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Luminosity measurements with dimuon and single muon reconstruction of Z 0 and W decays

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Luminosity measurements with

dimuon and single muon reconstruction of Z0 and W decays

M. Poli Lener

- OUTLINE:
- LHCb apparatus & trigger;
- Theoretical uncertainty of Z0 and W production cross section;
- Pythia settings and MC samples;
- Performance of the dimuon luminometer (Z0 );
- Performance of the single muon luminometer (W & Z0 );
- Conclusion

Details of this work are published in CERN-THESIS-2006-013

XI Spring School - "Bruno Touschek"

40 MHz

Level-0:

pT of

m, e, h, g

Rough pT ~ 20%

Calorimeters

Muon system

Pile-up system

380 mrad

1 MHz

Vertex Locator

Trigger Tracker

Level 0 objects

Level-1:

Impact parameter

15 mrad

40 kHz

HLT:

Final state

reconstruction

Full detector

information

2kHz output

LHCb spectrometer

- Two approaches have been investigated to perform luminosity measurements at LHCb by measuring:
- vertices of beam-gas interaction through the VELO detector (*)
- event rates of physical channels with a well known and sizeable cross section

(*) L. Ferro-Luzzi, CERN-PH-EP/2005-023

Luminosity measurements at LHCb

Motivations:

Relative Luminosity:

- Correct for systematic effects
Reconstruction and trigger efficiencies

- Control the stability of the hardware
- Stability of colliding beam conditions

Absolute luminosity:

- Measure (and publish) cross section:
- bb inclusive production

- prompt charm

- weak boson production

- constrain Parton Distribution Functions from

EW processes

- Measure absolute BR of Bs

MRST 99, 00 PDF sets

NNLO QCD

5%

x B.R. (nb)

x B.R. (nb)

5%

x B.R. (nb)

W B.R.(W) 10 xzB.R.(Z+-)

W.L.van Neerven et al., Nucl. Phys. B382 (2000) 11

W.J. Stirling et al., Eur. Phys. J. C18 (2000) 117

Theoretical uncertainty

Two physical channels are investigated to perform an “on-line” luminometerat LHCb due to theoretical accuracy (~ 4%) and sizeable cross sections at s = 14 TeV

V

annihilation

Compton scattering

V

V

QCD radiation

(LO)

(NLO)

(NLO)

(NLO)

V

QED radiation

Pythia settings

- The diagrams for the boson V (Z0 and W) production are:

- PDF CTEQ4Lis used
- The initial state radiation are switched off
- Only Z0 neutral current
interference in matrix elements of Z0/* and * are disabled

- A polar angle 400 mrad is required to the leptons decaying from bosons

The annual signal yield will be, assuming Lint =2 fb-1

(1 y =107s & <L > = 2x 1032 cm-2 s-1):

Sz = Lintx 2 tot x (Z x B.R.) 2

where:

2 tot= (genx recx selx trig) 2

(Z x B.R.) 2 2 nb

S1 = Lintx 1tot x ( x BR) 1

where:

1tot= (genx recx selx trig) 1

( x BR) 1tot= (Z x BR) +(W x BR) 22 nb

The performances of these two physical processes can be compared

Monte Carlo Samples

Single muon coming from W and Z0 decay (single-muon luminometer)

50 kevents of W± ±

5 kevents of Z0 (with a not reconstructed)

Z0 +- decay process

(dimuon luminometer)

25 kevents of Z0

Performance of the dimuon luminometer

Acceptance: 4 vs 400 mrad

Future work

For the next, I will assume the W and Z decays have

the same geometrical acceptance efficiency

In order to evaluate the generation efficiency (gen) in [0, 400] mrad a small pre-production of 4 kevents of Z0 +- have been generated in 4

1 vs 2

All the production cross section have been evaluated at NNLO (*)

Dimuon selection algorithm

The strategy of the selection algorithm is a compromise between

a high efficiency on the signal and a large rejection of the background sources

- The signal is represented by a couple of muons (lnL> -8) with:
- opposite charge
- low significance (IP/IP) < 5
- high pT > 10 GeV/c

Background processes

60

These cuts together with the large di-muon invariant mass are able to totally rejects~15x106 of minimum bias and ~8x106 of b inclusive events.

The first two physical channels (Z0 +- & ttW+ W-) are not yet generated: Z0 +- decay could be rejected requiring an IP cut due to c(tau) ~ 100 m, while the ttW+ W- contribution to the signal is at most ~4‰ considering their cross section x B.R.

8 pb against 2 nb of the signal

(*) N. Kidonanakis et al., hep-ph/0410367

380 mrad

16 mrad

Asymmetric distribution due to radiation in the final state

Dimuon invariant Mass GeV/c2

Dimuon luminometer efficiencies & results

The total signal efficiency 2 tot= (genx recx selx trig) 2 can be computed

Performance of the single muon luminometer

Background processes

All the production cross section have been evaluated at NNLO(*)

Single muon selection algorithm

The signal is given by single muon events coming from

W or Z0 (with a not reconstructed)

3520

320

The first three physical channels (W,Z0 +-, ttW+ W-) are not yet generated:

W and Z0 +- decays could be rejected requiring an IP cut due to c(tau) ~ 100 m, while the ttW+ W- contribution to the signal is at most ~ 4‰ considering their cross section x B.R.

~70 pb of the BG against ~ 22 nb of the signal

The minimum bias events are not taking into account because ~99% events are rejected with the previous “smooth” selection cuts

(*) N. Kidonanakis et al., hep-ph/0410367

- Signal
- Background

- Signal
- Background

To achieve a systematic uncertainty below 4%, a S/B ratio > 25 is needed

22 nb

pT spectra

IP/IP

500 b

conservative pT > 30 GeV/c

Single muon selection algorithm

The single muon selection algorithm is applied on

background (~ 8x106 bb inclusive) andsignalevents

Selection

- vertex reconstructed with IP/sigma < 3
- pT cut

Single muon luminometer efficiencies & results

The total signal efficiency 1tot= (genx recx selx trig) 1can be calculated

To perform an “on-line” luminosity measurement with an uncertainly < 4%, 700 events must be collected during data taking

~ 31/2 hours

~ 45 minutes

Comparison of luminosity measurements

The performances of thesetwo samples can be compared

S1 = Lintx 1tot x ( x BR) 1

with:

1tot= 6.1 %

( x BR) 1tot= 22.13 nb

Sz = Lintx 2 tot x (Z x B.R.) 2

with:

2 tot= 14.3%

(Z x B.R.) 2 1.86 nb

The final annual yield (Lint= 2 fb-1) is

5.3x105selected & triggered events

bandwidth of 53 mHz

Z0 +- event every ~ 20 s

2.7x106selected & triggered events

bandwidth of 270 mHz

Z0 or W muon decays every ~ 4 s

Spares

XI Spring School

A new L1 specific algorithm, based on a IP < 0.15 mm and a pT > 10 GeV, is introduced in the L1Decision package (v4r5)

L1 Trigger algorithms

XI Spring School

L1 Trigger algorithms & results

- The addition of the new L1 specific algorithm, called low IP muon
- reaches a L1 efficiency on the Z0 signal up to ~ 85% comparable tothat obtained with other dimuon processes such as theB0s→J/(µµ)
- requires a limited bandwidth in order to not upset the L1 streaming. The bandwidth can be computed looking at the muons coming from the bb inclusive events which pass L0&L1 trigger (without any selection cuts)
- a negligible value of ~ 50 Hzis obtained

XI Spring School

Used by the dimuon luminometer

HLT Trigger data flow

XI Spring School

Parton Distribution Function

- New PDF sets have been recently updated considering the more recent data from H1 and ZEUS at HERA and CDF and D0 at Tevatron:
- Alekhin(*)
- CTEQ6(**)
- MRST2004(***)
- ZEUS2005

All these PDFs estimate an uncertainty on the Z and W boson production cross sections of 2÷3 %

(*) S.I Alekhin, hep-ph/0508248

(**) J.Pumplin et al., A.D. Martin et al, hep-ph/0201195

(***) A.D. Martin et al, hep-ph/0507015

XI Spring School

Pythia results

==========================================================

I I I I

I Subprocess I Number of points I Sigma I

I I I I

I------------------------------------------------I---------------------------------I (mb) I

I I I I

I N:o Type I Generated Tried I I

I I I I

==========================================================

I I I I

I 0 All included subprocesses I 7047 120254 I 3.010E-05 I

I 1 f + fbar -> Z0 I 2088 10232 I 8.917E-06 I

I 15 f + fbar -> g + Z0 I 2596 72079 I 1.127E-05 I

I 19 f+ fbar -> gamma + Z0 I 29 743 I 1.260E-07 I

I 30 f + g -> f + Z0 I 2334 37200 I 9.787E-06 I

I I I I

==========================================================

XI Spring School

<x1> = 0.1

<x2> = 2.5*10-4

Parton momentum distributions

From S. de Capua PhThesis http://sdecapua.home.cern.ch/sdecapua/

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UP & UPbar distributions vs PDF sets

(Q2=104 GeV2)

XI Spring School

DOWN & DOWNbar distributions vs PDF sets

(Q2=104 GeV2)

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STRANGE & CHARM distributions vs PDF sets

(Q2=104 GeV2)

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BOTTOM & GLUON distributions vs PDF sets

(Q2=104 GeV2)

XI Spring School

- Signal
- Background

2 track

Single muon selection algorithm

The single muon selection algorithm is applied on

background (~ 8x106 bb inclusive) andsignalevents

- Pre-selection
- particles identified as muons lnL> -2 (standard lnL> -8 )
- well reconstructed tracks 2-track < 2.5

- Signal
- Background

lnL hypothesis

XI Spring School