QCD Physics - Lecture 6ppeferrando/lectures/feb2006/newsupa6.pdf · phenomenological eﬀorts. Examine & improve theoretical & experimental tools. Increase quantitative understanding

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


Summary

Outline

1 Recent Workshops

2 PDFs and LHC

3 Energy Flow

4 Heavy Quark Production

5 Summary


WorkshopsPDFs


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.


WorkshopsPDFs


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


WorkshopsPDFs


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?


WorkshopsPDFs


Summary


Future PDFMeasurements

Valence quark distributions expected to improve in HERA II


WorkshopsPDFs


Summary


Future PDFMeasurements

Sea quark and Gluon expected to improve from HERA II, relevantfor LHC processes


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


Sea Quark and Gluons

Huge improvement in sea-q and g uncertainties from HERA data


WorkshopsPDFs


Summary


PDF improvementsfrom LHC

Small improvement on uncertainties indicated by simulated data


WorkshopsPDFs


Summary


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)


WorkshopsPDFs


Summary


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+


WorkshopsPDFs


Summary


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.


WorkshopsPDFs


Summary


Impact of FL

on PDF uncertainties

Impact is very small except at low Q2


WorkshopsPDFs


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.


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary



CDF run1 data already gave evidence for MI


WorkshopsPDFs


Summary



Tunes to CDF run2 data already being done. Progress willcertainly affect Tevatron MC


WorkshopsPDFs


Summary


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.


WorkshopsPDFs


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...


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


Open BeautyProduction II

Beauty production well described in most of the kinematic regions


WorkshopsPDFs


Summary


Open CharmProduction I

Calculations less precise than in beauty

Theoretical precision much worse than experimental data


WorkshopsPDFs


Summary


Open CharmProduction II

Calculations less precise than in beauty

Theoretical precision much worse than experimental data


WorkshopsPDFs


Summary


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.


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


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


WorkshopsPDFs


Summary


Universality ofCharm Fragmentation

Results from different types of collisions are consistent.


WorkshopsPDFs


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


Documents

QCD Physics - Lecture 6ppeferrando/lectures/feb2006/newsupa6.pdf · phenomenological eﬀorts. Examine & improve theoretical & experimental tools. Increase quantitative understanding