In this presentation the equipment is briefly described and the method of analysis used for all events is reviewed. New results are then presented on the ratio of neutrino cross sections on neutrons to that on protons. One is reminded of the published event indicating an apparent $delta$S = -$delta$Q current. Finally, the parameters of all the other charged current events containing at least one strange particle are presented. (auth

Cazzoli, E. G.

Cnops, A. M.

Connolly, P. L.

Murtagh, M. J.

Palmer, R. B.

Samios, N. P.

Tso, T. T.

Williams, H. H.

UNT Digital Library

Presented at:Int'l Symposium on Lepton & PhotonInteractions at High EnergiesStanford, CaliforniaAug. 21-27, 1975BNL-20544m- 320Recent Results fromthe Brookhaven National Laboratory 7' Bubble Chamber*E.G. Cazzoli, A.M. Cnops, P.L. Connolly, M.J. Murtagh,R.B. Palmer, N.P. Samios, T.T, Tso and H.H. WilliamstBrookhaven National Laboratory, Upton, New York 11973In this presentation we will briefly describe the equipmentand review the method of analysis used for all events. We will thenpresent new results on the ratio of neutrino cross sections on neu-trons to that on protons* We will remind you of the published eventindicating an apparent AS--AQ current. Finally, we present theparameters of all the other charged current events containing atleast one strange particle.NOTICE- — • —I This report Wai prepared ai an account of wurhiponiored by the United Staid (riivnnmrni Nrithfi(he United States nni the United Slalcs Kneff-yRewaich and Ot^rlnprnent Ailmtnlttratlan, nur any ofthru employeei, nor any of thtit contractor*,tMbcontitctan, or theit rrnpfureri, makM anywarranty, t\ptea or implied, or auumtt any lt£aor uiefulnen of *ny inturrmtion. apparatut, puxtuci 01(iftiiesi dbclowd, or icprrwrii that its we wnulil notinfringe pnvir*!y nwncJ rl/chfi.* Work performed under the auspices of the Energy Research and De-velopment Administration.t Present Address: Department of Physics, University of Pennsylvania,Philadelphia, Pennsylvania 19174.- 1 -The neutrino beam is sketched in Fig. 1. A proton beam of125 x 10 protons per pulse strikes a water cooled sapphire target onceevery 1.5 seconds. Pions and kaons emerging from the target are fo-cused by a two horn (finger) system and allowed to decay inside a 57 inlong tunnel. Muons from the decays are stopped in 30 m of iron andneutrinos pass through this shield to the bubble chamber beyond.The bubble chamber is approximately 2 m (7 ft.) in diameter and3 m high with a visible volume of 7 m . Inside the chamber there arefour steel plates, each 5 cm thick and about 1.5 m x 1.5 m, locatedon the down stream side of the chamber. They are set on an angle of30 to the beam, such as to intersect the maximum number of negativetracks (see Fig. 1). The visible volume in front of the plates is5 m3.The data that will be presented has come from a sample of 62,000pictures taken with hydrogen in the chamber and 220,000 pictures withdeuterium.Events were initially selected if they contained at least twotracks. Three pronged events representing the reaction p + p -*• p + pwere removed if they fitted. The vector sum of momenta of all trackswas then computed and its direction plotted in azimuth and dip. Sucha plot for a partial sample of the data is shown in Fig. 2. A clearseparation is seen between beam related events (azimuth * 0, dip i* 0),and cosmic ray events (dip <\» 90 ). A cut was applied requiring thevector momentum to be within 40 of the beam direction and a furthercut required the sum of the vector momenta to be greater than A00 MeV.After these cuts, there remained 95 events from the hydrogen exposure- 2 -and 476 from the deuterium.Neutral current events were identified when there was no utrack candidate; the flux of antineutrinos that would give eventswith no v~ being negligible (IX). The u hypothesis could be re-jected whenever a track interacted in the chamber or plates. It wasalso rejected if the track made a IT - y - e decay or was observedto stop in the chamber with no decay electron. Fig. 3 shows the prob-ability for such negative track identification as a function of thetrack momentum. For comparison, the dashed curve shows the case hadthere been no plates. The advantage of the plates is clear. Back-ground for the neutral current sample from beam related neutrons wasdetermined by identifying events that fit the reaction n + p->-p + p +Only three such events were found, indicating a small background fromsuch neutrons and allowing, using known cross sections, ?n estimateof the magnitude of this background in individual channels. Analy-sis of the neutral current events is still in progress and will notbe further reported on here.We will now restrict ourselves to the charged current eventsand report on the ratio of cross sections on neutrons to that on pro-tons. In this charged current sample, the background from neutronsis negligible and the contamination from unidentified neutral cur-rent events is sufficiently small as to introduce errors that aremuch less than those due to statistics. In order to obtain the neu-trino energy and other parameters for each event in the sample, eachevent was examined by a physicist and the tracks identified byionization or range whenever possible. Events with multiple fit.*- 3 -were given appropriate fractional weights in all distributions. Inpractice, 90% of all events were unambiguously identified so thatany smearing of distributions from this cause is small. When thekinematics indicated that one or more TT°'S were missing, the eventwas calculated on the assumption of its being a single ir . Similarly,if « neutron with or without ir 's was missing, it was assumed thatonly the neutron was present. Since the number of events falls rap-idly as a function of track multiplicity, this treatment introduceslittle bias to the data. The distribution of the neutrino energycalculated in this way is shown in Fig. 4.We will now consider the method used to estimate the relative'ontribution from interactions of the neutrinos on the neutrons andprotons. For this purpose we will restrict ourselves to the deuter-ium data only. If in this data we see a number of events with aneven number of prongs (n ), then we identify them as interactionson neutrons in which the spectator protons were not seen. If, onthe other hand, we see events with an odd number of prongs (n j d),then they can be interactions on protons, or, alternatively, inter-actions on neutrons in which the proton spectators were visible. Ifwe define C to be the ratio of seen to unseen spectator protons,then it is trivial to conclude that the number of events on protons(n ) and on neutrons (n ) are given by:neven <* + C>np " nodd " neven(1)- 4 -We can determine C by observing the ratio of events that fitthe reaction v n (p ) •*• y p (p ) in which the spectator proton (p )s s sis either seen or not seen. The numbers of events observed were 42and 131 respectively, yielding a value of C:C = .32 + .06 . <2)We note that the use of this value of C in equation (1) assumes thatC is not dependent on the event type. Large variations are not ex-pected, but some increase in C as a function of multiplicity shouldoccur and is in fact indicated by a study of the reaction v n (p )-y v p v v (p ) with the spectator seen and unseen (12 and 24 eventssrespectively). If this data is used together with the above, we ob-tain a modified value of C:C • .35 + .06 . (3)These two results (equations (2) and (3)) agree within the errors.2We will here use the second (equation (3)) since it does, at leastto first order, allow for such variations of C with multiplicity.The observed distribution of charged current events as a func-tion of the number of observed prongs is:Number of Prongs Number of Events2 1713 2494 245 246 27 2- 5 -We have thus observed 197 events with even prongs and 275 withodd prongs. Using equations (1) and (3) and applying a correction2for a 13% contamination of hydrogen in the deuterium used, we obtainfor the ratio of neutrino total cross section on neutrons a(n) tothat on protons cr(p):± -This result may now be compared with various parton theoreticalexpectations. Barger and Phillips obtain a(n)/a(p) =2.05 whichdoes not agree with our result. However, their prediction for theratio of deeply inelastic electron scattering is also in disagree-4ment with that experiment. McElhaney and Tuan fit the electron dataand predict a(n)/a(p) =1.72 ' , where the errors reflect those ofthe data used. This is clearly in agreement with our result.It should be stressed at this point that the theories quotedgive predictions for this ratio at asymptotic energies, a situationfar from that present in this experiment. The only justification forcomparing such predictions with this experiment is that other asymp-totic predictions, such as those for the total v and v cross sec-tions, have been realized at low energies. This situation has beendescribed as "precocious scaling" , a concept that further impliesthat distributions of events at low energies in the variable2 2x' « q /(2Mv + M ) should approximate the asymptotic distributions2in the variable x « q /2Mv. (Here q is the 4 momentum transfer be-tween the neutrino and lepton, v the fourth component of q, and Mthe nucleon mass.) In order to further test this hypothesis we can- 6 -plot the distribution of all our events as a function of x' andcompare this distribution with the parton theory. Fig. 5 shows thisdistribution. The smooth line is a parton prediction based on theassumption that there are only p and n quarks in the nucleon. Thecurve is then obtained directly from the function vW (en) and vW (ep)derived experimentally from electron scattering . The line is dash-ed in the small x' region since the above assumption that ignores a"sea" of quarks and antiquarks renders the theory inapplicable tothis region. The agreement between the theory and the experimentalresults as seen in this figure represents further evidence for pre-cocious scaling.We can now present the ratio of a(n)/a(p) as a function of thevariable x' and again compare the result with the asymptotic predic-tions for :c obtained from the parton theory. This is shown in Fig. 6.The line is again obtained assuming no sea, but we have omitted todash the curve in the small x' region since charge invariance requiresthat the ratio be 1 at x1 = 0, thus assuring that the introductionof the sea cannot have a very large effect on the prediction evenfor small x'. It is seen that although the trend of the data is inagreement with the prediction, there is a suggestion of a departurein the high x1 region.In the last part of this paper we wish to discuss the chargedcurrent events containing strange particles. We will use for thisthe data from both the hydrogen and the deuterium exposures. T£.ble I2gives the event type3, constraint class, interpretations, q and x2(where x - q /2Mv, q and M, are defined as above) of the events.- 7 -The first event in the table has been reported in the literatureand is shown again in Fig. 7. It represents an apparent AS=-AQ cur-rent which would be an expected signature for the production and de-cay of a charmed baryon. The effective mass of the hadron system inthis event is 2426 + 13 MeV and this represents either the mass orthe upper limit on the mass for such a baryon. The probability thatthis event is a misinterpreted associated production event is calcu-lated to be 3 x 10 . This value must be compared to an estimatedprobability that a charmed baryon be produced and decay in a way suchthat it would be identified in this experiment. For this purpose weneed a cross section for charm production as a function of energy,the branching ratio of the charmed baryon into scates with no miss-ing neutral and the probability that at production no other neutralis produced. Obviously such data is unavailable. However, if we2assume the charm/noncharm ratio to be of order sin 6 (̂  3%) and coa-cstant above threshold O 4 GeV), take the branching ratio of charminto identifiable negative strangeness to be 50%, and further assumethat the ratio of charm events with and without neutrals is similarto that observed for associated strange particle production 50%, thenwe can obtain an estimate for the number of charmed particle eventsthat would be Identified by their 3C fits. Since we have 74 eventsabove 4 GeV, we would expect to have identified 0.6 (74 x .03 x .5ox .5) such events . The a priori probability that such events onceseen could be ascribed to associated production Is estimated to be of theorder of a few 10 . The much lower value of this probability inthe event observed arises primarily from the fact that its energy- 8 -(13.5 GeV) is near Che upper limit of the spectrum. On the basisof these assumptions, we would expect •v 10% of identified charm eventsto have this energy or higher.Returning now to Table I, we note five other events containingstrange particles, one of which 02) cannot be associated productionoand is consistent with a AS=AQ current. We note that since a AS»AQinteraction implies in parton theory that the initial interaction oc-cured on a X quark in the parton "sea," the x of such an event shouldtherefore be low. The observed value of .13 is consistent with thisexpectation.Of the remaining four events that are all candidates for assoc-iated production, two (#4 and #6) could in principle be AS—£Q. Theirhadronic masses on this assumption are less than or equal to ̂  3.5and 2.40 + .04 GeV respectively. The agreement between the secondmass and that for the identified AS—AQ event say be coincidental butis interesting. For the moment, however, we will assume that we haveobserved one AS—AQ, one AS*AQ, and four associated events. Theratios of observed strange particle events to the numbers of non-strange events are thenRatio Observed Strange to NonstrangeFor E > 4 GeV1.4 + 1.421.4 + 1.4Z5.4 + 2.7%Event TypeAS—AQAS-AQAssociatedProductionNumber ofEvents114REFERENCES1. R.B. Palmer, Proc. Informal Conference on Experimental NeutrinoPhysics, CERN, p. 141 (1965).2. The result actually presented at the conference was based onequations (1) and (2) which gave c(n)/a(p) - 1.40 + .14, however,the possible bias due to variations of C with multiplicity wasstressed. The result given here allows for this bias and isthus more reliable.3. V. Barger and R.J.S. Phillips, Nucl. Phys. B.73, 269 (1974).4. R. McElhaney and S.F. Tuan, Nucl. Phys. B.72, 487 (1974).5. iUoom and Gilman, Phys. Rev. Lett. £5, 1140 (1974); H. Deden,et al., Nucl. Phys. B85, 269 (1975).6. G, Miller, ££al., Phys. Rev. D5, 529 (1972).7. E.G. Cazzoli, etal., Phys. Rev. Lett. 34, H25 (1975).8. We can apply this same method of estimation to the FNAL bubblechamber experiment. Out of 450 events, we would expect 6 charmevents with identifiable negative strangeness, of which one-sixth(6 out of 36 strange particle events) would make 3C fits. Thuswe would expect FNAL to observe 1.1 (450 x .03 x .5 x .1*) iden-tified charm events.9. S. Triectan, private communication.TABLE IEvent TypeNeutrino 2Constraint Energy ^ _Class GeV Interpretations GeV/c x1 v p •*•U A IT TT TT Tf 3C 13.5 AS—AQ 2.2 .312 vp •*U ir p K 3C 5.2 AS-AQ .5 .133 vd •+3C 7AssociatedProduction 7 ,4 vd •»•y'ir"ir+p K°(K°)(ps) OC 6.5AssociatedProduction.4 .055 vd •*•3C 3.1 AssociatedProduction .3 .056 vd ->•OC -5<AssociatedProduction '19FIGURE CAPTIONS1. The neutrino beam.2. Distribution of a typical sample (̂  30%) of events in the dip(X) and azimuth (6) of the direction of the sum of all trackvector momenta.3. The efficiency for separating negative pions from unions as afunction of their momentum.A. fiistribution of all charged current neutrino events as a func-tion of calculated incoming neutrino energy. Events containinga strange particle are indicated in black.5. Distribution of charged current events in the variablex' • q /(2Mv + M ). The line represents a parton theory pre-diction that ignores the "sea."6. The ratio of neutrino cross sections on protons to that onneutrons as a function of x'. The curve indicates a partontheory prediction that ignores the "sea."7. Event #1 which fits the reaction v + p->uAirirTr;r and thusrepresents an apparent AS=-AQ current.60• *• • •* *oosoO Oa-IiIJ( rj /1" J1\I/•.«Ii£60l .o H O WJOFig.XFig. 5oc'Fig. 6* -I-V •\

Recent results from the Brookhaven National Laboratory 7' bubble chamber

https://digital.library.unt.edu/ark:/67531/metadc864796/m2/1/high_res_d/4108225.pdf

Recent results from the Brookhaven National Laboratory 7' bubble chamber

Abstract

Similar works

Full text

Available Versions

UNT Digital Library