the measurement of w ’s at the cern and fnal hadron colliders

RHIC-PV, April 27, 2007 M. Rijssenbeek 1

The Measurement of W’sat the CERN and FNAL hadron

colliders

• W’s at RHIC !• W’s at CERN – UA2• W’s at FNAL - CDF


W’s at RHIC !• Measurement of W’s in polarized-proton collisions at

RHIC:– measure the u and d-quark and anti-quark

contributions to the proton spin as function momentum fraction x.

– use the W charge and the V–A structure of W production & decay to eν and μν to select quark flavor and quark helicity.

( )d x ( )u x

For W–: u↔d


Non-Hermetic Detectors• RHIC detectors are not fully hermetic… • electron and muon acceptance of the RHIC

detectors varies strongly over the rapidity range…• Thus:

– missing transverse energy cannot be used to clean up the W signal

– Trigger has to rely exclusively on high pT electrons and muons

– The backgrounds to the W from Z→e/μ and QCD/fakes may be significant

– enough Z-statistics for measurement/tuning of corrections?

• Detailed simulations will be crucial to determine acceptance corrections, efficiencies, and backgrounds– limited Z acceptance makes tuning of the simulations with

Z→ee/μμ more difficult


a “Non-Hermetic” Detector: UA2 – vs.1• Central tracking + Preshower + EM

Calorimetry; |η|<1

• Forward spectrometer + PS + EM Calorimetry; 1<|η|<3

a non-hermetic detector…

UA2 collaboration: M Banner et al., Phys. Lett. 122B (1983) 476.

UA2 collaboration: P Bagnaia et al., Phys. Lett. 129B (1983) 130.


W and Z in UA2• strong quality selections on

electron candidates necessary:– isolation, shower shape,

preshower signal, track-PS-cluster match

– even so: cut on missing pT for final sample…

all good EM cluster pairs

+ Track & PS match

Z

W

for eTE p 0.8e

TT E

p e

TT E

eTM

eTM

W

vs and eTE


W and Z in UA2 – vs.2• 1987: UA2 Upgrade program with hermetic calorimetry

Tp

QCD

UA2 collaboration: J.Alitti et al., Z.Phys.C47 (1990) 11.


W‘s at Fermilab• the measurement of the W mass with a 0.05%

accuracy (50 MeV or 100×Me) requires the ultimate understanding of the detector!

simulations and cross checks & tuning with the data itself…


Simulations• the state of simulations of W and Z production

(and decay) has much advanced over the past decades– forced by very high statistics W/Z samples for mass

determination from LEP and Tevatron– much QCD calculational progress– improved detector simulations: showering – availability of raw computing power allows more detail

and increased sophistication

• State-of-the-Art: – RESBOS: NLO W and Z production – CTEQ6M: NLO pdf’s with uncertainties

Note: pTe/μ is quite sensitive to boson recoil

MT less so, but is sensitive to yW/Z

• CRUCIAL for all precision W/Z measurements


FNAL Example: CDF W Mass(from seminar by Dr. David Waters, UC

London)• CDF is a modern hermetic detector:

hermetic detection of e, γ, jets; less hermetic for μ

– recent MW measurement (e+μ) is single best in the world: MW

CDF = 80413 ± 34 (stat) ± 34 (syst) MeV

cfr: WA 2006: 80392±29 MeV

65 MT 90

2 (1 cos( ))e eT T TM p p

CDF Note 8665, Jan 17, 2007; CDF http://www-cdf.fnal.gov/


W/Z Production in pp & Decay Modeling

+ Corrections:Higher orders (EW,QCD)Non-perturbative

Leading Order picture:

dpp W / Z ll

[ f iq (x p ) f j

q (x p )i, ju,d ,s,(c,b ) f i

q (x p ) f jq (x p )]d

qq W / Z ll dx pdx p

rapidity distribution

dqq W / Z ll

(s , l,l ) couplings

1

(s - MW/Z

2 )2 (W/Z

s /MW/Z )2

angular & mass distributions :

p

p

W /Z

l

l

pT distribution


W Production Modeling: pT

• Use the best theoretical model on the market :– RESBOS NLO QCD + resummation + non-perturabtive.

• Constrain the parameters g1, g2, g3 and lineshape with the Z data:

W : 7 MeV

MW : 3 MeV

1 3 RE2 SBOS~ (1 ) ( , , )ZTZ

T

dp fB gg g

dp

g20.640.05

b 0.00140.0010 GeV-1

(Landry et al., 2003)


W→μν Backgrounds• mostly Z→μμ: easy to lose a muon (at CDF !)• But can estimate this background very reliably.


Decay-In-Flight Background in W→μν

• difficult background: very flat in transverse mass

• Use track quality: 2 and track impact parameter

fin

al

cu

t v

alu

e

/NDF

xxx

x

xx

x

x

K,fake

high-pT track

W : 27 MeV

MW : 5 MeV

Vary normalization &

shape:

Z provides the template for real

muons

High impact parameter cuts provide the DIF template


W→eν Backgrounds

Z’s ~ negligible

QCD background dominates


QCD Background in W→eν • Multijet events: large σ Rfake(jete)

Rfake(jetET)

fin

al

cu

t v

alu

e

QCD template from a

background-rich “anti-electron“

sample

W : 32 MeV

MW : 7 MeV

Vary normalization &

shape:


CDF W Mass 2007:• sample size analyzed: 200/pb

• event selection: pTe,μ>30 GeV, pT

ν>30 GeV

• Uncertainties in MW for fit to pTe/μ:


W/Z Physics result: Van Neerven Plot:

• MW/MZ with Mtop constrain the SM Higgs range; 200/pb

• Current data sets: 2/fb (CDF, DØ)

• FNAL Expectation: δMW≈ 30 MeV/expt


My Conclusions• A precise measurement of Δu and Δd will bring new

understanding of the spin structure of the baryons…

• However: in order to obtain the required precision, the measurement will need sophisticated simulations to understand and model the detector acceptance, efficiencies, and backgrounds – the physics and detector models must be tuned and

checked with measurements of the Z (<10% of W statistics)

– Dominant backgrounds must be measured with the data itself

these simulations must be done beforehand to prove the measurement capability with RHIC’s “non-hermetic” detectors…

the measurement of w ’s at the cern and fnal hadron colliders

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