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Observability of R-Hadrons at the LHC

Andrea Rizzi, ETH Zuerich, CMS Collaboration

Moriond QCD, 17-24/03/2007 O u t l i n e

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● Introduction to R-hadrons ● R-hadrons hadronic interactions ● LHC production ● Detector signature ● Trigger ● Slow particles mass measurement H e a v y S t a b l e C h a r g e d P a r t i c l e s

3 ● Some theories extending the SM predict new HSCP (hep-ph/0611040) – Gauge Mediated SuSY breaking => stau – Split Supersymmetry

● very high mass for scalars ● gluino is long lived – Other models: GDM, AMSB, extrDim, stable stop, 4th gen fermions

● To be clear: – heavy = mass in 100 GeV – few TeV region – stable = ctau > few meters (stable for LHC exp)

● LHC detectors are not designed for such particles I n t r o d u c t i o n t o R - h a d r o n s

● If new heavy stable particles are coloured they will form new 4 hadrons

● gluinos or stops are example of such particles

● New hadrons formed with new SuSY particles are called R-hadrons

● Several different states can be formed : Gluino: stop: baryons ~gqqq baryons ~tqq antibaryons ~g qbar qbar qbar antiparyons ~tbar qbar qbar mesons ~g q qbar mesons ~t qbar glue­balls ~g g antimesons ~tbar q ● Some are charged, some are neutral

● The charge can change in hadronic interactions with matter while crossing the detector (by “replacing” the light quarks bounded to the heavy parton)

● Simulation of hadronic interactions is crucial to understand LHC detectability H a d r o n i c i n t e r a c t i o n s

● Some models have been proposed to understand R-hadrons / matter interaction 5

● The common starting points are (see Moriond QCD 2005 talk A.Kraan):

– the heavy spectator is mainly a kinetic energy reservoir and does not actually take part in the interaction – the pion-nucleon interaction is used for cross section estimation (12mb per u/d quark and 6mb per s quark) [hep-ex/0404001]

● Results (energy loss per hadronic interaction) of different model are similar: hep­ph/0611040

hep­ph/0612161

Few GeV per interaction (tot Ekin > 100 GeV) => not showering in calorimeters A variant of the interaction model proposed in [hep­ex/0404001] has been implemented in Geant4 to allow LHC experiment simulations [hep­ph/0612161] H a d r o n i c I n t e r a c t i o n s I I

An interesting effect is predicted for r-hadrons suffering several hadronic 6 interactions (e.g. crossing calorimeters):

● The probability that after an interaction an R-baryon, R-meson or R- antibaryon emerge depends on the heavy parton type

● Since matter is build of quarks we have gluino that the following processes are suppressed: stop – gluino-baryons to gluino-mesons antistop

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– stop-baryons to stop-mesons 1

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– 6 antistop-mesons to antistop-baryons 0

/

h

● p The gluinos and the stop will tend to go to ­

p

R-baryons while the antistop will go to R- e

h meson

● Different energy release for baryon/mesons (different cross sections) => distinguish stop/antistop vs gluino L H C R - h a d r o n p r o d u c t i o n

● Up to now only stable gluino and stau scenarios have been fully simulated in LHC 7 experiment environment (stop hadrons are next)

● The fraction of neutral gluino-gluon ball is a free parameter (10% in our samples)

● The main process for gluino production at LHC gg->~g~g Cross sections @LHC: R.Mackeprang ~g Mass (GeV) Xsec 100 50nb 300 0,3nb 600 5pb 1000 0,1pb

Velocity and momentum distributions at generator level D e t e c t o r s i g n a t u r e

Summary of key features for LHC experiments: 8

● Long lived particle (c > few meters)

● Charged or neutral at production

● Suffering hadronic interaction (not showering) with ~1GeV energy loss per interaction (not stopped by calorimeters!)

● Low velocity: – high ionization – high time of flight

● Charge can change in every hadronic interaction – several interactions occur in calorimeters or other absorbers – charge in the inner tracking detector uncorrelated with charge in muon systems (outside calorimeters) – charge can change even inside the muon system C MS d e t e c t o r s i g n a t u r e

9 12.5ns late @  =0.5 , = 0

● The signature is similar to the one of muons ● Standard Model and cosmic muons are the main backgrounds E f f e c t s o n d e t e c t a b i l i t y

10 Features that we can exploit:

● Muon like detector crossing -> muon trigger

● High time of flight can be measured -> mass reconstruction

● High ionization: dE/dX can be measured -> mass reconstruction

● Charge flip -> typical feature / distinguish from e.g. stable stau Experimental problems (detectors not designed for it):

● High time of flight -> out of bunch crossing (25ns) reconstruction

● High time of flight -> trigger time cuts

● High time of flight -> signal attenuation in tracker

● Neutral in inner tracker -> no muon/tracker tracks association

● Charge flipping in muon system -> lower reconstruction efficiency

● Charge flipping in muon system -> bad momentum measurement T r i g g e r

● Muon triggers are natural candidates. Typical 11 thresholds (from CMS) are:

– single muon Pt > 37 GeV – single isolated muon Pt > 19 GeV – double isolated muon Pt > 7 GeV – additional constraint: delay wrt a muon t<12.5ns (i.e. beta >~0.6 for CMS)

● Muon calorimetric isolation can be an issue if hadronic interactions produce large calorimetric energy deposit

● Other triggers (jet, MET) can be used in model dependent/optimized analysis Main SM backgrounds are due to ● Muon only trigger are more model ●QCD (c/b ­> muon + X) independent (gluinos, stop, stau, ....) ●W production ●Drell­Yan muons ● A final trigger efficiency ~ 15% is predicted ● with CMS full simulation for gluinos with top production M=600 GeV using standard triggers A n a l y s i s a n d m a s s m e a s u r e m e n t

● By reconstructing the mass the standard model backgrounds (high Pt 12 muon production) can be suppressed

● The mass is also a free parameter in theories predicting new stable charged particles

● Mass can be determined measuring velocity and momentum

● The velocity can be measured with two independent methods: – t.o.f. measurement in muon drift ­2 tubes – dE/dX measurement in tracker or calorimeters

● The momentum can be measured with inner tracker detector or with muon spectrometers – charge flipping makes CMS muon measurement not very reliable – neutral in tracker R-hadrons can be measured only in muon spectrometer B e t a a n d ma s s r e s o l u t i o n

13 ● Preliminary investigation in CMS Golden beta region – tracker dE/dX 0.6 < beta < 0.85 – Drift Tube time information ● best beta resolution (better than 5%) ● Full simulation (G4 model) used ● highest trigger efficiency ● Similar t.o.f. measurement techniques used for stable stau (GMSB)

BETA (DT and Tk combined) MASS Ma s s p l o t

14 ● Backgrounds are removed by requiring low beta measurement both from dE/dX and from T.O.F.

● Different backgrounds for the two measurements

● Minimum Pt cut of 150 GeV is also applied

600 GeV L=30pb­1 L= 0.5 fb­1 C o n c l u s i o n s

15 ● LEP and Tevatron searches excluded stable gluinos up to ~300 GeV

● R-hadrons (as well as other Heavy Stable Charged Particles) can be observed at LHC exploiting their unique signature

● Trigger efficiency can still be optimized

● Mass measurement is possible using t.o.f. and ionization

● Detailed studies (reach with first data, etc..) going to be finalized this year

● Other models with similar signature can be studied in detail as well: – long lived stop – long lived stau – long lived KK states – long lived 4th generation fermions 16

BACKUP s - t a u v s r - h a d r o n s

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MUONS STAU RHADRONS T r a c k e r d E / d X

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100 GeV 600 GeV

300 GeV

3­4% beta resolution for beta in range [0.6­0.8] d E / d X c a l i b r a t i o n ( K & p ) 19

Calibration of module response can be obtained using K and / or protons S i g n a l a t t e n u a t i o n

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TOB TEC

TIB / TID

Fraction of signal lost @beta=0.5

●Too slow particle signal is attenuated because of the tracker electronics pulse shape. ●The dE/dX estimators weight more the lower signal hits (to cut Landau tails) ●Dedicated tool should be implemented to recover the effect D r i f t T u b e

21 ● R-hadron are late, so RecHit is further from wire than actual energy deposit Several ways to estimate tof:

● compute average distance of RecHit wrt reconstructed track (doesn't work if left and right hits are not the same number)

● make a fit where “tof” is a free parameter and hit position is function of “tof” (i.e. find the time of realignment of the hits) D r i f t t u b e r e s o l u t i o n

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t.o.f. from refitting segments in layers with T0 as free parameter. Minimized function is:

where P r e l i mi n a r y s t u d y

23 P t c u t s u s e d i n s e l e c t i o n

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S/N can be improved with a Pt cut (background rapidly decreasing)

– 50 GeV (100 GeV gluino) – 80 GeV (152 GeV stau) – 150 GeV (300 GeV gluino) – 200 GeV (600 GeV gluino)

W­> b­jets pt>170GeV S p l i t S U S Y

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