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

2. 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 CTEQ-MCNet school, Lauterbad

3. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad
Theory
Experiment
Prediction
Ronan McNulty CTEQ-MCNet school, Lauterbad

4. 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 CTEQ-MCNet school, Lauterbad

5. 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 CTEQ-MCNet school, Lauterbad

6. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

7. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1
LHC kinematic region
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

8. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1
LHC kinematic region
DGLAP evolution
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

9. Recap from Fred’s lecture…..
Ronan McNulty CTEQ-MCNet school, Lauterbad

10. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
LHC kinematic region
Production of
object of mass Q
at rapidity
DGLAP evolution
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

11. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad
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(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 CTEQ-MCNet school, Lauterbad

12. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad
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(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 CTEQ-MCNet school, Lauterbad

13. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
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.
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

14. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad
ALICE h:
|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 CTEQ-MCNet school, Lauterbad

15. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
LHC kinematic region
Alice:
Collision between two partons having similar momentum fractions.
But some far-forward muon converage.
Alice
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

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

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

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

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

20. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad
LHCb h:
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 CTEQ-MCNet school, Lauterbad

21. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
LHC kinematic region
LHCb:
Collision between one well understood parton and one unknown or large DGLAP evolved parton.
LHCb
LHCb
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

22. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
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*
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

23. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
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*
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

24. Accessing low-x through Vector Bosons
Ronan McNulty CTEQ-MCNet school, Lauterbad

25. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
LHC kinematic region
W,Z
DGLAP evolution
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

26. 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 CTEQ-MCNet school, Lauterbad

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

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

29. 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 CTEQ-MCNet school, Lauterbad

30. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

31. Theoretical predictions
Total cross-section and rapidity distributions known to NNLO. <1% precision
(FEWZ MonteCarlo) .
Ronan McNulty CTEQ-MCNet school, Lauterbad

32. 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 CTEQ-MCNet school, Lauterbad

33. 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 CTEQ-MCNet school, Lauterbad

34. 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 CTEQ-MCNet school, Lauterbad

35. 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 CTEQ-MCNet school, Lauterbad

36. Finding Vector Bosons at low-x.
Ronan McNulty CTEQ-MCNet school, Lauterbad

37. Ronan McNulty 2010 CTEQ-MCNet school, LauterbadZ:
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 CTEQ-MCNet school, Lauterbad

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

39. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

40. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

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

42. Ronan McNulty 2010 CTEQ-MCNet school, LauterbadW: 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 CTEQ-MCNet school, Lauterbad

43. W signal and background live in different regions
Cone isolation
Event isolation
Ronan McNulty CTEQ-MCNet school, Lauterbad

44. Muon transverse momentum spectrum
LHCb preliminary 59nb-1
Ronan McNulty CTEQ-MCNet school, Lauterbad

45. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

46. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

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

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

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

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

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

52. 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 CTEQ-MCNet school, Lauterbad

53. 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 CTEQ-MCNet school, Lauterbad

54. 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
Modest improvement with small amount of data
Ronan McNulty CTEQ-MCNet school, Lauterbad

55. 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 CTEQ-MCNet school, Lauterbad

56. 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 CTEQ-MCNet school, Lauterbad

57. 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 CTEQ-MCNet school, Lauterbad

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

59. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1 y:
LHC kinematic region
LHCb
LHCb
DGLAP evolution g*
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

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

61. 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 CTEQ-MCNet school, Lauterbad

62. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad

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

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

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

66. Why is this interesting9 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
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

67. Why is this interesting9 8 7 6 5
log10(Q2)
[GeV2] 4 3 2 1 -1
Why is this interesting
LHC kinematic region
BFKL evolution
log10(x)
Ronan McNulty CTEQ-MCNet school, Lauterbad

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

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

70. 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 CTEQ-MCNet school, Lauterbad

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

72. 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: and arXiv: (14th July 2010).
Ronan McNulty CTEQ-MCNet school, Lauterbad

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

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

75. Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad
Improvement to MSTW08 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV
Ronan McNulty CTEQ-MCNet school, Lauterbad

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

77. 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 CTEQ-MCNet school, Lauterbad

78. 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 CTEQ-MCNet school, Lauterbad

79. 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%
20%
20%
10%
10%
Significant improvements possible with modest amount of data
Ronan McNulty CTEQ-MCNet school, Lauterbad

80. Exclusive particle production
Ronan McNulty CTEQ-MCNet school, Lauterbad

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

82. ... 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)
700 events in 100 pb-1
SIMULATION!
Ronan McNulty CTEQ-MCNet school, Lauterbad

83. ... 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 CTEQ-MCNet school, Lauterbad

84. 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 CTEQ-MCNet school, Lauterbad