PHYSICS WITH LHC EARLY DATA

PHYSICS WITH LHC EARLY DATA

ONE OF THE LAST „PROPHETIC” TALKS ON THIS SUBJECT – HOPEFULLY

We may have some two month of the Machine operation in 2008

LONG HISTORY ...

I will extensively use:

 

 Fabiola GIANOTTI – CERN, ATLAS  2004  

  

Wouter VERKERKE – NIKHEF,  AOW Paris 2005

  Fabiola GIANOTTI – CERN, ATLAS 2007 

http://www.njp.org/

 10.1088/1367­2630/9/9/332

  Fabrice HUBAUT ­  CPPM/IN2P3, ATLAS & CMS I lepsza referencja

  Pamela FERRARI ­ATLAS & CMS asXiv hep­ex 0705.3021v2

  Albrert De ROECK – CERN,  KITF Seminar, 1 Apr 2008

  Anna KACZMARSKA – IFJ PAN – Institute Seminar 16 Feb 2008

What energy at the start ?

900 GeV

10 TeV

12 TeV

14 TeV

LHC machine status and schedule

A rather extensive meeting took place on 3

rd

March between the DG R Aymar, J Engelen

and ATLAS represented by M Nessi and PJ

During this meeting we were informed about the machine status, we had a chance to

explain our schedule, and to give input to the LHC start-up strategy discussions

The main conclusions, which are still valid, were:

- The current planning of the LHC is such that the machine is expected to be cold by

by mid-June, and first injections could start soon after, from end June onwards

- CERN Management will give ATLAS a warning signal 2 months before we have to start

closing, currently expected by mid-April (around RRB) when the progress on the machine

will be known from the cooldown status of further sectors

- Experience from power tests in Sector 4-5 showed that several magnets need training,

starting somewhere above 5 TeV equivalent

- The first physics run (typically 2 months) in 2008 will be at 10 TeV (or slightly above), the

energy will be defined in April when experience from Sector 5-6 will also be available

Similar statements were also made during this Council Week, in addition it was said that

One can follow cooling states of sectors almost on line ...

Some LHC parameters

COLLISION ENERGY

7

TeV

INJECTION ENERGY

450 GeV

PROTONS PER BUNCH

1.1

₁₀

11

NUMBER OF BUNCHES

2808

NOMINAL BUNCH SPACING

25

ns

NOMINAL LUMINOSITY

₁₀

34 cm

-2

s

-1

REVOLUTION TIME

88924

µs

(REVOLUTION FREQUENCY

11.2455 kHz) *

STORED BEAM ENERGY

336 MJ

BUNCH LENGTH

75.5 mm

LUMINOSITY LIFETIME

10 h

ACCELERATION TIME

20

min

WHAT IS THIS ALL FOR ?

      

 

Flagships:

 ­ Higgs searches

 ­ physics beyond the SM  

supersymmetry

extra dimensions

 ­ precision physics (W, top, Higgs mass, ....)

 ­ b­physics

 ­ quark­gluon plasma

BUT – it is NOT the first day physics

. These goals and unprecedented complexity

of detectors (ATLAS and CMS) require first:

 ­ commissioning of detectors and triggers, first calibrations and alignment with

minimum bias and QCD jet production,

 ­ measure of some exclusive channels (e.g. Z ­> lepton lepton) to set the absolute

electron and mion ECAL energy scale

 ­ measure tt events to determine the absolute jet energy scale and understand

b­tagging in the inner detectors

   

Recently, prof. Peter W. Higgs has visited

the ATLAS detector.

Some say, that it is going to be the only

Higgs seen in ATLAS

Muon system:

(precision

chambers + triggers in air

core toroids B=4T mean

value) ,

σ

/pT ~ 7 % at 1 TeV

standalone

(e.g. H,A



µµ, H



4µ

)

Hadronic Calorimeters: (Fe scint

Cu-liquid argon)

σ

/E ~ 50%/

√

E

⊕

0.03

Jet, E

_Tmiss

performance

(e.g.

H

→τ τ

, H

→

bb)

Inner Detector:

(Silicon

pixels + strips

+TRT



particle ID (e/

π

) )

B=2T ,

σ

/p

_T

~ 4x10-4 p

_T

⊕

0.01

(e.g. H

→

bb)

Electromagnetic calorimeter : (Pb, liquid argon)

σ /E ~ 10%/√E , Uniform longitudinal segmentation

Provides: e/γ identification, energy and angular resolution, γ/jet , γ /π0 separation (e.g. H→γγ)

ATLAS

Length ~45m weight ~7000 t

H

e

ig

ht

~

2

2

m

„business plan”

A solid candidate

for the very early

valuable result:

it is enough to collect

10 000 events to

Considerable statistics

„easily

vailable”

to study and estimate

absolute scales

for

electron and mion

ECAL measurements

as well as for the

absolute jet energy

10 pb

-1 ATLAS preliminary

1 pb

-1 ATLAS preliminary

J/

ψ → µ µ

Y

→ µ µ

→

alignment of mion spectrometer,

momenta calibrations for the ID and mion

After all cuts:

~ 160 Z

→

µ µ

ev./day at L = 10

31

(assume 30% data taking efficiency)

First picks: rediscovery of the „known physics” to demostrate the detector

understanding

After all cuts:

~ 4200 (800) J/

ψ

(Y)

→

µ µ

ev./day, L = 10

31

LHC 

 ~850 pb

σ

10% qq, 90% gg

Tevatron 

 ~7 pb

σ

85% qq, 15% gg

Top production and decay at LHC

FASCINATING ROLE OF ttbar EVENTS : ONE GETS TO INTERESTING PHYSICS

SIMULTANEOUSLY CALIBRATING MANY HIGHER LEVEL RECONSTRUCTION

CONCEPTS SUCH AS JET ENERGY SCALES, B­TAGGING AND MISSING ENERGY

I have learned that from Wouter Verkerke at Paris AOW in 2005 !

σ

tt

(

tot

)

=

759±100 pb

N

_evt

~ 700/hour

3 2 1 2 1

;

~

10

ˆ

=

sx

x

x

x

−

s

gg



tt

qq



tt

Top physics at LHC

Large ttbar production cross section at LHC

Production:

gluon

dominated at LHC,

quark

dominated at Tevatron

Top topology

Decay products are 2 W bosons

and two b quarks.

About 99.9% decays to Wb

For commissioning studies consider events

with one hadronic and one semi-leptonic

W decay (about 30% of the total ttbar cross

What can we learn from ttbar production?

–

Abundant clean source of b jets

●

2 out of 4 jets in event are b jets 

 O(50%) a priori purity

(need to be careful with ISR

 and jet reconstruction)

●

Remaining 2 jets can be kinematically

identified (should form W mass) ­>

possibility for further purification

What can we learn from ttbar production?

–

Abundant source of W decays into light jets

●

Invariant mass of jets should add 

up to well known W mass

●

Suitable for light jet energy scale 

calibration (target prec. 1%)

–

Caveat: should not use W mass in jet

assignment for calibration purpose

to avoid bias

●

If (limited) b­tagging is available,

W jet assignment combinatorics

greatly reduced

What can we learn from ttbar production?

–

Known amount of missing energy

●

4­momentum of single neutrino in each

event can be constrained from event

kinematics

–

Inputs in calculation: m(top) from Tevatron, 

b­jet energy scale and lepton energy scale

How to identify ttbar events?

4 hard jets

(P

_T

>40 GeV)

1 hard lepton

(Pt >20 GeV)

Missing E

_T

(E

_T

>20 GeV)

●

Restrict ourselves to basic (robust) quantities

–

Apply some simple cuts

–

Hard pT cuts really clean up

sample (ISR). 

–

Possible because

of high production rate

Combined efficiency of requirements

is ~5%      still have ~10 evts/hour

→

TOP CANDIDAT E

W CANDIDATE

m(top

_had

)

m(W

_had

)

Signal­only distributions (Full Simulation)

•

Clear top, W mass peaks visible

●

Background due to mis­assignment of jets

– Easier to get top assignment right than  to get W assignment right ●

Masses shifted somewhat low

– Effect of (imperfect) energy calibration

L=300 pb

-1 (~1 week of running)

S

m_top = 162.7±0.8 GeV MW = 78.1±0.8 GeV

Jet energy scale

calibration

possible from

shift in m(W)

S/B = 1.20

m(top

_had

)

m(W

had

)

Signal + Wjets background (Full Simulation)

S/B = 0.45

L=300 pb

-1 (~1 week of running) S/B = 0.27

Jet energy scale calibration possible from shift in m(W) ●

Plots now include W+jets background

–

Background level roughly triples

–

Signal still well visible

–

Caveat: bkg. cross section quite uncertain

TOP CANDIDAT E W CANDIDATE

L=300 pb

-1 (~1 week of running) TOP CANDIDAT E W CANDIDATE

m(top

_had

)

m(W_had)

m(top

_had

)

S/B = 0.45 S/B = 1.77 ●

Now also exploit correlation between 

m(top

_had

) and m(W

_had

)

–

Show m(top

_had

) only for events 

with |m(jj)­m(W)|<10 GeV

m(top

_had

)

m(top

had

)

S/B = 0.45 _{S/B = 1.11}

L=300 pb

-1 (~1 week of running)

|m(jl

ν

)-m

_t

|<30 GeV

•

Can also clean up sample by with 

requirement on 

m(jlν)

[semi­leptonic top]

– NB: There are two m(top) solutions for each  candidate due to ambiguity in reconstruction of pZ  of neutrino ●

Also clean signal quite a bit

– m(W) cut not applied here

Signal + Wjets background (Full Simulation)

TOP CANDIDAT E W CANDIDATE W+jets (background) ‘random jet’,  no b enhancement expected  ttbar (signal) ‘always b jet if all jet assignment are OK’ b enrichment expected and observed

AOD b-jet probability

AOD b-jet probability

Clear

enhancement

observed!

Exploiting ttbar as b­jet sample (Full Simulation)

●

Simple demonstration use of ttbar sample to 

provide b enriched jet sample

–

Cut on 

m(W

_had

)

 and m(top

_had

) masses

–

Look at b­jet prob for 

4

th

 jet

 (must be b­jet if 

If we are lucky we may have 

some interesting use of the LHC 

machine already by this winter.

Any way, come to 

Kraków EPS Conference 

next year