EMC search for higher-twist effects in the hadronic final state of deep inelastic muon scattering

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Nuclear Physics B (Proc. Suppl.) 7B (1.989) 200 20.1 North Ilollan,I, Amster,lam

EMC SEARCH FOR HIGHER-TWIST EFFECTS 1N THE HADRONIC FINAL STATE OF DEEP INELASTIC MUON SCATTERING Chafik BENCHOUK Centre de Physique des Particules de Marseille, Facultd des Sciences de Luminy, Case 907, F-13288 Marseille Cedex 09

The analysis of higher-twist effects in the hadronic final state made by the EMC Collaboration is presented. These effects were searched through the specific y-z, PT-Q2 and ~ -z correlations. The expected correlations were not observed.

1.

INTRODUCTION Following the suggestion of Berger 1, the EMC Collaboration carried a search for

higher-twist effects in the hadronic final state of deep inelastic muon-proton scattering. The prediction of Berger implies that the variables Q2 y, z, Pw and qb will be correlated. These variables are the standard variables of deep inelastic lepton scattering. _Q2 is the four-monlentunl squm'ed of the virtual photon. In the laboratory, system of reference, ~2 is the energy of the virtuuI photon ( "v = E~a-E'/.t,El.t and E'g being the energy of the incoming muon and that of tile outgoing muon), while y=(~) / E) is the fraction of the incoming lepton energy cm'ried by the virtual photon. z=(EtT~) ) is the fraction of the photon energy carried by the obse~,ed hadron of energy Eh. Besides the z v,'u'iable which describes the longitudinal properties of the hadron, we also study the transverse properties with PT' which is the transverse momentum of the hadron with respect to the photon direction and qb which is the azimuthal angle of the hadron relative to the plane which contains tile incoming and the outgoing muons. It is worth recalling that one of the advantages of the charged lepton scattering experiments is the conect measurement of all these variables, via the precise detem-tination of the incoming and outgoing lepton momenta. Taking into account tile fact that the elementary constituents are not free but always bound in hadronic wave functions, Berger 1 predicts the following fotIllklia for the cross-section of the process ~a p+ Ix' z X: dcr 1 z dzdydqb dP~- o¢ yp~Q2 (l-z)2 [ l+(~--Y)2] + 3 (1-z)(2-y)(1-y)t/2 cos qb~

0920-5632/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

j

+ ~ ( 1 - y ) Q2j

(1)

('. Benchouk / E,'~I(' search for higher-twist efl'ects

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The first term inside the brackets corresponds to the standard cross-section which is expected froth the quark-parton tnodel (in the Q2...+ oo limitl. The second and third terms are the higher-twist contributions which ,,,,'ill change the hadronic distributions for z ----) 1. More precisely, these higher-twist contributions lead to specific correlations: 1) y-z correlation: 2) PT-Q 2 correlation: 3) qb-z correkttion.

2.

E X P E R I M E N T A L DATA The search for these correlations was carried by' the European Muon Collaboration using the

data taken during the NA2 experiment (1978 to 1980) 2. Data ,,,,,ere taken in different runs v,,ith rnuons of energy I20, 200 and 280 GeV, scattering on a 6 m long hydrogen target. The hadrons were analysed in the EMC forward spectrometer °. At that stage of the experiment no hadron identificution was performed and all hadrons ,,,.'ere treated as pions, which are dominant in the forward region. Notice that, for fast hadrons, the mass rnisassignen~ent introduces no effect on the z value. Let us recall th'tt at a subsequent stage of the experiment (NA9), the hadron identification x~its made possible, but with too low statistics in the very forward region to allow fl)r such an analy'sis as the search for higher-twist effects. After the application of some cuts, choosen to avoid regions of small acceptance and to ensure small systematic errors, 143 (l()() events remained and were used for the following analysis,

_~.

y-z C O R R E L A T I O N ,

,')

.

.

One of the consequences of tile fomaula 1) is thut at high z (Q+ being fixed), the cross section should ,,'at}, its (1-I,'). An increase of the cross section ,.,,'ill then appear if y is decreased. In order to avoid effects conning fi'om scale breaking 4, one has to fix the values of and Q". The vuriation of ,',' is then experimentaly obtained by' ch:mging the beam energy (y= "9 / E). This was done for 70< ,,~ <100 OeV, the domain in "9 v.hich is common for the three data sets. The higher bound is clctermined by the 120 GeV data and the lower bouncl is that of the 280 GeV. i recall that a high cut in ,) in each of the data set is needed to maintain the radiative corrections lov,, and thut a low cut is irnposed to ensure a good detemlination of the high z spectrum (high values of the scattered muon energy leads to a large relative error in ",2 and hence in z). Fig. 1a shows the energy, scaled differential multiplicities of charged hadrons for data t ~ e n at each incident muon energy in the range (5
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C. Benchouk / EMC search for higher-twist etfocts

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FIGURE 1

4.

PT-Q 2 CORRELATION From previous studies on the transverse momentum properties in the hadronic final

state 5,6,7, we know that the average tranverse nlomentum squared is growing with z (the so-called "seagull effect"). A convenient parameterization of is: = z 2 + frag + QCD + (soft gluons) where frag and QCD are the contributions due to tile fragmentation and to gluon emission. The soft gluons contribution is needed to describe tile global event properties, in particular tile forward-backward PT balance6'7' but its effect is tile same as the intrinsic k T and, in particular for an inclusive study of the forward hemisphere, a value of =0.7 GeV is found to reproduce correctly the data (the quoted value including in fact the soft gluon contribution). One essential conclusion of those studies is that no dependence on Q2 was ever seen. The formula (1) also implies a dependence o f < PT ? > on z. But, in addition, it predicts a dependence on Q2. This formula shows that for (PT2/Q 2) << (l-z), (el cy/dPr2 )decreases as PT 4, but at large

('. Benchouk / EMC search for higher-twist effects

20:1

z, the higher-twist terms contribute with a PT -- component, which is proportional to Q2. At htrge z, the value of would then increase if Q2 decreases. Fig. 2 shows the ratio of in the two ranges indicated tot" the three muon energies. The measured ratios are consistent with 1 over the whole range of z and tile data are inconsistent with the rise at high z expected from the higher twist effects.

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~

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0.4

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FIGURE 2

5.

qb -z C O R R E L A T I O N Previous measurements of the moments of the azimuthal distributions showed that

is negative 8,9. Those measurements were in particular advocated as a clean test o f QCD 10 within which was predicted to be negative, with a ( l - z ) we dependence. It was found that the QCD contribution is washed out by the hadronization effects and that the dominant contribution to tnegativc) is due to tile intrinsic k T . Tile higher-twist contribution to is predicted to be positive with a (l-z) -1 dependence. Fig. 3 shows tile values of /fl(y), where ft(y) is a kinematic factor given by:

( 2 - y ) ( l - y ) 1/2 fl(Y) = >(1-}0

°

To compare the data to higher-twist predictions, the contribution from the higher-twist must be added to the QCD and intrinsic k w contributions. This was done following reference 11 with a value of kw=0.7 GeV, giving the smooth curves which are plotted. One sees that is always negative. The rise at high z of is not observed in the 280 GeV data but difficult to exclude in tile 200 GeV data.

C. Benchouk / EMC search for higher-twist etTect.~

204

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E = 280 OeV ( y ) = 0 55

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6.

CONCLUSIONS The experimental study of the higher-twist effects in the hadronic final state of deep inelastic

scatteringis clearly a dificult subject, these effects being confined to the highest range of z where the statistics are very low. The search for these effects, made by the NA2 experiment showed that: 1) the expected variation with y at high z of the cross section cannot be tested; 2) the variation of < PT 2 > with Q2 at high z is absent in the data.; 3) is always negative and doesn't have the expected z dependence. REFERENCES

i)

E.L. Berger, Z. Phys. C4 (1980) 289.

2)

EMC, J.J. Aubert et al., Z. Phys. C30 (1986) 23.

3)

EMC, O.C. Allkofer et al., Nucl. Instr. Meth. 179 (1981) 445.

4)

E]kIC, J.J. Aubert et al., Phys. Lett. I l a b (1982) 373.

5)

EMC, J.J. Aubert et al., Phys. Lett. 95B (1980) 306.

61) EMC, J.J. Aubert et a1., Phys. Lett. 119B (1982) 233. 7)

EMC, M. Arneodo et aI., Phys. Lett. 150B (1984) 45,g.

8)

EIk'IC,J.J. Aubert et al., Phys. Lett. 130B (1983) 1 I8.

9)

EMC, M. Arneodo et ill., Z. Phys. C34 (1987) 277.

10) H. Georgi and H. D. Politzer, Phys. Rev. Lett. 40 (1978) 3. 11) H. Koenig and P. Kroll, Z. Phys. C30 (1986) 23.