Diffractive W/Z Bosons

Andrew BrandtUTA

•Run I Diffractive W/Z Boson Production Recap•Run II Preliminary Search for Diffractive Z Bosons (Courtesy of Tamsin Edwards, Manchester)

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Why study Diffractive W/Z Boson Production?

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Diffractive processes at the Tevatron

Diffractive processes are mediated by color singlet exchange:

one or both protons remains intact

color singlet: referred to as Pomeron, nowusually thought of as a gluon pair or gluon ladder..

e.g. single diffraction:

in hard diffraction, this can be high pT jets, W, Z...

Signature of diffractive events:

Rapidity gap - absence of particles or energy above threshold in some region of rapidityProton track -detection of intact p/p in Forward Proton Detector

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Tracking

North South

Finding rapidity gaps

Luminosity Monitors Forward Calorimeter

• Two arrays of scintillators close to beam pipe

• 2.7 < |η| < 4.4• Detect proton break-up• Currently in Run II the output is ‘on/off’ for each side (in Run I multiplicity available)

• off for diffractive

• Use the electromagnetic and fine hadronic layers in region behind LM• e.g. sum energy or count number of cells above threshold

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Run I Data Samples

Z boson sample: Start with Run1b Z ee candidate sample

Central and forward electron W boson sample: Start with Run1b W e candidate sample

hep-ex/0308032;Accepted by Phys. Lett B

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Observation of Diffractive W/Z

• Observed clear Diffractively produced W and Z boson signals

• Background from fake W/Z gives negligible change in gap fractions

Sample Diffractive Probability Background All Fluctuates to Data Central W (1.08 + 0.19 - 0.17)% 7.7Forward W (0.64 + 0.18 - 0.16)% 5.3All W (0.89 + 0.19 – 0.17)% 7.5All Z (1.44 + 0.61 - 0.52)% 4.4

ncalnL0

Diffractive W and Z Boson Signals

Central electron W Forward electron W

All Z

ncalnL0

ncal

nL0

DØ Preliminary

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Run I DØ/CDF Comparison

CDF {PRL 78 2698 (1997)} measured RW = (1.15 ± 0.55)% for ||<1.1 where RW = Ratio of diffractive/non-diffractive W (a significance of 3.8)

This number is corrected for gap acceptance using MC giving 0.81 correction, so uncorrected value is (0.93 ± 0.44)% , consistent with our uncorrected data value:

We measured (1.08 +0.19 –0.17)% for ||<1.1

Uncorrected measurements agree, but corrections derived from MC do not…

Our measured(*) gap acceptance is (21 ± 4)%, so our corrected value is 5.1% !(*) : derived from POMPYT Monte Carlo

Comparison of other gap acceptances for central objects from CDF and DØ using 2-D methods adopted by both collaborations:DØ central jets 18% (q) 40%(g)CDF central B 22%(q) 37% overallCDF J/ 29%

It will be interesting to see Run II diffractive W boson results!

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Run II Rapidity Gap System

• Use signals from Luminosity Monitor and Veto Counters (designed at UTA) to trigger on rapidity gaps with calorimeter towers for gap signal• Work in progress (Mike Strang UTA, Tamsin Edwards U. Manchester);

LM: 2.5 < < 4.4

VC: 5.2 < < 5.9

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Z bosons in the muon channel at DØ

Z0 → μ+μ- analysis Emily Nurse & Paul Telford DØ Note 423114th Aug 2003

Dataset:

• February - June 2003• Before this run period the muon triggers required fastz, which vetoes diffraction

Excluded data:

• Runs declared bad by SMT, CFT and muon groups• Bad luminosity blocks

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3627 Z candidates

• Two central tracks matched to two ‘loose muons’ • ‘loose’ - refers to quality of tracks in muon chambers

• Both muons have pT > 15 GeV • Muons have opposite charge• Di-muon invariant mass Mμμ > 30 GeV

• At least one muon is isolated in calorimeter and tracking detectors

• Timing difference of hits in muon chambers |Δt| < 13ns• Distance of closest approach < 0.16cm for both muon tracks • Event fired one of the di-muon triggers• Both muons are within |η| < 1.8 and not in muon chamber gap

Z bosons in the muon channel at DØ

Event selection and cuts:

to exclude bb events

to exclude cosmic rays

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Luminosity Monitor categories

Fastz on (N+S both on)

Gap N • N off and S on and fastz off

Gap S• S on and N off and fastz off

Gap SN• S off and N off and fastz off

Single diffractive candidates:

Double Pomeron Exchange candidates:

Non-diffractive candidates:

N and S on and fastz off(this can happen due to haloor fastz inefficiency)

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Invariant mass: LM gaps

All Z

fastz on

Gap S

Gap SNN+S onfastz off

3627

3420 40

111

48

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Gap N

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North EM Energy Sum

All Z

fastz on

Gap S

Gap SNN+S onfastz off

Gap N

E (GeV)

If Gap is on southexpect energy onNorth only

Little energy onside of LM gap

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Run II Forward energy: noise sample

E (GeV)

High energy tail:• hot towers/cells• real energy deposits (e.g. if LM inefficient)

95% of noise sample has an energy sum of less than 3 GeV on at least one side

Empty crossing data sample: No LM hits, No vertex

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Invariant mass: LM + calorimeter gaps

All Z

fastz on

Gap S

Gap SNN+S onfastz off

3627

3420 18

111

21

3

Gap N

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Comparing Z candidates

However, the issue is not really whether these are Z’s but whether these are diffractive Z’s. Gap definition isstill being defined

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Diffractive Z candidate event

Strong candidate for diffractive Z event:• LM N gap• calorimeter N gap• track in FPD on N side

Red: EM energyBlue: Hadronic energyYellow: missing ET (not including muons)

RZ view

XY view

muon hits in 3 layers (overwrapped in φ here)

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Diffractive Z candidate event

N side = outgoing anti-protonFPD system includes a dipole spectrometer on this side:• Two scintillating fiber tracker detectors inserted into beam pipe • Situated after the dipole magnet outside DØ• If proton loses longitudinal momentum in a diffractive exchange, it is bent inwards by the dipole magnet

Track position in detector 1 vs detector 2:

Vertical plane Horizontal plane

unbent vertically

bent horizontally

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Current status and plans

Gap definition: • Highest priority: we need strong distinction between noise and real energy:• Trying new variables (multiplicity of cells, clusters, towers above threshold)• Fixing/killing bad calorimeter data such as hot cells• Once standardized: make available to DØ to apply to any (all?!) analyses (anything you can do we can do diffractively)

Diffractive Z analysis: • Cannot progress without gap definition• Manchester will continue with Z →μμ analysis

• update diffractive search alongside this• aim to present at Moriond conference early 2004