Table Of ContentElectroweak and beyond the Standard Model results from HERA
9
0
0 Z. Zhanga (for the H1 and ZEUS Collaborations)
2
aLaboratoire de l’Acc´el´erateurLin´eaire, Universit´e Paris-Sud 11 et IN2P3/CNRS, BP 34, 91898 Orsay
n
Cedex, France
a
J
0 The latest results from the e±p HERA collider both within the Standard (electroweak) Model and beyond
3 are reviewed. Most of the results are based on the full HERA data sample, which corresponds to an integrated
luminosity of about 0.5fb−1 perexperiment (H1or ZEUS).
]
x
e
-
p
e 1. INTRODUCTION 2. DOMINANT INCLUSIVE DIS PRO-
h CESSES
[ The electron-proton collider HERA ended its
data taking on June 30, 2007. Over its lifetime At HERA, both the neutral current (NC) and
2
ofmorethan15yearsofoperation,ithasallowed the charged current (CC) interactions can be
v
2 eachexperiment,H1andZEUS,tocollectaninte- studied. Thecrosssectionsoftheseprocessesmay
6 gratedluminosity of about100pb−1 and20pb−1 be written in the following simplified form:
6 in positron-proton and electron-proton collisions
4 α2
respectively in the first phase (HERA-1) from σ Φ (PDFs,v ,a ,P ), (1)
2. 1992to2000. Inthesecondphase(HERA-2)from NC∝Q4 NC q q e
081 2h0a0v3e tinoc2re0a0s7e,dthbeypaosfiatrcotonraonfd2elaenctdro1n0sraemsppelecs- σC±C∝G2F 11+±QP2e 2ΦCC(PDFs), (2)
: tively. In addition, spin rotators installed at the (cid:16) MW2 (cid:17)
v
H1 and ZEUS interaction regions have provided
where α and G are the fine structure constant
i F
X longitudinalelectronandpositronpolarizationat
andFermiconstantforelectromagneticandweak
HERA-2.
r interactionsrespectively,v anda arevectorand
a The results reviewed in the talk concern both q q
axial-vector couplings of the Z boson to light
theelectroweak(EW)measurementsintheStan-
quarksq (u,d),P isthepolarizationvalueofthe
e
dard Model (SM) and searches beyond the SM. electronbeam,Q2 isthefour-momentumtransfer
They are based either on the dominant inclusive
squared, M the propagator mass of the W bo-
W
Deep Inelastic Scattering (DIS) processesof neu-
son,andΦ arethestructurefunctionterms
NC,CC
tral and charged current interactions or on rare
forNCandCCprocessesrespectively,whichpro-
exclusive processes such as single W and lepton
vide the primary constraint on the parton distri-
pair productions. The advantage in the former
bution functions (PDFs) of the proton.
case is high statistics and in the latter case clean In NC interactions at high Q2, when Z ex-
and well defined final states.
change and γZ interference become increasingly
While the emphasis is put on presenting the
important, the NC cross section depends both
latest results, some early results are also shown
on v and a and on the beam polarization P .
q q e
in a few cases in order to illustrate the progress
However, these dependencies cannot be factor-
thathasbeenmadeovertheyearsandtheimpact
ized, contrary to the P dependence of the CC
e
of the increased data samples. cross section. In addition, from the Q2 depen-
dence and the normalizationof the CC cross sec-
tion, one can determine the propagator mass of
the W boson and the Fermi constant G respec-
F
1
2 Z. Zhang
tively. section in e−p interactions.
With an increased e+p data sample of
2.1. The propagator mass of the W boson 47.7pb−1 taken in 1994-1997 by ZEUS, a simul-
The first evidence of the massive W boson at
taneous fit of M and G was performed re-
W F
HERA wasobtainedfromthe observeddeviation sultinginM =80.8+4.9+5.0+1.4GeV,wherethe
W −4.5−4.3−1.3
of the CC cross section from a linear dependence
threeerrorscorrespondtothe statistical,system-
onthebeamenergybasedontheveryfirstdataof
aticandPDFsuncertaintyrespectively[4]. When
theH1collaborationtakenin1993corresponding
fixing G to the world average value [5], ZEUS
to an integrated luminosity of 0.35pb−1 [1]. The obtainedFM = 81.4+2.7 2.0+3.3GeV, where
electron beam of 26.7GeV in collision with the W −2.6 ± −3.0
the last error from the PDFs dominates over the
proton beam of 820GeV resulted in a center of
statistical and other systematic errors.
mass energy √s of 296GeV.1 This is equivalent
In all above results, the correlation between
toanelectronbeamwithenergyofalmost50TeV
the EW parameters with the PDFs was not fully
hitting a fixed target.
taken into account. A first analysis which con-
The first quantitative determination of the
sidered such a correlation was performed by H1,
propagator mass of the W boson, MW = 76 using the complete high Q2 NC and CC cross
±
16stat 13systGeV, was derived from the Q2 de- section data and all available low Q2 data from
±
pendence oftheCCcrosssectiondσ /dQ2 with
CC HERA-1 [6]. The result on the propagator mass
an e−p data sample of 0.54pb−1 [2]. Together wasM =82.87 1.82+0.30GeV, wherethefirst
with the NC cross section dσ /dQ2, it was W ± −0.16
NC error is experimental and the second the model
shownthattheQ2 dependenceoftheNCandCC
uncertainty [6]. Within the SM, a much more
cross sections are very different. At low Q2 the
precise W mass determination, M = 80.786
W
electromagneticinteractionismuchstrongerthan 0.205+0.063GeV,wasobtainedusingthe SMcon±-
−0.098
the weak interaction. The difference diminishes,
straintbetween M , G and other EW parame-
however, as Q2 increases, and the cross sections W F
ters
become comparablewhenQ2 values reachthe W
and Z boson mass squared, demonstrating the πα M2 1
G = Z , (3)
EW unification in DIS. F √2(M2 M2 )M2 1 ∆r
Z − W W −
The propagator mass determination in the
whereM isthemassoftheZ boson,and∆rrep-
space-like regime at HERA is complementary to Z
resentsradiativecorrectionswhichcontainamong
othermeasurementsusingtime-likeproductionof
othersaquadratictopquarkmasstermandalog-
W bosons at the Tevatron and at LEP and con-
arithmic Higgs mass term [6].
stitutes an important experimental consistency
The ten-fold increase of the e−p data from
check of the SM.
Based on an e−p (e+p) sample of 0.29pb−1 HERA-2 should substantially improve the final
(2.7pb−1), H1 has performed a similar deter- precision of the W mass determination from
mination of the W propagator mass: Me−p = HERA.
W
78+11+4GeV(Me+p =97+18+5 GeV),wherethe 2.2. Thelightquark couplingsto theZ bo-
−9−3 W −15−10
first error is statistical and the second system- son
atic[3]. Itisinterestingtonotethattheprecision Using the same HERA-1 data sample, H1 has
is better with the e−p data, albeit with a smaller also performeda combinedEW-PDFfit inwhich
integrated luminosity. This is because the W− the four light quark couplings vu, au, vd and ad
(W+) exchangeprobes mostly the u (d) quark in to Z have been determined taking into account
the proton resulting in a much larger CC cross theircorrelationwiththePDFs[6]. Theresulting
precisioniscomparablewiththosefromCDFand
1From1994on,theelectronbeamenergywasincreasedto
the combinedLEP/SLDEWfit. Inaddition, the
27.5GeV andfrom1998 on,the protonbeam energywas
alsoincreasedto920GeV resultingin√s=300GeV and HERA data can resolve the sign ambiguity seen
√s=318GeV. in some of the other determinations.
Electroweakand beyond the Standard Model results from HERA 3
The polarizedelectron beam at HERA-2 is ex- The preliminary results combining the H1 and
pectedto giveadditionalsensitivityto these cou- ZEUSHERA-2dataareshowninFig.2[10]. This
plings in particular the vector couplings. In- is the first observation of parity violation in NC
deed, including new preliminary NC cross sec- DIS at HERA.
tionmeasurementsbasedonpartofthe analyzed
HERA-2 data, a new fit has substantially im-
proved the precision [7]. The comparison for the
uquarkcouplingswiththeresultsofCDF[8]and HERA
LEP/SLD [9] is shown in Fig. 1. A 1
0.8 H1+ZEUS Combined (prel.)
0.6
ZEUS
u 0.4
v
11 ZtoEtaUlS u-pnocel-rvtu.-vd-au-ad-PDF (prel.) 0.2
uncorr. uncert.
0
H1 prel. (HERA I+II 94-05)
00..55 -0.2
-0.4
00 A+
-0.6 A-
H1 2000 PDF
--00..55 -0.8 ZEUS-JETS PDF
SM
CDF -1
68% CL LEP/SLD 103 104
--11 Q2 (GeV2)
--11 --00..55 00 00..55 11
a Figure2. MeasurementsofA± byH1andZEUS.
u
Theerrorbarsdenotethetotaluncertaintywhich
Figure 1. HERA results at 68% confidence level
is dominated by the uncorrelated error contribu-
(CL) on u quark couplings to Z in comparison
tions. The curvesdescribe the theoreticalpredic-
with those from CDF and LEP/SLD.
tions in NLO QCD as obtained in PDF fits to
the H1 inclusive data and to the inclusive and
jet ZEUS data, respectively. Both fits have been
2.3. Parity violation in NC DIS at high Q2 performed using the unpolarized HERA-1 data.
In the early HERA-1 data with unpolarized
electronbeam,differentNCcrosssectionsathigh
Q2 between e−p and e+p interactions were ob-
served, exhibiting the γZ interference and Z ex-
2.4. Limits on finite quark radius and con-
change contribution at high Q2. With the polar-
tact interactions
ized electron beam at HERA-2, additional dif-
NC interactions at high Q2 at HERA provide
ferences in the cross sections between the left-
an important test of the SM at the high energy
handed polarization (P ) and the right-handed
L frontier. Indeed, the SM Q2 spectrum is ex-
polarization(P )areexpectedfortheweakinter-
R
pected to be modified if the interacting electron
action of Z exchange. A polarization asymmetry
and quark have a finite size:
A± for e± beams can thus be introduced
2 σ±(P ) σ±(P ) dσ dσSM
A± = R − L . (4) NC = NCf2(Q2)f2(Q2), (5)
P P σ±(P )+σ±(P ) dQ2 dQ2 e q
R L R L
−
4 Z. Zhang
where f(Q2) = 1 hr2iQ2 is the electron or ZEUS
quark form factor, −r2 6being the mean squared 94-06 (prel.) e±p 330 pb-1
h i -1/L 2 best fit value +1/L 2 best fit value
radius of the EW charge distribution. Assum-
allowed ±1/L 2 range
ing a point-like electron, i.e. setting fe 1 as L - (TeV) L + (TeV)
≡
it has been well constrained elsewhere [11], and VV 8.0 6.3
using the full HERA (preliminary HERA-2 and AA 6.7 6.1
published HERA-1) data, an upper limit on the VA 3.2 3.5
X1 4.9 4.7
radiusofthe lightuanddquarks,Rq =qhrq2i< X2 5.5 4.7
0.75 10−18m, at 95% CL has been derived by X3 6.6 5.3
· X4 5.4 4.9
H1[12]. ThelimitR <0.62 10−18mfromZEUS
q X5 5.5 4.5
·
is similar [13]. X6 3.8 4.6
New interactions between e and q involving U1 5.8 5.4
mass scales Λ above the center of mass energy U2 6.5 5.3
U3 7.9 6.2
can also modify the SM NC cross section at high
U4 6.0 5.1
Q2 via virtual effects. Such interactions can be U5 6.6 5.0
modeled as four-fermion contact interactions as U6 4.1 5.1
aneffectivetheorybyadditionalterms inthe SM LL 4.7 4.2
LR 2.0 3.7
Lagrangian
RL 2.2 3.6
RR 4.5 3.7
= ηeq(e¯γµe )(q¯γ q ), (6)
L X ij i i j µ j -0.2 0 0.2
±1/L 2 (TeV-2)
where the sum runs over electron and quark he-
licities and quark flavors. In Eq.(6), only vector
Figure 3. Confidence intervals of 1/Λ2 at 95%
currents are considered as strong limits have al-
±
CL for the studied contact interaction scenarios.
ready been placed on scalar and tensor contact
interactions. The coupling ηeq may be written as Thenumbersattherightandleftmarginsarethe
ij corresponding lower limits on the mass scale Λ+
ηeq = ǫeq4π, where the coefficient ǫeq may take
ij ij Λ2 ij (η >0) and Λ− (η <0) respectively.
the values 1 or zero. In the HERA contact in-
±
teraction analyses, 19 different chiral structures
have been considered. The preliminary results
from ZEUS are shown in Fig. 3 [13]. The typical ticipates in the weak interaction, and the corre-
excluded scale is Λ> 5TeV at 95% CL. Compa- sponding cross sections thus vanish for P = 1
e
−
rable limits were achieved by H1 [14]. (e+p)andP =1(e−p). Theproductionofright-
e
handed CC would result in a deviationfrom zero
2.5. TotalCCcrosssectionsandconstraint
at these polarization values. The contribution of
on right-handed currents
aright-handedCCcanbe derivedfromlinearfits
According to Eq.(2), the CC cross section of
to the measuredcrosssectionsandfromextrapo-
purely weak interactions is expected to have a
lating the results of the fits to P = 1. The up-
e
linear dependence on the beam polarization P . ±
e per limit on the right-handed CC cross sections
The (preliminary) measurements of the total CC
can be further translated into lower mass limits
cross section for Q2 > 400GeV2 and inelasticity
on a right-handed W [15].
y < 0.9 with polarized HERA-2 data are shown
in Fig. 4 together with the corresponding cross 2.6. Other searches in inclusive DIS pro-
sections of the published HERA-1 data with un- cesses
polarized beams [15]. The measurements are in Many searches for new particles such as lepto-
good agreement with the expected linear depen- quarks(LQs),squarks(the supersymmetricpart-
dence. ners of quarks) and excited states of fermions
In the SM, only the left-handed W boson par- have been carried out at HERA, since the very
Electroweakand beyond the Standard Model results from HERA 5
most exciting moments happened in 1997 when
Charged Current e–p Scattering
bothH1andZEUSreportedanexcessofhighQ2
pb)pb)112200 e-p fi n X events basedon their 1994-1996data of less than
( (CC H1 2005 (prel.) 20pb−1 [19]. The excess generatedin a shortpe-
CC
110000 H1 98-99
ss riod of time a huge number of speculations on
ZEUS 04-05 (prel.)
ZEUS 98-99 possible new physics including resonant produc-
8800 e+p fi n X tion of LQs or squarks. However, the later high
H1 99-04
ZEUS 06-07 (prel.) statistics data samples collected by both experi-
ZEUS 99-00 mentsdidnotconfirmtheexcess,andconstraints
6600
on LQ production were derived.
CTEQ6D
An example for the existing constraints using
MRST 2004
4400
the full HERA data sample from H1 on a scalar
LQ,whichdecaysexclusivelyintoanelectronand
2200 Q2 > 400 GeV2 a quark,is presentedin Fig.5 [20]. The new pre-
liminary limitsignificantly improvesthe previous
y < 0.9
limit from HERA-1 [21]. For moderate Yukawa
00
--11 --00..55 00 00..55 11
coupling values λ of about 0.03 0.6, it also ex-
PPee tends wellbeyondthe directlimi−t fromthe Teva-
tron [22] and the indirect limit from LEP [23].
Figure 4. The dependence of the measured to-
tal CC cross sections on the e± beam polariza-
tion in comparison with the expectations from
CTEQ6D [16] and MRST 2004 [17].
Leptoquark Search, HERA I+II (449 pb-1)
l 11
beginning of data taking. Both H1 and ZEUS
haveperformedtheirfirstsearchusingthedataof
1ep99c2ololifdleerssisthananid3e0anlbm−a1ch[1in8e].tIonldoeoekd,fotrhreeHsoEnRanAt 1100--11 E xclu d e d S~1/2,L (e+d)
H1 prelim. single LQ
productionofnewparticles(LQs,squarks),which
H1 (94-00) single LQ
couple via Yukawa couplings directly to a lepton
and a parton, up to the center of mass energy. 2 D0 pair prod.
Thisisincontrasttothepair-productionine+e− 1100--22 OPAL indir. limit
at the LEP or in pp¯ at the Tevatron collider, in 220000 225500 330000 335500 440000
which case the mass reach for a direct search is M / GeV
LQ
up to half of the center of mass energy.
The searchfor the first generationLQs consid- Figure 5. Example of mass-dependent upper
ers its decay into an electron and a quark or into bounds on the Yukawa coupling λ of a first gen-
a neutrino and a quark. These LQ decays lead eration leptoquark coupling to ed (S˜ (ed)).
1/2,L
to final states identical to DIS events. One can-
not distinguish them on an event by event basis,
buthas to relyondifferentangulardistributions,
e.g.flatforscalarLQsandsteeplyfalling forDIS Searches for LQs coupling to first and sec-
events. ond generation fermions have also been per-
InthehistoryoftheHERArunning,oneofthe formed [24]. In this case, the final state lepton
is different from the incident electron(lepton fla-
2Beyond the kinematic limit,the search sensitivityarises
fromu-channelvirtualexchangeofnewparticlescoupling vorviolatingprocesses),andthesearchesarethus
tolepton-quarkpairs. essentially background free. The corresponding
6 Z. Zhang
constraints are also more stringentthan those on 3.1. Multi-lepton events at high PT
the first generation LQs. Using the full HERA data, H1 has com-
In supersymmetric (SUSY) models where the pleted [30] a study on the multi-lepton events of
so-called R-parity (R ) is not conserved, squarks which an excess was reported earlier based on
p
couldberesonantlyproducedatHERAsimilarto HERA-1 data [31]. Seven final state topologies
LQs. In addition to the LQ-like decays, squarks ee, µµ, eµ, eee, eµµ, eeµ and eeee have been an-
alsoundergodecaysintogauginos(thesupersym- alyzed. Inalltopologies,the predictedeventrate
metric partners of gauge bosons). A dedicated agrees with the number of observed events [30].
searchrequiresalargenumberoffinalstatestobe However, when a comparison is made requiring
analyzed. This has been pioneeredby H1 in [25], for the invariant mass of the two highest P lep-
T
wherethefullHERA-1datahavebeenusedtoset tons M > 100GeV, an excess is observed in
12
constraints on SUSY models. A similar analysis most of the topologies (Table 1), although the
looking for the SUSY partner of the top quark number of observed events remains statistically
has been performed by ZEUS [26]. limited. Demanding P >100GeVforthelep-
T
The observedreplicationof three fermion fam- tons, 5 (0) events arPe observed in the e+p (e−p)
ilies suggests the possibility that these fermions sample with 0.96 0.12 (0.64 0.09) expected.
± ±
may be composite particles, consisting of combi- Also shown in Table 1 are preliminary results
nations ofmore fundamental entities. The obser- from ZEUS on di-electron and tri-electron sam-
vationofexcitedstatesofleptonsorquarkswould ples [32]. In both samples no excess has been
be a clear signal that these particles are compos- observed.
ite rather than elementary. At HERA, excited
electrons, quarks and neutrinos (e∗,q∗,ν∗) with
masses up to the kinematic limit of the center
of mass energy could be produced directly via t-
channel exchange of a gauge boson.
The full HERA e±p data sample collected by 3.2. Search for doubly-charged Higgs
H1 has been used to search for excited elec- bosons
trons via the decays e eγ, e∗ eZ and Motivated by the observedexcess of the multi-
∗ → →
e∗ νW [27]. A similar preliminary search has lepton events discussed above, a search for
→
alsobeenperformedforexcitedquarksviathede- doubly-charged Higgs bosons (H±±) has been
cays q∗ qγ and q∗ qW [28]. The search for performed by H1 using the HERA-1 data [33].
→ →
excitedneutrinosdecayingviaν∗ νγ,ν∗ νZ Thesearchlooksforeventswithasinglyproduced
→ →
and ν∗ eW has been made using only e−p col- H±± decaying into a high mass pair of same
→
lisions as the corresponding cross section in e+p chargeleptons,oneofthembeinganelectron. No
is much smaller [29]. No evidence for excited evidence for H±± production is observed; out of
fermions has been found, and limits on the char- theobservedmulti-leptoneventsfromtheHERA-
acteristic couplings have been derived [27–29]. 1 data, only one di-electron event is compatible
with the H±± signature. Mass dependent up-
per limits are derived on the Yukawa coupling
3. RARE EXCLUSIVE PROCESSES
h ofthe Higgs bosonto anelectron-leptonpair.
el
The large data sample of HERA has made it Assuming that the doubly-charged Higgs decays
possible to use rare exclusive processes with well onlyintoanelectronandamuonviaacouplingof
measured and isolated final states for perform- electromagnetic strength h =√4πα =0.3, a
eµ em
ing EW measurements and for searching for de- lower limit of 141GeV on the H±± mass is ob-
viations from the SM. Two examples are multi- tained at 95% CL. For a doubly-charged Higgs
leptoneventswithhightransversemomenta(P ) decaying only into an electron and a tau and a
T
and isolated lepton events with large missing coupling h = 0.3, masses below 112GeV are
eτ
transverse momentum (P ). ruled out.
T
6
Electroweakand beyond the Standard Model results from HERA 7
Table 1
The number of observed events and SM expectations in different multi-lepton topologies for M >
12
100GeV. The numbers shown in parentheses correspond to the contribution from the dominant pair
production in γγ interactions.
Topology H1 ZEUS
Data SM (pair) Data SM (pair)
ee 3 1.34 0.20 (0.83) 2 1.7 0.2 (0.9)
± ±
eµ 1 0.59 0.06 (0.59)
±
eee 3 0.66 0.09 (0.66) 2 1.0 0.1 (1.0)
± ±
µµ 1 0.17 0.07 (0.17)
±
eµµ 2 0.16 0.05 (0.16)
±
Table 2
The number of observed events and SM expectations for three types of leptons in e±p data samples for
PX >25GeV. The SM signal (dominated by W production) is shown in percentage in parentheses.
T
Dataset Lepton H1 [36] ZEUS [37]
Data Exp (signal) Data Exp (signal)
e 9 4.32 0.71 (82%) 3 4.0 0.6 (77%)
± ±
e+p µ 8 3.70 0.63 (92%) 3 3.4 0.5 (81%)
± ±
τ 0 0.82 0.12 (46%)
±
e 1 3.18 0.58 (70%) 3 3.2 0.5 (69%)
± ±
e−p µ 0 2.40 0.41 (92%) 2 2.3 0.4 (85%)
± ±
τ 1 0.68 0.11 (31%)
±
3.3. Isolatedleptonevents at large missing responding signal contribution (purity) for the e
PT and µ channels is also shown in Table 2.
Sincethe observationofthefirstisolatedmuon
event with large P by H1 in 1994 [34], searches
6 T 3.4. Single W production cross section
havebeenpursuedandextendedbyH1andZEUS
From the previous section, we noticed that the
toallthreetypesofleptonsandwithincreasingly
isolated lepton events with large P are domi-
larger data samples [35]. The results [36,37] 3 6 T
nated by the SM W production. It is possible,
with the full HERA data are summarized in Ta-
usingthesamesample,tofurtherenhancetheW
ble 2. H1 has observed an excess in both the
signal with additional selection cuts and to mea-
e and the µ channel. The excess is, however,
sure the W production cross section.
not confirmed by ZEUS. ZEUS has, on the other
The extracted production cross section from
hand, observed 2 isolated τ lepton events with a ZEUS [37] is σ(ep lWX) = 0.89+0.25(stat)
hadronic final state (X) at large transverse mo- → −0.22 ±
0.10(syst)pb at √s = 316GeV corresponding to
mentum(PX >25GeV)for0.20 0.05expected,
T ± the luminosity weighted mean of center of mass
basedonthe HERA-1 data, of which49%is con-
energiesofthe differentdatasamples. The corre-
tributed by the SM signal events from single W
sponding theoretical expectation is 1.2pb which
productionwitha genuineisolatedτ andmissing
has anerrorof15%due to the uncertaintyof the
transversemomentuminthefinalstate. Thecor-
next to leading order corrections.
The corresponding measurement from H1
3The preliminary results of H1 shown at the workshop
σ(ep lWX)=1.14 0.25(stat) 0.14(syst)pb
have been replaced by the published one. The same is
→ ± ±
truefortheresultsshowninsections3.4and4. is quoted for √s = 317GeV and is in agreement
8 Z. Zhang
with the prediction of 1.27 0.19pb [36]. ment with the SM and compatible with anoma-
±
BothmeasurementsfromZEUSandH1havea lous single top production via flavor changing
significance of about five standard deviations. neutral current (FCNC, sec. 3.6).
3.5. W polarization fractions at HERA 3.6. Single top production
To test the compatibility of the observed W The production of single top quarks is kine-
decays with the SM, a measurement of the W matically possible at HERA. The signature of a
boson polarization is performed by H1 [36]. The top decay to b and W with subsequent decay of
measurement is based on the cosθ∗ distribution theW intheleptonicelectronandmuonchannels
in the decay W e/µ+ν, where θ∗ is defined wouldbealeptonandmissingtransversemomen-
→
as the angle between the W boson momentum tum. This signature coincides with that of the
in the lab frame and that of the charged decay observed isolated lepton events discussed above.
lepton in the W boson rest frame. The angu- Thesearchisperformedforanomaloustopquark
lar distribution is expected to have three distinct productioninanFCNCprocessinvolvingthecou-
terms, 3/4F ( 1 cos2θ∗), 3/8F (1 cosθ∗)2 pling κ . The dominant process for SM single
0 − tuγ
− − −
and 3/8F (1+cosθ∗)2 with F , F and F cor- top production at HERA has a cross section of
+ 0 − +
responding to the longitudinal, left-handed and less than 1fb and is treated as background.
right-handedfractionsrespectively,ofwhichonly The limits from HERA [38,39] are compared
two are independent as F +F +F 1. with other experiments [40,41] in Fig. 7. The H1
0 − +
≡
The results on the polarization fractions F limitisbasedonthefullHERAdata,whereasthe
0
and F are obtained from a simultaneous fit to ZEUSlimits areobtainedfromthe HERA-1data
−
the measured angular distribution and shown in taking into account not only κ but also v
tuγ tuZ
Fig. 6. The measurement is found in good agree- both in top production and decay. ZEUS also
studied the dependence of the limits on the top
mass. It is found that the limits are more strin-
gent for a smaller top mass value, as expected.
W Polarisation at HERA
1
F− H1 H1 Data
EPVEC 4. GENERIC SEARCH AT HIGH PT
ANOTOP Different from specific searches presented so
far, a generic and model-independent search for
deviations from the SM prediction for all high
68% CL
0.5 transversemomentumfinalstateconfigurationin-
N
ot volvingelectrons(e),muons(µ),jets(j),photons
all (γ)orneutrinos(ν)wasperformedbyH1firstus-
o
95% CL we ing the HERA-1 data [42] and now including the
d
HERA-2 data [43]. Events areclassifiedindiffer-
entclasses. Eacheventclasscontainsatleasttwo
0 finalstateobjects(e,µ,j,γ,ν)withP >20GeV
0 0.5 11 T
F in the polar angle range 10◦ < θ < 140◦. All
0
0.5 F eventclasses with observeddata events orwith a
0 SMexpectationgreaterthan0.01eventareshown
Figure 6. Preliminary W boson polarization for all H1 e+p data in Fig. 8. In a further step,
(point) at 1 and 2σ CL (contours). Also shown a statistical algorithm is used to search for devi-
are the values for the SM prediction (triangle) ations from the SM in the distributions of the
and anomalous single top production via FCNC scalar sum of transverse momenta or invariant
(square). mass of final state particles.
Electroweakand beyond the Standard Model results from HERA 9
H1 General Search at HERA (e+p, 285 pb-1)
jjj---jjj
| | H1 Preliminary (HERA I+II) mmmeee------jjjjjj
uZuZ nnn---jjj
vvtt eee---nnn
|| Excluded eee---eee
11 eee---mmm
mmm ---mmm
ggg---jjj
ggg---eee
ggg---mmm
Excl. by ZEUS ggg---nnn
ggg---ggg
jjj---jjj---jjj
Excl. by CDF mmmeee------jjjjjj------jjjjjj
nnn---jjj---jjj
eee---eee---jjj
eee---eee---nnn
eee---eee---eee
mmm ---mmm ---jjj
Excl. by L3 eee---mmm ---mmm
1100--11 mmmeee---mmm---mmm---nnn---jjj
k tcg = vtcZ = 0 mmmeee------nnnnnn------jjjjjj
mt = 175 GeV eee---mmm ---nnn
ggg---jjj---jjj
1100--11 11 ||kk ttuugg || gggggg------eeennn------jjjjjj
eee---jjj---jjj---jjj
nnn---jjj---jjj---jjj
ggg---nnn---jjj---jjj
eee---nnn---jjj---jjj H1
Figure 7. Exclusion limits at 95% CL on the ggg---eee---jjj---jjj
eee---eee---nnn---jjj
anomalous κtuγ and vtuZ couplings obtained by eee---mmmjjj---jjj------nnnjjj------jjjjjj H1 Data
the H1, ZEUS, CDF and L3 experiments. The eee---jjj---jjj---jjj---jjj SM
nnn---jjj---jjj---jjj---jjj
charm quark couplings κtcγ and vtcZ are ne- jjj---jjj---jjj---jjj---jjj
glected, and the limits are shown assuming a top 10-210-1 1 10 102 103 104 105
mass m =175GeV. Events
t
Figure 8. The data and the SM expectation for
all event classes with observed data events or a
5. SUMMARY SM expectation greater than 0.01 event. The er-
rorbandsonthepredictionsincludemodeluncer-
The HERA ep collider is first of all a precision
taintiesandexperimentalsystematicerrorsadded
QCDmachineprovidingdominantconstraintsfor
in quadrature.
partondistributionfunctionsatlowx. Itisalsoa
highenergycollidercomplementarytoLEPe+e−
and the Tevatron pp¯ colliders allowing rich elec-
troweak measurements as well as many searches observedbyH1ontheisolatedelectronandmuon
for new particles to be performed. eventswithlargemissingtransverseenergy,which
There is a good prospect to achieve much im- has around 2.4 standard deviations. This excess
proved electroweak results in the next two years is, however, not confirmed by a similar analy-
or so before the next Ringberg workshop. The sis from ZEUS. The various limits derived from
W propagator measurement will be mainly im- the HERA data are often comparable to or more
proved using the ten-fold increases in the e−p stringent than those from LEP and the Tevatron
data sample at HERA-2. The light quark cou- experiments.
plings to the Z boson will be further constrained
using the complete polarized HERA-2 data.
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