WUE-ITP-98-056

��

hep-

ph/9

8124

81

22 D

ec 1

998

WUE�ITP��

DAPNIA�SPP ��

High Q Physics at HERA and Searches for New

Particles

T� Matsushita�� E� Perez�� R� R�uckl�

� Department of Physics� University of Oxford� U�K�� ZEUS Collaboration� CEA�Saclay� DSM�DAPNIA�Spp� France� H� Collaboration� Institut f�ur Theoretische Physik� Universit�at W�urzburg� Germany

Abstract� Preliminary results fromH� and ZEUS on Deep Inelastic Scattering �DIS

at high momentum transfer squared Q� are presented� Used are all available e�p data

accumulated by the H� and ZEUS experiments between �� and �� corresponding

to integrated luminosities of �pb�� and ��pb�� respectively� The anomalies observed

at high Q� in the �� to �� data still remain� though with less signi�cance� Since

this high Q� domain represents a new frontier in DIS� the same data are used to

search for new particles possessing direct couplings to lepton�quark pairs� Assuming

that the slight excess of events observed in Neutral Current DIS is due to a statistical

�uctuation� preliminary limits on the production of leptoquarks and of squarks in

R�parity violating MSSM are presented�

Contribution to the �rd UK Phenomenology Workshop on HERA Physics� St� John�s

College� Durham� UK� September ��

High Q� Physics at HERA and Searches for New Particles

�� Introduction

Deep Inelastic Scattering �DIS� of leptons by nucleons at the ep collider HERA o ers

an unique possibility to probe the proton at very small distances �� cm�� The

domain of very high momentum transfer squared Q� is also a window where �rst signs

of physics beyond the Standard Model �SM� may manifest themselves�

Using �� to �� data� corresponding to a total integrated luminosity of about

�pb�� H� �� and ZEUS �� experiments reported an observation of an excess of

events in DIS at Q� � �� GeV�� This �nding stimulated a lot of experimental and

theoretical activity� Data accumulated during �� allowed to increase the statistics by

roughly a factor �� The measurements of DIS at high Q� based on all available e�p

data are presented and compared to the SM expectations in section � The same data

are then used to search for new particles possessing direct couplings to lepton�quark

pairs� namely leptoquarks and squarks in SUSY models where R�parity is violated � �Rp��

Preliminary results on leptoquark production are shown in section � together with

some remarks on particular phenomenological aspects at HERA� Analogous constraints

on resonant �Rp�squarks production are presented in section ��

�� Deep Inelastic Scattering at high Q� at HERA

�� Comparison of DIS data with the SM expectations

The full available e�p data have been used by H� and ZEUS experiments to update

the measurements of Neutral Current �NC� and Charged Current �CC� DIS presented

in �� The distribution in Q� of the NC DIS events measured by H� �� is shown

in Fig� �a �all data� L � ��pb�� and Fig� �c �� data alone� L � ��pb�� in

comparison to the SM expectation� Also shown in Fig� �b and Fig� �d are the ratios

of these measurements to theory� The curves above and below unity specify the ��

levels determined from the combination of statistical and systematic errors of the NC

DIS expectation� These errors are dominated by the uncertainty on the electromagnetic

energy scale of the calorimeter and vary between �� at low Q� and about �� at the

highest Q�� The NC DIS expectation agrees well with the data at Q� � �� GeV��

while at larger Q� deviations are observed with a slight de�cit of events between

�� GeV� and �� GeV� and a slight excess at Q� �� GeV�� The �� data

alone suggest similar deviations� although with a more marginal signi�cance� Table �

summarizes the numbers of events observed by both experiments and expected in the

SM with Q�z above various thresholds Q�min� At Q� � �� GeV�� ZEUS observes two

events while �� are expected� These events were reported previously �� In the ��

data� no additional events were found in this Q� domain�

z Q�e and Q�

�� mean Q� reconstructed by the electron and double angle method� respectively�

High Q� Physics at HERA and Searches for New Particles

Figure �� a Q� dis�

tribution of the observed

NC DIS events in com�

parison to the SM expec�

tation �histogram� �b

ratio of the observed

and expected number of

events as a function of

Q�� c�d same as �a�b

but for the �� data

alone�

Q2e (GeV2)

even

ts /

bin

H1 94 → 97 data

NC DIS (MRSH)

(a)

Q2e (GeV2)

dat

a / S

M (

MR

SH

)

H1 preliminary

94 → 97 data

(b)

Q2e (GeV2)

even

ts /

bin

H1 97 data

NC DIS (MRSH)

(c)

Q2e (GeV2)

dat

a / S

M (

MR

SH

)

H1 preliminary

97 data

(d)

1

10

10 2

10 3

10000 20000 30000

1

10

10000 20000 30000

10-1

1

10

10 2

10 3

10000 20000 30000

1

10

10000 20000 30000

Table �� Num�

ber of observed �Nobs

and expected �Nexp NC

DIS events with momen�

tum transfer squared Q�

above given thresholds

Q�min�

Q�min Exp� Nobs Nexp

Q�e � �� GeV� H� ��

Q�� GeV� ZEUS � ��

Q�e � �� GeV� H� � ��

Q�� GeV� ZEUS ��

�� Di�erential Cross Sections

Di erential cross sections in terms of Q� are derived by both experiments ��

For that purpose� the number of events in each Q� bin is corrected for acceptance and

migration� using Monte Carlo simulation� and for photon radiation and electroweak

e ects� using the HERACLES �� program� Fig� shows the NC and CC cross sections

d��dQ� obtained by H� and ZEUS�

For NC DIS� the rapid fall o of d��dQ� by about � orders of magnitude is on

the whole well described by the SM prediction� However� as already mentioned� the

High Q� Physics at HERA and Searches for New Particles �

Figure �� Dif�

ferential cross sections

d��dQ� for NC and

CC DIS� measured by

H� �squares and ZEUS

�points� using all avail�

able e�p data� The

curves represent the SM

predictions�

HERA e +p DIS cross section 94 - 97

Q2 (GeV 2)

dσ

/dQ

2 (

pb/G

eV2)

Preliminary

y < 0.9

Neutral Current

Charged Current

ZEUS

H110-7

10-6

10-5

10-4

10-3

10-2

10-1

1

103

104

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

103

104

0.5

0.6

0.70.80.9

1

2

3

4

103

104

105

Q2 / GeV2

Dat

a / S

M (

QC

D fi

t)

1

10

103

104

1

10

103

104

Q2 (GeV2)

dσ/d

Q2 /

dσ/d

Q2 (C

TE

Q4)

(b)

PDF band

0.8

0.9

1

1.1

1.2

103

1040.8

0.9

1

1.1

1.2

103

104

Figure �� Left� ratio of the NC DIS cross section measured by H� to the SM

prediction� Right� ratio of the CC DIS cross section measured by ZEUS to the SM

prediction� On both �gures� the inner bars represent the statistical error and the outer

ones the total error�

measurements slightly undershoot the SM expectation at Q� � �� GeV�� while

a slight excess of events is observed in the two highest Q� bins� The ratio of the

experimental �H�� and theoretical NC cross section is shown in Fig� �left�� Here� the

SM prediction is obtained from a NLO QCD �t performed by H� �� which includes the

new H� high Q� data as well as H� and �xed target charged lepton�nucleon data at lower

Q�� The inner error bars on the data points correspond to the statistical errors� while

the outer ones indicate the full error� the typical total systematic error being about ��

For CC DIS� the Standard Model also provides a very good description of the data

over most of the kinematic range as can be seen in Fig� � Only at the highest Q� the

data show a tendency to lie above the SM prediction� The ratio of the measured �ZEUS�


to the theoretical �CTEQ�� d��dQ� distribution for CC DIS is shown in Fig� �right��

The shaded region indicates the theoretical uncertainty due to the uncertainties in the

partons densities� As before� the outer error bars on the data points correspond to the

total error� The main source of systematic error is the calorimeter energy scale� It leads

to an error on the measured cross section of �� in most of the Q� range� except at

the highest Q�� where the error rises to about ��

The CC di erential cross section d��dx has also been measured by ZEUS and

compared to the SM prediction� One can see some tendency in the data to lie above

the expectation� If the CTEQ� parametrization of parton densities is modi�ed to give a

larger d�u ratio at high x �� the excess observed at high x relative to CTEQ� becomes

less signi�cant ��

�� Leptoquark Searches

�� Phenomenology

Leptoquarks �LQ� are scalar or vector bosons which carry both lepton and quark

quantum numbers� They appear in many extensions of the SM� At HERA� LQs may

be produced resonantly� via the fusion of the incident lepton with a quark or antiquark

coming from the proton�

A reasonable phenomenological framework is provided by the BRW model �� This

model is based on the most general Lagrangian that is invariant under SU��SU��U�� respects lepton and baryon number conservation� and incorporates dimensionless

couplings of LQs to left� and�or right�handed fermions� Under these assumptions LQs

can be classi�ed into �� isospin multiplets� half of which carry fermion number F � �

and couple to e� � q� while the other half carries F � and couples to e� � �q� The

F � � �F � � class contains �� scalar and �� vector multiplets� In order to

avoid stringent bounds from low energy experiments which would put LQs outside the

reach of present day colliders� couplings are allowed to only one combination of fermion

chiralities� and only within a single family� The couplings are generically called � and

are unknown parameters of the model� In the above model� �rst generation LQs only

decay into e� q and �e � q with �xed branching ratios� �eq�� q� � �� We will use

the nomenclature of �� to label the di erent LQ states�

In the following analysis� it is assumed that only one of the LQ multiplets is

produced at a time� and that the states within a given isospin multiplet are degenerate

in mass� At HERA� LQs can be resonantly produced in the s�channel and exchanged in

the u�channel� This is illustrated by the diagrams in Fig� �� Obviously� the s� and u�

channel processes do not interfere with each other� but each of these channels interferes

with the SM and Z exchange� The relevant matrix elements can be found in ��x�x The signs of the interference terms between standard model gauge boson contributions and fermion

number F � � leptoquark contributions given in �� are incorrect� The correct signs can be inferred

from �� and amount to multiplying the right�hand�side of Eqs ��d�e and ��d�e by ��


Figure �� Illustra�

tive diagrams for reso�

nant LQ production in

the s�channel �a and

u�channel LQ exchange

�b�

e+

d

LQ

e+

d(a)

e+ e+

LQ

d–

d–

(b)

Figure �� Ratio of the

cross section �� for s�

channel LQ production

and �nite LQ width �

to the narrow�width ap�

proximation�

λ = 1.0

λ = 0.1

λ = 0.5

MLQ (GeV)

σ Γ /

σ NW

A

e+ + d → S 1 / 2, L

~

λ = 1.0

λ = 0.1

λ = 0.5

MLQ (GeV)

σ Γ /

σ NW

A

e+ + u → S 0, L

–

1

10

100 150 200 250

1

10

10 2

100 150 200 250

For the allowed strength of the couplings and in the mass range interesting for

HERA� the decay width of a LQ into a lepton and a quark is generally small �

�S � ��V � ��M�� MeV for M � �� GeV and � � �� S and V referring

to scalar and vector states� respectively� In the narrow�width approximation �NWA��

the resonant production cross section is given by �NWA � ��s��q�x�� q�x� being the

probability to �nd the relevant quark q in the proton with a fraction x � M��s of the

proton momentum� In the above�ps � �� GeV is the collision energy in the ep centre

of mass and M is the LQ mass�

For high mass LQs� i�e� at high x� the parton densities in the proton are falling

very steeply� At the same time� the LQ width increases with M leading to important

tails at smaller values of x� As a result� the convolution of the parton density with the

corresponding Breit�Wigner distribution gives a considerable larger cross section than

the NWA� This is quanti�ed in Fig� �� for the typical range of couplings which can be

probed at HERA with the current luminosity� One can see that for F � � and � � ��

the deviation from NWA becomes noticeable at M � �� GeV� while for F � the

�nite width becomes non�negligible already at M � �� GeV�

Furthermore� the u�channel contribution can also become large especially for F �

LQs and e�p scattering where valence quarks are involved� A third e ect which plays

an important role at high M is the interference with SM and Z exchange� Depending

on the LQ type� the interference can be constructive or destructive� and it can get

comparable to or greater than the pure s� and u�channel contributions� In Fig� � the

relative magnitudes of the various terms in the high Q� domain are shown for the S��R�F � � and the �S��L �F � �� scalar leptoquarks� For the S��R� the u�channel and

the �positive� interference with SM DIS begin to dominate the s�channel cross section

at M � � GeV� For the �S��L� the u�channel contribution is negligible� but the


Figure �� Con�

tributions of the s�and

u�channel processes and

of their interference with

SM DIS for �a S��R�F � � and �b S��L�F � �� has been

�xed to �� a value

which is typical for the

experimental sensitivity

in this mass range�

σs + σu + σint

σs

σu

σint

(a)

F=2, Q2 > 10000 GeV2

MLQ (GeV)

σ (p

b)

σs + σu + σint

σs

- σint

(b)

F=0, Q2 > 25000 GeV2

MLQ (GeV)

σ (p

b)

10-1

1

200 220 240 260 280 30010

-2

10-1

1

10

10 2

200 220 240 260 280 300

�negative� interference becomes important for M � �� GeV�

�� Analysis

Resonant leptoquarks decaying into an electron and a quark jet lead to signatures which

resemble NC DIS events� However� the signal could be identi�ed as a resonance peak in

the invariant mass distribution� with events in the peak showing an angular distribution

speci�c to the spin of the leptoquark state� For example� the isotropic decay of a

scalar LQ in its CM frame leads to a �at spectrum in y� where y � �� cos ��

�� being the polar angle of the scattered lepton in this frame� This is remarkably

di erent from the steeply falling ��y� distribution expected at �xed x for the dominant

t�channel exchange in NC DIS events� Hence� a LQ signal should become more and

more prominent at high y �or high Q� � yM�� Consequently� it should be possible to

optimize the signal to NC DIS background ratio by a mass dependent lower cut on y

�or equivalently on cos �� Note that this optimal cut depends not only on the spin

of the LQ� but also on its fermion number� since the s� and u�channels� which lead to

di erent angular distributions� contribute di erently for F � � and F � �

The NC DIS candidates at Q� � �� GeV� observed by H� are shown in Fig� �a

in the �M y��plane� Here� the full dataset was re�analysed including a new in�situ

energy calibration for the electrons� This has led to slight migrations �within originally

quoted systematic errors� of individual events in �M y�� Fig� �b shows the projected

mass spectra� before and after applying the optimal y cut mentioned above� In the mass

window considered in �� of total width � GeV around �� GeV� and at y � �� events

are observed� while �� are expected from NC DIS� Out of these� � events originate

from the �� data and from the �� data� Hence� the �mass clustering

around � �� GeV observed previously is not con�rmed by the �� H� data alone�

ZEUS searched for a resonance in positron�jet mass Mej� without applying any

constraints� The Mej does not give the optimal resolution for LQ� however� it is free

from initial state QCD and QED radiation shifts� The mean reconstructed mass is

determined to be within �� of the generated value� whereas the peak is on average ��


Me (GeV)

y e

10000GeV 2Q2=2500GeV2

15000GeV 2

Q 2=25000GeV 2

H1 Preliminary

Data 97Data 97 Data 94 → 96

(a)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

50 75 100 125 150 175 200 225 250 275 30010

-1

1

10

10 2

10 3

75 100 125 150 175 200 225 250 275

Me (GeV)

evts

/ b

in

H1 data, ye > 0.1

H1 data, ye > ycut

NC DIS, ye > 0.1

NC DIS, ye > ycut

H1 preliminary

(b)

ZEUS 1994-97 Preliminary

Mej(GeV)

cos(

θ* )

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

(c)

250 300


Mej(GeV)

Eve

nts/

10G

eV

10-1

1

10

10 2

10 3

75 100 125 150 175 200 225 250 275

(d)

300

25

50

75

100

125

150

175

200

225

160 180 200 220 240 260 280 300

Figure � High Q� NC DIS candidates observed by the H� �a�b and ZEUS �c�d

experiments� �a Events in the �M� y�plane� Contours of �xed Q� are shown� �b Mass

spectra of the events before �triangles� dotted lines and after �circles� solid lines the

optimal y cut� �c Events in the �cos �� Mej�plane� Shaded is the removed region

which corresponds to the interface between calorimeters� Open circles show the high�x

and high�y events reported previously� �d Mass spectra of the events�

lower than the one obtained from a LQ MC� Fig� �c shows the distribution of ZEUS NC

DIS candidates in the �M cos ��plane� The �ducial cut applied on the positron which

excludes the interface region between the central and forward calorimeters is indicated

as the shadowed area� The projected spectrum of the invariant mass Mej is shown in

Fig� �d� It is interesting to note that the outstanding high Q� events discussed in ��

tend to cluster around Mej � �� GeV� At Mej � �� GeV and cos �� events

are observed which is in slight excess to the �� events expected from NC DIS� On the

other hand� for the events at Mej � �� GeV the angular distribution of the scattered

positron is found to be similar to that expected from the SM ��


MLQ (GeV)

λ 11

H1 Preliminary

S0, R

~

S0, RS1, LS0, L

SEA QUARKS

EXCLUDED

(a)

MLQ (GeV)

λ 11

S1/2, L

~

S1/2, L

S1/2, R

VALENCE QUARKS

EXCLUDED

H1 Preliminary

(b)

10-2

10-1

1

80 100 120 140 160 180 200 220 240 260

10-2

10-1

1

80 100 120 140 160 180 200 220 240 260

10-2

10-1

1

10

150 200 250

10-2

10-1

1

10

150 200 250

10-2

10-1

1

10

150 200 250

10-2

10-1

1

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150 200 250


SL0

SR0

S~ R

0

SL1

(a)

MLQ(GeV)

λ VL1/2

VR1/2

V~ L1/2

(b)

MLQ(GeV)

λ

SL1/2

SR1/2

S~ L1/2

(c)

MLQ(GeV)λ

VL0

VR0

V~ R

0

VL1

(d)

MLQ(GeV)

λ

Figure � Left � H� exclusion limits at �� CL on the Yukawa coupling � as a

function of the LQ mass �a for F � � and �b for F � � scalar leptoquarks� Right �

analogous limits from ZEUS �a for scalars with F � �� b vectors with F � �� c

scalars with F � �� and �d vectors with F � ��

�� Results

Assuming that the slight excess of NC events observed at high Q� is due to a statistical

�uctuation� H� and ZEUS have derived preliminary limits on the Yukawa coupling � as

a function of the LQ mass� for all LQ species classi�ed by BRW �� These are shown in

Fig� �� The discrepancy between the H� and ZEUS bounds for high mass scalar LQs is

due to the use of the NWA approximation by ZEUS in the calculation of the signal cross

section� while H� takes into account the LQ width by a Breit�Wigner distribution� For

Yukawa couplings of electromagnetic strength� �� EM � the existence of F � �

LQs is excluded up to M � �� GeV�

Relaxing the BRW assumptions� the H� Collaboration also obtained bounds in

the plane vs M � being the branching ratio for LQ � eq� for �xed values of the

Yukawa coupling� The results can be found in �� and in these proceedings �� For

� �� TeVatron experiments exclude �rst generation LQ masses below � � GeV ��

independently of any assumption on �� Nevertheless� for low � an important discovery

potential remains for HERA� For example� if � �� the D� experiment excludes

masses only up to about �� GeV� while H� rules out masses below �� GeV if � � ��

and �� GeV if � � �� The DELPHI �� and OPAL �� experiments at LEP

recently performed a search for single LQ production and exclude LQ masses below

�� GeV for � � �� which because of the smaller centre of mass energy cannot be

competitive with the sensitivity achieved at HERA�

High Q� Physics at HERA and Searches for New Particles ��

Figure �� Bounds on ��j�

j � �� against the S��L mass�

for several �xed values of ��

The grey domains are excluded at

�� con�dence level� The full

curve indicates the most stringent

indirect low energy limit on �� for

��

10-3

10-2

10-1

80 100 120 140 160 180 200 220 240 260

Indirect limit onλ31 (τ → π e)

for λ11 = 0.3λ11 = 0.03

λ11 = 0.1

λ11 = 0.3

MLQ (GeV)

λ 3j

e+ + u → S 1 / 2 , L → τ + qj

τ channel only

H1 preliminary

H� also carried out a search for Lepton Flavor Violating LQs �� looking for

LQs decaying into � � qjk and � � qj through the coupling ��j and ��j respectively�

Mass dependent bounds on ��j are shown in Fig� � for several �xed values of �� For

�� j � �� H� excludes LQs lighter than �� GeV� which signi�cantly improves

the limit of about �� GeV obtained from third generation LQ searches carried out at

the TeVatron �� unless � is much smaller� Indirect limits on �� are also improved

by typically one order of magnitude�

�� Searches for Rparity violating squarks

�� Phenomenology

The most general superpotential consistent with the gauge symmetry and supersymme�

try of the Minimal Supersymmetric Standard Model �MSSM� contains Yukawa terms of

super�elds which violate lepton �L�� and baryon �B��number� These interactions can

give rise to rapid proton decay� In order to avoid this problem� one introduces additional

discrete symmetries which forbid some or all of the dangerous Yukawa couplings� A rad�

ical solution is achieved by requiring R�parity conservation where R�parity is de�ned

by Rp � ��B�L��S� S being the spin of a given particle� so that the particle content

of the MSSM can be classi�ed in SM particles with Rp � �� and superpartners with

Rp � �� The L� and B�violating Yukawa interactions in the superpotential being

odd under Rp vanish� However� phenomenologically it is already su!cient to forbid

the B�violating interactions and to constrain the L�violating terms such that they are

k Note that the ep � � � jet � X events observed by H� and discussed in �� fail signi�cantly the

kinematical constraints that should hold for the process eq � LQ� �q�


Figure �� Illustrative example

of resonant squark production fol�

lowed by Rp�violating and conserv�

ing decays into e� and jets�

~

–

~

consistent with the remaining� much weaker experimental bounds�

In such a scenario one can have the term ��ijkLiQj�Dk in the superpotential� where

Li and Qj are the lepton and quark SU��L�doublet super�elds� respectively� �Dk is

the down�quark SU��L�singlet super�eld� and ��ijk denote the dimensionless Yukawa

couplings� The indices i j k refer to the fermion generations� Most interestingly

at HERA� the Rp�violating couplings with i � � can give rise to resonant squark

production �� In e�p scattering� the possible production channels are e��uj � ��dRkand e�dk � �uLj� In particular valence d quarks allow to probe �uLj squarks and ��j�couplings� The present indirect bounds on ��

�j� and M�q �� leave an interesting search

window for HERAwith the exception of �� in which case the limit from neutrinoless

decay precludes the observation of e ects at HERA� The possible squark interpretations

of the observed excess of events in NC DIS reported in �� have been studied in detail

in ��

In the following� we consider scenarios with a single nonvanishing coupling ��j��

Furthermore� we assume that gluinos are heavier than squarks� so that �q � �g � q is

kinematically forbidden� The resonantly produced squark can then decay into e � q

via the R�p�coupling ��j� resembling a scalar leptoquark� and into a neutralino ��i or

a chargino ��i plus a quark via Rp�conserving gauge interactions� In the latter cases�

the �nal state is determined by the subsequent neutralino and chargino decays� The

heavier gauginos may cascade down to the lightest one� while the lightest neutralino

may �nally decay into e�q�q� or �q�q� involving again the coupling ��j�� Similar �Rp� decay

channels exist for the �� with the exception of the e��mode� An illustrative example

is shown in Fig� �� In �� a classi�cation is given of all possible �nal states in terms

of several distinguishable event topologies� Recently� the SUSYGEN �� package has

been extended to include R�p�SUSY processes at HERA� The new version which will be

released shortly allows a complete event generation for all possible channels�

� Further studies of R�p�processes at HERA can be found in ��


Figure �� In�

variant mass spectrum

of e� � multijet �nal

states for data �symbols

and NC DIS expectation

�histogram�

Minv (GeV)

even

ts /

bin

H1 data 94 → 97

SM expectation(NC DIS + γp)

H1 Preliminary10

-1

1

10

10 2

80 100 120 140 160 180 200 220 240 260

�� Analysis

The H� Collaboration �� analysed �nal states containing an e� and several jets� Since

the lepton appears late in the cascade decay chain� it is expected to be degraded in

energy� Consequently� the main selection criteria require an e� candidate reconstructed

at high ye together with two forward jets� The invariant mass spectrum of the �� events

selected is shown in Fig� �� and compared with the distribution of the �� events

expected in the SM� The agreement is good� In the �wrong sign channel� e��p� e��

multijet� one candidate is observed� while �� are expected�

�� Results

Combining the LQ�like channel e�� jet with the e�� multijet channels� one can put

mass�dependent limits on the couplings ��j�� To this end� four di erent sets of values

have been chosen for the MSSM parameters �� M� and tan� leading to four typical

scenarios for the lightest neutralino � photino�like states with mass �� and �� GeV�

and a zino�like state with mass �� GeV� Given ��j� and the squark mass� the relevant

widths and branching ratios are �xed� The H� analysis shown in Fig� � excludes squark

masses up to � GeV for ��j� � �� As expected� in the case of �� the H� bound

is not competitive with the best indirect constraint coming from the non�observation of

neutrinoless decay� However� for �� and �� the best indirect limits from atomic

parity violation are improved by the H� direct bounds in a large region of the parameter

space� Especially� for large �� masses� the upper bounds on �� and �� are decreased

by as much as a factor ��

The R�p squark scenarios considered above are also constrained by the �rst

generation LQ searches at the TeVatron� However� these bounds become rather weak

when ��q � e � q� is small� a possibility which exists due to the presence of Rp�

conserving decays� Recently� the TeVatron experiments have performed a dedicated

�Rp search �� analysing �nal states with like�sign electrons plus jets� These may

originate from the decay of pair�produced squarks� Assuming �ve degenerate squark

�avours� squark masses below � GeV are excluded� If one squark �e�g� the lightest

stop� is much lighter than the others� the cross section is signi�cantly smaller� The


Figure ��

CL upper limits on the

coupling ��j� as a func�

tion of the squark mass�

for di�erent masses and

composition of the light�

est neutralino �� The

most stringent indirect

limits from neutrinoless

decay and atomic

parity violation are also

indicated�

H1 Preliminary

10-2

10-1

1

80 100 120 140 160 180 200 220 240 260

e+ + d → u

~

e+ + d → c, t

~ ~

APV (96)

APV (97)EXCLUDED

Msquark (GeV)

λ’1j

1γ 150 GeV~

γ 100 GeV~

γ 40 GeV~

Z 40 GeV~

INDIRECT LIMITS :

λ’111 ( ββ0ν )

λ’121, λ’131 (APV)

(PDG 96 / C.S.Wood et al. 97)

corresponding mass bound is estimated �� to decrease to about �� GeV�

� Conclusions

Results on neutral and charged current Deep Inelastic Scattering at high momentum

transfer derived from the e�p data taken by the H� and ZEUS experiments in ��

�� have been presented and compared to the Standard Model expectations� The

di erential cross sections in terms of Q� are found to be in good agreement with the

SM predictions� A slight excess at high Q� observed in the �� data still persists

with the �� data included� but with less signi�cance�

Using the same data� a search for leptoquarks and for squarks with R�parity

violating couplings has been performed� No evidence was found for the existence of

such species� The resulting bounds on leptoquark�squark masses and Yukawa couplings

to lepton�quark pairs improve the constraints from other colliders and from low energy

experiments in important regions of the parameter space�

Acknowledgments

TM would like to thank the British Council� Collaborative Research Project

TOK�� RR acknowledges support by the Bundesministerium f"ur Bildung�

Wissenschaft� Forschung und Technologie� Bonn� Germany� Contract �� WZ��P ��

�� References

�� H� Collaboration� C� Adlo� et al�� Z� Phys� C��

�� ZEUS Collaboration� J� Breitweg et al�� Z� Phys� C��


� � H� Collaboration� �Measurement of Neutral and Charged Current Cross�Sections at high Q�

Contributed paper ! � to the XXIX Int� Conf� on High Energy Physics �ICHEP��

Vancouver� Canada �� July ��

�� http��www�desy�de�quadt�talks�desy � � ��desy seminar�html�

ZEUS Collaboration� �Measurement of High�Q� Neutral�Current DIS Cross Sections at HERA

Contributed paper ! �� to ICHEP��

�� ZEUS Collaboration� �Measurement of High�Q� Charged�Current DIS Cross Sections at HERA

Contributed paper ! �� to ICHEP��

�� A� Kwiatkowski� H� Spiesberger and H��J� M�ohring� Comput� Phys� Commun� � ��

�� W� Melnitchouk and A�W� Thomas� Phys� Lett� B ��

W� Melnitchouk and J�C� Peng� Phys� Lett� B��

A� Bodek� U�K� Yang� �Parton distributions at high x � to be published in proceedings of the �th

International Workshop on Deep Inelastic Scattering and QCD� Brussels� Apr� ��

�� A� Bodek� E� Rizvi� WG� contribution� these proceedings�

�� W� Buchm�uller� R� R�uckl and D� Wyler� Phys� Lett� B��

�� B� Schrempp� Proc� of the Workshop Physics at HERA� DESY� Hamburg �� vol� � p� ��

and references therein�

�� J� Kalinowski� R� R�uckl� H� Spiesberger and P�M� Zerwas� Z� Phys� C��

�� H� Collaboration� �A Search for Leptoquark Bosons in DIS at High Q� at HERA Contributed

paper ! �� to ICHEP��

�� ZEUS Collaboration� �Search for Narrow High Mass States in Positron�Proton Scattering at

HERA Contributed paper ! �� to ICHEP��

�� D� Waters� plenary talk� these proceedings�

�� Leptoquark Limit Collaboration Working Group �for the CDF and D� collaborations� hep�

ex�� Oct� ��

�� DELPHI Collaboration� Searches for Single Leptoquark Production atps � �� GeV andp

s � �� GeV with the DELPHI Detector � Contributed paper ! �� to the International

Europhysics Conference on High Energy Physics� Jerusalem �EPS�� Isra�el �� August ��

�� OPAL Collaboration� Search for Single Leptoquark Production in Electron�Photon Scattering atp�s � �� and �� GeV� Contributed paper ! �� to EPS��

�� H� Collaboration� C� Adlo� et al�� DESY preprint �� May �May �� hep�ex�� See

also these proceedings�

�� D� Collaboration� B� Abbott et al�� FERMILAB preprint PUB��E �March �� pp�� eprint

hep�ex��

�� CDF Collaboration� F� Abe et al�� Phys� Rev� Lett� ��

�� H� Collaboration� �A Search for Squarks of R�Parity Violating SUSY at HERA � Contributed

paper ! �� to ICHEP��

�� J� Butterworth and H� Dreiner� Nucl� Phys� B � �� and references therein�

�� H� Collaboration� S� Aid et al�� Z� Phys� C��

�� E� Perez� �Recherche de Particules en Supersym"etrie Violant la R�parit"e dans H� #a HERA � Ph�D�

thesis� Universit"e Paris VI� DAPNIA�SPP report �� in French�

�� H� Dreiner� E� Perez� Y� Sirois Proceedings Workshop �Future Physics at HERA �

�� H� Dreiner� hep�ph�� To be published in $Perspectives on Supersymmetry$� Ed� by G�L�

Kane� World Scienti�c�

�� G� Altarelli et al�� Nucl� Phys� B��

�� D� Choudhury and S� Raychaudhuri� Phys� Lett� B��

�� H� Dreiner and P� Morawitz� Nucl� Phys� B��

� �� S� Katsanevas� P� Morawitz� �SUSYGEN �� A Monte Carlo Event Generator for MSSM

sparticle production at e�e� colliders � hep�ph�� pp� The SUSYGEN package has

been extended by E� Perez to �RpSUSY processes at HERA�

� �� D� Collaboration� �Search for R�parity Violating Supersymmetry in the Dielectron

http://www.desy.de~quadt/talks/desy_130398/desy_seminar.html

http://xxx.lanl.gov/abs/hep-ex/9810015




http://xxx.lanl.gov/abs/hep-ph/9707435

http://xxx.lanl.gov/abs/hep-ph/9711417


and Multijet Channel Contributted paper !�� to ICHEP�� see also http��www�

d��fnal�gov�public�new�analyses�rpareejjjj�welcome�html�

� �� M� Chertok� forD� and CDF Collaborations� to be published in the Proceed� of the rd Rencontres

de Moriond� QCD and High Energy Hadronic Interactions� Les Arcs� France� �March ��

��

http://www-d0.fnal.gov/public/new/analyses/rpareejjjj/welcome.html

http://www-d0.fnal.gov/public/new/analyses/rpareejjjj/welcome.html

Documents

WUE-ITP-98-056