uhh ss09: lhc the large hadron collider machine, experiments, physics sm physics (at the lhc)...

UHH SS09: LHC

The Large Hadron ColliderMachine, Experiments, PhysicsSM Physics (at the LHC)

Johannes HallerThomas Schörner-Sadenius

Hamburg UniversitySummer Term 2009

UHH SS09: LHCJH/TSS 2

SM MEASUREMENTS: OVERVIEWHeavy-flavourphysics (c,b)

BS, BC hadronsTop physicsQCD / jets,

strong coupling,Underlying event,

diffraction, forward physics, QGP, ET,miss

Prompt photons

- W,Z cross-sections- EW parameters (asymmetries, sin2θ)

- MW and Γ(W)

- Boson couplings- anomalous quark couplings?- PDFs?

- Lepton universality- Zll- Z’ll

Further gauge bosons?

QCD

Not shown: Importance of particles for BSM measurements (as signal or background)!Mostly results from CMS simulations (newer “Physics TDR” than ATLAS)!


QCD MEASUREMENTS: JET YIELD AT LHC

n gqqbaabn

abnS

pbn gqqba

abnpaRnS

pbban

anpanSjet

sd

dL

s

d

xfxfdxdx

fbf

,,,

1

,

2/,,,

,1/212

/,,

,/

0

1

Remember: Cross-section for production of jetsfrom quarks and gluons in pp collisions:

CDF data

Remember: parton-parton lumi at LHC and Tevatron:

sxxs 21ˆ

Huge pT values are reachable

Test of pQCD to highest scales! Large sensitivity to new physics!


JETS: ALGORITHMS, EXAMPLE EVENTS

Either “cone” algorithms or clustering a la kT algorithm.

e–

p remnant

jet

jet

jetneutrino

jet

jet

jet


JETS: EXAMPLE EVENTS


QCD MEASUREMENTS: UNCERTAINTIES

2tanln

2

1

Unfortunately (or luckily?): large theory uncertain-ties for predictions (up to 100%?):– scale μR of strong coupling αS(μR). Effect due to truncated perturbative expansion in powers of αS:

Often dominates the theory!– PDF uncertainty

4

323

222

/,,

,/

O

SR

RSRS

pbban

anpanSjet

d

d

CC

fbf

Other view on PDF uncertainties: Uncertainty when using best HERA PDFs, and possible improvements due to all HERA data:


UNCERTAINTIES: HERA VERSUS TEVATRON

Tevatron goes to high ET, but with large uncertainties – especially jet energy scale (yellow) and PDF (red lines).

HERA: much smaller ET, but with small uncertainties less than 10% (depending on observable).


Also at LHC: Large experimental uncertainties. Dominated by jet energy scale determination.

Remember that (multi)jets have high cross-sections (clearly higher than W,Z production) triggering no bigissue – but need to keep rate under control – also for background subtraction!

QCD MEASUREMENTS

Selection 2·1033 cm-2s-1 1034 cm-2s-1

J200 (290) 0.2 kHz 0.2 kHz

3J90 (130) 0.2 kHz 0.2 kHz

4J65 (90) 0.2 kHz 0.2 kHz

J60+xE60 (100)

0.4 kHz 0.5 kHz

EM25i(30) 11 kHz 22 kHz

QCD background to(high-mass) SUSY!

Example CMS


LHC covers much larger range in x and Q2 – but can this be used to learn more about the PDFs f(x,Q2)?

No simple answer – depends dramatically on experimentaland theoretical precision (remember that PDFs are ex-tracted in comparisons of (N)NLO theory with data).Lots of work done especially by Oxford fitters within theATLAS collaboration (M. Cooper-Sakar et al.).Lots of physics at the LHC play at (very) low x! But new heavy resonances require (very precise) high x!

QCD MEASUREMENTS: PDFS?

M = 10 TeV

M = 100 GeV

M = 1 TeV

Jets at the LHC might help themselves – but depends critically on uncertainties (jet energy scale to 1%???)


Then try to distuingish predictions for these processes using different PDF parametrisations (two very close blue curves in plot below):

There is some potential, but it requires extremely good photon ID, fake photon rejection, and a photon selection efficiency of above 90%.

Alternative ideas are to use Z+b events access to b PDF: ppb+gZ+b!Note that also determination of αS possible from jets!

Might also use prompt photons (cleaner than jetsbecause no hadronisation involved – but large backgrounds):

QCD MEASUREMENTS: PDFS?

323

222 CC RSRSjet


At the LHC, up to 25 proton-proton collisions will takeplace in one bunch-crossing (every 25 ns) – pile-up!Most of these events will be soft – will not involve a hard QCD scattering, but a rather soft distribution ofparticles with low transverse momenta – minimum bias!

Difficult to model (no hard scale for pQCD!), they are - an important background for all studies- interesting in themselves- … and a good tool to monitor detector performance!

Tevatron: charged particle flows and pT spectra:

QCD MEASUREMENTS: MINIMUM BIAS

Porting these findings to the LHC requires know-ledge of the energy behaviour of minimum bias:

Much larger discrepanies! So what is the real particle flow at the LHC? Will be among the first questions to be investigated at the LHC!

… but it gets worse: Underlying events!


QCD MEASUREMENTS: UNDERLYING EVENTS


A FULL EVENT


QCD MEASUREMENTS: UNDERLYING EVENTS

The underlying event (UE) is defined as everything in the event except the hardest scattering:

- Minimum bias.- Proton remants.- Initial- and final-state radiation.- multiple parton interactions (MPI)Investigation: transverse regions!

Experience from Tevatron: UE can be described!

But extrapolation to LHC fails drastically!


QCD MEASUREMENTS: UE AT HERA

Example: 3- and 4-jet cross-sections in photoproduction

With UE model

Without UE model


Prompt photons from QCD events are– powerful QCD test (cross-section calculations similar to jet cross-sections) PDFs?– difficult to measure: high backgrounds from QCD jets, and neutral mesons (π0).– Background to (and playground for) photons in Hγγ events.

Jet rejection based on HAC energy, shower shape:

QCD MEASUREMENTS: PROMPT PHOTONS

But we have high statistics and efficiency:

d2σ/d

ηd

pT [

pb

/GeV

]


Contact interactions and alike: expect modification of dijet cross especially at high scales / masses – where the uncertainties are large need high theoretical and experimental precision!

Also taking other observables than just ET helps!

Dijets from decay Xjetjet for BSM searches!Sensitivity depends on mass and cross-section:

Difficult to separate using only mass! Need very precise modelling of QCD background (NLO theory, control of uncertainties)!

QCD MEASUREMENTS: NEW PHYSICS

43212

2

ssssg

L

LLL

iab

iabCI

CISM

Example: Z’qq

δΦ(pT,miss,jet2) δΦ(pT,miss,jet2)

SUSY QCD


W and Z production ppW/Zll/ν is a high-rate, clean environment for - Luminosity determination (precise NNLO σ!)- PDF information.

- tests of couplings (consistency with SM?).- tests of higher-order QCD corrections

ELECTROWEAK PHYSICS: W,Z XSECTIONS

W

g

q

q’

q

q’

W

g

q’

O(s)

About 100 W/s at the LHC (1/s at the Tevatron!)


Theory predictions for W and Z boson productionare available in NNLO! How precise are thesepredictions?

So NNLO gives a rather precise (1%) answer – but the PDFs give a problem:

ELECTROWEAK PHYSICS: W,Z AND PDFs

So sometimes more difference between differentmeans than errors would allow – and newer results(CTEQ6.5) make it even worse – 8% difference!

Study: Inclusion of W,Z data in PDF fits would changethese substantially!

Electron rapidity spectrumfrom W decay: pseudodataversus prediction.

No ATLAS data

With ATLAS data


Relatively clear signatures:

ELECTROWEAK PHYSICS: Z XSECTIONS

Zll

Wlν A ~ 10%

Purity 98%

Tevatron results:

σZ = 264.9 ±3.9 stat ±9.8 syst ±17.2 lum (pb) (e)

σZ = 261.3 ±2.7 stat ±6.3 syst ±15.1 lum (pb) (μ)


Extract, from ppZee events, the couplings of the Z to u and d quarks:

SM fits!

ELECTROWEAK PHYSICS: Zqq COUPLINGSgZ=T3-sin2W 2gV 2gA

eL 1/2 1 1

eL -1/2+ sin2W -1+4 sin2W -1

eR 0 1 1

eR sin2W -1+4 sin2W -1

uL 1/2-2/3 sin2W 1-8/3 sin2W 1

dL -1/2+1/3 sin2W -1+4/3 sin2W -1

uR -2/3 sin2W 1-8/3 sin2W 1

dR 1/3 sin2W -1+4/3 sin2W -1

However, compared to LEP, the reach is rather limited:

3325 sin2

2

1TgTggg AWVAV


Measurement in small part of CDF data (by nowmore precise measurements out):

ELECTROWEAK PHYSICS: W XSECTIONS

So far all results are compatible with expectationsfrom the SM: data compared to NNLO theory!

Also couplings according to SM expectations!CDF: = 2719 ± 10stat ± 53sys ±165lum (pb)

A ~ 23(10)%

Purity 97/90%

2,,2

,, TlTTlTWT ppEEm


ELECTROWEAK PHYSICS: W XSECTIONS

Comparisons of various Wlν cross-section measurements:

CDF data compared to NNLO theory!

NNLO theory works very well at the Tevatron Good tool for studies at the LHC!

z

z

pE

pEy ln

2

1


Method for MW extraction: “Template method”:

- Final state neutrino no direct MW reconstruction.- Use correlation of MW with transverse mass:

- Fit distributions of MW,T(MW), pT,l etc. to data measurement on statistical basis only!

Relevance of MW:

– SM makes clear predictions for MW and its connection to Z mass:

So measurements of MW and MZ are stringent tests of SM!– But MW also sensitive to higher-order effects due to vacuum fluctuations:

These effects are entering mainly through rW.

Sensitivity to MH (especially when combined with top mass measurement) and to SUSY!

Plot on the right: Connection of masses of top, Higgs, and W. Current measurements prefer ratherlow values for MH!

EW PHYSICS: W MASS + WIDTH TEVATRON

2,,2

,, TlTTlTWT ppEEm

W

WZ

WwF

W

MM

rGM

cos

1sin

1

2

2/1



GeV025.0398.80 WM

Example of fit to MW,T: from CDF. Overview on various MW determinations:

Current world average of MW:

Aim LHC: Errors less than 15 MeV !



Example of fit to MW,T: from CDF. Overview on various ΓW determinations:

Aim of Tevatron Run 2: W < 40 MeV per experiment


Also indirect determination of W mass: Measure R:

This allows extraction of CMK parameter Vcs:

pQCD LEP SM EW

)(

)(

)(

)(

)(

)(

)(

)(

W

lW

llZ

Z

Z

W

llZBRXZpp

lWBRXWppR

CDF: R(e+μ) = 10.92 ± 0.15stat ± 0.14syst DØ : R(e) = 10.82 ± 0.16stat ± 0.28syst

Vcs = 0.967 ± 0.030

ΓW = 2078.8 ± 41.4 MeV (CDF) ΓW = 2118 ± 42 MeV (World)

ΓW = 2.0921 ± 0.0025 GeV (theory)

W WIDTH INDIRECT TEVATRON

topnoW

SSSWW

02

qq

320 V77.12409.1133


W MASS + WIDTH LHC

GeV

GeVM

W

W

06.0140.2

025.0398.80

Also MW and ΓW at the LHC: Huge statistics!

10-100 W bosons produced per second aim for precision measurement! Remember Tevatron:

Aim at LHC: <15 MeV errors.Needed (if top mass error <2 GeV) so that MW does not dominate error on Higgs mass determination!Systematics in electron, muon channels largelyuncorrelated; dominated by energy scale+resolution, and muon pT resolution.

Assumed electron ET

spectrum in Weν

That’s what a signal could look like!Achieve 20 MeV precision with first 10 fb-1?


Remember AFB in e+e- collisions: Asymmetry allowsimportant tests of couplings and Weinberg angle!

ELECTROWEAK PHYSICS: ASYMMETRIES (1)

qB

qF

qB

qFqq

FBA

221

2

VfAf

VfAff gg

ggA

The asymmetry is caused by the V-A structure of the EW Z interactions: eeγ/Zff

22

2

,0

sin411

sin412

4

3fefff

fefff

ffef

FBQ

QAAAA

fef

FB AAA4

3,0


Remember AFB in e+e-: The measurement was best done in eebb events because of the slow variationof Ab with sin2θ

Best way to access Ae and thus the effective Mixing angle for electrons!


fef

FB AAA4

3,0

Tevatron not really able to compete here – mainlybecause of statistics – would need 10 fb-1!How about the LHC? Study the phenomenon in Drell-Yan process ppZll:

A

BA

BAsd

d

FB 8

3

)0(cos)0(cos

)0(cos)0(cos

coscos12cos

22

p p

l+

l-

θ

AFB

- Sensitive to new physics via new terms or interference.- Possible access to PDFs (sea!)

221

2

VfAf

VfAff gg

ggA


Especially at Mee > LEPII AFB is very interesting – sensitivity to multi-TeV resonances (like Z’!). Example here: different Z’ couplings, masses!

Precise measurement of AFB will reveal structure beyond SM! First results from Tevatron Run II so far it looks like SM!


Rosner, J.L.: Phys. Rev. D 54, 1078 (1996) Rosner, J.L.: Phys. Rev. D 54, 1078 (1996)

Mee [GeV/c2]

AFB

uuee


Alternatively: Consider W charge asymmetry:

… with rapidity y:

Predictions for two different PDF sets:


uddu

uddu

dyddyd

dyddydyA

WW

WWW

//

//)(

z

z

pE

pEy ln

2

1

These differences can be explained in terms of the different valence quark parametrisations in the two PDF sets:

y y

A A

SVV

VV

qdu

duyA

2)(

ud

ud–

–

log(x)

xf(

x,

Q2)


CTEQ and MRST valence distributions at two different values of Q2 = MW

2:

This indicates potential to further constrain PDFs!


However, we only measure the lepton asymmetry, not directly the W charge asymmetry:

Convolution of production asymmetry with V-A structure

)(

)(~

//

//)(

xu

xd

dddd

ddddA

ll

lll


Newer measurement does not confirm discrepancybetween data and NLO theory (with full PDF uncertainty indicated as band):

Is there something interesting hidden? Only further data from Tevatron or input from LHC can clarify!

CDF measurement in small data sample:

Consider large data errors at high lepton rapidities! Can be improved by LHC! Also PDFs least constrained for high y Potential to constrain the PDFs there!!!



SM: triple and quartic gauge boson couplings (con-sequence of non-abelian structure of underlyinggauge group SU(2)LxU(1)Y):

… but no γZZ vertex!

In pp reactions, the following diagram with WZ final state is unique, since it allows separation of the WZZ and WWγ vertices!

EW PHYSICS: MULTI-BOSONS VERTICES

All this can be seen from the Lagrangian:

Write this in more generic way allow for SM extensions (only W vertices, simplest extension):

SM: g, κ, λ=1. measurement of κγ, λ,gZ is powerful test of SM.

Also: Relation W couplings and static W features:

WWZig

ZWWWWig

WWAig

AWWWWigL

W

W

W

Wgauge

ˆcos

ˆˆcos

ˆsin

ˆˆsin

WWW

WW

WeW

Me

MeQ

21:moment dipole mag.

:moment quadrupole el. 2


For studies of multi-boson vertices (TGC, triple gauge coupling) select events with two gauge bosons, especially via leptons (W,Z).

Examples (signals + backgrounds):

EW PHYSICS: MULTI-BOSONS EVENTS

Non-SM!

At LHC: mainly WZ and ZZ measurements!


Example 1: W+Z selection in about 2 fb-1 (CDF):

EW PHYSICS: MULTI-BOSONS: TEVATRON

Example 2: Zγ final states with Zll. Question: Do we observe a non-SM ZZγ contribution? Answer: No!

So far:

– TGC observed and measured. – Limits on non-SM couplings derived. – Cross-sections for diboson production measured with good accuracy.


Very precise samples can be selected already in verysmall data sets at the LHC (5σ statistical significancealready for 150 pb-1 of data in the WZ channel, includingsystematic uncertainties!)

LHC will allow for detailed analyses of the TGCs!.

EW PHYSICS: MULTI-BOSONS: LHC

Also photon channels will be included!


Remember V-A coupling of leptons in the SM with coupling strength g:

Question: Do all leptons e, μ, τ have the samevalue of g – as predicted by the SM? Measurablefor example in Zll or Wlν events – possibly withcorrections for τ mass etc.

LEPTON UNIVERSALITY

0.040.99 g

g

0.0120.998 g

g

e

e

.)0.011(syst)0.004(stat0.998 g

g

e

CDF preliminary results (note BR(Wlν ~ g2(ν)):

CDF preliminary from R(μ)/R(e)=g2(μ)/g2(e):

WLWR

LRW

QIcQc

ccg

i

gi

23

2

55

5

sinsin2

1

2

1

cos

2

1

2

Nature seems to obey lepton universality!


FUTURE OF EW SM MEASUREMENTS

Run I Run II 2 fb-1

Run II 15 fb-

1

LHC

Wlν 77k 2300k 17250k Huge

Zll 10k 202k 1515k Huge

ΓW [GeV] 2.158 ± 0.042

± 0.040

δsin2θW (AFB) ± 5.1x10-4 ±4x10-4 ±1.4x10-4

MW [GeV] 80.451 ± 0.033

± 0.030 ± 0.017 ± 0.01

δMH/MH (ind.) > 50% 35 % 25 % 18 %

uhh ss09: lhc the large hadron collider machine, experiments, physics sm physics (at the lhc)...

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