Observation of single-diffractive W production with CMS: a feasibility study

$Page 1: Observation of single-diffractive W production with CMS: a feasibility study$
Observation of single-diffractive W production with CMS: a feasibility study

Antonio Vilela Pereira

(Università degli Studi di Torino & INFN Torino)

On behalf of the CMS Collaboration

DIS 2008 - London

Outline

1. W selection

2. Observation of single-diffractive (SD) W signal

3. Background & Systematics

NB: No pile-up Effective luminosity for single-interactions – 100 pb-1

(Large Rapidity Gap)

08/04/2008

2 DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

Motivation

1. Rediscover hard-diffraction at LHC

2. Sensitive to quark component of diffractive PDF’s (dPDF’s)

3. Probe Rapidity Gap Survival Probability (S2) – connection to multiple partonic interactions and soft rescattering effects

Ratio of diffractive to inclusive W’s measured at Tevatron (~1%)

(Large Rapidity Gap)

dPDF

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…

(cf. K. Goulianos’s talk)

W selection

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Overall efficiency: Pomwig SD W: 34% (2.4k evts/100pb-1) Pythia W: 28% (600k evts/100pb-1)

W selection: (same as inclusive analysis)1 isolated pT > 25 GeV 50 < MT < 200 GeV|| < 2.0 = - (,ET

miss) < 1 radn jets(ET > 40 GeV) 3 n muons(pT > 20 GeV) < 2

NB: Diffractive W: Pomwig v2.0beta (dPDF: NLO H1 2006 Fit B)

Forward detectors

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DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

5

(5.2 < || < 6.6)

Hadronic Forward (HF)

CMS

(5.2 < || < 6.6)

(3.0 < || < 5.0)(3.0 < || < 5.0)

(|| > 8.1)

CASTOR

CASTOR

Services routing:

From Castor to Racks

Patch Panels

T2 Services routing:


Patch Panels

T2

TOTEM T2TOTEM T2ZDC ZDC

(|| > 8.1)

Hadronic Forward (HF)

TOTEM T1

TOTEM T1

Not present in start-up

TOTEM: Total Cross Section, Elastic Scattering and Diffraction Dissociation at the LHC

Selecting diffractive events

Generated particles – energy weighted

Diffractive sample generated with gap in –plus side

• Diffractive events historically selected through Large Rapidity Gaps (LRG’s) in (forward) hadronic final state

• Select diffractive events based on multiplicities in CMS Forward Hadron Calorimeter (HF) and/or CASTOR

• In addition, look at activity in central tracker

For selection: exploit different multiplicity distributions in forward and central regions (cf. Tevatron, HERA)

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Observation of diffractive signal

1. Selection of Rapidity Gap side:

• Observed gap side: side with lowest energy sum in HF (HF-plus vs HF-minus)

2. Pre-selection on central Track Multiplicity

• Maximum number of reconstructed tracks in central tracker (|| < 2.0, pT > 0.9 GeV)

3. HF tower multiplicity in “low ” vs. HF tower multiplicity in “ high ” region

4. HF tower multiplicity vs. CASTOR sector multiplicity in gap side

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CASTOR CASTOR

HFHF

CMS

ZDC (|| > 8.1)

ZDC

(3.0 < || < 5.0)

(5.2 < || < 6.6)

HF multiplicity at “low ” vs HF multiplicity at “high ”

“low ”: 3.0 < || < 4.0

“high ”: 4.0 < || < 5.0

Excess in low multiplicity visible

Non-diffractive background clusters in the low multiplicity (signal) region

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HF tower multiplicity vs CASTOR -sector multiplicity in gap side

• CASTOR: (5.2 < || < 6.6)

• no segmentation, 16 -sectors

• Generator level information: number of -sectors with energy above threshold (E > 10 GeV)

• For start-up (2008), installed only in one side (z-minus)

• Reject events in which observed gap side is not the z-minus (CASTOR) side

Potential signal increase by factor ~2 with CASTOR in both sides Clear peak at low

multiplicity

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Effect of central track pre-selection

Diffractive signal less visible when no track pre-selection is present – but still visible

~ 100 – 200 events in zero multiplicity bin (S/B ~ 6 - 20)

Increased non-diffractive background when loosening the track pre-selection

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Backgrounds

i) Non-diffractive W ii) Inclusive QCD events

- Applied full selection to sample of “-enriched” QCD sample (Pythia)

- Inclusive W selection: ~ 50k events/100pb-1 (< 10% of inclusive W rate)

- Negligible contribution at low HF/CASTOR multiplicity (< 1% level wrt diffractive W yields)

iii) Proton-dissociative events: ppXN (with X containing a W boson and N a low mass state into which the proton diffractively dissociates)

- Cross section expected to be of the same order as signal

- If N escapes detection (limited forward coverage), it’s an irreducible background (but it’s also diffraction)

- Around 50% can be rejected by vetoing on ZDC

- Expect enhancement in signal region by ~ 30% (cf. S. Ovyn’s talk)

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http://cms-physics.web.cern.ch/cms-physics/public/DIF-07-001-pas-v4.pdf

Systematics

i. Rapidity Gap Survival Probability (S2):

- Assumed 5% (as indicated by theory)

- Ndiff S2

- Extraction of diffractive yields provides S2 measurement

ii. W selection and reconstruction: from inclusive analysis (cf. S. Bolognesi’s talk)

- Same for diffractive and non-diffractive

- Negligible effect on acceptance

iii. UE description:

- Generator level study: Pythia Tune DWT (default) vs Tune A

- Selection applied on full simulated Alpgen W events

- Potential background enhancement by factor 3 – 5 in signal region

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http://cms-physics.web.cern.ch/cms-physics/public/EWK-07-002-pas-v8.pdf

Summary

• Procedure discussed to observe single-diffractive W production with an effective 100pb-1 for single interactions

• Diffractive selection based on forward hadron multiplicities in HF and CASTOR calorimeters, as well as multiplicity in central tracker

• For S2 = 0.05, expected O(100) reconstructed signal events per 100pb-1

• S/B of up to 20 with CASTOR

• This study emphasizes the need of CASTOR (ideally on both sides)

• TOTEM T1/T2 information (similar acceptance as HF/CASTOR) should add to the background rejection

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BACKUP

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MC samples

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Diffractive W samples:

• Pomwig v2.0beta• Proton PDF: CTEQ6M• dPDF: NLO H1 2006 Fit B (IP(0) = 1.111, ' = 0.06 GeV-2, BIP = 5.5 GeV-2) Non-diffractive samples:

• Pythia W• Pythia Inclusive QCD (-enriched)• Alpgen W

Cross sections:- Diffractive W: ~ 69 pb (NLO PDFs) (Assumes S2 = 5%)- Non-difractive W: ~ 21 nb (NLO cross section)

This gives a ratio of diffractive to inclusive cross sections of ~0.3%

distribution

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16

MX=(s)1/2

= p/p

Choosing the gap side

Fraction of times the gap side is chosen incorrectly

- Gap side: side with lowest energy sum in HF (HF-plus vs. HF-minus)

- Energy thresholds for counting HF towers minimizing the probability of choosing the gap side incorrectly

HF alone has limitedcoverage for gap sideselection

08/04/2008


HF tower multiplicity in gap side

• HF multiplicities in gap side for diffractive (Pomwig) vs non-diffractive (Pythia) W events

• Gap side: side with lowest energy sum in HF (HF-plus vs HF-minus)

• Pre-selection on number of reconstructed tracks in central tracker (|| < 2.0, pT > 0.9 GeV)

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Diffractive signal cannot be clearly distinguished in “data” sample

Effect on track pre-selection – HF “low ” vs HF “high ”

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19

Diffractive signal most visible when track pre-selection is tighter

> 200 events in zero multiplicity bin even for strict track pre-selection

S/B for zero multiplicity bin: O(1) or less

CMS Forward detectors

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20

(D. d’Enterria hep-ex/0708.0551)

(5.2 < || < 6.6)

CASTOR CASTOR

Services routing:


Patch Panels

T2 Services routing:


Patch Panels

T2

T2T2HFHF

CMS

(5.2 < || < 6.6)

(3.0 < || < 5.0)(3.0 < || < 5.0)

ZDC

(|| > 8.1)ZDC

(|| > 8.1)

Not present in start-up

CASTOR & ZDC

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CASTOR: Centauro And Strange Object Research

• Quartz tungsten sampling calorimeter

• 5.2 < || < 6.6 – charged & neutrals

• EM + HAD sections

CMS Zero Degree Calorimeter

• Quartz tungsten sampling calorimeter

• || > 8.1 - neutrals

• EM + HAD sections

Documents

Observation of single-diffractive W production with CMS: a feasibility study