Lowx physics at the LHC Ronan Mc Nulty

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

Outline n n n Overview W, Z production (x~10 -4) Sensitivity to PDFs g* production (x~10 -6) Exclusive processes Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 2

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

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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 4

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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 5

9 8 7 6 5 log 10(Q 2) [Ge. V 2] 4 3 2 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 6

9 8 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] ki ne m 4 LH C 3 2 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 7

9 8 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] ki ne m 4 2 1 LH C 3 DGLAP evolution 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 8

Recap from Fred’s lecture…. . Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 9

9 y: 8 6 4 2 0 2 4 6 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] ne m 4 ki Production of object of mass Q at rapidity 2 1 LH C 3 DGLAP evolution 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 10

ATLAS � : -8 -6 � -4 -2 0 2 Tracking 4 6 8 -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 Ge. V, (10 Ge. V), Pt(e) > 5 Ge. V, (12 Ge. V) Low mass DY (|η|<2. 5, or forward w. o. tracking) Low mass resonances (|η|<2. 5) Ronan Mc. Nulty 2010 CTEQ-MCNet Forward jet production (|η|<4. 5) school, Lauterbad 11

CMS : -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 -2. 5 < η < 2. 5 Trigger Pt(μ)>3. 5 Ge. V, Pt(calo)>4 Ge. V, + fwd. triggers Low mass DY (|η|<2. 5, or forward with limited tracking) Low mass resonances (|η|<2. 5) Ronan Mc. Nulty 2010 CTEQ-MCNet Forward jet production (|η|<6. 5) school, Lauterbad 12

9 y: 8 6 4 2 0 2 4 6 7 6 re log 10(Q 2) at ic [Ge. V 2] ki ne m 4 ATLAS & CMS: LH LA S C 3 &C MS gi on 5 AT 2 Collision between two partons having similar momentum fractions. PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP. 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 13

ALICE | |<0. 9 : -8 -6 -4 -2 0 2 4 6 Tracking -0. 9 < < 0. 9 Particle ID -0. 9 < < 0. 9 ECAL (HCAL) -0. 9 < < 0. 9 (| | > 8. 5) Muon -4 < < -2. 5 Counters -3. 4 < < 5 Trigger Pt (m)> 1 (2) Ge. V 8 Forward particle (muon) production (-4 < η < -2. 5) Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 14

9 y: 8 6 4 2 0 2 4 6 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] ne m 4 ki Alice: Collision between two partons having similar momentum fractions. Ali ce LH C 3 2 But some far-forward muon converage. 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 15

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

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

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

LHCb: a forward spectrometer y Interaction Point Tracking Stations Muon Chambers 250 mrad 10 mrad Vertex detector Magnet RICH detectors ECAL HCAL Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad l 20 mz 19

LHCb : -8 -6 -4 -2 0 2 4 6 8 Tracking 1. 8 < η < 4. 9 Particle ID 1. 8 < η < 4. 9 ECAL, HCAL 1. 8 < η < 4. 9 Muon 1. 8 < η < 4. 9 Trigger* Pt(μ) > 1 Ge. V, Pt(had) > 2. 5 Ge. V 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) Ronan Mc. Nulty 2010 CTEQ-MCNet Forward resonances (1. 8 < η < 4. 9) school, Lauterbad 20

9 y: 8 6 4 2 0 2 4 6 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] ki ne m 4 2 Cb LHCb: Collision between one well understood parton and one unknown or large DGLAP evolved parton. LH Cb LH LH C 3 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 21

9 y: 8 6 4 2 0 2 4 6 7 6 re log 10(Q 2) at ne m 4 ki W, Z ic [Ge. V 2] LH LA S ATLAS & CMS: 2 Collision between two partons having similar momentum fractions. AT g* C 3 &C MS gi on 5 PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP. 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 22

9 y: 8 6 4 2 0 2 4 6 7 6 gi on 5 re log 10(Q 2) at LHCb: ne m 4 Collision between one well understood parton and one unknown or large DGLAP evolved parton. g* 1 LH Cb LH LH 2 C 3 Cb ki W, Z ic [Ge. V 2] DGLAP evolution 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 Potential to go to very low x, where PDFs essentially unknown 23

Accessing low-x through Vector Bosons Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 24

9 y: 8 6 4 2 0 2 4 6 7 6 gi on 5 re log 10(Q 2) at ki ne m 4 2 1 C 3 LH W, Z ic [Ge. V 2] DGLAP evolution 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 25

W production at LHC 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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 26

hep-ph/9927031 Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 27

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

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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 29

Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 30

Theoretical predictions n n Total cross-section and rapidity distributions known to NNLO. <1% precision (FEWZ Monte. Carlo). . Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 31

• 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 Effect of PDF uncertainties on crosssections ~LHCb Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 32

But you can do better ! How do PDF uncertainties contribute? So ratio of Ws is sensitive to d to u ratio. (For LHCb d v/uv) Ratio of Z to W is almost insensitive to PDFs! Gold plated test of SM at the highest energies Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 33

But you can do better ! How do PDF uncertainties contribute? W asymmetry is sensitive to difference in u and d. (For LHCb u v-dv) Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 34

• 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. Percentage uncertainty on crosssection ratios due to PDF Effect of PDF uncertainties on crosssections • Even nicer, most experimental systematics (especially luminosity) cancel in the ratio. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 35

Finding Vector Bosons at low-x. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 36

Z: n n n Characteristic signature of two high transverse momentum muons (p. T~m. Z/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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 37

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

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Many more (higher-x) Z at CMS, Atlas. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 41

W: W n n One high transverse momentum muon (p. T~m. W/2) from hard scatter, plus some softer tracks. Isolation criteria: n Missing pt. n Note some sensitivity to higher order processes, to underlying event, and to pile-up. n Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 42

Cone isolation W signal and background live in different regions Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad Event isolation 43

Muon transverse momentum spectrum LHCb preliminary 59 nb-1 Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 44

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Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 46

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

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

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

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

How well can W, Z measurements constrain the PDFs? Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 51

How can W, Z constrain PDFs? From global fits, PDFs described by a set of orthogonal eigenvectors, which have a ‘central’ value e 0, and ‘uncertainties’ ei. is the value of the differential cross-section obtained using the central value. 1 is the value of the differential cross-section obtained moving 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 Mc. Nulty 2010 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 e 0, 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 MSTW 08, CTEQ 6. 5, ALEKHIN 2002, NNPDF 2. 0 studied. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 53

Uncertainty on PDF with 0. 1 fb-1 of LHCb data Uncertainty on PDF without LHCb data Improvement to MSTW 08 PDFs with 0. 1 fb-1 of high mass vector bosons at 7 Te. V 10 -5 10 -4 10 -3 10 -2 10 -1 1 10 -5 10 -4 10 -3 10 -2 10 -1 1 1 Modest improvement with small amount of data Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 54

Uncertainty on PDF with 0. 1 fb-1 of LHCb data Uncertainty on PDF without LHCb data Comparison with different PDFs using 0. 1 fb-1 of high mass vector bosons at 7 Te. V Similar sensitivity. Ability to distinguish models Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 55

Uncertainty on PDF with 1 fb-1 of LHCb data Uncertainty on PDF without LHCb data Improvement to CTEQ 66 PDFs with 1 fb-1 of high mass vector bosons at 14 Te. V More data and higher energy lead to larger improvements. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 56

Improvement to PDFs: n n Similar sized improvements for each PDF Ability to distinguish between models 0. 1 fb-1 of data at 7 Te. V will test the SM 1 fb-1 of data at 14 Te. V can improve PDFs by about 30%. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 57

Using g* to go to very low-x. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 58

9 y: 8 6 4 2 0 2 4 6 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] g* 1 LH Cb LH LH 2 C 3 Cb ki ne m 4 DGLAP evolution 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 59

Percentage uncertainty on cross-sections due to PDF Effect of PDF uncertainties on cross-sections ~LHCb Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 60

First data. Analyses using 37 nb-1 n g*->mm q q q Require two isolated muons pt>1 Ge. V 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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 61

Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 62

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

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

J/Y production (ALICE) Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 65

9 Why is this interesting 8 7 6 gi on 5 re log 10(Q 2) ic [Ge. V 2] at DGLAP evolution ki ne m 4 Up in Q 2 LH C 3 for given x 2 1 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 66

9 Why is this interesting 8 7 6 gi on 5 re log 10(Q 2) at ic [Ge. V 2] ki ne m 4 2 1 LH C 3 BFKL evolution 0 -1 -6 -5 -4 -3 -2 -1 Ronan Mc. Nulty 2010 CTEQ-MCNet log 10 Lauterbad (x) school, 0 67

Phase Change: Saturation. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 68

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

PDF uncertainties at low-x, low. MSTW 08. (Thanks to Graeme Watt) 2 Q 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 resummation effects. Possibly entering saturation regime. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 70

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

For q. T~m. V NLO calculations e. g. with MCFM generator (Ellis & Campbell) see Bozzi, Catani, Ferrera, de Florian, Grazzini, ar. Xiv: 0812. 2862 and ar. Xiv: 1007. 2351 (14 th July 2010). Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 72

But for q. T<<m. V Large logs gluon resummation required see Bozzi, Catani, Ferrera, de Florian, Grazzini, ar. Xiv: 0812. 2862 and Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 73

With resummation for Z Full calculation for low mass DY: hep-ph 0303021 Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 74

Improvement to MSTW 08 PDFs with 0. 1 fb-1 of low mass vector bosons at 7 Te. V 10 -5 10 -4 10 -3 10 -2 10 -1 1 Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 10 -5 10 -4 10 -3 10 -2 10 -1 1 75

Improvement to CTEQ 66 PDFs with 0. 1 fb-1 of low mass vector bosons at 7 Te. V Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 76

Improvement to NNPDF 2. 0 PDFs with 0. 1 fb 1 of low mass vector bosons at 7 Te. V Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 77

Uncertainty on PDF with 0. 1 fb-1 of LHCb data Uncertainty on PDF without LHCb data Improvement to different PDF sets with 0. 1 fb-1 of low invariant mass muons (10 -20 Ge. V) at 7 Te. V Similar improvements to MSTW, CTEQ and Alekhin PDFs. Sensitivity exists. Ronan to Mc. Nulty distinguish between models. 2010 CTEQ-MCNet school, Lauterbad 78

Current uncertainty on MSTW 08 PDFs and projections with 0. 1 fb-1, 1 fb-1 of very low invariant 20% 20% mass muons at 7 Te. V 10% 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 20% 10% 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 Significant improvements possible with modest amount of data Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 79

Exclusive particle production Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 80

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

. . . the flat, well understood part. . . (ATLAS, CMS, LHCb) CMS: S. Ovyn at DIS 08 ATLAS: B. Caron CERN-THESIS-2006 -024 LHCb: LHCb/2008/01 n n Important for luminosity determination Theoretically known to <1% (QED) Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 700 events in 100 pb-1 SIMULATION! 82

. . . exclusive resonances forward region. . . (CMS, LHCb, ALICE) 100 pb-1 n Produced at low x-Q 2 as for DY, but now probing photons or pomerons. Ronan Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 83

Summary n n 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 Mc. Nulty 2010 CTEQ-MCNet school, Lauterbad 84