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## Low-x physics at the LHC

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**Low-x physics at the LHC**Ronan McNulty University College Dublin 2010 CTEQ-MCNet School, Lauterbad. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Outline**• Overview • W,Z production (x~10-4) • Sensitivity to PDFs • g* production (x~10-6) • Exclusive processes Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Theory**Experiment Prediction Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Theory v Experiment at the LHC**Hadronic Cross-section PDFs Partonic Cross-section • Identify process with precise partonic cross-section • in a region where the PDFs are well understood • with a large enough cross-section • which can be cleanly identified by experiment Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Theory v Experiment at the LHC**Hadronic Cross-section PDFs Partonic Cross-section Test the SM at highest energies Check out that DGLAP evolution works (test QCD) Push theory into interesting regions with very soft gluons Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 LHC kinematic region -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 LHC kinematic region DGLAP evolution -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Recap from Fred’s lecture…..**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region Production of object of mass Q at rapidity DGLAP evolution -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**h: -8 -6 -4 -2 0 2**4 6 8 ATLAS Low mass DY (|η|<2.5, or forward w.o. tracking) Low mass resonances (|η|<2.5) Forward jet production (|η|<4.5) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**CMS**h: -8 -6 -4 -2 0 2 4 6 8 Low mass DY (|η|<2.5, or forward with limited tracking) Low mass resonances (|η|<2.5) Forward jet production (|η|<6.5) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region ATLAS & CMS ATLAS & CMS: Collision between two partons having similar momentum fractions. PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP. -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**|h|<0.9**h: -8 -6 -4 -2 0 2 4 6 8 ALICE Forward particle (muon) production (-4 < η < -2.5) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region Alice: Collision between two partons having similar momentum fractions. But some far-forward muon converage. Alice -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**ATLAS 3D view**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**CMS Longitudinal View**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**ATLAS 3D view**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**LHCb: a forward spectrometer**y Tracking Stations Muon Chambers Interaction Point 250mrad 10mrad Vertex detector l 20m z Magnet RICH detectors ECAL HCAL Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**h: -8 -6 -4 -2 0 2**4 6 8 LHCb Low mass forward DY (1.8 < η < 4.9) Forward Z/W production (1.8 < η < 4.9) Forward jet production (1.8 < η < 4.9) Forward resonances (1.8 < η < 4.9) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**9**8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region LHCb: Collision between one well understood parton and one unknown or large DGLAP evolved parton. LHCb LHCb -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W,Z**g* 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region ATLAS & CMS ATLAS & CMS: Collision between two partons having similar momentum fractions. PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP. -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W,Z**g* 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region LHCb: Collision between one well understood parton and one unknown or large DGLAP evolved parton. Potential to go to very low x, where PDFs essentially unknown LHCb LHCb DGLAP evolution -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Accessing low-x through Vector Bosons**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W,Z**9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 0 -1 y: 6 4 2 0 2 4 6 LHC kinematic region DGLAP evolution -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W production at LHC**n n W W W n W n n W u or d quark comes from one proton (valence or sea) u or d from gluon splitting in other. Naïve expectation that W+/W- = 2 (Note also some sensitivity to c,s quark.) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**hep-ph/9927031**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Boson Level**Lepton Level Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Z production at LHC**u or d quark comes from one proton (valence or sea) u or d from gluon splitting in other. But also some sensitivity to s quark. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Theoretical predictions**• Total cross-section and rapidity distributions known to NNLO. <1% precision • (FEWZ MonteCarlo) • . • . Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**~LHCb**Effect of PDF uncertainties on cross-sections • Region where the most precise EW tests can be made. • At highest rapidities, PDFs can be constrained. • Experimental statistical error <<1%. • Systematic error likely to be ~1% Percentage uncertainty on cross-sections due to PDF Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**But you can do better ! How do PDF uncertainties**contribute? So ratio of Ws is sensitive to d to u ratio. (For LHCb dv/uv) Ratio of Z to W is almost insensitive to PDFs! Gold plated test of SM at the highest energies Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**But you can do better ! How do PDF uncertainties**contribute? W asymmetry is sensitive to difference in u and d. (For LHCb uv-dv) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Effect of PDF uncertainties on cross-sections**• RWZ precise test of SM everywhere. • Difference in u and d quarks can be significantly improved by all experiments at the LHC. • Going forward, you increasingly constrain the u-valence to d-valence ratio. • Even nicer, most experimental systematics (especially luminosity) cancel in the ratio. Percentage uncertainty on cross-section ratios due to PDF Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Finding Vector Bosons at low-x.**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Z:**• Characteristic signature of two high transverse momentum muons (pT~mZ/2) from hard scatter, plus some softer tracks. • Isolation criteria: • Note some sensitivity to higher order processes, to underlying event, and to pile-up. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**37nb-1**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Many more (higher-x) Z at CMS, Atlas.**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W:**n W • One high transverse momentum muon (pT~mW/2) from hard scatter, plus some softer tracks. • Isolation criteria: • Missing pt. • Note some sensitivity to higher order processes, to underlying event, and to pile-up. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W signal and background live in different regions**Cone isolation Event isolation Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**Muon transverse momentum spectrum**LHCb preliminary 59nb-1 Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W/Z ratio.**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W asymmetry (ATLAS)**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W asymmetry (CMS)**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad**W asymmetry (LHCb)**Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad