Pair production of W Bosons at LEP2

kllll ELSEVIER

Nuclear Physics B (Proc. Suppl.)

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PROCEEDINGS SUPPLEMENTS

65 (1998) 109-113

Pair production of W Bosons at LEP2 P. Azzurria* aScuola Normale Superiore and INFN sezione di Pisa, Piazza dei Cavalieri 7, 56126 Pisa, Italy We report on the measurements of W pair productions achieved with the 1996 run by the four LEP Collaborations at centre-of-mass energies of 161 GeV and 172 GeV. Combining all possible leptonic and hadronic final states, partial and total WW cross-sections are measured and direct measurements of W Branching Ratios are extracted. Finally the W mass is derived in the framework of the Standard Model profiting from its remarkable dependence from the total cross-section at the kinematic production threshold of 161 GeV.

1. I N T R O D U C T I O N During the 1996 physics running period the LEP collider at CERN was operated above the nominal kinematic threshold of the e+e - - ~ w + w - reaction. The average integrated luminosities delivered to each Interaction Point were 12.1 pb -1 at V~ -- 161.33±0.05 GeV and 11.3 pb -1 at 172.11±0.06 GeV. With the collected data the first W pairs have been observed in all decay modes by the four LEP Collaborations [1-4]. The collected data samples are rather limited and correspond to approximately 30 and 100 W pairs per experiment at the two energies. From these samples the WW cross-sections at the two energy points have been derived and, combining the information from partial cross-sections, the first direct measurements of the W decay Branching Ratios have been obtained, in good agreement and comparable precision with previous indirect determinations at pp colliders [5]. The W Boson mass, row, is one of the key parameters of the electroweak theory. Combined direct measurements from single-W productions at pp colliders give a value m w = 80.35 ± 0.13 GeV/c2[6] , in agreement with the fit of all other electroweak data to the Standard Model which yields row= 80.352± 0.033 GeV/c2[7]. The Wpair production cross-section near its kinematic threshold depends strongly on mw so that men*On behalf of the ALEPH, DELPHI, L3 and OPAL Collaborations 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PlI S0920-5632(97)00985-7

suring aww in this region yields indirectly a measurement of mw , independent from the direct determinations that can be obtained by the kinematic reconstruction of the W decay products [8]. For a fixed luminosity and detector efficiency, the maximum statistical sensitivity to mw is obtained for a minimum av/'5-~-~lOmw/Oaww I factor, at the optimal threshold energy of V~ 2mw + 0.5GeV _~ 161GeV, that was chosen for this reason for the first LEP2 running period. In the following, the results given for the 161 GeV data are the published ones while the measurements determined at 172 GeV are still preliminary and subject to modifications. 2. W W P R O D U C T I O N SELECTIONS

AND

EVENT

To lowest order within the Standard Model, three diagrams contribute to W-pair production in e+e-annihilations, the s-channel ~/ and Zboson exchange and the t-channel ve exchange, as in figure 1 and referred to as CC03 diagrams.

w+ J / ' i

w*

h /,

e

L

Figure 1. W-pair production in e+e-collisions: Standard Model CC03 diagrams.

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P. Azzurri/Nuclear Physics B (Proc. Suppl,) 65 (1998) 109--113

Each W is expected to decay rapidly into a quark-antiquark pair (W- -+ ~d,~s,...) or a lepton-neutrino pair (W- -~ e-Ve, # - ~ , T-Pr). Therefore each CC03 diagram leads to an experimentally accessible four-fermion final state, but many other SM processes can lead to the same four-fermion final states as W-pair decays, interfering with the CC03 ones. In the following all cross-sections are to be interpreted as CC03 ones, defined by the production of four-fermion final states only through two resonating W bosons. The effect of the nonCC03 diagrams is corrected for with additive (ALEPH,OPAL) or multiplicative (DELPHI,L3) factors, obtained by comparing Monte Carlo simulations including only CC03 processes and the full four-fermion production. The selections of W-pair events are very similar at 161 and 172 GeV, as only some criteria are rescaled with the centre-of-mass energy. Up to v~ = 172 GeV, W's are produced with small boosts, so that the resulting four-fermion momenta are around 40 GeV each, and tend to be back-to-back in pairs. This peculiar kinematic configuration is fundamental for the selection criteria of all decay modes. 2.1. Fully leptonic decays When both W's decay in a lepton-neutrino pair (WW -4 g+ug-P) the event is characterised by two acoplanar energetic leptons and a large missing energy carried away by the corresponding neutrinos. In 5/9 of the events at least one lepton is a tau, giving rise to visible softer final states and additional missing energy. A low multiplicity of charged tracks is required in the selection as well as a large missing transverse momentum and acoplanarity A¢ in the plane transverse to the beam axis. Further cuts are applied to reduce radiative dileptons, 77 ~ g+g- and tau pairs backgrounds, leading to CC03 selection efficiencies around 70%. About 11% of W-pair decays are expected in this channel.

2.2. Semi-leptonic decays In this channel (WW ~ guq~) the primary neutrino carries about 40 GeV of energy and momentum, and in one third of the cases, when the

lepton is a tau, additional energy is lost in neutrinos from its decay. The key feature of these events is that the lepton, or the thin T decay jet, points in a roughly opposite direction to the missing momentum and is well separated from the two hadronic jets. In the event selection a large multiplicity of charged and calorimetric hadronic objects is required together with missing momentum out of the beam axis direction. Then an energetic identified isolated electron or muon is searched for, or a narrow tan-jet. Overall final CC03 selection efficiencies are 70-80%, where 44% of W-pair decays are expected to decay semi-leptonically. Main backgrounds are q~ events with energetic leptons from heavy flavour decays, and e+e - -~ ZZ*, Zee processes. 2.3. Fully hadronic decays

Figure 2. The first WW event seen at LEP at 161 GeV: a q'~q'~ decay in DELPHI.

About 45% of W-pair decays are expected to decay into four quarks. The resulting four jet events, as the one in figure 2, have no missing energy or momentum and are characterised by a large multiplicity of charged and neutral objects. The largely dominant background comes from non-radiative hadronic q~ events with hard QCD gluon emission and multi-jet spherical topologies.

P, Azzurri /Nuclear Physics B (Proc. Suppl.) 65 (1998) 109-113

After a simple preselection of hadronic nonradiative events with a four-jet topology, several variables axe built to discriminate signal events from QCD backgrounds. These variables are mostly based on the kinematics of the four jets, as inter-jet angles, y34 and di-jet masses, and also global topological variables, as the event sphericity. Since individual variables do not provide a good discrimination, different multivariable methods, as Neural Networks, have been employed to combine the information from each distribution and separate optimally signal events from backgrounds. For each method the WW hadronic cross-section is finally obtained by fitting the data output distribution shape as the one shown in figure 3. 20

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E]U.C. CQmkl 402 I M.C. b~,cground 15 30-

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sured one, which is subject to large fluctuations. Systematic errors are quite smaller that statistical fluctuations and come mainly from the hadronic channel, due to uncertainties on the amount of QCD background and on the distributions of the variables used to discriminate signal events.

Table 1 Summary of WW total cross-section measurements for the four LEP experiments. The first error is statistical and the second comes from systematic uncertainties. Experiment aww(pb) V~-- 161 GeV x/~ -- 172 GeV ALEPH 4.23 + 0.73 + 0.19 11 . . . . . .~+l.~y 1 . 2 4 5= 0.25 3 . ~7+0'97 5=0.19 11. . . .u~+1.54 DELPHI ... 0.85 . 1 . 4 3 5= 0.32 2 •~a+O.Sl 4- 0.14 12 "~9+1"5° 5= 0.38 L3 "'~--0.70 ~--1.41 3 . a9+o.93 5= 0.16 12.3 5= 1.3 ± 0.3 OPAL ,,,,_0.82 LEP comb. 3.69 4- 0.45 12.05 + 0.73

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0

In Y~

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,

0.4

0.6

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0.8

Neural---Netwo~ Output

~14

LEP Average

U12. Figure 3. One of the input variable distributions, Y34, and the final output for the Neural Net used by L3 to select WW -~ qqqq events at 172 GeV. Signal events contribution tend to have a higher output, close to 1.

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q ; 5 i~d" i ~ i i')6 i-)5 is0 3. W W CROSS-SECTIONS BRANCHING RATIOS

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W

The results from all WW decay channels have been combined by all experiments using maximum likelihood fits with Poisson statistics. The total WW cross-sections have been extracted assuming Standard Model W branching ratios in the fit. The results at the two energy points are summarised in table 1. The combination of the four LEP cross-sections was performed using the expected statistical error rather than the mea-

~/s lG-eV]

Figure 4. LEP combined WW cross-sections at 161 and 172 GeV compared to the GENTLE[9] theoretical predictions assuming the previous world average mw=80.356 GeV/c 2.

As it can be seen in figure 4, the combined LEP results for the total WW production crosssections are in good agreement with the Standard

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Model, given the previous electroweak determinations of the W mass. Alternatively the partial WW decay channels have been combined without assuming Standard Model branching fractions but constraining the sum of the hadronic and the three leptonic W branching Ratios to be unity. A fit to the ALEPH and L3 partial cross-sections, separated per lepton flavour, yields: B(W -~ eve) = 12.0 + 2.0 %,

(1)

B(W

(2)

pv.) = 10.3 4- 1.7 %,

Table 2 Summary of W mass measurements from the WW threshold cross-section for the four LEP experiments. Separated errors indicate statistics and systematics contributions. Experiment mw (GeV/c 2) ALEPH 80.14 4- 0.34 +0.09 DELPHI 80.40 + 0.44 + 0.09 L3 ~a aa+o.4s vv.vv_0.42 OPAL an aa+0.44 +0.09 ~'~v--0.41 --0.10 LEP comb. ~a t J v . An+o.~ 'zv_0.21 "4s = 161.33 :i:0.05 GeV

B(W -~ rv~) = 10.7 4- 2.2 %.

(3)

~-7

= 6

Assuming lepton coupling flavour universality to the W, the single hadronic W Branching Ratio has been measured by fitting the fully leptonic, semi-leptonic and fully hadronic WW cross-sections, as measured by ALEPH, DELPHI and L3, yielding: B(W ~ hadrons) = 67.0 4- 2.0 %.

o ~ = 3.69 ± 0.45 pb raw = 80.40_0~ GeV

~

~5

(4)

All such direct Branching Ratio determinations are in good agreement with the the Standard Model expectations B(W --+ hadrons) = 67.5%, B(W ~ tvl) = 10.8%, and with previous indirect measurements [5]. 4. W M A S S E X T R A C T I O N

The W mass has been extracted from the 161 GeV cross sections for the four LEP experiments using the theoretical Standard Model prediction for the WW cross-section as a function of the W mass, calculated with the GENTLE program [9]. The results for each experiment are showed in table 2. The LEP combined result has been extracted from the average 161 GeV cross-section, as showed in figure 5. The final result is

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79.5

80

kll 2 80.5

81

81.5

82

mw(GeV)

Figure 5. Total WW cross-section at threshold as a function of the W mass. The shaded band shows the LEP combined results.

MeV/c 2 to account for a 2% error on the theoretical GENTLE calculations. The sensitivity of the WW cross-section to the W mass at v/s = 172 GeV is not as pronounced as at threshold. Using the LEP averaged WW crosssection at 172 GeV to constrain the W mass with GENTLE, the result is mw = 80 •8+°'s -1.0 GeV/c 2. 5. C O N C L U S I O N S

~a Aa+O.22 GeV/c ~. mw = u~,--~v_0.21

(5)

The quoted error is dominated by statistics and includes 70 MeV/c 2 from the systematics on the cross-section determinations, 30 MeV/c 2 from the beam energy calibration uncertainty and 30

The successful increase of LEP energy above the W-pair production threshold has allowed the first observation of the e+e--~W+W - process. All standard W+W - decay channels have been observed and analysed, allowing the first direct

P. Azzurri /Nuclear Physics B (Proc. Suppl.) 65 (1998) 109-113

determinations of the W decay Branching Ratios. About 150 WW events have been collected in total at the threshold energy of 161 GeV allowing an indirect measurement of the W mass, through the total WW cross-section, with a precision comparable with previous p~ collider results. In the following years LEP2 is expected to provide much larger samples of W-pair events at higher centre-of-mass energies leading to major improvements in the determination of the W Branching Ratios and in the measurement of the W mass by the direct kinematic reconstruction of its decay products. REFERENCES

1. OPAL Collaboration, Phys. Lett. B389 (1996) 416. 2. DELPHI Collaboration, Phys. Lett. B397 (1997) 158. 3. L3 Collaboration, Phys. Lett. B398 (1997) 223. 4. ALEPH Collaboration, Phys. Lett. B401 (1997) 347. 5. CDF Collaboration, Phys.Rev. D52 (1995) 4784 6. Average presented by M.Rijssenbeek at ICHEP'96, Warsaw. 7. LEP/SLD Electroweak Working Group, CERN-PPE/96-183 (1996) 8. A.Tonazzo, these proceedings. 9. D.Bardin et al., Phys. Lett. B344 (1995) 383.

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