Ronan McNulty University College Dublin Low-x physics at LHC Ronan McNulty University College Dublin

Outline Introduction The LHC experiments Measurements sensitive to low x (x<~5x10-4) W,Z at LHCb Drell-Yan at ATLAS, CMS, LHCb Exclusive forward production: CMS, LHCb, ALICE Forward jet production CMS. LHCb Summary

9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 -6 -5 -4 -3 -2 -1 0 log10(x)

LHC kinematic region DGLAP evolution 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 -1 LHC kinematic region DGLAP evolution -6 -5 -4 -3 -2 -1 0 log10(x)

y: 6 4 2 0 2 4 6 LHC kinematic region Production of object of mass Q 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 -6 -5 -4 -3 -2 -1 0 log10(x)

fully instrumented ATLAS, CMS y: 6 4 2 0 2 4 6 Low-x physics: 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 y: 6 4 2 0 2 4 6 fully instrumented ATLAS, CMS Low-x physics: Low mass Drell-Yan Low mass resonances Forward jet production (some detectors) (some detectors) -6 -5 -4 -3 -2 -1 0 log10(x)

LHCb LHCb y: 6 4 2 0 2 4 6 Low-x physics: W,Z production 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 y: 6 4 2 0 2 4 6 Low-x physics: W,Z production Low mass Drell-Yan Low mass resonances Forward jets LHCb LHCb -6 -5 -4 -3 -2 -1 0 log10(x)

y: 6 4 2 0 2 4 6 some instrumentation ALICE some instrumentation 9 8 7 6 5 log10(Q2) [GeV2] 4 3 2 1 -1 y: 6 4 2 0 2 4 6 ALICE some instrumentation some instrumentation Low-x physics: Forward jet production Forward resonances -6 -5 -4 -3 -2 -1 0 log10(x)

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(m) > 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)

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(m)>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)

LHCb h: -8 -6 -4 -2 0 2 4 6 8 Tracking 1.8 < h < 4.9 Particle ID ECAL, HCAL Muon Trigger* Pt(m) > 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)

ALICE h: -8 -6 -4 -2 0 2 4 6 8 low mass DY (| η | < 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 low mass DY (| η | < 0.9) Forward particle (muon) production (-4 < η < -2.5)

Selected Physics Channels W and Z production LHCb (low-x sensitivity) Low mass Drell-Yan ATLAS CMS LHCb Exclusive particle production ALICE Forward jet production All preliminary studies!

Foward Z production (LHCb)

Z in forward region (LHCb) Partonic c/s known at NNLO to 1% One pdf well known; one 2-15% Experimental measurement Test SM Test DGLAP Constrain PDFs MCFM + NNPDF % error on σZ

Z->mm selection Z triggered requiring 2 high-pt muons (e~0.90) Z isolated quite easily using requirements on pT of muons impact parameter cuts hadronic energy cut mass window

Raw distribution (after trigger)

After selection requirements

Systematic Uncertainties bb

Z Cross-section measurement Luminosity Statistical Uncertainty Systematic Uncertainty 10pb-1 2.4% 10% (lumi*) 100pb-1 0.8% 5% (lumi*) 1000pb-1 0.3% 1% * Require independent luminosity measurement for normalisation e.g. p+p->p+p+m++m- or W or g* However information on PDF still present from shape

Foward W production (LHCb)

W in forward region (LHCb) RWZ W+ Z RWZ insensitive to PDF R+- sensitive to d A+- sensitive to uv-dv

W->mn selection W triggered requiring high-pt muon (e~0.65) W isolated quite easily using requirements on pT of muons little other activity in event

W->mn selection Momentum Asymmetry

W->mn selection Muon pt after trigger Muon pt after selection

W Cross-section measurement Luminosity Statistical Uncertainty Systematic Uncertainty 10pb-1 1% 10% (lumi) 100pb-1 0.3% 5% (lumi) 1000pb-1 0.1%

Drell-Yan production (ATLAS,CMS,LHCb)

Drell-Yan production (ATLAS,CMS,LHCb)

Drell-Yan production (ATLAS,CMS,LHCb) Dominant at Z but large backgrounds at low masses

Central Drell-Yan (ATLAS) ATLAS g* → e+e- : ATL-COM-2008-064 Trigger: pt(e)>10GeV, |h|<2.5 (low lumi) Event selection: 2 oppositely charged electrons Both electrons: pt(e)>10 GeV, |h|<2.5 Missing Et < 30 GeV Backgrounds: dijets(largest), tt, tt, diboson Systematic errors: acceptance correction (PDFs) largest, rest under investigation  stat. error ~ 7% (M(ee)=8-60GeV) 50 pb-1 Probes x → 10-4

Forward Drell-Yan (CMS) CMS g*→e+e- : CERN/LHCC 2006-039/G-124 No saturation Saturation included Trigger: E (em) >300 GeV, E (had) <5 GeV, n.charged≥1 5.2< h <6.5 (CASTOR + TOTEM) Event selection: M(ee)>4 GeV No background /full simulation yet. No systematic study yet. Probes x →10-6 Sensitive to saturation effects if M(ee)~4 GeV possible (~30% difference).

Forward Drell-Yan (LHCb) LHCb, DIS 2008: Proceedings: R.Thorne & R.McNulty Trigger: 2 muons ΣPt >1.6 GeV Event selection: 2 muons, impact parameter consistent with primary vertex Cuts to ensure balanced, isolated muons in empty event. Backgrounds: misid. muons, b, c decays considered Eff>10%, Pur>70% for M(mm)>8GeV Possibly to lower M: M(mm)>5GeV=>x>2x10-6 Full systematic study ongoing. Probes x →10-6 Sensitive to saturation effects

Forward Drell-Yan (LHCb) Order of magnitude improvement in pdf possible Constrain gluon Probes x->2x10-6 Become sensitive to saturation effects at lowest Q2 J. Stirling Experimental expectations with 1fb-1

Exclusive particle production

Recent CDF results Paper in preparation (402 events)

... 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) Selected as exclusive di-muon event Largest uncertainty from double pomeron exchange processes 700 events in 100 pb-1

... the exclusive resonances... (ATLAS, CMS, LHCb, ALICE) TDR 100 pb-1

J/Y production (ALICE) ALICE: courtesy K. Safarik Forward muon production (no selection or systematic errors available) Probes x →10-6 with J/Y production

Forward jet production

Forward jet production (CMS, LHCb) CMS: CERN/LHCC 2006-039/G-124 Trigger: Jet Et>10 GeV* 3<|h|<5 (HF calorimeter) Event selection: Cone jets R=0.5, Et >20 GeV Expected systematics: Energy scale (3%) Luminosity (5%) Energy correction (UE, hadronisation) (? -> 30%) *possible (L1 threshold) Probes x → 10-5 Needs full and careful systematic study

Summary 4 LHC experiments cover the x-Q2 plane Sensitive down to x~10-6 Central rapidities: Low Q2 Drell-Yan Forward rapidities: DY, W, Z, Jets, Exclusives Constrain PDFs Essential for understanding putative New Physics Investigations of QCD

Z PDF uncertainty for error sets 6% MSTW07* (MCFM-LO) *(DIS07 proceedings)

Z PDF uncertainty for errorsets 6% MSTW07 (MCFM-LO)

Forward Drell-Yan (LHCb)

6%

6%

6%