qcd physics - lecture 6ppeferrando/lectures/feb2006/newsupa6.pdf · phenomenological efforts....
TRANSCRIPT
WorkshopsPDFs
Energy FlowQ Production
Summary
QCD Physics - Lecture 6
James Ferrando
Department of Physics and AstronomyUniversity of Glasgow
8th February 2007
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
Outline
1 Recent Workshops
2 PDFs and LHC
3 Energy Flow
4 Heavy Quark Production
5 Summary
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
HERA-LHCTeV4LHCHERA/LHC workshop
http://www.desy.de/∼heralhc
Identify & prioritise measurements tobe made at HERA with an impact onthe physics reach of the LHC.
Encourage and stimulate knowledgetransfer between communities +establish ongoing interaction.
Encourage & stimulate theory andphenomenological efforts.
Examine & improve theoretical &experimental tools.
Increase quantitative understandingof the implications of HERAmeasurements on LHC physics.
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
HERA-LHCTeV4LHCTeV4LHC
Work on understanding how to use Tevatron data to improve eventmodelling and theoretical understanding of cross sections for thesignals and backgrounds at the LHC
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
PDFs
HERA will run until mid 2007
Will collect a large amount of new data
Can expect significant improvement before LHC turns onHERA Luminosity 2002 - 2007
0
25
50
75
100
125
150
175
200
225
0 50 100 150 200 250 300 350Days of running
Inte
gra
ted
Lu
min
osi
ty (
pb
-1)
How much improvement can we expect inPDFs?
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
Future PDFMeasurements
Valence quark distributions expected to improve in HERA II
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
Future PDFMeasurements
Sea quark and Gluon expected to improve from HERA II, relevantfor LHC processes
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
W and Z productionat the LHC I
Already seen that W /Z productionoccurs via qq̄ → W /Z at LOMomentum fraction of particlestaking part in the process isx1,2 = M√
sexp(±y)
y = 12
E+plE−pl
at central rapidity x ∼ 0.005|y | < 2.5 → 10−4 < x < 0.1Unlike TeVatron uv , dv not involvedQ2 ≈ M2
W → g dominant
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
W and Z productionat the LHC II
Before HERA I uncertainty in g and sea q was ∼ 16%
Post HERA I uncertainty improves to e ∼ 3.5%
Will improve from HERA II measurements
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
Sea Quark and Gluons
Huge improvement in sea-q and g uncertainties from HERA data
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
PDF improvementsfrom LHC
Small improvement on uncertainties indicated by simulated data
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
FL
In the QPM FL is zero.
In DGLAP QCD to lowest order FLis given by:
FL(x ,Q2) = αS4π x2
1∫x
dzz3 ·
[163 F2(z ,Q2) + 8
∑e2q
(1− x
z
)zg(z ,Q2)
]which can be, approximately solved to give a measurable gluonquantity:
xg(x) = 1.8[
3π2αS
FL(0.4x)− F2(0.8x)]' 8.3
αSFL
(determined by measurements of F2 and FL)
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
Extraction of FL
Recall that for unpolarised e±p scattering:
d2σe±p
dxdQ2 = 2πα2xQ4 [Y+F2(x ,Q2)− y2FL(x ,Q2)∓ Y−xF3(x ,Q2)]
Y± = 1± (1− y)2
For Q2 < 50 we can neglect xF3 and write an expression of thereduced cross section (σr )
σr =(
xQ4
2πα2Y+
)d2σe±p
dxdQ2 =[F2(x ,Q2)− y2
Y+FL(x ,Q2)
]Measurements at different y for the same x ,Q2 required.Solution? Use different beam energies
Choice of energies is made to maximise difference in y2
Y+
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
Measuring FL
at HERA
The ZEUS and H1 experiments haveproposed several low-energy runningscenarios to enable HERA data to beused to measure FL.Scenarios require approx 3 months ofrunning, and may take place laterthis year. 15 % of the nominalenergy running luminosity would besacrificed for this importantmeasurement.
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
PDFsW/Z at the LHCFL
Impact of FL
on PDF uncertainties
Impact is very small except at low Q2
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroUnderlying EventMultiple Parton Interactions
Underlying Events
Inelastic hadron-hadron collisions are dominated by soft (lowpT ) interactions. Occasionally we see a hard scatter
Important to model both the soft and hard components inMCs so that:
The impact on physics backgrounds can be assesedThe impact on detector performance can be evaluated
We also expect the effects of multiple parton interactions(MPI) to become significant at the LHC
Perturbative QCD is very succesful when applied to hardprocesses but cannot be applied to the soft interactions.
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroUnderlying EventMultiple Parton Interactions
Underlying Event
Break the activity in an event into two separate groups
The hard scattering component which consists only of thehigh transverse energy jets arising from the hard interactionitself and containing the information about the underlyingsubprocess
The Underlying Event which incorporates all remaininghadronic activity
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroUnderlying EventMultiple Parton Interactions
Multiple PartonInteractions
Multiple Parton Interactions
Becuase hadrons are composite objects it is possible for secondaryscatters to take place between partons in beam remnants in hadroniccollisions. This phenomenon is called Multiple Parton Interactions
The existence of MPI has a direct impact on the hadronicfinal state
The increased energy flow can form a hard partonic scatteringcomponent to the underlying event
Jet energies in hard scatter may be increased and other jetquantities may be altered
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroUnderlying EventMultiple Parton Interactions
Multiple PartonInteractions
CDF run1 data already gave evidence for MI
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroUnderlying EventMultiple Parton Interactions
Multiple PartonInteractions
Tunes to CDF run2 data already being done. Progress willcertainly affect Tevatron MC
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroUnderlying EventMultiple Parton Interactions
Underlying Eventat HERA
Can HERA help us to understand the underlying event?
Resolved γp at HERA is a hadron-hadron collision
Sensitivity to MPI has been observed at HERA
Fits to HERA data have been used to predict the LHC finalstate, results are close to CDF tune and stable.
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Heavy QuarkProduction
The production of Heavy Quarks in hadron-hadron collisions(and DIS) is directly sensitive to the gluon content of thehadron.
Heavy quarks will be copiously produced at future colliders asa background to the more exotic processes expected
A precise description of the production properties in QCD willaid in discovery of phsyics beyond the SM. An example of thisfrom the ATLAS TDR...
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
bb̄bb̄ backgroundat ATLAS
Irreducible background to supersymmetric Higgs production arisesfrom QCD. Dominant processes are gg and gb. Discovery in thischannel only possible with precise knowledge of this QCD process
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Recall hadron-hadron lecture. for generic collision between hadronswith heavy quark production:
Ha + Hb → QQ̄ + X
cross section at centre of mass energy S can be written as:
σ(S) =∑i ,j
∫dx1
∫dx2σ̂ij(x1x2S ,m2,µ2)f
Hai (x1, µ)f Hb
i (x2, µ)
σ̂ij is the short distance cross section
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Short DistanceCross Section
The short-distance cross section is calculable in QCD and is aperturbative expansion in the mass of the heavy quark m:
σ̂ij (s, m2, µ2) =
α2S (µ2)
m2
hf
(0)ij (ρ) + 4παS (µ2)
hf
1)ij (ρ) + f
(1)ij (ρ) log(µ2/m2)
i+O(αS )
iand ρ = 4m2/sThe larger the mass the faster the convergence → predictions forbeauty production should be more accurate than those for charm.3 Schemes for mass treatment
Fixed Order (FO) - valid for pT ≈ mQ
Resummed ot next-to-leading order (NLL) a.k.a. “massless”for pT � mQ
Matched FONLL valid for all scales
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Information NeededFor LHC
The information for LHC which can be provided by runningexperiments is:
The state of the description of heavy quark production bytheoretical predictions.
The gluon and heavy quark content of the proton PDFs
Details of hadronic fragmentation
Effect of the underlying event in heavy quark processes
HERA results (for example) can provide general informationon event and jet topologies which will be useful for designingalgorithms or triggers at the LHC
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Open BeautyProduction I
Remember from previous lecture that B production is now welldescribed at TeVatron.
FONLL and MC@NLO (Fixed Order) describe the measuredquantities well
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Open BeautyProduction II
Beauty production well described in most of the kinematic regions
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Open CharmProduction I
Calculations less precise than in beauty
Theoretical precision much worse than experimental data
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Open CharmProduction II
Calculations less precise than in beauty
Theoretical precision much worse than experimental data
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Open CharmProduction III
Measurements of charm photoproduction accompanied with jetspose a challenge for theory due to the extra scale of the jettransverse energy. Such complicated states will be commonplace atthe LHC → verification of theory with HERA data will aid inunderstanding these high rate QCD events.Dijet correlations in charm events have been measured, which (asin the D0 result shown in previous lecture) are sensitive to higherorder effects.At LO the two jets are back to back.
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Open CharmProduction IV
For resolved events description by NLO poor
HERWIG describes the shapes well
Indicates that higher order calculations or implementation ofadditional parton showers in current NLO needed
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Universality ofCharm Fragmentation
Heavy quark fragmentation has been extensively studied ine+e−
LEP was the ideal environment for such studies (clean withback-to-back jets)
The accurate measurements of fragmentation functions etc/are used as input to models and NLO calculations of hadroncollisions.
The validity of these functions in the hadronic environmentneeds to be verified
Measurements at HERA and the TeVatron are therefore veryinteresting to establish the universality of the fragmentationfunctions
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
IntroTheory/PhenomenologyInformation for LHCOpen Beauty ProductionOpen Charm ProductionCharm Fragmentation
Universality ofCharm Fragmentation
Results from different types of collisions are consistent.
James Ferrando QCD Physics - Lecture 6
WorkshopsPDFs
Energy FlowQ Production
Summary
Summary
As we near the turn on date for the LHC,much work isunderway to bridge the gap from the personnel of runningexperiments with experience of data analysis to the LHCexperimentsMany measurements are still to come from HERA and theTevatron which directly affect the discovery potential at theLHC.
PDF (esp. gluon) uncertaintiesHeavy Quark production
A key issue is the production of Monte Carlo generators andtools that can accurately describe behaviour of the LHC data:
MPI & Underlying eventsDefinition of safe Event Shape variablesSurvival of rapidity gapsMatching of detector to hadron level jets in large jetmultiplicity events
James Ferrando QCD Physics - Lecture 6