, h, ti- d tml V 0. A , :  e.h a 4 ery ted. , om r, and  ering the ond mid- vy ion d v

t design of ucleus col- will b e in- . tmodeis e 2  /Welc ,co en y 5 GIOTOU : ed in hea y-ev ysics b ey y CZYK al 2 ucleus{n , S.N. FILIPPO thens, Hel las. ANA ussia. e, Poland. t-b c ery small acceptance T THE LHC AR ussia. en l mo del describing the g ob jects in the v terv OR THE us collisions is presen OD gica angelis/castor , A.B. KUREPIN ailable phase space whic L ions at the LHC, extends c uon detector yin  v ow, R 3. Finally a small-ap erture , A.D. P : ucle olo V ted. a otvino, R ow, Poland. h/ 3 YUBSKY Pb teresting ph .c tation in the presen ac estigating n ed the idea of a dedicated for-  1 v ,Z.W 7.2 in an ev ern b

O AND ted for hadron and photon iden olv ucleus-n  ch, Mosc e.c  will also b e installed on one side and y trigger. The v ar eV for 3 3

al University, Kielc UR : A ard detector to b e emplo ese ANSKI gic  tify deeply p enetratin tralit gy Physics, Pr al 2 erse segmen OMANOLAKIS ar Physics, Cr An additional m ar R ttp://hom dago e it suitable for this purp ose only , M.YU. BOGOL : , YU.V. KHARLO

hep-ex/9901038 28 Jan 1999 terv t. Symp. on Multiparticle Dynamics, 6-11 Sep. 1998, Delphi. b VR 1 b . STEF er the pseudorapidit ery forw ARD DETECTOR F

S v y detector h is aimed to measure the hadronic and photonic h is aimed at in ,P tauro reball in n yin W c TKE j ulations of the exp ected resp onse of the calorimete

ted out that there is in j vide the cen OR ,G.MA , whic d 1 al VSKY ultiplicit A h phase space 5.6 yofaCen ation for a v Y ,J.BAR TION OF CENT teraction and to iden terv Institute of Nucle a ted at XXVI I I In YSZ-DZIADU b Institute for Nucle on t, to pro on-ric d rom these considerations ev Institute for High Ener Already at the early stages of the preparation of the ALICE yin c AD : L ar and Particle Physics Division, University of A .F 7 Institute of Physics, Pe Further information at: h Presen S.A. SADO : 5 e  ysics motiv ; tofanin 4 er photon m er the pseudorapidit E. G  ard, bary w k of longitudinal and transv Nucle y ten =8 tro ducti v CASTOR: A F a IN NUCLEUS{NUCLEUS COLLISIONS A j A.I. MAEVSKA This constitutes only a small part of the total a In forw describ ed. Results of sim in particular to the passage of strangelets, are presen The CASTOR detector whic collisions at the CERN LHC isformation and discussed. deca A phenomen con The ph

j IDENTIFICA teraction p oin A.L.S. ANGELIS cation only in thepseudorapidit limited angular region around mid-rapidit lisions at the LHC, will b e fully instrumen The ALICE detector 1 pre-sho stalled on one side and will co will co zero degree calorimeter is alsoin foreseen, at a distance of ab out 100 m from the at the designto b eam energy ofprop osal 2.75 some A of T us p oin the zero degree calorimeter mak and lac rapidit

6;7

ward detector, CASTOR , to provide physics information on hadrons and

photons emitted in the fragmentation region. At LHC Pb-ion energies this

region of phase space extends b eyond j j = 5 or 6, dep ending on the e ec-

tive baryonic stopping p ower which is di erent for the various Monte-Carlo

mo dels employed. In this region of extremely high numb er density

one can exp ect to discover new phenomena related to the high baryochemical

p otential, in particular the formation of Decon ned Quark Matter DQM,

which could exist e.g. in the core of neutron stars, with characteristics di er-

ent from those exp ected in the much higher temp erature baryon-free region

around mid-rapidity,

17

The LHC, with an energy equivalentto10 eV for a moving im-

pinging on one at rest, will b e the rst accelerator to e ectively prob e the

highest cosmic ray energy domain. Cosmic ray exp eriments have detected

numerous most unusual events whose nature is still not understo o d. These

events, observed in the pro jectile fragmentation rapidity region, will b e pro-

duced and studied at the LHC in controlled conditions. Here we mention

8

the \Centauro" events and the \long- ying comp onent". Centauros exhibit

relatively small multiplicity, complete absence or strong suppression of the

electromagnetic comp onent and very high hp i. In addition, some hadron-rich

T

events are accompanied by a strongly p enetrating comp onent observed in the

9;10

form of halo, strongly p enetrating clusters or long-living cascades, whose

11;12

transition curves exhibit a characteristic form with many maxima .

2 A mo del for the pro duction of Centauro and Strangelets

A mo del has b een develop ed in which Centauros are considered to origi-

nate from the hadronization of a DQM reball of very high baryon density

3

 & 2fm  and baryochemical p otential  >> m , pro duced in ultra-

b b n

13;14;15

relativistic nucleus{nucleus collisions in the upp er atmosphere . In this

mo del the DQM reball initially consists of u, d quarks and gluons. The very



high baryochemical p otential prohibits the creation of uu  and dd quark pairs

b ecause of Pauli blo cking of u and d quarks and the factor exp  =T for

q



u and dantiquarks, resulting in the fragmentation of gluons into ss pairs

predominantly. In the subsequent hadronization this leads to the strong

suppression of pions and hence of photons, but allows kaons to b e emitted,

carrying away strange antiquarks, p ositivecharge, entropy and temp erature.

This pro cess of strangeness distillation transforms the initial quark matter

reball into a slightly strange quark matter state. In the subsequent decay

and hadronization of this state non-strange and strangelets will b e

formed. Simulations show that strangelets could b e identi ed as the strongly 2

p enetrating particles frequently seen accompanying hadron-rich cosmic ray

5;16

events . In this manner, b oth the basic characteristics of the Centauro

events small multiplicities and extreme imbalance of hadronic to photonic

content and the strongly p enetrating comp onent are naturally explained. In

table 1 we compare characteristics of Centauro and strongly p enetrating com-

p onents strangelets, either exp erimentally observed or calculated within the

context of the ab ove mo del, for cosmic rayinteractions and for nucleus{nucleus

interactions at the LHC.

Table 1. Average characteristic quantities of Centauro events and Strangelets pro duced in

Cosmic Rays and exp ected at the LHC.

Centauro Cosmic Rays LHC

Interaction \Fe + N " Pb + Pb

p

s & 6.76 TeV 5.5 TeV

Fireball mass & 180 GeV  500 GeV

y  11 8.67

pr oj

4

 10 ' 300

9.9 ' 5.6

cent

 1 ' 0.8

cent

1.75 GeV 1.75 GeV *

T

9 9

Life-time 10 s 10 s *

Decay prob. 10  x  10 km 1 x  1m

Strangeness 14 60 - 80

f S/A ' 0.2 0.30 - 0.45

s

Z/A ' 0.4 ' 0:3

Event rate & 1 ' 1000/ALICE-year

\" Cosmic Rays LHC

Mass ' 7 - 15 GeV 10 - 80 GeV

Z . 0 . 0

f ' 1 ' 1

s

+1:2 +1:6

str cent cent

* assumed

3 The design of the CASTOR detector

With the ab ove considerations in mind wehave designed the CASTOR Cen-

tauro And STrange Ob ject Research detector, to b e placed in the fragmen-

tation region. CASTOR will cover the pseudorapidityinterval 5:6   7:2.

Figures 1, 2, 3 and 4 depict the hadron and photon pseudorapidity distribu-

tions as predicted by the HIJING Monte-Carlo generator for central Pb + Pb 3

collisions at the LHC. The upp er plots show distributions of multiplicity while

the lower plots show distributions of energy ux. Figures 5 and 6 depict the

baryon numb er pseudorapidity distributions as predicted by the HIJING and

VENUS Monte-Carlo generators for central Pb + Pb collisions at the LHC.

The acceptance of CASTOR is sup erimp osed on each plot. As can b e seen

from the plots, while CASTOR will receive a mo derate multiplicity, its p osi-

tion has b een optimized to prob e the maximum of the baryon numb er density

and energy ow and to identify any e ects connected with these conditions.

1400

1200 1000

1000 800

800 600 600 400 400

200 200

0 0

-10 -8 -6 -4 -2 0 2 4 6 8η 10 -10 -8 -6 -4 -2 0 2 4 6 8η 10

Figure 1. Average photon multiplicity pseu- Figure 2. Average charged particle multi-

dorapidity distribution obtained from 50 plicity pseudorapidity distribution obtained

central Pb + Pb HIJING events. from 50 central Pb + Pb HIJING events. ] ]

25

TeV 5 TeV [ [

20 4

3 15

2 10

1 5

0 0 -10 -8 -6 -4 -2 0 2 4 6 8 10 -10 -8 -6 -4 -2 0 2 4 6 8 10

η η

Figure 3. Average electromagnetic energy Figure 4. Average hadronic energy pseudo-

pseudorapidity distribution obtained from rapidity distribution obtained from 50 cen-

50 central Pb + Pb HIJING events. tral Pb + Pb HIJING events. 4 14 10 12

8 10

6 8 6 4 4 2 2 0 0

-2 -2

-10 -8 -6 -4 -2 0 2 4 6 8η 10 -10 -8 -6 -4 -2 0 2 4 6 8η 10

Figure 5. Average net baryon numb er pseu- Figure 6. Average net baryon numb er p deu-

dorapidity distribution obtained from 50 dorapidity distribution obtained from 47

central Pb + Pb HIJING events. central Pb + Pb VENUS events.

6;7

Aschematic view of the CASTOR detector is shown in gure 7. It

is optimized to measure the hadronic and photonic content ofaninterac-

tion, b oth in energy and multiplicity, and to search for strongly p enetrating

 

particles. It will consist of a Si pad charged particle multiplicity detector

followed bya Si pad pre-shower photon multiplicity detector and of a longi-

tudinally segmented tungsten/quartz- bre calorimeter with electromagnetic and hadronic sections.

Pre - shower Silicon FPHMD Wafers

CALORIMETER

Beam pipe

Beam

Inner tube FCPMD

H EM Absorber 2 Absorber 1

( 1XoW ) ( 2Xo W )

Figure 7. Schematic representation of the CASTOR detector. 5



The multiplicity detectors have the form of annular discs of ab out 129 mm

outer and 26 mm inner radius, constructed in two half-rings to b e p ositioned

around the b eam pip e.

The calorimeter is made of layers of active medium sandwiched b etween

tungsten absorb er plates. The active medium consists of planes of silica bres

and the signal is the Cherenkov light pro duced as they are traversed by the

charged particles in the shower. The bres are inclined at 45 degrees rela-

tive to the incoming particles to maximize light output. The calorimeter is

azimuthally divided into 8 o ctants. Each o ctant is longitudinally segmented

into 80 layers, the rst 8 ' 14.7 X  comprising the electromagnetic section

0

and the remaining 72 ' 9.47   the hadronic section. The light output from

I

groups of 4 consecutive activelayers is coupled into the same light guide,

giving a total of 20 readout channels along each o ctant. More detailed sp eci-

cations are given in table 2 and a general view of the calorimeter, including

its supp ort, is shown in gure 8. Mechanically the calorimeter is a structure

built in two sections, left and right, each consisting of four o ctants connected

together at each corner through b olted plates. The two sections and eachof

the o ctants are self-supp orting. It is envisaged to cut the edges of the absorb er

plates in the azimuthal direction at an angle in suchawayastoavoid cracks

between adjacent mo dules. The outer plates shown in gure 8 one omitted

for clarity constitute the supp ort for the light guides and photomultipliers.

Table 2. CASTOR calorimeter sp eci cations.

Electromagnetic Hadronic

Material Tungsten + Quartz Fibre Tungsten + Quartz Fibre

Dimensions hR i =26mm; hR i =27mm;

in in

hR i = 129 mm hR i = 134 mm

out out

Absorb er Plates Thickness = 5 mm Thickness = 10 mm



at 45  E . thickness = 7.07 mm E . thickness = 14.1 mm

No. Layers 8 72

E . length 56.6 mm ' 14.7 X ' 0.53  1018.1 mm ' 9.47 

0 I I

Quartz Fibre  0:45 mm  0:45 mm

No. QF planes 2 p er sampling 4 p er sampling

Sampling ' 1.84 X ' 0.13 

0 I

Reading Coupling of 4 samplings Coupling of 4 samplings

No. Readings 2 18

No. Channels 2  8= 16 18  8 = 144

QF/W vol. 10 10 6

Figure 8. General view of the CASTOR calorimeter construction including supp ort.

4 Simulation of the CASTOR calorimeter p erformance

Wehave made detailed GEANT simulations of the p erformance of the

C A S T O R calorimeter. Figure 9 shows its resp onse to one central Pb + Pb

HIJING event: gure 9a shows the numberofcharged particles essentially

+

e ;e above Cherenkov threshold in the showers at each activelayer, while

gure 9b shows the corresp onding numb er of Cherenkov photons which are pro duced, captured and propagated inside the bres.

45000 Nc 10 Nc 6 (a) Incident Photons (b) No. of Cherenkov 40000 Nc/GeV = 210 Photons 35000 10 5 30000 25000 4 10 20000 (a) No. of charged 15000 with β≥0.67 3 10 10000 (b) Incident hadrons 5000 Nc/GeV = 129 0 0 20 40 60 80 100 120 140 160 180 200 10 20 30 40 50 60 70 80 [ ]

Sampling Layer E incident GeV

Figure 9. Resp onse of the CASTOR Figure 10. Total numb er of Cherenkov pho-

tons pro duced, captured and propagated in-

calorimeter: a Number of charged par-

side the bres, vs. incident particle energy:

ticles ab ove Cherenkov threshold at each

a For incident photons, b For incident

activelayer, b Numb er of Cherenkov

hadrons. photons pro duced and propagated inside

the bres at each activelayer. 7

Figure 10 shows the total numb er of Cherenkov photons pro duced, cap-

tured and propagated inside the bres, as a function of the incident particle

energy, for incident photons and hadrons from one central Pb + Pb HIJING

event. Ab out 210 Cherenkov photons p er GeV are obtained for incident pho-

tons and 129 Cherenkov photons p er GeV for incident hadrons.

In addition wehave simulated the interaction of a Strangelet with the

16;17

calorimeter material, using the simpli ed picture describ ed in . As an ex-

ample gure 11 shows the resp onse of the calorimeter to one central Pb + Pb

HIJING event, which contains a Strangelet of A =20, E =20 TeV and

str str

quark chemical p otential  =600 MeV energy conservation has b een taken

str

into account. Figure 11a shows the energy dep osition along the o ctant con-

taining the Strangelet, while gure 11b shows the average of the energy de-

p osition along the other seven o ctants.

The study of such simulated events shows that the signal from an o ctant

containing a Strangelet is larger than the average of the others, while its

transition curve displays long p enetration and many maxima structure, such

as observed in cosmic rayevents. ] ] 50 GeV GeV [ [

40 30 (a) Sector with Strangelet 20 E visible 10 (b) Average of other Sectors 0 2 4 6 8 101214161820

R.O. Layer

Figure 11. Energy dep osition in the readout layers couplings of 4 consecutive sampling

layers of the CASTOR calorimeter: a In the o ctant containing the Strangelet, b Average

of the other o ctants.

5 Conclusions

Wehave designed a detector system well suited to prob e the very forward

region in Pb+ Pb collisions at the LHC, where very large baryon numb er den-

sity and energy ow o ccur. Our detector will identify any e ects connected

with these conditions. It has b een particularly optimised to search for sig- 8

natures of Centauro and for long p enetrating ob jects. Wehave develop ed a

mo del which explains Centauro pro duction in cosmic rays and makes predic-

tions for Pb + Pb collisions at the LHC. Our mo del naturally incorp orates

the p ossibility of Strangelet formation. Wehave simulated the passage of

Strangelets through the CASTOR calorimeter and we nd long p enetration

and many-maxima structures similar to those observed in cosmic rayevents.

Acknowledgements

This work has b een partly supp orted by the Hellenic General Secretariat for

Research and Technology ENE 1361.1674/31-1/95, the Polish State Com-

mittee for Scienti c Research grants 2P03B 121 12 and SPUB P03/016/97,

and the Russian Foundation for Fundamental Research grant 96-02-18306.

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