low-x physics at the lhc
DESCRIPTION
Low-x physics at the LHC. Ronan McNulty University College Dublin. 2010 CTEQ-MCNet School, Lauterbad. Outline. Overview W,Z production (x~10 -4 ) Sensitivity to PDFs g * production (x~10 -6 ) Exclusive processes. Theory. Experiment. Prediction. Theory v Experiment at the LHC. - PowerPoint PPT PresentationTRANSCRIPT
Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 1
Low-x physics at the LHC
Ronan McNultyUniversity College Dublin
2010 CTEQ-MCNet School, Lauterbad.
Ronan McNulty 2010 CTEQ-MCNet school, Lauterbad 2
Outline
Overview W,Z production (x~10-4) Sensitivity to PDFs * production (x~10-6) Exclusive processes
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Theory
Prediction
Experiment
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Theory v Experiment at the LHC
Hadronic Cross-section
Partonic Cross-section
PDFs
• 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 2010 CTEQ-MCNet school, Lauterbad 5
Theory v Experiment at the LHC
Hadronic Cross-section
Partonic Cross-section
PDFs
Test the SM at highest energies
Check out that DGLAP evolution works (test QCD)
Push theory into interesting regions with very soft gluons
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DGLAP evolution
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Recap from Fred’s lecture…..
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DGLAP evolution
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Production of object of mass Qat rapidity
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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
ATLAS
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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)
CMS
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ATL
AS
& C
MS
ATLAS & CMS:
Collision between two partons having similar momentum fractions.
PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP.
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||<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)
ALICE
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Alic
e
Alice:
Collision between two partons having similar momentum fractions.
But some far-forward muon converage.
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ATLAS 3D view
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CMS Longitudinal View
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ATLAS 3D view
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250mrad
10mrad
Interaction Point
Magnet
Vertex detector
Tracking Stations
ECALRICH detectors
y
z20mlHCAL
Muon Chambers
LHCb: a forward spectrometer
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LHCb
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)
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LHCb
LHCb LHCb:
Collision between one well understood parton and one unknown or large DGLAP evolved parton.
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ATL
AS
& C
MS
ATLAS & CMS:
Collision between two partons having similar momentum fractions.
PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP.
W,Z
*
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DGLAP evolution
y: 6 4 2 0 2 4 6
LHCb
LHCb
W,Z
*
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
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Accessing low-x through Vector Bosons
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W,Z
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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.)
W
W W
W
W
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hep-ph/9927031
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Boson Level
Lepton Level
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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.
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Theoretical predictions
Total cross-section and rapidity distributions known to NNLO. <1% precision
(FEWZ MonteCarlo) . .
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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%
Perc
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s d
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DF
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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
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But you can do better ! How do PDF uncertainties contribute?
W asymmetry is sensitive to difference in u and d. (For LHCb uv-dv)
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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.
Perc
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ross-
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rati
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Finding Vector Bosons at low-x.
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Z:
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.
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37nb-1
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Many more (higher-x) Z at CMS, Atlas.
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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.
W
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W signal and background live in different regions
Event isolation
Cone iso
lati
on
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Muon transverse momentum spectrum
LHCb preliminary 59nb-1
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W/Z ratio.
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W asymmetry (ATLAS)
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W asymmetry (CMS)
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W asymmetry (LHCb)
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How well can W,Z measurements constrain the PDFs?
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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
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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.
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Improvement to MSTW08 PDFs with 0.1fb-1 of high mass vector bosons at 7TeV
Unce
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b d
ata
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
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Comparison with different PDFs using 0.1fb-1 of high mass vector bosons at 7TeV
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Similar sensitivity. Ability to distinguish models
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Improvement to CTEQ66 PDFs with 1fb-1 of high mass vector bosons at 14 TeV
Unce
rtain
ty o
n P
DF
wit
h 1
fb-1
of
LHC
b d
ata
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wit
hout
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b d
ata
More data and higher energy lead to larger improvements.
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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%.
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Using * to go to very low-x.
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LHCb
LHCb
*
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Effect of PDF uncertainties on cross-sections
~LHCb
Perc
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s d
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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
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ATLAS (shown at ICHEP)
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CMS (shown at ICHEP)
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J/ production (ALICE)
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DGLAP evolution
Up in Q2
for given x
Why is this interesting
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BFKL evolution
Why is this interesting
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Phase Change: Saturation.
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Interlude
Some theoretical issues (as discussed extensively at this school).
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PDF uncertainties at low-x, low-Q2
LO
NLO
NNLO
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.
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From Fred:
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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).
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But for qT<<mV
see Bozzi, Catani, Ferrera, de Florian, Grazzini, arXiv:0812.2862 and
Large logs
gluonresummationrequired
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With resummation for Z
Full calculation for low mass DY:
hep-ph 0303021
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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
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Improvement to CTEQ66 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV
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Improvement to NNPDF2.0 PDFs with 0.1fb-1 of low mass vector bosons at 7TeV
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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.
Unce
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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% 20%
10%10%10%
10% 10%
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Exclusive particle production
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Recent CDF results pppp
(402 events)
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... 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!
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... exclusive resonances forward region... (CMS, LHCb, ALICE)
100 pb-1
Produced at low x-Q2 as for DY, but now probing photons or pomerons.
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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.