Focus on phenomenology, less on technical aspects of resummation

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Introduction

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The archetype: Drell-Yan LO :

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  • NLO correction:

“threshold logarithms”

  • higher orders:

. . .

  • for z->1 real radiation inhibited
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  • logs emphasized by parton distributions :

z = 1 relevant, in particular as τ→1

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Large logs can be resummed to all orders

Catani, Trentadue; Sterman; …

  • factorization of matrix elements
  • and of phase space when integral transform is taken:
  • they enhance cross section !

MS scheme

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to NLL (much more is known):

Catani,Mangano,Nason,Trentadue

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Inverse transform: “Matching” to NLO:

Catani,Mangano,Nason,Trentadue

“Minimal prescription”

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W boson production at RHIC

  • A. Mukherjee, WV
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Polarized pp collider RHIC

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unpol. u

W boson production:

 goal: probe proton’s helicity distributions  use Parity Violation:

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  • insight into QCD via models (large-Nc, chiral quark,

meson cloud,…)

  • so far, obtained from SIDIS:

DSSV: de Florian, Sassot, Stratmann, WV

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Recent NLO calculation:

de Florian, WV

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STAR (also Phenix)

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  • B. Surrow

(STAR) de Florian, WV

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W moderately large

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Introduce

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No dependence on near threshold:

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W+

Mukherjee, WV

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Drell-Yan process in πN scattering

  • M. Aicher, A.Schäfer, WV
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Drell-Yan is key focus in nucleon structure physics:

  • in pp, pN: probe of anti-quark distributions
  • in πN: probe of pion structure
  • probe of spin phenomena: TMDs, Sivers effect

Currently: E906

  • ngoing

RHIC, COMPASS near-term plans J-PARC, FAIR future possibilities

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µ+ µ-

  • Drell-Yan process has been main source of information on

pion structure:

E615, NA10

  • Kinematics such that data mostly probe valence region:

~200 GeV pion beam on fixed target

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  • LO extraction of uv from E615 data:

QCD counting rules

Farrar,Jackson; Berger, Brodsky; Yuan Blankenbecler,Gunion, Nason

Dyson-Schwinger

Hecht et al.

Holt,Roberts

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(Compass kinematics)

Aicher,Schäfer, WV (earlier studies: Shimizu,Sterman,WV,Yokoya)

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Hadron pair production

  • L. Almeida, G.Sterman, WV
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pair mass2

  • data: fixed target (NA24,E711,E706)

ISR (CCOR)

  • typically ok with NLO only if small scales are

chosen (~ M/3)

Owens, Binoth et al.

  • in some sense, a generalization of Drell-Yan to

“completely hadronic” situation

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Differences w.r.t. Drell-Yan:

  • color structure of hard scattering
  • fragmentation -> only part of parton pair mass is

converted to observed pair mass

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where Define

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Take moments :

  • > works only at LO
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Instead, write

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LO: NLO: true to all orders

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π π

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Kidonakis,Oderda,Sterman Bonciani,Catani,Mangano,Nason Banfi,Salam,Zanderighi Dokshitzer,Marchesini

 matrix problem this part depends on scattering angle !  algebra done numerically

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sets new scale

GeV

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Top quark charge asymmetry

  • L. Almeida, G.Sterman, WV
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Charge asymmetry: p p

_

p p

_

vs

Differential in rapidity : Integrated:

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charge asymmetry leads to forward-backward asym.: in :  also:

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 Less diluted for

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 D0: Tevatron :

not corrected for acceptance or reconstruction

SM expectation (MC@NLO): ~ 1%

 CDF:

fully corrected

SM expectation: ~ 6% SM expectation: ~ 4%

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LO symmetric in : no Ach  electroweak: tiny (no interference with QCD )  Tevatron: ~85% of cross section is from qq

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 however, at :

Brown,Sahdev,Mikaelian ‘79 Halzen,Hoyer,Kim ’87 Kühn,Rodrigo ‘98 QED: Berends,Gaemers,Gastmans ’73 Putzolu ‘61

 in QCD, effect involves color factor

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 diagrams are subset of full NLO, and therefore also included there

Beenakker et al., Ellis,Dawson,Nason, MCFM (Campbell,Ellis,et al.) MC@NLO (Frixione et al.)

 however, for asymmetric part, they are LO  as a result, loops are UV-finite  diagrams also collinear-finite:  single IR divergence that cancels between real & virtual

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Stability of this prediction ?

Why (might need to) worry:  only LO  NLO gives ~30% correction to cross section, significant scale uncertainty  NLO for charge-asymmetric part not available (would be part of NNLO for full cross sec.)

  • > investigate higher orders of perturbation theory
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 similar to dihadron resummation:

like Drell-Yan

depends on scattering angle  roughly:

Almeida,Sterman,WV

 leading-log part cancels in AFB

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Almeida,Sterman,WV

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 general trend is like CDF data, but less pronounced  stability of results confirmed to NNLL

Ahrens,Ferroglia,Neubert, Pecjak,Yang