Open charm reconstruction at mid-rapidity in the ALICE Experiment at LHC

Transcription

1 Open charm reconsrucion a mid-rapidiy in he ALICE Experimen a LHC Universia and INFN di Torino, ITALY dash@o.infn.i Open charmed mesons produced in ulra-relaivisic nuclear reacions are among he mos direc probes o invesigae he medium produced in hese collisions. I is imporan o measure he producion of as many charmed hadrons as possible, such as D, D +, D s, D and Λ c, because he measuremen of heir relaive yield can provide informaion on he hadronizaion mechanism. I will also reduce he sysemaic error on he oal charm cross-secion. The ALICE experimen a he LHC is designed o perform such measuremens a mid-rapidiy by means of is barrel racking deecors. The sudy on exclusive reconsrucion of D, D + and D + mesons hrough heir hadronic decay channels will be presened for pp collisions a 7 TeV. The firs resuls on D-meson specra and he D meson raios will also be shown. XLIX Inernaional Winer Meeing on Nuclear Physics January 211 BORMIO, Ialy Speaker. c Copyrigh owned by he auhor(s) under he erms of he Creaive Commons Aribuion-NonCommercial-ShareAlike Licence. hp://pos.sissa.i/

2 1. Inroducion The main moivaion for sudying heavy-ion collisions a high energy is he sudy of Quanum Chromodynamics (QCD) in exreme condiions of high emperaure and high energy densiy [1]. These collisions creae nuclear maer of high energy densiy over an exended volume, allowing QCD predicions o be esed in he laboraory. A high emperaure and densiy, QCD predics a phase ransiion from nuclear maer o a sae of deconfined quarks and gluons known as he quark gluon plasma (QGP). Heavy, long lived quarks, i.e. charm and boom, are frequenly sudied boh as ess of QCD and as probes for oher physics aspecs wihin and beyond sandard model. They are sensiive probes of he medium produced in nucleus-nucleus collisions. They are predominanly produced by iniial hard paron scaering processes and hus, he iniially produced heavy quarks experience he full collision hisory. Boh he iniial and final sae effecs of he medium are expeced o ener in he producion and propagaion of heavy quarks. Perurbaive QCD calculaions a fixed order plus nex o leading log (FONLL) describe well he cross secion of charm and boom paricles. The world daa (single elecrons from STAR/PHENIX experimens a RHIC and D mesons from CDF) are described by he upper limi of he FONLL calculaions [2, 3, 4, 5]. The measuremen of open charm in pp collisions a LHC energies is imporan o es he predicions of pqcd calculaions in he new energy domain. Furhermore, he yields of open charm will ac as reference for he sudy of in-medium effecs in he sysem produced in Pb Pb collisions. Beyond he oal charm cross-secion, open charmed mesons can be used o probe he medium produced in relaivisic heavy-ion collisions. The medium-induced suppression of high p paricles in nucleus-nucleus collisions can be characerized by he nuclear modificaion facor, R AA. For example, he suppression of R AA from semi-leponic D and B-meson decay in cenral Au Au collisions was expeced o be less severe for charm quarks han for ligher flavors because of he dead cone effec (where gluon radiaion is suppressed because of desrucive inerference for angles less han m/e, m and E are he mass and energy of he quark [6]). Consequenly a decrease of high p suppression is expeced from ligh o charm o boom quarks. In conras, a srong suppression of heavy flavour decay elecrons was discovered. Recen experimenal resuls from RHIC (STAR and PHENIX) experimen observed large yield suppression of heavy flavor decay elecrons in cenral Au Au collisions, consisen wih subsanial energy loss of heavy quarks in dense maer [4, 7, 5]. The suppression is larger han ha expeced from radiaive energy loss calculaions, suggesing ha oher processes conribue significanly o heavy quark energy loss. This unique sensiiviy o he energy loss mechanisms make he measuremen of heavy quark suppression an essenial componen of he sudy of dense maer. This paper is organized as follows. The secion 2 conains a brief descripion of he ALICE experimenal seup. The secion 3 describes he analysis sraegy followed by some preliminary resuls in secion 4. The resuls are summarized in secion The ALICE Experimenal seup A Large Ion Collider Experimen, ALICE is a general-purpose heavy ion deecor a he CERN LHC designed o address he physics of srongly ineracing maer and he quark gluon plasma a exreme values of energy densiy and emperaure in nucleus nucleus collisions. The ALICE 2

3 Figure 1: The ALICE experimenal seup. See ex for deails. seup consiss of a cenral barrel covering he full azimuhal angle in η <.9 and several forward sysems. The cenral barrel sysem is embedded in a magneic field, B of.5 T. The cenral deecors are, in he order from he ineracion verex o he ouside, he Inner Tracking Sysem (ITS) wih six layers of silicon deecors, he Time Projecion Chamber (TPC), which is he main racking deecor, he Transiion Radiaion Deecor (TRD) for elecron idenificaion and he Time Of Fligh deecor (TOF), which exends he momenum range of paricle idenificaion (PID) up o 4 GeV/c. The Time-Projecion Chamber (TPC) is he main racking deecor of he cenral barrel and is opimized o provide, ogeher wih he oher cenral barrel deecors, charged-paricle momenum measuremens wih good wo-rack separaion, paricle idenificaion, and verex deerminaion. Two oher deecors wih reduced accepance complemen he cenral barrel: he High-Momenum Paricle Idenificaion Deecor (HMPID) consising of an array of ring-imaging Cherenkov couners and he Phoon Specromeer (PHOS), which is an elecromagneic calorimeer. The las addiion o he ALICE design is he Elecromagneic Calorimeer (EMCal), dedicaed o he physics of jes and high-p phoons. The EMCal also measures he ligh mesons, such as π and η, decaying ino phoon. More echnical deains on he design of he ALICE deecor can be found in [8]. A schemaic of he ALICE experimenal seup is shown in Figure 1. The main deecors conribuing o he sudy of charm hadrons are he ITS, TPC and TOF. The ITS wih is impac parameer resoluion of around 75 µm a 1 GeV/c provides informaion abou he secondary verex, which is crucial for physics sudies dependen on displaced verex opologies. The resoluion of impac parameer as a funcion of p is shown in Figure 2. The TPC and TOF signals are used for paricle idenificaions as a funcion of ransverse 3

4 resoluion [µm] ALICE Performance 3/9/2 rφ d 15 5 Daa (LHCc period) MC, residual misal. 1 p [GeV/c] Figure 2: Transverse impac parameer resoluion as a funcion of p. momenum. The paricle idenificaion in he TPC is based on he specific ionizaion energy loss (de/dx) of each paricle in he fill gas of he TPC. This is shown in Figure 3 (Upper panel) as a funcion of momenum. The paricle idenificaion a higher p is assured by adding he informaion from he TOF deecor where paricles are idenified by measuring simulaneously heir momenum and velociy. The velociy of he paricle is obained from he reconsruced fligh pah and he ime of fligh. This is shown as a funcion of he momenum in Figure 3 (Lower panel). The bands corresponding o he charged pions, kaons and proons are clearly visible. 2.1 Trigger Selecion The main rigger selecion is he minimum-bias, MB, which is defined by he inclusion of a leas one signal in eiher of he wo scinillaor hodoscopes, he V deecors, posiioned in he forward and backward regions of he ALICE experimen or in he silicon pixel deecor, SPD. This rigger selecs evens wih a leas one charged paricle in he region of η <8. and is sensiive o abou 95% of he pp inelasic collisions. Mos of he daa is shown for 1.1X 8 minimum-bias evens. 3. Reconsrucion of Charmed mesons Open charm hadrons are no direcly observable as hey decay far oo quickly (wih cτ values of few hundred microns) o survive ino he deecors. In order o sudy open charm mesons, hey mus be reconsruced via heir hadronic decay channels. The selecion sraegy is based on he displaced verex opology of he D meson decay. A single rack is required o have a leas 7 spaial poins in TPC and should have a leas 3 clusers in he ITS. All racks ha are idenified as pions and kaons are combined ino riples or pairs wih he correc combinaion of charge signs. 4

5 Figure 3: (Upper panel) The measured energy-deposi of charged paricles versus he rack momenum. The lines correspond o he ALEPH parameerizaion of he Behe-Bloch curve. (Lower panel) The paricle velociy as a funcion of he momenum measured by he TOF deecor. Then an invarian mass specrum of all he candidae combinaions is calculaed o exrac he raw yield. One has o find some properies of he candidae pairs/riples ha allow disinguishing hem from he combinaorial background and applying cus on hese variables. The ask is o find opimum uning for he cu values. The disance beween primary verex of he collision and decay verex of he meson is a good example of such a variable because he ITS deecor of ALICE measures his disance wih a good accuracy separaing paricles coming direcly from primary verex from hose produced in secondary decays of shor-lived mesons. 5

6 3.1 Selecion Sraegy Figure 4: Kπ invarian mass specra for D candidaes in differen p bins. The firs sep of he selecion is applied a he level of single reconsruced racks before combining hem ino pairs/riples. A his sage, he selecion is based on ransverse momenum, p and he impac parameer, d. The racks ha have passed he previous selecions are combined ino pairs wih opposie charge sign. The verex of hese pairs of racks is calculaed using he sandard ALICE verexing algorihm. For he D reconsrucion in he channel D K + π (BR = 3.8±.5 %), he relevan cu variables are he produc of he impac parameers of he kaon and he pion rack, cosine of he poining angle, he difference beween he mass of D meson and he invarian mass of Kπ pair and he disance beween he found verex and he primary verex. The invarian mass disribuions obained in various p T bins for he D meson is shown in Figure 4. For he D + (D + D + π +, BR = 67.7±.5 %) meson, he invarian mass calculaion is performed wih a D candidae and a sof pion. The background is esimaed using he D candidaes wih invarian mass falling in he side-bands (4σ - 8σ range) of he D mass region. The corresponding invarian mass disribuions of M(Kππ) - M(Kπ) can be seen in Figure 5. 6

7 Figure 5: Kππ - Kπ invarian mass difference specrum for D candidaes for various p bins. The reconsrucion of D + mesons in he channel D + π + K π + (BR = 9.2±.5 %) requires furher cus on he candidae riples in order o enhance he signal o background raio. The cus are namely he disance among he racks and found verex, cos(θ poin ) where θ poin is he poining angle (he angle beween he direcion of he reconsruced D + momenum in he bending plane and he line connecing he primary and secondary verices), he sum of he squares of he hree racks impac parameers wih respec o he primary verex (his cu is based on he fac he signal racks are coming from he displaced secondary verex), disance beween he primary and secondary verices and he qualiy of he found secondary verex (based on he dispersion of hree racks around he verex). Afer applying hese rack pair and riple selecions, an invarian mass of he πkπ riple is calculaed for raw yield exracion. This is shown in Figure 6. The invarian mass disribuion of oher promising open charm signals seen in rare decay channels of D + s K+ K π +, Λ + c π pk + and D π + π π + K is depiced in Figure Efficiency Correcions The raw signal yield exraced from he invarian mass disribuion is a small fracion of he 7

8 Figure 6: Kππ invarian mass specra for D + candidaes in differen p bins. oal number of D mesons produced due o deecor accepance, verexing and racking inefficiencies and various selecion cus on racks. The D meson invarian yields as a funcion of p are hen correced for deecor accepance and reconsrucion efficiency. The efficiency facors were exraced from Mone Carlo simulaions wih deailed descripion of deecor response and experimenal condiions for pp collisions. The variaion of he efficiency wih p for D, D + and D + mesons is depiced in Figure 8. A LHC energies, a significan fracion of D meson comes from he decay of B mesons. This conribuion mus be esimaed and subraced from he raw yield in order o obain he producion cross secion for promp charm. In he presen analysis, he correcion facor for boom quark feed down is esimaed using he boom producion cross-secion predicions of FONLL calculaions wih deecor simulaion. This esimaion will be checked wih he daa driven mehod developed by he CDF Collaboraion [3] wih full available saisics. The sysemaic uncerainies on he D meson cross-secions were esimaed as follows: (1) An uncerainy on raw yield exracion, which was evaluaed by repeaing he fi, in each p inerval, in a differen mass range and wih a differen funcion describing he background. (2)The uncerainies due o efficiency correcion 8

9 Figure 7: The invarian mass specra showing D s, Λ c, and D (D π + π π + K ) signals. facors and he beauy feed down facors. The oal sysemaic uncerainies amouned o 2-25 %. No significan p dependence of he sysemaic uncerainies is observed for p > 3 GeV/c. The sysemaic uncerainies arising from he differen sources for he hree mesons are shown in Figure Resuls Afer he raw yield exracion and correcions for various facors menioned in he previous secion, he D meson cross-secion is calculaed using he following formula: dσ dp = 1 y < y(p ) 1 BR 1 f c (p ) ND Raw (p ) y < y(p ) ε c N o σminbias o (4.1) MinBias 9

10 Figure 8: (Upper panel) D and D + reconsrucion and selecion efficiencies versus p. (Lower panel) D + reconsrucion and selecion efficiencies versus p. where N D Raw (p ) is he raw exraced yield, ε c, he efficiency correcion facor, f c, he correcion facor for he beauy feed-down and σminbias o is he oal pp inelasic cross-secion for minimum bias evens. The BR corresponds o he branching raio of he decay channel sudied while y(p ) is he rapidiy accepance. N o MinBias is he oal number of minimum-bias evens. Figure shows he preliminary p differenial cross secion for D mesons compared wih he heoreical predicions from FONLL and GM-VFNS. The daa are well described by boh calculaions wihin heoreical uncerainies. Figure 11 illusraes a comparison of he D meson raios, D /D + and D /D + for p > 3 GeV/c, which are found o be in good agreemen wih previous measuremens a low energies [11, 3]. 5. Summary and Oulook The firs open charm producion measuremens performed by he ALICE deecor in pp colli-

11 Relaive Error.4.2 pp, - D K π + s = 7 TeV, 1.4 nb ALICE Performance 3/11/2 Toal (excl. norm.) Normalizaion (%) Feed-down from B Tracking efficiency Branching raio Yield exracion Cus efficiency PID efficiency MC p shape D = D Relaive Error D K π + π + pp, s = 7 TeV, 1.4 nb ALICE Performance 3/11/2 Toal (excl. norm.) Normalizaion (%) Feed-down from B Tracking efficiency Branching raio Yield exracion Cus efficiency PID efficiency MC p shape p [GeV/c] Relaive Error pp, * D D π + s = 7 TeV, 1.4 nb ALICE Performance 15/3/ p [GeV/c] p [GeV/c] Toal (excl. norm.) Normalizaion (7%) Feed-down from B Tracking efficiency Branching raio Yield exracion Cus efficiency PID efficiency MC p shape Figure 9: D, D + and D + mesons sysemaic errors corresponding o he preliminary cross-secion resuls shown in Figure. sions a 7 TeV a LHC is presened a cenral rapidiy. The excellen racking, verexing and paricle idenificaion capabiliies of he ALICE deecor allow he high precision measuremen of he D- meson cross secion hrough is hadronic decay channels. The preliminary resuls on he producion cross-secion of he D, D + and D + mesons are well described by pqcd calculaions wihin uncerainies. These measuremens will provide useful reference for he upcoming measuremens in heavy ion collisions a he LHC. References [1] J. Adams e al., STAR Collaboraion, Nucl. Phys. A 757 (25) 2. [2] M. Cacciari, P. Nason, and R. Vog, Phys. Rev. Le. 95 (25) [3] D. Acosa e al., CDF Collaboraion, Phys. Rev. Le. 91 (23) [4] J. Adams, Phys. Rev. Le. 91 (23) [5] A. Adare, Phys. Rev. Le. 98 (27)

12 Open Charm Reconsrucion a mid-rapidiy in ALICE Experimen a LHC µb/gev/c 3 2 *+ - D D π + K π + π + pp, s = 7 TeV, 1.4 nb µb/gev/c 3 2 pp, - D K π + s = 7 TeV, 1.4 nb dσ / dp y <.5 1 sa. unc. sys. unc. FONLL GM-VFNS ALICE Preliminary GeV/c σ MB p = 62.3 mb ± 7% global norm. unc. (no shown) PWG3-Preliminary-26 dσ / dp y <.5 1 sa. unc. sys. unc. FONLL GM-VFNS ALICE Preliminary GeV/c σ MB p = 71.4 mb ± % global norm. unc. (no shown) Figure : Preliminary p T differenial cross secion for D +, D and D + mesons, compared o predicions from FONLL([9]) and GM-VFNS([]) calculaions. [6] Y.L. Dokshizer and D.E. Kharzeev, Phys. Le. B 519 (21) 199. [7] J. Schaffner-Bielich, Phys. Rev. Le. 5 (2) [8] K. Aamod e al., ALICE Collaboraion, JINST 83 (28) S82. [9] M. Cacciari e al., Privae Communicaion. [] B.A. Kniehl e al., Privae Communicaion. [11] S. Chekanov e al., ZEUS Collaboraion, JHEP7 (27) 74. PWG3-Preliminary-24 12

13 Raio ALICE Preliminary + full: D /D open: D /D + - ALICE e e *+ ZEUS H1 CDF error bar: sa. only sa.+sys. sa.+sys. sa.+sys. sa. only +b.r. PWG3-Preliminary-22 Figure 11: D /D + and D /D + raios (in he range 3< p < 12 GeV/c) compared o previous experimens a lower collision energies. 13