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

2. 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

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

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

5. y: 6 4 2 0 2 4 6 LHC kinematic region Production of object of mass Q9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
LHC kinematic region
Production of
object of mass Q
at rapidity
log10(x)

6. 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:
fully instrumented
ATLAS, CMS
Low-x physics:
Low mass Drell-Yan
Low mass resonances
Forward jet production
(some detectors)
(some detectors)
log10(x)

7. LHCb LHCb y: 6 4 2 0 2 4 6 Low-x physics: W,Z production9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
Low-x physics:
W,Z production
Low mass Drell-Yan
Low mass resonances
Forward jets
LHCb
LHCb
log10(x)

8. y: 6 4 2 0 2 4 6 some instrumentation ALICE some instrumentation9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
ALICE
some instrumentation
some instrumentation
Low-x physics:
Forward jet production
Forward resonances
log10(x)

9. ATLAS h: 
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)

10. CMS h:
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)

11. LHCb h:
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)

12. 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)

13. 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!

14. Foward Z production (LHCb)

15. 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

16. 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

17. Raw distribution (after trigger)

18. After selection requirements

19. Systematic Uncertaintiesbb

20. 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

21. Foward W production (LHCb)

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

23. 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

24. W->mn selection
Momentum Asymmetry

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

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

27. Drell-Yan production (ATLAS,CMS,LHCb)

28. Drell-Yan production (ATLAS,CMS,LHCb)

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

30. Central Drell-Yan (ATLAS)
ATLAS g* → e+e- : ATL-COM
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

31. Forward Drell-Yan (CMS)
CMS g*→e+e- : CERN/LHCC /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).

32. 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

33. 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

34. Exclusive particle production

35. Recent CDF results
Paper in preparation
(402 events)

36. ... the flat, well understood part... (ATLAS, CMS, LHCb)
CMS: S. Ovyn at DIS08
ATLAS: B.Caron CERN-THESIS
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

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

38. 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

39. Forward jet production

40. Forward jet production (CMS, LHCb)
CMS: CERN/LHCC /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

41. 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

42. Z PDF uncertainty for error sets6%
MSTW07* (MCFM-LO)
*(DIS07 proceedings)

43. Z PDF uncertainty for errorsets6%
MSTW07 (MCFM-LO)

44. Forward Drell-Yan (LHCb)

45. 6%

46. 6%

47. 6%