Low-x physics at the LHC Ronan McNulty University College Dublin 2010 CTEQ-MCNet School, Lauterbad. Ronan McNulty 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

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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 LHC kinematic region -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad ATLAS h: -8 -6 -4 -2 0 2 4 6 8  Tracking -2.5 < η < 2.5 Counters (lumi) 5 < η< 6.1, -6.1 < η< 5, |η| > 8.1 ECAL, HCAL -4.5 < η < 4.5, |η| > 8.1 Muon -2.7 < η < 2.7 Trigger at low (high) lumi Pt(μ) > 4 GeV, (10 GeV), Pt(e) > 5 GeV, (12 GeV) 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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad CMS h: -8 -6 -4 -2 0 2 4 6 8 Tracking -2.5 < η < 2.5 TOTEM: 3.1 <  < 4.7, 5.2 <  < 6.5 ECAL, HCAL -6.5 < η < 6.5 Muon Trigger Pt(μ)>3.5 GeV, Pt(calo)>4 GeV, + fwd. triggers 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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad ALICE h: -8 -6 -4 -2 0 2 4 6 8 |h|<0.9 Tracking -0.9 < h < 0.9 Particle ID ECAL (HCAL) -0.9 < h < 0.9 (|h| > 8.5) Muon -4 < h < -2.5 Counters -3.4 < h < 5 Trigger Pt (m)> 1 (2) GeV Forward particle (muon) production (-4 < η < -2.5) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad ATLAS 3D view Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad CMS Longitudinal View Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad ATLAS 3D view Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

LHCb: a forward spectrometer y Tracking Stations Interaction Point Muon Chambers 250mrad 10mrad Vertex detector l 20m z Magnet RICH detectors ECAL HCAL Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad LHCb h: -8 -6 -4 -2 0 2 4 6 8 Tracking 1.8 < η < 4.9 Particle ID ECAL, HCAL Muon Trigger* Pt(μ) > 1 GeV, Pt(had) > 2.5 GeV 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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 y: 6 4 2 0 2 4 6 LHC kinematic region W,Z 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. g* -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -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 W,Z LHCb LHCb DGLAP evolution g* -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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 y: 6 4 2 0 2 4 6 LHC kinematic region W,Z DGLAP evolution -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad hep-ph/9927031 Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Boson Level Lepton Level Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

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

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

Effect of PDF uncertainties on cross-sections ~LHCb 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

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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 37nb-1 Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Many more (higher-x) Z at CMS, Atlas. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

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

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad W/Z ratio. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad W asymmetry (ATLAS) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad W asymmetry (CMS) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad W asymmetry (LHCb) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

How well can W,Z measurements constrain the PDFs? Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

How can W,Z constrain PDFs? From global fits, PDFs described by a set of orthogonal eigenvectors, which have a ‘central’ value e0, and ‘uncertainties’ ei. is the value of the differential cross-section obtained using the central value. is the value of the differential cross-section obtained moving one unit along eigenvector 1 1 is the change in the differential cross-section when I move one unit along eigenvector 1 is the change in the differential cross-section when I move 0.5 along e.v. 1 and 0.3 along e.v. 3 Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

How can W,Z constrain PDFs? From global fits, PDFs described by a set of orthogonal eigenvectors, which have a ‘central’ value e0, and ‘uncertainties’ ei. (where δi is #sigmas along ei) Current knowledge of PDFs mapped out by sampling δi from unit multinomial distribution. Perform pseudo-experiments, generating LHC data and fitting for δi, to see how eigenvector knowledge improves. Effect on MSTW08, CTEQ6.5, ALEKHIN2002, NNPDF2.0 studied. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to MSTW08 PDFs with 0.1fb-1 of high mass vector bosons at 7TeV Uncertainty on PDF with 0.1fb-1 of LHCb data Uncertainty on PDF without LHCb data 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 Modest improvement with small amount of data Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Comparison with different PDFs using 0.1fb-1 of high mass vector bosons at 7TeV Uncertainty on PDF with 0.1fb-1 of LHCb data Uncertainty on PDF without LHCb data Similar sensitivity. Ability to distinguish models Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to CTEQ66 PDFs with 1fb-1 of high mass vector bosons at 14 TeV Uncertainty on PDF with 1fb-1 of LHCb data Uncertainty on PDF without LHCb data More data and higher energy lead to larger improvements. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to PDFs: Similar sized improvements for each PDF Ability to distinguish between models 0.1 fb-1 of data at 7 TeV will test the SM 1fb-1 of data at 14 TeV can improve PDFs by about 30%. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Using g* to go to very low-x. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 y: 6 4 2 0 2 4 6 LHC kinematic region LHCb LHCb DGLAP evolution g* -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Effect of PDF uncertainties on cross-sections ~LHCb Percentage uncertainty on cross-sections due to PDF Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

First data. Analyses using 37 nb-1 g*->mm Require two isolated muons pt>1GeV consistent with primary vertex. b,c background taken from simulation Muon misidentified tracks taken from data: pions and kaons in Minimum Bias triggered event scaled with misidentification probability. Sample of DY events can be identified with reasonably high purity Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad ATLAS (shown at ICHEP) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad CMS (shown at ICHEP) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

J/Y production (ALICE) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Why is this interesting 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 Why is this interesting LHC kinematic region DGLAP evolution Up in Q2 for given x -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Why is this interesting 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 Why is this interesting LHC kinematic region BFKL evolution -6 -5 -4 -3 -2 -1 0 log10(x) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Phase Change: Saturation. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Interlude Some theoretical issues (as discussed extensively at this school). Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

PDF uncertainties at low-x, low-Q2 MSTW08. (Thanks to Graeme Watt) NLO LO NNLO Different behaviour and uncertainty with order of calculation. Gluon essentially unconstrained by data below 10-4 DGLAP evolution not trustworthy in this region. Gluon re-summation effects. Possibly entering saturation regime. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad From Fred: Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad For qT~mV NLO calculations e.g. with MCFM generator (Ellis & Campbell) see Bozzi, Catani, Ferrera, de Florian, Grazzini, arXiv:0812.2862 and arXiv:1007.2351 (14th July 2010). Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad But for qT<<mV Large logs gluon resummation required see Bozzi, Catani, Ferrera, de Florian, Grazzini, arXiv:0812.2862 and Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad With resummation for Z Full calculation for low mass DY: hep-ph 0303021 Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to MSTW08 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 10-5 10-4 10-3 10-2 10-1 1 Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to CTEQ66 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to NNPDF2.0 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Improvement to different PDF sets with 0.1fb-1 of low invariant mass muons (10-20GeV) at 7TeV Uncertainty on PDF with 0.1fb-1 of LHCb data Uncertainty on PDF without LHCb data Similar improvements to MSTW, CTEQ and Alekhin PDFs. Sensitivity exists to distinguish between models. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Current uncertainty on MSTW08 PDFs and projections with 0.1fb-1, 1fb-1 of very low invariant mass muons at 7TeV 20% 20% 20% 10% 10% 10% 10-6 10-5 10-4 10-3 10-2 10-1 1 10-6 10-5 10-4 10-3 10-2 10-1 1 10-6 10-5 10-4 10-3 10-2 10-1 1 20% 20% 10% 10% 10-6 10-5 10-4 10-3 10-2 10-1 1 10-6 10-5 10-4 10-3 10-2 10-1 1 Significant improvements possible with modest amount of data Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Exclusive particle production Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Recent CDF results (402 events) Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

... the flat, well understood part... (ATLAS, CMS, LHCb) CMS: S. Ovyn at DIS08 ATLAS: B.Caron CERN-THESIS-2006-024 LHCb: LHCb/2008/01 Important for luminosity determination Theoretically known to <1% (QED) 700 events in 100 pb-1 SIMULATION! Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

... exclusive resonances forward region... (CMS, LHCb, ALICE) 100 pb-1 Produced at low x-Q2 as for DY, but now probing photons or pomerons. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad Summary The low-x region can be probed in several ways at the LHC, allowing tests of the SM and of QCD. Significant improvements to gluon PDF are likely in the near future, probing the previously unexplored, and theoretically interesting region down to x=10-6. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad