neutrino results from the fermilab tevatron

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Nuclcar Physics B (Proc. Suppl.) 38 (1995) 188-197

P R O C E E D I N G S S U P P L E M E N T S

Neutrino Results from the Fermilab Tevatron

M.H. Shaevitz, C. Arroyo, K.T. Bachmann, A.O. Bazarko, R.E. Blair, T.A. Bolton, C. Foudas, B.J. King, W.C. Lefmann, W.C. Leung, S.R. Mishra,E. Oltman, P.Z. Quintas, S.A.Rabinowitz, F. Sciulli, W.G. Seligman; ~ F.S. Merritt, M.J. Oreglia, B.A. Schumm; b R.H. Bernstein, F. Borcherding, H.E. Fisk, M.J. Lamm, W. Marsh, K.W.B. Merritt, H. Schellman, D.D. Yovanovitch; c A. Bodek, H.S. Budd, P. de Barbaro, W.K. Sakumoto; d T.S. Kinnel, P.H. Sandler, W.H. Smith; e

~Columbia University, New York, NY 10027

bUniversity of Chicago, Chicago, IL 60637

CFermilab, Batavia, IL 60510

dUniversity of Rochester, Rochester, NY 14627

eUniversity of Wisconsin, Madison, WI 53706

Results from the high-energy, high-statistics studies of neutrino nucleon interactions by the CCFR collaboration at the Fermilab Tevatron are described. Using a data sample of over 3.7 million events with energies up to 600 GeV, precision measurements are presented for the weak mixing angle, sin 2 0w, the structure functions, FT(:c, Q2) and zFa(~, Q2), and the strange quark distribution, xs(x, Q2). Comparisons of these measurements to those obtained in other processes are made in the context of global electroweak and QCD tests. Prospects for the next generation measurements by the NuTeV collaboration at Fermilab are also presented.

1. I n t r o d u c t i o n

Neutrino scattering measurements offer a unique tool to probe the electroweak and strong interactions as described by the standard model (SM). Electroweak measurements are accessible through the comparison of neutrino neutral and charged current scattering. These measurements are complimentary to other electroweak measurements due to differences in the radiative corrections both within and outside the SM. For the neutrino measurements, these radiative corrections are particularly sensitive to non-standard Higgs or extra Z bosons.

Neutrino scattering measurements also provide a precise method for measuring the F2(x, Q~) and xF3(z, Q2) structure functions. The predicted Q2 evolution can be used to test QCD as well as to measure the strong coupling constant, as , and the patton distributions. In addition, neutrino charm production, which can be determined from the observed dimuon events, allows the strange- quark sea to be investigated.

0920-5632/95/$09.50 v 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(94)00746-2

2. N e u t r i n o D e t e c t o r a n d F l u x D e t e r m i n a - t i o n

The results reported here are from data taken in two runs, E744 and E770, with the Columbia - Chicago - Fermilab- Rochester (CCFR) detector using the Fermilab Tevatron quad-triplet neutrino beam. This wide-band beam was composed of v~, and V~'s with energies up to 600 GeV and a flatter energy spectrum than characteristic of horn-focused neutrino beams. The data sample of 3,700,000 triggers was reduced after flducial and kinematic cuts to 1,280,000 u~ and 270,000 V~, induced events for the structure function analyses and about 480,000 events for the sin s0w measurement.

The CCFR Lab E neutrino detector [1] con- sists of a target calorimeter with iron plates, scintillation counters, and drift chambers fol- lowed by a solid iron toroid muon spectrometer. The calorimeter is composed of 84 planes of 3m×3m×10cm steel absorber with a total mass of 695 metric tons and a mean density of

M.H. Shaevitz et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 18~197 189

4.2 g /cm 3. Scintillation counters of the same transverse dimension sample the energy every 10era (0.6)~, 5.9X0) of steel, permitting a hadron energy resolution of A E / E = 0.85/x/E. The scintillators also determine the event timing and establish the longitudinal vertex position. Trans- verse vertex position and muon tracking is pro- vided by double planes of drift chambers placed at 20 cm intervals of steel. The solid-iron toroidal magnetic spectrometer identifies muons and mea- sures their momenta with a resolution Ap = 0.11p.

The detector was calibrated using charged particle test beams directed into the appara- tus. Using this procedure, the absolute energy scale for hadrons and muons was determined to better than 1% and the resolution function was measured over three decades. The relative muon/hadron energy scale was determined to 1% directly from the data.

The neutrino flux determination is broken into two parts. First, the absolute flux is found from the observed events and the world average measured total cross-section, o ~'x = 0.676 zk 0.014 × 10-3Scm2E~(GeV). Next, the relative flux at different energies for both uu and ~u's is determined from the subset of events with low hadron energy, EHAD < 20 GeV. (The cross-section for these events has been shown[2], up to small corrections, to be independent of energy and the same for both uu and ~u interactions.) This technique is statis- tically limited and introduces an error of less than 1% for the relative flux determination.

8. lg lee t roweak M e a s u r e m e n t s

The standard model describes the unification of the electromagnetic and weak interactions in terms of a weak mixing angle[3]:

Mw 2 sin20w - I - - - (1)

MZ 2 '

where Mw and Mz are the masses of the W and Z bosons. In deep-inelastic neutrino-nucleon (uN) scattering, this weak mixing angle can be extracted from the ratio of neutral current (NC) to charged-current (CC) total cross-sections ,

u u + nucleon --~ uu + hadrons (NC),

v~ + nucleon --~ tz- + hadrons (CC).

Within the SM, the experimental determination of sin20w from vN scattering has very lit- tle dependence on Mtop or MHiggs[4]; in contrast, the SM prediction of sin20w from a, GF and Mz depends strongly on Mtop. Requiring the sin20w from vN scattering to agree with the prediction using Mz sets fimits on Mtop which are comparable with the best determinations from Z and W decay experiments at colliders[5]. From a more general perspective, the consistency of the Mtop determinations from different processes constrains possible physics processes beyond the SM. Neutrino-nucleon scattering is particularly sensitive to some proposed models with an extended Higgs sector or with extra Z's[6]. Comparing the SM prediction for Mw from vN scattering with the direct measurements at hadron colliders is a further test of the SM which is almost independent of Mtop and MHiggs.

For our measurement of sin 20w [7], we need to separate the observed events into CC and NC cat- egories. Both CC and NC interactions initiate a cascade of hadrons in the target that is registered by the drift chambers and scintillation counters. On the other hand, the muon produced in CC interactions typically penetrates well beyond the end of the hadron shower, appearing as a track of drift chamber hits with deposits of characteristic minimum-ionizing energies in the scintillation counters. In the analysis, three experimental quantities are calculated for each event: the event length, the transverse position of the event vertex, and the event energy. We define the event length, L, to be the number of scintillation counters spanned by the event from the event vertex to the last counter with a minimum-ionizing pulse height. The mean position of the hits in the drift chambers immediately downstream from the vertex determines the transverse vertex coordinates. A calorimetric energy, Ecah is calculated by sum- ming the energy deposited in the 20 counters immediately downstream from the vertex. We re- quire the event vertex to be more than 5 counters from the upstream end of the target and 34 counters from the downstream end and less than 76.2 cm from the detector center-fine. Requiring

190 M.H. Shaevitz et at/Nuclear Physics B (Proc. Suppl.) 38 (1995) 188-197

Ecaj > 30 GeV ensures complete efficiency of the energy deposition trigger.

The presence of a penetrating muon in CC interactions permits an approximate partition of CC and NC events by event length:

N o . ev t s w i t h L < 30 c n t r s

R . . . . ~ N o . ev t s w i t h L > 30 c n t r s (2)

where L > 30 counters implies a penetration greater than about 3.1m of iron. This experimental quantity was translated into a SM value for sin20w using a detailed Monte Carlo-based computer simulation (MC) of the experiment which modeled the integrated neutrino fluxes, the relevant physics processes and the response of the CCFR detector. The experimental value of sin20w was defined to be the input value to the MC which returned the same R . . . . as the E770 data. Our experimental determination, Rmeas = 147795/327832 = 0.4508, corresponds to

sin 2 8w = 0.2218 -4- 0.0025(stat.) +0.0036(exp.syst.) -4- 0.0040(theor.) (3)

The experimental and theoretical uncertainties were obtained from the MC by varying the model parameters within errors, and are itemized in Ta- ble 1. The da ta and MC distributions agree well for the three experimental quantities; length, Ec~j and vertex radial position. Figure 1 shows the length distribution of the final data sample and a MC simulated event sample. Events reaching the muon spectrometer, comprising 79% of the C C

interactions, have been left out for clarity. The events with length less than or equal to the 30 counter partition are predominantly true r,g (or V~) NC events, with 22.9% and 7.3% backgrounds from short v t, and ~ CC events and ve events, respectively.

The largest systematic uncertainties associated with the neutrino extraction of sin 2 Ow are related to the ve flux and charm production corrections as shown in Table 1. The future NuTeV experiment[9], scheduled to run in the next Fer- milab fixed-target run, will significantly reduce these uncertainties by making measurements sep- arately for vv's and Pv's using the sign-selected quadrupole triplet (SSQT) beam. These measurements can then be used to evaluate the

Table 1 Uncertainties in the sin 2 Ow measurement.

SOURCE ERROR data statistics 0.0024 Monte Carlo statistics 0.0006 TOTAL STATISTICAL 0.0025 vu, ~u flux 0.0005 transverse vertex 0.0009 cosmic ray subtraction (-t-25%) 0.0003 energy scale,resolution 0.0020 length determination 0.0014 v~ flux (-4-4.2%) 0.0023 TOTAL EXP. SYSTEMATIC 0.0036 structure functions 0.0003 non-isoscalarity 0.0004 RIo~g (-4-15%) 0.0019 charm prod.(mc = 1.31 ± 0.24 G e V ) 0.0030 strange sea (t¢ = 0.37 -4- 0.05) 0.0003 charm sea (~ = 0.10 -4- 0.15) 0.0015 higher twist 0.0005 radiative corrections 0.0007 TOTAL PHYSICS MODEL 0.0040

Paschos-Wolfenstein relation[8]:

R - - CrNC -- ~rNC - - p 2 ( __ sin 20w) (4)

This quanti ty is insensitive to sea-quarks and the charm mass, me. In addition, the SSQT beam uses a bend early in the beamline to make the sign selection which removes any sensitivity to the hard to calculate v , ' s from K~ong decay. The expected uncertainty for the NuTeV measurement is 6 sin 20w = 0.0021.

The most precise previous determinations of sin20w in vN scattering are from the CDHS[10], CHARM[I l l and our previous CCFR collaborations[12]. After adjusting to our theoretical assumptions for the charm quark mass, Mc = 1.31 ± 0.24 GeV/c 2 [13], and top quark mass,

Mtop = 150GeV/c 2, these experiments yield the values given in Table 2. The four measurements can be combined into an global neutrino average assuming that a common correlated theoretical uncertainty of 0.0040 is combined with the uncor- related statistical and experimental uncertainties associated with each experiment.

M.H. Shaevitz et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 188 197 191

m

c 14003 >

% 1200S,

E 10002,

8'3'00

6000

4000

2000

l N(

totol

1500 •

1500

l a o o

1 2 0 0

1 9 0 0

BOO

6OO

4 9 0

2 0 0

0 3,3 40 50 50 7D 8 0

0 10 20 30 40 50 60 70 80

event ler'gtq (counters)

Figure 1. Data and Monte Carlo (MC) event length distributions. The experimental data is represented by dots and the MC prediction by the sohd hne. Also shown are the MC contribu- tions from NC rg events ("NC"), CC ru events ("CC") and combined NC and CC interactions from re or ue ("re") . The inset shows the data, total MC and the NC contribution to the MC for the region L>25 counters.

by about two and a half sigma. Most of the experiments will have further running in the next several years which should reduce the uncertainties to below the 100 MeV level where the difference due to the Higgs mass may be observable. In addition, the NuTeV experiment will be able to isolate the p parameter which is related to the ap- parent strength of the neutral to charged current

2 2 interaction, p = gNC/gCC, and which is sensitive to a non-standard Higgs sector.

Table 3 Comparison of the Mw values derived from the various electroweak measurements asumming the s tandard model with Mz = 91.187-4-0.007GeV/c 2. (The uncertainties include a contribution for Higgs masses between 100 and 1000 GeV/c2.)

Measurement Equiv. Mw (GeV/c 2)

uN NC/CC Ratio C D F / U A 2 / D 0 Mw [15] Comb. LEP Results[5] S L D A L R

CDF Mtop = 174 =k 16 GeV/c 2

80.24 4- 0.25 80.23 4- 0.18 80.25 4- 0.10 80.77 4- 0.20 80.29 4- 0.19

The various electroweak measurements can be compared most easily by converting all measurements to an equivalent Mw value within the context of the s tandard model for a fixed value of Mz = 91.187-4- 0.007 GeV[14]. For example, our value ofsin20w gives Mw = 80.444-0.31 GeV/c 2. Table 3 list the Mw values for the recent precision measurements. The agreement is very good within the few hundred MeV uncertainties except for the SLD value which differs from the average

Table 2 sin 2 0w measurements from gN experiments.

Experiment sin ~ Ow CCFR (1994)[7] 0.2218 q- 0.0059 CDHS[10] 0.2225 4- 0.0064 CHARM[11] 0.2319 4- 0.0064 C C F R (1990)[12] 0.2363 4- 0.0114 Average r N 0.2259 4- 0.0048

4. S t r u c t u r e F u n c t i o n R e s u l t s

The F2(x ,Q 2) and ~F3(a:, Q2) structure functions which are associated, in the quark parton model, with the total quark (q + ~ ) and valence quark ( q - ~ ) momentum distributions, can be extracted from the differential cross-section for neutrino charged current scattering. The differential cross- section is given by:

d x d y - - 27r

e 1 + T 2 x F , ( z , 0 2) ± y(1 - ~)zF3(z, 0 2) (5)

We extract the structure functions from the measured number of vu(Yu) events and the in- cident neutrino flux. The da ta sample used in the extraction included 1,280,000 v~, and 270,000 ~u induced events after fiducial and kinematic cuts (Pu > 15 GeV, 0g < .150 , EHAD > 10

192 M.H. Shaevitz et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 188 197

GeV, Q2 > 1 GeV 2 and E~ > 50 GeV). The data were corrected by Monte Carlo techniques for acceptance and resolution smearing. In the analysis, we assume a parameterization of R determined from SLAC measurements[16] and ap- ply isoscaler corrections for the 6.85% excess of neutrons over protons in iron. Based on our measurements[13] of dimuon production, we include scattering offstrange sea quarks and a slow- rescaling threshold suppression for the production of charm quarks. Radiative corrections[17] are applied, and the cross-section is corrected for the massive W-boson propagator. The xF3 structure function[18] extracted with this procedure is shown in Figure 2 along with the next-to-leading fits described in the next section.

l

t

~L

:I

0 !I

i

10 100 10(~

X X X ~ - a_ X x 4~

zc ~ - ~ J- ~ ~ ~- ~ x ~ 0 1 2 5 ( ' 2 )

~ s ~ ~ x ~ O 175 I*l 5)

T "~ ~ 275

~ x = D 3 5 0

x = 0550 ~- ~ x = 0450

- - D ~ O QCD Fit ~ Ext~apolatio n of Fit :#

10 100 I(~00

QX (GeVZl

Figure 2. CCFR neutrino-iron structure functions zF3. The solid line is the NLO QCD prediction and the dotted line is an extrapolation to regions outside the kinematic cuts for the fit.

The zF3 structure function is related in leading order to the momentum distribution of the valence quark in the nucleon. The Gross-Llewellyn Smith (GLS) sum rule predicts that the integral over x ofzF3, weighted by 1/~ should equal three, the number of valence quarks, up to higher order

corrections.

= f01 Q0 Theoretical calculations of this sum rule have been made through next-next-to-leading order[19 ] and an estimate of the higher twist effects has also been presented[20]. To experimentally determine S(;LS, the values of xF3 were interpo- lated or extrapolated to Q02 = 3 GeV 2, which is approximately the mean Q2 of the da ta in the low-x region that contributes the most to the weighted integral. Using this procedure, we ob- tain the result:

SGLS(3 GeV 2) : 2.50 4- 0.02 4- 0.08 (7)

which is in agreement with the NNLO calculation o f SGL S ---- 2.63 ± 0.04119] with A~-7~ : 250 =i= 50MeV.

The predicted Q2 evolution of ~F3 is particularly simple since it is not coupled to the un- known gluon distribution and, therefore, can be used as a unambiguous test of perturbat ive Quan- tum Chromodynamics and measurement of AMs. To leading order, the zF3 evolution is given by:

d~rF3(:r, Q2) _ ot~?2) 2 x 2 dZz -- Pqq(Z) xF: , (z , Q )

As shown in Figure 2, the measured zF3 evolution agrees well with the predicted QCD scaling violations obtained from a next-to-leading order (NLO) fit[21]. The fit includes target mass corrections and cuts of Q2 > 15 GeV 2 to eliminate the non-perturbative region and x < 0.7 to remove the highest z bins where resolution corrections are sensitive to Fermi motion. The statistical precision of the fit can be improved by substituting F2 for xF3 at values of x > .5. The evolution of F~ should conform to that of a non-singlet structure function in this high-x region where the effects of antiquarks, gluons, and the longitudinal structure function are negligible. This combined fit yields the value (for four quark flavors):

A (4) = 2 1 0 ± 2 8 ~ : 4 1 M e V f o r Q2 MS > 15 GeV2(8)

where the first error is statistical and second error is systematic. The X ~ for the fit is 60 for 52

M.H. Shaevitz et al./Nuclear Physics B (Ptvc. Suppl.) 38 (1995) 188-197 193

degrees of freedom. This value of A!4")s " l yields the following value for the strong coupling constant in next-to-leading order:

a , ( Q "2 = M 2) = .111 ~ .002 4- .00Z (9)

The evolution of the F2 structure function is dependent on the gluon distribution as indicated in the leading order expression:

dV~(~,~2 ~) _ ,~.(O. ~) [ f l dtnQ2 - Pqq(Z) F2( x, Q2) dz T

Pqa(z) ~G(~, Q2) a~] (10) +

This dependence allows the gluon distribution to be determined from a simultaneous fit to e2 ( , , & ) and zFz(z , Q2). In this fit, the gluon distribution is parameterized by the functional form zG(z , Q2 o = 5 G e V ~) = Ag(1 - ~),7~ where Ag is fixed by the total momentum sum rule for the nucleon. Using the NLO fitting program[21], the simultaneous fit yields the values:

A ~ . = 225 ± 30-t- 40 MeV for Q2 > 15 GeV 2

and ~/g = 4.45 +1.65 -1.15 ± 1.25 at Q~ = 5GeV 2

where in both cases the first error is statistical and the second is systematic. The X 2 for the fit is 118 for 106 degrees of freedom and the value of A ~ . is consistent with the value from the non- singlet analysis indicating good agreement with the expected QCD phenomenology.

5. N e u t r i n o D i m u o n P r o d u c t i o n a n d t h e S t r a n g e S e a

The distinctive opposite sign dimuon signature serves as a unique and highly sensitive probe of the strange sea content of the nucleon through neutrino charm production.

(s) r'u + d ~ tt + c /z +

S /J#

The strange quark distribution function is of par- ticular theoretical interest in the exploration of higher order corrections and the threshold behav- ;or associated with the heavy charm mass is crit- ical to the extraction of the weak mixing angle,

sin 20w, from neutrino neutral current data. The heavy charm quark is expected to introduce an energy threshold in the dimuon production rate. This effect has been described in the past through the slow rescahng model, in which (, the momentum fraction carried by the struck quark, is related to the kinematic variable z = Q 2 / 2 M u by

2 2 the expression ( = z(1 + raG~ Q )(1 - z '2M2/Q2) . Representing the momentum distribution of the s and d quarks within the nucleon as ( s ( ( ) and (d( ( ) and neglecting Callen-Gross violations, the leading-order cross section for neutrino production of dimuons is given by:

d=<.N--.-.+X> G22M : ( d( )lV dl d~ dy = ¥

+ s( )lrc r ")) (1 m: Bc (11)

where the function D(z ) describes the fragmen- tation of charm quarks into charmed hadrons and Be is the semileptonic branching ratio for charmed hadrons. For D ( z ) , we have used the parameterization of Colhns and Spiller[22] with e = 0.81 ± 0.14 for the NLO fits and the parameterization of Peterson[23] with e = 0.20 4- 0.04 for the LO fits[13]. The analogous equation for an- tineutrinos is found by substituting d(() ~ d(() and s(() --~ ~(().

With the kinematic cuts P ~ > 9 GeV/c, Pu~ > 5 GeV/c, and 30 _< E . _< 600 GeV, a sample of 5030 vu and 1060 O~ induced #~: /z ± events is observed with (Q2} = 22.2 GeV2/c 2. Muonic decays of non-prompt ~r and K mesons comprise the pr imary dimuon background of 797 ± 118 vu and 118 ± 25 Or, events to the above sample[24]. A leading-order analysis of these data has been reported previously[13].

Recently, there has been much theoretical work[25] to extend the leading-order formalism of neutrino charm production to higher orders. Since a dominant contribution to this process is scattering off strange sea quarks, it is expected that the next-to-leading-order terms due to gluon quark-pair sphtting will be significant. Further, a next-to-leading order analysis of the dimuon data should produce results for m~ that are comparable to similar analyses of charm photo- and lepto- production.

194 M.t£ Shaevitz et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 188-197

Table 4 Next-to-leading-order and leading-order fit results, assuming zs(a~) = z~(x). The errors include both statistical and systematic uncertainties.

Fit ~ a Bc rnc GeV/c 2

NLO[26] 0.477 -4- 0.051 -0.02 ± 0.63 0.1091 =fi 0.0097 1.70 -4- 0.19 LO[13] 0.373 -4- 0.048 2.50 -4- 0.65 0.1050 -4- 0.0086 1.31 -4- 0.24

We have used the Aivasis, Collins, Olness, and Tung next-to-leading order programs [25] in a fit to the dimuon data to extract the relevant physics parameters[26]. The procedure involves comparing the number of data events in z and E~ bins with a prediction from a Monte Carlo simulation. The simulation uses the quark and antiquark momentum densities obtained from the structure function measurement described above. The strange quark z dependence is assumed to be related to the ~ distribution by zs(z) ~ (1 - z) ~ a~(z) with the magnitude set by the param-

eter ~ = 2 S / ( U -4- D) where S = f01 a:s(z) dz, etc. The overall normahzation is set by the ratio of data to Monte Carlo for the charged-current sin- gle muon events. The results of the fit are shown in Figures 3 and 4. The extracted parameters are presented in Table 4 along with the previous leading-order results[13].

The charm quark mass parameter from the NLO fit is 1.70,4, 0.19 GeV/c 2, which differs from the leading-order result, indicating a marked dependence of mc on the order to which the analysis is done. This result is in good agreement with the NLO analysis of charm photoproduction where the charm mass is found to be mc = 1.74 +0.13

- 0 . 1 8

[2% Several theoretical models have put forward the

proposition that the s and ~ quarks may have different momentum distributions in the nucleon. For example, postulating intrinsic strange quark states[28] leads to the prediction that the s quark momentum distribution will be harder than the

quark distribution [29]. We have performed a fit in which the momentum distributions of the s and ~ quarks are allowed to have different shape parameters a and a ' , respectively.

In order to reduce the number of free parameters, this fit constrains the average charmed

z

1250 I tO00 75O

500 t 250

°f 40O

300

200, J

IO0 0 0

U

///

0.2 0.4 0.6 0.8

Xvis

Figure 3. The z-distribution of the observed dimuon data for v u (left) and ~u (right) induced events. Also shown are the predictions of the NLO fit (sohd fine) and the r / K background (dotted line).

hadron branching ratio to the value obtained from other measurements, BIc -- 0.099 ± 0.012 as described later. We fit for ~, a, and A a - - a - a ' and the charm quark mass me. The result is:

~; = 0 .536± 0.030+ 0.089,

a = -0 .78 ±0.40,4, 1.24,

A a = -0 .46 + 0.42 -4- 0.76,

mc -- 1.66 -4- 0.16,4, 0.08; GeV/c 2, (12)

where the first error is statistical and the second

M.I~ Shaevitz et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 188 197 195

[

+

I

t3

0.015

0 0 1 0

0.005

0.000 0.015

÷

~- O.OLO t~

I

+ 0 .005

t~

0.000

. . . . I . . . . I . . . . I . . . . I ' ' ~ -

L/

- / _ 0 . . . .

/ NLO c u r v e

(±2o" e r r o r )

I -

/ NLO c u r v e

(±2o" e r r o r )

, , , I . . . . [ . . . . I . . . . I , , _ ~ _ 100 200 300 400 500

E~ (Gev)

Figure 4. The dimuon rate versus E. for u~, (left) and t9 u (right) data. The rates are corrected for acceptance, smearing, and kinematic cuts. Also shown are the predictions of the NLO fit (solid line) as described in the text.

systematic including the B~ and renormalizatiou scale uncertainty.

The value of Aa = -0 .46 ± 0.87 indicates that the momentum distributions of s and ~ are consistent and the difference in the two distributions is limited to -1 .9 < A a < 1.0 at the 90% con- fidence level. This is the first quantitative comparison of the components of the s and ~ quark sea.

If the CKM matrix elements are not assumed, then a four parameter NLO fit can be made for c~, mc and the following products:

IVcal2nc = (5.34 + 0.60) × 10 -3, t~

~+2[Vcsl2Bc = (2.00-t-0.16) × 10 -2 . (13)

These combinations can be used to extract I V~dl 2 and ~lVc~l 2 when B~ is determined from other data. B~ is determined by combining[30] the

charmed particle semileptonic branching ratios measured at e+e - colliders [31] with the neutrino- production fractious measured by the Fermilab E531 neutrino-emulsion experiment[32], using updated values of the charmed hadron lifetimes.

We find B]c = 0.099 + 0.012 and extract the value of the CKM matrix element

IVcdl = 0.232 +_ 0.02o°'°1s (14)

where the error includes both statistical and systematic uncertainties. This value is consistent with the PDG value, IVcu[ = 0.221-4-0.003, which is determined from measurements of the other matrix elements and the unitarity constraint on the CKM matrix assuming three generations.

6. C o m p a r i s o n s t o L e p t o n S c a t t e r i n g

The F2 structure function is accessible both through charged lepton and neutrino scattering. The lepton and neutrino structure functions are related to leading order by the "mean square charge" relationship:

S+q) f ~ N - 1 8 5 q +

Our CCFR results span the Q2 range from the low energy SLAC[33] (eD) region through the range covered by the NMC[34] and BCDMS[35] (/zD) measurements as shown in Figure 5. For this comparison, the deuterium data has been corrected to iron using the F~F~/F~ D ratio measured by SLAC[36] and NMC[34]. The results are in good agreement for x > .1 but show a 5 to 10% dis- crepancy between the neutrino and muon results for lower z. The source of this small disagree- ment is at present not known. Possible sources include experimental systematic errors related to normahzation and calibration or theoretical uncertainties associated with next-to-leading order and higher-twist effects. Recently, Donnachie and Landscho~37] have pointed out that the higher- twist effects may be different for neutrino and charged lepton scattering due to the extra ax- ial vector couphngs associated with W-boson ex- change. In contrast, the original global fits by the CTEQ [38] collaboration have attributed the muon versus neutrino difference to an enhanced

196 M.H. Shaevitz et al./Nuclear Physics B (Prec. Suppl.) 38 (1995) 188-197

strange sea at low x. This possibility is ruled out by our dimuon measurements as shown in Figure 6.

o i

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10

09

o.$

03

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T

Io NMc<4?,- T] ± = I * N M C 2 8 0 C ~ v x = 070ar~x=. ' . ,%"a!

I 10 100 10m

i . . . . . . . . i i

T - - - r - [

° NMC 90 GeV 4. NMC 280 Ge'¢ • SLAC 1 e. B C D M S

I Io ioo Imo

Figure 5. Comparison of the F2 values measured by CCFR (vFe), SLAC (eD), NMC (#D), and BCDMS (/~D) for • -- .080 and .275. The lepton- deuterium results have been converted to be equivalent to t~Fe scattering using the " 5/18ths rule "with the strange sea extracted from our dimuon data and the heavy target correction.

7. S u m m a r y a n d P r o s p e c t s

When the Fermilab Tevatron came on line in the mid-1980's, the increased energy available for fixed-target experiments made a qualitative im- provement for neutrino scattering studies. The increased flux and cross section allowed high statistical samples to be accumulated in the high- energy region where theoretical uncertainties are minimized. Over the last several years, the Teva- tron intensity has been significantly increased with upgrades to the Linac and other accelerator components. This increased intensity will directly translate into higher precision measurements for the next neutrino scattering experiment at Fermi- lab, NuTeV[9]. Table 5 summarizes the present

5~

0 . 3

0 . 2

0 . 1

0 . 0 0 . 0 1

MS ]z z= 4 . 0

\

0 . 0 5 0 A 0 . 5

X

Figure 6. The strange-sea a~ distribution extracted from the NLO fit to the CCFR dimuon da ta compared to the distribution of the C T E Q global fit. The band on the CCFR curve indicates the ltr uncertainty in the distribution.

CCFR results along with the expected sensitivity for the NuTeV experiment. By combining the high intensity of the upgraded Tevatron with the systematic cleanliness of the new SSQT beam, the NuTeV experiment will continue the long his- tory of precision neutrino scattering experiments at Fermilab.

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Table 5 Current and future Tevatron neutrino measurements

Measurement Current Expected CCFR NuTeV Value Error

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neutrino results from the fermilab tevatron

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