low-x physics at the lhc

Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 1

Low-x physics at the LHC

Ronan McNultyUniversity College Dublin

2010 CTEQ-MCNet School, Lauterbad.

Outline

Overview W,Z production (x~10-4) Sensitivity to PDFs * production (x~10-6) Exclusive processes

Theory

Prediction

Experiment

Theory v Experiment at the LHC

Hadronic Cross-section

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

Theory v Experiment at the LHC

Hadronic Cross-section

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

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

DGLAP evolution

Recap from Fred’s lecture…..

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

DGLAP evolution

y: 6 4 2 0 2 4 6

Production of object of mass Qat rapidity

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)

-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 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)

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

y: 6 4 2 0 2 4 6

ATLAS & CMS:

Collision between two partons having similar momentum fractions.

PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP.

-8 -6 -4 -2 0 2 4 6 8

||<0.9

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 ()> 1 (2) GeV

Forward particle (muon) production (-4 < η < -2.5)

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

y: 6 4 2 0 2 4 6

Alice:

But some far-forward muon converage.

ATLAS 3D view

CMS Longitudinal View

ATLAS 3D view

250mrad

10mrad

Interaction Point

Magnet

Vertex detector

Tracking Stations

ECALRICH detectors

z20mlHCAL

Muon Chambers

LHCb: a forward spectrometer

-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 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)

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

y: 6 4 2 0 2 4 6

LHCb LHCb:

Collision between one well understood parton and one unknown or large DGLAP evolved parton.

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

y: 6 4 2 0 2 4 6

ATLAS & CMS:

PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP.

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

DGLAP evolution

y: 6 4 2 0 2 4 6

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

Accessing low-x through Vector Bosons

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

DGLAP evolution

y: 6 4 2 0 2 4 6

W production at LHC

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.)

hep-ph/9927031

Boson Level

Lepton Level

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.

Theoretical predictions

Total cross-section and rapidity distributions known to NNLO. <1% precision

(FEWZ MonteCarlo) . .

Effect of PDF uncertainties on cross-sections

• 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%

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

But you can do better ! How do PDF uncertainties contribute?

W asymmetry is sensitive to difference in u and d. (For LHCb uv-dv)

• 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.

Finding Vector Bosons at low-x.

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.

37nb-1

Many more (higher-x) Z at CMS, Atlas.

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.

W signal and background live in different regions

Event isolation

Cone iso

Muon transverse momentum spectrum

LHCb preliminary 59nb-1

W/Z ratio.

W asymmetry (ATLAS)

W asymmetry (CMS)

W asymmetry (LHCb)

How well can W,Z measurements constrain the PDFs?

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.

1is 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 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

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.

Improvement to MSTW08 PDFs with 0.1fb-1 of high mass vector bosons at 7TeV

10-5 10-4 10-3 10-2 10-1 1

Modest improvement with small amount of 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

Comparison with different PDFs using 0.1fb-1 of high mass vector bosons at 7TeV

Similar sensitivity. Ability to distinguish models

Improvement to CTEQ66 PDFs with 1fb-1 of high mass vector bosons at 14 TeV

More data and higher energy lead to larger improvements.

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%.

Using * to go to very low-x.

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

DGLAP evolution

y: 6 4 2 0 2 4 6

First data. Analyses using 37 nb-1

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

ATLAS (shown at ICHEP)

CMS (shown at ICHEP)

J/ production (ALICE)

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

DGLAP evolution

Up in Q2

for given x

Why is this interesting

-6 -5 -4 -3 -2 -1 0

log10(x)

log10(Q2)

[GeV2]

BFKL evolution

Why is this interesting

Phase Change: Saturation.

Interlude

Some theoretical issues (as discussed extensively at this school).

PDF uncertainties at low-x, low-Q2

MSTW08. (Thanks to Graeme Watt)

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.

From Fred:

For qT~mVNLO 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).

But for qT<<mV

see Bozzi, Catani, Ferrera, de Florian, Grazzini, arXiv:0812.2862 and

Large logs

gluonresummationrequired

With resummation for Z

Full calculation for low mass DY:

hep-ph 0303021

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 10-1 110-5 10-4 10-3 10-2 10-1 1

Improvement to CTEQ66 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV

Improvement to NNPDF2.0 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV

Improvement to different PDF sets with 0.1fb-1 of low invariant mass muons (10-20GeV) at 7TeV

Similar improvements to MSTW, CTEQ and Alekhin PDFs.

Sensitivity exists to distinguish between models.

Current uncertainty on MSTW08 PDFs and projections with 0.1fb-1, 1fb-1 of very low invariant mass muons at 7TeV

Significant improvements possible with modest amount of data

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

10-6 10-5 10-4 10-3 10-2 10-1 110-6 10-5 10-4 10-3 10-2 10-1 1

20%20%

20% 20%

10%10%10%

10% 10%

Exclusive particle production

Recent CDF results pppp

(402 events)

... the flat, well understood part...(ATLAS, CMS, LHCb)

Important for luminosity determination

Theoretically known to <1% (QED)

CMS: S. Ovyn at DIS08

ATLAS: B.Caron CERN-THESIS-2006-024

LHCb: LHCb/2008/01

700 events in 100 pb-1

SIMULATION!

... exclusive resonances forward region... (CMS, LHCb, ALICE)

100 pb-1

Produced at low x-Q2 as for DY, but now probing photons or pomerons.

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.

low-x physics at the lhc

lauterbadronan mcnulty

gevronan mcnulty

soft gluonsronan mcnulty

low mass dy ronan mcnulty

mass qat rapidityronan

lhc kinematic regiony

z production x

production of object

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