RESULTS FROM THE UA2 EXPERIMENT AT THE CERN SPS COLLIDERThe UA2 Collaboration-Univ. Bern-CERN-NBI Copenhagen-LAL Orsay-Univ. and INFN Pavia-CEN Saclay

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RESULTS FROM THE UA2 EXPERIMENT AT THE CERN SPS COLLIDERThe UA2 Collaboration-Univ.

Bern-CERN-NBI Copenhagen-LAL Orsay-Univ. and INFN Pavia-CEN Saclay

J.-P. Repellin

To cite this version:

J.-P. Repellin. RESULTS FROM THE UA2 EXPERIMENT AT THE CERN SPS COLLIDERThe UA2 Collaboration-Univ. Bern-CERN-NBI Copenhagen-LAL Orsay-Univ. and INFN Pavia-CEN Saclay. Journal de Physique Colloques, 1982, 43 (C3), pp.C3-571-C3-578. �10.1051/jphyscol:1982376�.

�jpa-00221931�

RESULTS FROM THE UA2 EXPERIMENT AT THE CERN SPS COLLIDER

The UA2 C o l l a b o r a t i o n - U n i v . Bern-CERN-NBI Copenhagen-LAL O r s a y - U n i v . and INFN Pavia-CEN S a c l a y

Presented by J .-P. R e p e l l i n

Division EP,'CERN, CH-1211, Geneva 23, Switzerland

RESUME

Nous présentons des résultats concernant les sections efficaces inclusives de production de mesons ir°, ir+ et TT~ et les rapports de production K/ir et p+p/ir.

L'étude des états finals à grande énergie totale transverse montre clairement la production de jets à haute énergie.

ABSTRACT

We present results on inclusive cross sections of TT° , TT+ and IT" .production and particle ratios K/ir, p+p/fr.

High transverse total energy final states have been observed and show clear evidence for high energy jets.

EXPERIMENTAL APPARATUS

A longitudinal cross-section- of the UA2 detector is shown in Fig. 1. The colli- sion of the proton and antiproton beams occurs in the centre of the apparatus. A vertex detector surrounds the intersection. It consists of an assembly of cylindrical proportional and drift chambers. There are four proportional chambers with analog read-out of helicoidal cathode strips, and two JADE type drift chambers with multihit read-out of time and charge division. These chambers are operated at atmospheric pressure. There is no magnetic field in the vertex detector volume.

The polar angular range,40 to 140 to the beam, corresponding t o ± 1 unit of rapidity, is covered by the central calorimeter. It is segmented into 240 towers pointing to the interaction point. Each tower consists of an electromagnetic compart- ment of 17 radiation lengths and two hadronic compartments, adding up to 4.5 absorp- tion lengths. The electromagnetic and hadronic energies are measured with an accuracy a(E)/E = 0.14/vU and 0 . 6 / ^ respectively (E in GeV).

The polar angular range 20° to 37.5 and 142.5° to 160 is instrumented by a forward and a backward detector. Each corresponds to one unit of rapidity on both directions. The forward and backward detectors each consist of a toroidal magnet, drift chambers, proportional tubes, and electromagnetic calorimeters. The integral of the magnetic field is 0.38 T-m in average and allows with the drift chambers to measure the momentum of charged particles. In particular the sign of the electron charge in the case of W •+ eV decays can be determined in a region where the charge asymmetry is important. The forward-backward electromagnetic calorimeters are subdi- vided in 120 cells with two compartments each, 24 and 6 radiation lengths. The energy resolution for electrons is a(E)/E = 0.15//E (E in GeV).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982376

JOURNAL DE PHYSIQUE

p-p expenment UA2

-- -

-

FORWARD-BACKWARD CALDRIMETER

Figure 1 A transverse section of the apparatus at the intersection point is shown in Fig. 2.

Four out of 24 slices of the central calori- meter have been removed for the first opera-

,,,,,,,

tion of the collider to give room for a large angle magnetic spectrometer. It covers 30' in azimuth and 68O in polar angle (1.7 units of rapidity). The field integral is about 1 T-m.

The charged particle trajectories are measured after the plagnet in a set of 12 drift chambers, "

They are followed by scintillator-iron-

scintillator sandwich for time-of-flight measu=

rement and an array of 280 lead-glass blocks, 14.5 radiation lengths deep. The energy resalution is alE)/E = d11.6 + 32.5/E % (E in GeV).

Figure 2 LUMINOSITY AND TRIGGER COUNTERS

The data uqed for the results presented at this Conference have been taken in the first collider period between end November and end December 1981.

A minimum bias trigger was used : it consists of a coincidence between two arrays of scintillation counters located at k10.3 metres from the interaction point.

Each array covers a rapidity gap from 4.2 to 5.3 units.

The results presented here have been taken with four different triggers in coincidence with the minimum bias requirement.

In the large angle magnetic spectrometer the scintillation counters and the lead-glass array provided a "charged" and an "electromagnetic" trigger, respectively.

They cover k0.7 units of rapidity and 30' in azimuth.

A total electromagnetic transverse energy trigger resulted from a minimum value of the summed response of all the electromagnetic compartments of the central, for- ward and backward calorimeters. This trigger covers +-2 units of rapidity and about 80% of 2Tr.

In the same way a total transverse energy trigger was obtained from the energy deposited in the electromagnetic and hadronic compartments of the central calorime- ter. It covers 1 unit of rapidity and 300' in azimuth.

The last two triggers have been recorded for most of the running period and the corresponding integrated luminosity is about 80 inverse microbarns.

RESULTS ON INCLUSIVE PARTICLE PRODUCTION

The photons emitted in the acceptance of the large angle magnetic spectometer are observed in the lead-glass array. The energy deposited in the lead glass cells is reduced to clusters, two photonscan be resolved into separated clusters provided their separation exceeds 25 cm. From the energy and the centroid position of the clusters a two-photon mass is calculated. This mass spectrum is shown in Fig. 3. A clear s o peak appears over a combinatorial background of 20 to 30X depending on the transverse momentum. A signal due to the rl -t 2y decays is observed when stricter cuts better suited to this decay mode are applied to the data. This is shown in the inset of Fig. 3 . The amount of observed 17 is compatible with the ratio rl/so of 0.55 observed at the ISR.

From the sb signal we extract the inclusive cross-section from 1.5 to 4.5 GeV/c transverse momentum averaged over 20.7 units of rapidity. It is shown in Fig. 4. The solid line shows the so cross-section measured at the ISR at

&

= 53 GeV. At high transverse momentum we observe a large increase of the s o cross-section when tlie energy rises from 53 to 540 GeV.

l o O O c , , , , ,

M,

, (~ev/c'I 0 I I 2 I 1 ' 14 I 6 I

Figure 3 ^P, ( G e V / c )

a (n 'en*) / 2 LAMS Figure 4

+ and n- hi32ceunive c h o b b - n e d o n

Z---

The charged particle spectrum is ob- tained by measuring the momentum of char-

f

ged tracks in the large angle magnetic

*

-

₂ spectrometer flight measurement in the scintillator (LAMS). From the time of

4 counters located between the drift cham- i bers and the lead-glass array we can

separate the charged pions from the kaons and the protons and antiprotons up to 1.4 GeV/c Sransvezse momentum.

The n and n cross-sections are equal and the averaged value of the two is displayed in Fig. 5 between 0.4 and 1.4 GeV/c transverse momentum, and for

3 0 a rapidity range of 20.7 units.

PrlGeV/cl

Figure 5

JOURNAL DE PHYSIQUE

In Fig. 6 we present a fit to the (n+ +n-)/z and no cross-sections of the form E d3a/dp3 a l/(po + pT)n

A strong correlation between po and n is observed. The value of po resulting from the fit is close to 1 GeV/c. Whenthevalue of po is fixed to 1 GeV/c then the exponent determined in the fit is n = 8.31 f 0.06

Figure 6 Figure 7

The technique used in the lead-glass array to measure no has also been applied to the forward backward electromagnetic calorimeters. The inclusive cross-sectlon obtained from these data covers a p _{range of} 0.5 to 2.2 GeV/c and a rapidity between 1.1 and 1.7. Th$ meSsuremen'E is shown in Fig. 7 together with the fit we have performed on the n

,

ⁿ

,

and n o cross-section at 90'.

K/n

and

p,;/n p d & e W o n

...

The charged particle identification gives the possibility to measure separately the charged kaon spectrum from 0.4 to 1.2 GeV/c transverse momentum and the proton- antiproton spectrum form 0 . 4 to 1.4 GeV/c.

Cross-sections for particle production are found to be equal to those for the corresponding antiparticle and their average values are compared

0

u -

0.6 0.8 1.0 1.2 1.4 1.6

Transverse MassIGeV/cz\

1 5 to the pion cross-section : the result is shown in Fig. 8 for K/n and p(p)/n as a function of the transverse mass

5

⁼

v.

^{1 0}

The average value for K/n is 0.39 ? 0.04 to be compared to 0.23 at ISR energies. The average value for p/r is _0.5 1.02

+

0.07 similar to ISR

energies.

Figure 8

ds=540 ~ e ~ / c '

-

P+P

Average

- 1.02+0.05+0.05 S t a t Syst

K/n

-

+

^I

^I

Average 39%t2%-+3%

-Lr - - - +- ^-7-

^-

^{-- --}

S t a t Syst ISR 28%

OBSERVATION OF LARGE TRANSVERSE ENERGY FINAL STATES

The distribution of total transverse energy CET deposited in the central calorimeter is measured over the rapidity interval -1 to +l and the azimuthal range of 300°.

The corresponding cross-section is shown in Fig. 9. All the points follow an exponential fall-off up to 60 GeV. If the transverse energy sum is limited to an azimuthal angular range of 60° we obtain the second set of points shown in Fig. 9.

We observe a clear departure from an exponential law beyond 15 GeV. This suggests that as the transverse energy increases, it tends to concentrate in a limited solid angle.

\

^A Minimum bias

Etot (low threohold)

.

^{.... )}

30-c mc330m

Figure 9

JOURNAL DE PHYSIQUE

As an example we show in Fig. 10 the display of an event with CET = 26 GeV. The central map gives the distribution of the energy deposited in each tower of the central calorimeter. The lower and upper numbers are the energy deposited in the electromagnetic and hadronic compartments, respectively. The side maps correspond to the forward and backward calorimeters.

The energy appears to be distributed among many clusters of cells.

Figure 10

Fig. 11 shows a perspective display of the highest energy event observed in our sample. The total transverse energy of this event is 150 GeV and the energy is clearly concentrated in two clusters of about 70 GeV each.

Figure 11

A more quantitative evidence for the appearance of jets as the total transverse energy increases is given in Fig. 12. Energy clusters are defined and the fraction of total transverse energy contained in the highest and the two highest energy clusters is calculated. These average fractions are plotted as a function of the total transverse energy CE

.

It is clear that as CE increases one or two clusters carry most of it. It shou1;fi be emphasized that the average number of cells in a T single cluster never exceeds 5.

A p r e l i m i n a r y measurement of t h e j e t c r o s s - s e c t i o n i s given i n F i g . 13. The p o i n t s below 20 GeV can be a f f e c t e d by t h e 20 GeV t r i g g e r t h r e s h o l d , and t h e uncer-

t a i n t y i n t h e energy s c a l e i s about 2 GeV. The s o l i d l i n e r e p r e s e n t s a QCD predic- t i o n from Horgan and Jacob and i s a t a s i m i l a r l e v e l a s t h e d a t a .

@

UA2 preliminary

q

vs 9 E(cluster)>2OGeV

67 entries

0.

5

Fig. 14 shows a s c a t t e r p l o t of t h e 4

azimuthal a n g l e and t h e r a p i d i t y of a l l j e t s having an energy g r e a t e r t h a n 20 GeV.

The d e n s i t y of e v e n t s i s uniformly

3

d i s t r i b u t e d o v e r t h e acceptance of t h e a p p a r a t u s .

2 1 -

Figure 14

-

^{a .}

5

. . . _. . .

-

_{. - .}

_z •

. . .

^{0 .}

.

^{00 .*}

-

. . .. . .

^m

_.. . .

- _.. . . . . .

.

^a

JOURNAL DE PHYSIQUE

TWO JET EVENTS

We have observed 65 events with two clusters of at least 10 GeV. From the azimu- thal separation of the two j~ets (Fig. 15a) we observe that the two jets tend to be back-to-back in azimuth

.

The same observation applies to events with one jet in the central calorimeter and one jet of more than 5 GeV transverse energy measured in the forward or backward calorimeter (Fig. 15b).

A @ (degrees)

Figure 15 (b)

In Fig. 16 we show the mass distribution of the two jets of highest transverse energy contained in the sample of 65 events. To avoid the distortions due to the transverse energy cut we only show the masses beyond 30 G~v/c'. There are 5 events with masses greater than 50 G~V/C' and the highest mass observed is 140 G~v/c*.

Figure 16

In conclusion, we have observed that events with a transverse energy in excess of

-

60 GeV in a rapidity interval of -i1 unit have a dominant two-jet structure at

V '

S = 540 GeV, and we have measured the inclusive jet cross-section.