Measurement of Charged and Neutral Current e − p Deep Inelastic Scattering Cross Sections at High Q 2

Abstract

Deep inelastic e-p scattering has been studied in both the charged current (CC) and neutral current (NC) reactions at momentum transfers squared Q2 above 400 GeV2 using the ZEUS detector at the HERA ep collider. The CC and NC total cross sections, the NC to CC cross section ratio, and the differential cross sections dsigma/dQ2 are presented. From the Q2 dependence of the CC cross section, the mass term in the CC propagator is determined to be MW = 76+/-16+/-13 GeV.

Full text

VOLUME 75, NUMBER 6PHYSICAL REVIEW LETTERS 7AUoUsT 1995
Measurement of Charged and Neutral Current epDeep Inelastic Scattering Cross Sections
at High Q
M. Derrick, 'D. Krakauer, 'S. Magill, 'D. Mikunas, 'B.Musgrave, 'J. Repond, 'R. Stanek, 'R.L. Talaga, 'H. Zhang, '
R. Ayad, 2G. Barj,2M. Basile, L. Bellagamba, 2D. Boscherjnj, 2A. Brun&, G. Brunj, P. Brunj, 2G. Cara Romeo,
G. Castelljnj, M. Chjarjnj, L. Cjfarellj, F. Cjndolp, A. Contjn, M. Cprradj, I. Gjalas, P. Gjustj, G. Iacobuccj,
G. Laurentj, G. Levj, A. Margottj, T. Massam, R. Nania, C. Nempz, F. Palmpnari, A. Pol1nj, G. Sartorellj, 2
R. Timellini, Y. Zamora Garcia, A. Zichichi, A. Bargende, J. Crittenden, K. Desch, B.Diekmann, T. Docker,
M. Eckert, L. Feld, A. Frey, M. Geerts, G. Geitz, M. Grothe, T. Haas, H. Hartmann, D. Haun, K. Heinloth,
E. Hilger, H.-P. Jakob, U. F. Katz, S.M. Mari, A. Mass, S. Mengel, J. Mollen, E. Paul, Ch. Rembser,
R. Schattevoy, D. Schramm, J. Stamm, R. Wedemeyer, S. Campbell-Robson, "A. Cassidy, N. Dyce, B.Foster,
S. George, "R. Gilmore, G.P. Heath, 4H. F. Heath, T.J. Llewellyn, "C.J.S. Morgado, D. J.P. Norman, 4
J.A. O'Mara, R.J. Tapper, S.S. Wilson, R. Yoshjda, R. R. Rau, M. Arneodo, L. Iannotti, M. Schioppa,
G. Susinno, A. Bernstein, A. Caldwell, N. Cartiglia, J.A. Parsons, S. Ritz, F. Sciulli, P.B.Straub, L. Wai,
S. Yang, 7Q. Zhu, 7P. Borzemski, sJ. Chwastowski, sA. Eskreys, K. Piotrzkowski, sM. Zachara, sL. Zawiejski,
L. Adamczyk, B.Bednarek, K. Jelen, D. Kisielewska, T. Kowalski, E. Ruljkowska-Zargbska, L. Suszycki,
J. Zajyc, 9A. Kotanski, '0 M. Przybycjen, 'L.A. T. Bauerdick, "U. Behrens, "H. Beier, "J.K. Bienlein, "
C. Coldewey, "O. Deppe, "K. Desler, "G. Drews, "M. Flasjnskj, "D.J. Gilkinson, "C. Glasman, "P. Gott»cher, "
J. Grosse-Knetter, "B.Gutjahr, "W. Hain, "D. Hasell, "H. Hessling, "H. Hultschig, "Y. Iga, "P. Joos, "
M. Kasemann, "R. Klanner, "W. Koch, "L. Kopke, "U. Kotz, "H. Kowalski, "J~Labs, "A. Ladage, "B.Lohr, "
M. LOwe, "D. Liike, "O. Manczak, "J.S.T. Ng, "S. Nickel, "D. Notz, "K. Ohrenberg, "M. Roco, "M. Rohde, "
11 USchneeklpth 11 WSchulz 11 FSelpnke 11 FStjljaris 11 BSurrow 11 TVoss 11 DWestphal 11
G. Wplf, C. Youngman, J.F. Zhou, H. J. Grabosch, ~A. Kharchjlava, A. Lejch, M. Mattjngly, A. Meyer,
S. Schlenstedt, N. Wulff, G. Barbaglj, 3P. Pelfer, G. Anzjvjno, G. Maccarrpne, S. De Pasquale, "
L. Votano, 'A. Bamberger, 'S. Eisenhardt, 'A. Freidhof, 'S. Spldner-Rembold, '5 J. Schroeder, '5 T. Trefzger, '
N. H. Brook, 'P. J. Bussey, 'A. T. Doyle, 'J.I. Fleck, 'D. H. Saxon, 'M. L. Utley, 'A. S. Wilson, '
A. Dannemann, U. Holm, D. Hprstmann, T. NeUmann, R. Sjnkus, K. Wjck, E. Badura, B.D. Burpw, ~
L. Hagge, 'E. Lohrmann, 'J. Mainusch, 'J. Milewski, 'M. Nakahata, 'N. Pavel, 'G. Poelz, 'W. Schott, '
F. Zetsche, 'T.C. Bacon, 'I. Butterworth, 'E. Gallo, 'V.L. Harris, 'B.Y.H. Hung, 'K.R. Long, 'D. B.Miller, '
P.P. O. Morawitz, 'A. Prinias, 'J.K. Sedgbeer, 'A. F. Whjtfield, 'U. Mallik, E. McCliment, 0M. Z. Wang,
SMWang 20 JTWu YZhang PCloth DFilges 'SHAn SMHong S W Nam2 SKPark
M. H. Suh, ~S.H. Ypn, R. Imlay, S. Kartjk, H.-J. Kjm, R. R. McNejl, W. Metcalf, V. K. Nadendla, ~3
F. Barrejro, G. Cases, R. Gracjani, J.M. Hernandez, L. Hervas, L. Labarga, 4J. del Peso, J. Puga, 4
J. Terron, J.F. de Troconiz, 24 G. R. Smith, ~F. Corrjveau, ~6 D. S. Hanna, ~6 J. Hartmann, L.W. Hung, 6J.N. Lim, 26
C.G. Matthews, P. M. Patel, 6L.F. Sinclair, ~D. G. Stairs, M. St. Laurent, R. Ullmann, G. Zacek,
V. Bashkjrov, B.A. Dolgoshejn, A. Stjfutkjn, G. L. Bashjndzhagyan, P. F. Ermolpv, L. K. Gladilin,
Y.A. Golubkov, 2~ V. D. Kobrin, V. A. Kuzmjn, 2~ A. S. Proskuryakov, 28 A. A. Savin, 28 L.M. Shcheglova, ~
A. N. Solomin, ~N. P. Zotov, 8M. Botje, 9F. Chlebana, A. Dake, J. Engelen, 9M. de Kamps, P. Kooijman,
A. Kruse, H. Tjecke, W. Verkerke, M. Vreeswjjk, L. Wiggers, E. de Wplf, R. van Wpudenberg,
D. Acosta, B.Bylsma, 0L. S. Durkin, K. Honscheid, 0C. Li, T.Y. Ling, 0K.W. McLean, W. N. Murray,
I.H. Park, T. A. Romanowski, R. Seidlein, D. S. Bailey, 'G. A. Blair, 'A. Byrne, 'R. J. Cashmore, 3'
A. M. Cooper-Sarkar, D. Daniels, 'R.C.E. Devenish 31 NHarnew, 1M. Lancaster 31 PE. Luffman, '
L. Lindemann, 'J.D. McFall, 'C. Nath, 'V. A. Noyes, 'A. Quadt, 'H. Uijterwaal, 'R. Walczak, 'F.F. Wilson, '
T. Yip, 'G. Abbiendi, A. Bertolin, R. Brugnera, R. Carlin, F. Dal Corso, M. De Giorgi, U. Dosselli,
S. Ljmentanj, M. Morandjn, M. Ppsoccp, L. Stanco, R. Strpjlj, C. Vpcj, J. Bulmahn, J.M. Butterworth,
R.G. Feild, B.Y. Oh, J.J. Whitmore, G. D'Agostini, G. Marini, A. Nigro, E. Tassi, J.C. Hart,
N. A. McCubbin, K. Prytz, T.P. Shah, T. L. Short, 5E. Barberis, T. Dubbs, C. Heusch, M. Van Hook,
B.Hubbard 36 WLock~an 36 JTRahn 36 HFWSadrozjnskj 36 ASejden 36 JBjltzjnger 37 QSchwarzer 37
R.J. Seifert, A. H. Walenta, G. Zech, H. Abramowicz, G. Briskin, S. Dagan, A. Levy, T. Hasegawa,
M. Hazumi, T. Ishii, M. Kuze, S. Mine, Y. Nagasawa, M. Nakap, I. Suzuki, K. Tokushuku,
S. Yamada, Y. Yamazaki, M. Chiba, R. Hamatsu, T. Hirose, 4K. Homma, S. Kitamura, Y. Nakamitsu,
K. Yamauchi, R. Cirio, 'M. Costa, 'M. I. Ferrero, 'L. Lamberti, 'S. Maselli, 4' C. Peroni, "' R. Sacchi, '
A. Solano, 'A. Staiano, 'M. Dardo, D. C. Bailey, D. Bandyopadhyay, F. Benard, 4M. Brkic, M. B.Crombie,
D. M. Gingrich, "G.F. Hartner, K.K. Joo, G. M. Levman, J.F. Martin, R. S. Orr, C. R. Sampson, "
1006 0031-9007/95/75(6)/1006(6)$06. 00 1995 The American Physical Society

VOLUME 75, NUMBER 6PHYSICAL REVIEW LETTERS 7AUGUST 1995
R.J. Teuscher, 43 C. D. Catterall, ""T.W. Jones, 44 P. B. Kaziewicz, 44 J.B.Lane, R.L. Saunders, 44 J. Shulman, 44
K. Blankenship, J. Kochocki, B.Lu, L. W. Mo, W. Bogusz, K. Charchula, J. Ciborowski, 6J. Gajewski,
G. Grzelak, M. Kasprzak, M. Krzyzanowski, K. Muchorowski, R.J. Nowak, J.M. Pawlak, 6
T. Tymieniecka, A. K. Wroblewski, 6J.A. Zakrzewski, A. F. Zarnecki, M. Adamus, Y. Eisenberg,
U. Karshon, ~D. Revel, D. Zer-Zion, I. Ali, 49 W. F. Badgett, B.Behrens, S. Dasu, C. Fordham,
C. Foudas, A. Goussiou, R.J. Loveless, D. D. Reeder, S. Silverstein, W. H. Smith, A. Vaiciulis,
M. Wodarczyk, 9T. Tsurugai, S. Bhadra, 'M. L. Cardy, 'C.-P. Fagerstroem, 'W. R. Frisken, 'K.M. Furutani, '
M. Khakzad st and W.B. Schmidke~'
(ZEUS Collaboration)
'Argonne National Laboratory, Argonne, Illinois 60439
University and INFN Bologna, Bologna, Italy
Physikalisches Institut der Universitat Bonn, Bonn, Federal Republic of Germany
H H Wills .Ph. ysics Laboratory, University of Bristol, Bristol, United Kingdom
Brookhaven National Laboratory, Upton, New York 11973
Calabria University, Physics Department and INFN, Cosenza, Italy
Columbia University, Nevis Laboratory, Irvington on Hudson, New York 10533
Institute ofNuclear Physics, CracowPol, and
9Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland
'Department of Physics, Jagellonian University, Cracow, Poland
"Deutsches Elektronen Synchrot-ron DESY, Hamburg, Federal Republic of Germany
'2DESY Zeuthen, -Institut fur Hochenergiephysik, Zeuthen, Federal Republic of Germany
'University and INFN, Florence, Italy
'INFN, Laboratori Nazionali di Frascati, Frascati, Italy
'5Fakultiit fur Physik der Universitat Freiburg iBr,Frei.bur. giBr ,Fed. era. lRepublic of Germany
'6Department of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
'Hamburg University, IInstitute of
.Experimental Physics, Hamburg, Federal Republic of Germany
'8Hamburg University, II Institute of.Experimental Physics, Hamburg, Federal Republic of Germany
'High Energy Nuclear Physics Group, Imperial College London, London, United Kingdom
Physics and Astronomy Department, University of Iowa, Iowa City, Iowa 52242
~'Institut fur Kernphysik, Forschungszentrum Julich, Julich, Federal Republic of Germany
Korea University, Seoul, Korea
Department of Physics and Astronomy, Louisiana State University, Baton RougeLouisia, na 70803
Depto de FIsica Teonca, Universidad Autonoma Madrid, Madrid, Spain
25Department of Physics, University ofManitoba, Winnipeg, Manitoba, Canada
"Department of Physics, McGill University, Montreal, Quebec, Canada
Moscow Engineering Physics Institute, Mosocw, Russia
"Moscow State University, Institute of Nuclear Physics, Moscow, Russia
NIKHEF and University ofAmsterdam, Amsterdam, The Netherlands
Physics Department, The Ohio State University, Columbus, Ohio 43210
3'Department of Physics, University of Oxford, Oxford, United Kingdom
Dipartimento di Fisica dell' Universita and INFN, Padova, Italy
3Department of Physics, Pennsylvania State University, University Park, Pennsylvania I6802
Dipartimento di Fisica, Univ. 'La Sapienza' and INFN, Rome, Italy
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom
6University of California, Santa Cruz, California 95064
Fachbereich Physik der Universita tGesamthochs'c-hule SiegenFederal ,Republic of Germany
School of Physics, Tel Aviv University, T-el Aviv, Israel
Institute for Nuclear Study, University of Tokyo, Tokyo, Japan
Department of Physics, Tokyo Metropolitan University, Tokyo, Japan
'Universita di Torino, Dipartimento di Fisica Sperimentale and INFN, Torino, Italy
II Faculty of Sciences, Torino University and INFN, Alessandria, Italy
Department of Physics, University of Toronto, Toronto, Ontario, Canada
Physics and Astronomy Department, University College London, London, United Kingdom
Physics Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
Institute of Experimental Physics, Warsaw University, Warsaw, Poland
71nstitute for Nuclear Studies, Warsaw, Poland
Nuclear Physics Department, 4'eizmann Institute, Rehovot, Israel
Department of Physics, University of Wisconsin, Madison, Wisconsin 53706
Faculty of General Education, Meiji Gakuin University, Yokohama, Japan
'Department of Physics, York University, North York, Ontario, Canada
(Received 31 March 1995)
1007

VOLUME 75, NUMBER 6PH YSICAL REVIEW LETTERS 7AUGUsT 1995
Deep inelastic epscattering has been studied in both the charged current (CC) and neutral current
(NC) reactions at momentum transfers squared Qabove 400 GeV using the ZEUS detector at the
HERA ep collider. The CC and NC total cross sections, the NC to CC cross section ratio, and the
differential cross sections do./dg2 are presented. From the Qdependence of the CC cross section,
the mass term in the CC propagator is determined to be M~ =76 ~16 ~13 GeV.
PACS numbers: 13.60.Hb, 12.15.Ji, 14.70.Fm
Lepton-nucleon scattering is an important technique for
studying the constituents of the nucleon and their inter-
actions. In the standard model [1],electron-proton (ep)
scattering occurs via the exchange of gauge bosons (y,
Zo, W—
). At long wavelengths (small momentum trans-
fers), photon mediated interactions dominate over the ex-
change of the heavy gauge bosons. However, at the ep
storage ring HERA, for the first time, scattering can be ob-
served at sufficiently short wavelengths (large momentum
transfers) that the "weak" and "electromagnetic" scatter-
ing amplitudes are comparable in magnitude.
The differential Born cross section for deep inelastic
scattering (DIS) with unpolarized epbeams can be
expressed as [2]
0
dx dg2
2'7T AHl +(1 y) )Si
+(I —(1 —y) ]x$3j,
where the g(x, Q)functions describe the proton struc-
ture and couplings. In this equation Qis the negative
square of the four-momentum transfer, yis the fractional
energy transfer from the lepton in the proton rest frame,
nis the electromagnetic fine-structure constant, and xis
the proton momentum fraction carried by the struck par-
ton. These variables are related by Q=sxy, where ~s
is the center-of-mass energy. The +can be expressed
as sums over quark flavors fof the proton's quark den-
sities qf(x, Q)weighted according to the gauge structure
of the scattering amplitudes. For the neutral current (NC)
reaction, ep~eX, mediated by yand Zexchange,
they can be written as
+2 pqf [ef +2vvf ef 2z
+(v. +a)(vf +af) 7z]
xj3 gqf [2aeaf ef &z—
+4veaevf af 2zJ]
f
YACC
yCC
where qf =(xqf(x, Q)~xqf(x, Q)), a, and v, are
the axial and vector couplings of the eto the Z, and af
and vf are the analogous couplings for aquark of Aavor
f, which has electric charge ef [1]. 2z is the ratio of Zo-
to-photon propagators, given by 2z —
—Q/(Q2 +Mz),
where Mz is the mass of the Zboson.
For charged current (CC) scattering, ep~v,X, in
which a8' boson with mass M~ is exchanged, the
functions are
where kand Iare the generation indices of up-type
quarks ut. (x, Q)and down-type antiquarks d(x, Q), V
is the Cabibbo-Kobayashi-Maskawa quark mixing matrix,
tliv is the weak mixing angle, and gatv =Q/(Q +
Mtv). At lowest order, GFMtv =em/csin Otv, where
GF is the Fermi constant.
In 1993, HERA collided 26.7GeV ewith 820 GeV
p, giving ~s =296 GeV. At this high center-of-mass
energy, DIS can be investigated at much higher Qthan
in fixed target experiments. The predicted DIS cross
sections at fixed xover alarge Qrange depend both on
the electroweak theory for the propagators and couplings
and on quantum chromodynamics (QCD) for the parton
density evolution. The structure functions +2 have been
measured [3] in epand p, pscattering for Qup to
—
5(150) GeV at x=0.03 (0.3). The parton density
distributions [4,5] inferred from those measurements were
extrapolated to our Qregion using the next-to-leading-
order QCD evolution equations [6]. At xof 0.03 (0.3),
the up-quark density is predicted to change by 21%
(—
39%) as Qincreases from 5to 16000 GeV2. The
NC propagator varies by 7orders of magnitude over the
same Qinterval.
This paper reports measurements of integrated and
differential cross sections do./dg for NC and CC DIS
with Q)400 GeV using aluminosity of 0.540 ~
0.016 pb '. ZEUS [7] and H1[8] have previously
reported on NC DIS cross section measurements at lower
Q.Hl has also measured the CC total cross section [9]
and demonstrated that the CC propagator mass is finite.
In ZEUS [7], charged particles are tracked by drift
chambers operating in an axial magnetic field of 1.43 T.
The superconducting solenoid is surrounded by acom-
pensating uranium-scintillator calorimeter (CAL) with
an electromagnetic (hadronic) energy resolution of
18%/QF(GeV )[35%/QE(GeV )] and asubnanosecond
time resolution. The CAL covers the polar angles
between 2.2and 176.5', defined with respect to the
incident proton direction. The CAL is segmented in
depth into electromagnetic and hadronic sections, with a
total thickness of 4to 7interaction lengths. Surrounding
the CAL is an iron magnetic return yoke instrumented for
muon detection. The luminosity is measured by the rate
of high-energy photons from the reaction ep ~epy.
Data were collected with athree-level trigger. The
first-level trigger was based on electromagnetic energy,
transverse energy, and total energy deposits in the CAL
[7]. The thresholds, between 2and 15 GeV, were well be-
low the off-line selection cuts. The second-level trigger
rejected p-gas events (proton interactions with residual
1008

VOLUME 75, NUMBER 6PH YSICAL REVIEW LETTERS 7A~GUs~ 1995
gas in the beam pipe upstream of the detector) recognized
by CAL energy deposited earlier than the ep crossing.
The third-level trigger selected events as NC DIS candi-
dates if E—P, exceeded 25 GeV, where Eand P, are
the total energy and zcomponent of the momentum mea-
sured in the calorimeter. If no energy escapes through the
rear beam hole, E—P, =2E„where E, is the electron
beam energy. Events were selected as CC DIS candidates
if g, ,the absolute value of the missing transverse momen-
tum measured by the calorimeter, exceeded 9GeV, and
there was either more than 10 GeV deposited in the for-
ward part of the CAL or at least one track reconstructed
in the drift chambers.
The acceptances and measurement resolutions for sig-
nal and background events were calculated using Monte
Carlo methods. Simulated CC and NC DIS events, gen-
erated using IEPTo [10] interfaced to HERAcLEs [11]by
DJANGO [12], were passed through aGEANT [13] based
detector simulation, and subsequently analyzed with the
same reconstruction and off-line selection procedures as
the data. The efficiencies and acceptances were found to
have negligible dependences on either the model of the
hadronic final state [10,14] or the proton parton density
parametrizations [4] used in the simulation.
As the ZEUS detector is nearly hermetic, it is possible to
reconstruct the kinematic variables (x,y, Q)for NC DIS
using different combinations of the angles and energies
of the scattered lepton and hadronic systems [7]. Three
reconstruction methods were relevant to this analysis. The
electron (e) method uses E,
'and 0„ the energy and
polar angle of the scattered electron. The hadronic, or
Jacquet-Blondel (JB) [15],method reconstructs yand Q
yJB (Eh Pzh)/2E and QJB Pt h/(I yJB)
where Eh, P, ~, and P, hare the energy, the zcomponent of
momentum, and the transverse momentum of the hadronic
system. The double angle (DA) method uses 0, and yh,
the polar angle of the struck quark given by cosyh =
[P, h—(2E,yJB) ]/fP, h+(2E,yJB) ]. The DA method,
which estimates Qwith small bias and good resolution,
was used for NC events [7]. For CC DIS, the hadronic
(JB) method was used.
The off-line NC DIS event selection required an electron
candidate with E,
'~10 GeV in the calorimeter and E-
P, )35 GeV. To reject backgrounds from photoproduc-
tion events with afake electron (mostly vr 's at small
polar angles) the electron candidate was required to have
amatching track and to satisfy y, (0.95. Cosmic-ray
triggers were rejected by requiring I/, /~E, (2GeV'i .
Afinal cut required Q, as reconstructed by the DA and
emethods, to be consistent: 0.7(Q/QDA (1.2. Af-
ter these selections, 436 events with QDA )400 GeV re-
mained. The photoproduction background is less than 2%.
More than 85% of all Monte Carlo NC DIS events with
Qz )400 GeV zpass all of the above cuts. The spectra
of xand Qfor the data and the Monte Carlo simulation
are shown in Figs. 1(a) and 1(b).
@150
LL1
ZEUS NC (a) ZEUS NC (b)
100
50
10
10 10
iII&II IIIIIIIII I
10 10
Q'(GeV')
C
I' 30
LLJ
ZEUS CC (c)
LLI
ZEUS CC
10 10
I
ir&&&I I I I IIIIII II
10 10
Q(GBV )
FIG. 1. (a) xfor NC events, (b) Q2 for NC events, (c) xfor
CC events, and (d) Q2 for CC events. The points with error
bars are ZEUS data. The histograms are the predicted numbers
of events from the absolutely normalized simulation.
The NC DIS cross sections in five bins of Qbetween
400 GeV and the kinematic limit of 87500 GeV are
given in Table I. The Born cross section was calculated
for each bin as rrNc =NNcBrNc/L WNc. JVNc is the
number of NC DIS events reconstructed in the bin, 6rNc
is the radiative correction factor, and 5is the luminosity.
The acceptance for the bin, A.Nc, was calculated from
the NC DIS Monte Carlo event sample, as the ratio of
the number of events which passed all cuts and were
reconstructed in the Qbin to the number of events with
the true Qin the bin. A.Nc varies between 0.79 and 0.85.
HERAcLEs [11]was used to calculate 6rNc, which was in
the range 0.88 to 0.95, with an uncertainty of less than 4%%uo.
The systematic errors on A.Nc include 4% due to the
uncertainty of the calorimeter energy response, 3% for the
efficiency of the calorimeter-track matching for the elec-
tron, 4% for the efficiency of the electron identification
algorithm, and 5% in the lowest Qbin for the efficiency
of the Q, /QDA cut.
CC DIS events are characterized by alarge g, due
to the final-state neutrino. The 36000 CC triggers are
mainly from upstream p-gas or cosmic-ray interactions.
The off-line CC DIS selection required P, )12 GeV and
avertex, formed from two or more tracks, within 45 cm
of the nominal interaction point. Events with more than
40 tracks not associated with the vertex were rejected. To
reduce the remaining p-gas background, for which the
reconstructed transverse energy was concentrated at small
polar angles, events with pat"'" (0.7$, were rejected,
where g;"'" is the missing transverse momentum in the
calorimeter excluding the 1.0X1.0mregion around the
forward beam pipe. The 117 candidates remaining were
mostly cosmic-ray events, including cosmic-ray muons
coincident with ap-gas interaction. Single muons were
1009

VOLUME 75, NUMBER 6PH YS ICAL REVIEW LETTERS 7AUGUsT 1995
TABLE I. Events observed and integrated Born cross sections for NC and CC DIS. Errors shown are statistical, followed by
systematic (including 3.5% luminosity uncertainty). The standard model cross sections o. are calculated with LEPTO [10] using
the MRSD' parton distributions [4]. The predictions for an infinite mass in the CC propagator o.cc are shown. In the last three
rows the NC and CC total cross sections and their ratios are given for Q2 )Q;„.
Q;„(GeV )
Q~ „(GeV2)
NNc
o'Nc (pb)
~oNc (pb)
o'Nc (pb)
Ncc
occ (pb)
~~cc (pb)
occ (pb)
occ (pb)
oNc (pb)
~cc (pb)
oNc/o cc
400
1000
328
629
~38 ~73
644
2
5.8
-3:8 —o9
+4.6
13.3
17.5
837 ~100
57 +20
14.7
1000
2500
86
163
+18+ 16
167
7
16.8
+6.7
17.1
28.3
209 ~27
50+ 13
4.20'9
+1.3
2500
6250
18
36
+9+ 4
41
5
12.3
53 1.7
+5.8
15.9
41.8
Cross sections for Q)Q
46 +12
34 ~10
1.4p4
+0.6
6250
15625
3
5.80.6
+3.6
8.8
7
16.8
+6.7~2.2
8.0
46.0
8.0~4.1
21 3.1
04 pi
+0.3
15625
87500
1
2.0
+29 ~03
1.1
2
3.4
+27 ~04
1.6
21.2
2.0~1.7
3.4~2.7
+j.3
07-0.6
rejected on the basis of their characteristic spatial distri-
bution of energy deposition in the calorimeter or muon
chambers. Additionally, the times of all energy deposits
in the calorimeter were required to be consistent with an
epinteraction.
The events passing all selection criteria were scanned,
and one cosmic-ray event was removed, leaving 23 events
with Q)400 GeV in the final CC DIS sample. From
Monte Carlo simulations, we expect fewer than one
background event from photoproduction. Figures 1(c)
and 1(d) show the reconstructed xand Q2 distributions
for the data sample with Q2 )400 GeV2 compared to
the Monte Carlo simulation for CC DIS.
The CC DIS Born cross sections, oc~ =
Ncc 6rcc/5 A.cc, are shown in Table I. The ac-
ceptances Mcc are in the range 0.67—
0.80, except for
the bin at largest Q, where it is 1.10. 75% of the Monte
Carlo CC DIS events generated with Q)400 GeV
pass all selection cuts. The radiative correction factors
Brcc are in the range 1.02—
1.03. The systematic errors
on A.cc include 5% due to the dependence on the P,
and the g;"'"/g, thresholds, 5% for the efficiency to
reconstruct avertex, 8% in the lowest Qbin due to the
calorimeter energy scale, and 9% (20%) on the lower
four bins (highest Qbin) due to the hadronic energy
reconstruction.
The differential Born cross sections do./dg for both
NC and CC scattering, shown in Fig. 2, agree with the stan-
dard model predictions. The ratios of the NC to CC total
cross sections for Q)Q;„are listed in Table I. From
the lowest bin in Q2 to the highest, the ratio of do Nc/dg
to do.cc/d Qdecreases by 2orders of magnitude to
around unity, thus demonstrating the equal strengths of the
weak and electromagnetic forces at high Q2.
Because of the massless photon propagator, the NC
cross section decreases rapidly with Q.The Qde-
pendence of the CC cross section depends on the mass
M~ in the CC propagator. The CC cross sections ex-
pected in the limit of infinite propagator mass, o~~,
are inconsistent with the data, as shown in Table I. Fit-
ting do.cc/dg with M~ as the free parameter, and
GF fixed, we find M~ =76 ~16(stat) ~13(syst) GeV,
which agrees with the W—mass, M~ =80.22 ~0.26
GeV [I],measured at hadron colliders.
We express our deep gratitude to the people from
DESY and outside institutes whose dedicated efforts made
possible the successful operation of the ZEUS experiment.
The inventiveness of the HERA machine group is appre-
ciated. We thank the DESY directorate for strong sup-
port and encouragement. This work was supported by
the Natural Sciences and Engineering Research Council of
1
CL 1
10
t&0
zEUs epDls
~NC Data
oCC Data
NC SM
---- CC SM
—
3
10
—
4
10
—
5
10
Q' (GeV')
FIG. 2. do./dQ2 for CC and NC DIS. The points with errors
are the data, and the curves are the standard model Born cross
sections. The data are plotted at the average Q2 of the events
in each bin.
1010

VOLUME 75, NUMBER 6PH YSICAL REVIEW LETTERS 7AUGUsT 1995
Canada, FCAR of Quebec, German Federal Ministry for
Research and Technology (BMFT), Volkswagen Foun-
dation, MINERVA Gesellschaft fiir Forschung, Israeli
Academy of Science, German Israeli Foundation, Italian
National Institute for Nuclear Physics (INFN), Japan-
ese Ministry of Education, Science and Culture, Korean
Ministry of Science and Korea Science and Engineer-
ing Foundation, Netherlands Foundation for Research on
Matter, Polish State Committee for Scientific Research,
Foundation for Polish-German Collaboration, Deutsche
Forschungsgemeinschaft (DFG), Spanish Ministry of Ed-
ucation and Science, U.K. Particle Physics and Astronomy
Research Council (PPARC), U.S. Department of Energy
(DOE), and U.S. National Science Foundation (NSF).
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1011